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Abstract Naphthenic acids (NAs) are one of the constituents of concerns in oil sands process water (OSPW) because of their persistence and recalcitrance. Herein, we investigated the degradation of five model NA compounds by UV-activated persulfate (UV/persulfate) process under medium-pressure UV lamp irradiation at pH 8.0. UV/persulfate process showed higher degradation efficiency towards cyclohexanoic acid (CHA) compared to UV/H 2 O 2 process under the same experimental conditions. CHA (0.39 mM) was completely removed within 30 min when 2 mM persulfate was used as oxidant, while more than 60 min were needed for the UV/H 2 O 2 process. The removal of CHA decreased from 100% to 10% when 300 mM tert-butyl alcohol (TBA) was used as the scavenger, indicating that hydroxyl radical (OH) was responsible for the CHA degradation in the UV/persulfate process. Sulfate (SO 4 −) radicals reacted slowly with CHA in the UV/persulfate process with a second-order rate constant of k = 5.3 × 107 M−1s−1. Relative kinetics studies using binary mixtures of model NA compounds showed similar structure-reactivity to that under UV/H 2 O 2 process. NAs with long carbon chain, cyclic ring, and aromatic ring were more reactive in the UV/persulfate process. The presence of high concentration of chloride ions dramatically inhibited the reaction. The OH radicals in the UV/persulfate process were generated by capturing OH− in solutions, as evidenced by the decrease of the pH value from 8.0 to 2.8 before and after treatment, respectively, in a pure water matrix. Primary intermediate products (oxy-CHA, hydroxyl-CHA, and dihydroxyl-CHA) of UV/persulfate process were confirmed by UPLC-MS. Highlights • UV activated persulfate oxidation process was used for the degradation of model NAs. • SO 4 − radical reacted slowly with CHA in UV/persulfate process compared to BA. • OH generated from SO 4 − and OH− were the main radicals responsible for the CHA removal. • Relative kinetics of model NA compounds in UV/persulfate process were studied. • The presence of chlorine ions was adverse to CHA removal and related chlorinated by-products were formed. [ABSTRACT FROM AUTHOR]

Naphthenic acids (NAs) are a highly complex mixture of organic compounds naturally present in bitumen and identified as the primary toxic constituent of oil sands process-affected water (OSPW). This work investigated the degradation of cyclohexanoic acid (CHA), a model NA compound, and natural occurring NAs during the UV photolysis of Fe(III)-nitrilotriacetic acid (UV-Fe(III)NTA) and UV-NTA-Fenton processes. The results indicated that in the UV-Fe(III)NTA process at pH 8, the CHA removal increased with increasing NTA dose (0.18, 0.36 and 0.72 mM), while it was independent of the Fe(III) dose (0.09, 0.18 and 0.36 mM). Moreover, the three Fe concentrations had no influence on the photolysis of the Fe(III)NTA complex. The main responsible species for the CHA degradation was hydroxyl radical ( OH), and the role of dissolved O 2 in the OH generation was found to be negligible. Real OSPW was treated with the UV-Fe(III)NTA and UV-NTA-Fenton advanced oxidation processes (AOPs). The removals of classical NAs (O 2 -NAs), oxidized NAs with one additional oxygen atom (O 3 -NAs) and with two additional oxygen atoms (O 4 -NAs) were 44.5%, 21.3%, and 25.2% in the UV-Fe(III)NTA process, respectively, and 98.4%, 86.0%, and 81.0% in the UV-NTA-Fenton process, respectively. There was no influence of O 2 on the NA removal in these two processes. The results also confirmed the high reactivity of the O 2 -NA species with more carbons and increasing number of rings or double bond equivalents. This work opens a new window for the possible treatment of OSPW at natural pH using these AOPs. [ABSTRACT FROM AUTHOR]