ISSN 0012-5008, Doklady Chemistry, 2007, Vol. 412, Part 2, pp. 55–58. © Pleiades Publishing, Ltd., 2007.Original Russian Text © G.N. Fedotov, Yu.D. Tret’yakov, V.I. Putlyaev, E.I. Pakhomov, A.I. Kuklin, A.Kh. Islamov, 2007, published in Doklady Akademii Nauk, 2007, Vol. 412,No. 6, pp. 772–775.

55

The electron microscopic investigation of soil solu-tions carried out in [1–3] discovered that colloidal par-ticles in soils are fixed on a substrate and form struc-tures that visually resemble fractal clusters. Theseresults implied that soil colloids have a fractal organi-zation. However, it was impossible to definitely con-clude that fractal structures exist in soil solutions or theoozy fraction of soils: similar structures can likewiseappear during sample preparation for electron micro-scopic experiments.

It became necessary to find techniques for the obser-vation of colloidal structures directly in soils. Small-angle neutron scattering (SANS) and X-ray scatteringcan be such techniques [4].

The SANS investigations carried out in [5, 6] con-firmed the existence of a fractal organization in soil col-loids, but its origin was not quite clear.

The goal of this work was to elucidate the origin ofthe fractal organization in soil colloids.

Electron microscopic experiments were carried outon a Carl Zeiss Supra 50 VP scanning electron micro-scope (SEM).

It was shown in [7] that, when predried and capil-lary-moistened aggregates taken from the alluvial lay-ers of some soils are transferred to water, a film appearson the water surface. Test samples for electron micros-copy were prepared as follows. Soil aggregates 3–5 mmin size were transferred on paper filters to petri dishes,and the filters were moistened to capillary saturate theaggregates with water. The water level in the petri dishwas raised after 2–3 min; films separated from theaggregates and buoyed to the water surface. Then, thefilms were transferred to the atomically smooth surface

of a fresh mica cleave: the mica surface was brought incontact with the water surface covered with the film.

After drying, the samples prepared thus werecoated with carbon in a Leybold Univex 300 thermalevaporator.

SANS was used to determine the fractal dimensionof the test samples and the maximal scattering intensity.Fractal objects are known to have specific SANS pat-terns; namely, the log–log plots of the scattering inten-sity versus pulse energy are straight lines over a fairlywide range of pulses [8]:

For mass fractals, the

ı

value, i.e., the Porod index,coincides with their fractal dimension

D

, while, for sur-face fractals,

x

= 6 –

D

.The measurements were carried out on a YuMO

SANS spectrometer in the town of Dubna. Because atwo-detector system was used (Fig. 1), the range of thescattering vector magnitude was from 0.007 to

0.6

Å

–1

for neutron wavelengths from 0.7 to

5

Å and detector–sample distances of 3.6 and 12.97 m for the nearer andthe farther detectors, respectively.

Test samples were placed in Hellma cells with a use-ful thickness of 2 mm. The beam size was 14 mm. Thecells were mounted inside a thermobox maintained at25

°

C.Rough data processing was performed using SAS

software [9]. The data were normalized to a vanadiumreference in order to obtain the spectra in absolute val-ues. The fractal dimension and scattering intensity weremeasured.

For a better understanding of the relevance of usingSANS, let us accentuate its utility for soil investiga-tions. Colloidal soil particles scatter neutrons at smallangles. When colloidal particles respond as autono-mous emitters to irradiation (i.e., when they are at dis-tances longer than the neutron wavelength from oneanother), the fractal dimension of the sample is lessthan three. When colloidal particles are in contact withone another and cannot behave as autonomous emitters,the fractal dimension is greater than three.

I k( )log x k.log–∼

Origin of Fractal Organization in Soil Colloids

G. N. Fedotov

a

,

Academician

Yu. D. Tret’yakov

b

, V. I. Putlyaev

b

,E. I. Pakhomov

a

, A. I. Kuklin

c

, and A. Kh. Islamov

c

Received November 10, 2006

DOI:

10.1134/S0012500807020085

a

Moscow State Forestry University, ul. Pervaya Institutskaya 1, Mytishchi-5,Moscow oblast, 141005 Russia

b

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

c

Joint Institute for Nuclear Research, Dubna,Moscow oblast, 141980 Russia

CHEMISTRY

56

DOKLADY CHEMISTRY

Vol. 412

Part 2

2007

FEDOTOV et al.

Let us analyze the fractal characteristics observedfor contrasting soils (Table 1). It is noteworthy that thefractal dimension for dry soil samples is less than three,while, for moistened, fresh precipitates of silver bro-mide and barium sulfate, the fractal dimension is3.2

−

3.3. This means that colloidal particles in air-drysoils are not in contact with one another, which may bedue to their stabilization in the humus molecular net-work.

In the electron micrographs of films removed from achernozem and a soddy-podzolic soil, there are regions

containing inorganic particles (Fig. 1).

