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The Development and Characterisation of Sorbents from Irish Sphagnum Moss Peat.

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Dev.Chem.Eng. Mineral Process., 5(3/4), pp.251-266, 1997.
The Development and Characterisation of
Sorbents from Irish Sphagnum Moss Peat
L.J. Whitten and S.J. Allen*
Department of Chemical Engineering, The Queen 's University of
Belfast, The David Keir Building, Stranmillis Road, Belfast BT9 5AG,
Northern Ireland, UK
A range of carbon chars was produced @om sphagnum moss peat by heating in a
furnace with a reduced oxygen atmosphere.
A series of heating times and
temperatures were selected for the preparation of the chars. Comparative porosity
and surface area studies, based on nitrogen sorption isotherms and mercury
porosimetry data, were carried out to characterise the chars. Further
characterisationstudies were conducted by electron microscopy techniques. Optimum
charring conditions, yielding maximum surface area for Nt ahorption, were
observed at a charring temperature of 800°C and charring time 30 minutes. Using
the optimum charring conditions, a further series of chars was produced Prior to
charring the raw peat was chemically pre-treated with a range of metal salt solutions
of varying percentage concentration. The resulting chars were characterised as
above.
The ahorptive capacity of these chars for N2 was investigated on the
Sorptomatic 1900 (Fisons Instruments). The ahorption isotherms were compared to
the standard isotherms of Langmuir. Ahorbate uptake was seen to vary with surface
area availability,pore volume, pore type and the nature of the ahorbate.
Introduction
A lot of work has been done on activated carbon with respect to preparation and
characterisationthat has enabled it to reach a significant level of achievement and usage.
Physical and chemical properties differ between materials (Mattson and Mark [ 11) due
* Author for correspondence.
251
LJ. Whirren and S.J. Allen
to the differences in the base materials and the way they have been prepared. Due to
this variation, not all activated carbons are suitable for a specific application. Potential
application of an activated carbon 6om a particular source can be realised by detailed
studies of its properties. The activated carbon is characterised in order to identify its
properties and the ability to match a particular application.
The rapid development of gas adsorption processes in industry has necessitated the
parallel development of a fundamental understanding of the mechanisms involved in gas
phase adsorption systems [2]. Adsorption occurs when any clean solid surface is
exposed to a gas or vapour [3]. The amount adsorbed at the gadsolid interface is
dependent on the nature of the molecular interactions, the surface area and porosity of
the adsorbent and the experimental conditions. Following the collision of a gas
molecule with a solid surface only a limited number of outcomes are possible. The
molecule may rebound from the surface or be adsorbed chemically or physically. The
molecular interactions, either physical or chemical will lead respectively to
chemisorption or physisorption (specific or non-specific).
In gaseous adsorption
systems below 2OO0C physical adsorption predominates [4,5].
Comparison of the
energetics of adsorption provides a usefil indication of the relative affinity of an
adsorbent for different molecules (polar, non-polar, etc.).
Adsorbents are generally seen as materiais of high surface area with a highly porous
structure [5,6,7]. Traditional adsorbents (e.g. activated carbons, silica gels, clays and
aluminas) generally exhibit a high degree of surface and textural heterogeneity, whereas
the newer adsorbents (e.g. zeolites, carbon molecular sieves and modified silicas) are
more uniform. Most adsorbents of technological importance are highly ‘active’. The
required large surface is generated either by the production of very small particles or,
more usually, by the formation of a pore structure [6]. For many purposes, the latter is
advantageous since it reduces the risk of ageing (i.e. the loss of surface area). However,
the internal surface located inside a porous material does not have exactly the same
properties as the corresponding external surface. Furthermore, the adsorptive properties
are controlled by the pore size and shape distribution. Industries employing adsorbents,
which for a long time have used the Brunauer, Emmett and Teller [8,9] (BET)surface
252
Development and characterisationof sorbentsfrom Irish sphagnum moss peat
area determinations for research and control, have recently been adding pore structure
analysis as an important tool both in research and in operation.
