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Investigations on the adsorption properties and porosity of natural and thermally treated dolomite samples
POWDER TECHNOLOGY ELSEVIER
Powder Technology 92 (1997) 253-257
Investigations on the adsorption properties and porosity of natural and thermally treated dolomite samples P. Staszczuk a,., E. Stefaniak a, B. Bilifiski a, E. Szymafiski a R. Dobrowolski u, S.A.A. Jayaweera c a Department of Physical Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, M. Curie-Sklodowska Square 3, 20-031 Lublin, Poland b Central Laboratory, Maria Curie-Sklodowska University, M. Curie-Sklodowska Square 3, 20-031 Lublin, Poland c Division of Chemical and Biotechnological Sciences, School of Science and Technology, University ofTeeside, Middlesbrough, Cleveland TSI 3BA, UK Received 30 June 1996; revised 23 December 1996; accepted 10 March 1997
Abstract Adsorption properties with respect to certain toxic substances as well as porosity investigations of raw and thermally modified dolomite are presented. After the partial decomposition of dolomite at 800°C a new so-called 'dolomitic' adsorbent of the calcium carbonate skeleton and pumice structure was obtained. Its physicochemical properties are significantly different from those of the raw material, e.g. the specific surface area increases 37 times and the adsorption capacity for SO2 increases approximately 15 times, which makes the new adsorbent useful in the protection of the environment. Keywords: Adsorption; Porosity; Dolomite
1. Introduction
2. Experimental
Dolomite is a mineral which has received a great deal of attention in recent years, resulting in numerous papers considering its various properties [ 1-7]. This mineral is a major and cheap source of magnesium. The problem of magnesium deficiency occurs not only in Poland, but also in the whole of Europe. This mineral can be used as a food and fodder additive in order to compensate for magnesium deficiency. Dolomite usage is increasingly important in different branches of industry such as the food and pharmaceutical industries, production of fertilizers, glass, building materials, and even the kinescopes (picture tubes) for color television. Unfortunately, dolomite is now used mainly as a building material or as a filler for glass, plastics and colors, in spite of its ability to adsorb certain poisonous substances [7]. As can be seen from the literature, the physicochemical properties of dolomite have not been completely investigated. This paper is aimed at presenting the influence of thermal modification on the porosity and adsorption properties of dolomite, especially in regard to the adsorption of some of the toxic substances which are a major problem in environmental protection.
2.1. Materials and reagents
* Corresponding author. Tel.: +48 81 375 646; fax: +48 81 375 102; e-mail: piotr @hermes.umcs.lublin.pl 0032-5910/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved PIIS0032-5910(97)03246-4
The experiments were carried out on dolomite originating from Otdrzychowice-Romanowo deposits (Poland). A grain fraction of 0.12-0.25 mm in size was used. The samples were dried at 150°C or heated at 800°C for 12 h (in air), and then washed with borax buffer (pH 8.6) in order to remove MgO from the newly created pores. 2.2. Methods The percentages of the main elements in the raw material were determined by means of an atomic absorption spectrophotometer (model AAS-3, Carl Zeiss, Jena, Germany) and are as follows: CaO, 30.61; MgO, 18.4; Fe203, 0.29; BaO, below 0.028%. The thermal decomposition of the raw dolomite sample and that of the modified one were performed with a Derivatograph (model Q-1500 D, MOM, Hungary). The specific surface area and porosity were measured by a Sorptomatic apparatus (Surfamat 5x, Poland). The adsorption of benzene was analyzed by the chromatographic peak maximum method using a gas chromatograph
254
P. Staszczuket al. / PowderTechnology 92 (1997) 253-257
(model GCHF 18.3, Germany). The sample of 7 g was placed in a 20 cm glass column (i.d. 3 mm) and conditioned with dry hydrogen at a temperature of 29.9°C which was also the measurement temperature• The procedure for these measurements and the method by which to calculate the adsorption isotherms are described in detail elsewhere [8]. The adsorption of SO2 from the gas phase was carried out by means of equipment consisting of a cylinder of SO2, suitable manometers, a column in which was the sample to be studied, a conductometer and a recorder•
3. Results a n d d i s c u s s i o n
Fig. 1 presents the results of the thermal decomposition of the dolomite. According to the literature data [4], when this process occurs in air, dolomite decomposes in two steps, as follows: First peak:
CaMg(CO 3)2 --->CaCO3 + MgO + CO2
Second peak:
CaCO3 ~ CaO +CO2
1000
900
• ..... -.
