The behavior of polyprotic anions in ion-exchange resins

After observing that pH dropped dramatically when chloride-form strong-base anion resins were contacted with neutral and alkaline solutions of bicarbonate and arsenate anions, three representative strong-base resins were tested for their ability to produce pH changes. It was discovered that all three chloride-form resins could convert HCO3-(-) to CO32-, and H2AsO4- to HAsO42- with the expulsion of protons, a decrease in pH, and an increase in the ionic concentration of the aqueous phase. For bicarbonate which can give up and take up protons, the following reaction was observed: 2R(4)N(+)Cl(-) + HCO3- double left right arrow (R4N+)(2)CO3- + Cl- + H+Cl- Depending on the local concentration of excess bicarbonate, the HCl produced either remained as HCl or reacted to form carbonic acid in a reaction which tended to buffer the system. The resin characteristics related to its preference for divalent ions and its ability to produce higher-valent anions by proton expulsion were (a) appropriate functional group charge spacing, (b) ability to reorient functional groups to satisfy polyvalent ions, and (c) presence of hydrogen-bonding hydrophilic groups such as carbonyl and hydroxyl. IRA-458, the resin that was clearly superior in its ability to produce higher-valent ions, exhibits the highest divalent selectivity, is hydrophilic, and carries its quaternary amine functional groups at the end of a long flexible pendant arm containing an amide bond with hydrogen-bonding potential. Implications of the experimental findings as applied to arsenate removal from water an discussed.