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Colloids and Surfaces A: Physicochemical andEngineering Aspects
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dsorption kinetics of methylene blue onto Fe-doped sulfated titania
ing Yanga,∗, Congxue Tiana, Xiangpo Zhaob
College of Biological and Chemical Engineering, Panzhihua University, Panzhihua 617000, ChinaPetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China
r t i c l e i n f o
rticle history:eceived 6 August 2011eceived in revised form 2 September 2011ccepted 4 September 2011vailable online 10 September 2011
eywords:
a b s t r a c t
Fe-doped sulfated titania (FST) photocatalysts with high photocatalytic activity were prepared fromindustrial titanyl sulfate solution and characterized using N2 adsorption–desorption technique. Adsorp-tion kinetics and mechanism of methylene blue onto FST samples were studied at different temperatures(298, 303 and 308 K). The kinetic experimental data appropriately correlate with the pseudo-secondorder model. The overall rate of the adsorption process appears to be influenced by both bound-ary layer diffusion and intraparticle diffusion. The low adsorption activation energy (in the range of
−1
dsorptione-doped sulfated titaniaethylene blue
inetics
15.59–19.31 kJ mol ) suggests that the adsorption of methylene blue onto FST samples was conformed tothe physisorption mechanism. With calcination temperature increases from 400 to 600 ◦C, sulfur speciesgradually decomposes and desorbs from the surface of FST samples, which can enhance the affinitybetween methylene blue and FST samples. Moreover, the specific surface decreases and the pore volumeand pore diameter increase with rise in calcining temperature. All these have a significant influence onthe adsorption properties of FST samples.
. Introduction
In recent years, semiconductor photocatalysis has attracted con-iderable scientific and practical interest because it’s promisingpplications in environmental pollution remediation such as theemoval of inorganic or organic pollutants from air or wastew-ter. Among various semiconductor metal oxides, titania hasroven to be the most potential photocatalyst due to its highhotoreactivity, cost effectiveness, nontoxicity, chemical and bio-
ogical inertness, long-term stability against photocorrosion [1–4].ike other semiconductors, titania has some fatal drawbacks (theelatively large band gap, the rapid recombination rate of photo-enerated electron–hole pairs, etc.) [5]. Thus, many modificationethods including metal or non-metal doping, surface sensiti-
ation, semiconductor coupling, precious metal deposition andncreasing crystal defects have been developed in order to enhancehe photocatalytic activity and efficiencies of titania photocatalystsspecially under visible light in the past few decades [6–9]. How-ver, little information about the absorption in photocatalysis haseen reported especially in photodegradation of organic pollutants
n wastewater, although absorption of organic pollutants onto the
urface of photocatalysts is a critical step in photocatalysis [10].
In the previous work of our group, using low-cost industrialitanyl sulfate solution as raw material, which contains abundant
iron and sulfate, Fe-doped sulfated titania (denoted as FST) pho-tocatalysts were prepared by one-step thermal hydrolysis method,and the effects of the volume ratio of pre-adding water to TiOSO4 onthe photocatalytic activity of FST photocatalysts have been studied[11,12]. In this work, it is aimed to investigate the adsorption kinet-ics of methylene blue onto FST photocatalysts at three differenttemperatures (298, 303 and 308 K). This study was also undertakento evaluate kinetic parameters of the adsorption process at differ-ent temperatures above for each FST photocatalyst. In addition, theeffects of the preparation condition (calcination temperature) ofFST samples on its adsorption kinetics were carried out as well.
2. Experimental
2.1. FST samples preparation and characterization
FST samples were prepared through one-step thermal hydrol-ysis method using industrial titanyl sulfate solution as materials.The more detailed procedure can be found in references [11,12].
In a typical synthesis, 150 ml TiOSO4 solution and 34.5 ml pre-adding water were preheated to 96 ± 1 ◦C, respectively. The heatedTiOSO4 solution was dropped into the pre-adding water under stir-ring and reflux at a feeding speed of 8.55 ml/min. After feeding off,the mixture solution was heated to boiling point (called the first
boiling point) at a heating rate of 0.82 ◦C/min. Heating and stirringwere stopped immediately when the mixture reached a gray color(called gray point) and then were turned on again after 30 min,then the mixture was heated to the boiling point again (called the
Table 1Pseudo-first order kinetic model rate constants for adsorption of methylene blueonto different FST samples at different temperatures.
Samples 298 K 303 K 308 K
Y. Yang et al. / Colloids and Surfaces A:
econd boiling point). Maintaining slight boiling 2.5 h after theecond boiling point, the hydrolysis process is then finished. Thelurry was promptly cooled to room temperature, then filtered andashed. The as-prepared metatitanic acid was dried at 80 ◦C for
4 h, then calcined at different temperatures (400, 500 and 600 ◦C)or 1.5 h in static air, Fe-doped sulfated titania photocatalysts werehen obtained, denoted as FST(t), and the value in parentheses indi-ates the calcining temperature, ◦C.
