WATER POLLUTION CONTROL RESEARCH SERIES • 17020—06/70
EFFECT OF SURFACE GROUPS ON
ADSORPTION OF POLLUTANTS
ENVIRONMENTAL, PROTECTION AGENCY • WATER QUALITY OFFICE
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WATER POLLUTION CONTROL RLSEARCH SERIES
The Water Pollution Control Research Series describes the
results and progress in the control and abatement of pollu-
tion of our Nation's waters. They provide a central source
of information on the research, development, and demon-
stration activities of the Water Quality Office, Environ-
mental Protection Agency, through inhouse research and grants
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Inquiries pertaining to the Water Pollution Control Research
Reports should be directed to the Head, Project Reports
System, Office of Research and Development, Water Quality
Office, Environmental Protection Agency, Washington, D.C. 20242.
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EFFECT OF SURFACE GROUPS ON ADSORPTION OF POLLUTANTS
by
Robert W. Coughlin
Department of Chemical Engineering
Lehlgh University, Bethlehem, Pa. 18015
for the
WATER QUALITY OFFICE
ENVIRONMENTAL PROTECTION AGENCY
Program #17020 06/70
Grant No. WP-00969-01
June 1970
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 40 cents
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EPA Review Notice
This report has been reviewed by the Water Quality Office,
EPA, and approved for publication. Approval does not signi-
fy that the contents necessarily reflect the views and poli-
cies of the Environmental Protection Agency, nor does mention
of trade names or commercial products constitute endorsement
or recommendation for use.
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ABSTRACT
It has been shown by experiment that acidic surface
oxides on active carbon can profoundly influence the sorp-
tion of various pollutant-type molecules from aqueous
solution. Not only is the equilibrium sorption capacity
of the carbon affected but the rate of sorption is also
changed. However, these changes are reversible, for re-
moval of the acidic surface oxides can restore the carbon
to its original sorption capacity or beyond. In the
cases of sorption of phenol, nitrobenzene, sodium ben-
zenesulfonate and dextrose, surface oxides reduced the
sorption capacity of the carbon as well as the speed of
sorption. In the case of urea sorption the sorption
capacity of the carbon was increased by the presence
of acidic surface oxides. It appears that the influence
of these surface oxides depends on the relative strength
of their interactions with both the water solvent and
the solute to be adsorbed.
This report was submitted in fulfillment of project
No. 1 R01 WP00969-01 under the sponsorship of the Federal
Water Quality Administration.
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CONTENTS
Section Page
Abstract
Contents
List of Figures
List of Tables
I Introduction
II Summary And Conclusions 6
III Recommendations 7
IV Experimental 8
V Discussion 11
VI References 28
VII Patents And Publications 29
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FIGURES
Page
1. Possible Structures of Acidic Surface 5
Oxides on Carbon
2. Adsorption Isotherms of Phenol on Active 12
Carbon
3. Adsorption Isotherms of Nitrobenzene on 13
Active Carbon LC325
4. Percentage of Carbon Surface Area Occupied 14
by Phenol as a Function of Total Surface
Acidity
5. Adsorption Isotherms of Phenol on Active 16
Carbon S51
6. Adsorption Isotherms of Sodium Benzene- 17
sulfonate on Active Carbon S51
7. Adsorption Isotherms of Dextrose on Active 18
Carbon LC-325-per gram basis
8. Adsorption Isotherms of Dextrose on Active 19
Carbon LC-325-per m2 basis
9. Adsorption Isotherms of Urea on Active 21
Carbon LC-325
10. Rate of Adsorption of Dextrose on Active 22
Carbon CF 300
11. Rate of Adsorption of Dextrose on Oxidized 23
Active Carbon CF 300
12. Influence of Oxidant Concentration on Rate 24
Constant for Adsorption of Dextrose on Active
Carbon CF 300 at 19°C
13. Activation Energies for Adsorption of Dextrose 25
on Oxidized and Untreated Carbons
14. Activation Energies for Adsorption of Phenol 26
on Oxidized and Untreated Carbons
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TABLES
Number Paqe
1. Specific Surface Area And Base Consumption 9
For Several Carbons
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SECTION I
INTRODUCTION
Tertiary treatment of secondary sewage effluent is a
process that will assume ever greater importance as pop-
ulation and water use continue to grow. The aim of this
kind of treatment is to remove from water not only those
impurities which can be removed by ordinary biological
treatment, but also to remove other organic substances
which are oxidized only with difficulty and are, there-
fore, frequently called refractory or perdurable pollu-
tants . Of the numerous processes which have been con-
sidered and evaluated for tertiary treatment of waste-
water, adsorption onto active carbon has emerged as one
of the most efficient for removing organic impurities
and one of the most promising from the standpoint of
cost.
