The document describes research on using exfoliated graphitic nanoplatelets (xGnPs) to adsorb the azo dye Acid Orange 7 from aqueous solutions. The effects of temperature, pH, and initial dye concentration on the adsorption process were studied. Kinetic data fit the pseudo-first-order and pseudo-second-order models well. Adsorption isotherms followed the Langmuir model. Analysis of the kinetics and isotherms provided insight into the adsorption mechanism and rate determining steps. The xGnPs were found to efficiently and effectively remove the acid dye from solutions through an exothermic adsorption process governed by chemical reaction kinetics.

1. Indian Journal of Chemical Technology
Vol. 20, January 2013, pp. 7-14
Film-pore diffusion modeling for sorption of azo dye on to exfoliated
graphitic nanoplatelets
T Maiyalagan1 & S Karthikeyan2,*
1
School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639 798
2
Department of Chemistry, Chikkanna Government Arts College, Tirupur 641 602, India
Received 8 September 2011; accepted 8 May 2012
Exfoliated graphitic nanoplatelets (xGnPs) have been utilized as a potential adsorbent for toxic textile dye Acid Orange
7 (acid dye). The effects of major variables governing the efficiency of the process, such as temperature, initial dye
concentration and pH are studied. The kinetic measurements have been used for determining the specific rate constant,
confirming the applicability of pseudo first-order rate expression. Plausible mechanism of ongoing adsorption process
involved is obtained by carrying out kinetic measurements. To identify whether the ongoing process is particle diffusion or
film diffusion, the treatments given by Boyd and Reichenberg have been employed. The influence of different factors on the
adsorption of Acid Orange 7 from solution is explained in terms of electrostatic interaction by considering the dye species
and the surface character of the xGnPs. The developed system for the removal of acid dye is found to be very useful,
economic, rapid and reproducible.
Keywords: Acid Orange 7, Adsorption, Film diffusion, Graphitic Nanoplatelets, Particle diffusion
The adverse effects of discharge of organic pollutants of catalytic degradation and hypo chloride treatment of
dyeing industry waste on health have been already dye waste effluents2,3. Among these approaches,
proved. Textile effluents are known toxicants, which adsorption is regarded as an easy and economic
inflict acute disorders in aquatic organisms. Uptake of process. This is attributed to its easy availability,
textile effluents through food chain in aquatic organisms simplicity of design, ease of operation, various
may cause various physiological disorders like materials, such as commercial activated carbon, natural
hypertension, sporadic fever, renal damage and cramps1. materials, bio adsorbents and wastes from agriculture,
The release of colored waste water into the eco-system have been used for such processes4.
is a dramatic source of the aesthetic pollution, The rapid development in nanotechnology sheds
eutrophication and perturbation in aquatic life. light on the waste water treatment. Nano materials
Brightly coloured and water soluble acid dyes, being have been studied for the adsorptions of metal
sodium salts of organic sulphonic acids, are composed ions5, dyes6, and antibiotics7. Exfoliated graphitic
of ionisable anionic groups such as sulphonates, nanoplatelets (xGnPs) and graphite nano sheets8 have
carboxylates or sulphates. They have direct affinity for been successfully utilized as sorbents to extract oils9
polyamide and protein in an acidic bath and hence are and dyes10 from their aqueous solutions. In the present
commonly used for dying polyamide, as well as nylon, work, exfoliated graphitic nanoplatelets (xGnPs)
silk, wool and modified acrylics; also used to some have been used as an adsorbent for Acid Orange
extent for paper , leather and cosmetics. Acid dyes 7 (AO7) removals and the adsorption capacity of
with higher molecular weight are one of the most xGnPs is regulated by many influencing factors,
problematic groups of dyes which tend to pass through such as temperature, pH variations and initial
conventional treatment system unaffected. Various dye concentrations.
