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Geochemical Decoupling of Iron and Zinc during Transformation of Zn-Bearing Ferrihydrite in Reducing Sediments
Journal article   Open access   Peer reviewed

Geochemical Decoupling of Iron and Zinc during Transformation of Zn-Bearing Ferrihydrite in Reducing Sediments

Pierre Lefebvre, Andrew R. C. Grigg and Ruben Kretzschmar
Environmental science & technology, Vol.58(45), pp.20224-20234
03/11/2024
DOI: https://doi.org/10.1021/acs.est.4c09261
PMID: 39491537
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Abstract

mineral transformation Mössbauer spectroscopy X-ray absorption spectroscopy environmental speciation green rust zinc sulfide zinc carbonate
The transformation of the mineral ferrihydrite in reducing environments, and its impact on the mobility of incorporated trace metals, has been investigated in model laboratory studies, but studies using complex soil or sediment matrices are lacking. Here, we studied the transformation of zinc (Zn)-bearing ferrihydrite labeled with 57Fe and mixed with natural sediments, incubated in reducing conditions for up to six months. We tracked the evolution of Fe and Zn speciation with 57 Fe Mossbauer spectroscopy and with bulk and micro-X-ray absorption spectroscopy. We show that Fe was readily reduced and incorporated into a poorly crystalline mixed-valence Fe(II)-Fe(III) phase resembling green rust. In parallel, Zn was released in the surrounding porewater and scavenged by precipitation with available ligands, particularly as zinc sulfide (ZnS) or Zn-carbonates. Early in the mineral transformation process, the chemical behavior of Fe was decoupled from Zn, suppressing the impact of Zn on the rates and products of the ferrihydrite transformation. Our results underline the discrepancy between model experiments and complex field-like conditions and highlight the importance of sediment and soil geochemistry and ligand competition on the fate of divalent metal contaminants in the environment.

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