Correlation of physical characterization parameters of bituminous coal and its activated carbon in
the removal of organic pollutants from wastewater
S.P. Langwenyaa, B.B. Mambaa, S.B. Mishraa*, M. Balakhrishnanb
a
University of Johannesburg, Faculty of Science, Department of Chemical Technology, PO Box
17011, Doornfontein 2028, RSA
b
The Energy and Resource Institute (TERI), Habitat Place, Lodhi Road, New Delhi 100003,
India
*Corresponding Author Email: smishra@uj.ac.za
ABSTRACT
In this work, the particle size and morphology of activated carbon derived from coal was
correlated in the removal of phenolic compounds from wastewater. Activated carbon was
prepared from bituminous coal of the particle size ranges: C1 (>106 µm), C2 (106-75 µm), C3
(75-53 µm), C4 (53-38 µm) and C5 (< 38µm) via physical activation in steam at temperatures
700˚C and 1000˚C. Characterization was achieved by proximate analysis, apparent density,
Braunner-Emmet-Teller (BET) surface area analysis and scanning electron microscopy (SEM).
ACs with superior characteristics were obtained at 1000˚C. Their surface area increased with
decreasing particle size up to C4 from 59.3 m2/g -223m2/g then decreased for C5 to 112m2/g.
SEM micrographs presented a rough and eroded surface for C1 to C4 and for C5 some
agglomeration was noted. In adsorption studies, the trend observed was an increase in the
adsorption capacity of the ACs with decreasing particle size up to C4 from 12.9mg/g-19.0mg/g
then lowered for C5 to 18.6mg/g which was in agreement with the recorded BET surface area.
Key words: activated carbon, adsorption, morphology, particle size, surface area
1. Introduction
1
In the preparation of activated carbon (AC), the characteristics of the resulting activated carbon
are usually determined by the activation process and the nature of the precursor (Kopac and
Toprak, 2007). Coal, for example, has been reported to produce ACs with specific beneficial
properties such as very well developed surface area, high mechanical and chemical resistance,
ease of degradation of the used product and good ion-exchange properties (Pietrzak et al, 2007;
Schobert and Song, 2007). The particle size of the precursor has also been reported to have an
effect on product quality. When Dalai et al (2008), activated different coal particle sizes by steam
in a fixed bed reactor the finding was that pellets and grains (1.25-2.5mm) had higher iodine
numbers and surface areas compared to the fines (0-1.25mm) for lignite, sub-bituminous and
bituminous coal. However, literature is scant regarding activation of fines of different particle
sizes.
Increasing the surface area of the precursor in AC preparation may be one way of influencing the
characteristics of the resulting AC. Particle size reduction has been reported to increase the
surface area for chemical reaction or to liberate valuable minerals held within particles (Rhodes,
1998). In flash pyrolysis systems particle size reduction affects product distribution and char
characteristics. During devolatilization, particle size influence is significant because of the
variation of the heat and mass transfer inside coal particles (Zhu et al, 2008; Wutti et al, 1996).
Zhu et al, further stated that the particle size effect may also be dependent of coal type, the
reactor used as well as the range of particles used. According to Gao et al, (2006), the rate of
volatile evolution during the early stages of carbonization plays an important role in coal
conversion processes and high volatile contents need more time to release from relatively larger
particles under laser heating conditions.
Phenols are high priority pollutants because of their high toxicity and possible accumulation in
the environment (Lin and Juang, 2008). The Environmental Protection Agency has set a water
purification standard of less than 1ppb of phenol in surface waters (Busca et al, 2008). Adsorption
process has proved to be an effective method for their removal especially on activated carbons as
2
shown by several workers (Gryglewicz et al, 2002; Stavrolpoulos et al, 2008; Tseng et al, 2005;
Podkoscielny et al, 2006; Salame and Bandosz, 2003; Hameed and Rhman, 2008; Tseng et al,
2003; Tancredi et al, 2004). The adsorption process, specifically physical adsorption, may be
affected by amongst other parameters the properties of the surface area of the adsorbent such as
polarity, pore size and spacing (El Qada et al, 2006). Hence the objective of the study was to
correlate the particle size and morphology of AC derived from bituminous coal in the removal of
phenolic compounds from wastewater.
2. Experimental
2.1 Materials
Coal samples from South Africa (Soweto Coal Deposits) were obtained for this study. The coal
samples were first crushed then ground in a high pressure ball mill to obtain small-sized particles
which were sieved into five different fractions namely C1 (>106 µm), C2 (106-75 µm), C3 (7553 µm), C4 (53-38 µm) and C5 (< 38µm) and stored in propylene bags. 2-nitrophenol (98%) was
supplied by Sigma-Aldrich, Germany.
