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Organophilic bentonites as adsorbents for radionuclides
Applied Clay Science 16 Ž2000. 1–13
Organophilic bentonites as adsorbents for radionuclides I. Adsorption of ionic fission products J. Bors a
a,)
, S. Dultz
b,1
, B. Riebe
a,2
Center of Radiation Protection and Radioecology, UniÕersity of HannoÕer, Herrenhauser Str. 2, ¨ D-30419 HannoÕer, Germany b Institute of Soil Science, UniÕersity of HannoÕer, Herrenhauser Str. 2, D-30419 HannoÕer, ¨ Germany
Abstract The retardation of radionuclides by engineered clay barriers is primarily controlled by the sorption potential of the mineral constituents. Adsorption and desorption experiments were performed with Iy, TcO4y, Csq and Sr 2q on MX-80 Wyoming-bentonite treated with hexadecylpyridinium ŽHDPyq. in amounts equivalent to 20%–400% of the cation exchange capacity ŽCEC.. Bidistilled water, synthetic ground water ŽSGW., and sea water with half of the ionic strength ŽSear2. were used as equilibrium solutions, and 125 I, 95m Tc, 134Cs and 85Sr were employed as tracers. In HDPy-bentonite, Iy and TcO4y exhibited increasing adsorption Žcharacterized by the distribution ratio, R d ., while Csq and Sr 2q ions showed decreasing adsorption with increasing organophilicity. The extent of the adsorption as well as the reversibility of the binding processes was influenced by the chemical composition of the equilibrium solutions. These effects were more pronounced in the case of the cationic fission products compared to the anions investigated. Generally, sorption and desorption were linear over a wide concentration range of the carrier ions investigated, indicating that adsorption was independent from the sorbate concentrations Žup to ; 10y1 mmol gy1 organo-bentonite.. This suggests ion exchange as the principal sorption mechanism. Adsorption capacities for the anions investigated were estimated to be ; 0.5 mol c kgy1, for the cations to be ; 0.1 mol c kgy1 depending on the HDPyq loading of the samples. In case of cationic radionuclides, Csq was preferentially adsorbed in competition with the bivalent Sr 2q ions in both the untreated and modified samples. The results of this study may
)
Corresponding author. Fax: q49-511-762-3008; E-mail: [email protected] Fax: q49-511-762-5559; E-mail: [email protected]. 2 E-mail: [email protected]. 1
0169-1317r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 4 1 - 1
2
J. Bors et al.r Applied Clay Science 16 (2000) 1–13
be applicable to predict the long-term behaviour of fission products in waste disposal sites equipped with clay barriers. q 2000 Elsevier Science B.V. All rights reserved. Keywords: adsorption; iodide; pertechnetate; cesium; strontium; organophilic bentonite
1. Introduction In the construction of repositories for the disposal of spent nuclear fuel and fission products, bentonite clay has been proposed Ž e.g., in Sweden, Finland, Switzerland. as host or as principal component of engineered barrier material ŽPusch and Karnland, 1990; Muurinen, 1994; McCombie et al., 1995. . In a recent paper, Madsen Ž1998. summarized the mineralogical and geotechnical investigations of bentonite with respect to nuclear waste disposal. Bentonite is characterized by a low hydraulic conductivity and excellent sorption capabilities for cationic radionuclides, but is generally ineffective in adsorbing anionic contaminants. This is especially of concern in the case of the long-lived 129 I and 99 Tc, which exist predominantly as anions in aqueous environments Ž Lieser and Bauscher, 1987; Lieser and Steinkopff, 1989; Oscarson et al., 1994. . Extensive studies have shown, however, that the sorptive capabilities of clay minerals for anionic radionuclides can be improved substantially by replacing natural inorganic interlayer cations with quaternary alkylammonium ions of the form wRNŽ CH 3 . 3 xq, where R is an alkyl or aromatic hydrocarbon. The resulting organo-clays are capable of sorbing non-ionic organic compounds Ž Mortland et al., 1986; Stockmeyer, 1991; Boyd and Janes, 1993. as well as iodide Ž Bors, 1990.. After modification of bentonite, vermiculite and Cretaceous clay by the long C-chain hexadecylpyridinium Ž HDPyq. and benzethonium Ž BEq., these organo-clays exhibited sorption capabilities for iodide, which are several orders of magnitude higher than those of untreated samples. Moderate increases of the sorption parameters were found after cation exchange with hexadecyltrimethylammonium ŽHDTMAq., while the treatment with trimethyl-phenylammonium ŽTMPAq. and tetramethylammonium Ž TMAq. were ineffective in this respect. Furthermore, in experiments with the hexadecylpyridinium Ž HDPyq.-vermiculite system, it was shown that the partial replacement of the inorganic cations makes the organo-clays capable to sorb comparable amounts of anions Ž Iy. and cations ŽCsq, Ca2q . simultaneously ŽBors et al., 1994, 1997. . In the present work, the sorption behavior of the anionic radionuclides iodide and technetium as pertechnetate and of the cesium and strontium cations on MX-80 bentonite with different HDPyq saturations, was studied using equilibrium solutions with different ionic strengths. Another matter of interest was the selective sorption of cations such as cesium and strontium ions on original and HDPy-modified bentonites. The mineralogical and chemical characterization of the organophilic bentonites, which can help to explain adsorption mechanisms,
J. Bors et al.r Applied Clay Science 16 (2000) 1–13
3
and can give indications on thermal stabilities of the samples, is described in Dultz and Bors Ž2000. of this special issue.
