Actinide speciation in aquatic systems

Abstract

Nuclear test explosions and reactor wastes have deposited an estimated 16 × 10 15 Bq of plutonium into the world's aquatic systems. However, plutonium concentration in open ocean waters is on the order 10 − 5 Bq/kg, indicating that most of the plutonium is quite insoluble in marine waters and has been incorporated into sediments. Actinide ions often are not in a state of thermodynamic equilibrium and their solubility and migration behavior is related to the form in which the nuclides were introduced into the aquatic system. Actinide solubility depends on such factors as the pH (hydrolysis), Eh (oxidation state), reaction with complexants (e.g., carbonate, phosphate, humic acid, etc.), sorption to surfaces of minerals and/or colloids etc., in the water. The most significant of these variables is the oxidation state of the metal ion. The simultaneous presence of more than one oxidation state for some actinides (e.g., plutonium) in a solution complicates actinide environmental behavior. Both Np(V)O 2 + and Pu(V)O 2 +, the most significant states in natural, oxic waters are relatively noncomplexing and resistant to hydrolysis and subsequent precipitation. The solubility of NpO 2 + can be as high as 10 − 4 M while that of PuO 2 + is limited by reduction to the insoluble tetravalent species, Pu(OH 4), (pK sp = 56). The net solubility of hexavalent UO 2 2+ in sea water is also limited by hydrolysis; however, it has a relatively high concentration due to formation carbonate complexes. The insoluble trivalent americium hydroxocarbonate, Am(CO) 3(OH), is limiting species for the solubility of Am(III) in sea water. Thorium is found exclusively as the tetravalent species and its solubility is limited by the formation of quite insoluble Th(OH) 4. The chemistry of actinide ions in the environment is reviewed to show the spectrum of reactions that can occur in natural waters which must be considered in assessing the environmental behavior of actinides. While much is understood about sorption of actinides on surfaces, the mode of migration of actinides in such waters and the potential effects of these radioactive species on marine bioto, much more is needed for a satisfactory understanding of the behavior of the actinides in the environment.