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1
MOBILITY OF HEAVY METALS FROM THE UNIVERSITY OF NIGERIA SEWAGE-
SLUDGE DISPOSAL SITE TO THE SURROUNDING SOILS AND PLANTS
BY
EZEUDO VICTOR C.
REG NO: PG/M.Sc/10/52369
B. AGRIC. TECH. (FUTO)
A DISSERTATION SUBMITTED IN PARTIAL FULFILMENT OF THE
REQUIREMENTS FOR THE AWARD OF THE DEGREE OF MASTER OF SCIENCE
(M.Sc) IN SOIL SCIENCE (SOIL CHEMISTRY AND MINERALOGY)
DEPARTMENT OF SOIL SCIENCE
UNIVERSITY OF NIGERIA, NSUKKA
FEBRUARY, 2014
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CERTIFICATION
EZEUDO, VICTOR C. a postgraduate student of the department of soil science, with the
registration number PG/M.Sc/10/52369, has satisfactorily completed the research work for the
Award of the Master of Science Degree in Soil Science. The work embodied in this dissertation is
original and has not been submitted in part or full for any other diploma or degree of this or any
other institution.
---------------------------------- -------------------------------
PROF. C.L.A. ASADU Dr. P.I EZEAKU
Supervisor Head of Department
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DEDICATION
This work is dedicated to Our Lady of Perpetual Help
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ACKNOWLEDGEMENT
I am highly grateful to God Almighty and to Our Lady of Perpetual Help whose out of
their assistance and protection has made this work a success. I appreciate the support, assistance,
encouragement and the guidance of my supervisor, Prof. C.L.A. Asadu.
I will ever remain grateful and indebted to all the lecturers as well as the administrative
and the laboratory staff of the Department of Soil Science, University of Nigeria, Nsukka. I
acknowledge the assistance of Dr Obalum, Rev Fr. Dr. David Omego and the entire community of
Our Lady of Perpetual Help College Orba whose assistance has made this work success.
Finally, I hereby express my profound gratitude to every member of my extended family
more especially, Mr & Mrs Chris Ezido (KSM), Oluchi, Chinwendu, Ozioma, Ogochukwu,
Ugwumsinachi and my friend Oluchi who had suffered much to ensure that this work comes to a
successful end. May the good Lord reward you abundantly.
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TABLE OF CONTENTS
Content Page
Title page- - - - - - - - - - - i
Certification- - - - - - - - - - - ii
Dedication- - - - - - - - - - - iii
Acknowledgement- - - - - - - - - - iv
Table of Contents- - - - - - - - - - v
List of Tables- - - - - - - - - - viii
List of Plates - - - - - - - - - - ix
Abstract- - - - - - - - - - - x
CHAPTER ONE
INTRODUCTION- - - - - - - - - - 1
CHAPTER TWO
LITERATURE REVIEW- - - - - - - - - 5
2.1 Sewage Sludge - - - - - - - - - 5
2.2 Heavy Metal Toxicity and its Mobility - - - - - - 7
2.3 Effect of pH and Organic Matter in Heavy Metal Mobility - - - 9
2.4 Effect of Sewage Sludge Application in Soil - - - - - 10
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2.5 Sludge-Soil Interaction - - - - - - - - 20
2.6 Implications of Trace Metal in Soil and Plant - - - - - 23
2.7 Plant Factors Influencing Metal Uptake - - - - - - 25
CHAPTER THREE
MATERIALS AND METHODS - - - - - - - 30
3.1 Site description - - - - - - - - - 30
3.2 The Sewage System - - - - - - - - 30
3.3 Field Sampling - - - - - - - - - 35
3.4. Routine Analysis - - - - - - - - - 35
3.5 Determination of the soil content of heavy metals (Pb, Cu, Cd and Hg) - - 36
3.5 Sample Preparations and Digestion of crop plant for Heavy Metal Determination 36
3.6 Data analysis - - - - - - - - - - 37
CHAPTER FOUR
RESULTS- - - - - - - - - - - 38
4.1 Sewage sludge - - - - - - - - - - 38
4.2 Physicochemical properties at various distances away from sewage site - 40
4.3 Physicochemical Properties at Various Soil Depths - - - - 42
4.4 The Simple matrix Distance and Depth on Soil Physicochemical Properties- - 44
4.4.1 Soil pH as influenced by sewage disposal - - - - - 44
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4.4.2 The Organic Matter Concentration and Nitrogen Content of the Soil - - 44
4.4.3 Soil Exchangeable Bases - - - - - - - 44
4.4.4 Total Exchangeable Acidity (TEA) - - - - - - 45
4.4.5 Available phosphorus - - - - - - - - 45
4.5 Heavy Metals in Soil - - - - - - - - - 49
4.5.1 Effect of distance on heavy metal contents of the soil- - - - 49
4.5.2 The Main Effects of Depth on Selected Heavy Metal Content of the Soil - 51
4.5.3 The Simple matrix of Distance and Depth on the Concentration of the Selected
Heavy Metal in Soils - - - - - - - - - 53
4.6 Heavy Metal concentration in Plant - - - - - - 55
4.6.1 Cadmium - - - - - - - - - - 55
4.6.2 Copper - - - - - - - - - - 55
4.6.3 Mercury- - - - - - - - - - 56
4.6.4 Lead- - - - - - - - - - - 56
4.7 The Bioconcentration Factor (BCF) of Cassava - - - - - - 58
4. 8 The Bioconcentration Factor (BCF) of Maize - - - - - 60
4.9 The Bioconcentration Factor (BCF) of Pannicum maximum` - - - 62
CHAPTER FIVE
DISCUSSION- - - - - - - - - - 65
5.1 Physicochemical Properties of the Soil - - - - - `65
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5.2 Heavy metals in soil - - - - - - - - 70
5.3 Heavy Metal in Plant - - - - - - - - 73
5.4 The bioconcentration factor (BCF) - - - - - - 75
CHAPTER SIX
SUMMARY AND CONCLUSION - - - - - - - 76
REFERENCES- - - - - - - - - - 79
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LIST OF TABLES
Tables Pages
1. Mean Values of Some Chemical Properties and Heavy Metal Contents of the Sewage
Sludge - - - - - - - - - - 39
2. Effects of Distances from the Sewage Site on Soil Physicochemical Properties. 41
3. Effects of Depth from the Sewage Site on Soil Physicochemical Properties. 43
4a. The Mean Simple Matrix of Distance and Depth on the Soil Physicochemical Properties 47
4b. The Mean Simple Matrix of Distance and Depth on the Soil Physicochemical Propertie
(contd) - - - - - - 48
5. Effects of Distance on Soil Heavy Metals Distributions - - - 50
6. Effects of Depth on Soil Heavy Metals Distribution - - - 52
7. The Mean Simple Matrix Of Distance And Depth On The Soil Heavy Metal Content 55
8. The Main Effects of Distance, Plant and Plant parts on heavy metal accumulation- 57
9. Bioconcentration Factor (BCF) in root and leaf of cassava - - - 59
10. Bioconcentration Factor (BCF) in root and leaf of Maize - - - 61
11. Bioconcentration factor (BCF) in root and leaf of Pannicum Maximum - 63
12. Guideline for safe limits of heavy metals in soil and plant - - - 64
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LIST OF PLATES
Plates Pages
1. The sewage system - - - - - - - - 31
2. The settling chamber - - - - - - - - 32
3. The oxidation pond - - - - - - - - 33
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ABSTRACT
This study was carried out at the University of Nigeria, Nsukka sewage disposable site. The study
was to quantify the mineral contents of sewage sludge and assess their mobility in the soil and
plants around UNN disposal site. The experimental layout was a 4x3 factorial in RCBD in which
distance from the sewage disposal site and soil depth were the two factors under consideration.
The distances from sewage pond were 0 (edge), 100, 300 and 2000 m away from the sewage pond
with 2000 m serving as control while soil depths were 0 – 40, 40 – 80, and 80 – 20cm. Soil and
plant sampling was carried out from three distances in both sewage site and at the control site.
Physico-chemical properties of the soil were determined. Four heavy metals (Cd, Cu, Hg and Pb)
were determined from the soil, plant samples as well as the sewage sludge. Results showed that
the effects of distance and soil depth were significant (P < 0.05) on pH, exchangeable bases,
cation exchange capacity, organic matter, total nitrogen, exchangeable acidity and available
phosphorous. At the control site there were no differences among soil properties except organic
matter that decreased significantly (P < 0.05) with increase in depth. All the chemical properties
determined decreased with increase in depth, except exchangeable acidity which increased with
increase in depth. Cadmium, Cu, Hg, and Pb, significantly (P < 0.05) decreased as both distance
and depth increased. There was an obvious high heavy metal content in the soil of disposal site
when compared with the control soil. In the tissues of the three plants (Manihot spp, Zea mays,
Pannicum maximum) studied, Cd, Cu, Hg, Pb concentrations were significantly different (P <
0.05) at 0, 100, 300 and 2000 m distances. Heavy metal content observed in plant tissues at the
disposal site was relatively high when compared to that of the control site. Therefore, it follows
that the studies at the University of Nigeria Nsukka sewage disposal site results in an increased
concentration of soil property, such as organic matter, macro and micro nutrients. The mobility of
heavy metals (Cd, Cu, Hg, and Pb) from soil to leaves suggest that all these metals were mobile
from soil to plant components. Heavy metal concentrations varied among the tested crop plant,
which reflects their differences in their uptake capabilities and their further translocation to the
shoot portion of the plant. The bioconcentration factor at both root and leaf tissues of crop plant
did not show any abnormal accumulation of heavy metals at sewage site. From the study the
heavy metal contents of the sludge, soil and plant species were below toxicity levels when
compared to WHO/FAO standards.
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CHAPTER ONE
1.0 INTRODUCTION
Sewage sludge is a solid, semisolid, or liquid muddy looking residue that results after human and
other waste from households, industries and run-off from road is treated at a sewage plant.
Quantitative and qualitative composition of the sewage sludge is very complicated. It is rich in
organic matter, nitrogen, phosphorus, calcium, magnesium, sulphur and other microelements
necessary for the survival of plants and soil fauna. Of the microelement commonly found in
sewage sludge heavy metals such as, cadmium, lead, mercury and arsenic are toxic to humans
whereas zinc, copper, and nickel are harmful to plants (Jane-Hope, 1986). There release to soil
liquid phase may cause leaching through the soil profile to the ground water table, and also
facilitate plant uptake of heavy metals thus posing a risk to environment.
Heavy metals have a density greater than 5.0 g cm-3
(Seaward and Richardson, 1990) and metal
transfer from sewage sludge to soil and subsequently to groundwater and plants represents
potential health and environmental risks (McBride et al., 1997; Bhogal et al., 2003). The greater
the mobility the higher the toxicity risk of these metals. Heavy metal mobility mainly depends on
soil properties (Rowell, 1994).
One important process affecting heavy metal mobility in soil is sorption. The importance of pH in
metal solubility is well-known as it influences heavy metal adsorption, retention and movement
(Sauve et al., 1997).Organic matter is another important soil component that influences metal
mobility. It has a nutritional function by serving as a source of N, P and S, and has a high binding
capacity for cations and organic contaminants (Oste et al., 2002). It has been shown that heavy
metals are accumulated in surface organic layers in agricultural and urban soils. It therefore
appears that soil properties such as sorptivity, organic matter and soil pH may have a major effect
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on metal mobility. The addition of organic matter may also affect metal mobility, by increasing
soluble organic matter and Cu mobility may be enhanced, especially in sandy soils of high pH
(Udom et al., 2000, McBride et al., 1999).
There are several indices of heavy metal mobility in soils, the retardation factor, Rd, being a
straight forward, unitless one (Kookana et al., 1994). The meaning of Rd is that it is the ratio of
the rate of movement of the heavy metal to the rate of movement of water in which the metal is
dissolved.
The fate of heavy metals in sewage sludge is mainly related to their mobility in soil. A
traditionally held view on the mobility of heavy metals in sewage sludge-amended soils is that the
metals’ migration is insignificant and that the metals tend to remain in the site of input, i.e. in the
topsoil (Chang et al., 1984; Schirado et al., 1986). Heavy metals are dangerous because they are
non-biodegradable, bioavailable and toxic to different crops (Mahler, 2003; Garcia and
Dorronsoro, 2005).
Distribution of heavy metals in plant body depends upon availability and concentration of the
heavy metal as well as upon the particular plant species and its population ( Punz and Seighardt,
1973). Plants have a natural ability to extract elements from the soil and to distribute them
between roots and shoot depending on the biological processes in which the element is involved
(Ximénez-Embun et al., 2002). In addition to the uptake of nutrients, toxic compounds such as
heavy metal can also be taken up by the plants. The plants store high levels of toxic heavy metals
within their roots, shoots, and leaves. The growth and metabolism of many plant species were
reported to be affected adversely by excess supply of heavy metals (Foy et al, 1978, Wong and
Lau 1985).
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Potentially toxic metal concentrations are not the same in all portions of the plant. A survey of a
number of food crops revealed that the tuber, seed, or fruit tissue had a lower amount of Cd than
plant leaf tissue. Comparisons between crop species show wide variations in their ability to absorb
potentially toxic trace elements from the sludge-soil system. Cereals and legumes accumulated
less Cd in shoots than leafy vegetables, like curly cress (LepidiumsativamL.), lettuce (Lactuca
sativa L.), and spinach (Spinaciaoleracea L.) (Bingham et al., 1975). An earlier study in the site
of present study showed that the build-up of heavy metals to critical level could be phyto-toxic
and might result in reduced plant growth (Udom, 2000, and Asadu et al, 2008). There is a concern
that heavy metals in the composted product may transfer from soil and accumulate in edible
plants. Some of these heavy metals can be detrimental to human, plant or animal life if they are
present above certain limits.
