Transuranium elements, hydrolysis

The redox chemistry of the actinide elements, especially plutonium, is complex (Katz et al., 1980). Disproportionation reactions are especially important for the +4 and +5 oxidation states. Some of the equilibria are kinetically slow and irreversible. All transuranium elements undergo extensive hydrolysis with the +4 cations reacting most readily due to their large charge/radius ratio. Pu (IV) hydrolyzes extensively in acid solution and forms polymers. The polymers are of colloidal dimensions and are a serious problem in nuclear fuel reprocessing. 

Despite the extremely low concentrations of the transuranium elements in water, most of the environmental chemistry of these elements has been focused on their behavior in the aquatic environment. One notes that the neutrality of natural water (pH = 5-9) results in extensive hydrolysis of the highly charged ions except for Pu(V) and a very low solubility. In addition, natural waters contain organics as well as micro- and macroscopic concentrations of various inorganic species such as metals and anions that can compete with, complex, or react with the transuranium species. The final concentrations of the actinide elements in the environment are thus the result of a complex set of competing chemical reactions such as hydrolysis, complexation, redox reactions, and colloid formation. As a consequence, the aqueous environmental chemistry of the transuranium elements is significantly different from their ordinary solution chemistry in the laboratory. 

Colloids are always present in natural waters containing the transuranium elements. (Colloids are defined as particles with sizes ranging from 1 to 450 nm. These particles form stable suspensions in natural waters.) Colloids of the transuranium elements can be formed by hydrolysis of transuranium ions, or by the sorption of transuranium elements on the naturally occurring colloids. The naturally occurring colloids include such species as metal hydroxides, silicate polymers, organics (such as humates), and the like. The mobility of the transuranium elements in an aquifer is determined largely by the mobility of its pseudocolloids, that is, those colloidal species formed by the adsorption of the transuranium ions upon the naturally occurring colloids. 

The speciation of the transuranium elements in waters is thus a complex function of hydrolysis, colloid formation, redox reactions, and complexation with available ligands. The solubility (mobility) is, thus, highly dependent on the particular aquatic environment and its characteristics. 

Hydroxides of transuranium elements (TUE) can be obtained as amorphous precipitates from aqueous solutions with certain pH values. This process is preceded by hydrolysis that can be considered as a stepwise removal of H from H2O molecules from the first coordination sphere of the TUE ions. In order to characterize hydrolytic behavior of the TUE ions, one can use hydrolysis constants according to the following reactions ... 

While not the most toxic, plutonium is the most likely transuranium element to be encountered. Plutonium commonly exists in aqueous solution in each of the oxidation states from III to VI. However, under biological conditions, redox potentials, complexa-tion, and hydrolysis strongly favor Pu(IV) as the dominant species (27, 28). It is remarkable that there are many similarities between Pu(IV) and Fe(III) (Table I). These include the similar charge per ionic-radius ratios for Fe(III) and Pu(IV) (4.6 and 4.2 e/k respectively), the formation of highly insoluble hydroxides, and similar transport properties in mammals. The majority of soluble Pu(IV) present in body fluids is rapidly bound by the iron transport protein transferrin at the site which normally binds Fe(III). In liver cells, deposited plutonium is initially bound to the iron storage protein ferritin and... 

The information about foe hydrolysis of trivalent transuranium elements other than Am is limited because of foe redox instability of foe trivalent oxidation state (Pu and Np) or foe restricted availability of foe isotope (elements beyond Cm). The available data refer mostly to foe older literature, given in Ref. 25. 

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