1. technology and computing

GLUCOSE AND CELLOBIOSE ADSORPTION
ONTO ACTIVATED CARBON
A Thesis
Submitted to the School of Graduate Studies and Research
in Partial Fulfillment of the
Requirements for the Degree
Master of Science
Yu Sun
Indiana University of Pennsylvania
August 2013
Indiana University of Pennsylvania
School of Graduate Studies and Research
Department of Chemistry
We hereby approve the thesis of
Yu Sun
Candidate for the degree of Master of Science
August 28, 2013
Signature on File_____________________
John C. Ford, Ph.D.
Associate Professor of Chemistry, Advisor
August 28, 2013
Signature on File_____________________
Jaeju Ko, Ph.D.
Associate Professor of Chemistry
August 28, 2013
Signature on File_____________________
Nathan R. McElroy, Ph.D.
Associate Professor of Chemistry
August 28, 2013
Signature on File_____________________
Keith Kyler, Ph.D.
Assistant Professor of Chemistry
ACCEPTED
___________________________________
Timothy P. Mack, Ph.D.
Dean
School of Graduate Studies and Research
______________
ii
_____________________
Title: Glucose and Cellobiose Adsorption onto Activated Carbon
Author: Yu Sun
Thesis Advisor: Dr. John C. Ford
Thesis Committee Members: Dr. Jaeju Ko
Dr. Keith Kyler
Dr. Nathan R. McElroy
Glucose and cellobiose are the simplest model compounds for cellulose.
Glucose is itself a valuable commodity. It is a food as well as a biorenewable
starting material for a variety of other materials.
However, certain fundamental data for this common material are scarce or
unavailable. For example, although activated carbon has long been used to
purify carbohydrates, published adsorption isotherms for glucose on carbon are
rare. We are unaware of any published adsorption isotherm for cellobiose on
carbon.
In this research, adsorption isotherms for glucose and cellobiose on
activated carbon were determined. The experiment was carried out using HPLC
and a derivatization method to quantify the saccharide concentrations before and
after adsorption. The Brauner-Emmet-Teller equation best fit the isotherm data.
For glucose, over the temperature range of 20-35°C, the adsorption equilibrium
constants gave a nonlinear van't Hoff plot, which can be rationalized by
consideration of the conformational equilbria involved.
iii
ACKNOWLEDGMENTS
The author wishes to express his gratitude to Dr. Ford for his patience,
guidance, support and encouragement. He would also like to thank his parents
for their financial support and encouragement.
iv
TABLE OF CONTENTS
Chapter
Page
1
INTRODUCTION ....................................................................................... 1
2
EXPERIMENTAL ....................................................................................... 8
2.1 Chemicals..................................................................................... 8
2.2 Preparation of Activated Carbon ................................................. 10
2.3 HPLC System ............................................................................. 10
2.4 Other Equipment ........................................................................ 12
2.5 Procedure ................................................................................... 12
2.5.1 Preparing saccharide samples ........................................ 12
2.5.2 Derivatization procedure ................................................. 13
3
RESULTS AND DISCUSSION ................................................................. 15
3.1 Equilibration Time ........................................................................ 16
3.2 Preparation of Carbon ................................................................. 17
3.3 Isotherms of glucose and cellobiose ........................................... 20
3.3.1 Best-fit model ................................................................... 21
3.3.2 Comparison of fitted model parameters ........................... 28
3.4 Effect of temperature and thermodynamics of adsorption ........... 31
4
SUMMARY AND CONCLUSION ............................................................. 35
REFERENCES ................................................................................................... 37
APPENDICES .................................................................................................... 41
Appendix A - Langmuir parameters for glucose and cellobiose
adsorbed to washed Darco G-60 activated carbon. ............. 41
Appendix B - BET parameters for glucose and cellobiose
adsorbed to washed Darco G-60 activated carbon. ............. 41
Appendix C - Freundlich parameters for glucose and cellobiose
adsorbed to washed Darco G-60 activated carbon. ............. 41
v
LIST OF TABLES
Table
Page
1
Characterizations of Darco G-60 Activated Carbon. ................................... 9
2
The Suppliers and Grades of all the Chemicals Used in
This Research .......................................................................................... 10
3
The HPLC working conditions ...................................................................11
4
All Equipment Used In This Research .......................................................11
5
Langmuir parameters for glucose and cellobiose adsorbed
to unwashed and washed Darco G-60 activated carbon .......................... 20
6
Statistical results for fitting model equations to
adsorption isotherm data .......................................................................... 27
7
Brauner-Emmet-Teller parameters for glucose and cellobiose
adsorbed to washed Darco G-60 activated carbon................................... 29
vi
LIST OF FIGURES
Figure
Page
1
Structure of glucose .................................................................................. 1
2
Structure of cellobiose. ............................................................................. 1
3
Solution concentrations of glucose and cellobiose in
equilbrium with Norit SX carbon as a function of time............................. 17
4
Absorbance of MilliQ water, water from unwashed
carbon, and water from washed carbon.................................................. 18
5
Adsorption isotherms of glucose on washed and unwashed
Darco G-60 activated carbon at 20°C, 25°C and 30°C. .......................... 19
6
Adsorption isotherm of glucose on Darco G-60
activated carbon at 20°C. ....................................................................... 23
7
Adsorption isotherm of glucose on Darco G-60
activated carbon at 25°C ........................................................................ 23
8
Adsorption isotherm of glucose on Darco G-60
activated carbon at 30°C. ....................................................................... 24
9
Adsorption isotherm of glucose on Darco G-60
activated carbon at 35°C. ....................................................................... 24
10 Adsorption isotherm of cellobiose on Darco G-60
activated carbon at 20°C. ....................................................................... 25
11 Adsorption isotherm of cellobiose on Darco G-60
activated carbon at 25°C. ....................................................................... 25
12 Adsorption isotherm of cellobiose on Darco G-60
activated carbon at 30°C ........................................................................ 26
13 Two possible ways of sugar molecules staying on the
surface of activated carbon. (a) Sugar molecules sidewards
lying on the surface of carbon. (b). Sugar molecules
end-on, sticking up on the surface of carbon .......................................... 30
14 Adsorption isotherms of glucose on Darco G-60
activated carbon at 20°C, 25°C, ,30°C and 35°C. .................................. 32
15 Adsorption isotherms of cellobiose on Darco G-60
activated carbon at 20°C, 25°C, and 30°C. ............................................ 32
vii
16 Plot of lnKa1 and 1/T for glucose and cellobiose
adsorption on G-60 activated carbon. ..................................................... 34
viii
CHAPTER 1
INTRODUCTION
Glucose (C6H12O6) is a simple monosaccharide which is also called Dglucose (the other stereoisomer, L-glucose is almost nonexistent in nature),
dextrose, blood sugar, corn sugar or grape sugar. Glucose, fructose and
galactose are three dietary monosaccharides, i.e., they can be absorbed directly
into the blood and metabolized. Glucose is the primary source of energy for cells
(1).
