- Email: [email protected]
Study of adsorption of phenol on titanium oxide (TiO2)
DESALINATION Desalination 166 (2004) 355-362
ELSEVIER
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Study of adsorption of phenol on titanium oxide (TiO2) S. Bekkouche*, M. Bouhelassa, N. Hadj Salah, F.Z. Meghlaoui Department of Industrial Chemistry, Faculty of Science of the Engineer, Mentouri University, Constantine, Algeria TeL~Fax: +213 (31) 631906; email: [email protected]
Received 16 February 2004; accepted 27 February 2004
Abstract
The application of photocatalysis in the treatment of organic micropollutants in wastewater is an interesting alternative and is the object of a great interest over the last years by many researchers. The stage of adsorption of the micropollutant on the photocatalyst, mainly the titanium oxide anatase form, is a determining stage in the process of photodegradation. We present an experimental study of the adsorption of phenol, chosen as the model pollutant, on a photocatalyst, titanium oxide anatase (Degussa P25). The measure of the quantity of phenol adsorbed was made by UV spectroscopy. The equilibrium of adsorption was reached after 1 h; the kinetics of adsorption were slow and obeyed the Lagergrein model. The adsorption was chimisorption in a monolayer and obeyed the Langmuir model. The survey also showed that there was an advantage to operate at great velocity of agitation and a natural pH. The agitation by ultrasonic drives to a light increase of phenol quantity adsorbed (5%) because this mode of agitation reduces the phenomenon of agglomeration of titanium oxide particles and therefore increases the interracial area of the catalyst. Keywords; Adsorption; Titanium oxide; Phenol; Photocatalysis; Wastewater
1. I n t r o d u c t i o n
Over the last 20 years heterogeneous photocatalysis has been the subject o f great interest as a means o f organic micropollutant elimination in water treatment processes. This technique is based on the excitation of a semiconductor, *Corresponding author.
mainly the oxide of titanium TiO2 (anatase form), by a luminous radiance of wavelengths less than 400 nm. An electron passes then from the valence band to the conduction band, thus creating a site of oxidization (a hole h +) and a site of reduction (an electron e-). Holes h + react with the donors of electrons such as water, anions OH- and organic products, adsorbed at the surface of the semiconductor forming the hydroxyl radicals R.
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l 'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi; 10.1016/j.desal.2004.06.090
356
S. Bekkouehe et aL / Desalination 166 (2004) 355-362
Electrons react with the electron acceptors such as dissolved oxygen to form superoxyd radicals, preventing the recombination of charges. Most works published in this domain underlined that the key stages in the process of photodegradation are reactions to the surface of the photocatalyst between adsorbed species (substrats, reducers and oxidants). But very little research has been made on the adsorption of these species themselves. However, we can note the works ofOllis et al. [1 ], Bideau et al. [2], Chen et al. [3-5], Zahraa et al. [6], Robert et al. [7], Tunesi et al. [8], Cunningham et al. [9,10] and Piscope et al. [ 11 ]. The degradation of the phenol by photocatalysis has also been studied by some authors including Sykora et al. [12], Okmoto et al. [13,14], and Augugliaro [15], but very few works were interested in the survey of the adsorption on TiO2. In this work we propose a study on the adsorption of phenol on oxide titanium (TiO2) in its anatase form.
2. Materiels and methods
2. I. Materials
The adsorbant used was TiO2 constituted of a mixture of 80% anatase and 20% ruffle. The anatase form was identified by x-ray diffractometry and the use of the JCPDS data bank. Its specific surface is 50 m2/g, as given by the manufacturer. The diameter of the particles is 20 nm given, also as reported by the manufacturer and verified under MEB. The phenol adsorbant was provided by Cheminova International, and the water used was bidistilled and filtered on a Millipore filter, 0.2 ~m.
absorption band corresponding to the transition n-~n * to a wavelength of 270 nm. The indicated absorbance is proportional to the concentration in phenol, according to the law of Beer Lambert in the studied concentration domain 0-300 ppm or 0-3.2 mmoles. 2.3. Experiment
The adsorption of the phenol on TiO2 was done in a Pyrex vessel with a double envelope with a capacity of 500 ml. To maintain the temperature constant, water was recirculated through the double envelop from a Lauda RC6 thermostated bath, provided for a pump. Mixing was done with a magnetic stirrer. All studies were made at a temperature of 25°C. The samples were taken at regular intervals by syringes on which filters of 0.2 ~tm were adapted to eliminate particles of TiO2. Their absorbance was then measured and transformed in concentration (C) by a standardization absorbanceconcentration previously made. The quantity of phenol adsorbed by a mass of TiO2 at time t is:
Q - _C_o - C m
where Q is the quantity of phenol adsorbed at time t by a mass ofTiO2 (mg/l.g Ti O2), Co is the concentration initial of phenol (mg/l), C is the concentration of phenol at time t (mg/1) and m is the concentration of the catalyst (g/l).
3. Results and discussion
3.1. Effect o f the agitation velocity 2.2. Analysis
The method of analysis of the phenol was UV spectroscopy, using a Chimadzu 130 A spectrophotometer to sweep. The specter of absorption of the phenol indicates the existence of an
The results are represented in Fig. 1. For speeds between 100 and 1200 rpm and one initial concentration in phenol equal to 80 ppm, the increase of the agitation velocity acts favourably on the adsorption.
