Effect Of Organic Matter And Illite On The Se(Ⅳ) And Re(Ⅶ)diffusion In Compacted Montmorillonite

79Se and99 Tc are the high-level radioactive and long-lived nuclides which are of great concern in geological disposal saftey acessment. In an aqueous solution,79 Se exists mainly in the form of Se O32- and HSe O3-, and the predominate species of 99 Tc is Tc O4- under aerobic condition. As anion, both of Se(IV) and Re(VII) are diffcult to be absorbed by backfill material and easy to transport from repository to geosphere.Since GMZ bentonite with negative surface is low permeability, high adsorption and swelling, it is chosen as the backfill materia of deep geological disposal of high level radioactive waste(HLW) of the main composition is montmorillonite. In shale, mudstone and other sedimentary rocks, montmorillonite can be transformed into illite. With increasing of age of the repository, the illite content is bound to gradually increase, which would seriously affect the migration behavior of radionuclides. Therefore, it is necessary to study of radionuclides migration behavior in mixed-layer clay minerals of illite and montmorillonite.Humic acid is a high molecular polymer and widely exists in the environment. It alters the migration of cations by forming the complexation. In recent years, the effect of humic acid on radionuclide migration has attracted more and more attention. However, as anions, little investigation has been published on the diffusion behavior of79 Se and99Tc and their influence factor. The complexity of the diffusion system makes difficult to explain the diffusion mechanism of radionuclides. Humic acid contains a lot of carboxyl and hydroxyl, in order to learn more about the effect of those functional groups on the radionuclide. Citrate, oxalate and formate was chosen in this study to investigate the effect of humic acid on diffusion behavior of stable nuclides Se(instead of Se-79) and Re(instead of Tc-99), three of them are small organic molecules containing a carboxyl group.The interaction mechanism was discussed, the influence factor on diffusion was investigated, and the corresponding migration parameters was obtained. Moreover, the diffusion behavior of Se(IV) and Re(VII) in the mixture of illite and montmorillonite was studied to obtained the diffusion parameters. The conclusion can be found as follows:(1)In present of organic matter, such ascitrate, oxalate, formate and humic acid, the effective diffusion coefficient(De) and rock capacity factors( ?) of Se(IV) in montmorillonite were increased. At p H 9.0, De was 2.13 ? 10-12 m2/s and ? was 0.12,whereas,De was 3.89 ? 10-12 m2/s and ? was 1.96 at p H 3.0. In the presence of organic matter, De were(4.25 ~ 20.8)? 10-12 m2/s and ? were 0.31 ~ 1.45, at p H 9.0. At p H 3.0, De were(30.7 ~ 40.4) ? 10-12 m2/s and ? were 2.28 ~ 2.82. It showed that organic matter improves the accumulated mass of Se(IV) and diffusion flux through the 1 cm length of montmorillonite sample plug. Under the conditions of p H 9.0, De of Se(IV) are less than p H 3.0, It can be explained that the predominate species of Se(IV) is Se O32-under alkaline conditions, whereas, the main species of Se(IV) is HSe O3- under acidic conditions.Therefore, the diffusion of Se O32- is slower than that of HSe O3-. Under acidic conditions,the distribution coefficient of Se(IV) was 9.68 ~ 14.9, indicating that HSe O3-can be adsorbed on the surface of bentonite. It may be explained that the organic matters enhance the De and ? of Se(IV) in bentonite, because the protonation reaction on the variable potential of the surface of bentonite reduces its surface electronegativity under acidic condition. However, the apparent diffusion coefficient change was insignificant. Therefore,the apparent diffusion coefficient Da was unaltered by those organic matters, excepted that the accumulated mass and diffusion flux of Se(IV) were increased.(2)The effective diffusion coefficient and rock capacity factors of Re(VII) in montmorillonite is insignificant in present of organic matter, such as citrate, oxalate,formate and humic acid. Under neutral condition(p H = 7.0), De of Re(VII) was 9.1 ? 10-12m2/s and?was 0.05. Whereas, De of Re(VII) were(5.1 ~ 5.9) ? 10-12 m2/s and ? were0.024 ~ 0.044 in present of those organic matter. The results showed that the small organic molecules like citrate, oxalate, formate containing a carboxyl group partly hindered the diffusion of Re O4- in montmorillonite. This is probably due to the organic matters change the surface properties of montmorillonite. Under alkaline conditions(p H = 9.0), De of Re(VII) was 10.7 ? 10-12 m2/s and ? was 0.087. Whereas, the De value of Re(VII) was 12.2 ?10-12 m2/s and ? was 0.070 in present of humic acid. The results showed that humic acid on diffusion of Re(VII) in montmorillonite was insignificant. The diffusion of Re(VII) in montmorillonite could be described by Arrhenius formula. The activation energies of Re,Re-oxalate and Re-formate were 23.05 ? 1.69, 20.69 ? 1.76 and 26.49 ? 1.17 k J/mol,respectivley.(3)The montmorillonite illitization significantly improve the the diffusion of Se(IV)(4)and Re(VII). At p H 7.0, De of Se(IV) was 3.89 ? 10-12 m2/s and ? was 1.99. When increasing the ratio of illite montmorillonite(I-M)mixture from 1:9 to 3:7, t, De were(6.45 ~ 7.45) ? 10-12 m2/s and ? were 2.62 ~ 4.26, indicating that he effect on diffusion of Se(IV) is insignificant. Whereas, the De value of Re(VII) was 2.85 ? 10-12 m2/s and ?was 0.058 at p H 7.0. The effect on diffusion of Re(VII) is insignificant with increasing the ratio of illite-montmorillonite mixture from 1:9 to 3:7, De were(5.26 ~ 6.03) ? 10-12 m2/s,? were 0.051 ~ 0.143. Both of the De values of Se(IV) and Re(VII) were reduce with increasing of dry density at I-M mixture ratio of 1:9. At p H 9.0, the De values of Se(IV)and Re(VII) were(1.32 ~ 4.32) and(23.1 ~ 50.5) ? 10-12 m2/s at the dry density in the range of 1400 ~ 1800 kg/m3. In case of Re(VII), the ? values could not be measured accurately because of the short time to reach the steady-state.