J Therm Anal Calorim (2013) 114:1191–1199 DOI 10.1007/s10973-013-3152-x
Thermal behavior and dehydroxylation kinetics of naturally occurring sepiolite and bentonite Mu¨ge Sarı Yılmaz • Yasemen Kalpaklı Sabriye Pis¸ kin
•
Received: 30 October 2012 / Accepted: 20 March 2013 / Published online: 23 April 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013
Abstract Sepiolite and bentonite have a wide range of industrial applications based on their physicochemical properties such as surface area, thermal behavior, chemical composition, and mineralogic composition. The thermal behavior and kinetics of naturally occurring sepiolite and bentonite were determined in order to give an idea about the potential use of naturally occurring clay minerals in possible applications. Naturally occurring sepiolite and bentonite samples were heated to the temperature that was achieved at the end of the dehydroxylation process. Mineralogic and thermal characteristics of raw and heat treated samples were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen adsorption/desorption analyses. Changes in the structure following heat treatment were used for the evaluation of the dehydroxylation properties of the samples. The dehydroxylation properties of the minerals are strongly affected by the crystal structure. Kinetic analyses, which were related to the dehydroxylation of naturally occurring sepiolite and bentonite, were conducted using dynamic thermogravimetry/derivative thermogravimetry analysis under nitrogen atmosphere. Flynn–Wall–Ozawa, Kissenger–Akahira–Sunose, and Friedman isoconversional methods were used to determine the activation energies of the dehydroxylation reactions of the samples. The results indicate that the activation energy of naturally occurring sepiolite showed a little variation at a particular conversion rate (0.3–0.7), while the activation energy of naturally occurring bentonite showed a significant variation within the range of variation of the conversion rate. The present study shows that the dehydroxylation reactions of naturally M. Sarı Yılmaz Y. Kalpaklı S. Pis¸ kin (&) Department of Chemical Engineering, Yıldız Technical University, Davutpasa Campus, Topkapı, Istanbul, Turkey e-mail:
[email protected]
occurring sepiolite and bentonite were single mechanism reaction and complex mechanism reaction, respectively. Keywords Naturally occurring sepiolite Bentonite Thermal treatment Dehydroxylation Kinetics Thermal behavior
Introduction Clay minerals are widely used in various industrial applications owing to their physicochemical properties such as high surface area and porosity, low specific gravity, adsorption and ionic exchange capacities, crystal morphology, composition hydration and swelling abilities, as well as their catalytic and other properties. These minerals are widely used in many industrial processes such as effective sources of protons in the paper and ceramic industry; as the suspending medium in saltwater drilling fluids, paints, and pharmaceuticals; as absorbents in pet litter, agricultural chemicals, water, and oil sorption industries; as well as in the cosmetic sector, among many more [1, 2]. Most of these features of clay minerals can be improved and changed by making use of acid activation, soda activation, ion exchange, and thermal treatment processes [3–8]. Turkey has large clay mineral reserves; especially of bentonite and sepiolite, which are generally located in the Central Anatolian Region. Turkish sepiolites are located in Eskisehir, Konya, Bolu, and Ankara regions in northwestern Anatolia [9–11]. The main bentonite deposits are located in Edirne-Enez, Cankırı, Tokat-Resadiye, AnkaraKalecik, and Giresun-Tirebolu [12]. Sepiolite (Si12Mg8O30(OH)4(OH2)4nH2O) is a hydrated magnesium hydrosilicate with a fibrous texture, which
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belongs to a group of layered silicates [13]. Talc-like tetrahedral–octahedral–tetrahedral (T–O–T) ribbons expand infinitely along the z direction, resulting in the unusual fibrous morphology of the sepiolite crystals. The ribbon is linked to the next one with inverted Si–O–Si bonds, producing the continuous tetrahedral sheets and discontinuous octahedral sheets. The terminal Mg2? at the edges of the octahedral sheet, complete their coordination with the oxide surface of the neighboring silica layer [14]. Bentonite is colloidal, alumino-silicate clay derived from weathered volcanic ash which is generally consisting mostly of montmorillonite [15]. It has a 2:1 layer structure and is composed of one Al octahedral sheet placed between two Si tetrahedral sheets, with substitutions of some tetrahedral Si atoms by Al atoms and/or of octahedral atoms (Al3? or Mg2?) by atoms with lower oxidation number. Sepiolite and bentonite minerals may be subjected to high temperatures in some applications including foundry, porous ceramic materials [16], catalysts [17], treatment of aqueous solution [18], civil engineering, and others [19, 20]. The thermal treatment changes the physical and mechanical properties of these clays depending on their mineralogy and crystal structures. Therefore, the utilization of these minerals for being used as raw materials in possible applications requires knowledge of the response of bentonite and sepiolite to variations in temperature. The properties of the resulting anhydrous clay minerals and their broad range of possible uses depend strongly on the dehydroxylation conditions; therefore, the kinetic mechanism of the dehydroxylation of these minerals needs to be known. Although a number of studies were available in the literature regarding the thermal behavior of bentonite and sepiolite [5, 21–25], to the best of the authors’ knowledge, no study, in which both the thermal behavior and the dehydroxylation kinetics of naturally occurring sepiolite and bentonite were investigated together, was available. The objective of this study was to determine the mineralogic and structural changes in naturally occurring sepiolite and bentonite by thermal treatment at the temperature, which was achieved by the end of the dehydroxylation process. In addition, the study also focuses on the decomposition kinetics of the natural clay samples during the dehydroxylation stage with the aim of revealing the kinetic mechanisms that were involved.
