ARSENIC REMOVAL BY FERRIC-CHLORIDE

Bench-scale studies were conducted in model freshwater systems to inve stigate how various parameters affected arsenic removal during coagula tion with ferric chloride and arsenic adsorption onto preformed hydrou s ferric oxide. Parameters included arsenic oxidation state and initia l concentration, coagulant dosage or adsorbent concentration, pH, and the presence of co-occurring inorganic solutes. Comparison of coagulat ion and adsorption experiments and of experimental results with predic tions based on surface complexation modeling demonstrated that adsorpt ion is an important (though not the sole) mechanism governing arsenic removal during coagulation. Under comparable conditions, better remova l was observed with arsenic(V) [As(V)] than with arsenic(III) [As(III) ] in both coagulation and adsorption experiments. Below neutral pH val ues, As(III) removal-adsorption was significantly decreased in the pre sence of sulfate, whereas only a slight decrease in As(V) removal-adso rption was observed. At high pH, removal-adsorption of As(V) was incre ased in the presence of calcium. Removal of As(V) during coagulation w ith ferric chloride is both more efficient and less sensitive than tha t of As(III) to variations in source water composition.