Effects of silicate, sulfate, and carbonate on arsenic removal by ferric chloride

Effects of silicate, sulfate, and carbonate on the removal of arsenite [As(III)] and arsenate [As(V)] by coprecipitation with ferric chloride were studied. Silicate significantly decreased As(III) removal when Si concentration was higher than 1 mg/l and the pH was greater than 5. The removal of As(V) was decreased moderately by silicate in 0.04 M KNO3 solution. Addition of Ca2+ and Mg2+ to the KNO3 solution reduced the adverse affect of silicate on As(V) removal. In the presence of 10 mg/l Si and at a pH of approximately 6.8, the adsorption capacity of ferric hydroxide for As(V) and As(III) was reduced from 864 and 116 μg/mg to 274 and 23 μg/mg Fe, respectively. Sulfate and carbonate had a negligible effect on the removal of As(III) and As(V). The triple layer model was used with site binding reactions to describe As(III) and As(V) removal by ferric hydroxide in the presence of silicate. The results obtained in the present study suggest that silicate in natural water can significantly decrease the efficiency of As(III) removal by coagulation treatment with ferric chloride.