ISSN 0012-5008, Doklady Chemistry, 2006, Vol. 411, Part 1, pp. 212–214. © Pleiades Publishing, Inc., 2006.Original Russian Text © G.N. Fedotov, Yu.D. Tret’yakov, V.I. Putlyaev, E.I. Pakhomov, A.I. Pozdnyakov, 2006, published in Doklady Akademii Nauk, 2006, Vol. 411, No. 2,pp. 203–205.

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Experiments intended to study the stickiness, elec-trical conductivity, enzymatic activity, structural-mechanical, and some other soil properties as a func-tion of the time elapsed since moistening of dry soil,confirmed that there are interactions between colloidalsoil particles and that these interactions result in gelstructures incorporating the soil solution.

Scanning electron and transmission electronmicroscopy and small-angle neutron scattering showedthat colloidal particles in soils are fixed in an organicjellylike matrix at some distance from one another.

Our previous results [1–3] imply that organomineralsoil gels, which coat and bind soil particles, consist ofa jellylike soil humus reinforced with organic and inor-ganic colloidal particles. The reinforced humus gelbehaves like many polymers when interacting withwater: it swells, absorbing water and enlarging, andshrinks upon drying. Various actions change the state ofthe reinforced polymeric humus gel, which in turnchanges soil properties.

However, the difficulty of isolating the structures inquestion in an unchanged form and of separating themfully is a problem in this type of investigation. Experi-ments confirmed the existence of reinforced humus gelsin soils, but they did not provide unambiguous evidenceof the nonexistence of other types of gel structures insoils [3].

This work was undertaken in order to refine themodel of soil-gel structures.

Soils for the study were collected from the humus-rich horizons of leached Kuban chernozem and thesoddy-podzolic soil from the floodplain of theYakhroma River. The properties of the soils, whichwere determined by routine procedures, are found else-where [4, 5].

Test samples were prepared as follows. Soil aggre-gates 3–5 mm in size were placed onto paper filter inPetri dishes and moistened (thus, the aggregates werecapillary saturated with water). After 2–3 min, thewater level in the Petri dish was raised; as a result, thefilms separated from the aggregate and rose to the watersurface. Then, the films were transferred onto an atom-ically smooth fresh cleave of mica; to do this, the micasurface was brought in contact with the water surfaceon which the film floated.

Carbon was sputtered onto the thus-prepared sam-ples after they were dried on a Leybold Univex 300thermal evaporator.

The electron microscopic examination was carriedout on a Carl Zeiss LEO Supra 50 VP electron micro-scope.

Grossman and Lynn [6] found that, when theyplaced into water predried and then capillary-moist-ened aggregates taken from the illuvial horizons ofseveral soils, a film appeared on the water surface.Grossman and Lynn [6] concluded that these filmsconsisted of clay mineral particles with sizes rangingfrom several micrometers to tens of micrometersbound through an inorganic gel. Grossman and Lynn’sconclusion was based on the fact that they failed tooxidize the organic substance of the gel with hydrogenperoxide.

We suggested that such films are formed of piecesof the gel layer that flaked off soil particles upon dry-ing of soil aggregates. This means that such filmsshould also be observed for samples taken fromhumus-rich horizons, in which gels are formed ofhumus molecules. Experiments carried out on aggre-gates from humus-rich horizons of various soils con-firmed our suggestion.

Thus, an electron microscopic examination of thegel films that separate when dried and then capillary-moistened aggregates are exposed to water offers ameans for isolating soil gels in an unchanged form and

Refined Model of the Soil-Gel Structure

G. N. Fedotov

a

,

Academician

Yu. D. Tret’yakov

b

, V. I. Putlyaev

b

, E. I. Pakhomov

a

, and A. I. Pozdnyakov

b

Received July 5, 2006

DOI:

10.1134/S001250080611005X

a

Moscow State Forestry University, Mytishchi-5,Moscow oblast, 141005 Russia

b

Moscow State University, Vorob’evy gory,Moscow, 119992 Russia

CHEMISTRY

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REFINED MODEL OF THE SOIL-GEL STRUCTURE 213

gaining quite extensive information about the gel struc-ture.

Figure 1a displays a representative microstructureof films isolated from the soddy-podzolic soil. Thesefilms are analogous to the films we isolated from cher-nozem and Grossman and Lynn [6] isolated from illu-vial horizons: they consist of a gel reinforced withmicrometer-sized mineral particles.

At higher magnifications, two types of regions aredistinguished in gel films. In one type of region, parti-cles several tens of nanometers in size are arranged inthe immediate vicinity of each other or are clusteredaround microparticles in the humus gel matrix(Figs. 1b, 2a; regions A). This is not unexpected: manyresearchers have observed colloidal particle gelsaround inorganic microparticles [7–9]. In particular,when clay mineral particles in water were exposed to adye, a transparent layer with an increased dye concen-tration surrounding such particles was observed with anoptical microscope [7]. This observation was explained

by the existence of a gel coating of colloidal particlesaround a clay mineral microparticle.

In the other type of region, the humus gel films,which are as a rule in between microparticles, are prac-tically free of nanoparticles or contain them in far lowerconcentrations (Figs. 1b, 2a; regions B).

In film samples isolated from chernozem, brokenand bent regions were observed (Fig. 2b). This allowedus to estimate the film thickness at about 0.7

µ

m.

The examination of surface images of microparti-cles shows that microparticles are incorporated into thefilm; i.e., microparticles are coated with the humus gel,as we observed previously [2].

The results obtained in this work prove that gel filmsare inhomogeneous. The films consist of two types ofregions: some regions are indeed the humus gel rein-forced with colloidal particles, while, in the otherregions, the humus gel is almost free of colloidal parti-cles. Therefore, we may infer that the humus gel playsa key role in gel formation in humus-rich horizons: the

100

µ

m

2

µ

m

(‡)

B

Ä

(b)

Fig. 1.

Electron micrographs of a film on a mica substrate separated from the surface of aggregates of the soddy-podzolic soil. Mag-nification: (a) 500

×

and (b) 10 000

×

. Notation: A, humus-gel regions reinforced with colloidal particles; B, humus-gel regions freeof colloidal particles.

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FEDOTOV et al.

humus gel binds regions that contain micro- and nano-particles.

ACKNOWLEDGMENTSThis work was supported by the Russian Foundation

for Basic Research (project nos. 03–04–48216a and04–04–48586a).

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2

µ

m

1

µ

m

(‡)

B

B

Ä

(b)

Fig. 2.

Electron micrographs of a film on a mica substrate separated from the surface of soil aggregates of chernozem. Magnifica-tion: (a) 10 000

×

and (b) 25 000

×

. Notation: A, humus-gel regions reinforced with colloidal particles; B, humus-gel regions free ofcolloidal particles.