Role of co-existing anions in non-radical and radical processes of carbocatalyzed persulfate activation for acetaminophen degradation

Abstract

We investigated the effect of co-existing anions of Cl⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, and HCO₃⁻ on potassium persulfate (PS) activation by multiwalled carbon nanotubes (MWCNTs) and N-doped MWCNTs (N-MWCNTs) for acetaminophen (ACP) degradation. The results of radical quenching studies and electron paramagnetic resonance (EPR) analyses affirmed that non-radical (¹O₂) and radical (O₂˙⁻, HO˙, and SO₄˙⁻) species initiated the ACP degradation. Catalytic performance studies demonstrated that CO₃²⁻ and HCO₃⁻ provided higher ACP degradation compared to Cl⁻, SO₄²⁻, and NO₃⁻. The observed beneficial and adverse impacts of Cl⁻, SO₄²⁻, and NO₃⁻ depend on their concentration. EPR analysis confirms that the radical pathway was modified in the presence of CO₃²⁻ or HCO₃⁻, which proved that HO˙ and SO₄˙⁻ were scavenged by CO₃²⁻ and HCO₃⁻. Non-radical TEMP–¹O₂ adduct signals were not changed by co-existing anions, inferring that the high selectivity ¹O₂ species were resistant to the impact of co-existing anions. An ACP degradation mechanism via electron transfer, acetamide cleavage, mono- and dihydroxylation was proposed based on liquid chromatography-mass spectroscopy analysis. The co-existing anions did not noticeably affect the formation of degradation products via the electron transfer mediative pathway, however, Cl⁻, SO₄²⁻ and CO₃²⁻ notably limited the di-hydroxylation pathways.