Reactivity of Fe(II) Species Associated with Clay Minerals

Abstract

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.

Gravity-driven transport of three engineered nanomaterials in unsaturated soils and their effects on soil pH and nutrient release

“…Sorption can occur via electrostatic attraction between charged clay surfaces and oppositely-charged ENMs (Zhou et al, 2012) or chemically (for metal oxides or metals with an outer oxide layer) through a dehydration reaction similar to the binding of phosphate or iron oxides to clays (Horst et al, 2010;Hofstetter et al, 2003). Sorption to organic matter (Duester et al, 2011) and organisms (Mudunkotuwa and Grassian, 2010) in soil may take place through similar mechanisms.…”

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“…The role of the 2:1 smectite clays may be significantly different for NDMA degradation, since there is no direct link between NDMA degradation and adsorbed ferrous iron (which there is for TCE). In a similar study, reduction of nitroaromatics was caused both by adsorbed iron on edge surfaces of clays as well as structural iron in the 2:1 smectite clays (Hofstetter et al 1999(Hofstetter et al , 2003 There may be a small amount of H 2 production at magnetite surfaces in the reduced sediment, which could catalyze NDMA degradation rate through reaction with H 2 . Our previous research has shown that limited quantities of H 2 can be produced in the natural subsurface abiotically by release from mafic minerals (Stevens and McKinley 2000).…”

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“…Basing risk criteria on elemental speciation is superior to elemental composition alone because it identifies the unique set of reactions available to each species. For example, suspended zero-valent Fe nanoparticles have been shown to catalyze degradative ability has been shown for structural Fe 2+ (higher oxidation state than zero-valent Fe but different speciation in terms of its complexation environment) domains at clay-edge and -interlayer nano-sites in soil [18]. The Cd 2+ cation in quantum dots exhibits no toxicity to organisms as long as it remains complexed with Se [11].…”