Cellobiose ((HOCH2CHO(CHOH)3)2O) is a disaccharide which is consisted
of two glucose units linked by a β bond. Cellobiose can be hydrolyzed to glucose
with specific enzyme or acid (2). The structures of glucose and cellobiose are
given below in Figure1 and Figure2.
Figure 1. Structure of glucose. The structure shown is β-D-glucose in the
pyranose form. This is the preferred form in aqueous solution at 25°C.
Figure 2. Structure of cellobiose. Because of the flexibility of the structure,
cellobiose has many possible configurations. The structure shown here is one
possible configuration: α-D-glucopyranosyl β-D-glucopyranoside.
1
Cellulose is the most abundant, renewable biopolymer (3). In industry, it is
an important raw material. It plays a central role in the global carbon cycle and
has the potential to be more widely used as a source of fuels and commodity
chemicals in the future. Insoluble cellulose is a linear polymer of anhydroglucose
units joined by β(1-4) linkages, with degrees of polymerization (DP, the number of
repeating units) from 100 to 20000. While cellulose is frequently called a
polymer of glucose, cellobiose is the actual monomeric unit of cellulose (4). The
DP of a cellulosic material is an important measurement for functionally based
models of enzymatic cellulose hydrolysis as well as for paper-making and other
applications (5).
Short fragments of cellulose whose DP are from 2 to 12 are termed
cellodextrins. The cellodextrins are soluble when DP≤6 and slightly soluble when
6 < DP <12. The most common cellodextrins are cellobiose (DP=2), cellotriose
(DP=3), cellotetraose (DP=4), cellopentaose (DP=5), and cellohexaose (DP=6).
(6) These oligosaccharides have special properties which include solubility in
nonaqueous or partially aqueous solvents and a melting point which increases
with increasing molecular weight (8). Cellodextrins are intermediates in the
production of glucose from cellulose.
Cellulose has the potential to serve as a renewable carbon or energy
resource for the microbial production of fuels and chemical feedstocks in the
future but it is hard to utilize cellulose directly in many industrial processes. As
one of the United States' most abundant renewable resources, about one billion
tons of cellulose-containing residues are generated annually (7) and glucose can
2
be produced from cellulose (8). If these residues could be utilized well, there is a
potential for yielding over 6×1011 pounds of a valuable chemical feedstock,
glucose, from which fuel alcohol and other fermentation derived chemicals can
be made (7). Over 109 tons of “waste” cellulose is generated annually in the
United States, which is a huge waste. The cellulose has the potential to be
reused but much of it is discarded directly (9).
Activated carbon is a form of carbon which has been processed, usually
with oxygen or steam, to create many small pores of low pore volume and thus
produce a very large surface area available for adsorption or chemical reactions
(1). “The use of special manufacturing techniques results in highly porous carbon
that has surface areas of 300-2,000 square meters per gram.” (10) Activated
carbon is a widely used adsorbent for the removal of a wide range of
contaminants from liquids and gases. It is also used to adsorb a product to purify
it. For example, the activated carbon can be used to adsorb a solvent from a
process stream, and the adsorbed product can be subsequently desorbed on-site
for reuse (2).
The meaning of “adsorb” is: when an adsorbent adsorbs an adsorbate, the
adsorbate attaches to the adsorbent by chemical attraction. The surface area is
an important factor for an adsorbent. Activated carbon is a good adsorbent
because of its huge surface area which gives it many bonding sites. When
certain chemicals pass next to the carbon surface, they attach to the surface and
are trapped (11). Activated carbon is good at adsorbing organic chemicals. Many
other chemicals can not be adsorbed by activated carbon. In other words, under
3
appropriate conditions, an activated carbon filter can adsorb target chemicals
and ignore others. It also means that, if all of the bonding sites are filled, an
activated carbon filter doesn’t work anymore. At that point the filter must be
replaced (6).
Activated carbon is commonly used for purification of sugar liquors. Sugar
liquors include solutions of starch hydrolyzate which contains a mixture of mono-,
di-, oligo- and higher polysaccharides, as well as sugar solutions derived from
cane, beet and corn sources. The term, oligosaccharide, refers to a carbohydrate
containing from 2 to 8 simple sugars linked together, while the term,
“polysaccharide,” refers to a carbohydrate containing more that 8 simple sugars.
A starch hydrolyzate is an aqueous mixture of sugar components derived from
acid, enzyme or other treatment of starchy materials (6).
