Oxalate-enhanced reactivity of nanoscale zero-valent iron under different conditions of O2, N2 or without aeration

Being that the influence of organic ligands on contaminant removal by nanoscale zero valent iron (nZVI) has been explored in quite a few studies, there is little information on reaction mechanisms of the ZVI-ligand system under different aeration conditions. The degradation of nitrobenzene (NB) by nZVI-oxalic acids (OA) was investigated under N2, O2 and without aeration conditions. From the findings, it was evident that OA dramatically improved NB degradation by nZVI with all of the aeration conditions. Approximately 100% of NB was degraded by nZVI-OA+N2 system after 60 min of reaction, followed in effectiveness by nZVI-OA+ without aeration (95%) as well as nZVI-OA+O2 (85%). Operation parameters including OA concentration and pH had significant effect on the removal of NB in the nZVI-OA system. 3 mM OA concentration and pH 3.0 were found to be favourable. The Fe2+, total dissolved iron concentrations, pH and ORP values in the nZVI-OA systems had also been monitored. Intermediates and trapping experiments showed that reduction played a fundamental role in the NB degradation by nZVI-OA under N2 or without aeration conditions. The formation of Fe(OA)0 and Fe(OA)22− complexes had lower standard ORP, which could improve the strength of reductant in the systems, and thus resulting in an increased degradation efficiency of NB by nZVI-OA+N2. Under O2 aeration condition, the NB degradation process could be regarded as an oxidation. In the meantime, OA promoted the production of reactive oxidants species by speeding up the oxidation rates of Fe2+ by O2 and H2O2. Summarily, the improved reactivity of nZVI was largely as a result of the strong complexation ability of OA.