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

Abstract 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) Fe 3+ 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 Fe 3+ ( 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,…