physical and chemical methods of treatment of
industrial waste water have been suggested. These Experimental Procedure
include adsorption method, coagulation process, photo xGnPs (with average diameter of 15 µm and
average length of <0.01 µm) were procured from
——————
*Corresponding author. xG Sciences Inc., USA. Detailed information on
E-mail: skmush@rediffmail.com fabrication, geometrical and surface characteristics

2. 8 INDIAN J. CHEM TECHNOL., JANUARY 2013
of this material is already available11. The textile C.I. Number : 15510
dye (AO7) was purchased from Sigma - Aldrich Natural pH : 6.1
(Germany), and characterization of the dye are Molecular formula : C16H11N2NaO4S
summarized in Table 1. All the chemicals used were Molecular weight, g mol-1 : 350.33
obtained as research grade chemicals and used pKa : 8.86
without purification.
Molecular volume, Å3 molecule-1 : 231.95
Characterization Molecular dimension, nm : 1.24×0.68×0.22
Morphological structure of as-received xGnPs was Batch adsorption studies were carried out in
characterized by the scanning electron microscopy. 250 mL tight lid glass bottle (Borosil R). Standard
The sample was directly coated on the conductive stock solution (1000 mg/L) containing Acid Orange 7
surface and SEM images were obtained with was prepared by dissolving appropriate amount of it
a field-emission scanning electron microscope in water. 50 mg of adsorbent was added to 100 mL of
(FESEM, JEOL JSM-6700F). BET measurements aqueous dye solution, initial concentration of AO7
were performed by using ASAP 2020 volumetric ranging from 20 mg/L to 60 mg/L. The contents of the
adsorption analyzer (Micrometrics, USA). The flasks were agitated by placing them in temperature
surface functional groups on the adsorbents were controlled orbital shaker. The mixture was withdrawn
quantitatively measured by Boehm’s titration at specified intervals then centrifuged using electrical
method12. The Boehm titration is based on the centrifuge (universal make) at 3000 rpm for 10 min
principle that oxygen groups on graphite surface have and un adsorbed supernatant liquid was analyzed
different acidifies being neutralized by bases of for residual dye concentration using Elico make Bio
different strengths. In our procedure, 20 mg of xGnPs UV-Visible spectrometer (BL-198) at a wave length
(as-received) were stirred in 10 mL of 0.05 M base of 484 nm. All the experiments were conducted in
solution (NaOH, Na2CO3, and NaHCO3), aqueous duplicate and mean of the two values were taken for
solution under Ar for 48 h, (in order to equilibrate calculation. Maximum deviation is 4%. The amount
with the NaHCO3 solution). The mixtures were of AO7 adsorbed in mg/L at time t was computed by
filtered (on 0.20 µm pore size membrane filters), using the following equation:
10 mL volume from each mixture being further C0 − Ct
titrated with 0.05 M hydrochloric acid. Three samples qt = ×V … (1)
ms
of each base solution were titrated; a blank sample
without xGnPs is being titrated with the same where C0 and Ct are the AO7 concentration in mg/L
procedure. NaOH solution neutralizes all acidic sites initially and at a given time t respectively; V, the
(carboxyl, lactonic and phenols) from the surface of volume of the AO7 solutions in mL; and ms, the
xGnPs; NaHCO3 neutralizes only carboxyl groups, weight of the xGnPs. The removed AO7 (%) in
Na2CO3 reacts with carboxyl, lactonic groups. solution was calculated using the following equation:
The quantity of the possible surface groups is C0 − Ct
estimated through the difference between the % Removal = ×100 … (2)
calculated amount of surface functionality. The pH C0
of the point of zero charge (pHpzc) was determined Adsorption dynamics and equilibrium studies
using the pH drift method13. The Acid Orange 7 dye The study of adsorption dynamics describes the
