In-situ RBS Studies on Dissolution of Pb Atoms from the SiO2 Surface into Water Solutions K. Morita, J. Yuhara, R. Ishigami, B. Tsuchiya, D. Ishikawa, K. Soda, K. Saitoh1), T. Ohnuki2), S. Yamamoto3), Y. Aoki3), K. Narumi2) and H. Naramoto2) Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan National Institute of Advanced Industrial Science and Technology (AIST Chubu)1) 2266-98 Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan Tokai Establishment2) and Takasaki Establishment3) JAERI, Watanuki-cho, Takasaki 370-12, Japan An in-situ RBS system has been developed to study the dissolution of Pb layers deposited physically on the SiO2 surface of Si(100) crystal into water solutions with different pH values. It is found that Pb atoms are not dissolved into alkaline water, but into acid water, and that the dissolution in the latter case is the zero-th order reaction kinetics and the rate constant is 0.67×1013 atoms cm-2s-1, which corresponds to 1.04×10-11 mol. cm-2s-1. The dissolution mechanism is discussed based on the experimental results. is irradiated with a probing MeV ion beam, and to determine the rate constants of sorption and dissolution of various nuclides at the liquid-solid interface [7]. In this paper, we report the experimental data on the rate constants of dissolution of Pb atoms deposited on the SiO2 surface into water solutions with different values of pH in which HNO3 or NH3OH are weakly dissolved. It is shown that Pb atoms are not dissolved into alkaline water, but into acid water. INTRODUCTION Understanding of adsorption and desorption processes of atomic and molecular species at the liquid-solid interface is of both fundamental and applied interest. In the technology for geologic radioactive waste disposal, adequate containment is estimated to result in principal from low solubility-limited release rates of the radioactive reactor products, even if low-velocity flowing ground water is present. The partition coefficients of various radioactive nuclides between the surface of minerals and the modeled ground water have been obtained using the radioactivity measurement of nuclear waste products [1,2]. For estimation of the period of time for the radioactive material to reach the biosphere, reliable data on the rate constants for the adsorption and desorption at the liquid-solid interface are primarily required. Fundamentally, the information on the adsorption sites at the solid surface in contact with a liquid is also interesting in relation to the stability of nuclides adsorbed there. The ion beam analysis techniques have been applied for in-situ measurements of elements adsorbed at the liquid-solid interfaces. The usefulness of RBS and NRA for analysis of metal deposition, oxide formation and ingress of elements in membrane of liquid-solid interfaces has been demonstrated by several authors [3~6]. In the JAERI-Universities collaboration project for advanced application of MeV ion beam materials analysis, we have developed an in-situ RBS system in a chamber connected to a beam line of a 3 MV Tandem Accelerator in TIARA facilities in order to measure the depth distribution of stable heavy nuclides adsorbed at the inner surface of a thin film window of a liquid container, which EXPERIMENTAL The thin window of the silicon specimen was fabricated using preferentially slow etching of heavily B-implanted silicon. The starting materials were (100)-oriented silicon disks of 0.4 mm in thickness and of 10mm in diameter, one face being mirror-polished. 4 MeV B ions, of which the projected range is 5.5m, were uniformly implanted into the mirror-polished surface to a The dose of 1×1016 ions cm-2 at random direction. implanted disks were annealed at 950°C for 1 hour to reduce damage produced by the implantation and to form an oxide layer of about 0.25m on the surface. The thinning process was described in detail elsewhere [8]. The silicon disk with a thinner window of 3mm in diameter was used for the wall of the sample assembly, which is installed in the small vacuum chamber of the experimental apparatus shown in Fig.1. The sample assembly was made of stainless steel and quartz glass, and sealed with O-rings. The inner surface of stainless steel components of the sample assembly was covered with the tubes and plates of quartz. The vacuum chamber was connected to a beam line of the 3MV Tandem accelerator, in TIARA facilities of JAERI, CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan © 2003 American Institute of Physics 0-7354-0149-7/03/$20.00 404 FIGURE 1. Experimental arrangement of an in-situ RBS system for measuring nuclides absorbed at the liquid-solid interface through a differentially pumped line equipped with an emergent shatter moving quickly and electrically for vacuum