CESIUM SORPTION/DESORPTION ON SALIGNY GEOLOGIC FORMATIONS

Transcription

1 CESIUM SORPTION/DESORPTION ON SALIGNY GEOLOGIC FORMATIONS C. BUCUR 1, M. OLTEANU 1, N. DULAMA 1, M. PAVELESCU 2 1 Institute for Nuclear Research, P.O. Box 78, Pitesti, Romania, crina.bucur@nuclear.ro 2 Academy of Scientists, Bucharest, Romania Received March 10, 2010 Batch sorption/orption experiments were performed to evaluate the ability of Saligny geologic formations to retard radiocesium. Experimental results were fitted to different sorption models in order to estimate sorption characteristics. Results suggest that cesium sorbs strongly on all geologic samples investigated, samples with higher clay content were the most sorptive; red clay (the higher clay content) has the higher distribution coefficient and loess the lower one. Less than 1% of the presorbed cesium was orbed suggesting that cesium may sorb strongly and almost irreversibly on the Saligny geologic formations. Key words: waste disposal, cesium sorption, batch experiment, sorption isotherm, orption. 1. INTRODUCTION Low and intermediate level (LIL) waste generated from Cernavoda NPP operation and decommissioning will be disposed of in a surface repository located on Saligny site. These wastes contain a variety of radionucli, but almost all of the disposed activity is found in relatively short-lived radionucli, including 137 Cs, 60 Co, 90 Sr, 3 H, and 55 Fe. In order to be accepted for surface disposal, LIL waste will contain long-lived radionucli in limited concentrations. Geological units identified on the Saligny site that are significant for the long-term safety assessment are the silty loess, the clayey loess, the Quaternary red clay, the pre-quaternary clay, and the Barremian limestone. They contain the potential contamination pathways of the receptors. Water table is located at around 40 m depth in the Pre-Quaternary clay [1]. Disposal of radioactive waste containing 137 Cs is a concern because it persists in the environment for almost 300 years (10 half lives) as a highly radioactive element (by its disintegration a strong gamma ray is emitted MeV emitted by 137m Ba) and bioaccumulates through the food chain. Most transport model co used to assess the transport of contaminants in the subsurface environment require parameters cribing the distribution of the contaminants between the aqueous and solid interfaces. These parameters are usually derived from sorption isotherms. Rom. Journ. Phys., Vol. 56, Nos. 5 6, P , Bucharest, 2011

2 770 C. Bucur, M. Olteanu, N. Dulama, M. Pavelescu Cs is present in the radioactive waste only as Cs + at any ph values. Its sorption is affected by the groundwater salinity, concentration at which this radionuclide is released in the groundwater and by the presence of the competitive ions (such as Na +, Ca 2+ and specially K + ). It is very soluble in water, but its mobility in natural environments is believed to be highly retarded by its strong interaction with soil minerals especially with clay minerals. Being one of the major short-lived radionuclide that is present in LIL waste produced at Cernavoda NPP, an experimentally work was performed to evaluate the sorptivity of cesium onto Saligny geologic formations. 2. BATCH SORPTION/DESORPTION EXPERIMENTS Most transport co that attempt to incorporate chemical reactions to predict the contaminants migration in the subsurface environment require parameters cribing the distribution of the contaminants between the aqueous and solid interfaces; these co use parameters derived from sorption isotherms to incorporate surface chemical reactions into numerical models. In considering the reversibility of sorption, if the sorption uptake of a contaminant is reversible, although the initial sorption may be high, the initially sorbed ions may be released from the solid phase into the aqueous phase, thereby enabling the migration of contaminants. Batch sorption experiments were performed to evaluate the cesium sorptivity on Saligny geologic formations. This method is commonly used in many laboratories and it consists in contacting a certain volume of tracer solution with a known mass of crushed material [2]. Saligny simulated water, containing the main anions and cations that are naturally presented in the Saligny groundwater [3], was used for batch sorption/orption experiments. This simulated water was prepared in the laboratory using g of calcium chloride (CaCl 2 ), g of magnesium sulphate (MgSO 4 ), g of sodium bicarbonate (NaHCO 3 ), g of potassium chloride (KCl) and g of sodium chloride (NaCl) dissolved in one liter of demineralized water and stirred using a magnetic stirring for complete dissolution. The chemicals used for Saligny simulated water preparation were weighed using an analytical balance with a precision error of ± 0.1 mg. A laboratory ph-conductometer, type MPC 227 was used to measure the ph, conductivity and total dissolved salt of the simulated water (Table 1). The geologic samples (sediments) used in these experiments were first preequilibrated with uncontaminated Saligny simulated water in order to isolate the radionuclide sorption reaction from the large number of reactions that may occur while sediments and aqueous solutions equilibrate, thus maximizing the chance that adsorption is the dominant mechanism controlling the fate of the radiocesium.

