Super-stable mineralization of multiple heavy metal ions from wastewater for utilization in photocatalytic CO2 reduction and trace precious metal recovery

The treatment and recycling of heavy metal pollutants are vital for the restoration of the ecological environment and the recovery and reuse of resources. Herein, we report an atomic-economic method to synthesize a novel CaCr-layered double hydroxide (LDH) by extracting toxic Cr3+ (more than 1000 mg/L) from industrial wastewater, by which the wastewater can be purified to discharge standards. The obtained CaCr-LDH can be reused for the super-stable mineralization of multiple heavy metal ions with a high capacity, and the removal capacities of Ni, Co, Cu and Zn ions are 306.7 mg/g, 578.0 mg/g, 657.9 mg/g, and 352.1 mg/g, respectively. Furthermore, CaCr-LDH has the advantages of a large treatment range, anti-interference, alkali corrosion resistance, low cost, and less sludge. In the above mineralization process, CaCr-LDH converts to MCaCr-LDH (M = Co, Ni, Cu, and Zn) through an isomorphous substitution process. The obtained NiCaCr-LDH products show excellent performance in photocatalytic CO2 reduction with a high CH4 selectivity of 34.8% under visible light. The MCaCr-LDH products can selectively adsorb gold ions with approximately 90% adsorption efficiency and reduce them with approximately 80% reduction efficiency from printed circuit board leaching wastewater, offering promising functionalities for improving environmental pollution, resource waste, and energy crises.