At the University of Nigeria Nsukka (UNN) the sewage sludge discharged from the administrative
blocks, hostels and surrounding staff quarters are continuously disposed at the sewage sludge site.
Heavy metal content of the sewage sludge is likely to be high due to the usage of cosmetics,
detergents, fats, oils, chemicals and cigarette ends flushed down the toilets. There are vegetation
and farmlands with different kinds of crops surrounding the sewage disposal site. Heavy metals in
sewage sludge and the toxicological implications are of concern in agricultural production due to
the adverse effects on food quality (safety and marketability) and crop growth (phytotoxicity).
Metals such as lead, mercury, cadmium and copper are cumulative poisons. These metals cause
environmental hazards and are exceptionally toxic. Crops and vegetations around take up metals
by absorbing them from contaminated soils, animal graze upon this vegetation and this heavy
metal enter into the food chain. This exposes the consumers of these crops and animals to
bioaccumulation of heavy metals with time. It has been reported that nearly half of the mean
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ingestion of lead, cadmium and mercury through food is due to plant origin (Mohsen and Mohsen
2008).
Therefore, the major objective of this work was to assess the mobility of some heavy metals from
sewage sludge in soils and plants around UNN disposal site. The specific objectives were to:
1 determine the chemical composition of sewage sludge;
2 determine the chemical compositions of the soils around the pond;
3 quantify the accumulation of the heavy metals laterally away from the pond to the soil
around; and
4 determine the accumulation of heavy minerals in the roots, and leafs systems of selected
crops/plants so as to compare with safety standards.
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CHAPTER 2
LITERATURE REVIEW
2.1 Sewage Sludge
Sewage sludge is a solid, semisolid, or liquid muddy looking residue that results after human and
other waste from households, industries and run-off from road is treated at a sewage plant.
The composition of sewage sludge varies from one treatment to another, depending on the extent of
treatment and on the differences in the original sewage make-up. Domestic and municipal waste
water and sewage are treated so that the final products of sludge and effluents may be disposed of
with minimum adverse effect on the soil and environment.
Sewage sludge contains an abundance of trace elements necessary for plant growth (boron, copper,
iron, manganese, molybdenum, and zinc) as well as others considered to be nonessential (e.g.,
cadmium, chromium, mercury, lead). The trace elements in sludge are sometimes called the heavy
metals, because most of them (boron being the major exception) have a density greater than 5.0
(Page, 1974). Industrialized sludges, of course, can contain high concentrations of trace elements.
Source control with limits on discharges of toxic trace elements is practiced by cities that use their
sludge for agricultural purposes. But even the tightest source control is unlikely to reduce the trace-
element content much below the median value (Dean and Smith, 1973). This is because household
products contain trace elements. The trace elements likely to cause toxicities to plants in soils
treated with large amounts (for example, 400 t ha–1
) of domestic sludge for a number of years (15
yr) are cadmium, copper, zinc, boron, and possibly nickel (Page, 1974). In industrialized sludges, in
addition to the elements listed, concentrations of arsenic, barium, cobalt, chromium, manganese,
lead, and vanadium may be sufficiently high to reduce crop yields when applied in large amounts.
17
Plants can grow normally and yet contain concentrations of selenium, cadmium, molybdenum, and
possibly lead that are toxic to man and animals.
Cadmium is the element of most concern in sludge, because it poses the greatest threat to human
health (Sommers and Barbarick, 1986). Food obtained from plants grown on sludge treated soil
might contain concentrations of cadmium toxic to man and animals. Cadmium is used in
electroplating, pigments, chemicals, batteries, alloys, photographic supplies, fungicides, as well as
other products. Even though industrial sources of cadmium in sludge can be controlled, domestic
sources cannot. For example, cigarette ends flushed down toilets raises the cadmium concentration
in sludge, because tobacco has a high concentration of cadmium (Juste and Mench, 1992). Long-
term studies suggest that cadmium is a problem in sludge-treated soils (Juste and Mench, 1992).
Nine years after a one-time sludge addition to a fine sandy loam, Bell et al. (1991) found that the
cadmium was more extractable (up to 60%) than zinc, manganese, and iron (less than 4%), which
indicated a greater potential mobility for cadmium than for the other metals.
Dowdy et al., (1991) reported that after 14 years of large additions of sludge (765 t ha–1
total
added), sludge-born cadmium moved out of the tillage zone into the subsoil. At the 83–100 cm
depth, the concentration of extractable cadmium in the soil was 2.5 times higher in the sludge
treated plots than in control plots. Kirkham (1975) showed that, at the 30–61 cm depth, a silt-loam
soil spread with moderate amounts of sludge (28 t ha–1
yr–1
) for 35 years had 46 times more total
cadmium than a control soil, and leaves of corn plants growing on that soil had concentrations of
cadmium higher than normal.
In addition, synergistic and antagonistic interactions occur between elements in sludge and soil that
affect the absorption of elements by plant roots. For example, high phosphorus levels in soil often
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inhibit uptake of trace elements. Therefore, it is possible to decrease uptake of one element by
supplying another element to the soil. After application to soil, trace elements tend to remain in the
plow layer with the sludge residue, although cadmium appears to be an exception. Trace elements
are often associated with the organic-rich sludge fractions Oxides of iron and manganese absorb
trace elements and limit their movement (Bell et al., 1991).
Citric acid, present in sewage sludge (e.g., from washing fluids), forms complexes with metals,
which may play a part in migration of metals away from disposal sites (Francis et al., 1992).
Chelating agents such as organic acids produced in the rhizosphere by root exudation or by
microbial activity also affect the mobility of heavy metals in soils and their transfer to plants. In
addition, microbes can absorb heavy metals and prevent their transfer to plants (Munier-Lamy et al.,
1991). However, microbes are short-lived, and when they die the metals will again be available for
uptake by plants.
2.2 Heavy Metal Toxicity and its Mobility
The sludge contains heavy metals which have potential ecological, biological and health impact.
The heavy metal contents of sludge from various countries has been reported Berrow and Webber,
1972; Fergusson, 1990). Heavy metals like As, Cd, Co, Cu, Ni, Zn, and Cr is phytotoxic either at
all concentrations or above certain threshold levels. Toxic metals are biologically magnified through
the food chain. They infect the environment by affecting soil properties, its fertility, biomass and
crop yields and ultimately human health.
Depending on the origin and composition, however, sludge may contain substantial amounts of
toxic metals as well as beneficial nutrients (Wong et al., 2007). However, the mentioned application
can pose a threat to environment and the major concern arises from the fact that sewage sludge,
especially those from the heavily urbanized and industrialized areas, contains a relatively high
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concentration of heavy metals (Selivanovskaya and Latypova, 2003; Singh et al., 2004). However,
sludge’s often contain appreciable amounts of metals, e.g. Zn, Cu, Ni, Cd and Pb (Chander and
Brookest, 1991). Metal transfer from sewage sludges to soil and subsequently to groundwater and
plants represents potential health and environmental risks (McBride et al., 1997; Bhogal et al., 2003
Heavy metal ions may have toxic effects on plants, animals or humans, and their toxicity is linked
to their mobility in soil. The greater the mobility the higher the toxicity risk of these metals. Heavy
metal mobility mainly depends on soil properties (Rowell 1994). One important process affecting
heavy metal mobility in soil is sorption. Sorption is the phenomenon in which metal ions, which
typically bear a positive charge, are attracted to solid particles in the soil such as clay and organic
matter, which mostly bear a net negative charge.
This binding is often reversible, and metals bound onto the solids are in equilibrium with metals in
the soil water. This means that strongly retained metal ions are removed from the soil water and
become less mobile than weakly retained ions. It has long been thought that heavy soils, that is, high
clay content soils, tend to bind and immobilize heavy metals, so that many industries feel safe when
disposing of their wastes in clayey soils (Sharma and Lewis 1994). Yet, this simplification is not
always true, since the transport of contaminants even in clayey soils can be significant and needs to
be addressed and quantified.
There are several indices of heavy metal mobility in soils, the retardation factor, (Rd) being a
straightforward, unitless one (Kookana et al. 1994). The natural meaning of Rd is that it is the ratio
of the rate of movement of the heavy metal to the rate of movement of water in which the metal is
dissolved. To give a rather simplistic example, if Ni is dissolved in water, and the water moves 100
m in the soil, but Ni moves 5 m, then Ni Rd is 20 (20=100/5). It is easy to see that the greater the
metal Rd, the higher the metal retention by the soil, and, thus, the lower the metal mobility.
20
The distribution and bioavailability of heavy metals vary, depending on the soil fraction. Iyengar et
al., (1981) reported that water soluble, exchangeable, and absorbed fractions are readily available to
plants; while primary and secondary soil minerals are relatively unavailable to plants. The metal
species present and their relative bioavailability within the soil are of greater importance than the
total metal concentration in the environment that determines the overall physiological and toxic
effects on biological systems (Bernhard et al., 1986; Hughes et al., 1989; Morrison et al., 1989).
Heavy metal contamination due to sludge application has received much attention due to concerns
regarding uptake by plants and contamination of groundwater or surface waters (Cunningham et al.,
1975).
2.3 The Effect of pH and Organic Matter in Heavy Metal Mobility
The importance of pH in metal solubility is well-known as it influences heavy metal adsorption,
retention and movement (Sauve et al., 1997). Organic matter is another important soil component
that influences metal availability. It has a nutritional function by serving as a source of N, P and S,
and has a high binding capacity for cations and organic contaminants (Oste et al., 2002). It has been
shown that heavy metals are accumulated in surface organic layers in agricultural and urban soils.
However, the extent to which topsoil heavy metals can be mobilized to labile forms remains
uncertain (Mico et al., 2006). Some of the heavy metals emitted by this sludge are known to be
toxic to human and plants, even at low concentrations (Pendias and Mukherjee, 2007).
The fate of heavy metals in sewage sludge is mainly related to their mobility in soil. A traditionally
held view on the mobility of heavy metals in sewage sludge-amended soils is that the metals’
migration is insignificant and the metals tend to remain in the site of input, i.e. in the topsoil (Parker
et al., 1978, Chang et al., 1984; Schiradoe et al., 1986; Dowdy et al., 1991). However, in several
21
field-base experiments the apparent migration of heavy metals down the soil depth has been
observed (Bell et al., 1991; McBride et al., 1997).
Furthermore, in column leaching (Ashworth and Alloway, 2004) and batch experiments, (Burton et
al., 2003). It has been shown that heavy metal ions may leach more easily in the presence of sewage
sludge than in its absence. They ascribed such elevated mobility to the high contents of dissolved
organic matter (DOM) in sewage sludge (Liu et al., 2007). Due to its net negative charge at typical
soil pH, DOM generally moves very easily through soil system (Dunnivant et al., 1992). In
addition, metal sorption on soils is strongly related to soil properties. Researches have shown that
metal sorption by soils increased with increasing the pH (Naidu et al., 1994), organic matter
(Gerriste and Driel, 1984; Udom et al., 2004), cation exchange capacity (Buchter et al., 1989), and
the contents of iron and manganese oxides. However, there is a lack of information concerning the
adsorption of sludge-borne heavy metals on different soils (Sigua, 2005).
In case DOM in sewage sludge decreases metal sorption, and thereby increases their mobility in
soil, there might be a greater potential for adverse impacts on the surrounding environment. The
most direct and obvious impact is the threat to ground water quality. Therefore, when we calculate
the sludge application rate to agriculture land, it is necessary to consider the effects of DOM derived
from sewage sludge on heavy metal sorption and also of the soil physicochemical properties (Liu et
al., 2007).
In a study, pH of the sewage sludge was 7.19. The total concentrations of Cu, Ni and Pb were
189.86, 27.24 and 8.90 mg/kg, respectively. The sludge sample contained 53.4% organic matter,
24.9 g/kg DOC and 1.3 cmolc/kg CEC (Centi mole charge per kilogram CEC). Speciation of Ni, Cu
and Pb in the sludge filtrate obtained, showed that they were present mainly as dissolved organic
metallic complexes with 98.93% for Ni, 99.96% for Cu, and 99.98% for Pb, respectively. The
22
sewage sludge filtrate has a relatively high content of dissolved organic carbon (1245.6 mg/L), of
which 53% was within the hydrophilic DOC fraction. The large proportion of hydrophilic DOC
could contribute to the composting processes (Liu et al., 2007).
Soil organic matter can not only provide sorption sites for metals directly, but also combine with
soil minerals and increase the sorption sites (Kaiser and Guggenberger, 2003). Regarding to
environmental risks, it is worthwhile mentioning that the real sludge application risk has been
certainly determined by many other factors except for the sludge matrix and soil properties, such as
land usage, microbial activity and plant root-soil interactions (Qureshi et al., 2003; Qureshi et al.,
2004; Seguin et al., 2002; Vervaeke et al., 2004).