protection. The sample was irradiated with a 9MeV He++ ion beam through a carbon aperture slit with a hole of 2mm in diameter in order to reduce the background scattering from high-Z elements compared with carbon. The back scattered ions were measured with an annular-type SSB detector. The introduction of liquid into assembly was performed through an inlet pipe form the bottom, which was some what inclined to the horizontal level. Deposition of Pb layers onto the SiO2 surface of the silicon window and measurement of the Pb thickness were done in the other chamber which is connected to a 2MeV Van de Graaff accelerator in the Nagoya University [9]. The pH value of water solutions contacted with the sample was varied by weak dissolution of HNO3 or NH3OH into water in ultra high purity. FIGURE 2. Typical RBS spectra of 9 MeV He++ ion beam from the silicon window of the sample assembly in which Pb layers as-deposited on the backsurface, before injection of water solution (a) and 10min after injection water solution with pH5(b) the RBS technique with 2 MeV He+ ion beam. The thickness (coverage) of the Pb layers was measured as a function of time elapsing after injection of water solutions with different pH values into the inner chamber. In Fig.3 is shown the Pb coverage as a function of time elapsing after injection of deionized water into the inner chamber. It is clearly seen from Fig.3 that the Pb coverage does not change at all, namely Pb atoms on the SiO2 surface are not dissolved into deionized water. Fig.4 shows the Pb coverage as a function of time elapsing after injection of water solution with pH higher than 9 which includes dilute NH3. It is also clearly seen from Fig.4 that Pb atoms are not dissolved into alkaline water. Fig.5 shows the Pb coverage as a function of time after injection of water solution with pH 5 which includes dilute HNO3. It is clearly seen from Fig.5 that the Pb coverage decreases linearly with elapse time from 24 ML to 3ML. The rate constant of dissolution was obtained to be 0.67×1013 atoms cm-2·s-1. This fact indicates that the dissolution of Pb atoms is the zero-th order reaction kinetics. Therefore, it is concluded the Pb atoms are dissolved uniformly from the whole surface. EXPERIMENTAL RESULTS AND DISCUSSION Dissolution behavior of Pb layers deposited on the oxidized backsurface of the silicon (100) window into the different water solutions has been measured under the in-situ condition. Typical RBS spectra of the 9 MeV He++ ion beam from the silicon window with Pb layers of 24 ML in thickness on the backsurface are shown in Fig.2, where (a) represents the RBS spectrum before injection of water solutions and (b) represents the RBS spectrum at 10 min after start of injection of water solutions with pH=5. In Fig.2 the backscattering yields at the channel number between 320 and 420 are from the silicon window, of which multiple peaks are ascribed to the resonance nuclear reactions and a sharp peak yield, marked by PbB, at channnel number of 600 is from the Pb layers. The total peak area corresponds to the 24 monolayers in thickness which has been determined by 405 Here, the dissolution mechanism of Pb from the SiO2 surface is discussed. The sorption of Pb on the SiO2 surface is considered to be controlled by two different mechanisms. One is the precipitation on the surface as PbO. The other is the surface complex formation of Pb with the silanol expressed by (2SiO)Pb. In such cases, The dissolution of PbO and the desorption of Pb2+ are described as follows: Dissolution: PbO+2H+→Pb2++H2O (1) Desorption: (2SiO)Pb+2H+→2SiOH+Pb2+ (2) Direct in-situ X ray adsorption measurement shows that lead is inner-spherically bound to the surface of γ–Al2O3[10]. No direct evidence of the sorption of Pb2+ on SiO2 has been reported. If the sorption of Pb2+ on SiO2 is similar to that on γ–Al2O3, Pb layer on SiO2 should be mono-layer. Since thickness of the Pb layers is 24 ML and the specimen has been exposed to normal air, most of Pb are precipitated as PbO. The dissolution of PbO in the solution depends on the thermodynamic stability of PbO [11]. Since the thermodynamic stability is examined by the saturation index which is expressed by log (Q/K), where Q is the activity product of species in the PbO dissolution reaction, and K is the stability constant of PbO. The saturation index of PbO in the different pH water solutions are calculated using the EQ3NR software package [12]. The thermodynamic data for PbO is taken from the data base in EQ3NR. If the water saturation index is higher than 0, PbO is considered to form thermodynamically in the solution. The relationships between the saturation index of PbO and the Pb2+ concentration at the different values of pH are shown in Fig.6. The dissolution