Singlet Oxygen-Mediated Degradation of 17β-Estradiol by Peracetic Acid Activated over Co Species Confined in Carbon Nanotubes

17β-Estradiol (E2), a highly bioactive steroid estrogen, poses potential ecological risks even at trace concentrations in aquatic environments. In this study, a nanoconfined Co-based catalyst (Co@Cin) was employed to activate peracetic acid (PAA) for efficient E2 degradation. Microscopic and spectroscopic characterizations indicated that Co-containing species were highly dispersed within the CNT-based architecture, while the graphitic tubular framework was largely preserved. Compared with PAA alone, CNTs, Co@Cinalone, and the externally loaded Co@Cout/PAA system, Co@Cin/PAA exhibited substantially faster E2 degradation. Pseudo-first-order kinetic analysis further demonstrated the enhanced degradation kinetics of the internally confined system. Importantly, ICP analysis showed comparable Co loadings for Co@Cinand Co@Cout, while Co@Cinretained a markedly higher Co-normalized apparent kinetic activity, indicating that its enhanced performance cannot be explained simply by differences in total Co loading. Moreover, Co@Cinexhibited substantially lower Co leaching than Co@Coutand maintained considerable catalytic activity over five consecutive cycles, demonstrating improved stability of the confined Co species. The degradation performance was influenced by initial E2 concentration, catalyst dosage, PAA concentration, and pH, while Cl−and NO3−showed negligible effects and humic acid and CO32−caused only moderate inhibition. Scavenging experiments and controlled TEMP-EPR measurements with appropriate blank and control systems supported a dominant contribution of singlet oxygen (1O2), with radical pathways playing only minor roles. The enhanced performance is therefore associated with the nanoconfined reaction environment, which promotes efficient PAA activation while stabilizing the Co species. This work extends nanoconfinement-regulated PAA oxidation to the treatment of the highly bioactive steroid estrogen E2 and provides a quantitative assessment of the activity–stability advantages of internally confined Co species.