Some of the organic compounds that can be removed
from water in this way are phenols, cresols, alkylben-
zenesulfonates, nitrochlorobenzenes, chlorinated paraf-
fins, butadiene, synthetic dyes, insecticides, fungicides,
etc. At present, active carbon is one of the most pro-
mising solid adsorbents for this purpose, owing to its
commercial availability, high adsorptive capacity, and
affinity for a broad spectrum of chemical compounds.
Important tertiary treatment plants are in operation
(12,13) at Lake Tahoe and at Pomona, California. The
Lake Tahoe plant prevents undesirable nutrients and im-
purities from reaching the lake by removing most of the
phosphate and much of the organic material from secondary
sewage effluent. In this plant the secondary effluent
from an activated sludge process is chemically coagulated
and filtered before treatment with active carbon. The
Pomona plant treats effluent directly from an activated
sludge process, with no intermediate coagulation or
settling step. Here less effective reclamation is
achieved, but at far lower expense. A related plant in
operation (11) at Nitro, West Virginia, treats river
water with active carbon to purify it for domestic use.
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In each of these plants saturated or spent carbon,
that through use has become ineffective for adsorbing
additional impurities, is regenerated by using direct
fired furnaces. This regeneration is a three-step pro-
cess in which the carbon is dried, baked, and activated.
After removal of water in the drying step, the second
step, baking, carbonizes the organic adsorbate on the
surface of the carbon and contained within its porous
structure. Baking takes place within a temperature
range of 212° to about 1,500°F., and presumably it is
accompanied by evolution of gaseous decomposition pro-
ducts and formation of free carbon residue within the
pores of the active carbon. The final step, activation,
is accomplished by contacting the carbon directly with
flue gas which contains additional steam in variable
quantity. Activation takes place above about 1,500°F.,
and during this process the material is oxidized to re-
develop a porous structure of large surface area from
the spent, baked carbon in which the pore system contains
solid decomposition products from the carbonization
reaction.
SHORTCOMINGS OF REGENERATION
As might be intuitively expected, this regeneration
process does not restore spent, active carbon to its ori-
ginal adsorptive capacity. This shortcoming occurs in
addition to actual loss of carbon resulting from burn-
off, mechanical attrition, and other purely physical
losses. Thus, after regeneration, the carbon does not
possess the original specific adsorptive capacity, and
this may be ascribed to possibly diminished specific
surface area, possible alterations in the pore structure
of the carbon adsorbent, and possible chemical change in
the nature of the adsorbing surface. Presumably, all
three of these phenomena play a role in altering the ad-
sorptive capacity of regenerated, active carbon. To date
no quantitative assessment exists as to the relative con-
tributions from changes in pore structure, specific sur-
face area, and surface chemistry in bringing about this
reduction of adsorptive capacity.
It is clear that lowering the specific surface area
and narrowing the pores of a carbon adsorbent tend to re-
duce its adsorptive capacity, but the influence on adsorp-
tion of chemical alteration of the carbon surface is some-
what more remote from intuitive understanding. The puv-
pose of the work reported below is to shed more light on
the possible importance of the nature of the carbon surface
in adsorption, as distinct from the influence of pore
structure and specific surface area.
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SURFACE CHEMISTRY OF CARBON ADSORBENTS
Active carbon is one form of black, microcrystalline
(sometimes called amorphous) carbon. Its underlying struc-
ture is fundamentally graphitic, as was shown by Hofmann
and Wilm (1) using X-ray diffraction. A visualization of
this structure consists of packets of graphite-like layer
planes of some three to thirty layers about 10 to 100 A
thick. A graphitic layer, which may be regarded as anal-
ogous to a very large, polynuclear aromatic molecule,
contains carbon atoms joined by a bonds to three neighbor-
ing carbon atoms with the fourth electron of each atom
participating in a TT bond (sp2 hybridization) . In graphite,
the resulting layers are stacked with a separation of about
3.35 A in the sequence ABAB, although some ABCABC sequence
may also occur (2). In microcrystalline carbon, the inter-
layer spacing is larger than in graphite, that is, about
3.6 A, and the stacking sequence is greatly perturbed with
the result that many graphitic layer planes are tilted with
respect to one another. The layers of both graphite and
microcrystalline carbon are held together relatively weak-
ly by van der Waals forces. In addition, there may be
present in microcrystalline carbon a considerable content
of disorganized tetrahedrally bonded carbon (3,4) often
crosslinking different layers. Moreover, foreign atoms
are always present in differing amounts, and they may be
bound at the edges of the crystallites to form functional
groups or incorporated within the graphitic layers to
form heterocyclic ring systems.
