Uranium Content in Natural Freshwater Sources and Oceans

5 URANIUM CONTENT IN NATURAL FRESHWATER SOURCES AND OCEANS 

Modern analytical techniques usually have sufficient sensitivity to determine the concentration of uranium in aqueous environmental samples and in most cases mass spectrometric techniques can also provide isotopic composition data. However, in some samples, especially where the precise content of minor uranium isotopes is required then preconcentration, separation, and purification can improve the accuracy of the measurement. Several methods have been developed for this purpose based on solid phase extraction (SPE), electro-analytical selective absorption techniques, liquid-extraction, ion-exchange and chromatographic columns, co-precipitation, and selective sorption. Other methods, like single-drop microextraction, are being developed and may serve for microanalysis (Jain and Verma 2011). Some of these techniques are discussed in the context of the specific sample preparation procedures throughout the book, so in this section only a few select methods will be discussed. 

An SPE method for online preconcentration of uranium in water and effluent samples is based on amberlite XAD-4 resin functionalized with p-nitroso-a-naphtol (Lemos and Gama 2010). A preconcentration factor of 10 was achieved and with a colorimetric arsenazo-III spectrophotometry a detection limit 1.8 pg L was reported. The online system included a mini-column through which the solution was passed for 180 s. In the elution step, the uranium was desorbed from the column and mixed with the colorimetric reagent and introduced into the spectrometer. The parameters were optimized and the effects of several interferences were examined. Recovery of spiked tap water, well water, and effluent samples was close to 100% (Lemos and Gama 2010). 

Calix[6]arene-modifled electrodes (bound to a gold surface by cysteamine 4-sul-fonic) were also used for preconcentration and detection of uranyl ions in aqueous solutions (Becker et al. 2008). The calixarene derivative has a strong affinity toward uranyl ions (super uranophile) and selectively extracts the uranium (see bottom frame of Eigure 3.13). 

Another SPE procedure for preconcentration of uranium in natural waters used octadecyl silica membrane disks modified by tri-n-octylphosphine oxide (TOPO) (Shamsipur et al. 1999). The high extractive properties of organic phosphorus compounds for uranyl ions are well known (e.g., TBP in the nuclear fuel cycle). 

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