Arsenate (As(V)) anions are readily removed from drinking water by anion exchange with chloride-form strong-base resins. However, the process is not simple ion exchange, because anion resins can convert monovalent arsenate (H2AsO4-) to divalent arsenate (HAsO42-) with the expulsion of a proton and a lowering of pH. A similar but more significant pH change results from the conversion of bicarbonate (HCO3-) to carbonate (CO32-) within the resin, and these pH changes can affect process design. Another surprising aspect of the chloride-for-arsenate ion-exchange process is that spent NaCl brine, heavily contaminated with arsenic, may be reused directly to regenerate the exhausted ion-exchange bed. Finally, in spite of the low level (less than or equal to 2 mu g/L) to which arsenic must be treated, empty bed contact times (EBCTs) as short as 1.5 minutes may be employed in the process. In lab studies at the University of Houston and in field studies carried out in McFarland, California, Hanford, California, and Albuquerque, New Mexico, several versions of the anion-exchange process for arsenic removal have been tested at bench and field scale. Currently, the optimal process comprises the use of a type 2 polystyrene divinylbenzene, strong-base anion resin, 1.5 minutes EBCT, 1 M NaCl regenerant, and regenerant reuse up to twenty five times with the chloride concentration maintained at approximately 1.0 M. Typically, 400 to 800 bed volumes of water can be treated before arsenic breakthrough, and the arsenic concentration in the process effluent is consistently below 2 mu g/L. The process will be tested at full scale in Albuquerque in the near future.
