Phosphate Adsorption Properties and Modeling of Iriki Kaolinite

Abstract

Phosphate adsorption on kaolinite is affected by pH and the electrolyte concentration, but the reasons underlying these effects are not fully understood. The reason why the amount of phosphate adsorption has a maximum at a certain pH is also unclear. Here we used Iriki kaolinite (from Japan's Kagoshima Prefecture) as a material, and we measured the phosphate adsorption at different pH values and electrolyte concentrations by the batch method. Sodium chloride was used as the supporting electrolyte. The amount of phosphate adsorption at ≤1 mM equilibrium phosphate showed a maximum value at pH 6.0. The adsorption increased with the increase in the electrolyte concentration at pH 4.0-6.0, and it decreased with the increase in the electrolyte concentration at pH 3.0. We propose a simplified surface complexation model represented by a competitive Langmuir adsorption equation. The values calculated using the model agreed well with the measured values. Based on the model and the measured zeta potential, we speculate that the characteristic phosphate adsorption reaction was caused by a protonation/deprotonation reaction of aluminol groups and phosphate ions, changes in the potential of the adsorption surface due to the pH, and changes in the potential by the phosphate adsorption itself.