Lanthanum-modified shale powder/alginate composite hydrogel beads for fluoride-contaminated groundwater treatment

Excessive fluoride (F^-) in groundwater poses a serious threat to human health, necessitating the development of efficient defluorination materials. In this study, a novel composite hydrogel bead (La-SP/SA) was fabricated by incorporating shale powder (SP) into sodium alginate (SA) matrix followed by crosslinking with lanthanum chloride. The synthesis conditions were determined as SP dosage 4.0%, La³⁺ concentration 0.2 mol L^-1, and crosslinking time 12 h. Under adsorbent dosage of 0.7 g L^-1, initial F^- concentration of 9.0 mg L^-1, and pH of 5.0, La-SP/SA achieved an excellent removal efficiency of 94.33% (12.18 mg g^-1), with a residual F^- concentration meeting both the WHO guideline and Chinese standard. Kinetic analysis revealed that the adsorption process followed the pseudo-second-order model, characteristic of chemisorption-dominated systems. The composite hydrogel beads exhibited excellent selectivity toward F^- in the presence of coexisting anions (Cl^-, SO₄ ^2-, HCO₃ ^-, and NO₃ ^-) and maintained over 98.01% of initial adsorption capacity over 5 days across pH 4.0-9.0. Mechanistic investigations using SEM-EDS, FTIR, XRD, and XPS demonstrated that F^- removal occurred via a synergistic mechanism involving electrostatic interaction, ion exchange, inner-sphere complexation, chemical precipitation, and chemisorption. This work provides a promising strategy for fluoride-contaminated groundwater remediation while enabling the resource utilization of shale waste.