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 (Cr⁶⁺) to trivalent chromium (Cr³⁺) 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 Cr⁶⁺-contaminated groundwater. The chemical liquid-phase deposition and co-reduction method was applied to produce BNPs. Cr⁶⁺ removal rate by Fe/Al BNPs was directly proportional to the saturation concentration and reactive sites, which caused a higher Cr⁶⁺ removal rate. The pseudo-first-order kinetic model could be used to describe the Cr⁶⁺ adsorption mechanism by Fe/Al BNPs. Results show that Fe/Al BNPs and Al/Fe BNPs could reduce Cr⁶⁺ to Cr³⁺, and the removal efficiencies for Cr⁶⁺ were 1.47 g/g BNP and 0.07 g/g BNP, respectively. Detection of Cr³⁺ in the aqueous phase was observed during the Cr⁶⁺ removal process. Results from X-ray diffraction (XRD) analysis confirmed that Cr(OH)₃ was present on the surface of BNPs. Main mechanisms caused Cr⁶⁺ removal included reduction, precipitation, and adsorption. The reduction of Cr⁶⁺ produced OH^-, which created alkaline environment and facilitated the formation of chromium hydroxide precipitates [Cr(OH)₃]. Thus, the migration of Cr³⁺ was prevented and the environmental risk was reduced. BNP had a higher activity and stability, and it was applicable for Cr⁶⁺-contaminated site remediation.