Colloidal silica (CS) is being considered as an injectable low viscosity fluid for creation of impermeable barrier containment of low level radioactive waste at the Savannah River Site (SRS), South Carolina. The sorption behavior of cesium and strontium on Savannah River Site Soils impregnated with Colloidal Silica was studied using a batch experimental method. The samples were prepared by addition of CS and an aqueous solution of CaCl{sub 2} to the soil materials. Sorption studies were conducted after the gelation of the CS samples had occurred. The variation of the sorption ratio, R, as a function of cesium or strontium concentration was examined. The Freundlich isotherm was used to fit the data and very good results were obtained

Al Mahamid, I.

Apps, J.

Hakem, N.

Moridis, G.

UNT Digital Library

LBNL-39498 1 UC-600 ERNEST ORLANDO LAWRENCE ERKELEY NATI 0 NAL hABCJ RAT0 RY Sorption of Cesium and Strontium on Savannah River Soils Impregnated with Colloidal Silica N. Hakem, I. AI Mahamid, J. Apps, MAE 2 5 1997 0 s,T a and G. Moridis Earth Sciences Division , . .  . .  . .  . , _  .. , . .  .. * . ,  January 1997 Presented at the . . . . .  . .  . . 1997 Intiirmtional . .  . .  .. , ~. .-. , DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information. neither the United States Government nor any agency thereof, nor The Regents of the University of California. nor any of their employees, makes any warranty, express or implied, o r  assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or The Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, or The Regents of the University of California. ! 1 . I  I Ernest Orlando Lawrence Berkeley National Laboratory is an equal opportunity employer. Portions of this document may be illegible in electronic image product& Images are produced from the best available original dOl.ument. LBNL-39498 UC-600 Sorption of Cesium and Strontium on Savannah River Soils Impregnated with Colloidd Silica N. Hakem, I. Al Mahamid, J. Apps, and G. Moridis Earth Sciences Division Ernest Orlando Lawrence Berkeley National Laboratory University of California Berkeley, California 94720 January 1997 This work was supported by the Office of Environmental Management, Office of Technology Development, Subsurface Contamination Focus Area, of the U.S. Department of Energy under Contract No. DE-AC03- 76SF00098. LBNL-39498 UC-600 Sorption of Cesium and Strontium on Savannah River Soils impregnated with Colloidal Silica N. Hakeml; 1. AI Mahamidl ; J. Appsl, G. Moridis'. Abstract The sorption behavior of cesium and strontium on Savannah River Site Soils impregnated with Colloidal Silica was studied using a batch experimental method. The samples were prepared by addition of CS and an aqueous solution of CaClZ to the soil materials. Sorption studies were conducted after the gelation of the CS samples had occurred. The variation of the sorption ratio, R, as a function of cesium or strontium concentration was examined. The Freundlich isotherm was used to fit the data and very good results were obtained. Introduction Colloidal silica (CS) is being considered as an injectable low viscosity fluid for creation of impermeable barrier containment of low level radioactive waste at the Savannah River Site (SRS ), South Carolina. To determine the effectiveness of the barrier, it is desirable to predict the transport behavior of radionuclides through the barrier. This transport can be modified by several mechanisms such: co- precipitation, adsorption or diffusion (Adeleye et a/. 1994). If precipitation or strong adsorption occurs, radio-contaminants will be immobilized and the CS barrier will retard radionuclide migration. If the radionuclides diffuse through the CS barrier, the barrier will be less effective in preventing groundwaters contamination (Persoff et a/. 1994). The purpose of this work is to study the sorption of cesium and strontium on the colloidal silica and soil from SRS. 137Cs and 9%r ions, respectivelly, are present in the radioactive waste and potentially mobile as dissolved species under the conditions prevailing at the SRS. The sorption experiments were performed using a batch method, at different concentrations of radionuclides and at pH constant . The sorption is measured by gamma counting for cesium, by Inductively Couple Plasma (ICP), and by liquid scintillation counting (LSC) for strontium. Experimental Reagents Reagents used in the experiments were: CsCI, ultra pure reagent, CaCI2.2 &O 99 % pure and SrC12. 