Chemistry of inorganic arsenic in soils: kinetics of arsenic adsorption–desorption

The influence of ionic strength, index cations and competing anions on arsenate (AsV) adsorption–desorption kinetics was studied in an Alfisol soil. A flow-through reactor system similar to that developed by Carski and Sparks (Soil Sci Soc Am J 49:1114–1116, 1985) was constructed for the experiments. Arsenate adsorption kinetics for all the treatments were initially fast with 58–91% of AsV adsorbed in the first 15 min. Beyond 15 min, AsV adsorption continued at a slower rate for the observation period of the experiments. Changes in the solution composition had differing effects on the cumulative amount of AsV adsorbed by the soil. Ionic strength and different index cations had little effect on the amount of AsV adsorbed, while the presence of phosphate decreased the amount of AsV adsorbed from 169 to 89 and 177 to 115 g AsV μg−1 in 0.03 M sodium nitrate and 0.01 M calcium nitrate, respectively. Considerably less AsV was desorbed than was adsorbed, with only between 2 to 17% of the adsorbed AsV desorbed. The presence of phosphate increased the amount of AsV desorbed by 17%, but other changes in the solution ionic strength or index cation had little effect on the amount of AsV desorbed.