EPA/600/A-94/038
601 Wythe Street, Alexandria VA 22314
Water Environment Federation
65th Annual Conference & Exposition
New Orleans, Louisiana
September 20-24,1992
Oxidative Coupling of Phenolics on the
GAC Surface
R.D. Vidic, M.T. Suidan, R.C. Brenner
AC92-002-001

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OXIDATIVE COUPLING OF PHENOLICS ON THE GAC SURFACE
Radisav D. Vidic, Research Assistant
Makram T. Suidan, Professor
University of Cincinnati, Cincinnati, OH 45221-0071
Richard C. Brenner, Environmental Engineer
RREL, U.S. EPA, Cincinnati, OH 45268
ABSTRACT
Previously reported results by the authors revealed that the presence of molecular oxygen
(oxic conditions) in the test environment can, in some instances, cause an almost threefold
increase in the adsorptive capacity of granular activated carbon (GAC) for phenolic compounds.
It was discovered that these compounds undergo oxidative coupling on the carbon surface under
oxic conditions. The polymers formed as a result of these chemical reactions are very difficult
to desorb from the surface of GAC. This led to significant irreversible adsorption in the
presence of molecular oxygen. On the other hand, when the same compounds are adsorbed on
the carbon surface under anoxic conditions, essentially all of the adsorbate can be recovered from
the carbon surface by solvent extraction.
The ionized species of phenolic compounds showed even higher susceptibility towards
polymerization on the surface of GAC than the parent neutral molecules. GAC particle size did
not influence the extent of polymerization. Oxygen uptake measurements revealed significant
consumption of molecular oxygen during the adsorption of phenolic compounds. The amount
of molecular oxygen consumed in these experiments was found to be linearly proportional to the
amount of irreversibly adsorbed compound. This study also showed that some phenolic
compounds undergo oxidative coupling in the presence of molecular oxygen even in the absence
of the catalytic GAC surface.
KEY WORDS
Adsorption, Activated Carbon, Phenols, Oxidative Coupling, Polymers
INTRODUCTION
Presence of low concentrations of various refractory compounds can be a major obstacle
to the use and reuse of water streams. Phenolic compounds can cause objectionable taste and
odor problems in drinking water and can exert adverse effects on various biological treatment
processes. Some phenolic compounds originate from natural sources while others are
manufactured. The urine of some animals and decay of vegetation release phenol to water
bodies. Several industrial sources such as coal gasification sites, coke-ovens, oil refineries, town
gas sites, and petrochemical units, generate large quantities of phenolic compounds that can, if
improperly managed, cause long-term contamination of both surface and groundwater bodies.
Several treatment methods for the removal of phenolic compounds that have been
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investigated in the past include both aerobic (Adams, 1974) and anaerobic biodegradation
(Suidan , 1983), chemical oxidation with ozone (Gould and Weber, 1976; Chrostowski et aL,
1983), adsorption by ion exchange resin (Kim et aL, 1976; van Vliet et aL, 1981), and activated
carbon (Weber and Morris, 1964; Coughlin and Ezra, 1968; Snoeyink et aL, 1969; Zogorski et aL,
1976; Crittenden and Weber, 1978; Peel and Benedek, 1980; Seidel et aL, 1985; Magne and
Walker, 1986).
One of the key parameters in designing the activated carbon adsorption process for the
removal of organic compounds is the capacity of carbon for the retention of the compounds of
interest. This capacity is usually expressed in terms of adsorption isotherms, which represent a
relationship between the concentration of the organic compound in the aqueous phase and the
mass of that compound on the carbon surface that is in equilibrium with the aqueous phase
concentration. The experimental protocol widely used for obtaining adsorption equilibrium data
is the bottle point technique. Some of the parameters that affect the adsorption equilibrium
determined from this test are discussed by Martin and Al-Bahrani (1978) and Randtke and
Snoeyink (1983). Due to the lack of a unified procedure for conducting this test, many different
adsorption isotherms for the same adsorbent-adsorbate pair can be found in the literature (Peel
and Benedek, 1980).
