Low-molecular weight organic acids can enhance the microbial reduction of iron oxide nanoparticles and pollutants by improving electrons transfer.

The combined application of dissimilatory iron-reducing bacteria (DIRB) and Fe(III) nanoparticles has garnered widespread interest in the contaminants transformation and removal. The efficiency of this composite system relies on the extracellular electron transfer (EET) process between DIRB and Fe(III) nanoparticles. While modifications to Fe(III) nanoparticles have demonstrated improvements in EET, enhancing DIRB activity also shows potential for further EET enhancement, meriting further investigation. In this study, we demonstrated that the addition of low-molecular organic acids (LMWOAs) (oxalate, pyruvate, malate, citrate, or fumarate) can improve the reduction of Fe2O3 nanoparticles by Geobacter sulfurreducens PCA through three pathways: increasing intracellular electron production, enhancing the reductive activity of extracellular metabolites, and improving the electron-donating capacity of extracellular polymeric substances. The maximum reduction of Fe2O3 nanoparticles reached up to 72 %. Our results further showed that LMWOAs significantly boosted the removal rate and ratio of Cr(VI) and hexachlorobenzene (HCB) by accelerating the EET process. Following the introduction of LMWOAs, the maximum reduction ratio of Cr(VI) reached 98 ± 0.05 % within 24 h, while the degradation efficiency of HCB reached 92 ± 0.06 % within 9 h. Overall, our study provided a precise mechanism of the role of LMWOAs on the EET process and a new strategy for reductive bioremediation of pollutants.