Lanthanum substitution enhances the intrinsic phosphate-adsorption capacity of hydrated ferric oxide via increasing Fe electron density

Abstract

Fe (hydr)oxides are low-cost phosphate adsorbents for aqueous phosphate removal, but restricted by the unsatisfactory adsorption capability due to the mild binding between ≡Fe(III) and phosphate. Herein, we demonstrated that La substitution increased electron density on ≡Fe(III) sites in hydrated ferric oxide (HFO), thus strengthening the binding of phosphate on ≡Fe(III) sites and enhancing the intrinsic phosphate-adsorption capacity of ≡Fe(III) sites. Lanthanum substituted HFO (La-HFO) with 5 wt% dopant displayed a 3.4-fold increase in phosphate-adsorption capacity and 6.0-fold increase in adsorption rate, as well as the exceptional cyclability and stability. The increased electron density on ≡Fe(III) sites favored surface hydroxyl group dissociation, inducing more positively charged surface of HFO to attract the negatively charged phosphate anions, and resulting in the phosphate-adsorption mode transition from bidentate binuclear to bidentate mononuclear, more favorable for the utilization efficiency of ≡Fe(III) sites for phosphate-adsorption. Moreover, this strategy exhibits broad applicability beyond La, as evidenced by similar performance enhancements achieved with Ce substitution, highlighting its potential as a cost-effective strategy to enhance the intrinsic phosphate-adsorption performance of metal (hydr)oxides for advanced phosphate removal by regulating electron properties of the metal centers.