Enhanced bio-reduction of Cr(VI) using Shewanella putrefaciens CN32 mediated by Fe(III) minerals and riboflavin synergistically.

Iron minerals and the coupling of electron shuttle media can effectively overcome the problem of the insolubility of iron minerals and the higher cross-medium resistance consequently to enhance the bio-reduction rate of Cr(VI) by dissimilatory metal-reducing bacteria (DMRB). This study explored the potential synergistic enhancement of Cr(VI) bio-reduction by Shewanella putrefaciens CN32 in combination with three iron minerals (ferrihydrite, goethite and hematite) and riboflavin (RF). The addition of RF accelerates the transfer of electrons from bacterial cells to Fe minerals, which in turn promotes the production of large amounts of Fe(II). The results indicated that compared to the control group, the Cr(VI) reduction rates in the CN32/RF/hematite, goethite, ferrihydrite systems increased to 93.03%, 91.07%, and 86.83%, hematite was capable of generating 2.24 mM Fe(II) due to its stable structure and efficient synergy with riboflavin. Enhancement factor(EF) was used to quantify the synergistic effect of RF and iron minerals on the bio-reduction of Cr(VI). At all three reaction times, the FEF (KCN32+RF+Fe/KCN32) of three Fe(III) minerals were all greater than 1. XPS analysis revealed that the primary reduction products of Cr(VI) were identified as Cr(CH3C(O)CHC(O)CH3)3, Cr2O3 and Fe(II)-Cr(III) hydroxide, were predominantly deposited on both bacterial and mineral surfaces, thereby influencing their synergistic interactions. This study unveiled the dynamic synergistic mechanism changes of Cr(VI) reduction in different iron minerals environment,which offers new ideas for the remediation of Cr(VI) pollution.