Riboflavin as a non-quinone redox mediator for enhanced Cr(VI) removal by Shewanella putrefaciens

Abstract

Bioreduction of Cr(VI) has proven to be cost-effective and environmentally friendly, and has attracted widespread interest. However, the slow bioreduction rate of Cr(VI) has been a bottleneck for practical applications. Riboflavin (RF) is a widely distributed micronutrient in the aquatic environment and can function as an endogenous redox mediator to enhance the extracellular electron transfer (EET) rate, thereby promoting the bioreduction rate of Cr(VI). However, research on the role of RF during Cr(VI) removal by dissimilatory iron-reducing bacteria (DIRB) remains scarce. In this study, we investigated the use of RF as a redox mediator to facilitate the bioreduction of Cr(VI) by DIRB. The results showed that RF can serve as a redox mediator for Shewanella putrefaciens (SP) to significantly enhance EET efficiency and accelerate Cr(VI) reduction. Under optimal conditions, 40 mg/L Cr(VI) could be completely removed by SP/RF within 7 d, whereas only 24.10% of Cr(VI) was removed by the SP strain alone. Cu2+, Mn2+, and Cd2+ significantly inhibited Cr(VI) removal; Zn2+, Fe3+, and Pb2+ demonstrated negligible inhibitory effects on Cr(VI) removal, whereas different concentrations of NO3– exhibited different inhibitory effects on Cr(VI) removal. In the method herein, the electron transfer process between the DIRB, RF, and electrodes during the EET process was confirmed. Mechanistic studies indicated that Cr(VI) was partially reduced to Cr(III); subsequently, Cr(III) was immobilized by secretion of the SP strain. Meanwhile, the surface functional groups of SP cell participated in Cr adsorption, reduction and complexation. These findings provide valuable insights into the role of RF in the remediation of Cr(VI)-contaminated water in natural aquatic environments.