Degradation of nitrophenols by Fenton and photo-Fenton processes

The degradation of four nitrophenols (NPs), namely 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (DNP), 2,4,6-trinitrophenol (TNP) with Fenton process, solar- and UV-assisted Fenton processes were investigated and compared for their treatment efficiency. The maximum mineralization observed for mono-NP, DNP and TNP was 32%, 25% and 21%, respectively, i.e., only one-third to one-fifth of NPs were mineralized to CO2 by Fenton process. More than 92% of NPs were mineralized by solar- and UV-assisted Fenton processes. The formation of nitrate as a result of mineralization of organically bounded nitrogen was identified during the treatment of NPs studied. Near-stoichiometric accumulation of nitrate was observed during the degradation of NPs studied in both Fenton and photo-Fenton processes. A short-chain aliphatic carboxylic acid like oxalic and acetic acids were produced as final products during degradation of NPs studied by Fenton process. In both photo-Fenton processes, these acids were found as short-lived intermediate during the degradation process and were almost completely eliminated from the aqueous media. Ferrous ion regenerated during photo-Fenton processes accounted for more than 82% of Fe2+ ion for NPs studied. The relative degradation of NPs obeys a pseudo-first-order kinetics, and the treatment efficiency for the NPs follows the sequence: NP > DNP > TNP, irrespective of the processes studied. The reactivity follows Hammettʹs law with regard to the effect of nitro groups on phenolic functionality for Fenton and photo-Fenton processes. The results of the study showed that photo-Fenton processes was an effective treatment processes for NPs under acidic conditions by producing higher mineralization efficiency in a relatively short radiation time compared to Fenton process.