Environmental Technology
ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20
Photocatalytic degradation of resorcinol, an endocrine disrupter, by TiO2 and ZnO suspensions Sze-mun Lam , Jin-chung Sin , Ahmad Zuhairi Abdullah & Abdul Rahman Mohamed To cite this article: Sze-mun Lam , Jin-chung Sin , Ahmad Zuhairi Abdullah & Abdul Rahman Mohamed (2013) Photocatalytic degradation of resorcinol, an endocrine disrupter, by TiO2 and ZnO suspensions, Environmental Technology, 34:9, 1097-1106, DOI: 10.1080/09593330.2012.736538 To link to this article: https://doi.org/10.1080/09593330.2012.736538
Accepted author version posted online: 10 Oct 2012. Published online: 31 Oct 2012. Submit your article to this journal
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Environmental Technology, 2013 Vol. 34, No. 9, 1097–1106, http://dx.doi.org/10.1080/09593330.2012.736538
Photocatalytic degradation of resorcinol, an endocrine disrupter, by TiO2 and ZnO suspensions Sze-mun Lam, Jin-chung Sin, Ahmad Zuhairi Abdullah and Abdul Rahman Mohamed∗ School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Pulau Pinang, Malaysia (Received 5 January 2012; final version received 24 September 2012 ) In the work presented here, photocatalytic systems using TiO2 and ZnO suspensions were utilized to evaluate the degradation of resorcinol (ReOH). The effects of catalyst concentration and solution pH were investigated and optimized using multivariate analysis based on response surface methodology. The results indicated that ZnO showed greater degradation and mineralization activities compared to TiO2 under optimized conditions. Using certain radical scavengers, a positive hole, together with the participation of hydroxyl radicals, were the oxidative species responsible for ReOH degradation on TiO2 whereas, the ZnO photocatalysis occurred principally via hydroxyl radicals. Some hitherto unreported pathway intermediates of ReOH degradation were identified using gas chromatography-mass spectrometry. A tentative reaction mechanism for the formation of these intermediates was proposed. Moreover, the figure-of-merit electrical energy per order was employed to estimate the electrical energy consumption. Keywords: photocatalysis; resorcinol; degradation intermediate; electrical energy consumption; response surface methodology
1. Introduction There is increasing evidence that endocrine disruption is impacting wildlife and humans adversely on a global scale. The endocrine disruption is defined as a hormonal imbalance initiated by exposure to a pollutant, which leads to alterations in development, growth, and/or reproduction in an organism or its progeny [1–3]. The causative chemicals of endocrine disruption in wildlife populations are wide ranging and include natural and synthetic steroids, pesticides, and a plethora of industrial chemicals. Resorcinol (ReOH) is one of the endocrine disrupting chemicals (EDCs), which used in many industrial processes including adhesives, dyes, tanneries, food processing, agricultural research, pharmaceuticals and cosmetics. The exposure to ReOH can occur at its production sites, in the effluent stream, through its use in pharmaceutical applications and in cigarettes [4]. It was also found as a pollutant in surface water and filtered ground water. In fact, numerous in vivo tests of rats and epidemiological studies of humans have demonstrated that ReOH can interfere with triiodothyronine (T3) and thyroxine (T4) metabolism, causing disruptions to thyroid activity [5]. Various chemical, physical, and biological treatment processes are currently proposed for removing and destructing the organic pollutants in effluents. However, the conventional water and wastewater treatment plants using activated sludge and/or charcoal adsorption systems are ineffective and non-destructive for many EDCs [6]. In recent years, an alternative to the conventional methods ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
is the ‘advanced oxidation processes’ (AOPs) that have shown promising techniques for the purpose of removing EDCs from effluents of biological treatment plants and surface water [1,7,8]. In particular, heterogeneous photocatalytic oxidation (HPO) has garnered much attention as an efficient method for degrading the persistent pollutants in water due to the organic compounds can be nearly mineralized by irradiation energy in the near-ultraviolet (UV) range. The HPO is a process in which the destruction of recalcitrant pollutants is governed by the combined actions of an energetic irradiation source, an oxidizing agent, and a semiconductor photocatalyst. When semiconductor particles are illuminated with near-UV irradiation, electron–hole pairs are generated within the semiconductor photocatalyst. Dissolved oxygen in the solution can scavenge excited electrons restraining the electron-hole recombination. Organic compounds may undergo oxidation directly at the hole. The hole can also undergo charge transfer with adsorbed water molecules or hydroxide anions forming • OH radicals. Previously, the photocatalytic oxidation of ReOH was investigated mostly in titanium dioxide (TiO2 ) dispersions [4,9]. Even though zinc oxide (ZnO) is less studied, it presented promising outcomes due to its higher photocatalytic performance in comparison with TiO2 [10]. With respect to the photocatalytic degradation of ReOH by TiO2 and ZnO, few studies reported in the literature have placed emphasis on the influence of operating variables in their suspension reaction systems [5,11]. Nevertheless, studies
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S.-m. Lam et al.