Because free valences at the edges of the graphitic
layer planes of microcrystalline carbon are very reactive
and form compounds with any suitable foreign atoms present,
functional groups or surface compounds can be expected
almost exclusively at the layer edges. Foreign atoms or
molecules can be only weakly adsorbed on the basal faces
by means of the graphitic ir electron system, except where
they are bound at lattice defects. Most important and
best known among the surface compounds of carbon are those
with oxygen and sulfur, although other elements, such as
chlorine and hydrogen, can also combine with elemental
carbon. Of these compounds the surface oxides of carbon
have received the most study, and the role of these oxides
in adsorption is the principal topic of this report. In
particular, the concern here is the acidic surface oxides
of carbon, which are formed under the most usual conditions
of treatment and manufacture of microcrystalline carbon
products like active carbon. Basic surface oxides also
occur, but less frequently, and their nature and structure
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have not yet been elucidated very thoroughly. Garten,
Weiss, et al. (5) claim that basic oxides are produced
during oxidation at high temperatures, but Boehm (6) has
pointed out that they are formed after an outgassed car-
bon surface comes into contact with oxygen after cooling
in an inert atmosphere.
Of the many techniques for characterizing the acidic
surface oxides of carbon, the method used extensively by
Boehm, Diehl, et al. (7) has been employed in the present
work. In an extension of earlier work (5,8), these inves-
tigators used typical identification reactions of organic
chemistry to characterize oxygen chemisorbed on carbon as
comprising four different types of acidic surface groups:
a strongly acidic carboxyl group, a more weakly acidic
carboxyl group, a phenolic hydroxyl group, and a carbonyl
group. Figure 1 shows a schematic structure representation
of these groups in which the difference between the two
kinds of carboxyl groups is related to their ability to
form a lactone or lactol. These acidic functional groups
can be identified by their reaction (or failure to react)
with bases of different strength. Thus, group I is neu-
tralized by each of the bases sodium bicarbonate, sodium
carbonate, and sodium ethoxide; group II is neutralized
by sodium carbonate or stronger bases but not by sodium
bicarbonate, etc. Accordingly, simple titration with
different bases serves to identify the acidic surface
oxides present on a given sample of carbon.
These acidic functional groups can be produced by
oxidation in air or pure oxygen or by mixing the carbon
sample with aqueous solutions of oxidizing agents like
sodium hypochlorite, potassium permanganate, or ammonium
persulfate. It is also possible to remove partially the
acidic functional groups by reduction or vacuum out-gassing
at elevated temperatures. In addition, these groups can be
made to react in other ways like esterification, formation
of acid chlorides, acetylation, etc.
It was the purpose of the work reported here to attempt
to learn more about how the nature of the carbon surface in-
fluences adsorption of typical organic compounds from aque-
ous solutions. In particular, because acidic surface oxides
are commonly present on most commercial carbons and because
they are rather easy to form on as well as remove from the
surface of carbon, the present work focused on these groups
and upon their influence on the adsorption process. la
this work, the adsorptive capacity and the rate of adsorp-
tion for different carbons treated in different ways were
observed for several different adsorbates of importance
for tertiary waste treatment.
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COOH
COOH
OH
OPEN FORM LACTONE FORM
FIGURE 1 POSSIBLE STRUCTURES OF CARBON SURFACE OXIDES
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SECTION II
SUMMARY AND CONCLUSIONS
It has been shown by experiment that acidic surface
oxides on active carbon can profoundly influence the sorp-
tion of various pollutant-type molecules from aqueous
solution. Not only is the equilibrium sorption capacity
of the carbon affected but the rate of sorption is also
changed. However, these changes are reversible, for re-
moval of the acidic surface oxides can restore the carbon
to its original sorption capacity or beyond. In the cases
of sorption of phenol, nitrobenzene, sodium benzenesulfon-
ate and dextrose, surface oxides reduced the sorption
capacity of the carbon as well as the speed of sorption.