2. Materials and methods 2.1. Preparation of organophilic bentonites Commercial Wyoming-bentonite MX-80, originally saturated with Naq to 86% of the CEC, was used. Its CEC is 76 cmol c kgy1 and the interlayer charge per half unit cell is 0.30. The characterization and composition of MX-80 are described in more detail elsewhere ŽMuller-Vonmoos and Kahr, 1983; Madsen, ¨ 1998.. Ca- and Sr-saturated bentonites were produced by repeated washing with a 0.1-M CaCl 2 or SrCl 2 solution. After dispersion of 20.0 g of the clay in 1.0 l of distilled water, the chloride salt of the quaternary alkylammonium ion of HDPyq was added. In order to prepare HDPy-bentonites of different HDPyq loadings, the organic ion was added in amounts corresponding to different levels Ž 20% to 400%. of the CEC of the bentonite; hereafter this is referred to as HDPy-20, or HDPy-400 bentonite. The suspensions were stored for 18 h and filtered. The filter residues were then washed at least eight times with water Ž 1.0 l total. to remove excess organic cations. The samples were freeze-dried, and the uptake of HDPyq was determined by measuring the C content with a LECO C-determinator Ž IR 12. . 2.2. Procedure Sorption of the elements under concern was carried out with ; 37 kBq of I, 95m Tc, 134Cs and 85Sr Ž; 5 = 10y12 M. and the corresponding carriers KI, CsCl and SrCl 2 in concentrations ranging between 10y8 and 1 M. As no stable isotope of technetium exists, ReO4y was used as carrier for TcO4y in the same concentrations. According to experimental results, size, structure and adsorption behavior of ReO4y are similar to those of TcO4y Ž Kang et al., 1998. . The investigations on anion Ž Iy and TcO4y. and cation Ž Csq and Sr 2q . adsorption were performed applying the batch technique, where adsorption is commonly characterized by the distribution ratio Ž R d-value.. R d Ž l kgy1 . is defined as the ratio between the concentration of solute adsorbed on the solid matrix Ž mol kgy1 . and the concentration of the solute in the equilibrium solution Ž M. . In batch experiments, 0.2 g of the organo-clays were dispersed in 10 ml of bidistilled water or in synthetic ground water Ž SGW.. The chemical composition of SGW is summarized in Table 1. In the experiments with strontium, SO42y was replaced by Cly in equivalent amounts, in order to avoid SrSO4 precipitation. The solid material remained within the 30 ml centrifuge tubes throughout the sequences: Ž1. Pretreatment without tracers for three-times for 5 days, Ž 2. 125
J. Bors et al.r Applied Clay Science 16 (2000) 1–13
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Table 1 Composition of equilibrium solutions Žg ly1 . Ion 2q
Ca Mg 2q Naq Kq HCOy 3 SO42y Fy Cly NOy 3 Total dissolved solids Ion strength ŽM.
SGW
Sear2
0.28 0.04 0.15 0.10 – 0.17 – 0.49 0.55 1.78 0.037
0.21 0.65 5.39 0.20 0.07 1.36 – 9.68 – 17.54 0.346
adsorption for 7 days with the tracers, and Ž 3. desorption for 7 days, under shaking at 228C. After these treatments, the solid and liquid phases were separated by centrifugation Ž 15000 rpm, 20000= g for 15 min. and filtration Ž 0.2 mm.. Sorption R d-values Ž R d,so . were calculated from the measured radioactivities Žmeasured by gamma-spectroscopy. in the solution before and after incubation. Sorption isotherms were calculated using the formulae: M ls M
0 l
M s s Rd M
A lrA0 l
Ž1. Ž2.
where w M x l is the equilibrium concentration of the sorbate in the solution in M and w M x 0l the initial sorbate concentration in the solution Ž M .. A l is the radioactivity of the tracers in the liquid at the end of adsorption experiment in cpm mly1, and A0 is the total radioactivity at the beginning of the experiments in cpm mly1. w M xs is the amount of the nuclide adsorbed in mol kgy1. To investigate the desorption, the centrifuged and decanted wet minerals were dispersed again in 10 ml of the electrolytes without tracer and shaken for 7 days at 228C. After separation of the two phases, the activity of the liquid was measured, and desorption R d-values Ž R d,de . were calculated taking into account the remaining activity in the solution of the wet samples by determining the volume of the liquid after sorption and decantation. All experiments were carried out in triplicates. The overall mean error was - 10%. In a double-labeling experiment with Csq and Sr 2q, the competing adsorption of the two cations by the clays was tested. The sum of the ion concentrations in equivalents was kept constant Ž 10y1 or 10y2 M. , but the relation of Csq to Sr 2q was varied Žequivalents Cs:Srs 1:4, 2:3, 3:2 and 4:1. . After
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attaining equilibrium, exchange isotherms were determined using the following equations: ECs ,s s ECs ,l s
CCs ,s CCs ,s q 2CSr ,s aCs ,l aCs ,l q 2 aSr ,l
,
Ž3. Ž4.
where CCs,s s Csq sorbed wmol kgy1 x, CSr,s s Sr 2q sorbed wmol kgy1 x, aCs,l s chemical activity of Csq in the equilibrium solution w M x, aSr,l s chemical activity of Sr 2q in the equilibrium solution w M x. From thermodynamic equilibrium constants, Vanselow-selectivity coefficients, K v , can be calculated ŽSposito, 1981. : Kvs
2 Ž CCs ,s . aSr ,l 2
CSr ,s Ž aCs ,l . .
Ž5.
3. Results and discussion 3.1. Variation of R d-Õalues for anions and cations with the amounts of HDPy q applied The different sorption behavior of anions Ž Iy, TcO4y. and cations Ž Csq, Sr 2q . as influenced by the concentrations of HDPyq applied is illustrated in Fig. 1.
Fig. 1. Sorption, R d,so-values for the anions Iy and TcO4y ŽReO4y . and for the cations Csq and Sr 2q using SGW and the corresponding carriers Ž1=10y5 M. on MX-80 bentonite as a function of the amounts of hexadecylpyridinium ŽHDPyq . applied.
