Reaction pathways of heat-activated persulfate oxidation of naphthenic acids in the presence and absence of dissolved oxygen in water

Dissolved oxygen has been found to participate in the mineralization of naphthenic acids (NAs) by heat-activated persulfate (PS) oxidation. However, the reaction mechanisms involved needs further study. This work reports on the reaction pathways of heat (80 °C)-activated PS oxidation of model NAs in the presence and absence of molecular oxygen from the analyses of the reaction byproducts. The NAs selected include saturated and aromatic ring-bearing structures. The analyses were performed by gas chromatography with mass spectrometer (GC–MS), liquid chromatography with quadrupole time-of-flight mass spectrometer (LC-QTOF-MS), gas chromatography with flame ionization detector (GC-FID) and ion chromatography (IC). The results suggest that decarboxylation of NAs is the initial step of PS-oxidation under air and inert atmosphere. Cyclohexanone-like compounds are one kinds of the main intermediates, indicating the occurrence of oxygenation upon the reaction. The formation of those compounds relies on the dissolved oxygen since it was significantly hindered in inert atmosphere. This hindering effect was less significant in the case of aromatic ring-bearing NAs than for the saturated ones. Meanwhile, condensation byproducts were formed in the absence of oxygen. The corresponding degradation and ring-opening mechanisms were postulated on the basis of analytical results.