Sorptive Removal of Cesium and Cobalt Ions in a Fixed bed Column Using Lewatit S100 Cation Exchange Resin

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1 Sorptive Removal of Cesium and Cobalt Ions in a Fixed bed Column Using Lewatit S100 Cation Exchange Resin M.R. El-Naggar, H.A. Ibrahim, and A.M. El-Kamash Hot Lab. Center, Atomic Energy Authority, P.O , Inshas, Cairo, Egypt. Received: 5/9/013 Accepted: 0/10/013 ABSTRACT The sorptive removal of cesium and cobalt ions from aqueous solutions in a fixed bed column packed with Lewatit S100 cation exchange resin has been investigated. A preliminary batch studies were performed to estimate the effect of ph and contact time on the sorption process. Results indicated that Cs + and Co + could be efficiently removed using Lewatit S100 at a ph range of -7 with more affinity towards Cs + than Co +. Kinetic models have been applied to the sorption rate data and the relevant parameters were determined. The obtained results indicated that the sorption of both Cs + and Co + on Lewatit S100 followed pseudo second-order rather than pseudo first-order or Morris-Webber model. Fixed bed experiments were conducted at a constant initial concentration of 100 mg/l whereas the effect of bed depth (3,.5 and 6 cm) and volumetric flow rate (3 and 5 ml/min.) on the breakthrough characteristics of the fixed bed sorption systems were determined. The experimental sorption data were fitted to the well-established column models namely; Thomas and BDST models to compute the different model parameters. The higher column sorption capacities were obtained at bed depth of 3 cm with a flow rate of 3 ml/min., for both Cs + and Co +. The BDST model appeared to describe experimental results better than Thomas model. Results indicate that Lewatit S100 is an efficient material for the removal of cesium and cobalt ions from aqueous solutions. Key Words: Cesium/ Cobalt/ Lewatit S100 / Fixed bed/ Breakthrough modeling INTRODUCTION Radioactive waste arises from the generation of nuclear power plants and the use of radioactive materials in medicinal, industrial, agricultural, research and educational applications (1). The grandness of the safe management of radioactive wastes for the protection of human health and the environment has long been recognized and considerable experience has been gained in this field (). Cesium and cobalt are two of the most hazardous pollutants identified in liquid radioactive wastes of low and intermediate levels. 137 Cs is a serious radionuclide with a relatively long radioactive half-life (T 1/ = 30 y), high activity and high solubility. 60 Co (T 1/ = 5.7 y) also is one of the most problematic waste nuclides in radioactive liquid waste streams. The treatment of such radioactive effluents requires concentration of the dissolved metal ions followed by recovery and secure disposal (3). Treatment of liquid radioactive waste involves the application of several steps such as filtration, precipitation, sorption, ion exchange, evaporation and/or membrane separation to meet the requirements for both the release of decontaminated effluents into the environment and the conditioning of waste concentrates for safe disposal (). Ion exchange is one of the most widely used techniques, both in the nuclear industry and in the conventional chemical industry, for the purification, separation and partitioning of particular nonradioactive and radioactive species with different chemical properties. Organic and inorganic, naturally occurring (5-8) and synthetic ion exchangers (9-1) have found their specific fields of application in different purification and liquid waste treatment processes (13). In many cases ion exchange is the most appropriate and most efficient method for the treatment of a variety of low and 77

