Trace Cr(VI) Removal: Evidence of Redox-Active Ion Exchange by a Weak-Base Anion Exchanger

The presence of Cr(VI) in water, even in trace concentrations, is toxic to numerous animals, plants, and microorganisms. Remediation efforts require selective removal of trace concentrations of Cr(VI) anions from the background of other competing ions present in concentrations at least 2 orders of magnitude greater than Cr(VI). The objective of this study was to evaluate the trace Cr(VI) removal capacities of commonly available anion exchangers. Batch and column studies revealed that Duolite A7 showed extraordinarily high Cr(VI) removal capacity compared to other anion exchangers. For a synthetic influent solution containing trace Cr(VI) at pH 5, Duolite A7 removed Cr(VI) for 30 000 bed volumes (BVs), while the columns having other anion exchangers reached breakthrough almost instantaneously less than 1000 BVs. Equilibrium and kinetic studies were performed on Duolite A7 at varying pH and contact time, coupled with characterization studies on the raw and exhausted ion exchanger to elucidate the mechanism behind such a significantly higher Cr(VI) removal. Based on the results, it was concluded that there was an occurrence of redox reaction within the resin in addition to ion exchange because of the unique resin composition. Duolite A7 is a weak-base anion exchange resin with a phenol formaldehyde matrix, and both secondary and tertiary amine functional groups. The redox reactions resulted in the perpetual regeneration of ion exchange sites and 30× greater Cr(VI) removal capacity of Duolite A7. This finding may pave the way to design a new class of redox-active ion exchangers for various water and wastewater treatment applications.