Carbon nanotubes mediate electron transfer between Shewanella oneidenis MR-1 and nano zero-valent iron to enhance the elimination of Cr(VI) from aqueous media

Nano zero-valent iron (nZVI) has been widely employed for the elimination of heavy metals from wastewater. However, the practical application of nZVI is greatly limited due to its fast aggregation and surface passivation. In this study, a carbon nanotube-supported nZVI composite (nZVI@CNTs) was synthesized and coupled with Shewanella oneidensis MR-1 to enhance the removal of Cr(VI) from aqueous media. The results demonstrated that the coupling of nZVI@CNTs and MR-1 (nZVI@CNTs/MR-1) exhibited superior performance in eliminating Cr(VI) compared to using nZVI@CNTs or MR-1 alone. Electrochemical characterization demonstrated that CNTs acting as electron shuttles improve the electron transfer from MR-1 to nZVI, thus facilitating the passivation layer dissolution. XRD analysis proved that MR-1 can effectively promote the dissolution of the Fe(III) passivation layer with the existence of CNTs. FTIR and XPS analyses confirmed that CNTs contain many functional groups (such as C − C and C = O) related to extracellular electron transfer, thus promoting the electron transfer of MR-1 to nZVI. Three-dimensional excitation-emission matrix (3D-EEM) analysis showed that the extracellular polymeric substances (EPS) generated from MR-1 participated in the Cr(VI) elimination. The mechanism analysis demonstrated the Fe(II) dissolved from the nZVI passivation layer reduces Cr(VI) to Cr(III) and then removes it by co-precipitation, and the EPS secreted by MR-1 also participates in the Cr(VI) adsorption and reduction. Generally, the coupling of nZVI@CNTs and MR-1 successfully addresses the drawbacks of nZVI and demonstrates a remarkable ability to remove Cr(VI).