In the case of urea sorption, the sorption capacity of
the carbon was increased by the presence of acidic sur-
face oxides. It appears that the influence of these
surface oxides depends on the relative strength of their
interactions with both the water solvent and the solute
to be adsorbed.
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SECTION III
RECOMMENDATIONS
Further experimental and theoretical work is needed
to elucidate the mechanism by which surface functional
groups on adsorbents influence the adsorption of various
substances from aqueous solution. Hopefully, such work
may ultimately lead to the knowledge required to predict
by theory the behavior that is now accessible only by
experiment.
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SECTION IV
EXPERIMENTAL PHASE
The carbon adsorbents used in this project were:
Union Carbide's Columbia Carbon LC325 and NXC 12/28X,
Atlas Chemical's Darco S51, Calgon's Filtrasorb 300W,
and Cabot Corporation's Black Pearls 607. The adsorbates
used in this project were: sodium benzenesulfonate, phenol,
nitrobenzene, dextrose, and urea; these represent respect-
ively a detergent, two "refractory" organic compounds, a
sugar commonly present in secondary municipal sewage ef-
fluent and a molecule of relatively large dipole moment.
Chemical analysis of aqueous adsorbate solutions was
performed using ultraviolet spectrophotometry at 270.0 my
for phenol, at 216.0 my for sodium benzenesulfonate, and
at 267.8 my for nitrobenzene. It was necessary to use
differential refractometry for solutions containing large
concentrations of phenol. A standard spectrophotometric
technique (9) was employed for determining urea. Concen-
trations of dextrose were determined by measuring the
optical rotation of plane polarized light passed through
the solutions, using a polarimeter.
Acidic oxide functional groups were formed on the car-
bon surfaces by room temperature oxidation in aqueous solu-
tions of ammonium persulfate. These surface oxides were
removed from the carbon by exposure to hydrogen gas produced
by the action of HC1 on Zn amalgam, or by heating the carbon
samples to temperatures up to 890°C in vacuo. The nature
and concentrations of surface oxides on the carbon surfaces
were determined by titrating the samples using bases of
different strength using the technique of Boehm (7). Speci-
fic surface areas of the carbon adsorbents were determined
by the standard BET technique (10) using nitrogen as the
adsorbate. Specific surface areas and base consumption for
the carbon samples ar<; recorded in Table 1.
Adsorption expeTiments were carried out by equilibrating
samples of carbon adsorbent with the various analyzed solu-
tions of adsorbate for periods up to several hours. These
mixtures were stirred or shaken during the exposure. After
equilibration, the carbon was filtered from the solution
which was then re-analyzed. Kinetic experiments were per-
formed by adding carbon of known, large particle size to a
well-stirred vessel of adsorbate solution; periodically,
stirring was stopped and, after the carbon particles had
settled, a sample of clear solution was withdrawn for chemi-
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Table 1
SPECIFIC SURFACE AREA AND BASE CONSUMPTION FOR SEVERAL CARBONS
Carbon Sample
Identification*
LC325
LC325 0
LC325 OR
P607
P607 0
P607 R
P607 OR
P607 OROROR
S51
S51 0
S51 OR
S51 OD
LC325 -T
LC325 O -T
LC325 OR -T
CF300
CF300 0
NXC
NXC 0
BET(N2)
Area
mVgm
1200
556
40
665
668
663
5.3
562
586
596
836
725
1076
861
840
950
Base Consumption Meq/gm
NaOE t ( tot . acidity)
0.38
4.15
1.87
2.40
3.06
1 86
2.42
1.56
1.08
4.21
2.15
0.60
0.59
3.10
2.46
0.70
3.50
0.65
3.72
NaOH
0.12
4.22
1.92
1.88
2.25
1.31
1.94
1.26
0.71
3.57
1.81
0.60
0.48
2.92
2.10
0.35
4.47
Na2C02
0.11
3.10
2.04
1.34
1.69
0.97
1.42
0.90
0.48
2.45
1.12
0.25
0.28
2.05
1.29
0.07
3.24
NaHCO 3
0.11
2.20
0.84
0.85
0.97
0.48
0.88
0.80
0.40
1.85
0.80
0.20
0.30
1.50
0.92
0.04
2.32
*The suffix 0 means oxidized, R means reduced, D means outgassed at 890°C under
vacuum, OR means oxidation followed by reduction, etc.
No suffix means the carbon sample was simply wasted with no further treatment.
The suffix -T has been used to distinguish one set of LC325 samples from
another set.