Thus sphagnum moss peat was selected to identify the characteristic features of the
chars and the activated carbons prepared from this particular species, in order to gain a
new source of activated carbon. Furthermore, peat is an abundant natural resource in
Northern Ireland, and other countries, and large-scale production could potentially
enhance the economy. Results are presented for a range of novel chars and activated
carbons produced from sphagnum moss peat. Each char and carbon was characterised,
thus generating important information regarding the development of pore structures in
the sorbents according to their various preparation techniques.
Materials and Methods
Materiak Sphagnum moss peat was chosen as the raw material to produce a range of
chars and carbons. ZnCl,, KCI, Fq(SO,)+H,O and FeSO4.7H,O were the main metal
salts chosen to chemically pre-treat the raw peat prior to charring.
Elemental Analysis of the Moss Peat
Carbon: 49.0 1wt?h Hydrogen: 6.17wt?h Nitrogen: 1.93wt?! Sulphur: 0.12Wtoh
Oxygen (by difference): 4 2 . 7 7 d h
LO1 (loss on ignition): 98.1%
Trace metals detected: Ni, Fe, Mg, Ca, Al, Na
Peat is a natural material whose composition is dependent on its source. The peat used
was prepared by dry milling for horticultural use. The composition analysis was carried
out for random samples selected from the peat. The values reported are average values
for the available supply of peat.
Peat Chars Peat chars were produced by pyrolysis of au-dried raw peat. Peat was
prepared for charring by sieving it to a particle size range of 1-1.7mm; 8Og of the
particle size range were placed in a sealed air-tight metal box, which was placed in a
253
LJ. Whitten and S.J. Allen
constant temperature furnace. A series of heating times and temperatures were selected
for the preparation of the chars, as follows:
Charring Time (&)
Charring Temperature ("C)
Char Code
10
400
10
600
800
10
10
900
20
400
20
600
20
800
20
900
30
400
30
600
30
800
(CPR- C h r e d Peat Raw, i.e. not chemically pre-treated
CPR104
CPR 106
CPRlO8
CPRl09
CPR204
CPR206
CPR208
CPR209
CPR304
CPR306
CPR308
The chars were characterised by N2 adsorption isotherms, porosimetry and scanning
electron microscopy techniques, which are described in detail elsewhere [ 10-131.
Chemically Pre-treated Peai Chars 80g of 1-1.7mm peat was stirred mechanically for
1 hour with 8OOml of a metal salt solution. The slurry was then filtered in a Buchner
flask and funnel. The sluny was washed, during filtering, with 8OOml of distilled water
to remove any residual metal salt solution. The peat was then placed in a low
temperature oven, set at 60°C, to dry. 8Og of the chemically pre-treated peat was
pyrolysed in the h a c e for 30 minutes at 80OoC. A range of solution concentrations
was used to prepare a series of chemically pre-treated peat chars: Fq(SO4~.xH2O
(10,
20,25wto/o), FeS04.7H20(10,20,25Wh), KC1 (10,20,25wt?h), ZnClz (10,2owto/o).
These chars were characterised as for the raw peat chars.
Burn-@
Bum-off is a measure of the material in the raw material which has either
been driven off in the form of volatiles or 'burnt' off during the carbonisation process.
It is determined by calculating the mass loss of the material after charring. Therefore:
YOBum-off
254
-
= [(initial final mass)/ (initial mass)] x 100
Development and characterisation of sorbentsfrom Irish sphagnum moss peat
Nz Adsorption, Sorptomatic 1900 (Fisons Instruments)
Char specific surface area,
pore specific volume and total adsorbed volume were determined on the Sorptomatic
1900. The samples were outgassed at 80°C for 12 hours and then exposed to N2
adsorptioddesorption at 77K. During the tests the instrument recorded the various
parameters to construct the adsorptioddesorption isotherm (relative pressure against N,
uptake) and to calculate the aforementioned surface properties.
Mercury Porosimeter (Fisons Instrumen&) The char pore sue distribution and pore
structures were investigated in a mercury porosimeter. Samples were placed in a
dilatometer and evacuated. The dilatometer was filled, under vacuum, with mercury to
a pre-determined level. The mercury was exposed to high pressure (2000 bar) and the
pore volume was determined by mercury displacement methods.
Scanning Electron Microscopy (SEM)
SEM was used to investigate the surface
topography of the chars. Samples were set in epoxy and then coated in carbon.
