Comparative study on sorption of individual and coupling cesium and selenium on mudrock

This study focused on sorption mechanism of Cs and Se in mudrock by batch techniques. Batch kinetics tests have been conducted with carriers 10^{− 4}M CsCl and SeO₂ by using individual and coupling radiotracers of ¹³⁷Cs and ⁷⁵Se. The distribution coefficients (K _d ) of Cs in synthetic groundwater (GW) and seawater (SW) showed that there is no interference between individual and coupling value (13 and 6 ml/g). Moreover, individual and coupling K _d ) (80 and 40 ml/g) of Se in GW and SW are in agreement with Cs. It is quite clear that the sorption mechanisms of Cs and Se on mudrock are independent and different by using radiotracer’s technique.     

Evaluation of sorption and diffusion behavior of selenium in crushed granite by through-diffusion column tests

Distribution coefficients (K _d), apparent diffusion coefficients (D _a) and retardation factor (Rf) in this work obtained by batch and through-diffusion experiments have been performed, respectively. The accumulative concentration method developed by Crank (The mathematics of diffusion, 12) was applied to realize apparent and effective diffusion coefficient (D _a and D _e) of Se. Besides, a non-reactive radionuclide, HTO, was initially conducted in through-diffusion experiment for assessing the ability of radionuclide retardation. The distribution coefficients (K _d) obtained by batch tests in 14 days under aerobic and anaerobic systems were 6.98 ± 0.35 and 5.21 ± 0.25 mL/g. Moreover, Rf^cal and K _d ^cal of Se obtained from accumulative concentration’s method in through-diffusion test showed an obvious discrepancy with the increase of length/diameter (L/D) ratio. However, it presented an agreement of Rf^H/Se and K _d ^H/Se in a various L/D ratio by comparison of apparent diffusion coefficient’s (D _a) between HTO and Se. It appears that the Rf^H/Se and K _d ^H/Se obtained from the through-diffusion experiments are lower than those derived from the batch experiments. Therefore, it demonstrates that reliable Rf and K _d of Se by through-diffusion experiments could be achieved at a non-reactive radiotracer (HTO) prior to tests and will be more confident in long-term performance assessment of disposal repository.     

Simulation of a 2-site Langmuir model for characterizing the sorption capacity of Cs and Se in crushed mudrock under various ionic strength effects

The sorption capacity of cesium (Cs) and selenium (Se) in crushed mudrock was demonstrated in this study through a 2-site Langmuir model. To employ such a numerical analysis, batch tests were applied in this study in synthetic seawater and groundwater with sorption/desorption kinetic experiments (time-dependent) and different concentrations (10^?2–10^?7 M). The 2-site sorption models, which correspond to two rate constants (λ ₁ and λ ₂), might be more adequate than 1-site sorption models in characterizing Cs and Se sorption/desorption according to the least square errors between the numerical analysis and the results of the batch tests. The fitting results showed that a 2-site Langmuir model is capable of appropriately describing Cs and Se sorption in mudrock. Consequently, the sorption capacity was calculated at about 0.06 mol/kg for Cs and at 0.015 mol/kg for Se.     

Sorption and diffusion processes with134Cs and85Sr in natural sorbents

Sorption and diffusion processes with¹³⁴Cs and⁸⁵Sr in natural bentonites have been investigated. The distribution coefficients (K _d ) have been determined by a batch method. Various factors affecting theK _d value as water-to-bentonite ratio, concentration of the competitive cations in the aqueous phase or bentonite-to-sand ratio in the mixed sorbents have been evaluated. A comparison of the sorption and diffusion data has been made.     

Plutonium-239 sorption and transport on/in unsaturated sediments: comparison of batch and column experiments for determining sorption coefficients

Sorption (distribution) coefficients of plutonium were most often derived by static batch experiments. However, it is not clear how unsaturated flow conditions including moisture content and pore water velocity change the sorption coefficients. Transport experiments of plutonium through the unsaturated sediments packed into the columns were then performed in order to determine the sorption coefficients (column-K _ds). Static batch experiments were also conducted to obtain batch-K _ds and then compare the differences between batch-K _ds and column-K _ds. The results show that unsaturated flow conditions had no significant effect on column-K _ds, and the average column-K _d value was 1.74 ± 0.02 m³/kg. By comparison, batch-K _d values spanned several orders of magnitude, regardless of the specified liquid–solid conditions. Moreover, the batch-K _d (22.7 m³/kg) at the standard L/S (4 mL/g) recommended by ASTM D 4319 was over an order of magnitude larger than the average column-K _d.     

