Lanthanum-Modified Activated Alumina for Selective Removal of Fluoride from Phosphogypsum Leachate

ABSTRACT: Fluoride-rich phosphogypsum leachate is a challenging industrial wastewater because of its low pH, high ionic strength, and the presence of competing anions. In this study, lanthanum-modified activated alumina (La-AA) was prepared and evaluated as a selective adsorbent for the removal of fluoride from complex phosphate-industry wastewater. The results showed that La modification improved the adsorption performance of activated alumina, and the 10% La-AA sample exhibited the best fluoride removal behavior among the tested materials. Kinetic and thermodynamic analyses indicated that fluoride adsorption on La-AA was more favorable than on pristine activated alumina and proceeded spontaneously and endothermically under the tested conditions. In competitive adsorption tests, La-AA maintained better fluoride uptake than unmodified activated alumina in the presence of chloride, sulfate, and phosphate, demonstrating improved matrix tolerance. When applied to authentic phosphogypsum leachate, the optimized La-AA reduced fluoride concentration from 9.67 mg·L⁻¹ to 0.58 mg·L⁻¹, below the WHO guideline value. These results suggest that La modification is a practical strategy to improve the selectivity and applicability of activated alumina for fluoride removal in complex industrial process water.