Enhanced removal of fluoride ions using lanthanum-doped coconut shell biochar in PVDF ultrafiltration membranes

Fluoride contamination in surface and ground water poses significant health risks, necessitating a low-energy, cost-effective removal method. Herein, an ultrafiltration (UF) adsorptive membrane doped with lanthanum (La)-based coconut shell biochar was synthesized to effectively remove fluoride ions. Based on the results of adsorption and membrane separation performance, it was found that La-based biochar showed a significantly higher adsorption capacity (126.7 mg F/g) compared to bare biochar (27.41 mg F/g), due to the oxygen-containing functional groups from La₂O₂CO₃ that enhance binding sites and electrostatic attraction for fluoride ions. Furthermore, the addition of La-modified adsorbent to the polyvinylidene fluoride (PVDF) UF membrane markedly altered its structure and surface properties, changing the pore structure to porous, narrowing pore size, increasing active adsorption sites, and improving hydrophilicity. These changes boosted fluoride ion rejection from 9.53 % to 62.07 % with tiny effect on membrane permeability. More important, this UF adsorptive membrane exhibited excellent antifouling ability with the lowest flux decline (J/J₀ =0.450) compared to pristine PVDF membrane (J/J₀ =0.142), which was owing to the better improved surface properties. In all, this study provides a novel strategy for efficient, economic and selective separation of fluoride ions and further extends the application of UF to the field of ion removal.