Selenium reduction on Ni-Fe bimetallic nanoparticles: effect of process variables on reaction rates

The use of bimetallic nanoparticles (BNPs) for contaminant reduction has gained prominence in recent years. In this work, the effect of initial selenium concentration, BNP loading, dissolved oxygen and pH on selenium reduction using Ni-Fe BNPs was investigated in batch kinetic and equilibrium studies. Equilibrium studies revealed that maximum selenium uptake was obtained at near neutral pH. Reaction kinetics of Se(VI) and Se(IV) reduction on Ni-Fe BNPs was best described Langmuir-Hinshelwood kinetics, with first order at low Se concentrations and zero order at high Se concentrations. For both selenium species, complete removal was obtained within 1 h of contact time. Reaction rates for Se(VI) were higher than that of Se(IV), indicating a two-step reduction process. With increase in Ni-Fe loading (i.e., increase in number of active surface sites), reaction rates also increased linearly, suggestive of an adsorption-controlled rate-limiting step. Deviation from linearity was observed due to the deactivation of the Ni-Fe catalysts. The presence of dissolved oxygen did not significantly affect Se reduction rates. Reduction of selenate, Se(VI), and selenite, Se(IV), on Ni-Fe BNPs occurred primarily due to: (1) adsorption of Se species on the active surface sites; (2) chemical reduction reactions on the surface and (3) sorption of the reduced Se species.