2 intermediate level liquid waste streams. With respect to economy and efficiency, ion exchange stands between the other two major liquid waste treatment processes of evaporation and chemical precipitation (13). The development of new ion exchangers is narrowing the gap in decontamination factors between evaporation and ion exchange. Chemical precipitation, however, is often less expensive but is not always effective in removing radionuclides from solution (13). A wide range of natural and synthetic inorganic and organic materials is available for the ion exchange treatment of liquid radioactive waste. These materials are available in a variety of forms and have different chemical and physical properties (1). The main advantages of synthetic organic ion exchange resins are their high capacity, broad applicability, wide versatility and low cost relative to some synthetic inorganic media (15). Many researchers investigated the removal of heavy metals and pollutants from aqueous solutions using various types of Lewatit (16-) but few research studies have been reported for utilization of such resin types for removal of cesium and cobalt from liquid waste (3-5). The Lewatit MonoPlus S100 is a strongly acidic gel-type cation exchange resin of uniform particle size (monodispersed) based on a styrene-divinylbenzene copolymer (6). The main objective of the current study is to examine the feasibility of using Lewatit S100 for removal of cesium and cobalt ions from single and binary solutions. For this aim a batch and fixed bed column sorption studies were performed. Preliminary investigations were carried out to obtain the relevant kinetic data using simple kinetic models. Effects of flow rate and bed depth on sorption of both ions were studied, in single- and binary-component systems. The experimental results were analyzed using Thomas and BDST models. EXPERIMENTAL 1. Chemicals and Reagents All chemicals were of analytical grade and were used without further purification. Cesium chloride and cobalt chloride were purchased from Sigma-Aldrich Company. Stock solutions of Cs + and Co + ions were prepared by dissolving CsCl and CoCl in distilled water. Lewatit S100 cation exchange resin was obtained from Bayer Chemicals, Germany. The main features, chemical and physical properties of the resin are shown in Table (1).. Batch Kinetic Sorption Studies Table (1): Chemical and physical properties of Lewatit S100. Ionic form Na + Functional group sulfonic acid Matrix cross-linked polystyrene Structure gel type beads Appearance brown, translucent Density 1.8 g/l Stability at ph range 0-1 Mean bead size 0.58 (± 0.05) mm The effect of ph on the sorption of Cs + and/or Co + onto Lewatit S100 was studied. Batch sorption experiments were constructed by adding 10 ml of 100 mg/l CsCl and/or CoCl solutions to a series of 60 ml bottles, each containing 0.01 g resin. ph values were adjusted to be in the range of - 11 using dilute solutions of hydrochloric acid or sodium hydroxide. Kinetic studies were performed at room temperature (5 º C) using concentration of 00 mg/l for Cs + and/or Co +. Batch sorption experiments were carried out by shaking 0.03 g of the resin with 10 ml of Cs + and/or Co + solutions in a series of 60 ml bottles at ph of value 6. At different contact 78

3 time intervals the supernatant solutions were taken for measurement of metal ion concentrations. The amount sorbed by Lewatit S100 was calculated using the following equation: Where, Q t, C o, C t, V and m are the metal uptake (mg.g -1 ), initial metal ion concentration (mg.l -1 ), metal ion concentration at time t, volume of solution (l) and mass of resin (g), respectively. 3. Column Studies A glass column of 1.0 cm internal diameter and 15.0 cm length was used to perform the fixed bed column studies. The initial aqueous solution (100 mg/l for both ions) was fed continuously in down-flow mode. A series of experiments were performed with different bed depths (3,.5 and 6 cm) and at flow rates of 3 and 5 ml/min. Samples were regularly collected at determined time intervals and analyzed for concentrations of Cs + and Co + using atomic absorption spectrophotometer (Buck Scientific, VGP 10). 1. Effect of ph RESULTS AND DISCUSSION The ph of solution has significant impact on the uptake of metal ions, since it determines the surface charge of the adsorbent, the degree of ionization and specification of the adsorbate. The variation of ph affects the effectiveness as hydrogen ion itself is a tough competing ion. The sorption of Cs + and/or Co + was examined over a ph range from to 11 at room temperature (5 ± º C) (Fig.1- a,b). Sorption of both metal ions increased with increasing ph over the range of -7 at which the competition between hydrogen and metal ions for sorption sites decreased. On the other hand, in the acidic medium the surface of the adsorbent becomes more positively charged at a high H + concentration due to the high solubility and ionization of metal ions so that the attraction between adsorbents and metal cations is reduced (7). This trend can be illustrated by Eqs. (-5) for Co + and Cs +. Enhanced sorption of Co + ions on Lewatit S100 with increasing ph may be explained either by a mechanism involving the prior hydrolysis of the metal ions in the solution to give a hydrolysis product (Eq. ), which is more strongly adsorbed (Eq. 3) or by a mechanism involving the direct exchange of the un-hydrolyzed ions (Eq. ) with a specific group on the adsorbing surface in relatively low ph values (8). The latter trend is also observed in the case of sorption of cesium ions on Lewatit S100 (Eq. 5). The ph dependence of metal ion sorption may suggest that metal ion is sorbed by ion exchange mechanism (9). The maximum uptake was observed at ph range from to 7. It was observed during adjusting ph of samples that beyond ph 7 Co + begins to precipitate as CoCl. Therefore the optimum ph was chosen as 6 for further experiments. percentage removal of cesium (%) (a) PH Percentage Removal of Cobalt (%) Fig. (1) : Effect of ph on the sorption of (a) Cesium (b) Cobalt onto Lewatit S (b) (1) ph 79