The FST samples were characterized using N2 adsorption–esorption technique and the photocatalytic efficiencies of FSTamples were evaluated by the photooxidation of methylene bluen aqueous solution under UV irradiation. More details can be foundn Refs. [11,12].
.2. Adsorption of methylene blue on FST samples
The adsorption experiments were carried out in a 250 ml three-eck flask containing 200 ml methylene blue solution (6 mg l−1)nd 0.2 g FST sample and the three-neck flask was placed in a ther-ostatic electric jacket with a magnetic stirrer. After ultrasonic
ispersion for 3 min, the adsorption experiments were conductedn dark under stirring at a rate of 150 rpm to prevent bulk diffusions a controlling step of adsorption kinetics. At defined time points,0 ml solution was taken out from the three-neck flask and immedi-tely centrifuged for 5 min at 4000 rpm to separate the supernatend the FST sample. And then small amounts of the supernatantere taken to be analyzed by recording the absorbance at a wave-
ength of maximum absorbance of methylene blue (666 nm) using spectrometer. The concentration of methylene blue remaining inolution at time t = t (Ct, mg l−1) can be calculated according to thealibration curve of methylene blue (A = 0.0068 + 0.1514C) and theorresponding adsorption capacity (qt, mg/g) was obtained usinghe following equation:
t = (Ci − Ct) · V
mass FST(1)
where Ci represent the initial concentration of methylene bluemg l−1). The Ct is the concentration of methylene blue in solutionmg l−1) at time t = t· V (L) and mass FST are the solution volume andhe number of grams of FST sample used, respectively. Each exper-ment was performed individually at three different temperatures298, 303 and 308 K) for each FST sample. But the same procedureas used for each as detailed above.
. Results and discussion
.1. Adsorption kinetics model
The adsorption kinetics of methylene blue onto FST samplesay be described by the pseudo-first order model suggested by
agergren [13]. The equation is as follows:
dqt
dt= k1(qe − qt) (2)
ntegrating Eq. (2) with the boundary conditions t = 0 to t = t andt = 0 to qt = qt gives the linearized form:
n(qe − qt) = ln qe − k1t (3)
here qt and qe are the adsorption capacity (mg/g) of methylenelue onto FST samples at time and at equilibrium, respectively. k1
s the rate constant (min−1) of pseudo-first order sorption model.lthough this mode is the simplest one, it was extensively used
o describe the kinetics of sorption of solutes from a liquid solu-ion. If the adsorption kinetics follows a pseudo-first order model,he plot of ln(qe − qt) versus t should be linear. At the same time,he rate constant, k1 and the coefficient, R2
1 can be calculated from
Fig. 2. Linear plots of the pseudo-first order kinetic mode of methylene blue adsorp-tion onto different FST samples at 303 K.
the plot. Figs. 1–3 show the plots ln(qe − qt) versus t for adsorptionof methylene blue onto different FST samples (FST(400), FST(500)and FST(600)) at different temperatures (298, 303 and 308 K),respectively. The rate constants, k1 for different FST samples wereobtained from Figs. 1–3 at different temperatures and presented inTable 1 along with the corresponding correlation coefficients, R2
1.Another model for the analysis of sorption kinetics is the
pseudo-second order model put forward by Ho et al. [14]. The ratelaw for this system is expressed as
1/T for the adsorption of methylene blue onto different FST sam-
ig. 3. Linear plots of the pseudo-first order kinetic mode of methylene blue adsorp-ion onto different FST samples at 308 K.
ntegrating Eq. (4), for the boundary conditions t = 0 to t = t and qt = 0o qt = qt gives the following equation:
1qe − qt
= 1qe
+ k2t (5)
q. (5) can be rearranged to obtain a linear form,
t
qt= 1
k2q2e
+ 1qe
t (6)
here k2 is the rate constant (g mg−1 min−1) of pseudo-secondrder sorption model. The meanings of the other parameters asentioned above. A plot of t/qt against t gives a straight line with
he slope of 1/qe and the intercept of 1/(k2q2e ). So the sorption rate
onstant, k2 can be calculated from the slope and intercept. Figs. 4–6how the plots of t/qt against t for adsorption of methylene bluento different FST samples (FST(400), FST(500) and FST(600)) atifferent temperatures (298, 303 and 308 K), respectively. The rateonstants, k2 for different FST samples were obtained from Figs. 4–6
t different temperatures and presented in Table 2 along with theorresponding correlation coefficients, R2
2.Based on the correlation coefficients for k1 and k2 as presented in
ables 1 and 2, respectively, the adsorption of methylene blue onto
ig. 4. Linear plots of the pseudo-second order kinetic mode of methylene bluedsorption onto different FST samples at 298 K.
