soil remediation and plants || soil contamination, remediation and plants

525Soil Remediation and Plants. http://dx.doi.org/10.1016/B978-0-12-799937-1.00018-8Copyright © 2015 Elsevier Inc. All rights reserved.

Soil Contamination, Remediation and Plants: Prospects and Challenges

M.S. AbdullahiDepartment of Chemistry, Federal College of Education, Kontagora, Nigeria

INTRODUCTION

Vegetables constitute parts of the human diet since they contain carbohydrates, proteins, vitamins, minerals and trace elements. Their consumption is on the increase, particularly among the urban community, due to increased awareness of their food values as a result of exposure to other cultures and education. However, they contain both essential and toxic elements over a wide range of concentrations. Metals accumulation in vegetables may thus pose threat to human health. Heavy metals are contaminants in the tissues of vegetables. Heavy metals such as cadmium, copper, lead, chromium and mercury are pol-lutants, particularly in areas under irrigation using wastewater. Investigations on water, soil, sediments and vegetables irrigated with wastewater are available in literatures from different researchers of different parts of Africa.

Agriculture is one of the major sources of environmental pollution (Gucel et al., 2009; Ashraf et al., 2010 a, b, 2012; Ozturk et al., 2010; Hakeem et al., 2013). Efforts were made and are still being made to improve the productiv-ity of the low-nutrient status of soil in tropical Africa so as to enhance food production in order to sustain the projected population growth necessitate irrigated farming system. To ensure the success of this objective, the use of inorganic fertilizers had tripled. Superphosphate fertilizers contain not only the necessary elements for plant nutrients and growth, but impurities such as Cd, Pb or Hg. Thus fertilizer application is a source of soil contamina-tion. As such, the intake of these heavy metals by plants, especially the leafy vegetables, is an avenue of their entry into the human food chain with harm-ful effects on health. The level of heavy metals in soils through phosphate fertilizer application depends on the origin of phosphate rocks from which the fertilizer is manufactured.

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526 Soil Remediation and Plants

Industrial or municipal wastewaters are used for the irrigation of crops in a particular ecosystem, due to their availability and due to the scarcity of fresh water (Ozturk et al., 2011). Irrigation with wastewater contributes to heavy metal contents in soil. Heavy metals are harmful because of their non-biodegradable nature, long biological half-lives and their potentials to accumulate in different body parts. Heavy metals are toxic because of their solubility in water. Low concentrations of heavy metals have damag-ing effects on humans and animals because there is no mechanism for their elimination from the body. Wastewater contains substantial amounts of toxic metals which create problems. A high accumulation of these metals in agri-cultural soils through wastewater irrigation does not only result in soil con-tamination, but also affects food quality and safety. Food and water are the sources of essential metals: these are also the media through which we are exposed to toxic metals and their accumulation in the edible parts of leafy vegetables.

Environmental pollution is the undesirable change in the atmosphere, hydro-sphere and lithosphere. Advanced industrialization processes have provided a comfort to man but have also resulted in the indiscriminate release of gases and liquids which pollute the environment. Large amounts of untreated sewage and industrial water have been discharged into surface bodies of rivers for dis-posal and caused their contamination. With water shortage, farmers use waste-water to irrigate their fields. Such irrigation practices give good crop yields as the water contains organic material and some inorganic elements essential for their growth. It may also contain non-essential metals which, when present in large amounts, could be transferred to man and animals through the food chain (Saeed and Oladeji, 2013).

Long-term use of polluted water for cultivation of vegetables has resulted in the accumulation of trace metals in agricultural soils, affecting microbial activities as well as their transfer to various crops under cultivation with levels of contamination that exceed permissible levels. The uptake and bio-availability of trace metals to plants is controlled by factors associated with soil and climatic conditions, plant genotype, and agronomic management, activities / passive transfer processes, sequestration and speciation, soil PH, soil organic matter content, redox states, type of root system, the response of plants to the elements, seasonal cycles and competitive metal interactions (Tukura et al., 2013).

Similarly, the uptake and bioavailability of heavy metals in vegetables are influenced by many factors such as climate, atmosphere dispositions, the con-centrations of heavy metals in soil, the nature of soil and the degree of maturity of the plant at harvest. Air pollution may pose a threat to post-harvest vegetables during transportation and marketing, causing elevated levels of heavy metals in vegetables (Sale et al., 2013).

Rapid urbanization and industrial development during the last decades have provoked some serious concerns for the environment (Ozturk et al.,

527Chapter | 18 Soil Contamination, Remediation and Plants: Prospects

2010a, b). Heavy metal contaminations are a very serious health issues in many cities of the world especially the highly concentrated industrial areas. The sources of heavy metals in the environment are natural or anthropogenic. Usually the concentration of heavy metals in a natural environment is very low and is mostly derived from the mineralogy and the weathering material components. Main anthropogenic sources of heavy metal contamination are mining, disposal of untreated and partially treated effluents, indiscriminate use of heavy-metal-containing fertilizer and pesticides in agricultural fields. The natural sources are volcanic eruptions, erosion and weathering. Trace metal concentrations are important due to their potential toxicity to the envi-ronment and humans. Some of the metals like Cu, Fe, Mn, Ni and Zn are essential micronutrients for the life processes in animals and plants while many other metals such as Cd, Cr, Pb Hg and As have no known physiologi-cal activities. Metals are non-degradable and can accumulate in the human body causing damage to the nervous system and internal organs (Ibiam and Ekpe, 2013).

