Removal of Fluoride from Water Using a Calcium-Modified Dairy Manure-Derived Biochar

This study investigated the removal of fluoride from water using a calcium-modified dairy manure-derived biochar (Ca-DM500). The Ca-DM500 showed a 3.82 - 8.86 times higher removal of fluoride from water than the original (uncoated) manure-derived biochar (DM500). This is primarily attributed to strong precipitation/complexation between fluoride and calcium. The Freundlich and Redlich-Peterson sorption isotherm models better described the experimental data than the Langmuir model. Additionally, the removal kinetics were well described by the intraparticle diffusion model. The Ca-DM500 showed high reactivity per unit surface area [0.0001, 0.03, 0.16 mg F per m² for Douglas fir-derived biochar (DF-BC), DM500. and Ca-DM500, respectively] for retention of fluoride reflecting the importance of surface complexation. The copresence of anions reduced removal by Ca-DM500 in the order ${\text{SO}}_4^{2-}\approx {\text{PO}}_4^{3-}>{\text{NO}}_3^{-}$ . The sorption behavior of fluoride in a continuous fixed-bed column was consistent with the Thomas model. Column studies demonstrated that the Ca-DM500 shows a strong affinity for fluoride, a low release potential, and a stable (unreduced) removal capacity through regeneration and reuse cycles.