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Co-precipitation induces changes to iron and carbon chemistry and spatial distribution at the nanometer scale

Authors :
James J. Dynes
Michael J. Zachman
Johannes Lehmann
Lena F. Kourkoutis
Tom Regier
Angela R. Possinger
Source :
Geochimica et Cosmochimica Acta. 314:1-15
Publication Year :
2021
Publisher :
Elsevier BV, 2021.

Abstract

Association of organic matter (OM) with mineral phases via co-precipitation is expected to be a widespread process in environments with high OM input and frequent mineral dissolution and re-precipitation. In contrast to surface area-limited adsorption processes, co-precipitation may allow for greater carbon (C) accumulation. However, the potential sub-micrometer scale structural and compositional differences that affect the bioavailability of co-precipitated C are largely unknown. In this study, we used a combination of high-resolution analytical electron microscopy and bulk spectroscopy to probe interactions between a mineral phase (ferrihydrite, nominally Fe2O3•0.5H2O) and organic soil-derived water-extractable OM (WEOM). In co-precipitated WEOM-Fe, nanometer-scale scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) revealed increased Fe(II) and less Fe aggregation relative to adsorbed WEOM-Fe. Spatially distinct lower- and higher-energy C regions were detected in both adsorbed and co-precipitated WEOM-Fe. In co-precipitates, lower-energy aromatic and/or substituted aromatic C was spatially associated with reduced Fe(II), but higher-energy oxidized C was enriched at the oxidized Fe(III) interface. Therefore, we show that co-precipitation does not constitute a non-specific physical encapsulation of C that only affects Fe chemistry and spatial distribution, but may cause a bi-directional set of reactions that lead to spatial separation and transformation of both Fe and C forms. In particular, we propose that abiotic redox reactions between Fe and C via substituted aromatic groups (e.g., hydroquinones) play a role in creating distinct co-precipitate composition, with potential implications for its mineralization.

Details

ISSN :
00167037
Volume :
314
Database :
OpenAIRE
Journal :
Geochimica et Cosmochimica Acta
Accession number :
edsair.doi...........9cc3cc13841b592e75461d4dd901bc6a