Partitioning of Fe(II) in reduced nontronite (NAu-2) to reactive sites : Reactivity in terms of Tc(VII) reduction

Clay minerals impart important chemical properties to soils, in part, by virtue of changes in the redox state of Fe in their crystal structures. Therefore, measurement of Fe(III)/Fe(II) and partitioning of Fe(II) in different reactive sites in clay minerals (during biological and chemical Fe(III) reduction) is essential to understand their role and their relative reactivity in terms of reduction and immobilization of heavy metal contaminants such as technetium. This study had three objectives: (1) to understand the degree of dissolution of nontronite (Fe-rich smectite) as a result of chemical and biological reduction of Fe(III) in the structure; (2) to quantify partitioning of chemically and biologically produced Fe(II) into different reactive sites in reduced nontronite, including aqueous Fe, ammonium chloride-extractable Fe(II) (mainly from the ion-exchangeable sites, denoted as Fe(II)), sodium acetate-extractable Fe(II) (mainly from the surface complexation sites, denoted as Fe(II)), and structural Fe(II) (denoted as Fe(II)); and (3) to evaluate the reactivity of these Fe(II) species in terms of Tc(VII) reduction. Chemical and biological reduction of Fe(III) in nontronite (NAu-2) was performed, and reduced nontronite samples with different extents of Fe(III) reduction (1.2-71%) were prepared. The extent of reductive dissolution was measured as a function of the extent of Fe(III) reduction. Our results demonstrated that chemically and biologically produced Fe(II) in NAu-2 may be accommodated in the NAu-2 structure if the extent of Fe(III) reduction is small (∼30%, dissolution of nontronite occurred with a corresponding decrease in crystallinity of residual nontronite. The Fe(II) produced was available for partitioning into four species: Fe , Fe(II) Fe(II), and Fe(II). The increase in Fe(II) during the early stages of Fe(III) reduction indicated that the Fe(II) released had the greatest affinity for the surface-complexation sites, but this site had a limited capacity (∼60 μmol of Fe(II)/g of NAu-2). The subsequent increase in Fe(II) indicated that the released Fe(II) partitioned into the exchangeable sites once the amount of Fe at the surface-complexation sites reached half of its maximum site capacity. The fraction of Fe(II) decreased concomitantly, as a result of Fe(II) release from the NAu-2 structure, from 100% when the extent of Fe(IIl) reduction was