Degradation of a model naphthenic acid by nitrilotriacetic acid – modified Fenton process

• NTA-modified Fenton process was very efficient in degrading CHA at pH 8.
• The second-order rate constants of OH with CHA and with NTA at pH 8 were obtained.
• CHA degradation was closely related to the dose of H2O2 rather than FeIIINTA dose.
• OH radical was mainly responsible for the CHA degradation, while O2− radical played a minor role.
• Mechanisms for NTA-modified Fenton process were proposed and discussed for the first time.

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 FeIIINTA (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 FeIIINTA complex primarily influenced the reaction time. The second-order rate constants of hydroxyl radical (OH) with CHA and with NTA were investigated using a competition-kinetics method and obtained as 4.09 ± 0.39 × 109 M−1 s−1 and 4.77 ± 0.24 × 108 M−1 s−1, respectively. OH radical was the main responsible species for CHA degradation, while O2− radical played a minor role. The reaction of CHA with O2− 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 FeIINTA was detected in the system.