Reduction pathway-dependent formation of reactive Fe(II) sites in clay minerals

Katherine A Rothwell*, Martin Pentrak, Linda Pentrak, Joseph Stucki, Anke Neumann*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

2 Citations (Scopus)

Abstract

Structural Fe in clay minerals is an important, potentially renewable source of electron equivalents for contaminant reduction, yet our knowledge on how clay mineral Fe reduction pathways and Fe reduction extent affect clay mineral Fe(II) reactivity is limited. Here, we used a nitroaromatic compound (NAC) as a reactive probe molecule to assess the reactivity of chemically-reduced (dithionite) and Fe(II)-reduced nontronite across a range of reduction extents. We observed biphasic transformation kinetics for all nontronite reduction extents ≤5 % Fe(II)/Fe(total) regardless of the reduction pathway, indicating that two Fe(II) sites of different reactivities form in nontronite at environmentally relevant reduction extents. At even lower reduction extents, Fe(II)-reduced nontronite completely reduced the NAC whereas dithionite-reduced nontronite could not. Our 57Fe Mössbauer spectroscopy, UV-vis spectroscopy, and kinetic modelling results suggest that the highly reactive Fe(II) entities likely comprise di/trioctahedral Fe(II) domains in the nontronite structure regardless of reduction mechanism. However, the second Fe(II) species, of lower reactivity, varies and for Fe(II)-reacted NAu-1 likely comprises Fe(II) associated with an Fe-bearing precipitate formed during electron transfer from aqueous to nontronite Fe. Both our observation of biphasic reduction kinetics and the non-linear relationship of rate constant and clay mineral reduction potential EH have major implications for contaminant fate and remediation.
Original languageEnglish
Pages (from-to)10231–10241
Number of pages11
JournalEnvironmental Science and Technology
Volume57
Issue number28
Early online date7 Jul 2023
DOIs
Publication statusPublished - 18 Jul 2023

Bibliographical note

Funding Information:
The authors thank Dr. Wojciech Mrozik for help with the HPLC analyses, Dr. James Entwistle for help with the Full Static Hamiltonian site analysis of Mössbauer samples, and David Earley and Philip Green for technical help in the lab. This work was supported by the UK Engineering and Physical Sciences Research Council (Ph.D. studentship to K.A.R., 1516946) and the Mineralogical Society of the UK and Ireland (Postgraduate Student Bursary Award to K.A.R.). The authors also thank Chris Gorski and the four anonymous reviewers for their insightful comments that improved the final version of the manuscript.

Publisher Copyright:
© 2023 The Authors. Published by American Chemical Society

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