1

There are twotypes of clusters of colloidal particles. One type of clus-ter has a core, a micron-sized inorganic particle withcolloidal particles grouped around it. In the other type,colloidal particles are grouped in the same manner, butthere is no micron-sized core. The existence of suchclusters in humus films allows us to hypothesize rea-

1

We showed in [13] that these films consist of two types ofregions: regions containing inorganic particles are disseminatedin space free of them.

2

µ

m

200 nm 200 nm

2

µ

m(a) (c)

(d)(b)

Fig. 1.

SEM micrographs of films isolated from aggregates of (a, b) soddy-podzolic and (c, d) chernozem soils and transferred to amica substrate.

DOKLADY CHEMISTRY

Vol. 412

Part 2

2007

ORIGIN OF FRACTAL ORGANIZATION IN SOIL COLLOIDS 57

sons for the fractal organization of soil colloids. Likely,micron-sized particles are fixed in the humus gel. Envi-ronmental factors induce the chemical degradation ofminerals to colloidal particles, and these particles dif-fuse through the gel from the cluster. The humus gelcontains many polar groups, and colloidal particles arefixed to humus macromolecules. The longer the dis-tance from the center of the cluster, the fewer moleculescan travel it. As a result, the concentration of colloidalparticles decreases from the center to the periphery ofthe cluster. In some cases, a micron-sized particle isfully disintegrated and electron micrographs show acluster built of colloidal particles without a micron-sized core. The structures appearing in this way are typ-ical fractals [10].

It is known that many minerals are coated with a gelfilm of colloidal particles resulting from the environ-mental degradation of minerals [11, 12]. It is alsoknown that colloidal particles can diffuse through poly-mer gels [14]. The model we propose is in fact a naturalcombination of the aforementioned two processes:mineral particles are fixed in the humus gel, and colloi-dal particles forming on their surfaces diffuse throughthe gel.

Let us consider our results in the context of ourhypothesis of the appearance of colloidal structureswith fractal properties. The distance to which colloidalparticles can travel through the humus gel is inverselyproportional to the concentration of functional groupsin the humus. The humus content in soddy-podzolicsoils is far lower than in chernozems or krasnozems. Inaddition, this is a fulvate-type humus; i.e., it contains

more functional groups than a chernozem humus. As aresult, the colloidal particle concentration in the clus-ters of soddy-podzolic soil decreases more rapidly thanin the clusters of chernozem. Therefore, the fractaldimension of colloids in soddy-podzolic soil should behigher than in chernozem. When the humus content islow, colloidal particles can come in contact with oneanother upon removal of water and the mass fractal cantransform into a surface fractal. Humus containinghigher concentrations of functional groups should swellmore strongly in contact with water, and the fractaldimension of colloids in soddy-podzolic, brown forest,and krasnozem soils should decrease in contact withwater to a greater degree than in chernozem. Weobserved this in our SANS experiments.

In addition to the concentration and type of humus,the fractal dimension is affected by the alkalinity of thesoil, i.e., the exchangeable sodium concentration. Theswelling capacity of the humus gel should increase inassociation with this fact, which is apparently the rea-son for the changes in fractal dimension observed forlight chestnut soil samples.

The behavior of the scattering intensity, which char-acterizes the colloidal particle concentration in thesample, is worth noting (Table 1). In soddy-podzolicand brown forest soils, the scattering intensitydecreases upon interaction with water. In chestnut soils,the scattering intensity does not change, and in theother soils, it increases. This may be explained by theinterplay of two processes in a soil exposed to waterthat affect the fractal dimension in opposite directions.First, the concentration of organic colloidal particles in

Table 2.

Layer-by-layer SANS data for an alkali soil

Layer, cmAir-dry soils Soil pastes

fractal dimension scattering intensity, cm

–1

fractal dimension scattering intensity, cm

–1

0–3 2.9

±

0.02 242 2.73

±

0.02 324

3–10 2.93

±

0.02 251 2.74

±

0.02 500

10–20 3.02

±

0.02 256 2.73

±

0.02 490

20–30 3.05

±

0.02 301 2.76

±

0.02 564

Table 1.

SANS data for the high-humus layers of contrasting soils

SoilAir-dry soils Soil pastes

fractal dimension scattering intensity, cm

–1

fractal dimension scattering intensity, cm

–1

Soddy-podzolic soil 3.22

±

0.03 400 2.69

±

0.03 268

Brown forest soil 3.07

±

0.02 590 2.64

±

0.02 483

Leached chernozem 2.90

±

0.02 200 2.77

±

0.02 260

Typical chernozem 2.90

±

0.09 322 2.65

±

0.09 800

Dark chestnut soil 2.71

±

0.02 707 2.40

±

0.02 946

Light chestnut soil 2.97

±

0.02 500 2.47

±

0.02 500

Krasnozem 2.82

±

0.08 1159 2.53

±

0.02 1205

58

DOKLADY CHEMISTRY

Vol. 412

Part 2

2007

FEDOTOV et al.

a humus can decrease due to their transition to anextended molecular form. This process, likely, domi-nates in soddy-podzolic and brown forest soils. Second,colloidal aggregates in a water-exposed soil can trans-form as a result of the swelling of the humus gel insidethem and an increase in the separation between colloi-dal particles.

