Iron(III) reduction in clay minerals and its application to technetium immobilization.

摘要

The iron redox biogeochemistry, especially in clay minerals, is poorly understood. As a result, applicability of this redox reaction or the products to environmental problems such as remediation of metal contaminants is largely unexplored. This dissertation, therefore, seeks to develop a fundamental understanding of bacterial Fe(III) reduction in clay minerals and reactivity of resulting Fe(II) in reducing and immobilizing technetium (99Tc).; The first subproject involves research on iron biogeochemistry in different clay minerals [nontronite (NAu-1, NAu-2), Muloorina illite (Mu-Il) and chlorite (CCa-2)]. The results reveal that the extent of Fe(III) reduction depends on the difference in the crystal structure, layer charge and Fe(III) content. Among different Fe(III) species, Fe(III) in the tetrahedral site is the most reducible and that in the cis-octahedral sites the least. Fe(II) produced from Fe(III) reduction speciates into four chemical environments in clay minerals: aqueous, structural, complexed to amphoteric surface sites and at exchangeable sites. The Fe(II) sorbed both to cell and mineral surfaces exerts a strong inhibitory effect for further Fe(III) reduction. The interplay of these factors can be modeled by considering the reactive surface site concentration, surface saturation, Fe(II) production and its surface inhibition.; The second subproject involves the use of Fe(II), produced from microbial and chemical reduction of Fe(III) in nontronite, for the long-term immobilization of technetium, a significant contaminant in several U.S. DOE sites. The Fe(II) at different chemical environments in reduced nontronite (NAu-2) is found to reduce Tc(VII) into Tc(IV) with the following order of decreasing relative reactivity: Fe(II) at surface complexation sites Fe(II) at exchangeable sites > Fe(II) at structural sites. EXAFS results suggest that ∼70% of Tc(IV) is surface precipitated and the rest forms surface complex with NAu-2. Because the reduced Tc(IV) is associated with clay particle aggregates, it is very recalcitrant to reoxidation in the presence of thermodynamically more favorable electron acceptors/oxidants such as oxygen, nitrate, Fe(III) and Mn(III/IV) oxides. Since the clay aggregates are relatively stable and are less likely perturbed by any transient changes in the aquifer geo-, hydro- and biochemistry, these findings may provide an important step toward the long term immobilization of Tc.