The purification of sugar liquors including corn syrup, cane sugar and
relatively impure solutions of dextrose is one of the oldest established industrial
chemical procedures. Aqueous solutions of certain sugars such as glucose are
produced industrially in the hydrolysis of amylaceous or cellulosic materials. For
example, large amount of glucose solutions are producing by the hydrolysis of
starch in the manufacture of corn syrup, corn sugar and dextrose. These
solutions contain minor but significant amounts of other sugars which can’t be
removed by conventional refining procedures. One use of activated carbon is for
the decolorization of sugar liquors. Typically the powdered activated carbon is
slurried with the impure liquor one or more times followed by filtration of the
decolorized liquor. Decolorization is also accomplished by passing the liquors
4
through a column of granular activated carbon. By these procedures, colorcausing impurities can be removed, which are small amounts of oligosaccharides
present in the liquor (6).
In addition to its industrial uses, activated carbon has been important in
laboratory purification for many years. It is frequently used to remove impurities
from synthetic products, and is known as “decolorizing carbon.” It has been an
important adsorbent in chromatography as well (12), and is frequently used in
carbohydrate and oligosaccharide purification (6).
In adsorbent-adsorbate interaction research, the ability to predict the
“absorbability” for a given adsorbent from a specific adsorbate is an important
objective (13). Isotherm data is important for a better understanding of the
purification process of glucose as well as improving preparative separations of
cellodextrins using activated carbon. Isotherm data can reflect the adsorption
ability well. The adsorption isotherm is a curve giving the functional relationship
between adsorbate and adsorbent in a constant-temperature adsorption process.
Isotherm data of cellodextrins adsorption onto carbon is important because it can
give a better understanding of the purification process of cellodextrins solution,
which may help people to make a better industrial purification plan. Although the
purification of sugar liquors is one of the oldest established industrial chemical
procedures (11), very little relative isotherm data is published. Relative isotherm
data is useful in understanding the process more exactly and may help to
increase the working efficiency. The adsorption of cellodextrins onto carbon may
also be a good method to produce cellodextrins from cellulosic waste. Isotherm
5
data would be useful in modeling such a method. For example, studies of the
competitive adsorption of cellodextrins onto carbon need the individual isotherms
of each component as basic supporting information. And isotherm data are
invaluable for studies of adsorption energetics.
Activated carbon is extensively used in the purification of sugars, both
industrially and in the laboratory. Little or no published information is available
concerning the thermodynamics of the interaction between the carbon surface
and simple carbohydrates. Here, the isotherms of glucose and cellobiose
adsorption onto activated carbon under different temperatures are presented and
the adsorption of the simplest cellodextrin is studied. The important part of this
research is how to quantify the concentration of sugar before and after
adsorption. A derivatization method was used for the determination of the
concentration of sugar. The reducing ends of glucose and cellobiose can react
with 4-aminobenzoic acid ethyl ester. The amount of product can be used to
determine the original concentration of sugar. The concentration of product is
determined by a HPLC system. The method is available for oligosaccharides.
Brauner-Emmet-Teller (BET) isotherm equation was fit to the data. The BET
equation can be mathematically represented by:
Here, Γmono is the surface concentration of the sugar that corresponds to
monolayer coverage of the interface.Ka1 is the equilibrium constant for adsorption
of the sugar on the solid, and Ka2 is the equilibrium constant for adsorption on
sites that are already occupied by adsorbed molecules (14).
6
The adsorption process is temperature sensitive. Isotherms of glucose and
cellobiose were obtained under 20°C, 25°C and 30°C respectively. There is
obvious difference between the isotherms under different temperature. The
influence of temperature was studied. Basically the adsorption ability increased
with the increasing of temperature in this temperature range. And the isotherms
of glucose were compared with the isotherms of cellobiose. While the molecule
size of cellobiose is larger than that of glucose, approximately equal numbers of
molecules are adsorbed at monolayer coverage.
7
CHAPTER 2
EXPERIMENTAL
Isotherms were determined using the static method (15). Basically a known
mass of adsorbent was equilibrated with a known volume of adsorbate solution of
known concentration. After equilibration, the concentration of adsorbate
remaining was measured, and the amount adsorbed calculated from the
difference in the initial and final concentrations.
Glucose or cellobiose is not readily detected by the UV absorbance detector
of the HPLC. Two detection methods were employed: early work was performed
using a pulsed-amperometric detector (PAD). PAD is a well-known method of
detecting carbohydrates;(16) the carbohydrates are chromatographed in a
strongly basic mobile phase, rendering them anions. They are detected by
oxidization at a gold electrode. Unfortunately, an instrumental failure caused us to
adopt an alternative quantitation method.
Subsequently, a derivatization method was used. This method employed for
derivatization of glucose and cellobiose at their reducing end with paminobenzoic ethyl ester (ABEE). (17) The details of the method are given below.
All work on washed carbon was performed using this derivatization method.
2.1 Chemicals
The adsorbents used were Norit SX and Darco G-60 activated carbons (both
Norit Company). All reported isotherms were measured using the Darco G-60
activated carbon, as that material is frequently used in saccharide purification
8
and analysis (6). The manufacturer's characterization of Darco G-60 is provided
in Table 1, taken from the Norit data sheet. The water used in this research was
obtained from a Millipore Direct-Q® 8 UV-R (EMD Millipore, Billerica, MA). The
grades and suppliers of the other chemicals used are listed in Table 2.
