Reducing and electron-transfer capacities of municipal solid waste compost-derived fulvic acid during structural evolution.

The reducing capacity (RC) and microbially mediated electron-transfer capacity (ETC) of compost-derived fulvic acid (FA) are closely related to its molecular structure and may influence Fe biogeochemical cycling. However, how these redox functions evolve with FA structure during composting remains poorly understood. Using Fe(III) as a prevalent environmental electron acceptor, this study innovatively quantified the reducing capacity(RC) and microbially mediated electron-transfer capacity(ETC) of FA throughout a full-scale municipal solid waste (MSW) composting process and linked these functions to structural evolution characterized by multidimensional spectroscopy. Compost-derived FA exhibited Fe(III)-form- and electron-transfer-role-dependent redox behavior. Its inorganic reducing capacity decreased significantly during composting, whereas its organic reducing capacity remained relatively stable and was consistently higher (mean IRC: 1.89 mmol e- g-1C, mean ORC: 3.21 mmol e- g-1C). In contrast, ETCs mediated by Shewanella oneidensis MR-1 and SP200 showed no significant temporal trends, with FA facilitating electron transfer more effectively to inorganic than to organically complexed Fe(III) in both bacterial systems. Multidimensional spectroscopic analyses revealed that variations in RC and ETC were associated with the evolution of fluorescent components, functional groups, and hydrogen-containing structures, while the structural correlates of ETC differed between MR-1 and SP200. These findings establish a structure-redox linkage for compost-derived FA under full-scale composting conditions and provide a functional perspective beyond conventional compositional characterization, supporting the evaluation of its potential roles in Fe cycling and other environmentally relevant redox processes.