6H2O 99.8 % pure (J.T. Baker Inc., Phillipsburg, NJ). The isotopes 137Cs and Sr (Isotopes Products Laboratories, Burbank, CA) were received respectively as CsCl and SrC12 in 0.1 M HCI. An Ecolite cocktail liquid (ICN Biomedicals, Inc Irvine, CA) was used as scintillation solution for the radioactive counting. We used Nyacol DP5110 Colloidal Silica (PQ Corporation, Ashland, MA) with the characteristics described in Table 1. Earth Sciences Division, MS 70A-1150, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA, 94720, USA Table 1. Characteristics of Colloidal Silica Used in our- Experiments Density (g/mL) 1.20 I Si09 30 % - - L  I Na9O I co.1 % - L  I Also3 I c 0.25 % All solutions were prepared with freshly deionized distilled C02-free water, even if we don't suspect any presence of carbonates, and were stored under argon atmosphere. Apparatus and methods The concentration of 1S7Cs was measured using the 661.7 KeV gamma-ray of la7Ba and the concentration of 90Sr was measured by liquid scintillation counting. We used a high purity germanium (HPGe) detector designed at LBNL and a liquid scintillation counter, RackBeta, from Pharmacia Inc, Finland. The pH measurements were performed using a semimicro Ross combination pH electrode calibrated with NIST-traceable standard buffers solutions. The reference solution was 3 M KCI. All experiments were performed at ambient temperature (23 k lo  C). The experiments using radiotracers were conducted in an argon atmosphere glove box. Polyethylene vials were used to contain the samples and were kept on a low speed shaking table during the experiment. Samples were collected from each vial and filtered through 4.1 nm Centricon-30 filters (Amicon Corporation, Beverly, MA). Filters were pre-saturated with the same solutions. Sample preparation We carried out three series of experiments using cesium with lS7Cs as radiotracer, three series with strontium and three series using strontium with 90Sr as radiotracer. For the cesium sorption study, the initial concentration of CsCl was varied from 1 O3 to 1 0-8 M. We added 3.0 x 1 0-9 M of the radioactive isotope lS7Cs to each solution in order to use a y analysis technique that provides a lower detection limit than chemical methods. For the strontium study, the initial concentration of SrC12. 6 H 2 0  was varied from 10-2to My ICP being used to determine the concentration of strontium in the solution. For the system with the concentration of strontium as SrC12. 6H20 was varied from lo4 to 10" M , we also added M of 90Sr to each solution to obtain a better detection limit using the liquid scintillation counter. A fourth series with vials containing only Cs or Sr solutions with the different concentrations mentioned above have used as blank samples to correct for any sorption on the walls of the vials. The sorption study of cesium was carried out over a period of 97 days. Table 2 describes the material present in each experimental vial. 2 For each vial of each series we added 30 mL of the corresponding cesium or strontium solution. The mixtures of series 1,2, and 3 were prepared two days before the addition of any solutions to allow for the colloidal silica gelling process to occur. - The soil consists mainly of quartz(60%) and clay (28%), with minor prsence of geothite (6%) and hematite (6Yo). The clay mineral in the e2 mm size fraction is primarily kaolinite (99Y0) with a very minor amount of vermiculite (1%). The soils were dried in an oven for 16 h at 100' C before addition of colloidal silica and calcium chloride. Results and Discussion To evaluate the sorption of each element, we used the sorption ratio coefficient which is defined as: R =  co -cf where : CO CO = initial concentration of the element of concern (Sr or Cs) (M) Cf = final concentration of the element of concern (Sr or Cs) (M) a) Cesium Sorption For cesium, the sorption is significant for all systems as R is generally higher than 0.9 and occasionally reached a value of 1. As can be seen in figure 1, the curve is almost flat in the range froml0" to M. This part of the curve represents the maximum uptake and is considered as the fixation capacity of the materials studied. At higher concentrations of cesium to 10-3 M) the sorption decreases due to incomplete saturation of the free sites on the substrate by cesium ions (Hakem eta/. 