Abstract
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.
Highlights
Iron (Fe) is a ubiquitous, redox-active element that constitutes a significant fraction of the Earth’s crust and plays an important role in controlling the fate of numerous nutrients and toxic elements [1]
The biogenic magnetite, 7 nm magnetite, and 13 nm magnetite displayed similar sizes and morphologies as the particles described in the used protocols [29, 33, 34], whereas the 500 nm magnetite was larger than the particles reported by [28]
The Fe(II) release was most pronounced for the biogenic and 13 nm magnetite and the drop of ca. 500–800 μM Fe2+ and concurrent incorporation into the solid phase resulted in an apparent increase in solid-phase Fe(II)/Fe(III) ratio from 0.40 ± 0.01 to 0.43 ± 0.011
Summary
Iron (Fe) is a ubiquitous, redox-active element that constitutes a significant fraction of the Earth’s crust and plays an important role in controlling the fate of numerous nutrients and toxic elements [1]. HS are redox-active [4, 5] with multiple redox-active functional groups including quinone and phenolic groups [6,7,8,9,10] and can donate electrons to a number of dissolved and solid Fe(III) compounds [2, 11,12,13,14,15] resulting in the reduction and subsequent dissolution of minerals. Dissolved and solid-phase HS can serve as electron acceptors or donors for microorganisms [4, 16], resulting in reduced HS whose prevalence vary with the microbial community, but is expected to be abundant in environments such as reduced sediments and water logged soils. Whereas HS have been shown to reduce several Fe(III) minerals, similar electron transfer reactions have not been demonstrated between humic
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