The impact of the structural transformation mechanism of fulvic acid on redox capacity during composting with different biowastes.

Fulvic acid (FA) derived from composting functions can act as electron shuttle, facilitating and expediting the redox reaction during the composting process. However, limited research has been conducted on the redox capacity and structural transformation of FA during composting with different biowastes. The Fe (II) production quantity of the single S. oneidensis MR-1 (MR-1), MR-1 with FA derived from lignocellulose-rich and lignin-rich composting after 300 h inoculation were up to 2.28, 3.67 and 2.52 mmol/L, indicating the redox capacity of FA in lignocellulose-rich composts was stronger than that in lignin-rich composting. Furthermore, two-dimensional correlation spectroscopy revealed that FA derived from lignocellulose-rich composting exhibited a layer-by-layer structure, characterized by aromatic functional groups and other groups. In contrast, FA obtained from lignin-rich composting displayed an arrangement where the inner aromatic functional groups were obstructed by the methyl group. The structural equation model revealed that the high relative abundance and aromatic functional groups of FA derived from lignocellulose-rich composting process exert a direct influence on the reduction of Fe (III)-citrate, and the methyl groups of FA obtained from lignin-rich composting process also directly linked to the reduction of Fe (III)-citrate. The present study thus posited that the quantity of aromatic functional groups within outer of FA structure during composting constituted a crucial factor influencing its redox capacity. The findings offer novel insights into the transformation mechanism of FA derives from diverse biowastes and its redox characteristics, thereby providing significant guidance for the application of FA in mitigating environment pollutants.