800 ........
"
.
~ _=
DTG
t ,[
".!
,
\
.
700
.=
."
DTA
, .
i',
.:4
,
II
"~
i'_' /
~1
\':
and the course of its decomposition strongly depends on the CO2 partial pressure. The mechanism of dolomite decomposition is widely discussed in the literature and the suggested theories mainly consider the reactions referring to the first endothermal peak. They may be divided into two categories: direct formation of CaCO2 or recarbonization of freshly formed CaO to CaCO3. However, it is well established that the second peak represents the decomposition of the CaCO3 formed [ 5 ]. The partial decomposition of dolomite at 800°C leads to changes in the chemical composition of the surface and in the porosity of this mineral. Generally, the product of partial decomposition of dolomite (a so-called 'dolomitic' sorbent) contains calcium carbonate and magnesium oxide which may be confirmed by further thermal decomposition (only onestep decomposition at about 950°C). The results of porosity investigations presented in Table 1 show the significant increase of the specific surface area and total pore volume• Moreover, these parameters enlarge much more after the partially decomposed sample has been washed with borax buffer in order to remove MgO from the newly created pores. The total specific surface area of the obtained 'dolomitic' sorbent increased approximately 37 times when compared with the raw material• In general, this agrees with the results presented by Glasson [9]. The adsorption of benzene on the raw dolomite (dried at 150°C) as well as on the thermally modified dolomite (heated at 800°C) was investigated by a peak profile chromatographic method. The adsorption isotherms a =f(P/Po) were calculated from the retention data. The amount adsorbed, a ( Izmol / m2), was calculated by integration of the experimentally determined relationship VR =f(P/Po) according to the equation [ 10] p/po
~-
Po f a = m,RTA
500
(1)
VR d(p/po)
0 400
•
.
where m, is the adsorbent mass, P/Po the relative pressure of the adsorbate vapor, VR the retention volume, T the temperature, R the gas constant, and A the specific surface area. The relative vapor pressure P/Po was calculated from the recorded chromatogram as follows:
"
500
.
200
:
" 600
700
800
go0
1000
tO0
"I'E/'4PERATURE,Ln"C Fig. 1. Thermogravimetric(TG), differential thermogravimetric(DTG) and differentialthermoanalysis (DTA) curvesof the thermaldecomposition of natural dolomite.
p/po=h/hpo
(2)
where h is the peak height for the pressure p and hpo is the adequate height corresponding to the saturated vapor pressure Po and determined by the detector calibration.
Table l Changes in porosityof the investigatedsamplesdue to the thermalmodification Sample
Specificsurface area (m2/g)
Total pore volume (cm3/g)
Averagepore radius (/~)
Raw dolomitedried at 150°C Dolomite heated at 800°C Dolomite heated at 800°C and washedwith borax buffer
0.7 11.3 18.3
0.01 0.05 0.07
2 ! .2 81.5 43.3
P. Staszczuk et al. / Powder Technology 92 (1997) 253-257
The adsorption isotherms of benzene on the raw dolomite dried at 150°C (curve 1) and on that heated up to 800°C (curve 2) are presented in Fig. 2. They are typical isotherms of type II according to BET theory. The adsorption capacity (the amount adsorbed at P/Po= 1) for these two adsorbents is 21 and 29 p.mol/m 2, respectively. For a known value of a, the film pressure H of benzene on the surfaces of the examined samples can be calculated from the integral form of the Gibbs adsorption isotherm equation, called the Bangham-Razouk equation [ 11 ] :
255
"d G
4
3
p
/7=RTfa
2
(3)
d(ln p)
0'.s
',
~'.s Negr
O
where p is the equilibrium pressure of the adsorbate. The value o f / 7 reflects the changes of free energy of the surface upon adsorption. The relationships between the film pressure and the adsorbed amount of benzene for the raw dolomite (curve 1) and the modified dolomite (curve 2) are presented in Fig. 3. Fig. 4 shows the derivative of the film pressure with respect to the amount adsorbed, GH/Ga,as a function of the number of monolayers, NBET, of benzene 30-
20
10
0
i 0.2
0.4
= '-
0.6
0.8
P/Pu Fig. 2. Adsorption isotherms o f benzene on raw dolomite (curve ] ) and on the dolomite heated at 800°C (curve 2).
