Zero-valent iron coupled calcium hydroxide: A highly efficient strategy for removal and magnetic separation of concentrated fluoride from acidic wastewater

The removal of concentrated fluoride in acidic wastewater by the conventional Ca(OH)2 method is challenged by the insufficient efficiency and difficult separation of fine CaF2 precipitate. Herein, we construct a strategy to tackle these challenges by coupling zero-valent iron (ZVI) with Ca(OH)2. ZVI reduces fluoride concentration from 12,000 to 3980 mg L-1 under optimal conditions primarily through the in-situ growth of porous FeF2·4H2O shell on its surface, which simultaneously assists fluoride removal via adsorption. The residual fluoride after ZVI treatment then decreases to 6.74 mg L-1 via precipitation with Ca(OH)2. Interestingly, the iron ions dissolved from ZVI also participate in the precipitation to form magnetite. This co-precipitation reinforces the fluoride removal and meanwhile endows the resulted precipitates with magnetism, thus enabling the perfect solid-liquid separation by the magnetic field before discharge. The application prospect of this coupling strategy is further verified by its ability in decreasing the concentrations of fluoride and other coexisting heavy metals (Zn2+, Cd2+ and Pb2+) in real smeltery wastewater below their discharge limitations. This study provides a promising strategy for the treatment of concentrated fluoride in acidic wastewater and also highlights ZVI as a good candidate to couple with conventional methods for enhanced pollution control.