Mechanistic insights into fluoride removal by P-loaded calcite

Abstract

Phosphate (P) and fluoride commonly coexist in aquatic systems. Although P loading restructures calcite through Ca P phase formation, its influence on subsequent fluoride removal remains unclear. Calcite conditioned with high P concentrations at pH 8.3, 7.7, and 7.3 developed P-rich surface layers, with 56–95% higher specific surface areas and Ca P signatures in FTIR and XPS. SEM–EDS further revealed the development of Ca P coatings with uniform elemental distributions. Fluoride-removal experiments revealed a concentration-dependent response. At low fluoride concentrations (3–20 mg·L −1 ), P-loaded calcite consistently outperformed pristine calcite, achieving 33–60% higher removal efficiencies without measurable changes in solution pH or Ca 2+ concentration, indicating adsorption-dominated immobilization. XPS F 1 s spectra (∼684.4 eV) support fluoride association with Ca P surface domains under these conditions. At 20 mg·L −1 fluoride, HRTEM/SAED identified calcite lattice and diffraction features but no self-consistent evidence of crystalline CaF 2 in the examined regions. DFT yielded stronger F − adsorption on a representative Ca₃(PO 4 ) 2 (010) surface than on calcite (104) (−5.926 versus −3.655 eV) and showed adsorption-induced interfacial charge redistribution. In contrast, at high fluoride concentration (100 mg·L −1 ), fluoride removal was accompanied by pronounced Ca 2+ depletion and pH increases, and F 1 s binding energies (∼684.7–685.0 eV) characteristic of CaF 2 , demonstrating precipitation-dominated control irrespective of P conditioning. These results show that P loading creates chemically distinct Ca P surface environments that enhance fluoride adsorption only within a concentration window where precipitation is not thermodynamically dominant. This study clarifies the mechanistic basis for repurposing P-loaded calcite in fluoride control applications.