Fig. 4. Values of gH/Ga as a function of the number of monolayers, Nser, of benzene adsorbed on the raw dolomite (curve 1) and on the dolomite heated at 800°C (curve 2).
adsorbed on the raw dolomite (curve 1) and on the dolomite heated at 800°C (curve 2). The shape of each curve allows us to estimate the mechanism of benzene adsorption on the investigated surface. The extrema and inflection points on the curves suggest that the adsorption mechanism is quite complicated in both cases and should be attributed to surface heterogeneity. The calculations were performed based on the smooth data. Fig. 5 presents the dependence of the parameter HS/kT on the number of benzene monolayers, NBET. The parameter/TS/kT, where S is the average area per adsorbed molecule, appears from the simplest two-dimensional equation of state, and in the ideal case it should be equal to unity. This parameter, calculated from the relationship H=f(a) shown in Fig. 5, is used to estimate the intermolecular interaction forces [ 12]. It is worth noticing that the calculated values are significantly higher then unity and they are also connected to certain changes in surface heterogeneity of the examined samples. The intermolecular interactions within the benzene monolayer adsorbed on the raw dolomite appear stronger (curve 1 ) below a coverage of 0.5N~ET than those on the heated material. In order to fully characterize the intermolecular interactions and the distribution of adsorption sites on the examined 1.8
50
b. t
"~ ,to -
1.6
301.4
2O 1.2 I0 [ t
i
i
5
10
15
~
20
i
25 a,
30 pmol/m z
F i g 3. Relationship between the film pressure H and the amount adsorbed a on the raw dolomite (curve 1) and on the modified dolomite (curve 2).
0.5
1
1.5
2 Niter
Fig. 5. Relationship between the value of HS/kTand the number of adsorbed monolayers, NBET, of benzene on the raw dolomite (curve 1 ) and on the thermally modified dolomite (curve 2).
256
P. Staszczuk et al. / Powder Technology 92 (1997) 253-257
surfaces, a so-called adsorption energy distribution function X =f(E) was determined according to the ACCA (asymptotically corrected condensation approximation) method [ 12]. This method used to be applied very often in investigations on surface heterogeneity based directly on retention data. This function may be calculated from the equation [ 13 ] j P 2 X= --(~mm)(~-~ ( ~ p R)
1.5
,
,
,
.
,
O
A
10
(4)
wherej is the James-Martin compressibility factor, a m is the monolayer capacity, and p is the equilibrium pressure of the adsorbate. The value of function X allows us to estimate the changes in the number of adsorption sites on the surface. The interaction energy ~ can be calculated for known p from the equation [ 12,13]
0.5
GO
10
20
30
40
50
60
70
80
T IM E . i. m i .
