No abstract

Investigation of the effects of ferrous iron (Fe(II)) on the ability of aged (iron oxide coated) Fe(0) to degrade trichloroethylene (TCE) has revealed that, while neither aged Fe(0) nor Fe(II) separately were able to degrade TCE, approximately 95% of the TCE present was degraded after exposure to a mixture of aged Fe(0) and Fe(II) for 21 days. The rates of TCE degradation increased with an increase in Fe(II) concentration from 0 to 1.6 mM and then reached a relative plateau. Results of Fe(II) "adsorption" studies revealed that the equilibrium pH decreased significantly with an increase in Fe(II) concentration. Proton release during adsorption of Fe(II) to iron oxide coatings was identified as being responsible for promotion of surface dissolution and, concomitantly, enhancement in extent of TCE reduction by aged Fe(0). Results of open circuit potential analysis and Tafel plot measurement showed that the corrosion potential of aged Fe(0) (E(corr)) in the presence of Fe(II) decreased to levels similar to that of Fe(0)/Fe(2+), while significant increase in corrosion current (I(corr)) and decrease in polarization resistance (Rp) were found with an increase in Fe(II) concentration. The fact that the effects of different Fe(II) concentrations on the E(corr), I(corr), and Rp was decoupled from their effects on TCE degradation by aged Fe(0) suggested that the enhancement of TCE degradation in the presence of Fe(II) was attributable to the dissolution of the Fe(III) oxyhydroxide layer coating the aged Fe(0). While the presence of Fe(II) may also lead to transformation of the Fe(III) (oxy)hydroxide coating to more crystalline phases, the rate of reduction of compounds such as TCE by Fe(II) associated with the Fe(III) (oxy)hydroxide coating is substantially slower than that mediated by Fe(0). These findings provide new insight into the molecular-scale interaction of aged Fe(0) and ferrous iron with particular implications for sustaining the reactivity of Fe(0)-mediated degradation of contaminants in iron-bearing environments.

No abstract

No abstract

No abstract

The commonly held assumption that photodependent processes dominate HO production in natural waters has been recently questioned. Here, we present evidence for the unrecognized and light-independent generation of HO in groundwater of an alluvial aquifer adjacent to the Colorado River near Rifle, CO. In situ detection using a sensitive chemiluminescent method suggests HO concentrations ranging from lower than the detection limit (<1 nM) to 54 nM along the vertical profiles obtained at various locations across the aquifer. Our results also suggest dark formation of HO is more likely to occur in transitional redox environments where reduced elements (e.g., reduced metals and NOM) meet oxygen, such as oxic-anoxic interfaces. A simplified kinetic model involving interactions among iron, reduced NOM, and oxygen was able to reproduce roughly many, but not all, of the features in our detected HO profiles, and therefore there are other minor biological and/or chemical controls on HO steady-state concentrations in such aquifer. Because of its transient nature, the widespread presence of HO in groundwater suggests the existence of a balance between HO sources and sinks, which potentially involves a cascade of various biogeochemically important processes that could have significant impacts on metal/nutrient cycling in groundwater-dependent ecosystems, such as wetlands and springs. More importantly, our results demonstrate that reactive oxygen species are not only widespread in oceanic and atmospheric systems but also in the subsurface domain, possibly the least understood component of biogeochemical cycles.

Despite the importance of exogenous electron shuttles (ESs) in extracellular electron transfer (EET), a lack of understanding of the key properties of ESs is a concern given their different influences on EET processes. Here, the ES-mediated EET capacity of Shewanella putrefaciens 200 (SP200) was evaluated by examining the electricity generated in a microbial fuel cell. The results indicated that all the ESs substantially accelerated the current generation compared to only SP200. The current and polarization parameters were linearly correlated with both the standard redox potential (E(ES)(0)) and the electron accepting capacity (EAC) of the ESs. A thermodynamic analysis of the electron transfer from the electron donor to the electrode suggested that the EET from c-type cytochromes (c-Cyts) to ESs is a crucial step causing the differences in EET capacities among various ESs. Based on the derived equations, both E(ES)(0) and EAC can quantitatively determine potential losses (ΔE) that reflect the potential loss of the ES-mediated EET. In situ spectral kinetic analysis of ES reduction by c-Cyts in a living SP200 suspension was first investigated with the E(ES), E(c-Cyt), and ΔE values being calculated. This study can provide a comprehensive understanding of the role of ESs in EET.

No abstract