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 Ta