Fig. 5. Linear plots of the pseudo-second order kinetic mode of methylene blueadsorption onto different FST samples at 303 K.
FST samples is best described by the pseudo-second order model.This is quite consistent with the theoretical analysis results, i.e.,the sorption of solute from the solution obeys pseudo-first orderkinetics model at high initial concentration of solute, while it obeyspseudo-second order kinetics model at low initial concentration ofsolute [15].
The pseudo-second order rate constant for the adsorption ofmethylene onto FST samples could be expressed as a function oftemperature by the Arrhenius type relationship, as shown in thefollowing equation:
ln k2 = ln A − EaRT
(7)
where A is the frequency factor (g mg−1 min−1), Ea is the Arrheniusactivation energy of adsorption (J/mol), representing the minimumenergy that reactants must have for the reaction to proceed, R isthe universal gas constant (8.314 J mol−1 K−1), and T is the absolutetemperature of solution (K).
As shown in Fig. 7, the slopes of the linear plots of ln k2 versus
ples were constructed to calculate the adsorption activation energy,which are 17.31, 15.59 and 19.31 kJ mol−1 for FST(400), FST(500)and FST(600), respectively. Low activation energies (5–40 kJ mol−1)
re characteristic of physical adsorption, while higher activationnergies (40–800 kJ mol−1) suggest chemical adsorption [16]. Thushe adsorption of methylene blue onto FST samples may be con-ormed to physisorption mechanisms.
.2. Adsorption mechanism
The adsorption of methylene blue onto FST samples from aque-us solution may involve the following steps: (i) migration ofethylene blue from the bulk solution to the external surface of
ST samples (bulk diffusion), (ii) film diffusion of methylene bluehrough a hypothetical boundary layer to the external surface ofST samples (film diffusion or boundary layer diffusion or exter-al diffusion), (iii) adsorption of methylene blue at an active siten the surface of FST samples (adsorption), and (iv) the diffusionf methylene blue within the pore volume of FST samples and/orlong the pore wall surface (pore diffusion or intraparticle diffusionr internal diffusion). The rates of bulk diffusion and adsorption areenerally considered to be very fast and they cannot be the rateetermining step. Therefore, film and intraparticle diffusion maye the rate controlling steps in the adsorption of methylene bluento FST samples. Following equations were used to ascertain theate controlling step [17].
or film diffusion : Rt = − ln(1 − F) (8)
or intraparticle or pore diffusion : Bt = − ln(1 − F) − 0.4977 (9)
here F = qt/qe; qt and qe are the adsorption capacity (mg/g) ofethylene blue onto FST samples at time and at equilibrium,
espectively. R is the rate constant for film diffusion; B = �2 Di/r2
Di is the inter diffusion coefficient and r is the particle radius).
lots of −ln(1 − F) and Bt versus time, t, according to Eqs. (8) and9) for FST samples at 308 K, are shown in Figs. 8 and 9, respectively,hich are similar to the results at 298 and 303 K (not shown here).
traight lines were obtained when −ln(1 − F) was plotted against
ig. 7. Arrhenius plots of the pseudo-second order kinetic mode for the adsorptionf methylene blue onto different FST samples.
Fig. 8. Plots of −ln(1 − F) versus t for the adsorption of methylene blue onto differentFST samples at 308 K.
time, t (Fig. 8) which did not pass through the origins. This indicatesthat film diffusion is not limiting step of the overall adsorption pro-cess kinetics. Fig. 9 indicates that straight lines were obtained onplotting Bt versus time, t which nearly passes through the origins.This shows that intraparticle diffusion may be the rate controllingstep [18].
Adsorption kinetic data was further processed to confirmwhether intraparticle diffusion is the rate limiting and to find outthe rate parameter for intraparticle diffusion. For such purpose
system temperatures increase distinctly. This is probably a conse-
ig. 10. Morris–Weber plots of methylene blue onto FST(400) sample at differentemperatures.
