Reac Kinet Mech Cat (2010) 99:201–208 DOI 10.1007/s11144-009-0098-x
Photocatalytic decolorization kinetics of diazo dye Congo Red aqueous solution by UV/TiO2 nanoparticles ´ urkovic´ • Davor Ljubas • Hrvoje Juretic´ Lidija C
Received: 20 July 2009 / Accepted: 29 August 2009 / Published online: 22 December 2009 Ó Akade´miai Kiado´, Budapest, Hungary 2009
Abstract The efficiency of color removal from aqueous Congo Red dye (CR) solution has been investigated in TiO2 suspensions irradiated with artificial UV light. Batch photocatalytic tests were carried out by varying the amount of TiO2 and the irradiation time using the same initial CR concentration. The experimental results indicated that the decolorization rate follows pseudo first-order kinetics with respect to CR concentration. The doses of TiO2 were 0.25, 0.5 and 1.0 g L-1 and the wavelength of incident ultraviolet light was predominantly 254 nm. CR adsorption on the surface of TiO2 is also investigated and described. Keywords Photocatalytic decolorization kinetics UV radiation TiO2 Congo Red dye Adsorption
Introduction The first usage of dyes belongs to the textile industry for dyeing of textile materials. Nowadays, dyes are widely used in industries such as textile, rubber, paper, plastic, cosmetic etc. Dyes can be divided into several categories, based on their chemical nature: anionic or cationic and basic or reactive dyes. Azo dyes are the largest group of the synthetic colorants known and the most common group released into the environment. Wastewater containing dyes may be toxic, carcinogenic and mutagenic [1, 2]. Therefore, the removal of synthetic dyes with azo aromatic groups is extremely important. The development of new technologies for wastewater purification tends to reach complete destruction of the contaminants. Recently, advanced oxidation processes (AOPs) have been proposed as alternative methods for water purification. Among AOPs, heterogeneous photocatalysis using TiO2 as a photocatalyst is commonly used as destructive
´ urkovic´ (&) D. Ljubas H. Juretic´ L. C Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lucˇic´a 5, 10000 Zagreb, Croatia e-mail:
[email protected]
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technology, starting with pioneering research of Fujishima and Honda [3]. TiO2 is a widely used photocatalyst due to its stability during UV light exposure, high chemical resistance, non-toxicity and low cost [4, 5]. Titanium dioxide can be used as a photocatalyst in various applications, such as degradation of air pollutants (NOx, aromates, chlorofluorocarbons) or aerosols [6–9] and in the water purification processes, like degradation of contaminants in wastewater [10, 11], removal of biofilms [12] or remediation of drinking and surface water [13, 14]. Additional benefit for using TiO2 as the photocatalyst is a possibility for using solar radiation for activation of the photodegradation process instead of using artificial UV sources [15, 16]. In this paper, photocatalytic color removal of aqueous solution of CR has been investigated using a commercial form of titanium dioxide from Degussa (P-25). CR is a brownish red powder having an absorbance maximum between 497.0 and 500.0 nm in aqueous medium. It is a water soluble secondary diazo dye and contains an azo (–N=N–) chromophore and an acidic auxochrome (sulfonate: –SO3H) associated with the benzene structure. CR is used in medicine (as a biological stain) and as an indicator since it turns from red-brown (in basic medium) to blue in acid [17]. It is also used to color textiles and it could also be used as a gamma-ray dosimeter since its coloration decays with the intensity of the irradiation [18].
Experimental Materials and apparatus Titanium dioxide (TiO2) catalyst from Degussa (P-25), with purity 99.9%, was used as received, without further modification. It is mostly in the anatase form (75–80% anatase and 20–25% rutile), nonporous, with a reactive surface (BET) area of 50–54 m2 g-1, corresponding to a mean particle size of around 30 nm [16, 17]. CR was supplied by ACROS, as high purity biological stain, and used as a model compound without further purification. All solutions were prepared using double distilled deionized water.
