Adsorption Capacity of Expansion Graphite for Xylenol Orange

doi:10.4236/msce.2013.11001

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Journal of Materials Science and Chemical Engineering, 2013, 1, 1-5

http://dx.doi.org/10.4236/msce.2013.11001 Published Online February 2013 (http://www.scirp.org/journal/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: *[email protected]

Received 2013

Abstract

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

Adsorption

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

model.

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

X. Y. PANG ET AL.

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.

Expansion

volume

Total

volume

cm3/g

Total pore

area

m2/g

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

equation

436 Y= 0.01922+

0.01501x

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-

tion

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

X. Y. PANG ET AL.3

At the same time, adsorption free energy change (

⊿

G°)

is calculated according to Equation (4), the value is -

8.388 kJ/mol, negative

⊿

G° indicates that adsorption of

XO on EG is spontaneous.

⊿

G°=

﹣

RTlnb (4)

b: Langmuire equation constant;

⊿

G°: 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

Figure 3. Influence of initial concentration and temperature on

adsorption kinetics

X. Y. PANG ET AL.

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

[10].

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(qe−q) 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/L

℃

qe,exp

mg/g qe,cal

mg/g

min -1 r qe,cal

mg/g

k/10-5

g/(mg

·min)

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

100

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.

mg/L

°C

mg/(g·min)

t1/2

min

kJ/mol r

15 0.0743 572.96

25 0.0976 416.67

100

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

X. Y. PANG ET AL.5

u=kq (7)

1/2 e

t=1/(k q) (8)

u: Initial adsorption rate, (mg/g·min); t

1/2:

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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