Effect of Al‐F Complexation on Advanced Fluoride Removal by Zirconium Oxide Nanocomposite: Performance, Mechanism, and Mitigation Strategy

Conventional fluoride removal strategies predominantly target free F^- adsorption, while the impact of aluminum-fluoride (Al-F) complexation from residual aluminum ions (Al³⁺) in coagulation effluents is an often-overlooked factor compromising treatment efficiency. This study investigates how Al-F speciation regulates the performance of nano-hydrous zirconium oxide embedded in anion exchange resin (HZO-201), revealing adsorption inhibition by Al³⁺ and establishing mitigation strategies. The adsorbent achieved a dynamic adsorption capacity of 2000 bed volumes at pH 3.5 but failed completely with coexisting Al³⁺ (Al³⁺ = 6.4 mg/L, F/Al = 1:1) due to dual inhibition mechanisms: electrostatic repulsion of positively charged Al-F complexes ([AlF]²⁺ and [AlF₂]⁺) by quaternary ammonium groups on D201 and competitive site occupation. Thermodynamic and experimental analyses showed pH and F/Al ratio-controlled speciation: cationic [AlF]⁺ dominated at F/Al ≤ 1 and pH 3.5, while neutral/anionic species ([AlF], [AlF]^-) prevailed at F/Al ≥ 2 and pH ≥ 5, enabling effective adsorption. EDS mapping confirmed pore-confined nano-HZO as active sites with ligand-exchange dominating under sulfate-shielded conditions. XPS resolved distinct F1s signatures (683~686 eV for F^-, 688 eV for Al-F complexes) and Zr3d perturbation differences (Δ1.0 vs. 0.7 eV), confirming coordination-dependent adsorption pathways. This study bridges molecular-level Al-F coordination chemistry to process optimization, providing mechanistic insights into the role of Al-F charge states in defluoridation and advancing from singular fluoride targeting to multispecies cooperative regulation paradigms.