Fate of Neptunium in an Anaerobic, Ethanogenic Microcosm
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EXPERIMENT Origin of live sediments Lake DePue, a backwater lake of the Illinois River, is located in Northern Illinois, east of LaSalle-Peru. Industrial chemical operations, including zinc smelting and sulfuric acid/diammonium phosphate fertilizer production, began in 1903 and continued through 1992. Several residual sources of contamination, identified by the Environmental Protection Agency, include: residue and waste piles, lithopone waste material ridges, cinder fill areas, contaminated soils, lagoons, cooling ponds, and gypsum stack ponds. All of these sources have been found to contain elevated levels of zinc, lead, arsenic, cadmium, chromium, and copper. These residuals have also led to metal contamination of the aquatic system. Contaminants flow into Lake DePue through surface water runoff and groundwater migration. The lake is shallow (approximately 2 m) and is characterized by high primary productivity and high suspended solids. Since Lake DePue is a backwater lake, it often has drastic volume changes during periods of high precipitation. The water column remains oxic during most of the year, but on some occasions the bottom waters became oxygen depleted. Average concentrations of total metals present in the water column are: Zn = 7000 nM, Pb = 140 nM, Cd = 20 nM, and Cu- 700 nM. The ranges of metal concentrations in the sediments are: Zn = 1000-350,000 ppm, Pb 120-2000 ppm, Cd = 50-1000 ppm, and Cu = 300-100,000 ppm.
Growth conditions and microcosm setup To prevent contamination by foreign microorganisms, sediments were sampled using sterile techniques in late November 1997 and sealed anaerobically in glass containers. In an anaerobic chamber, 125 ml, of sediment slurry was inoculated into 1 liter of defined growth medium containing (in g/L) 0.8 NaC1, 1.0 NH 4CI, 0.1 KCI, 0.1 KHPO4, 0.165 MgC12-6HO, 1.361 NaAcetate, 1.5 N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid (TES) buffer, and 10 mL/L of a Wolin vitamin solution [21]. For the non-actinide experiments, NaAcetate (10 mM) and MnO\ (15 rmM), prepared by permanganate oxidation of Mn(II) to yield mixed Mn(III/IV) oxide [22], were added to the sediments at regular intervals as electron-donor and acceptor substrates, respectively. The microcosms were kept at a constant temperature of 30"C. For the Np experiments, two microcosms (one control and one receiving Np additions) were prepared in a similar manner. In an anaerobic chamber, approximately 5 mL of sediment slurry was inoculated into 95 mL of the growth medium described above. Both microcosms were supplemented with NaAcetate and MnO, for several weeks, and then MnOx additions were suspended; after that point NaAcetate was added periodically as the sole substrate. Filtersterilized, anoxic Np(V), prepared as a stock solution by electrolytic reduction of Np(VI), was added periodically to one microcosm. The other microcosm received no Np addition. Both microcosms received identical acetate additions. Due to the special requirements necessary for handling vessels containing actinides, all vessels
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