Studying the Effect of Crystal Size on Adsorption Properties of Clay M. M. Abdellatif Nuclear and Radiological Regulatory Authority, 3 Ahmed El Zomer st. Nasr City, 11762 Egypt. Email: magdadel200@hotmail.com Received: 23/1/2011 Accepted: 23/4/2011 ABSTRACT Sorption of radionuclides on mineral surfaces strongly affects their fate and mobility in the geosphere. Therefore using of clay minerals as a barrier In LLW repositories can delay the dispersion of radionuclides into environment. That is of fundamental importance for maintaining environmental quality and for the safety and long-term performance of waste repositories. In this study XRD analysis was applied to investigate three different types of clay minerals for quantitative analysis of each type and the MudMaster program for the measurement of the crystallite thickness distribution (CTD) according to of the BWA (Bertaut-Warren Averbach) technique. Six sample s of the three types of clay (Kaolin, Aswan clay and Ball clay) were studied. XRD and MudMaster were used to investigate the relation between CTD and Cs -137 uptake mechanism onto the clay. It was found that the best adsorption capacity related to the kaolinite content and the lowest CTD. Key Words: CTD/ sorption of radionuclides/clay minerals INTRODUCTION To prevent radionuclides spreading into soil and water there is a need to place geochemical barriers which main function is to convert the contaminant into not mobile forms. While studying radioelement mobility the attention is focused on sorption properties of clay minerals (up to 65-80% of all sedimentary minerals volume fall to their share), owing to their penetration and high sorption capacity such minerals can be the effective geochemical barriers. Therefore evaluation of sorption of radionuclides on clay surface is of fundamental importance for maintaining environmental quality and for assessing the long-term performance of waste repositor ies. Clay minerals are hydrous alumino-silicates broadly defined as those minerals that dominantly make up the colloid fraction (<2µm) of soil, sediments and rocks (1). The high specific surface area, chemical and mechanical stability, layered structure, high cation exchange capacity etc have made the clay excellent adsorbent material (2). Although they are considered the most complicated objects in quantitative mineral analysis of sedimentary rocks. So Powder X-ray diffraction (XRD) analysis is a fundamental mineralogical method. In addition to qualitative identification of mineral phases, XRD is able to provide other important data such as the crystal size and many other things. It can be applied to study weathering processes and the change of pyrophyllite during grinding among other application (3-7). The presented contribution will focus on crystallite thickness distribution (CTD) and quantitative analysis of the different clay type. The CTD can be measured by XRD because the widths 40
of the XRD peaks broaden as crystallite size decreases. MudMaster program, written by EBERL et al. (1996) (8), calculates crystallite thickness distribution (CTD) according to Bertaut-Warren-Averbach (BWA) theory (3). BWA technique has been applied to measure CTD of kaolin mineral (9), to explore crystal growth mechanisms for illite and smectite (4). The presented study aimed to find the relation between the CTD of different clay type and Cs-137 uptake onto different clay type. The kaolinite content was also determined through the XRD which varies in each type of clays and affects their ability as a good barrier. Preparation of the materials MATERIALS AND METHODS Three types (kaolin, Aswan clay and Ball clay) represent different type of clay minerals. Kaolin is the primary type of clay formed by the disintegration of the parent rock mainly feldspars, so it is a pure type mainly kaolinite mineral. In the other hand Ball clay is secondary type of clay formed by transportation of kaolin by wind or water so it mixed with other clay and rock types. It may contain other minerals such as quartz or mica. Aswan clay is considered also as secondary type contain different impurities depending on the localities and the parent rock. The three types are slightly different in color due to their contents. Prior to analyses, < 2 mm fractions were separated from the bulk samples by sieving. Three samples were exposed to Cs-137 solution with concentration of 22,469 ppm for 30 days in order to reach to the maxim adsorption of the Cs onto the surface of clay particles. XRD analysis Separated fine fractions were used for XRD analysis of oriented specimens. All specimens were analyses by XRD using a Philip PW 1710 diffract meter equipped with Cu radiation with graphite monochromatic. The step size was 0.02 2? with a counting time of 4s for the oriented specimens. The resulting basal reflections of the clay minerals were used for the determination of the mean crystallite thickness (crystallite = X-ray scattering domain) and thickness distribution by means of the BWA techniques (DRITS et al., 1998) using the MudMaster program (EBERL et al., 1996). The XRD method of crystallite size determination is based on the observation that XRD peaks broaden regularly as a function of decreasing crystallite size. The first basal reflection for all samples was subjected the BWA analysis in the recommended two theta intervals between 6 and 13 for kaolinite mineral. Adsorption experiment: Sorption experiment was carried out using the batch method. The solution was prepared with concentration of Cs (22469 ppm) as CsCl solution. In order to investigate duly the adsorption properties of different clays as a function of time, 5 grams of each type was immersed in 10ml of Cs solution and shacked for 15 min (500rpm) then left for three months ( 90 days). The adsorption percentage (Ads %) was calculated as: Ads % = A i - A f / A i 100 Where A i and A f are the initial and final concentrations of the solution (ppm). 41