M. Sarı Yılmaz et al.
study. All samples were ground and sieved to a particle size below 100 lm. The chemical compositions of the naturally occurring clays after being dried at 105 °C for 2 h were obtained by X-ray fluorescence (XRF, Spectro X-LabPro) analysis (Table 1). The raw bentonite and sepiolite samples were heated up to 850 and 900 °C (by the end of the dehydroxylation process) at a rate of 10 °C min-1, respectively, and the samples were thermally treated at these temperatures for 2 h in a furnace. X-ray diffraction (XRD) patterns of the raw and the thermally treated samples were obtained using a Philips PANanalytical X’Pert Pro diffractometer, with Cu Ka radiation operating at 45 kV, 40 mA. Fourier transform infrared (FTIR) spectra were acquired from the KBr pellets using a Perkin-Elmer Spectrum One FTIR spectrophotometer. The adsorption/desorption isotherm of nitrogen at 77 K was performed for samples degassed at 373 K using Micromeritics ASAP 2020 instrument. The thermal characteristics and the thermal dehydroxylation kinetics of the naturally occurring clay minerals were studied using a Perkin-Elmer Diamond thermogravimetry/derivative thermogravimetry (TG/DTG) instrument. In the non-isothermal studies, the reaction temperatures were varied between 30 and 900 °C, whereas the dynamic experiments were carried out at heating rates of 5, 10, 15, and 20 °C min-1. The kinetic parameters were computed using a specifically designed program in MS Excel, which directly receives the TG and DTG data from the equipment software. Theoretical approach Recently, the interest in determining the rate-dependent parameters of solid-state non-isothermal dehydroxylation reactions of clay and clay–sludge mixtures using thermal analysis techniques increased. Generally, non-isothermal methods were used in order to analyze TG/DTG curves and in order to obtain the values for the kinetic parameters [26–30]. In this study, the isoconversional Kissenger– Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and Friedman (FR) methods were used to determine the dehydroxylation kinetics of bentonite and sepiolite. Kissinger–Akahira–Sunose (KAS) method
Materials and methods
Kissenger–Akahira–Sunose is an integral isoconversional method, which was given by: ! bi Aa Ea Ea ln 2 ¼ ln ð1Þ Rgðai Þ RTa;i Ta;i
Two natural Turkish clay minerals, sepiolite from Eskisehir and bentonite from Ankara, were used as received in this
where a was the conversion rate, b was the heating rate (°C min-1), E was the activation energy (kJ mol-1), A was
Experimental
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Thermal behavior and dehydroxylation kinetics
1193
Table 1 Chemical analyses in oxides % for naturally occurring sepiolite and bentonite samples Al2O3
Clay minerals
SiO2
Sepiolite
30.70
4.20
Bentonite
64.38
13.22
Fe2O3
TiO2
CaO
MgO
1.01
–
18.25
17.95
0.70
0.05
2.12
2.53
(a)
K2O
Loss of ignition
–
–
27.89
0.11
0.61
16.28
(b)
Raw
M
D
Raw
F D
S
S S DS S
S SS
D S
S
DD D D
Di
Heat treated
Di E E E E E DiE Di EDi E Di 10
20
30
40
50
Intensity/a.u.
Intensity/a.u.
