Reduction and Reoxidation of Nontronite: The Data Reassessed

Heller-Kallai, L.

doi:10.1346/ccmn.1997.0450316

Cited by 11 publications

(2 citation statements)

References 22 publications

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“…Note that in our experiments, the pH values of the suspensions increased considerably during the oxidation of structural or surface-bound Fe(II) of the clays in the buffered suspension (0.2 M Na or KHCO3, Table 1). This effect suggests that processes may have occurred that resulted in the consumption of H + and/or release of OHfrom the modified clay structure (50,52,53).…”

Section: Resultsmentioning

confidence: 99%

Reactivity of Fe(II) Species Associated with Clay Minerals

Hofstetter

Schwarzenbach

Haderlein

2002

Environ. Sci. Technol.

234

189

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Mineral-bound Fe(II) species represent important natural reductants of pollutants in the anaerobic subsurface. At clay minerals, three types of Fe(II) species in fundamentally different chemical environments may be present simultaneously, i.e., structural Fe(II), Fe(II) complexed by surface hydroxyl groups, and Fe(II) bound by ion exchange. We investigated the accessibility and reactivity of these three types of Fe(II) species in suspensions of two different clay minerals containing either ferrous iron-bearing nontronite or iron-free hectorite. Nitroaromatic compounds (NACs) exhibiting different sorption behavior on clays were used to probe the reactivity of the various types of reduced iron species. The clay treatment allowed for a preparation of nontronite and hectorite surfaces with Fe(II) adsorbed by surface hydroxyl groups at the edge surfaces. Furthermore, hectorite suspensions with additional Fe(II) bound to the ion exchange sites at the basal siloxane surfaces were set up. We found that both structural Fe(II) and Fe(II) complexed by surface hydroxyl groups of nontronite reduced the NACs to anilines. An electron balance revealed that more than 10% of the total iron in nontronite was reactive Fe(II). Fe(II) bound by ion exchange did not contribute to the observed reduction of NACs. Reversible adsorption of the NACs at the basal siloxane surface of the clays strongly retarded NAC reduction, even in the presence of high concentrations of Fe(II) bound by ion exchange to the basal siloxane surfaces. Our work shows that in natural systems a fraction of the total Fe(II) present on clays may contribute to the pool of highly reactive Fe(II) species in the subsurface. Furthermore, this work may help to distinguish between Fe(II) species of different reactivity regarding pollutant reduction. Although structural iron in clays represents only a small fraction of the total iron pool in soils and aquifers, reactive Fe(II) species originating from the reduction of structural Fe(III) in clays may contribute significantly to the biogeochemical cycling of electrons in the subsurface since it is not subject to depletion by reductive dissolution.

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Section: Resultsmentioning

confidence: 99%

Reactivity of Fe(II) Species Associated with Clay Minerals

Hofstetter

Schwarzenbach

Haderlein

2002

Environ. Sci. Technol.

234

189

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“…Some studies show a decrease of specific surface area, interlayer distances and swelling upon reduction, and an increase in layer charge and CEC. The CEC increase is not stoichiometrically related to the number of moles of Fe reduced, possibly because of the loss of hydroxyls from the clay (Heller‐Kallai, 1997), and re‐organization of the reduced clay structure (Manceau et al ., 2000). Stucki et al .…”

Section: Introductionmentioning

confidence: 99%

Iron reduction and changes in cation exchange capacity in intermittently waterlogged soil

Favre

Tessier²,

Abdelmoula

et al. 2002

European J Soil Science

108

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Summary The long‐term effects of intermittent flooding on soil properties were studied in field experiments on a Vertisol cropped with rice in Senegal. The dominant clay minerals were smectite and kaolinite. When the soil was reduced after flooding, its cation exchange capacity (CEC) increased to twice that of its oxidized, unflooded state. Mössbauer spectroscopy showed an increase in smectite structural FeII upon reduction, which explained a part of the increase in CEC. The rest of the increase was attributed to the removal of iron oxyhydroxide coatings by reductive dissolution. The reduction and dissolution of oxides under the field conditions were substantiated by analysis of the surfaces of vermiculites buried in the Ap horizons of the cropped and the non‐cropped soils. The redox‐induced CEC changes were found to be reversible after 22 cycles of rice cropping. Nevertheless, the structural Fe and free Fe contents of the rice field Ap horizon were less than those of soil in uncropped neighbouring land, suggesting that inundation induced weathering and eluviation of the minerals. The observed changes in CEC and related redox reactions may substantially modify proton, anion and cation balances in intermittently flooded soils.

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