Humic substances (HS) are redox-active compounds that are ubiquitous in the environment and can serve as electron shuttles during microbial Fe(III) reduction thus reducing a variety of Fe(III) minerals. However, not much is known about redox reactions between HS and the mixed-valent mineral magnetite (Fe3O4) that can potentially lead to changes in Fe(II)/Fe(III) stoichiometry and even dissolve the magnetite. To address this knowledge gap, we incubated non-reduced (native) and reduced HS with four types of magnetite that varied in particle size and solid-phase Fe(II)/Fe(III) stoichiometry. We followed dissolved and solid-phase Fe(II) and Fe(III) concentrations over time to quantify redox reactions between HS and magnetite. Magnetite redox reactions and dissolution processes with HS varied depending on the initial magnetite and HS properties. The interaction between biogenic magnetite and reduced HS resulted in dissolution of the solid magnetite mineral, as well as an overall reduction of the magnetite. In contrast, a slight oxidation and no dissolution was observed when native and reduced HS interacted with 500 nm magnetite. This variability in the solubility and electron accepting and donating capacity of the different types of magnetite is likely an effect of differences in their reduction potential that is correlated to the magnetite Fe(II)/Fe(III) stoichiometry, particle size, and crystallinity. Our study suggests that redox-active HS play an important role for Fe redox speciation within minerals such as magnetite and thereby influence the reactivity of these Fe minerals and their role in biogeochemical Fe cycling. Furthermore, such processes are also likely to have an effect on the fate of other elements bound to the surface of Fe minerals.

中文翻译：

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磁铁矿与腐殖质之间的相互作用：氧化还原反应和溶解过程。

腐殖质（HS）是一种在环境中无处不在的氧化还原活性化合物，可在微生物还原Fe（III）期间充当电子飞梭，从而减少了各种Fe（III）矿物。但是，关于HS与混合价矿物磁铁矿（Fe3O4）之间的氧化还原反应知之甚少，这种氧化还原反应可能会导致Fe（II）/ Fe（III）化学计量的变化甚至溶解磁铁矿。为了解决这一知识空白，我们将非还原（天然）和还原HS与四种类型的磁铁矿进行了温育，这些磁铁矿的粒径和固相Fe（II）/ Fe（III）的化学计量比各不相同。我们随时间跟踪溶解和固相的Fe（II）和Fe（III）浓度，以定量HS和磁铁矿之间的氧化还原反应。HS的磁铁矿氧化还原反应和溶解过程取决于初始磁铁矿和HS特性。生物磁铁矿和还原的HS之间的相互作用导致固体磁铁矿矿物的溶解以及磁铁矿的总体还原。相反，当天然和还原的HS与500 nm磁铁矿相互作用时，观察到轻微的氧化并且没有溶解。不同类型磁铁矿的溶解度，电子接受能力和供电能力的这种变化可能是其还原电势差异的影响，这与磁铁矿的Fe（II）/ Fe（III）化学计量，粒径和结晶度有关。我们的研究表明，具有氧化还原活性的HS对于矿物质（如磁铁矿）中的铁氧化还原形态起着重要作用，从而影响这些铁矿物质的反应性及其在生物地球化学铁循环中的作用。此外，