Seawater, uranium content

Russell AD, Emerson S, Mix AC, Peterson LC (1996) The use of foraminiferal U/Ca as an indicator of changes in seawater uranium content. Paleoceanography 11 649-663 Rutherford E, Soddy F (1902) The cause and nature of radioactivity Part 11. Phil Mag Ser 6 4 569-585 Sacked WM (1960) Protactnium-231 content of ocean water and sediments. Science 132 1761-1762 Sacked WM (1958) Ionium-uranium ratios in marine deposited calcium carbonates and related materials. 

The use of foraminiferal uranium/calcium ratios as an indicator of changes in seawater uranium content. Paleoceanography 11(6), 649—663. 

Uranium content in seawater very low compared with other deposits seawater 0.003 ppm... 

The average uranium-content in seawater is at 0.003 ppm significantly lower than in conventional uranium ores (350 to 5000 ppm) or in phosphates (100 to 200 ppm). About 27 10- t of uranium is fed annually into the sea via rivers. 

Although seawater contains oniy 3.34 /rg of uranium/liter, the oceans of the world are so vast that their total uranium content is estimated to be around 4 billion MT [Dl]. Extraction of uranium from seawater is discussed in Sec. 8.8. 

Highlights The main interest in studies of uranium content in oceans or seawater is to determine the effect of anthropogenic activities. Therefore, the presence and abundance of the minor isotopes, particularly U, must be accurately determined, and this requires preconcentration and separation of the uranium and the use of accelerator mass spectrometry (AMS) for the analysis. Other mass spectrometric techniques (ICPMS or TIMS) can also be used but with inferior performance. The high salinity of ocean water introduces a matrix effect that could bias ICPMS measurements of the uranium content, so the separation and preconcentration methods described earlier may be needed for precise quantification of uranium (an internal standard can also be used for this purpose). 

Table 1 gives the average metal content of the earth s cmst, ore deposits, and concentrates. With the exceptions of the recovery of magnesium from seawater and alkaU metals from brines, and the solution mining and dump or heap leaching of some copper, gold, and uranium (see Uranium and uranium compounds), most ores are processed through mills. Concentrates are the raw materials for the extraction of primary metals. 

Thorium has a wide distribution in nature and is present as a tetravalent oxide in a large number of minerals in minor or trace amounts. Thorium is significantly more common in nature than uranium, having an average content in the earth s cmst of approximately 10 ppm. By comparison, Pb is approximately 16 ppm. Thorium has a seawater concentration of <0.5 x 10 . Thorium refined from ores free of uranium would be almost... 

Table 4 Speciation, Concentration, and Relative Content of Uranium in Seawater at 5 = 25 C and pH = 8.1...

One such proxy makes use of element ratios. For example, the ratio of Cd to Ca in foraminiferal tests is a proxy for the PO4 content of the past water mass in which the foraminifera formed and therefore provides information on the past thermohaline circulation. This tracer is explained in detail in the article on trace elements in foraminiferal tests. Similarly, the ratio of the intermediate uranium decay products Pa and °Th, measured in bulk sediment, may under certain conditions provide information on the advection of water masses in the overlying water column in the past see Cosmogenic Isotopes and Uranium-Thorium Series Isotopes in Ocean Profiles). Isotope ratios are ideally suited as long-term proxy tracers. Some of these isotope ratios are characteristic of certain seawater properties and... 

Limits of detection (LODs) reached were 0.4 ng/1 of uranium and 2.8 ng/1 of thorium. The reproducibility of the LOV—MSFIA—ICP-MS was 1.7% expressed as relative standard deviation (RSD). Moreover, a high sensitivity, a wide working range, e.g. 0—200 pg/1 for both U and Th, and an injection frequency up to 9/h (depending on the sample volume) should be highlighted. Different water sample matrices (seawater, well water, freshwater, tap water and mineral water), a phosphogypsum sample with natural uranium and thorium content and a channel sediment reference material were satisfactorily analyzed with the proposed method. 

---