Nano Fe3O4 improved the electron donating capacity of dissolved organic matter during sludge composting

The effect of Fe3O4 nanoparticles (Fe3O4 NPs) on the electron transfer process in aerobic composting systems remains unexplored. In this study, we compared the electron transfer characteristics of DOM in sludge composting without additives (group CK) and with the addition of 50 mg/kg Fe3O4 NPs additive (group Fe). It was demonstrated that the electron transfer capacity (ETC) and electron donating capacity (EDC) of compost-derived DOM increased by 13%–29% and 40%–47%, respectively, with the addition of Fe3O4 NPs during sludge composting. Analyzing the composition and structure of DOM revealed that Fe3O4 NPs promoted the formation of humic acid-like substances and enhanced the aromatic condensation degree of DOM. Correlation analysis indicated that the increase in EDC of DOM was closely associated with the phenolic group in DOM and influenced by quinone groups and the degree of aromatization of DOM. The higher EDC and the structural evolution of DOM in group Fe reduced the bioaccessibility of Cu, Cr, Ni, Zn. This study contributes to a deeper understanding of the redox evolutionary mechanism of DOM in sludge composting and broadens the application of iron oxides additives. [Display omitted] •For the first time, effect of Fe3O4 additive on ETC of DOM was studied.•Fe3O4 improved ETC and EDC of DOM by 13%–29% and 40%–47%.•Fe3O4 facilitated the conversion of protein to humic acid rather than fulvic acid.•Higher EDC of DOM in group Fe was mainly related to the structural evolution of DOM.•Fe3O4 NPs promoted the decrease of bioaccessibility of Cu, Cr, Ni, Zn.