Na2O
M MF
Heat treated
E Di E Di 60
70
80
10
20
2θ /°
30
40
2θ /°
Fig. 1 XRD patterns of raw and heat treated naturally occurring clay samples a sepiolite; S sepiolite, D dolomite, E enstatite, Di diopside, b bentonite; M montmorillonite, F feldspar
the pre-exponential factor (min-1), and R was the gas constant (8.314 J K mol-1). The knowledge on the conversion function (f(a)) was not required for this method. The process was accepted to follow the same mechanism of decomposition for a given conversion rate. The slope of the plot of the left side of Eq. (1) versus 1/Ta,i at constant conversion rate for the tested heating rates allowed the calculation of the Ea value [31, 32]. Flynn–Wall–Ozawa (FWO) method Flynn–Wall–Ozawa was derived from the integral method based on the following expression: Aa Ea Ea log ðbi Þ ¼ log ð2Þ 2:315 0:4567 Rgðai Þ RTa;i From this equation, the Ea value was determined by plotting log(bi) against 1/Ta,i at constant conversion degree [33, 34]. Friedman (FR) method Friedman is a differential isoconversional method, represented by the following equation: " # da Ea ln bi ð3Þ ¼ ln ðAa f ðai ÞÞ dT a;i RTa;i
where the subscript a denoted the value at a particular conversion rate and the subscript i denoted the data from a given heating rate run. The value of E for each given conversion rate, was obtained from the slope of the graph of ln[bi(da/dT)a,i] against 1/Ta,i [35].
Results and discussion Characterization The XRD pattern that was obtained using naturally occurring sepiolite (Fig. 1a) showed two sharp strong reflections ˚ (dolomite). The results at 12.03 (sepiolite) and 2.88 A obtained by XRD and XRF analyses indicated that naturally occuring sepiolite contains sepiolite and dolomite. Follow˚ completely ing heat treatment, peaks at 12.03 and 2.88 A disappeared, implying a thorough destruction of their crystal structures and the formation of new structures. The results obtained by XRD and TG/DTG analyses (Figs. 1a, 2a) indicated that following dehydroxylation, sepiolite was completely transformed to new forms; namely enstatite (Mg2Si2O6) and diopside (CaMgSi2O6), in agreement with the previous study [36]. New peaks at 3.19, 3.00, 2.52, 2.93, ˚ indicate the formation of enstatite and diopside. and 2.02 A The XRD pattern that was obtained using naturally occurring bentonite (Fig. 1b) showed three sharp and
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1194 Fig. 2 TG/DTG curves of naturally occurring clay samples a sepiolite, b bentonite
M. Sarı Yılmaz et al. (a)
100 5 °C min–1 10 °C min–1
95
15 °C min–1 20 °C min–1
85 DTG/% min–1
Maas/%
90
80
75
70 30
100
200
300
400
500
600
700
800
900
Temperature/°C
64.77 30
100
200
300
400
600
500
700
800
900
Temperature/°C
(b)
100 98
DTG/% min–1
96 94
Maas/%
92 90 88
30
100
200
300
86
400
500
600
700
800
600
Temperature/°C
84 5 °C min–1 10 °C min–1 15 °C min–1 20 °C min–1
82 79.76 30
100
200
300
400
500
600
700
800
900
Temperature/°C
˚ strong reflections at 3.18 (feldspar), 4.47 and 15.24 A (montmorillonite) [37]. After heating of the sample, all the diffraction peaks disappeared, indicating the complete destruction of the crystal lattice, and an amorphous structure was observed. The results of the XRD and TG/DTG analyses indicated that following dehydroxylation, bentonite was completely transformed to attain an amorphous phase (Figs. 1b, 2b). The FTIR spectra of the raw and the thermally treated naturally occurring sepiolite (Fig. 3a) revealed that the bands at 3,420 cm-1 corresponded to the zeolitic water stretching vibrations, and the band at 3,564 cm-1 was assigned to the hydroxyl stretching of the octahedral Mg3(OH) units [38]. The band at 1,632 cm-1 was attributed to the deformation vibration of water within the sepiolite structure. The absorption peak at 1,025 cm-1 corresponded to the Si–O stretching vibrations and the bands at 730 and 880 cm-1 attributed to the CO-2 3 (dolomite) impurities. The -1 bands at 789 and 648 cm corresponded to the Mg–OH inner bending vibrations. The band at 472 cm-1 was assigned to the Si–O–Si in plane vibrations [39]. Following the thermal treatment, the vibration of Mg coordinated hydroxyl group at 3,564 cm-1 disappeared and the vibrations due to the presence of zeolitic water at 3,436 and 1,640 cm-1 decreased. The band at 1,456 cm-1 was still present due to the presence of the CO-2 3 (dolomite) impurities, which did not practically change position following heat treatment, but the band showed reduced intensity [40]. New