rate should depend on the saturation index or the Gibbs free energy when the dissolution is congruent [11]. In the present study, PbO dissolution is congruent because of no secondary mineral formation. The Gibbs free energy is expressed by ∆G=RT ln (Q/K) , where R is the gas constant and T is the absolute temperature. With decreasing ∆G, the dissolution of the primary mineral increases [10]. The saturation index at pH=9 is higher than 0, even at the Pb concentration of 10-8 mol. cm-3. Thus, no dissolution of Pb at pH 9 results from the stability of PbO in the solution. The saturation index of PbO at the values of pH=5 and 7 are lower than 0, when the concentration of Pb2+ is below 10-6 mol. cm-3, In the present study, the rate constants of PbO dissolution at the values of pH=5 and 7 are 1.04×10-11 and 0 mol. cm-2s-1, respectively. Since the dissolution reaction of PbO is expressed by Eq. (1), the dissolution rate should depend on the square of the H+ concentration. The concentration of H+ at pH=7 is lower by 100 times than that at pH 5. If the dissolution rate is directly related to the square of the H+ concentration, the rate constant of PbO dissolution at pH=7 is calculated to be 1.04×10 - 15 mol. cm - 2 s - 1 . This value leads the dissolution rate constant of PbO layer to be 6.7×108 FIGURE 3. Pb coverage as a function of time elapsing after injection of deionized water FIGURE 4. Pb coverage as a function of time elapsing after injection of alkaline water FIGURE 5. Pb coverage as a function of time elapsing after injection of acid water 406 atoms do not dissolute into alkaline water, but acid water and that the dissolution in the latter case is the zeroth order reaction kinetics and the rate constant is 0.67×1014 atoms cm-2s-1. ACKNOWLEDGMENTS This study is partly supported by Grant-in-Aid for Scientific Research (B) in the Ministry of Education, Science, Sports and Culture. The authors are grateful to Dr. S. Tajima and the staff members of Electrostatic Accelerator Group for cooperation in the case of the 3MV Tandem accelerator in the TIARA facilities. REFERENCES 1. S. Sato, H. Furuya, Y. Sakamoto, O. Okuno and I. Yabe, Proc. 5th Int. Symp. Water-Rock Interaction ed. by H. Aromaunsson et al (1986, Reykjavik). 2. S. Sato, H. Furuya, S. Araya and K. Matsumoto, Sci. Basis Nucl. Waste Management IX ed. by L. Werme (1986, MRS, Pittsburgh) p.763. 3. B. Kötz et al, Electrochem. Acta, 31 (1986) 169. 4. K. R. Padmanabhan et al, Appl. Phys. Rett. 48 (1986) 578. 5. J. S. Foster et al, Nucl. Instr. Meth. B28 (1987) 385. 6. J. S. Foster et al, Nucl. Instr. Meth. B89 (1994) 153. 7. K. Morita, J. Yuhara, R. Ishigami, B. Tsuchiya, K. Soda, K. Saitoh, S. Yamamoto, P. Goppelt-Lauger, Y. Aoki, H. Takeshita and H. Naramoto, Rad. Phys. Chem. 49, (1997) 603. 8. K. Saitoh, H. Niwa, S. Nakao and S. Miyagawa, Proc. 10th Int. Conf. Ion. Implantation Technology ed. by S. Coffa et al (1995) P.998. 9. J. Yuhara, R. Ishigami and K. Morita, Surf. Sci. 326, (1995) 133. 10. A. L. Roe et al., Langmuir, (ed:) Langmuir 7, 367-373 (1991). 11. A. C. Lasaga, Journal of Geophysical Research, 89, (1987) 4009. 12. T. J. Wolely, EQ3NR, A Computer program for geochemical aqueous speciation-solubility calculations: Theoretical manual, usersguide, and related documentation (version 7.0), UCRL-MA-110662 PT III, 246p.Lawrence Livermore Laboratory, University of California(1992). 13. T. Murakami, T. Kogure, H. Kadohara and T. Ohnuki, American Mineralogist, 83 (1998) 1209. FIGURE 6. The relation ship between the saturation index of PbO and the Pb++ concentration calculated at different values of pH atoms cm-2s-1. This indicates that it takes 280 hours for the 1 ML PbO to be dissolved into the water solution at pH=7. Thus, in the present study no dissolution was observed at pH=7. The dissolution rate constant of PbO has not been reported. The dissolution rate constant of anorthite (CaAlSiO) is measured at pH=4.56 by Murakami et al [13], at the different temperatures of 90, 150 and 210°C. Extrapolation of this result gives the dissolution rate constant at 25°C to be 4.9x10-9 mole. cm2s. Since the crystal chemistry of PbO differs from that of anorthite, the rate constant of PbO may differ from anorthite. However, the dissolution rate constant of PbO at pH=5 (1.04×10-11 mol. cm-2s-1) is lower than that of anorthite at pH=4.56. Since lower pH gives higher dissolution rate constants, the dissolution rate constant of PbO obtained in the present study is reasonable. SUMMARY The dissolution of Pb layers deposited physically on the SiO2 surface of Si(100) crystal into water solutions with different pH values has been measured using the in situ RBS system developed. It has been found that Pb 407
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