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.     

Determination of sorption and diffusion parameters of <Superscript>99</Superscript>Tc in crushed granite using through-diffusion experiments

Diffusion and sorption are important processes associated with radionuclides migration in crystalline rock. These processes are studied in the laboratory using borehole core samples. In this study, we obtained distribution coefficients (K _d), apparent diffusion coefficients (D _a) and retardation factor (R _f) using batch and through-diffusion experiments. The \( R_{\text{f}}^{\text{H/Tc}} \) and \( K_{\text{d}}^{\text{H/Tc}} \) values obtained using the accumulative concentration method were lower than those derived in the batch experiments. These findings demonstrate that reliable R _f and K _d values of ⁹⁹Tc can be obtained using through-diffusion experiments with a non-reactive radiotracer, thereby bolstering confidence in the assessment of the long-term performance of disposal repositories.     

Determination of sorption and diffusion parameters of Se(IV) on crushed granite

Sorption and diffusion are important processes for the transport of radionuclides through geomedia from a radioactive disposal facility. In this study, batch tests and through-diffusion experiments were performed to investigate the sorption and diffusion of Se(IV) in crushed granite. Different column lengths of 2, 4 and 8, were employed in the through-diffusion experiments to study the effect of the column length on the apparent diffusion coefficients. Synthetic groundwater (GW) and synthetic seawater (SW) were the liquid phases in these experiments. A two-site kinetic linear model and the Freundlich isotherm were applied to match the sorption/desorption batch data quite closely for both of GW and SW conditions. The retardation factors (R _f ) were estimated by comparing the breakthrough curve of Se(IV) with that of HTO. Experimental results indicated that whether in GM or SW solution, the column lengths did not significantly affected D _a or R _f of Se in crushed granite.     

Effect of grain size on the sorption and desorption of 137Cs in crushed granite columns and groundwater system under dynamic conditions

In this work, the sorption of ¹³⁷Cs⁺ dissolved as ¹³⁷CsNO₃ in the solution of 10^?6?M CsNO₃ in SWG, and its desorption by solution mentioned, were investigated under dynamic conditions in columns with crushed granitic materials of various grain sizes, namely, of pure granite and, of corresponding filling materials. It aims at the quantification of the influence of grain size on the retardation and distribution coefficients of ¹³⁷Cs, as well as on the other transport parameters (Peclet numbers and dispersion coefficients). For their determination, model based on erfc-function was used, assuming reversible equilibrium linear or non-linear sorption/desorption isotherms. By means of both model approaches, the experimental BTC _S were fitted using non-linear regression procedure, in the course of which the parameters mentioned were sought. The obtained results also validated the applicability of the linear equilibrium isotherms of the ¹³⁷Cs⁺ sorption/desorption in the studied transport processes and systems. Depending on the grain size, the retardation coefficients were between 40?C93 in pure granite and 140?C200 in filling materials. These values correspond to distribution coefficients of 11?C34?cm³/g and 40?C69?cm³/g, respectively. It was found that both retardation and distribution coefficients increase with decreasing grain size.     

Numerical analysis for characterizing the sorption/desorption of cesium in crushed granite

The reactive mechanism of cesium in crushed granite was demonstrated in this study through a numerical analysis or a model of the results of sorption/desorption kinetic tests. To employ such numerical analysis, this study applied batch kinetic tests with different solid to liquid ratios (1: 20 and 1: 30) for the characterization of sorption/desorption reaction of Cs and the calibration/validation of hypothesized reactive models. Based on the least square errors (LSE) between numerical analysis and results of batch tests, the two-site sorption models, which are corresponding to two decay constants (λ ₁ and λ ₂), might be more adequate than one-site sorption models in characterizing Cs sorption/desorption. Moreover, a two-site Langmuir kinetic model has been found to be capable of appropriately describing Cs sorption/desorption under test conditions.     

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.     

Migration characteristics study of iodine in crushed Beishan granite column under dynamic flow condition

Journal of Radioanalytical and Nuclear Chemistry - The migration of iodine on Beishan granite was studied by the dynamic column method using an UV detector. Migration parameters, such as...     

A comparative study on the sorption and desorption of Hg,Th and U on clay

The sorption and desorption of uranium, thorium and mercury on a western Anatolian montmorillonite, obtained from the deposit located in Kula, were studied by application of a batch technique. The clay used is a tertiary clay originally found in a rather large geological formation of west Anatolia. It is nearly pure montmorillonite. Its cation exchange capacity (CEC) determination was performed for ammonium acetate by the Mehlich procedure. The mean CEC was found to be 83 meq/100 g, which, taking into account that CEC determinations were carried out on unfractionated material, is in good agreement with previously reported data. The concentration ranges were between 70–1500 ppm for mercury and 100–2000 ppm for thorium and uranium. The relative importance of test parameters, e.g., pH, clay particle size, groundwater composition, contact time and solid/water ratio, which determine the distribution coefficients was studied. The sorption coefficients varied between 2.7–6.4 ml/g for U, 0.22–1.59 ml/g for Th and 152.4–427.2 ml/g for Hg. The differences of distribution coefficients are discussed. The data could be fitted to Freundlich and Langmuir isotherms. The quantities of the sorbed and desorbed Th were much lower than its theoretical CEC's. This attitude was attributed to the blocking of montmorillonite by cation islands sorbed in the interlayer. Hg is sorbed most strongly. The experimental results indicate that the montmorillonites studied should be effective components of the buffer and backfill material and lead to eventual immobilization of these elements, which are environmentally dangerous.     