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cal analysis. In the case of dextrose adsorption, it was
found that the entire experiment could be carried out in
the polarimeter cell. All adsorption experiments were
carried out at constant temperature. For kinetic experi-
ments a narrow size range of carbon particles was employed
in each case - this narrow size range was obtained by
sieving through U. S. Standard sieves.
More detailed descriptions of analytical procedures
and techniques may be found in the various reports and
publications which resulted from this project and which are
listed in Section 7.
10
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SECTION V
DISCUSSION
The influence of surface oxides on sorption of phenol
from aqueous solution is clearly shown by the adsorption
isotherms plotted in Figure 2. The uppermost isotherm in
this Figure represents the adsorption of phenol on acid-
washed but otherwise untreated active carbon LC325; the
lowest isotherm in this Figure represents adsorption of
phenol on carbon oxidized, as outlined in the Experimental
Section, on order to produce a large population of surface
functional groups or oxides on the carbon surface. The
intermediate isotherm is for oxidized carbon that was
subsequently reduced with hydrogen before adsorption ex-
periments. These carbons are designated "0" for oxidized
and "OR" for oxidized followed by reduction; no such
designation indicates that the carbon was merely washed.
Figure 3 shows the same kind of behavior for the adsorp-
tion of nitrobenzene on the same carbons as for Figure 2.
Figures 2 and 3 show that oxidation of the carbon sor-
bent lowers considerably its adsorption capacity for phenol
and nitrobenzene on a "per gram of carbon" basis. Quite
similar behavior is also observed if the isotherms are
plotted on the basis of amount adsorbed per unit BET speci-
fic surface area of the carbon. It is clear that reduction
of the oxidized carbon surface using hydrogen partially
restores its adsorption capacity for phenol and probably
also for nitrobenzene. Figure 4 shows the influence of
surface acidity on sorption capacity expressed as percentage
of BET surface area occupied by phenol (assuming the phenol
molecule lies flat on the carbon surface) at concentrations
corresponding to the plateau of isotherms similar to those
of Figures 2 and 3; here percentage of area occupied is
plotted versus total surface acidity measured as meq of
NaOCH2CH3 neutralized per gram of carbon. Data for both
the carbon black P607 and active carbon LC325 are included
on Figure 4.
Although the data of Figure 4 show that a threefold
increase in surface acidity from about 1 meq/gm to 3 meq/gm
produces a corresponding threefold decrease (from about 15%
to 5%) in the percentage surface area occupied by the ad-
sorbate, it should not be concluded therefrom that the loss
in adsorptive capacity results from direct steric inter-
ference by surface acidic groups which occupy surface area
that would otherwise be available to the adsorbate. On the
11
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-Q
L.
O
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FIGURE 3.
ADSORPTION ISOTHERMS OF NITROBENZENE ON ACTIVATED
COLUMBIA CARBON LC325 FROM AQUEOUS SOLUTION, 30 °C
100 200
180O-
o
E
L.
O
0)
O
£
1200 -
600
LC3250«x,1wk,
O.OSg sample
LC325,1wk.
O.O1g sample
LC325,2wks.
0.02 g sample
100
200
Concentratration , M.mo\es / liter
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o
c
4>
JC
Q_
CD
•O
O
,.__
Q_
3
O
o
O
20
15
10
P607 Samples
LC325 Samples
I
1 2 3
FIGURE 4. Total Acidity meq / g
"/.AREA OCCUPIED BY PHENOL VS. TOTAL AMOUNT OF ACIDITY
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basis of the model proposed herein for the carbon surface,
the acidic groups would be expected to be concentrated at
the edges of the hexagonal layer planes, whereas the major
portion of surface active for physical adsorption would be
located on the flat, basal surfaces - not the edges - of
the layer planes. It may be that the acidic surface groups
merely block small pores and thereby the surface area within
such pores becomes inaccessible. Another explanation could
be based on the ability of oxygen to attract electrons from
the ir-electron system of the layer planes toward the edges
of these planes and thereby affect the sorptive properties
of the layer planes. It will be seen below that the pre-
sence of acidic functional groups actually raises the ad-
sorptive capacity of the layer planes for urea and this
fact would tend to support the electronic interpretation.
Other interpretations could be advanced based on the rela-
tive strength of interaction of the acidic groups with
water or phenol on the one hand and with water or urea on
the other hand. Unfortunately, neither the chemical nature
nor the concentrations of the acidic groups are known with
sufficient certainty or accuracy to make a firm interpreta-
tion of these results.
