Naphthenic acids (NAs) are reported to be the main species responsible for the oil sands process-affected water (OSPW) toxicity. In this study, the degradation of cyclohexanoic acid (CHA) as a model compound for NAs by an ethylenediamine-N,N'-disuccinic acid (EDDS)-modified Fenton process was investigated at pH 8. Optimum dose for Fe-EDDS (EDDS:Fe =2:1) was 0.45 mM, and 2.94 mM for hydrogen peroxide (H2O2). The time profiles of the main species in the process were studied, including CHA, H2O2, Fe(II), total Fe, and Fe-EDDS (in the main form of (FeEDDS)-E-III). The second-order rate constant between EDDS and hydroxyl radical ((OH)-O-center dot) at pH 8 was obtained as 2.48 +/- 0.43 x 10(9) M-1 s(-1). (OH)-O-center dot was proved to be the main species responsible for the CHA degradation, while superoxide radical (O-2(center dot-)) played a minor role. The consecutive addition of H2O2 and Fe-EDDS led to a higher removal of CHA compared to that achieved by adding the reagents at a time. The half-wave potential of (FeEDDS)-E-III/II was measured at pH 7-9. The EDDS-modified Fenton process is a promising alternative to degrade NAs. (C) 2016 Elsevier B.V. All rights reserved.