4 Co H O CoOH H () CoOH S O S O CoOH Co S OH ) ( S O) Co H ( () Cs S OH S O Cs H (5). Effect of contact time The effect of contact on sorption of Cs + and Co + at 00 mg/l initial concentration onto Lewatit S100 is given in Fig. (1-c). It is clear that the sorption process appears to be fast where about 80% of the total amount of metal ions were sorbed in the first 30 min., while equilibrium was attained within min. Sorption of Cs + was higher than Co + indicating that the affinity of Lewatit S100 resin towards Cs + is more than Co +. This may be more due to the smaller hydrated ionic radii of monovalent Cs + than divalent Co + (.1 and.3 º A, respectively) making Cs + moves freely into and out resin channels. (3) (c) q t, mg/g Cs + Co Time, min. Fig. (1-c): Effect of contact time on the sorption of Cs + and Co + onto Lewatit S100 at ph 6. MATHEMATICAL MODELING 1. Sorption Kinetics It had been recognized that the characteristics of the surface of sorbent and hence its diffusion resistance have played an important role in the rate of sorption and accordingly the overall transport of the solute. In order to investigate the sorption mechanism of Cs + and Co + onto Lewatit S100, various kinetic models were used Pseudo first-order kinetic model Pseudo first-order assumes that the rate of change of sorbate uptake with time is directly proportional to the difference in the saturation concentration and the amount of solid uptake with time. Lagergren equation is the most widely used rate equations in liquid phase sorption which can be expressed as (30) : 80

5 dq t dt = k 1(q e q t ) (6) Where, k 1, q e and q t are the pseudo-first-order rate constant (min 1 ), concentration of the ion sorbed at equilibrium (mg.g -1 ) and concentration of ion sorbed at time t (mg.g -1 ), respectively. The linear form of pseudo first-order equation is given as follows (31) : log (q e q t ) = logq e k 1 (7).303 Fig. (a) shows the plotting of log(q e q t ) vesus t. Values of k 1 and q e were determined from the slope and intercept, respectively. It is observed that the sorption of both ions follows the Lagergren equation over the entire period of investigation. The calculated values of the first order model for both studied metal ions are given in Table (). 1.. Pseudo second-order kinetic model Pseudo second-order kinetic model developed by Ho and McKay ( 3) is based on the amount of sorbed sorbate on the sorbent. If the rate of sorption is governed by a second-order mechanism, the pseudo second-order chemisorption kinetics rate equation can be expressed as: dq t dt = k (q e q t ) (8) Where, k is the pseudo second-order rate constant (g.mg -1.min -1 ). The linear form of pseudo second-order equation is given as follows (33) : t = 1 q t k q + t e q e The linear plot obtained between t/q t and t (Fig b) was used to determine rate constant k and sorption capacity q e. The initial sorption rate, h, can be regarded as: h = k q e (10) The computed sorption capacity q e,calc values for pseudo second-order model was much closer to experimental values (Table ). Therefore, it is concluded that the pseudo second-order kinetic model better describes sorption of Cs + and Co + onto Lewatit S Weber-Morris kinetic model Weber and Morris model (3) is based on the assumption that the rate of intraparticle diffusion is varied proportionally with the half power of time and can be expressed as: q t = K d t + A (11) Where, K d is the rate constant for the intraparticle transport (mg. g -1.min -0.5 ) and A is a constant related to Webber-Morris kinetic model. Equation (11) is a general representation of the kinetics, where the intercept is related to the mass transfer across the boundary layer and the expected value of the exponent is 0.5. According to this model, a graphical plot for q t versus t could predict the sorption mechanism. Sorption is diffusion controlled process if a straight line, passing through the origin, is obtained. When the plots do not pass through the origin, this is indicative of some degree of boundary layer control and this (9) 81

