Application of Al-Fe Co-modified Rice-Straw Biochar to Fluoride Removal: Synthesis, Optimization, and Performance

Fluoride-contaminated water is a major problem worldwide, and modified biochar is a promising low-cost adsorbent for fluoride removal. In this study, synthesis, optimization, and performance of Al-Fe co-modified rice-straw biochar (Fe-Al-RSBC) for fluoride removal were investigated. The Fe-Al-RSBC, containing amorphous Fe and Al oxides, efficiently adsorbed fluoride from liquid phase. The fluoride adsorption on Fe-Al-RSBC was a spontaneous process and was better fitted by the pseudo-second-order kinetic model than the first-order kinetic model. The Freundlich isotherm model better described the characteristic of fluoride adsorption than the Langmuir isotherm model. Fourier transform infrared spectra analysis showed that fluoride adsorption on Fe-Al-RSBC was mainly driven by electrostatic attraction and ion exchange. The maximum fluoride adsorption capacity of Fe-Al-RSBC reached 6.85 mg/g with a C-Fe-Al molar ratio of 1:0.25:0.75 at pH 7.0. Higher initial fluoride concentrations induced higher adsorption capacities of Fe-Al-RSBC, while the increasing pH from 6.0 to 9.0 decreased its adsorption capacity. The coexisting anions decreased fluoride adsorption capacities of the Fe-Al-RSBC following the order: PO43- > HCO3- > Cl- > SO42-. The current results suggest that Fe-Al co-modified rice straw biochar can be a promising fluoride adsorbent for remediation of the fluoride contamination in the aquatic systems.