HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 51(1) pp. 9–13 (2023) hjic.mk.uni-pannon.hu PRELIMINARY STUDY OF CESIUM IMMOBILIZATION IN A GEOPOLYMER MATRIX ERIKA KOCSIS1, MÁTÉ FEHÉRVÁRI1, MÁTÉ NOVÁK1, EDIT TÓTH-BODROGI1, TIBOR KOVÁCS1* 1 Department of Radiochemistry and Radioecology, University of Pannonia, Egyetem u . 10, Veszprém, 8200, HUNGARY Geopolymers are modern synthetic materials that have many beneficial characteristics, e.g. excellent mechanic al properties, fire- and heat-resistance, minimal shrinkage and can be molded into shape. Given their aforementioned characteristics, geopolymers could be applicable as conditioning materials, e.g. to handle hazardous waste and radioactive materials. Howev er, their long-term applicability needs to be investigated. The aim of this study is to investigate the mobility of Cs -137 radionuclides in embedding materials. Different geopolymeric matrices - namely kaolinite, bentonite, zeolites and red mud - were tested. The leach test was performed according to the ASTM C1308-08(2017) standard and the activity concentration of Cs -137 isotopes was measured by gamma spectrometry using a high-purity germanium (HPGe) semiconductor detector. According to our preliminary results, the matrices that resulted in the most significant immobilization effect were those in which bentonite, cement, fly ash and 8M NaOH were used, releasing approximately 10% of the cesium. Keywords: geopolymer, immobilization, radioactive waste, NORMs 1. Introduction In order to prevent any dispersion of radionuclides from liquid radioactive waste into the environment, this type of waste has been incorporated into various embedding materials. The aim of this process is to make these materials leach-resistant, so that the release of radionuclides is significantly reduced should they come into contact with groundwater. The other reason is to transform these materials into a mechanically, chemically and physically stable form for transport and emplacement [1]. Nowadays, the most commonly used solidifying materials are cement, bitumen and various types of polymers. Of these materials, cement is the most commonly used because it is widely available, cheaper than the other materials and easy to handle. However, it has been shown that cement is not the best material for immobilizing Cs-137 radionuclides. Several studies have shown that the sorption capacity of Cs-137 is low and the diffusion capacity in hydrated cements is high [2]-[3]. The immobilization properties of cement as an embedding material can be enhanced by using different types of additives that have a higher sorption capacity, e.g. bentonite and zeolites [4]-[5]. Given the low hydraulic conductivity and high sorption capacity of cations, bentonite is a promising material with regard to disposing of radioactive waste. Zeolites, particularly clinoptilolite, are used worldwide in ion exchange processes with a great degree of success in the removal Received: 8 Feb 2023; Revised: 16 Feb 2023; Accepted: 17 Feb 2023 *Correspondence: kovacs.tibor@mk.uni-pannon.hu of Cs-137 radionuclides from various types of wastewater. Both are natural materials, which means that their extraction has a huge impact on the environment [6]-[7]. As previously mentioned, since they are mixed with cement to obtain a mixture that has good radionuclide immobilization properties, the mining of zeolites and bentonites is not the only activity with a huge environmental impact. The cement manufacturing process is also known to have a huge environmental impact. The process releases several gases into the air, e.g. CO2, NO2, NO3, SO2, etc., which have major impacts on global warming [8]-[9]. As a result, research is now being conducted to reduce the amount of raw materials used in the industry and identify replacement materials. Testing NORMs (naturally occurring radioactive materials) - which are available in large quantities worldwide as well as could be used as potential additives in commonly used cement, zeolites and bentonite - could be promising. One of the ways in which radioactive waste solidifies is through geopolymerisation. NORMs, e.g. red mud and fly ash, can be used as additives [10]-[12] and are usually deposited in huge amounts, which has a significant environmental impact due to the heavy metals they contain, moreover, can of course increase the dose rate to passers-by or workers near these deposits [13]. Therefore, a method is needed to reduce the environmental impact of NORMs. However, the following question may be raised: why are they unsuitable as replacements for geopolymers nor as embedding materials for liquid radioactive waste. These DOI: 10.33927/hjic-2023-02 mailto:kovacs.tibor@mk.uni-pannon.hu https://doi.org/10.33927/hjic-2023-02 KOCSIS, FEHÉRVÁRI, NOVÁK, TÓTH-BODROGI AND KOVÁCS Hungarian Journal of Industry and Chemistry 10 materials are widely used in construction, water treatment and agricultural fertilization, some of which have good adsorption and water-binding capacities. Furthermore, using these materials as additives can reduce the environmental impact of mining and reduce the cost of production. The solidification of liquid radioactive waste by geopolymerisation is one of the cheapest methods of radioactive waste management. The structure of the resulting final product is stable as well as exhibits extremely good heat- and fire-resistance capabilities, moreover, the concentration of boric acid in liquid radioactive waste does not destroy the structure of geopolymers and the volume of the resulting final product is significantly smaller compared to that of waste conditioned by cementation [14]-[15]. The aim of this study is to investigate the immobilization of Cs-137 in different kinds of geopolymeric materials using NORM additives. 