Journal of Materials Science and Chemical Engineering, 2013, 1, 1-5 Published Online February 2013 (
Copyright © 2013 SciRes. MSCE
Adsorption Capacity of Expansion Graphite for
Xylenol Orange*
Xiuyan Pang1*, Chunyan Yang1,2, Shuxia Ren1
1College of Chemistry and Environmental Science, Hebei University, Baoding, China
2Baoding Tianwei Wind Power Blade Co. Ltd, Baoding, China
Email: *
Received 2013
Expansion graphite (EG) processing of an expanded volume of 400 mL/g was prepared with 50 mesh crude graphite
after chemical oxidation intercalation of potassium permanganate and vitriol, and its adsorption kinetics and thermody-
namics characteristics for xylenol orange (XO) was investigated. In thermodynamic study, adsorption isotherm and free
energy change (G°) were detected and calculated, respectively. Influence of ionic strength on adsorbance was inves-
tigated. Kinetic studies were carried out with a series of XO concentration under different temperatures, and the data
were simulated with pseudo first-order and second-order kinetic model, respectively. Results illustrate: adsorption of
EG for XO is a spontaneous process, and adsorption isotherm is type II; equilibrium adsorbance increases with the in-
crease of ionic strength. Kinetic studies show that the kinetic data can be delineated by pseudo second-order kinetic
model. Initial adsorption rate increases with the increase of temperature. Adsorption activation energy is less than 20
kJ/mol; physical adsorption is the major mode of the overall adsorption process.
Keywords: Expansion Graphite; Xylenol Orange; Adsorption Kinetics; Adsorption Thermodynamics; Physical
1. Introduction
Expansion graphite (EG) is a kind of porous material
prepared through chemical oxidation or electric chemical
oxidation [1,2]. EG had attracted attention of scientists
and engineers as an adsorbent of organic substance, such
as heavy oil [3-7]. As for the adsorption study for dyes,
Wang pressed the worm-like particles into a low-density
plate of 0.1g/cm3 [8], then the plate was used to treat dye
waste-water from woolen mill, and the optimum apply-
ing condition was tested. Wang investigated the influ-
ence factors in adsorption process [9], and concluded that
the adsorption capacity was affected by EG expansion
volume and dosage, primary concentration of dyes, con-
tact time, pH and temperature. But they neglected the
influence of pH both on absorbency and adsorption ca-
pacity, and improper pH was used. At the same time,
high EG dosage along with dyes low initial concentration
caused adsorption isotherms of the tested dyes were all
type I. Pang investigated the adsorption kinetic charac-
teristics of Acid Red 3B on EG [10], and the kinetic data
could be delineated by pseudo second-order kinetic
Xylenol orange (XO) is one kind of dye with triphe-
nylmethane structure, it is widely used as chemical indi-
cator and dye, and then causes plenty of wastewater. In
its decoloration with coal powder [11] as adsorbent, the
influence of adsorbent dosage, XO primary concentration,
contact time and pH was investigated. Under the condi-
tion of pH 4, dosage of coal powder 8.0 g/L and XO ini-
tial concentration of 7.6 mg/L, A decoloration rate of
85% was gained after a 2.0 h decoloration treatment.
With chitosan microsphere as adsorbent [12], the opti-
mum decoloration condition of XO was gained as: pH 5,
2.0 h, dosage of chitosan microsphere with a diameter of
74 μm 0.3 g/L and XO initial concentration of 32 mg/L.
As an adsorbent, EG adsorption capacity for XO has not
been reported. The aim of the study is to investigate the
adsorption characteristics of EG-XO system in water
solution, and discuss the effect of ionic strength, concen-
tration, temperature, and do further evaluation of appli-
cability of common isotherm model (i.e., Langmuir and
Freundlich) and pseudo-second-order rate model.
2. Experimental
2.1. Adsorbent
EG was prepared according to [13]. Its pore distribution
was detected with Micromeritics Instrument Corporation
TriStar II 3020 V1.02, and pore structure parameters
were calculated with BET method and shown in Table 1.
2.2. Adsorbate
XO molecular structure and molecular weight is shown
in Table 2. Its simulated wastewater was prepared by
dissolving the dye in distilled deionized water at various
concentrations. Color measurements were made with T6
New Century UV spectrophotometry (Puxi Tongyong
Instrument Limited Company of Beijing) operating in the
visible range on absorbance mode. Absorbance values
were recorded at the wavelength for maximum absorb-
ance (λmax), and XO solution was initially calibrated for
concentration in terms of absorbance units.
