Mechanistic insights into chelator-assisted washing of fluoride-contaminated soil using EDTA and biodegradable HIDS, and phosphate-induced immobilization

Fluoride (F – )-contaminated soil (FCS) poses a risk to groundwater, requiring decontamination methods that remove mobile F – while avoiding remobilization. We investigated chelator-assisted washing with ethylenediaminetetraacetate (EDTA) (nonbiodegradable) and 3-hydroxy-2,2′-iminodisuccinate (HIDS) (biodegradable), followed by CaO- and H₃PO₄-assisted immobilization. This study provides mechanistic insights into fluoride mobilization and suppression during chelator-assisted washing under pH-varied conditions. Mechanistic analysis showed that, in acidic solutions, chelation enhances mineral dissolution and likely inhibits F – readsorption on CaCO₃ via chelator-induced surface complexation (pH 3). Under basic solutions, deprotonated chelators promote metal–chelate complexation, while hydroxide competition, electrostatic repulsion, and surface complexation on FeOOH further suppress F – adsorption; dissolution of Ca-bearing phases was particularly effective at pH 11. These findings demonstrate a pH-dependent shift in the dominant fluoride removal mechanisms. Under optimum conditions (100 mM EDTA or HIDS, pH 11, 10 mL/g, 3 h), washing removed ~69% of the acid (1.0 M HCl)-leachable F – fraction in the original FCS (27.0 ± 5.4 mg/kg), corresponding to ~18.6 mg/kg, and reduced total F from ~180 to ~160–165 mg/kg. Subsequently, rinsing with CaO followed by H₃PO₄ immobilized residual F – and suppressed its elution to <0.08 mg/L, suggested to be consistent with the possible formation of fluorapatite based on thermodynamic calculations and complementary solid-phase analyses. Post-immobilization soil characterization, including pH, cation exchange capacity, and weak-acid-leachable F – , indicated a transformation of residual fluoride toward less mobile forms. By integrating mechanistically informed washing with post-treatment immobilization, this study highlights the environmental and engineering trade-offs between biodegradable and conventional chelators, offering a practical strategy for mitigating FCS risks. • Mechanistic analysis clarified pH-dependent mechanisms of F – removal in FCS. • F– removal occurred via mineral dissolution and chelator-inhibited readsorption. • Washing removed ~69% of acid-leachable F – total F fell from ~180 to ~160 mg/kg. • CaO then H 3 PO 4 rinsing immobilized F – , consistent with possible FAp formation. • Integrated washing + immobilization workflow mitigated environmental risks of FCS.