Bimetallic nickel-iron nanoparticles for groundwater decontamination: effect of groundwater constituents on surface deactivation.

The incorporation of catalytic metals on iron nanoparticles to form bimetallic nanoparticles (BNPs) generates a class of highly reactive materials for degrading chlorinated hydrocarbons (e.g., trichloroethylene, TCE) in groundwater. Successful implementation of BNPs to groundwater decontamination relies critically on the stability of surface reactive sites of BNPs in groundwater matrices. This study investigated the effect of common groundwater solutes on TCE reduction with Ni-Fe (with Ni at 2 wt.%) bimetallic nanoparticles (herein denoted as Ni-Fe BNPs). Batch experiments involving pre-exposing the nanoparticles to various groundwater solutions for 24 h followed by reactions with TCE solutions were conducted. The results suggest that the deactivation behavior of Ni-Fe BNPs differs significantly from that of the well-studied Pd-Fe BNPs. Specifically, Ni-Fe BNPs were chemically stable in pure water. Mild reduction in TCE reaction rates were observed for Ni-Fe BNPs pre-exposed to chloride (Cl(-)), bicarbonate (HCO3(-)), sulfite (SO3(2-)) and humic acid solutions. Nitrate (NO3(-)), sulfate (SO4(2-)) and phosphate (HPO4(2-)) may cause moderate to severe deactivation at elevated concentrations (>1 mM). Product analysis and surface chemistry investigations using high-resolution X-ray photoelectron spectroscopy (HR-XPS) reveal that NO3(-) decreased particle reactivity mainly due to progressive formation of passivating oxides, whereas SO4(2-) and phosphate elicited rapid deactivation as a result of specific poisoning of the surface nickel sites. At similar levels, phosphate is the most potent deactivation agent among the solutes examined in this study. While our findings point out the desirable quality of Ni-Fe nanoparticles, particularly their greater electrochemical stability compared to Pd-Fe BNPs, its susceptibility to chemical poisoning at high levels of complexing ligands is also noted. Groundwater chemistry is therefore an important factor to consider when choosing appropriate material(s) for decontaminating the complex environmental media.