3 3 Cesium sorption/orption on Saligny geologic formations 771 Table 1 Chemical composition and characteristics of the Saligny simulated water Chemical composition mg/l Concentration meq/l Ca ± ±0.1 Mg ± ±0.1 K ± ±0.1 Na ± ±0.1 Sum of cations ±0.1 - HCO ± ± SO ± ±0.1 Cl ± ±0.1 Sum of anionions ±0.1 ph 7.8 ± 0.01 Conductivity (µs/cm) 1049 ± 0.1 TDS (g/l) 0.83 ± SORPTION KINETIC To estimate the time needed for reaching the adsorption equilibrium, parallel method was applied [2]. Experiments were performed for three different geological samples characteristic to Saligny site: loess, clayey loess and clay, in 50 ml polycarbonate centrifuge tubes. For each sample, 12 test tubes were prepared using the same cesium concentration (8E+04 Bq/L) and the same solid: liquid ratio (1:10). Cesium concentrations in aliquots sampled from the aqueous phase separated after the centrifugation of the test tubes was determined at 2, 4, 8, 12, 24 and 28 hours of equilibration. Solid-solution separation was attained using a Universal 32 Type centrifuge by centrifugation at 4,000 rpm for 30 minutes and a 10 ml aliquot of the supernatant was removed for cesium analysis. Each experiment was performed in duplicate; one control sample with only the Cs solution (cesium dissolved in synthetic groundwater) was run in the same conditions in order to check the possible adsorption on the surfaces of the test tubes. Also, a blank sample was run for each soil type (4 g of soil and 40 ml of synthetic groundwater) to determine the 137 Cs background. 137 Cs quantification in each aliquot was accomplished with a gamma spectrometer with HpGe detector having a counting error of less than 3%. The amount of contaminant removed by the solid matrix during the batch investigations was computed using mass balance equation (1) and values are plotted versus time in order to estimate when the equilibrium is attained:

4 772 C. Bucur, M. Olteanu, N. Dulama, M. Pavelescu 4 V Aeq = ( C0 Ceq ) (1) m where: A eq is the radionuclide concentration sorbed on solid matrix (Bq/kg), C 0 is the initial aqueous concentration of radionuclide (Bq/L), C eq is equilibrium contaminant concentration in solution (Bq/L), V is solution volume (L) and M represents the mass of crushed solid phase present in the batch experiments (kg) SORPTION ISOTHERMS Four cesium concentrations were used (1.5E+05, 8E+04, 4E+04 and 1.7E+04 Bq/L) to estimate the cesium sorption isotherm at the room temperature (21±3 C). The soil/solution ratio was kept constant for all experiments (1/10) and the contact time, chose based on the kinetic experiments previously performed was 12 hours. Distribution coefficient (K d ) is defined as the ratio of quantity of contaminant sorbed per unit mass of solid to the equilibrium concentration of contaminant in solution [4]: K d Aeq = (2) C eq where: K d is distribution coefficient (L/kg). Equation (2) is applicable assuming that the system is reversible and it is independent of the contaminant concentration in the aqueous phase. To evaluate the effects of various parameters on K d, contaminant concentration is varied while other parameters are held as constant as possible resulting so called sorption isotherms. More frequently used isotherms are linear [4], Freundlich [5, 4] and Langmuir [6, 4] sorption models. Also Temkin and Dubinin-Radushkevich sorption isotherms [7, 8] are used to get additional information regarding the sorption process DESORPTION KINETIC/ISOTHERMS Desorption experiments were performed to determine if cesium is reversibly or irreversibly sorbed on geologic samples characteristic to Saligny site. This information is important since not only sorption but also orption are key processes in the contaminant fate in geologic formations. Desorption is defined as ratio between the percentage of the contaminant which is orbed to the quantity of contaminant previously adsorbed.