SOURCES OF HEAVY METALS IN SOIL

Heavy metals occur naturally in the soil environment from the pedogenetic processes of weathering of parent materials at levels that are regarded as trace (< 1000 mg kg−1) and rarely toxic (Pierzynski et al., 2000; Kabata-Pendias and Pendias, 2001). Due to the disturbance and acceleration of nature’s slowly occurring geochemical cycle of metals by man, most soils of rural and urban environments may accumulate one or more of the heavy metals above at defined background values high enough to cause risks to human health, plants, ani-mals, ecosystems or other media (Raymond and Felix, 2011). The heavy metals essentially become contaminants in the soil environments because (1) their rates of generation via man-made cycles are more rapid relative to natural ones, (2) they become transferred from mines to random environmental locations where higher potentials of direct exposure occur, (3) the concentrations of the metals in discarded products are relatively high compared to those in the receiving environment and (4) the chemical form (species) in which a metal is found in the receiving environmental system may render it more bioavailable.

Heavy metals in the soil from anthropogenic sources tend to be more mobile, hence bioavailable than pedogenic or lithogenic ones (Kuo et al, 1993; Kaasalainen and Yli-Halla, 2003). Metal-bearing solids at contaminated sites can originate from a wide variety of anthropogenic sources in the form of metal mine tailings, disposal of high metal wastes in improperly protected landfills, leaded gasoline and lead-based paints, land application of fertilizer, animal manures, biosolids (sewage sludge), compost, pesticides, coal combustion resi-dues, petrochemicals and atmospheric deposition are discussed below ( Alloway, 1990; Adriano, 2001; Nicholson et al., 2003; Zhang, 2006; Huang et al., 2007; Raymond and Felix, 2011).


Fertilizers

Historically, agriculture was the first major human influence on the soil. To grow and complete the lifecycle, plants must acquire not only macronutrients (N, P, K, S, Ca and Mg), but also essential micronutrients. Some soils are defi-cient in the heavy metals (such as Co, Cu, Fe, Mn, Mo, Ni and Zn) that are essential for healthy plant growth, and crops may be supplied with these as an addition to the soil or as a foliar spray. Cereal crops grown on Cu-deficient soils are occasionally treated with Cu as an addition to the soil, and Mn may similarly be supplied to cereal and root crops. Large quantities of fertilizers are regularly added to soils in intensive farming systems to provide adequate N, P and K for crop growth. The compounds used to supply these elements contain trace amounts of heavy metals (e.g. Cd and Pb) as impurities, which, after con-tinued fertilizer application, may significantly increase their content in the soil (Raymond and Felix, 2011). Metals, such as Cd and Pb, have no known physi-ological activity. Application of certain phosphatic fertilizers inadvertently adds Cd and other potentially toxic elements to the soil, including F, Hg and Pb.

Pesticides

Several common pesticides used fairly extensively in agriculture and horticulture in the past contained substantial concentrations of metals. For instance, in the recent past, about 10% of the chemicals approved for use as insecticides and fun-gicides in United Kingdom were based on compounds which contain Cu, Hg, Mn, Pb or Zn. Examples of such pesticides are copper-containing fungicidal sprays such as Bordeaux mixture (copper sulphate) and copper oxychloride. Lead arse-nate was used in fruit orchards for many years to control some parasitic insects. Arsenic-containing compounds were also used extensively to control cattle ticks and to control pests in banana in New Zealand and Australia, timbers have been preserved with formulations of Cu, Cr and As (CCA), and there are now many derelict sites where soil concentrations of these elements greatly exceed back-ground concentrations. Such contamination has the potential to cause problems, particularly if sites are redeveloped for other agricultural or nonagricultural pur-poses. Compared with fertilizers, the use of such materials has been more local-ized, being restricted to particular sites or crops (McLaughlin et al., 2000).

Biosolids and Manures

The application of numerous biosolids (e.g. livestock manures, composts and municipal sewage sludge) to land inadvertently leads to the accumulation of heavy metals such as As, Cd, Cr, Cu, Pb, Hg, Ni, Se, Mo, Zn, Tl, Sb and so forth, in the soil (Raymond and Felix, 2011). Certain animal wastes such as poultry, cattle and pig manures produced in agriculture are commonly applied to crops and pastures either as solids or slurries. Although most manures are seen as valuable fertilizers, in the pig and poultry industry, the Cu and Zn added


to diets as growth promoters and As contained in poultry health products may also have the potential to cause metal contamination of the soil (Chaney and Oliver, 1996). The manures produced from animals on such diets contain high concentrations of As, Cu and Zn and, if repeatedly applied to restricted areas of land, can cause considerable build-up of these metals in the soil in the long run.