In soils which had received metal-contaminated sewage sludge (high-metal soils), the amounts of
biomass were decreased by up to 50%. Also, there was no relationship between biomass C and total
soil organic C in the high-metal soils. Downward migration of heavy metals was observed 7 years
after sludge application, where soluble Cu, Zn and Cd were higher at 40 to 60 cm depth in the
sludge treated soils (Campbell and Beckett, 1988). The decrease in soil pH following sludge
application was a major reason for the higher mobility of heavy metals in sludge-treated soils
(Robertson et al., 1982). However, leaching of Cd, Cu and Pb in sludge-treated soils was also
observed in soils with higher pH values in some other field experiments (Darmody et al., 1983).
The incorporation of C-rich sludge to soils has been shown to increase the amount of dissolved
organic matter (DOM), which can facilitate metal transport in soil and groundwater by acting as a
“carrier” through formation of soluble metal–organic complexes (McCarthy and Zachara, 1989).
The leaching of metals was facilitated by higher amounts of dissolved organic matter in the soil/soil
solution. Furthermore, metal mobility might have been substantially higher shortly after sludge
application than many years later because laboratory-determined distribution coefficients Kd
23
(Distribution Coefficient), for the metals in newly prepared sludge/soil mixtures were lower than Kd
values of the aged sludge-treated field soil (McBride et al., 1999). It is suggested that under the field
condition soluble organic matter from organic wastes decomposition and preferential flow paths
were the two dominant factors enhancing increasing metal movement in soil (Wong et al., 2007).
However, some researchers revealed that Zn and Cd have a weak ability to bind to DOM compared
with Cu or Pb. The increased concentrations of soluble Zn and Cd in soil with long term sludge
application is not only determined by the total input of metals, but also strongly related to soil pH.
The lower metal solubility in soil of high pH was more obvious for metals like Zn and Cd that tend
not to complex strongly with soluble organics (McBride et al., 1997).
The concentrations of Cr, Hg, Cu, and As in soil percolates were positively correlated with DOM.
In contrast, Cd and Zn mobilization depend on soil pH and soluble elemental content, but not on
DOM. Moreover, DOM is of minor importance in the mobilization of heavy metals in soils with a
low soil pH (Zhou and Wong, 2001). It has also been shown that the addition of sludge DOM, to a
certain extent, reduced the sorption of both Cd and Zn by acidic sandy loam, calcareous clay loam
and calcareous sandy loam. (Wong et al., 2007).
The environmental risk caused by metal leaching and phytotoxicity due to the heavy application of
sewage sludge in agricultural soils should be taken into consideration especially in near-neutral and
alkaline soils, and anaerobically digested sludge with relatively high pH, because DOM derived
from sludge could significantly reduce metal sorption and increase its mobility through the
formation of soluble DOM–metal complexes in these soils (Wong et al., 2007). Thus, in basic soils
metal persistency is expected to be generally very high.
A number of studies on acid soils have examined the leaching of heavy metals in sewage sludge
amended-soil columns. While some studies have shown that the migration of heavy metals is
24
negligible (Schiradoe et al., 1986; Dowdy et al., 1991; Camobreco et al., 1996), other studies have
demonstrated that significant amounts of Zn, Cr, Cu and Cd (among others) were readily leached
(Frenkel et al., 1997; Karathanasi et al., 2005). It therefore appears that soil properties such as
organic matter and soil pH may have a major effect on metal mobility. The addition of organic
matter may also affect metal mobility, for instance, by increasing soluble organic matter, Cu
mobility may be enhanced, especially in sandy soils of high pH (McBride et al., 1999).
Basic soils showed a marked pH-decrease after the application of sewage sludges, the acid loam soil
showed a pH-increase that was even larger than the decrease in basic soils.
The low mobility of metals in the basic soils may be related to high CaCO3 and organic matter
content. Furthermore increased mobilization of these elements in the acid loam may be related to
the lower organic matter, clay, and CaCO3 content of this soil (Toribio and Romanya, 2006).
When considering different kinds of contaminants, heavy metals are especially dangerous because
of their persistence and toxicity (Adriano, 2001). Agricultural used of sewage sludge and inorganic
phosphate fertilizers can cause substantial accumulation of cadmium in soils (Nicholson et al.,
1994; Wiseman, 1994). In most soils, adsorption of Cd increases strongly with increasing organic
matter and pH (Boekhold and Zee; 1992; Hinz and Selim, 1994).
The allowable amount of applied sludge that has been calculated without the interference of
dissolved organic matters is higher than that of interference. This issue indicated that the effect of
dissolved organic matter is a factor of great importance for estimating the allowable amount of
application of sewage sludge. Nevertheless, the real allowable amount of applied sludge depends on
not only dissolved organic matter content and physicho-chemical soil properties, but also to other
factors including land use and microbial activities as well as the interaction between the soil and
plants roots (Liu et al., 2007; Toribio and Romanya, 2006).
25
In all types of several applications, heavy metals accumulation existed in plants that their ultimate
destinations were human beings and living creatures as well as the environmental risks posed to
both soil and water (Nicholson et al., 2003; Pendias and Mukherjee, 2007; Adriano, 2001). With an
increase in concentration of dissolved organic matter, movement and translocation of heavy metals
in soil increases while, with an increase in soil organic matter (Bounded with soil particles) and pH
as well, their mobility decrease (Liu et al., 2007; McBride et al., 1999). With an increase in soil pH
as well as the amount of clay in the soil, the mobility of heavy metals present in applied sewage
sludge decrease (Wong et al., 2007;)
2.4 Effect of Sewage Sludge Application in Soil
Application of sewage sludge to agricultural soil is a common practice because of low costs and
recycling of nutrients achieved (Sigua et al., 2005). Disposal of municipal sewage sludge and
effluents has recently received much publicity because of the in- circling amounts of these wastes
produced by urban and industrial activities. Due to growing concern over disposal of sewage sludge
in the oceans and the high cost of its incineration, land application and land tilling are becoming
more common. Disposal of sewage sludge to land can be beneficial because it contains plant
nutrients (especially N and P) and organic matter, which can be of agronomic benefit (Chander and
Brookest, 1991). Agricultural application has become a common waste treatment alternative for
sewage sludge because of practical and economic reasons (Cunningham et al., 1975). Sewage
sludge contains organic matter and nutrients that have the potential to enhance forest productivity
and several soil characteristics (Henry and Cole, 1997; Losada et al., 2001).
High CEC organic soils have been observed as good application sites for sewage sludge and
effluents. Current guidelines (Keeney et al; 1975) set heavy metal limits based on soil pH and
cation exchange capacity (CEC). Studies have shown that heavy metal availability was related to
26
the soil CEC and organic matter content (Robertson et al., 1982), while another work has shown
CEC to be a poor prediction of heavy mental availability (Dowdy and Volk, 1984) stressing that
although heavy metal availability was reduced by high organic matter content , extensive leaching
of mineral occurred where disposal rates of sewage sludge were excessive and the soil were coarse-
textured with low concentrations of hydrous oxides.
With respect to the improvement of soil fertility, many studies have shown that soil fertility increase
after land application of sewage sludge and effluence (Keeley and Quin, 1979; Hart and Speir,
1992). For example , study on the effects of over 80 years of application of sewage effluents to
limore silt in Canterbury , New Zealand (Hart and Spier, 1992) shown considerable increase in soil
N, P, K, S, Ca, Mg, organic carbon , pH , and base saturation. Short-term studies (Bernial et al.,
1992) also showed significant increase in soil N, P, K, and micro nutrient concentrations, 8 months
after application of sludge to a calcareous soil in Spain. Augers and N’Dayegamiye (1991) observed
that organic carbon content of a sandy loamy soil increase from 1.5% to 2.2% after 7 years of
applying sewage sludge at the rate of 5 t ha-1
year-1
.
An increase in soil organic matter content resulting from the application of sludge and effluent can
produce a concomitant increase in soil cation exchange capacity. Bernal et al. (1992) observed that
the application of sewage waste-water at rates from 200 to 1000 m3
ha-1
year-1
resulted in significant
increase in soil organic carbon and cation exchange capacity. Similarly, Stadelmmann and Furrer
(1985) observed that 7 years of application of sewage sludge at 5 t ha-1
year-1
to a sandy loam soil
increased the CEC from 172 Cmol kg-1
in the control plots to 23.7 and 22.2 Cmol kg-1
in the treated
plots, respectively. Significant increase in base saturation was also observed following application
of sewage sludge that contained significant quantities of the exchangeable cations, Ca2+
, Mg2+,
Na+,
and K+. However, Guisquiani et al., (1995) observed that though CEC increased significantly in
27
sewage treatment soils it was lower than expected. This was most likely due to the fact that the
transition metals added to the soil could have been complexed , thus causing a decrease in the
negative surface charge of the organic matter.
Significant increase in soil nitrogen has also been observed as a result of land application of sewage
sludge and effluents. Studies by Kelling et al., (1977) on the changes with time in the concentration
of soil organic N, inorganic N and available P in the soil profile of two sites treated with liquid,
digested sewage sludge shown that application of large amount of inorganic N, organic N, and
available P in a sandy loam and a silty loam soil in south-central Wisconsin. However, substantial
losses of sludge –applied N were observed to occur by leaching, denitrification, volatilization or a
combination of these.
Mbagwu and Piccolo (1990) observed that the application of sewage sludge at the rate of 200 t ha-1
increase the total N and available P content of soil by 57% and 642%, respectively, whilst
application of other types of organic waste such as pig slurry (40 t ha-1
) and cattle slurry (8 t ha-1
)
increased the soil N content by 18 and 13% and available P content by 430 and 372%, respectively.
Ross et al. (1982) also observed that application of untreated meat processing effluents over a
period of over 80 years (1899-1982) increasing soil organic carbon from 5.6 to 6.8% as well as
mineralized N, and available P.
High N content in most sewage sludge and effluents can cause a decline in soil and plant
concentration of K over a period of time. For example, palazzo and Jenkins (1979) observed a dec
line in plant and soil concentration of K over a 4-year period of land application of sewage waste
–water at the site of treatment , and related it to the K:N ratio of the sewage waste-water applied
because the sewage waste-water contain more than twice as much N as K.
28
With respect to the movement of P with soil depth, Bond et al., (1995) observed no increase in P
concentration of the soil water at a depth of 1.2 m in a cropped clay soil receiving 5 cm of effluents
per week for 8 years. The P concentration of the effluent ranged from 4.1 to 9.9 ppm while the P
concentration of the soil water at the 1.2 m depth was less than 0.10 ppm.
In addition to the beneficial effects of land disposal of sewage sludge on the soil chemical properties
a number of detrimental effects and risks associated with the agronomic utilization of sewage sludge
and effluents have been highlighted by Summer and Mclaughian (1996) and Cameron et al., (1996).
Such detrimental effects and /or risks include increased concentration of dissolved p in runoff,
build-up of heavy metals, nitrate leaching to ground water, salt concentration and elevated or
extremely low pH.
Studies of Sabey et al., (1977) on the influence of sewage sludge and wood waste mixtures
application on land indicated that a good combination and proper management of the mixture was a
valuable resource that governed nitrate supply, controlled nitrate leaching and enhanced physical
and chemical properties of the soil. But Agbim et al. (1977) observed that application of sewage
sludge alone to the soil resulted in inefficient utilization of soil nitrogen by plants and/or toxicity
due to high salt content on micro-organism. However, metal toxicity according to Lieffering and
Mclay (1996) was alleviated by liming the soil; moreover, continued crop production required a
near-neutral soil pH, even year after sludge application was discontinued, in order to reduce
mobility of heavy metals that were added to the soil.
Significant increase in soil pH and elevated salt concentration has been observed in soil when
secondary –treated sewage effluent with a high sodium adsorption ratio (SAR) was applied
(Summer and Mclaughlan, 1996). John and Mcconchie (1994) observed that the application of
29
secondary treated dilute sewage effluents to soil growing bananas at Woolgoola more than double
the soil sodium concentration from 0.11 to 0.31 cmol kg-1
. Despite the low electrical conductivity of
the effluents (0.44 d sm-1) the soil exchangeable sodium percentage (ESP) value reached 4% during
the trail.
Similarly, land disposal of treated sewage effluents to a waimakakiriri sandy loam in Canterbury,
New Zealand significantly increased soil increase salinity. The electrical conductivity of the soil
increased from 0.4 mm cm-1
to 5.5 mm cm-1
; reduce growth and yields of many crops were
observed (Balks et al; 1996). A high concentration of Na in soil is of concern because it can cause a
decrease in infiltration rate and an increase in the risk of runoff.
High concentration of boron, sodium, chloride, carbonates, and total dissolve solids in sewage water
could damage plant because the advantage use of effluents on land depends on the quality of waste
water, soil type, crop, and climate. Crops vary in their tolerance to salts. For example, Balk et al.
(1996) observed that the beans, red clover, groundnut and citrus were more sensitive to salts and
organic compounds associated with sewage waste-water than grasses, barley, and cotton.
High concentration of Na+ and C1
- was also observed in the top 15 cm of soils treated with beef
feedlot effluents (Tiarks et al., 1974). The major factor responsible for that development, according
to the study, was the high Na+
content of the feedlot effluents. The application to the soil of effluent
from a pulp and paper mill in New Zealand, increase the sodium adsorption ratio (SAR) from 2 to
16 and increase the sodium concentration in ground water (Johnson and Ryder, 1988). Brechin and
McDonald (1994) also detected increase sodium content in soil fertilized with pig slurry in South
Australia and suggested that this could become a problem in the long-term.