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The distribution ratios in all cases were obtained from experiments with carrier concentrations of 1 = 10y5 M using SGW as equilibrating solution. It can be seen that iodide and pertechnetate sorptions increase, while cesium and strontium sorptions decrease with increasing HDPyq saturation. The increased adsorption of cesium in HDPy-20 and HDPy-50 bentonites compared to the untreated material is remarkable. Most probably, structural Ž e.g., increased basal spacing. andror electrochemical changes of the montmorillonite crystals are responsible for this result, which has been repeatedly observed. It needs, however, further clarification. Considering the construction of engineered barriers, it is important to realize that bentonite saturated with HDPyq to about 70%–90% of the CEC is capable to sorb both anions as well as cations in comparable amounts. 3.2. Adsorption of iodide and pertechnetate The distribution ratios resulting from experiments with Na-bentonite, treated with HDPyq in amounts corresponding to 200% of the CEC are related to the sorbed carrier concentrations in the equilibrium solutions of bidistilled water, SGW and Sear2 water for iodide ŽFig. 2a. and for pertechnetate ŽFig. 2b., respectively. Generally, lower distribution ratios are found for the SGW Ž R d ; 2000–4000 l kgy1 . than for bidistilled water Ž R d ; 4000–14000 l kgy1 . , and even lower ones when using Sear2 water, stressing the importance of the chemical composition of the equilibrium solution. One possible explanation for the effects of the electrolytes with higher ionic strengths on anion sorption is, that part of the organic cations may be desorbed which have been bound only slightly because of the relatively high HDPyq-loading of 341 g kgy1 Ž see Part II.. The clarification of this question is expected from running DOC measurements and estimations of the HDPyq concentrations in the liquids before and after treatments. The distribution ratios remain nearly constant up to the relatively high sorbed ion concentrations of ; 1 = 10y1 mol kgy1 indicating saturation of sorption sites. At higher sorbate concentrations, a sharp decrease of the R d-values can be observed. With bidistilled water, a sorption maximum at iodide and pertechnetate concentrations of about 5 = 10y2 mol kgy1 occurred. This repeatedly observed phenomenon is not well understood yet and needs further attention, taking into account different particle size fractions. According to the experimental results, basal spacing expansions and ion exchange reactions are dependent on particle sizes Ž von Reichenbach, 1973. . The possible role of the two different montmorillonite types, detected recently for MX-80 bentonite ŽKasbohm et al., 1998., should also be taken into consideration in this respect. On the basis of the R d-values, adsorption and desorption isotherms Ž fractional loading of the sorbent vs. equilibrium concentration in the liquid. were determined for the HDPy-200 bentonite samples. Fig. 3 shows the sorption and
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Fig. 2. Iodide Ža. and pertechnetate Žb. sorption Ž R d-values. on MX-80 bentonite treated with HDPyq in amounts corresponding to 200% of the CEC using bidistilled water, SGW and Sear2 solutions.
desorption isotherms for iodide ŽFig. 3a. , and for pertechnetate Ž Fig. 3b. with bidistilled water, SGW and Sear2 water, respectively. The log–log plot of the isotherms revealed linearity up to ion concentrations of about 5 = 10y3 M or 1 = 10y1 mol kgy1, confirming ion exchange as the major sorption mechanism ŽFreundlich type isotherm with slope ; 1.. At higher concentrations, the shapes of the curves shift to non-linearity suggesting that besides ion exchange other types of mechanisms may be involved in iodide fixation. Most likely, the existence of binding sites with different reactivities is responsible for the observed phenomenon. The similarity of the sorption and desorption curves with SGW and Sear2 for pertechnetate, and with Sear2 water for iodide, suggests reversible sorption processes, whereas with bidistilled water, and for iodide also with SGW, the sorption seems to be partially reversible only.
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Fig. 3. Sorption and desorption isotherms for iodide Ža. and pertechnetate Žb. in MX-80 bentonite treated with HDPyq in amounts corresponding to 200% of the CEC using different equilibrium solutions.
3.3. Adsorption of cesium and strontium The effects of the ionic strength of the different electrolyte solutions on cesium and strontium adsorption was investigated with MX-80 bentonite treated with HDPyq in amounts corresponding to 50% CEC. The sorption of the cations decreases gradually with increasing ionic strength of the electrolyte solutions ŽFig. 4.. Compared to bidistilled water two orders of magnitude lower R d-values are observed for cesium using SGW Ž Fig. 4a. . Strontium adsorption seems to be affected by the chemical composition of the solutions even more than that of cesium Ž Fig. 4b.. Obviously, the bivalent cations Ž Ca2q and Mg 2q . of the equilibrium solutions, rather than the monovalent ones Ž Naq and Kq. interfere with the bivalent Sr 2q ions. With Sear2 water, the adsorption of cations decreases further Ž R d ; 100 l kgy1 for cesium.. Strontium adsorption with R d - 10 l kgy1 was found to be particularly sensitive to the ionic constituents of Sear2 water. Fig. 5 shows the adsorption and desorption isotherms for the HDPy-50 bentonite samples using bidistilled water, SGW and Sear2 water Žno desorption data are available with bidistilled water. . Generally, the log–log plot of the
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Fig. 4. Cesium Ža. and strontium Žb. sorption Ž R d-values. on MX-80 bentonite treated with HDPyq in amounts corresponding to 50% of the CEC using bidistilled water, SGW and Sear2 solutions.
isotherms showed linearity over a broad range of carrier concentrations indicating ion exchange adsorption mechanisms. The agreement of the sorption and desorption curves with SGW and Sear2 water for cesium suggests reversible sorption processes Ž Fig. 5a., whereas in the case of strontium with Sear2 water a hysteresis can be detected ŽFig. 5b.. More generally, a rather complicated
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Fig. 5. Sorption and desorption isotherms for cesium Ža. and strontium Žb. in MX-80 bentonite treated with HDPyq in amounts corresponding to 50% of the CEC using different equilibrium solutions.
sorption mechanism can be assumed for the bivalent Sr 2q, taking ion exchange and surface related processes into account.
Fig. 6. Cs–Sr exchange isotherms for Sr 2q and Csq-saturated MX-80 bentonite and HDPy-bentonite. Dashed lines represent the non-preference exchange isotherms ŽSposito, 1981..
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Fig. 7. Vanselow selectivity coefficients, K v , for the Cs–Sr exchange in original and HDPy-bentonite.
