Mechanistic insights for defluoridation of industrial wastewater by MIL-96(Al) filters

The rapid development of semiconductor and electronics industries have urged novel adsorbents for fluoride removal from wastewater. In this study, we developed a metal-organic framework filter by in-situ implanting MIL-96(Al) onto Al mesh, which demonstrated effective and stable fluoride removal over a wide range of pH (3−11). The solid state nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) results revealed that the different structural changes to which Al3 sites are subjected upon pH modifications trigger fluoride adsorption through Lewis interactions at acidic pH values and through ligand exchange mechanism at neutral/alkaline pH values. Moreover, the filter demonstrated limited Al ions leakage and satisfactory adsorption selectivity towards fluoride among chloride, nitrate, and sulfate, at concentrations up to 50-times higher than fluoride. As a proof of concept for practical applications, the developed filters were sealed in tandem into a home-made filter holder and tested for fluoride removal from real industrial wastewater. When 12 filters were used, fluoride concentrations decreased from 5.8 mg/L to below 1.5 mg/L after 10 min of hydraulic retention time in the filter holder over 5 consecutive cycles. We hope this work may provide new insights for the design of novel absorbents for fluoride removal.