Journal of Colloid and Interface Science 257 (2003) 173–178
www.elsevier.com/locate/jcis
Oxidation of activated carbon: application to vinegar decolorization
Francisco López,a,∗ Francisco Medina,b Marin Prodanov,c and Carme Güell a
a Departament d’Enginyeria Química, Unitat d’Enologia del CeRTA (Generalitat de Catalunya), Facultat d’Enologia,
Universitat Rovira i Virgili, Ramón y Cajal 70, 43005 Tarragona, Spain
b Departament d’Enginyeria Química, ETSEQ, Universitat Rovira i Virgili, Av. Païssos Catalans 26, 43007 Tarragona, Spain
c Departamento de Ciencia y Tecnología Agroforestal, ETSIA, Universidad de Castilla-La Mancha, 02071 Albacete, Spain
Received 5 December 2001; accepted 22 October 2002
Abstract
This article reports studies on the feasibility of increasing the decoloring capacity of a granular activated carbon (GAC) by using oxidation
with air at 350 ◦ C to modify its surface activity and porosity. The GAC, obtained from olive stones, had a maximum decolorization capacity
of 92% for doses of 20 g/l, while the maximum decolorization capacity of the modified granular activated carbon (MGAC) was about 96%
at a dose of 10 g/l. The increase in decoloring capacity is thought to be due to an increase in mesopore area (from 129 to 340 m2 /g) in the
MGAC. The maximum decoloring values and the doses needed to attain them are very close to values obtained in previous studies using
coconut shell powder-activated carbon (94 and 98% for red and white vinegar for a dose of 10 g/l, respectively).
 2003 Elsevier Science (USA). All rights reserved.
Keywords: Vinegar; Decolorization; Activated carbon; Adsorption; Oxidation
1. Introduction
In the production of some foods (molasses, fruit juices,
vinegars, oils) the phenolic compounds must be removed
[1–5]. These phenolic compounds are responsible for the
color, the astringency, and the bitter taste, but in high
concentrations they can produce oxidative reactions that
diminish the initial quality [6]. The phenolic compounds
must sometimes be removed to increase the stability of some
foods and, therefore, their shelf life.
It is common practice in the vinegar industry to decolor
a fraction of vinegar and blend it with colored vinegar to
obtain a standard final product of the same characteristics
and quality.
The method used for vinegar decolorization consists of
mixing powder-activated carbon (PAC) with the vinegar,
stirring, and then separating the activated carbon by filtration
or settling. This process is a semicontinuous operation and
small producers generally separate the activated carbon by
settling, which means that considerable amounts of vinegar
are lost and solid residue is generated. The settling process
normally takes about 48 h. In the next step the vinegar is
* Corresponding author.
E-mail address: [email protected] (F. López).
filtered to remove the remaining residues of the activated
carbon. The decoloring process uses as much as 10–20 g/l
PAC, which increases production costs. Achaerandio et
al. [7] obtained a maximum decolorization of 94 and 98%
for red and white vinegar, respectively using coconut shell
powder-activated carbon at minimum doses of 10 g/l.
Powder-activated carbon is used because its large surface area is effective at adsorbing large molecules and it
also reduces diffusional problems. On the other hand, diffusional problems restrict the use of granular activated carbon
(GAC). Nevertheless, activated carbon is a porous material
that is used for its structural properties and surface functional
groups. Carbons can be oxidized by heat treatment in carbon
dioxide [8–11], steam [11–15], or air [8,14,16–18]. The oxidation can also be wet using hydrogen peroxide [13,16,18,
19], nitric acid [12,13,19–25], or phosphoric acid [26].
The main aim of oxidizing an activated carbon (AC) is
to modify its chemical characteristics. If the AC is oxidized
with nitric or phosphoric acid, a proportion of the final
oxygen content of the AC can be attributed to surface
phosphate or nitro groups [25,26]. If functional groups are
inserted into the ACs the application of these modified
carbons to the treatment of food may be limited.