3.4. Cs–Sr selectiÕity The Cs–Sr-cation exchange chemistry of MX-80 bentonite is illustrated in Figs. 6 and 7. In Fig. 6, the equivalent mole fraction of Csq on the sorbent w ECs xs is related to the equivalent mole fraction of Csq in the liquid w ECs x l . The dashed curve is the thermodynamic non-preference isotherm for a binary monovalent-bivalent cation exchange Ž Sposito, 1981. . The Cs–Sr exchange curves of both original and HDPy-bentonite are positioned clearly above the non-preference isotherms indicating the preferential sorption of Csq over Sr 2q. The calculation of the Vanselow selectivity coefficients Ž K v . for Cs–Sr resulted in K v s ln 4.0, confirming the implications of Fig. 6. Higher K v-values exist for the HDPy-bentonite than for the untreated samples Ž Fig. 7. . The preferential adsorption of Csq can be explained by the greater ionic diameter and lower hydration energy of Csq.
4. Conclusions In organophilic MX-80 bentonite, Iy and TcO4y exhibited increasing adsorption, while Csq and Sr 2q showed decreasing adsorption with increasing alkylammonium ion ŽHDPyq. incorporation into the clay. Especially for Sr 2q ions, adsorption is affected by the composition of the equilibrium solutions. Reversibility of the processes was found for TcO4y and Csq with SGW, and Sear2 water, and for Iy with Sear2 water, while for the adsorption and desorption
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behavior of Sr 2q a hysteresis was observed. In both, original and HDPy-bentonite, Csq is preferentially adsorbed over Sr 2q, which can be explained by the greater ionic diameter and the lower hydration energy of Csq. Additional information concerning the effectivity of HDPy-bentonite as engineered barrier is expected from a series of presently running diffusion experiments. Attempts will also be made to investigate, if organo-bentonites can be used as effective sorbents for anionic chemical pollutants, such as CNy, CrO42y and AsO43y. As the organo-bentonites showed thermal stability up to 2008C, their use in nuclear waste isolation should be taken into consideration.
Acknowledgements The studies on organophilic bentonite are funded by the Nuclear Fission Safety Programme of the European Commission under the Project No. FI4WCT950012. The skilled technical assistance of Ms. G. Erb-Bunnenberg and Mr. K.-H. Iwannek is gratefully acknowledged.
References Bors, J., 1990. Sorption of radioiodine in organo-clays and soils. Radiochim. Acta 51, 139–143. Bors, J., Gorny, A., Dultz, St., 1994. Some factors affecting the interactions of organophilic clay minerals with radioiodine. Radiochim. Acta 66r67, 309–313. Bors, J., Gorny, A., Dultz, S., 1997. Iodide, caesium and strontium adsorption by organophilic vermiculite. Clay Miner. 32, 21–28. Boyd, S.A., Janes, W.F., 1993. Role of layer charge in organic contaminant sorption by organo-clays. In: Mermut, A.R. ŽEd.., Layer Charge Characteristics of 2:1 Silicate Clay Minerals, Vol. 6. CMS Workshop Lectures. The Clay Minerals Society, Boulder, CO, USA. Dultz, S., Bors, J., 2000. Organophilic bentonites as adsorbents for radionuclides: II. Chemical and mineralogical properties of HDPy-montmorillonite. Appl. Clay Sci. 16, 15–29, Žthis issue.. Kang, M.J., Chun, K.S., Rhee, S.W., Moon, H., Fanghanel, Th., Neck, V., 1998. Sorption of ¨ MO4y ŽM s Tc, Re. on MgrAl layered double hydroxide and characterization of reconstructed LDH with ReO4y. Radiochim. Acta, in press. Kasbohm, J., Henning, K.-H., Herbert, H.-J., 1998. Tranmissionselectronenmicroskopische Untersuchungen am Bentonit MX-80. In: Henning, K.-H., Kasbohm, J. ŽEds.., Tone in der Geotechnik und Baupraxis. Berichte der Deutschen Ton- und Tonmineralgruppe e.V. -DTTG-, Band 6, pp. 228–236. Lieser, K.H., Bauscher, Ch., 1987. Technetium in the hydrosphere and in the geosphere. Radiochim. Acta 42, 205–213. Lieser, K.H., Steinkopff, T.H., 1989. Chemistry of radioactive iodine in the hydrosphere and in the geosphere. Radiochim. Acta 46, 49–55. Madsen, F.T., 1998. Clay mineralogical investigations related to nuclear waste disposal. Clay Miner. 33, 109–129.
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McCombie, C., McKinlay, I.G., Lambert, A., Thury, M., Birkhauser, P., 1995. The swiss strategy ¨ for HLW siting; parallel investigation of two alternative host rocks. Scientific Basis for Nuclear Waste Management 18, 1363–1370. Mortland, M.M., Shaobai, S., Boyd, S.A., 1986. Clay-organic complexes as adsorbents for phenol and chlorophenols. Clays Clay Miner. 34, 581–585. Muller-Vonmoos, M., Kahr, G., 1983. Mineralogische Untersuchungen von Wyoming Bentonit ¨ MX-80 und Montigel. NTB 83-12, pp. 1–15. Muurinen, A., 1994. Diffusion of anions and cations in compacted sodium bentonite. PhD Thesis, Univ. Helsinki. VTT Publications, Finnland, 168 pp. Oscarson, D.W., Hume, H.B., Choi, J.-W., 1994. Diffusive transport in compacted mixtures of clay and crushed granite. Radiochim. Acta 65, 189–194. Pusch, R., Karnland, O., 1990. Preliminary report on longevity of montmorillonite clay under repository-related conditions. Report SKB-90-44. Swedish Nuclear Fuel and Waste Management, Stockholm, ISSN 0284-3757, 47 pp. Sposito, G., 1981. The thermodynamic theory of ion exchange. The Thermodynamics of Soil Solutions, Chap. 5. Clarendon Press, Oxford. Stockmeyer, M.R., 1991. Adsorption of organic compounds on organophilic bentonites. Appl. Clay Sci. 6, 39–57. von Reichenbach, G.H. 1973. Exchange equilibria of interlayer cations in different particle size fractions of biotite and phlogopite. In: Serratosa, J.M. ŽEd.., Proc. Intern. Clay Conf. Madrid, 1972 Div. De Ciencias, CSIC, Madrid, pp. 457–466.