e=RTIn(K)
(5)
where K is a constant dependent on the assumed local adsorption isotherm equation. In the case of the Langmuir equation, (6)
K = ( 2Flm ) ] /2 ( kT) 5/Zq~ h3qa
where m is the molecular mass, k is Boltzmann's constant, h is Planck's constant, and qa and qg are the partition functions connected with the internal degrees of freedom of molecules in an adsorption layer and in the gas phase, respectively. It is usually assumed that q, = qg [ 13]. Fig. 6 shows the distribution functions of the adsorption energy for benzene on the raw (curve 1 ) and thermally modified (curve 2) dolomite samples. Both curves suggest three dominating kinds of adsorption sites on both materials; however, the distribution of these sites differs significantly. The heated dolomite possesses more adsorption sites between 32 and 41 kJ/mol, while the very characteristic maximum at the lowest energy of interaction is shifted towards lower values 60, o
E <
40
2o
0
t 26
30
34
i
Fig. 7. Kineticsof SO2adsorptionon the raw dolomite (curve 1) and on the dolomite heated at 800°C (curve 2) and then washed with borax buffer (curve 3). of e. This evidently results from the partial decomposition of dolomite. Curve 2 possesses a very typical bimodal shape for hydrocarbons exhibiting only the dispersive interaction on the surfaces of oxide minerals [ 12]. In the case of curve 1 there are three maxima of X for the adequate values of E. This suggests the presence of three different types of active sites for benzene adsorbed molecules, probably due to the occurrence of certain ions on the mineral surface. The thermal activation of natural dolomite provides different physicochemical properties of the obtained 'dolomitic' sorbent. The adsorption energy distribution function for benzene suggests the increase of a number of adsorption sites at higher values of adsorption energy after the thermal treatment. The distinct differences in the surface properties of the three dolomite sorbents, i.e. natural, thermally modified, and subsequently washed with borax buffer, may be confirmed by investigation of the sorption of SOz. Fig. 7 shows the kinetic curves of the adsorption of this gas for the natural dolomite (curve 1 ), the dolomite heated up to 800°C (curve 2) and that washed with borax buffer after decomposition (curve 3). From this figure it appears that the partial decomposition of dolomite and the subsequent removal of MgO formed in this process bring about a significant change in the surface properties, in comparison with those of the natural sorbent. Obviously, this modification has a dominant influence on the rate, the capacity and the mechanism of the sorption. The amount adsorbed on the surface of the washed sample is approximately 15 times higher than on the raw one. This makes the new sorbent very promising in its application to environmental protection [ 14 ].
i 38
42
4. Conclusions
E, k J / t o o l
Fig. 6. Adsorption energy distributions for benzene on the raw dolomite (curve 1) and on the thermallymodifieddolomite (curve 2).
From the presented results it appears that the partial decomposition of dolomite is a very simple and successful mode of
P. Staszczuk et al. / Powder Technology 92 (1997) 253-257
the surface modification of natural dolomite, mainly due to the two-step decomposition. The first step leads to the decomposition of magnesium carbonate only, while calcium carbonate remains as a stable and mechanically resistant structure of pumice type. Depending on the thermal activation mode followed by removal of the formed MgO, the obtained material possesses quite new surface properties, both structural (the total specific surface area increased 37 times) and chemical (dolomite is converted to porous calcium carbonate). The result of the modification is a so-called 'dolomitic' adsorbent of completely different surface properties, e.g. its higher adsorption capacity in comparison with that of the raw material (the sorption of SO2 increased about 15 times). The physicochemical surface properties of the examined samples were also characterized by the adsorption energy distribution functions with respect to the energy of interaction. These relationships may be used to estimate the energetic heterogeneity of the surface which is directly influenced by the presence of the energetically different active sites. For the thermally activated dolomite the maximum value of the function X = f ( e ) is shifted to lower values of the interaction energy, which suggests the appearance of new active sites in the process of modification. These results seem to be prom-
257
ising in regard to the application of the 'dolomitic' adsorbents as a cheap and useful material for the retention of certain toxic compounds, especially SO2.
References [ 1] R.J. Reeder and S.A. Markgraf,Am. Mineral., 71 (1986) 795. [2] F. Prossokand G. Lehman, Phys. Chem. Miner., 13 (1986) 331. [3] S.J. Gaffey,Am. Mineral., 71 (1986) 151. [4] H.G. Wiedemanand G. Bayer, Thermochim. Acta, 121 (1987) 479. [5] R. Otsuka, Thermochim. Acta, 100 (1986) 69. [6 ] P. Staszczuk,B. Bilifski, E. Stefaniakand E. Szymarski,Adv. Compos. Mater., 4 (1992) 251. [7] P. Staszczuk, E. Stefaniak and E. Szymafiski, Przem. Chem., 74 (1995). [8] B. Bilifiski, Thesis, Maria Curie-Sklodowska University, Lublin, Poland, 1994. [9] D.R. Glasson, J. Appl. Chem., 14 (1964) 121. [ 10] B. Biliriski and E. Chibowski, Powder TechnoL, 35 (1983) 39. [ 11] F.M. Fowkes, Ind. Eng. Chem., 56 (1964) 40. [ 12] B. Bilirski, Powder Technol., 81 (1994) 241. [13] W. Rudzitiski and D.H. Everett, Adsorption of Gases on Heterogeneous Surfaces, AcademicPress, London, 1992. [ 14] P. Staszczuk,B. Bilifiski, E. Stefaniakand E. Szymafiski,Polish Patent Application P 306 526 (1994).