was used without purification. The characteristics solute uptake rate, and evidently this rate controls the
of dye are shown below: residence time of adsorbate uptake at the solid-
Table 1 –– Kinetic parameters for the adsorption of AO7 onto xGnPs
Concentration Pseudo first-order values Elovich values Pseudo second-order values
mg/L -2
kLager × 10 R 2
α Β R 2
qe k2 × 10-2 h R2
min-1 mg/g min g/mg g/mg min
20 1.7306 0.986 0.869 0.606 0.826 18.712 7.458 0.804 0.962
40 1.7186 0.982 0.768 0.271 0.899 19.038 1.438 0.558 0.980
60 1.7094 0.985 0.667 0.199 0.923 46.283 0.656 0.655 0.975

3. MAIYALAGAN & KARTHIKEYAN: FILM-PORE DIFFUSION MODELING FOR SORPTION OF AZO DYE 9
solution interface. The kinetics models of AO7 influenced by the two factors, namely (i) distribution
adsorption on the xGnPs were analyzed using pseudo of the dye ionized species in the solution phase, and
first-order14, pseudo second-order15, kinetic models (ii) overall charge of the adsorbent. Therefore, the
and Elovich equation16. The isotherm models of AO7 interaction between dye molecule and adsorbent is
adsorption on the xGnPs were analyzed using basically a combined result of charges on the dye
Langmuir and Freundlish equation17. molecules and the surface of the adsorbent18. The
effect of pH on the adsorption of AO7 by as-received
Results and Discussion xGnPs has been evaluated in pH range 2-11. Which
Characterization of xGnPs reveals that the removal of dye slightly decreases,
The dye adsorption on xGnPs depends on many when the pH is increased from 2-4 and then remain
factors, such as surface functional groups, specific almost constant up to pH 8. A large decrease in
surface area and composition of the solution, the most adsorption capacity for this dye is observed as the
important factor being the surface area. BET surface pH approaches pKa of AO7 under basic condition.
area measurement of xGnPs asserts the large When the solution pH is above the pKa of dye
hysteresis area of N2 adsorption-desorption isotherm (pKa for AO7 is 8.86), the adsorption decreases due
(Fig.1), suggesting the wide distribution of pores. The to the electrostatic repulsion between dissociated
specific surface area of xGnPs calculated using BET adsorbate and adsorbent surface. Below the isoelectric
equation is found to be 112.67 m2/g. Large hysteresis point (pHpzc of the xGnPs is 8.1), surface of adsorbent
area indicates a near uniform distribution of pores and may acquire positive charge leading to an increased
large surface area of xGnPs, suggesting the high anionic dye adsorption due to electrostatic
quality of graphene sheets. The SEM image reveals attraction19.
that the flat surface of xGnPs is homogeneous Effect of initial concentration of dye solution
(Fig. 2), the property being responsible for this The initial concentration of AO7 solution was
selectivity. The large layer of carbon containing varied (20 40 and 60 ppm) and batch adsorption
delocalized π electrons can explain the different experiments were carried out with 100 mg of
retention mechanisms such as electron transfer, the adsorbent at 30oC and pH 7. An increased
ion-pairing and hydrophobic interaction. The surface percentage removal of AO7 from 75 to 90 is observed
area and pH pzc are consistent with the literature18. The with 100 mg of the adsorbent, when the initial
properties of xGnPs specific surface area (SBET), concentration of the AO7 solution is increased
pHpzc, carboxylic acid, lactone groups, and phenolic from 20 ppm. The higher uptake of AO7 at low
groups values are 112.67 m2 g-1, 8.1, 0.25 meq g-1 concentration may be attributed to the availability
0.48 meq g-1 and 0.36 meq g-1 respectively. of more active centers on the surface of the adsorbent
Effect of pH
for lesser number of adsorbate species.
