ISSN 0012-5008, Doklady Chemistry, 2008, Vol. 420, Part 1, pp. 125–128. © Pleiades Publishing, Ltd., 2008.Original Russian Text © G.N. Fedotov, G.G. Omel’yanyuk, O.N. Bystrova, E.A. Martynkina, V.V. Gulevskaya, M.V. Nikulina, 2008, published in Doklady Akademii Nauk, 2008,Vol. 420, No. 3, pp. 346–350.

125

Many regions of the world being heavily polluted,the geochemical cycles of heavy metals in the bio-sphere are considerably affected by anthropogenic fac-tors. Studies and assessment of environmental pollutionwith heavy metals have become an integral part of envi-ronmental protection problems. In this context, soil isof great interest as a deposition medium [1].

The behavior of heavy metals in soils has beenwidely studied [2-7], but the consideration of soils interms of a physical model [8] limited the ability ofresearchers to elucidate the interactions of heavy metalswith soil components. Many heavy metals (zinc, cop-per, molybdenum, manganese, cobalt, and others) areessential microelements for plants.

Recent experimental results [9–11] indicate that col-loidal soil particles reside in the cells of a three-dimen-sional net of organic humus molecules. The gel struc-ture of the soil can be considered as a humus gel rein-forced by colloidal particles. Interacting with water,this reinforced humus gel (RHG) behaves as manypolymers; it absorbs water and increases in volume,forming a unified organomineral gel network.

This work studies the mechanism of binding ofheavy metals by soils, gaining and analyzing data onthe heavy-metal distribution in various fractions of soilmacroaggregates.

Test samples were taken from high-humus horizonsof a cultured soddy-podzolic soil from the vicinity ofthe Klyazma River floodplain, a gray forest soil fromVladimir oblast, and Kursk chernozem.

Air-dry soils were sieved into the following frac-tions (mm): >10, 7–10, 5–7, 3–5, 2–3, 1–2, 0.5–1,0.25–0.5, and <0.25. Fractionated samples were pestledin a mortar to particle sizes of less than 1 mm, divided

into halves, and treated to extract heavy metals. Onehalf was treated with 1 N nitric acid and the other withammonium acetate buffer (AAB) using routine proce-dures [12]. The heavy metals extracted with 1 N nitricacid are classified as potentially mobile compounds. Itis thought that the ions contained in the form ofexchangeable cations in the absorbing complex of thesoil and the ions adsorbed by the mineral soil compo-nents complexed with soil organics are solubilized.Ammonium acetate buffer extracts mobile heavy metalspecies that are capable of migrating through the soilprofile. A high degree of correlation was found betweenthe concentrations of heavy metals extracted by AABfrom test soils, on one hand, and the concentrations ofthese elements in plants growing on these soils [12]. Itis thought that AAB (pH 4.8), unlike 1 N nitric acid,does not extract the heavy-metal compounds that aremore firmly bound to the soil and, thus, less mobile.

Heavy metals in extracts were determined onPerkinElmer AAnalyst 200 and AAnalyst 600 atomicabsorption spectrophotometers. The measurement errorfor the 95% significance level did not exceed 2.5%for zinc, 2.6% for copper, 1.5% for manganese, 3.1%for lead, 5.5% for cobalt, 5.1% for nickel, and 4.5% forcadmium.

Let us consider the results with allowance for theRHG, which coats and binds soil particles (the gellayer).

The soil mass of the soddy-podzolic soil is inhomo-geneous [13]. This soil mass includes coarse-grainedparticles coated with a thin gel layer and blocks consti-tuted by finer particles and the gel. The soil mass isdestroyed upon sifting. Coarse aggregates retain thestructural state of the starting soil. Microscopic obser-vations show that aggregates with sizes of 1–2 mmmostly consist of fine particles and the gel. Aggregateswith sizes of 0.25–1 mm contain many gel-coatedcoarse-grained particles. The <0.25-mm aggregatefraction is enriched in the gel. Thus, RHG concentra-tions in aggregate fractions should be different.

Heavy-Metal Distribution in Various Types of Soil Aggregates

G. N. Fedotov, G. G. Omel’yanyuk, O. N. Bystrova, E. A. Martynkina, V. V. Gulevskaya, and M. V. Nikulina

Presented by Academician Yu.D. Tret’yakov January 30, 2008Received January 30, 2008

DOI:

10.1134/S0012500808050042

Russian Federal Center of Forensic Science, Ministryof Justice of the Russian Federation,Prechistenskaya nab. 15, Moscow, 119034 Russia

CHEMISTRY

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FEDOTOV et al.

