Modeling a novel ion exchange process for arsenic and nitrate removal

Arsenate and nitrate can be removed quantitatively from drinking water by anion exchange. However, if the raw water contains substantial concentrations of sulfate or nitrate, the resin becomes exhausted quickly, and the requirements for regenerant (brine) can make the process unattractive. Previously, we described a modified ion exchange operating procedure for arsenic removal from solutions containing sulfate that could overcome this problem. This paper extends that work to solutions containing nitrate, and presents a mathematical model for the process. The selectivity coefficient for sulfate over nitrate of a strong base anion exchange resin increased dramatically with increasing ionic strength, partially counteracting the decrease in SO4/NO3 separation factor predicted from mass action considerations. The value of this selectivity coefficient in different solutions can be used in conjunction with mass balances and solid/liquid equilibrium considerations to explore the brine requirement when the modified treatment process is applied to influent waters with various compositions. The modeling results indicate that, for relatively low influent nitrate concentrations, the volume of water treated per unit volume of brine used can be increased greatly by using the modified ion exchange process. At higher influent nitrate concentrations, the modified process remains advantageous, but is less so. The use of separate brine solutions to regenerate the upstream and downstream columns magnifies the benefits of the modified process significantly. If the sulfate in the brine is precipitated as CaSO4(s) rather than BaSO4(s), the brine usage rate increases by only 30–40%, even though the former solid is orders of magnitude more soluble than the latter.