2.2 Activated Carbon Preparation
2.2.1 Thermal activation
Known amounts of coal samples were filled in cylindrical stainless steel (SS) containers with a
tight lid. The containers were placed in another larger SS container and sealed with a tight lid. In
order to ensure an inert atmosphere (except for the air trapped in the voids of the adsorbent) the
annular space between the two containers was filled with sand to the brim of the larger container
and this arrangement also provided uniform and efficient heat. This was heated in a muffle
furnace at temperature ranges 550˚C -1000˚C.
2.2.2 Steam activation
3
A fixed amount of the precursor was mixed with distilled water to form slurry and the same
arrangement was prepared as described earlier and heated at the same temperature range.
3. Characterization
Coal samples and ACs were characterized by proximate analysis according to ASTM D31733175, ASTM D2867, ASTM D5832. Surface area was determined in by N2 adsorption at 77K
using a surface area analyzer (TriStar 3000, Micrometrics Instrument Corporation, USA). The
surface texture and development of porosity was studied by means of scanning electron
microscope (JEOL JSM-5600) with an accelerating voltage of 15kV. In addition, the surface
functional groups of the prepared activated carbon were detected by FTIR spectroscopy and the
spectra was recorded from 4000 to 400cm-1.
4. Adsorptive Properties
Adsorptive properties were measured using methylene blue (MB), iodine and phenol as
adsorbates. MB number is a measure of the extent of MB adsorption by the activated carbon
sample. It is expressed in terms of milligrams of MB adsorbed by 1 g of activated carbon. Iodine
number is a relative indicator of the porosity in activated carbon. MB and Iodine numbers were
determined according to ASTM D 2330 and ASTM D 4607, respectively. The equilibrium
adsorption isotherms for phenol were determined at room temperature in the concentration range
100-300mg/L. Known amounts of AC were weighed and added to stoppered elernmeyer flasks
containing 100ml of known concentrations of phenol solutions. These were agitated at 150rpm
for 24h which was sufficient to reach equilibrium. After filtration, the residual phenol was
determined by UV-spectroscopy using the aminoantipyrine method.
4. Results
The characteristics of the coal fractions used as precursors are presented in Table 1.
4
Table 1: Proximate Analysis of Coal Samples
Properties
>106 µm
106-75µm
75-53µm
53-38µm
< 38 µm
C1
C2
C3
C4
C5
Volatile matter (Vm) (%)
30.4
33.2
32.8
33.7
33.3
Ash (%)
17.7
18.9
19.4
20.1
17.6
Moisture (M)(%)
3.6
2.8
1.7
1.4
1.5
Fixed Carbon (Fc) (%)
48.5
45.1
46.1
44.8
47.6
4.1 Effect of particle size
From the proximate analysis data, of noticeable difference is the variation in ash content which
increases as the particle size decreases then decreases for smallest size fraction and the increasing
surface area with decreasing particle size. Table 2 records the characteristics of the resulting ACs.
It is worth mentioning that the surface area recorded is for the activated carbons activated at
1000˚C since they demonstrated superior properties.
Table 2: Characteristics of Activated carbon
Coal fraction
BET of coal (m2/g)
BET of activated coal
Ash content (%)
(m2/g)
C1
0.93
59.3
70
C2
1.33
123
68
C3
2.12
9.67
70
C4
4.40
223
70
C5
6.72
112
73
5
Decreasing the particle size of the starting material resulted in increased surface area of the
resulting AC. This may have been caused by an increase in the amount of volatile matter released
during carbonization due to the increased surface area of the smallest particles. Morris (1990)
found that in pyrolysis of low ash coal, the volatile yield increased with increasing particle size.
However, for the finer fraction (>38µm), the surface area decreased. This, according to Kizgut
and Yilmaz (2004) was due to fine particles presenting considerable thermoplastic properties that
were easy to lead to agglomeration thus lowering the surface area. No relationship was seen
between the ash content and particle size of the prepared ACs which is contrary to data obtained
for the inactivated samples.
4.2 Morphology
Figure 1-6, present the SEM images of the raw coal and activated samples at varying
magnifications due to the differences in particle size. Activation destroyed the original structure
of coal.
Fig. 1 SEM image of raw coal
Fig. 2 SEM image of activated C1
6
Fig. 3 SEM image of activated C2
Fig. 5 SEM image of activated C4
Fig. 4 SEM image of activated C3
Fig. 6 SEM image of activated C5
From the images, with decreasing particle size, the activated coal presented a rough and
somewhat eroded surface. From C2-C5, observed were smaller particles adhered on the larger
ones. With C4 and C5 agglomeration was evident, more pronounced with C5. Such could have
led to the lowering of surface area. This is in agreement with BET data, whereby the surface
decreased for C5. Obvious porous structures were not detected which might mean some of the
pores were formed inside the coal particles. The ash content increased drastically for all the
activated samples. This is also evident in the scanning electron micrographs shown by more white
structures on the activated samples compared to the raw coal.
