Uranium concentrations in sea

The conclusions of Hurt s study of year-by-year oxygen isotope ratios in 72 years of S. gigantea are thus supportive of the conclusions of the CIAP study [49] that solar variations influence the abundances of many kinds of chemical species in the stratosphere, and therefore influence the.amount of solar energy they absorb and re-radiate to earth, and therefore influence the surface temperature of the earth and especially the surface temperatures of the oceans. It is the surface temperature of the oceans which produces the phenomena we have discussed the isotope ratio variations in rain and hence in tree rings, the isotope ratio variations in the Greenland ice cap, in the organic carbon and uranium concentrations in sea cores, and furthermore variations of the sea surface temperature produces variations in the carbon-14 to carbon-12 ratio fractionation at the sea air interface and hence in the carbon-14 content of atmospheric carbon dioxide and hence in the carbon-14 content of tree rings. 

The oceans contain about 4.5 billion tons of dissolved uranium, almost a thousandfold of the reasonably assured and estimated terrestrial uranium resources in the western world 101). The concentration of uranium in sea water appears to be remarkably constant at about 3.3 pg/liter 120-122). Very recent measurements of uranium concentrations in sea water samples taken in the Arctic and South Pacific Ocean down to depths of more than 5000 m confirm this mean value 123). However, with increasing salinity of sea water a slight increase of uranium concentration is observed 124). The molar concentration of uranium in sea water is nearly 8 orders of magnitude lower than the total concentration of the major ions 125). Marine uranium displays no detactable deviation from the normal terrestrial U-235/U-238 isotope ratio, 03>126). 

Sea water is actually a very low grade uranium source, however, the advantage of the dissolved state and the almost inexhaustible quantities of uranium should be kept in mind. Moreover, it should be emphasized that the uranium concentration in sea water is relatively high compared to other heavy metals as for instance gold or thorium. Common metals like chromium, manganese, copper, or cobalt occur in sea water in lower molar concentrations than uranium (Table 1). 

The actinides (Th, Pa, U) are the three naturally occurring elements with the highest atomic masses. None of these three has a stable isotope, which makes Th and Pa rare elements in the Earth s crust and sea water. In contrast, the uranium concentration in sea water is 12.6 nM, present as U(VI) in an oxyanionic soluble form. This oxyanion may serve as an electron acceptor for anaerobic respiration in bacteria (Lovley et al. 1991, Payne et al. 2002, Holmes et al. 2002). The resulting U(IV) is insoluble and can be precipitated by microbial cells (Yong and Macaskie 1998), which in turn opens up a biotechnological route for the bioremediation of uranium-contaminated environments. 

Uranium concentration in sea water ranges from 1 to 4 ppb, except in inland seas, such as the Caspian Sea, where uranium content ranges from 3 to lOppb. The total amount dissolved in the oceans is estimated at 5000000000 ton uranium. Research on extraction has been conducted by several countries, including Japan, the United Kingdom, the U.S.A. and West Germany. 

The abundance of niobium in the earth s crust is estimated to be in the range 20 mg/kg and its average concentration in sea water is 0.01 mg/L. The metal also is found in the solar system including the lunar surface. Radionucleides niobium-94 and -95 occur in the fission products of uranium-235. 

Figure 9.18 Depth profile of uranium concentration in the Sea of Galilee. (L. Halicz, ]. S. Becker et al. Int. J. Mass Spectrom., 249-250, 296 (2006). Reproduced by permission of Elsevier.)...

Uranium occurs in sea water in its highest oxidation state +6 owing to the carbonate content of sea water, uranium predominantly should exist in sea water as the tricarbonato uranylate anion [U02(C03)3]4 , an extremely stable complex with a formation constant of log ji = 22.6. However, there is no experimental evidence for the occurrence of this complex ion in natural sea water due to its extremely low concentration. According to equilibrium constants also other uranium species are expected to occur in sea water (Table 2). 

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. 

Global uranium flux calculations have typically been based on the following two assumptions (a) riverine-end member concentrations of dissolved uranium are relatively constant, and (b) no significant input or removal of uranium occurs in coastal environments. Other sources of uranium to the ocean may include mantle emanations, diffusion through pore waters of deep-sea sediments, leaching of river-borne sediments by seawater," and remobilization through reduction of a Fe-Mn carrier phase. However, there is still considerable debate... 

Anderson RF (1987) Redox behavior of uranium in an anoxic marine basin. Uranium 3 145-164 Anderson RF, Fleisher MQ, LeHuray AP (1989) Concentration, oxidation state, and particulate flux of uranium in the Black Sea. Geochim Cosmochim Acta 53 2215-2224 Back W, Hanshaw BB, Pyler TE, Plummer LN, Weiede AE (1979) Geochemical significance of groundwater discharge in Caleta Xel Ha, Quintana Roo, Mexico. Water Res 15 1521-1535 Barnes CE, Cochran JK (1990) Uranium removal in oceanic sediments and the oceanic U balance. Earth. Planet. Sci. Lett 97 94-101... 

Kalil, E. K., Kaplan, I. R., Uranium and Organic Carbon Concentrations in a Deep Sea Core, PhD Dissertation, Dept, of Geochemistry, University of California at Los Angeles, 1976. 

The concentration of uranium in the sea is relatively constant and is equal to 3 pg/1 at a salinity of 35%. A slight increase of uranium concentration has been observed to occur upon increase of the general salinity of seawater in numerous deep-water studies [169]. 

Although strontium is one of the major metal ion constituents in sea water, its concentration of 8.1 mg/1 is about 50 times lower than that of calcium. No procedures are known for a commercial recovery of strontium from sea water. Strontium can be extracted from sea water along with uranium and other elements by hydrous titanium oxide. However, only 120-fold concentration has been reached... 

Table 3. Uranium loading of selected inorganic, organic, and biological sorbents in natural sea water with a uranium concentration of about 3.3 jxg/1 the loadings refer to the dry sorbent or to its metal content U9)... 

A process for recovery of uranium from sea water comprises the regeneration of the sorbent by elution of uranium. Elution should proceed rapidly and with high yield in order to maximize the overall eficiency of the recovery process, which is measured in terms of the increase of the effective concentration, i.e. the concentration of uranium in the eluate compared to the initial concentration in the sea water. Finally, a high selectivity of the elution process is desirable 107,130). A further concentration of uranium in the eluate up to the precipitability in the form of yellow cake can be attained by ion exchange, ion flotation, or electrodialysis. 

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