Detachment of colloidal particles from bentonites in water

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1 Available online at Applied Clay Science 39 (2008) Detachment of colloidal particles from bentonites in water S. Kaufhold, R. Dohrmann BGR Bundesanstalt für Geowissenschaften und Rohstoffe, LBEG Landesamt für Bergbau, Energie und Geologie, Stilleweg 2, D Hannover, Germany Received 13 September 2006; received in revised form 18 April 2007; accepted 19 April 2007 Available online 27 April 2007 Abstract Bentonites are currently investigated as geotechnical barrier for sealing radioactive waste e.g. in HLRW repositories (HLRW = high level radioactive waste). Their favourable properties are low hydraulic permeability, cation exchange capacity as well as swelling capacity in contact with aqueous solutions. The pre-requisite for this application is the stability of bentonite under the conditions expected. Accordingly, several studies are available dealing with different scenarios of possible alteration processes (e.g. high ph, high salinity, extensive drying, ). In most of the studies either only a few different bentonites are investigated or a number of bentonites are compared based on only a few parameters. Different bentonites perform rather different in most fields of applications. These differences cannot always be explained by the dominating exchangeable cation only. Therefore, comparative studies considering various different bentonites sometimes allow for the identification of relations between properties and performance. This study was conducted in order to identify the differences of stability (detachment of colloidal particles and/or dissolution) of bentonites in contact with deionized water, representing the simplest aqueous solution. Some of the bentonites release ultrafine colloidal particles upon shaking of suspensions which cannot be centrifuged even by using an ultracentrifuge with 46,000 g. After centrifugation the supernatant containing the colloidal particles was separated. ESEM, XRD, and IR prove that the colloidal particles are mainly montmorillonites. Considering the stochiometrical composition of montmorillonites suggests that approximately 10% of the elemental concentration measured in the supernatant stems from dissolution of octahedral sheet. However, detachment of colloidal particles was found to be the dominating mechanism. As expected, the amount of released colloidal particles strongly depends on the amount of exchangeable Na +. However, in the present study we show that exchangeable Na + and ph show a good correlation. In a separate test the facilitating effect of alkalinity (at a given Na + content) on detachment of colloidal particles was proved. This can be either due to the increase of solubility of alumosilicates at phn 10 and/ or due to the stabilization of the dispersion. In advance of this study it was expected, that bentonites showing a high degree of intergrowth of the fine constituents (e.g. montmorillonite intergrowth with relict volcanic glass) do not release the same amount of colloidal particles at a given exchangeable Na + content and ph compared to bentonites showing a loose microfabric. This was not confirmed in this study. We, conclude that alkaline Na + bentonites generally are less suitable for the application as geotechnical barrier in HLRW repositories since the probability that colloidal particles are released is higher than in the case of ph neutral Ca 2+ bentonites. It is conceivable that such colloidal particles are able to transport strongly adsorbed radionuclides. On the other hand detachment of colloidal particles reduces the thickness of the geotechnical barrier itself Elsevier B.V. All rights reserved. Keywords: Bentonite; Radioactive waste disposal; Detachment of colloidal particles; Erosion stability Corresponding author. address: s.kaufhold@bgr.de (S. Kaufhold) /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.clay

2 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) Introduction Bentonites are believed to represent a suitable geotechnical barrier element in high level radioactive waste (HLRW) repositories (e.g. Pusch, 1992; NAGRA, 2002). The general concept is that compacted bentonite blocks (geotechnical barrier) separate the canister containing the radioactive waste from the host rock (clay, granite, or even salt). The desired bentonite properties are low hydraulic permeability, high swelling (= sealing) capacity, and sorption capacity (Pusch, 1997). In order to investigate the durability of a waste repository several long term studies are conducted dealing with the stability of all barrier components including host rock. Bentonites, since they contain comparably reactive minerals (swelling clay minerals, commonly montmorillonite), are