Date of Award
Doctor of Philosophy (PhD)
George T. F. Wong
Gregory A. Cutter
Ronald E. Johnson
Uranium has been used extensively for quantifying geochemical processes in the marine environment. Due to its low concentration, about 3.3 μg/1, some form of preconcentration is required for analysis. The two most widely used pre-concentration schemes are co-precipitation with iron hydroxide, and chelating ion exchange chromatography. These methods were developed by following the behavior of inorganic uranium isotopes and seemed to give identical results. However, when used for trace metal analysis, these methods have been shown to extract different fractions from the same sample. Chelex-100 resin removes only inorganic or weakly bound trace metals from a solution, while the precipitates of iron hydroxide have been shown to remove variable amounts of dissolved organic matter as well.
Although uranium is predicted to be in the inorganic form in marine waters, there is some indirect evidence to suggest that organic and colloidal uranium are present. Uranium is well known for its association with organics material, such as humic and fulvic acids. The indirect measurement of organic uranium has also been reported in surface waters of the Pacific Ocean.
The comparability of the concentration of dissolved uranium in marine waters by pre-concentration with chelating ion exchange chromatography (Chelex-238U) and by iron-hydroxide co-precipitation (Fe-238U) was investigated. A method for the determination of "organic" uranium by incorporating UV-oxidation into the analytical scheme was also developed (UV-238U).
Samples were collected from the surface of open ocean and coastal waters. In open ocean water, no organic uranium, taken as the difference between the UV-238U sample and the Chelex-238U sample, was observed within the analytical uncertainty of the method. No difference was also observed between the two preconcentration methods. In coastal waters, however, organic uranium was found in significant quantities, constituting 25 to 45 percent of the UV-238U uranium. Samples in coastal waters increased in the following order: Chelex-238U < Fe-238U < UV-238U. This order is probably due to the incomplete removal of organic compounds by the Fe-238U method. Organic uranium was found to have a much stronger correlation to the concentration of D.O.C. than to salinity. The existence of organic uranium may explain the observed non-conservative behavior exhibited by uranium in certain coastal waters.
The concentration of organic uranium was determined with depth in the North Atlantic and the Chesapeake Bay. As with surface waters, no significant quantities were observed with depth in the open ocean. However, in the Chesapeake Bay, organic uranium was found down to a depth of 10 meters. The concentration of organic uranium was also found to be higher during the winter months than early spring. This may indicate that the formation of organic uranium is controlled more by the binding of inorganic uranium with organic ligands from rivers than by biological mediation.
Surface water samples were filtered through ultrafiltration membranes of varying nominal molecular weights (NMW) and processed using ultrafiltration techniques. The nominal molecular weight (NMW) distribution of Chelex-238U, Fe- 238U, UV-238U , and organic uranium was determined in a variety of surface waters. No colloidal uranium was found in the open ocean. In coastal samples, organic colloidal uranium was primarily associated with high molecular weight (HMW) compounds. The Chelex-238U and Fe-238U fractions were primarily associated with low molecular weight compounds (LMW). Inorganic colloidal uranium was measured and reported here for the first time. The concentration of colloidal uranium was found to decrease with salinity in the esturine samples, possibly indicating the flocculation of HMW colloidal compounds during mixing.
Mann, Darrin K..
"The Speciation of Dissolved Uranium in Marine Waters"
(1994). Doctor of Philosophy (PhD), dissertation, Ocean/Earth/Atmos Sciences, Old Dominion University, DOI: 10.25777/gp8e-sk64