Biosolids (sewage sludge) are primarily organic solid products, produced by wastewater treatment processes that can be beneficially recycled (Raymond and Felix, 2011). Land application of biosolids materials is a common practice in many countries that allow the reuse of biosolids produced by urban popula-tions (Weggler et al., 2004). The term sewage sludge is used in many refer-ences because of its wide recognition and its regulatory definition. However, the term biosolids is becoming more common as a replacement for sewage sludge because it is thought to reflect more accurately the beneficial characteristics inherent to sewage sludge (Raymond and Felix, 2011). It is estimated that in the United States, more than half of approximately 5.6 million dry tons of sewage sludge used or disposed of annually is land applied, and agricultural utilization of biosolids occurs in every region of the country. In the European community, over 30% of the sewage sludge is used as fertilizer in agriculture (Weggler et al., 2004). In Australia over 175,000 tons of dry biosolids are produced each year by the major metropolitan authorities, and currently most biosolids applied to agricultural land are used in arable cropping situations where they can be incor-porated into the soil (McLaughlin et al., 2000).

There is also considerable interest in the potential for composting biosolids with other organic materials such as sawdust, straw or garden waste. If this trend continues, there will be implications for metal contamination of soils. The potential of biosolids for contaminating soils with heavy metals has caused great concern about their application in agricultural practices (Raymond and Felix, 2011). Heavy metals most commonly found in biosolids are Pb, Ni, Cd, Cr, Cu and Zn, and the metal concentrations are governed by the nature and the intensity of the industrial activity, as well as the type of process employed dur-ing the biosolids treatment (Mattigod and Page, 1983). Under certain conditions, metals added to soils in applications of biosolids can be leached downwards through the soil profile and can have the potential to contaminate groundwater. Recent studies on some New Zealand soils treated with biosolids have shown increased concentrations of Cd, Ni and Zn in drainage leachates (Raymond and Felix, 2011).

Wastewater

The application of municipal and industrial wastewater and related effluents to land dates back 400 years and is now a common practice in many parts of the world. Worldwide, it is estimated that 20 million hectares of arable land are irri-gated with wastewater. In several Asian and African cities, studies suggest that agriculture based on wastewater irrigation accounts for 50% of the vegetable


supply to urban areas (Bjuhr, 2007). Farmers generally are not bothered about environmental benefits or hazards and are primarily interested in maximizing their yields and profits. Although the metal concentrations in wastewater efflu-ents are usually relatively low, long-term irrigation of land with such can even-tually result in heavy metal accumulation in the soil.

Metal Mining, Milling Processes and Industrial Wastes

Mining and milling of metal ores coupled with industries have bequeathed many countries the legacy of wide distribution of metal contaminants in soil. During mining, tailings (heavier and larger particles settled at the bottom of the flotation cell during mining) are directly discharged into natural depressions, including on-site wetlands resulting in elevated concentrations (Raymond and Felix, 2011). Extensive Pb and Zn ore mining and smelting have resulted in contamination of soil that poses risk to human and ecological health. Many reclamation methods used for these sites are lengthy and expensive and may not restore soil productivity. Soil heavy metal environmental risk to humans is related to bioavailability. Assimilation pathways include the ingestion of plant material grown in (food chain), or the direct ingestion (oral bioavailability) of contaminated soil (Basta and Gradwohl, 1998).

Other materials are generated by a variety of industries such as textile, tan-ning, petrochemicals from accidental oil spills or utilization of petroleum-based products, pesticides, and pharmaceutical facilities and are highly variable in composition. Although some are disposed of on land, few have benefits to agri-culture or forestry. In addition, many are potentially hazardous because of their contents of heavy metals (Cr, Pb and Zn) or toxic organic compounds and are seldom, if ever, applied to land. Others are very low in plant nutrients or have no soil-conditioning properties (Raymond and Felix, 2011).

Airborne Sources

Airborne sources of metals include stack or duct emissions of air, gas, or vapor streams, and fugitive emissions such as dust from storage areas or waste piles. Metals from airborne sources are generally released as particulates contained in the gas stream. Some metals such as As, Cd and Pb can also volatilize dur-ing high-temperature processing. These metals will convert to oxides and con-dense as fine particulates unless a reducing atmosphere is maintained (Smith et al., 1995). Stack emissions can be distributed over a wide area by natural air currents until dry and / or wet precipitation mechanisms remove them from the gas stream. Fugitive emissions are often distributed over a much smaller area because emissions are made near the ground. In general, contaminant concen-trations are lower in fugitive emissions compared to stack emissions. The type and concentration of metals emitted from both types of sources will depend on site-specific conditions. All solid particles in smoke from fires and in other


emissions from factory chimneys are eventually deposited on land or sea; most forms of fossil fuels contain some heavy metals and this is, therefore, a form of contamination which has been continuing on a large scale since the industrial revolution began. For example, very high concentrations of Cd, Pb and Zn have been found in plants and soils adjacent to smelting works. Another major source of soil contamination is the aerial emission of Pb from the combustion of petrol containing tetraethyl lead; this contributes substantially to the content of Pb in soils in urban areas and in those adjacent to major roads. Zn and Cd may also be added to soils adjacent to roads, the sources being tyres and lubricant oils (USEPA, 1996).

