No abstract

The toxicity of Al to Desulfovibrio desulfuricans G20 was assessed over a period of 8 weeks in a modified lactate C medium buffered at four initial pHs (5.0, 6.5, 7.2, and 8.3) and treated with five levels of added Al (0, 0.01, 0.1, 1.0, and 10 mM). At pH 5, cell population densities decreased significantly and any effect of Al was negligible compared to that of the pH. At pHs 6.5 and 7.2, the cell population densities increased by 30-fold during the first few days and then remained stable for soluble-Al concentrations of <5 ؋ 10 ؊5 M. In treatments having total-Al concentrations of >1 mM, soluble-Al concentrations exceeded 5 ؋ 10 ؊5 M and limited cell population growth substantially and proportionally. At pH 8.3, soluble-Al concentrations were below the 5 ؋ 10 ؊5 M toxicity threshold and cell population density increases of 20-to 40-fold were observed. An apparent cell population response to added Al at pH 8.3 was attributed to the presence of large, spirilloidal bacteria (accounting for as much as 80% of the cells at the 10 mM added Al level). Calculations of soluble-Al speciation for the pH 6.5 and 7.2 treatments that showed Al toxicity suggested the possible presence of the Al 13 O 4 (OH) 24 (H 2 O) 12 7؉ "tridecamer" cation and an inverse correlation of the tridecamer concentration and the cell population density. Analysis by 27 Al nuclear magnetic resonance spectroscopy, however, yielded no evidence of this species in freshly prepared samples or those taken 800 days after inoculation. Exclusion of the tridecamer species from the aqueous speciation calculations at pHs 6.5 and 7.2 yielded inverse correlations of the neutral Al(OH) 3 and anionic Al(OH) 4 ؊ monomeric species with cell population density, suggesting that one or both of these ions bear primary responsibility for the toxicity observed.Aluminum, the most abundant metal and the third most abundant element in the earth's crust (37), finds surprisingly little use in biological systems. Although the inherent chemistry of Al certainly is a factor, the chemical conditions existing on the earth at the time the earliest organisms were developing may also have played a role (42, 43). These conditions, which include neutral pH and the absence of oxygen, were such that the solubility of Al was minimal relative to that of divalent metals such as Mg, Ca, and Fe(II). Even if availability were not limiting, however, several other chemical properties of Al strongly militate against its use in cellular metabolism (5). The inherently slow rate of inner-sphere ligand exchange for Al 3ϩ (10 5 to 10 8 times slower than for Mg 2ϩ , Ca 2ϩ , or Fe 2ϩ [41]) makes its complexes relatively inert. Further inertness stems from the lack of any electron transfer chemistry. Moreover, Al 3ϩ forms complexes with greater thermodynamic stability than that of those formed by the divalent metals because of its smaller size and higher valence. Thus, the ability of Al to form strong, relatively inert complexes not only prevents its use by the cyclic feedback-regulated processes in cellu...

Hydrolysis is a ubiquitous abiotic degradation reaction for organic compounds, driven by reaction of the compound with water and other nucleophiles, that generally yields innocuous degradation products. If the rate of hydrolysis for a given compound is known, it can be incorporated into remediation planning, and, if significantly large, can be used to minimize the expense of active-remediation. At the Hanford Site in eastern Washington, two common organic contaminants, chloroform (CF) and carbon tetrachloride (CT), have been measured in groundwater plumes. Because their rates of hydrolysis are very slow, the only data available in the literature were from experiments conducted at temperatures of 70ºC and above. Extrapolation of these rates to groundwater temperatures yielded unacceptable margins of error. This project, therefore, was initiated in FY 2006 to help address these uncertainties and to explore the possible effects of contact with minerals and sediment (i.e., heterogeneous hydrolysis) on the overall rates of hydrolysis.

Between 1955 and 1973, an estimated 920,000 kg of carbon tetrachloride (CT) were discharged to the soil in the 200 West Area of the Hanford Site as part of the plutonium production process. Of this amount, some CT reached the groundwater more than 70 m below the ground surface and formed a plume of 10 km 2 extent. Remediation of the CT plume is underway using a pump-and-treat approach to reduce concentrations of CT and other contaminants such as chloroform (CF) and radionuclides in the aquifer over a projected 25-year operational period. Following the period of pump-and-treat operation, monitored natural attenuation will be the continuing remedy to reach groundwater remediation goals. Multiple mechanisms of natural attenuation can reduce CT and CF concentrations over time during pumpand-treat and the subsequent monitored natural attenuation remedy phases. Of these attenuation mechanisms, hydrolysis reactions are expected to contribute as a degradation process for CT and CF. This report documents the results of long-term experiments to quantify the homogeneous and heterogeneous hydrolysis rates for CT and CF needed to estimate the contribution of hydrolysis to natural attenuation of the contaminant plumes under the subsurface conditions at the Hanford Site (e.g., temperature and sediment mineralogy). Experiments were conducted across a temperature range of 20°-93°C for periods as long as 6 years, and the Arrhenius equation was used to estimate activation energies and calculate half-lives for typical Hanford groundwater conditions (temperature of 16°C and pH of 7.75). A half-life of 630 years for hydrolysis for CT under these conditions was estimated, where CT hydrolysis was unaffected by contact with sterilized, oxidized minerals or Hanford sediment within the sensitivity of the experiments. In contrast to CT, hydrolysis of CF was generally slower and very sensitive to pH due to the presence of both neutral and base-catalyzed hydrolysis pathways. A half-life of 3400 years was estimated for hydrolysis of CF in homogeneous solution at 16°C and pH 7.75. Experiments with CF in suspensions of Hanford sediment or smectite, the dominant clay mineral in Hanford sediment, equilibrated to an initial pH of 7.2, yielded calculated half-lives of 1700 years and 190 years, respectively, at 16°C. Experiments for CF with three other mineral phases at the same pH (muscovite mica, albite feldspar, and kaolinite) showed no change from the homogeneous solution results (i.e., a half-life of 3400 years). The strong influence of Hanford sediment on CF hydrolysis was attributed to the presence of smectite and its ability to adsorb protons, thereby buffering the solution pH at a higher level than would otherwise occur.