Fractal Colloidal Structures in Soils of Various Zonalities

Abstract

It has been demonstrated [1][2][3][4][5] that soil solutions are structured colloids and that they affect many properties of soils. Attempts to isolate a soil solution in order to study it using electron microscopy were inefficient [6]. Therefore, the need appeared for techniques to observe colloidal structures directly in soils rather than separate fragments of colloidal structures isolated from the soil and, thus, exposed to external factors.Small-angle neutron and X-ray scattering may be classified as such techniques [7]. In particular, these techniques provide information on the fractal dimension of soils, which is an integral characteristic of soil colloids [8].Here, we consider the fractal dimension of soil colloidal structures as dependent on the formation conditions and functioning of soils under various exposures.We employed small-angle neutron scattering (SANS) to find the fractal dimensions of the test soils. Fractal objects are known to have specific SANS patterns. Their log-log plots of the scattering intensity versus pulse energy are straight lines over a fairly wide range of pulse energies [9]:(1)For mass fractals, x (the Porod exponent) coincides with the fractal dimension D ; for surface fractals, x = 6 -D .The measurements were carried out on a YuMO small-angle neutron spectrometer installed at the fourth channel of an IBR-2 pulsed reactor. Since a two-detector system was used (Fig. 1), the range of the scattering vector magnitude Q was from 0.007 to 0.6 Å -1 for neutron wavelengths from 0.7 to 5 Å and detector-to-sample distances of 3.6 and 12.97 m for the nearer and the farther detectors, respectively.Test samples were placed in Hellma cells with a useful thickness of 2 mm; the beam size was 14 mm. The cells were mounted inside a thermobox maintained at 25°ë . I k ( ) logx k. log -∼ Primary data processing was performed using the SAS software [10]. The data were normalized to a vanadium reference in order to obtain the spectra in absolute values.The fractal dimensions and the fractal range (i.e., the self-similarity range) were determined.The test samples were various air-dry soils and soil pastes.The results of the SANS analysis are displayed in Tables 1-3.The results show that the fractal characteristics of the soil colloidal structures are substantially controlled by the soil zonality and profile and by external factors.Soil colloidal structures are most frequently mass fractals (Tables 1-3). It is only in air-dry soddy-podzolic and brown forest soils that the plot of the scattering intensity as a function of wave vector magnitude obeys the law characteristic of surface fractals. Note that the data obtained for these soils may be interpreted in a different way: an exponential particle size distribution of colloids can exist in these soils.There are cases where the fractal dimension reaches three, which is the limiting value for fractal objects. This value obtained in SANS analysis may characterize the transition from mass to surface fractals [11].The data compiled in Tables 1 and 2 show tha...

Application of Rock Weathering and Colonization by Biota for the Relative Dating of Moraines from the Arid Part of the Russian Altai Mountains

“…The absence of a deviation of the dΣ(q)/dΩ curve from the power law (the onset of the Guinier regime) at small values of q means that the upper self-similarity limit ξ of the surface fractal is larger than the maximum size R max of the inhomogeneities (pores) that can be detected in the experiment with a given resolution: R max = 3.5/q min ≈ 60 nm. Thus, the observed scattering patterns are typical for scattering from system possessing a disordered porous (solid phase-pore medium) structure with fractal phase boundaries [83][84][85] including natural systems represented by rock fragments from shallow soils [86] and soil aggregates from mature soils [87].…”

Possible ways of nanostructure development in soil gels

“…Insight about the structural organization of pri mary HS formations into larger objects (we shell denote them as clusters 4 several tens to several hun dreds of nanometers in size can be obtained by com paring the data on the study of HS solutions [57,62,63] and soils [35][36][37] by the SANS method. The frac tal organization of HS clusters in solutions and the soil colloidal component was noted in both cases.…”

“…The frac tal organization of HS clusters in solutions and the soil colloidal component was noted in both cases. In the soils, the fractal organization of the soil gels [35][36][37] was attributed to the specific arrangement of the humic gel of inorganic particles in the homogeneous matrix. However, some data contradicted this inter pretation of the SANS study of soils.…”

Fractal Clusters is a Form of Existence of Humic Substances in Soils

“…Studying humic acid solutions, Osterberg showed using the small-angle neutron scattering method that primary humic acid particles with a size of 2-10 nm form fractal clusters with a size of 100-200 nm in solutions (1). Fedotov found that in soils, as well as in solutions of humic acids, there are also primary particles of humic substances of 2-10 nm in size, forming fractal clusters of 100-200 nm in size (2), which in turn, together with mineral particles, form soil gels.…”