2.3. Methods
Static adsorption of XO: 0.200 g of EG is mixed in con-
ical glass flasks with 100 mL solution at the desired XO
concentration and ionic strength. Ionic strength is ad-
justed with NaCl or Na2SO4 solution. Adsorption ad-
sorbance was calculated according to Equation (1).
qVC CM (1)
q: Accumulative adsorbance of XO on EG at the moment
of t, mg/g; Ct: Concentration of XO in solution at the
moment of t, mg/L; M: mass of EG
Adsorption kinetics experiments of XO: Adsorption
kinetics experiments were carried out using a HZS-D
shaking water bath with a shaking speed of 100 rpm/min.
A series of desired XO concentration and a fixed volume
of 100 mL were placed in vessels, where they were
brought into contact with 0.200 g EG at 15 °C, 25 °C and
35 °C, respectively. Amount of XO captured by EG at
different time is determined according to Equation (1).
Table 1. Structural parameter of EGa.
Total pore
Adsorption average
pore width
(4V/A) nm
400 mL/g 0.101 34.3 11.61
a. Analysis adsorptive: N2; Sample mass: 0.1451 g; Equilibration interval:
10 s; Surface area or pore volume of pores between 1.7000 nm and
300.0000 nm diameter
Table 2. Chemical structure and quantitative method of XO.
Structure λmax/nm Working curve
436 Y= 0.01922+
3. Results
3.1. Adsorption Thermodynamics
Investigation of adsorption isotherm and thermodynamic
parameters: Static adsorption capacity of XO was de-
tected as Figure 1. It is a typical II type isotherm, multi-
layer adsorption occurs on EG surface. In the condition
of monolayer adsorption, the thermodynamic data were
treated with Langmuir and Freundlich isotherm Equa-
tions (2) and (3), respectively. As shown in Table 3,
Langmuir isotherm gives better results than Freundlich
isotherm, and the monolayer saturation adsorbance of
XO is 18.15 mg/g. But the total adsorbance increase with
the increase of XO initial and equilibrium concentration
due to its multilayer adsorption.
Langmuir equation: (2)
e0 0e
1/q= 1/q + A / (qC)´
Freundlich equation: (3)
lnq= lnK + (1/ n) lnC
q0: Saturation adsorption amount of XO in forming com-
plete monolayer coverage on EG pore surface, mg/g; A:
Equilibrium concentration of XO corresponding to half
saturation adsorbance, mg/mL; KF: Freundlich equation
constant; 1/n: Adsorption intensity for Freundlich equa-
Figure 1. Adsorption isotherm of XO at 15°C.
Table 3. Langmuir and Freundlich isotherm constants of
XO at 15 °C.
Langmuir Freundlich
qo mg/gA r KF 1/n r
18.15 0.0301 0.999 4.1915 0.31970.84
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At the same time, adsorption free energy change (
is calculated according to Equation (4), the value is -
8.388 kJ/mol, negative
indicates that adsorption of
XO on EG is spontaneous.
RTlnb (4)
b: Langmuire equation constant;
: The free energy
change in the adsorption, kJ/mol
Influence of ion strength on adsorption capacity: NaCl
and Na2SO4 were used respectively to adjust solution
ionic strength in the range of 0 to 0.6 mol/L with MO
concentration keeping 200 mg/L and 500 mg/L, respec-
tively. Influence of ionic strength on adsorbance (shown
in Figure 2) indicates that presence of salt ions can im-
prove the adsorption capacity of EG for XO, and it might
be caused by the increase of hydrophobic attraction of
XO due to the “salting-out” effect. Under the same mass
concentration, the influence of Na2SO4 is higher than that
of NaCl.
(a) MO concentration 200 mg·L-1
(b) MO concentration 500 mg·L-1
Figure 2. Influence of ionic strength on adsorption capacity of EG
for XO.
3.2. Adsorption Kinetics
Equilibrium time: Influences of XO concentration and
temperature on adsorption equilibrium time were de-
tected and shown in Figure 3. Results suggest that ad-
sorbance is the function of XO concentration, tempera-
ture and adsorption time. In the beginning, adsorption
rate increases with the increase of temperature, but tem-
perature has no obvious influence on adsorption equilib-
rium time. In kinetic experiment, all adsorption could
reach equilibrium within 24.0 h.
