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Shelf life stability of diatomites
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Applied Clay Science 41 (2008) 158 – 164 www.elsevier.com/locate/clay
Shelf life stability of diatomites S. Kaufhold a,⁎, R. Dohrmann a,b , Ch. Ulrichs c a
c
BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany b LBEG, Landesamt für Bergbau, Energie und Geologie, Stilleweg 2, D-30655 Hannover, Germany LGF, Landwirtschaftlich-Gärtnerische Fakultät, Humboldt-Universität zu Berlin, Unter den Linden 6, D-10099 Berlin, Germany Received 21 August 2007; received in revised form 8 October 2007; accepted 19 October 2007 Available online 12 November 2007
Abstract Diatomites or diatomaceous earths are biogenic sediments mainly consisting of porous siliceous algae skeletons. Some diatomites are industrially processed and used for different applications. For some applications and sometimes for the authorization of products for new applications it is important to prove the ‘shelf life stability’ which means that the properties of the product should not change upon storage. Freshly prepared silica gel is known to alter upon aging e.g. with respect to the specific surface area and porosity, respectively. This can be relevant for diatoms as well since recent skeleton consist of XRD amorphous silica which – upon aging – crystallizes, finally forming microcrystalline silica. The age of most of the diatomites being mined is higher than 1 million years. Accordingly, one could expect that storage for some years does not alter the diatomite properties. However, it is at least conceivable that prolonged open storage (exposure to air) can lead to the adsorption of molecules from air which in turn would block reactive adsorption sites. The aim of this study is to assess the shelf-life-stability of diatomite and perlite based on long term tests. Therefore, two different diatomite samples and a perlite sample (XRD amorphous volcanic alumosilicate glass) were stored at different conditions for 1 year. All materials were investigated with respect to structural aspects as well as adsorption capacity. Using the common mineralogical methods X-ray diffraction (XRD) and infrared spectroscopy no structural changes could be observed. In contrast, the determination of the amount of soda soluble silica turned out to be a valuable tool for the identification of structural changes. Using this method a slightly higher reactivity of the perlite surface after storage at 60 °C in water was found which possibly can be explained by the beginning devitrification process. At the same conditions, the diatomite sample which mostly consists of XRD amorphous silica showed a slight decrease of the amount of soda soluble silica which was interpreted as the beginning (re)crystallization process. The diatomite sample containing swellable clay minerals lost some water adsorption capacity upon extensive drying. Systematic changes of material properties after 1 year storage could only be observed under exaggerated conditions (120 °C or storage in water at 60 °C). It is concluded that the investigated materials are stable under common storage conditions. © 2007 Elsevier B.V. All rights reserved. Keywords: Diatomite; Diatomaceous earth; Perlite; Shelf-life stability; Storage
⁎ Corresponding author. E-mail address: [email protected] (S. Kaufhold). 0169-1317/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2007.10.007
S. Kaufhold et al. / Applied Clay Science 41 (2008) 158–164
1. Introduction Diatomites or diatomaceous earths are biogenetic sediments consisting of fossilised skeletal diatoms. Diatoms (Kieselalgen) in turn are one type of algae which adsorb silica from water, metabolize and deposit it as an external skeleton (Iler, 1979). Early in 1930 more than 10.000 different types were known (Calvert, 1930). The siliceaous skeleton of living diatoms is a microporous silica gel with a N2-BET specific surface area of up to 100 m2 /g (Iler, 1979). However, after death, sedimentation, and diagensis the surface area decreases because of (re)crystallization of the silica gel. Therefore the silica in recent deposits is XRD amorphous in contrast to the microcrystalline silica in several million years old deposits. The (re)crystallization of the silica gel is described e.g. by Helmcke (1954) and Darley (1974). Generally, diatoms are macroporous according to the IUPAC classification (IUPAC, 1994). However, even small mesopores (20 Å) were reported (Iler, 1979). Diatomites (or diatomaceous earths) consist mainly of the porous diatoms which determine the physical properties of this natural raw material. According to USGS (http:// minerals.usgs.gov/minerals/pubs/commodity/diatomite/) diatomites are chalk-like, soft, friable, fine-grained, usually light in color, porous, light (floating on water unless water saturated), chemically inert in most liquids and gases, possess low thermal conductivity, and a high fusion point. Because of these properties diatomites are of great industrial importance. They are frequently used as filter aid. Surface characteristics are described e.g. by AlGhouti et al. (2003). Well known applications are pet litter, filler, mild abrasive, liquid ad/absorption, and construction material (USGS, http://minerals.usgs.gov/ minerals/pubs/commodity/diatomite/). An additional innovative application of diatomaceous earths as insecticide is described by Mewis & Ulrichs (2001a, 2001b). For some applications it is important to prove that prolonged storage does not affect material properties. Hence, the ‘shelf life stability’ has to be proved. In Germany this is particularly important with respect to the authorization of new products which are supposed to
159
be used in the food industry. On the other hand, the shelf life stability is also important for the producers in order to ensure product quality. From mineralogical studies it is well known that amorphous silica tends to (re)crystallize which in turn reduces the porosity. This was proved e.g. by Kaufhold (2001) who investigated the shelf life stability of bleaching earths which are mainly used for purification of vegetable oils. Bleaching earths are produced by reacting strong mineral acids with bentonites. The product consists of a relict clay mineral structure and mesoporous silica gel. Kaufhold (2001) could show that aging a freshly prepared bleaching earth causes condensation of Si–O bonds of the silica gel which resulted in a slight decrease of the specific surface area and a decrease of Si-solubility in soda solution. Expectedly, changes were minor but they could be proved unambiguously. In contrast to bleaching earths, the age of most of the diatomite sediments exceeds 1 million years by far. Accordingly, one expects that storage over some years would not significantly alter the diatomite properties. However, it is at least conceivable that prolonged open storage (exposure to air) leads to the adsorption of molecules from air which in turn can lead to blocking of the reactive adsorption sites. The aim of this study is to assess the shelf-lifestability of diatomite and perlite based on long term tests (1 year). Kadey (1983) mentions the chemical stability of diatomites in general. However, a systematic investigation of the shelf-life-stability of diatomites and perlites is not available in literature, yet. 2. Materials and methods Two different diatomites and a perlite (amorphous volcanic alumosilicate glass) were stored at different conditions for 1 year; samples are listed in Table 1. All materials were investigated with respect to structural aspects as well as adsorption capacity. The aim of this study is to assess the shelf life stability of diatomites. In this respect material properties have to be investigated regarding structural changes ((re)crystallization followed by changes of the pore system) as well as changes of adsorption capacity. It is worth mentioning that structural
Table 1 Materials applied for the storage tests Sample
Perlite 30®
Diamol®
Celite®
Product name Production Company Location of deposit Geological age (appr.)
