Lanthanum‑calcium modified coal gangue-alginate composite hydrogel beads for efficient fluoride removal from groundwater

Coal gangue (CG) was encapsulated into lanthanum alginate and calcium alginate to synthesize bimetallic modified coal gangue-alginate composite hydrogel beads (La-Ca/CG-SA), and their adsorption potential towards fluoride (F − ) from groundwater was investigated. Under optimal conditions (preparation parameters: CG dosage 6 %, La/Ca ratio 1:2, and crosslinking time 24 h; operational parameters: adsorbent dosage 0.3 g L −1 , pH 6.0, and F − concentration 3 mg L −1 ), the maximum adsorption capacity of F − was achieved (15.7 mg g −1 and 87.8 %). Kinetic results indicated that the adsorption process of F − by La-Ca/CG-SA followed the pseudo-second-order model (R 2  > 0.95). Coexisting ion experiments suggested that La-Ca/CG-SA exhibited high selectivity for F − . Additionally, La-Ca/CG-SA maintained superior F − removal performance (81.5 %) after five cycles. Electrostatic interaction, inner-sphere complexation, chemical precipitation, and ion exchange were demonstrated to drive the F − immobilization. This study provides novel insights into the resource utilization of CG and the development of high-performance hydrogel adsorbents, offering potential application value in treating fluoride-contaminated groundwater.