Combined Effects of Biochar and Riboflavin on the Reduction of Hexavalent Chromium by Shewanella oneidensis MR-1

• • Biochar alone (28.85 μm mean particle size) and high-concentration riboflavin (1 mmol·L−1) each accelerate Cr(VI) bioreduction by S. oneidensis MR-1, enhancing indirect electron transfer; however, their combination does not significantly increase the fast-phase reaction rate (rf0) compared to single amendments, indicating antagonism. • • The combined action factor quantitatively demonstrates antagonistic inhibition between biochar and riboflavin in the Cr(VI) bioreduction system, meaning that co-application may be counterproductive for remediation efficiency. • • At high riboflavin concentration (1 mmol·L−1), biochar adsorption reaches saturation (equilibrium concentration 0.96±0.04 mmol·L−1), blocking biochar's active sites and suppressing its electron-shuttle function, which is a key mechanistic insight for process optimization. • • With bacterial density at 3.4×10^7 cells·mL−1, the inter-bacterial distance (30.87 μm) exceeds biochar's particle size, and the per-cell riboflavin concentration (2.9×10−2 pmol·cell−1) is sufficient for riboflavin to dominate as the primary electron shuttle, explaining why biochar's contribution becomes negligible in the presence of high riboflavin.