POTENTIAL RISK OF HEAVY METALS TO SOIL

The most common heavy metals found at contaminated sites, in order of abun-dance are Pb, Cr, As, Zn, Cd, Cu and Hg (USEPA, 1996). These metals are important since they are capable of decreasing crop production due to the risk of bioaccumulation and biomagnification in the food chain. There is also the risk of superficial and groundwater contamination. Knowledge of the basic chemistry, and environmental and associated health effects of these heavy metals is neces-sary in understanding their speciation, bioavailability and remedial options. The fate and transport of a heavy metal in soil depend significantly on the chemical form and speciation of the metal. Once in the soil, heavy metals are adsorbed by initial fast reactions (minutes, hours), followed by slow adsorption reactions (days, years) and are, therefore, redistributed into different chemical forms with varying bioavailability, mobility and toxicity. This distribution is believed to be controlled by reactions of heavy metals in soils such as (1) mineral precipitation and dissolution, (2) ion exchange, adsorption and desorption, (3) aqueous com-plexation, (iv) biological immobilization and mobilization, and (5) plant uptake (Raymond and Felix, 2011).

SOIL CONCENTRATION RANGES AND REGULATORY GUIDELINES FOR SOME HEAVY METALS

The specific type of metal contamination found in contaminated soil is directly related to the operation that occurred at the site. The range of contaminant concentrations and the physical and chemical forms of contaminants will also depend on activities and disposal patterns for contaminated wastes on the site. Other factors that may influence the form, concentration and distribution of metal contaminants include soil and ground-water chemistry and local transport mechanisms (GWRTAC, 1997).

Soils may contain metals in the solid, gaseous, or liquid phases, and this may complicate the analysis and interpretation of reported results. For example, the most common method for determining the concentration of metal contaminants in soil is via total elemental analysis (USEPA Method 3050). The level of metal


contamination determined by this method is expressed as mg metal kg−1 soil. This analysis does not specify requirements for the moisture content of the soil and may therefore include soil water. This measurement may also be reported on a dry soil basis. The level of contamination may also be reported as leachable metals as determined by leach tests, such as the toxicity characteristic leaching procedure (TCLP) (USEPA Method 1311) or the synthetic precipitation-leach-ing procedure, or SPLP test (USEPA Method 1312). These procedures measure the concentration of metals in leachate from soil contacted with an acetic acid solution (TCLP) (DPR-EGASPIN, 2002) or a dilute solution of sulfuric and nitric acid (SPLP). In this case, metal contamination is expressed in mg l−1 of the leachable metal. Other types of leaching tests have been proposed includ-ing sequential extraction procedures (Tessier et al., 1979; Ure et al., 1993) and extraction of acid volatile sulfide (DiToro et al., 1992). Sequential procedures contact the solid with a series of extractant solutions that are designed to dissolve different fractions of the associated metal. These tests may provide insight into the different forms of metal contamination present. Contaminant concentrations can be measured directly in metal-contaminated water. These concentrations are most commonly expressed as total dissolved metals in mass concentrations (mg l−1 or g l−1) or in molar concentrations (mol l−1). In dilute solutions, a mg l−1 is equivalent to one part per million (ppm), and g l−1 is equivalent to one part per billion (ppb).

Riley et al. (1992) and NJDEP (1996) have reported soil concentration ranges and regulatory guidelines for some heavy metals (Table 18.1). In Nigeria, in the interim period, whilst suitable parameters are being developed, the Department of Petroleum Resources has recommended guidelines for remediation of con-taminated land based on two parameters intervention values and target values (Table 18.2) below.

REMEDIATION OF CONTAMINATED SOIL BY HEAVY METALS

Remediation of soils contaminated by heavy metals is another attempt to clean-up the soils used for agricultural practices, reduce risk associated with heavy metals, improve soil nutrients and increase food production. Soil remediation is a necessary tool for the enhancement of food security. In general, it is very difficult to eliminate metals from the environment. Once metals get into the soil they remain there and do not degrade like organic molecules. However, metals like mercury and selenium are reported to be transformed and volatilized by microorganisms.

The overall objective of any soil remediation approach is to create a final solution that is protective of human health and the environment. Remedia-tion is generally subject to an array of regulatory requirements and can also be based on assessments of human health and ecological risks where no legis-lated standards exist or where standards are advisory. The regulatory authori-ties will normally accept remediation strategies that centre on reducing metal

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bioavailability only if reduced bioavailability is equated with reduced risk, and if the bioavailability reductions are demonstrated to be long term (Martin and Ruby, 2004). For heavy-metal-contaminated soils, the physical and chemical form of the heavy metal contaminant in soil strongly influences the selection of the appropriate remediation treatment approach. Information about the physical characteristics of the site and the type and level of contamination at the site must be obtained to enable accurate assessment of site contamination and remedial alternatives. The contamination in the soil should be characterized to establish the type, amount and distribution of heavy metals in the soil. Once the site has

Chapter | 18 Soil Contamination, Remediation and Plants: Prospects

TABLE 18.1 Soil Concentration Ranges and Regulatory Guidelines for Some Heavy Metals

Metal Soil Concentration Range (mg / kg)Regulatory Limits (mg / kg)

Pb 1.00–69,000 600

Cd 0.10–345 100

Cr 0.05–3950 100

Hg < 0.01–1800 270

Zn 150–5000 1500

Sources: Riley et al. (1992); NJDEP (1996).