The maximum heavy-metal yield from the 1–2 mmfraction treated with 1 N nitric acid and the low heavy-metal yields from the 0.25–0.5 and 0.5–1 mm fractions(Fig. 1, Table 1) correlate with the gel concentrations inthese fractions determined from microscopic data;likely, this signifies that 1 N nitric acid extracts heavymetals from the all of the gel contained in aggregates.

A slightly different pattern is observed for the AABextracts of heavy metals from various aggregate frac-tions of the soddy-podzolic soil.

Our results (Fig. 2, Table 1) show that the heavymetals can conventionally be classified into twogroups: one group comprises lead and copper, and theother comprises zinc and the other tested elements. Forcopper and especially for lead, the maximum concen-tration is contained in the 0.25–0.5 mm fraction; theconcentrations of the elements of the second group areminimal in this fraction. Recall that this fraction is richin sand-sized particles.

For interpretation, we needed data on the retentionof heavy metals by the components of the RHG.

EXAFS spectroscopy provided us with these data forseveral heavy metals [3]. Lead is mostly retained by thesoil organics, being preferably chelated by aromaticfunctionalities and forming bidentate complexes [14].Zinc is mostly retained by soil clay minerals and ironhydroxides [14].

Summing up EXAFS data [3] and our results, wecan suggest that the RHG is not homogeneous and itsparts in contact with various minerals differ in charac-ter. This is quite logical, as different minerals shouldpreferentially adsorb organic molecules of a certainstructure [15], forming different gels.

The RHG coating sand particles, apparently, is moreprone to weakly retain copper and, especially, lead cat-ions. From this, we can infer that this RHG is relativelyricher in organics and poorer in clay minerals (whichreinforce the humus gel). This inference agrees withearlier data [4]. The humus gel in other soil aggregatefractions is apparently enriched in clay minerals andiron compounds, increasing its ability to retain otherheavy metals.

1

10 2 3 4 5 6

Fraction, mm

2

3

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Ni

Co

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(b)

10

0

20

35

40

Concentration in soil, ppm

(a)

30

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8

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1

(a)

(b)

Concentration in soil

Zn

Cu

Pb

NiCdCo

Fig. 1.

(a) Copper and lead concentrations in nitric acidextracts from various aggregate fractions of the soddy-pod-zolic soil. (b) The same for nickel, cobalt, and cadmium.

Fig. 2.

(a) Copper and zinc concentrations in AAB extractsfrom various aggregate fractions of the soddy-podzolic soil.(b) The same for lead, nickel, cadmium, and cobalt.

DOKLADY CHEMISTRY

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HEAVY-METAL DISTRIBUTION IN VARIOUS TYPES OF SOIL AGGREGATES 127

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128

DOKLADY CHEMISTRY

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Part 1

2008

FEDOTOV et al.

Having studied various aggregate fractions of thesame soil (with similar compositions of the RHG), wecan compare the results obtained with the use of 1 Nnitric acid and AAB. It follows that the increase in theratio of the amounts of heavy metals extracted by 1 Nnitric acid and AAB from various aggregate fractions ofthe soddy-podzolic soil (Table 1) correlates with theincrease in their concentrations in the nitric acidextracts from these fractions. Thus, the richer the frac-tion in the RHG, the lower the heavy-metal fractionextracted by AAB. This is possible if the RHG enrichedin clay minerals and iron compounds retains heavymetals in greater amounts and more strongly than theRHG coating sand-sized particles.

Gray forest soils and chernozem do not differ con-siderably in the yields of heavy metals from variousaggregate fractions, and the ratio of the heavy-metalyields to nitric acid and AAB is virtually constant(Table 1). These results can be explained by the nonac-cumulation of sand-sized particles in separate aggre-gate fractions.

Noteworthy is the effect of the nature of the humuson the retention of heavy metals by RHGs in varioussoils. In the soddy-podzolic soil, the cadmium yield tonitric acid is comparable to the cadmium yield to AAB;in the gray forest soil and chernozem, the respectivevalues differ by more than two orders of magnitude.However, it is yet impossible to find definite correla-tions or draw any conclusions.

To summarize, the heavy-metal concentration inaggregates of various sizes depends on the proportionof the RHG in the sample, its nature, and the concentra-tion and composition of the reinforcing particles. Theability of the RHG to adsorb heavy metals will increasewith increasing RHG proportion in the sample.

In soils in which aggregate fractions contain differ-ent RHGs, complex correlations are observed betweenthe extraction of heavy metals and the aggregate size.

Reinforced humus gels in various soils can differ intheir ability to retain some heavy metals.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research (project no. 07–04–00144a).

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