2. Experimental During the laboratory work, different kinds of geopolymers containing zeolites, bentonite, kaolinite and red mud were prepared, which were mixed in different ratios as presented in Table 1 below: Table 1. Mixing ratio of the used materials Experiment Used materials Mixing ratio I. Zeolites – Cement 1:1 Zeolites – Bentonite – Cement 1:1:1 Zeolites – Kaolinite – Cement 1:1:1 Bentonite – Cement 1:1 Bentonite – Kaolinite – Cement 1:1:1 Kaolinite – Cement 1:1 II. Cement 1 Zeolites – Cement 1:1 Zeolites – Cement 2:1 Bentonite – Cement 1:2 III. Red mud – Bentonite – Cement 1:1:1 Red mud – Zeolites – Cement 1:1:1 Red mud – Bentonite – Cement 2:1:1 Red mud – Zeolites – Cement 2:1:1 Cement as a binder as well as an 8M NaOH solution as a wetting agent were added to the mixtures and mixed until a homogeneous mass was obtained. A radioactive Cs-137 solution was mixed with the already moldable mass before being placed in a sample container. The molded samples were then dried for 27 days at room temperature in accordance with the requirements of the ASTM C 1308-08 standard [16]. After the 27 days, the already solidified samples were placed in a 1L plastic container in such a way that they were attached to the lid of the container by means of a thread to prevent them from dropping to the bottom of the container. 250 ml of distilled water was poured into the vessel used for the dissolution test. During the leaching tests, the distilled water in the containers was replaced at the time intervals specified by the standard (Figure 1) before the amount of leached radionuclides dissolved from the test samples was measured by gamma spectrometry using a high-purity germanium (HPGe) semiconductor detector. According to the ASTM standard, the Incremental Fraction Leached (IFL) was calculated using the results from gamma spectrometry measurements as follows: 𝐼𝐹𝐿 = 𝑎𝑛𝑖 𝐴0𝑖 (1) where: ian denotes the quantity of species i measured in the leachate from the nth test interval, iA0 stands for the quantity of species i in the specimen at the beginning of the test The cumulative fraction of species i leached over the jth time interval, referred to as the Cumulative Fraction Leached (CFLj), was also calculated: 𝐶𝐹𝐿𝑗 = ∑ 𝑎𝑛𝑖 𝑖 𝑛=0 𝐴0𝑖 = ∑ 𝐼𝐹𝐿𝑗 𝑖 𝑛=0 (2) In the case of radionuclide i, both terms must be corrected in light of radioactive decay at the beginning of the test. 3. Results and Analysis During the first part of the experiments, zeolites, bentonite, kaolinite and, of course, cement were used, moreover, the leached amount of radionuclides was measured by gamma spectrometry using a HPGe semiconductor detector. These values were used to calculate the CFL values of each matrix, which are shown in Figure 2. Figure 1. Time intervals according to which the water was changed in accordance with the ASTM C 1308-08 standard [17] PRELIMINARY STUDY OF CESIUM IMMOBILIZATION IN A GEOPOLYMER MATRIX 51(1) pp. 9–13 (2023) 11 Based on the results obtained, it can be seen that during the 10-day-long dissolution tests, the amount of dissolved Cs-137 was lowest in the case of the geopolymers containing zeolites – cement and zeolites – bentonite – cement in 1:1 and 1:1:1 ratios, respectively. Regarding the sample containing zeolites – cement, the dissolution rate was 9.8%, while for the one consisting of zeolites – bentonite – cement, it was 10.1%. The highest dissolution rate, that is, 17.4%, was measured in the sample containing kaolinite – cement in a ratio of 1:1. The high dissolution rate of kaolinite is due to the structure of kaolin which, like illite, is a layered phyllosilicate in which the absorption of Cs ions is not as high when compared to its montmorillonite counterparts due to its crystal structure. On the basis of the results obtained, it can also be seen that the CFL values of the zeolites – cement and zeolites – bentonite samples constantly increase, moreover, after 7 days, the dissolution rate already increased much more slowly compared to the other samples, where a continuous increase was observed. During the initial phase of the dissolution tests, i.e. in the first few days, not only were the Cs-137 isotopes embedded in the geopolymer dissolved but also those on the surface of the test specimens. As a result, a larger increase in the CFL value was recorded during the initial phase. Once the Cs-137 on the surface has completely dissolved, only the dissolution of Cs-137 inside the geopolymer affects the obtained values. Of course, the amount of dissolved Cs-137 is also affected by the small microcracks in the samples that may have occurred during their preparation, possibly increasing the amount dissolved. In the second part of our experiments, ratios resulting in the best CFL values from the previous measurements were used. Cement matrices consisting of zeolites mixed in different ratios were measured and the CFL values obtained can be seen in Figure 3. It can be seen that the leachability of the Cs-137 radionuclide can be significantly reduced using zeolite additives. Depending on the quality and quantity of such additives, the dissolution rate may be reduced. As a result of ion exchange processes, the cesium ions