The pH value of the solution being an important
controlling parameter in adsorption is mainly
Fig. 1 –– Nitrogen adsorption–desorption isotherm of xGnPs Fig. 2 –– SEM image of exfoliated graphitic nanoplatelets (xGnPs)

4. 10 INDIAN J. CHEM TECHNOL., JANUARY 2013
Effect of temperature it is thus necessary to perform multiple regressions on
Temperature influences the AO7 adsorption different ranges of the data. The kinetics could not be
properties on xGnPs. The temperature effect on the approximated using Elovich model.
sorption capacity of xGnPs was examined at 30, 45
Pseudo second-order model
and 60°C using initial dye concentration of 20 mg/L The same data are shown as pseudo second-order
at pH 7. The adsorption capacity of the xGnPs equations in Fig. 5. These plots show that the data fits
increases with decreasing temperatures from 60°C has good correlation coefficients (>0.962) when the
to 30°C, which indicates that the adsorption process pseudo second-order equation is employed. It is
is exothermic. The optimum temperature for dye possible to ascertain from them whether the rate
adsorption of the adsorbent, within the temperature determining process is a chemical reaction. Thus,
range studied, is found to be 30°C. increasing the initial dye concentration from 20 mg/L
Kinetic modeling to 60 mg/L the AO7 sorbed at any contact time
increases. This is obvious for higher initial
Pseudo first-order model
concentration values, as a more efficient utilization of
Figure 3 shows a plot of pseudo first-order the sorption capacities of the adsorbent would be
equation for the results of adsorption of AO7 from expected due to greater sorption driving force.
20 mg/L to 60 mg/L between log (qe-qt) and agitation
time over whole sorption period with high correlation Isothermal modeling
coefficient (>0.986) for all the lines (Table 1). It is The Langmuir adsorption isotherm obtained in
clear that the pseudo first-order equation may be used 160 min of agitation time is shown in Fig. 6.
to describe the kinetics of sorption of AO7 on to
xGnPs. Although the pseudo first-order equation does
not provide any mechanistic evidence, it has been
proved suitable for highly heterogeneous systems of
which the adsorption of AO7 onto xGnPs is
undoubtedly such a case.
Elovich model
The results of the sorption of AO7 on to xGnPs
have been represented in the form of Elovich equation
in Fig. 4 at various initial dye concentrations (20, 40,
and 60 mg/L). From the plot a linear relationship
between the amount of AO7 adsorbed, qt and ln(t) are
established. These plots show different distinct linear
regions within individual sets of data. In these cases,
Fig. 4 –– Elovich plot for adsorption of AO7 onto xGnPs
Fig. 3 –– Pseudo first-order plot for adsorption of AO7 onto xGnPs Fig. 5 –– Pseudo second-order plot for adsorption of AO7 onto xGnPs

5. MAIYALAGAN & KARTHIKEYAN: FILM-PORE DIFFUSION MODELING FOR SORPTION OF AZO DYE 11
The values of RL obtained in this study lie within best-fit description for the sorption of AO7 on to
the range 0.127-0.151 indicating the favorable case xGnPs relative toFreundlish isotherm model.
of adsorption for the present adsorbent-adsorbate Table 3 shows the adsorption capacity of some
system. The Freundlich adsorption isotherm obtained of the adsorbents used for the adsorption of Acid
in 160 min of agitation is shown in Fig. 7. The values Orange 720-25. It is observed that the adsorption
of absorption intensity 1/n<<1 reveal the applicability capacity of exfoliated graphitic nanoplatelets is
of the Freundlich adsorption isotherm in Fig. 7. comparatively good when compared with some of the
The values of 1/n and kf are given in the Table 2. adsorbents already reported in the literature for the
The study of temperature effects on the Freundlich adsorption of Acid Orange 7 in aqueous solution.
parameters reveals a decreasing trend in the The differences in maximum adsorption efficiencies
adsorption capacity with increase in temperature. of various sorbents might be due to different
However, the variation in the adsorption intensity is structures and sorption mechanisms of various
negligible. These data are useful for practical design sorbents and experimental conditions.
purposes. Langmuir adsorption isotherm provides a
Mechanism for sorption of AO7 onto xGnPs
Because of the high correlation coefficients
obtained using pseudo first-order, pseudo second-
order and Elovich kinetic models, it is impossible to
conclude which adsorption mechanism actually occur
and is responsible for the ability of adsorbent to
review other sources of information in an attempt to
identify the specific adsorption mechanism.