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bands were observed at 1076, 999, 939, and 889 cm-1, as well as at 517 and 502 cm-1 due to the presence of Si–O stretching and Si–O–Si bending vibrations of enstatite, respectively [41]. The IR spectra of the raw and the thermally treated naturally occurring bentonite (Fig. 3b) revealed that the bands at 3,616 cm-1 corresponded to structural O–H stretching vibrations, and the band at 3,430 cm-1 was assigned to stretching and bending vibrations of water. The band at 1,638 cm-1 was attributed to deformation vibration of water in the bentonite structure. The sharp absorption peak at 1,039 cm-1 corresponded to the Si–O stretching vibrations, and those at 522 and 467 cm-1 were attributed to Si–O–Al (octahedral Al) and Si–O–Si bending vibrations, respectively. Following the thermal treatment, the O–H stretching vibrations at 3,616 cm-1, the OH bending bands observed at 917 cm-1 (Al2OH), at 843 cm-1 (AlMgOH) [42], and the peak at 792 cm-1 corresponded to the t(OH) stretching vibration of feldspar [43] disappearing as a result of the heat treatment. In addition, the stretching and bending vibrations of water at 3,430 cm-1 were decreased. New band was observed at 797 cm-1 due to vibrations of amorphous silica/quartz [44]. The nitrogen adsorption/desorption isotherms of raw and heat treated naturally occurring sepiolite and bentonite were shown in Fig. 4. The hysteresis loop of naturally occurring bentonite was similar to Type H3 on the basis of IUPAC classifications. The adsorption branch appeared to have the
Thermal behavior and dehydroxylation kinetics Fig. 3 FTIR spectra of naturally occurring clay samples a sepiolite, b bentonite
1195 (a)
Raw 789 648
730 3564
Transmittance/%
3420
1632
472
880 1025 1456 Thermal treated 1640
3436
1076
517 999
4000
3600
3200
2800
2400
2000
1800
1600
1400
1200
478
889 502
939
1000
800
600
450
cm–1
(b) a 792 843 1638
917
Transmittance/%
3616 522 3430 467
1039
797 570
1639 465 b
1092 3435
4000
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
450
cm–1
same typical Type II character and the desorption branch followed a different path until a critical p/p0 was reached. Moreover, there is a substantial decrease in the adsorbed volume of heat treated bentonite due to contraction or collapse of pore. The shape of isotherms of the naturally occurring and heat treated sepiolite possessed a Type II with hysteresis loop Type H3, typical for solids with slit-shaped pores [45]. It may be observed that calcination caused a shift in the hysteresis loop to higher relative pressure/wider pores. The results of the BET analysis indicated that heat treatment changed the specific surface area (SBET) of the natural clay minerals considerably (Table 2). For naturally occurring sepiolite, SBET decreased starting with the value of 98.53 m2 g-1 down to 16.74 m2 g-1, while for naturally occurring bentonite, this change was from 63.98 to 5.24 m2 g-1. These results showed that the structures of the naturally occurring sepiolite and the bentonite were altered by the heat treatment. Thermal decomposition of naturally occurring sepiolite Thermogravimetry/derivative thermogravimetry curves of the thermal decomposition of naturally occurring sepiolite at different heating rates (Fig. 2a) revealed that the thermal decomposition of naturally occurring sepiolite occurred within the temperature range of 33–822 °C with two
dehydration stages followed by one dehydroxylation stage. The first dehydration stage was initiated at approximately 33 °C and lasted until 98 °C with a mass loss of 2.09 % and the stage was attributed to the loss of hygroscopic and zeolitic water. The second dehydration stage, corresponding to a mass loss of 4.91 %, was completed at 564 °C and the stage was attributed to the loss of bound water. The third stage, which occurred as a result of the calcination of dolomite and the dehydroxylation of sepiolite, progressed up to 822 °C at 5 °C min-1 heating rate. The corresponding mass loss was 23.98 % while it was 8.8 % in the previous study [36]. Mass losses of decomposition stages of naturally occurring sepiolite at all heating rates were detailed in Table 3. Thermal decomposition of naturally occurring bentonite Thermogravimetry/derivative thermogravimetry for the thermal decomposition of naturally occurring bentonite (Fig. 2b) revealed that the dehydration and the dehydroxylation stages were observed in two regions: 38–173 and 173–814 °C at 5 °C min-1 of heating rate. The corresponding mass losses were 12.75 and 4.95 % with the total mass loss being 17.70 %. The mass loss of dehydroxylation reaction was in a good agreement with the previous study [24]. The dehydration of the naturally
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M. Sarı Yılmaz et al.
