The influence of silica and calcium on arsenate sorption to oxide surfaces

Arsenic can be effectively removed from water by sorption to amorphous or granular iron or aluminum oxides. however, silica is known to interfere with arsenic sorption at pH >7.5. The goal of this work was to examine arsenic sorption to amorphous and granular media oxides at various levels of silica, calcium and pH, and to closely monitor the progress of sorption in these systems. At pH 8.5, calcium dramatically increased arsenic sorption to amorphous iron hydroxide in the presence of silica over relatively short reaction times, but the effects gradually disappeared over longer reaction times. Additionally, batch tests indicated that activated alumina granular media was more sensitive to silica interference than granular ferric hydroxide. However, relatively low levels of calcium could reduce silica interference to arsenic sorption onto activated alumina. While future confirmation research is needed, severe silica interference can occur in practice due to exclusion of arsenate from interstitial granular media pores.