Application of iron/aluminum bimetallic nanoparticle system for chromium-contaminated groundwater remediation.

When the nanoscale zero valent iron (nZVI) is used for the reduction of hexavalent chromium (Cr) to trivalent chromium (Cr) in groundwater, the reduction efficiency is decreased due to the passivation of reactive sites by precipitation. The bimetallic nanoparticle (BNP) can be created with the addition of the second metal to achieve a higher activity and reduce the occurrence of the ferrous/ferric hydroxide precipitation. In this study, the iron-coated aluminum (Fe/Al) BNP and aluminum-coated iron (Al/Fe) BNP systems were designed for remediating Cr-contaminated groundwater. The chemical liquid-phase deposition and co-reduction method was applied to produce BNPs. Cr removal rate by Fe/Al BNPs was directly proportional to the saturation concentration and reactive sites, which caused a higher Cr removal rate. The pseudo-first-order kinetic model could be used to describe the Cr adsorption mechanism by Fe/Al BNPs. Results show that Fe/Al BNPs and Al/Fe BNPs could reduce Cr to Cr, and the removal efficiencies for Cr were 1.47 g/g BNP and 0.07 g/g BNP, respectively. Detection of Cr in the aqueous phase was observed during the Cr removal process. Results from X-ray diffraction (XRD) analysis confirmed that Cr(OH) was present on the surface of BNPs. Main mechanisms caused Cr removal included reduction, precipitation, and adsorption. The reduction of Cr produced OH, which created alkaline environment and facilitated the formation of chromium hydroxide precipitates [Cr(OH)]. Thus, the migration of Cr was prevented and the environmental risk was reduced. BNP had a higher activity and stability, and it was applicable for Cr-contaminated site remediation.