Additional isotherms for phenol adsorption appear in
Figure 5. However the data in Figure 5 are for a different
activated carbon, Darco S51, and isotherms are shown here
for the acid washed carbon (S51), oxidized carbon (S51 O),
oxidized and reduced carbon (S51 OR) and for oxidized carbon
that was subsequently outgassed at high temperature (S51 OD).
Note in this Figure that the ranking of adsorption capacity
for S51, S51 0 and S51 OR is identical to that for LC325,
LC325 0 and LC325 OR shown in Figures 2 and 3 and discussed
above. However, it is clear from Figure 5 that oxidation
followed by vacuum outgassing at elevated temperature (S51 OD)
produces a carbon of larger adsorption capacity than the
starting material (S51) . This behavior can be attributed to
two differences between S51 and S51 OD: the S51 OD had a
larger BET specific surface area and a lower population of
acidic surface sites (on either a per gram or per unit sur-
face area basis) than did the untreated S51 active carbon.
Note, however, that the isotherm of Figure 5 is expressed on
a surface concentration basis (y moles/m2) and, therefore,
the sorption capacity difference between S51 and S51 OD
evident from Figure 5 may be attributed to a difference in
population of acidic surface sites on the carbon adsorbent.
Figure 6 shows that the adsorption of sodium benzene-
sulfonate on active carbon LC321 is affected in the same way
by oxidation and reduction as is the adsorption of nitro-
benzene and phenol. Figures 7 and 8 show similar behavior
15
AWBERC LIBRARY U.S. EPA
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FIGURE 5 ADSORPTION ISOTHERMS OF PHENOL ON ACTIVATED CARBON "DARCO S5J"
IN AQUEOUS SOLUTION, 30°C, 3 DAYS EQUILIBRIA
cn
o
2
<
• S5I
S5I OD
SSI OR
S5I 0
CVJ
CO
LJ
o I n —
Q
LJ
GO
cr
o
CO
Q
0
100
CONCENTRATION ,
200
MOLES/ LITER
300
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FIGURE 6 ADSORPTION ISOTHERMS OF SODIUM BENZENE SULFONATE ON ACTIVATED
CARBON "COLUMBIA LC325" IN AQUEOUS SOLUTION, 30°C, 3 DAYS
EQUILIBRIUM TIME
O)
CO
LU
_l
O
2
4.
LJ
CD
tr
O
CO
O
200
100
• LC325-T
• LC325-0-T
A LC325-OR-T
50
CONCENTRATION , p. MOLES/LITER
100
-------
FIGURE 7 ADSORPTION ISOTHERMS OF DEXTROSE ON ACTIVATED CA.PW (COLIWA LC325)
IN AQUEOUS SOLUTION AT 30,0°C FOR 1,0 HOUR EQUILIBRIA TIMF ON A
PER GRAM BASIS
00
1.0
cr
CO
LJ
0.5
Q
LjJ
QD
(T
O
CO
Q
Z)
O
• LC325-0-T
• LC325-OR-T
A LC325-T
100 200
CONCENTRATION MILLI MOLES/LITER
300
-------
FIGURE 8 ADSORPTION ISOTHERMS OF DEXTROSE ON ACTIVATED CARBON (COLUMBIA LC325)
IN AQUEOUS SOLUTION AT 30,0°C FOR 1,0 HOUR EQUILIBRIUM TIME,
(PER UNIT BET SURFACE AREA BASIS)
CM
a:
LU
H
LJ
CO
LJ
O
a:
o
o
LJ
oo
en
o
CO
o
ID
O
0.5
LC325-0-T
LC325-T
LC325-OR-T
100
200
300
-------
for the sorption of dextrose on samples of LC325 treated in
the same way. In Figure 7 the adsorption isotherms are
plotted on a per-gram basis whereas in Figure 8 the quan-
tities adsorbed are expressed on a per unit BET specific
surface area basis.
The adsorption isotherms for urea on LC325 and LC325 0,
which appear in Figure 9, show a ranking behavior which is
opposite to that for all the other adsorbates discussed •
above. Oxidizing the carbon surface, therefore, appears to
increase its sorption capacity for urea from aqueous solu-
tion in contrast to the behavior of phenol, nitrobenzene,
sodium benzenesulfonate and dextrose. The reason for this
behavior may lie in the relative magnitudes of attractive
forces between carbon and water in comparison with the
attractive forces between carbon and adsorbate. Based on
the expected strength of hydrogen bonding between water
and acidic sites on the carbon surface, the water may
block part of the carbon surface and interfere with adsorp-
tion thereon by phenol, nitrobenzene, sodium benzenesulfon-
ate and dextrose. On the other hand, urea which can also
strongly hydrogen bond to surface oxides on carbon may be
able to compete with water for these sites more effectively
than can the other adsorbates.
