Although there are various options for bulk Cr(VI) removal from industrial wastewater, due to the non-selectivity of the adsorbents towards the target ions in the presence of other competing anions, fewer options are available to remove trace Cr(VI) from contaminated drinking water. It was observed that a weak base anion exchange resin, Duolite A7, showed an unusually high capacity for trace Cr(VI) removal, which would not be possible by the ion exchange process alone. Previous research has demonstrated that some organic adsorbents derived from plants could remove Cr(VI), but none of the existing adsorption models could appropriately describe the adsorption process. After a detailed investigation, it was proved that the enhanced Cr(VI) removal by Duolite A-7 and Cr(VI) removal by the natural substances were both due to two processes occurring sequentially - ion exchange or adsorption followed by redox reactions. Ion exchange or adsorption is a fast process through which Cr(VI) anions were first attached to the surface of ion-exchanger or the adsorbent at respective functional groups having an affinity towards Cr(VI) anions. Cr(VI) anions have strong oxidation potential. When sitting close to the matrix of the ion-exchanger/adsorbent, they initiated redox reactions with the matrix of the adsorbent through which Cr(VI) got converted to Cr (III), which precipitated as Cr(OH)<sub>3</sub> at neutral to alkaline pH. Conversion of Cr(VI) to Cr(III) acted almost like perpetual auto-regeneration of active adsorption or ion exchange sites, making it free to adsorb more Cr(VI) anions. The redox-active ion exchange phenomenon enabled Duolite A-7 to show a high selective Cr(VI) removal capacity. Unfortunately, the phenol-formaldehyde matrix of  Duolite A7 produced a formaldehyde leakage into the water, limiting its application for drinking water treatment. The plant-derived organic adsorbents could not show high Cr(VI) removal capacity because of their low ion-exchange capacity. Motivated by the phenomenon of redox-active ion exchange, we synthesized an adsorbent by immobilizing polyethyleneimine (PEI) polymer on the surface of a hydrophobic substrate which demonstrated highly selective Cr(VI) removal capacity - even better than Duolite A-7 at the neutral pH. The role of hydrophobicity of the matrix was also important, which was quantified by DFT calculations. The redox-active ion exchange phenomenon can motivate various essential applications in water and wastewater treatment.

Redox-active ion exchange process for enhanced Cr(VI) removal

ACS Spring 2022

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