Kinetics of Arsenic Adsorption on Goethite in the Presence of Sorbed Silicic Acid

Abstract

The potential toxicity and availability of As in the environment is dependent on several factors including redox potential, pH, and the presence of ligands that can compete for adsorption sites on mineral surfaces. Silicic acid is a ligand ubiquitous in natural systems and strongly chemisorbs to Fe oxides. However, there are relatively few studies examining its influence on As adsorption on Fe oxides. This study examined the influence of silicic acid (0.10 and 1.0 mM) on the adsorption kinetics of arsenite and arsenate (0.10 mM) on goethite over a range of common soil pH values (4, 6, and 8). The rate of arsenic (III and V) and silicic acid adsorption was greatest at pH values near their pK1 value. However, silicic acid sorption was characterized by biphasic kinetics with rapid adsorption followed by a much slower adsorption reaction. The rate and total quantity of arsenite adsorption decreased in the presence of silicic acid at all pH values and concentrations of silicic acid. Approximately 40% less arsenite was adsorbed in the presence of 1.0 mM silicic acid at all pH values. At 0.10 mM, silicic acid had less of an effect on arsenite adsorption. In contrast, only the rate and not the total quantity of arsenate was reduced in the presence of silicic acid. The rate of arsenate adsorption decreased as pH and silicic acid concentration increased. This was attributed to a decrease in the goethite's surface potential upon specific adsorption of silicic acid and deprotonation of the arsenate molecule creating an unfavorable electrostatic field. These results demonstrate the importance of evaluating As speciation, reaction kinetics, and the influence of naturally occurring ligands on the adsorption of As on variable charge surfaces.