Organophilic bentonites as adsorbents for radionuclides I. Adsorption of ionic fission products J. Bors a
a,)
, S. Dultz
b,1
, B. Riebe
a,2
Center of Radiation Protection and Radioecology, UniÕersity of HannoÕer, Herrenhauser Str. 2, ¨ D-30419 HannoÕer, Germany b Institute of Soil Science, UniÕersity of HannoÕer, Herrenhauser Str. 2, D-30419 HannoÕer, ¨ Germany
Abstract The retardation of radionuclides by engineered clay barriers is primarily controlled by the sorption potential of the mineral constituents. Adsorption and desorption experiments were performed with Iy, TcO4y, Csq and Sr 2q on MX-80 Wyoming-bentonite treated with hexadecylpyridinium ŽHDPyq. in amounts equivalent to 20%–400% of the cation exchange capacity ŽCEC.. Bidistilled water, synthetic ground water ŽSGW., and sea water with half of the ionic strength ŽSear2. were used as equilibrium solutions, and 125 I, 95m Tc, 134Cs and 85Sr were employed as tracers. In HDPy-bentonite, Iy and TcO4y exhibited increasing adsorption Žcharacterized by the distribution ratio, R d ., while Csq and Sr 2q ions showed decreasing adsorption with increasing organophilicity. The extent of the adsorption as well as the reversibility of the binding processes was influenced by the chemical composition of the equilibrium solutions. These effects were more pronounced in the case of the cationic fission products compared to the anions investigated. Generally, sorption and desorption were linear over a wide concentration range of the carrier ions investigated, indicating that adsorption was independent from the sorbate concentrations Žup to ; 10y1 mmol gy1 organo-bentonite.. This suggests ion exchange as the principal sorption mechanism. Adsorption capacities for the anions investigated were estimated to be ; 0.5 mol c kgy1, for the cations to be ; 0.1 mol c kgy1 depending on the HDPyq loading of the samples. In case of cationic radionuclides, Csq was preferentially adsorbed in competition with the bivalent Sr 2q ions in both the untreated and modified samples. The results of this study may
)
Corresponding author. Fax: q49-511-762-3008; E-mail: [email protected] Fax: q49-511-762-5559; E-mail: [email protected]. 2 E-mail: [email protected]. 1
0169-1317r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 4 1 - 1
2
J. Bors et al.r Applied Clay Science 16 (2000) 1–13
be applicable to predict the long-term behaviour of fission products in waste disposal sites equipped with clay barriers. q 2000 Elsevier Science B.V. All rights reserved. Keywords: adsorption; iodide; pertechnetate; cesium; strontium; organophilic bentonite
1. Introduction In the construction of repositories for the disposal of spent nuclear fuel and fission products, bentonite clay has been proposed Ž e.g., in Sweden, Finland, Switzerland. as host or as principal component of engineered barrier material ŽPusch and Karnland, 1990; Muurinen, 1994; McCombie et al., 1995. . In a recent paper, Madsen Ž1998. summarized the mineralogical and geotechnical investigations of bentonite with respect to nuclear waste disposal. Bentonite is characterized by a low hydraulic conductivity and excellent sorption capabilities for cationic radionuclides, but is generally ineffective in adsorbing anionic contaminants. This is especially of concern in the case of the long-lived 129 I and 99 Tc, which exist predominantly as anions in aqueous environments Ž Lieser and Bauscher, 1987; Lieser and Steinkopff, 1989; Oscarson et al., 1994. . Extensive studies have shown, however, that the sorptive capabilities of clay minerals for anionic radionuclides can be improved substantially by replacing natural inorganic interlayer cations with quaternary alkylammonium ions of the form wRNŽ CH 3 . 3 xq, where R is an alkyl or aromatic hydrocarbon. The resulting organo-clays are capable of sorbing non-ionic organic compounds Ž Mortland et al., 1986; Stockmeyer, 1991; Boyd and Janes, 1993. as well as iodide Ž Bors, 1990.. After modification of bentonite, vermiculite and Cretaceous clay by the long C-chain hexadecylpyridinium Ž HDPyq. and benzethonium Ž BEq., these organo-clays exhibited sorption capabilities for iodide, which are several orders of magnitude higher than those of untreated samples. Moderate increases of the sorption parameters were found after cation exchange with hexadecyltrimethylammonium ŽHDTMAq., while the treatment with trimethyl-phenylammonium ŽTMPAq. and tetramethylammonium Ž TMAq. were ineffective in this respect. Furthermore, in experiments with the hexadecylpyridinium Ž HDPyq.-vermiculite system, it was shown that the partial replacement of the inorganic cations makes the organo-clays capable to sorb comparable amounts of anions Ž Iy. and cations ŽCsq, Ca2q . simultaneously ŽBors et al., 1994, 1997. . In the present work, the sorption behavior of the anionic radionuclides iodide and technetium as pertechnetate and of the cesium and strontium cations on MX-80 bentonite with different HDPyq saturations, was studied using equilibrium solutions with different ionic strengths. Another matter of interest was the selective sorption of cations such as cesium and strontium ions on original and HDPy-modified bentonites. The mineralogical and chemical characterization of the organophilic bentonites, which can help to explain adsorption mechanisms,
J. Bors et al.r Applied Clay Science 16 (2000) 1–13
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and can give indications on thermal stabilities of the samples, is described in Dultz and Bors Ž2000. of this special issue.
