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• The photochemical reactions of ferric citrate were used to treat OSPW. • NAs could be effectively degraded by ferric citrate under visible light irradiation. • Photocatalytic activity of ferric citrate was greatly affected by the initial pH of OSPW. • Hydroxyl radical was the dominant reactive species in the process. • The photochemical reactions of ferric citrate could produce both Fe2+ and H 2 O 2 in situ. An environmental-friendly treatment method based on photochemical reactions of ferricarboxylate complexes was developed for the treatment of oil sands process water (OSPW). Without using H 2 O 2 , the classical naphthenic acids (NAs), one of the organic compounds present in OSPW, can be effectively degraded by ferric citrate under visible light irradiation. Photochemical reaction activity of ferric citrate was greatly affected by the initial pH of OSPW. In acidic conditions, ferric citrate exhibited an extremely high degradation ratio of classical NAs, 98.3% (initial pH 3) and 96.8% (initial pH 5). Once initial pH exceeded 5, the degradation ratio of classical NAs in OSPW decreased quickly, and the degradation ratio of classical NAs could reach 65.7% (initial pH 7) and 42.4% (without adjusting pH, initial pH 8.9), respectively. It was found that the structure of NAs had little impact on the removal efficiency of NAs. It was also confirmed through scavenger experiments and reaction mechanism study that hydroxyl radical was the dominant reactive species in the process. The study demonstrated that ferric citrate possesses excellent visible light absorption ability and the photochemical reactions of ferric citrate can produce both Fe2+ and H 2 O 2 in situ , Fenton reagent, which non-selectively degrade NAs of OSPW. [ABSTRACT FROM AUTHOR]