S. Bekkouche et al. / Desalination 166 (2004) 355-362 14-
Ca(phenol)(ppm)
10 ~
'5
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8
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357
150140
12. 10,
CO
~e
"5
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0
8.
C
J
r"
o
64.....................
o
i
i
i
i
i
i
i
0
200
400
600
800
1000
1200
agitation velocity (rpm)
20 0
20 .
80 . 40 . . 60 t i m e of c o n t a c t (min)
100
Fig. 1. Variation o f equilibrium adsorbed quantities vs. agitation velocity (C o (phenol) = 80 ppm, r = 1 g/l, pH = 5.72, T = 25°C).
Fig. 2. Effect o f initial concentration o f adsorbed phenol (V = 1200 rpm, r = 1 g/l, pH = 5.72, T = 25°C).
It is obvious that the effect o f the agitation speed cannot be indefinitely beneficial, quantities adsorbed at equilibrium Qe seem tender toward the values constant for greater speed. Indeed, after certain speed limits, the effect must become constant when the phenomenon o f mass transfer is not limited by diffusion.
literature where the adsorption of the phenol on TiO z was studied [16]. The different values of phenol adsorbed by the mass o f the catalyst obtained at equilibrium Qe will serve to establish the adsorption isotherm.
3.2. Effect of the initial concentration of phenol The effect o f the initial concentration of phenol is represented in Fig. 2. The increase of the initial concentration in phenol gives a substantial increase of the quantity of retained phenol for the same quantity o f TiO 2. The increase of the initial concentration o f phenol for the same mass of catalyst creates a great increase o f molecules of phenol adsorbed so that at the free sites o f the surfaces of particles of TiO2, when all sites are occupied, this concentration adsorbed becomes constant, and there is formation o f a monolayer. Curves of adsorption are superimposed then. The time of equilibrium observed in any case of initial concentration was reached after 60 min; it can be lower at low concentrations. This result is in agreement with certain results of the
3.3. Effect of the reported solid~liquid To examine the influence of the reported solid/liquid, we varied the quantity of the support while keeping the concentration of the solution constant and equal to 80 ppm. If one draws a curve that expresses (Co-Ce), the quantity adsorbed at equilibrium is in ppm or (rag/l) (but not the quantity adsorbed at equilibrium by mass of catalyst), according to the concentration of catalyst in g/1 of TiO 2 (Fig. 3). We note that it presents a maximum at r equal to 0.8, whereas one should have a landing; indeed if one increases r indefinitely, (Co-Ce) should remain constant. We conclude that for a given concentration, there is an optimal value for r. In our case it is equal to 0.8 for a concentration o f phenol equal to 80 ppm. For elevated quantities o f TiO2, the adsorption of phenol is disfavoured because particles of TiO2
358
S. Bekkouche et aL / Desalination 166 (2004) 3 5 5 - 3 6 2 ",4.
C,,(TiO~)(g/I)
/ - / ,
,2-
V-
E o
"6 10-
7
~
/
/#
~/
-
....
US 0.8 ,
us08
.____~.o ~ , ---o--= ~..._~e~e~,~
----L~ Us I ~MS 1
O
~
t~
~ 5 o, 4 C ~: 3
-
,, 6 ~ ,,~
0.0 0.2 0,4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
report solid/liquidr(g/I) Fig. 3. Equilibrium adsorbed quantities of phenol (C0- Ce, m g/l) vs. reported solid/liquid r (Co (phenol) = 80 ppm, V= 1200 rpm, pH = 5.72, T= 25°C).
have the tendency to agglomerate and are conducted to the reduction of the specific surface and therefore the reduction of the retention capacity. This interesting result explains why (in addition to the opacity of the luminous radiance to the strong concentrations of photocatalyst) all studies ofphotocatalysis note the existence of an optimal concentration of TiO2, especially as we know that the kinetics of adsorption plays a primordial role in these chemical reaction types.
3.4. Effect of aggregation of Ti02 The analysis of the TiO 2 powder in aqueous suspension shows the existence of micrometric aggregates. The dimension of these aggregates seems to be the same size that the one observed on the dry powder. Indeed, the gunpowder of TiO 2 presents itself to dry as an association of particles to the scale of a few tens of micrometers. This structure seems to preserve itself in aqueous suspension [ 17]. We studied the effect of this aggregation on its property of adsorption. We tested two modes of agitation: simple agitation with a magnetic stirrer and specific agitation using ultrasound. The adsorption was followed for three initial concen-
20
o
2'o
. . . . .
us1.5
4'0
8'o
~'o
time of contact (rnin)
Fig. 4. Effect of mode of agitation. US, ultrasonic; MS, magnetic stirrer. trations of TiO 2 according to the two modes of agitation. Results presented in Fig. 4 show that an adsorbed quantity of phenol is better in the presence of ultrasound. The effect was much more important than the quantity of TiO 2. The increase was on the order of 5%. This result also confirmed the one found above, that an increase of the concentration of TiO2 beyond a certain optimal value would decrease an adsorbed quantity. Chen [18] found, in the presence of ultrasound, an increase of 29% of the quantity of adsorbed chloroethane in the range of 0.6 to 3 g/1 of TiO 2. Rideh [19] found, in the presence of ultrasound, an increase of 12% of the quantity of chlorophenol in the range of 0.8 to 2 g/1 of TiO2. The increase of the adsorbed phenol in the presence of ultrasound is caused by the fragmentation of agglomerates. The fragile agglomerates at great concentrations of TiO2 are easily destructible by ultrasound; on the other hand, at low concentrations these agglomerates formed are resistant to ultrasound [17].
