Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter

Abstract

Despite substantial experimental evidence of electron transfer, atom exchange, and mineralogical transformation during the reaction of Fe(II) aq with synthetic Fe(III) minerals, these processes are rarely investigated in natural soils. Here, we used an enriched Fe isotope approach and Mossbauer spectroscopy to evaluate how soil organic matter (OM) influences Fe(II)/Fe(III) electron transfer and atom exchange in surface soils collected from Luquillo and Calhoun Experimental Forests and how this reaction might affect Fe mineral composition. Following the reaction of 57 Fe-enriched Fe(II) aq with soils for 33 days, Mossbauer spectra demonstrated marked electron transfer between sorbed Fe(II) and the underlying Fe(III) oxides in soils.Comparing the untreated and OM-removed soils indicates that soil OM largely attenuated Fe(II)/Fe(III) electron transfer in goethite, whereas electron transfer to ferrihydrite was unaffected. Soil OM also reduced the extent of Fe atom exchange. Following reaction with Fe(II) aq for 33 days, no measurable mineralogical changes were found for the Calhoun soils enriched with high-crystallinity goethite, while Fe(II) did drive an increase in Fe oxide crystallinity in OM-removed LCZO soils having low-crystallinity ferrihydrite and goethite. However, the presence of soil OM largely inhibited Fe(II)-catalyzed increases in Fe mineral crystallinity in the LCZO soil. Fe atom exchange appears to be commonplace in soils exposed to anoxic conditions, but its resulting Fe(II)-induced recrystallization and mineral transformation depend strongly on soil OM content and the existing soil Fe phases.

Organic Matter Counteracts the Enhancement of Cr(III) Extractability during the Fe(II)-Catalyzed Ferrihydrite Transformation: A Nanoscale- and Molecular-Level Investigation

“…In addition, ubiquitous dissolved organic matter (DOM) can be involved in Fe­(III) and Cr­(III) coprecipitation to form ternary coprecipitates [Cr­(III)-containing organo-ferrihydrite coprecipitates (OFCs)]. ,− The incorporated organic matter (OM) can inhibit the Fe­(II)-catalyzed ferrihydrite transformation through lowering electron transfer or hindering the crystal growth of secondary Fe (oxyhydr)­oxides, depending on the Fe/C molar ratios and OM types. ,,, Accordingly, the re-distribution of HMs caused by ferrihydrite transformation is affected by the incorporated OM. ,,, Moreover, the OM fraction of OFC could also be modified by the ferrihydrite transformation, ,, which may further complicate the behavior of HMs during the transformation because OM fraction could bind HMs or mask the HMs’ binding sites on the Fe (oxyhydr)­oxides. ,− In paddy soils, the returned crop straw is one of the most important sources of DOM in soils, , which can contribute to ∼40% soil DOM . Compared to humic acid or fulvic acid from soils, the crop straw-derived DOM without humification displays a lower aromaticity, which mainly consists of carbohydrates, amino acids, and aliphatic acids. , Our previous studies also suggested that the rice or rape straw-derived DOM could be involved in the coprecipitation of Fe­(III) and Cr­(III). , However, the effect of crop straw-derived DOM on the ferrihydrite transformation under redox conditions and subsequent Cr­(III) behavior remains unclear.…”

Unrecognized Role of Organic Acid in Natural Attenuation of Pollutants by Mackinawite (FeS): The Significance of Carbon-Center Free Radicals

“…Under ambient conditions, various environmental factors may profoundly influence the oxidation activity of FeS-mediated reaction systems . For instance, research by Wang et al showcased that a simple pH adjustment can regulate both the oxidation process of FeS and the associated pollutant transformation mechanism. , The presence of environmental components, such as organic matter, often disturb the mineral-associated electron transfer reactions. − Compared with the impact of solution pH, the interaction between FeS and organic matter concerning pollutant transformation remains less clear. Recognizing the primary determinants, especially organic matter, and the corresponding influencing mechanism is vital for a comprehensive grasp of the biogeochemicals of pollutants in natural processes.…”

Biotic and Abiotic Regulations of Carbon Fixation into Lacustrine Sediments with Different Nutrient Levels

“…However, the balance between crystalline and morphous Fe fractions could be broken, and the precipitation and oxidation of comparatively more crystalline FeO occurred along with the C retention under oxic and even reducing conditions . Higher concentrations of crystalline FeO than free and amorphous phases were also observed in soils and peatlands, suggesting the supply potential and subsequent formation of more crystalline oxides from the amorphous phase. , Higher TOC FeO concentrations were detected in the northern region (Figure l), indicating stronger adsorption of OC to FeO under anoxic condition. In addition, a lower molar ratio of C/Fe was found, indicating that the adsorbed OM had higher chemical stability together with lower C/Fe ratios.…”

“…Generally, the cycle of Fe and C in aquatic ecosystems is a dynamic and complex redox-driven process controlled by O 2 condition . Many previous studies have considered the mechanisms by which association of OM with FeO can stabilize OM against microbial degradation . However, coupling mechanisms and turnover processes between Fe and C in the sediments need further exploration in response to the changed O 2 diffusion level in different nutrient scenarios in lakes.…”