5 5 Cesium sorption/orption on Saligny geologic formations 773 Desorption experiments were performed after sorption isotherm experiments were finished and liquid were removed from the test tubes by centrifugation and decantation and replaced by 40 ml of Saligny simulated water. Test tubes were intermittent shacked until orption equilibrium was reached. After orption equilibration test tubes were centrifuged at 4,000 rpm for 30 minutes and a 10 ml aliquot of the supernatant was removed for cesium analysis. The apparent orption coefficient (K ) is defined by [2]: K ads s aq aq m m V = (3) m m where: m aq is mass of contaminant orbed from the solid matrix at orption ads equilibrium and m s is the mass of contaminant sorbed on the solid matrix in the sorption step, V (L) is the total volume of aqueous phase in contact with the sediment during the orption experiment and m is the sediment mass (kg). Desorption kinetics were run using parallel method to estimate the time needed for orption equilibrium. To obtain orption isotherms, the amount of the contaminant orbed is calculated and the content of the contaminant remaining sorbed on soil at orption equilibrium is plotted against the contaminant equilibrium concentration in solution. 3. RESULTS AND DISCUSSION 3.1. ADSORPTION KINETIC AND ADSORPTION ISOTHERMS Sorption coefficients have to be measured at equilibrium and consequence a previous kinetic study is necessary to know the time required to attain equilibrium. The percentage adsorption was calculated according to the following relation: A ti ti ms 100 = (%) (4) m where: m o represent the mass of contaminant in the test tube, at the beginning of ti the test (Bq); m s is mass of contaminant on soil at the time t i when analysis is performed (Bq). Results obtained during the sorption kinetic experiment are presented in Figure 1. As it can be seen from this figure, sorption equilibrium was reached after relatively short time, no significant changes in the percentage adsorption values o

6 774 C. Bucur, M. Olteanu, N. Dulama, M. Pavelescu 6 were observed after 8 hours. Consequently, the following sorption experiments using similar experimental conditions (liquid/solid ratio, ph, temperature, tracer concentrations) were performed for a contact time of 24 hour. Percentage adsorption [%] clay clayey loess loess Equilibration time [h] Fig. 1 Percentage adsorption vs. equilibration time. Sorption experiments showed that cesium is strongly sorbed on investigated samples. Over 99% from cesium quantity was removed from solution by all type of sediments investigated (loess, clayey loess and clay). Percentage adsorption is larger for clay sample than for loess one (see Figure 1), confirming that clay presented in the sample brings more sorption sites in the sorption system. Since all geologic formation presented in Saligny unsaturated zone has significant percent of clay (between ~16% in loess horizon and ~ 37% in red clay horizon) they play an effective role in cesium retention ensuring that this contaminant will not reach the water table in concentration that could produce a dose higher than the constrain one (0.3mSv/yr). Simulations using FEHM (Finite Element Heat and Mass transport) code showed that the cesium concentration in the pore-water is strongly decreased by its high sorption on the clay fraction and in 500 years the plume does not reach the red clay layer, considered the main barrier against radionuclide migration at the site [9]. To estimate the distribution coefficient over the contaminant concentration range used in sorption experiments, experimentally data were fitted to linear sorption isotherm. Sorption linear isotherm (equation 3) obtained for the investigated samples are presented in Figure 2, while distribution coefficients estimated from these isotherms and the regression coefficients of determination (R 2 ) are presented in Table 2.

7 7 Cesium sorption/orption on Saligny geologic formations 775 The conformity between experimentally data ( the sorption model ( ξ are also presented in Table 2. exp A eq ) and values predicted by estim A eq ) was expressed by the total mean error (ξ %). Values for (Aeq) Amount sorbed [Bq/kg] y = x R 2 = y = x R 2 = y = x R 2 = (C eq ) Equilibrium conc. in solution [Bq/L] clay loess clayey loess Fig. 2 Linear sorption isotherms. The regression coefficients of determination for linear isotherms are better for clay and clayey loess samples than for the loess one, and the total mean error were lower for clay and clayey loess samples and a little higher for loess sample, indicating that for samples more reach in clay sorption could be approximated by the linear model. It have to be noted that values for distribution coefficients obtained from linear isotherms are valid only for the concentration range used in the experiments (1.7E04 1.5E05Bq/L.). Using these values for higher concentrations could overestimate the cesium sorption and consequently its retardation on Saligny geologic formations. Freundlich adsorption model was used to estimate the adsorption intensity of the contaminant on the sediment surface. Freundlich constants were determined using the following linearized form of the Freundlich equation [4, 5]: 1 log Aeq = log KF + log Ceq (5) n where: A eq is the amount of contaminant sorbed on the solid matrix (Bq/kg); C eq is the equilibrium contaminant concentration in solution (Bq/L), K F is Freundlich adsorption constant (L/Bq) and 1/n is a constant related both to the relative magnitude and diversity of energies associated with a particular sorption process.