Table 1. Characterizations of Darco G-60 Activated Carbon. This is the carbon
most commonly used for cellodextrin purification. These data are from the data
sheet supplied by Norit America, the manufacturer
Characterization Tests
Results of Tests
Methylene blue adsorption, g/100 g
15 min
Iron, Zacher method, ppm as Fe
200 max
Moisture, % as packed
12max
Water solubles, %
0.50 max
Acid soluble ash, %
0.7
pH, water extract
6
Ash, %
4
Bulk density, tamped, g/mL
0.40
3
Bulk density, tamped, ib/ft
25
Particle size, laser, d5, um
5.5
Particle size, laser, d50, um
34
Particle size, laser, d95, um
125
Food Chemical Codex
Passes
9
Table 2. The Suppliers and Grades of all the Chemicals Used in This Research
Compound
Grade
Supplier
Acetonitrile
HPLC
Fisher Scientific
Methanol
Laboratory
Fisher Scientific
Acetic Acid Glacial
Laboratory
Emdchemicals
ABEE
Laboratory
Fisher Scientific
Sodium
Laboratory
MP Biomedicals
Cyanoborohydride
Cellobiose
Chemical Purpose Eastman Organic Chemicals
Glucose
Reagent
Fisher
2.2 Preparation of Activated Carbon
Prior to use, 4 g of activated carbon powder was stirrred 1000ml MillliQ
water for 4 hours, and the water exchanged three times. The carbon was then
collected by filtration and dried prior to use. Several batches of washed carbon
were combined to provide the material used for isotherm determinations.
2.3 HPLC System
Initially, the carbohydrate concentrations were determined by high
performance anion exchange (HPAE) liquid chromatography with A Shimadzu
LC-20, consisting of a LC-20AT pump, a SIL-20ADvp auto-sampler, and a CTO20Acvp column oven, will provide the basic chromatography. A Bioanalytical
Systems Model SPD-10A(V)vp was used as a pulsed-amperometric detector
(PAD). The quadrupole-potential waveform recommended by Rocklin et al (18).
was used.
10
As mentioned above, an equipment problem caused us to change to a
derivatization-based determination, using reductive amidation to form UVfluorescent derivatives according to the reaction scheme show in Figure 4. This
aminobenzoic acid ethyl ester (ABEE) derivatization is common in carbohydrate
analysis (19). The derivatized products are separated from one another and the
unreacted ABEE by reversed-phase liquid chromatography using the HPLC
working conditions given in Table 3. For this work, a RF-20A Fluorescence
Detector (Shimadzu) was used.
Table 3. The HPLC working conditions
Mobile phase
70% of MilliQ water mixing with 30% of ACN.
Flow rate
1.0mL/min
Column
40°C
Injection volume
5μl
Table 4. All Equipment Used In This Research
Equipment
Supplier
Dry Bath
Fisher Brand
Centrifuge
Micro Centrifuge Model 235B
Balance
Lab companion
SI-300R
Lab companion
RF-20A Fluorescence Detector
Shimadzu
Pulsed Amperometric Detector
Bioanalytical Systems, Inc, West Lafayette,
IN USA
LC-20AT pump
SIL-20 ACHT Auto Sampler
CTO-20A Column Oven
Shimadzu
Shimadzu
Inc, Rescek Corporation, IN USA
11
2.4 Other Equipment
Other equipment included a dry bath instrument, a thermostatted shaker, a
centrifuge, and an analytical balance, also listed in Table 4.
2.5 Procedure
Initial experiments with the PAD showed us that the linear range of the
detector allowed concentrations at least as high as 10 mM but that the lower limit
of quantitation (LOQ) was affected by the choice of high concentration. This is
because the PAD detector signal is sent to an analog-to-digital converter (ACD)
in the Shimadzu system and the resolution of the ADC determines the LOQ.
Adjusting the amplification to allow a smaller LOQ reduces the maximum
concentration represented without clipping. Consequently, the upper limit of
concentration used to some extent affected the lowest concentration used in a
given experiment.
2.5.1 Preparing saccharide samples
Saccharide solutions were prepared with concentrations ranging from 0.0 to
the 5.0mM, with. 11 different concentrations used for each trial: 0.0mM, 0.5mM,
1.0mM 1.5, 2.0mM, 2.5mM, 3.0mM, 3.5mM, 4.0mM, 4.5mM, 5.0mM.
The isotherms were measured as follows:
(i) 0.1 gram of activated carbon were placed in stoppered glass tubes.
(ii) 8mL of each saccharide solution was added to each tube.
12
(iii) The tubes were incubated with shaking at a fixed temperature for at least 8
hours to make sure the saccharide has enough time to adsorb carbon.
(Preliminary experiments indicated that the adsorption of glucose is essentially
complete in 1 hour at room temperature.)
(iv) The carbon was allowed to settle, and an aliquot taken for analysis.
2.5.2 Derivatization procedure
The derivatization reagent was prepared by mixing 7.0mL methanol and
820μL of glacial acetic acid, then adding 3.30 gram of ABEE and, 0.7 gram of
sodium cyanoborohydride were added. The ABEE did not dissolve well at room
temperature, so water bath was used to make solvent warm (around 50°C).
After the adsorption process was finished, the 10μL aliquot of each
saccharide solution before adsorption (11 samples) and after adsorption (11
samples) was put in separated 6×50mm tube. Next, 40μL of ABEE reagent ( the
method of making it was described before) was added to each tube, and the
tubes were placed in a dry bath instrument for 1 hour at 80°C. After 1 hour, the
reaction mixtures were cooled to room temperature, 200 μL of MillliQ water and
200 μL of chloroform were added to each tube, the tubes vortexed for 5 seconds,
and allowed to stand for 1 minute. The mixture separated into two layers—an
upper clear aqueous layer and a lower muddy layer. Fifty μL of the clear upper
aqueous layer was put in a HPLC vial and 400 μL of MilliQ water was added. The
saccharide concentrations before and after adsorption were then measured by
13
HPLC. The only change to collect data at different temperatures was to change
the incubation temperature.