extensively investigated with respect to their stability under different conditions. Particularly possible reactions with Fe 0 or Fe 2+ containing solutions, highly saline solutions, and high ph solutions as cement pore water are considered. Depending on the conditions applied a variety of alteration products can be observed. Herbert et al. (2004) and Kasbohm et al. (2004) investigated the reaction of bentonites with highly saline solutions and identified different alteration mechanisms of the montmorillonites depending on the starting material and whether the system is closed or open. They concluded that dissolution rates should differ from one bentonite to another. However, the investigation of the dissolution of montmorillonites always bears the problem of distinguishing between detachment of ions (dissolution) and detachment of colloidal particles. In the framework of HLRW repository research the possibility and extent of detachment of colloidal particles from compacted bentonites are investigated. In this context Pusch (1999) identified a critical water flow rate with respect to the detachment of colloidal particles from compacted bentonite. He, additionally, discusses the probabilities of the possible fate of released colloidal particles. According to Missana et al. (2002) released colloidal particles are only able to significantly promote radionuclide transport if the radionuclides are irreversibly adsorbed. Additionally, Missana et al. (2003) discuss the effect of different ionic strength and ph on the degree of colloidal particle detachment. All of the available studies dealing with detachment of colloidal particles concentrate on very few different bentonite types. It is well known that bentonite properties from different deposits or even from different parts of a deposit vary significantly (Grim and Güven, 1978). They, therefore, perform rather different in most fields of applications. These differences cannot always be explained by the dominating exchangeable cation. Other parameters like montmorillonite content, layer charge density (distribution), crystal chemistry of montmorillonites, morphology of (quasi)crystals, and intergrowth between different components (amongst others) are supposed to additionally influence bentonite properties. However, the determination of each of these parameters still is a challenge. Therefore, comparative studies considering various different bentonites sometimes allow for the identification of relations between properties and performance. The subject of this paper is to compare 38 different bentonites with respect to the tendency to release colloidal particles and to explain the reason(s) for the differences observed. The aim of this study was to identify the most stable bentonites with respect to dissolution and/or detachment of colloidal particles in water (most stable = least tendency to release colloidal particles or ions in contact with water). The tendency to release colloidal particles and/or ions is considered as one major quality determining parameter of HLRW repository bentonites. It is expected to be an important process particularly in case of water being able to flow along the interface between the geotechnical barrier and the host rock which is a typical scenario in case of granite as host rock (Pusch, 1999). In salt or clay as host rocks flowing water at the interface host rock/bentonite is not expected. It is well known that montmorillonites containing Na + as dominant exchangeable cation are able to almost completely delaminate in water. When dispersed in water Na + -montmorillonite tactoids consist of only 1 2 TOT layers whereas dispersed Ca 2+ -montmorillonites consist of 6 10 TOT layers per tactoid (Banin and Lahav, 1968). Additionally, a low ionic strength and high ph are known to stabilize a dispersion (e.g. Missana et al., 2003). 2. Materials and methods In order to study detachment of colloidal particles bentonite powder was subjected to water, shaken for 48 h and afterwards centrifuged and decanted (4000 U/min, 30 min). The decanted supernatants were then ultracentrifuged (Heraeus Sepatech Suprafuge 22, 46,000 g, 10 min). These supernatants were analyzed with respect to ph and chemical composition (ICP- AES). The water of three supernatants was evaporated in order to investigate the solid. The sample pre-treatment was suspected to influence the results. Therefore, in order to ensure comparability of the different bentonites, the sample pre-treatment was kept constant: Except for samples B24 B28 (industrially dried and ground) only industrially unprocessed materials were considered, none of the samples contained any soda ash. Most of the materials were kindly provided by bentonite producing companies and not all allowed for the publication of localities.

