Rapid and efficient removal of phosphate by La-doped layered double hydroxide/biochar from aqueous solution

Abstract

Layered double hydroxide (LDH) is frequently used for phosphate removal in water, while a desirable LDH adsorbent should have sufficiently high adsorption capacity and selectivity. To enhance the removal performance and selectivity of phosphate by LDH, La-doped layered double hydroxide/biochar (La–MgAl–LDH/BC) was synthesized by using a co-precipitation method. Investigation results showed that lanthanum (La) metal loading can significantly enhance the phosphate adsorption performance of La–MgAl–LDH. Then, the introduction of La–MgAl–LDH into BC improved the adsorption capacity of phosphate by the adsorbent, which is 1.8 times that of pure La–MgAl–LDH. The optimum La to Al molar ratio was 0.2 : 0.8 in La–MgAl–LDH materials, and the La–MgAl–LDH to BC mass ratio was 9.3 : 10. Furthermore, both La–MgAl–LDH and La–MgAl–LDH/BC exhibited an enhanced ability to resist interference from co-existing anions (Cl⁻, SO₄²⁻, NO₃⁻ and CO₃²⁻) compared to LDH. For batch experiments, the kinetic results indicated that the adsorption process of phosphate by La–MgAl–LDH/BC follows a pseudo-second-order kinetics model, reaching equilibrium quickly within 1 h. Furthermore, film diffusion and intra-particle diffusion simultaneously took place. The maximum monolayer phosphate adsorption capacity of La–MgAl–LDH/BC could reach 249.3 mg PO₄³⁻ g⁻¹ (513.6 mg PO₄³⁻ g⁻¹, converting to an equivalent La–MgAl–LDH). After four cycles, La–MgAl–LDH/BC showed good reusability, and over 63.5% of phosphate uptake was retained with minimal metal leaching. Additionally, phosphate could be rapidly removed from actual water and simulated water at low concentrations. The electrostatic attraction, ligand exchange, ion exchange and Lewis acid action jointly facilitate the adsorption of phosphate.