Perlite 30 Industrially ground and expanded Damolin Hamburg GmbH Island Milos, Greece b1 Ma
Diamol Dl 10 G Industrially ground Damolin Hamburg GmbH Island Fur, Denmark 50 Ma
Celite 266 Industrially ground Lehmann&Voss & Co Alicante, Spain 8 Ma
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S. Kaufhold et al. / Applied Clay Science 41 (2008) 158–164
changes likely affect adsorption capacity as well. On the other hand, adsorption of specific molecules from air would reduce the adsorption capacity without any evidence of structural changes. In order to separately test both aspects of stability the three samples were portioned and stored at different conditions: 1) ‘Closed’ (samples in PE flasks = closely sealed; 1 year) 2) ‘Open’ (samples in open ceramic cups in laboratory without sealing; 1 year) 3) ‘60 °C’ (samples in open ceramic cups in oven at 60 °C; 1 year) 4) ‘120 °C’ (samples in open ceramic cups in oven at 120 °C; 1 year) 5) ‘H20’ (samples mixed with water (solid/liquid = 1/10) in oven at 60 °C, 6 months). The mineralogical composition of the three samples was determined by XRD (Phillips PW 3710, Cu radiation, fixed primary slit system, secondary monochromator). XRD was not supposed to be suitable to study the expected small structural changes of the relatively old diatomites because only the new formation and/or disappearance of significant amounts of crystalline phases can be detected. This was not expected in case of the relatively old natural raw materials. Therefore, more sensitive methods had to be selected. Amorphous silica is known to be partly soluble in soda solution. Kaufhold (2001) could show that storing a bleaching earth at elevated temperature (60–105 °C) for 18 months leads to a reduction of the amount of soda soluble silica from appr. 12 down to 9 wt.%. This was accompanied by a shift of the infrared active Si–Ostretching vibration towards higher wavenumbers caused by the increased ratio of out of plane / in plane vibrations (loose layers condense three-dimensionally). Therefore, both methods, infrared spectroscopy and determination of the amount of
soluble silica were regarded as appropriate for the identification of minor structural changes caused by storage. For measuring mid infrared (MIR) spectra the KBr pellet technique (1 mg sample/200 mg KBr) was applied. Spectra were collected on a Thermo Nicolet Nexus FTIR spectrometer (beam splitter: KBr, detector DTGS TEC, resol. 2 cm− 1). In order to determine the soda soluble silica 0.100 and 0.500 g sample (predried for 24 h at 105 °C) were subjected to 50.0 mL 4% Na2CO3 solution and shaken end over end for 72 h at ambient conditions. After centrifugation the Si concentration of the supernatant was measured using ICP–AES. In addition the BET method was used in order to study possible changes of the specific surface area which significantly depends on porosity. N2-measurements were performed by a Micromeritics Gemini III 2375 surface area analyzer (appr. 300 mg weight). The samples were degassed for 24 h at 105 °C and for 24 h in vacuum. The specific surface area was calculated using the BET equation. Changes of adsorption capacities were studied by adsorption tests using water and ethylene glycol, respectively. Water adsorption was performed by weighing 500 mg sample in aluminum cups (diameter 4 cm) and the adsorption capacity was measured gravimetrically at 50, 60, and 70% relative humidity (r.h.) provided by a climate oven (Binder APT.Line KBF). Samples were maintained at each relative humidity for 1 week and weighed outside the oven after each week. Finally, the samples were dried at 105 °C in order to determine the initial dry mass. Ethylene glycol adsorption was studied by storing 500 mg sample in a desiccator over 500 mL ethylene glycol. The maximum adsorption capacity was attained after appr. 4 weeks and determined gravimetrically. The accuracy of all methods applied was estimated by measuring five replicates using the raw material (prior to the storage tests).
Fig. 1. XRD powder pattern of the three samples (‘fsp’ = feldspar, ‘qtz’ = quartz, ‘cc’ = calcite, ‘ara’ = aragonite, ‘jar’ = jarosite).
