Removal of hexavalent chromium from groundwater using Shewanella oneidensis MR-1 modified micron-sized zero-valent iron to enhance the Kirkendall effect

This study introduces a novel modification method for micron-sized zero-valent iron, utilizing Shewanella oneidensis MR-1 to simultaneously colonize its surface, disrupt the passivation layer, and alter the crystal structure of the core. This process yields a composite that retains its reducing activity even in high concentrations of pollutants (500 ppm Cr(VI)) and under neutral to slightly acidic conditions. Our findings indicate that the modified system induces the Kirkendall effect in zero-valent iron, leading to the formation of amorphous zero-valent iron with a non-spherical core structure, in contrast to conventional direct mixing approaches. The modified system achieves complete reduction of 100 ppm Cr(VI) to Cr(III) within 8 h. Notably, within the first 4 h of reaction, the reduction rate is approximately three times higher than that of the conventional direct mixing system. XPS, XRD, SEM, EEM and TEM analyses reveal that the surface passivation layer of the biochemically modified zero-valent iron is disrupted, leading to changes in its crystal structure, the formation of hollow spherical structures, and the development of a biological protective film on its surface. Interestingly, electrochemical and ETSA analyses indicate that the electron transfer rate of the biochemically modified composite is significantly enhanced. This study provides fundamental insights into the removal of Cr(VI) from groundwater using biochemically modified micron-sized zero-valent iron (mZVI).