Mechanisms of 4-phenylazophenol elimination in micro- and nano-ZVI assisted-Fenton systems

The 4-phenylazophenol (4-PAP), was treated with two different sources of metallic iron (ZVI): commercial micrometric powder (pZVI) and nanoparticles synthetized by the borohydride reduction method (nZVI). 4-PAP degradation was studied both in the absence and in the presence of H2O2 at different pHs. The degradation products of 4-PAP in each treatment were followed by LC-MS and CG-MS. Results showed that, in the absence of H2O2, the azo bond reduction of 4-PAP with the formation of amines was the main mechanism involved for both ZVI sources and nZVI exhibited a faster substrate removal than pZVI. In the presence of H2O2, an additional mechanism involving the oxidation mediated by hydroxyl radicals takes place. For pZVI, the addition of H2O2 produced a complete inhibition of the reduction pathway, being the oxidation the main degradation mechanism. In the case of nZVI, the system behavior showed an important dependence on the working pH. At pH 3.00, oxidative transformation pathways prevailed, whereas at pH 5.00 an almost negligible degradation-mainly driven by 4-PAP reduction-was observed. The assessment of the involved reaction mechanisms under different conditions allows the selection of the most suitable source for a specific treatment.