Modification of the structure of natural bentonite and study on the sorption of Cs+

Natural bentonite clay was treated in order to remove impurities to increase the cation exchange capacity of the montmorillonite and to obtain a more effective radioactive cesium sorption. It was found that the treatment of the clay determines the amount of sorbed cesium. On the other hand it was shown that montmorillonites may retain cesium through several mechanisms which provide strongly retained cations occupying cationic sites into the clay structure or sorbed cesium which may be lost by purification treatments.     

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.     

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.     

Water sorption on and water diffusion in chitin and chitosan

The kinetics of the sorption of water vapor on powders of crab-shell chitin and chitosan are studied via the methods of static sorption, thermography, and X-ray structural analysis. Sorption isotherms are obtained in the range of humidity from 10 to 95%. S-Shaped water-sorption isotherms observed for all chitin and chitosan samples are approximated via superpositioning of Langmuir and Flory-Huggins isotherms. The water-polysaccharide interaction parameters and the maximum sorption capacities of water located in chitin and chitosan are determined. The cluster integral is calculated, and the moisture values corresponding to water-cluster formation are determined. The water-diffusion coefficients are determined, and the effective activation energies of water diffusion are estimated: 70 kJ/mol in chitosan and 60 kJ/mol in chitin. The data on the concentration dependences of the coefficients of diffusion of water in the powdered chitin and chitosan are summarized.     

Experimental study and modelling of 137Cs sorption behaviour in the Baltic Sea and the Curonian Lagoon

Sorption–desorption behaviour of ¹³⁷Cs in the Baltic Sea and the Curonian Lagoon was studied in 1997–2009 with the aim to better understand processes responsible for redistribution and sink of ¹³⁷Cs in the system. Data obtained from several sampling campaigns were analyzed and short and long-term kinetic tracer experiments using natural water and bottom sediments were carried out with particles of various sizes from 0.2 to 50 μm. Samples of suspended particles and bottom sediments collected during two sampling campaigns were fractionated according to the size, and association of ¹³⁷Cs with solid phase was studied using sequential extraction. The difference in ¹³⁷Cs behaviour observed between expeditions in 1999 and 2001 was attributed to seasonal variations in chemical composition of suspended particles entering the system and consequent differences between the sorption (in 1999) and the desorption (in 2001) of ¹³⁷Cs in sea water. Data obtained from tracer kinetic sorption experiments with ¹³⁴Cs and bottom sediment fractions of different grain size were used for finding a suitable kinetic sorption model, kinetic constants and the corresponding equilibrium K _d values. It has been found that the modelled data best conform to the mechanism of ion diffusion through the so-called inert layer on the surface of the sediment particles.     

A PGSE diffusion and electrophoretic NMR study of Cs+ and Na+ dynamics in aqueous crown ether systems

Multinuclear pulsed gradient spin-echo (PGSE) NMR diffusion and linewidth measurements were used to probe binding and transport in aqueous Na+-15-crown-5, Na+-18-crown-6, Cs+-15-crown-5 and Cs+-18-crown-6 systems. Since direct PGSE observation of many alkali cations is precluded by either low inherent sensitivity or rapid relaxation (or both), the feasibility of proton-detected electrophoretic NMR (ENMR) measurements to complement PGSE data was investigated. ENMR measurements were performed on aqueous Cs+-, Li+-, Na+-, K+-, and Rb+- 18-crown-6 systems. The data analysis is based on a two-site binding model and its corresponding association constants. Cs+ was found to bind considerably more tightly to 18-crown-6 (K=8 M-1) than to 15-crown-5 (K approximately 2 M-1), whereas Na+ had almost equal affinity (K approximately 4.5 M-1) for 15-crown-5 and 18-crown-6. The difficulties encountered in analysing the NMR parameters, methodological limitations and the implied need for more complicated binding models are discussed.     

A model incorporating reversible immobilization for sorption and diffusion in glassy polymers

A model incorporating reversible, bimolecular immobilization for diffusion and sorption in glassy polymers is developed. Sorption is considered to occur by two distinct mechanisms: ordinary diffusion-controlled sorption and sorption resulting from the immobilization of diffusing gas molecules by prexisting sites in the polymer. Expressions are obtained for equilibrium sorption, transient sorption, and time lag. The effects of kinetic parameters of the model are illustrated and discussed.     

Effect of humic acid on nickel(ii) sorption to Ca-montmorillonite by batch and EXAFS techniques study

The influence of humic acid (HA) on Ni(ii) sorption to Ca-montmorillonite was examined by using a combination of batch sorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy technique. The sorption of Ni(ii) on HA-montmorillonite hybrids is strongly dependent on pH and temperature. At low pH, the sorption of Ni(ii) is mainly dominated by Ni-HA-montmorillonite and outer-sphere surface complexation. The EXAFS results indicate that the first coordination shell of Ni(ii) consists of ～6 O atoms at the interatomic distances of ～2.04 ? in an octahedral structure. At high pH, binary Ni-montmorillonite surface complexation is the dominant sorption mechanism. EXAFS analysis indicates the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5, while a Ni-Al layered double hydroxide (LDH) phase at the Ca-montmorillonite surface formed with pH 8.5. At pH 10.0, the dissolved HA-Ni(ii) complexation inhibits the precipitation of Ni hydroxide, and Ni-Al LDH phase forms. The rise of temperature increases the sorption capacity of Ni(ii), and promotes Ni-Al LDH phase formation and the growth of crystallites. The results are important to evaluate the physicochemical behavior of Ni(ii) in the natural environment.     

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.