Selective fluoride removal on LaHAP/3D-rGO composite electrode by capacitive deionization

Excessive fluoride in water is severely hazardous to environment and human health. It is of great significance to exploit effective methods for defluoridation from water. In this paper, lanthanum-doped hydroxyapatite modi-fied three-dimensional reduced graphene oxide (LaHAP/3D-rGO) was successfully synthesized with LaHAP uniformly distributed on the 3D network. Electrochemical characterization demonstrates the improvement of the conductivity and ion diffusion rate after doping appropriate amount of LaHAP. With LaHAP/3D-rGO as positive electrode (activated carbon as negative electrode), the corresponding capacitive deionization (CDI) system is successfully applied for the selective adsorption of fluoride. With the appropriate LaHAP/3D-rGO ratio (50%), the obtained LaHAP/3D-rGO electrode achieves the optimal F- adsorption amount of 1132 mu mol g-1. The cor-responding CDI cell shows excellent selectivity and high cycle stability for adsorption of fluoride. When applied to the fluoride removal in the fluoride-containing groundwater through a 3-cell CDI stack, the electrode attains the high F- removal rate of 90.5%, with the effluent concentration down to 0.32 mg L-1, much lower than the permissible limit of World Health Organization. This LaHAP/3D-rGO based CDI system is much promising for water purification of the high F- groundwater.

Automated summary

In this study, a new method for defluoridation was developed by synthesizing lanthanum-doped hydroxyapatite-modified three-dimensional reduced graphene oxide. The resulting electrode showed excellent selectivity and high cycle stability for fluoride removal in a capacitive deionization system, achieving a fluoride removal rate of 90.5% with an effluent concentration below the permissible limit of the World Health Organization.