Comparative study on retardation behavior of Cs in crushed and intact rocks: two potential repository host rocks in the Taiwan area

This study investigates sorption and diffusion of Cs in two potential host rocks (granite from Kinmen Island and basalt from Penghu Island) by using batch and through-diffusion methods in order to establish a reliable safety assessment methodology. These methods were applied to crushed and intact rock samples to investigate the actual geological environment. According to solid-phase analysis, including X-ray diffraction, elemental analysis, auto radiography, and polar microscopy, the sorption component primarily contained iron?Cmagnesium (Fe?CMg) minerals in basalt and granite. Moreover, the distribution coefficient (K _d) of Cs in various concentrations (~10^?2?C10^?7?M) obtained from batch tests indicated a higher sorption capacity in basalt than that in granite because of the 10% Fe?CMg mineral content. The diffusion of Cs in both crushed granite and basalt reached steady state after 110?days, and the apparent diffusion coefficients (D _a) were 2.86?×?10^?11 and 1.82?×?10^?12?m²/s, respectively. However, the value of D _a for Cs in intact rocks was estimated to be 1.45?×?10^?12?m²/s in granite and 0.56?×?10^?12?m²/s in basalt, lower than the values obtained using crushed rocks. In addition to the microporous structure (major sorption minerals), it showed that the major retardation of Cs depended on the porosity (??) of compacted media, according to through-diffusion tests. In fact, the solid/liquid (S/L) ratio decreased as is the case when switching from batch to column experiments and the sorption effect on minerals became even more negligible in retardation of radionuclide migration.     

Sorption of Cs,Pu and Am on clay minerals

Performance assessment of radioactive waste disposal requires modeling of long-term migration of radionuclides through the engineered barriers and the geological environment. The chemical complexity of sorption-desorption processes is usually reduced to integrated parameter distribution coefficients (K _d ). There are a great number of publications on K _d determination, however, the existing data on K _d of radionuclides on different geological materials are for general understanding only and are not very useful for performance assessment, since changes of the geological conditions result in variability of K _d values by two orders of magnitude. In order to obtain realistic sorption data sets for safety relevant radionuclides present in a cement/concrete based repository some preliminary studies were carried out. The development of sorption database for the near-surface repository was started with measurements of cesium, plutonium and americium K _d values. Several experiments were performed in order to determine the chemical composition of cement water which could originate from infiltration of precipitation and from contact of groundwater with concrete. More than 100 batch sorption experiments were conducted with two clay samples. Cs, Pu and Am K _d values were determined for rainwater, groundwater and cement-water of different chemical compositions. Cs, Pu, Am K _d values ranged from 450 to 9700, from 15000 to 21000 and 15000 to 80000 ml/g, respectively. Changes in the geochemical conditions resulted in the variability of Cs, Pu and Am K _d values.     

The influence of pH on diffusion of 75Se(IV) in Beishan granite

The through-diffusion method and batch sorption experiments were used to explore the influence of pH on the diffusion behavior of ⁷⁵Se(IV) in matrix Beishan granite (BsG). In the pH range of 2.0–8.5, the D_e values of ⁷⁵Se(IV) in BsG decreased first and then increased with pH increasing, while the changing trend of K_d was nearly opposite. It was speculated that the influence of pH on the diffusion of ⁷⁵Se(IV) in BsG was due to the joint effects of different species distribution of Se, change in surface charge of BsG and change in ionic strength at various pH values.

     

Retention of uranium(VI) by laumontite, a fracture-filling material of granite

Retention of U(VI) by laumontite, a fracture-filling material of granite was investigated by conducting dynamic and batch sorption experiments in a glove-box using a granite core with a natural fracture. The hydrodynamic properties of the granite core were obtained from the elution curve of a non-sorbing tracer, Br⁻. The elution curve of U(VI) showed a similar behavior to Br⁻. This reveals that the retention of U(VI) by the fracture-filling material was not significant when migrating through the fracture at a given condition. From the dynamic sorption experiment, the retardation factor R _a and the distribution coefficient K _a of U(VI) were obtained as about 2.9 and 0.16 cm, respectively. The distribution coefficient (K _d ) of U(VI) onto laumontite obtained by conducting a batch sorption experiment resulted in a small value of 2.3±0.5 mL/g. This low K _d value agreed with the result of the dynamic sorption experiment. For the distribution of uranium on the granite surface investigated by an X-ray image mapping, the fracture region filled with laumontite showed a relatively lower content of uranium compared to the surrounding granite surface. Thus, the low retention of U(VI) by the fracture-filling material can be explained by following two mechanisms. One is that U(VI) exists as anionic uranyl hydroxides or uranyl carbonates at a given groundwater condition and the other is the remarkably low sorption capacity of the laumontite for U(VI).     