[y/n] Infrared spectroscopy Changes v (Si–O)
‘Fresh’ = before the experiment; ‘clo’ = 1 year closed; ‘op’ = 1 year open in laboratory; ‘60’ = 1 year open in oven at 60 °C; ‘120’ = 1 year open in oven at 120 °C; ‘H2O’ = half a year closed with excess water at 60 °C.
n n n n n n n n n n n n n n n n n n
10.7 10.0 10.1 10.3 11.6 11.1 45.1 41.3 36.2 42.0 46.7 47.0 3.2 3.0 3.0 2.7 3.1 2.3 [m2/g] Specific surface area N2-Adsorption (BET)
±10%
3.0 1.6 3.5 2.0 3.6 2.0 3.6 2.0 3.6 2.0 3.7 2.1 3.0 1.2 3.0 1.4 2.5 1.3 2.9 1.3 2.9 1.3 3.0 1.1 1.0 0.5 0.9 0.3 0.9 0.3 0.9 0.3 0.9 0.3 0.9 0.3 ±0.1 wt.% ±0.1 wt.% [wt.%] [wt.%]
60 °C op clo Fresh Fresh Precision
clo
op
60 °C
120 °C
H2O
Fresh
clo
op
60 °C
120 °C
H2 O
Celite® Diamol® Perlite30® Estimate
Table 2 Results of shelf life stability tests with respect to structural changes
The XRD patterns of the three samples are given in Fig. 1. All samples mainly consist of a XRD-amorphous phase (likely the siliceous diatom skeletons in case of Diamol® and Celite® and an Al–Si-dominated volcanic glass in case of Perlite30®). Diamol® additionally contains jarosite and quartz and a poorly ordered swelling clay mineral (assigned ‘swellable clay mineral’). Minor constituents of Celite® are calcite, aragonite, and quartz. The perlite sample contains traces of quartz and possibly some feldspar. According to e.g. Helmcke (1954) diatoms in general recrystallize throughout a long period of time finally forming microcrystalline quartz which should be evident from XRD. Expectedly, Celite® and Perlite30® possess a pronounced broad XRD peak representing XRD amorphous silica. However, even in case of the 50 Ma old Diamol® only minor quartz reflections were found. This means that the XRD-amorphous silica present in all samples conceivably could (re)crystallize upon aging. The results of the investigation of structural parameters before and after storage are shown in Table 2. It is necessary to mention that the extraction values depend on the solid/liquid ratio used, with higher extraction values as this ratio decreases. The values obtained in the present study represent a parameter describing the reactivity of the surface of these materials. The precision of the extraction values is appr. ±0.1 wt.% SiO2 (determined by five replicates of Si extractions of sample Perlite30®). In order to be able to identify systematical changes, the values of both solid / liquid ratios have to be considered. In contrast to the other samples Diamol® shows nonsystematic variation of extraction values. This might be because of the existence of clay minerals being particularly sensitive towards slight changes of experimental conditions. Accordingly, no systematic changes could be observed in case of sample Diamol®. The value 2.5 (Diamol®, 60 °C, 0.1 g/50 mL) is considered as outlier, since this significant decrease could not be confirmed by the 0.5 g/50 mL-value. On the other hand, the soda soluble silica values of Perlite30® and Celite® stored in water are believed to reveal minor structural changes. In case of Perlite30® a slightly higher amount of soda soluble silica was found after storage at 60 °C in water. Volcanic glass, the main component of Perlite30®, is known to be instable in contact with water. In geological time scales clay minerals and/or zeolites may form due to devitrification. Here, it is possible that the slightly increased values reflect the beginning of the volcanic glass / water reaction. In case of sample
120 °C
3. Results and discussion
161
Extraction Silica (0.1 g/50 mL) Silica (0.5 g/50 mL)
H2 O
S. Kaufhold et al. / Applied Clay Science 41 (2008) 158–164
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S. Kaufhold et al. / Applied Clay Science 41 (2008) 158–164
Fig. 2. Infrared spectra displaying the Si–O-stretching vibration.
Celite® a decrease of appr. 15% (±3%) soda soluble silica with respect to the fresh sample was found after storage in water which possibly indicates (re)crystallization. The specific surface area (SSA) values of all samples vary non-systematically. For example, the decrease of the Diamol® SSA should be greater for 120 °C than for 60 °C since the alteration mechanisms induced by dry heating should be similar. The discussion of different mechanisms occurring at 60 °C and 120 °C, respectively, is hypothetical. In case of sample Celite® stored in water a decrease of the specific surface area caused by (re)crystallization was expected by considering the extraction values. This could not be confirmed. In contrast, the specific surface area of this sample almost exactly fits with the mean value of all Celite® specific surface areas. This means that (re)crystallization of the amorphous silica – if it occurred at all – did not affect the SSA. The SSA study did not allow to detect meaningful changes.
By infrared spectroscopy particularly the Si–O-stretching vibration was considered which can be used for the determination of changes of chemical bonds. As can be seen in Fig. 2 no systematic changes could be observed. The water and ethyleneglycol adsorption capacity of all samples was investigated before and after storage. Generally, it is conceivable that the adsorption capacity is reduced either by adsorption of specific molecules from air (should result in a pronounced decrease in case of the open sample) or by reduction of pore volume caused by structural changes as (re)crystallization. Before the storage period the reproducibility of both adsorption experiments was determined. The reproducibility of ethyleneglycol adsorption was found to be significantly worse than the reproducibility of water adsorption. This is likely caused by the different experimental setup. In case of ethyleneglycol adsorption samples were stored under ethyleneglycol atmosphere (in desiccator in laboratory). In contrast, water adsorption was performed in a climate
Fig. 3. Comparison of ethyleneglycol (EG)- and water adsorption values of all three fresh (before storage) samples (mean values, three replicates).