3.5. Effect of pH pH plays a major role in the phenomena of
359
S. Bekkouche et aL / Desalination 166 (2004) 355-362 11.0~ 10.5-" I-"5 -~
10,0-4D
9.5£ 9.0-
~" 8.58,00
;,5~ 6.5' pH
Fig. 5. Effect ofpH on adsorbed equilibrium quantity of phenol (Co (phenol) = 80 ppm, r = 1 g/l, V= 1200 rpm, T = 25oc).
adsorption, particularly when the adsorbant is a semiconductor such as TiO 2. It exists at an isoelectric point; over this point the surface of TiO2 is charged negatively and under it is positive. The state of the phenol molecule and the load of the surface of the catalyst play a large role in the phenomenon of adsorption. These two states are bound directly to the value of the pH. It is therefore necessary to examine the influence of the pH of the solution on the adsorption. In our experience the pH varied from 2 to ! 1.5, which is to say from very acidic to very basic. All other operative conditions remained constant: concentration of the phenol, concentration of the catalyst, temperature and speed of agitation. The results in Fig. 5 represent quantities of phenol adsorbed, at equilibrium, for different pHs. The adsorption of the phenol presents an optimum for a pH understood to be between 5 and 6 (natural pH). The adsorbed quantity is bound to the distribution of species of the phenol and the state of the surface of TiO2 according to pH and to cations and anions present in solution. At a strongly acidic pH (pH = 2), the adsorption is relatively low, followed by a strong increase until a pH of 5 where the adsorption is
maximal. At a low pH the molecule of phenol is non-dissociated (neutral) and the surface of TiO2 is also at a neutral state (TiOH). In addition, when the pH is adjusted with HC1, the C1- anions are also adsorbed at the surface of TiOz. There is competition between the adsorption of the anions and phenol. At pH 5, the surface of TiO2 is positively charged; just before the isoelectric point, the adsorption is at its maximum, and the quantity of CI- ions is lower. Beyond a pH of 6.5 to 9, the surface of TiO2 is charged negatively in a state of TiO. An apparition ofphenolat ions, there is a phenomenon of repulsion, in addition to the adsorption of the Na + ions and OH-, creating competition with the phenolat ions. Beyond a pH of 10, the phenol is entirely at the state ofphenolat and the surface of TiO2 is at the state of TiOH where the quantity adsorbed was constant. In conclusion, we note that the adsorption is favoured around the isoelectric point either at pH 5 or 6. 3.6. Adsorption isotherm
At a constant temperature, the variation of the adsorbed quantity of the phenol at equilibrium vs. the equilibrium concentration in liquid at different initial concentrations represents the adsorption the isotherm shown in Fig. 6. The isotherm is represented better by the Langmuir model (Fig. 6 and Table 1). Table 1 Constants of adsorption isotherm Langmuir model Freundlich model a.b. C e l+b.C~
Equations
Qe - - -
Qe = a. Cb
a b
19.16565 0.01599 0.99137
1.12009 0.50761 0.95142
R2
R2, correlation coefficient.
360
S. Bekkouche et al. / Desalination 166 (2004) 355-362 14 10.
12-
,:5. '5
6-
o~
4..
o3
2-
j,,,
9-
I-- 10, "6 ¢3b 0 8-
...."
7'
o
//
- - - model of langmuir
/
0
model of freundlich
o"
'
'
20
7""
E 6. ~5
/.,°
4o
' "
6o ' 8o C,~(mg/I)
'
loo'
'
~20
'
...'" .¢~qlj"r j..,""
3. 210
20
t40
Fig. 6. Isotherm of adsorption of phenol by TiO2 (r =
40
60
80
120
100
10
C~. ( m g / l )
Fig. 7. Linear form of the Langmuir isotherm.
1 g/l, V= 1200 rpm, pH = 5.72, T = 25°C). Table 2 Constants of the linear form of the Langmuir isotherm Model Equation
Langmuir
1/(KQmJ 1/Q,,a×
3.45893 0.05116 0.99283
R
300
D. Robert and all,F]
250
/
~
1
e
2O0
(Ce/Q,)= 1/~(Q=~.~)+(Ce/Qm~x)
Og10050/
0
The linear form o f the equation (Fig. 7 and Table 2) gives the best linearity for the Langmuir model. The linear representation permits us to calculate values of the adsorption constant K and the specific maximal quantity, Qmax. We find:
(Ce/Qe)= l/~Qmax) + (f e/Omax) K = 0 . 0 1 4 7 9 1.mg -] = 1.39 m m o l e . 1 - 1
Qm~x = 19.54652 mg.g -1 = 0.2079 mmole.g
A comparison of our experimental results with those of Robert et al. [7] and Fig. 8 shows agreement in the domain of our studied concentrations.
o
r
,
experimentalresults
i
loo
•
2;0
a;o
Co(mg/I)
Fig. 8. Equilibrium adsorbed concentration of phenol vs. initial concentration of phenol. For constants of adsorption we do not dispose values of comparison with regard to the phenol; however, the value of K found was lower than that given in the literature for certain organic compounds (Table 3). This result shows that the phenol itself was adsorbed with difficulty on TiO 2.
