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

The removal of CrIII 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.