2. Materials and methods 2.1. Preparation of organophilic bentonites Commercial Wyoming-bentonite MX-80, originally saturated with Naq to 86% of the CEC, was used. Its CEC is 76 cmol c kgy1 and the interlayer charge per half unit cell is 0.30. The characterization and composition of MX-80 are described in more detail elsewhere ŽMuller-Vonmoos and Kahr, 1983; Madsen, ¨ 1998.. Ca- and Sr-saturated bentonites were produced by repeated washing with a 0.1-M CaCl 2 or SrCl 2 solution. After dispersion of 20.0 g of the clay in 1.0 l of distilled water, the chloride salt of the quaternary alkylammonium ion of HDPyq was added. In order to prepare HDPy-bentonites of different HDPyq loadings, the organic ion was added in amounts corresponding to different levels Ž 20% to 400%. of the CEC of the bentonite; hereafter this is referred to as HDPy-20, or HDPy-400 bentonite. The suspensions were stored for 18 h and filtered. The filter residues were then washed at least eight times with water Ž 1.0 l total. to remove excess organic cations. The samples were freeze-dried, and the uptake of HDPyq was determined by measuring the C content with a LECO C-determinator Ž IR 12. . 2.2. Procedure Sorption of the elements under concern was carried out with ; 37 kBq of I, 95m Tc, 134Cs and 85Sr Ž; 5 = 10y12 M. and the corresponding carriers KI, CsCl and SrCl 2 in concentrations ranging between 10y8 and 1 M. As no stable isotope of technetium exists, ReO4y was used as carrier for TcO4y in the same concentrations. According to experimental results, size, structure and adsorption behavior of ReO4y are similar to those of TcO4y Ž Kang et al., 1998. . The investigations on anion Ž Iy and TcO4y. and cation Ž Csq and Sr 2q . adsorption were performed applying the batch technique, where adsorption is commonly characterized by the distribution ratio Ž R d-value.. R d Ž l kgy1 . is defined as the ratio between the concentration of solute adsorbed on the solid matrix Ž mol kgy1 . and the concentration of the solute in the equilibrium solution Ž M. . In batch experiments, 0.2 g of the organo-clays were dispersed in 10 ml of bidistilled water or in synthetic ground water Ž SGW.. The chemical composition of SGW is summarized in Table 1. In the experiments with strontium, SO42y was replaced by Cly in equivalent amounts, in order to avoid SrSO4 precipitation. The solid material remained within the 30 ml centrifuge tubes throughout the sequences: Ž1. Pretreatment without tracers for three-times for 5 days, Ž 2. 125
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Table 1 Composition of equilibrium solutions Žg ly1 . Ion 2q
Ca Mg 2q Naq Kq HCOy 3 SO42y Fy Cly NOy 3 Total dissolved solids Ion strength ŽM.
SGW
Sear2
0.28 0.04 0.15 0.10 – 0.17 – 0.49 0.55 1.78 0.037
0.21 0.65 5.39 0.20 0.07 1.36 – 9.68 – 17.54 0.346
adsorption for 7 days with the tracers, and Ž 3. desorption for 7 days, under shaking at 228C. After these treatments, the solid and liquid phases were separated by centrifugation Ž 15000 rpm, 20000= g for 15 min. and filtration Ž 0.2 mm.. Sorption R d-values Ž R d,so . were calculated from the measured radioactivities Žmeasured by gamma-spectroscopy. in the solution before and after incubation. Sorption isotherms were calculated using the formulae: M ls M
0 l
M s s Rd M
A lrA0 l
Ž1. Ž2.
where w M x l is the equilibrium concentration of the sorbate in the solution in M and w M x 0l the initial sorbate concentration in the solution Ž M .. A l is the radioactivity of the tracers in the liquid at the end of adsorption experiment in cpm mly1, and A0 is the total radioactivity at the beginning of the experiments in cpm mly1. w M xs is the amount of the nuclide adsorbed in mol kgy1. To investigate the desorption, the centrifuged and decanted wet minerals were dispersed again in 10 ml of the electrolytes without tracer and shaken for 7 days at 228C. After separation of the two phases, the activity of the liquid was measured, and desorption R d-values Ž R d,de . were calculated taking into account the remaining activity in the solution of the wet samples by determining the volume of the liquid after sorption and decantation. All experiments were carried out in triplicates. The overall mean error was - 10%. In a double-labeling experiment with Csq and Sr 2q, the competing adsorption of the two cations by the clays was tested. The sum of the ion concentrations in equivalents was kept constant Ž 10y1 or 10y2 M. , but the relation of Csq to Sr 2q was varied Žequivalents Cs:Srs 1:4, 2:3, 3:2 and 4:1. . After
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attaining equilibrium, exchange isotherms were determined using the following equations: ECs ,s s ECs ,l s
CCs ,s CCs ,s q 2CSr ,s aCs ,l aCs ,l q 2 aSr ,l
,
Ž3. Ž4.
where CCs,s s Csq sorbed wmol kgy1 x, CSr,s s Sr 2q sorbed wmol kgy1 x, aCs,l s chemical activity of Csq in the equilibrium solution w M x, aSr,l s chemical activity of Sr 2q in the equilibrium solution w M x. From thermodynamic equilibrium constants, Vanselow-selectivity coefficients, K v , can be calculated ŽSposito, 1981. : Kvs
2 Ž CCs ,s . aSr ,l 2
CSr ,s Ž aCs ,l . .
Ž5.
3. Results and discussion 3.1. Variation of R d-Õalues for anions and cations with the amounts of HDPy q applied The different sorption behavior of anions Ž Iy, TcO4y. and cations Ž Csq, Sr 2q . as influenced by the concentrations of HDPyq applied is illustrated in Fig. 1.
Fig. 1. Sorption, R d,so-values for the anions Iy and TcO4y ŽReO4y . and for the cations Csq and Sr 2q using SGW and the corresponding carriers Ž1=10y5 M. on MX-80 bentonite as a function of the amounts of hexadecylpyridinium ŽHDPyq . applied.