1995). Figure 2 shows clearly that the sorption is very fast and occured within the few fisrt days. A complete sorption equilibrium was established in 8 days for all series. b) Strontium Sorption The sorption data obtained by ICP showed that the presence of calcium, silica, and alumina in the samples interfered with the strontium measurement leading to higher values of Sr concentrations. In addition, the detection limit of the ICP is about 1.25 x10" M, which is higher than some of our experimental concentrations. Figure 3 shows a general tendancy of decrease in sorption as a function of Sr concentration, however, the errors in the measurements are high. We are repeatin this study with gost-. The presence of 90Sr allows us to achieve a detection limit less than 10- B M with the liquid scintillation counter. This work still in progress. Data Analysis We used the Freundlich isotherm to interpret the experimental results. This isotherm is described by the equation :Qab = k C$ (Sposito 1989) Qads and Ceq are the concentrations of the nuclide in the solid and liquid phases respectively after the sorption. n and k are positive values determined by plotting log Qads against log C,. Logk and n are the y intercept and the slope, respectively, of the resulting straight line. The curves obtained for cesium and strontium are plotted in figures 4 and 5, respectively. We obtained straight lines for all experimental systems indicating that the Freundlich isotherm fits our data very well. In table 3, we report the values of n and k for each element. We obtained values 3 of n between 0 and 1 which suggest that the structure of the adsorbant surface is heterogenous with an exponential energy distribution of the adsorption sites (Sposito 1989). Cesium Strontium System n -i- 0.05 logK -i- 0.1 n Itr 0.05 logK rt 0.1 Soil, 5-10 ft depth 0.700 -2.90 0.970 -2.99 Soill 0-20 ft  depth 0.810 -2.60 0.930 -3.21 cs 0.820 -2.40 1.190 -2.39 - Conclusion The high sorption ratio, R, for both of cesium and strontium suggests that they can be immobilized in the soil at SRS. Generally cesium sorption is higher than that of strontium because cesium ions have a lower hydration energy which enables the cations to shed their hydration shell upon entering the clay interlayers (28% of soil composition) (Cornell, R.M. 1992). The sorption behavior on geological material is highly specific and each measured sorption is valid only for specific conditions including pH, concentration of the element considered, presence and concentration of competing ions, temperature,. time for equilibrium and method of separation of solid and liquid phases. Therefore all realistic conditions prevailing at the site of interest must be taken into account. For a better understanding of the role of CS as gel liquid barrier, both desorption and diffusion studies must be done on such material. Acknowledgement This work was supported by the  U.S. Department of Energy, Office of Environmental Management, Office of Technology Development, Subsurface Contamination Focus Area, under Contract No. DE-AC03-76SF00098. References Adeleye, S A ,  P.G. Clay and M. 0. A. Oladipo. (1994) Sorption of caesium, strontium and europium ions on clay mineral. J. of Materials Science ., 29, 954-958. Cornell, R.M. (199 Adsorption of cesium on mineral: a reveiw. jof Radioanalytical and Nuclear Chemistry, 2) Articles., 171 (2). 483-500. Hakem, N, B. Fourest, and R. Guillaumont. (1995) Sorption of radionuclides at tracer level on mineral colloids. Sixth Annual lnternational Conference on H@h level Radioactive Waste Management., 237-239. Persoff, P, G.J. Moridis, J. Apps, K. Pruess, and S.J. Muller. (1994) Designing injectable colloidal silica barriers for waste isolation at the Hanford Site, Thirty-third Hanford Symposium on Health and the Environmant., 87-1 00. Sposito, G. (1 989) The chemistry of soils, pp.153-154. Oxford University Press. 4 1x10" 1 x 1 ~ '  i r l o b  1 x 1 0 ~  1 x 1 0 ~  1x10" Cesium Concentration (M) Figure 1. Sorption of Cesium as a Function of its Concentration 0 0 1x10' 1XlOb l X l 0 d  1x104 1 x 1 6  1x10' 1x10' Strontium Concentration (M) Figure 3. Sorption of Strontium as a Function of its Concentration 5 0 0 9 0 cs+soil10-20n ,,,, ,,,, ,,,, ,),, , I , ,  ,,,,~-', ,,(, , , G  0 Time (days) Figure 2 Sorption of Cesium at 1 E4 M as a Function of Time 6 0 CS+soilSlOft -1 1 ° 4  1 -11 -10 -9 -8 -7 .6 -5 4 -3 log (Cesium Equilibrium Concentration) (M) Figure4. Variation of the Amount Adsorbed as a Function of Cesium's Equilibrium Concentration , a 5  -7:: -55 -5 4.5 -4 -3.5 9 -25 log (Strontium Equilibrium Concentration) (M) Figure 5. Variation of the Amount Adsorbed as a Function of Strontium Equilibrium Concentration 7 

Sorption of cesium and strontium on Savannah River soils impregnated with colloidal silica

https://digital.library.unt.edu/ark:/67531/metadc676207/m2/1/high_res_d/463590.pdf

Sorption of cesium and strontium on Savannah River soils impregnated with colloidal silica

Abstract

Similar works

Full text

Available Versions

UNT Digital Library