Arsenic Removal during Precipitative Softening

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Abstract
Because utilities with hard waters tend to have higher concentrations of arsenic, removal of arsenic via precipitative softening processes was investigated in the context of the more stringent proposed arsenic regulation. Arsenic removal can be facilitated by a variety of solids formed during softening including CaCO3, Mg(OH)2, Mn(OH)2, and Fe(OH)3. The extent of As(V) removal is decreased in the presence of orthophosphate and carbonate. As(III) removal is much lower than As(V) removal. At typical solids concentrations, arsenic removal followed a linear isotherm for CaCO3, Mg(OH)2, and Fe(OH)3, with constant percentage arsenic removal regardless of initial arsenic concentrations. However, for Mn(OH)2 solids arsenic removal was sensitive to arsenic concentrations. A framework for predicting arsenate removal when multiple solids form during softening is presented.