Abstract
Dispersion is an important issue for clay leaching in soils. In this study, effects of various anions (Cl−, SO42−, acetate, oxalate and citrate) on dispersion of a kaolinitic soil clay were determined at different pH values and ionic strengths by dynamic light scattering and test tube experiments. Adsorption of anions on clay samples was characterized by the zeta potential (ζ) in a pH range of 2 to 11. At a pH range between 2 and 6, the effects of different anions on decreasing ζ were obvious and followed the order oxalate>citrate>SO42−>Cl−>acetate, while fluctuated changes in ζ were observed at pH>6. Based on a comparison of hydrodynamic radii (rh) obtained from dynamic light scattering and of transmission of 50% (T50 values) from the test tube experiments, the ability of anions to facilitate the dispersion of the clay fraction followed the sequence of oxalate>citrate>acetate>SO42−>Cl−. It implies that adsorption of anions on positively charged edge sites of kaolinite resulting in a decrease in ζ is a key factor for dispersion of the clay fraction. Also, the results suggested that the dynamic light scattering can be used in combination with the test tube experiments in order to evaluate the effect of anions on dispersion at broader ranges of pH, ionic strength and clay concentration.
Highlights
Clay loss is common in bare soils subjected to rainfall or sprinkler irrigation
Coagulation of the clay fraction was still observed in the presence of Cl− or SO42− where the rh values increased from 225 to 502 nm and 222 to 447 nm, respectively
Preliminary determinations of the dynamic light scattering conducted at pH b 3 and pH N 4 did not show different effects in rh values among anions
Summary
Clay loss is common in bare soils subjected to rainfall or sprinkler irrigation. Clays can be transported by the surface runoff. Frenkel et al (1992) reported that anions interact with 1:1 clay minerals, e.g., kaolinite, and facilitate dispersion. We can infer that the presence of dissolved anions might be an important factor for clay loss in tropical soils, where kaolinite is the most dominant clay mineral. Dispersion properties of the pure clay minerals under the influence of anions have received much attention (Kretzschmar et al, 1998; Obut, 2005; Xu et al, 2004). The effect of anions on making surface charge more negative and dispersion properties of such kaolinite-rich soil clays has been neglected
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