30
Heavy metal contamination due to sludge application has received much attention due to concerns
regarding uptake by plants and contamination of groundwater or surface waters (Cunningham et al.,
1975). Thus application of sewage sludge to agricultural soil may result in elevated concentrations
of toxic metals, which may then threaten ground water quality and lead to food chain contamination
(Selivanovskaya and Latypova, 2003; Singh et al., 2004).
Heavy metals are often highly persistent in soil, with residence times as long as thousands of years
(Alloway, 1990). Metals applied with sewage sludge may be retained in the soil as a result of their
adsorption on hydrous oxides, clays, and organic matter; the formation of insoluble salts; or the
presence of residual sewage sludge particles (Alloway and Jackson, 1991). Moreover, soil CaCO3
has often been found to increase soil metal retention (Raikhy and Takkar, 1983). Heavy metal
accumulation in soils can result in a loss of soil functions leading to concerns about environmental
quality protection, maintenance of human health and productivity. Soil pollution can have
implications in phytotoxicity at high concentrations and result in the transfer of heavy metals to the
human diet from crop uptake or soil ingestion by grazing livestock (Pendias and Pendias, 2001;
Nicholson et al., 2003; Pendias and Mukherjee, 2007). Industrial waste incinerators also are being
sources of heavy metal and organic compound pollution (Hu et al., 2003).
Application of sewage sludge to cropland could result in soil contamination, phytotoxicity, and
accumulation of trace elements in the food supply. The magnitude of the problem depends on the
interrelationships of a number of factors, such as the composition of sludge, the rate and frequency
of applications, soil characteristics, and plant species. Sludge type and sludge-soil interactions
influence the chemical forms of a metal which determine its availability for plant uptake. However,
additional plant and soil factors further modify the uptake and the concentration of elements in
crops.
31
2.5 Sludge-Soil Interaction
Limited information is available on the reactions of metals in municipal sewage sludge with soils,
so reactions between metals in sludges and soils are not completely understood. Many researchers
have reported that after municipal sewage sludge is incorporated into soil, portions of the metals
revert to non-extractable, less available chemical forms (Beckett et al., 1979; Wollan and Beckett,
1979; Lagerwerff et al., 1977;). The variability in the results of incubation studies suggested to
Beckett et al. (1979) that the initial stages of the sludge-soil interaction are microbial rapid
decomposition of metal compounds in sewage sludge with microbial immobilization of the released
ions. On the one hand, with time, these ions form inorganic compounds or stable organic complexes
which resemble the "native" soil forms of these metals [Beckett et al., 1979; Wollan and Beckett,
1979; Lagerwerff et al., 1977; Cunningham et al., 1975).
On the other hand, after addition to the soil, some metals change only slightly in chemical character.
Chromium, which is present as a colloidal precipitate in municipal sewage sludge, changed little in
its chemical form. It remained mostly insoluble after incorporation of sludge in soil (Grove and
Ellis, 1980).
The chemical and physical properties of soils that receive sludge influence metal conversions
(Stover et al., 1976). Soil properties, such as CEC, pH, organic matter content, sesquioxide content,
redox potential, texture, and presence of other elements affect plant uptake, solubility, and mobility
of these metals (Soon, 1981; Zwarich and Mills,1979; Street et al., 1977, 1978; CAST, 1976;
MacLean et al., 1969). Not all metals react similarly to changes in these soil properties. For
instance, soil pH levels which decrease the availability of metal cations to plants increase the
32
availability of Mo (La Hann, 1976). Dowdy and Larson (1975) mixed municipal sewage sludge into
two topographically associated soils with different pH levels of 5.9 and 7.9 (with free carbonates).
After incubating the sludge-soil samples for one growing-degree year, the researchers grew barley
(Hordeum vulgare L.) to measure the quantities of plant-available potentiaily toxic metals.
Incubation, sludge application rate, and soil pH affected metal accumulation within the roots and
tops of barley seedlings. As sludge application rates increased, an increase in the removal of Cr, Ni,
Zn, and Pb occurred when barley was grown on the acid soil.
However, there was no change in the uptake of these metals when barley was grown on the high pH
soil. Thus, with different levels of soil pH, metals released by the decomposition of sewage sludge
either increased or decreased in availability for plant uptake. Lagerwerff et al., (1977) suggest that
the formation or the strengthening of metal complexes with higher soil pH levels and incubation
affects their availability. The presence and the concentrations of other metals can have a
pronounced effect on the uptake of the metal in question (Ham and Dowdy, 1978; Cunningham et
al., 1975). Cadmium concentration increased in plant tissue as soil Cu levels increased
(Cunningham, 1975); the levels of Cr, Ni, and Zn also affected the magnitude of the increase. These
interactions are extremely complex and are not well understood
Land application of sewage sludge can have many beneficial effects. Supplying nutrients (N, P,
secondary nutrients, and micronutrients) to the crops, improving soil physical properties, and
increasing soil organic matter content are several advantages of land application of municipal
sewage sludge. Although these are obvious benefits, there are also concerns that must be addressed
to insure a safe, economical, and environmentally sound approach to applying sewage sludge to the
soil. The most commonly voiced concerns include: 1) the potential for nitrate or phosphate
33
contamination of waters; 2) the potential for damage to soils, plants, animals, and humans because
of possible toxic metal applications; and 3) the potential for pathogen transfer.
Municipal sewage sludge added to soil can improve soils biological, chemical, and physical
properties. Changes in the properties of sludge amended soils vary with the characteristics of the
sludge and soil (Mitchell et al., 1978; Gupta et al., 1977). The faunal and microfloral components of
soils were altered when sewage sludge was added (Mitchell et al., 1978). A rapid increase in
nematode (predominantly Rhabditidae spp.) populations and a slower increase in enchytraeid
populations occurred in sludge amended soil. Both of these organisms feed on soil microbial flora
(bacteria for nematodes, fungi for enchytraeids).
A change in soil pH can result from application of sewage sludge. Increased soil pH occurred when
municipal sewage sludge was added to soils (Silviera and Sommers, 1977; Gaynor and Halstead,
1976). Other researchers noted a decrease in soil pH in sludge-amended soils (Silviera and
Sommers, 1977; Epstein et al., 1976; LaHann, 1976). Silviera and Sommers (1977) related changes
in soil pH to the calcium carbonate content of sludges and the acid production during sludge
decomposition. Whether a sludge-soil mixture increases or decreases pH is an important
consideration in relation to potentially toxic metal uptake by crops, since these metals generally are
more available to plants at lower pH levels. The addition of relatively high rates of sludge increases
the cation exchange capacity (CEC) of soils (Soon, 1981; Mitchell et al., 1978). This increase in
CEC results in additional cation binding sites which retain essential plant nutrients within the
rooting zone. Possibly the CEC increase causes more complexing of heavy metals in an unavailable
form for plant uptake (Kladivko and Nelson, 1979).
34
Organic matter, which constitutes approximately 50 percent of the solid fraction of most sludges,
improves the physical condition of soils (Khaleel et al., 1981; Kladivko and Nelson, 1979;). An
increase in organic matter content decreases bulk density (Kladivko and Nelson, 1979), increases
aggregate stability, increases water holding capacity and promotes greater water infiltration
(Kladivko and Nelson, 1979, 1979; Kelling et al., 1977).
Improving the physical properties of soil increases soil productivity (Kelling et al., 1977). In coarse-
textured soils, higher productivity occurs from the increase in the amount of water available to
crops. In fine textured soils, productivity increases because reduction in bulk density increases
infiltration, porosity, and aeration by aggregation of soil particles. The improvement of soil
productivity by treatment with sewage sludge is shown dramatically in experiments using
drastically disturbed lands (Garcia et al., 1981; Stucky et al., 1980; Stucky and Newman, 1977).
2.6 Implications of Trace Metal in Soil and Plant
Trace elements have been defined as the elements that occur in natural systems in small amounts
and when present in excessive concentrations, are toxic to living organisms (Kirkham, 1977). The
plants oil system has three protective mechanisms that can limit these potentially toxic trace
elements in the aerial portions of a plant, and so minimize health problems to animals or humans.
The ''Soil-Plant Barrier,'' as it is called by Chaney (1980), includes: Elements that are insoluble in
soil and do not accumulate in the plant (Pb, Hg, Cr, F, Ag, Au, Ti, Sn, Si, and Zr), elements that are
absorbed into the root but are insoluble in the root or have limited translocation to the shoot (Fe, Al,
and occasionally Hg and Pb); and elements which when applied in excess cause phytotoxicity, so
plants are not consumed by man or domestic animals (Zn, Cu, Ni, Co, Mn, As, and B). Not all trace
elements present in municipal sewage sludges fall into one of these three categories. Important
35
exceptions are Cd, Se, and Mo which can cause toxicities in animals and humans. Molybdenum and
Se are present in sewage sludges in low concentrations, thus these elements normally do not limit
the rate of sludge application to soil. Cadmium, however, has caused adverse health effects in
humans who ingested plants grown on soils contaminated with high levels of Cd (Chaney, 1980).
Ingestion of soil or a sludge-soil mixture by grazing animals provides a direct pathway to adding
excessive trace elements (especially Pb) to the food chain (Chaney, 1980). The Council for
Agricultural Science and Technology (CAST, 1976) classified Cd, Cu, Mo, Ni, and Zn as potential
hazards in land application of municipal sewage sludges. These metals tend to accumulate in plants
and cause either reduced yields or health problems to animals or humans that ingest the plants. A
number of investigations have been conducted in the greenhouse and in the field to assess the
phytotoxicity and availability of potentially toxic elements to plants grown under three different soil
conditions: soils treated with sludge, soils treated with sludges supplemented with metal salts, or
soils treated with metal salts alone. The concentrations of Cd, Cu, Mn, Ni, Pb, and Zn in the edible
parts of lettuce tops (Lactuca sativa L.) and onion bulbs (Allium cepa) were generally much higher
when the plants were grown on sludge-amended soils in the greenhouse rather than when the plants
were grown in the field (DeVries and Tiller, 1978). Moreover, reduced yields and higher Cr, Cu,
and Zn concentrations were observed in plants grown in soils treated with additions of inorganic
salts than were observed in plants grown in soils treated with equivalent amounts of these metals
from sludge additions (Cunningham et al., 1975).
Therefore, plant data from studies in the greenhouse or from additions of inorganic metal salts
should not be used as the only indicator of the potential adverse effects of applying municipal
sewage sludge under field conditions. Practices that promote good soil aeration, such as drainage
and structure development, lead to decreasing solubility of trace elements. Also, soils with high
36
cation exchange capacities are desirable for sludge disposal, because they have a great ability to
hold and immobilize trace elements. Because of the nitrification reaction and the microbial
production of carbon dioxide, sewage sludge usually lowers the pH of soils; thus, liming is
recommended at sludge-disposal sites, but only when the soil pH is acidic (i.e., < pH 6). Liming
also controls the uptake of trace elements of concern in sludge (cadmium, nickel, and zinc). Trace
elements can form inert and insoluble compounds with clays and organic compounds. Therefore,
they may be less available to plants than total concentrations of these elements in the soil indicate.
Trace elements tend to move little with percolating water and remain at the site of application,
unless they are transported away on eroded sediments.
2.7 Plant Factors Influencing Metal Uptake
The increased availability of heavy metals for plant uptake may lead to phytotoxicity, whereas less
phytotoxic elements may accumulate in higher amount in plants. This later process enhances their
transfer into the food chain, thus posing serious environmental hazard to the population. Once
present in agricultural fields, the availability and uptake of heavy metals by plants roots is
determined by what happens at or near the soil plant root, stem and leaves interface.
The myriad of parameters regulating the chemical fate of specific elements in soils determine their
solubility and availability for plant uptake. The plant uptake of chemical species in soil solution is
also dependent on a number of plant factors. These include: physical processes such as root
intrusion, water, and ion fluxes and their relationship to the kinetics of metal solubilization in soils;
biological parameters, including kinetics of membrane transport, ion interactions, and metabolic fate
of absorbed ions; and the ability of plants to adapt metabolically to changing metal stresses in the
environment.
37
The accumulation of metal ions by root systems is a key function in terrestrial plants, which exhibit
extensive ramifications through soil. Distribution of heavy metals in plant body depends upon
availability and concentration of heavy metals as well as particular plant species and its population
(Punz and Seighardt, 1973). For instance, roots usually show higher heavy metal concentration than
shoots, because they are the origin, which comes into contact with the toxic metals present in the
soil (Breckle, 1991). In recent past, Bunzl et al. (2001) investigated soil to plant transfer of heavy
metals like, Cu, Pb and Zn by vegetables. Studies on heavy metal uptake revealed that vegetables
grown at environmentally contaminated sites in Addis Ababa, Tanzania, could take up and
accumulate metals at levels that are toxic to human health
Metal uptake differences by leafy vegetables are attributed to plant differences in tolerance to heavy
metals (Itanna, 2002). Cadmium, copper and nickel levels in vegetables from industrial and
residential areas of Lagos City, Nigeria was studied by Yusuf et al. (2002), which revealed that
levels of Cd, Cu and Ni in different edible vegetables along with its soils on which they were grown
were higher in industrial areas than those of the residential areas due to pollution. Trace element and
heavy metal concentrations in fruits and vegetables of the Gediz River region were intensively
studied by Delibacak et al. (2002). Also edible portions of five varieties of green vegetables viz.
Amaranth, Chinese Cabbage, Cowpea leaves, Leafy Cabbage and Pumpkin leaves collected from
several areas in Dar Es Salaam, Africa were analyzed for Pb, Cd, Cr, Zn, Ni and Cu. There was a
direct positive correlation between Zn and Pb levels in soils with levels in vegetables. The relation
was absent for other heavy metals (Othman et al., 2002).
