Water-stable zirconium and iron-based metal-organic frameworks (MOFs) as fluoride scavengers in aqueous medium

Metal-organic frameworks (MOFs) of different water-stable classes were prepared hydrothermally and studied for fluoride (F^-) capture in aqueous medium. In the screening test, the (F^-) removal for MIL-53 (Fe), UIO-66, AP -UIO-66, and MOF-235 were 95.6%, 92.3%, 74.1% and 61.5%, respectively. MIL-53 (Fe) further explored by Box–Behnken design (BBD) approach to develop a mathematical model for the prediction of F^- removal under determined environmental conditions. The model demonstrated that the mixing time was the most significant operational variable in the process. To maximize the F^- capture, model optimization carried out and the best operational condition obtained as pH 4, mixing time of 60 min, and MIL-53 (Fe) dosage 0.25 g/L. Monolayer adsorption onto energetically equivalent sorption sites described the F^- capture by MIL-53 (Fe). The qmax obtained by non-linear Langmuir model was 3.82 mmol F^- per g MIL-53 (Fe). The sorption was favorable at the different F^- concentrations in the range of 10–30 mg/L, based on the separation factor. Moreover, the kinetic models revealed that the rate of adsorption controlled by the migration of F^- ions through the boundary layer.