Samples were placed in the sample chamber and evacuated to high vacuum (2 xlOd
Torr). The sample is bombarded with a finely focused eleciron beam. A threedimensional topographic image (SEM micrograph) is formed by collecting the
secondary electrons generated by the primary beam.
Micrographs at various
magnifications were obtained for all the chars produced.
Results and Discussion
(0 Determination of optimum charring conditions
As previously mentioned, the adsorbents are generally seen as materials of high surface
area with a highly porous structure. Accessibility of these pores to an adsorptive
molecule is dependant on such factors as the molecule size and shape, the area of the
pores and the volume enclosed by the pores. Therefore, the chaning process is
engineered in such a way to maximise the surface area and pore volume of the adsorbent
and to encourage the development of micro and meso pore structures.
255
L J. Whitten and S.J. Allen
Char
Furnace Charring
temp.
time
ec,
(mi4
Raw Peat
(500-71Opm)
CPR104
CPR204
CPR304
CPR 106
CPR206
CPR306
CPR108
CPR208
CPR308
CPRlO9
CPR209
Table I .
400
Su~ a c e
area (BET)
(m?d
35.13
Pore speclfc
volume (xld)
(cm3/d
7.56
Total
adsorbed
voi. (cm3/d
64.68
22.88
22.49
26.01
64.57
1 1 1.46
167.01
196.25
211.36
240.13
196.34
15.84
3.76
5.37
5-22
8.18
10.06
12.33
13.36
14.75
16.02
14.01
1.60
37.60
53.01
56.67
61.51
86.31
99.07
112.35
1 18.74
124.04
110.21
3 1.09
10
20
30
10
20
30
10
20
30
10
20
600
800
900
Characterisation results for peat chars +om the Sorptomatic 1900 (Fisons
Instruments).
0
0.2
0.4
0.6
0.8
1
Rel. Pressure (plp,)
Figure I. Nitrogen ahorptioddesorption isotherms for peat chars (Sorptomatic
1900). Closed symbols - ahorption; open symbols - &sorption.
256
Development and characterisation of sorbents from Irish sphagnum moss peat
On examination of Table 1 and Figure 1 it can been seen clearly that common trends
are exhibited in surface area, pore specific volume and the total adsorbed volume of the
raw peat chars. With each furnace temperature the aforementionedcharacteristics of the
chars increase with increasing length of the charring times. It is also observed that as the
furnace temperature is raised, the characteristic parameters increase steadily. However,
as the h a c e temperature is increased from 800°Cto 900°C, for as little as 10 minutes,
a rapid decrease in surface area and total adsorbed volume is observed from 240 to
196m2/gand 124 to 1 10cm3/grespectively. A slight decrease in pore specific volume is
noted (0.16 to 0.14cm3/g) but the extent of this decrease is not as pronounced as that
observed for the other two parameters. These observations indicate the optimum
charring conditions of 30 minutes at 8OO0C resulting in maximum values for char
characteristics, i.e. surface area (240m2/g), pore specific volume (0. 16cm3/g)and total
adsorbed volume (124cm’/g). These can be explained by considering the structural
changes which occur within the peat during the carbonisation process.
0
0.2
0.4
0.6
ReI. P r e s s u r e (plp,)
0.8
1
Figure 2. Nitrogen adsorptioddesorption isothermsfor peat chars to compare effects
of charring temperature fSorptomatic 1900).
Closed symbols - dorption; open symbols - desorption.
257
L.J. Whitten and S.J. Allen
(ii) Bum-offin the chars
Peat is a material composed mainly of water. The portion that is not water, usually only
about 10-2070 of the mass, is the partially decomposed residue of dead plants.
Combined with the remains of these plants are those of the decay microorganisms.
Given et al., (1979-1983) [14,15,16], identified plant materials such as cellulose and
lignin derivatives in the large scale and fine-grained humic materials in peat. Hence,
during carbonisation the mainly volatile water and aromatic structures, based on
benzene and naphthalene rings, and some tar products are driven off the char. Burn-off
therefore is a measure of the material in the raw peat, which has either been driven off in
the form of volatiles or 'burnt' off in the carbonisation process. Figure 3 illustrates the
fact that the higher the charring temperature and the longer the charring time the greater
the degree of bum-off.
