Effective catalytic hydrodechlorination of o-, p- and m-chloronitrobenzene over Ni/Fe nanoparticles: Effects of experimental parameter and molecule structure on the reduction kinetics and mechanisms

Abstract

Zero-valent iron (ZVI) reductive dechlorination is a promising approach for the abatement of chlorinated organic pollutants in the environment. In this study, ZVI and Ni/Fe bimetal nanoparticles were synthesized and applied for the treatment of chloronitrobenzenes (CNBs) in water. The pre- and post-reaction nanoparticles samples were characterized by SEM and XRD respectively. Here we show that ZVI nanoparticles can effectively perform the selective reduction of p-CNB to p-chloroaniline (p-CAN) with a selectivity of ca. 100% in 10min without dechlorination throughout the research. Catalytic reduction of p-CNB over Ni/Fe nanoparticles could be divided into two sequential processes: the first is the conversion of p-CNB to p-CAN, and the second is the reductive hydrodechlorination of p-CAN with aniline formation. Direct catalytic hydrodechlorination of p-CNB as a side route was also observed. Results indicate that ZVI initiates the electron transfer process for the reduction of p-CNB to p-CAN, whereas Ni catalyst is in favor of hydrogen transfer as the dominant pathway for the dechlorination reactions. The complete reduction mechanisms of p-CNB over ZVI and Ni/Fe nanoparticles were discussed, and the kinetics of dechlorination of CNBs were found to be pseudo-first order. The effects of different experimental parameters, such as solution pH, dosage of Ni/Fe nanoparticles, initial concentration of p-CNB and the Ni loading on the catalytic reduction of p-CNB were carefully investigated. Under the several reaction conditions tested, a complete dechlorination could be attained in 2h. The dechlorination reactivity for mono-CNBs decreases in the order: p-CNB>o-CNB>m-CNB. The energy gap between HOMO and LUMO is firstly indicated as a descriptor to evaluate the dechlorination reactivity of mono-CNBs, which may provide a new insight in predicting the dechlorination reactivity for other organic chlorides.