Resin oxidization phenomenon and its influence factor during chromium(VI) removal from wastewater using gel-type anion exchangers

Anion exchange resins have been widely used to effectively remove hexavalent chromium (Cr(VI)) from Cr(VI)-containing wastewater. However, Cr(VI) is a powerful oxidizing agent that is capable of oxidizing almost all organic materials under acidic conditions, resulting that it is possible for Cr(VI) to oxidize the polymeric resins during the Cr(VI) adsorption process and change the sorption properties of the resins. This study investigated the resin oxidization phenomenon and its influence factors during the 10-cycle column operation of two strong-base gel-type resins (201 × 4 and 201 × 7) for Cr(VI)-containing wastewater. With the development of 10-cycle column operation, approximately 20-34% and 18-31% of Cr(VI) was reduced to trivalent chromium (Cr(III)) during each cycle for the 201 × 4 and 201 × 7 resins. As a result, the adsorption capacity and the nitrogen content of the resins decreased, and microcracks were observed on both resins, indicating that both the functional groups and the polymer matrix of the resins have been oxidized by Cr(VI) ions. Further investigations found that the resin oxidization was highly dependent on pH and initial Cr(VI) concentration of the solutions and more substantial with the increasing initial Cr(VI) concentration and contact time, and decreasing solution pH and temperature.