The catalytic role of nascent Cu(OH)2 particles in the sulfite-induced oxidation of organic contaminants

Abstract In the recent years, sulfite-induced oxidation of various contaminants using oxysulfur radicals (SO 4 − , SO 5 − and SO 3 − ) has been explored to establish novel advanced oxidation processes (AOPs). Although transition metals, such as cupric ions (Cu(II)), have been demonstrated to be capable of activating sulfite (S(IV)), the catalytic mechanism of the specific Cu(II) species is still not very clear. In this work, N-acetyl- para -aminophenol (APAP) was used as a basic substrate to investigate the process, mechanism and stability of the Cu(II)-S(IV) system with low dosages of Cu(II) (0.010 mmol L −1 ) in an alkaline solution (pH 10). The results show that 0.010 mmol L −1 Cu(II) can efficiently activate S(IV) for the oxidation of APAP and other contaminants (e.g., metoprolol, sulfanilamide, aniline, 4-chloroaniline, N,N-dimethylaniline, p -nitrophenol and 3-nitrophenol). Simulation calculations for the distribution of the Cu(II) species and pH-dependence of APAP oxidation in the Cu(II)-S(IV) system showed that Cu(OH) 2 particles are the major active species in the catalysis of S(IV). The surface complexation of S(IV) with the Cu(OH) 2 particles prior to the production of oxysulfur radicals is the basic mechanism of the alkaline Cu(II)-S(IV) system. Radical inhibition with scavengers (ethanol, tert-butyl alcohol or aniline) demonstrates that SO 5 − , SO 4 − and hydroxyl radicals (HO ) are responsible for the contaminant oxidation. Moreover, SO 5 − is believed to be the main active species in the Cu(II)-S(IV) system, which has a second-order rate constant of 2.98 × 10 8  M −1  s −1 with APAP. This work renews our recognition of the Cu(II)-S(IV) system applications and further understanding of the catalytic role of the nascent Cu(OH) 2 particles in the activation of S(IV).