Degradation of the antibiotic ornidazole in aqueous solution by using nanoscale zero-valent iron particles: kinetics, mechanism, and degradation pathway

Degradation of ornidazole (ONZ) by nanoscale zero-valent iron (nZVI) particles was investigated for the first time in this work. The results showed that ONZ was almost completely degraded within 30 min by 0.1 g L-1 nZVI at pH 5.8 and 25 degrees C. The effects of the nZVI dose, initial ONZ concentration, pH, and temperature on ONZ removal were systematically investigated, and removal of ONZ was followed by a pseudo-first-order kinetics model. Experimental results demonstrated that higher nZVI doses, lower initial ONZ concentrations, and lower pH levels could increase the pseudo-first-order rate constant (k(obs)) of ONZ removal. While higher temperatures favored removal, the activation energy results suggested that mass transfer was the limiting step during the removal process. The possible effect of oxygen was ruled out by introducing hydroxyl radical scavengers into the experiment. The variation of ONZ concentrations and total organic carbon (TOC) contents in the solution indicated that adsorption was not the main mechanism. The possibility that precipitation was the main mechanism was also excluded by the results for the change in pH and effect of pH. The characterization of nZVI before and after the reaction indicated that ONZ was reduced on the surface of nZVI, which was the main mechanism. Three intermediates and two final products were detected based on the results of UV-vis and high performance liquid chromatography/mass spectrometry (HPLC-MS) analyses. Dechlorination, nitro reduction, N-denitration, and cleavage were all involved in the entire reaction process, and therefore a complicated potential degradation pathway was proposed.