Selective removal of nitroaromatic compounds from wastewater in an integrated zero valent iron (ZVI) reduction and ZVI/H2O2 oxidation process

Abstract

In this study, an integrated system comprised of zero-valent iron (ZVI) reduction and ZVI-based Fenton oxidation processes (ZVI-ZVI/H₂O₂) was applied for the selective removal of nitroaromatic compounds (NACs) from 2,4-dinitroanisole (DNAN) producing wastewater. For the ZVI reduction process, at a hydraulic retention time (HRT) of 6 h and neutral pH of 7.2, removal efficiencies of 2,4-dinitroanisole (DNAN), 2,4-dinitrophenol (DNP) and 2,4-dinitrochlorobenzene (DNCB) were as high as 81.3 ± 3.6%, 80.6 ± 1.8% and 90.9 ± 3.5%, respectively, demonstrating the excellent performance of ZVI. For the ZVI/H₂O₂ oxidation process, the optimal pH and H₂O₂ dosage were found to be 3.0 and 100 mmol L⁻¹, respectively. Under these optimal conditions, NACs and their degradation intermediates could be removed selectively and effectively in the coupled ZVI reduction and ZVI/H₂O₂ oxidation process, as was indicated by the low UV₂₅₄ value of 0.104 ± 0.003 and the low TOC removal efficiency of 32.4 ± 0.7% in the effluent. Ferrous ions could be generated in situ through the corrosion of the metal iron in both the ZVI reduction process and the ZVI/H₂O₂ oxidation process, giving rise to a potent Fenton-type reaction. In addition, the enhanced Fenton reaction with the aid of reaction between Fe⁰ and Fe³⁺ was probably due to the presence of Fe⁰ in the ZVI/H₂O₂ oxidation process, which promoted the utilization efficiency of the Fenton catalyst, i.e., Fe²⁺. Compared to the sequential ZVI reduction and homogeneous Fenton oxidation process (ZVI-Fe²⁺/H₂O₂), the low consumption of iron shavings, the reduced H₂O₂ consumption and the low yield of ferric sludge made the integrated ZVI-ZVI/H₂O₂ process promising for the treatment of NAC containing wastewater.