Enhanced degradation of naphthalene in persulfate-based systems coupled with calcium sulfite: Elucidation of degradation mechanisms and pathways

Na2SO3 and NaHSO3 have been widely used as the source of SO32− to participate in the reproduction of Fe(II) in iron-based advanced oxidation processes. In this work, CaSO3 with low solubility was innovatively introduced and compared with Na2SO3 and NaHSO3 to investigate their effects on naphthalene (NAP) degradation in Fe(II)-activated peroxydisulfate (PDS) and peroxymonosulfate (PMS) processes. The results showed that CaSO3 displayed a better performance due to its sustained-release of SO32−, and NAP removal increased from 57.6% and 77.7% to 91.4% and 98.5% in PDS/Fe(II) and PMS/Fe(II) processes, respectively, with the addition of CaSO3. The enhancement mechanisms of CaSO3 were illustrated by measuring the variation of iron and SO32− concentrations and by quantitatively determining the production of reactive oxygen species (ROS). The dominant ROS generated in CaSO3-enhanced systems was confirmed by scavenging tests. Moreover, NAP degradation intermediates and pathways, as well as the toxicological properties of intermediates, were accordingly elucidated. Finally, CaSO3-enhanced systems had a wide application range of pH, and exhibited a great performance on the tolerance of various water matrixes. The significant removal of various contaminants in CaSO3-enhanced processes confirmed that these techniques are suitable for the remediation of organic contaminated groundwater.