Effects of Chelators on Fluoride Removal by AlCl₃ and Al₁₃ Coagulation: Performance and Mechanism

Aluminum (Al) coagulation is one of the most widely used techniques for the defluoridation of industrial effluents. This study examined the influence of two common chelating agents, i.e., ethylenediaminetetraacetic acid (EDTA) and citric acid (CA), on fluoride removal during coagulation using monomeric Al (AlCl₃) and polymeric aluminum species [Al₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺, Al₁₃]. The results revealed that EDTA reduced the fluoride removal efficiency by up to 95.4% with AlCl₃ and by 28.3% with Al₁₃. In contrast, CA inhibited fluoride removal by up to 100% with AlCl₃ and 90.6% with Al₁₃. Both chelators impaired fluoride removal primarily by disrupting floc formation. Advanced analytical techniques, including ¹⁹F/²⁷Al NMR spectroscopy combined with DFT calculations, demonstrated that this interference was due to the formation of EDTA-Al-F ternary complexes and Al-CA binary complexes. Additionally, the formation of a bridging fluoride (μ-F) in the Al₁₃ structure enhanced its thermodynamic stability against chelators. Specifically, Al₁₃ maintained structural stability even in the presence of up to 1.50 mmol/L EDTA, whereas CA progressively destabilized the Al₁₃ structure by forming soluble Al-CA complexes. These findings are believed to help guide the rational design of Al-based coagulants and optimize water treatment strategies aimed at defluoridation of industrial effluents.