Radionuklidien sorptio ja spesiaatio pohjoisessa metsämaassa

In biosphere safety assessment of spent nuclear fuel, the importance of radionuclides increases with their possibility to induce radiation dose for humans and other organisms in the future. The surface environment migration and sorption of 135Cs, 129I, 93m,94Nb and 79Se is of great importance since these radionuclides have been assessed to contribute to the potential radiation dose in the most realistic biosphere calculation cases. This doctoral thesis aimed to investigate the retention and behaviour of cesium (Cs+), iodine (I- and IO3-), niobium (Nb(OH)5) and selenium (SeO32- and SeO42-) in humus and rather undeveloped mineral soil of boreal forest on Olkiluoto Island when abiotic factors affecting the sorption reactions were varied. Factors affecting species transformations of iodine and selenium were also examined for the same soil samples and under the same experimental conditions. Cesium retention was affected by e.g. incubation conditions, soil depth, pH, humus and mineralogy. Humus exhibited lower sorption of cesium than mineral soil, which was caused by mineral soil s relatively high muscovite content and the presence of Cs selective FES sites in muscovite interlayer spaces. Formation of slightly reducing soil conditions decreased soil retention of cesium, presumably caused by the formation of NH4+ ions and arisen competition of the FES sites. Increase in soil pH accelerated the retention of cesium on negatively charged surface sorption sites. The highest retention of inorganic iodine forms iodide and iodate were observed in humus, as caused by sorption processes and speciation changes leading to presumable formation of organo iodine compounds in microbially mediated reactions. Iodine sorption on mineral soil was very low in aerobic and anaerobic soil conditions, even though acidic pH values increased the retention. Decrease in pH had similar effect for selenium (selenite) and niobium. For these two elements inorganic soil components, and especially weakly crystalline aluminium and iron oxides are considered important retentive phases due to inner sphere complexation with surface Al and Fe atoms. Speciation of iodine showed considerable dependence on soil environment. Iodate was reduced to iodide especially in anaerobic soil conditions, low pH and in the presence of organic matter and microbial activity. No oxidation of iodide to iodate was detected. Formation of unidentified, presumable organo iodine compounds was observed in humus and mineral soil of low pH or varying incubation time. Inorganic selenium forms selenite and selenate proved to be persistent in the experimental conditions as no changes in selenium liquid phase speciation was observed irrespective of variation in incubation time, pH or redox potential.Radionuklidien merkitys käytetyn ydinpolttoaineen biosfäärin turvallisuusarvioinnissa kasvaa niiden ihmiselle sekä muille organismeille tulevaisuudessa mahdollisesti aiheutuvan säteilyannoksen todennäköisyyden kasvaessa. Radionuklidien 135Cs, 129I, 93m,94Nb ja 79Se pidättyminen eli sorptio sekä kulkeutuminen maapallon pintaympäristössä ovat merkittäviä prosesseja johtuen kyseisten nuklidien arvioidusta vaikuttavuudesta mahdolliseen säteilyannokseen kaikista realistisimmissa biosfäärin mallinnustapauksissa. Tässä väitöskirjatyössä tarkastellaan cesiumin (Cs+), jodin (I- ja IO3-), niobiumin (Nb(OH)5) sekä seleenin (SeO32- ja SeO42-) pidättymistä ja käyttäytymistä Olkiluodon pohjoisen metsämaan humuskerroksessa sekä melko kehittymättömissä mineraalimaakerroksissa kun sorptioon vaikuttavia abioottisia tekijöitä muunneltiin. Jodin ja seleenin kemiallisten muotojen pysyvyyteen vaikuttavia tekijöitä tutkittiin samoille maaperänäytteillä samoissa kokeellisissa olosuhteissa. Cesiumin pidättymiseen maaperässä vaikutti mm. inkubaatio-olosuhteet, syvyys, pH, orgaanisen aineksen pitoisuus sekä mineralogia. Cesiumin sorptio humukseen oli alhaisempaa kuin mineraalimaahan johtuen mineraalimaan suhteellisen korkeasta muskoviittipitoisuudesta sekä Cs-selektiivisten FES-paikkojen läsnäolosta muskoviittikerrosten välitiloissa. Lievästi pelkistävien maaperäolosuhteiden syntyminen heikensi cesiumin pidättymistä, mikä todennäköisesti oli seurausta NH4+ ionien muodostumisesta sekä FES-paikoista kehkeytyneestä kilpailusta. Maaperän pH:n kasvu tehosti cesiumin pidättymistä negatiivisesti varautuneille pintasorptiopaikoille. Epäorgaanisten jodin muotojen jodidin ja jodaatin tehokkain pidättyminen havaittiin humuksessa johtuen sorptioprosesseista sekä oletettavien organojodiyhdisteiden muodostumisesta mikrobivälitteisissä kemiallisen esiintymismuodon muutoksissa. Jodin sorptio mineraalimaahan oli hyvin alhaista sekä hapellisissa että hapettomissa maaperäolosuhteissa, vaikkakin tehokkaampaa pidättymistä havaittiin alhaisissa pH-arvoissa. Liuoksen pH:n alenemisella oli samanlainen vaikutus myös seleenille (seleniitti) sekä niobiumille. Näille kahdelle alkuaineelle maaperän epäorgaanisten komponenttien, ja erityisesti heikosti kiteytyneiden alumiinin ja raudan oksidien, ajatellaan olevan tärkeitä pidättäviä faaseja johtuen spesifisestä pidättymisestä sisäkompleksimuodossa pinnan Al- ja Fe-atomien kanssa. Jodin spesiaatio osoitti huomattavaa riippuvuutta maaperäolosuhteista. Jodaatti pelkistyi jodidiksi erityisesti hapettomissa olosuhteissa, alhaisissa pH-arvoissa sekä orgaanisen aineksen pitoisuuden ja mikrobiaktiivisuuden kasvaessa. Jodidin hapettumista jodaatiksi ei havaittu. Tunnistamattomien, oletettujen organojodiyhdisteiden muodostumista havaittiin humuksessa sekä mineraalimaassa vaihtelevalla inkubaatioajalla tai alhaisissa pH-arvoissa. Seleenin epäorgaaniset muodot seleniitti ja selenaatti osoittautuivat kokeellisissa olosuhteissa pysyviksi, sillä muutoksia seleenin liuosfaasin spesiaatiossa ei havaittu huolimatta vaihtelusta inkubaatioajassa, pH-arvossa ja redox-potentiaalissa

