Clay mineral surfaces play an important role for the fate of organic and organic contaminants. In addition to contaminant sorption, clay minerals also contribute to contaminant transformation, for example of organic contaminants such as nitroaromatic compounds or halogenated hydrocarbons, in the presence of structural Fe(II) in clay minerals. Most previous studies have focussed on investigating the redox reactivity of chemically or biologically reduced Fe(II)-bearing clay minerals. We have, however, recently shown that aqueous Fe(II), an abundant reductant in anoxic environments, is also capable of transferring electrons to structural Fe in clay minerals, resulting in the formation of clay mineral Fe(II) and an Fe(III) oxidation product (1, 2). It is unclear how reduction with Fe(II) and the presence of an in situ-formed solid Fe(III) oxidation product will affect the reactivity of clay minerals towards organic contaminants. To assess the reactivity of Fe(II)-containing clay minerals, we used nitroaromatic compounds as reactive probe molecules. Although less than 15% of the clay mineral Fe was present as Fe(II), we found biphasic reduction kinetics for both dithionite and Fe(II)-reduced Fe-rich clay mineral NAu-1 with the same Fe(II)/Fe(III) ratio. We previously assigned the presence of highly reactive Fe(II) sites in Fe-rich smectites to clustering of Fe(II) (3), which should only occur at Fe(II)/Fe(tot) ratios higher than observed here ( &gt;50%). We are currently investigating Fe(II)- and dithionite-reduced clay minerals with a range of Fe(II)/Fe(tot) ratios to establish whether the biphasic reduction kinetics are caused by the same mechanism, i.e. the presence of different structural Fe(II) sites, rather than the presence of the Fe(III) oxidation product. To further evaluate whether the in situ-formed solid Fe(III) oxidation product was contributing to the biphasic reduction kinetics, we are conducting similar transformation experiments with Fe-poor clay mineral SWy-2, which should not exhibit any structural Fe-clustering (4). 1. Neumann, A. et al., Environ Sci Technol, 2013, 47, 6969-6977. 2. Schaefer, M. V. et al., Environ Sci Technol, 2011, 45, 540-545. 3. Vantelon, D. et al, Phys Chem Minerals, 2003, 30, 44-53. 4. Neumann, A. et al. Geochim Cosmochim Acta, 2011, 75, 2336-2355.

Organic contaminant reduction at the clay mineral-water interface

251st National Meeting (2016)

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