8 776 C. Bucur, M. Olteanu, N. Dulama, M. Pavelescu 8 Freundlich sorption isotherms obtained for clay, loess and clayey loess used in the batch sorption experiments are presented in Figure 3. Table 2 Sorption parameters estimated from sorption isotherms and statistical parameters associated to the models used (regression coefficients of determination R 2 and the total mean error - ξ %) Sorption model used to fit experimentally data Linear sorption isotherm Freundlich sorption isotherm Langmuir sorption isotherm Temkin sorption isotherm D-R sorption isotherm Sorption and statistical loess clayey loess clay parameters R K d, (L/kg) ξ (%) R K F, (L/kg) /n ξ (%) R K L, (L/kg) 5.0E E E-04 A m, (Bq/kg) 3.33E E E+06 G 0 (KJ/mol) ξ (%) R K T, (L/Bq) 1.28E E E-02 b T, (J/mol) 5.24E E E-03 ξ (%) R K D-R, (mol 2 /J 2 ) 5E-08 7E-08 5E-08 A m, (Bq/Kg) 8.77E E E+09 E (kj/mol) ξ (%) The regression coefficients of determination (R 2 ) from the linearization of the Freundlich sorption isotherms are higher than those obtained from the linear fitting for loess and clayey loess samples, but lower for clay sample, showing once more that for clay sample, for the contaminant concentration range used in the experiments is more appropriate to use linear approximation than Freundlich one. On the other hand, from the Freundlich isotherm values of 1/n (see Table 2) indicate that sorption is slightly nonlinear and significant sorption takes place at low contaminant concentration but the increase in the amount contaminant sorbed with concentration becomes less significant at higher concentrations. In this situation, the contaminant mobility in sediments can be significantly greater for the higher concentrations and consequently errors may be introduced by assuming a linear sorption isotherm for a concentration range extended over the one used in the sorption experiments.

9 9 Cesium sorption/orption on Saligny geologic formations 777 (Aeq) Amount sorbed [Bq/kg] 2.E+06 2.E+06 8.E+05 6.E+05 4.E+05 2.E+05 0.E+00 y = x R 2 = y = x R 2 = y = 4999x R 2 = clay loess clayey loess (C eq ) Equilibrium conc. in solution [Bq/L] Fig. 3 Freundlich sorption isotherms. Langmuir isotherm was used to estimate the maximum adsorption capacity of the investigated sediments corresponding to complete monolayer coverage on the solid matrix surface. Representing the experimentally data in form of Langmuir linearized equation (13), 1/A m is obtained from the slope of the best fit line and K L is the ratio of isotherm slope to isotherm intercept [4, 6]. Linearized Langmuir isotherms (C eq /A = f(c eq ) obtained for the investigated sediments are presented in Figure 4. Ceq = + Ceq (5) A K A A eq L m m where: K L is the Langmuir adsorption constant (related to the adsorption/ orption energy) and A m is the maximum amount of contaminant that can be sorbed on the solid matrix (assuming monolayer adsorption). Correlation coefficient for Langmuir isotherm is acceptable only for loess sample (R 2 = 0.95) but it is lower than those obtained by Freundlich model and it is quite low (0.73 and 0.32 respectively) for clayey loess and clay samples indicating a poor correlation of experimentally data to Langmuir isotherm. Consequently, this type of sorption model is not suitable to be used in cesium transport modeling, but information regarding maximum sorption capacity and sorption energy can be estimated from this model. For loess, maximum sorption capacity estimated from Langmuir isotherm is 3.33E+06 Bq/kg, and it increase for clayey loess and clay to 5E+06 Bq/kg.

