Photodegradation of naphthenic acids induced by natural photosensitizer in oil sands process water

Utilization of renewable solar energy driven advanced oxidation processes for oil sands process water (OSPW) remediation has received extensive attention. Naturally existing inorganic photosensitizer in OSPW was investigated in this work to provide information about the application of indirect photolysis treatment of organic contaminants in OSPW. OSPW and OSPW organic fraction were exposed under UV irradiance with fluence of 10 J/cm2 to investigate the effect of OSPW inorganic fraction (OSPW-IF) on the non-catalytic photolysis of naphthenic acids (NAs) in OSPW. The results indicated that the inorganic fraction in OSPW enhanced the photodegradation of NAs, with 24.3% of total NA removal in OSPW, while only 12.4% of total NAs were removed in OSPW organic fraction. Moreover, the photodegradation of 1-adamantanecarboxylic acid (ACA) dissolved in OSPW-IF or carbonate buffer was conducted to verify the enhanced photodegradation of NAs by OSPW-IF. The results showed that 30.9% of ACA was removed in the OSPW-IF, while no ACA degradation was observed in carbonate buffer after 60 min of UV exposure, indicating that the OSPW-IF induced the photodegradation of ACA. In addition, nitrate was identified to be the photosensitizer in OSPW-IF responsible for the indirect photolysis of ACA. In the presence of nitrate, both hydroxyl radicals (•OH) and reactive nitrogen species were generated, where •OH was the dominant reactive species that contributed to the degradation of ACA. Ten possible by-products ranging from single to multiple hydroxyl, nitroso, nitro and carbonyl substituted products were proposed to be produced from the nitrate-induced photodegradation process through three different pathways. This study demonstrated that the photolysis of NAs in OSPW due to the presence of natural photosensitizers and nitrate could act as a natural photosensitizer for the remediation of OSPW by the photo-oxidation process.