+400'C
+800'C
Char +6OO'C
Char + S O O T
Char
Char
70
1 4 0 4
10
I
I
15
20
CHARRING TIME(m ins)
Figure 3. Burn-offfor raw peat chars.
258
25
3c
Development and characterisation of sorbents from Irish sphagnum moss peat
As the peat is charred, volatiles and tars are driven off to open a pore structure within
the char. Initially microporous structures are opened and the surface material of the char
is burnt off. As the charring times increase and the temperature rises a greater
proportion of the off products are expelled from the internal pores. At elevated
temperatures the micropore walls are burnt off and these pores open into one another to
generate mesopore structures. Eventually at very high temperatures the majority of
micropore volume is lost due to meso and macro pore formation resulting in a rapid
decrease in surface area. This is what is observed in the 900°C chars. There is a rapid
decrease in the surface area compared to CPR308 (optimum conditions, 30 mins at
800°C) hence a decrease in the total adsorbed volume. Overall, there is only a slight
decrease in volume. This loss is due to the pores at the surface being burnt off and
although the pores change physically (i.e. micropores open to form mesopores,
mesopores open to form macropores), there is negligible internal volume change.
The charring process is best illustrated by examining the following scanning electron
micrographs for the series of peat chars.
.
Figure 4. Raw Peat (x 1900).
Figure 5. CPR206 - Raw peat char 20 mins
at 600°C (x 550).
259
L J. Whitren and S.J. Allen
Figure 6. CPR308 - Raw peat char
30 mins at 800°C (x 1400).
Figure 7. CPRIO9 - Raw peat char
10 mins at 900°C (x 500).
The surface of raw peat is quite smooth in appearance (Figure 4) with a few ‘organ
pipe’ type pores of approximate diameter 10pm. These visible pores are the water
transport pores within the peat. The internal ‘finger-like’ structures observed in the
pores are the higher band strengthening structures for the pores to make them rigid. As
the peat is charred the smooth surface is burnt off to expose the internal pore structures.
The higher band strengthening observed adds internal surface to the chars. In Figure 5 it
can be seen that the pore development has become well defined and regular in nature.
The majority of pores have diameters of low.There is evidence in the left hand comer
of surface diminishment due to bum-off, revealing the higher band strengthening
structures. The development of pore Structures over the surface of the char gives a
regular honeycomb appearance. Figure 6 shows the pore structures at the optimum
charring conditions. The external pores are approximately 20pm with a regular figureof-eight formation. They enclose many higher band strengthening structures and
internal pores of diameter 2-5pm. Charring beyond the optimum conditions results in
loss of sorbent surface as is seen in Figure 7. The pores have been opened to form
macropores with diameters of 20-3Opm and in extreme cases of bum-off only the higher
band strengthening is visible on the surface.
260
Development and characterisation of sorbents from Irish sphagnum moss peal
(ui)
Determination of pore srructures present in the chars
Shapes of isotherms arising from gaseous adsorption have often been associated and
identified with specific pore structures. One structure which is clearly evident in both
samples, raw peat and CPIUOI, in Figure 8 is the hysteresis loop. Hysteresis appearing
in the multi-layer range of the physiorption isotherm is usually associated with capillary
condensation occuning in the residual pore spaces left in the mesoporous structures
within the particles after monolayer-multilayercoverage has taken place. The hysteresis
displayed is low pressure hysteresis, i.e. the desorption branch of the loop does not close
onto the adsorption branch but instead extends to low relative pressures, which indicates
the presence of microporous structures. Low pressure hysteresis is thought to be
associated with the swelling of a non-rigid porous structure or with the irreversible
uptake of molecules in pores (or through pore entrances) of about the same width as that
of the adsorbate molecule [lo]. This phenomenon is more evident in CPR308 than in
the raw peat indicating that the char has a greater presence of micropores. Micropores
are also indicated in the char due to the isotherm’s initial Type 1 character. The initial
portion of the graph @/p00.05-0.33) is also an indication of micropores. This is the
section of the graph which is used in the calculation of surface area (BET Method).
