Co nanoparticle-embedded N,O-codoped porous carbon nanospheres as an efficient peroxymonosulfate activator: singlet oxygen dominated catalytic degradation of organic pollutants

In this study, Co nanoparticle-embedded N,O-codoped porous carbon nanospheres (C@Co) with abundant N and O doping, high graphitization, large specific surface area (319 m² g^-1) and a well-developed mesoporous structure were synthesized and characterized thoroughly, and were applied to activate peroxymonosulfate (PMS) for the degradation of methylene blue (MB). Various influential factors affecting the catalytic performance including C@Co dosage, PMS dosage, MB concentration, initial pH, temperature, and co-existing common anions and humic acid (HA) on the MB degradation were systematically investigated. The increase of the C@Co dosage (15-60 mg), PMS dosage (25-100 mg) and reaction temperature (278-308 K) promoted the MB degradation in the C@Co/PMS system. The best performance of the C@Co/PMS system was observed under weakly acidic or nearly neutral conditions. Both the MB concentration (25-100 mg L^-1) and Cl^- (5-100 mM), NO₃^- (10-500 mM), CO₃^2- (10-300 mM), HCO₃^- (1-30 mM) and HA (2-40 mg L^-1) had an inhibitory effect on MB degradation, and the degree of decrease in MB degradation increased as their concentrations were enhanced. Interestingly, HPO₄^2- (1-100 mM) had an overall inhibitory effect on the degradation process of MB; however, in comparison with lower concentrations (1-10 mM), an attenuation of the inhibitory effect at higher concentrations (50-100 mM) could be observed. Moreover, the C@Co/PMS system also exhibited general applicability in eliminating various organic pollutants from water such as methyl orange, malachite green, safranine T, Congo red, Rhodamine B, ofloxacin and tetracycline. Classical radical-quenching tests and EPR measurements showed that both the non-radical pathway (major route, involving ¹O₂) and radical pathway (minor route, involving ˙OH, ˙SO₄^- and ˙O₂^-) contribute to the MB degradation. DFT calculations disclosed that the combination of Co-C interactions with graphitic N doping brought in catalytically active sites in C@Co where the charge states of some C atoms were significantly increased. The degradation intermediates of MB during the catalytic reaction were also identified by HPLC-MS and the possible degradation pathway was proposed. Overall, the resultant C@Co can be developed as a novel and efficient heterogeneous catalyst for activating PMS to degrade organic pollutants, and has potential application in environmental remediation.