Efficient calcined lanthanum-doped magnesium-aluminum layered double hydroxides for advanced phosphate removal:...

Eutrophication, caused by excessive phosphate accumulation in aquatic systems, presents a persistent environmental challenge. To address this issue, we developed novel lanthanum-doped magnesium-aluminum layered double hydroxides (La/Mg-Al LDHs) with systematically varied Al/La molar ratios through a co-precipitation method. Comprehensive material characterization revealed that La doping preserved the characteristic hydrotalcite-like layered structure while strategically substituting Al sites. This substitution enhanced electron transfer between Mg-O and Al-O bonds and significantly improved the material's physicochemical properties, including specific surface area, pore characteristics, and point of zero charge (pH_pzc). Among the synthesized materials, La/Mg-Al LDHs with an Al/La molar ratio of 2 demonstrated superior phosphate removal performance, achieving 96.3-99.0 % removal efficiency at initial phosphate concentrations of 2-10 mg/L. Calcination treatment further enhanced the adsorption capacity to 79.9 mg/g. The adsorption process followed pseudo-second-order kinetics and Langmuir model, suggesting monolayer chemisorption. The material exhibited robust performance across a broad pH range (3-11) and demonstrated selective phosphate removal in the presence of competing anions. Mechanistic investigations revealed that phosphate removal occurred primarily through ligand exchange and electrostatic attraction, involving oxygen vacancies, metal hydroxyl groups, and carbonate ions. These findings establish calcined La-Mg-Al LDHs as promising advanced materials for phosphate removal in water treatment applications.