A surface complexation model was developed to predict arsenate removal by ferric hydroxides in the presence of silica and natural organic matter. Modeling of arsenate removal in the presence of silica is improved somewhat by explicit consideration of both monomeric and dimeric silica species. Consistent with previous observations, the model simplifies to a linear isotherm under conditions present in many natural waters at pH ≤ 7.8 and if silica concentrations are relatively low. In waters at higher pHs with relatively high levels of silica, the model predicted that arsenate removal would be relatively insensitive to coagulant dose, but decreasing coagulation pH would produce very significant improvements to arsenate removal, as has been confirmed in practice.

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