Nature of the clay - cation bond affects soil structure as verified by X-ray computed tomography

Abstract

Non-destructive X-ray computed tomography (µCT) scanning was used to characterise changes in pore architecture as influenced by the proportion of cations (Na, K, Mg, or Ca) bonded to soil particles. These observed changes were correlated with measured saturated hydraulic conductivity, clay dispersion, and zeta potential, as well as cation ratio of structural stability (CROSS) and exchangeable cation ratio. Pore architectural parameters such as total porosity, closed porosity, and pore connectivity, as characterised from µCT scans, were influenced by the valence of the cation and the extent it dominated in the soil. Soils with a dominance of Ca or Mg exhibited a well-developed pore structure and pore interconnectedness, whereas in soil dominated by Na or K there were a large number of isolated pore clusters surrounded by solid matrix where the pores were filled with dispersed clay particles. Saturated hydraulic conductivities of cationic soils dominated by a single cation were dependent on the observed pore structural parameters, and were significantly correlated with active porosity (R2 = 0.76) and pore connectivity (R2 = 0.97). Hydraulic conductivity of cation-treated soils decreased in the order Ca > Mg > K > Na, while clay dispersion, as measured by turbidity and the negative charge of the dispersed clays from these soils, measured as zeta potential, decreased in the order Na > K > Mg > Ca. The results of the study confirm that structural changes during soil–water interaction depend on the ionicity of clay–cation bonding. All of the structural parameters studied were highly correlated with the ionicity indices of dominant cations. The degree of ionicity of an individual cation also explains the different effects caused by cations within a monovalent or divalent category. While sodium adsorption ratio as a measure of soil structural stability is only applicable to sodium-dominant soils, CROSS derived from the ionicity of clay–cation bonds is better suited to soils containing multiple cations in various proportions.

Potassium and magnesium in irrigation water quality assessment

“…Total porosity and pore connectivity were influenced by cation valence, as might be anticipated from the Schulze-Hardy Rule (Sposito, 2008). A well-developed pore structure was evident in soils having a sufficiently high concentration of exchangeable Ca and Mg, whereas soils dominated by Na and K had isolated pore clusters surrounded by pores filled with dispersed clay particles (Marchuk et al, 2012). Clay dispersion decreased in the order: Na > K > Mg > Ca and, correspondingly, hydraulic conductivity decreased in the reverse order, as reported originally by Quirk and Schofield (1955).…”

“…Scanning electron micrographs showed that enrichment with K resulted in the formation of dense, continuous networks of clay particles filling the pore spaces between sand particles (Chen et al, 1983). Marchuk et al (2012) applied computed X-ray tomography to characterize changes in pore architecture as influenced by exchangeable Na, K, Mg, and Ca. Total porosity and pore connectivity were influenced by cation valence, as might be anticipated from the Schulze-Hardy Rule (Sposito, 2008).…”

Clogging Mechanism of Stormwater Filter Media by NaCl as a Deicing Salt

“…Both flocculation and dispersion may occur under certain conditions that could result in enlargement of pores with simultaneous blocking of pore throats, thus reducing the pore connectivity and active porosity. For studying such complex changes in pore structure, X-ray CMT appears to be more appropriate than MIP (Marchuk et al, 2012).…”

“…However, they are not sufficient to understand the clogging mechanism of SFM by NaCl as a deicing salt. Marchuk et al (2012) used x-ray computed microtomography (CMT) and studied the permeability reductions due to permeation with water containing different cations, namely, Na, K, Mg, or Ca. They concluded that CROSS serves as a better indicator of pore structure stability than SAR, particularly for soils containing Ca, Mg, and K, in addition to Na.…”

Accounting for potassium and magnesium in irrigation water quality assessment

“…However, the high salinity and sodium (Na) concentrations characteristic of recycled wastewaters pose a significant challenge to their sustainable reuse for crop production (Assouline et al 2015;Laurenson et al 2012;Grismer 2014a, 2014b). Adding to this challenge, several recent studies (Arienzo et al 2012;Buelow et al 2015;Marchuk et al 2013;Rengasamy and Marchuk 2011;Smith et al 2015) have documented deleterious effects on soil hydraulic properties caused by high concentrations of potassium (K) and magnesium (Mg), which are typical of recycled wastewaters (Buelow et al 2015;Laurenson et al 2012;Weber et al 2014). The potential consequences include negative impacts on infiltration, water availability and plant growth.…”