The Investigation Of Radiounclides Sorption At Solid-Water Interface

The safety and reasonable treatment of radio-waste is a key factor for the Chinese nuclear power program. The speciation in environmental medium (include aqueous and solid speciation) could strongly affect radiounclides' dissolvability, biotoxicity, sorption and diffusion and so on. The studies on physiochemical behaviors of radionuclides on solid-water interface are of importance for the performance and safety assessment of high-level radioactive repository. In this work, the sorption of radionuclides on attapulgite (ATP) and calcareous soil (CS) was investigated under various physiochemical conditions (such as pH, ion strengthen, humic substances, temperature etc.) to discuss the sorption mechanism and speciation of radionuclides on solid-water interface combined advanced spectroscopies (EDS, XPS and EXAFS).(1) Sorption site densities and intrinsic acidity constants of sorbents were studied here using continuous potential titration and surface complexation model. The results indicated that sorption sites of intrinsic ATP involved in sorption process were main ion exchange site (=XNa/K), strong site (=SSOH) and weak site (=SWOH), while only strong site (=SSOH) and weak site (=SWOH) were predominant for activated ATP.(2) Influences of pH and ionic strength on radionuclides sorption onto solid-water interface were discussed in detail. Th(IV), Eu(III) and U(VI) sorption on solid-water interface were strongly dependent on pH and ionic strength indicating that the sorption mechanisms were actually complex, which may be attributed to the synergistic effects of ion exchange, surface complexation and surface precipitation. Cs(I) sorption on CS was influenced significantly by ionic strength, however, hardly independent of pH; and the foreign cations competed with Cs(I) in the following order: K+>Mg2+>Ca2+≈Na+>Li+(3) Ion exchange reaction and inner-sphere complexes dominated Th(IV) sorption on ATP, but only inner-sphere complexes could be found for Th(VI) sorption on activated ATP. The sorption edges of U(â…¥) on ATP could be described well by monodentate complexes (=X2UO20 and=SWOUO2(OH)0) or the combination of four sorption speciation (=X2UO20,=SSOUO2+,=SWOUO2CO3-and=SWOUO2(CO3)23-). Compared with ATP, both sorption mechanism and speciation of U(VI) changed obviously, the prevalent speciation were both=SSOUO2+ and=SWOUO2(CO3)23- on ATP/IOM composites in the observed pH range. The sorption of Eu(III) might be expressed as =X3Eu0=SWOHEu3+(outer-sphere complexes) and=SOEu-OOC-/HA in the ternary Eu/HA/ATP system.(4) Humic substances could influence radionuclides speciation in both aqueous and solid phases, and affect sorption process and mechanism of radionuclides at soli/water interface. The sorption of Th(IV) on activated ATP was obviously enhanced in ternary system (HA/ATP/Th(â…£)); and both=SSOH-HA-Th and=SSOTh were the predominant speciation. In the presence of FA and HA. Eu(â…¢) sorption was enhanced at pH<4.0. decreased at pH range of 4.0-6.0. and then raised again above pH～7.0. It was very interesting to note that the humic acid could enhance the desorption of U(â…¥) from ATP and decarbonated calcareous soil surface.(5) Eu Lâ…¢edge EXAFS analysis demonstrated that the interatomic distance R (Eu-O) (first coordination shell) in binary HA-Eu(â…¢) system decreased from 2.415 to 2.360 (?) with increasing pH from 1.76 to 9.50. and the coordination number (N) also reduced from 9.94 to 8.56. It can be attributed to the contribution of the hydration sphere Eu-OH2 and the carboxylate groups Eu-O(HA). The measured distances of 2.415 (?) (N=9.94) at pH 1.76 and 2.395 (?) (N=9.97) at pH 2.85 are attributed to the hydration inner-sphere Eu-OH₂, which suggests that HA does not participated in forming complexes with Eu3+. As pH increasing, the coordination positions of H2O were replaced by HA molecule resulting in the decreasing of N and R.(6) The influence of humic acid addition sequence on radionuclides sorption mechanism is always controversy. The mean R(Eu-O) were 2.411 (?) (N=11.91) for batch 1 (ATP+HA—Eu(â…¢)),2.399 (?) (N=11.66) for batch 2(Eu(III)+HA-ATP).2.321 (?) (N=8.866) for batch 3 (Eu(III)+ATP-HA), and 2.314 (?) (N=8.24) for binary system(Eu(III)+ATP) at pH 4.50. The mean R(Eu-O) and N decreased slightly from batch 1 to batch 3. N and d (Eu-O) of batch 1 and 2 were very close to each other, whereas those of d (Eu-O) and N of batch 3 and binary Eu/attapulgite system were similar because Eu(â…¢) was adsorbed on attapulgite firstly in batch 3, which suggested that the configuration of Eu(â…¢) were different in various addition sequences of HA.(7) Above pH～6.0, the CO2 and carbonates component of calcareous soil had obvious influence and attribution on U(â…¥) sorption speciation and mechanism.