BACKGROUDIn the current study, two-step persulfate and Fenton oxidation has been investigated for the mineralization of naphthenic acids at 80 degrees C and initial pH approximate to 8. This pH evolves during the persulfate oxidation step towards the optimum for Fenton oxidation (approximate to 3). The effects of persulfate and H2O2 doses, iron concentration, duration of the persulfate oxidation step and operating temperature have been assessed. RESULTSThe combined treatment allowed up to approximate to 80% mineralization of cyclohexanoic acid using fairly low relative amounts of reagents (20 and 30% of the stoichiometric for persulfate and H2O2, respectively). For mineralization of cyclohexanoic acid, 115 and 87 kJ mol(-1) were obtained as representative values of the apparent activation energy for the persulfate and Fenton oxidation steps, respectively. The system was also successfully tested with other naphthenic acids, including cyclohexanebutyric acid, 2-naphthoic acid and 1,2,3,4-tetrahydro-2-naphthoic acid. Treatment of the naphthenic acids tested by this system gave rise to easily biodegradable effluents consisting mainly of short-chain organic acids. The biodegradability was confirmed by the BOD5/COD ratio and respirometric tests. CONCLUSIONThe results show the potential application of this approach as a promising cost-effective solution for the treatment of naphthenic acids-bearing aqueous wastes. This approach has significant advantage compared with the single thermally-activated persulfate or Fenton oxidation, since it allows a high mineralization at reduced reagent cost upon replacing part of the persulfate by less expensive H2O2. (c) 2018 Society of Chemical Industry