The concentration of each saccharide solution before adsorption was known,
so a linear relationship between saccharide solution concentration and the HPLC
peak area could be determined, which was used to calculate the final
concentration of each saccharide solution after adsorption. Because the volume
of each solution and the weight of carbon were known, the amount of saccharide
adsorbed per gram of carbon (in units of μg/g) could be calculated. Thus, a
series of adsorption data points were determined. Langmuir, Freundlich and BET
equations were fit to the data by nonlinear least squares regression using the
Solver tool in Microsoft Excel.
14
CHAPTER 3
RESULTS AND DISCUSSION
For each experiment, a calibration curve relating the HPLC-determined peak
areas to the prepared saccharide prior to exposure to carbon was prepared.
These calibration curves were described by linear equations. Next, the
saccharide concentration of each solution following adsorption was determined
using the calibration curve. Because the volume of each solution and the weight
of carbon powder were also known, the adsorption capacity of carbon (CS) could
be calculated and expressed in μmol/g .
Then, the isotherm could be determined with CS as the dependent variable
and CM (equilibrium solution concentration) as the independent variable. However,
fitting of adsorption isotherm equations to experimental data is an important
aspect of data analysis. An adsorption isotherm gives the functional relationship
between adsorbate and adsorbent in a constant-temperature adsorption process
(20). A large number of such functional relationships have been applied (14).
One of the more widely used models is the Langmuir model. This model is
based on a simple statistical consideration involving adsorption onto a surface
with a finite number of equivalent sites (21). The Langmuir model has been
applied extensively, even in situations where non-equivalent populations of
adsorption sites are clearly evident (22). While it is highly unlikely that activated
carbon contains a homogeneous population of adsorption sites, the Langmuir
model is so appeallingly simple that it is included here.
15
In contrast, the Freundlich isotherm model is usual considered as empirical,
although it can be derived from an adsorption model where the adsorption
energy decreases exponentially as the number of adsorbed molecules increases.
The Freundlich model has found widespread use and has previously been
applied to mono- and oligosaccharides adsorbed onto activated carbons (23).
Adsorption is also frequently described by the Brauner-Emmet-Teller (BET)
isotherms model. The BET model allows multi-layer adsorption, with each layer
following Langmuir-like behavior (24).
3.1 Equilibration Time
Adsorption isotherms require that the solution concentration of the adsorbate
comes to equilibrium with the adsorbed concentration. Thus, it is necessary to
first determine the length of time required for the establishment of that equilibrium.
Figure 3 shows the solution concentrations of glucose and cellobiose in
equilbrium with Norit SX carbon as a function of time. In each case, 1.00 mL of
10 mM sugar was mixed with 10.0 mg of the carbon and shaken for the indicated
period of time at rom temperature. Following centrifugation and HPLC-PAD
analysis, the remaining solution concentrations were determined. The figure
shows that equilbrium was reached quickly, and that two-hour equilibration times
were adequate. To ensure equilibrium, all isotherm measurements were
performed using overnight (8-12 hours) equilibration times.
16
Figure 3. Solution concentrations of glucose and cellobiose in equilbrium with
Norit SX carbon as a function of time. Ten mL of 10.0 mM aqueous solute was
mixed with 100 mg of carbon, and samples withdrawn at the indicated times.
Analysis with performance by high-performance anion exchange chromatography
using pulsed-amperometric detection.
3.2 Preparation of Carbon
Initial studies were performed using Norit SX carbon, and quantifying the
adsorbed sugar by HPLC. For the temperature studies, the carbon was changed
to Darco G-60 which the carbon most often recommended for the separation of
sugars (25). Both these carbons were used as supplied.
However, an instrumental issue caused us to attempt to perform some work
using absorbance at 191 nm to quantify the amount of sugar adsorbed. Figure 4
shows the absorbance spectrum of the carbon blank, i.e., 100 mg of carbon were
shaken in MilliQ purified water for 12 hours and centrifuged. Figure 4 also shows
the absorbance spectrum of the water itself, the absorbance spectrum obtained
when the carbon was first washed with water and dried before weighing.
17
Figure 4. Absorbance of MilliQ water, water from unwashed carbon, and water
from washed carbon. The absorbance of the MilliQ water was recorded directly,
without contact with carbon. The other two curves represent the absorbances of
water which had been in contact with unwashed or washed Darco G-60 activated
carbon for 8 hours, then recovered by filtration.
While the washing procedure did not remove all water-soluble material,
washing reduced it considerably, as revealed by the difference in the spectra of
water from unwashed carbon and water from washed carbon. More extensive
washing did not significantly reduce the absorbance from this unknown watersoluble material, nor did washing with glacial acetic acid prior to washing with
water (data not shown). Washing with boiling water was also effective at reducing
the amount of water soluble material, but did not eliminate the absorbance
observed in subsequent washes (data not shown).
18
Figure 5. Adsorption isotherms of glucose on washed and unwashed Darco G-60
activated carbon at 20 °C, 25°C and 30°C. Data fit with Langmuir equations.
Figure 5 shows adsorption isotherms obtained on washed and unwashed
Darco G-60 activated carbon at several temperatures. Each data set is fit to the
Langmuir Equation:
qm 
QK
1  QKC
Where K is the equilibrium constant, Q is surface concentration of the adsorbate
on the carbon, C is final concentration. The resultant best-fit parameters are
given in Table 5. As received, the Darco G-60 activated carbon had a watersoluble, UV-active substance adsorbed to the surface. This material increased
the affinity of the surface for glucose, and significantly decreased the saturation
capacity of the surface for glucose, as calculated by the Langmuir equation.
These results strongly suggest that work performed on unwashed adsorbent
does not necessarily reflect the adsorption properties of that adsorbent. And it
19
must be noted that, while the washing employed here reduced the amount of
water-soluble substance, it was not eliminated. Hence, these isotherms, while
likely more correct than those obtained on unwashed activated carbon, may not
completely reflect adsorption to the carbon surface.