It is generally accepted that oxidation of carbons produces oxygen-containing functional groups at the edge sites
0021-9797/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S0021-9797(02)00040-1
174
F. López et al. / Journal of Colloid and Interface Science 257 (2003) 173–178
of the graphitic planes. Furthermore the surface oxygen
functional groups are mainly at the entrance to micropores [27,29]. It is well known that the adsorption capacity
of ACs is related to their functionality.
The adsorption of phenol on oxidized activated carbons
is considerably lower than on nonoxidized activated carbons
[17,21,23,24,28,29]. This is because the presence of oxygen
surface groups can hinder the adsorption of phenol under
conditions in which the phenol molecules are thought to
be adsorbed in a planar position on the basal planes. It is
proposed that aromatic compounds with a functional group
capable of H bonding, such as phenol, do so with carboxylic
and carbonyl oxygen groups on the surface. Then, increasing
surface oxygen content leads to a higher adsorption capacity
for phenol since the number of sites available for H bonding
is increased [29]. This is in agreement with Teng and Hsieh’s
[10] results, which show an increase in phenol adsorption
with an oxidized AC.
Nevskaia and Guerrero-Ruíz [24] found that the adsorption of nonylphenol is greater because a large molecule is
hydrogen-bonded to the oxidized carbon surface by means
of acidic groups. The nonyl group is rather voluminous
and probably does not allow phenol ring interactions with
the carbon surface because of steric hindrance. Tamon and
Okazaki [28] observed hysteresis of adsorption and desorption in the low concentration of different adsorbates. This
hysteresis suggested irreversible adsorption in aqueous solution. On the other hand, aromatic compounds with electronattracting groups showed no hysteresis and the adsorption
was reversible. The influence of acidic surface oxides on
the desorption characteristics of nitrobenzene and phenol
was experimentally studied with activated carbon [28]. Although the amounts of nitrobenzene and phenol adsorbed decreased due to wet oxidation, the desorption characteristics
improved.
Such adsorption conditions as pH, temperature, and ionic
strength can affect the adsorption process between the
adsorbate and the activated carbon [10,26,29–31].
The second aim of oxidizing AC is to modify its structural
characteristics, that is, BET surface area, pore volume, and
pore distribution. There are many articles in the literature on
how different treatments affect the physical characteristics
of activated carbon. The oxidation of activated carbon with
steam increases BET surface area, external surface area,
total pore volume, and micropore volume [14,15]. In the
wet oxidation of AC, BET surface area normally decreases
and pore diameter normally increases, while micropore
volume increases or decreases depending on the acid used
for the treatment [13,18,24]. Furthermore, oxidation with
air increases BET surface area, pore diameter, micropore
volume, and, more clearly, pore size distribution [13,14,18,
29]. As a result of the treatment with air more mesopores,
and many more micropores, are generated on the activated
carbon.
In a previous study, Achaerandio et al. [32] oxidized a
GAC (coconut shell) with an air/Ar mixture. This oxidized
GAC was used to decolor wine vinegar and considerably
more color was removed than when untreated GAC was
used (86 and 14%, respectively). The aim of this article is
to study the feasibility of obtaining a new modified GAC by
air oxidation that is capable of removing color from vinegar
as efficiently as powder-activated carbon. The decolorization
capacity of ACs is related to porosity, superficial area, and
functionality.
2. Materials and methods
2.1. Materials
Granular activated carbons (1–2 mm), obtained from
olive stones, were provided by Productos Agrovin S.A.,
Alcázar de San Juan, Ciudad-Real (Spain). Badia Vinagres
S.L., Mollerusa (Spain) supplied the white wine vinegar.
2.2. Vinegar characterization
The color of the vinegar samples was characterized
with modified color intensity (MCI), which is the sum of
the absorbances at wavelengths of 420, 520, and 620 nm
(1-cm pass). These wavelengths were measured using a
Hitachi U-2000 spectrophotometer. To normalize the values
for vinegar, decoloring efficiency (DE) was defined as:
DE = 100(MCI0 − MCI)/MCI0, where MCI0 is the MCI of
vinegar before it is treated in batch studies.
