Materials Science Forum Vols. 587-588 (2008) pp 844-848
online at http://www.scientific.net
© (2008) Trans Tech Publications, Switzerland
Different Ways to Regenerate an Activated Carbon: Comparison
between an Activated Carbon from Cork and a Commercial Carbon
P.A.M. Mourão1, a, P.J.M. Carrott1, b, M.M.L. Ribeiro Carrott1, c and L. Marques1, d
1
Centro de Química de Évora and Departamento de Química, Universidade de Évora
a
pamm@uevora.pt, bpeter@uevora.pt, cmanrc@uevora.pt, dluisa.s.marques@hotmail.com
Keywords: Regeneration, Activated carbon, Adsorption, Cork.
Abstract. Activated carbons are materials extensively used to remove a variety of pollutants in
different media. The porous structure, the surface chemistry and the type of adsorptive determine
the potential of activated carbons as an adsorbent material. However, in a more global perspective it
is necessary to consider other factors such as the economical viability of production and reuse. This
last factor depends on the capacity of regeneration of the activated carbons after each application.
Following this guideline, the objective of this work was to evaluate the capacity of regeneration of
some specific activated carbons prepared from cork oak wastes, used in the adsorption of phenolic
compounds in the liquid phase. Simultaneously, the performance of our carbons are compared with
the behaviour of a commercial carbon (Norit SX plus). The experimental procedure consists in the
adsorption of one phenolic compound, followed by the regeneration of the adsorbent, succeeding a
series of adsorption/regeneration processes. The methods of regeneration used were simple washing
with distilled water under magnetic stirring, washing with ultrasound and thermal regeneration
under inert atmosphere. The performance of the activated carbons was strongly dependent on their
nature and the regeneration method applied.
Introduction
In the present days one of the greatest challenges to humanity is to diminish or remediate the
pollution of the earth’s environment produced by human activities [1]. In this context, phenolic
compounds represent one of the most important pollutants present in the environment. The major
problems are the contamination of surface and ground waters, with very strong implications for
human health. Fundamental sources of this pollution are petroleum refining and the plastics and ink
industries. In this perspective the adsorption of this type of compound represents one of the most
important and more efficient techniques used to retain the contamination [1-3].
Amongst the materials commonly employed as adsorbent, activated carbons are one of the most
used materials [3-5]. Their efficiency for applications in the liquid-phase is due to their high pore
volume and pore size distribution, conjugated with surface chemistry. Nevertheless, the
inconvenience of many carbon materials is their high cost. This disadvantage can be minimised by
the regeneration of the activated carbon allowing an increase in the number of times that it can be
used. Concerning these two aspects, it was decided to produce activated carbon form cork oak
wastes (precursor of low cost). By the use of physical activation or chemical activation it was
possible to achieve activated carbons with different structural and chemical characteristics. The
performance of these carbons, not only in the gas-phase but also in the liquid-phase, was found to be
similar to that of some commercial carbons [6-8]. Beyond those advantages and in a perspective of
their employment in real cases, it will be important to study the behaviour of some of these carbons
in successive applications.
An effective use of the adsorbent material in that case must be attached to a no less efficient
process of regeneration. The literature shows different ways to achieve this objective, but the most
common are thermal, chemical and biological treatments [9]. Others like microwave irradiation,
liquid water under subcritical conditions and ultrasound, are being tested in different laboratories
with differentiated results [10]. All of them have advantages and disadvantages, that depend on each
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publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 129.215.66.3-13/06/08,11:51:26)
Materials Science Forum Vols. 587-588
845
adsorbent-adsorptive system in concrete. These aspects comprise the energetic costs, the yield in
terms of recovered mass of activated carbon, the efficiency of each step of the regeneration process,
the structural and chemical changes caused during the process, amongst others.
Experimental
Materials. In this study we used a set of samples prepared from cork oak (Quercus suber L.) wastes
and a commercial carbon (Norit SX Plus, NSP). The activated carbons prepared in our laboratory
were activated by physical activation with carbon dioxide (CD) or steam (S). In a common first
stage the samples were carbonised in a flux of dry nitrogen, subsequently in a second step the
activation was made by switching the flux to CO2 (carbon dioxide activation) or bubbling the
nitrogen in water (steam activation). All the process is controlled by using a specific heating rate,
final temperature and time resident at this temperature. In chemical activation the precursor was
impregnated with phosphoric acid with one specific absolute concentration and mass ratio
(chemical/precursor). After impregnation the sample was pyrolysed in similar conditions to the
carbonisation step of physical activation. Subsequently, the material was washed for removing the
excess of impregnate and finally dried (C). The phenolic compounds used were p-nitrophenol
(PNP), p-clorophenol (PCP), p-cresol (PC) and phenol (P), all from Aldrich with purity greater then
99%.
