The degradation of cyclohexanoic acid (CHA) as a model compound of naphthenic acid (NA) was investigated in this study by a nitrilotriacetic acid (NTA) - modified Fenton process at pH 8. The optimum doses of hydrogen peroxide (H2O2) and Fe(III)NTA (NTA:Fe = 2:1) were 4.41 mM and 0.27 mM, respectively. CHA degradation reached a plateau within 4 h under these conditions. The degradation of CHA was influenced primarily by the dose of H2O2, while the concentration of Fe(III)NTA complex primarily influenced the reaction time. The second-order rate constants of hydroxyl radical ((OH)-O-center dot) with CHA and with NTA were investigated using a competition-kinetics method and obtained as 4.09 +/- 0.39 x 10(9) M-1 s(-1) and 4.77 +/- 0.24 x 10(8) M-1 s(-1), respectively. (OH)-O-center dot radical was the main responsible species for CHA degradation, while O-center dot(2)- radical played a minor role. The reaction of CHA with O-center dot(2)- radical generated by potassium superoxide (KO2) in dimethyl sulfoxide (DMSO)/water solution was investigated for the first time. The reaction mechanisms of the NTA-modified Fenton process were proposed and discussed based on the discovery that no Fe(III)NTA was detected in the system. (C) 2016 Elsevier B.V. All rights reserved.