Insights into the Superstable Mineralization of Chromium(III) from Wastewater by CuO

The removal of Cr^III ions from contaminated wastewater is of great urgency from both environmental protection and resource utilization perspectives. Herein, we developed a superstable mineralization method to immobilize Cr³⁺ ions from wastewater using CuO as a stabilizer, leading to the formation of a CuCr layered double hydroxide (denoted as CuCr-LDH). CuO showed a superior Cr³⁺ removal performance with a removal efficiency of 97.97% and a maximum adsorption capacity of 207.6 mg/g in a 13000 mg/L Cr³⁺ ion solution. In situ and ex situ X-ray absorption fine structure characterizations were carried out to elucidate the superstable mineralization mechanism. Two reaction pathways were proposed including coprecipitation-dissolution and topological transformation. The mineralized product of CuCr-LDH can be reused for the efficient removal of organic dyes, and the adsorption capacities were up to 248.0 mg/g for Congo red and 240.1 mg/g for Evans blue, respectively. Moreover, CuCr-LDH exhibited a good performance for photocatalytic CO₂ reduction to syngas (H₂/CO = 2.66) with evolution rates of 54.03 μmol/g·h for CO and of 143.94 μmol/g·h for H₂ under λ > 400 nm, respectively. More encouragingly, the actual tanning leather Cr³⁺ wastewater treated by CuO showed that Cr³⁺ can reduce from 3438 to 0.06 mg/L, which was much below discharge standards (1.5 mg/L). This work provides a new approach to the mineralization of Cr³⁺ ions through the "salt-oxide" route, and the findings reported herein may guide the future design of highly efficient mineralization agents for heavy metals.