Methods. All the samples were structurally and chemically characterised, by nitrogen adsorption at
77K, elemental analysis (C, H, N, O and S), FTIR spectroscopy and point of zero charge. The batch
and kinetic studies of adsorption were carried out at 25ºC and in acid pH medium. In these studies a
fixed quantity of each activated carbon was added to an aqueous solution of the mono-phenolic
compound (with specific concentrations). The Erlenmeyers were placed in thermostated shaker bath
for different times. The filtrate was analysed by UV/visible spectrophotometry at appropriate
wavelength (399, 298, 276 and 269nm for PNP, PCP, PC and P, respectively). The amount of
phenolic compound was fixed in appropriate conditions of pH that ensured only one of the forms of
the compound was present.
The regeneration process was made in one of three ways. Firstly by a simple wash with distilled
water, secondly by washing the samples with ultrasound and thirdly by carbonisation at 800ºC. For
the first situation the samples were washed at room temperature by simple contact between the
activated carbon post adsorption and distilled water. In the second case, the first procedure was
carried out with the use of ultrasound equipment. The experiments of thermal regeneration were
conducted in a horizontal furnace under nitrogen atmosphere. The thermal conditions were
10ºCmin-1, 800ºC and 30min of dwell time.
Results
All the samples gave nitrogen adsorption isotherms of type I (IUPAC classification) with some
variation in terms of pore volume, external surface area and micropore size. The elemental analysis
and point of zero charge also reflect differences between the activated carbons, namely the acid or
basic character and the content in carbon and oxygen. In Table 1 a summary of these results is
given. The PNP isotherms are characterised by a large increase in the amount adsorbed at low
concentrations, decreasing around an equilibrium concentration of 2mmoldm-3 (Fig. 1). For higher
concentrations a tendency to establish a plateau emerges. The kinetic studies for all the phenolic
compounds reveal a similar profile. As an example in the same Figure we show the kinetics of PNP
adsorption on different samples. The variation of the normalised concentration presents a tendency
to stabilise for a contact time superior to 24h. The initial concentration used in this test was
2mmoldm-3, which justifies a reduction to values near zero.
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Advanced Materials Forum IV
Table 1. Structural and chemical parameters of carbon samples obtained by different techniques (Asurface area, V-pore volume, L0-mean pore width; BET, S and 0 subscripts correspond to BET, αS
and DR methods, respectively). * Estimated value.
N2
Adsorption
Activated
Carbon
2 -
ABET/[m g
2 -1
3 -1
3 -1
Elementa
Analysi
l
s
PZC
AS/[m g ]
VS/[cm g ]
V0/[cm g ]
L0/[nm]
N/[%]
C/[%]
H/[%]
O/[%]
pH
1
]
NSP
833
299.0
0.24
0.31
1.06
0.4
76.1
0.4
15.2
7.2
CD
957
36.0
0.38
0.36
0.95
0.7
57.0
0.8
*(15)
10.0
S
660
64.8
0.26
0.25
0.74
0.9
79.8
0.5
4.6
10.2
C
618
96.1
0.27
0.22
1.92
0.2
74.1
2.0
*(20)
2.2
3,5
Based on the literature and on the kinetic
results we decided to do all the equilibrium
2,5
studies with an equilibrium time of one
week [11]. After this, some samples were
2,0
selected and we studied their behaviour in a
1,5
multiadsorption-regeneration process. The
1,0
regeneration methodology used in this
0,5
preliminary study was the simple wash of
activated carbon with distilled water.
0
0
5
10
15
20
25
30
It was possible to see distinct and variable
Ceq / mmol dm
adsorption capacities for the activated
Fig. 1. Experimental adsorption isotherms and carbons in that process. In a global analysis,
there was a decrease in adsorption capacity
kinetics of adsorption on activated carbons.
around 20% and 40% after the first and
second regeneration phase, respectively. In general the best results emerged with phenol and pchlorophenol adsorbates. In particular p-chlorophenol shows a very regular behaviour and uniform
adsorption capacities for all the samples In terms of samples, the activated carbons prepared by
physical activation by steam and CO2 show the best values. Contrarily, the highest decrease in
adsorption capacity is observed with the sample activated with phosphoric acid. The adsorbentadsorbate pairs of phenol or p-cresol with the steam activated carbon deserve prominence by the
high values of adsorption along the process.
