Investigation of reaction mechanism and stability of nanoscale zero-valent iron in bromate reduction

Abstract

Treatment of bromate-contaminated water and wastewater is of particular concern as bromate is known for its potential cancerous health hazards. In this study, the reduction of bromate to bromide was successfully catalyzed by mono and bimetallic catalysts based on NZVI. The catalytic tests carried out in batch and continuous-flow reactors showed complete removal of bromate and production of bromide as a single by-product. The stability of bromate reduction over longer periods of time by NZVI supported catalyst was enhanced by a set of optimization experiments. Iron corrosion and sequential rapid passivation of the catalyst were decelerated due to the variation of Cu loading (0.5% wt), Pd loading (1.5%wt), hydrogen gas supply (40 mL/min) and bromate removal (>98%) was observed for 11 h. The lifetime of NZVI supported bimetallic catalyst was also tested by variations in operational parameters such as HRT, catalyst loading and initial bromate concentrations. The durability test of the catalyst at optimized conditions revealed that complete bromate reduction has been sustained for 24 h with further reactivity loss (from 100 to 20%) over the next 100 h. Several analytical techniques (XRD, TEM/EDX and XPS) were used to identify the reaction mechanism for bromate reduction and efficiency of the catalyst. Both XRD and XPS analysis showed that the reactive NZVI support was oxidized to $Fe^{2+}$ and $Fe^{3+}$ along with Cu0 to CuO, while the oxidation state of Pd0 did not change. Therefore, bromate reduction occurred at the surface of interacting reactive support (NZVI) and metal particle (Cu), while noble metal particle (Pd) acted as hydrogenation catalyst that prolonged the lifetime of the bimetallic catalyst.