3 52 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) Table 1 Chemical composition (main elements) of all bentonite samples Sample Si0 2 TiO 2 A1 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O P 2 O 5 LOI [% w/w] [% w/w] [% w/w] [% w/w] [% w/w] [% w/w] [% w/w] [% w/w] [% w/w] [% w/w] [% w/w] B B B B B B B B B Bl B B B B B B B B B B B B B B B B B B B B B B B B B B B B Therefore, localities are not provided in this study. The samples were dried at air, broken to approximately 5 20 mm, dried at 60 C and afterwards ground to b0.25 mm by a hammer mill. The water content was determined by oven drying at 105 C up to constance of weight (approximately 1 week). For the experiment 1.00 g 60 C dried material plus the respective weight loss between 60 and 105 C was weighed (e.g g 60 C in case of 5% water content between 60 and 105 C) and subjected to 50.0 ml deionized water. The chemical composition of the bentonites was determined by XRF (PANalytical Axios XRF spectrometer) and XRD pattern were recorded by a Phillips PW 3710 (Cu radiation, fixed primary slit system, secondary monochromator). The quantitative mineralogical composition of the raw bentonites was determined by the Rietveld technique including the new approach of Ufer et al. (2004). The solids gained from evaporation of three supernatants were analysed using IR spectroscopy, XRD, and ESEM. For XRD oriented clay aggregates were analysed air dried and after intercalation of ethylene glycol. Mid-infrared spectra of KBr pellets were recorded on a Thermo Nicolet Nexus. An ESEM FEI Quanta 600 FEG scanning electron microscope was used to evaluate the morphology of the colloidal particles. CEC was determined by the Cu-triene method (Meier and Kahr, 1999). 3. Results and discussion The chemical composition of the bentonites is given in Table 1. The quantitative mineralogical composition was

4 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) calculated with the Rietveld program AutoQuan (Bergmann and Kleeberg, 1998) applying the montmorillonite structural model developed by Ufer et al. (2004). The values are provided by Ufer et al. (submitted for publication). All samples are composed mainly of montmorillonite, quartz, cristobalite, illite/muscovite, feldspar, and carbonates. The presence of illite/montmorillonite mixed layer minerals was proved in sample B30 and B35. Zeolites (heulandite, clinoptilolite) were found in samples B8, B9, B12, B28, B31, and B32. Only sample B12 contains some apatite. Pyrite was found in samples B5 and B30. The cation exchange capacity of all samples is given in Table 2. The CEC, representing the amount of index cation which was adsorbed by the clay, is given for both techniques: ICP-AES ( CEC ICP ) and visible spectroscopy ( CEC VIS ). Deviations between both methods represent analytical precision. The percentage of Na + occupation was calculated by referring the amount of exchangeable Na + (in meq/100 g) to the CEC determined by ICP-AES. In the CEC experiment the index cation (here Cutriene) is adsorbed by the clay and the exchangeable cations as present in natural state are desorbed. Theoretically, the sum of exchangeable cations ( sum of cations in Table 2) should equal the amount of adsorbed index cation ( CEC in Table 2). However, almost half of the samples show a higher sum of cations which can be Table 2 Cation exchange capacity incl. exchangeable cations of all bentonite samples Na + ICP Na + K + ICP Mg 2+ ICP Ca 2+ ICP Sum of cations CEC ICP Sum CEC CEC VIS Calcite Gypsum [meq/100 g] [%] [meq/100 g] [meq/100 g] [meq/100 g] [meq/100 g] [meq/100 g] [meq/100 g] [meq/100 g] B B B B B B B B B Bl B B B B B B B B B B B B B B B B B B B B B B B B B B B B

5 54 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) attributed to mineral phases which are soluble in the exchange solution. The difference between both values is given as sum CEC in Table 2. In most cases partial dissolution of calcite and/or gypsum is responsible for the increase of Ca 2+ concentration which leads to an overestimation of exchangeable Ca 2+.Inthatcaserather different Ca 2+ values depending on sample mass (solid/ liquid ratio in the CEC experiment) are obtained. As an example, for sample B13 (sum CEC=32 meq/100 g) Ca 2+ values ranging from 68 meq/100 g (sample weight 120 mg/60 ml) to 82 meq/100 g (sample weight 80 mg/ 60 ml) were observed which can be explained by calcite solubility depending on solid/liquid ratio. Both values for exchangeable Ca 2+ are incorrect. These dependencies and a method for the measurement of correct Ca 2+ despite the presence of calcite is presented by Dohrmann (2006a,b). In Table 2 most of the sum CEC values can be explained by the presence of calcite and/or gypsum. However, sample 