RESULTS AND DISCUSSION Complete XRD analysis of each type of clay is shown in Fig. (1-3). The interpretation of XRD pattern of bulk samples of fine caly fraction (< 2µm) led to the identification of the following minerals Kaolinite, Illite and Quartz. The quintitative analysis of the three types of clay represented in Table (1). It was found that the kaolinite mineral represented by 78%, 40.7% and 16.1% in kaolin, Aswan clay and Ball clay respectively. Table (1) mineral content of different clay type. Clay type Kaolinite % Illite % Quartz% Kaolin 78% 4.4% 17.5 Aswan clay 40.7% - 54.3% Ball clay 16.1% - 82.2% As we can see the kaolinite content in kaolin is greater than that in the other type. That is matching the origin because the kaolin is primary type and formed of pure kaolinite mineral through the disintegration of the parent rock, which usually feldspar and plagioclase. This primary rock had formed in-situe of its parent rocks and had not been transported. But ballclay contains kaolinite mineral less than kaolin rock and Aswan, this is due the weathering processes and long transportation of it, as they are secondary type. So they contain different mineral with different percentage according to their localities and their parent rock. Typical XRD pattern of each type of clays shown in Figures (1-3) it shows the basal reflection of the clay minerals which were used in the determination of the mean crystallite thickness (crystallite= X-ray scattering domain). Fig. (1): XRD analysis of Aswan clay. 42
Fig. (2): XRD analysis of Kaolin clay. Fig. (3): XRD analysis of Ball clay. Typical XRD pattern of each type of clays shown in figures (4-6) it shows the basal reflection of the clay minerals which were used in the determination of the mean crystallite thickness (crystallite= X-ray scattering domain). Fig. (4): Typical XRD pattern of kaolin clay. Fig. (5): Typical XRD pattern of Aswan clay. 43
Fig. (6): Typical XRD pattern of Ball Clay. The results of the BWA measurements of kaolin (ka), Aswan clay (A.C) and Ball clay (B.C) samples are shown in Table (2). Table (2): List of clays used for BWA-analysis and the input and output data. Sample Position of maximum peak (?) d-spacing (A ) Analysis area (?) Best mean nm, extrapolated size KA 12.34 7.16 6-18.68 3.1 B.C 12.12 7.17 6-18.24 2.2 A.C 12.12 7.3 6-16.68 1.9 As we can see from the table the crystal size of the kaolinite (KA) is larger than that of ball cla y (B.C) and A.C. this is confirming the origin of kaolin as a primary mineral which does not affected by transportation or severe weathering. On the other hand A.C and B.C which are secondary clay type affected by long transportation and heavy weathering processes. Also the low mean crystallite thicknesses of these clays generally indicate weak crystallized materials due to the presence of more than two generations of kaolinite particles. Figures (7-9) show the crystallite thickness of different type of clays. Fig. (7): The crystallite size distribution of B.C obtained by BWA technique. 44
Fig. (8): The crystallite size distribution of kaolin obtaine d By BWA technique. Fig. (9) The crystallite size distribution of A.C. obtained by BWA technique. Adsorption of Cs cation on different clay types (A.C, B.C and K) Fig.(10), was determined through the difference in its concentration with time. It was found that the total adsorption capacity was 89.577%, 88.22% and 86.1% for Aswan clay (A.C), Ball clay (B.C) and kaoinite (K) respectively. It was noticed that the different between Ball clay and Kaolinite very little although the kaolinite has high percentage of kaolin mineral. On the other hand Aswan clay (A.C.) has the higher adsorption capacity than Ball clay (B.C) and kaolinite (K.). This is probably refereed to the lowest CTD of A.C. and its relatively higher content of kaolinite mineral. 45
25000 20000 15000 Ads % 10000 A.C. B.C. K. 5000 0 0 10 20 30 40 50 60 70 80 90 100 Time Fig. (10) Adsorption of Cs-137 by different clays CONCLUSION Three different types of clays (A.C, B.C and K) were examined by XRD analysis. MudMaster program was applied to determine CTD of kaolinite mineral according to BWA techniques. From the previous results, it was found that A.C. has higher adsorptive capacity than KA and B.C. This is could be due to its relatively high content of kaolinite mineral with low CTD. So the adsorption of Cs cation probably related to the lowest CTD since kaolin clay which has the highest kaolinite content does not have the same capacity as A.C. REFRENCES (1) T.J.Pinnavaia; Science; 220, 365 (1983). (2) K.Tanabe, Catalysis Science and Technology Spring, New York, (1981). (3) D.D.Eberl, V. Drits, J. Srodon and R. Nuesch Mud Master: a program for calculating crystal size distribution and strain from the shapes of X-ray diffraction peakes U.S. Gealogical Survey, Open file Report, 96 (1996). (4) V. Drits, D.D.Eberl and J. Srodon; Clays and Clay Mineral; 45, 461 (1998). (5) V. Sucha, I. Kraus, J. Samajova and L. Puskeluva; Periodico di Mineralogia; 68,82 (1999). (6) J. Srodon, D.D.Eberl V. Drits; Clays and Clay Minerals; 48, 446 (2000). (7) K. Mystjowski and J. Srodon; Clay Minerals; 35, 545 (2000). (8) M. Kotarba and J. Srodon; Clay minerals; 35, 383 (2000). (9) V. Sucha; Clay Minerals; 36, 403 (2001). 46