Plants growing on polluted soils may contain elevated levels of heavy metals (Gallego et al., 2002;
Zornoza et al., 2002). Heavy metal ions such as zinc, manganese and nickel are essential
micronutrients for plants but, when present in excess, these, and also non-essential heavy metals
38
such as cadmium, can accumulate in plant parts used for human or animal nutrition to undesirably
high contents. At even higher levels, they can become toxic to the plant (Williams et al., 2000).
Comparisons between crop species show wide variations in their ability to absorb potentially toxic
trace elements from the sludge-soil system. Cereals and legumes accumulated less Cd in shoots than
leafy vegetables, like curlycress (Lepidium sativam L.), lettuce (Lactuca sativa L.), and spinach
(Spinacia oleracea L.) (Bingham et al., 1975). Tomato plants (Lycopersicon esculentum Mill.)
accumulated more Cd from municipal sewage sludge-amended soils than either barley or bean
plants (Phaseolus spp.) did (Bradford et al., 1975). Furr et al. (1980) grew bush bean (Phaseolus
vulgaris), cabbage (Brassica oleracea var. capitata), carrot (Daucus carota var. sativa), Japanese
millet (Echinochloa crusgalli var. frumentacea), onion (Allium cepa), potato (Solanum tuberosum)
and tomato plants on sludge-ash amended soils and analyzed plant tissue for B, Cd, Mo, Ni, Se, and
Zn. These plants exhibited differing affinities for the absorption of specific elements. Differences in
accumulation of these metals in plant tissues occured even within plant species, varieties [Boggess
et al., 1978], and hybrids (Hinesly et al., 1978).
This finding suggests that the capacity to accumulate metals may be under genetic control (Hinesly
et al., 1978). Sensitivity of plants to metal toxicity can be associated with the tendency to
accumulate the metal in shoots (Boggess et al., 1978; Bingham et al., 1975). Bingham et al. (1975)
observed that spinach, soybeans, lettuce, and curlycress were injured by soil Cd levels of 4 to 13,
ug/g soil. These plants tended to accumulate Cd in shoots; whereas, lower shoot accumulators like
tomato and cabbage could tolerate soil Cd levels of 170, ug/g without injury. Some plant species
can tolerate high levels of metals in tissue or can confine accumulated metals to the roots and
provide resistance to toxicity (Davis and Beckett, 1978). Potentially toxic metal concentrations are
not the same in all portions of the plant. A survey of a number of food crops revealed that the tuber,
39
seed, or fruit tissue had a lower amount of Cd than plant leaf tissue had (Ritter and Eastburn, 1978;
Giordano and Mays, 1977; Bingham et al., 1975).
Hyperaccumulating plants must have the ability to accumulate heavy metals, have a high biomass
production, and disease and pest resistance in order to flourish within toxic heavy metal
contaminated soil. However, concentration of heavy metals in the roots of clover and wheat were
much higher than their shoots. These disproportional metal concentrations within the roots may be
due to the plants not reaching maturity. Comparisons between crop species show wide variations in
their ability to absorb potentially toxic trace elements from the sludge-soil system. This finding
suggests that the capacity to accumulate metals may be under genetic control (Hinesly et al., 1978).
Sensitivity of plants to metal toxicity can be associated with the tendency to accumulate the metal in
shoots (Boggess et al., 1978; Bingham et al., 1975).
The plants that are not deep rooted are highly affected by the heavy metals because the metals
remain in the top soils and do not leached deeper (Yin et al., 2009; Zupan i et al., 2009). Some plant
species can tolerate high levels of metals in tissue or can confine accumulated metals to the roots
and provide resistance to toxicity (Davis and Beckett, 1978).
In conclusion, it has become obvious from the literature that the majority of research into land
treatment or disposal of sewage and effluents has been carried out. Heavy metal contamination due
to sludge application has received much attention due to concerns regarding uptake by plants and
contamination of groundwater or surface waters. More studies are needed on sites that have
received sludge for many years to evaluate the mobility and bioavailability of minerals in soils and
crop plants. However, heavy minerals in sewage sludge and the toxicological implications are of
concern in agricultural production due to the adverse effects on food quality (safety and
40
marketability) and crop growth (phyto-toxicity). There is need for this study in developing
countries, because it will represent one of the long term problems associated with disposal of
sewage sludge and effluents in Nigeria.
41
CHAPTER THREE
MATERIALS AND METHOD
3.1 Site Description
The study was carried out at the University of Nigeria Nsukka (UNN) sewage disposable site.
Ecologically, Nsukka belongs to derived savannah zone of Nigeria and is located on latitude
06◦52ʹN and longitude 07
◦ 24ʹE. It is on an average elevation of 447 m above mean sea level
and is characterized by tropical wet climate usually from April to October and dry climate usually
from the month of November to March. The mean annual rainfall is about1600mm. The relative
humidity is rarely below 60% (Asadu et al., 2002). Nsukka has high temperature throughout the
year. The mean maximum and minimum temperature are about 35◦ and 22
◦C, respectively.
The soils of the area have been classified using key to soil taxonomy as arenic kandiustult based
on USDA Soil Taxanomy (Soil Survey Staff, 1984), derived from false bedded sandstone
(Akamigbo and Igwe, 1990). Its clay mineralogy is composed mainly of kaolinite and quartz
(Akamigbo and Igwe, 1990). The soil of the site has been subjected to heavy application of
partially treated sewage sludge and effluent for over 40 years and the farming community has
been taking advantage of the sewage sludge and effluent to grow various crops like vegetable,
cassava (manihot spps), maize (zea mays)etc.
3.2 The Sewage System
The sewage sludge and effluent are received through a network of underground pipes from the
residential and administrative area of the UNN. This is because the sewage site is at almost the
lowest elevation place within the university area. The effluent moves gravitationally through the
network of underground pipes to where it is received at the central collecting centre. The sewage
42
sludge and effluent from any part of the university at lower elevation than the central collecting
centre were conveyed by the conveyor (tractor tank) and emptied at the central reservoir at the
sewage site.
The central sewage system is constructed so well that no seepage is allowed; it has concrete down
to the base (Plate 1). The walls are thickly sealed with concrete down the base. The sewage
sludge is made to pass through the primary treatment path way. Primary treatment pathway
involves sedimentation of the biosolid. Settling is hastened by gravity and chemical flocculation
with aluminum and hydrated lime treatment. The sewage does not pass through further
stabilization involved in the secondary and tertiary pathway before they are discharged as partly
treated sewage sludge and effluent through open gutter to oxidation pond. These sewage sludge
and effluent are allowed to seep freely into the soil profile and some of the farmers make use of
the water from the oxidation pond to irrigate their crops.
43
Plate 1: The central sewage system (Photo courtesy of Ezeudo VC.)
44
Plate 2: The settling chamber (Photo courtesy of Ezeudo VC.)
45
Plate 3: The oxidation pond (Photo courtesy of Ezeudo VC.)
46
3.3 Field Sampling
Sludge samples were collected in two different position at the central sewage system. Auger soil
samples (0 – 40, 40 – 80 and 80 – 120 cm depths) were collected along two transects around the
oxidation pond at disposal site (0(edge), 100 and 300 m) and approximately 2000 m (control)
away from the disposal site. A total of 24 soil samples were collected. The sludge and soil
samples were air-dried (the sludge sample were additionally ground) and passed through 2mm
sieve and stored in black polyethylene bags preparatory for analysis.
Three samples of most common plants (cassava, maize, and Pannicum maximum) were collected
at 0(edge), 100, 300 and approximately 2000 m away from the disposal site.
3.4. Routine Analysis
The particle size analysis of the soil was determined using the Bouycous hydrometer method as
described by Gee and Or (2002). Soil pH was determined potentiometrically using pH meter in a
soil-water/KCl ratio of 1:2.5. The exchangeable acidity was done using 1N KCl displacement
method. Organic carbon concentration in the sludge and soil samples were determined by the
dichromate wet oxidation method of Walkley and Black as described by Nelson and Sommer
(1982). This was converted to organic matter concentration by multiplying with a correction
factor of 1.724. The total nitrogen was determined using the Kjeldhal method as described by
Bremner (1996). Available phosphorus was extracted using Bray II extractant as described by
Bray and Kurtz (1945) and determined using photo-electric calorimeter. Exchangeable calcium,
magnesium, sodium and potassium were extracted with NH4OAc at pH of 7.0. Calcium and
magnesium were determined using Ethylene diamine tetra-acetic acid (EDTA) titration method
47
while Potassium and sodium were determined by flame photometer (Rhoades, 1982). Cation
exchange capacity was determined titrimetrically using 0.01N NaOH. Exchangeable acidity (EA)
will be determined 1NKCl (Mclean, 1995)
3.5 Determination of the soil content of heavy metals (Pb, Cu, Cd and Hg)
The methods developed by the United States Environmental Protection Agency for total available
heavy metals in soil, sediments and sludge (USEPA SW-846, method 3050) (USEPA 1986), were
used in the preparation of the soil samples for the determination of their contents of total available
metals content in this study.
Cadmium was determined using xylenol orange indicator as described by Vogel (1965). Copper
was determined by ferro cyanide method as described by Alexegev (1969). Mercury was
determined ferrocyanide method as described by Alexegev (1969). Lead was determined by
sulphide method as described by Vogel (1965).
3.6 Sample Preparations and Digestion of crop plant for Heavy Metal Determination
All crops/plants were rinsed in distilled water gently; moisture and water droplets were removed
with the help of blotting paper. The plant samples were separated into roots, and leaves, and dried
before grinding to fine powder. The drying was done in an oven at 80◦C for 72 hours. The dry
samples were blended in a blending machine.
Metal concentration in plant tissues was determined by using the wet digestion method. The metal
accumulation of each plant species was determined by spectrophotometer method.
The levels of metal concentration in plants were compared with safety standard (WHO/FAO).
48
3.7 Data analysis
The design was a 4x3 factorial in which distance from the sewage disposal site and soil depth
were the two factors under consideration, factor A - Distances from sewage pond (0 edge, 100, 300 and
2000 m (control) away from the sewage pond) and factor B – soil depths (0-40, 40-80, and 80-120 cm).
Data obtain were subjected to two-way analysis of variance. Significant treatment mean were separated
using fisher’s least significant difference (LSD) at 5% probability level.
Bioconcentration Factor (BCF) was calculated to understand the extent of risk and associated
hazard due to the mobility of the heavy metals in soil and metal accumulation in each plant part.
The BCF was calculated as the ratio of concentration of metal in plant part to the concentration of
metal in soil. BCF = ������������� � ����� �� ����
������������� � ����� �� ����
Descriptive statistics was also employed in some aspects of data analysis.
49
CHAPTER FOUR
RESULTS
4.1 Sewage sludge
The results of the analysis of the sludge showed that the sludge was slightly acidic (pH of 6.5)
and was composed of high content of organic matter and total nitrogen, high level of available
phosphorous and potassium, and high concentrations of Cd, Cu, Hg, and Pb.
50
Table 1: Mean values of some elemental characteristics of sewage sludge and the corresponding
FAO/WHO standards.
FAO/WHO standards
pH in water 6.5 -
Organic matter (%) 6.10 -
Nitrogen (% ) 3.50 -
Phosphorous ( ppm) 35.00 -
K (cmol/kg) 2.52 -
Cd (mg/kg) 4.33 3000 – 6000
Cu (mg/kg) 26.01 140000
Hg (mg/kg) 5.01 23000
Pb (mg/kg) 1.50 300000
51
4.2 Physicochemical properties at various distances away from sewage site
The main effect of distance from sewage sludge on soil chemical properties shows that there were
significant differences (P < 0.05) in soil pH, organic matter concentration, nitrogen,
K+,Ca
2+,Mg
2+, total exchangeable base, cation exchange capacity and available phosphorous.
These soil parameters gradual decreased from the edge (0 m) of the disposal site through the 300
m distance to the control site (2000 m).Notably, the exchangeable sodium and total exchangeable
acidity though not significant (P < 0.05) showed negligible decrease as distance increased from
the edge to 2000 m from the soil(control) (Table 2).
52
Table 2: Effect of distance from the sewage site on soil physicochemical properties
Soil property Distance (m)
LSD(0.05)
0 (edge) 100 300 ~2000 (control)
Clay (g/kg) 95 95 111 171 NS
Silt (g/kg) 129 116 94 186 NS
F. sand (g/kg) 256 251 238 232 NS
C. sand (g/kg) 561 556 561 387 NS
pH in H2O 4.3 4.5 4.5 4.7 0.3
OM (g/kg) 1.7 1.5 0.6 0.40 0.2
N (g/kg) 0.3 0.2 0.1 0.06 0.06
Na+
(cmol/kg) 0.17 0.16 0.16 0.07 NS
K +
(cmol/kg) 0.19 0.17 0.13 0.10 0.04
Mg2+
(cmol/kg) 2.6 2.3 1.9 0.9 0.6
Ca2+
(cmol/kg) 2.8 2.6 1.9 0.9 0.6
TEB (cmol/kg) 5.7 4.8 3.6 2.1 0.8
CEC (cmol/kg) 22.6 20.8 18.2 9.94 1.5
H +(cmol/kg) 1.5 1.4 1.3 1.3 NS
Al2+
(cmol/kg) 1.4 1.3 1.3 1.3 NS
TEA(cmol/kg) 2.7 2.6 2.5 2.5 NS
P(mg/kg) 29.4 25.6 22.3 8.1 5.6
F. sand – fine sand; C. sand – coarse sand
TEB – total exchangeable base; TEA – total exchangeable acidity; CEC – cation exchange capacity
NS – non significant
53
4.3 Physicochemical Properties at Various Soil Depths
The main effect of depth on chemical property of the soil results in high accumulation of soil chemical
properties at 0 – 40 cm depth when compared to corresponding soil depths of 40 – 80 cm and 80 –120 cm.