S. Kaufhold et al. / Applied Clay Science 41 (2008) 158–164
163
Table 3 Results of ethyleneglycol and water adsorption tests before and after storage at different conditions Estimate
Perlite30®
Diamol®
Celite®
Precision Fresh clo op 60 °C 120 °C H2O Fresh clo op 60 °C 120 °C H2O Fresh clo op 60 °C 120 °C H2O EG adsorption Ethyleneglycol [wt.%] ±0.3 wt.% 0.8
0.5 0.4 0.6
0.3
0.6
5.8
5.7 7.4 7.4
6.0
7.0
2.6
1.7 2.0 1.9
1.8
2.0
Water adsorption 50% rh. [wt.%] ±0.1 wt.% 0.5 60% rh. [wt.%] ±0.1 wt.% 0.7 70% rh. [wt.%] ±0.1 wt.% 0.9
0.5 0.6 0.5 0.7 0.8 0.7 0.9 1.0 1.0
0.5 0.7 0.9
0.4 0.6 0.8
5.3 5.8 6.9
5.2 5.6 4.8 6.0 6.4 5.5 6.6 7.0 6.1
4.9 5.6 6.2
5.8 6.6 7.2
1.7 2.3 2.9
1.8 1.8 1.8 2.3 2.3 2.3 2.9 2.9 3.0
1.8 2.3 3.0
1.6 2.1 2.7
‘Fresh’ = before the experiment; ‘clo’ = 1 year closed; ‘op’ = 1 year open in laboratory; ‘60’ = 1 year open in oven at 60 °C; ‘120’ = 1 year open in oven at 120 °C; ‘H2O’ = half a year closed with excess water at 60 °C.
oven providing constant temperature (30 °C) and minor relative humidity gradients due to ventilation. Therefore, etheyleneglcol adsorption values (standard deviation appr. 0.3 wt.%, n = 5) are not as significant as water adsorption values (standard deviation appr. 0.1 wt.%, n = 5). Moreover, it was expected that water and ethyleneglycol adsorption mechanisms are related since both molecules are small and polar. This is confirmed in Fig. 3. The results of the adsorption experiments before and after the storage period are given in Table 3. No systematic differences of the EG adsorption values could be detected (Table 3). In contrast, water adsorption values indicate some systematic changes in case of samples Diamol® and Celite®. Despite a slightly higher content of soda soluble silica (Table 2) Perlite30® did not reveal any changes with respect to water adsorption (Table 3). In case of Diamol® slightly lower water adsorption values were found for the samples stored at elevated temperature without water. Probably this is caused by changes of the swellable clay minerals (minor constituents of sample Diamol®, Fig. 1) since it is well known that swellable clay minerals can loose their swelling capacity upon extensive drying (Heller-Kallai, 2006). Therefore it is likely that the slight decrease of water adsorption capacity of Diamol® is caused by the loss of swelling capacity of clay minerals. In case of Celite® a slightly lower water adsorption capacity was found for the sample stored in water at 60 °C. From Table 2 it is evident that the amount of soda soluble silica of this sample decreased. This, conceivably, can be related to (re)crystallization processes which in turn would explain the slight decrease of water adsorption capacity. 4. Conclusions The aim of this study was to investigate the shelf life stability of diatomites. In addition, a perlite sample
which was considered as comparably instable particularly in contact with water, was investigated. However, both types of materials were supposed to be stable under ambient conditions. Therefore, it was expected that any changes of material properties could only be proved after prolonged storage under exaggerated conditions. Hence, the temperature and amount of available water were increased in a way which does not reflect realistic ambient conditions with respect to shelf-life-stability. On the other hand, exaggerated conditions allow for the identification of the specific conditions revealing the first systematic changes. As expected, systematic changes were only observed under exaggerated conditions. Therefore, it is concluded that the investigated materials are stable under common storage conditions. However, in the present study we present and discuss methods being able to even detect minor changes of materials properties caused by storage (under exaggerated conditions). Using these methods we could prove that the XRD amorphous silica of some diatomites tends to (re)crystallize. References Al-Ghouti, M.A., Khraisheh, M.A.M., Allen, S.J., Ahmad, M.N., 2003. The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth. Journal of Environmental Management 69, 229–238. Calvert, R., 1930. Diatomaceous earth. Chemical Catalog Co., New York. 250 pp. Darley, W.M., 1974. Silicification and calcification. In: Werner, D. (Ed.), The Biology of Diatoms. Bot. Mon., 10, pp. 655–675. Heller-Kallai, L., 2006. Thermally modified clay minerals. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of clay science. Elsevier, Amsterdam. 1224 pp. Helmcke, J.-G., 1954. Die Feinstruktur der Kieselsäure und ihre physiologische Bedeutung in Diatomeenschalen. Die Naturwissenschaften 10, 254–255. Iler, R.K., 1979. The Chemistry of Silica. Wiley, New York. 866 pp. IUPAC, 1994. In: Rouquérol, J., Avnir, D., Fairbridge, C.W., Everett, D.H., Haynes, J.H., Pernicone, N., Ramsay, J.D.F., Sing, K.S.W., Unger, K.K.
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(Eds.), Recommendations for the Characterisation of Porous Solids (Recommendations 1994). Pure & Appl. Chem., vol. 66, p. 1739. Kadey, F.L., 1983. Diatomite, In: LeFond Jr., S.J. (Ed.), (5th ed.). Industrial Rocks and Minerals, 1. American Institute of Mining, Metallurgical, and Petroleum Engineers, New York, pp. 677–708. Kaufhold, S., 2001. Untersuchungen zur Eignung von natürlich alterierten sowie mit Oxalsäure aktivierten Bentoniten als Bleicherde für Pflanzenöle. PhD study. RWTH Aachen. 188 pp., online available: http://134.130.184.8/opus/frontdoor.php?source_opus= 256.
Mewis, I., Ulrichs, C., 2001a. Treatment of rice with diatomaceous earth and effects on the mortality of the red flour beetle tribolium castaneum (Herbst). Journal of Pesticide Science 74, 13–16. Mewis, I., Ulrichs, C., 2001b. Action of amorphous diatomaceous earth against different stages of the stored product pests tribolium confusum, tenebrio molitor, sitophilus granarius and plodia interpuncttella. Journal of Storted Products Research 37, 153–164.