21 and 24 (both from the same deposit), do not contain soluble Ca minerals. Additionally, no dependency of cation values on solid/liquid ratio was observed. Such results can only be explained by electrolyte-rich pore water or if trace amounts of readily soluble minerals not containing Ca 2+ (e.g. NaCl) are present. In this particular case it is likely that the Na + value, and in turn the percentage of Na +, are erroneous. Table 3 Electrical conductivity, ph-value, and chemical composition (ICP-AES) of the supernatant after ultracentrifugation (Al removed is the percentage of Al removed from the solid either by dissolution or detachment of colloidal particles) Conductivity ph K Na CI Mg Ca SO 4 Al SiO2 Fe Al removed Si removed Fe removed [μs] [mg/l) [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [% w/w] [% w/w] [% w/w] B B B B B B B B B Bl B B B B B B B B B B B B B B B B B B B B B B B B B B B B

6 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) In Table 3 the element concentrations as well as the percentage of Al, Si, and Fe which were removed from the solid and remained in the supernatant after ultracentrifugation are given. The values assigned removed were calculated based on the chemical composition of the respective bentonite considering 1.00 g/50 ml. In order to investigate the effect of ultracentrifugation time, the ultracentrifugation time was increased which expectedly slightly decreased the element concentrations but did not affect the trends. This means that the ratio of element concentrations of the supernatants of the different bentonite samples did not change significantly. Accordingly, the absolute values presented in Table 3 are physically meaningless since they depend on experimental parameters. However, all results presented in this study were obtained by treating all samples exactly the same way. Therefore, the values can be used for the comparison of different bentonites and in turn for the identification of reasons for solubility and/or detachment of colloidal particles, respectively. It was one goal of the study to find out whether montmorillonites of the bentonite samples are partly dissolved or if colloidal particles are detached from the montmorillonite quasicrystals in contact with water. One indication for detachment of colloidal particles is provided by the SiO 2 concentration (Table 3) which in some instances by far exceeds the solubility maximum of SiO 2 in water (ortho silica acid approximately 200 mg/l; Iler, 1979). It is possible to explain these concentrations by the presence of colloidal particles or (alumo)silicate polymers. For the detailed interpretation of the data set in Table 3 the correlation coefficients are given in Table 4. From Table 4 a good correlation between the concentrations of the elements forming the montmorillonite structure, particularly Al and Si, is evident. The ratio Si/(Al + Mg + Fe) found in solution is approximately 1.5 which is approximately 10% lower than the typical Si/(Al + Mg + Fe) ratio of montmorillonite. This indicates congruent dissolution of montmorillonites or the presence of montmorillonite colloidal particles in the supernatants caused by detachment of colloidal particles. 10% of these elements can be explained by approximately 10% dissolution of octahedral cations. The electric conductivity of solutions is determined by the ionic strength. Despite the high concentration of the structural elements of montmorillonite (Si, Al, Fe, ) obviously the lower Na + concentration determines electric conductivity which is clear if the structural elements are arranged as polymers or colloidal particles carrying significantly less charges than the respective ions. In order to clarify this question the supernatant of three samples showing a high concentration of Al and Si was selected, the water evaporated at 60 C, and the solid investigated by ESEM, XRD, and IR. The results are presented in Fig. 1. All three methods prove the dominance of montmorillonite (Fig. 1) in the evaporated supernatants. It is remarkable that even the initially observed cornflake like morphology of sample B20 (Wyoming) was preserved. The results indicate that the dominating mechanism was detachment of colloidal particles rather than dissolution. However, stochiometrical data indicate approximately 10% incongruent dissolution. Interestingly, the four selected samples all contain cristobalite (4.04 Å reflection in Fig. 1). Pusch (2000) describes the precipitation of silica gel by steam treatment. Therefore, the possibility was considered, that the cristobalite might represent a reaction product. Dissolution of montmorillonites in combination with the Table 4 Correlation coefficients of the data presented in Tables 1 and 2 ( %Na + = percentage of exchangeable Na + of montmorillonites, Na + meq = exchangeable Na + in meq/100 g, Na = amount of Na + which was determined in the supernatant by ICP-AES, Cond. = electrical conductivity) ph %Na + Na + meq Cond. K Na Cl Mg Ca S Fe Al Si ph %Na Na + meq Conductivity K Na Cl Mg Ca S Fe Al 1 Si