All subsequent work was done using Darco G-60 carbon which had been
washed extensively with MilliQ-purified water, dried at 60C, and stored in a
sealed container.
Table 5. Langmuir parameters for glucose and cellobiose adsorbed to unwashed
and washed Darco G-60 activated carbon
Solute
Adsorbent
Temp
K
Q (μg/g)
Glucose
Unwashed carbon 20
0.30
207.64
Glucose
Unwashed carbon 25
0.40
172.22
Glucose
Unwashed carbon 30
0.32
218.17
Glucose
Washed carbon
20
0.03
1028.27
Glucose
Washed carbon
25
0.01
2818.68
Glucose
Washed carbon
30
0.03
920.59
Glucose
Washed carbon
35
0.06
478.46
Cellobiose
Washed carbon
20
0.98
210.02
Cellobiose
Washed carbon
25
0.52
333.77
Cellobiose
Washed carbon
30
0.85
279.31
3.3 Isotherms of glucose and cellobiose
Figures 6-9 show the adsorption isotherm data for glucose adsorbed to
Darco G-60 carbon at 20, 25, 30 and 35°C, respectively. In each case, the
experimental data is shown as points, while lines corresponding to best-fit BET,
Freundlich, and Langmuir equations are also shown. Figures 10-12 show similar
20
adsorption isotherm data for cellobiose,also adsorbed to Darco G-60, at 20, 25,
and 30°C, respectively.
3.3.1 Best-fit model
To select the model in best agreement with the experimental data, the
reduced sum of squared residuals (RSSν), Akaike’s information criterion(AIC)
(26), and the mean absolute percentage error (MAPE) are shown in Table 6. The
RSSν is approximately the reduced chi-squared of the fit, that is, it is the sum of
the square of the distances of each point from the fitted line, divided by the
degrees of freedom of the fit:
N
 (y
fit
RSSv 
- yi) 2
i 1
N  np  1
Where yi is experimental amount adsorbed, yfit is calculated amount adsorbed, N
is the number of data, and np is the number of parameters in model. It is widely
used in fitting; the smaller the RSSν,the better the fit. The AIC is a similar statistic,
but derived from information theory. It is given by:
 SS 
AIC  N  ln   2K
N
Where N is the number of data points, K is the number of parameters fit by the
regression plus one, and SS is the sum of the square of the vertical distances of
the points from the curve. Again, the smaller value of the AIC corresponds to the
better fit. The advantage of using the AIC is that it allows direct comparison of
different models, while, strictly speaking, the RSSν should only be used to
21
compare nested models, such as the Langmuir and BET models. MAPE is
commonly used in quantitative forecasting methods because it produces a
measure of relative overall fit. The absolute values of all the percentage errors
are summed up and the average is computed (27). It is given by:
MAPE 
1 n At  Ft

n t 1 At
Where At is experimental amount adsorbed, Ft is calculated amount adsorbed, n
is the number of data.
Comparing the RSSν values, the fit of the BET equation model has given the
smallest values in 6 of 7 isotherms; for the AIC model statistic, the fit of the BET
equation has gave smaller values in 6 of 7 isotherms. In the case of cellobiose at
20°C, the Freundlich equation fits the data slightly better than the BET equation.
In comparing the MAPE values, the fit of the BET equation model gave the
smallest values in all of 7 isotherms.The BET and the Freundlich equations both
fit the data better than the Langmuir equation. So the BET model is generally the
best-fit model. However, in the present work, the BET equation is used sole due
to it providing the best fit of the data points, not for any implied mechanistic
information.
22
Figure 6. Adsorption isotherm of glucose on Darco G-60 activated carbon at
20°C. Data fit with Langmuir, Freundlich and BET equations.
Figure 7. Adsorption isotherm of glucose on Darco G-60 activated carbon at
25°C. Data fit with Langmuir, Freundlich and BET equations.
23
Figure 8. Adsorption isotherm of glucose on Darco G-60 activated carbon at
30°C. Data fit with Langmuir, Freundlich and BET equations.
Figure 9. Adsorption isotherm of glucose on Darco G-60 activated carbon at
35°C. Data fit with Langmuir, Freundlich and BET equations.
24
Figure 10. Adsorption isotherm of cellobiose on Darco G-60 activated carbon at
20°C. Data fit with Langmuir, Freundlich and BET equations.
Figure 11. Adsorption isotherm of cellobiose on Darco G-60 activated carbon at
25°C. Data fit with Langmuir, Freundlich and BET equations.
25
Figure 12. Adsorption isotherm of cellobiose on Darco G-60 activated carbon at
30°C. Data fit with Langmuir, Freundlich and BET equations.
26
Table 6. Statistical results for fitting model equations to adsorption
Solute
Model
Temp (C)
RSS1
AIC2
Glucose
Langmuir
20
22.60
32.95
BET
20
8.78
22.15
Freundlich
20
16.50
29.80
Langmuir
25
48.38
121.09
BET
25
14.53
82.68
Freundlich
25
41.54
116.37
Langmuir
30
57.49
85.85
BET
30
36.22
76.94
Freundlich
30
54.19
84.61
Langmuir
35
41.59
79.05
BET
35
36.65
77.19
Freundlich
35
38.48
77.42
Cellobiose Langmuir
20
129.46
50.40
BET
20
99.43
48.43
Freundlich
20
98.58
47.68
Langmuir
25
100.36
97.55
BET
25
14.09
57.12
Freundlich
25
29.18
71.61
Langmuir
30
223.29
114.34
BET
30
104.11
99.12
Freundlich
30
158.42
107.13
N
 (y
fit
1. RSSv 
- yi) 2
i 1
N  np  1
, reduced sum of squared residuals.