Applied Clay Science 41 (2008) 158 – 164 www.elsevier.com/locate/clay
Shelf life stability of diatomites S. Kaufhold a,⁎, R. Dohrmann a,b , Ch. Ulrichs c a
c
BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany b LBEG, Landesamt für Bergbau, Energie und Geologie, Stilleweg 2, D-30655 Hannover, Germany LGF, Landwirtschaftlich-Gärtnerische Fakultät, Humboldt-Universität zu Berlin, Unter den Linden 6, D-10099 Berlin, Germany Received 21 August 2007; received in revised form 8 October 2007; accepted 19 October 2007 Available online 12 November 2007
Abstract Diatomites or diatomaceous earths are biogenic sediments mainly consisting of porous siliceous algae skeletons. Some diatomites are industrially processed and used for different applications. For some applications and sometimes for the authorization of products for new applications it is important to prove the ‘shelf life stability’ which means that the properties of the product should not change upon storage. Freshly prepared silica gel is known to alter upon aging e.g. with respect to the specific surface area and porosity, respectively. This can be relevant for diatoms as well since recent skeleton consist of XRD amorphous silica which – upon aging – crystallizes, finally forming microcrystalline silica. The age of most of the diatomites being mined is higher than 1 million years. Accordingly, one could expect that storage for some years does not alter the diatomite properties. However, it is at least conceivable that prolonged open storage (exposure to air) can lead to the adsorption of molecules from air which in turn would block reactive adsorption sites. The aim of this study is to assess the shelf-life-stability of diatomite and perlite based on long term tests. Therefore, two different diatomite samples and a perlite sample (XRD amorphous volcanic alumosilicate glass) were stored at different conditions for 1 year. All materials were investigated with respect to structural aspects as well as adsorption capacity. Using the common mineralogical methods X-ray diffraction (XRD) and infrared spectroscopy no structural changes could be observed. In contrast, the determination of the amount of soda soluble silica turned out to be a valuable tool for the identification of structural changes. Using this method a slightly higher reactivity of the perlite surface after storage at 60 °C in water was found which possibly can be explained by the beginning devitrification process. At the same conditions, the diatomite sample which mostly consists of XRD amorphous silica showed a slight decrease of the amount of soda soluble silica which was interpreted as the beginning (re)crystallization process. The diatomite sample containing swellable clay minerals lost some water adsorption capacity upon extensive drying. Systematic changes of material properties after 1 year storage could only be observed under exaggerated conditions (120 °C or storage in water at 60 °C). It is concluded that the investigated materials are stable under common storage conditions. © 2007 Elsevier B.V. All rights reserved. Keywords: Diatomite; Diatomaceous earth; Perlite; Shelf-life stability; Storage
⁎ Corresponding author. E-mail address: [email protected] (S. Kaufhold). 0169-1317/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2007.10.007
S. Kaufhold et al. / Applied Clay Science 41 (2008) 158–164
1. Introduction Diatomites or diatomaceous earths are biogenetic sediments consisting of fossilised skeletal diatoms. Diatoms (Kieselalgen) in turn are one type of algae which adsorb silica from water, metabolize and deposit it as an external skeleton (Iler, 1979). Early in 1930 more than 10.000 different types were known (Calvert, 1930). The siliceaous skeleton of living diatoms is a microporous silica gel with a N2-BET specific surface area of up to 100 m2 /g (Iler, 1979). However, after death, sedimentation, and diagensis the surface area decreases because of (re)crystallization of the silica gel. Therefore the silica in recent deposits is XRD amorphous in contrast to the microcrystalline silica in several million years old deposits. The (re)crystallization of the silica gel is described e.g. by Helmcke (1954) and Darley (1974). Generally, diatoms are macroporous according to the IUPAC classification (IUPAC, 1994). However, even small mesopores (20 Å) were reported (Iler, 1979). Diatomites (or diatomaceous earths) consist mainly of the porous diatoms which determine the physical properties of this natural raw material. According to USGS (http:// minerals.usgs.gov/minerals/pubs/commodity/diatomite/) diatomites are chalk-like, soft, friable, fine-grained, usually light in color, porous, light (floating on water unless water saturated), chemically inert in most liquids and gases, possess low thermal conductivity, and a high fusion point. Because of these properties diatomites are of great industrial importance. They are frequently used as filter aid. Surface characteristics are described e.g. by AlGhouti et al. (2003). Well known applications are pet litter, filler, mild abrasive, liquid ad/absorption, and construction material (USGS, http://minerals.usgs.gov/ minerals/pubs/commodity/diatomite/). An additional innovative application of diatomaceous earths as insecticide is described by Mewis & Ulrichs (2001a, 2001b). For some applications it is important to prove that prolonged storage does not affect material properties. Hence, the ‘shelf life stability’ has to be proved. In Germany this is particularly important with respect to the authorization of new products which are supposed to
159
be used in the food industry. On the other hand, the shelf life stability is also important for the producers in order to ensure product quality. From mineralogical studies it is well known that amorphous silica tends to (re)crystallize which in turn reduces the porosity. This was proved e.g. by Kaufhold (2001) who investigated the shelf life stability of bleaching earths which are mainly used for purification of vegetable oils. Bleaching earths are produced by reacting strong mineral acids with bentonites. The product consists of a relict clay mineral structure and mesoporous silica gel. Kaufhold (2001) could show that aging a freshly prepared bleaching earth causes condensation of Si–O bonds of the silica gel which resulted in a slight decrease of the specific surface area and a decrease of Si-solubility in soda solution. Expectedly, changes were minor but they could be proved unambiguously. In contrast to bleaching earths, the age of most of the diatomite sediments exceeds 1 million years by far. Accordingly, one expects that storage over some years would not significantly alter the diatomite properties. However, it is at least conceivable that prolonged open storage (exposure to air) leads to the adsorption of molecules from air which in turn can lead to blocking of the reactive adsorption sites. The aim of this study is to assess the shelf-lifestability of diatomite and perlite based on long term tests (1 year). Kadey (1983) mentions the chemical stability of diatomites in general. However, a systematic investigation of the shelf-life-stability of diatomites and perlites is not available in literature, yet. 2. Materials and methods Two different diatomites and a perlite (amorphous volcanic alumosilicate glass) were stored at different conditions for 1 year; samples are listed in Table 1. All materials were investigated with respect to structural aspects as well as adsorption capacity. The aim of this study is to assess the shelf life stability of diatomites. In this respect material properties have to be investigated regarding structural changes ((re)crystallization followed by changes of the pore system) as well as changes of adsorption capacity. It is worth mentioning that structural
Table 1 Materials applied for the storage tests Sample
Perlite 30®
Diamol®
Celite®
Product name Production Company Location of deposit Geological age (appr.)