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The distribution ratios in all cases were obtained from experiments with carrier concentrations of 1 = 10y5 M using SGW as equilibrating solution. It can be seen that iodide and pertechnetate sorptions increase, while cesium and strontium sorptions decrease with increasing HDPyq saturation. The increased adsorption of cesium in HDPy-20 and HDPy-50 bentonites compared to the untreated material is remarkable. Most probably, structural Ž e.g., increased basal spacing. andror electrochemical changes of the montmorillonite crystals are responsible for this result, which has been repeatedly observed. It needs, however, further clarification. Considering the construction of engineered barriers, it is important to realize that bentonite saturated with HDPyq to about 70%–90% of the CEC is capable to sorb both anions as well as cations in comparable amounts. 3.2. Adsorption of iodide and pertechnetate The distribution ratios resulting from experiments with Na-bentonite, treated with HDPyq in amounts corresponding to 200% of the CEC are related to the sorbed carrier concentrations in the equilibrium solutions of bidistilled water, SGW and Sear2 water for iodide ŽFig. 2a. and for pertechnetate ŽFig. 2b., respectively. Generally, lower distribution ratios are found for the SGW Ž R d ; 2000–4000 l kgy1 . than for bidistilled water Ž R d ; 4000–14000 l kgy1 . , and even lower ones when using Sear2 water, stressing the importance of the chemical composition of the equilibrium solution. One possible explanation for the effects of the electrolytes with higher ionic strengths on anion sorption is, that part of the organic cations may be desorbed which have been bound only slightly because of the relatively high HDPyq-loading of 341 g kgy1 Ž see Part II.. The clarification of this question is expected from running DOC measurements and estimations of the HDPyq concentrations in the liquids before and after treatments. The distribution ratios remain nearly constant up to the relatively high sorbed ion concentrations of ; 1 = 10y1 mol kgy1 indicating saturation of sorption sites. At higher sorbate concentrations, a sharp decrease of the R d-values can be observed. With bidistilled water, a sorption maximum at iodide and pertechnetate concentrations of about 5 = 10y2 mol kgy1 occurred. This repeatedly observed phenomenon is not well understood yet and needs further attention, taking into account different particle size fractions. According to the experimental results, basal spacing expansions and ion exchange reactions are dependent on particle sizes Ž von Reichenbach, 1973. . The possible role of the two different montmorillonite types, detected recently for MX-80 bentonite ŽKasbohm et al., 1998., should also be taken into consideration in this respect. On the basis of the R d-values, adsorption and desorption isotherms Ž fractional loading of the sorbent vs. equilibrium concentration in the liquid. were determined for the HDPy-200 bentonite samples. Fig. 3 shows the sorption and
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Fig. 2. Iodide Ža. and pertechnetate Žb. sorption Ž R d-values. on MX-80 bentonite treated with HDPyq in amounts corresponding to 200% of the CEC using bidistilled water, SGW and Sear2 solutions.
desorption isotherms for iodide ŽFig. 3a. , and for pertechnetate Ž Fig. 3b. with bidistilled water, SGW and Sear2 water, respectively. The log–log plot of the isotherms revealed linearity up to ion concentrations of about 5 = 10y3 M or 1 = 10y1 mol kgy1, confirming ion exchange as the major sorption mechanism ŽFreundlich type isotherm with slope ; 1.. At higher concentrations, the shapes of the curves shift to non-linearity suggesting that besides ion exchange other types of mechanisms may be involved in iodide fixation. Most likely, the existence of binding sites with different reactivities is responsible for the observed phenomenon. The similarity of the sorption and desorption curves with SGW and Sear2 for pertechnetate, and with Sear2 water for iodide, suggests reversible sorption processes, whereas with bidistilled water, and for iodide also with SGW, the sorption seems to be partially reversible only.
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Fig. 3. Sorption and desorption isotherms for iodide Ža. and pertechnetate Žb. in MX-80 bentonite treated with HDPyq in amounts corresponding to 200% of the CEC using different equilibrium solutions.
3.3. Adsorption of cesium and strontium The effects of the ionic strength of the different electrolyte solutions on cesium and strontium adsorption was investigated with MX-80 bentonite treated with HDPyq in amounts corresponding to 50% CEC. The sorption of the cations decreases gradually with increasing ionic strength of the electrolyte solutions ŽFig. 4.. Compared to bidistilled water two orders of magnitude lower R d-values are observed for cesium using SGW Ž Fig. 4a. . Strontium adsorption seems to be affected by the chemical composition of the solutions even more than that of cesium Ž Fig. 4b.. Obviously, the bivalent cations Ž Ca2q and Mg 2q . of the equilibrium solutions, rather than the monovalent ones Ž Naq and Kq. interfere with the bivalent Sr 2q ions. With Sear2 water, the adsorption of cations decreases further Ž R d ; 100 l kgy1 for cesium.. Strontium adsorption with R d - 10 l kgy1 was found to be particularly sensitive to the ionic constituents of Sear2 water. Fig. 5 shows the adsorption and desorption isotherms for the HDPy-50 bentonite samples using bidistilled water, SGW and Sear2 water Žno desorption data are available with bidistilled water. . Generally, the log–log plot of the
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Fig. 4. Cesium Ža. and strontium Žb. sorption Ž R d-values. on MX-80 bentonite treated with HDPyq in amounts corresponding to 50% of the CEC using bidistilled water, SGW and Sear2 solutions.
isotherms showed linearity over a broad range of carrier concentrations indicating ion exchange adsorption mechanisms. The agreement of the sorption and desorption curves with SGW and Sear2 water for cesium suggests reversible sorption processes Ž Fig. 5a., whereas in the case of strontium with Sear2 water a hysteresis can be detected ŽFig. 5b.. More generally, a rather complicated
10
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Fig. 5. Sorption and desorption isotherms for cesium Ža. and strontium Žb. in MX-80 bentonite treated with HDPyq in amounts corresponding to 50% of the CEC using different equilibrium solutions.
sorption mechanism can be assumed for the bivalent Sr 2q, taking ion exchange and surface related processes into account.
Fig. 6. Cs–Sr exchange isotherms for Sr 2q and Csq-saturated MX-80 bentonite and HDPy-bentonite. Dashed lines represent the non-preference exchange isotherms ŽSposito, 1981..
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Fig. 7. Vanselow selectivity coefficients, K v , for the Cs–Sr exchange in original and HDPy-bentonite.
