A Novel Phosphate-Rich Biogenic Manganese Oxide for the Efficient Rare Earth Metal Ions Adsorption: Performance, Mechanism, and Applicability

Abstract

The removal and recovery of rare earth metal ions (RE³⁺) from wastewater by an adsorption method are significant for both environmental protection and sustainable resource development. However, the RE³⁺ extraction efficiency and selectivity still need to be improved in the adsorption process. Herein, a phosphate functionalization biogenic manganese oxide (PR-BMO) was fabricated by a biomineralization method for effective and selective adsorption of RE³⁺. PR-BMO was a complex of biogenic manganese oxide and Mn₂P₂O₇, which exhibited a stacked lamellar structure, had rich surface phosphate groups, and carried a large number of negative charges. The adsorption capacity of PR-BMO for model RE³⁺ (La³⁺) was up to 537.92 mg·g^–1, which was significantly higher than that of BMO without phosphate functionalization and chemical manganese oxides. Adsorption kinetic, isotherm, thermodynamic studies, and spectroscopic techniques revealed the adsorption of RE³⁺ by PR-BMO was a complex but feasible endothermic process, coexisting electrostatic attraction, and coordinated complexation. Owing to the high affinity of phosphate groups to RE³⁺, PR-BMO showed outstanding selectivity for RE³⁺ adsorption despite the presence of typical coexisting ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^–) and possessed great adsorption capacities toward various RE³⁺ including La³⁺, Ce³⁺, Nd³⁺, Pr³⁺, Y³⁺, and Dy³⁺. Notably, PR-BMO could reduce the concentration of RE³⁺ in low-concentration wastewater to nearly 0 mg·L^–1 and concentrate the RE³⁺ from 10 to 1811.5 mg·L^–1 through enrichment treatment. This study gives new insight into the treatment of rare earth industrial effluents and expands the application of BMO.