Catalytic properties of transition metals modified nanoscale zero-valent iron for simultaneous removal of 4-chlorophenol and Cr(VI): Efficacy, descriptor and reductive mechanisms

Abstract

Since chlorophenols (CPs) and Cr(VI) are two types of common pollutants in the environment, developing an effective approach to remove these contaminants has important benefits for public health. However, few efforts have been made so far. In this study, we prepared nanoscale zero-valent iron (nZVI) and a series of bimetallic nanoparticles (transition-metal modified nZVI) to investigate their catalytic properties for the simultaneous removal of 4-chlorophenol (4-CP) and Cr(VI). While nZVI enabled a fast removal of Cr(VI), it had a poor dechlorination ability. However, effective simultaneous removal of 4-CP and Cr(VI) was achieved with the transition metal modified nZVI, especially in the Pd/Fe bimetallic system. The enhanced catalytic activity of transition metal modified nZVI was primarily attributed to the formations of numerous nano-galvanic cells and atomic hydrogen species that facilitated electron transfer in the reaction system and played a key role in triggering the C-Cl bond cleavage, respectively. According to the dechlorination ability, the transition-metal catalysts examined in this study can be divided into three groups in descending order: the first being Pd and Ni, the second including Cu and Pt, while the last consisting of Au and Ag. The catalytic hydrodechlorination activity of bimetals can be well described by the volcano curve and rationally explained by the hydrogen adsorption energies on the metals, and was severely impaired by increasing Cr(VI) concentrations. Characterization results validated the formations of Fe(III)-Cr(III) hydroxide/oxyhydroxide on the bimetals surface after reacting with 4-CP and Cr(VI). This work provides the first insight into the catalytic properties of transition-metal modified nZVI for the effective removal of combined pollutants.

Keywords: Catalytic hydrodechlorination; Combined pollutants; Hydrogen transfer; Nano-galvanic cell; Nanoscale zero-valent iron (nZVI); Transition-metal modification.