Technologies and Costs for Removal of Arsenic From Drinking Water

applicability of IX at a particular site. Typically, strong-base anion exchange resins are used inarsenic removal. Strong-base anion resins tend to be more effective over a larger range of pH thanweak-base resins. The order of exchange for most strong-base resins is given below, with theadsorption preference being greatest for the constituents on the far left.HCrO 4-> CrO 42-> ClO 4-> SeO 42-> SO 42-> NO 3-> Br - > (HPO 42-, HAsO 42-, SeO 32-, CO 32-) > CN - >NO 2-> Cl - > (H 2 PO 4- , H 2 AsO 4- , HCO 3- ) > OH - > CH 3 COO - > F -2-These resins have a relatively high affinity for arsenic in the arsenate form (HAsO 4 ),however, previous studies have shown that high TDS and sulfate levels compete with arsenate andcan reduce removal efficiency (AWWA, 1990). In general, ion exchange for arsenic removal is onlyapplicable for low-TDS, low-sulfate source waters. Previous studies have confirmed thisgeneralization; the low-sulfate/low-TDS source water in a Hanford, CA study proved to be amenableto IX treatment whereas the high-sulfate/high-TDS source water in a San Ysidro, NM study provedto be impractical for IX treatment (Clifford and Lin, 1986; Clifford and Lin, 1995).If nitrate removal is being performed concurrent with arsenic removal, sulfate level can alsobe an important factor in arsenic removal. Clifford and others (1998) have shown that when sulfatelevels are low (about 40 mg/L), the number of BV to exhaustion is limited by nitrate breakthrough.If the sulfate level is high (about 100 mg/L), however, the number of BV to exhaustion is limited byarsenic breakthrough. In other words, sulfate competes with both nitrate and arsenic, but competesmore aggressively with arsenic than nitrate.The presence of iron, Fe(III), in feed water can also affect arsenic removal. When Fe(III) ispresent, arsenic may form complexes with iron. These complexes are not removed by IX resins andtherefore arsenic is not removed. Utilities with source waters high in Fe(III) may need to address thisissue for IX use or evaluate other treatment techniques for arsenic removal (Clifford, et al., 1998).When an ion is preferred over arsenate, higher arsenic levels in the product water than existin the feed water can be produced. If a resin prefers sulfate over arsenate, for example, sulfate ionsmay displace previously sorbed arsenate ions, resulting in levels of arsenic in the effluent which aregreater than the arsenic level in the influent. This is often referred to as chromatographic peaking.As a result, the bed must be monitored and regenerated well in advance of the onset of this peaking.Clifford and Lin (1995) recommend operating the bed to a known BV setpoint to avoid peaking.2-21