TABLE 18.2 Target and Intervention Values for Some Metals for a Standard Soil

Metal Target Value (mg / kg)Intervention Value (mg / kg)

Ni 140.00 720.00

Cu 0.30 10.00

Zn — —

Cd 100.00 380.00

Pb 35.00 210

As 200 625

Cr 20 240

Hg 85 530

Source: DPR-EGASPIN (2002).


been characterized, the desired level of each metal in soil must be determined. This is done by comparison of observed heavy metal concentrations with soil quality standards for a particular regulatory domain, or by performance of a site-specific risk assessment. Remediation goals for heavy metals may be set as total metal concentration or as leachable metal in soil, or as some combination of these (Raymond and Felix, 2011).

Several technologies exist for the remediation of metal-contaminated soil. Gupta et al. (2000) classified remediation technologies of contaminated soils into three categories of hazard-alleviating measures:

1. gentle in situ remediation; 2. in situ harsh soil restrictive measures; and 3. in situ or ex situ harsh soil destructive measures.

The goal of the last two harsh alleviating measures is to avert hazards either to man, plant or animal while the main goal of gentle in situ remediation is to restore the malfunctionality of soil (soil fertility), which allows safe use of the soil. At present, a variety of approaches have been suggested for remediating contaminated soils. USEPA (2007) has broadly classified remediation technologies for contaminated soils into:

1. Source control and 2. Containment remedies.

Source control involves in situ and ex situ treatment technologies for sources of contamination. In situ or in place means that the contaminated soil is treated in its original place; unmoved, unexcavated; remaining at the site or in the subsurface. In situ treatment technologies treat or remove the contaminant from the soil with-out excavation or removal of the soil. Ex situ means that the contaminated soil is moved, excavated or removed from the site or subsurface. Implementation of ex situ remedies requires excavation or removal of the contaminated soil. Containment remedies involve the construction of vertical engineered barriers, caps and liners used to prevent the migration of contaminants.

Another classification places remediation technologies for heavy-metal-contaminated soils into five categories of general approaches to remediation (Table 18.3): isolation, immobilization, toxicity reduction, physical separation and extraction (USEPA 2007). In practice, it may be more convenient to employ a hybrid of two or more of these approaches for more cost effectiveness. The key factors that may influence the applicability and selection of any of the avail-able remediation technologies are:

1. cost, 2. long-term effectiveness / permanence, 3. commercial availability, 4. general acceptance, 5. applicability to high metal concentrations,


6. applicability to mixed wastes (heavy metals and organics), 7. toxicity reduction, 8. mobility reduction, and 9. volume reduction.

Excess heavy metal accumulation in soils is toxic to humans and other animals. Exposure to heavy metals is normally chronic (exposure over a longer period of time), due to food chain transfer. Acute (immediate) poisoning from heavy metals is rare through ingestion or dermal contact, but is possible. Chronic problems associated with long-term heavy metal exposures are:

l Lead – mental lapse; l Cadmium – affects kidney, liver and GI tract; and l Arsenic – skin poisoning, affects kidneys and central nervous system.

Hazardous heavy metals such as lead, cadmium, chromium, mercury and arsenic are pervasive in much of the environment and it may not be possible to completely avoid exposure. There are many safe, organic and affordable ways that can help detoxify and eliminate these toxins from the body. One such way is the use of EDTA (ethylenediaminetetraacetic acid). Of all the metals, lead is the most common soil contaminant. Unfortunately, plants do not accumulate lead under natural conditions. A chelator such as EDTA has to be added to the soil as an amendment. The EDTA makes the lead available to the plant (USEPA, 1993). The most common plant used for lead extraction is Indian mustard (Brassica juncea). Phytotech (a private research company)

TABLE 18.3 Technologies for Remediation of Heavy-Metal-Contaminated Soils

Category Remediation Technologies

Isolation (i) Capping (ii) subsurface barriers.

Immobilization (i) Solidification / stabilization (ii) vitrification (iii) chemical treatment.Toxicity and / or mobility reduction; physical separation (i) Chemical treatment (ii) permeable treatment walls (iii) biological treatment bioaccumulation, phytoremediation (phytoextraction phytostabilization and rhizofiltration), bioleaching, biochemical processes.

Extraction (i) Soil washing, pyrometallurgical extraction, in situ soil flushing and electrokinetic treatment.

Source: Raymond and Felix (2011).


has reported that they have cleaned up lead-contaminated sites in New Jersey to below the industrial standards in one to two summers using Indian mustard (Wantanabe, 1997).

Therefore, background knowledge of the sources, chemistry and potential risks of toxic heavy metals in contaminated soils is necessary for the selection of appropriate remedial options. Remediation of soil contaminated by heavy metals is necessary in order to reduce the associated risks, make the land resource available for agricultural production, enhance food security and scale down land tenure problems. Immobilization, soil washing and phytoremedia-tion are frequently listed among the best available technologies for clean-ing up heavy-metal-contaminated soils but have been mostly demonstrated in developed countries. These technologies are recommended for field applica-bility and commercialization in the developing countries also where agricul-ture, urbanization and industrialization are leaving a legacy of environmental degradation.

PREVENTION OF HEAVY METAL CONTAMINATION

Preventing heavy metal pollution is critical because cleaning contaminated soils is extremely expensive and difficult. Applicators of industrial waste or sludge must abide by the regulatory limits set by the US Environmental Protection Agency (EPA) (see Table 18.4).