Photocatalytic experiments All experiments were carried out in a 0.2 L batch glass photoreactor with 60 mm diameter. The radiation source was a mercury UV lamp, kmax = 254 nm, model Pen-Ray 90-001201, manufactured by UVP. The UV lamp was placed in the center of the reactor. The removal of CR was investigated at the temperature (25 ± 0.2) °C, with continuous purging with air (O2) using three different conditions: (i) adsorption in the dark (TiO2 without UV), (ii) UV-irradiation (without TiO2), and (iii) UV/TiO2 by varying the amount of TiO2. The reaction temperature was controlled by circulating cooling water. The initial concentration of CR was 55 mg L-1 and concentrations of TiO2 were 0.25, 0.5 and 1.0 g L-1. The mixtures of aqueous solution of CR and TiO2 nanoparticles were stirred with a magnetic stirrer at a constant speed of 300 rpm. Before turning on the UV lamp, the solution was placed in the dark, covered with aluminum foil, and kept stirring for 15 min to reach the adsorption/desorption equilibrium of the suspension. The samples were taken from the reactor for analysis at certain reaction intervals (60, 120, 240, 360 and 480 min),
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centrifuged (SIGMA-LABORZENTRIFUGEN, Model 2-15) and the remaining dye concentration was analyzed with UV–Vis spectrophotometer (HEWLETT PACKARD, Model HP 8430) at 498 nm using a 1 cm quartz cell. For each condition, repetition tests were made to ensure reproducibility.
Adsorption The adsorption process of dissolved CR onto TiO2 nanoparticles was investigated. Sorption experiments were performed in the above-described photoreactor. During the process of adsorption, all the parameters were same as the parameters during the process of the photocatalytic degradation except that the UV lamp was switched off. The amount of TiO2 was constant (1 g TiO2 L-1) and the concentrations of the CR varied from 2 to 55 mg L-1. After 90 min of adsorption, the aqueous phase was separated from TiO2 nanoparticles by centrifugation and concentration of CR (mg L-1) was determined by means of absorbance measurement at 498 nm. The amount of CR adsorbed on the TiO2 nanoparticles was calculated from the following equation: qe ¼
ðc0 ce Þ V m
ð1Þ
where qe is the equilibrium CR concentration adsorbed on the TiO2 nanoparticles (mg g-1), V is the initial volume of CR solution used (L), m is the mass of TiO2 used (g), c0 is the initial concentration of CR in the solution (mg L-1) and ce is the equilibrium concentration of CR in the solution (mg L-1).
Results and discussion Adsorption isotherm studies The results of adsorption study of CR onto TiO2 nanoparticles are shown in Fig. 1. Several isotherm models are available to describe the equilibrium sorption distribution in which two models are used to fit experimental data: Langmuir’s and Freundlich’s models [19]. The Langmuir isotherm is expressed by: qe ¼
qm KL ce 1 þ KL ce
ð2Þ
The above equation can be rearranged to the following linear form: 1 1 1 ¼ þ qe qm KL qm ce
ð3Þ
where ce is the equilibrium concentration (mg L-1), qe is the amount of CR sorbed (mg g-1), qm is the maximum amount of TiO2 required to give a complete monolayer on the surface (mg g-1), and KL is an indication of the sorption capacity of the sorbent (L mg-1). A plot of 1/qe versus 1/ce results in a straight line with a slope of (1/KL qm) and
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204 14 12
qe, mg/g
10 8 6 4 2 0 0
5
10
15
20
25
30
35
40
ce, mg/L Fig. 1 CR adsorption isotherm on the TiO2 nanoparticles (1 g L-1 TiO2) at 298 K
intercept of 1/qm (not shown here). The Langmuir isotherm parameters are summarized in Table 1. The good agreement (R2) with the experimental data suggests that the CR sorbed forms a monolayer coverage on the adsorbent surface of TiO2 nanoparticles. Freundlich’s model presents an empirical sorption isotherm for non-ideal sorption on heterogeneous surfaces [20] as well as multilayer sorption and is expressed by the equation: qe ¼ KF c1=n e
ð4Þ -1
where ce is the equilibrium concentration of CR in the solution (mg L ), qe is the equilibrium CR concentration adsorbed on the TiO2 nanoparticles (mg g-1), KF (mg g-1)/ (mg L-1)1/n and n are the Freundlich constants related to sorption capacity and sorption intensity, respectively. The Langmuir isotherm data together with the Freundlich isotherm data for the investigated CR adsorption on TiO2 nanoparticles are summarized in Table 1. Comparing the correlation coefficients listed in Table 1, we can draw the conclusion that the adsorption of CR on TiO2 nanoparticles is better fitted with the Langmuir isotherm
Table 1 Langmuir and Freundlich isotherm parameters for the system CR-TiO2 nanoparticles with TiO2 load of 1 g L-1 Langmuir isotherm -1
qm (mg g )
Freundlich isotherm -1
2
KL (L mg )
R
19.47
0.9151
RL
-1
c0 (mg L )
n
KF ((mg g-1)/ (mg L-1)1/n)
R2
5.24
6.28
0.8567
Congo Red dye 8.56
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0.02102
2.4
0.00963
5.3
0.00775
6.6
0.00387
13.2
0.00195
26.3
0.00128
40.0
0.00095
54.0
Photocatalytic decolorization kinetics
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equation under the concentration range studied. It indicates the monolayer coverage of TiO2 nanoparticles by the CR dye. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter RL, which is defined by Eren and Acar [21] as: RL ¼ 1=ð1 þ KL c0 Þ
ð5Þ -1
where RL is dimensionless separation factor, c0 is the initial concentration of CR (mg L ) and KL is Langmuir constant, i.e. indication of the sorption capacity of the sorbent (L mg-1). The RL values (Eq. 5) dictate favorable adsorption for 0 \ RL \ 1, as it is proposed in [21]. As it is shown in Table 1, RL value decreases with the concentration increase and indicates favorable adsorption for CR on the surface of TiO2 nanoparticles.
