Effects of Bioleaching Pretreatment on Humus Fractions and Electron Transfer Capacity During Aerobic Composting of Dewatered Sludge

Compost-derived humic acids (HAs) and fulvic acids (FAs) play an essential role in enhancing soil microbial diversity and activity by facilitating metabolic processes through electron transfer. Herein, the effect of bioleaching dewatered sludge (BDS) in comparison with filter press dewatered sludge (FDS) on the electron transfer capacity (ETC) of humic substances during composting was investigated as a novel attempt. A variety of characterization methods including UV-Vis, FTIR, 3D-EEM, and electrochemical measurements, were used to explore the change in humic substances during composting. The results indicated that bioleaching treatment significantly influenced the organic matter composition and hindered the accumulation of redox-active functional groups during composting. Notably, the ETC of HA increased by 24.07% in the FDS group but declined by 40.62% in the BDS group. This divergence stemmed from the organic matter loss during bioleaching, leading to reduced quinone-like and tryptophan-like substances associated with electron transfer in HA during composting. Furthermore, BDS showed lower pH, water content, and organic matter, but higher concentrations of ammonium nitrogen (NH4+-N) and ammonia nitrogen NH3−-N, all of which potentially influenced humification efficiency. These findings not only clarify the electron-transfer dynamics of humic fractions but also highlight the importance of optimizing sludge pretreatment for improved composting performance and resource recovery.