Prevention is the best method to protect the environment from contami-nation by heavy metals. With the above table, a simple equation is used to show the maximum amount of sludge that can be applied. For example, sup-pose city officials want to apply the maximum amount of sludge (kg ha−1) on some agricultural land. The annual pollutant-loading rate for zinc is 140 kg ha−1 / yr (from Table 18.4). The laboratory analysis of the sludge shows a zinc concentration of 7500 mg kg−1 (mg kg−1 is the same as parts per million). How much can the applicator apply (tons A−1) without exceed-ing the 140 kg ha−1 / yr?

Solution:

1. Convert mg to kg (1,000,000 mg = 1 kg) so all units are the same: 7500 mg × (1 kg / 1,000,000 mg) = 0.0075 kg.

2. Divide the amount of zinc that can be applied by the concentration of zinc in the sludge: (140 kg Zn ha−1) / (0.0075 kg Zn per kg sludge) = 18,667 kg sludge ha−1.

3. Convert to lb A−1: 18,667 kg ha−1 × 0.893 = 16,669 lbs A−1. Convert lbs to tons: 16,669 lb A−1 / 2000 lb T−1 = 8.3 T sludge per acre.

TRADITIONAL REMEDIATION OF CONTAMINATED SOIL

According to Jacob et al. (2013), traditional treatments for metal contamination in soils are expensive and cost prohibitive when large areas of soil are contami-nated. Treatments can be done in situ (on-site), or ex situ (removed and treated


off-site). Different methods exist for the remediation of heavy metals in the environment. Some of these methods include mechanical, chemical and biore-mediation. Some treatments that are available include:

1. High-temperature treatments (produce a vitrified, granular, non-leachable material).

2. Solidifying agents (produce cement-like material). 3. Washing process (leaches out contaminants).

MANAGEMENT OF CONTAMINATED SOIL

Soil and crop management methods can help prevent uptake of pollutants by plants, leaving them in the soil. The soil becomes the sink, breaking the soil–plant–animal or human cycle through which the toxin exerts its effects (Brady and Weil, 1999). The following management practices will not remove the heavy metal contaminants, but will help to immobilize them in the soil and reduce the potential for adverse effects from the metals. Note that the kind of metal (cation or anion) must be considered.

1. Increasing the soil pH to 6.5 or higher. Cationic metals are more soluble at lower pH level, so increasing the pH makes them less available to plants and therefore less likely to be incorporated in their tissues and ingested by humans. Raising the pH has the opposite effect on anionic elements.

TABLE 18.4 Regulatory Limits on Heavy Metals Applied to Soils

Heavy Metal

Maximum Concentration in Sludge (mg / kg)

Annual Pollutant Loading Rates

Cumulative Pollutant Loading Rates

(kg / ha / yr) (lb / A / yr) (kg / ha) (lb / A)

Arsenic 75 21.9 1.8 41 36.6

Cadmium 85 1.9 1.7 39 34.8

Chromium 3000 150 134 3000 2679

Copper 4300 75 67 1500 1340

Lead 420 21 14 420 375

Mercury 840 15 13.4 300 268

Molybdenum 57 0.85 0.80 17 15

Nickel 75 0.90 0.80 18 16

Selenium 100 5 4 100 89

Zinc 7500 140 125 2800 2500

Adapted from USEPA (1993).


2. Draining wet soils. Drainage improves soil aeration and will allow metals to oxidize, making them less soluble. Therefore, when aerated, these metals are less available. The opposite is true for chromium, which is more avail-able in oxidized forms. Active organic matter is effective in reducing the availability of chromium.

3. Applying phosphate. Heavy phosphate applications reduce the availability of cationic metals, but have the opposite effect on anionic compounds like arsenic. Care should be taken with phosphorus applications because high levels of phosphorus in the soil can result in water pollution.

4. Carefully selecting plants for use on metal-contaminated soils. Plants translocate larger quantities of metals to their leaves than to their fruits or seeds. The greatest risk of food-chain contamination is in leafy vegetables like lettuce or spinach. Another hazard is forage eaten by livestock.

The Use of Plants for Environmental Clean-Up

Research has demonstrated that plants are effective in cleaning up contami-nated soil (Wenzel et al., 1999). Phytoremediation is a general term for using plants to remove, degrade, or contain soil pollutants such as heavy met-als, pesticides, solvents, crude oil, polyaromatic hydrocarbons and landfill leacheates. For example, prairie grasses can stimulate breakdown of petro-leum products. Wildflowers were recently used to degrade hydrocarbons from an oil spill in Kuwait. Hybrid poplars can remove ammunition com-pounds such as TNT as well as high nitrates and pesticides (Brady and Weil, 1999).

The Use of Plants for Treating Metal-Contaminated Soils

Plants have been used to stabilize or remove metals from contaminated soil and water. The three mechanisms used are phytoextraction, rhizofiltration and phytostabilization. This technical note will define rhizofiltration and phytosta-bilization but will focus on phytoextraction.