2.3. Batch experiments
The performance of the activated carbons was evaluated
in batch experiments, which were conducted at room temperature. In these experiments 20-ml volumes of vinegar
were mixed with different amounts of activated carbon. The
amber flasks containing the vinegar and the activated carbon
were placed in a shaker and agitated for 72 h. Before analysis, the samples containing the activated carbon and vinegar
were filtered through a 0.45-µm cellulose acetate Millipore
membrane. Control samples were run using the same amount
of vinegar without activated carbon. All experiments were
run in triplicate.
2.4. Activated carbon modifications
Modified granular activated carbon (MGAC) was prepared by placing 10 g of GAC in a tubular quartz glass reactor under an air flow of 5 ml/s. It was heated at a rate of
5 ◦ C/min up to 350 ◦ C, and this temperature was then kept
constant for 4 or 8 h. We refer to the modified granular activated carbons as MGAC-4h and MGAC-8h for the 4- and
8-h treatments, respectively.
F. López et al. / Journal of Colloid and Interface Science 257 (2003) 173–178
175
2.5. Determination of specific surface areas and pore-size
distributions
BET specific surface areas were determined by nitrogen
adsorption at 77 K with a Micromeritics ASAP 2000 surface
analyzer and taking a value of 0.162 nm2 for the cross
sectional area of the nitrogen adsorbed molecule. The t-plot
approach was used to determine micropore volumes and
external area. The accuracy of these measurements was 5%.
The same equipment was used to calculate the distribution of
pore sizes using Barrett, Joyner, and Halenda’s method [33].
2.6. FTIR measurements
FTIR measurements were made with a Bruker Equinox
55, with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) MCT detection. FTIR spectra were
recorded in the range 400–4000 cm−1 , at a resolution of
4 cm−1 and with 200 scans per sample. Activated carbon
was placed in a microsample holder. A previously recorded
background spectrum was subtracted automatically from the
spectrum of each sample.
Fig. 1. N2 adsorption isotherms at 77 K on activated carbons. Solid symbols: adsorption; open symbols: desorption.
3. Results and discussion
3.1. Textural characteristics of activated carbon
The characteristics of the activated carbon samples used
in this study are given in Table 1. The oxidation of GAC
increases BET and mesopore surface areas because micropores are transformed to mesopores. Micropore surface area
decreases, but less drastically than the mesopore surface
area increases. This may be because closed pores open, the
existing micropores become wider or deeper [8,9], and new
micropores are formed. At the same time the mesopore is
formed as a result of micropore opening.
The adsorption isotherms for nitrogen on the three activated carbons are given in Fig. 1. The isotherms are essentially Type IIa in the IUPAC classification. The oxidation
treatment increases the hysteresis, indicating a more heterogeneous pore size distribution.
Because of its high reactivity, oxygen tends to react at the
entrance of the pores and does not penetrate into the narrow pores. As a consequence, oxidizing carbonized material
with air produces an oxidized carbon with a predominantly
macroporous structure. Petrov et al. [14] managed to in-
crease BET surface area by oxidizing with air. They also reported an increase in pore volume (micro, meso, and macro).
Their treatment was less drastic (350 ◦ C, air flow rate 8 l/h
for 1 h). Mangun et al. [18] obtained an increase in micropore size and micropore volume by oxidation in air (400 ◦ C
for 1 h), but, for the same GAC, aqueous oxidation with sulfuric acid, nitric acid, and hydrogen peroxide had very little
effect on average micropore size and decreased micropore
volume and BET surface area. Achaerandio et al. [32] also
obtained an increase of nearly 100% in BET surface area
with a coconut shell and a treatment of 2 h at 350 ◦ C and an
air flow rate of 18 l/h. They also reported that the formation
of micropores increased and that pore volume was larger.
For treatments under 2 h micropores did not transform into
mesopores.
In the present study, oxidation times were higher and
mesopore surface area increased notably (87 and 165% for 4
and 8 h of oxidation, respectively). This increase represents
increased accessibility of vinegar polyphenols to the active
point in GAC and, consequently, its adsorption capacity.