Powder Technology 92 (1997) 253-257
Investigations on the adsorption properties and porosity of natural and thermally treated dolomite samples P. Staszczuk a,., E. Stefaniak a, B. Bilifiski a, E. Szymafiski a R. Dobrowolski u, S.A.A. Jayaweera c a Department of Physical Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, M. Curie-Sklodowska Square 3, 20-031 Lublin, Poland b Central Laboratory, Maria Curie-Sklodowska University, M. Curie-Sklodowska Square 3, 20-031 Lublin, Poland c Division of Chemical and Biotechnological Sciences, School of Science and Technology, University ofTeeside, Middlesbrough, Cleveland TSI 3BA, UK Received 30 June 1996; revised 23 December 1996; accepted 10 March 1997
Abstract Adsorption properties with respect to certain toxic substances as well as porosity investigations of raw and thermally modified dolomite are presented. After the partial decomposition of dolomite at 800°C a new so-called 'dolomitic' adsorbent of the calcium carbonate skeleton and pumice structure was obtained. Its physicochemical properties are significantly different from those of the raw material, e.g. the specific surface area increases 37 times and the adsorption capacity for SO2 increases approximately 15 times, which makes the new adsorbent useful in the protection of the environment. Keywords: Adsorption; Porosity; Dolomite
1. Introduction
2. Experimental
Dolomite is a mineral which has received a great deal of attention in recent years, resulting in numerous papers considering its various properties [ 1-7]. This mineral is a major and cheap source of magnesium. The problem of magnesium deficiency occurs not only in Poland, but also in the whole of Europe. This mineral can be used as a food and fodder additive in order to compensate for magnesium deficiency. Dolomite usage is increasingly important in different branches of industry such as the food and pharmaceutical industries, production of fertilizers, glass, building materials, and even the kinescopes (picture tubes) for color television. Unfortunately, dolomite is now used mainly as a building material or as a filler for glass, plastics and colors, in spite of its ability to adsorb certain poisonous substances [7]. As can be seen from the literature, the physicochemical properties of dolomite have not been completely investigated. This paper is aimed at presenting the influence of thermal modification on the porosity and adsorption properties of dolomite, especially in regard to the adsorption of some of the toxic substances which are a major problem in environmental protection.
2.1. Materials and reagents
* Corresponding author. Tel.: +48 81 375 646; fax: +48 81 375 102; e-mail: piotr @hermes.umcs.lublin.pl 0032-5910/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved PIIS0032-5910(97)03246-4
The experiments were carried out on dolomite originating from Otdrzychowice-Romanowo deposits (Poland). A grain fraction of 0.12-0.25 mm in size was used. The samples were dried at 150°C or heated at 800°C for 12 h (in air), and then washed with borax buffer (pH 8.6) in order to remove MgO from the newly created pores. 2.2. Methods The percentages of the main elements in the raw material were determined by means of an atomic absorption spectrophotometer (model AAS-3, Carl Zeiss, Jena, Germany) and are as follows: CaO, 30.61; MgO, 18.4; Fe203, 0.29; BaO, below 0.028%. The thermal decomposition of the raw dolomite sample and that of the modified one were performed with a Derivatograph (model Q-1500 D, MOM, Hungary). The specific surface area and porosity were measured by a Sorptomatic apparatus (Surfamat 5x, Poland). The adsorption of benzene was analyzed by the chromatographic peak maximum method using a gas chromatograph
254
P. Staszczuket al. / PowderTechnology 92 (1997) 253-257
(model GCHF 18.3, Germany). The sample of 7 g was placed in a 20 cm glass column (i.d. 3 mm) and conditioned with dry hydrogen at a temperature of 29.9°C which was also the measurement temperature• The procedure for these measurements and the method by which to calculate the adsorption isotherms are described in detail elsewhere [8]. The adsorption of SO2 from the gas phase was carried out by means of equipment consisting of a cylinder of SO2, suitable manometers, a column in which was the sample to be studied, a conductometer and a recorder•
3. Results a n d d i s c u s s i o n
Fig. 1 presents the results of the thermal decomposition of the dolomite. According to the literature data [4], when this process occurs in air, dolomite decomposes in two steps, as follows: First peak:
CaMg(CO 3)2 --->CaCO3 + MgO + CO2
Second peak:
CaCO3 ~ CaO +CO2
1000
900
• ..... -.
