Degradation of a Model Naphthenic Acid, Cyclohexanoic Acid, by Vacuum UV (172 nm) and UV (254 nm)/H2O2

The mechanism of hydroxyl radical initiated degradation of a typical oil sands process water (OSPW) alicyclic carboxylic acid was studied using cyclohexanoic acid (CHA) as a model compound. By use of vacuum ultraviolet irradiation (VUV, 172 nm) and ultraviolet irradiation in the presence of hydrogen peroxide UV(254 nm)/H2O2, it was established that CHA undergoes degradation through a peroxyl radical. In both processes the decay of the peroxyl radical leads predominantly to the formation of 4-oxo-CHA, and minor amounts of hydroxy-CHA (detected only in UV/H2O2). In UV/H2O2, additional 4-oxo-CHA may also have been formed by direct reaction of the oxyl radical with H2O2. The oxyl radical can be formed during decay of the peroxyl-CHA radical or reaction of hydroxy-CHA with hydroxyl radical. Oxo- and hydroxy-CHA further degraded to various dihydroxy-CHAs. Scission of the cyclohexane ring was also observed, on the basis of the observation of acyclic byproducts including heptadioic acid and various short-chain carboxylic acids. Overall, the hydroxyl radical induced degradation of CHA proceeded through several steps, involving more than one hydroxyl radical reaction, thus efficiency of the UV/H2O2 reaction will depend on the rate of generation of hydroxyl radical throughout the process. In real applications to OSPW, concentrations of H2O2 will need to be carefully optimized and the environmental fate and effects of the various degradation products of naphthenic acids considered.