Significance differences (P < 0.05) were observed among soil pH, organic matter content, total N,
exchangeable K, Mg and Ca, cation exchange capacity CEC and available phosphorous (Table 3).
54
Table 3: Effects of depth from the sewage site on soil physicochemical properties
Depth (cm)
Soil property 0-40 40-80 80-120 LSD0.05
Clay( g/ kg) 99 111 143 NS
Silt(g/ kg) 132 131 130 NS
F. sand(g/ kg) 260 228 246 NS
C. sand(g/ kg) 534 513 501 NS
pHinH2O 4.4 4.6 4.5 0.3
OM (g/ kg) 1.54 1.20 0.68 0.3
N (g/ kg) 0.21 0.13 0.11 0.05
Na+
(cmol/kg) 0.27 0.2 0.2 0.10
K +
(cmol/kg) 0.21 0.13 0.11 0.03
Mg2+
(cmol/kg) 2.1 1.9 1.7 0.3
Ca2+
(cmol/kg) 2.4 2.1 1.7 0.5
TEB (cmol/kg) 4.4 4.6 4.5 0.7
CEC(cmol/kg) 19.0 17.7 15.8 1.3
H +( cmol/kg) 1.5 1.2 1.2 0.3
Al2+
( cmol/kg) 1.3 1.3 1.3 NS
TEA(cmol/kg) 2.8 2.6 2.6 NS
P(mg/kg) 25.4 24.3 14.4 4.9
F. sand – fine sand; C. sand – coarse sand
TEB – total exchangeable base; TEA – total exchangeable acidity; CEC – cation exchange capacity
NS – non significant
55
4.4 Simple Matrix of Distance and Depth on Soil Physicochemical Properties
4.4.1 Soil pH as influenced by sewage disposal
At 0 m(edge) 100 m and 300 m of the disposal site soil pH gradually increased from 0 – 40 cm
depth to 80–120 cm depth. Notably the soil pH increased significantly (P ʹ 0.05) with depth at
the disposal site (0 m – 300 m) but decreased significantly with depth at the control site (2000 m)
Irrespective of the distance and depth the range of soil pH was 4.0 – 5.2 (Table 4a).
4.4.2 The Organic Matter Concentration and Nitrogen Content of the Soil
At all distances the organic matter concentration tends to decrease with increase in depth. The
organic matter content of the soil decreased with a definite pattern as distance increased from edge
to the control site. The result shows that organic matter content of the soil was significant (P ʹ
0.05) irrespective of the distance and depth. Total Nitrogen has a similar trend as that of organic
matter being much higher at the 0 m (edge) and decreased through 100 m and 300 m distance to
the control site (2000 m). Nitrogen content also decreased as soil depth increased from 0 – 40 to
80 – 120 cm interval (Table 4a).
4.4.3 Soil Exchangeable Bases
The results of simple matrix of distances and depths on the exchangeable bases show that
exchangeable sodium made no obvious difference. There were negligible decrease as distance and
soil depth increased from 0 to 2000 m and 0 to 120 cm respectively, then soil exchangeable
sodium ranges from 0.2 – 0.7 cmol/kg irrespective of distance and soil depth. Exchangeable
potassium consistently decreased from the 0 – 40 cm depth through 40 - 80 and 80 -120 cm soil
depth at each distance from the disposal site with exception of control (2000 m) where
56
exchangeable potassium concentration increased with increase in the soil depth (Table 4b). There
was significant difference (P ʹ 0.05) along four distances. Exchangeable calcium and magnesium
showed similar trends where 0 – 40 cm soil depth of each distance at the sewage disposal site
showed highest values for exchangeable bases while 2000 m or the control shows reverse
trend(Table 4b).
4.4.4 Total Exchangeable Acidity (TEA)
The soil total exchangeable acidity increased with depth in a definite pattern at 2000 m(control
site) (Table 4b) but varied considerably at the sewage site. The effects of distance and depth on
TEA were significant (P ʹ 0.05). Low accumulation of total exchangeable acidity was observed
at different soil depths at each distance at the sewage site (0 m 100 m and 300 m) compared with
similar soil depths at 2000 m(control) from the site (Table 4b).
There were significant (P ʹ 0.05) effects of distance and depth on cation exchange capacity
(CEC) content of the soil. The CEC values of the soils ranged from 8.5 - 24.8 cmol/kg with the
highest value 24.8 recorded at 0 – 40 cm soil depth at 0 m distance. CEC consistently decreased
from 0 –120 cm soil depth at each distance. Notably the soil CEC concentration of the disposal
site was about two times higher than that observed from the control (Table 4b).
4.4.5 Available Phosphorous
Available phosphorous distribution in the soil ranged from 7.5 to 34.4 mg/kg, available
phosphorous showed a gradual decrease from edge (0 m) through the 300m distance to the
control (2000 m), though different soil depth at each distance varied considerably along the four
distances (Table 4b). However, there were significant (P ʹ 0.05) effects of distance and depth on
57
available phosphorous. The available phosphorous concentration was significantly higher at
sewage site when compared to the control site (Table 4b).
58
Table 4a: The Mean Simple matrix of Distance and Depth on the Soil Physicochemical Properties
Distance
(m)
Depth (cm) Clay Silt F.sand C.sand pH in H2O OM% N% Na
(cmol/kg)
K
(cmol/kg)
0-40 80 133 27.6 564 4.0 2.24 0.37 0.22 0.29
0(edge) 40-80 80 125 24.14 551 4.3 2.0 0.21 0.16 0.16
80-120 124 130 25.41 568 4.5 0.80 0.2 0.13 014
0-40 7.5 13.9 27.7 54.1 4.2 1.77 0.24 0.18 0.27
100 40-80 8.9 11.1 22.8 57.7 4.6 1.50 0.16 0.13 0.14
80-120 12.2 9.8 24.7 54.9 4.8 1.11 0.11 0.11 0.11
0-40 9.7 9.5 25.4 56.9 4.1 1.22 0.16 0.16 0.16
300 40-80 9.7 8.7 22.8 56.9 4.5 0.98 0.10 0.11 0.11
80-120 14.0 10.1 23.1 54.4 4.7 0.51 0.1 0.08 0.08
0-40 14.4 16.0 23.4 46.4 5.2 0.87 0.1 0.06 0.9
2000 40-80 17.7 20.0 21.4 35.5 4.8 0.64 0.07 0.07 0.12
80-120 18.8 19.3 24.9 34.3 4.0 0.40 0.04 0.08 0.12
FLSD0.05 NS NS NS NS 0.6 0.6 0.11 NS 0.06
F. sand – fine sand; C. sand – coarse sand
TEB – total exchangeable base; TEA – total exchangeable acidity; CEC – cation exchange capacity
NS – non significant
59
Table 4b: The Mean Simple matrix of distance and depth on the soil physicochemical properties (contd.)
Distance
(m)
Depth
(cm)
Mg
(cmol/kg)
Ca
(cmol/kg)
TEB
(cmol/kg)
CEC
(cmol/kg)
H
(cmol/kg)
Al
(cmol/kg)
TEA
(cmol/kg)
P
(ppm)
0-40 3.12 3.01 6.57 24.8 1.4 1.11 2.59 34.4
0(edge) 40-80 2.65 3.05 5.78 22.2 1.07 1.41 2.41 34.4
80-120 2.05 2.5 4.74 20.7 1.41 1.38 2.76 19.31
0-40 2.81 3.2 6.55 21.4 1.52 1.08 2.60 29.2
100 40-80 2.23 2.6 5.64 20.1 1.41 1.33 2.74 30.6
80-120 1.90 2.02 4.05 18.5 1.36 1.41 2.76 17.08
0-40 1.94 2.50 4.25 19.6 1.7 1.07 2.76 28.6
300 40-80 1.84 2.01 4.09 19.3 1.23 1.31 2.54 27.9
80-120 1.07 1.30 2.33 13.6 1.47 1.45 2.72 13.7
0-40 0.55 0.80 1.58 10.3 1.95 1.30 3.01 9.2
2000 40-80 1.06 0.81 1.92 9.4 1.9 1.40 3.20 7.5
80-120 1.25 1.04 2.79 8.5 1.85 1.8 3.65 7.59
FLSD0.05 0.69 1.02 1.4 2.5 0.41 0.35 0.42 9.8
60
4.5 Heavy Metals in Soil
4.5.1 Effect of distance on selected heavy metal contents of the soil
The main effects of distance on selected heavy metals contents of the soil are shown in Table 5. It
is evident from the table that the 0 m (edge) distance accumulated much of the heavy metals.
These heavy metals significantly (P ʹ 0.05) decreased consistently as distance increased from the
edge (0 m) to the control (2000 m). However, the result of the study reveals that the sewage site
distances (0, 100 and 300 m) had higher concentrations of the soil heavy metal when compared
with the 2000 m or control distance.
61
Table 5: Effects of distance on soil heavy metals distributions
0 m 100 m 300m 2000 m FLSD0.005
Cd (mg/kg) 1.3 1.1 0.7 0.3 0.07
Cu (mg/kg) 28.6 26.9 21.4 12.6 1.0
Hg (mg/kg) 4.1 3.1 2.4 1.1 0.32
Pd (mg/kg) 1.3 1.3 1.0 0.3 0.12
62
4.5.2 The Main Effects of Depth on Selected Heavy Metal Content of the Soil
The main effects of soil depth on selected heavy metal content shown in (Table 6) indicate that
irrespective of distance from the sewage site, there was significantly higher accumulation of
heavy metal at the top soil (0 – 40 cm). Heavy metal concentration significantly (P ʹ 0.05)
decreased in a definite pattern as soil depth interval increased from 0–40 cm to 80–120 cm. It was
observed that copper had the maximum concentration at each soil depth interval while cadmium
was the least at each interval.
63
Table 6: Effects of depth on soil heavy metals distribution
Depth (cm) Cd ( mg/kg) Cu ( mg/kg) Hg ( mg/kg) Pb ( mg/kg)
0-40 1.1 26.4 3.2 1.3
40-80 1.0 25.9 1.5 1.1
80-120 0.5 14.8 0.3 0.5
FLSD 0.05 0.06 1.0 0.7 0.1
64
4.5.3 The Simple Matrix of Distance and Depth on the Concentration of the Selected
Heavy Metal in Soils
The simple matrix of distance and depth on the concentration of the selected heavy metal in soil
are shown in Table 7. The result show that there was greater accumulation of heavy metals at
different soil depth of sewage disposal site distances (0, 100, and 300 m) when compared with the
value obtained from the control. There were significant interaction (P ʹ 0.05) effects of cadmium
concentration. The highest cadmium concentration of disposal site soil (1.84 kg/mg) was obtained
from 0 m (edge) distance at 0–40 cm soil depth while the lowest (0.14 kg/mg) was from 300 m
distance at 80 – 120 cm soil depth. The control (2000 m) varied inconsistently among the soil
depth. However, the other three heavy metals ( Cu, Hg and Pd) content of the disposal site gave
similar pattern as that of cadmium by decreasing significantly (P ʹ 0.05) in a definite pattern at 0
m distance with soil depth interval while the same heavy metals considerably varied at the soil
depth of the control. Similar trend was maintained at 100, 300 and 200 m. Irrespective of soil
depths and distance Cd, Cu, Hg, and Pd ranges from 0.11-1.84, 8.84-33.7, 0.9-4.7 and 0.29-1.7
mg/kg respectively. Copper had the highest concentration among the four heavy metals studied at
both the sewage soil and the control soil while lead was the lowest.
65
Table 7: The mean simple matrix of distance and depth on the soil heavy metal content
Distance (m) Depth(cm) Cd ( mg/kg) Cu ( mg/kg) Hg ( mg/kg) Pb ( mg/kg)
0-40 1.84 33.7 4.7 1.70
0(edge) 40-80 1.41 30.5 4.6 1.40
80-120 0.75 20.6 2.9 0.9
0-40 1.59 30.9 3.5 1.8
100 40-80 1.06 26.6 4.4 1.6
80-120 0.67 19.9 1.3 1.7
0-40 0.59 27.8 3.2 1.40
300 40-80 0.25 24.6 3.0 1.30
80-120 0.24 9.9 0.9 0.30
0-40 0.2 14.6 1.5 0.23
2000 40-80 0.28 8.9 1.41 0.43
80-120 0.40 14.9 1.7 0.29
FLSD0.05 0.3 2.1 0.6 0.1
66
4.6 Heavy Metal concentration in Plant
Table 8 show the main and interaction effects of the three factors considered (distance, plant and
plant parts) on heavy metal content of the soil. Generally the heavy metal concentration obtained
from the test plants decreased significantly (P ʹ 0.05)with increase in distance indicating high
accumulation of heavy metals in plants at the sewage areas compared to the control (2000 m).