7 56 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) Fig. 1. ESEM, XRD, and IR investigation of the supernatant after evaporating the water (XRD pattern air dried: black, XRD pattern ethyleneglycol: grey). generation of silica is also known from the acid activation process (e.g. Siddiqui, 1968). Here montmorillonite reacts with a strong mineral acid producing etched acid montmorillonite and porous silica gel. This silica gel, unlike the observed cristobalite, is XRDamorphous and XRD-intensities of activated samples after drying at 60 C between 3.5 and 4.5 Å are only diffuse (Fahn, 1973; Kaufhold, 2001). This indicates that it is unlikely that the cristobalite was produced by dissolution and precipitation. Accordingly it must be assumed that very fine cristobalite particles are present as colloidal particles after ultracentrifugation. From element concentrations listed in Table 3 a significant variation of the degree of detachment of colloidal particles of the different bentonites is evident. As an example, the amount of Si which was removed (removed=colloidal detachment+dissolution) varies from 0.1 to 7.0% w/w. As expected, the percentage of exchangeable Na + plays an important role (correlation coefficient Na + /structural cations= ). This is consistent with the well known behaviour of Na + occupied montmorillonites which tend to delaminate in diluted aqueous dispersions. However, almost the same correlation was found for ph and amount of Al and Si removed (Fig. 2). This observation easily can be explained by the correlation of ph and Na + content (Fig. 3). From Fig. 2 it is evident that the fraction of removed Al and Si depends on ph in a way similar to

8 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) Fig. 2. Dependence of removed Al and Si (colloidal+dissolved) on ph. the well known qualitative dependence of Al 2 O 3 and SiO 2 solubility on ph. Additionally, the high ph stabilizes the dispersion. Both mechanisms, increased solubility and stabilization of the colloidal system, are believed to play a role with respect to detachment of colloidal particles from bentonites. Interestingly, a correlation of ph with percentage of exchangeable Na + was found (Fig. 3). In this context it is not clear how Na + occupation in the interlayer can lead to the high ph values observed for Na + rich bentonites. This still open question is currently investigated and not subject of this paper. However, because of this correlation it is impossible to decide whether Na + or ph is the determining parameter with respect to detachment of colloidal particles. In order to investigate the effect of ph we repeated the test and systematically varied ph and ionic strength of a Ca 2+ and a Na + bentonite suspension. In order to prevent cation exchange processes the ionic strength of the Na bentonite was adjusted with NaCl and CaCl 2 in case of the Ca 2+ bentonite, respectively. The results are given in Fig. 4. At ph 10 a slight increase of detachment of colloidal particles (given as Si removed ) was found for the Na + bentonite (Fig. 4a). On the other hand, in case of the Ca 2+ bentonite even a decrease of the amount of SiO 2 removed was found. Probably this is caused by the ionic strength (Ca(OH) 2 concentration) which was added in order to raise the ph to 10. The effect of ionic strength Fig. 3. Correlation of amount of exchangeable Na + with ph (2% w/w clay/water).

9 58 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) Fig. 4. Effect of ph (a) and ionic strength (b) on colloid detachment (given as Si removed ) from a Na + and a Ca 2+ bentonite. (Fig. 4b) was frequently studied and therefore expected (e.g. Le Bell, 1978; Missana et al., 2003). However, the pronounced difference of SiO 2 removed between the Na + and the Ca 2+ bentonite compared to the minor effect of changing the ph suggests that delamination of montmorillonites caused by Na + occupation is the most important colloidal particle detachment mechanism at least in case of low ionic strength. 