3.7. Kinetics o f adsorption The kinetics o f adsorption were determined by the mass transfer at the liquid/solid interface
X Bekkouehe et al./Desalination 166 (2004) 355-362 3,0
"//~
Co(phenol ) mgll
2.5 -
2.o-. l s[
/,i/20
1.o
o.s"
/if
/.
o5
1.0
361
where Q is the adsorbed quantity at time t, Qethe adsorbed quantity at equilibrium, and K~ is the kinetic constant of mass transfer or conductance of transfer. By integration we obtain:
ln(O-Oe)/(Oe-Po
) - = g a'
•
In this particular case where the adsorption takes place on a nuded surface, Q0 is null, thus:
-1.5 -2.0 ] -25 -30
•
.5
i
a
•
i
•
5
i
to
,
~
•
i
,
i
,
i
,
i
15 20 25 30 35 time of contact (min)
,
|
40
•
i
•
45
i
•
50
55
Fig. 9. Adsorption kinetics of phenol by TiO2. Table 3 Values of adsorption constants o n T i O 2 and quantities maximal adsorbed for different organic compounds Compound
K Qm~x Ref. (1 mmole-1) (mmole g-i)
Salicylic acid 4-aminobenzoic acid 3-chloro-4-hydro benzoique acid Dichloroethane acid 2-chlorophenol Phenol
10.5 6.60
0.07 0.071
4.57
0.047
2.59
0.225
3.6 1.39
0.107 0.207
[ 18] [ 18] [ 18] [ 17]
Q
=
- -Kat ) Qe(1 e
Variations of In (Qe-Q) vs. time for different initial concentrations of phenol in the liquid phase are represented in Fig. 9. We obtained one satisfactory linearity. Therefore, this kinetic law well represents our experimental results. The constant kinetic Ka practically does not vary with the concentration of phenol; there is an average value of 0.086 min -1. The kinetics of adsorption of the phenol on TiO 2 is relatively slow since one can associate this with a constant of time of 1/K~ of the order of 11.63 min, independent of the concentration of phenol in the studied domain.
[ 19] Our results 4. Conclusions
where all resistance to mass transfer is localized. The fundamental equation to apply is the one that governs the phenomena of mass transfer, in general, between two phases; the flux of adsorption is proportional to the gradient between the quantity adsorbed Q at time t and the adsorbed quantity Qe at equilibrium and a constant K a (conductance of transfer):
dQ/dt=Ka(Q~-Q )
The phenol was weakly adsorbed on TiO 2. This adsorption is a chimisorption, and it is represented well by the Langmuir model in the studied concentration domain. The aggregation effect of TiO 2 in aqueous solution had an optimal concentration of the catalyst for a concentration of phenol. The adsorption was optimal for a pH between 5 and 6 in the neighborhood of the isoelectric point of YiO2.
362
S. Bekkouche et al. / Desalination 166 (2004)355-362
References [1] D.F. Ollis, C.Y. Hsiao, L. Budiman and C.L. Lee, J. Catalysis, 88 (1984) 89-96. [2] M. Bideau, B. Clandel and M. Otterbein, J. Photochem., 14 (1980) 291-302. [3] H.Y. Chen, O. Zahraa, M. Bouchy, F. Thomas and J.Y. Bottero, J. Photochem. Photobiol. A. Chem., 85 (1995) 179-186. [4] H.Y. Chen, O. Zahraa andM. Bouehy, J. Photochem. Photobiol. A. Chem., 108 (1997) 37-44. [5] D.W. Chen, A. Jay and K. Ray, App. Catalysis B. Enve., 23 (1999) 143-157. [6] O. Zahraa, C. Dorian, S.M. Ould-mame and M. Bouehy, Poster, 1lth International Congress on Catalysis, 1996. [7] D. Robert, S. Parra, C. Pulgarin, A. Krzton and J.V. Weber, Appl. Surf. Sci.., 167 (2000) 51-58. [8] S. Tunesi and M. Anderson, J. Phys, Chem., 95 (1991)3399. [9] J. Cunningham and G. A1-Sayyed, J. Chem. Sot., Faraday Trans., 86 (1990) 3935.
[10] J. Cunningham and P. Sedlak, Catal. Today, 29 (1996) 309. [11] A. Piseopo, D. Robert and J.V. Weber, App. Cata. B. Enve., 35 (2001) 117-124. [12] J. Sykora, Coordination Chem. Rev., 159 (1997) 95-108. [13] K.L. Okamoto, Y. Yamamoto, H, Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Soc. Jpn., 58 (1985) 2015-2022. [14] K.L. Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Soe. Jpn., 58 (1985) 2023-2028. [15] V. Augugliaro, L. Palmisano and A. Sclafani, Toxicol. Environ. Chem., 16 (1987) 89-109. [16] V. Brezova and A. Blazkova, J. Photochem. Photobiol. A: Chem., 109 (1997) 177-183. [17] H.Y. Chen, Etude compar6e de l'adsorption et de la d6gradation photocatalytique de polluants des eaux, Th6se INPL, Nancy, France, 1994. [18] J. Cunningham and G. Al-Sayyed, J. Chem. Soc. Faraday Trans., 86 (1990) 3935-3941. [19] L. Rideh, Thbse INPL, Nancy, France, 1997.