Recent studies by Vidic et aL (1990) and Vidic and Suidan (1991) revealed that the
adsorptive capacity of granular activated carbon (GAC) for several phenolic compounds is highly
influenced by the presence of molecular oxygen (oxic conditions) in the test environment. The
GAC adsorptive capacity for o-cresol that was attainable under oxic conditions was as much as
200% above that obtained in the absence of molecular oxygen (Figure 1). A similar phenomenon
was demonstrated for the adsorption of phenol, 2-chlorophenol, and 3-ethylphenol as well as
natural organic matter (Vidic and Suidan, 1991).
o oxic
• anoxic
10°
C, mg/L
Figure 1. Adsorption Isotherms for o-Cresol
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Extraction of the carbon preloaded with o-cresol revealed that almost 100% of the
originally adsorbed compound can be recovered from the surface of GAC by solvent extraction
if the adsorption was carried out in the absence of molecular oxygen (Figure 2). Conversely, only
10-30% of the adsorbed compound was recovered from GAC loaded under oxic conditions
(Figure 2). Similar behavior was also documented for phenol and 3-ethylphenol (Vidic and
Suidan, 1991).
This study was designed to further evaluate the role of molecular oxygen on the adsorption
of phenolic compounds by activated carbon and to provide possible explanation for the observed
phenomena.

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Figure 2. Extraction Efficiency for GAC Used in the Isotherm Tests with o-Cresol
METHODOLOGY
All adsorption experiments were performed at pH 7.0 using autoclaved Milli-Q water
(deionized water passed through Millipore purification system, Millipore Corp., Bedford, MA)
to prepare the adsorbate solutions. Water was buffered with 0.01M phosphate buffer, and the
pH was adjusted with a 10M NaOH solution. The adsorbent used in this study was 16x20 U.S.
Mesh Size Filtrasorb 400 (Calgon Carbon Corp., Pittsburgh, PA). Prior to use, carbon was
thoroughly washed with Milli-Q water, dried at 105°C, and stored in a desiccator until use. All
the experiments described in this study were performed using the same batch of GAC.
Adsorbates used in this study were phenol; 2-, 3-, and 4-methylphenoI; 2-ethylphenol; 2-
and4-chlorophenol;4-hydroxybenzoicacid;2,4-dimethylphenol;2,4,6-trichlorophenol;aniline;and
3

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p-anisidine. All the adsorbates were reagent grade or better. Adsorbate concentration
measurements were performed on a Hewlett Packard (HP) 8452 Diode-Array Spectrophotometer
(Hewlett-Packard Co., Palo Alto, CA) using both 1-cm and 5-cm quartz cells. The extracts from
the GAC used in the adsorption isotherm tests were analyzed using an HP 5890A Gas
Chromatograph equipped with a DB-1 30-m fused silica capillary column (J&W Scientific,
Folsom, CA) and a flame ionization detector (FID). Gas Chromatography/Mass Spectroscopy
(GC/MS) analyses were performed on an HP 5985A GC/MS using the electron-impact positive
ion mode. The gas chromatograph was equipped with a DB-1 30-m fused silica capillary column
(J&W Scientific, Folsom, CA). The oven temperature was programmed from 40-280°C at
10°C/min with a 5 min hold at 40°C.
Adsorption Equilibrium: Detailed description of the experimental protocol used to
evaluate GAC adsorptive capacity in the presence and absence of molecular oxygen is provided
elsewhere (Vidic and Suidan, 1991) and will not be repeated here.
