Carbon nanotubes promote Cr(VI) reduction by alginate-immobilized Shewanella oneidensis MR-1

•Impregnating NWCNTs into alginate beads for Shewanella oneidensis MR-1 immobilization.•NWCNTs significantly accelerated electron transfer from cells to Cr(VI).•NWCNTs-contained beads showed higher Cr(VI) reduction rates than non-CNT ones.•Optimal MWCNTs content, temperature and pH for Cr(VI) reduction were evaluated. Bioreduction of hexavalent chromium (Cr(VI)) into trivalent one (Cr(III)) based on microbial immobilization techniques has been recognized as a promising way to remove Cr contaminants from wastewater. However, such a bioreduction process is inefficient due to limited electron transfer through the immobilization matrix. In this study, a modified immobilization process was proposed by impregnating carbon nanotubes (CNTs) into Ca-alginate beads, which were then used to immobilize Shewanella oneidensis MR-1 for enhanced Cr(VI) reduction. Compared with the free cells and the beads without CNTs, the AL/CNT/cell beads showed up to 4 times higher reduction rates, mainly attributed to an enhanced electron transfer by the CNTs. In addition, the dose of CNTs greatly improved the stability of beads, suggesting a high feasibility of the AL/CNT/cell beads for repeated use. The optimized CNT concentration, temperature and pH for Cr(VI) reduction by the AL/CNT/cell beads were 0.5%, 30°C and 6.0–7.0, respectively.