Sorption of Se species on mineral surfaces, part I : Batch sorption and multi-site modelling

The sorption behavior of Se(IV) on Grimsel granodiorite and its main minerals, plagioclase, K-feldspar, quartz and biotite, were investigated in Grimsel groundwater simulant in a large Se concentration range (from 1.66 x 10(-10) M to 1 x 10(-3) M). Experimental results show that the distribution coefficients (K-d values) of Se (IV) on the rock and mineral samples increased with the decreasing of Se(IV) concentration. The sorption of Se (IV) on biotite has the largest K-d value in low concentration area (<10(-7) M) stabilizing between 0.0595 +/- 0.0097 m(3)/Kg and 0.0713 +/- 0.0164 m(3)/Kg. The Kd value of Se(IV) on K-feldspar was the second largest (0.0154 +/- 0.0019 m(3)/Kg in 10(-9) M) while the sorption on quartz was negligible. The sorption behavior of Se(IV) on Grimsel granodiorite followed the same trend as plagioclase, the most abundant mineral in Grimsel granodiorite, with K-d values of 0.0078 +/- 0.0010 m(3)/Kg for Grimsel granodiorite and 0.0085 +/- 0.0016 m(3)/Kg for plagioclase, when Se(IV) concentration was 10(-9) M. HPLC-ICP-MS results show that all the Se(IV) remained in + IV oxidation state after more than 1 month experimental time and speciation modelling proved that the main species in Grimsel groundwater simulant were HSeO3- and SeO32-. Multi-site surface complexation modelling was performed by PHREEQC with the help of molecular modelling techniques which was performed with the CASTEP code implemented into Materials Studio. The modelling results predict that there are three kinds of sorption sites on the surface of biotite mineral, with sorption site densities differing in three magnitudes.Peer reviewe

Sorption and speciation of selenium in boreal forest soil

Sorption and speciation of selenium in the initial chemical forms of selenite and selenate were investigated in batch experiments on humus and mineral soil samples taken from a 4–m deep boreal forest soil excavator pit on Olkiluoto Island, on the Baltic Sea coast in southwestern Finland. The HPLC–ICP-MS technique was used to monitor any possible transformations in the selenium liquid phase speciation and to determine the concentrations of selenite and selenate in the samples for calculation of the mass distribution coefficient, Kd, for both species. Both SeO3 2− and SeO4 2− proved to be resistant forms in the prevailing soil conditions and no changes in selenium liquid phase speciation were seen in the sorption experiments in spite of variations in the initial selenium species, incubation time or conditions, pH, temperature or microbial activity. Selenite sorption on the mineral soil increased with time in aerobic conditions whilst the opposite trend was seen for the anaerobic soil samples. Selenite retention correlated with the contents of organic matter and weakly crystalline oxides of aluminum and iron, solution pH and the specific surface area. Selenate exhibited poorer sorption on soil than selenite and on average the Kd values were 27–times lower. Mineral soil was more efficient in retaining selenite and selenate than humus, implicating the possible importance of weakly crystalline aluminum and iron oxides for the retention of oxyanions in Olkiluoto soil. Sterilization of the soil samples decreased the retention of selenite, thus implying some involvement of soil microbes in the sorption processes or a change in sample composition, but it produced no effect for selenate. There was no sorption of selenite by quartz, potassium feldspar, hornblende or muscovite. Biotite showed the best retentive properties for selenite in the model soil solution at about pH 8, followed by hematite, plagioclase and chlorite. The Kd values for these minerals were 18, 14, 8 and 7 L/kg, respectively. It is proposed that selenite sorption is affected by the structural Fe(II) in biotite, which is capable of inducing the reduction of SeO3 2− to Se(0). Selenite probably forms a surface complex with Fe(III) atoms on the surface of hematite, thus explaining its retention on this mineral. None of the minerals retained selenate to any extent