Redox-noninnocent surface sulfur enables fast yet selective peracetic acid activation on CoFe₂O₄/red mud

Sulfidation modification creates a redox-noninnocent sulfur interface on cobalt ferrite/red mud (CoFe₂O₄/RM, and its sulfidated form is denoted as S-CoFe₂O₄/RM), enabling both fast and selective peracetic acid (PAA) activation for sulfamethazine (SMT) degradation. The S-CoFe₂O₄/RM/PAA system removes 95.1 % of SMT within 5 min, with a rate constant 12.77 times that of unmodified CoFe₂O₄/RM. Using X‑ray absorption near-edge structure (XANES), time-resolved X‑ray photoelectron spectroscopy (XPS), operando attenuated total reflectance Fourier‑transform infrared spectroscopy (ATR-FTIR), and density functional theory (DFT) calculations, we reveal that surface sulfur acts as a dynamic interfacial cofactor. First, low-valence sulfur species (S^2-, SO₃^2-) can serve as electron donors that continuously regenerate Co²⁺/Fe²⁺, sustaining rapid metal redox cycling. Second, sulfur alters the adsorption configuration of PAA and steers asymmetric O-O bond cleavage, switching the dominant reactive oxygen species from non-selective ^•OH to highly selective R-O^• and ¹O₂. Nine degradation intermediates are identified, with pathways including hydroxylation, S-N bond cleavage, and N-centered radical coupling. Most intermediates are predicted to exhibit lower toxicity than SMT. The system shows strong resistance to anions and humic substances, excellent recyclability, and stable performance in real water matrices. This work establishes sulfur as a redox-noninnocent interfacial cofactor that decouples activation rate from indiscriminate radical chemistry, offering a new design principle for selective PAA activation in antibiotic wastewater treatment.