GAC Extraction: GAC samples from the adsorption isotherm bottles were first separated
from the liquid phase by filtering through 0.45-/im nylon filters (Micron Separations, Inc.,
Westboro, MA) and transferred into cellulose extraction thimbles (Whatman International, Ltd.,
Maidstone, England). The thimbles were then placed into a soxhlet extractor apparatus
minimizing any contact with ambient air. The extraction was first carried out for one day using
methanol (Fisher Scientific, Fair Lawn, NJ) as a solvent. Extraction was then continued for an
additional three days with methylene chloride (Fisher Scientific, Fair Lawn, NJ) since it allows
for the extraction to be performed at a lower temperature, thus minimizing alteration of the
organics due to excessive heat. After the extraction phase, methylene chloride was allowed to
evaporate from the round bottom flasks and the extract, dissolved in methanol, was transferred
into a volumetric flask where the volume was complemented to 100 mi with fresh methanol.
Oxygen Uptake Measurements: The amount of molecular oxygen consumed during the
adsorption of organic compounds was measured using a computerized respirometer (N-CON
Comput-OX, Model WB512, N-CON Systems Co., Inc., NY). GAC was first wetted in air
saturated buffered water with-DO concentration of 8J mg/L. GAC was then transferred
together with the buffered water to a 500-mL respirometer bottle where it was complemented
to a total volume of 400 mL with air saturated buffered water that contained known
concentrations of the organic compound to be tested. The bottle was then connected to the
oxygen supply system that maintains a constant partial pressure of oxygen in the bottle headspace.
Data on the amount of oxygen supplied to the bottle were automatically collected and stored in
the computer. These experiments were carried out for a period of at least two weeks, which was
the equilibration period for most of the adsorption isotherm tests.
RESULTS AND DISCUSSION
GC/MS analyses performed on the extracts from the GAC used in the oxic isotherm tests
with o-cresol revealed the presence of appreciable amounts of dimers, trimers, and even
tetramers of o-cresol. Therefore, a reasonable conclusion is that the increase in the GAC
adsorptive capacity under oxic conditions is a result of adsorbate polymerization on the carbon
surface. It is generally believed that the mechanism of phenolic compounds coupling involves
4

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radicals of these compounds (Musso, 1967 and Nonhebel and Walton, 1974), although some
researchers proposed that nonradical oxidative coupling of phenolics can also occur (McDonald
and Hamilton, 1973). The first step in the oxidative coupling of phenolics is usually the
formation of phenoxy radicals from the phenol molecule or phenolate ion. The radical formation
is generally initiated by the removal of a hydrogen atom from each phenolic molecule to form
phenoxy radicals. These radicals can then participate in direct coupling with other radicals,
homolytic aromatic substitution with phenol molecules, or heterolytic coupling with phenolate
ions to form dimers (McDonald and Hamilton, 1973). Electron localization in the radicals
determines the coupling position (ortho or para position to hydroxyl group). Coupling is
predominantly achieved through carbon-carbon bonding and less frequently through carbon-
oxygen bonding (Musso, 1967).
Several studies have shown that molecular oxygen can act as an initiator in oxidative
coupling reactions of phenols (Denisov, 1980). Molecular oxygen can react directly with phenol
according to the following reaction (Shibaeva et al., 1969):
(1)
PhOH + 02 - PhO + H02
In addition, the phenolate ion can also react with oxygen:
PhOH ** PHO- + H*
PhO' + 02 - PhO + 02	(2)
0~2 + H* - H02
The radicals that are formed according to Equations (1) or (2) can react with another phenol
molecule according to the following equation:
(3)
PhOH * H02 - PhO + H202
Hydrogen peroxide reacts with another phenol molecule according to the following equations:
PhOH + H202 - PHO * H20 + HO	(4)
PhOH * HO - PhO ~ H20
The above reactions were demonstrated to take place at elevated pressures (35 atm) and
temperatures (180-210°C) and pressures (35 atm) (Shibaeva et ai, 1969) indicating a high
activation energy of radical formation while Hay et al (1959) reported oxidative coupling to take
place at room temperature in the presence of copper (I) salt. The results of this study indicate
that such reactions are also feasible at room temperatures with the surface of GAC serving as
a catalyst.
