Uranium complexes groundwaters

The speciation of dissolved uranium in groundwater has been modeled in a study of uranium oxide fuel dissolution. The calculated ],-pH diagram for a typical groundwater composition is shown in Figure This again shows the importance of carbonate complexes but also... 

Colorimetric Method (Photometry) A rapid, simple colorimetric method for the determination of uranium in groundwater and drinking water that can be nsed in the laboratory and in the field was described (Ratliff 2008). The first step is selectively trapping the uranium on a chromatographic resin (U/TEVA-2 in this example) followed by the formation of a colored complex with a pyridylazo indicator dye (Br-PADAP). At neutral pH, the complex absorbs light at 578 nm and is clearly visible. Qnantiflcation can be achieved with a spectrophotometer. The method can detect nraninm concentrations above the US-EPA guideline for drinking water of 30 pg L In Section 4.4.1, another colorimetric method that is snitable for the determination of nraninm in water and nrine based on the formation of a complex with Arsenazo-III is presented. 

A comprehensive review of uranium determinations in sea water was given by Rogers and Adams. Ocean water contains uranium at a broadly uniform concentration (0.001-0.004 ppm). The average uranium concentration in stream water is less than 1 ppb U. Groundwater shows remarkable variability of concentration as a result of, for example, the presence of enriched mineralization, the time of contact of the water with the source rocks and the concentration of ligands that either form soluble uranium complexes or insoluble uranium compounds. 

Uranium is readily mobilized in the meteoric environment, principally as the highly soluble uranyl ion (U02 ) and its complexes, the most important of which are the stable carbonate complexes that form in typical groundwaters (pH > 5, pC02 = 10 bar) (Gascoyne 1992b Grenthe et al. 1992 see also Langmuir (1997) for review). Uranium is... 

Lenhart JJ, Cabanis SE, McCarthy P, Honeymann BD (2000) Uranium (VI) complexation with citric, humic and fulvic acids. Radiochim Acta 58 5455-5463 Lienert C, Short SA, Von Gunten HR (1994) Uranium infiltration from a river to a shaUow groundwater. Geochim Cosmochim Acta 58 5455-5463... 

Field measurements in sedimentary fluvial-type calcrete deposits also suggest that present-day groundwater in these areas may also display potential to both dissolve and precipitate uranium in the near surface. Chemical dilatancy and evaporation-driven diffusion that promote de-complexing, diffusion, and reprecipitation mechanisms are seen to play integral parts in the continued chemical reworking and modification of these calcrete-hosted carnotite deposits. 

According to the technology developer, geochemical fixation can treat dissolved hexavalent chromium and other metals in groundwater at concentrations ranging from the detection limit to several hundred parts per milhon. The developer asserts that geochemical attenuation can treat most of the common heavy metals, trace elements, and namral radionuclides that occur in groundwater, such as metal-cyanide complexes, arsenic, cadmium, chromium, copper, lead, selenium, uranium, and radium. 

The dissolution time for the unreprocessed fuel would be at least 1 million years due to the limited water supply, even if a rapid oxidation of uranium to the hexavalent state and a subse-guent formation of water soluble carbonate complexes are assumed (15). Since the conditions are reducing in the groundwater (see beTow) the dissolution time would probably be several orders of magnitude larger. The unsignificant dissolution of uranium and fission products observed in the Oklo-deposit (16) is an example of a similar extremely slow leaching process in the natural environment. 

Uranyl carbonate complexes have attracted considerable interest in recent years as they are intermediates in the processing of mixed oxide reactor fuels and in extraction of uranium from certain ores using carbonate leaching more topically they can be formed when uranyl ores react with carbonate or bicarbonate ions underground, and can be present in relatively high amounts in groundwaters. The main complex formed in carbonate leaching of uranyl ores is 8 coordinate [1102(003)3], but around pH 6 a cyclic trimer [(002)3(003)6] has been identified. 

Uranium deposited by wet or dry precipitation will be deposited on land or in surface waters. If land deposition occurs, the uranium can be reincorporated into soil, resuspended in the atmosphere (typically factors are around 10 ), washed from the land into surface water, incorporated into groundwater, or deposited on or adsorbed onto plant roots Gittle or none enters the plant through leaves or roots). Conditions that increase the rate of formation of soluble complexes and decrease the rate of sorption of labile uranium in soil and sediment enhance the mobility of uranium. Significant reactions of uranium in soil are formation of complexes with anions and hgands (e.g., COj, OH ) or humic acid, and reduction of U" " to U. Other factors that control the mobility of uranium in soil are the oxidation-reduction potential, the pH, and the sorbing characteristics of the sediments and soils (Allard et al. 1979, 1982 Brunskill and Wilkinson 1987 Herczeg et al. 1988 Premuzie et al. 1995). 

The mobility of uranium in soil and its vertical transport (leaching) to groundwater depend on properties of the soil such as pH, oxidation-reduction potential, concentration of complexing anions, porosity of the soil, soil particle size, and sorption properties, as well as the amount of water available (Allard et al. 

Other factors also affect the mobility of uranium in soil. A field study performed near an active carbonate leach uranium mill showed that uranium in an alkali matrix can migrate to the groundwater (Dreesen et al. 1982). Uranium mobility may also be increased due to the formation of soluble complexes with chelating agents produced by microorganisms in the soil (Premuzie et al. 1995). 

Groundwater Separation and concentration on two HPLC columns complexation with Arsenazo III Spectrophotometry (total uranium) 1-2pg/L No data Kerr et al. 1988... 

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