Monolayer coverage and micropore filling are proceeding between plpovalues of 0.050.33 [8] and it is desirable for the plot to hug the y-axis until as high a possible value of
volume adsorbed is reached. The steeper this initial section is the greater the surface
area value will be. This is demonstrated well in Figure 8 from the different isotherm
shapes, with raw peat having a surface area of 35m2/g and CPR308 having a surface
area of 249m2/g. Finally, both samples exhibit limiting adsorption at high p/po. The
steep rise of the isotherm at p/po approaching unity indicates the presence of
macroporous structures in both samples.
261
LJ. Whitten and S.J. Allen
0.2
0
0 .4
0.6
ReI. Pressure (plp,)
0.8
1
Figure 8. Nitrogen ahorptioddesorption isotherms for raw peat (500-71Opm) and
CPR308for shape comparison.
Closed symbols: adsorption; open symbols: desorption.
Char
ZnClz
ZnC1,
KCl
KCI
KCI
Fe2(S04h*f120
F%(S04)3.f120
F%(S04)3*f120
FeS04.7H20
FeS04.7H20
FeS0,.7H,O
Table 2.
262
10%
20%
10%
20%
25%
10%
20%
25%
10%
20%
25%
Sur$ace
Area
(BET)
(m2&
198.27
557.06
1.95
1 1.97
19.89
70.30
243.03
358.00
88.83
412.29
333.21
10.46
23.18
5.39
6.46
7.50
5.39
18.0 1
27.95
5.22
27.46
22.86
86.0 1
203.4 1
46.74
68.0 1
58.22
38.92
152.00
194.23
35.63
187.65
154.17
Characterisation results for chemically pre-treated peat chars fiom the
Sorptomatic I999 (Fisons Instruments).
All chars produced at the optimum conditions of 800°Cfor 30 mins.
Development and characterisation of sorbents from Irish sphagnum moss peat
(iv)
Bum-offfor chemicallypre-treated chars
The first observation for bum-off for chemically pre-treated chars compared to raw peat
chars is that the bum-off values are greater than those observed for the low temperature
chars and slightly higher than those observed for the high temperature chars. Also, as
the solution concentration for pre-treatment is increased, bum-off increases. Chemically
pre-treating the peat prior to charring is carried out to enhance the pore development in
the chars. The metal salt penetrates deeply into the internal pore structure of the raw
peat and is driven off under the charring conditions initiating pore propagation.
Characterisation of these chars shows that the surface area tends to increase with
increasing concentration of pre-treatment solution. The exception being FeS0,.7HZ0
where it was observed that the 20% solution yielded the maximum surface area (see
Table 2). The pre-treatment producing the best results, as compared to the raw peat and
the raw peat char, was the 20% solution of ZnCI,. Chemically pre-treating the peat with
20% ZnC1, prior to charring yielded a specific surface of 557mZ/gwhich is comparable
to some commercially available active carbons.
Considering the micropore
development in the chars, again, the 20% ZnC1, gives the highest micropore volume
value of 0.23mVg as compared to 0.OImVg for raw peat (particle size range 5007 1Ow).
80
-
+Fe2(S04)3.xH20
&KCL
+FeS04.7H20
75
---t Zn Ct2
--
3
m
I
15
20
SOLN. CONCENTRATION (X)
lo
251
Figure 9. Burn-offor chemically pre-treated chars.
263
L J. Whitten and S.J. Allen
Char
Micropore Volume
( x l d ) (mug,
~ a Peat
w
(500-7 10pm)
CPR 1 04
CPR204
CPR304
CPR106
CPR206
CPR306
CPRlO8
CPR208
CPR308
CPRlO9
1.01
0.74
1.04
0.82
3.10
5.27
6.93
9.26
9.38
9.46
9.15
ZnC1,
KCl
KCl
KCI
Fq(S0,)3.xH20
Fe,(S0,)3.xH20
20%
10%
20%
25%
20%
25%
23.04
0.09
0.49
0.80
10.93
15.63
Table 3. Micropore volumesfor the peat chars (Dubinin, Radushkevich, St0eck.h) [ I 71.
(v)
Scanning electron micrographsfor chemicallypre-treated chars
Figure 10.
264
20% FeSO, 7H20(x 1900).
Figure 11. 10% KCI (x 350).