The oxidation treatment led to an increase in average pore
diameter and a decrease in micropore volume due to the
Table 1
Characteristics of the activated carbons
Activated
carbon
GAC
MGAC-4h
MGAC-8h
BET area
Meso area
(m2 /g)
(m2 /g)
791
873
915
129
240
340
Micro area
(%)
(m2 /g)
16.3
27.5
37.2
662
633
574
Micropore volume
Mesopore volume
Total volume
Average pore
(%)
(cm3 /g)
(cm3 /g)
(cm3 /g)
diameter (nm)
83.7
72.5
62.8
0.308
0.291
0.261
0.074
0.155
0.220
0.401
0.468
0.509
2.0
2.1
2.2
176
F. López et al. / Journal of Colloid and Interface Science 257 (2003) 173–178
Fig. 2. FTIR spectra for granular activated carbon (GAC) and modified
granular activated carbon (MGAC-8h).
increase in the porosity of activated carbon and the increase
in mesopore volume.
3.2. Chemical functionality of activated carbon
To determine the changes in functionality of the MGAC
and GAC, we recorded and compared the FTIR spectra.
The FTIR spectra summarized in Fig. 2 show three main
bands. The band between 1550 and 1750 cm−1 was assigned to free carboxyl groups, esters, lactone groups, and
carbonyl groups near the hydroxyl groups [13,34,35]. The
oxidation treatment leads to an increase in the intensity of
this band centered at about 1610 cm−1 . Moreno-Castilla
et al. [13] in the region 1860–1650 cm−1 obtained bands
more pronounced with the oxidized carbons than the original ones. Some functional groups identified were lactone,
carbonyl, and carboxylic groups. This increase is larger for
MGAC-8h. Between 1400 and 1530 cm−1 , functionality decreased with oxidation treatment. This band can be assigned
to carboxylic groups [36,37]. In the band between 1200 and
1400 cm−1 oxidation treatment increased functionality. This
band shows the presence of lactones, quinones, and/or carbonyl groups near hydroxyl groups due to the transformation
of carboxylic groups [36,37]. The intensity of both bands
increased because carboxylic groups in the band between
1400 and 1530 cm−1 transformed into conjugated ketones
and quinones, which led to a decrease in the band [36]. The
same tendency was observed by Achaerandio et al. [32] with
a coconut shell-activated carbon and Petrov et al. [14] with
activated carbon obtained from furfural.
3.3. Adsorption equilibrium
Figure 3 shows the decoloring efficiency (DE) of granulated activated carbons (modified and nonmodified) for
white vinegar. Each point is the average of three repeated experiments and the error bars are the standard deviations. The
lines are the best fit to the experimental points. The max-
Fig. 3. White vinegar adsorption isotherm on activated carbons.
imum DE for GAC was 92%, while those for MGAC-4h
and MGAC-8h were 94 and 96%, respectively. The minimum doses of MGAC-8h and GAC required for the maximum DE to be reached were approximately 10 and 20 g/l,
respectively. A dose of 10 g/l MGAC-8h means that it is
possible to run 1 l of vinegar with 10 g of MGAC-8h and
obtain a DE of 96%.
The Freundlich equation is an empirical one employed to
describe the isotherm data by
1/n
qe = KF Ce ,
were KF and n are empirical constants that determine the
curvature and steepness of the isotherm.
The experimental data were fitted with the Freundlich
model, and the parameters are listed in Table 2. It can be seen
that 1/n is less than unity, revealing that significant adsorption takes place at low concentrations, being notably smaller
with MGAC-8h, indicating small amounts of this adsorbent
are necessary to reach the maximum decolorization. The parameter KF is a measure of the adsorptive capacity of the
adsorbent. Table 2 shows that the values of KF increase with
oxidation treatment, reflecting the fact that the surface area
and/or functionality are related to KF .
Therefore, the treatment applied to modify the GAC
and obtain the MGAC increased DE from 92 to 96%.
However, more important than this is the decrease in the total
amount of activated carbon needed to reach the maximum
decolorization, because it means that the decolorization
capacity of activated carbon is doubled.