(a) XO initial concentration 100 mg/L
(b) XO initial concentration 200 mg/L
(c) XO initial concentration 300 mg/L
Figure 3. Influence of initial concentration and temperature on
adsorption kinetics
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Copyright © 2013 SciRes. MSCE
Adsorption kinetic models: Both pseudo first- and sec-
ond-order adsorption models were used to describe the
adsorption kinetics data according to Equation (5) and (6)
[14, 15].
Table 4. Not only the line curve fit, but also qe,cal,
second-order model agrees more well with experimental
data than first-order model. Second-order model is more
suitable to describe XO kinetics data. Similar results
were observed in the adsorption of EG for Acid Red 3B
First-order model (5)
ln(q- q)=lnq- kt
Second-order model (6)
t/q=1/(k q) + t/qBased on the second-order model, initial adsorption
rate and half-adsorption time were estimated according
to Equations (7) and (8). As shown in Table 5, initial
adsorption rate u increases with the increase of initial XO
concentration and temperature. But there is no obvious
relativity between half-adsorption time t
1/2 and tempera-
ture, XO concentration, respectively. The results are
consistent with experiment data.
k: Adsorption rate constant (min-1 for first-order adsorp-
tion, g/(mg·min) for second-order adsorption); t: Ad-
sorption time, min
Since q reached qe at equilibrium, q values smaller
than 0.9qe were used for analysis. Plots of ln(qeq) ver-
sus t and t/q versus t were used to test the first- and sec-
ond-order models, and the fitting results were given in
Table 4. Adsorption kinetics model comparison of EG for XO.
First-order Second-order
mg/g qe,cal
min -1 r qe,cal
15 25.24 21.88 0.0028 -0.985 24.77 6.9 0.996
25 27.40 22.95 0.123 -0.977 26.38 8.7 0.993
35 30.57 30.89 0.12 -0.979 30.33 10.0 0.984
15 30.40 28.31 0.118 -0.979 31.39 4.5 0.999
25 31.07 28.77 0.118 -0.970 32.69 6.0 0.999 200
35 36.56 35.74 0.138 -0.983 35.02 7.2 0.999
15 30.40 32.63 0.211 -0.986 30.43 10.6 0.995
25 36.06 36.95 0.147 -0.987 36.23 14.0 0.997 300
35 42.73 33.19 0.152 -0.943 43.34 14.0 0.992
Table 5. The second-order model parameters of EG for XO.
kJ/mol r
15 0.0743 572.96
25 0.0976 416.67
35 0.1219 321.43
14.28 -0.994
15 0.0783 719.41
25 0.1081 445.97 200
35 0.1219 454.59
17.50 -0.999
15 0.1218 307.77
25 0.2231 201.07 300
35 0.2932 171.39
9.059 -0.991
u=kq (7)
1/2 e
t=1/(k q) (8)
u: Initial adsorption rate, (mg/g·min); t
Half-adsorption time (min)
To judge the sorption belongs to physical adsorption or
chemical adsorption, the second-order rate constants are
used to estimate activation energy of XO adsorption on
EG using Arrhenius Equation (9). Plots of lnk versus 1/T
is used to evaluate Ea, and it is found less than 20.0
kJ/mol (as shown in Table 5). So, the adsorption between
EG and XO is mainly physical adsorption.
Lnk=LnA - Ea/(RT) (9)
A: The re-exponential factor, g/(mg·min); Ea: The ad-
sorption activation energy, kJ/mol
4. Conclusion
This study has provided an insight into the adsorption
characteristics of EG for XO.
Thermodynamics study illustrates: adsorption of XO on
EG is a spontaneous process, and the adsorption isotherm
is type II. Adsorption process is affected by multifactor,
not only the initial concentration, temperature, but also
the ionic strength of solution. High XO initial concentra-
tion and ionic strength are propitious to adsorbance.
Kinetic study illustrates: adsorption kinetics can be well
described by the pseudo second-order kinetic model.
Initial adsorption rate increases with the increase of
temperature. Active energy of adsorption is less than 20
kJ/mol; and the sorption between EG and XO belongs to
physical adsorption.
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