The influence of L-cysteine, a common aliphatic amino acid, on the zero-valent iron (nZVI)/O-2 photo-Fenten degradation of rhodamine B (RhB) was investigated in this study. The oxidation rate of RhB in the nZVI/O-2/hv system was 91.2% after 40 min under the illumination and oxygen conditions and pH of 3, but when cysteine was introduced into the system, the oxidization process was inhibited. The removal of RhB was only about 50% after 40 min at a cysteine concentration >= 50 mu M. It was shown experimentally that, under dark conditions, only 40.5% and 19.8% RhB was removed by the nZVI/O-2 and nZVI/O-2/cysteine systems, respectively. Electron paramagnetic resonance (EPR) and iron dissolving experiments revealed that the addition of cysteine clearly reduced the production of hydroxyl radicals ((OH)-O-center dot) and Fe2+ and Fe3+. In addition, Fourier transform infrared spectroscopy (FTIR) demonstrated that cysteine could form hydrogen bonds on the iron surface. These results indicated that the main inhibition mechanism of cysteine was the alleviation of the oxidation of nZVI to Fe2+ and Fe3+ through wrapping the nZVI particles. Moreover, cystine (the oxidized form of CYS) could partly react with (OH)-O-center dot to regenerate cysteine, which resulted in competition with RhB for (OH)-O-center dot. Another possible reason for the inhibitory effect of cysteine was the prevention of light utilization. These findings indicate a non-negligible inhibitory trait for heterogeneous Fenton process in wastewater treatment when amino acids are present. (C) 2020 Elsevier Inc. All rights reserved.