Perlite 30 Industrially ground and expanded Damolin Hamburg GmbH Island Milos, Greece b1 Ma
Diamol Dl 10 G Industrially ground Damolin Hamburg GmbH Island Fur, Denmark 50 Ma
Celite 266 Industrially ground Lehmann&Voss & Co Alicante, Spain 8 Ma
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changes likely affect adsorption capacity as well. On the other hand, adsorption of specific molecules from air would reduce the adsorption capacity without any evidence of structural changes. In order to separately test both aspects of stability the three samples were portioned and stored at different conditions: 1) ‘Closed’ (samples in PE flasks = closely sealed; 1 year) 2) ‘Open’ (samples in open ceramic cups in laboratory without sealing; 1 year) 3) ‘60 °C’ (samples in open ceramic cups in oven at 60 °C; 1 year) 4) ‘120 °C’ (samples in open ceramic cups in oven at 120 °C; 1 year) 5) ‘H20’ (samples mixed with water (solid/liquid = 1/10) in oven at 60 °C, 6 months). The mineralogical composition of the three samples was determined by XRD (Phillips PW 3710, Cu radiation, fixed primary slit system, secondary monochromator). XRD was not supposed to be suitable to study the expected small structural changes of the relatively old diatomites because only the new formation and/or disappearance of significant amounts of crystalline phases can be detected. This was not expected in case of the relatively old natural raw materials. Therefore, more sensitive methods had to be selected. Amorphous silica is known to be partly soluble in soda solution. Kaufhold (2001) could show that storing a bleaching earth at elevated temperature (60–105 °C) for 18 months leads to a reduction of the amount of soda soluble silica from appr. 12 down to 9 wt.%. This was accompanied by a shift of the infrared active Si–Ostretching vibration towards higher wavenumbers caused by the increased ratio of out of plane / in plane vibrations (loose layers condense three-dimensionally). Therefore, both methods, infrared spectroscopy and determination of the amount of
soluble silica were regarded as appropriate for the identification of minor structural changes caused by storage. For measuring mid infrared (MIR) spectra the KBr pellet technique (1 mg sample/200 mg KBr) was applied. Spectra were collected on a Thermo Nicolet Nexus FTIR spectrometer (beam splitter: KBr, detector DTGS TEC, resol. 2 cm− 1). In order to determine the soda soluble silica 0.100 and 0.500 g sample (predried for 24 h at 105 °C) were subjected to 50.0 mL 4% Na2CO3 solution and shaken end over end for 72 h at ambient conditions. After centrifugation the Si concentration of the supernatant was measured using ICP–AES. In addition the BET method was used in order to study possible changes of the specific surface area which significantly depends on porosity. N2-measurements were performed by a Micromeritics Gemini III 2375 surface area analyzer (appr. 300 mg weight). The samples were degassed for 24 h at 105 °C and for 24 h in vacuum. The specific surface area was calculated using the BET equation. Changes of adsorption capacities were studied by adsorption tests using water and ethylene glycol, respectively. Water adsorption was performed by weighing 500 mg sample in aluminum cups (diameter 4 cm) and the adsorption capacity was measured gravimetrically at 50, 60, and 70% relative humidity (r.h.) provided by a climate oven (Binder APT.Line KBF). Samples were maintained at each relative humidity for 1 week and weighed outside the oven after each week. Finally, the samples were dried at 105 °C in order to determine the initial dry mass. Ethylene glycol adsorption was studied by storing 500 mg sample in a desiccator over 500 mL ethylene glycol. The maximum adsorption capacity was attained after appr. 4 weeks and determined gravimetrically. The accuracy of all methods applied was estimated by measuring five replicates using the raw material (prior to the storage tests).
Fig. 1. XRD powder pattern of the three samples (‘fsp’ = feldspar, ‘qtz’ = quartz, ‘cc’ = calcite, ‘ara’ = aragonite, ‘jar’ = jarosite).