800 ........
"
.
~ _=
DTG
t ,[
".!
,
\
.
700
.=
."
DTA
, .
i',
.:4
,
II
"~
i'_' /
~1
\':
and the course of its decomposition strongly depends on the CO2 partial pressure. The mechanism of dolomite decomposition is widely discussed in the literature and the suggested theories mainly consider the reactions referring to the first endothermal peak. They may be divided into two categories: direct formation of CaCO2 or recarbonization of freshly formed CaO to CaCO3. However, it is well established that the second peak represents the decomposition of the CaCO3 formed [ 5 ]. The partial decomposition of dolomite at 800°C leads to changes in the chemical composition of the surface and in the porosity of this mineral. Generally, the product of partial decomposition of dolomite (a so-called 'dolomitic' sorbent) contains calcium carbonate and magnesium oxide which may be confirmed by further thermal decomposition (only onestep decomposition at about 950°C). The results of porosity investigations presented in Table 1 show the significant increase of the specific surface area and total pore volume• Moreover, these parameters enlarge much more after the partially decomposed sample has been washed with borax buffer in order to remove MgO from the newly created pores. The total specific surface area of the obtained 'dolomitic' sorbent increased approximately 37 times when compared with the raw material• In general, this agrees with the results presented by Glasson [9]. The adsorption of benzene on the raw dolomite (dried at 150°C) as well as on the thermally modified dolomite (heated at 800°C) was investigated by a peak profile chromatographic method. The adsorption isotherms a =f(P/Po) were calculated from the retention data. The amount adsorbed, a ( Izmol / m2), was calculated by integration of the experimentally determined relationship VR =f(P/Po) according to the equation [ 10] p/po
~-
Po f a = m,RTA
500
(1)
VR d(p/po)
0 400
•
.
where m, is the adsorbent mass, P/Po the relative pressure of the adsorbate vapor, VR the retention volume, T the temperature, R the gas constant, and A the specific surface area. The relative vapor pressure P/Po was calculated from the recorded chromatogram as follows:
"
500
.
200
:
" 600
700
800
go0
1000
tO0
"I'E/'4PERATURE,Ln"C Fig. 1. Thermogravimetric(TG), differential thermogravimetric(DTG) and differentialthermoanalysis (DTA) curvesof the thermaldecomposition of natural dolomite.
p/po=h/hpo
(2)
where h is the peak height for the pressure p and hpo is the adequate height corresponding to the saturated vapor pressure Po and determined by the detector calibration.
Table l Changes in porosityof the investigatedsamplesdue to the thermalmodification Sample
Specificsurface area (m2/g)
Total pore volume (cm3/g)
Averagepore radius (/~)
Raw dolomitedried at 150°C Dolomite heated at 800°C Dolomite heated at 800°C and washedwith borax buffer
0.7 11.3 18.3
0.01 0.05 0.07
2 ! .2 81.5 43.3
P. Staszczuk et al. / Powder Technology 92 (1997) 253-257
The adsorption isotherms of benzene on the raw dolomite dried at 150°C (curve 1) and on that heated up to 800°C (curve 2) are presented in Fig. 2. They are typical isotherms of type II according to BET theory. The adsorption capacity (the amount adsorbed at P/Po= 1) for these two adsorbents is 21 and 29 p.mol/m 2, respectively. For a known value of a, the film pressure H of benzene on the surfaces of the examined samples can be calculated from the integral form of the Gibbs adsorption isotherm equation, called the Bangham-Razouk equation [ 11 ] :
255
"d G
4
3
p
/7=RTfa
2
(3)
d(ln p)
0'.s
',
~'.s Negr
O
where p is the equilibrium pressure of the adsorbate. The value o f / 7 reflects the changes of free energy of the surface upon adsorption. The relationships between the film pressure and the adsorbed amount of benzene for the raw dolomite (curve 1) and the modified dolomite (curve 2) are presented in Fig. 3. Fig. 4 shows the derivative of the film pressure with respect to the amount adsorbed, GH/Ga,as a function of the number of monolayers, NBET, of benzene 30-
20
10
0
i 0.2
0.4
= '-
0.6
0.8
P/Pu Fig. 2. Adsorption isotherms o f benzene on raw dolomite (curve ] ) and on the dolomite heated at 800°C (curve 2).
