Soil contamination with heavy metals and possibility for its remediation
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ISSN 1064-2293, Eurasian Soil Science, 2006, Vol. 39, Suppl. 1, pp. S63S68. Pleiades Publishing, Inc., 2006.
INTRODUCTIONSoil is an important natural resource supporting the
ecological equilibrium on the planet. Soil fertility,which is partly formed due to the activity of microor-ganisms, is one of the primary soil characteristics. Soilcontamination with industrial pollutants leads tochanges in the microbiological activity of soils and to adecrease in their fertility.
Ions of heavy metals, being important micronutri-ents up to certain concentrations, become contaminantsin heavily polluted soils of industrial regions. Ions ofZn
, and Co
are the most wide-spread soil contaminants. In this relation, the study ofthe effect of increased concentrations of heavy metal(HM) ions on the chemical and physicochemical prop-erties and fertility of soils is an important task.
It is known that chemical elements are accumulatedin the biomass of organisms differently; for instance,the accumulation of Fe
ions in the biomassexceeds the accumulation of Cu
, and Mo
ions by many times. As follows from Table 1, the Fe :Mn ratio in sedimentary rocks and in ocean water isapproximately (89) : 1, whereas it is approximately1 : 1 in vegetation and 18.8 : 1 in soils. A significantaccumulation of Fe in the pedosphere points to the greatrole of soil microorganisms in this process.
A predominant accumulation of Fe in the pedo-sphere makes it possible to suggest that iron electrodesinstalled in the soil may be indicative of the soil micro-biological activity. It is known that iron is present insoils in the forms of Fe
that are dif-ficultly available to organisms in contrast to Fe
ionsthat may participate in the biochemical processes.These ions are formed on the surface of iron elec-trode in the outer layer of the double electric layer:
. Many microorganisms (bacteria,bacilli, microscopic algae, and fungi) can transformHM ions into complex organic compounds. This trans-
formation proceeds in the course of metabolism in par-allel with the metabolic transformation of macronutri-ents P, N, K, S, and Ca that are present in the substrate.Therefore, microorganisms and plants are used for soilpurification purposes .
In 1988, Japanese scientist Teruo Higa studied about3000 microorganisms and found that their regenerativeand degenerative functions are interrelated. He alsofound that only about 5% of pathogenic microorgan-isms are leaders. Other microorganisms can changetheir orientation towards leaders. Teruo Higa selected86 leading regenerative strains, which perform all thefunctions related to the nutrition of plants, their protec-tion from diseases, and improving soil conditions.
Soil Contamination with Heavy Metals and Possibility for Its Remediation
N. I. Melekhova, S. V. Semashko, E. A. Gorskaya, and A. V. Troshina
Tula State University, pr. Lenina 92, Tula, 300600 Russia
Received March 15, 2003
The problems of changes in soil fertility under the impact of industrial contaminants and of reme-diation of soils polluted with heavy metals are discussed. It is shown that Fe(II) ions may serve as soil remedi-ators. It is also shown that changes in the soil microbiological activity can be estimated by the electrochemicalmethod from data on the potential of the biologically active iron electrode.
Distribution of heavy metals between the sedimen-tary mantle of the lithosphere (Tt10
t), vegetation of con-tinents, oceanosphere (Mt10
t), and pedosphere (Dobro-volskii, 1998)
Metal Sedimentarymantle, Tt
Fe 60721 4658 500 1550Mn 7520 548 600 93V 171 3.75 9.3Cr 132 274 4.5 12.4Zn 129 6850 75 76Ni 92 685 5.0 12.4Cu 56 1233 20 9.3Pb 32 41.1 3.13 6.2Co 22 41.1 1.3 3.1Mo 3.3 1.2 1.5Cd 0.4 151 0.09 0.9Hg 0.6 206 0.03 0.3
EURASIAN SOIL SCIENCE
MELEKHOVA et al.
These microorganisms were referred to as effectivemicroorganisms.
A considerable part of soil microorganisms belongsto the group of effective microorganisms, which con-sume iron ions together with other microelements. Thenecessary microelements can be recovered from min-eral crystals and other difficultly soluble compounds bycell enzymes. This fact suggests that the microbiologi-cal activity of soil can be controlled electrochemically(by ionometry), via measuring changes in the stationarypotential of a biologically active iron electrode
additionally to direct methods of cell counting and thestudy of decomposition of substrates (e.g., cotton fab-ric) applied into the soil (known as the applicationmethod in Russian literature). Earlier observationsdemonstrated the agreement between the resultsobtained by the method of ionometry and by Mishus-tins application method . The principle of ionome-try is based on a thermodynamic equation describingchanges in the electrode potential depending on theproperties of the electrode material, enclosing medium,and potential-determining components. The equilib-rium potential of the double electric layer on a metal(
) is described by the following equation :
is the standard electrode potential,
is theuniversal gas constant,
is the valence of metal ions, and
is theactivity of metal ions.