10 778 C. Bucur, M. Olteanu, N. Dulama, M. Pavelescu 10 Ceq/Aeq [(Bq/L)/(Bq/Kg)] 1.E-03 1.E-03 1.E-03 8.E-04 6.E-04 4.E-04 2.E-04 0.E+00 y = 3E-07x R 2 = y = 2E-07x R 2 = loess y = 2E-07x R 2 = clayey loess clay C eq [Bq/L] Fig. 4 Linearized Langmuir sorption isotherms. From the Langmuir adsorption constant (K L ) adsorption energy can be calculated using the following relation [10]: 0 G = RT ln K (6) Values for Gibbs energy of spontaneity are 18.6 KJ/mol for loess and clayey loess and 17.9 KJ/mol for clay samples, a little higher than the typical bonding energy for ion exchange mechanism (8-16 KJ/mol), but kipping account of high fitting errors these values could suggest that the sorption mechanism of cesium ion on investigated samples is ion exchange. It is recognize that Cs + sorbs via ion exchange reactions as a hydrated cation on planar sites on expansible clays like smectite with selectivity trend approximated by the lyotropic series [11, 12]. This type of sorption sites is denoted as low affinity sites. Also, Cs + sorbs in highly selective fashion to wedge or fried edge sites (FES) developed along the weathered periphery of micas and their immediate weathering products such as hydrous mica and illite. Since the loess and clays sampled from Saligny site contain smectite and also mica and illite [10], cesium sorption could occur on the both types of sorption sites and retention of 137 Cs in the subsurface will be strong influenced by the relative proportion and abundance of expansible clays and mica and its weathering products (especially illite). Good fits with experimentally data were also obtained with Themkin and Dubinin-Radushkevich sorption models. Temkin adsorption potential (K T ) obtained fitting experimentally data to Temkin adsorption model showed that the investigated samples have low ion potential for Cs +. From Dubinin-Radushkevich model free energy of sorption was calculated. These data are presented in Table 2. L

11 11 Cesium sorption/orption on Saligny geologic formations DESORPTION KINETIC AND DESORPTION ISOTHERMS For the orption experiments, the solution used in the sorption ones was replaced by tracer-free synthetic groundwater. To estimate time needed to attain the equilibrium, sufficient test tube were prepared (parallel method) and the kinetics of orption from the bulk sample were followed until steady state was reached (cesium concentration in liquid were measured after 4, 8, 12, 24, 48, 60 and 70 hours). The percentage of orption was calculated using the following relation [2]: maq D = 100 (%) (7) m ads s where: D is orption percentage (%), m is the contaminant mass orbed ads from the sediment (Bq), m s is the contaminant mass sorbed on sediment at sorption equilibrium (Bq). Results obtained during orption kinetic experiments are presented in Figure 5. As it can be seen from this figure no significant changes in the percentage orption values were observed after 48 hours. Consequently, the following orption experiments using similar experimental conditions (liquid/solid ratio, ph, temperature, tracer concentrations) were performed for a contact time of 48 h (double time for equilibration compared with the sorption experiments). aq 1.2 loess Desorption [%] clayey loess clay Time [h] Fig. 5 Desorption percentage vs. equilibration time. The content of the contaminant remaining sorbed on soil at orption equilibrium was computed using the following equation [2]:

12 780 C. Bucur, M. Olteanu, N. Dulama, M. Pavelescu 12 C s ads s aq m m = (8) m sedim ent Experimentally results were plotted against the contaminant equilibrium concentration in solution in order to get the orption isotherms. For the investigated sediments, linear orption isotherms are presented in Figure 6 while Freundlich isotherms are presented in Figure 7. The coefficients of correlation (R 2 ) and the apparent orption coefficients (distribution coefficients) are presented in Table 3. Usually sorption is considered reversible when orption equilibrium is attained even within twice the time of the sorption equilibrium, and the total orption is more than 75% of the amount sorbed. Since in our orption experiments orption percentage was of only 0.5% for clay, 0.8 for clayey loess and 1.13% for loess, and the high distribution coefficients were determined from the sorption experiments, it can be concluded that cesium is strongly sorbed and almost irreversible on sediments collected from Saligny site (loess, clayey loess and clay). Linear sorption isotherms fit well experimentally data for clay samples (R 2 = ) while Freundlich sorption isotherms are more adequate for cesium sorption on loess (R 2 =0.9912) and clayey loess (R 2 =0.9921). The values for the distribution coefficients obtained for Cs from the orption experiments are very similar with those obtained from sorption experiments (see Table 3) indicating that equilibrium was really reached both in the sorption experiments and in the orption ones. Cs conc remaining sorbed [Bq/g] 2.E+06 2.E+06 8.E+05 6.E+05 4.E+05 2.E+05 0.E+00 y = x y = x R 2 = R 2 = clay y = x loess R 2 = clayey loess Cs conc in sol afted orption [Bq/L] Fig. 6 Linear orption isotherms.