Fig. 4. Values of gH/Ga as a function of the number of monolayers, Nser, of benzene adsorbed on the raw dolomite (curve 1) and on the dolomite heated at 800°C (curve 2).
adsorbed on the raw dolomite (curve 1) and on the dolomite heated at 800°C (curve 2). The shape of each curve allows us to estimate the mechanism of benzene adsorption on the investigated surface. The extrema and inflection points on the curves suggest that the adsorption mechanism is quite complicated in both cases and should be attributed to surface heterogeneity. The calculations were performed based on the smooth data. Fig. 5 presents the dependence of the parameter HS/kT on the number of benzene monolayers, NBET. The parameter/TS/kT, where S is the average area per adsorbed molecule, appears from the simplest two-dimensional equation of state, and in the ideal case it should be equal to unity. This parameter, calculated from the relationship H=f(a) shown in Fig. 5, is used to estimate the intermolecular interaction forces [ 12]. It is worth noticing that the calculated values are significantly higher then unity and they are also connected to certain changes in surface heterogeneity of the examined samples. The intermolecular interactions within the benzene monolayer adsorbed on the raw dolomite appear stronger (curve 1 ) below a coverage of 0.5N~ET than those on the heated material. In order to fully characterize the intermolecular interactions and the distribution of adsorption sites on the examined 1.8
50
b. t
"~ ,to -
1.6
301.4
2O 1.2 I0 [ t
i
i
5
10
15
~
20
i
25 a,
30 pmol/m z
F i g 3. Relationship between the film pressure H and the amount adsorbed a on the raw dolomite (curve 1) and on the modified dolomite (curve 2).
0.5
1
1.5
2 Niter
Fig. 5. Relationship between the value of HS/kTand the number of adsorbed monolayers, NBET, of benzene on the raw dolomite (curve 1 ) and on the thermally modified dolomite (curve 2).
256
P. Staszczuk et al. / Powder Technology 92 (1997) 253-257
surfaces, a so-called adsorption energy distribution function X =f(E) was determined according to the ACCA (asymptotically corrected condensation approximation) method [ 12]. This method used to be applied very often in investigations on surface heterogeneity based directly on retention data. This function may be calculated from the equation [ 13 ] j P 2 X= --(~mm)(~-~ ( ~ p R)
1.5
,
,
,
.
,
O
A
10
(4)
wherej is the James-Martin compressibility factor, a m is the monolayer capacity, and p is the equilibrium pressure of the adsorbate. The value of function X allows us to estimate the changes in the number of adsorption sites on the surface. The interaction energy ~ can be calculated for known p from the equation [ 12,13]
0.5
GO
10
20
30
40
50
60
70
80
T IM E . i. m i .
e=RTIn(K)
(5)
where K is a constant dependent on the assumed local adsorption isotherm equation. In the case of the Langmuir equation, (6)
K = ( 2Flm ) ] /2 ( kT) 5/Zq~ h3qa
where m is the molecular mass, k is Boltzmann's constant, h is Planck's constant, and qa and qg are the partition functions connected with the internal degrees of freedom of molecules in an adsorption layer and in the gas phase, respectively. It is usually assumed that q, = qg [ 13]. Fig. 6 shows the distribution functions of the adsorption energy for benzene on the raw (curve 1 ) and thermally modified (curve 2) dolomite samples. Both curves suggest three dominating kinds of adsorption sites on both materials; however, the distribution of these sites differs significantly. The heated dolomite possesses more adsorption sites between 32 and 41 kJ/mol, while the very characteristic maximum at the lowest energy of interaction is shifted towards lower values 60, o
E <
40
2o
0
t 26
30
34
i
Fig. 7. Kineticsof SO2adsorptionon the raw dolomite (curve 1) and on the dolomite heated at 800°C (curve 2) and then washed with borax buffer (curve 3). of e. This evidently results from the partial decomposition of dolomite. Curve 2 possesses a very typical bimodal shape for hydrocarbons exhibiting only the dispersive interaction on the surfaces of oxide minerals [ 12]. In the case of curve 1 there are three maxima of X for the adequate values of E. This suggests the presence of three different types of active sites for benzene adsorbed molecules, probably due to the occurrence of certain ions on the mineral surface. The thermal activation of natural dolomite provides different physicochemical properties of the obtained 'dolomitic' sorbent. The adsorption energy distribution function for benzene suggests the increase of a number of adsorption sites at higher values of adsorption energy after the thermal treatment. The distinct differences in the surface properties of the three dolomite sorbents, i.e. natural, thermally modified, and subsequently washed with borax buffer, may be confirmed by investigation of the sorption of SOz. Fig. 7 shows the kinetic curves of the adsorption of this gas for the natural dolomite (curve 1 ), the dolomite heated up to 800°C (curve 2) and that washed with borax buffer after decomposition (curve 3). From this figure it appears that the partial decomposition of dolomite and the subsequent removal of MgO formed in this process bring about a significant change in the surface properties, in comparison with those of the natural sorbent. Obviously, this modification has a dominant influence on the rate, the capacity and the mechanism of the sorption. The amount adsorbed on the surface of the washed sample is approximately 15 times higher than on the raw one. This makes the new sorbent very promising in its application to environmental protection [ 14 ].