4. Conclusions Some bentonites tend to release colloidal particles which cannot be removed from dispersion even by ultrazcentrifugation. In batch or flow-trough experiments such very fine colloidal particles might pretend montmorillonite dissolution. In this study we could show that these fine colloidal particles which remained in dispersion after ultracentrifugation still are mostly montmorillonites. It is well known that detachment of colloidal particles of bentonites depends on the amount of exchangeable Na +, ph, and ionic strength. In the present study we could show that Na + and ph are related. We conclude, based on the results of systematic variation of ph at given interlayer occupation and ionic strength, that the amount of exchangeable Na + represents the most important parameter with respect to detachment of colloidal particles at low ionic strength. In advance of this study it was expected, that bentonites showing a high degree of intergrowth of the fine constituents (e.g. montmorillonite intergrowth with relict volcanic glass) would not release the same amount of colloidal particles at given Na + content and ph as bentonites showing a loose microfabric. This could not be confirmed in this study. In conclusion, the probability of detachment of colloidal particles from bentonites in a given environment is higher in case of Na + bentonites than in case of Ca 2+ bentonites. Therefore, the selection of a Ca 2+ bentonite as a technical barrier in a HLRW repository would reduce the probability of detachment of colloidal particles. Acknowledgements We like to kindly acknowledge all bentonite trading and producing companies which supported us in establishing the sample set. References Banin, A., Lahav, N., Particle size and optical properties of montmorillonite in suspension. Israel Journal of Chemistry 6, Bergmann, J., Kleeberg, R., Rietveld analysis of disordered layer silicates. Proc. 5th European Conf. on Powder Diffraction (EPDIC 5), Parma, Italy, May 24 28, Mat. Sci. Forum, vol , pp part 1. Dohrmann, R., 2006a. Cation exchange capacity methodology I: An efficient model for the detection of incorrect cation exchange capacity and exchangeable cation results. Applied Clay Science 34, Dohrmann, R., 2006b. Cation exchange capacity methodology III: Correct exchangeable calcium determination of calcareous clays using a new silver-thiourea method. Applied Clay Science 34, Fahn, R., Influence of the structure and morphology of bleaching earths on their bleaching action on oils and fats. Fette, Seifen, Anstrichmittel 75, Grim, R.E., Güven, N., Bentonites Geology, Mineralogy, Properties and Uses. Developments in sedimentology, vol. 46. Elsevier, New York. 254 pp. Herbert, J.-J., Kasbohm, J., Moog, H.C., Henning, K.-J., Longterm behaviour of the Wyoming bentonite MX-80 in high saline solutions. Applied Clay Science 26, Iler, R.K., The Chemistry of Silica. Wiley, New York. 866 pp.

10 S. Kaufhold, R. Dohrmann / Applied Clay Science 39 (2008) Kasbohm, J., Pusch, R., Henning, K.-H., In: Nüesch, R., Emmerich, K. (Eds.), Short term experiments with different bentonites in saline solutions, vol. 10. Berichte der DTTG, Karslruhe, p. 47. ISSN Kaufhold, S., Untersuchungen zur Eignung von natürlich alterierten sowie mit Oxalsäure aktivierten Bentoniten als Bleicherde für Pflanzenöle. PhD study, RWTH Aachen, 188 pp., online available: source_opus=256. Le Bell, J.C., Colloid chemical aspects of the confined bentonite concept. SKB Technical Report No. 97 ID Meier, L.P., Kahr, G., Determination of the cation exchange capacity (CEC) of clay minerals using the complexes of Copper (II) ion with triethylenetetramine and tretraethylenepentamine. Clays and Clay Minerals 47, Missana, T., Garcia-Gutiérrez, M., Alonso, U., Kinetics and irreversibility of cesium and uranium sorption onto bentonite colloids. Andra: Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, Reims, 2002, Program and Abstracts. Missana, T., Alonso, U., Turrero, M.J., Generation and stability of bentonite colloids at the bentonite/granite interface of a deep geological radioactive waste repository. Journal of Contaminant Hydrology 61, NAGRA, Projekt Mont Terri Synthese der geowissenschaftlichen Untersu-chungsergebnisse. Nagra technischer Bericht, vol pp. Pusch, R., Use of bentonite for isolation of radioactive waste products. Clay Minerals 27, Pusch, R., Deep disposal of radioactive waste. Schriftenreihe Angewandte Geowissenschaftliche 1, Pusch, R., Clay colloid formation and release from MX-80 buffer. SKB Technical Report No: TR Pusch, R., On the effect of hot water vapor on MX-80 clay. SKB Technical Report No: TR Siddiqui, M.K.H., Bleaching earths. Pergamon Press, Oxford. 86 pp. Ufer, K., Roth, G., Kleeberg, R., Stanjek, H., Dohrmann, R., Bergmann, J., Description and quantification of powder X-ray diffractograms of turbostratically disordered layer structures with a Rietveld compatible approach. Zeitschrift fuér Kristallographie 219, Ufer, K., Stanjek, H., Roth, G., Dohrmann, R., Kleeberg, R., Kaufhold, S., submitted for publication. Quantitative phase analysis of bentonites by the Rietveld method. Clays and Clay Minerals.