4.6.1 Cadmium
There were significant (P ʹ 0.05) effect of distances on the cadmium concentration in plants at
both sewage site (0,100, and 300 m) and control (2000 m) The highest cadmium concentration
(1.34mg/kg) was obtained from the edge of the disposal site (Table 8) while the lowest was found
to be at the control distance (0.1mg/kg). Data presented in Table 8 also show that the differences
between cadmium content of plant (cassava, maize and pannicum) were significant (P ʹ 0.05).
Pannicum gave the maximum accumulation of cadmium (0.8mg/kg), while cassava recorded the
lowest concentration of Cd (0.6 mg/kg). Significant (P ʹ 0.05) differences were obtained among
plant part (leaf and root), with the root showing highest accumulation of cadmium.
4.6.2 Copper
Significant (P ʹ 0.05) differences were obtained among distances with respect to the
concentration of copper in plants. Table 8 shows that the highest concentration of copper (23.8
mg/kg) in plant was obtained at 0 m distance and the value consistently decreased through 300 m
distance to the control site (2000 m). As shown in Table 8, the copper content differed
significantly among cassava, maize and pannicum. Pannicum had the highest concentration of
copper while the lowest concentration was obtained from Cassava. The effects of plant part on the
67
copper content were significant (P ʹ 0.05). The leave had higher (16.3mg/kg) accumulation of
copper than the root (15.9mg/kg).
4.6.3 Mercury
Analysis of variance (ANOVA) for the concentration of mercury reveals a significant (P ʹ 0.05)
effect on the plant due to distance. Maximum concentration of mercury was obtained at 0 m
distance (3.2 mg/kg) and the value decreased progressively through 300m distance, (1.7 mg/kg) of
the sewage site to the 2000 m distance (control) (0.8 mg/kg). However, the mercury contents in
plants (cassava maize and pannicum maximum) were significant (P ʹ 0.05). Pannicum maximum
had the maximum accumulation of the mercury content (2.3 mg/kg) while the lowest
concentration (1.8 mg/kg) was observed in cassava (Table 8). Table 8 reveals that mercury
concentration in plant parts (root and leaf) were also significant (P ʹ 0.05).
4.6.4 Lead
The results of effect of distance on the concentration of lead in plants were significant (P ʹ 0.05).
The concentration of lead in plant at 0 m (edge) was highest and decreased gradually through
100m (1.2 mg/kg), 300 m (1.0 mg/kg) and then to the control (0.4 mg/kg). The control (2000 m)
was about 3times lower than values obtained at each distance at the sewage disposal site (Table 8).
Cassava, maize and pannicum plant were significant (P ʹ 0.05). Pannicum had the highest
concentration of lead while maize plant was the least on lead concentration (Table 6). The leaf and
root was significant (P < 0.05) and the root recorded the highest concentration.
68
Table 8: Mean effects of distance, plant and plant parts on heavy metal
accumulation
Cd ( mg/kg) Cu ( mg/kg) Hg ( mg/kg) Pb ( mg/kg)
Distance (m)
0 1.34 23.8 3.2 1.2
100 0.98 18.1 2.4 1.0
300 0.43 15.3 1.7 1.3
2000 0.1 7.3 0.8 0.4
FLSD 0.05 0.03 0.5 0.1 0.1
Plant
Cassava 0.6 14.3 1.8 0.9
Maize 0.7 15.1 2.0 0.8
Pannicum 0.8 18.9 2.3 1.1
FLSD 0.05 0.03 0.43 0.1 0.1
Plant part
Leaf of all plants 0.7 16.3 1.9 0.8
Root of all plants 0.8 15-9 2.2 1.1
FLSD0.05 0.1 0.4 0.1 0.1
69
4.7 The Bioconcentration Factor (BCF) of Cassava
There were significant (P ʹ 0.05) effects of distance on the four heavy metals (Cd, Cu, Hg, and Pb). The
highest BCF value of the heavy metals was obtained at 0 m (edge) distance (Table 8) while the lowest was
found to be at the 2000 m (control). Data presented in Table 9 also show that the BCF of the leaf portion
were significant (P ʹ 0.05) among the four heavy metals, with 0 m (edge) recording the highest BCF
value while the control was the lowest. With exception of lead, the BCF value of the other three
heavy metals at the 2000 m (control site) appears to be less compared to the other distance at
sewage site (Table 9). Data on the table shows that Cu had the maximum accumulation in the leaf
rather than in the root of cassava plant. However, the result on table 9 reveals no hyper
accumulation of heavy metals in both root and leaf of cassava.
70
Table 9: Bioconcentration factor (BCF) in root and leaf of cassava
Cd ( mg/kg) Cu ( mg/kg) Hg ( mg/kg) Pb ( mg/kg)
Root
0 m(Edge) 0.83 0.77 0.85 0.80
100 m 0.71 0.62 0.83 0.71
300 m 0.82 0.63 0.67 0.65
2 km(control) 0.36 0.57 0.66 0.80
FLSD 0.05 0.03 0.03 0.03 0.02
Leaf
0 m(Edge) 0.76 0.88 0.67 0.72
100 m 0.66 0.71 0.81 0.65
300 m 0.66 0.73 0.62 0.60
2 km(control) 0.28 0.52 0.60 0.55
FLSD 0.05 0.03 0.03 0.03 0.02
BioconcentrationFactor(BCF) = ������������� � ����� �� ����
������������� � ����� �� ����
71
4.8 The Bioconcentration Factor (BCF) of Maize
The results of the effect of distance on the heavy metals were significant (P ʹ 0.05) (Table 10).
The BCF value of Cd, Cu, Hg and Pb had their maximum accumulation at both root and leaf of
maize at 0 m distance (edge) while the lowest accumulation of heavy metals in maize was at 2000
m (control) distance respectively (Table 9). The level of accumulation of heavy metals in maize
consistently decreased from 0 m (edge) through 300 m to the control (2000 m). The BCF value of
maize was higher in the root than in the leaf tissue of maize. Notably, there was no hyper
accumulation of heavy metal at both root and leaf of maize (Table13).
72
Table 10: Bioconcentration factor in root and leaf of maize
Cd ( mg/kg) Cu ( mg/kg) Hg ( mg/kg) Pb ( mg/kg)
Root
0 m(Edge) 0.69 0.65 0.78 0.71
100 m 0.66 0.53 0.63 0.88
300 m 0.60 0.50 0.69 0.82
2 km(control) 0.32 0.46 0.53 0.84
FLSD 0.05 0.03 0.03 0.03 0.02
Leaf
0 m(Edge) 0.59 0.71 0.74 0.40
100 m 0.60 0.56 0.36 0.50
300 m 0.43 0.57 0.61 0.67
2 km(control) 0.20 0.50 0.39 0.60
FLSD 0.05 0.03 0.03 0.03 0.02
Bioconcentration Factor (BCF) = ������������� � ����� �� ����
������������� � ����� �� ����
73
4.9 The Bioconcentration Factor (BCF) of Pannicum maximum
Significant (P ʹ 0.05) differences were obtained among heavy metals accumulation with respect
to distances at both roots and leaves of plant respectively. The result showed maximum
accumulation of cadmium, mercury and lead in the root than the leaf system of panncium
maximum plants at the sewage site (Table 11). The BCF value of Cd, Hg and Pb had their
maximum accumulation at both root and leaf of panncium maximum at 0 m distance, while the
lowest accumulation of heavy metals in pannicum maximum was at their respective 2000 m
(control) distance. Cadmium *had the BCF value of 1.00, but only in the root tissue of pannicum
maximum at 0 m distance.
74
Table 11: Bioconcentration factor in root and leaf of pannicum maximum
Cd ( mg/kg) Cu ( mg/kg) Hg ( mg/kg) Pb ( mg/kg)
Root
0 m(Edge) 1.00 0.58 0.71 0.87
100 m 0.67 0.52 0.54 0.58
300 m 0.83 0.55 0.62 0.64
2 km(control) 0.14 0.44 0.67 0.60
FLSD 0.05 0.03 0.03 0.03 0.03
Leaf
0 m(Edge) 0.94 0.61 0.67 0.68
100 m 0.60 0.58 0.28 0.41
300 m 0.94 0.57 0.50 0.24
2 km(control) 0.40 0.46 0.61 0.54
FLSD 0.05 0.03 0.03 0.03 0.02
Bioconcentration Factor (BCF) = ������������� � ����� �� ����
������������� � ����� �� ����
75
Table 12: Guideline for safe limits of heavy metals in soil and plant
Samples Standards Cd Cu Hg Pb
Soil (mg kg-1
) WHO/ FAO
commission
3000 – 6000 140000 23000 300000
Plant(mg kg-1
) WHO/ FAO
commission
200 40000 20000 – 30000 50000
76
CHAPTER FIVE
DISCUSSION
5.1 Physicochemical Properties of the Soil
Most of the changes in soil chemical properties observed in this study are largely explained by
differences in distances away from the sewage pond and their corresponding soil depths. Close
distances to the sewage pond had an increased, exchangeable bases, soil organic matter, total
nitrogen, available phosphorous at the disposal site. These properties decreased as distance and
soil depth increased from 0 m to 2000 meters away from the pond.
The soil pH differed significantly among the three distances (0 m, 100 m 300 m) and three soil
depths. These values are expected as most of the observed increase in pH with increase in distance
and depth could be attributed to accumulation of organic acid generated during the process of
organic matter decomposition (Veerseh et al., 2003). Under condition of reduced oxygen,
decomposition would result in accumulation of more organic acid due to incomplete breakdown
of organic matter. The values obtained at the sewage site were however, lower on average when
compared to the value on the control. This may be attributed to the buffering effect of soil organic
matter against pH change in addition to the release of basic cation during the organic matter
decomposition (Oyedele et al., 2008).
The textural composition of the soil did not differ significantly with distance and among the three
depths. This is expected as soil texture is mainly inherited from soil forming parent materials. The
soil percentage of sand and silt generally decreased with increase in depth while the clay content
increased down the profile (Kadeba, 1978).
77
Higher soil OM level was typically observed in the soils closer to the sewage site. This higher soil
OM content at the edge (0 meter distance) was associated with sewage pond seepage into the soil,
leading to more organic matter accumulation at 0m distance relative to other two distances. This
confirms an earlier indication by (Asadu et al., 2008, Udom et al., 2000) that sewage sludge
effluents are utilized for irrigation purposes and also increases soil fertility as well. The soil
organic matter content of the control (2000 m) was lower compared to the study site. Higher
organic matter content of the sewage site might be due to sewage waste application (Bhati and
Singh 2005). In general the suitability of soil for receiving waste water without deterioration
varies widely, depending on their infiltration capacity, permeability, CEC, phosphorous
adsorption capacity, texture, structure and type of clay minerals(Ivan and Ear)1972. The organic
matter contents of the sub soil plays important role in the absorption reaction in the soil thereby
preventing pollutants from reaching ground water (Alloway and Aryes 1997).
Like the organic matter content total nitrogen content showed significant differences among the
four distances and three soil depths. Increased soil nitrogen content was expected to result from
the sewage sludge site. This increase was evident on crops but appeared to decline slightly as
distance and depth increases. This decline in soil nitrogen content apparently resulted from
reduction of sewage sludge as we move away from the disposal site,this were in agreement with
the findings of (Gelsomins et al., 2006) Matovi et al., 2005 and Weber et al., 2007. The high
nitrogen content in the soils of sewage sludge could sometimes be detrimental to crop production,
particularly leguminous crops in view of interference in plants absorption of nutrients (Asadu et
al., 2008).
Exchangeable bases (Sodium, Potassium, Calcium and Magnesium) decreased as distance and
depth increased in the study site. These high concentrations of basic cations at 0 m and top soil
78
was apparently because the cations were concentrated in the organic matter rich surface soil that is
mixed with waste materials at different stages of decomposition which continuously released the
cation (Oyedele et al., 2008).
The significantly higher concentration of K at the edge (0 m distance) and top soil (0–40 cm) at
the sewage site may be explained by the release of K from the sewage sludge.
The top soil (0 – 40 cm) at the sewage sludge site had significantly higher concentration of Ca at
all distances this might be due to high contents of sewage sludge and surface application of
sewage sludge at the study site.
The significant concentration of Mg at all distance and depth at the study site was consistent with
the idea that the main source of Mg is the exchangeable Mg found in the clay humus complex
(Brady and Weil, 2004) which was believed to be influenced by sewage sludge.
As expected the exchangeable Ca was higher than exchangeable Mg apparently as a result of high
level Ca in sewage sludge. Sewage sludge inputs are not only important for agricultural
productivity, but also for the supply of some internal nutrient elements.
Generally, the results show higher content of exchangeable base at sewage site relative to the
control site. Exchangeable bases also decreased consistently with distance and depth. These
results agree with some of the earlier work comparing sewage sludge soil and unamended soil
(Mbagwu et al., 1991, Anikwe and Nwobodo 2002, Asadu et al., 2008).
The concentration of cation exchange capacity in the soil differed significantly among three
different distances and three different depths at sewage site. The effect of the CEC was more
pronounced at 0 – meter distance and 0 - 40cm depth in sewage site. The decrease in CEC as both
79
distance and depth increased might have resulted from differences in soil texture and organic
matter. High soil organic matter accounts for great percentage of the total cation in exchange site
at the sewage site. The higher CEC at the sewage site confirms the findings of Soon 1981, Asadu
and Nweke 1999, and Asadu et al.,(2008), that sewage sludge increases the CEC and organic
matter of soil. This also confirms the earlier indication by Udom et al., (2000) that the sewage soil
was more fertile than the control soil.
