Adsorption of chromate on variable charge soils as influenced by ionic strength

The adsorption behavior of chromate on two variable charge soils (Oxisol and Ultisol) was investigated through batch experiments at different ionic strengths and pH values. The adsorption of chromate on the variable charge soils was found to be strongly dependent on the pH of the soil solutions. A characteristic pH was observed, which corresponds to the intersection of the chromate adsorption—pH curves at different ionic strengths. The characteristic pH values are 5.50 for Oxisol and 5.04 for Ultisol, close to the point of zero salt effect (PZSE) of these soils. The zeta potentials measured for these soils provide the evidences to support the interpretation of the effect of ionic strength on the adsorption of chromate on these variable charge soils. The adsorption behavior of chromate was interpreted by a schematic representation of chromate distribution at increasing ionic strength. The chromate desorption–pH curves were also found to intersect at pH of 5.15 and 4.89 for the Oxisol and Ultisol, respectively. It is considered that chromate adsorption by the variable charge soils was mainly determined by the electrostatic potential on the adsorption plane, which was controlled by the ionic strength of the soil solutions.