Insight into heterovalent metal modification for lanthanum carbonate to enhance phosphate removal at trace levels

Reducing phosphate concentrations from low levels to ultra-low levels (e.g., below 0.01 mg P/L) is crucial for preventing and treating eutrophication. Adsorption is promising but still challenged by the limited adsorption capacity at low phosphate concentrations, which leads to high costs. Herein, we propose a heterovalent metal modification strategy using Fe(II) to improve the adsorption capacity of lanthanum (La) carbonate for low-level phosphate removal. A La:Fe(II) molar ratio of 7:1 enabled the partial substitution of Fe(II) for La, resulting in a La/Fe carbonate with a high adsorption capacity. La/Fe carbonate exhibited fast adsorption kinetics, excellent applicability in a pH range of 5-7, and good reusability at low phosphate concentrations. Its adsorption capacity at low phosphate levels surpassed previously reported adsorbents. La/Fe carbonate demonstrated high resistance to complex water matrices and cost-effectiveness, achieving an effluent concentration below 0.01 mg P/L in real wastewater treatment. Mechanism analysis revealed that Fe(II) substitution led to super-saturated coordination and Fe(II) -* Fe(III) conversion for charge compensation. The self-conversion of Fe(II) -* Fe(III) donated electrons to neighboring La active sites, activating these active centers, which caused a positive shift in the d-band center of La active sites and enhanced interfacial electron transport between La/Fe carbonate and phosphate. This work offers an efficient, easy-to-operate, and low-cost approach to achieve ultra-low phosphate concentrations through adsorption, and fills the research gap on the mechanism of heterovalent metal modulation to enhance the phosphate adsorption capacity.