A novel magnetic adsorption–coagulation system using lanthanum carbonate/Fe3O4 composite adsorbent for simultaneous efficient removal of turbidity and fluoride

The reduction of fluoride concentration in wastewater to ultra-low levels remains a significant challenge, as conventional aluminum-based coagulation suffers from low efficiency at trace levels and high chemical consumption. Herein, a novel magnetic adsorption-coagulation (MAC) process was developed in this study to address this limitation by integrating a lanthanum carbonate/Fe 3 O 4 composite (LC-Fe 3 O 4 ) as a magnetic seed with polyaluminum chloride (PAC). Sole LC-Fe 3 O 4 adsorbent exhibited a high fluoride adsorption capacity of 215.3 mg F/g (pH 7, 298 K, 0.1 g/L, 24 h), fitting well with Langmuir and Freundlich models and following the pseudo-second-order kinetic model. It maintained robust performance across a wide pH range (5.0–9.0) and exhibiting strong selectivity toward fluoride even in the presence of competing HCO 3 − , CO 3 2− , SO 4 2− , and NO 3 − anions, whereas PO 4 3− significantly interfered. This study revealed that the mechanisms of fluoride removal during LC-Fe 3 O 4 adsorption process mainly included ligand exchange, electrostatic attraction, and inner-sphere complexation with La 3+ sites. When applied for treating real secondary effluent, this novel MAC process achieved simultaneous and efficient removal of both fluoride and turbidity. Under optimized conditions, fluoride and turbidity removal efficiencies reached 69.12% and 95.2%, respectively, significantly outperforming sole coagulation. This enhancement might be attributed to the synergistic effects of adsorption on the magnetic seeds and the formation of denser and separable flocs. Furthermore, the LC-Fe 3 O 4 seed showed excellent regenerability and reusability over multiple cycles, underscoring the practical potential of the MAC strategy for advanced wastewater treatment.