 SS 
2. AIC  N  ln   2K , Akaike’s information criterion.
N
3. MAPE 
1 n At  Ft

, mean absolute percentage error.
n t 1 At
27
isotherm data
MAPE3
6.64%
5.13%
7.41%
11.35%
6.33%
13.54%
10.89%
9.35%
9.95%
11.00%
10.97%
11.17%
8.52%
5.46%
8.28%
11.03%
2.42%
4.38%
9.71%
7.51%
8.65%
3.3.2 Comparison of fitted model parameters
Although the BET equation accounts for multi-layer adsorption and contains
a parameter which characterizes the attractive interaction between the adsorbed
solution and the free solute in solution (24), we do not propose that the
adsorption of glucose or cellobiose is mechanistically explained by the BET
model. Nonetheless, the Ka1 term in the BET model represents the affinity of the
adsorbent for the adsorbate, and is in the limit of infinite dilution, the Henry's Law
constant for that system. Thus, the fitted Ka1 values should be, if not identical to,
proportional to the true equilibrium constants of interest.
The values of the best-fit BET equations are shown in Table 6. Comparing
the Ka1 values for glucose at different temperatures, there is not a obvious
tendency regarding to the increasing of temperature from 20 to 35°C.
On the other hand, with the increasing of temperature, the K a1 values of
cellobiose increase, and the Γmono values decreases. This implied that with the
increasing of temperature, the affinity of the carbon for the cellobiose is
increasing and the adsorption capacity is decreasing.
Comparing the Ka1 and Γmono values of glucose with those of cellobiose, we
can see, the Ka1 and Γmono values of cellobiose are significant larger than those of
glucose, which can indicate that the affinity of activated carbon for cellobiose is
much stronger than that for glucose. And the adsorption capacity of activated
carbon for cellobiose is also larger than that of glucose, when expressed in μg/g.
However, when the adsorption capacity is expressed in μmol/g, it is nearly
identical for the two sugars.
28
Table 7. Brauner-Emmet-Teller parameters for glucose and cellobiose adsorbed
to washed Darco G-60 activated carbon
1
Adsorbate Temperature Ka11 Ka21 Γmono1 (μg/g) Γmono (μmol/g)
0.75 0.16 47.98
0.27
Glucose
20
1.34 0.19 38.26
0.21
Glucose
25
1.36 0.17 44.79
0.25
Glucose
30
0.84 0.13 55.39
0.31
Glucose
35
2.55 0.12 118.89
0.35
Cellobiose 20
3.03 0.19 112.11
0.33
Cellobiose 25
4.27 0.21 108.97
0.32
Cellobiose 30
The average Ka2 value of glucose (excepting 35C) is 0.17 +/- 0.02 while the
average Ka2 value of cellobiose is0.17 +/- 0.05. Ka2 is the equilibrium constant for
adsorption on the second layer of carbon. The Ka2 values of glucose seem to be
the same as the Ka2 values of cellobiose. Of course this does not prove that the
adsorption equilibrium of glucose and cellobiose on the second layer are identical,
but does suggest that behavior. However, in the concentration range of these
experiments, an entire curve for the second layer of any isotherm was not
measured, and the existence of a second layer is not firmly established, so the
similarity comparation of Ka2 values is suggestive but not conclusive.
As we can see in Table 7, the Γmono values of cellobiose with unit μg/g are
almost twice of glucose. However, in units of μmol/g, the Γmono values are almost
constant. This implies similarity in the conformation of sugar molecules on the
surface of carbon. In general, one can imagine two ways for the sugar molecules
29
to adsorb the carbon surface, either sidewards lying on the surface or end-on,
sticking up on the surface.
(a)
(b)
Figure 13. Two possible ways of sugar molecules staying on the surface of
activated carbon. (a) Sugar molecules sidewards lying on the surface of carbon.
(b). Sugar molecules end-on, sticking up on the surface of carbon.
Γmono represents the surface concentration of the adsorbate on the carbon
which corresponds to a monolayer - full coverage of the interface. If the glucose
and cellobiose are sidewards lying on the surface when adsorbed (Figure 13a),
the Γmono values (in molar terms) of glucose would be expected to be different
30
from those of cellobiose because the length and surface area of glucose are
different from cellobiose. Similarly, if glucose is adsorbed flat while cellobiose is
adsorbed sticking up, the Γmono values would likely be different as well. However,
if glucose and cellobiose are both sticking up on the surface of carbon (Figure
13b), the interface area of each sugar molecule occupying should be the same,
and the Γmono values of glucose and cellobiose would be the same in terms of
moles adsorbed per unit area. The Γmono values of glucose and cellobiose in
Table 6 are almost constant, consistant with the analysis above that both glucose
and cellobiose are adsorbed end-on, sticking up from the surface of activated
carbon.
3.4 Effect of temperature and thermodynamics of adsorption
Figure 14 shows the isotherms of glucose at 20°C, 25°C, and 30°C. The
amount adsorbed increased with increased temperature in this temperature
range. Interestingly, the isotherm of glucose at 35°C showed less adsorption than
the lower temperatures. The 35°C isotherm shown represents two independent
sets of data, acquired on different days using the same reagents and techniques
as were used for the data acquired at lower temperatures. (Actually, each
isotherm in Figures 14 and 15 represent two independent experiments.) Hence,
this decrease in adsorption is most likely real, not some odd experimental error. A
possible explanation for this oddity will be presented below.
For cellobiose, the isotherms were similar in that the amount adsorbed
increased with the increasing of temperature. The isotherms of cellobiose are
31
shown in Figure15.
Figure 14. Adsorption isotherms of glucose on Darco G-60 activated carbon at
20°C, 25°C, 30°C and 35°C. Original data points are omitted to make the curves
clear.
Figure 15. Adsorption isotherms of cellobiose on Darco G-60 activated carbon at
20°C, 25°C, and 30°C. Original data points are omitted to make the curves clear.