In adsorption process of dye on the solid surface,
the dye species migrate towards the surface of the
adsorbent. This type of migration proceeds till the
concentration of the adsorbate species, adsorbed on to
the surface of the adsorbent. Once equilibrium is
Fig. 6 –– Langmuir plot for adsorption of AO7 onto xGnPs attained, the migration of the solute species from the
solution stops. Under this situation, it is possible to
measure the magnitude of the distribution of the
solute species between the liquid and the solid phases.
The magnitude of this kind of distribution is a
measure of the efficiency of the chosen adsorbent and
the adsorbate species.
When graphene platelets are made to contact
with a solution containing dyes, the dyes first migrate
from the bulk solution to the surface of the liquid
film. This surface exerts a diffusion barrier. This
barrier may be very significant or less significant. The
involvement of a significant quantum of diffusion
barrier indicates the dominant role taken up by the
Fig. 7 –– Freundlich plot for adsorption of AO7 onto xGnPs film diffusion in the adsorption process. Furthermore,
Table 2 –– Parameters of Langmuir and Freundlich adsorption isotherm models for AO7 on xGnPs
Temperature Langmuir isotherm Freundlich isotherm
o
C
b Q0 RL R 2
1/n n kf R2
L/mg mg / g
30 0.06826 85.172 0.151 0.987 0.5939 1.485 1.484 0.897
45 0.06637 92.132 0.121 0.979 0.6381 1.694 4.132 0.902
60 0.06571 117.332 0.127 0.981 0.6524 1.7034 6.345 0.887

6. 12 INDIAN J. CHEM TECHNOL., JANUARY 2013
Table 3 –– Comparison of maximum adsorption capacity for (iii) adsorptions of the ingoing ion (adsorbate) on the
Acid orange 7 on other different adsorbents. interior surface of adsorbent.
Adsorbent Adsorption capacity Reference Out of these three processes the third process is
mg/g considered to be not the limiting step in the uptake of
Canola stalks 25.06 20 dyes on to xGnPs28. The remaining two steps impart
Beech wood sawdust 5.06 21 the following three possibilities:
Spent brewery grains 30.5 22 Case I − External transport < internal transport, where
Soil 3.47 23 rate is governed by particle diffusion.
Waste Brewery’s yeast 3.56 24
Case II − External transport > internal transport,
Untreated S. marginatum 35.62 25
Exfoliated graphitic 85.172 Present study
where rate is governed by external diffusion.
nanoplatelets Case III − External transport ≈ internal transport,
where the transport of the adsorbate ions to the
the rate of an adsorption process is controlled either
boundary may not be possible with significant rate,
by external diffusion, internal diffusion or by both
this may result into a possibility of formation of a
types of diffusion.
liquid film surrounded by the adsorbent particles with
The adsorption of adsorbent on graphene layer is a proper concentration gradient.
remarkably different from other conventional porous
carbons in several aspects. First, due to their two- In the present study, the quantitative treatment of
dimensional nano structure, the external surface the sorption dynamic is found in accordance with the
available for adsorption is considerably larger than the observation of Reichenberge29, as described by the
surface area arising from inner cavities. The following equation:
predominance of outer cavity surface area to inner cavity 6 ∞
1
surface area determines the adsorption characteristics of F =1−
π 2 ∑n
N −1
2
exp [ − n 2 β t ] … (3)
dyes on xGnPs. The adsorption on the external surface
of graphene nanoplatelets is more important than the where F is the fractional attainment of equilibrium at
adsorption inside micro/mesoporous cavities. Another time t; and n, the constant30.