ELSEVIER
www.elsevier.com/locate/desal
Study of adsorption of phenol on titanium oxide (TiO2) S. Bekkouche*, M. Bouhelassa, N. Hadj Salah, F.Z. Meghlaoui Department of Industrial Chemistry, Faculty of Science of the Engineer, Mentouri University, Constantine, Algeria TeL~Fax: +213 (31) 631906; email: [email protected]
Received 16 February 2004; accepted 27 February 2004
Abstract
The application of photocatalysis in the treatment of organic micropollutants in wastewater is an interesting alternative and is the object of a great interest over the last years by many researchers. The stage of adsorption of the micropollutant on the photocatalyst, mainly the titanium oxide anatase form, is a determining stage in the process of photodegradation. We present an experimental study of the adsorption of phenol, chosen as the model pollutant, on a photocatalyst, titanium oxide anatase (Degussa P25). The measure of the quantity of phenol adsorbed was made by UV spectroscopy. The equilibrium of adsorption was reached after 1 h; the kinetics of adsorption were slow and obeyed the Lagergrein model. The adsorption was chimisorption in a monolayer and obeyed the Langmuir model. The survey also showed that there was an advantage to operate at great velocity of agitation and a natural pH. The agitation by ultrasonic drives to a light increase of phenol quantity adsorbed (5%) because this mode of agitation reduces the phenomenon of agglomeration of titanium oxide particles and therefore increases the interracial area of the catalyst. Keywords; Adsorption; Titanium oxide; Phenol; Photocatalysis; Wastewater
1. I n t r o d u c t i o n
Over the last 20 years heterogeneous photocatalysis has been the subject o f great interest as a means o f organic micropollutant elimination in water treatment processes. This technique is based on the excitation of a semiconductor, *Corresponding author.
mainly the oxide of titanium TiO2 (anatase form), by a luminous radiance of wavelengths less than 400 nm. An electron passes then from the valence band to the conduction band, thus creating a site of oxidization (a hole h +) and a site of reduction (an electron e-). Holes h + react with the donors of electrons such as water, anions OH- and organic products, adsorbed at the surface of the semiconductor forming the hydroxyl radicals R.
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l 'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi; 10.1016/j.desal.2004.06.090
356
S. Bekkouehe et aL / Desalination 166 (2004) 355-362
Electrons react with the electron acceptors such as dissolved oxygen to form superoxyd radicals, preventing the recombination of charges. Most works published in this domain underlined that the key stages in the process of photodegradation are reactions to the surface of the photocatalyst between adsorbed species (substrats, reducers and oxidants). But very little research has been made on the adsorption of these species themselves. However, we can note the works ofOllis et al. [1 ], Bideau et al. [2], Chen et al. [3-5], Zahraa et al. [6], Robert et al. [7], Tunesi et al. [8], Cunningham et al. [9,10] and Piscope et al. [ 11 ]. The degradation of the phenol by photocatalysis has also been studied by some authors including Sykora et al. [12], Okmoto et al. [13,14], and Augugliaro [15], but very few works were interested in the survey of the adsorption on TiO2. In this work we propose a study on the adsorption of phenol on oxide titanium (TiO2) in its anatase form.
2. Materiels and methods
2. I. Materials
The adsorbant used was TiO2 constituted of a mixture of 80% anatase and 20% ruffle. The anatase form was identified by x-ray diffractometry and the use of the JCPDS data bank. Its specific surface is 50 m2/g, as given by the manufacturer. The diameter of the particles is 20 nm given, also as reported by the manufacturer and verified under MEB. The phenol adsorbant was provided by Cheminova International, and the water used was bidistilled and filtered on a Millipore filter, 0.2 ~m.
absorption band corresponding to the transition n-~n * to a wavelength of 270 nm. The indicated absorbance is proportional to the concentration in phenol, according to the law of Beer Lambert in the studied concentration domain 0-300 ppm or 0-3.2 mmoles. 2.3. Experiment
The adsorption of the phenol on TiO2 was done in a Pyrex vessel with a double envelope with a capacity of 500 ml. To maintain the temperature constant, water was recirculated through the double envelop from a Lauda RC6 thermostated bath, provided for a pump. Mixing was done with a magnetic stirrer. All studies were made at a temperature of 25°C. The samples were taken at regular intervals by syringes on which filters of 0.2 ~tm were adapted to eliminate particles of TiO2. Their absorbance was then measured and transformed in concentration (C) by a standardization absorbanceconcentration previously made. The quantity of phenol adsorbed by a mass of TiO2 at time t is:
Q - _C_o - C m
where Q is the quantity of phenol adsorbed at time t by a mass ofTiO2 (mg/l.g Ti O2), Co is the concentration initial of phenol (mg/l), C is the concentration of phenol at time t (mg/1) and m is the concentration of the catalyst (g/l).
3. Results and discussion
3.1. Effect o f the agitation velocity 2.2. Analysis
The method of analysis of the phenol was UV spectroscopy, using a Chimadzu 130 A spectrophotometer to sweep. The specter of absorption of the phenol indicates the existence of an
The results are represented in Fig. 1. For speeds between 100 and 1200 rpm and one initial concentration in phenol equal to 80 ppm, the increase of the agitation velocity acts favourably on the adsorption.