Quantity adsorbed/cm3 g–1
Kinetic analysis
Raw bentonite Thermal treated bentonite Raw sepiolite Thermal treated sepiolite
200
150
100
50
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative pressure/p/p°
Fig. 4 The nitrogen adsorption/desorption isotherms of raw and heat treated naturally occurring clay samples
Average apparent activation energies of the dehydroxylation of naturally occurring sepiolite and bentonite were calculated by the FR, KAS, and FWO isoconversional methods. The isoconversional methods allow the attaining of the dependence of the kinetic parameters using the conversion information from TG and DTG curves that were determined through the measurements at different heating rates without making any assumptions regarding the reaction function and the order of the reaction. The isoconversional methods together with other multi-heating rate methods were the most reliable techniques for describing the kinetic analysis of thermal data, as reported in the ICTAC Kinetic Project [46]. No reports were available in the literature on the application of isoconversional methods on the data obtained from the thermal decomposition of naturally occurring sepiolite and bentonite.
Decomposition kinetics of naturally occurring sepiolite Table 2 BET analysis results for the raw and heat treated naturally occurring samples Sample
SBET/m2 g-1
Raw sepiolite
98.53
Treated at 900 °C
16.74
Raw bentonite
63.98
Treated at 850 °C
5.24
occurring bentonite was complex with two overlapping stages, while its dehydroxylation was observed in a single stage reaction, all of which were represented in the DTG peaks. Thermal decomposition data obtained by the TG and DTG analyses conducted on naturally occurring bentonite were listed for all heating rates in Table 3.
The differential conversion curves for the non-isothermal decomposition process of naturally occurring sepiolite under air atmosphere at different heating rates (Fig. 5) revealed that the rate of decomposition reaction increased with increased rate of heating. The E values may be computed from the slopes of the KAS, FWO, and FR plots at a specific conversion range, a = 0.2–0.9 (Fig. 6). The dependence of E on the conversion rate for the dehydroxylation reaction of naturally occurring sepiolite (Fig. 7) indicated that the E values change slightly in the range of 0.30 B a B 0.70, indicating that the dehydroxylation reaction of sepiolite should be a single kinetic mechanism. The following mean values of E were determined as E(KAS) = 204.05 ± 3 kJ mol-1, E(FWO) = 209.28 ± 3 kJ mol-1, and E(FR) = 198.69 ± 4 kJ mol-1. The E
Table 3 Thermal decomposition characteristics of naturally occurring sepiolite and bentonite Clay minerals Sepiolite
Bentonite
a
Heating rates/°C min-1
Dehydration temperature range/°C
Mass loss/%
Mass loss/%
5
33–564
7.00
564–822
23.98a
10
35–567
6.82
567–841
23.48a
15
36–571
6.66
571–853
23.90a
20
36–579
6.47
579–874
23.50a
5
38–173
12.75
173–814
4.95
10
38–190
13.17
190–819
4.91
15 20
38–196 39–199
13.16 13.12
196–822 199-828
4.99 4.92
The value is a result of the calcination of the dolomite and dehydroxylation of the sepiolite
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Dehydroxylation temperature range/°C
Thermal behavior and dehydroxylation kinetics
1197
differential method (E(FR)), (DE = 10.59 kJ mol-1).