noteworthy difference should be ascribed to the Qt
interstitial space between individual graphene sheets. F= … (4)
The dimension of this space is determined by the Q∞
relative positions among individual graphene sheets. where Q1 and Q∞ are the amounts adsorbed after time
In the batch mode contact time adsorption t and after infinite time respectively.
experiments, rapid stirring is maintained. This induces
AO7 from the solution to the external surface of
π 2 Di
B= = time constant … (5)
the adsorbent material and this step may control the r02
rate of the adsorption process26. To interpret the where Di is the effective diffusion coefficient of
experimental data it is necessary to recognize the adsorbate in the adsorbent phase; and ro, the radius of
steps involved in the process of adsorption that adsorbent particles.
govern the overall rate of removal of dye. The For energy observed values of F, corresponding
ingenious mathematical treatments recommended by values of Bt are derived from Reichenberg̕ s table38. In
Boyd et al.27 have been applied. These mathematical each case the plot of Bt vs time distinguishes between
treatments are found to be useful to distinguish the processes involved film diffusion and particles-
between particles diffusion and film diffusion. diffusion controlled rate of adsorption.
The successive steps in the adsorption dyes by Typical Bt vs time plots at the concentration
adsorbents are: 20 mg/L of AO7 adsorbed on xGnPs at different
(i) transport of adsorbates to the external surface of temperature are represented in Fig. 8. It is found to be
adsorbent (film diffusion); non-linear throughout the temperature 30, 45 and
(ii) transport of adsorbates within the pores of the 60°C, thus the process involved can be represented as
adsorbent, except for a small amount of film diffusion. At 30°C the adsorbent exhibits linearity
adsorption, which occurs on the external surface in Bt vs time plots in the entire concentration range,
(particle diffusion); and but the straight lines obtained do not pass through

7. MAIYALAGAN & KARTHIKEYAN: FILM-PORE DIFFUSION MODELING FOR SORPTION OF AZO DYE 13
values of ∆ S# reflect that no significant change
occurs in the internal structure of chosen adsorbent
using the adsorption process.
Conclusion
The study shows that xGnPs is an effective
adsorbent for the removal of AO7 from aqueous
solution. The adsorption of AO7 is dependent on the
initial concentration and agitation time. Equilibrium
of AO7 adsorption reaches at 160 min.
The pseudo first- and second-order equations
provide a best fit description for the sorption of
AO7 onto xGnPs related to Elovich model, but the
Fig. 8 –– Time vs Bt plots at different temperature of AO7 - xGnPs pseudo first-order correlation coefficient has better
adsorption correlation value than pseudo second-order equation,
Table 4 –– Values of energy of activation (Ea), entropy of Pseudo first-order equation is consider to be the
activation ( S#), effective diffusion coefficient (Di) and most appropriate due to high correlation coefficient
pre-exponential factor (Do) when compared to pseudo second-order equation, and
Parameter Value adsorption takes place via film diffusion process.
Di, cm2s-1 Langmuir and Freundlich adsorption isotherms
30° C 1.4687 × 10-11 correlate the equilibrium adsorption data. The
45° C 1.313 × 10-11 adsorption of AO7 onto xGnPs is an exothermic
60° C 1.093 × 10-11 reaction based on enthalpy change values.
Ea, kJmol-1 -9.7153
S#, JK-1mol-1 -179.53 Acknowledgement
Do, cm2s-1 9.4932 × 10-12 The authors acknowledge with thanks the support
origin, revealing thereby that the rate-determining of Department of Chemistry, Chikkanna Govt.
process is film diffusion at this temperature for Arts College, Tirupur and Sophisticated Analytical
chosen adsorbent. Instrument Facility, Indian Institute of Technology,
The Di values were also calculated for each Madras for characterization process.
adsorbent material at the three different temperatures
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