focusing on the theory behind the operational parameters and their interactions are insufficient and present intricate tasks for process optimization. In addition, the photocatalytic degradation pathway of ReOH has not been entirely elucidated. Earlier work by Duczmal and Sobczynski [12] obtained three oxidation intermediates in the TiO2 photocatalytic degradation of ReOH and explained the role of the oxidizing species in the process. The efforts to identify degradation intermediates of ReOH over ZnO and S-doped ZnO catalysts using high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) were reported only to have two oxidation intermediates [11,13]. Hence, the present work aims to optimize the operational parameters for the degradation of ReOH using TiO2 and ZnO suspensions using a multivariate approach based upon a response surface methodology (RSM). Important parameters such as catalyst concentration and solution pH were investigated in the experimental design. To understand better the role of the participation of the oxidizing species in the reaction mechanism, radical scavengers were used. The study also investigated the reaction pathway and disclosed new stable ReOH intermediates. Finally, the electrical energy consumption and related treatment costs were calculated. 2.
Materials and methods
2.1. Materials Titania P25 having 80% anatase and 20% rutile (mean diameter 30 nm and surface area 50 m2 /g) was obtained from Degussa. ZnO having a hexagonal phase (mean diameter 180 nm and surface area 5 m2 /g) and ReOH were purchased from ACROS Organics. All other chemicals used for analytical methods were reagent grade. Chemical standards include ethanol and acetonitrile from Fischer Chemicals, p-benzoquinone from Sigma-Aldrich and sodium iodide from Hayashi Pure Chemical. 2.2. Photocatalytic experiments All experiments were carried out in a batch-mode immersion well photoreactor. The photoreactor was made of Pyrex glass with dimensions of 200 × 100 × 60 mm (height × outer diameter × inner diameter). In the centre of the cylindrical photoreactor, a 15 W UV Pen-ray (UVP, Inc.) lamp with a maximum emission at about 365 nm was applied as the UV source. The average UV output intensity at 10 mm away from UV light, measured by radiometer (Cole Parmer, Series 9811) was 1.060 mW/cm2 . The temperature of the system was maintained at 26 ± 2◦ C by cooling the water jacket. In a typical experiment, the desired amount of photocatalyst was added to the photoreactor containing 350 mL of a 20 mg/L ReOH solution. Where required, pH adjustment was done using equimolar NaOH and HCl solutions. During all experiments, the heterogeneous mixture was magnetic stirred and bubbled with air at a fixed flow rate of 4 mL/min.
The mixture was equilibrated for 5 min in the dark and subsequent by the UV irradiation. 2.3. Analytical methods The solution pH of all test samples was measured using a calibrated Mettler Toledo 320 pH meter. An atomic absorption spectrophotometer (Shimadzu, AA-6650) was used to estimate the dissolution of ZnO in the course of photoreaction. Standard solutions bracketing the Zn2+ content of samples were prepared and ran at the same time as the test samples. The concentrations of the ReOH in the reaction mixture at different reaction times were monitored by HPLC (Perkin Elmer, Series 200) equipped with an isocratic gradient pump, a 20 L injection circuit and a variable wavelength UV detector. The detection wavelength was set at 238 nm. A C18 column (length 150 mm × inner diameter 4.6 mm × particle size 5 μm) was used in the sample analysis with acetonitrile–water mobile phase in the ratio of 30:70 (v/v) at a flow rate of 1 mL/min. A sample of about 5 mL was drawn at given time interval of irradiation. The liquid samples were centrifuged at 5600 rpm for 10 min and then filtered through 0.20 μm millipore filters to separate the particles. GC-MS was employed to identify the potential intermediates during the degradation of ReOH. The GC-MS equipment used for the analysis was a Perkin Elmer Clarus 600 gas chromatograph. Prior to instrumental analysis, photoproducts were acidified with diluted HCl and extracted three times from 20 mL catalyst free test samples with dichloromethane (CH2 Cl2 ). The extract was then dried with anhydrous Na2 SO4 and then evaporated nearly to dryness using rotary evaporator. Separation was carried out in an Elite-5MS column (length 30 m × inner diameter 0.25 mm × particle size 0.25 μm). The injector temperature was 280◦ C and the sample was injected in the split less mode for 1 min. The column temperature was held at 65◦ C for 1 min and then increased at 20◦ C/min to 280◦ C, where it was held for 10 min. Helium was used as the carrier gas. For the MS detection, standard electrospray ionization conditions (70 eV) were used with a source temperature of 280◦ C and the mass range from 20 to 300 m/z units. To monitor the extent of mineralization during the ReOH degradation, changes in total organic carbon (TOC) were measured using a TOC analyser (Shimadzu, TOC-VCPH ). In order to determine the reproducibility of the results, at least duplicated runs were carried out for each condition for averaging the results and the experimental error was found to be within ±4%. 2.4.