6 further show that the intraparticle diffusion is not only the rate controlling step but also other processes control the rate of the sorption process (35). The kinetics of sorption for both ions onto Lewatit S100 resin were also computed by applying the Morris-Weber equation (36) : q t = K d t (1) As shown in Fig. (c,d) the Weber-Morris plots did not pass through the origin indicating that the mechanism of sorption was influenced by two or more steps of sorption process. This also indicates that intraparticle diffusion is not the only rate-controlling step. The values of rate constants for Cs + and Co + sorbed onto Lewatit S100 obtained from the slope of the linear plot are presented in Table ().. Column Performance Fixed bed column performance is conveniently described through the concept of the breakthrough curve that derived by plotting the normalized concentration C t/c o (where C t and C 0 are the influent and effluent metal ions concentration respectively) against either effluent volume or time. The shape of breakthrough curve and time of breakthrough curve are important factors for determining the operation and dynamic response of sorption column. The value of total sorbed metal ion quantity of a given influent concentration and flow rate is equal to the area under the plot of sorbed metal ions (Cs + or Co + ) (Fig. 3) concentration versus effluent according to the following equation (37) : Cs + Co (b) 0.6 log(q e -q t ) R = (a) R = Time, min. 65 t/q t, min.g.mg Cs + Co R = Time, min. R = (c) 60 (d) q t, mg/g 5 0 q t, mg/g 5 0 R = R = Cs + Co Cs + Co t 1/, min t 1/, min. Fig. (): a) pseudo first-order, b) pseudo second-order, c) and d) Morris-Webber kinetic modeling for sorption of Cs + and Co + ions onto Lewatit S100. 8