13 13 Cesium sorption/orption on Saligny geologic formations 781 Cs conc remaining sorbed [Bq/g] 2.E+06 2.E+06 8.E+05 6.E+05 4.E+05 2.E+05 0.E+00 y = x y = x R 2 = R 2 = clay y = x loess R 2 = clayey loess Cs conc in sol afted orption [Bq/L] Fig. 7 Freundlich orption isotherms. Sorption model used to fit experimentally data Linear sorption isotherm Freundlich sorption isotherm Table 3 Sorption data obtained from the orption experiments Sorption and statistical loess clayey loess clay parameters R K d, (L/kg) R K F, (L/kg) /n CONCLUSIONS Sorption/orption experiments performed showed that cesium is strongly and almost irreversible sorbed on the loess and clay samples from Saligny site. The high distribution coefficients determined for 137 Cs is explained by the presence in relatively high percent of clay minerals that are well recognized for their high cation exchange capacity (CEC). Between clay minerals presented in the investigated samples, smectite has the higher CEC ( meq/100g) and it is present in all investigated samples. Also, illite (CEC between 10 and 40 meq/100g) and mica cold contribute to the overall sorption process [11]. Values obtained from Langmuir Model for Gibbs energy of spontaneity is closed to the typical bonding energy range for ion exchange mechanism. It is recognize that Cs + sorbs via ion exchange reactions as a hydrated cation on planar sites on expansible clays such as smectite but it is also sorbed in highly selective way to wedge or fried edge sites (FES) that develop along the weathered periphery

14 782 C. Bucur, M. Olteanu, N. Dulama, M. Pavelescu 14 of micas (biotite and muscovite) and their immediate weathering products (such as hydrous-mica and illite) [12, 11]. For Saligny site smectite fraction will contribute to the overall cesium sorption together with illite and mica. Since in Saligny unsaturated zone, the smectite content is high in all geologic formation and increase with depth (average smectite content varies between 8.6% in loess layer, 12.98% in clayey loess and 16.66% in red clay layer) and the content of mica and illite is also appreciable [13, 14], Saligny geologic units will act as an efficient natural barrier against cesium migration. Desorption experiments suggest that cesium sorption is almost irreversible. Cs + is weakly hydrated and lose its shell of hydration more easily than other cations, thus it may enter interlayer of illite preferentially. Once within the interlayer this cation may become fixed over a long time scale and consequently it will not be exchangeable. Sorption isotherms used to fit experimentally data showed that cesium sorption is slightly non-linear (exponential constant of Freundlich isotherm was 0.77 for loess sample, 0.85 and 0.86 for clayey loess and clay sample respectively). Freundlich model is more suitable than the linear one to be incorporated in the transport co to model the cesium interaction with Saligny geologic units. Using linear sorption isotherms to model cesium interaction with geologic units could underestimate its mobility at concentrations higher than those used to obtain these sorption isotherms. REFERENCES 1. I. Durdun, C. Marunteanu, V. Andrei, The adaptation of natural (geologica) barriers for Radioactive LILW near surface disposal in Romania, Proc. Waste Management Conference, Tucson, USA, *** Fate, Treanport and Transformation Test Guidelines, EPA 712-C , United States Environmental Protection Agency, 20 39, M. Olteanu, A. Popa, C. Bucur, The properties of groundwater from Saligny site the main parameters in migration process of radionucli from radioactive waste, Proceedings of ICEM 01: The 8 th International Conference on Radioactive Waste Management and Environmental Remediation, Bruges, Belgium, C.W. Fetter, Contaminant Hydrology, University of Wiscounsin, Oshkosh, H. Freundlich, Colloid and Capillary Chemistry, Methuen, London, England, I. Langmuir, The adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum, Journal of The American Chemical Society, 40, , *** Understanding variation in Partition Coefficient, K d, values, EPA 402-R , United States Environmental Protection Agency, M.M. Dubidin, L.V. Radushkevich, Equation of the Characteristic Curve of Activated Charcoal, Proc. Of the Academy of Science, Physical Chemistry Section, U.S.S.R, 55, D. Diaconu, K. Birdsell, G. Zyvolosky, Natural and Engineered Barriers in a Romanian Disposal Site for Low and Intermediate Level Waste, Proceedings of ICEM 03: The 9 th International Conference on Radioactive Waste Management and Environmental Remediation, September 2003, Oxford, England.

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