0.3
were
slightly
lower
0.25
dα /dt/min–1
Decomposition kinetics of naturally occurring bentonite 0.2 5 °C/min 10 °C/min
0.15
15 °C/min 20 °C/min
0.1 0.05 0 800
900
1000
1100
1200
T/K Fig. 5 The experimental differential conversion (da/dT vs T) curves obtained from the integral conversion curves, for the decomposition of naturally occurring sepiolite at different heating rates
values, which were determined by means of integral isoconversional methods (E(KAS) and E(FWO)), were in a good agreement, while the values, which were determined by the Friedman
The degree of conversion rate as a function of temperature for naturally occurring bentonite was obtained from the experimental data (Fig. 8). Using these data, the three isoconversional methods were used to determine the activation energies. The mean E values obtained by the KAS, FWO, and FR methods were comparable providing similar values (Table 4). The E values increased with increasing conversion in the range of 0.2–0.8, whereas, at higher conversions, they were observed to be decreasing. This trend of the kinetic parameters was similar for all the isoconversional methods, implying the reliability of the results. In the present case, the E depended on a, which was the indication of the presence of a complex dehydroxylation reaction. In such a reaction, a change in the E would be observed as the extent of reaction, a, increased. As a summary, the increasing dependence of E on the extent of reaction may be explained by the competing or consecutive reactions and the decreasing dependence may be explained by the presence of reversible reactions [47–49]. It was observed
(a)
(b) 1.4 1.00
0.95
1.05
1.15
1.10
1.2
log β
In ( β / T 2)
–10.5
1
11.5 0.8
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
(1/T ) × 10–3 /K –2
(c)
0.2 0.3 0.4 0.5 0.6 0.8 0.9
0.6 0.95
–12.5
1
1.05
1.1
1.15
(1/T ) × 10–3 /K –2
0.95
1
1.05
1.1
In (dα /dt)
–0.75
–1.75
–2.75
–3.75 0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
–4.75
(1/T ) × 10–3 /K –2
Fig. 6 Typical isoconversional plots for naturally occurring sepiolite a KAS, b FWO, and c FR
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M. Sarı Yılmaz et al. 280
Table 4 Activation energies of naturally occurring bentonite obtained by the KAS, FWO, and FR methods
FR KAS FWO
E /kJ/mol
260
240
220
200
180 10
30
50
90
70
a
EaKAS/ kJ mol-1
EaFWO/ kJ mol-1
EaFR/ kJ mol-1
0.2
124.52
127.96
140.85
0.3
141.65
145.50
153.81
0.4 0.5
144.32 224.89
147.93 219.87
166.12 234.99
0.6
249.22
249.80
264.55
0.7
297.92
297.43
270.21
0.8
308.50
307.84
284.32
0.9
294.20
294.70
278.11
Mean Ea values
223.15 ± 72
223.88 ± 70
224.12 ± 57
Conversion rate/% Fig. 7 The dependence of the calculated Ea as a function of a for the dehydroxylation of naturally occurring sepiolite utilizing the three kinetic methods 1
0.8
α
0.6
0.4
0.2 5 0 400
500
600
700 T/K
10 800
15 900
20 1000
Fig. 8 The conversion (a–T) curves for the dehydroxylation of naturally occurring bentonite at different heating rates
in previous experiments that the activation energy for the dehydroxylation of white calcium bentonite was calculated as 48.84 kJ mol-1 using Coats and Redfern method [25].
Conclusions The heat treated naturally occurring sepiolite and bentonite were characterized by XRD, FTIR, and BET analyses, and the irreversible dehydroxylation was observed to alter their structure. Naturally occurring sepiolite was completely transformed into new phases of enstatite and diopside, while naturally occurring bentonite was transformed into an amorphous structure. The heat treatment changed the surface area of naturally occurring sepiolite and bentonite
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as a result of the dehydration and the dehydroxylation. The thermal behavior and the dehydroxylation kinetics of naturally occurring sepiolite and bentonite were studied using the TG/DTG techniques. The activation energies of naturally occurring sepiolite, which were determined by three isoconversional methods, were practically independent of the conversion rate in a range from 0.30 to 0.70. The observed dependency of the activation energy on the rate of conversion was consistent with a single stage reaction mechanism. The activation energies of naturally occurring bentonite were calculated through the utilization of different isoconversional methods, and the values were found to be strongly dependent on the rate of conversion. The results showed an increase followed by a decrease in the activation energy with increasing conversion rate and hence with temperature. As a result of this, the dehydroxylation reaction of naturally occurring bentonite was determined to be a complex reaction. The thermal behavior and the dehydroxylation kinetics of naturally occurring sepiolite and bentonite provide an idea about the suitability of the use of natural clay minerals in possible applications.
References 1. Murray HH. Overview-clay mineral applications. Appl Clay Sci. 1991;5:379–95. 2. Breen C, Watson R, Madejova J, Komadel P, Klapyta Z. Acidactivated organoclays: preparation, characterization and catalytic activity of acid-treated tetraalkylammonium-exchanged smectites. Langmuir. 1997;13:6473–9. 3. Mahmoud S, Saleh S. Effect of acid activation on the de-tertbutylation activity of some Jordanian clays. Clay Clay Miner. 1999;47:481–6. 4. Barker RM. Shape-selective sorbents based on clay minerals: a review. Clay Clay Miner. 1989;37(5):385–95. ¨ nal M, Baran B, Alemdarog˘lu T. The effect of 5. Sarıkaya Y, O thermal treatment on some of the physicochemical properties of a bentonite. Clay Clay Miner. 2000;48:557–62.