7 Table (): Kinetic model parameters fitted to the sorption of Cs + and Co + onto Lewatit S100. Kinetic models Parameters Pseudo first-order q e,calc (mg.g -1 ) k 1 (min -1 ) Pseudo second-order q e,calc (mg.g -1 ) k (g.mg -1.min -1 ) R H Morris-Webber K d (mg.g -1.min -1/ ) R A R Cs+ Co q e,exp (mg.g -1 ) Where, Q and C ads are the volumetric flow rate (ml.min -1 ) and concentration of sorbed metal ion (mg.l - 1 ), respectively. The total amount of ion sent to the column, X (mg), is calculated from the following equation: The total percent removal of the ion by the column, i.e. the ratio of the maximum capacity, q tot (mg), to the total amount of the metal ion amount sent to the column can be calculated from the following equation:.1 Effect of bed depth The breakthrough curves of sorption of Cs + and Co + from a single solution at bed depths (3,.5, and 6 cm) at a flow rate of 3 and 5 ml/min are shown in Fig. 3(a-d). As shown in Fig. 3(a, b) the shape of breakthrough curves of sorption of cesium at both 3 and 5 ml/min flow rates are significantly different as depth changed from 3 to 6 cm. But for sorption of cobalt Fig. 3(c, d) the shape of breakthrough are slightly different with variation of bed depth. At higher bed depths a larger volume of metal solution could be treated (Table 3) due to the increase in the amount of the resin where more active binding sites are available for proceeding the sorption process. The decreasing in uptake capacities by increasing the bed height is due to the change in volume to mass ratio of ion exchanger. The higher column capacity may be due to the fact that a continuously large concentration gradient occurred at the interface as it passes through the column while the concentration gradient decreased with time in the batch experiment (5).. Effect of flow rate As shown in Fig. 3(a-d), the effect of flow rate on sorption of Cs + and Co + from single solutions onto Lewatit S100 was investigated at 3 and 5 ml/min flow rates for each examined bed height (3,.5, and 6 cm). While in case of binary solutions the effect of the two flow rates on the sorption processes was examined at constant bed depth (.5 cm) and illustrated in Fig. 3(e). The breakthrough curves of sorption of Cs + and Co + from binary solution became steeper at both studied flow rates and the breakpoint time decreased. If these breakthrough curves are compared with those of single solutions, it is clear that the presence of other component developed a competition for ion exchange sites and some sites were occupied by the second metal ion. It can be deduced that the uptake of cobalt was retarded by the presence of cesium (13) (1) (15) 83

8 at both studied flow rates. Moreover, the sorption capacity of Lewatit S100 for both ions decreased in case of sorption from binary solution than from single solution. The results indicated that the affinity of the studied resin toward Cs + is greater than that of Co +. This may due to the smaller hydrated ionic radius of Cs + (3.9 º A) than Co + (.3 º A) leading to more free movement of Cs + into and out of the resin beads Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm Bed Depth: 3.0 cm Bed Depth:.5 cm Bed Depth: 6.0 cm Flow rate = 5 ml/min. 0.6 (a) 0.6 (b) C/C 0 C/C o Cesium 0. Cesium Cobalt 1.0 Cobalt C/C o (c) C/C (d) 0. Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm Bed depth, 3.0 cm Bed depth,.5 cm Bed depth, 6.0 cm C/C o (e) Cs + at flow rate = 3 ml/min. 0. Co + at flow rate = 3 ml/min. Cs + at flow rate = 5 ml/min. Co + at flow rate = 3 ml/min. Bed depth =.5 cm Fig. (3): Effect of flow rate and bed depth on the breakthrough curves of Cs + and Co + in singlecomponent systems (a-d) and the effect of flow rate on the breakthrough curves of Cs + and Co + in binary-component systems of metal chlorides(e). 8

9 Table (3): Fixed bed data of Cs + and Co + ions onto Lewatit S100 at different process parameters. Metal ions Q (ml.min -1 ) Z (cm) X (mg) qtot (mg) Total metal removal (%) Bed capacity (mg.g -1 ) Cs Co Binary system Cs Co Fixed Bed Column Models 3.1. Thomas model The Thomas model is one of the most general and widely used models in column performance theory (38). Thomas model is based on the assumption that the process follows Langmuir kinetics of adsorption-desorption with no axial dispersion. So that the driving force for sorption obeys secondorder reversible reaction kinetics. The shapes of the breakthrough curve and breakthrough time are important characteristics for determining the operation of the column and its dynamic response. Successful design and operation of an industrial sorption column requires the ability to predict its concentration-time profile or breakthrough curve. The main advantages of this model are its simplicity of application and adequate consistency in predicting the breakthrough curves under various operating conditions (39). This model can be represented by: (16) Where, C, K Th, Q, M and V eff are the effluent metal ion concentration (mg.l -1 ), Thomas rate constant (ml.min -1.mg -1 ), volumetric flow rate (ml.min -1 ), mass of resin (g) and effluent volume (ml), respectively. 3.. Bed depth service time model The bed depth service time model is a simple model for predicting the relationship between the bed height and the service time in terms of process concentrations and adsorption parameters and is generally exploited for the evaluation of different column design parameters. This model can be used to predict breakthrough of a packed bed during ion exchang 85