Additivity of the distribution ratio of Cs and Se on bentonite/quartz sand mixture in seawater

Summary To provide comprehensive information for assessing the safety of geological disposal of radioactive waste, the additivity of sorption properties during different reaction times for various bentonite/quartz sand mixtures was investigated. Se and Cs were the nuclides of interest. Synthetic seawater (SW) was employed as the liquid phase tosimulate the possible groundwater conditions while the disposal site is an island area. Batch sorption experiments were conducted to measure the distribution ratio, R_d. Regardless of whether Cs and Se were used, the additivityat the reaction time of 7 and 14 days was better than that of 28 and 56 days.Interactions between bentonite and quartz sand might occur in synthesized seawater in reactions beyond 14 days. Under the experimental conditions,the additivity equation could be corrected by the composition ratio and reaction time. Long-term prediction of the R_dby short term batch sorption experiments would be helpful while assessing buffer materials mixed by bentonite and quartz sand.     

Sorption and diffusion of HTO and cesium in crushed granite compacted to different lengths

In this study, batch and through-diffusion experiments have been performed in order to determine the distribution coefficients (K _d ), apparent diffusion coefficients (D _a ) and retardation factor (Rf), respectively. Both apparent and effective diffusion coefficient (D _a and D _e ) of Cs were obtained by accumulative concentration method developed by Crank (1975). In addition, a non-reactive radionuclide, HTO, was initially conducted in through-diffusion experiment for assessing the ability of radionuclide retardation. The distribution coefficients (K _d ) obtained by batch tests in 14 days under aerobic and anaerobic systems were 2.06 and 3.52 ml/g. Moreover, it is found in through-diffusion test that Rf = 4.12 and 4.40 and K _d = 0.97 and 1.06 of Cs did not have an obvious discrepancy in a length/diameter/ (L/D) ratio of 0.44 and 1.78. However, Rf and K _d revealed a larger difference in an L/D ratio closing to 1 due to the geometric change of one-dimension diffusive hypothesis. Therefore, it demonstrates that Rf and K _d obtained by through-diffusion experiments only could be achieved at a lower or larger L/D ratio and would be reliable for long-term performance assessment.     

Adsorption and kinetic behavior of uranium and thorium in seawater-sediment system

The adsorption and kinetic mechanism of uranium (U) and thorium (Th) in seawater-sediments system of Mumbai Harbour Bay (MHB) has been studied by K _d values of two sets of experimental determinations using a batch experiment. After equilibrium time (7 days), under static conditions, K _d for U and Th ranged from 25,030 to 55,662 mL/g (mean: 42,140 mL/g) and 24,926 to 38,561 mL/g (mean: 34,256 L/kg), respectively. Extraction studies showed that both U and Th were strongly bound to sediment components due to insignificant difference in their mean concentration in equilibrium solution. Rate constants (k) for transfer between seawater and the exchangeable fraction were found to be similar for the sediments as 1.02 ± 0.03 day^?1 for uptake of U and Th. The resulting adsorption data were fitted to Freundlich, linear and Langmuir isotherm models. All the three models showed a significant correlation (R ² >0.9), indicating that there is more complex relationships with adsorption behavior of U and Th on sediment surface. Since, the Freundlich constant (n) for U and Th was found to be closer to unity. Therefore linear model was observed to be highly suitable. Based on the linear model, the distribution coefficient (k _d) of Th was about 1.5 times higher than U. According to Freundlich model, sorption of U in sediments observed to be higher than Th. However, Langmuir model considered in opposite to Freundlich and showed a higher value of K _L constant for Th than U. The pH (water) of the sediments ranged from 7.8 to 8.2 and the estimated total carbon (determined by C H N S–O elemental analyser) ranged from 1.7 to 3.4 %.     

