Technologies and Costs for Removal of Arsenic From Drinking Water

2.6 ALTERNATIVE TECHNOLOGIES2.6.1 Iron Oxide Coated SandIron oxide coated sand (IOCS) is a rare process which has shown some tendency for arsenicremoval. IOCS consists of sand grains coated with ferric hydroxide which are used in fixed bedreactors to remove various dissolved metal species. The metal ions are exchanged with the surfacehydroxides on the IOCS. IOCS exhibits selectivity in the adsorption and exchange of ions present inthe water. Like other processes, when the bed is exhausted it must be regenerated by a sequence ofoperations consisting of rinsing with regenerant, flushing with water, and neutralizing with strong acid.Sodium hydroxide is the most common regenerant and sulfuric acid the most common neutralizer.Several studies have shown that IOCS is effective for arsenic removal. Factors such as pH,arsenic oxidation state, competing ions, EBCT, and regeneration have significant effects on theremovals achieved with IOCS.Effect of pHpH appears to have an effect on arsenic adsorption by IOCS. Benjamin et al. (1998)conducted isotherm and column studies with IOCS to investigate the removals of As(V) at various pHlevels. Results indicated that increasing the pH from 5.5 to 8.5 decreased the sorption of As(V) byapproximately 30 percent.Effect of Arsenic Oxidation StateAs with other processes, the oxidation state of arsenic plays a role in its removal: As(V)appears to be more easily removed than As(III). Benjamin et al. (1998) showed that As(V) sorptiononto IOCS was much more rapid than As(III) sorption during the first few hours of exposure andslower thereafter. The ratio of As(V) adsorption densities at 2 and 24 hours was approximately 60percent, whereas the ratio of As(III) adsorption densities was only about 50 percent.Effect of Competing IonsConcentrations of competing ions will be an important consideration for arsenic removal withIOCS. Benjamin et al. (1998) evaluated the effect of sulfate and chloride on IOCS arsenic adsorption.They found that increasing sulfate from 0 to 100 mg/L had only slight impact on the sorption of As(V),and the presence of chloride did not appear to affect As(V) removal. Organic matter, however, did2-43