Study On The Chemical Behavior Of Pu Under The Special Geological Environment

This paper presents a systematic experimental study on the chemical behavior of Pu, a transuranic element possessing the long-term potential risk for the environment, under the geological repository. Studies are the basic chemical reactions possibly happening in groundwater, including solubility, complexation, redox reaction, colloidal formation, and adsorption.Results of the measurements of the solubility of Pu in BS03 well underground water (from the Beishan region, Gansu province, which is being investigated as a potential candidate of high-level radioactive waste repository site) indicate that when stabilizing agent(NaNO₂, FeO powder) added in the groundwater, the main valence state of Pu in the underground water is Pu(â…£), the effect of pH(6. 98,7. 95,8. 78,9. 92) in solution is little effect on the solubility of Pu(â…£) in the BS03 well underground water, the main species of Pu(â…£), which existed in the BS03 well underground water, may be Pu(OH)₄, Pu(OH)₅^-, and a series of the complex of Pu(â…£) with carbonate ion. When none stabilizing agent added in the underground water, the main valence state of Pu in the underground water is Pu(â…£), and co-existed with the Pu(â…¢) and Pu(â…¤). The calculated results of the geochemical modeling code EQ3/6(Version 7.2a) indicate that solubilities are highest at low pH values (pH<5) and decrease to minimum at pH values in the range of 5 to 8. When the redox potential in the range -0.35V≤Eh<0.6V, the main species of Pu in the BS03 well underground water is Pu(OH)₅^-.Results of the kinetics of the reduction of PuO₂⁺ by hydrogen peroxide indicate that the reduction of PuO₂⁺ is first order with respect to hydrogen peroxide concentration and inverse first order with respect to hydrogen ion concentration. With these results and assuming a first order dependence with respect to the concentration of PuO₂⁺, the reaction can be described by the following rate equation:dc(Pu(V) {3.93±\.93)xlQ-*[Pu(V)][H2O2] dt [H+]The reduction rate of PuOi* increases with the increase of temperature. The activation energy is deduced to be about 84KJ/mol. The existences of Fez+, F" , HCCVand SO42" accelerate the reduction of Pu(V) obviously. The reduction behavior of Pu(>2+ with hydrogen peroxide in BS03 well groundwater has been studied. The reduction rate is obviously faster than the PuO2+ with none hydrogen peroxide in BS03 well groundwater. Results of the measurements of the surface charges of granite samples indicate that with the increase of depth of sample point, the surface charges of granite samples increase. Results of the kinetics of the reduction of PuC>2+ by granite indicate that the reduction of PuO2+ is first order reaction with respect to granite concentration and inverse first order with respect to hydrogen ion concentration. With these results and assuming a first order dependence with respect to the concentration of PuC>2+? the reaction can be descrided by the following rate equation: dc(Pu(V) = (746±077)xl0-.2 VxV () dtThe reduction rate of PuC>2+ increases with the increase of temperature. The activation energy is deduced to be about 29KJ/mol.Results of the complex formation of Pu(0H)4(am) with carbonate indicate that 1:2 complex of Pu(0H)4(am) with bicarbonate is obtained and the complex formation , constants determined in the experiments.Pu(OH)A(am) + 2HCO; o Pu(OH)4(HCO3)2 K=l0"2 6I±018 orPu(OH)4(am) + 2HC0; Pu(OH)2(CO,)2 + 2H2O K=l0"261 ±018At high pH(10.28 13.08) where carbonate ion is dominant, 1:2 complex of Pu(OH)4(am) with carbonate ion is obtainedPu(OH)4(am) + 2COi2 o Pu(OH)4(CO32')24' K-10'352±0"Results on the study of the nature colloid, which may be existed in the underground water,and the colloid formation of Pu in the BS03 well underground water indicate that the colloid particle size of the granite changed with increasing the pH value of the underground water, and reached to maximum at pH values in the range of 7 to 8, and the ï¿¡ potential decreased. The particle fraction was formed for Pu in the pH-range of interest (3-10).Results of the adsorption behavior of Pu on Fe, FeO, Fe2C>3, Fe3C>4 powder, granite and its major component minerals(quartz and feldspar, biotite, hornblende, pyrite, sphene, and clay minerals) indicate that the adsorption mechanism are formation of surface complex, and can be described by the Freundlich equation of the adsorption isotherm. The sorption ability of Pu on them is increased with increasing the aqueous phase pH value, and decreased with increasing the aqueous phase carbonate ion concentration(4.0 X 10"3 1.0 X 10^'mol/L). The oxygen concentration of the experimental condition is less influence on the sorption of Pu on them. The main species of Pu in the aqueous phase and on the surface of the solid phase is quadri-valence after the adsorption equilibrium. The pyrite and clay mineral are the key minerals, which affect the adsorption of Pu on the granite. The adsorption of Pu on the mixture of quartz with feldspar, and sphene are linear adsorption, and on biotite, hornblende, pyrite, and clay mineral are nonlinear adsorption. On the basis of the present study, some proposals for the further work are suggested.