[y/n] Infrared spectroscopy Changes v (Si–O)
‘Fresh’ = before the experiment; ‘clo’ = 1 year closed; ‘op’ = 1 year open in laboratory; ‘60’ = 1 year open in oven at 60 °C; ‘120’ = 1 year open in oven at 120 °C; ‘H2O’ = half a year closed with excess water at 60 °C.
n n n n n n n n n n n n n n n n n n
10.7 10.0 10.1 10.3 11.6 11.1 45.1 41.3 36.2 42.0 46.7 47.0 3.2 3.0 3.0 2.7 3.1 2.3 [m2/g] Specific surface area N2-Adsorption (BET)
±10%
3.0 1.6 3.5 2.0 3.6 2.0 3.6 2.0 3.6 2.0 3.7 2.1 3.0 1.2 3.0 1.4 2.5 1.3 2.9 1.3 2.9 1.3 3.0 1.1 1.0 0.5 0.9 0.3 0.9 0.3 0.9 0.3 0.9 0.3 0.9 0.3 ±0.1 wt.% ±0.1 wt.% [wt.%] [wt.%]
60 °C op clo Fresh Fresh Precision
clo
op
60 °C
120 °C
H2O
Fresh
clo
op
60 °C
120 °C
H2 O
Celite® Diamol® Perlite30® Estimate
Table 2 Results of shelf life stability tests with respect to structural changes
The XRD patterns of the three samples are given in Fig. 1. All samples mainly consist of a XRD-amorphous phase (likely the siliceous diatom skeletons in case of Diamol® and Celite® and an Al–Si-dominated volcanic glass in case of Perlite30®). Diamol® additionally contains jarosite and quartz and a poorly ordered swelling clay mineral (assigned ‘swellable clay mineral’). Minor constituents of Celite® are calcite, aragonite, and quartz. The perlite sample contains traces of quartz and possibly some feldspar. According to e.g. Helmcke (1954) diatoms in general recrystallize throughout a long period of time finally forming microcrystalline quartz which should be evident from XRD. Expectedly, Celite® and Perlite30® possess a pronounced broad XRD peak representing XRD amorphous silica. However, even in case of the 50 Ma old Diamol® only minor quartz reflections were found. This means that the XRD-amorphous silica present in all samples conceivably could (re)crystallize upon aging. The results of the investigation of structural parameters before and after storage are shown in Table 2. It is necessary to mention that the extraction values depend on the solid/liquid ratio used, with higher extraction values as this ratio decreases. The values obtained in the present study represent a parameter describing the reactivity of the surface of these materials. The precision of the extraction values is appr. ±0.1 wt.% SiO2 (determined by five replicates of Si extractions of sample Perlite30®). In order to be able to identify systematical changes, the values of both solid / liquid ratios have to be considered. In contrast to the other samples Diamol® shows nonsystematic variation of extraction values. This might be because of the existence of clay minerals being particularly sensitive towards slight changes of experimental conditions. Accordingly, no systematic changes could be observed in case of sample Diamol®. The value 2.5 (Diamol®, 60 °C, 0.1 g/50 mL) is considered as outlier, since this significant decrease could not be confirmed by the 0.5 g/50 mL-value. On the other hand, the soda soluble silica values of Perlite30® and Celite® stored in water are believed to reveal minor structural changes. In case of Perlite30® a slightly higher amount of soda soluble silica was found after storage at 60 °C in water. Volcanic glass, the main component of Perlite30®, is known to be instable in contact with water. In geological time scales clay minerals and/or zeolites may form due to devitrification. Here, it is possible that the slightly increased values reflect the beginning of the volcanic glass / water reaction. In case of sample
120 °C
3. Results and discussion
161
Extraction Silica (0.1 g/50 mL) Silica (0.5 g/50 mL)
H2 O
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Fig. 2. Infrared spectra displaying the Si–O-stretching vibration.
Celite® a decrease of appr. 15% (±3%) soda soluble silica with respect to the fresh sample was found after storage in water which possibly indicates (re)crystallization. The specific surface area (SSA) values of all samples vary non-systematically. For example, the decrease of the Diamol® SSA should be greater for 120 °C than for 60 °C since the alteration mechanisms induced by dry heating should be similar. The discussion of different mechanisms occurring at 60 °C and 120 °C, respectively, is hypothetical. In case of sample Celite® stored in water a decrease of the specific surface area caused by (re)crystallization was expected by considering the extraction values. This could not be confirmed. In contrast, the specific surface area of this sample almost exactly fits with the mean value of all Celite® specific surface areas. This means that (re)crystallization of the amorphous silica – if it occurred at all – did not affect the SSA. The SSA study did not allow to detect meaningful changes.
By infrared spectroscopy particularly the Si–O-stretching vibration was considered which can be used for the determination of changes of chemical bonds. As can be seen in Fig. 2 no systematic changes could be observed. The water and ethyleneglycol adsorption capacity of all samples was investigated before and after storage. Generally, it is conceivable that the adsorption capacity is reduced either by adsorption of specific molecules from air (should result in a pronounced decrease in case of the open sample) or by reduction of pore volume caused by structural changes as (re)crystallization. Before the storage period the reproducibility of both adsorption experiments was determined. The reproducibility of ethyleneglycol adsorption was found to be significantly worse than the reproducibility of water adsorption. This is likely caused by the different experimental setup. In case of ethyleneglycol adsorption samples were stored under ethyleneglycol atmosphere (in desiccator in laboratory). In contrast, water adsorption was performed in a climate
Fig. 3. Comparison of ethyleneglycol (EG)- and water adsorption values of all three fresh (before storage) samples (mean values, three replicates).
