Graphical AbstractVerifiedCombined Effects of Biochar and Riboflavin on the Reduction of Hexavalent Chromium by Shewanella oneidensis MR-1
The biological reduction of Cr(VI) to less hazardous Cr(III) is a promising strategy for remediating Cr(VI)-contaminated sites. Both biochar and riboflavin can act as electron shuttles to accelerate this bioreduction process, yet their combined effects remain poorly understood. Using Shewanella oneidensis MR-1 as a model reducing bacterium, we investigated the joint influence of biochar (average particle size 28.85 μm) and riboflavin at high (1 mmol·L−1) and low concentrations on Cr(VI) bioreduction. Individually, biochar and high-concentration riboflavin enhanced indirect electron transfer, accelerating Cr(VI) removal. However, when combined, the fast-phase reaction rate (rf0) did not significantly improve compared to single amendments. The combined action factor revealed an antagonistic inhibition between biochar and riboflavin. Mechanistically, high-concentration riboflavin saturated biochar's adsorption sites (equilibrium concentration 0.96±0.04 mmol·L−1), hindering biochar's role as an electron conduit. With a bacterial density of 3.4×10^7 cells·mL−1, the inter-bacterial distance (30.87 μm) exceeded biochar's particle size, and the per-cell riboflavin concentration (2.9×10−2 pmol·cell−1) was sufficient for riboflavin to dominate as the primary electron shuttle, while biochar's surface became coated, reducing its efficacy. These findings reveal the complex interplay between biochar and soluble organic matter in Cr(VI) bioreduction, underscoring the need to consider such antagonistic effects when designing bioremediation strategies for multi-component contaminated environments.
