Abstract
The redox activity of natural organic matter (NOM) is crucial for contaminants transformation in soils. Soil micropores (<2.5 nm) have limited accessibility for microorganisms and large NOM molecules; therefore, insoluble organic pollutants and heavy metals trapped in these micropores are usually reached by low molecular weight fractions (LMWF) of NOM. However, the mechanism of spatial electron transfer via electron shuttle of LMWF remains unclear. In this study, we separated low molecular weight fractions (LMWF < 3500 Da and LMWF < 14,000 Da) of Leonardite humic acids (LHA) and measured its acceleration of microbial ferrihydrite reduction. The results show that LMWF, as an electron shuttle, significantly accelerates the reduction in Fe (III), among which 3500-LMWF is the main fraction contributing to the acceleration. Additionally, 3D-EEM shows that quinone content was positively correlated with reduction efficiency, supporting its role as the key functional group. Based on the accelerating experiments, we determined an electron shuttling critical distance of 117.2 nm for LMWF LHA. These findings establish LMWFs as effective natural electron shuttles, providing a theoretical basis for understanding pollutant dynamics in soil micropores.