Uranium in sea water

Uranium in Sea Water by Pulse Polarography. J. Electroanal. Chem. 2, 25 (1961). 

Nagj M, Makjanic J, Orlic I, et al. 1986. Determination of uranium in sea-water by X-ray fluorescence spectroscopy. J Radioanal Nucl Chem 97 373-380. 

Hart S. R. and Staudigel H. (1982) The control of alkalies and uranium in sea water by ocean emst alteration. Earth Planet Set Lett 58, 202-212. 

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). 

The capability of any sorbent to bind uranium in sea water presupposes that the functional group of the sorbent forms a strong uranyl complex at the natural pH of sea water. The stability of such a complex should be comparable with that of [U02(C03)3]4 occuring in sea water. The binding mechanism seems to proceed mostly via direct carbonate substitution by the functional group. 

A distribution study has been carried out on the extraction of uranium in sea water using solutions of trioctylphosphine oxide in cyclohexane, and the optimum conditions for a quantitative recovery determined. The effect of added trioctylphosphine oxide on the distribution of a number of P-diketones between various solvents and aqueous perchlorate solution has been investigated. ... 

A. Katakai, N. Seko, T. Kawakami, K. Saito and T. Sugo, Adsorption of Uranium in Sea Water Using Amidoxime Adsorbents Prepared by Radiation-Induced Cografting, J. Atom. Energy Soc. Jpn., 40 (1998) 878. 

An essential step in the safety analysis of potential waste repositories is the prediction of what chemical species are formed in the actual water. For exanq>le, the relatively high solubility of uranium in sea water is due to this strong carbonate complexation which forms 1102(003)3. Figure 22.6 shows the variation of uranyl species in a surface water under nor atnnspheric pressure of OO2 (Pco2 Iog[CC ]=2pH-18.1 +logPc02)-... 

U is chemically equivalent to U and occurs with a 235 /238 activity ratio of 0.046. As a result of the preferential mobilization of " U during chemical weathering, the river supply of " U activity exceeds the supply of U, causing a " U/ U ratio in the ocean greater than unity. The isotopic composition of uranium in sea water with salinity 35 is shown in Table 1. 

All niarine phosphorites consist mostly of microcrystalline apatite (carbonate fluorapatite) in the form of laminae, pellets, oolites, nodules and skeletal or shell debris. Uranium, considered syngenetic, may be present in carbonate fluorapatite as a substitute for calcium. Uranium in sea water was probably incorporated during or shortly after precipitation, and it is usually disseminated rather uniformly throughout a given bed or horizon. Primary uranium minerals are rarely present, but secondary uranium minerals (tyuyamunite, autunite, torber-nite) have been identified in a few localities. 

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. 

In 1994, Dadfarnia and McLeod described the analysis of uranium in surface waters and sea water using a simple FI system with an alumina column for preconcentration.77 Species eluted from this column were delivered to an ICP-MS as the detector. Also in 1994, Hollenbach et al. described the automation of extraction chromatographic methods based on TRU-Resin and TEVA-Resin to separate and preconcentrate U, Th, and Tc from soil samples, using ICP-MS for detection.49125 In 1996, Aldstadt et al. described the use of FI and extraction chromatography using TRU-Resin to determine U in environmental samples by ICP-MS.78... 

The location of uranium from sea water taken up by the alga Dunaliella was achieved by EDAX and the microscope in the STEM mode (Spry and Bochem, 1981). 

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... 

In view of the anticipated exhaustion of terrestrial uranium reserves in the western world at the beginning of the next century 101), the recovery of uranium from sea water has received much attention over the past three decades 102 U9). First studies on uranium extraction from sea water were carried out as early as 1953 by the Atomic Energy Research Establishment in Harwell (AERE), United Kingdom. Extensive efforts have been made in Japan since the early 1960s from the People s Republic of China activities in this direction have been known since 1970. In the Federal Republic of Germany continuous research on the recovery of uranium from sea water has been in progress since about 1973. Investigations have also been carried out in France, Italy, Soviet Union, Finland, India, and more recently also in the United States and Sweden. 

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). 

The uranium atom is eight-coordinate the linear uranyl group is equatorially surrounded by six oxygen atoms of the three bidentate carbonate groups. In the equatorial plane the radius of the complex ion amounts to 4.85 A, thus, it is one of the largest inorganic ions existing in sea water 119). 

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). 

A suitable sorbent must be available in large quantities and at low cost. Its performance should not deteriorate in service, that is, it should be almost insoluble in sea water and eluants, and highly stable against physical, chemical, and biological degradation in order to permit long-term recycling procedures and to avoid contamination of the sea. Further, since most of the uranium is sorbed only on the surface of the sorbent particles, any loss due to attrition would mean a serious loss of uranium. 

Lead compounds such as lead naphthalene tetracarboxylate, lead pyrophosphate, lead stannate, lead sulfide, and others which are not listed in Table 3, were proved to be effective uranium sorbents in sea water 131). However, sorbent materials containing toxic metals must be excluded with respect to sea contamination which would be inevitable even in the case of slight solubility. On the other hand, many metal compounds are decomposed in sea water by hydrolysis and carbonate formation. Among the inorganic sorbents presented in Table 3, hydrous titanium oxide and... 

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. 

Several technical processes have been suggested for the recovery of uranium from sea water using solid sorbents, in particular hydrous titanium oxide. The processes have partially been tested in preliminary model plants. Generally, fixed and fluidized sorbent bed operations can be distinguished. 

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