S. Bekkouche et al. / Desalination 166 (2004) 355-362 14-
Ca(phenol)(ppm)
10 ~
'5
O_
8
I.-0
357
150140
12. 10,
CO
~e
"5
&4 E 2 0 v
0
8.
C
J
r"
o
64.....................
o
i
i
i
i
i
i
i
0
200
400
600
800
1000
1200
agitation velocity (rpm)
20 0
20 .
80 . 40 . . 60 t i m e of c o n t a c t (min)
100
Fig. 1. Variation o f equilibrium adsorbed quantities vs. agitation velocity (C o (phenol) = 80 ppm, r = 1 g/l, pH = 5.72, T = 25°C).
Fig. 2. Effect o f initial concentration o f adsorbed phenol (V = 1200 rpm, r = 1 g/l, pH = 5.72, T = 25°C).
It is obvious that the effect o f the agitation speed cannot be indefinitely beneficial, quantities adsorbed at equilibrium Qe seem tender toward the values constant for greater speed. Indeed, after certain speed limits, the effect must become constant when the phenomenon o f mass transfer is not limited by diffusion.
literature where the adsorption of the phenol on TiO z was studied [16]. The different values of phenol adsorbed by the mass o f the catalyst obtained at equilibrium Qe will serve to establish the adsorption isotherm.
3.2. Effect of the initial concentration of phenol The effect o f the initial concentration of phenol is represented in Fig. 2. The increase of the initial concentration in phenol gives a substantial increase of the quantity of retained phenol for the same quantity o f TiO 2. The increase of the initial concentration o f phenol for the same mass of catalyst creates a great increase o f molecules of phenol adsorbed so that at the free sites o f the surfaces of particles of TiO2, when all sites are occupied, this concentration adsorbed becomes constant, and there is formation o f a monolayer. Curves of adsorption are superimposed then. The time of equilibrium observed in any case of initial concentration was reached after 60 min; it can be lower at low concentrations. This result is in agreement with certain results of the
3.3. Effect of the reported solid~liquid To examine the influence of the reported solid/liquid, we varied the quantity of the support while keeping the concentration of the solution constant and equal to 80 ppm. If one draws a curve that expresses (Co-Ce), the quantity adsorbed at equilibrium is in ppm or (rag/l) (but not the quantity adsorbed at equilibrium by mass of catalyst), according to the concentration of catalyst in g/1 of TiO 2 (Fig. 3). We note that it presents a maximum at r equal to 0.8, whereas one should have a landing; indeed if one increases r indefinitely, (Co-Ce) should remain constant. We conclude that for a given concentration, there is an optimal value for r. In our case it is equal to 0.8 for a concentration o f phenol equal to 80 ppm. For elevated quantities o f TiO2, the adsorption of phenol is disfavoured because particles of TiO2
358
S. Bekkouche et aL / Desalination 166 (2004) 3 5 5 - 3 6 2 ",4.
C,,(TiO~)(g/I)
/ - / ,
,2-
V-
E o
"6 10-
7
~
/
/#
~/
-
....
US 0.8 ,
us08
.____~.o ~ , ---o--= ~..._~e~e~,~
----L~ Us I ~MS 1
O
~
t~
~ 5 o, 4 C ~: 3
-
,, 6 ~ ,,~
0.0 0.2 0,4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
report solid/liquidr(g/I) Fig. 3. Equilibrium adsorbed quantities of phenol (C0- Ce, m g/l) vs. reported solid/liquid r (Co (phenol) = 80 ppm, V= 1200 rpm, pH = 5.72, T= 25°C).
have the tendency to agglomerate and are conducted to the reduction of the specific surface and therefore the reduction of the retention capacity. This interesting result explains why (in addition to the opacity of the luminous radiance to the strong concentrations of photocatalyst) all studies ofphotocatalysis note the existence of an optimal concentration of TiO2, especially as we know that the kinetics of adsorption plays a primordial role in these chemical reaction types.
3.4. Effect of aggregation of Ti02 The analysis of the TiO 2 powder in aqueous suspension shows the existence of micrometric aggregates. The dimension of these aggregates seems to be the same size that the one observed on the dry powder. Indeed, the gunpowder of TiO 2 presents itself to dry as an association of particles to the scale of a few tens of micrometers. This structure seems to preserve itself in aqueous suspension [ 17]. We studied the effect of this aggregation on its property of adsorption. We tested two modes of agitation: simple agitation with a magnetic stirrer and specific agitation using ultrasound. The adsorption was followed for three initial concen-
20
o
2'o
. . . . .
us1.5
4'0
8'o
~'o
time of contact (rnin)
Fig. 4. Effect of mode of agitation. US, ultrasonic; MS, magnetic stirrer. trations of TiO 2 according to the two modes of agitation. Results presented in Fig. 4 show that an adsorbed quantity of phenol is better in the presence of ultrasound. The effect was much more important than the quantity of TiO 2. The increase was on the order of 5%. This result also confirmed the one found above, that an increase of the concentration of TiO2 beyond a certain optimal value would decrease an adsorbed quantity. Chen [18] found, in the presence of ultrasound, an increase of 29% of the quantity of adsorbed chloroethane in the range of 0.6 to 3 g/1 of TiO 2. Rideh [19] found, in the presence of ultrasound, an increase of 12% of the quantity of chlorophenol in the range of 0.8 to 2 g/1 of TiO2. The increase of the adsorbed phenol in the presence of ultrasound is caused by the fragmentation of agglomerates. The fragile agglomerates at great concentrations of TiO2 are easily destructible by ultrasound; on the other hand, at low concentrations these agglomerates formed are resistant to ultrasound [17].