The influence of surface oxides on the rate of adsorp-
tion of dextrose on carbon is revealed by Figures 10 and 11.
In each of these Figures the concentration of dextrose solu-
tion is plotted versus the square root of time measured from
the introduction of a known amount of carbon into the solu-
tion. Figure 10 gives these results for untreated carbon
at 19°C and 40°C whereas Figure 11 shows results for oxidized
carbon at 40°C. Based on the extremely simple relationship:
%
rate of adsorption = K t
the rate constant K can be evaluated from the slopes of the
curves in Figures 10 and 11. At 40°C K for adsorption of
dextrose on untreated carbon is 867 y moles/gm, hr^ (Figure
10) whereas K = 476 y moles/gm, hr^ for adsorption of dex-
trose on oxidized carbon at the same temperature (Figure 11).
Further evidence for the inhibiting effect of surface oxides
on rate of adsorption appears in Figure 12 where the rate
constant K for dextrose adsorption is plotted versus the
concentration of oxidizing agent used to treat the carbon.
By plotting the logarithm of the rate constant versus
the reciprocal of absolute temperature it is possible to
estimate an apparent activation energy. Figure 13 shows
these plots for the adsorption of dextrose on CF300 and
CF300-W and Figure 14 for the adsorption of phenol on active
20
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FIGURE 9 ADSORPTION ISOTHERM OF UREA ON ACTIVATED CARBON (COLUMBIA LC325)
IN AQUEOUS SOLUTION AT 30,0°C FOR 0,5 HOURS EQUILIBRIUM TIME ON
A PER GRAM BASIS
s
<
o
fe 18
2
o
£3
UJ
m
a:
8
m
<
I I
LC325-T
LC325 0-T
<
UJ
cc
ID
U_
o
CO
cs 4
10-
\ Concentration Range In a Normal Human
I I
10
20 40 50
UREA CONCENTRATION MILLIGRAMS/100 ML.
80
-------
FIGURE 10 RATES OF ADSORPTION OF DEXTROSE ON CF300 W IN AQUEOUS SOLUTION,
PARTICLE SIZE RANGE 2,38-2,83 MM,
NJ
fO
cr
LU
CO
o:
z
o
h-
<
LJ
o
•z.
o
o
T = I9.0 °C, K = 6e Micromoles/Gram(hr.)'/2
= 4O.O°C, K=867 Micromoles/Gram
0
TIME
-------
FIGURE 11 RATE OF ADSORPTION OF DEXTROSE ON CF300-0 IN AQUEOUS SOLUTION
AT 40,0°C, PARTICLE SIZE RANGE 2,38-2,83 MM,,
o:
LJ
CO
cr
CD
o
5
LJ
o
o
o
60
48
K = 476 Micromoles/Gram
0.5
TIME (HR.)lx2
1.0
-------
NJ
oc
e>
\
CO
LJ
O
or
o
FIGURE 12 EFFECT OF (NH/|)2S208 CONCENTRATION ON THE RATE CONSTANT K IN
AQUEOUS SOLUTION, PARTICLE SIZE RANGE 2.38-2,83 MM,, AT 19,0°C
CF300 CARBON
^ 1000
of
500
100
0
Oxidation Time = 3.0 days
0.5
1.0
MOLAR CONCENTRATION OF (NH4)2S208
-------
FIGURE 13 ARRENHIUS PLOT FOR THE ADSORPTION OF DEXTROSE ON CF300 IN
AQUEOUS SOLUTION, PARTICLE SIZE RANGE 2,38-2,83 MM,
-3.0
to
en
-3.4
A CF300-0,0.4M. (NH4) 2S208 ,3.0 Days
• CF300
E=785 cal/mole
E = 2290 cal /mole
3.2
3.7
1000/T ,(°K)
-I
-------
3.8
3.0
I
H
ct:
2.0
o:
ID
• NXC
A NXC o
E = 2330 cal /mole
E = 10,800 cal /mole
Curve I Data obtained by Jere
Curve 2 Data obtained by Cofman
3.1 3.2
VALUES OF I/T x I03 (°K~I)
FIGURE 14 DETERMINATION OF ACTIVATION ENERGIES
3.3
3.36
26
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carbons NXC and NXC-0. In each case, it is evident that
oxidation increases the apparent activation energy.
