Protein-derived structures determines the redox capacity of humic acids formed during hyperthermophilic composting.

Humic acid (HA) in compost has received widespread attention for its high redox activity, which can mediate the degradation of organic pollution and the passivation of heavy metals in the environment. Hyperthermophilic composting (HTC) can accelerate HA formation. However, few studies have examined whether and how the structures of different organics affect the formation of the HA and HA redox structure at the molecular level in HTC. Detailed molecular information and the redox capacity (electron transfer capacity, ETC) of HA in HTC and thermophilic composting (TC) were characterized using pyrolysis gas chromatography/mass spectrometry and the electrochemical method, respectively. HTC promoted the formation of redox structure, leading to the improvement of the ETC of HA. Aromatics and N-containing compounds were mainly derived from protein components, and the rate at which they were transferred into HA was accelerated in HTC, while the relative abundance of lipids decreased. Partial least squares regression and correlation analysis demonstrated that protein-derived compounds were the key factor determining the HA redox capacity. Finally, partial least squares path modeling suggested that the influence mechanism of protein-derived structures on HA redox capacity might differ in HTC and TC. HTC may promote the relative abundance of N-containing components into the C-skeleton and accelerate the accumulation of the aromatic products, thereby improve the HA redox capacity. These findings provided new insight into how the redox capacity of the HA in compost could be improved and how compost products could be prepared for use in environmental remediation.