The results of the extraction experiments (Figure 2) indicate that o-cresol polymers are
5

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difficult to desorb from the surface of GAC since a very low extraction efficiency was achieved
for the carbons used in the oxic isotherm procedure. Weight measurements conducted on these
carbons after extraction and drying confirmed that essentially all of the non-desorbed adsorbate
is still retained on the carbon surface. The amount of irreversibly adsorbed o-cresol was
calculated from the data collected from the oxic adsorption isotherm tests and extraction
experiments. The results are given in Figure 3 together with oxic and anoxic Freundlich
adsorption isotherms for o-cresol. It is important to note that the amount of irreversibly
adsorbed o-cresol remained essentially constant over a three orders of magnitude range of
aqueous phase concentrations. This indicates that the capacity of GAC to promote the chemical
reaction and, consequently, the capacity of GAC for the products of that reaction are practically
constant.
Oxygen uptake measurements conducted using an aerobic respirometer were always
performed on triplicate samples to obtain statistically significant results. The oxygen consumption
rate measured for the adsorption system with the initial o-cresol concentration of 1957.7 mg/L
and 2.0 g of GAC is depicted in Figure 4. The total amount of oxygen consumed during the
equilibration period ranged from 82.8 to 88.0 mg. Based on the equilibrium liquid phase
concentration measurements and the data presented in Figure 3, the corresponding amount of
irreversibly adsorbed o-cresol ranged from 532.2 to 535.8 mg. The results of this test together
with several other tests involving different masses of GAC and different initial o-cresol
concentrations are summarized in Figure 5. The results of similar experiments performed with
2-ethylphenol and 2-chlorophenol are presented in Figures 6 and 7, respectively.
3
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2
10
reversible
irreversible
i
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10
10
c. rng/g
Figure 3. Reversibly and Irreversibly Adsorbed o-Cresol
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1
135.33
210.98
2
112.17
220.82
3
133.19
205.34
10	is
Time, days
Figure 4. Rate of Oxygen Uptake During the Adsorption of o-Cresol
o.o
0	1	2	3	4	s
Irreversible Adsorption (mmol)
Figure 5. Total Oxygen Consumption for Polymerization of o-Cresol
7

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Figure 6. Total Oxygen Consumption for Polymerization of 2-EthylphenoI
3.0
2	3
Irreversible Adsorplion (mmol)
Figure 7. Total Oxygen Consumption for Polymerization of 2-ChIorophenoI

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As is apparent from Figures 5, 6, and 7, the amount of oxygen consumed during the
adsorption of phenolic compounds is linearly proportional to the amount of irreversibly adsorbed
compound. The coefficient of proportionality was 0.5026 for o-cresol, 0.6519 for 2-ethylphenol,
and 0.4198 for 2-chlorophenol. Equations 1 to 4 suggest that the proportionality coefficient
should be 0.25. However, that value pertains only to the first step of oxidative coupling (radical
formation) of phenol in aqueous solution. The system employed in this study was much more
complex than that and involved oxidative coupling of substituted phenols. Consequently, the
coefficient of proportionality was above that suggested by Shibaeva et al (1969).
All the organic compounds tested for the effect of molecular oxygen on the adsorptive
capacty of GAC belong to different classes of organic compounds and differ in their affinity
towards oxidative coupling. In most cases, the relative affinity of different adsorbates towards
polymerization on the surface of GAC can be related to their tendencies to undergo oxidative
coupling as solutes. Reactivity of these compounds in oxidative coupling reactions can be
characterized by "critical oxidation potential" (COP). The notion of the COP was introduced by
Fieser (1930). Fieser (1930) tested several phenols and amines with oxidizing agent of known
redox potentials to determine the threshold oxidation potential required to oxidize each
compound in a water-alcohol mixture at 35°C. COP is used in this study to determine the
relative order of susceptibility of a compound towards polymerization on the surface of GAC.
