Calcium-rich biochar/lanthanum alginate hydrogel beads for fluoride removal from groundwater

Fluoride (F^-) pollution in groundwater has become a global environmental issue and a significant threat to human health. In this study, calcium-rich biochar (CRB) was introduced into the lanthanum (La) alginate matrix to prepare calcium-rich biochar/lanthanum alginate hydrogel beads (La/CRB@SA). Adsorption tests showed that composite hydrogel beads (pyrolysis temperature 600 °C, CRB dosage 0.2%, and La³⁺ content 0.2 M) achieved excellent F^- removal (95.57%) under optimal operation conditions (F^- content 9.0 mg/L, adsorbent dosage 0.7 g/L, and pH 5.0). Kinetic results suggested that the F^- adsorption process by La/CRB@SA conformed to the pseudo-second-order model (R² > 0.99). La/CRB@SA exhibited high selectivity towards F^- and maintained 96.5% of its initial capacity after 120 h of desorption. Electrostatic interaction, ion exchange, surface precipitation, inner-sphere complexation, and chemisorption were demonstrated as the main mechanisms for F^- removal. This study provides a new insight for the resource utilization of crab shell waste and the development of high-performance hydrogel adsorbents, demonstrating considerable application potential in efficient F^- removal from groundwater.