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Uranium can exist in natural waters in four different oxidation states; U(III), U(IV),
<br />U(V) and U(VI), only U(IV) and U(VI) are important. U(III) tends to very quickly
<br />oxidize to U(IV) in natural waters, while U(V) is unstable and quickly converts to U(IV)
<br />and U(VI). U(VI) aqueous concentrations are almost always much higher in natural
<br />waters than U(IV) due to the very low solubility of U(IV) minerals, such as uraninite.
<br />Speciation refers to the form in which a dissolved element is present in a solution. In
<br />solution, a given element has either a positive or negative charge and tends to associate
<br />with other elements of opposite charge to form a complex or chemical species.
<br />Complexes can be either positive (cations), negative (anions), or neutral. The most
<br />common uranium species found in natural waters are shown in Table 2.
<br />Table 2
<br />Common Uranium Comnlex* Found in Natural Waters
<br />0
<br />Name Chemical Formula
<br />Uran 1 ion U02 2+
<br />Uranylhydro species UGH3+, U(OH)22+, U(OH)3+, U(OH)40, U(OH)s,
<br />U6(OH)159+, UO2OH+, (UO2)2(OH)22+, (UO2)3(OH)s
<br />Uranylcarbonate species U02CO30, U02(CO3)22-, UO2(CO3)3¢
<br />Uranylsulfate species USO42+, U02SO4°, U02(CO+) _, U(SO4)20
<br />Uranylphosphate species UHPO42+, U(HP04)2°, U(HPO4)32-, U(HP04)44-,
<br />UO2HPO41, U02(HP04)22-, UO2H2PO4+
<br />Uranylfluoride species UF3+, UF22+, UF40, UFs , UF62-, U02F+, UQF2° UO2F3-,
<br />U F 2-
<br />Uran Ichloride species UO2C1+, UCP+
<br />Uranylsilicate species UO2H3SiO4+
<br />*Note that "species" and "complexes" are used interchangeably
<br />The speciation of an element is important when determining its solubility. Because
<br />some elements form complexes with other elements preferentially, addition of the
<br />complex-forming element to the water tends to increase the solubility of the mineral
<br />phases and increase the mobility of the element. Table 2 indicates that an increase in
<br />water pH (addition of hydroxide), or addition of carbonate, sulfate, phosphate,
<br />fluoride, chloride or silicate tends to increase the solubility of uranium minerals and the
<br />overall mobility of uranium in water. The formation of the complexes shown in Table 2
<br />are favored by higher concentrations of the lignads (carbonate, phosphate, etc.) from
<br />which they are formed. For example, the more phosphate that is present in the water,
<br />the more stable the phosphate complex, with all other factors being equal. Given that
<br />the formation of the complex increases the solubility of uranium, it follows that adding
<br />phosphate, carbonate, etc. increases the mobility of uranium.
<br />• Uranium solubility in natural waters range from as low as 0.0001 mg/Lunder reducing
<br />conditions to several grams/L under highly oxidizing conditions. Natural groundwater
<br />concentrations typically range from 0.0001 mg/L to 0.005 mg/L.
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