as applied to the kinetic data. The parameter kid is the rateonstant for intraparticle diffusion (mg g−1 min−1/2). Values of Img g−1) give an idea about the thickness of the boundary layer,.e., the larger the intercept, the greater is the boundary layer effect20]. Plots of qt against (t)0.5 are shown in Figs. 10–12 for FST(400),ST(500) and FST(600), respectively. Figs. 10–12 indicate that twoistinct regions were observed for all FST samples. The initial lin-ar portion was ascribed to the boundary layer diffusion effects andhe second linear portion was due to the intraparticle diffusion [17].he values of kid1, kid2 and I1, I2 for methylene blue adsorption onto
ifferent FST samples at different temperatures are listed in Table 3,hich obtained from the slopes and intercepts of the two portion
traight lines, respectively. It was also observed that all lines do notass through the origin, indicating that there is a boundary layer
64 10 8 12 14
0.04
0.08
0.12
0.16
0.20
0.24 FST(500)
298 K303 K308 K
qt (m
g.g-1
)
t1/2 (min1/2 )
ig. 11. Morris–Weber plots of methylene blue onto FST(500) sample at differentemperatures.
resistance and the magnitude of the intercepts are proportional tothe extent of the boundary layer thickness. Similar observations ofdouble nature plots were also reported previously by other work-ers on various adsorbate–adsorbent systems studied [20–22]. Thechange in the intercepts of the plots suggests that the mechanismof the adsorption of methylene blue onto FST samples is predomi-nantly diffusion, and the intraparticle diffusion played a significantrole in rate determining, but it was not the sole rate-controlling stepthrough out the adsorption process. Namely, both intraparticle andboundary layer diffusion seem significant in the rate determiningstep. Initially, the methylene blue was adsorbed by the exterior sur-face of FST samples at the beginning, so the adsorption rate was veryfast. Upon the saturation of the exterior surface due to the adsorp-tion, the methylene blue entered into the particle of FST samplesthrough pores and was adsorbed by the interior surface of the par-ticle. As a result of diffusion resistance, the intraparticle diffusionrate become slow and is therefore the rate determining step.
3.3. Effect of calcination temperature on kinetics
Fig. 13 shows the effect of calcination temperature on adsorp-tion activation energy, Ea, and adsorption rate constant, k2 ofpseudo-second order kinetic model. This figure indicates that alladsorption rate constant (k2) at different system temperatures havethe similar variation trend with increasing the calcination tem-perature. With the calcination temperature increases from 400 to500 ◦C, all adsorption rate constant (k2) at different system tem-peratures decreases slightly, which may be due to the decreasein specific surface (158.3 m2 g−1 for FST(400) and 120.7 m2 g−1
for FST(500)). However, with the calcination temperature furtherincreases to 600 ◦C, all adsorption rate constant (k2) at different
quence of the rise in pore volume and pore diameter (0.4076 ml g−1
and 12.21 nm for FST(500) and 0.5793 ml g−1 and 33.84 nmfor FST(600)), which can lower the resistance of intra-particle
64 10 8 12 14
0.04
0.08
0.12
0.16
0.20
FST(600)
298 K303 K308 K
qt (m
g.g-1
)
t1/2 (min1/2 )
Fig. 12. Morris–Weber plots of methylene blue onto FST(600) sample at differenttemperatures.
Y. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12– 17 17
iffusion. At the same time, with the calcination temperaturencreases from 400 to 600 ◦C, the adsorption activation energy (Ea)lso decreases first, and then increases obviously. This is in goodgreement with the photocatalytic efficiencies (94.32, 98.08 and0.92% for FST(400), FST(5400) and FST(600), respectively), i.e., the
arger the adsorption activation energy, the lower is the photocat-lytic efficiencies. One possible reason responsible for this is thathe sulfur species gradually decomposes and desorbs from the sur-ace of FST samples with the calcining temperature increases, whichan enhance the affinity between methylene blue and FST samples23].
. Conclusions
Adsorption kinetics and mechanism of methylene blue ontoST samples were studied at different temperatures (298, 303 and08 K). The kinetics experimental data appropriately correlate withhe pseudo-second order model. The overall rate process appearso be influenced by both boundary layer diffusion and intraparticleiffusion. The adsorption activation energy calculated from Arrhe-ius equation was 17.31, 15.59 and 19.31 kJ mol−1 for FST(400),ST(500) and FST(600), respectively, suggesting that the adsorp-ion of methylene blue onto FST samples follows the physisorption
echanism. With calcination temperature increases from 400 to00 ◦C, sulfur species gradually decomposes and desorbs from theurface of FST samples, which can enhance the affinity betweenethylene blue and FST samples. Moreover, the specific surface
ecreases and the pore volume and pore diameter increases withise in calcining temperature. All these have a significant influencen the adsorption properties of FST samples.
cknowledgements
This work was supported by the National Natural Sci-nce Foundation of China (50804025), the Applied and Basicesearch Program of Sichuan Province, China (2008JY0140),
he Youths Foundation of Sichuan Province, China (09ZQ026-67), the Talents Innovation Project of Panzhihua City, China2009TX-5(1)) and the Promoting Industrialization Program ofanzhihua City, China (2011CY-G-23).
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