i 38
42
4. Conclusions
E, k J / t o o l
Fig. 6. Adsorption energy distributions for benzene on the raw dolomite (curve 1) and on the thermallymodifieddolomite (curve 2).
From the presented results it appears that the partial decomposition of dolomite is a very simple and successful mode of
P. Staszczuk et al. / Powder Technology 92 (1997) 253-257
the surface modification of natural dolomite, mainly due to the two-step decomposition. The first step leads to the decomposition of magnesium carbonate only, while calcium carbonate remains as a stable and mechanically resistant structure of pumice type. Depending on the thermal activation mode followed by removal of the formed MgO, the obtained material possesses quite new surface properties, both structural (the total specific surface area increased 37 times) and chemical (dolomite is converted to porous calcium carbonate). The result of the modification is a so-called 'dolomitic' adsorbent of completely different surface properties, e.g. its higher adsorption capacity in comparison with that of the raw material (the sorption of SO2 increased about 15 times). The physicochemical surface properties of the examined samples were also characterized by the adsorption energy distribution functions with respect to the energy of interaction. These relationships may be used to estimate the energetic heterogeneity of the surface which is directly influenced by the presence of the energetically different active sites. For the thermally activated dolomite the maximum value of the function X = f ( e ) is shifted to lower values of the interaction energy, which suggests the appearance of new active sites in the process of modification. These results seem to be prom-
257
ising in regard to the application of the 'dolomitic' adsorbents as a cheap and useful material for the retention of certain toxic compounds, especially SO2.
References [ 1] R.J. Reeder and S.A. Markgraf,Am. Mineral., 71 (1986) 795. [2] F. Prossokand G. Lehman, Phys. Chem. Miner., 13 (1986) 331. [3] S.J. Gaffey,Am. Mineral., 71 (1986) 151. [4] H.G. Wiedemanand G. Bayer, Thermochim. Acta, 121 (1987) 479. [5] R. Otsuka, Thermochim. Acta, 100 (1986) 69. [6 ] P. Staszczuk,B. Bilifski, E. Stefaniakand E. Szymarski,Adv. Compos. Mater., 4 (1992) 251. [7] P. Staszczuk, E. Stefaniak and E. Szymafiski, Przem. Chem., 74 (1995). [8] B. Bilifiski, Thesis, Maria Curie-Sklodowska University, Lublin, Poland, 1994. [9] D.R. Glasson, J. Appl. Chem., 14 (1964) 121. [ 10] B. Biliriski and E. Chibowski, Powder TechnoL, 35 (1983) 39. [ 11] F.M. Fowkes, Ind. Eng. Chem., 56 (1964) 40. [ 12] B. Bilirski, Powder Technol., 81 (1994) 241. [13] W. Rudzitiski and D.H. Everett, Adsorption of Gases on Heterogeneous Surfaces, AcademicPress, London, 1992. [ 14] P. Staszczuk,B. Bilifiski, E. Stefaniakand E. Szymafiski,Polish Patent Application P 306 526 (1994).