Very interesting results were obtained for solonetzes(Table 2).

Increased sodium ion concentrations, apparently,decrease the number of links between macromoleculesin the humus gel and, thus, decrease the strength of thehumus matrix. The increase in the absorbed sodiumconcentration with depth [15] is clearly demonstratedby the change in the fractal parameters of soil colloidsin solonetzes. In the 0–10 cm topsoil, the humus matrixis strong enough to keep colloidal particles at some dis-tance from one another upon drying. At higherabsorbed sodium concentrations, however, the massfractal transforms to a surface fractal; that is, the num-ber of contacts between colloidal particles increases.The scattering intensity also increases with depth,matching the increase in the colloidal particle concen-tration along the profile of the alkali soil.

In summary, from our results we infer that(1) colloidal particles in soils are within the humus

gel matrix, whose strength is due to the ions of theabsorbing complex of the soil, and

(2) the fractal properties of soil colloids appearbecause of the degradation of minerals contained in thehumus host, the diffusion of the resulting colloidal par-ticles through the humus matrix, and the anchorage ofthese particles to the host.

ACKNOWLEDGMENTSThis work was supported by the Russian Foundation

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

REFERENCES

1. Fedotov, G.N., Tret’yakov, Yu.D., Putlyaev, V.I., Gar-shev, A.V., and Pakhomov, E.I.,

Dokl. Chem.,

2005,vol.

403, part 1, pp. 129–131 [

Dokl. Akad. Nauk,

2005,vol. 403, no. 2, pp. 205–207].

2. Fedotov, G.N., Tret’yakov, Yu.D., Dobrovol’skii, G.V.,Putlyaev, V.I., Garshev, A.V., and Pakhomov, E.I.,

Dokl.Chem

., 2005, vol. 404, part 1, pp. 182–184 [

Dokl. Akad.Nauk

, 2005, vol. 404, no. 3, pp. 352–354].3. Fedotov, G.N.,

Extended Abstract of Doctoral (Biol.)Dissertation

, Moscow: Mosk. Gos. Univ., 2006.4. Svergun, D.I. and Feigin, L.A.,

Rentgenovskoe i neitron-noe malouglovoe rasseyanie

(X-ray and Neutron Small-Angle Scattering), Moscow: Nauka, 1986.

5. Fedotov, G.N., Tret’yakov, Yu.D., Ivanov, V.K., Kuk-lin,

A.I., Islamov, A.Kh., Putlyaev, V.I., Garshev, A.V.,and Pakhomov, E.I.,

Dokl. Chem.,

2005, vol. 404, part 2,pp. 199–202 [

Dokl. Akad. Nauk

, 2005, vol. 404, no. 5,pp. 638–641].

6. Fedotov, G.N., Tret’yakov, Yu.D., Ivanov, V.K., Kuk-lin,

A.I., Pakhomov, E.I., Islamov, A.Kh., and Pochatk-ova, T.N.,

Dokl. Chem.

, 2005, vol. 405, part 1, pp. 240–242 [

Dokl. Akad. Nauk

, 2005, vol. 405, no. 3, pp. 351–354].

7. Grossman, R.B. and Lynn, W.C.,

Soil Sci. Soc. Am.Proc.

, 1967, vol. 31, pp. 259–262.8. Rothschild, W.G.,

Fractals in Chemistry,

New York:Wiley, 1998.

9. Solov’ev, A.G., Solov’eva, T.M., Stadnik, A.V., et al.,

SAS.

Programma dlya pervichnoi obrabotki spektrovmalouglovogo rasseyaniya. Versiya 2.4. Opisanie i ruk-ovodstvo pol’zovatelya. Soobshchenie OIYaI R10-2003-86

(SAS. Program for Primary Processing Small-AngleScattering Spectra, Vers. 2.4, Description and User’sGuide, Report of Joint Inst. for Nuclear Research),Dubna: Ob. Inst. Yad. Issled., 2003.

10. Tence, J.P. and Chevalier, R.,

J. Phys.

, 1986, vol. 47,pp.

1989–1998.11. Vershinin, P.V.,

Pochvennaya struktura i usloviya eeformirovaniya

(Soil Structure and Its Formation Condi-tions), Moscow: Izd. AN SSSR, 1958.

12. Lipson, G.A. and Kolodieva, T.S.,

Kolloidn. Zh.

, 1972,vol. 34, p. 235.

13. Fedotov, G.N., Tret’yakov, Yu.D., Putlyaev, V.I., Pakho-mov, E.I., and Pozdnyakov, A.I.,

Dokl. Chem.

, 2006,vol.

411, part 1, pp. 212–214 [

Dokl. Akad. Nauk,

2006,vol. 411, no. 2, pp. 203–205].

14. Voyutskii, S.S.,

Rastvory vysokomolekulyarnykh soedine-nii

(Solutions of High-Molecular-Weight Compounds),Moscow: Goskhimizdat, 1960.

15.

Pochvovedenie

(Soil Science), Kaurichev, I.S., Ed.,Moscow: Kolos, 1975.