Uranium organic matter complexes

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

Surface analytical techniques. A variety of spectroscopic methods have been used to characterize the nature of adsorbed species at the solid-water interface in natural and experimental systems (Brown et al, 1999). Surface spectroscopy techniques such as extended X-ray absorption fine structure spectroscopy (EXAFS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) have been used to characterize complexes of fission products, thorium, uranium, plutonium, and uranium sorbed onto silicates, goethite, clays, and microbes (Chisholm-Brause et al, 1992, 1994 Dent et al, 1992 Combes et al, 1992 Bargar et al, 2000 Brown and Sturchio, 2002). A recent overview of the theory and applications of synchrotron radiation to the analysis of the surfaces of soils, amorphous materials, rocks, and organic matter in low-temperature geochemistry and environmental science can be found in Fenter et al (2002). 

Another relevant general review summarizes the knowledge on the behavior of series radionuclides in soils and plants and is intended to provide a comprehensive source of information for environmental impact studies (Mitchell et al. 2013). The summary of the data on plant to soil concentration ratios that depends on the specific soil and type of plant and the distribution of uranium within the parts of the plant is especially important. The dependence of the sorption of dissolved uranium compounds on the type of soil (like the clay content) and the parameters mentioned earlier (pH, complex forming agents, anions, presence of iron, organic matter, etc.), based mainly on studies of the (distribution factor) of spiked soil samples, is discussed. It is noted that in general the uranium concentration in plants is several orders of magnitude lower than in soil, but some plants may efficiently absorb uranium and translocation within the plant is quite common (Mitchell et al. 2013). These features, and especially the soil-to-plant transfer factors, will be discussed in Section 3.4 that deals with the uranium content in plants and soil and the relation between them. 

Coffinite was first described as a new mineral by Stieff and co-workers from several localities in the sandstones of the Colorado Plateau deposits often intimately associated with asphaltic material. It was also found in vein-type deposits in Spain by Arribas and has since been found in almost all types of deposits. The composition has been reported as U(SiOi4)i AOrganic matter was always present and organometallic complexes of uranium may have accounted for the excess uranium rather than requiring excess (OH) to account for the U Si ratio deviating from unity. USi04 has been prepared by Fuchs and Gebert with no evidence of OH substitution. 

The reduced mobility of radium in comparison with uranium is explained by the solubility difference between the two elements, which occur in nature as sulphates and carbonates at 18°C radium sulphate = 1.410" g/1 uranyl sulphate = 205 g/1 radium carbonate is insoluble and uranyl carbonate = 60g/l. With the acidity and alkalinity of water, however, radium solubility changes. The radium content of water also depends on the salt concentration of certain elements—mainly alkaline chloride (radium replaces sodium). Radium precipitates with complexes of barium (S04Ba) and with calcium carbonates (travertine). Radium is also fixed by clay, organic matter, iron and manganese hydroxides. 

Owing to the stability of the uranyl carbonate complex, uranium is universally present in seawater at an average concentration of ca. 3.2/rgL with a daughter/parent activity ratio U) of 1.14. " In particulate matter and bottom sediments that are roughly 1 x 10 " years old, the ratio should approach unity (secular equilibrium). The principal source of dissolved uranium to the ocean is from physicochemical weathering on the continents and subsequent transport by rivers. Potentially significant oceanic U sinks include anoxic basins, organic rich sediments, phosphorites and oceanic basalts, metalliferous sediments, carbonate sediments, and saltwater marshes. " ... 

---