Equation (1) can be used for calculating the equilib-rium potential
of the double electric layer and fordetermining the concentration of the potential-formingcomponent by the change in the electrode potential.
The stationary potential
is formed in the soil onthe metal surface at the expense of stationaryquasiequilibrium processes running on the surface ofthe electrode in the double electric layer:
+ [L] [FeL]
, (3)where [L] is the concentration of ligands or microor-ganisms in the soils near the electrode surface,mmol/kg.
According to Eqs. (2) and (3), changes in the con-centration of electro active Fe
ions in the near-elec-trode layer result in the formation of the electrodepotential
Changes in the concentration of Fe
ions on themetal surface in the soil may be due to the microbiolog-ical activity and various complexing reactions.
The value of the stationary potential of iron elec-trode in the contaminated soil can be calculated as fol-lows:
= + (0.059/2) , (4)Estex Est0 aFe2+log
where and are the values of the electrodepotential under experimental conditions and in the con-trol, and is the activity of iron ions in the near-electrode layer.
If the complex-forming processes in the studiedsoils have the same intensity (soils of the same type),then changes in the electrode potential are only due tochanges in the activity of microorganisms upon the soilpollution:
= ( ). (5)When the biological activity of soil is low or absent,
the stationary electrode potential
is formed on thesurface of the electrode only under the impact of com-plexing reactions. The microbiological activity of soilincreases the expenditure of iron ions, which shifts theequilibria in reactions (2) and (3). The released elec-trons should displace the potential of the iron electrodetoward the zone of negative values .
The aim of this work was to study the effects ofstress doses (up to 10 MPCs) of industrial contaminants(including HM ions) on the physicochemical and bio-logical properties of soils and soil fertility and the influ-ence of Fe (II) ions as remediators of contaminatedsoils.
OBJECTS AND METHODSThe soils of the reserved zone of Bogoroditskii dis-
trict, arable chernozems of Kireevskii district, gray for-est soils of Arsenevskii district, and soddy-podzolicsoils of Belevskii district of Tula oblast were the objectsof the study.
The changes in soil acidity (pH), redox potential(Eh), microbiological activity (
), and fertility (
)were measured in model laboratory experiments anddirectly in the field [6, 7].
Measurements of E
Heavy metal ions in doses corresponding to tenfoldmaximum permissible concentrations (MPCs) wereadded to the soils in the form of water solutions of salts.The contaminated soil samples were carefully mixedand incubated for 7 days, and then plant seeds weresown. The measurements of Eh,
, and pH in the soilsand in the rhizosphere were performed on the 14th dayafter germination, and the degree of plant developmentwas recorded; the experiment was performed in tripli-cate. Average values are presented in the tables. Thedifference between the electrode potentials in parallelsoil samples did not exceed 1015 mV. The values of
(mV) in the control plowed gray forest soil and inthe same soil contaminated with Cu
and inthe rhizosphere of barley seedlings developing in thecontrol and contaminated soils are presented in Table 2.
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SOIL CONTAMINATION WITH HEAVY METALS AND POSSIBILITY S65
As seen from Table 2, copper and zinc ions had toxiceffects on the soil microorganisms, because they dis-placed
to the region of more positive values. Theaddition of Cu
ions at the rate of 5 MPC increased the
value by 55 mV (up to 606 mV) in comparisonwith the control (661 mV) on the 60th second. The
value under the influence of Zn
ions added in thesame concentration increased by 26 mV (in comparisonwith the control) during the same time interval. Thetoxic effects of copper and zinc were also observed inthe rhizosphere of barley seedlings, but to a lesserextent, which may be explained by the additional com-plex formation between iron ions and root exudates; thecorresponding changes in the electrode potential com-prised 33 mV for copper and 16 mV for zinc (on the60th second). With an increase in the time (duration) ofmeasurements, the values of the iron electrode potentialin the contaminated soils gradually approached those inthe control soils. This fact attests to the activating effectof iron ions from the double electric layer on the soilmicrobiological activity. It can be supposed that theaddition of iron ions into the contaminated soils (in theform of chemical ameliorants) should result in the res-toration of the soil microbiological activity. It is obvi-ous that a positive effect of ameliorants produced fromthe powdered metallurgic slag on the soil fertility was
caused by several factors, including the bioremediationof the soil by Fe (II) ions in the blast furnace slag [8, 9].