absorbed by the zeolites are exchanged for the calcium ions in the cement. The cesium ions are then released from the matrix into the environment by diffusion. The rate at which cesium diffuses away from the matrix is determined by the environment, ion exchange process and rate of diffusion. The ion exchange process can be slowed down by increasing the additive content of the matrices so that the ability of the clay minerals to bind to the calcium hydroxide ions is enhanced, preventing displacement of the cesium ions from the structure of the matrix to such an extent that results in significantly less cesium being released from the matrix. In addition to the calcium ions in the cement, this process can also be influenced by the amount of such ions in the solvent, because if the solvent is rich in calcium, the calcium ions it contains can also displace the cesium ions, thereby increasing the rate at which they escape from the matrix. For this reason, distilled water was chosen as the solvent, as it cannot interfere with the ion exchange processes taking place in the test samples [18]. The results obtained during the dissolution tests were developed in accordance with the aforementioned ion exchange processes. In the case of the cement – zeolites samples in the ratios of 1:1 and 1:2, it can be seen that the amount of dissolved cesium was smaller than in the samples where only cement was present or in larger quantities compared to that of zeolites. In contrast with cement and cement – zeolites specimens, the dissolution rate of Cs-137 dissolved from samples containing zeolites – cement in a ratio of 1:1 was 9.8%, while those containing zeolites – cement in a ratio of 2:1 was only Figure 2. Representation of the change in CFL values obtained during the I. experiment as a function of time 0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0 2 4 6 8 10 C L F Time [days] Zeolites Zeolites - Bentonite Zeolites - Kaolinite Bentonite Bentonite - Kaolinite Kaolinite Figure 3. Representation of the change in the CFL values obtained with regard to cement – zeolites matrices in different ratios 0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20 0 2 4 6 8 10 C L F Time [days] Cement Cement - Zeolites 1:1 Cement - Zeolites 2:1 Cement - Zeolites 1:2 KOCSIS, FEHÉRVÁRI, NOVÁK, TÓTH-BODROGI AND KOVÁCS Hungarian Journal of Industry and Chemistry 12 8.3%. In the case of samples consisting of a ratio of 2:1, the amount of dissolved Cs-137 was 17.5 and 16.2 %. NORMs containing bentonite and zeolites were also used as additives during leaching tests. Based on recommendations from the literature, red mud was selected as the NORM for the experiment. The leaching rates of Cs-137 from samples containing bentonite and red mud in the ratios of 1:1 and 1:2 were 5.90 and 8.04%, respectively, whereas those of Cs-137 from samples containing zeolites and red mud in the ratios of 1:1 and 1:2 were 10.20 and 11.73%, respectively. The CFL values recorded can be seen in Figure 4. Based on the results obtained during the dissolution tests, the specimens containing bentonite proved to have a better immobilization effect on Cs-137. Based on the results obtained, it can be seen that ion exchange processes can be influenced by some parameters such as the slightly alkaline chemical effect resulting from the composition of bentonite and the Si/Al ratio of the red mud which created favorable conditions for the cesium ions to bind much better in the matrix of the specimens containing bentonite than in those containing zeolites. However, it can also be seen that if the Si/Al ratio of the samples is increased even more by adding additional red mud to the samples, the rate of dissolution increases. This can be explained by the fact that the Al rings in the test specimens become smaller due to the lower Si/Al ratio, so the rate at which cesium ions escape from the structure reduces. These results are supported by the study written by Quanzhi Tian, Shingo Nakama and Keiko Sasaki in which cesium leaching tests were also carried out on specimens with different compositions and Si/Al ratios [19]. 4. Conclusions The aim of this research was to investigate the applicability of NORMs with regard to the immobilization of radionuclides in liquid radioactive waste. During the experiments, various compositions with different raw materials were investigated in order to determine the basic material whereby the NORM, that is, red mud, is used as an additive. Based on the results of the laboratory measurements, it can be said that the best CFL value was obtained with a test batch containing a 1:1 mixture of bentonite and red mud. It can also be concluded that the chemical composition of the NORM used as an additive, namely red mud, significantly contributes towards the reduction in ion exchange processes. However, this should be treated with caution as the 1:1 mixture of bentonite and red mud exhibited favorable CFL and diffusion rate values compared to the sample containing twice as much red mud. All in all, it can be stated that the reuse of NORMs can reduce our impact on the environment. It was observed that NORMs, especially red mud, can be successfully used as additives to manage liquid radioactive waste. This is an opportunity that must be seized and a field in need of much more thorough research in the future. REFERENCES [1] Plecas, I.: Immobilization of 137Cs and 60Co in concrete matrix, Ann. Nucl. 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