Interactions between magnetite and humic substances: redox reactions and dissolution processes

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. Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-017-0044-1) contains supplementary material, which is available to authorized users.

Effects of Modified Magnetite Nanoparticles on Bacterial Cells and Enzyme Reactions

“…Dissolution of the solid phase and an increase in dissolved Fe 2+ and/or Fe 3+ as a result of incubation of MNPs with humic substances was demonstrated by Sundman et al in [71]. Authors revealed that the magnetite incubated with native humic substances becomes more oxidized as compared with a control magnetite [71].…”

“…However, remarkable variations in zeta potential are not observed likely due to the lower sensitivity of the electrophoretic light scattering method in comparison with the cellular sensitivity. Sensitive Mossbauer spectroscopy showed minor changes in composition of solid Fe 3 O 4 -HA due to iron ions release [71,72].…”

“…This effect might be explained with a partial hydrolyzing of HA coating in the Bq solutions and, as a result, higher content of oxidized iron (Fe 3+ , surface and/or free) in the suspensions of Fe 3 O 4 /HA, as compared to bare Fe 3 O 4 . Peculiarities of these processes are previously discussed in [71][72][73][74][75][76][77][78], as summarized below:…”

Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

“…Such humic substances have been shown to be able to shuttle electrons to and from both biogenic and chemically synthesized magnetites. 236 Furthermore, the identity of the mineral phases produced by the Fe(III)-reducing strain Shewanella oneidensis MR-1 has been shown to be dependent upon other physico-geochemical factors including geometric orientation of the bottles that the bacteria and minerals are incubated in, with magnetite formation favoured when the ferrihydrite pellet was most dense. 235 In the environment, microbial magnetite has been shown to form at hydrocarboncontaminated field sites and was even used by its magnetic signal to locate the hotspot of microbial activity.…”

Frontiers and advances in environmental soil chemistry: a special issue in honor of Prof. Donald L. Sparks

“…By experimental analysis and density functional theory (DFT) calculations, Kubicki et al simulate interactions between chromate and a ferrihydrate nanoparticle [4]. Then, several articles by Sowers et al [5], Stuckey et al [6], Sundman et al [7], and Zhu et al [8] discuss how iron and manganese (hydr)oxides interact with organic compounds undergoing redox reactions and dissolution. In addition, Cade-Menun et al [9] and Hamilton et al [10] investigate the fate and transport of phosphate as a nutrient in soil systems by using inductively coupled plasma spectroscopy, P-nuclear magnetic resonance, and X-ray absorption spectroscopy.…”