Based on these preliminary results we started a new analysis of multiadsorption-regeneration
processing. Others methods were tested with the aim of improving the process of regeneration.
These were the washing regeneration with ultrasounds and the thermal regeneration in inert
atmosphere. On the other hand, one commercial carbon was introduced in this new study. With this
carbon from Norit it was possible to compare the behaviour of our carbons not only in equilibrium
studies but also in the regeneration process. A detailed observation of Fig. 1 shows that the kinetic
process of adsorption takes place fundamentally in the first 24h. Knowing this, we changed the
batch adsorption process to 1 day instead of one week. Based on this new procedure we advanced to
tests with the adsorptive that presented the worst results.
The results of the second study present large differences when compared to the first regeneration
experiments involving one week for equilibrium adsorption time. The achieved values for all the
adsorbent-adsorbate pairs were highest. Sometimes this superiority was expressed in excellent
results that exceed the initial adsorption capacities reached in the first adsorption of the adsorptionregeneration cycle. This improvement in the amount adsorbed was also noted by others authors [12,
CD
S
NSP
C
nads / mmol g
-1
3,0
1,0
0,8
C /C0
0,6
0,4
C
0,2
CD
S
0,0
0
200
400
600
t / m in
-3
6000
12000
Materials Science Forum Vols. 587-588
847
1,0
1,0
0,8
0,8
Relative adsorption
Relative adsorption
13]. In Fig. 2 is possible to see that the best results were obtained by washing the carbons with
ultrasounds.
0,6
0,4
Adsorption
1ª
2ª
3ª
0,2
0,0
Ultrasounds
Water
0,6
0,4
Adsorption
1ª
2ª
3ª
0,2
0,0
Thermal
Ultrasounds
Regeneration Method
1,0
1,0
0,8
0,8
0,6
0,4
Adsorption
1ª
2ª
3ª
0,0
Ultrasounds
Water
Thermal
b)
Relative adsorption
Relative adsorption
a)
0,2
Water
Regeneration Method
0,6
0,4
Adsorption
1ª
2ª
3ª
0,2
0,0
Thermal
Ultrasounds
Regeneration Method
Water
Thermal
Regeneration Method
c)
d)
Fig. 2. Relative adsorption capacity for samples NSP, C, CD and S, respectively in a), b), c) and d),
before and after regeneration by different methods.
Probably these better results were achieved due to the energetic mechanical action of these
vibrations. In the end of each process of regeneration, especially in the first, a reduction of the
original particles to others of smaller size was noted. All these factors can explain the increase in the
adsorption amount and the stabilisation achieved in the next step of the cycle. From all the activated
carbons the commercial carbon from Norit presented a more uniform and stable behaviour. The
analysis of the carbons prepared in our laboratory show a more consistent behaviour for the carbon
activated with CO2. Otherwise, the sample activated by steam presents the larger reduction in the
adsorption capacity, with exception for the regeneration by ultrasounds for the reasons we presented
above. With the chemically activated carbon the adsorbed quantity increased from the first to the
second adsorption and decreased always in the third, to similar values to the ones achieved in the
initial stage.
Transmittance / %
Transmittance / %
original
1ª ads
2ª ads
original
1ª ads
2ª ads
3ª ads
2000
3ª ads
1500
1000
-1
Wavenumber / cm
500
2000
1500
1000
500
-1
Wavenumber / cm
Fig. 3. FTIR spectra of samples C e S before and after regeneration by ultrasounds and water,
respectively
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Advanced Materials Forum IV
The infrared spectra show a similar profile between the regenerated carbon and the original
sample. Some differences emerge only with the chemical activated carbon. From this analysis we
can ensure with some confidence that the reduction in the adsorption capacity of the different
adsorbents must be due not to the retention of the phenolic compounds but to structural changes of
the carbon.
Conclusion
In this work we show that the regeneration performance of some of the activated carbons prepared
from cork oak wastes are similar and sometimes superior to that of the most used commercial
carbon. Furthermore, an improvement of their behaviour was achieved when the contact time
between the phenolic compound and the carbon was diminished from one week to one day. The
carbons prepared by physical activation present the best results and the chemical activated the worst.
The preliminary results indicate a reduction of amount adsorbed due to structural changes and not to
the retention of phenolic compounds in appreciable quantity on the adsorbent. With some
confidence it is possible to suggest a dominant physical adsorption in our systems, with exception to
the sample activated with phosphoric acid. In this moment we are doing more structural and
chemical studies to explore in a detailed way these first conclusions.
Acknowledgements
This work was supported by the FCT with national and European community funds (Project No.
POCTI/CTM/38255/2001).
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