Resorcinol - formaldehyde adsorption of cesium from Hanford waste solutions

Batch equilibrium measurements were conducted at 25±2 °C with a granular resorcinol-formaldehyde (RF) resin to determine the distribution coefficients (K _d ) for cesium. In the tests, Hanford Site actual waste sample containing radioactive cesium (¹³⁷Cs) and a pretreated waste sample that was spiked with non-radioactive ¹³³Cs were used. Initial concentrations of non-radioactive cesium in the waste sample were varied to generate an equilibrium isotherm for cesium. Majority of the tests were conducted at 25±2 °C using a liquid to solid phase ratio of 100 ml/g and at a contact time of 72 hours. Two additional tests were conducted at a liquid to solid phase ratio of 10 and at contact time of 120 hours. The measured distribution coefficient (K _d ) for radioactive cesium (¹³⁷Cs) was 948 ml/g, the K _d for non-radioactive cesium (¹³³Cs) was 1039 ml/g. The K _d for nonradioactive cesium decreased from 1039 to 691 ml/g with increased initial cesium concentration from 8 to 64 g/ml. Very little change of the K _d was observed at initial cesium concentration above 64 g/ml. The maximum sorption capacity for cesium on granular RF resin was 1.17 mmole/g dry resin. This value was calculated from the fit of the equilibrium isotherm data to the Dubinin-Radushkevich equation. Previously, a total capacity of 2.84 mmole/g was calculated by Bibler and Wallace for air-dried RF resin.This work was conducted at the Savannah River Technology Center in Aiken, SC, which is operated for the U. S. Department of Energy (DOE) by Westinghouse Savannah River Company under Contract DE-AC09-96SR18500. The Hanford River Protection Project-Waste Treatment Plant (RPP-WTP) funded this work. The authors are very grateful to Karen Palmer, Betty Mealer, and Yvonne Simpkins for their assistance in the experimental work.     

Resorcinol - formaldehyde adsorption of cesium from Hanford waste solutions

Batch equilibrium measurements were conducted at 25±2 °C with a granular resorcinol-formaldehyde (RF) resin to determine the distribution coefficients (K _d ) for cesium. In the tests, Hanford Site actual waste sample containing radioactive cesium (¹³⁷Cs) and a pretreated waste sample that was spiked with non-radioactive ¹³³Cs were used. Initial concentrations of non-radioactive cesium in the waste sample were varied to generate an equilibrium isotherm for cesium. Majority of the tests were conducted at 25±2 °C using a liquid to solid phase ratio of 100 ml/g and at a contact time of 72 hours. Two additional tests were conducted at a liquid to solid phase ratio of 10 and at contact time of 120 hours. The measured distribution coefficient (K _d ) for radioactive cesium (¹³⁷Cs) was 948 ml/g, the K _d for non-radioactive cesium (¹³³Cs) was 1039 ml/g. The K _d for nonradioactive cesium decreased from 1039 to 691 ml/g with increased initial cesium concentration from 8 to 64 g/ml. Very little change of the K _d was observed at initial cesium concentration above 64 g/ml. The maximum sorption capacity for cesium on granular RF resin was 1.17 mmole/g dry resin. This value was calculated from the fit of the equilibrium isotherm data to the Dubinin-Radushkevich equation. Previously, a total capacity of 2.84 mmole/g was calculated by Bibler and Wallace for air-dried RF resin.This work was conducted at the Savannah River Technology Center in Aiken, SC, which is operated for the U. S. Department of Energy (DOE) by Westinghouse Savannah River Company under Contract DE-AC09-96SR18500. The Hanford River Protection Project-Waste Treatment Plant (RPP-WTP) funded this work. The authors are very grateful to Karen Palmer, Betty Mealer, and Yvonne Simpkins for their assistance in the experimental work.     

Experimental investigation of radiocesium sorption on ceramic clay using a batch method

In this study, radiocesium sorption on ceramic clay was investigated as a function of particle size and initial ¹³⁷Cs concentration using a batch method. Ceramic clay samples taken from the Sö?üt(?nisar) clay deposit were composed of kaolinite, dickite and quartz. The equilibrium time and the liquid–solid ratio were determined as 60 min and 250 mL g^?1, respectively. The distribution coefficients (K _d) for variable liquid–solid ratio and the percentage adsorption (P _Ad) were calculated. The values of K _d and P _Ad ranged from 483 to 3165 mL g^?1 and 34–93%, respectively. The K _d and P _Ad values increased with increasing particle size, but decreased with increasing initial concentration. The sorption data were interpreted in terms of a Langmuir isotherm. The results indicated that the Sö?üt(?nhisar) ceramic clay has good sorption capacity for cesium.     