3.4. Cs–Sr selectiÕity The Cs–Sr-cation exchange chemistry of MX-80 bentonite is illustrated in Figs. 6 and 7. In Fig. 6, the equivalent mole fraction of Csq on the sorbent w ECs xs is related to the equivalent mole fraction of Csq in the liquid w ECs x l . The dashed curve is the thermodynamic non-preference isotherm for a binary monovalent-bivalent cation exchange Ž Sposito, 1981. . The Cs–Sr exchange curves of both original and HDPy-bentonite are positioned clearly above the non-preference isotherms indicating the preferential sorption of Csq over Sr 2q. The calculation of the Vanselow selectivity coefficients Ž K v . for Cs–Sr resulted in K v s ln 4.0, confirming the implications of Fig. 6. Higher K v-values exist for the HDPy-bentonite than for the untreated samples Ž Fig. 7. . The preferential adsorption of Csq can be explained by the greater ionic diameter and lower hydration energy of Csq.
4. Conclusions In organophilic MX-80 bentonite, Iy and TcO4y exhibited increasing adsorption, while Csq and Sr 2q showed decreasing adsorption with increasing alkylammonium ion ŽHDPyq. incorporation into the clay. Especially for Sr 2q ions, adsorption is affected by the composition of the equilibrium solutions. Reversibility of the processes was found for TcO4y and Csq with SGW, and Sear2 water, and for Iy with Sear2 water, while for the adsorption and desorption
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behavior of Sr 2q a hysteresis was observed. In both, original and HDPy-bentonite, Csq is preferentially adsorbed over Sr 2q, which can be explained by the greater ionic diameter and the lower hydration energy of Csq. Additional information concerning the effectivity of HDPy-bentonite as engineered barrier is expected from a series of presently running diffusion experiments. Attempts will also be made to investigate, if organo-bentonites can be used as effective sorbents for anionic chemical pollutants, such as CNy, CrO42y and AsO43y. As the organo-bentonites showed thermal stability up to 2008C, their use in nuclear waste isolation should be taken into consideration.
Acknowledgements The studies on organophilic bentonite are funded by the Nuclear Fission Safety Programme of the European Commission under the Project No. FI4WCT950012. The skilled technical assistance of Ms. G. Erb-Bunnenberg and Mr. K.-H. Iwannek is gratefully acknowledged.
References Bors, J., 1990. Sorption of radioiodine in organo-clays and soils. Radiochim. Acta 51, 139–143. Bors, J., Gorny, A., Dultz, St., 1994. Some factors affecting the interactions of organophilic clay minerals with radioiodine. Radiochim. Acta 66r67, 309–313. Bors, J., Gorny, A., Dultz, S., 1997. Iodide, caesium and strontium adsorption by organophilic vermiculite. Clay Miner. 32, 21–28. Boyd, S.A., Janes, W.F., 1993. Role of layer charge in organic contaminant sorption by organo-clays. In: Mermut, A.R. ŽEd.., Layer Charge Characteristics of 2:1 Silicate Clay Minerals, Vol. 6. CMS Workshop Lectures. The Clay Minerals Society, Boulder, CO, USA. Dultz, S., Bors, J., 2000. Organophilic bentonites as adsorbents for radionuclides: II. Chemical and mineralogical properties of HDPy-montmorillonite. Appl. Clay Sci. 16, 15–29, Žthis issue.. Kang, M.J., Chun, K.S., Rhee, S.W., Moon, H., Fanghanel, Th., Neck, V., 1998. Sorption of ¨ MO4y ŽM s Tc, Re. on MgrAl layered double hydroxide and characterization of reconstructed LDH with ReO4y. Radiochim. Acta, in press. Kasbohm, J., Henning, K.-H., Herbert, H.-J., 1998. Tranmissionselectronenmicroskopische Untersuchungen am Bentonit MX-80. In: Henning, K.-H., Kasbohm, J. ŽEds.., Tone in der Geotechnik und Baupraxis. Berichte der Deutschen Ton- und Tonmineralgruppe e.V. -DTTG-, Band 6, pp. 228–236. Lieser, K.H., Bauscher, Ch., 1987. Technetium in the hydrosphere and in the geosphere. Radiochim. Acta 42, 205–213. Lieser, K.H., Steinkopff, T.H., 1989. Chemistry of radioactive iodine in the hydrosphere and in the geosphere. Radiochim. Acta 46, 49–55. Madsen, F.T., 1998. Clay mineralogical investigations related to nuclear waste disposal. Clay Miner. 33, 109–129.
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McCombie, C., McKinlay, I.G., Lambert, A., Thury, M., Birkhauser, P., 1995. The swiss strategy ¨ for HLW siting; parallel investigation of two alternative host rocks. Scientific Basis for Nuclear Waste Management 18, 1363–1370. Mortland, M.M., Shaobai, S., Boyd, S.A., 1986. Clay-organic complexes as adsorbents for phenol and chlorophenols. Clays Clay Miner. 34, 581–585. Muller-Vonmoos, M., Kahr, G., 1983. Mineralogische Untersuchungen von Wyoming Bentonit ¨ MX-80 und Montigel. NTB 83-12, pp. 1–15. Muurinen, A., 1994. Diffusion of anions and cations in compacted sodium bentonite. PhD Thesis, Univ. Helsinki. VTT Publications, Finnland, 168 pp. Oscarson, D.W., Hume, H.B., Choi, J.-W., 1994. Diffusive transport in compacted mixtures of clay and crushed granite. Radiochim. Acta 65, 189–194. Pusch, R., Karnland, O., 1990. Preliminary report on longevity of montmorillonite clay under repository-related conditions. Report SKB-90-44. Swedish Nuclear Fuel and Waste Management, Stockholm, ISSN 0284-3757, 47 pp. Sposito, G., 1981. The thermodynamic theory of ion exchange. The Thermodynamics of Soil Solutions, Chap. 5. Clarendon Press, Oxford. Stockmeyer, M.R., 1991. Adsorption of organic compounds on organophilic bentonites. Appl. Clay Sci. 6, 39–57. von Reichenbach, G.H. 1973. Exchange equilibria of interlayer cations in different particle size fractions of biotite and phlogopite. In: Serratosa, J.M. ŽEd.., Proc. Intern. Clay Conf. Madrid, 1972 Div. De Ciencias, CSIC, Madrid, pp. 457–466.