Advanced oxidation processes for removal of monocyclic aromatic hydrocarbon from water: Effects of O₃/H₂O₂ and UV/H₂O₂ treatment on product formation and biological post-treatment

Several oxidative treatment technologies, such as ozonation or Fenton reaction, have been studied and applied to remove monocyclic hydroaromatic carbon from water. Despite decades of application, little seems to be known about formation of transformation products while employing different ozone- or ^∙OH-based treatment methods and their fate in biodegradation. In this study, we demonstrate that O₃/H₂O₂ treatment of benzene, toluene, ethylbenzene (BTE), and benzoic acid (BA) leads to less hydroxylated aromatic transformation products compared to UV/H₂O₂ as reference system – this at a similar ^∙OH exposure and parent compound removal efficiency. Aerobic biodegradation tests after oxidation of 0.15 mM BA (12.6 mg C L⁻¹ theoretical DOC) revealed that a less biodegradable DOC fraction > 4 mg C L⁻¹ was formed in both oxidative treatments compared to the BA control. No advantage of ozonation over UV/H₂O₂ treatment was observed in terms of mineralization capabilities, however, we detected less transformation products after oxidation and biodegradation using high-resolution mass spectrometry. Biodegradation of BA that was not oxidized was more complete with minimal organic residual. Overall, the study provides new insights into the oxidation of monocyclic aromatics and raises questions regarding the biodegradability of oxidation products, which is relevant for several treatment applications.