Detailed investigation of effective trace Cr(VI) removal mechanism by anion exchange resin with phenol–formaldehyde matrix

• The extraordinarily high Cr(VI) removal capacity by Duolite A7 was investigated. • Cr(VI) removal capacity of Duolite A7 significantly increased with decreasing pH. • No breakthrough observed for column run with influent pH 3 even over 131,000 BVs. • Ion exchange coupled redox mechanism motivates design of Cr(VI) selective sorbents. Prolonged exposure to trace Cr(VI) concentrations in potable water can cause serious health problems in living beings. A weak base anion exchange resin, Duolite A7, showed a large capacity for trace Cr(VI) removal from a background of competing anions present at much higher concentrations, while conventional ion exchangers and adsorbents become ineffective due to lack of selectivity under similar conditions. The objective of this study was to find out the mechanism behind such significant capacity shown by the resin. Fixed-bed column studies showed that the resin was fully exhausted below 4,000 bed volumes (BVs) for pH 7, whereas no breakthrough was observed for 25,000 and 131,000 BVs at pH 5 and 3, respectively. Extensive characterization studies revealed that redox reactions were also occurring inside the resin in addition to ion exchange, where Cr(VI) oxidized the amine functional groups and phenol–formaldehyde matrix while itself getting reduced to Cr(III). Cr(III) formed was either precipitated inside the resin as Cr(OH) 3 or bound with the oxidation products such as carboxylic acid groups. Analysis of treated water showed that formaldehyde, a carcinogen, was formed due to oxidative attack of Cr(VI) on the resin and was released in the effluent at trace concentrations. A detailed understanding of the mechanism would motivate the development of such redox-active sorbents for selective trace Cr(VI) removal from contaminated drinking water.