10 e or adsorption and can be expressed by (0) : (17) Where, C b, K, N o, Z and v are the breakthrough metal concentration (mg.l -1 ), bed depth service time rate constant (l.mg -1.min -1 ), adsorption capacity of the bed (mg.l -1 ), bed height (cm) and linear velocity (cm.h -1 ), respectively.. Applications of Kinetic Models for Breakthrough Analysis.1 Thomas model The linearized form of the Thomas model is given by Eq. (18) and was used to fit the experimentally obtained data by plotting against the effluent volume. With regard to sorption of Cs + and Co + from the single component solutions, Figs. (-5) show the experimental data fitted by Thomas model at 10% and 90% saturation. While, the fitted data of binary component solutions are shown in Fig. 6. (18) The Thomas rate constant and the maximum ion exchange capacity were determined from the slope and intercept respectively. The values of these two parameters are given in Table () for Cs + and Co + at 10% and 90% saturation. The rate constant K Th for Cs + was found to increase by increasing both bed depth and flow rate. For Co +, K Th increased by increasing both bed depth (3-6 cm ) and flow rate (3-5 ml/min.) in binary solution experiments... Bed depth service time (BDST) model The linear relationship between the bed height and the service time is given by (1) : (19) The model constants K and N o can be determined from the plot of Z against t in Eq. (19). Fig. (7) shows that the experimental data follow BDST model. Tables 5&6 present the values of BDST model parameters obtained from the slopes and intercept of the linear plots. It is obvious that an increase in flow rate results in decreasing in N 0 for cesium but increase for cobalt and increase in K for Cesium and cobalt. At 90% the logarithmic term yields a negative value of K. The BDST parameters are calculated at 50%,, and where the logarithmic term in Eq. (19) is reduced to zero giving the following equation: (0) 86

11 ln[(c o )-1] (a) R = 0.9 R = % saturation R = Bed Depth 3.0 cm Bed Depth.5 cm Bed Depth 6.0 cm ln[(c o )-1] (b) R = 0.95 R = 0.97 Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm 10% saturation R = % saturation (c) % saturation Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm ln[(c o )-1] 0 R = 0.98 R = 0.99 R = Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm ln[(c o )-1] R = 0.96 (d) R = 0.93 R = Fig. (): Effect of different flow rates and bed depths on Thomas model plots for sorption of Cs + in single-component systems onto Lewatit S100 at 10 (a-b) and 90% saturation (c-d). 87

12 ln[(c 0 )-1] R =0.97 R = % Saturation (a) R =0.91 ln[(c 0 ) -1] % Saturation R = 0.97 (b) R = 0.93 R = Bed Depth: 3.0 cm Bed Depth:.5 cm Bed Depth: 6.0 cm Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm R = % Saturation (c) R = % Saturation (d) R = 0.97 ln[(c 0 ) -1] 1 0 R = 0.96 R = 0.9 ln[(c 0 ) -1] 1 0 R = Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm - -3 Bed depth: 3.0 cm Bed depth:.5 cm Bed depth: 6.0 cm Fig. (5): Effect of different flow rates and bed depths on Thomas model plots for sorption of Co + in single-component systems onto Lewatit S100 at 10 (a-b) and 90% saturation (c-d). 88

13 ln[(c 0 ) -1] % Saturation R = 0.96 Cesium Cobalt (a) R = ln[(c 0 ) -1] % Saturation (b) R = 0.99 Cesium Cobalt R = Flow rate:3 ml/min. 3 90% Saturation (c) 6 90% Saturation (d) ln[(c 0 ) -1] R = 0.98 R = 0.98 ln[(c 0 ) -1] 0 R = 0.98 R = Cesium Cobalt - Cesium Cobalt Fig. (6): Effect of flow rate on Thomas model plots for sorption of Cs + and Co + in binary-component systems onto Lewatit S100 at 10 (a-b) and 90% saturation (c-d). 89