According to van 't Hoff equation:
, where K is the
equilibrium constant, ΔHo is standard enthalpy change, T is temperature, ΔSo is
32
standard entropy change. Thus, -lnK should have linear relationship with 1/T if
ΔHo keeps constant. Figure 16 shows ln Ka1 vs 1/T for the BET parameters fit to
the adsorption data of glucose and cellobiose. While three data is hardly
definitive, the plot for adsorption of cellobiose onto carbon does appear
reasonably linear. But the adsorption data for glucose onto carbon does not show
a linear relationship. The limited number of data do allow for experimental error
as being the originating factor of this curvature in the van't Hoff plot, but it is also
possible that ΔHo is temperature sensitive in this regime. Again, the isotherm at
each temperature represents two entirely independent experiments. Thus it
seems most likely that -lnK and 1/T don’t have a linear relationship because ΔHo
is changing with temperature. One possible explanation for this nonlinearity is the
temperature-dependence of the structure of glucose. While we refer to the
adsorbate as “glucose,” it is in fact an equilibrium mixture of α-glucopyranose, βglucopyranose, the corresponding glucofuranoses, and the open-chain form (1).
The equilibria among these forms are temperature dependent (28). Thus, the
adsorption being modeled as a single component system is actually a complex,
competitive adsorption system, which may account for the van't Hoff nonlinearity
as well as, e.g., the decreased adsorption of glucose at 35°C.
33
Figure 16. Plot of lnKa1 and 1/T for glucose and cellobiose adsorption on G-60
activated carbon. The upper tendency line is for three cellobiose data points at
20°C, 25°C and 30°C. The other tendency with the same length and similar slop
is for glucose data points at 20°C, 25°C and 30°C. The longest tendency line is
for all the four glucose data points at 20°C, 25°C, 30°C and 35°C.
The data for glucose in Figure 16 show a strong deviation from linearity, if
taken as a whole. Over the 20-35°C range, the least-squares best straight line is
y = -760x + 2.6,
corresponding to a weakly endothermic adsorption process. However, if we
consider Figure 16 to represent two distinct adsorption processes for glucose,
then the adsorption processes for glucose and cellobiose are both endothermic
from 20°C to30°C, and they have very similar ΔHo values, while for glucose, it is
exothermic from30°C to 35 . This interpretation also implies a significant change
for glucose in the entropy associated with the adsorption process, i.e.,
significantly different mechanisms above and below 30°C. Given the energetics
associated with hydrogen bonding, and the large numbers of possible hydrogen
bonds, this is not impossible. However, additional data should be gathered to
substantiate the behavior before making any such interpretation.
34
CHAPTER 4
SUMMARY AND CONCLUSION
In this research, the adsorption behaviors of glucose and cellobiose on
activated carbon was examined. The experiment was carried out using HPLC to
measure the concentrations of solutions after adsorption using a derivatization
method for the detection of glucose and cellobiose by HPLC. Langmuir,
Freundlich, and BET equations were fit to the adsorption data; the BET equation
gave the best fit by a number of statistical criteria.
From the isotherms, the amount of cellobiose adsorbed increases with
increasing temperature in the temperature range examined. The amount of
glucose adsorbed onto carbon is almost the same as the amount of cellobiose
adsorbed, when measured in moles per gram, which means they are apparently
adsorbed to the surface in the same way, standing on the surface of carbon with
the same cross-sectional area. The thermodynamic analysis shows the
adsorption process for glucose from 20°C to 30°C is exothermic and from 30°C
to 35°C is endothermic; for cellobiose from 20°C to 30°C the process is
exothermic. The adsorption behavior is likely complicated by the conformational
equilibria for these species.
This study indicates areas for further research: (a) Additional measurements
of glucose isotherms in the 10-30°C region, to better refine the thermodynamic
parameters associated with this adsorption. (b) Additional measurements of
these isotherms beyond 30°C, which appear to be controlled by different
thermodynamic parameters. Of course, the temperature sensitive conformational
35
equilibria for glucose must be considered. (c) Isotherm determinations for
additional cellodextrins, e.g., cellotriose, cellotetraose, etc. (d) Competitive
adsorption isotherms. This last is particularly interesting as providing insight into
the chromatographic conditions best suited for preparative isolation of individual
cellodextrins.
36
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40
APPENDIX
Table A, B, C give the least-squares, best-fit parameters for Langmuir, BET
and Freundlich isotherm equations for each set of adsorption data. Each set of
adsorption data represents two separate trials, performed on different days.
Appendix A. Langmuir parameters for glucose and cellobiose adsorbed to
washed Darco G-60 activated carbon.
Glucose onto washed carbon Cellobiose onto washed
carbon
Temp
K
Q
K
Q
20
0.026
1028.30
0.98
210.02
25
0.010
2818.69
0.52
333.77
30
0.033
920.59
0.85
279.31
35
0.064
478.46
Appendix B. BET parameters for glucose and cellobiose adsorbed to washed
Darco G-60 activated carbon.
Glucose onto washed carbon Cellobiose onto washed
carbon
Temp
Ka1
Ka2
Γmono
Ka1
Ka2
Γmono
20
0.75
0.16
47.98
2.55
0.12
118.89
25
1.34
0.19
38.26
3.03
0.19
112.11
30
1.36
0.17
44.80
4.27
0.21
108.97
35
0.84
0.13
55.39
Appendix C. Freundlich parameters for glucose and cellobiose adsorbed to
washed Darco G-60 activated carbon.
Glucose onto washed carbon Cellobiose onto washed
carbon
Temp
K
1/n
K
1/n
20
24.09
0.96
97.51
2.06
25
24.658
0.90
111.16
1.65
30
30.57
1.09
120.85
1.83
35
29.90
1.17
41