RhizofiltrationRhizofiltration is the adsorption onto plant roots or absorption into plant roots of contaminants that are in solution surrounding the root zone (rhizosphere). Rhizofiltration is used to decontaminate groundwater. Plants are grown in greenhouses in water instead of soil. Contaminated water from the site is used to acclimatize the plants to the environment. The plants are then planted on the site of contaminated groundwater where the roots take up the water and contaminants. Once the roots are saturated with the contaminant, the plants are harvested including the roots. In Chernobyl, Ukraine, sunflowers were used in this way to remove radioactive contaminants from groundwater (USEPA, 1998).


PhytostabilizationPhytostabilization is the use of perennial, non-harvested plants to stabilize or immobilize contaminants in the soil and groundwater. Metals are absorbed and accumulated by roots, adsorbed on to roots, or precipitated within the rhizo-sphere. Metal-tolerant plants can be used to restore vegetation where natural vegetation is lacking, thus reducing the risk of water and wind erosion and leaching. Phytostabilization reduces the mobility of the contaminant and pre-vents further movement of the contaminant into groundwater or the air and reduces the bioavailability for entry into the food chain.

PhytoextractionPhytoextraction is the process of growing plants in metal-contaminated soil. Plant roots then translocate the metals into above-ground portions of the plant. After plants have grown for some time, they are harvested and incinerated or composted to recycle the metals. Several crop growth cycles may be needed to decrease contaminant levels to allowable limits. If the plants are incinerated, the ash must be disposed of in a hazardous waste landfill, but the volume of the ash is much smaller than the volume of contaminated soil if dug out and removed for treatment. See the following example of disposal.

Example of Disposal

Excavating and landfilling a 10-acre contaminated site to a depth of 1 foot requires handling roughly 20,000 tons of soil. Phytoextraction of the same site would result in the need to handle about 500 tons of biomass, which is about {1/40} of the mass of the contaminated soil. In this example, if we assume the soil was contaminated with a lead concentration of 400 ppm, six to eight crops would be needed, growing four crops per season (Phytotech, 2000).

Phytoextraction is done with plants called hyperaccumulators, which absorb unusually large amounts of metals in comparison to other plants. Hyperaccu-mulators contain more than 1000 mg kg−1 of cobalt, copper, chromium, lead or nickel; or 10,000 mg kg−1 (1%) of manganese or zinc in dry matter (Baker and Brooks, 1989). One or more of these plant types was planted at a particular site based on the kinds of metals present and site conditions. Phytoextraction is easiest with metals such as nickel, zinc and copper because these metals are preferred by a majority of the 400 hyperaccumlator plants. Several plants in the genus Thlaspi (pennycress) have been known to take up more than 30,000 ppm (3%) of zinc in their tissues. These plants can be used as an ore because of the high metal concentration (Brady and Weil, 1999).

Preventive Steps

The best solution for combating heavy metal toxins is to avoid them in the first place, today’s modern world makes this task almost impossible, but significant


improvements can be accomplished using the following guidelines (Ibiam and Ekpe, 2013):

l Avoid mercury–silver dental amalgams; l Avoid fish and shellfish; l Clean water; l Clean food; l Natural products; l Thimerosal-free vaccines; and l Avoid industrial areas.

CLASSIFICATION OF HEAVY METALS

Heavy metals can be classified into four major groups based on their health implications:

l Essential: Cu, Zn, Cr (III), Mn and Fe. These metals are also called micronu-trients and are toxic when taken in excess of requirements.

l Nonessential: Ba, Al and Zn. l Less toxic: Sn and Al. l Highly toxic: Pb, Cd, As, Cr(VI) and Hg.

SOURCES OF HEAVY METALS IN THE ENVIRONMENT

Heavy metals originate from either natural or anthropogenic sources. The pri-mary anthropogenic sources of heavy metals are point sources such as mines, foundries, smelters and coal-burning power plants, as well as diffuse sources such as combustion by-products and vehicle emissions. Humans also affect the natural geological and biological redistribution of heavy metals by altering the chemical form of heavy metals released to the environment. Natural erosion and weathering of crusted materials take place over long periods of time and the amount of heavy metals released is small.

Major sources of toxic metals in the environment are by human activities both from domestic and industrial wastewaters, effluents and their associated solid waste; cadmium, chromium, copper, lead, and mercury are used exten-sively in industries and the effluents from these industries contribute to pol-luting the environment. Table 18.5 gives a list of some metals used in major industries.

Solid waste or sewage sludge is commonly disposed of in landfills or sold as fertilizers. Heavy metals can be released through leaching of sewage sludge in landfills. Sewage sludge also contains plant nutrients and compares favour-ably to other fertilizers in crop production. The amount of sewage sludge that can be applied to cropland is regulated by the USEPA and depends on the concentrations of heavy metals in the sludge and the soil chemistry of the cropland.


BENEFITS OF HEAVY METALS TO PLANTS

Heavy metals are required in trace amounts by plants, because they are essen-tial for certain metabolic functions while others are essential components of enzymes and pigments in living systems. Such heavy metals include iron, cobalt, copper, manganese, molybdenum, vanadium, strontium and zinc; but in excess, they can be detrimental to organisms (Harada, 1994). Cobalt is essential for nitrogen fixation by rhizobium in legume noodles. In practice, it has the largest significance to animal nutrition and is observed to be a component of vitamin B12 (cobalamine) molecule.