Franz et al. [29] concluded that surface oxygen groups,
particularly carboxylic groups, adsorb water, creating water
clusters through H bonding, which reduces accessibility and
affinity to aromatic adsorbates and, thus, reduces adsorption
capacity. Oxygen groups can enhance adsorption capacity
in the absence of water, by forming H bonds with the
aromatics. These authors, in their study of adsorption at
pH 11.6, say that the majority of the carboxylic surface
oxygen groups exist in the dissociated carboxylate form
(COO− ), and water adsorption is increased, resulting in a
F. López et al. / Journal of Colloid and Interface Science 257 (2003) 173–178
177
Table 2
Parameters of the Freundlich equation of the activated carbons
Carbon
KF
1/n
r
GAC
MGAC-4h
MGAC-8h
36.7
43.3
65.5
0.259
0.228
0.106
0.930
0.936
0.975
lower adsorption capacity with an oxidized carbon, while
at an acid pH 3, the capacity decreases less with surface
oxygenation, because a smaller amount of water adsorption
is expected, since the carboxylic groups on the surface are
essentially in neutral form.
Vinegar is a product with a pH between 2.5 and 3.0 for
60 g/l acetic acid (commercial form) and about 2.0 for
100 g/l acetic acid (most common result of industrial production). The low pH of vinegar diminishes water adsorption according to Franz et al. [29], and favors the adsorption of polyphenols with surface oxygen groups of activated
carbons. This fact could explain the increase in capacity of
MGAC-8h with respect to GAC. However, in our previous
study using carbon from coconut shell [32] the increase in
decolorization capacity for vinegar was notably higher. Carbon obtained from olive stone (used in the present study)
does not show such a high increase in decolorization capacity, mainly due to the fact that the initial carbon (before oxidation) was fairly acidic, as can be seen by its FTIR
(Fig. 2). The adsorption capacity increases in the present
work mainly because the surface area of the GAC also increases, from 791 to 915 m2 /g. It should be pointed out
that the surface area of the mesopores increases by about
150% (from 129 to 340 m2 /g), which means that the oxidation is responsible for the microporous material changing to
a mesoporous material (see Table 1). This change decreases
the diffusional problems and increases the effective area for
polyphenol adsorption. The main advantage of using oxidation instead of nitric or phosphoric acid is that functional
groups such as nitrates and phosphates are not introduced
into the carbon matrix, which may contaminate the vinegar
and make it impossible to use this type of modified carbon
commercially.
The FTIR spectra in Fig. 4 are for MGAC-8h before
and after the vinegar decolorization process. It can be seen
that the intensity of the band between 1550 and 1750 cm−1
decreases. This band is assigned to the stretching vibration
of carboxyl groups on the edges of the layer planes [38]
or to conjugated carbonyl groups. This indicates that the
carboxyl and conjugated carbonyl groups participate in the
interaction between the polyphenols and the activated carbon
by forming oxygen complexes. The band around 1300 cm−1
also diminishes (Fig. 4). This band was attributed to C–O
single bonds, such as those in ethers, phenols, and hydroxyl
groups and the decrease can also be assigned to hydrogen
bonds between these groups and polyphenols from vinegar.
Finally, it should be pointed out that there is a new band
in the range 1750–1800 cm−1 because the formation of
hydrogen bonds weakens and displaces the carbonyl bond.
Fig. 4. FTIR spectra for modified granular activated carbon (MGAC-8h)
and used modified granular activated carbon (MGAC-8h, used).
4. Conclusions
The oxidation treatment applied to the granular activated
carbon from olive stones increased BET surface area. This
increase is mainly the result of an increase in mesopore area
and average pore diameter, which in turn makes it easier for
the compounds to access the active sites. The functionality
of the modified granular activated carbon also increased.
The decoloring efficiency of the MGAC (96%) is similar
to that of the powder-activated carbon (98%), which is the
adsorbent that is most widely used in the vinegar industry.
This makes it possible to modify the decolorization process
currently carried out in the vinegar industry by transforming
the batch process into a continuous one using packed
columns.
Acknowledgments
We are grateful to Badia Vinagres S.L. (Mollerussa,
Spain), Productos Agrovin S.A. (Alcázar de San Juan,
Spain), and the MCYT (AGL2000-0237-P4-04).
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