Preparation of coffee residual based biochar and its removal of fluoride ions from mine water

In recent years, the problem of excessive fluoride in mine water is particularly prominent, which not only affects the water quality of reuse after treatment, but also endangers human health.Therefore, the study herein developed a novel adsorbent for fluoride -coffee residual based biochar(CRB)by applying the waste coffee residue as raw material and chemical activation, to realize the purpose of solid waste resource utilization and fluoride removal from mine water at the same time.The authors firstly optimized the preparation protocols of CRB through single factor, including the type of activator, activator concentration and activation temperature; then evaluated the adsorption efficiency of fluoride for CRB before and after optimization by kinetic and isothermal adsorption experiments; finally investigated the adsorption mechanism of fluoride for CRB after optimization(O-CRB)by surface physicochemical characterization.The results showed that the more favorable CRB for fluoride was prepared at the ZnCl₂ concentration of 4 mol/L and the activation temperature of 400 ℃.The maximum adsorption capacity of O-CRB for fluoride was 2.20 mg/g, which was 1.30 times higher than that of CRB before activation.In addition, compared with CRB, O-CRB exhibited higher amount of mesopore volume and specific surface area, as well as more oxygen containing functional groups(-COOH)and surface Zeta potential.Thus, the amount of chloride on the O-CRB surface decreased obviously(3.27 % to 0.61 %)after fluoride adsorption, whereas the amount of fluoride increased significantly(0.76 % to 10.34 %).This suggested that the adsorption of fluoride for O-CRB was mainly attributed to the electrostatic adsorption and ion exchange.