Technologies and Costs for Removal of Arsenic From Drinking Water

was reduced to 0.05 mg/L when the initial concentration was 0.35 mg/L or lower, while As(III) wasreduced to 0.05 mg/L when the initial concentration was less than 0.1 mg/L.McNeill and Edwards (1997b) also found that As(V) removal by manganese hydroxide solidsis sensitive to As(V) initial concentrations. At pH of 10.5, there was about 80 percent removal in thesystem with 75 µg/L of As(V) versus about 30 percent of removal in the 150 µg/L As(V) solution.Effect of Arsenic Oxidation StateAs(V) was generally more effectively removed by LS than As(III). Sorg and Logsdon (1978)conducted several LS pilot studies for the removal of both As(III) and As(V). Two of the tests wereperformed at pH 9.5 and 11.3. At a pH of 11.3, 99 percent of an initial As(V) concentration of 0.58mg/L was removed, whereas only 71 percent of an initial As(III) concentration of 0.34 mg/L wasremoved. At a pH of 9.5, 53 percent of an initial As(V) concentration of 0.42 mg/L was removed,whereas only 24 percent of an initial As(III) concentration of 0.24 mg/L was removed.Effect of pHThe optimum pH for As(V) removal by LS is approximately 10.5, and the optimum pH forAs(III) removal is approximately 11 (Logsdon, et al., 1974; Sorg and Logsdon, 1978). Logsdon, etal. (1974) studied the effectiveness of excess LS on the removal of arsenic in jar tests. The test waterwas a well water that contained 300 mg/L hardness as CaCO 3 spiked with 0.4 mg/L As(V). The pHvaried between 8.5 and 11.5. At pH 10.5 and above, nearly 100 percent arsenic removal wasobtained. Below the optimum pH, the removals decreased with decreasing pH. When the water wasspiked with As (III), removals were only around 75 percent in the optimum pH range. Below theoptimum pH range, removals sharply decreased to less than 20 percent. Removals of oxidized As(III),however, were almost identical to removals of As(V).Effect of Type of Precipitative Solids FormedArsenate removal during softening is controlled by formation of three solids including calciumcarbonate, magnesium hydroxide, and ferric hydroxide. Calcium carbonate and magnesium hydroxideare produced from reactions which remove hardness from water after addition of lime, caustic soda,and soda ash. Ferric hydroxide can be formed by precipitation of iron naturally present in treatmentplant influent or by addition of iron coagulant during softening.2-11