Mechanism of FeCe bimetallic doped-biochar material regulating fluoride adsorption: Fe(II)-mediated maintenance of Ce(III) sites and the key role of terminal/bridging hydroxyl groups

Excessive fluoride (F − ) in water environment seriously endangers ecological security. This study constructed a series of dual-metal loaded biochar adsorbents (CeFe@BC) by modifying cerium-based materials through Fe doping and coupling with straw-derived biochar substrates. Ce 9 Fe 1 @BC 11 exhibited a remarkable F − adsorption capacity of 110.32 mg/g and had broad operational pH range (q e(pH=2 – 12)  ≥ 82.61 mg/g). It showed exceptional selectivity and affinity toward F − even in multicomponent systems. The adsorption process was successfully characterized by pseudo-second-order kinetic (R 2  ≥ 0.999) and Temkin isotherm model (R 2  ≥ 0.952), indicating a chemisorption-dominated mechanism. Mechanistic studies revealed that Fe(II) doping maintained the quantity of Ce(III) active adsorption sites through the Fe(II)/Ce(IV) redox reactions, enabling the inverse transformation from Ce(IV) to Ce(III), rather than participating directly in adsorption. This regeneration mechanism increased the number of adsorption active sites of the material by re-reducing Ce(IV) back to Ce(III) during fluoride adsorption or desorption, thereby significantly enhancing adsorption performance. Unlike other anions that adsorb solely via substitution of terminal hydroxyl groups (T-OH) on metal oxides, fluoride onto CeFe@BC exhibited a dual-site adsorption behavior, involving both T-OH substitution and occupancy of bridging hydroxyl (B-OH) sites. Consequently, the T-OH and B-OH groups coordinated with Ce(III) constitute the primary active adsorption sites. The hierarchical porous structure of the straw biochar exposed abundant adsorption sites and also enhanced electron transfer during redox cycles due to its superior conductivity, enabling regeneration of Ce(III) active sites. This work provided a novel strategy for the synergistic structural-functional modulation of high-performance fluoride adsorbents, advancing the rational design of materials for environmental remediation.