Development and characterisation of sorbents from Irish sphagnum moss peat
Chemical pre-treatment is carried out to encourage the process of burn-off during
and to promote the pore development. Crystals from the pre-treatment solutions are
visible in Figures 10 and 11. Figure 10 shows the regular honeycomb surface
topography with pores of diameter 20pm which enclose visible internal pores (2-5 pm).
However, inspection of Figure 11 shows poor surface burn-off and pore propagation.
The low surface yielded for this char could be associated with the poor surface
topography.
Conclusions
1.
The optimum conditions for producing peat chars were found to be; charring time
30 minutes, charring temperature 800°C.
2.
Charring the peat at the optimum conditions increased the potential adsorption
surface area from 35m2/gto 240m’/g.
3. Charring produces micro and meso pore structures within the peat.
4.
Chemically pre-treating the peat prior to charring increases the potential adsorption
surface area, e.g. for 20% ZnC12from 35m2/gto 557m2/g.
5.
Chemical pre-treatments increase the development of micropores.
6. The best pre-treatment was 20% ZnCI, yielding a surface of 557m2/g and a
micropore volume of 0.23mVg.
Acknowlegements
Thanks are due to the QUESTOR Centre for funding this project and for all their
support and guidance. The basis of this paper was originally presented at the 5th Irish
IChemE Research Symposium in Belfast, March 1996. The authors would like to
thank the research symposium organisers for their assistance with further publication
of this research.
265
LJ. Whitten and S.J. Allen
References
I.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Manson, J.S. and Mark, H.B., Jr. 1976. Activated Carbon: Surface Chemistry and Adsorption from
Solution. Marcel Decker, New York.
O’Shea, J.D. and Liapis, A.I. 1990. Evaluation of Simple and Complex Models for Mass Transfer
in the Non-Isothermal Gas Adsorption of Multiple Adsorbates in a Single Adsorbent Particle. Trans.
IChemE, 68, Part A, 242-250.
Gregg, S.J.and Sing, K.S.W. 1967. Adsorption Surface Area and Porosity. Academic Press.
Young, D.M.and Crowell, A.D. 1962. Physical Adsorption of Gases. Butterworth Publishers.
Ross, S. and Olivier, J.P. 1964. On Physical Adsorption. Interscience Publishers.
Rodriguez-Reinaso, F. and Molina-Sabio, M. 1992. Activated Carbons from Lignocellulosic
Materials by Chemical and/or Physical Activation: An Overview. Carbon, 30 (7), 1111-1118.
Zulkamain, Z., Hussein, M.Z. and Badri, M. 1993. Activated Carbon from Mangrove Wood
(Ruophoruupiculofu):Preparation and Characterization. Pertanika. J. Sci. Technol., 1 (2), 169-177.
Brunauer, S., Emmet, P.H., and Teller, E. 1938. Adsorption of Gases in Multimolecular Layers. J.
h e r . SOC.,60,309-319.
British Standard 4359: Part 1 (1969).
Sing, K.S.W. 1982. Int. Union Pure and App. Chem., Subcommittee on Reporting Gas Adsorption
Data, Provisional: Reporting physisorption data for gadsolid systems, 2202-22 18.
ASTM Designation: D 464 1-87
Van Brakel, J., Modry, S.and Svata, M. 1981. Mercury Porosimetry: State of Art. Powder Technol.,
29, 1-12.
ASTM Designation: D 4284-88
Casagrande, P.J, and Given, P.H. 1980. Metals in Okefenokee Peat-Forming Environment: Relation
to Constituents Found in Coal. Geochim. Cosmochim. Acta., 44 (lo), 1493-1507.
Given, P.H., 1983. Nature of the contribution of polymers of cell walls of the higher plants to coal
formation. DOEiEW10988-T.
Given, P.H. 1984. Biochemical Aspects of Early Stages of Coal Formation. Adv. Org. Geochem.,
6,399407.
Dubinin, M.M., and Stoeckli, H.F. 1980. Homogeneous and Heterogeneous Micropore Structures in
Carbonaceous Adsorbents. J. Colloid Interface Sci., 75 (l), 34.
Received: 1 1September 1996; Accepted ajer revision: 19 May 1997.
266
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