Degradation of a model naphthenic acid, cyclohexanoic acid, by vacuum UV (172 nm) and UV (254 nm)/H_2O_2

摘要

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)/H_2O_2, 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/H_2O _2). In UV/H_2O_2, additional 4-oxo-CHA may also have been formed by direct reaction of the oxyl radical with H_-O _2. 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/H _2O_2 reaction will depend on the rate of generation of hydroxyl radical throughout the process. In real applications to OSPW, concentrations of H_2O_2 will need to be carefully optimized and the environmental fate and effects of the various degradation products of naphthenic acids considered.