It is interesting to speculate how a detailed know-
ledge of the kind of phenomena reported here might be har-
nessed to produce more efficient waste treatment. For
example, when more knowledge becomes available as to the
specific chemical components in primary and secondary
effluent it may become possible to use carbons treated in
different ways to selectively remove different classes of
chemical compounds. On the basis of the present results,
it would appear desirable to contact effluent with non-
acidic carbon (say vacuum outgassed or reduced) to remove
compounds similar to phenol and nitrobenzene as well as
with oxidized carbon to remove compounds similar to urea.
The results reported here should serve as motivation to
study the influence of acidic groups on the sorption of
other classes of compounds like proteins, amino acids,
fats, sugars, tannins, lignins, etc.
27
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SECTION VI
REFERENCES
1. Hofmann, U., and D. Wilm, Z. Elektrochem.Angew.Phys.
Chem. , 4_2_, 504 (1936) .
2. Boehm, H.P., and R.W. Coughlin, Carbon, 2_, 1 (1964).
3. Hofmann, U., and E. Groll, Ber.Deut.Chem.Ges., 65,
1257 (1932) .
4. Alexander, L. E., and E. C. Sommer, J. Phys. Chem., 60,
1646 (1956) .
5. Garten, V. A., D. E. Weiss, and J. B. Willis, Australian
J. Chem., 10_, 295 (1957).
6. Boehm, H. P., Advan. Catalysis, 16, 179 (1966).
7. , E. Diehl, W. Heck, and R. Sappok, Angew. Chem.
Intern. Ed. Engl., 3_, No. 10,669 (1964).
8. Studebaker, M.L., K.W.D. Huffman, A.C. Wolfe, and L.G.
Nabors, Ind. Eng. Chem., 48, 162 (1956).
9. Fister, H. J., Manual of Standardized Procedures for
Spectrophotometric Chemistry, Urea Nitrogen Method U-lOa,
Standard Scientific Supply Corp. (1950) .
10. Brunauer, S., P.H. Emmett, and E. Teller, J. Am. Chem. Soc,
6£, 309 (1938) .
11. Hager, D.G. and Flentje, M.E., J. Am. Waterworks Assn,
5J7, 1440 (1965) .
12. Masse, A.N., Chairman, Symposium on Wastewater Treat-
ment By Activated Carbon, AIChE National Meeting,
Atlanta, Georgia (February 1970).
13. Masse, A.N., Technical Seminar On Advanced Waste Treat-
ment, Chapter III, Federal Water Quality Control Adminis-
tration (December 1967).
28
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SECTION VII
PATENTS AND PUBLICATIONS
1. R. W. Coughlin and F. S. Ezra, Role of Surface Acidity in
the Adsorption of Organic Pollutants on the Surface of
Carbon, Environmental Science and Technology 2^ 291 (1968).
2. R. W. Coughlin and R. N. Tan, Role of Functional Groups
in Adsorption of Organic Pollutants on Carbon, Chemical
Engineering Progress Symposium Series 64, (90), 207 (1968).
3. R. W. Coughlin, F. S. Ezra and R. N. Tan, Influence of
Chemisorbed Oxygen in Adsorption onto Carbon from Aqueous
Solution, Journal of Colloid and Interface Science 28,
386 (1968).
4. R. W. Coughlin, Carbon As Adsorbent And Catalyst, Industrial
And Engineering Chemistry Product Research And Development
Quarterly, 8_, 12 (1969)
5. R. W. Coughlin, Preparation Of Active Carbon, U. S. Patent
Application, Serial No. 816,933, Interior Case No. FWP-1225
6. R. N. Tan, Effects of Acidic Surface Oxides on the Adsorp-
tion Properties of Carbon, M. S. Research Report, Lehigh
University 1968.
7. E. H. Jere, Effects of Acidic Surface Oxides on the Kinetics
of Adsorption of Phenol by Active Carbon, M. S. Research
Report, Lehigh University 1970.
8. C. A. Burchett, Adsorption of Secondary Effluent and
Carbohydrates by Carbon, M.S. Research Report, Lehigh
University 1970.
9. R. W. Coughlin, Predicting the Activity of Carbon
Catalysts, paper (with reprint) presented at IVth Inter-
national Congress on Catalysis, Moscow (1968).
29
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