Bench-scale studies were conducted with synthetic freshwaters to assess the applicability of coagulation processes to meet a lower standard for arsenic in drinking water. The parameters examined for their effects on arsenic removal include initial arsenic concentration and oxidation state, pH, coagulant dose.Studies conducted in synthetic freshwaters indicated that arsenic removal by coagulation with alum was generally consistent with removal being governed by adsorption onto freshly-precipitated amorphous aluminum hydroxides. This hypothesis is supported by observations that As(V) removal by alum was more efficient than As(III) removal and that, for both As(III) and As(V), removal was independent of initial As concentration (over a limited range) and increased with coagulant dose. In addition, the negligible As (III) removal by alum, as compared with moderate removal by ferric chloride, suggested adsorption alone could not account for observed removal. Under optimal pH conditions, however, excellent removal of As (V) could be achieved with alum, which suggests that, for influent As(V) concentrations typical of U.S. source waters, residual, dissolved As (V)concentrations < 2 μg/L in product water can be achieved by coagulation.