14 Service time, h (a) R = 0.97 R = 0.99 R = % Saturation 50 % Saturation 90 % Saturation Bed depth, cm Service time, h (b) R = 0.9 R = 0.99 R = % Saturation 50 % Saturation 90 % Saturation Bed depth, cm 11 0 (c) R = (d) R = 0.96 Service time, h R = 0.99 R = % Saturation 50 % Saturation 90 % Saturation Bed depth, cm Service time, h R = 0.95 R = % Saturation 50 % Saturation 90 % Saturation Bed depth, cm Fig. (7): Effect of flow rate on BDST model plots for sorption of Cs + (a-b) and Co + (c-d) onto Lewatit S100 at 10, 50 and 90% saturation. 90

15 Table (): Thomas model parameters for Cs + and Co + sorption on Lewatit S100 at different bed depths and flow rates. Metal ions % Saturation Flow rate Bed depth ml.min -1 KTh cm ml.min -1.mg -1 qo mg.g -1 R Cesium Cobalt Binary system Cesium Cobalt Table (5): BDST model parameters at 10% saturation of Cs + and Co + ions sorption on Lewatit S100 at different flow rates. Flow rate Metal ions ml.min -1 Cesium 3 5 Cobalt 3 5 No g.l -1 No mg.g -1 K L.mg -1 h -1 R

16 Table (6): BDST model parameters at 50% saturation of Cs + and Co + ions sorption on Lewatit S100 at different flow rates and bed depths. Metal ions Flow rate Bed depth No No t1/ ml.min -1 cm g.l -1 mg.g -1 h R Cesium Cobalt CONCLUSION Following conclusions can be drawn from the present study of sorption of cesium and cobalt ions on Lewatit S100 cation exchanger: The sorptive removal of Cs + and Co + from single and mixed aqueous solutions in a fixed bed column using Lewatit S100 cation exchange resin has been investigated. The experimental results indicate that this resin can be successfully used for the sorption of the studied metal ions in the ph range -7. The optimum ph chosen in this study was at value of 6. Batch kinetic studies show that a rapid uptake occurred within the first 30 min and the equilibrium is attained within 10 min. The sorption process follows the pseudo second-order rather than the pseudo first-order or Morris-Webber model. The results obtained show that the intraparticle diffusion is not the only rate controlling step, but also other processes may control the rate of sorption. Column experiments show that Cs + and Co + could be successfully removed by Lewatit S100 ion exchange resin. The column breakthrough curves were analyzed at different flow rates and bed depths. Data of ion exchange were well represented by Thomas and BDST models and the model parameters were estimated. BDST is the best fitting model with experimental data. The sorption data obtained using a mixed solution of Cs + and Co + indicated that the presence of second metal ion decreases both capacities. This may be due to the competition of the two metal ions for the same sorption sites on the resin. The sorption capacity and time of breakthroughs were dependent on the flow rate and bed height. Results also indicate that the affinity of the resin towards Cs + is greater than Co +. This may due to the smaller hydrated ionic radius of Cs + than Co +. REFERENCES (1) Disposal of radioactive waste, IAEA Safety standards, No. SSR-5, 011. () IAEA Safety standards for protecting people and the environment, DS390, (006). (3) IAEA Handling and treatment of radioactive aqueous wastes, Tech. Doc. No. 65, Vienna (199). () IAEA Combined methods for liquid radioactive waste treatment, Tech. Doc. No. 1336, Vienna (003). (5) D. Lua, Q. Cao, X. Cao, and F. Luo; J. Hazard. Mater.; 166, 39 (009). (6) S. Dultz, J.H. An, and B. Riebe; Appl. Clay Sci.; 67, 15 (01). 9

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