Spin-State Switching Modulates Lewis Acidity in Ferrihydrite for Enhanced Phosphate Capture

Iron-based adsorbents are promising candidates for phosphorus removal, whereas the current progress suffers from the effectiveness of their Lewis acid sites. Herein, an innovative strategy is proposed to modulates Lewis acidity by switching high-spin (HS) Fe³⁺ to activate t_2g→e_g orbital electron transitions. Results demonstrate that the weak field ligand effect of sulfur (S) reduces the orbital splitting energy in ferrihydrite (Fh), inducing the generation of HS Fe³⁺ (e_g filling ≈0.983). Compared to pristine Fh, the HS S-Fh exhibits an elevated number of unpaired d electrons (2.36→3.45), thereby significantly increasing its Lewis acidity. Mechanistic studies reveal that improved electron transfer between P-O bonds and Fe centers, together with strengthened d-p orbital hybridization, promotes phosphate adsorption, resulting in a 146-fold improvement in adsorption kinetics. Remarkably, S-Fh continuous-flow reactor maintains ≈100% phosphate removal after treating over 1200 bed volumes of wastewater. This work emphasizes the crucial role of spin state in regulating Lewis acidity and provides a new design strategy for highly efficient adsorbents.