La₂O₃-modified MCM-41 for efficient phosphate removal synthesized using natural diatomite as precursor

In this study, an ordered mesoporous silica modified with lanthanum oxide was synthesized using diatomite as silica source and applied for adsorption of phosphate from aqueous solution. By taking cost-effectiveness for practical application into consideration, the adsorbent with a theoretical La/SiO₂ molar ratio of 0.2 (La_0.2M41) possessed a promising performance. In the batch adsorption tests, the adsorbents with La₂O₃ loading possessed markedly enhanced adsorption capacities. Phosphate uptake by La_0.2M41 was pH-dependent with the highest sorption capacities observed over a pH range of 3.0-6.0. Coexistent anions displayed an adverse effect on phosphate adsorption following the order of CO₃ ^2-  > F^-  > NO₃ ^- > Cl^- > SO₄ ^2-. In the kinetic study, phosphate adsorption onto La_0.2M41 followed the pseudo-second-order equation better than the pseudo-first-order, suggesting chemisorption. The Langmuir isothermal model well described the adsorption isotherm data, showing a maximum adsorption capacity for phosphate of up to 263.16 mg/g at 298 K. In a real treated wastewater effluent with phosphate concentration of 2.5 mg P/L, La_0.2M41 efficiently reduced the phosphate concentration to 28 µg P/L.