Preparation of low-cost aluminum-loaded longan shell adsorbent for fluoride removal: Experimental and modeling studies

Low-cost adsorbents for efficient defluoridation from drinking water have been an issue of great concern. Aluminum-loaded longan shell (LGS-Al) was prepared by a chemical co-precipitation method and was optimized following the conditions of longan shell of 5.12 g/200 mL, aluminum chloride concentration of 0.34 mol/L and water bath temperature of 68.3 degrees C. The as-prepared LGS-Al exhibited the amorphous structure, which is beneficial to developing functional groups of -Cl and -OH, and high reactivity of coordinatively unsaturated aluminum. The adsorption kinetic and isotherm data were better described by the pseudo-second-order and Sips models, respectively. The maximum adsorption capacity was 21.49 mg/g, which was higher than numerous reported biomass-based adsorbents. LGS-Al showed over 89.7% fluoride removal and limited leaching of aluminum and dissolved organic carbon in the initial pH range of 6.0-8.0. The exchange action of -OH and -Cl between fluoride and the formation of Al-F bonds were also confirmed by attenuated total reflectance fourier transform infrared (ATR-FTIR) spectra and X-ray photoelectron spectroscopy (XPS). The adsorption energy (E-ads) of the Al-OH site for F- (-826.7 kJ/mol) was lower than that of Al-Cl, illustrating that Al-OH dominated the removal of F-. On the contrary, the E-ads of Al-Cl for HF was lower (-121.8 kJ/mol), suggesting Al-Cl was a priority to react with HF. These findings indicated that LGS-Al could be a promising low-cost material for fluoride removal from drinking water.