S. Kaufhold et al. / Applied Clay Science 41 (2008) 158–164
163
Table 3 Results of ethyleneglycol and water adsorption tests before and after storage at different conditions Estimate
Perlite30®
Diamol®
Celite®
Precision Fresh clo op 60 °C 120 °C H2O Fresh clo op 60 °C 120 °C H2O Fresh clo op 60 °C 120 °C H2O EG adsorption Ethyleneglycol [wt.%] ±0.3 wt.% 0.8
0.5 0.4 0.6
0.3
0.6
5.8
5.7 7.4 7.4
6.0
7.0
2.6
1.7 2.0 1.9
1.8
2.0
Water adsorption 50% rh. [wt.%] ±0.1 wt.% 0.5 60% rh. [wt.%] ±0.1 wt.% 0.7 70% rh. [wt.%] ±0.1 wt.% 0.9
0.5 0.6 0.5 0.7 0.8 0.7 0.9 1.0 1.0
0.5 0.7 0.9
0.4 0.6 0.8
5.3 5.8 6.9
5.2 5.6 4.8 6.0 6.4 5.5 6.6 7.0 6.1
4.9 5.6 6.2
5.8 6.6 7.2
1.7 2.3 2.9
1.8 1.8 1.8 2.3 2.3 2.3 2.9 2.9 3.0
1.8 2.3 3.0
1.6 2.1 2.7
‘Fresh’ = before the experiment; ‘clo’ = 1 year closed; ‘op’ = 1 year open in laboratory; ‘60’ = 1 year open in oven at 60 °C; ‘120’ = 1 year open in oven at 120 °C; ‘H2O’ = half a year closed with excess water at 60 °C.
oven providing constant temperature (30 °C) and minor relative humidity gradients due to ventilation. Therefore, etheyleneglcol adsorption values (standard deviation appr. 0.3 wt.%, n = 5) are not as significant as water adsorption values (standard deviation appr. 0.1 wt.%, n = 5). Moreover, it was expected that water and ethyleneglycol adsorption mechanisms are related since both molecules are small and polar. This is confirmed in Fig. 3. The results of the adsorption experiments before and after the storage period are given in Table 3. No systematic differences of the EG adsorption values could be detected (Table 3). In contrast, water adsorption values indicate some systematic changes in case of samples Diamol® and Celite®. Despite a slightly higher content of soda soluble silica (Table 2) Perlite30® did not reveal any changes with respect to water adsorption (Table 3). In case of Diamol® slightly lower water adsorption values were found for the samples stored at elevated temperature without water. Probably this is caused by changes of the swellable clay minerals (minor constituents of sample Diamol®, Fig. 1) since it is well known that swellable clay minerals can loose their swelling capacity upon extensive drying (Heller-Kallai, 2006). Therefore it is likely that the slight decrease of water adsorption capacity of Diamol® is caused by the loss of swelling capacity of clay minerals. In case of Celite® a slightly lower water adsorption capacity was found for the sample stored in water at 60 °C. From Table 2 it is evident that the amount of soda soluble silica of this sample decreased. This, conceivably, can be related to (re)crystallization processes which in turn would explain the slight decrease of water adsorption capacity. 4. Conclusions The aim of this study was to investigate the shelf life stability of diatomites. In addition, a perlite sample
which was considered as comparably instable particularly in contact with water, was investigated. However, both types of materials were supposed to be stable under ambient conditions. Therefore, it was expected that any changes of material properties could only be proved after prolonged storage under exaggerated conditions. Hence, the temperature and amount of available water were increased in a way which does not reflect realistic ambient conditions with respect to shelf-life-stability. On the other hand, exaggerated conditions allow for the identification of the specific conditions revealing the first systematic changes. As expected, systematic changes were only observed under exaggerated conditions. Therefore, it is concluded that the investigated materials are stable under common storage conditions. However, in the present study we present and discuss methods being able to even detect minor changes of materials properties caused by storage (under exaggerated conditions). Using these methods we could prove that the XRD amorphous silica of some diatomites tends to (re)crystallize. References Al-Ghouti, M.A., Khraisheh, M.A.M., Allen, S.J., Ahmad, M.N., 2003. The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth. Journal of Environmental Management 69, 229–238. Calvert, R., 1930. Diatomaceous earth. Chemical Catalog Co., New York. 250 pp. Darley, W.M., 1974. Silicification and calcification. In: Werner, D. (Ed.), The Biology of Diatoms. Bot. Mon., 10, pp. 655–675. Heller-Kallai, L., 2006. Thermally modified clay minerals. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of clay science. Elsevier, Amsterdam. 1224 pp. Helmcke, J.-G., 1954. Die Feinstruktur der Kieselsäure und ihre physiologische Bedeutung in Diatomeenschalen. Die Naturwissenschaften 10, 254–255. Iler, R.K., 1979. The Chemistry of Silica. Wiley, New York. 866 pp. IUPAC, 1994. In: Rouquérol, J., Avnir, D., Fairbridge, C.W., Everett, D.H., Haynes, J.H., Pernicone, N., Ramsay, J.D.F., Sing, K.S.W., Unger, K.K.
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(Eds.), Recommendations for the Characterisation of Porous Solids (Recommendations 1994). Pure & Appl. Chem., vol. 66, p. 1739. Kadey, F.L., 1983. Diatomite, In: LeFond Jr., S.J. (Ed.), (5th ed.). Industrial Rocks and Minerals, 1. American Institute of Mining, Metallurgical, and Petroleum Engineers, New York, pp. 677–708. Kaufhold, S., 2001. Untersuchungen zur Eignung von natürlich alterierten sowie mit Oxalsäure aktivierten Bentoniten als Bleicherde für Pflanzenöle. PhD study. RWTH Aachen. 188 pp., online available: http://134.130.184.8/opus/frontdoor.php?source_opus= 256.
Mewis, I., Ulrichs, C., 2001a. Treatment of rice with diatomaceous earth and effects on the mortality of the red flour beetle tribolium castaneum (Herbst). Journal of Pesticide Science 74, 13–16. Mewis, I., Ulrichs, C., 2001b. Action of amorphous diatomaceous earth against different stages of the stored product pests tribolium confusum, tenebrio molitor, sitophilus granarius and plodia interpuncttella. Journal of Storted Products Research 37, 153–164.