Figure 8 shows the amount of irreversibly adsorbed compound as a function of the COP. The
values of the COP for most of the compounds presented in Figure 8 are as measured by Fieser
(1930), while several COP values were calculated using the approximate values that Fieser
established for substituents.
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.• 2.4.6-lcp
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• p-chlorophenol
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= 10
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4-hydroxybenzoic acid • p-nitrophenol
0.9
1.0	1.1	1.2
COP [V]
1.3
1.4
1.5
Figure 8. Irreversible Adsorption vs. Critical Oxidation Potential
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In general, unsaturated groups increase the COP while the saturated ones decrease it.
Furthermore, the modified hydroxyl and amino groups cause the greatest decrease in the COP.
Among the compounds investigated, the nitro group caused the highest increase in the COP,
which can explain the observed lack of polymerization of nitrophenols on the GAC surface. The
other functionality that significantly increases the COP is the carboxyl group. However, the
magnitude of the increase is lower than that measured for the nitro group. Since 4-
hydroxybenzoic acid polymerized on the carbon surface, it can be concluded that the threshold
potential encountered in the adsorption system is somewhere between the COP of the nitro and
the carboxyl substituted phenols.
Another important observation of Fieser (1930) is that the multiple substitution of the
same functional group further emphasize the change in COP in the same direction. Since the
methyl functional group decreases the COP, the amount of irreversible adsorption for 2,4-
dimethylphenol should be above that observed for 2-methylphenol or 4-methylphenoI. Such
behavior is documented in Figure 8. Similar behavior was demonstrated for the adsorption of
2-chlorophenol and 2,4-chlorophenol but in the opposite direction of the relationship observed
for the methylphenols since the chlorine functionality belongs to the group of substitutes that
increase the COP. However, the average amount of irreversible adsorption for 2,4,6-
trichlorophenol was found to be above that computed for 2,4-dichlorophenol. This can be
attributed to the fact that the value of the dissociation constant, pKg, is 7.9 for 2,4-dichlorophenol
and 5.99 for 2,4,6-trichlorophenol (Drahanovsky and Vacek, 1971), indicating that most of 2,4-
dichlorophenol was present in the neutral form at pH 7.0 while the predominant species in the
case of 2,4,6-trichlorophenol was the phenolate ion. Shibaeva et al. (1969) and Denisov (1980)
found that the reaction of phenolic anions with oxygen is much faster than the reaction of the
corresponding phenols. Furthermore, Chin et al (1985) found the rate of aerobic coupling of
phenol using cuprous chloride as a catalyst to be second order with respect to the solution pH
when the pH values were below the pKg value. These studies suggest that phenoxy radicals are
formed much more readily from phenolate ions than from the neutral species, which explains the
observed behavior of polysubstituted chlorophenols.
CONCLUSIONS
This study clearly delineates the important role of molecular oxygen on the adsorption of
phenolic compounds by activated carbon. The presence of molecular oxygen induced
polymerization of these compounds on the carbon surface resulting in a significant increase in
the adsorptive capacity of GAC when compared to that attainable under anoxic conditions. The
capacity of GAC to promote polymerization reactions and the adsorptive capacity for the reaction
products remained constant over three orders of magnitude of equilibrium aqueous phase
concentrations. Solvent extraction of GAC loaded with adsorbate in the presence of molecular
oxygen yielded very low extraction efficiencies while almost 100% of adsorbate can be recovered
from the carbon surface if the adsorption phase was carried out under anoxic conditions.
The amount of oxygen consumed during the adsorption of three different adsorbates was
linearly proportional to the amount of irreversibly adsorbed compound. Critical oxidation
potential was successfully used to establish relative susceptibility towards polymerization and,
consequently, the amount of irreversible adsorption among the compounds tested in this study.
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ACKNOWLEDGEMENTS
Funding for this work was provided by the U.S. Environmental Protection Agency under COE-
UC/RREL Cooperative Agreement CR-816700. The views expressed are entirely those of the
authors and do not necessarily reflect the views of the agency. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
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