Photocatalytic degradation Fig. 2 shows the time-dependent UV–Vis spectra of CR aqueous solution with concentration 55 mg L-1 as starting solution during the photocatalytic oxidation process. The spectrum of CR in the visible region has a maximum absorbance at 498 nm. The concentration of TiO2 during the experiment was 1 g L-1 and the solution was irradiated with UV-radiation and continuously bubbled with air (O2). It is clear from this figure that the intensity of the absorption peaks decreases with time of UV-irradiation exposure, which implies that the CR is degraded. The initial pH value of the solution was 7.13, and after 480 min it decreased to the value of 5.92. Almost complete degradation of CR dye was observed after 480 min. Experiments performed ‘‘in the dark’’ confirmed that the effects of
3 a
2.5
Absorbance (A)
b
2 c
1.5 d
1
e f
0.5
0 200
g
250 300
350
400 450
500 550
600
650 700
750
800 850
900
Wavelength, nm Fig. 2 UV-Vis spectra: (a) starting concentration of CR in aqueous solution 55 mg L-1, (b) after 15 min of dark adsorption in the presence of 1 g L-1 TiO2, (c) after 60 min of photocatalytic oxidation, (d) after 120 min of photocatalytic oxidation, (e) after 240 min of photocatalytic oxidation, (f) after 360 min of photocatalytic oxidation and (g) after 480 min of photocatalytic oxidation
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206 0.25 g/L TiO2+UV+O2 0.25 g/L TiO2 0 g/L TiO2+ UV+O2
0.5 g/L TiO2+UV+O2 0.5 g/L TiO2
1.0 g/L TiO2+UV+O2 1.0 g/L TiO2
1.00
c/ co
0.80
0.60 0.40 0.20
480
450
420
390
360
330
300
270
240
210
180
150
120
90
60
30
-15 0
0.00
t, min Fig. 3 Time dependence of normalized concentration (c0 is the initial dye concentration and c is the dye concentration at time t). Time interval marked as ‘‘-15’’ to time ‘‘0’’ suggests that the solution was not irradiated 15 min at the beginning of each experiment (i.e. UV lamp was switched off)
adsorption of CR on the surface of TiO2 catalysts were negligible in the overall color removal in comparison to photocatalytic oxidation process (Fig. 3). The effect of the amount of photocatalyst in suspension on the photodegradation of CR can be seen in Fig. 3, too. Obtained results indicate that the amount of TiO2, as well as the duration of UV exposure influence the degree of CR dye photodegradation. The best result of the photodegradation process is obtained using a TiO2 concentration of 1.0 g L-1 ? UV radiation ? O2 bubbling. Solely the UV-C radiation (predominantly by 254 nm) is capable of continuous degradation of CR. This degradation rate is lower (about 25% of the initial concentration of the CR is degraded after 480 min) than the degradation rate with presence of the photocatalyst, but it is still worth considering, especially in the case of trying to design a real-scale reactor, because one of the major operational costs for photocatalyst removal (or photocatalyst loss) could be avoided. The above results indicate that UV irradiation exposure of TiO2 particles during 480 minutes continuously ensure degradation of CR dye. It is in accordance with the theoretical background that explains energetics of electron processes in photocatalytic systems based on dispersed semiconductors [6–9, 22]: when TiO2 is continuously illuminated by light k \ 400 nm, electrons are continuously promoted from the valence band to the conduction band to give electron–hole pairs. The valence band potential is positive enough to generate hydroxyl radicals at the surface, and the conduction band potential is negative enough to reduce molecular oxygen. The hydroxyl radical is a powerful oxidizing agent and attacks organic pollutants that are present at or near the surface of TiO2. Decolorization kinetics The photocatalytic decolorization of CR follows the Langmuir–Hinshelwood kinetics model given by the following equation [23]:
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Photocatalytic decolorization kinetics Table 2 The pseudo first-order rate constant (k0 ) for the decolorization CR in aqueous solution under different experimental conditions
207
Parameters in reactor
k0 (min-1)
R2
0 g TiO2 ? UV ? O2
60 9 10-5
0.9730
0.25 g TiO2
8 9 10-5
0.9827
0.5 g TiO2
10 9 10-5
0.9342
1.0 g TiO2
20 9 10-5
0.9554
0.25 g TiO2 ? UV ? O2
2.1 9 10-3
0.9146
0.5 g TiO2 ? UV ? O2
3.2 9 10-3
0.9837
1.0 g TiO2 ? UV ? O2
4.7 9 10-3
0.9767
dc kKc ¼ dt 1 þ Kc
ð6Þ
where c is the CR concentration (mg L-1) at time t (min), k the reaction rate constant (mg L-1 min-1), and K is the adsorption coefficient of CR (L mg-1). After integration of Eq. 6 it is transformed to: c 1 1 0 ln þ ðc0 cÞ ð7Þ t¼ Kk k c As the initial concentration (c0) is a millimolar solution (c0 \ 0.1 mmol L-1), the second term on the right-hand side of Eq. 7 is negligible [5, 23]. Therefore, Eq. 7 can be further simplified to give an apparent first-order equation: c 0 ffi kKt ¼ k0 t ð8Þ ln c where k0 is the pseudo first-order rate constant in min-1. The pseudo first-order rate constant k0 from Eq. 8 is evaluated through the linear regression of -ln (c/c0) versus t and is in compliance with other research data [24, 25]. The corresponding values of the pseudo first-order rate constant k0 as well as the correlation coefficient R2 are given in Table 2. The values of the pseudo first-order rate constants are higher for all amounts of TiO2 in the presence of UV irradiation comparing to the same amounts of TiO2 without UV irradiation. Also, when introducing an increasing amount of TiO2, an increase of the values of the pseudo first-order rate constants is observed (Table 2). Obtained values of the correlation coefficient R2 (Table 2) indicate that the pseudo firstorder rate constant is suitable for describing photodegradation process of CR photocatalytic degradation with TiO2 as the photocatalyst.
Conclusions This paper presents the results of photodegradation process of diazo congo red dye (CR) aqueous solution with UV radiation and TiO2 nanoparticles. It was observed that the degree of CR photodegradation increased with increasing dose of the photocatalyst. The highest color removal rate was achieved using a TiO2 dose of 1.0 g L-1 ? UV radiation ? O2 bubbling. Adsorption processes on the TiO2 surface are better described using Langmuir’s isotherm model than Freundlich’s model.
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The values of RL (dimensionless separation factor) for adsorption of CR on the surface of TiO2 indicate that adsorption of CR is favorable on the adsorbent and the adsorption is more favorable at higher CR initial concentrations than at lower ones. The kinetics of photocatalytic degradation of CR follows a pseudo first-order equation. The values of the pseudo first-order rate constants were found to increase with increase in photocatalyst concentration. The ranking of the degree of CR removal/degradation of different used methods in 480 min is as follows: 0:25 gL1 TiO2 \0:5 gL1 TiO2 \UV ð254 nmÞ 1:0 gL1 TiO2 \0:25 gL1 TiO2 þ UV\0:5 gL1 TiO2 þ UV\1:0 gL1 TiO2 þ UV Further investigation will include immobilizing TiO2 catalyst on the inner surface of a batch glass photoreactor by thin spray or sol–gel coatings and therewith facilitate the process of separation of the photocatalyst from the suspension/solution. Acknowledgements This study was supported by Ministry of Science, Education and Sports of Republic of Croatia within the framework of the Projects no. 120-1253092-30 and 120-1201833-1789:10106-797300-1.
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