3.5. Effect of pH pH plays a major role in the phenomena of
359
S. Bekkouche et aL / Desalination 166 (2004) 355-362 11.0~ 10.5-" I-"5 -~
10,0-4D
9.5£ 9.0-
~" 8.58,00
;,5~ 6.5' pH
Fig. 5. Effect ofpH on adsorbed equilibrium quantity of phenol (Co (phenol) = 80 ppm, r = 1 g/l, V= 1200 rpm, T = 25oc).
adsorption, particularly when the adsorbant is a semiconductor such as TiO 2. It exists at an isoelectric point; over this point the surface of TiO2 is charged negatively and under it is positive. The state of the phenol molecule and the load of the surface of the catalyst play a large role in the phenomenon of adsorption. These two states are bound directly to the value of the pH. It is therefore necessary to examine the influence of the pH of the solution on the adsorption. In our experience the pH varied from 2 to ! 1.5, which is to say from very acidic to very basic. All other operative conditions remained constant: concentration of the phenol, concentration of the catalyst, temperature and speed of agitation. The results in Fig. 5 represent quantities of phenol adsorbed, at equilibrium, for different pHs. The adsorption of the phenol presents an optimum for a pH understood to be between 5 and 6 (natural pH). The adsorbed quantity is bound to the distribution of species of the phenol and the state of the surface of TiO2 according to pH and to cations and anions present in solution. At a strongly acidic pH (pH = 2), the adsorption is relatively low, followed by a strong increase until a pH of 5 where the adsorption is
maximal. At a low pH the molecule of phenol is non-dissociated (neutral) and the surface of TiO2 is also at a neutral state (TiOH). In addition, when the pH is adjusted with HC1, the C1- anions are also adsorbed at the surface of TiOz. There is competition between the adsorption of the anions and phenol. At pH 5, the surface of TiO2 is positively charged; just before the isoelectric point, the adsorption is at its maximum, and the quantity of CI- ions is lower. Beyond a pH of 6.5 to 9, the surface of TiO2 is charged negatively in a state of TiO. An apparition ofphenolat ions, there is a phenomenon of repulsion, in addition to the adsorption of the Na + ions and OH-, creating competition with the phenolat ions. Beyond a pH of 10, the phenol is entirely at the state ofphenolat and the surface of TiO2 is at the state of TiOH where the quantity adsorbed was constant. In conclusion, we note that the adsorption is favoured around the isoelectric point either at pH 5 or 6. 3.6. Adsorption isotherm
At a constant temperature, the variation of the adsorbed quantity of the phenol at equilibrium vs. the equilibrium concentration in liquid at different initial concentrations represents the adsorption the isotherm shown in Fig. 6. The isotherm is represented better by the Langmuir model (Fig. 6 and Table 1). Table 1 Constants of adsorption isotherm Langmuir model Freundlich model a.b. C e l+b.C~
Equations
Qe - - -
Qe = a. Cb
a b
19.16565 0.01599 0.99137
1.12009 0.50761 0.95142
R2
R2, correlation coefficient.
360
S. Bekkouche et al. / Desalination 166 (2004) 355-362 14 10.
12-
,:5. '5
6-
o~
4..
o3
2-
j,,,
9-
I-- 10, "6 ¢3b 0 8-
...."
7'
o
//
- - - model of langmuir
/
0
model of freundlich
o"
'
'
20
7""
E 6. ~5
/.,°
4o
' "
6o ' 8o C,~(mg/I)
'
loo'
'
~20
'
...'" .¢~qlj"r j..,""
3. 210
20
t40
Fig. 6. Isotherm of adsorption of phenol by TiO2 (r =
40
60
80
120
100
10
C~. ( m g / l )
Fig. 7. Linear form of the Langmuir isotherm.
1 g/l, V= 1200 rpm, pH = 5.72, T = 25°C). Table 2 Constants of the linear form of the Langmuir isotherm Model Equation
Langmuir
1/(KQmJ 1/Q,,a×
3.45893 0.05116 0.99283
R
300
D. Robert and all,F]
250
/
~
1
e
2O0
(Ce/Q,)= 1/~(Q=~.~)+(Ce/Qm~x)
Og10050/
0
The linear form o f the equation (Fig. 7 and Table 2) gives the best linearity for the Langmuir model. The linear representation permits us to calculate values of the adsorption constant K and the specific maximal quantity, Qmax. We find:
(Ce/Qe)= l/~Qmax) + (f e/Omax) K = 0 . 0 1 4 7 9 1.mg -] = 1.39 m m o l e . 1 - 1
Qm~x = 19.54652 mg.g -1 = 0.2079 mmole.g
A comparison of our experimental results with those of Robert et al. [7] and Fig. 8 shows agreement in the domain of our studied concentrations.
o
r
,
experimentalresults
i
loo
•
2;0
a;o
Co(mg/I)
Fig. 8. Equilibrium adsorbed concentration of phenol vs. initial concentration of phenol. For constants of adsorption we do not dispose values of comparison with regard to the phenol; however, the value of K found was lower than that given in the literature for certain organic compounds (Table 3). This result shows that the phenol itself was adsorbed with difficulty on TiO 2.