Cesium, strontium, europium(III) and plutonium(IV) complexes with humic acid in solution and on montmorillonite surface

The effect of Aldrich humic acid (HA) on the mobility of¹³⁷Cs,⁸⁵Sr,¹⁵²Eu and²³⁹Pu radionuclides was studied in Ca-montmorillonite suspensions. Verified 2-sites-2-species (2s2s) models correspond to an intensive interaction of all elements with humificated surface, what is in a remarkable contrast with the weak complexation of cesium and even strontium in solutions — the neutral ligand interaction constants β (l/mol) are log β<−9.9 and 7.56±0.21 for Cs and Sr, respectively. The result for europium complexation in solution, log β=12.49±0.18 is in a good agreement with literature data. For plutonium(IV) not only a high proton competitive constant in solution was obtained, log β β=(−0.67±0.32)+3pH, but also a strong chemisorption, which at high concentrations of humic acid (above 0.05 g/l) indicates the formation of bridge humate complexes of plutonium on the humificated surface. Logarithms of heterogeneous interaction constants ( ₂₄ l/g) of the elements with surface humic acid are 4.47±0.23, 4.39±0.08, and 6.40±0.33 for Cs, Sr, and Eu(III), respectively, and the logarithm of the proton competitive constant ( ₂₄, l/g) for Pu(IV) −3.80±0.72. Distribution coefficients of humic acid and metal humates between 0.01 g HA/l solution and montmorillonite were derived as logK _d(AH)=−1.04±0.11, logK _d(EuA)=1.56±0.11 and logK _d(PuA)=2.25±0.04, while the values for Cs and Sr were obtained with very high uncertainty. Speciation of the elements on montmorillonite surface is illustrated as a function of equilibrium concentration of humic acid in solution and of pH.     

Batch-type study of Cs,Co, and Tc binding with hydrated cement under hyperalkaline conditions

In the presented study the binding of Cs, Co, and Tc with hardened cement paste (HCP) under hyperalkaline conditions, with respect to sorption reversibility, has been investigated using batch-type experiments. The Cs, Co, and Tc sorption kinetics were determined and the distribution coefficients K _d , representing the experimental systems, were obtained. The reversibility of the element sorption has been discussed in accordance with the published data. It was shown that the sorption kinetics was rapid and the equilibrium was attained within 1–5 days. The K _d values clearly demonstrated the sorption dependence on both the chemical composition of the HCP and the element speciation.     

Estimation of distribution coefficient of polonium in geological matrices around uranium mining site

In case of ground (groundwater) contamination or contaminants release from the disposal modules (facilities) to the geo-environment, the fate of contaminant transport is mainly governed by the parameter called distribution (partition) coefficient, K _d. It is a measure of sorption of contaminants to soils. For that the sorption of polonium in soil were carried out using laboratory batch method in different soil samples collected from different places around Turamdih uranium mining site. The kinetics of polonium sorption were also carried out at different time intervals which clearly indicates that sorption equilibrium for polonium achieved at around 72 h. The K _d for polonium varies from 1,443 to 7,501.3 L/kg in soil samples. Chemical characterization of soil and ground water samples were carried out to know the effect of various chemical parameters with distribution coefficient of polonium.     

Sorption behavior of selenide on montmorillonite

Batch sorption experiments were performed to investigate the sorption mechanism of Se on montmorillonite under reducing conditions in deep geological environments. Based on E_h–pH diagrams and ultraviolet–visible spectra, Se was dissolved as selenide (Se(–II)) anions under the experimental conditions. The distribution coefficients (K_d; m³ kg^?1) of Se(–II) indicated ionic strength independence and slight pH dependence. The K_d values of Se(–II) were higher than those of Se(IV), which also exists as an anionic species. X-ray absorption near edge spectroscopy showed that the oxidation state of Se-sorbed on montmorillonite was zero even though selenide remained in the solution. These results suggest that Se(–II) was oxidized and precipitated on the montmorillonite surface. Therefore, it is implied that a redox reaction on the montmorillonite surface contributed to high K_d values for Se(–II).

     

Validity of using modified capillary column with larger diameter to study the Cs diffusion in local Taiwan laterite

We have examined the working diameter of capillary columns with diameter of 5, 7, 10 and 20 mm. These modified capillary columns were carefully filled with local Taiwan laterite (LTL). The porosity and density of these packed columns was 0.51±0.02 g/g and 1.27±0.05 g/cm³, respectively. The diffusion experiments were then carried out in synthetic groundwater with Cs loading of 0.1mM at room temperature. Experimental results have shown that the diffusion profiles of modified capillary columns fit Fick’s second law very well. This result revealed that the working diameter of a capillary column can be expanded to at least to 20 mm without affecting the validity of the derived diffusion coefficients. Among these columns, the ones with 5 mm diameter show the most consistent results of the derived K _d , apparent and effective diffusion coefficients. Although the derived distribution and effective diffusion coefficients slightly decrease as the diameter of these columns increases due to the increase of the solid/liquid ratio. These values are still informative of the Cs diffusion in local Taiwan laterite. Moreover, our results clearly demonstrate the potential of using “modified capillary method” to study the diffusion behaviors of concerned radionuclide because columns with large diameter enable the filling with more versatile geological substances.