Thermal behavior and dehydroxylation kinetics 6. JrAS Reis, Ardisson JD. Exchangeable ion and thermal treatment effects on basal spacings of Al-hydroxy pillared montmorillonites. Clay Clay Miner. 2003;51:33–40. 7. Komadel P. Chemically modified smectites. Clay Miner. 2003;38:127–38. 8. Yıldız N, Sarıkaya Y, C¸alımlı A. The effect of the electrolyte concentration and pH on the rheological properties of the original and Na2CO3-activated Ku¨tahya bentonite. Appl Clay Sci. 1999; 14:319–27. ¨ . Sepiolite–Palygorskite from the Hekimhan 9. Yalc¸in H, Bozkaya O region (Turkey). Clay Clay Miner. 1995;43(6):705–17. 10. Karakaya N, C¸elik Karakaya M, Temel A, Ku¨peli S¸ , Tunaog˘lu C. Mineralogical and chemical characterization of sepiolite occurrences at Karapınar (Konya Basin, Turkey). Clay Clay Miner. 2004;52(4): 495–509. 11. Kadir S, Bas¸ H, Karakas¸ Z. Origin of sepiolite and loughlinite in a neogene volcano-sedimentary lacustrine environment, Mihallic¸–Eskis¸ ehir, Turkey. Can Miner. 2002;40:1091. 12. Gu¨nister E. Na-Aktif bir bentonit-su sisteminde partiku¨l etkiles¸ imleri u¨zerine DTABr ve BDTDACl katyonik yu¨zey aktiflerinin etkisinin reolojik ve elektrokinetik yo¨ntemlerle incelenmesi. Master thesis, University Istanbul Technical, ˙Istanbul, Turkey (In Turkish) 2003. 13. Alvarez A. Sepiolite: properties and uses, developments in sedimentology. New York: Elsevier; 1984. 14. Weir MR, Rutinduka E, Detellier C, Feng CY, Wang Q, Matsuura T, Le VanMao R. Fabrication, characterization and preliminary testing of all-inorganic ultrafiltration membranes composed entirely of a naturally occurring sepiolite clay mineral. J Membr Sci. 2001;182:41–50. 15. Grimm RE, Gu¨ven N. Bentonites geology, mineralogy, properties, and uses. Amsterdam: Elsevier; 1978. 16. Gunay E, Ozkan TO. Production of porous ceramics from sepiolite based minerals. Ind Ceram. 2001;21:145–9. 17. Blanco J, Yates M, Avila P, Bahamonde A. Characterization of alumina:sepiolite monoliths for use as industrial catalyst supports. J Mater Sci. 1994;29:5927–33. 18. Gonzalez-Pradas E, Socias-Viciana M, Urena-Amate MD, Cantos-Molina A, Villafranca-Sanchez M. Adsorption of chloridazon from aqueous solution on heat and acid treated sepiolites. Water Res. 2005;39:1849–57. 19. Wang MC, Benway JM, Arayssi AM. The effect of heating on engineering properties of clays physicochemical aspects of soil and related materials. Philadelphia: ASTM STP 1095; 1990. p. 1139–58. 20. Abu-Zreig MM, Al-Akhras NM, Attom MF. Influence of heat treatment on the behavior of clayey soils. Appl Clay Sci. 2001; 20:129–35. 21. Cebulak S, Langier-Kuzniarowa A. Some remarks on the methodology of thermal analysis of clay minerals. J Therm Anal Calorim. 1998;53:375–81. 22. Perez-Rodriguez JL, Galan E. Determination of impurity in sepiolite by thermal analysis. J Therm Anal Calorim. 1994;42:131–41. 23. Frost RL, Ding Z. Controlled rate thermal analysis and differential scanning calorimetry of sepiolites and palygorskites. Thermochim Acta. 2003;397:119–28. ¨ nal M, Sarıkaya Y. Thermal behavior of a bentonite. J Therm 24. O Anal Calorim. 2007;90:167–72. ¨ nal M, Yılmaz H, Sarıkaya Y. Thermal analysis of a 25. Bayram H, O white calcium bentonite. J Therm Anal Calorim. 2010;101:873–9. 26. Koc S, Toplan N, Yildiz K, Toplan HO. Effects of mechanical activation on the non-isothermal kinetics of mullite formation from kaolinite. J Therm Anal Calorim. 2011;103:791–6. 27. Cheng FKH, Sahoo NG, Lu X, Li L. Thermal kinetics of montmorillonite nanoclay/maleic anhydride modified polypropylene nanocomposites. J Therm Anal Calorim. 2012;109:17–25.
