Microbial iron-(hydr)oxide reduction: Effects on zinc speciation and interactions with nitrate reduction

Subsurface inorganic contaminants such as heavy metals and nitrate pose a threat to the quality of groundwater and ultimately to human health through the consumption of well water. Iron and nitrate-reducing bacteria are seen as possible solutions to these problems via bioremediation of the contaminants. Iron-reducing bacteria have been shown to promote the formation of new minerals via the production of Fe(II). These new minerals have the potential to incorporate previously sorbed heavy metals into their crystalline structure effectively immobilizing them in the sediments. This process, though, is mitigated by the many components of natural sediments. Fe(II) has also been shown to sorb to the surface of bacteria and react with nitrite, an intermediate of nitrate reduction, to form an Fe(III) coating on the cell surface. We hypothesize that a Fe(III)-hydroxide coating is forming on the cell surfaces that acts as a physical barrier preventing the entry of soluble electron acceptors into the cells. This has implications for the remediation of not only nitrate, but many other contaminates tied to the microbial reduction of electron acceptors. Using batch system experiments, this research examined (1) the factors influencing the immobilization of Zn in natural sediments under environmentally relevant conditions; and (2) the formation of a Fe(III)-oxyhydroxide cell-coating and its effect on the reduction of a number of soluble electron acceptors. It was found that components in sediments that act as complexing agents (clays, NOM) inhibit the immobilization of Zn, but that Fe-(hydr)oxide surface area and identity can promote Zn immobilization. The inhibition of nitrate reduction was found to occur using a number of Fe(III)-(hydr)oxide sources, and the inhibition of a number of environmentally significant soluble electron acceptors occurred after the reaction of Fe(II) and NO2 - on the cell surface. Electron micrographs were taken to support the hypothesized formation of Fe(III) cell coatings.