Harmless treatment and selective recovery of acidic Cu(II)-Cr(VI) hybrid wastewater via coupled photo-reduction and ion exchange

Herein, we report a simple two-step method combining adsorption and photoreduction toward the effective removal, separation, and recovery of Cu(II) and Cr(VI) from their hybrid wastewater. A solid solution photocatalyst consisting of TiO2-ZrO2 was utilized to photoreduce Cr(VI) in the absence of a sacrificial agent. The Cr (VI) removal rate reached 95%, whereas Cu(II) was substantially retained in the solution. Conversely, a commercial cation exchange resin, Lewatit TP207, only adsorbed Cu(II) without any notable change in the Cr(VI) concentration. After combining these two selective removal processes, both Cu(II) and Cr(VI) in wastewater can be removed to reach the wastewater discharge standard regardless of the treatment sequence used. However, the most effective sequence was photocatalysis after ion exchange. The residual Cu(II) concentration was 0.06 mg/L and the total Cr concentration was 0.74 mg/L without Cr(VI). The Cu(II)/Cr(VI) separation efficiency was 11,949 with 99.17% Cu(II) and 96.29% Cr(VI) recovered. This work provides a promising referable process for hybrid heavy metal ion wastewater treatment.