Plutonium processing hydrolysis

For plutonium in the tri- and tetravalent state, when hydrolysis would dominate the solution chemistry, most sorption phenomena in geologic systems can be looked upon largely as physical adsorption processes. Ion exchange processes, as defined above, would be... 

The complex interrelationships of three types of chemical equilibria, namely oxidation-reduction, hydrolysis, and complexa-tion, as well as polymerization, a nonequilibrium process, determine the nature and speciation of plutonium in aqueous environmental systems. This paper presents a selective, critical review of the literature describing these processes. Although most research has been conducted under non-environmental conditions— that is, macro concentrations of plutonium and high acidities—the results in some cases are applicable to environmental conditions. In other cases the behavior is different, however, and care should always be exercised in extrapolating macro data to environmental conditions. 

The dominant oxidation state of plutonium in the dissolved phase of seawaters has been shown to be Pu(V). Data are presented that indicate a significant role by humic materials which cause rapid reduction of Pu(Vl) to Pu(lV), The latter leaves the solution phase via hydrolysis. The humic material also seems to reduce Pu(V) but at a much slower rate and, in sunlight, this reduction may be negated by an oxidation process of unknown origin at this time. 

The sorption process and the attainment of apparent equilibrium may be regarded then as involving essentially two kinds of sorbing species. There are a very small number of ionic plutonium species, including monomeric and low-molecular-weight polymeric hydrolysis products (1) which sorb relatively quickly and perhaps are involved in a true equilibrium, such as by ion exchange with silanol sites at the silica surface. There is evidence of such sorption of various types of univalent and multivalent cations on silica, and both chemisorption and physical adsorption processes have been deduced (13, 14, 15). Filtration of the desorbing plutonium with a 15-40-micron porous silica disc indicated that the very first material to desorb was essentially small, unfilterable Pu(IV). 

The tendency toward hydroly of some of these elements can be used to advantage in separation processes. For example, in the Redox process for separating uranium and plutonium from fission products, the aqueous feed to the separation plant is made acid-deficient to promote hydrolysis of zirconium to a less extractable species, probably a colloidal hydrate [B5]. 

The hydrolysis of Pu(IV) can result in the formation of polymers which are rather intractable to reversal to simpler species. This has led often to incorrect conclusions about the nature of the plutonium species present and the validity of their equilibrium constants. The kinetics of depolymerization take a different course as the polymer ages such that while freshly prepared hydroxides are easily decomposed, aged polymers require quite rigorous conditions. A reasonable model for this process involves initial formation of aggregates with OH bridging which dehydrate with aging (Choppin 1983). 

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