Effect of anions on dispersion of a kaolinitic soil clay: A combined study of dynamic light scattering and test tube experiments

- Anions can be sorbed on positive edge sites of kaolinitic soil clay.
- Resulting decrease in ζ is a key factor for dispersion of the clay fraction.
- Effects of anions on dispersion were as follows: oxalate > citrate > acetate > SO42 − > Cl−.
- Clay suspension may occur at various concentrations in realistic conditions.
- It can be investigated by light scattering in combination with test tube experiments.

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.