Phosphorous shows a significantly higher concentration at the three distances and three depths in
the soils sewage site, with P concentration decreasing as distance and depth increases. The
difference in the mineralogical composition of soil may influence the changes in concentration of
available phosphorous. With the edge (0m distance) having more of sewage sludge its ability to
fix P may be more than soil of other corresponding distances. This may explain the higher P
content observed in sewage site distances (0 m, 100 m and 300 m) compared to 2000m of the
control site. The study by Gelsomino, et al., (2006) also found P increased in soil with high
organic amendment.
The high concentration of P at 40-80 cm depth at the sewage site is in agreement with earlier
finding by Asadu et al., 2008 that there was greater P mobility in the sewage soil indicating
possible leaching into the soil where it became more fixed. Phosphorous though beneficial to
crop, an excessive content of mobile P is eventually unfavourable because it might represent a risk
factor in water eutrophication (Karboulewsley 2002, Penn and Slino, 2002).The study also found
that there was threefold increase in P concentration in soils of the sewage site relative to the soils
of the control. This may be due to higher Fe content associated with the control soil may result in
the formation of insoluble phosphate further reducing P available Maguire et al., (2000).
80
Exchangeable acidity of 0 meter distance have the lowest acidity measurement at the disposal site,
there was a significant increase in soil acidity as both distance and depth increase. The 0-meter
distance producing the lowest measurement of acidity may be explained by the larger
concentration of base cations and organic matter that are present in the sewage sludge such as
calcium, resulting in a nutrient upliftment which increased basic cations in the soil and decreased
acid cations, thereby decreasing the soil acidity of disposal site.
Furthermore, a low acidic environment will decrease the leaching of Ca because solubility of
aluminium and iron decreases, thus competing with acid cations for the binding sites. At the
control site the acidity decrease with increase in depth. This may be due to intense decomposition
taking place in the control site may possibly increase the release of H+ and Al
3+ ion from the
decomposing plant matter. These ions tend to kick base cations off the binding site, thus
increasing the acid cations on the binding sites causing the soil acidity to increase at the control
site.
81
5.2 Heavy metals in soil
Soil is a dynamic and complex system where any changes in its physiochemical properties would
severely alter the fate of heavy metals within its body. Sewage sludge due to its heavy metal
content might be a threat to. the soil and the environment, if it is used for a long time (Epstein,
2003, Gasco et al., 2005). In general, the suitability of the soils for receiving waste without
deterioration varies widely depending on their infiltration capacity, permeability, Cation exchange
capacities, phosphorus adsorption capacity, texture, structure and type of clay minerals.
Using approximately 2000 m distance as a baseline for comparison, over 40 year disposal of
sewage sludge on to the soil of the sewage site resulted in an increment of the soil concentration
heavy metal in the different soil depth intervals, compared with soil depth intervals at the control
(2000 m).
The sequence of heavy metals in soil sample from sewage sludge site was in order Cu > Hg ʹ Cd
ʹ Pb. The concentration of heavy metal show progressive variations which might be ascribe to
the variation in heavy metals sources and the quantity of heavy metals in sewage sludge. In the
present study, concentration of heavy metal decreased significantly with the increase in distances
and soil depth at sewage sludge site. These results are in agreement with the findings obtained by
(Yada et al., 2002, Mascoid Tabari 2008). Since the surface is rich in heavy metal than the
underlying layer greater accumulation in the top soil is probably due to soil texture, low mobility
of heavy metal in soil (Afonia et al., 1998) and surface application of sewage sludge, (Asadu et
al, 2008).
The inconsistent pattern in the variation of heavy metal content with soil depths at the control
(2000 m) was in contrast with the consistent trend in the sewage site( 0 m 100 m 300
82
m),suggesting the presence of discontinuities within sewage site due to low mobility of heavy
metal in soil.
The comparison of concentration of Cu, Cd, Hg and Pb in soil with WHO/FAO standard (Table
12) shows that they are below safe limits of heavy metals in soils. Continuous cultivation and
regular absorption by plants possibly keep the concentration of heavy metals in the soil within
safe limit at sewage site. Hence, the proximity of the sewage site to the cultivated land does not at
the present pose any threat to the soil.
Furthermore, since soil macro organisms and their activities are crucial to the maintenance of soil
fertility, there is considerable concern that these heavy metals could have permanent adverse
effect on soil quality and crop production. The solubility of the metals in soil and ground water is
predominantly controlled by pH, organic matter and CEC. From this present study heavy metal
mobility decreases with increase in soil pH. This was in agreement with the; Smith and Giller
(1992). The result shows that the soils of the study site are slightly acidic which favours the
precipitation and mobilization (Battachry et al., 2002).
The seepage of these heavy metals through the soil of sewage site can infiltrate directly through
unsaturated zones to cause severe pollution problems. Their presence in ground water can cause a
long term health risk to humans through food chain (Earth et al., 2000 Ogbonna et al., 2006).
Although metals are essential to plant, at high concentration they become toxic and present
different problems to soil micro organism because they cause oxidative stress by formation of free
radicals. They can also replace essential metals in pigment or enzymes, thus disrupting their
function (Henry 2000) and may render the land unsuitable for plant growth and destroy the
biodiversity.
83
However, it was suggested that soil contamination may be considered high when concentrations
of an element in soil were two to three times greater than the average background level. There is
an indication that this metal are higher in sewage site, which was also reflected in the low level of
these metals obtained from the control. The degree of heavy metal contamination may be
considered to be high at the sewage site.
Therefore, the soil of sewage soil may be considered contaminated based on the fact that heavy
metal content in all control soil in this study was low. Although the mean concentration of the
heavy metal obtain from the sewage site fall within the acceptable safe limits proposed
WHO/FAO standard. Thus, sewage sludge seepage and anthropogenic input of these heavy metals
seemed to be less pronounced in the area of investigation.
84
5.3 Heavy Metal in Plant
There was a progressive decrease in heavy metal content of selected plants at the disposal site as
the distance increased from edge (0 m) to control (2000 m). The low levels of heavy metals in the
test plants at the control (2000 m) might be due to low mobility of heavy metals. The high
concentration of Cu in leaves of the selected plants as compared with the roots is not consistent
with literature (Romana et al., 2005). The high concentration of Cu in the leaves of plant in this
study might be partly due to surface irrigation with the sewage water and partly to the fact that
copper was the most abundant metal in the soil at the three different distances in the sewage site.
With exception of Cu the roots have higher concentration of heavy metals than the leaves.
Ademoroti (1966) reported that plants accumulated considerable amount of heavy metals
especially Pb, Cu, and Zn in the root than the leaves. The concentration of heavy metals in all
plant sample analyze were lower than FAO/WHO guide lines values (Table 12). Liu, et al. 2005
and Sharma et al., 2007 demonstrates that plant grown on waste water irrigated soil are
contaminated with heavy metal and pose health concerns.
This study has shown little accumulation of Cd and Pb. This may be due to the reason that Cd and
Pb poses some toxic effect on plant (Anderson et al., 2004). Another possible explanation would
be unavailability of Cd and Pb due to high affinity of these metals to organic matter (Merrit and
Erich 2003, Migam et al., 2001, Strawn and Sparks 2000). The overall low concentration of the
Pb and Cd was observed in the both root and leaf of test plants.
The absorbance and accumulation of heavy metals in the plant tissue depends on metal
concentration and chemical forms in soil (Shen et al. 2000). Very little amount of lead was
extracted from the soil. These factor involved in the phyto-availability of this metal are organic
85
matter, soil pH, plant root, and other soil condition (Zimdahl and Hassett, 1997). The limited
potentials of Phyto availability were due to low soil mobility and little tendency for the uptake Pb
into the root (Lasat, 2002) (low bio availability in the root).
The unconventional behaviour of Pb uptake is well renowned (Reeves and Brooks, 1983). It may
be due to this reason that plant respond differently to soil Pb as compared to other metal, (Huang
et al, 1997). Some other reason might be the high susceptibility of lead to change into sorption
form in soil matrix. Moreover root membrane barrier cannot be overlooked because mechanism of
Pb transport from soil to root tissue is not clear Blaylock et al. (1997).
The concentration of the heavy metal in the root and leaves of selected plant differed significantly
at different distance among different test plant and plant parts. Marked different in heavy metal
content may be due increased seepage and irrigation with water from the oxidation pond. This
result was in agreement with Singh and Bhati (2005) and Aghabarati et al. (2003) where
substantial greater concentration of these heavy metals were observed in the plants at the disposal
site and plants irrigated with sewage sludge compared to the plant at control site. However, the
presence sewage sludge and irrigation with sewage waste water did not result in toxicity of the
test plants.
Further more, in the present study many soil factors such as pH, organic matter interacted to
impact in the uptake of heavy metal by plant. The acidic range of soil is known to increase the
mobilization of heavy metals, thus increasing their uptake. The field data also support that soil pH
was acidic. The presence of organic matter has been reported to increase the uptake of Cu, Pb Cd
and Zn in the wheat Plant (Rupa et al., 2003). Earlier study on the site have it that the build up of
86
these metals to critical level can be phyto toxic and may result in reduced plant growth and or
increase within the food chain (Asadu et al ., 2008)
5.4 The bioconcentration factor (BCF)
The bio concentration factor (BCF) which is the relationship between total metal content in plant
and soil shown in table 9, 10 and 11 had a significant effect among distances and among plants
parts. The BCF recorded in this study is an indication of the potential of heavy metal in the
sewage site to be transferred into the food chain through the consumption of edible plant on the
site by either animal or man. Variation in bioconcentration factor among different root and leafs
of plant may be attributed to differences in the concentration of heavy metal in the soil and
differences in element uptake by different plant (Cui et al., 2004, Zheng et al., 2007).
Notably, high accumulation of metal in the root than in the leaf was consistent with other work
(Bidar et al., 2007; Margiues et al., 2009). Metal in plant did not show any hyper accumulation
with those of soil. The BCF show that there is a higher accumulation of Cd and Pb in root of
Panincum maximum plant than in the leaves. The high concentration of heavy metal in the root of
Pannicum maximum was in agreement with findings of Yin et al. 2009 and Zupan et al. 2009 that
the plants that are not deep rooted are highly affected by the heavy metals because these metals
remain in the top soils and do not leached deeper in to soil.
87
CHAPTER SIX
6.0 SUMMARY AND CONCLUSION
The study at the University of Nigeria Nsukka disposal site found that cation exchange capacity
pH, organic matter content, total N, P, K, Ca and Mg were increased in soil around the sewage
sludge site. The extents of improvement were highly significant with distance and depth. The
concentration of the soil properties was pronounced at the edge (0 m distance) and decreased with
distance through 100 m to 300m.There was higher concentration of these properties at 0 - 40 cm
depth and decreased gradually with depth, with the exceptions of soil pH that increased with soil
depth. The mobility of these soil properties was pronounced at the sewage site than the control
(2000 m).
The level of soil accumulation of heavy metal was high at the sewage site due to sewage water
free seepage in to the soil and probably due to irrigation with water from the oxidation pond
within this area. The level of heavy metal increased significantly (P ʹ 0.05)with distance and
depth. However, level of heavy metal in the soil was lower than those recommended by
WHO/FAO joint safe limit. Hence has not reached hazardous concentration even though their
concentration in the soil of the sewage site has increased about twice compared with the control.
Plants grown around the sewage site have increased heavy metal content. Heavy metal
accumulation in plant decreased with distance. The presence of heavy metals in plant has been
shown to be influenced by sewage pond around the sewage site. Among the root and leave tissues
of plant studied, the root has highest concentration of heavy metals compared with the leaves. The
concentrations of heavy metal in both root and leaves of plant are below WHO/FAO guide line
values.
88
Therefore it follows that, free seepage and anthropogenic sources of heavy metal increase the
heavy metal content of the sewage sludge. Toxic heavy metals accumulate faster in the soil and
plant around sewage sludge due to their proximities to sewage pond. However, with an increase in
concentration of soil organic matter, soil exchangeable bases, movement and translocation of
heavy metals in soil decreased while, with a decrease in soil organic matter (Bounded with soil
particles) and pH as well, their mobility increase. The low mobility of metals in the sewage soils
may be related to high Ca and organic matter content. Furthermore increased mobilization of these
elements in the sewage may be related to the lower organic matter, clay, and Ca content of the
soil.
Conclusions drawn from this study are that;
� The University of Nigeria Nsukka sewage disposal site resulted in an increased
concentrations cation exchange capacity, pH, organic matter content, total N, P, K, Ca and
Mg in soil around the sewage sludge site.
� The study has also revealed presence of Cd, Cu, Hg, and Pb in the soils around sewage
sludge site but they were below the maximum allowable limit.
� The mobility of heavy metals (Cd, Cu, Hg, and Pb) from soil to leaves suggesting that all
these metals were mobile from soil to plant components
� Heavy metal concentrations varied among the tested crop/plant, which reflect the
differences in their uptake capabilities and their further translocation to the shoot portion
of the plant.
� The level of heavy metal in both crop/plant will not place the consumer of this crop/plant
grown within the vicinity of the sewage site at health risk with time.
89
� The bioconcentration factor at both root and leaf tissues of crop plant did not show any
abnormal accumulation of the heavy metal at the sewage site.
It is therefore suggested that the regular monitoring of heavy metal in soil and plant grown around
sewage sites is essential since heavy metal is considered a serious agricultural concern.
.
90
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