4.3 Spectral analysis
7
Representative FTIR spectra for raw and activated coal are shown in fig. 7. Expected
characteristic bands for activated carbon were 1735cm-1, 1590 cm-1 and 1250 cm-1 attributed to
carbonyl groups, aromatic structures and carbon-oxygen or phenoxy absorption. However, these
were not observed in our spectrum. According to Friedel and Hofer (1970), the spectrum shows
nearly complete absorption under ordinary instrument condition; a rescan with wider slits and
slower spectral scanning speeds must be used to observe clear peaks. Visible peaks were around
3500cm-1, 1600 cm-1 and 1040 cm-1, which could be attributed to –OH, C=C and C-O (also Si-O).
Other peaks disappear due to possible carbonization.
Activated
Raw
2.6
2.4
2.2
2.0
1.8
absorbance
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4000
3500
3000
2500
2000
1500
1000
500
wavenumber cm-1
Fig. 7 FTIR spectra of raw and activated coal
4.4 Adsorption studies
The relationship between particle size and the amount of 2-nitrophenol adsorbed is presented in
Fig 8 and the actual values are presented in the Table3. A clear relationship was observed
between the surface area of the different particle sizes of AC and the amount of 2-nitrophenol
adsorbed. It is worth mentioning that adsorption was carried out on as-prepared and washed
8
(several washes with warm distilled water) ACs. For the unwashed sample, the adsorption
capacity increased from 6.74mg/g to 11.4mg/g between C1 and C2. It then dropped significantly
to 5.00mg/g with C3 followed by a sharp increase up to 14.51mg/g for C4. Lastly, there was a
decrease in the adsorption capacity of C5 recording a value of 11.36mg/g. The results obtained
amount adsorbed (mg/g)
here were in agreement with BET surface area as shown earlier.
20
15
washed AC
10
unwashed AC
5
0
0
1
2
3
4
coal fraction
5
6
Fig. 8 A comparison on the adsorption capacity of washed and unwashed AC
Table 3: Adsorption capacities of as-prepared and washed ACs
Activated fraction
Unwashed AC -amount
Washed AC -amount
adsorbed (mg/g)
adsorbed (mg/g)
% Removal
(washed AC)
C1
6.74
12.9
65
C2
11.4
17.3
86
C3
5.00
11.7
59
C4
14.5
19.0
95
C5
11.4
18.6
91
After washing the AC, its adsorption capacity increased by over 50% except for C4 whereby, the
increment was slightly above 30%. Washing probably removes the water-soluble substances
9
which were blocking the pores of the ACs. However, though the increment was lower for C4 it
still recorded the highest adsorption capacity of 19.0mg/g. Even though the adsorption capacity of
the prepared ACs is lower compared with the usual AC adsorption capacity (100-300mg/g), up to
95% removal of nitrophenol was achieved. Furthermore, iodine and methylene blue number tests
were inapplicable for the ACs due to low porosity development when compared to commercial
ACs which normally record 1000m2/g surface areas.
Effect of Initial concentration
A graphical presentation on the effect of increasing the initial nitrophenol concentration on the
adsorption capacity of C4 (as it recorded the highest BET surface area) is shown in fig. 9. A
linear relationship existed between the amount of nitrophenol adsorbed and initial concentration
whereby increasing initial concentration led to an increase in adsorption capacity hence higher
initial concentration was more favourable for the adsorption of nitrophenol by the activated
amount adsorbed (mg/g)
carbon.
60
50
40
amount
adsorbed
30
20
10
0
0
100
200
300
400
conce ntration(ppm)
Fig. 9 The effect of initial concentration on adsorption capacity
5. Conclusion
10
Activated carbon was prepared using bituminous coal of different particle sizes. Particle size and
morphology of the resulting AC were correlated in the adsorption of 2-nitrophenol in synthetic
wastewater. The surface area of AC increased with decreasing particle size from C1 to C4 then
decreased with C5 probably due to agglomeration as shown in the micrographs. Adsorption
capacity increased with increasing surface area which showed dependence of nitrophenol
adsorption on porosity. The highest surface area recorded was 223m2/g for C4 with a
corresponding adsorption capacity of 19.0mg/g. Hence, a correlation existed between surface
area, morphology and the adsorption capacity of the coal-based activated carbons.
Acknowledgements
My sincere gratitude goes to my supervisors listed as co-authors for their support and
encouragement, the National Research Fund (NRF), Nanotechnology Innovation Centre (NIC)
and the University of Johannesburg for Financial support, and finally The Energy and Resource
Institution for collaborative research.
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