1199 28. Sahnoune F, Saheb N, Khamel B, Takkouk Z. Thermal analysis of dehydroxylation of Algerian kaolinite. J Therm Anal Calorim. 2012;107:1067–72. 29. Kantu¨rk Figen A, I˙smail O, Pis¸ kin S. Devolatilization non-isothermal kinetic analysis of agricultural stalks and application of TG-FT/IR analysis. J Therm Anal Calorim. 2012;107:1177–89. 30. Sarı Yılmaz M, Kasap S, Piskin S. Preparation, characterization and thermal dehydration kinetics of titanate nanotubes. J Therm Anal Calorim. 2012. doi:10.1007/s10973-012-2688-5. 31. Kissinger HE. Reaction kinetics in differential thermal analysis. Anal Chem. 1957;29(11):1702–6. 32. Akahira T, Sunose T. Joint convention of four electrical institutes. Res Rep Chiba Inst Technol. 1971;16:22. 33. Flynn JH, Wall LA. General treatment of thermogravimetry of polymers. J Res Natl Bur Stand A Phys Chem. 1966;70A(6): 487–523. 34. Ozawa T. A new method of analyzing thermogravimetric data. Bull Chem Soc Jpn. 1965;38:1881–6. 35. Friedman HL. Kinetics of thermal degradation of char-forming plastics from thermogravimetry. Application to a phenolic plastic. J Polym Sci Part C. 1964;6:183–95. ¨ nal M, U ¨ stu¨nıs¸ ık G, Sarıkaya Y. Thermal behavior of 36. Yener N, O a mineral mixture of sepiolite and dolomite. J Therm Anal Calorim. 2007;88(3):813–7. 37. Harris WG, White GN. X-ray diffraction techniques for soil mineral identification. In: Ulery A, Drees R, editors. Methods of soil analysis: part 5 mineralogical methods. Soil Sci Soc Am, Madison; 2008. 38. Casal B, Merino J, Serratosa J, Ruiz-Hitzky E. Sepiolite-based materials for the photo- and thermal-stabilization of pesticides. Appl Clay Sci. 2001;18:245–54. 39. Vicente-Rodriguez M, Suarez M, Banares-Munoz M, LopezGonzalez J. Comparative FT-IR study of the removal of octahedral cations and structural modifications during acid treatment of several silicates. Spectrochim Acta A. 1996;52:1685–94. 40. Frost RL, Locos OB, Kloprogge HTR. Near-infrared and midinfrared spectroscopic study of sepiolites and palygorskites. Vib Spectrosc. 2001;27:1–13. 41. Kalinkina EV, Kalinkin AM, Forsling W, Makarov VN. Sorption of atmospheric carbon dioxide and structural changes of Ca and Mg silicate minerals during grinding-II. Enstatite, akermanite and wollastonite. Int J Miner Process. 2001;61:289–99. 42. Madejova J. FTIR techniques in clay mineral studies. Vib Spectrosc. 2003;31:1–10. 43. Davarcıog˘lu B, C¸iftc¸i E. Investigation of central Anatolian clays by FTIR spectrocopy (Arapli-Yesilhisar-Kayseri, Turkey). Int J Nat Eng Sci. 2009;3(3):167–74. 44. Davarcıog˘lu B. Investigation of central Anatolian region NigdeDikilitas (Turkey) clays by FTIR spectroscopy. Mater Technol. 2010;62:55–60. 45. Rouquerol F, Rouquerol J, Sing K. Adsorption by powders and porous solids. London: Academic Press; 1999. 46. Brown ME, Maciejewski M, Vyazovkin S, Nomen R, Sempere J, Burnham A, Opfermann J, Strey R, Anderson HL, Kemmler A, Keuleers R, Janssens J, Desseyn HO, Li CR, Tang TB, Roduit B, Malek J, Mitsuhashi T. Computational aspects of kinetic analysis part A: the ICTAC kinetics project-data, methods and results. Thermochim Acta. 2000;355:125. 47. Hong J, Guo G, Zhang K. Kinetics and mechanism of non-isothermal dehydration of nickel acetate tetrahydrate in air. J Anal Appl Pyrol. 2006;77:111–5. 48. Vyazovkin S, Wight CA. Kinetics in solids. Annu Rev Phys Chem. 1997;48:125–49. 49. Vyazovkin S. Kinetic concepts of thermally stimulated reactions in solids: a view from a historical perspective. Int Rev Phys Chem. 2000;19:45.
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