Measurements of Eh and pH
The values of redox potential (Eh) and soil acidity(pH) were measured by routine methods . Theresults of the measurements at the depths of 3 and10 cm in two pits with stable parameters of the micro-biological activity are presented in Table 3 and in thefigure.
Changes in the stationary potential (
, mV) in the plowed gray forest soil, in the soil samples contaminated withCu
(5 MPC) and Zn
(5 MPC) ions, and the in rhizosphere of barley seedlingsVariant
= 15 30 60 90 120 180 s
4Rhizosphere of barley seedlings
Variant = 15 30 60 90 120 180 s
Control 640 5 679 6 701 3 713 7 723 6 730 4MPC Cu2+ 635 9 653 7 668 9 680 9 691 7 701 8MPC Zn2+ 638 6 667 5 685 4 698 7 709 9 717 5
Table 3. Est and Eh (mV) values as dependent on time in the chernozem and gray forest soil of the reserve zone at the depthsof 3 and 10 cm
, s Eh, 3 cm Eh, 10 cm Est, 3 cm Est, 10 cm Eh, 3 cm Eh, 10 cm Est, 3 cm Est, 10 cm
Chernozem Gray forest soil0 272 250 800 785 210 120 700 685
15 275 251 800 787 197 120 710 71030 277 252 803 790 191 115 740 75045 279 252 804 790 189 110 750 77060 280 253 805 795 182 108 760 778
120 280 255 805 797 179 105 760 780
16 18 20 22 24 1 2 3 4 5%
Fig. 1. Changes in the microbiological activity MBA in thesoil profile as judged from the weight loss (%) of cotton fab-ric 10 10 cm2 applied into (a) typical chernozem and(b) gray forest medium loamy soil.
EURASIAN SOIL SCIENCE Vol. 39 Suppl. 1 2006
MELEKHOVA et al.
It is seen from Table 3 that chernozems have higherEh values than gray forest soils; in the latter, the Eh val-ues at the depth of 10 cm are by 2225 mV lower thanthose as the depth of 3 cm. In chernozems, the Ehremain relatively stable during the time of measure-ments. In the gray forest soils, they somewhat decreasefrom the beginning to the end of measurements. Cher-nozems are characterized by more negative Est in com-parison with gray forest soils. In the latter, the distribu-tion of both Eh and Est values in the soil profile and intime is more contrasting than that in chernozems.
Microbiological Activity as Assessedby the Decomposition of Cotton Fabric in Soil
Cellulolytic microorganisms were selected as indi-cators of soil microbiological activity . The degree ofdecomposition of uncolored cotton fabric of 10 10 cm2 with known weight placed for 14 days into theA (020 cm), AB (2045 cm), and BA (4564 cm) hori-zons of chernozems and gray forest soils in the reservedzone of Bogoroditskii district was studied. Overall,more than 50 samples were analyzed. Data on themicrobiological activity (as expressed in percent of theweight loss of cotton fabric) are presented in the figure.
It is seen from the figure that the microbiologicalactivity slightly varies in the profile of chernozems andcomprises 2025%. In the gray forest soils, someincrease in the microbiological activity is seen at thedepth of 1527 cm. In general, the microbiological(cellulolytic) activity is the gray forest soils does notexceed 5%.
DISCUSSIONA comparison of the activity of cellulolytic microor-
ganisms with the electrochemical parameters (redoxpotential) of the soils points to correlative relationshipsbetween them.
High Eh values in chernozems (270280 mV) incomparison with gray forest soils (120210 mV) corre-late with the high microbiological activity (2025% inthe entire profile of chernozems). The differencebetween Eh values in chernozems and gray forest soilsaverages 100 mV (E = Ehchern Ehgray soil 100 mV, = 120 s).
Therefore, the increase in the microbiological activ-ity per unit of the redox potential (Eh) comprises (25 5) % : 100 mV = 20 : 100 = 0.2% per 1 mV.
The increased microbiological activity in cher-nozems also corresponds to the lower values of Est thatcomprise (800805) mV in chernozems (for the upper3 cm) and (700760) mV in gray forest soils. The dif-ference in Est values between the two soils decreasesdown the soil profile. For example, Est values on the120th second at a depth of 10 cm are equal to 780 mVin chernozems and to 797 mV in gray forest s...