Raymond E. Wildung

Technetium reduction in sediments of a shallow aquifer exhibiting dissimilatory iron reduction potential

Pertechnetate ion [Tc(VII)O(4) (-)] reduction rate was determined in core samples from a shallow sandy aquifer located on the US Atlantic Coastal Plain. The aquifer is generally low in dissolved O(2) (<1 mg L(-1)) and composed of weakly indurated late Pleistocene sediments differing markedly in physicochemical properties. Thermodynamic calculations, X-ray absorption spectroscopy and statistical analyses were used to establish the dominant reduction mechanisms, constraints on Tc solubility, and the oxidation state, and speciation of sediment reduction products. The extent of Tc(VII) reduction differed markedly between sediments (ranging from 0% to 100% after 10 days of equilibration), with low solubility Tc(IV) hydrous oxide the major solid phase reduction product. The dominant electron donor in the sediments proved to be (0.5 M HCl extractable) Fe(II). Sediment Fe(II)/Tc(VII) concentrations >4.3 were generally sufficient for complete reduction of Tc(VII) added [1-2.5 micromol (dry wt. sediment) g(-1)]. At these Fe(II) concentrations, the Tc (VII) reduction rate exceeded that observed previously for Fe(II)-mediated reduction on isolated solids of geologic or biogenic origin, suggesting that sediment Fe(II) was either more reactive and/or that electron shuttles played a role in sediment Tc(VII) reduction processes. In buried peats, Fe(II) in excess did not result in complete removal of Tc from solution, perhaps because organic complexation of Tc(IV) limited formation of the Tc(IV) hydrous oxide. In some sands exhibiting Fe(II)/Tc(VII) concentrations <1.1, there was presumptive evidence for direct enzymatic reduction of Tc(VII). Addition of organic electron donors (acetate, lactate) resulted in microbial reduction of (up to 35%) Fe(III) and corresponding increases in extractable Fe(II) in sands that exhibited lowest initial Tc(VII) reduction and highest hydraulic conductivities, suggesting that accelerated microbial reduction of Fe(III) could offer a viable means of attenuating mobile Tc(VII) in this type of sediment system.

Comparative metabolic behavior and interrelationships of Tc and S in soybean plants

The comparative behavior of sulfur (S) and technetium (Tc) in soybean seedlings shows gross subcellular distributions to be similar for these oxyanions. More than 75% of the tissue-deposited Tc remains soluble and extractable. Differences in Tc fixation/incorporation were noted for the nuclear and chloroplast fractions of leaf and root cells. Pulse studies showed that soluble protein and nitrate reductase levels rose in response to Tc accumulation by sink leaves but not source leaves. In vitro assay of chloroplast-based S reduction and incorporation systems showed Tc to be reduced and incorporated into amino nitrogen-containing products. A hypothesis related to the metabolic behavior of Tc in plants is presented.

Alkylpyridines in surface waters, groundwaters, and subsoils of a drainage located adjacent to an oil shale facility.

Soil extracts, surface waters, and groundwaters were analyzed for the presence of water-soluble organic compounds in a drainage located adjacent to the retorted shale disposal pile a t the Department of Energy Anvil Points Oil Shale Facility, Rifle, CO. The c3-C~ alkylpyridines were positively identified in water from one of several alluvial wells, and in a surface seep. Surface waters of the stream below the seep contained alkylpyridines but in lower concentration. Alkylpyridines were detected in a moist subsoil sampled adjacent to the well, in retort water, and in aqueous extracts of shale oil. They were not detected in aqueous extracts of raw shale, retorted shale, or Prudhoe Bay crude oil. The absence of the alkylpyridines in a petroleum suggests that the compounds may be unique to shale oils, perhaps allowing their use as diagnostic indicators of water in contact with shale oils a t sites of oil shale production and processing.

The role of soil and plant metabolic processes in controlling trace element behavior and bioavailability to animals

Metabolic and physiological processes play important roles in regulating the transfer and behavior of trace elements in the soil/plant/animal system. The behaviors of Ni, Cd, Cr, T1, Np, Pu and Tc are used to illustrate important aspects of these processes. Microbial metabolism has both indirect and direct effects on trace element solubility in soils. Once non-nutrient trace elements are solubilized, the ability of plant roots to actively accumulate them is dependent on chemical activity of the element in soil solution, the presence of competing ions and the redox potential and absorption capacity of the root. After absorption in the plant, trace elements are translocated, metabolized and stored; fate and behavior varies with the properties of the element, but is generally analogous to nutrient elements. These processes can dramatically affect the availability of individual elements to animals consuming plants.

Technetium Sorption in Surface Soils

Technetium from a number of sources has the potential for entering soils in initially volatile or solubilised forms [1,2]. Several assessments [3] have suggested that subsequent soil solubility and uptake by plants will be important factors in governing Tc radiation dose to man. However, prediction of Tc behaviour in soils and availability to plants is complicated by its complex chemistry relative to soil processes.

Plant root absorption and metabolic fate of technetium in plants

Technetium-99 (Tc), produced in the fission of U-235 and Pu-239, has a long half-life (2·1 × 105 year) and has been shown to exhibit a relatively high degree of bioavailability. While Tc can enter the environment from a number of sources [1], the nuclear fuel cycle is the major contributor from the standpoint of point sources. Concentration ratios (CR = μg Tc/g dry wt vegetation per μg Tc/g dry wt soil) for transfer from soils to plants span three orders of magnitude from 1 to 1000 [2–7]. This broad range of CR values presents some difficulty in assessment of dose [8].

Concentration of orally administered and chronically fed 95mTc in Japanese quail eggs.

A chronic feeding study using 95mTc incorporated into alfalfa and an acute study where 95mTc was amended to alfalfa showed that about 8.4% of ingested Tc was transferred to eggs. After 10 days of chronic feeding, 80% of the Tc was in yolk, 20% in albumin and less than 1% in shell and associated membranes. At necropsy, technetium concentrations in the three largest oocytes were nearly equal. The biological half-time for Tc was about one to two days in acute studies. Results from the chronic feeding study also indicated that Tc levels in albumin reach a maximum between three and five days while maximum yolk concentration is attained in about six to seven days. Albumin concentrations declined about 20-50% after Day 6.