Iron is an essential element in plants and a micronutrient required in a relatively small quantity. It has a major role in a host of biochemical reactions, particularly in connection with the electron transport chain (cytochromes). However, iron is not part of the chlorophyll molecule but is essential in plants for the synthesis of chlorophyll. Some of the enzymes and carriers that function in the respiratory and photosynthesis mechanisms of living cells are iron compounds, for example cytochromes and ferrodoxin. The principal iron-containing pigments in animals are the red hemoglobin of vertebrate blood, the red erythrocruorin found in many annelids and moulds, and the green chlorocruorin of certain polycheate worms.

TABLE 18.5 Selected Heavy Metals Presently or Formerly Used in Major Industries

Industries Cd Cr Cu Pb Hg

Machinery product X X X X X

Paint pigment and ink X X X X X

Electroplating X X X X

Textile mill X X

Wood, pulp and paper X X X X

Organic chemicals X X X X

Inorganic chemicals X X X X X

Rubber manufacturing X X

Iron and steel foundries X X

Nonferrous metal foundries X X X X

Leather processing X

Petroleum refining X X

Steam-generating power plants X X

X denotes the heavy metal that is used in the industry.


Chromium is required for normal carbohydrate, lipid and nucleic acid metabolism. Insufficient dietary chromium leads to impaired glucose and lipid metabolism and may ultimately lead to maturity onset diabetes and / or cardio-vascular diseases. The essentiality of chromium is seen in mice, chicken and sheep. In humans, it acts as a defence mechanism against weight loss, glucose intolerance and impaired energy metabolism.

Nickel is required to maintain health in animals. Small amount of nickel are probably essential for humans, although a lack of nickel has not been found to have any effect.

Copper and zinc are micronutrients essential for the normal growth and development of the body. Copper is found to be essential in hemoglobin forma-tion, production of RNA, cholesterol utilization among other processes.

Zinc is essential for protein synthesis, carbon dioxide transport, and sexual function, among other processes.

Arsenic, lead, mercury and cadmium have no known beneficial importance in living organisms.

FUTURE PROSPECTS

Phytoremediation has been studied extensively in research and at small-scale demonstrations, but in only a few at full-scale applications. Phytoremediation is moving into the realm of commercialization (Watanabe, 1997). It is predicted that the phytoremediation market will reach $214 to $370 million by the year 2015 (Environmental Science & Technology, 1998). Given the current effec-tiveness, phytoremediation is best suited for clean-up over a wide area in which contaminants are present at low to medium concentrations. Before phytoreme-diation is fully commercialized, further research is needed to ensure that tissues of plants used for phytoremediation do not have adverse environmental effects if eaten by wildlife or used by humans for things such as mulch or firewood (USEPA, 1998). Research is also needed to find more efficient bioaccumula-tors and hyperaccumulators that produce more biomass, and to further monitor current field trials to ensure a thorough understanding. There is the need for a commercialized smelting method to extract the metals from plant biomass so they can be recycled. However, phytoremediation is slower than traditional methods of removing heavy metals from soil but much less costly. Prevention of soil contamination is far less expensive than any kind of remediation and much better for the environment.

CHALLENGES

The high concentration of heavy metals in agricultural soils affects the crop output and quality. It also deteriorates the growth, morphology and metabo-lism of microorganisms in soils. These effects can be considered a real threat to human food safety. In recent years, rapid industrialization and urbanization


have caused heavy metal contamination to become a serious concern in many countries (Hani and Pazira, 2011). For any African country to introduce and practice the remediation technologies widely employed in the developed coun-tries, the background knowledge of the sources, chemistry, and potential risks of toxic heavy metals in contaminated soils is necessary for the selection of appropriate remedial options. However, the financial resources needed to effec-tively adopt these remediation technologies are lacking in Africa. Remedia-tion of soil contaminated by heavy metals is necessary in order to reduce the associated risks, make the land resource available for agricultural production, enhance food security, and scale down land tenure problems. Immobilization, soil washing and phytoremediation are frequently listed among the best avail-able technologies for cleaning up heavy-metal-contaminated soils but have been mostly demonstrated in developed countries. These technologies are recommended for field applicability and commercialization in the developing countries also where agriculture, urbanization and industrialization are leav-ing a legacy of environmental degradation. Developing countries of the world (mostly in Africa) need to experiment with some of these remediation options and find ways to subsidize the possible ones from peasant farmers in order to reduce poverty among the citizens who cannot afford to adopt the remediation technology in their farmlands which are already contaminated by heavy metals and the resultant effect is low yield.

CONCLUSIONS

Soils may become contaminated by the accumulation of heavy metals and metal-loids through fertilizer application, use of pesticides, herbicides, biosolids and manures, wastewater, mining, milling processes, industrial effluents, airborne and many other sources not mentioned here. However, some of these heavy met-als can be removed and prevented if remediation technologies are adopted and practiced. The benefits are that food security will be enhanced, farmers can be rest-assured of a bumper harvest each year and more revenue will be generated in the country. The literature reports on agricultural soil contamination by heavy metals in Africa and the world at large further suggests that baseline values for heavy metals should be proposed in Africa. This is necessary to facilitate the iden-tification of soil contamination processes over the whole continent as a basis for undertaking appropriate action to protect the soil resource quality for improved food production and reduce the health risks associated with heavy metals.

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