Circular chromium economy: Electrolytic recovery of pigment-grade Cr₂O₃ from plating industry effluent

Cr(VI) plating effluent poses a long-standing environmental problem. In this study, a two-compartment electrolysis membrane reactor was designed to achieve the complete reduction of Cr(VI) to Cr(III) in chrome plating effluent. The reactor comprised a Ti/IrO₂–RuO₂–TiO₂ anode and a Ti expanded-mesh cathode separated by an H⁺-selective cation-exchange membrane. The two-compartment membrane electrolysis system enabled anodically generated H⁺ ions to participate in the cathodic reduction of Cr(VI). Treatment of the chrome plating effluent resulted in the complete reduction of Cr(VI) (2392 mg L⁻¹). Approximately 92.96 wt% of the reduced Cr(III) was recovered as Cr₂O₃ from the effluent and subsequently reused for potential pigment applications in paints. Performance evaluation showed that the recovered Cr₂O₃ pigment performed comparably to commercial Cr₂O₃ in both salt-spray and acid-resistance tests, confirming its suitability for coating applications. The findings demonstrate that membrane-assisted electroreduction offers an effective and sustainable route for detoxifying and recycling chromium-containing industrial effluents.