Effect of the soil water content on the fractal properties of soil colloids
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ISSN 0012-5008, Doklady Chemistry, 2006, Vol. 409, Part 1, pp. 117119. Pleiades Publishing, Inc., 2006.Original Russian Text G.N. Fedotov, Yu.D. Tretyakov, E.I. Pakhomov, A.I. Kuklin, A.Kh. Islamov, T.N. Pochatkova, 2006, published in Doklady Akademii Nauk, 2006, Vol. 409,No. 2, pp. 199201.
Organomineral soil gels are the soil humus in the jellystate reinforced by organic and inorganic colloidal parti-cles (the reinforced humus gel; hereafter, RHG) [1, 2].Colloidal particles situated in the humus gel obey a cer-tain order. A fractal organization of soils was discoveredusing small-angle neutron scattering (SANS) [3, 4].
On one hand, the state of organomineral gel struc-tures controls many properties of soils [5, 6]. On theother hand, the water content is an essential character-istic that influences almost all soil properties .Therefore, the changing soil water content is expectedto alter the colloidal structure of soil.
The goal of this work was to study how the soilwater content affects the fractal properties of soil.
Soil samples were collected from the high-humuslayers of contrasting soils: leached chernozem, asoddy-podzolic soil, and krasnozem.
Test samples were prepared as follows: an air-drysoil sample was moistened to its least water capacity,exposed in this state for at least two weeks, and thendried to the required water content. To elucidate howthe soil gels are altered in response to the changingwater content, we determined the fractal dimensions ofsoils with different water contents using SANS.
Fractal objects are known to have specific SANSpatterns. Their loglog plots of the scattering intensityversus the pulse energy are straight lines over a fairlywide range of pulse energies :
The measurements were carried out on a YuMOsmall-angle neutron spectrometer installed at a channel
I k( )log x k.log
of an IBR-2 pulsed reactor and equipped with a two-detector system. Therefore, the range of the scatteringvector magnitudes
was from 0.007 to
for neu-tron wavelengths from 0.7 to 5 and detector-to-sam-ple distances of 3.6 and 12.97 m for the nearer and thefarther detectors, respectively.
Primary data processing was performed using theSAS software . The data were normalized to a vana-dium reference in order to obtain absolute spectra. Atest sample was placed in a cell with a useful thicknessof 2 mm; the beam size was 14 mm.
Only the krasnozem sample shows a monotonicdecrease in the fractal dimension (Fig. 1a) and a mono-tonic increase in the scattering intensity (Fig. 1b). Thesoddy-podzolic soil and chernozem samples showmore intricate correlations.
We will consider the bonding character of water insoils in order to interpret our results. It is believed that soil water is adsorbed on soil particles while thesoil water content is low. When the soil water contentincreases, low-mobility fluid water films appear on thesurface of soil particles.
It is difficult to interpret our results in these terms(assuming the existence of a polydisperse system). Forthe krasnozem and soddy-podzolic soil samples, themaximal change in the fractal dimension occurs in theregion where film water exists. For the chernozem sam-ple, it occurs in the region covering the existence ofcapillary water. In addition, while the monotonic curvefor the krasnozem sample is explainable, the nonmono-tonic curves obtained for the chernozem and soddy-podzolic soil samples cannot be explained in theseterms.
If we postulate that soil colloids exist in the RHGform, the interaction of soil with water is considereddifferently. The increasing soil water content shouldfirst induce the hydration of the active sites of thehumus molecules and colloidal particles. Then, fullhydration occurs (water fills the contracted RHG).Here, the RHG structure and the disposition of the col-loidal particles remain almost unchanged. At still
Effect of the Soil Water Content on the Fractal Properties of Soil Colloids
G. N. Fedotov
Yu. D. Tretyakov
, E. I. Pakhomov
A. I. Kuklin
, A. Kh. Islamov
, and T. N. Pochatkova
Received March 24, 2006
Moscow State Forestry University, Mytishchi-5, Moscow oblast, 141005 Russia
Moscow State University, Vorobevy gory, Moscow, 119992 Russia
Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia
FEDOTOV et al.
higher water contents, the RHG should start to swelldue to captured water. This evolution of the RHG willcover the film water region and the onset of the capil-lary water region since small capillaries can be fullyfilled with the swollen RHG. The resulting colloidalsystem should have the ultimate shear strength and ahigh viscosity. Indeed, low mobilities were observedfor film water and, in many cases, for water enclosed incapillaries with diameters smaller than 10
m [10, 11].The results cited in  in the discussion of Darcys lawwell illustrate the existence of low-mobility gel layersin capillaries: the permeability coefficient jumps uponreaching a certain pressure (the stress exceeds the ulti-mate shear strength of gel structures, the gel structuresdegrade, and the effective capillary diameters increase),and there is no permeation at low pressures. These datafind explanation from the standpoint of the existence ofsoil colloids in the form of the RHG.
When the soddy-podzolic soil reaches a 78% watercontent, its RHG expands with an attendant increase inthe distances between reinforcing colloidal species
(colloidal particles and colloidal-size aggregates of col-loidal particles). The decrease in the fractal dimension(Fig. 1a), which may be treated as an increase in theaverage distance between neutron-scattering particles,proves that the RHG expands. It is noteworthy thatrestructuring in the soddy-podzolic soil occurs within avery narrow range of water contents. The scatteringintensity increases in the same range of water contents(Fig. 1b). The increasing scattering intensity means thatthe concentration of colloidal particles increases; likely,they are generated by degrading aggregates.
The chernozem samples show a more complex cor-relation. First, the fractal dimension decreases inresponse to the increasing soil water content with asmall increase in the scattering intensity (Fig. 1).Therefore, the RHG is swollen, the average distancebetween colloidal-size species (particles and aggre-gates) increases, and an insignificant fraction of theaggregates degrade to colloidal particles or colloidal-size aggregates. When the water content is about 17%,the fractal dimension curve shows an inflection(Fig. 1a) and the scattered intensity curve shows a jump(Fig. 1b). These features may be related to the degrada-tion of aggregates to colloidal particles and to adecrease in the average distance between scatteringparticles at these water contents.
In the krasnozem, the increasing water contentdecreases the fractal dimension and increases the scat-tering intensity (Fig. 1). This means that the RHG isswollen with an attendant increase in the distancebetween scattering colloidal species; the concentrationof the scattering species continuously increases as aresult of the degradation of aggregates. However, theRHG swelling (which increases the distance betweenparticles) prevails over the aggregate degradation(which increases the concentration of colloidal speciesand decreases their average separation).
From the above, we infer that the soil colloidalstructures are restructured in a complex way wheninteracting with water. The following two processesoccur at the same time: the humus gel reinforced withcolloidal particles and colloidal aggregates swells, andthe aggregates degrade to smaller colloidal species.
ACKNOWLEDGMENTSThis work was supported by the Russian Foundation
for Basic Research (project nos. 040448586 and 050448655).
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Plots of (a) the fractal dimension and (b) the scat-tered neutron intensity vs. soil water content for (
) soddy-podzolic soil, (
) chernozem, and (
) krasnozem samples.
EFFECT OF THE SOIL WATER CONTENT ON THE FRACTAL PROPERTIES OF SOIL COLLOIDS 119
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