Novel imaging techniques, integrated with mineralogical, geochemical and microbiological characterizations to determine the biogeochemical controls on technetium mobility in FRC sediments

[摘要] The objective of this research program was to take a highly multidisciplinary approach to define the biogeochemical factors that control technetium (Tc) mobility in FRC sediments. The aim was to use batch and column studies to probe the biogeochemical conditions that control the mobility of Tc at the FRC. Background sediment samples from Area 2 (pH 6.5, low nitrate, low {sup 99}Tc) and Area 3 (pH 3.5, high nitrate, relatively high {sup 99}Tc) of the FRC were selected (http://www.esd.ornl.gov/nabirfrc). For the batch experiments, sediments were mixed with simulated groundwater, modeled on chemical constituents of FRC waters and supplemented with {sup 99}Tc(VII), both with and without added electron donor (acetate). The solubility of the Tc was monitored, alongside other biogeochemical markers (nitrate, nitrite, Fe(II), sulfate, acetate, pH, Eh) as the 'microcosms' aged. At key points, the microbial communities were also profiled using both cultivation-dependent and molecular techniques, and results correlated with the geochemical conditions in the sediments. The mineral phases present in the sediments were also characterized, and the solid phase associations of the Tc determined using sequential extraction and synchrotron techniques. In addition to the batch sediment experiments, where discrete microbial communities with the potential to reduce and precipitate {sup 99}Tc will be separated in time, we also developed column experiments where biogeochemical processes were spatially separated. Experiments were conducted both with and without amendments proposed to stimulate radionuclide immobilization (e.g. the addition of acetate as an electron donor for metal reduction), and were also planned with and without competing anions at high concentration (e.g. nitrate, with columns containing Area 3 sediments). When the columns had stabilized, as determined by chemical analysis of the effluents, we used a spike of the short-lived gamma emitter {sup 99m}Tc (50-200 MBq; half life 6 hours) and its mobility was monitored using a {gamma}-camera. Incorporation of low concentrations of the long-lived 99Tc gave a tracer that can be followed by scintillation counting, should the metastable form of the radionuclide decay to below detection limits before the end of the experiment (complete immobilization or loss of the Tc from the column). After the Tc was reduced and immobilized, or passed through the system, the columns were dismantled carefully in an anaerobic cabinet and the pore water geochemistry and mineralogy of the columns profiled. Microbial community analysis was determined, again using molecular and culture-dependent techniques. Experimental results were also modeled using an established coupled speciation and transport code, to develop a predictive tool for the mobility of Tc in FRC sediments. From this multidisciplinary approach, we hoped to obtain detailed information on the microorganisms that control the biogeochemical cycling of key elements at the FRC, and we would also be able to determine the key factors that control the mobility of Tc at environmentally relevant concentrations at this site.

[发布日期] 2009-02-03

[学科分类] 核物理和高能物理

[关键词] ACETATES;ANIONS;BINDING ENERGY;CHEMICAL ANALYSIS;ELECTRONS;GEOCHEMISTRY;HALF-LIFE;MICROCOSMS;MICROORGANISMS;MINERALOGY;PROBES;RADIOISOTOPES;RESEARCH PROGRAMS;SCINTILLATION COUNTING;SEDIMENTS;SENSITIVITY;SOLUBILITY;SYNCHROTRONS;TECHNETIUM;VALENCE