3.7. Kinetics o f adsorption The kinetics o f adsorption were determined by the mass transfer at the liquid/solid interface
X Bekkouehe et al./Desalination 166 (2004) 355-362 3,0
"//~
Co(phenol ) mgll
2.5 -
2.o-. l s[
/,i/20
1.o
o.s"
/if
/.
o5
1.0
361
where Q is the adsorbed quantity at time t, Qethe adsorbed quantity at equilibrium, and K~ is the kinetic constant of mass transfer or conductance of transfer. By integration we obtain:
ln(O-Oe)/(Oe-Po
) - = g a'
•
In this particular case where the adsorption takes place on a nuded surface, Q0 is null, thus:
-1.5 -2.0 ] -25 -30
•
.5
i
a
•
i
•
5
i
to
,
~
•
i
,
i
,
i
,
i
15 20 25 30 35 time of contact (min)
,
|
40
•
i
•
45
i
•
50
55
Fig. 9. Adsorption kinetics of phenol by TiO2. Table 3 Values of adsorption constants o n T i O 2 and quantities maximal adsorbed for different organic compounds Compound
K Qm~x Ref. (1 mmole-1) (mmole g-i)
Salicylic acid 4-aminobenzoic acid 3-chloro-4-hydro benzoique acid Dichloroethane acid 2-chlorophenol Phenol
10.5 6.60
0.07 0.071
4.57
0.047
2.59
0.225
3.6 1.39
0.107 0.207
[ 18] [ 18] [ 18] [ 17]
Q
=
- -Kat ) Qe(1 e
Variations of In (Qe-Q) vs. time for different initial concentrations of phenol in the liquid phase are represented in Fig. 9. We obtained one satisfactory linearity. Therefore, this kinetic law well represents our experimental results. The constant kinetic Ka practically does not vary with the concentration of phenol; there is an average value of 0.086 min -1. The kinetics of adsorption of the phenol on TiO 2 is relatively slow since one can associate this with a constant of time of 1/K~ of the order of 11.63 min, independent of the concentration of phenol in the studied domain.
[ 19] Our results 4. Conclusions
where all resistance to mass transfer is localized. The fundamental equation to apply is the one that governs the phenomena of mass transfer, in general, between two phases; the flux of adsorption is proportional to the gradient between the quantity adsorbed Q at time t and the adsorbed quantity Qe at equilibrium and a constant K a (conductance of transfer):
dQ/dt=Ka(Q~-Q )
The phenol was weakly adsorbed on TiO 2. This adsorption is a chimisorption, and it is represented well by the Langmuir model in the studied concentration domain. The aggregation effect of TiO 2 in aqueous solution had an optimal concentration of the catalyst for a concentration of phenol. The adsorption was optimal for a pH between 5 and 6 in the neighborhood of the isoelectric point of YiO2.
362
S. Bekkouche et al. / Desalination 166 (2004)355-362
References [1] D.F. Ollis, C.Y. Hsiao, L. Budiman and C.L. Lee, J. Catalysis, 88 (1984) 89-96. [2] M. Bideau, B. Clandel and M. Otterbein, J. Photochem., 14 (1980) 291-302. [3] H.Y. Chen, O. Zahraa, M. Bouchy, F. Thomas and J.Y. Bottero, J. Photochem. Photobiol. A. Chem., 85 (1995) 179-186. [4] H.Y. Chen, O. Zahraa andM. Bouehy, J. Photochem. Photobiol. A. Chem., 108 (1997) 37-44. [5] D.W. Chen, A. Jay and K. Ray, App. Catalysis B. Enve., 23 (1999) 143-157. [6] O. Zahraa, C. Dorian, S.M. Ould-mame and M. Bouehy, Poster, 1lth International Congress on Catalysis, 1996. [7] D. Robert, S. Parra, C. Pulgarin, A. Krzton and J.V. Weber, Appl. Surf. Sci.., 167 (2000) 51-58. [8] S. Tunesi and M. Anderson, J. Phys, Chem., 95 (1991)3399. [9] J. Cunningham and G. A1-Sayyed, J. Chem. Sot., Faraday Trans., 86 (1990) 3935.
[10] J. Cunningham and P. Sedlak, Catal. Today, 29 (1996) 309. [11] A. Piseopo, D. Robert and J.V. Weber, App. Cata. B. Enve., 35 (2001) 117-124. [12] J. Sykora, Coordination Chem. Rev., 159 (1997) 95-108. [13] K.L. Okamoto, Y. Yamamoto, H, Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Soc. Jpn., 58 (1985) 2015-2022. [14] K.L. Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Soe. Jpn., 58 (1985) 2023-2028. [15] V. Augugliaro, L. Palmisano and A. Sclafani, Toxicol. Environ. Chem., 16 (1987) 89-109. [16] V. Brezova and A. Blazkova, J. Photochem. Photobiol. A: Chem., 109 (1997) 177-183. [17] H.Y. Chen, Etude compar6e de l'adsorption et de la d6gradation photocatalytique de polluants des eaux, Th6se INPL, Nancy, France, 1994. [18] J. Cunningham and G. Al-Sayyed, J. Chem. Soc. Faraday Trans., 86 (1990) 3935-3941. [19] L. Rideh, Thbse INPL, Nancy, France, 1997.