Chemical Behavior of Transuranium Elements in Underground Environments

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Speciation, colloidal behavior, solubility/precipitation, and retardation phenomena of neptunium were experimentally studied. These are essential to clarify TRU migration in geologic media relating to safety assessment of high level radioactive wastes.1. Paper electrophoresis was found to be a useful method for studying speciation of the species which had opposite charge each other at low concentration and also for studying solubility of precipitate if the dissolved species had a charge. Using paper electrophoresis, hydrolysis constants for NpO_2OH^O and NpO_2(OH)^-_ were obtained. Solubility product of undissolved Np(IV) species was also obtained from experiments under anoxic conditions.2. Chemical behavior of neutral Np(V) species, under oxidizing conditions, and effects of coexisting SiO_2OH^O and Fe(III) colloids were studied using ultrafiltration and centrifugation; (1)NpO_2OH^O,NpO_2OH^O, and NpO_2OH^O+Fe(III) were partially trapped by ultrafiltration with the trappea fractions i … More ncreasing with respect to time. (2)Ultrafiltration trapped fractions were found to be higher than centrifuging precipitated fractions which indicates the possibility of adsorption onto the filter or the formation of small particles that were not centrifuged. (3)A Np-FE(III) pseudocolloid was formed and the trapped ratio was higher at increased Fe(III) concentrations.Under reducing conditions the chemical behavior of undissolved amorphous NpO_2.H_2O(am) was studied. Ultrafiltration was determined to be a difficult method to study the chemical behavior because of adsorption on the filter and the non-trapping of some undissolved species.3. From column experiments using quartz powder, the following conclusions were obtained; (1)The adsorption mechanism of NpO^+_ was made clear. NpO_2 is thought to be adsorbed electrostatically on the quartz surface. (2)Reiardation of NpO_2OH^O was found to be different from that of NpO^+_. Filtration of aggromeate was found to play an important role in the retention mechanism. In addition, sorption of NpO_2OH^O is also thought to be effective. (3)Retardation of megatively charged carbonate complexes was found to be little. (4)Neptunium(IV) undissolved species (NpO_2.xH_2O(am)) retarded in comparison with the Np(V) dissolved species, and the retardation factor increased with increasing pH and with decreasing particle size of quartz. Less