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Abstract
Humic acids (HAs) can act as electron shuttles and mediate biogeochemical cycles, thereby influencing the transformation of nutrients and environmental pollutants. HAs commonly complex with metals in the environment, but few studies have focused on how these metals affect the roles of HAs in extracellular electron transfer (EET). In this study, HA-metal (HA-M) complexes (HA-Fe, HA-Cu, and HA-Al) were prepared and characterized. The electron shuttle capacities of HA-M complexes were experimentally evaluated through microbial Fe(III) reduction, biocurrent generation, and microbial azoreduction. The results show that the electron shuttle capacities of HAs were enhanced after complexation with Fe but were weakened when using Cu or Al. Density functional theory calculations were performed to explore the structural geometry of the HA-M complexes and revealed the best binding sites of the HAs to metals and the varied charge transfer rate constants (k). The EET activity of the HA-M complexes were in the order HA-Fe > HA-Cu > HA-Al. These findings have important implications for biogeochemical redox processes given the ubiquitous nature of both HAs and various metals in the environment.
Highlights
Humic acids (HAs), which are ubiquitous in natural environments, are organic macromolecules primarily derived from the degradation of plant and microbialmaterials[1]
Microbial Fe(III) reduction, biocurrent generation and microbial azoreduction experiments were conducted to observe the effects of the HA-M complexes on the electron transfer (EET) of S. oneidensis
The fluorescence intensity as recorded by the stopped-flow spectrophotometer decreased rapidly as the reaction of HA-M complexation proceeded (Fig. 1), which was expected because previous reports showed that HA-M complexation occurs very rapidly, usually within tens of seconds[15]
Summary
Humic acids (HAs), which are ubiquitous in natural environments, are organic macromolecules primarily derived from the degradation of plant and microbialmaterials[1]. Numerous studies have indicated that HAs reduce the toxic effects of metals, hydrocarbons and other chemicals on microorganisms, but can directly catalyze the oxidation of toxic compounds by both anaerobic and aerobic microorganisms[2,3]. This is accomplished through HAs acting as electron shuttles. Shewanella sp., and donate electrons to Fe(III) minerals[4] or other spatially distant electron acceptors that are not directly accessible[5] Their electron shuttling capacity has been mainly attributed to the enriched quinone groups in HAs that interconvert between the three oxidation states of quinone, semiquinone, and hydroquinone via reversible single-electron transfer reactions[6]. Microbial Fe(III) reduction, biocurrent generation and microbial azoreduction experiments were conducted to observe the effects of the HA-M complexes on the EET of S. oneidensis
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