Investigation of photocatalytic reaction mechanism To elucidate the roles of the photogenerated hole, • OH radical and superoxide radical in the photocatalytic
Environmental Technology reactions with the TiO2 and ZnO under UV light irradiation, the ReOH degradation with the interference of 1 mM scavenging species such as ethanol, sodium iodide and p-benzoquinone were investigated. In a separate experiment, acetonitrile was used instead of water in order to confirm the roles of • OH radicals in both photocatalytic systems. 2.5.
Experimental design
The multivariate approach was performed using a rotatable central composite design (CCD). Analysis of the experimental data was supported by statistical graphics software system Design Expert version 8.0.6 (STAT-EASE Inc., Minneapolis, USA).
3. Results and discussion 3.1. Optimization of ReOH degradation The design consisted of three series of experiments: (i) a two-level rotatable CCD 22 in which the factor levels are coded to the usual low (−1) and high (+1) values; (ii) four axial or star points localized on the axis of each variable at a distance ±α = 1.41 from the designed centre and (iii) three centre points that can be replicated to give an estimation of experimental error variance. Experiments were undertaken in random order to provide protection against the effects of lurking variables. 3.1.1. Optimization via TiO2 Table 1 shows the experimental results for the ReOH degradation after 80 min UV irradiation using TiO2 suspensions. The catalyst concentration was ranged between 2.5 and 6.0 g/L and the pH ranged from 4.0 to 10.0. These values were selected according to our previous experiments Table 1. Codified variable levels from the CCD for ReOH degradation by TiO2 photocatalysis. Level of variables and codes Run no. 1 2 3 4 5 6 7 8 9 10 11 12 13
XT (g/L)
XpH
2.5 (+1) 4.0 (+1) 6.0 (−1) 4.0 (+1) 2.5 (+1) 9.0 (−1) 6.0 (−1) 9.0 (−1) 1.78 (+1.41) 6.5 (0) 6.72 (−1.41) 6.5 (0) 4.25 (0) 2.96 (−1.41) 4.25 (0) 10.04 (+1.41) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0)
Response after 80 min, Y (%) Measured Calculated 63.8 67.8 67.9 70.2 65.1 70.2 62.3 69.4 69.9 69.2 68.7 69.3 68.9
62.9 67.2 67.9 70.4 65.6 70.4 62.3 69.2 69.2 69.2 69.2 69.2 69.2
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and also from the data in the literatures for similar laboratory experiments [6,7,9,14]. The figures in parenthesis represented the codified values. The results reported in Table 1 revealed that a high coincidence was obtained when comparing the two last columns, demonstrating a good description between the mathematical model and experiment. The response factor of ReOH was best explained by a second degree polynomial expression as shown in Equation (1), where Y was the percentage of compound degraded, XT was the TiO2 concentration in g/L and XpH was the solution pH. Each coefficient of a variable in the equation determined the change in mean response per unit increase in the associated independent variable while the other variables were kept constant. Y (%) = 69.20 + 1.69XT + 2.07XpH − 0.61XT2 2 − 1.51XpH − 0.43XT XpH
(1)
The statistical analysis results expressed in terms of the regression coefficient, degrees of freedom (DF), standard error, F and p values are shown in Table 2. From the statistical analysis, it was evident that all the linear and quadratic terms of solution pH were statistically significant (p < 0.05, where p denotes the probability). The regression coefficients and the analysis of variance (ANOVA) results further suggested that the solution pH was the parameter exhibiting most significant effect on the degradation rate. For the regression coefficients, both the magnitude and sign are important, as the former indicates the importance or influence of the variable on the response factor, whereas the sign determines its effect direction. A positive sign of the coefficient represents a synergistic effect, while a negative sign indicates an antagonistic effect. The quadratic term of solution pH has a negative significant relationship with the ReOH degradation process, whereas the quadratic term of the TiO2 concentration (T 2 ) and the interactive term of the TiO2 concentration and the pH (T∗ pH) exhibited a negative sign but a less significant effect on the ReOH degradation. The response surface diagram in Figure 1(A) indicates that the degradation of ReOH was not favoured in acidic media (pH < 4). An increase in solution pH enhanced the degradation, which reached a maximum at pH 8.8. This may be due Table 2. The ANOVA results for degradation of ReOH by TiO2 photocatalytic process. DF
Standard error
F-value
p-value
1.69 2.07
1 1
0.25 0.25
46.68 69.95
0.0002 0.0001
XT2
−0.61
1
0.27
5.23
0.0561
2 XpH
−1.51
1
0.27
32.28
0.0007
XT XpH
−0.43
1
0.35
1.48
0.2635
Variable XT XpH
Coefficient
1100
Figure 1.
S.-m. Lam et al.
Response surface showing the percentage of ReOH degradation using TiO2 (A) and ZnO (B).
to the TiO2 has a zero point charge (zpc) closed to the neutrality (6.25), while the ReOH presented pKa values at 9.3 and 9.8 [15]. At pH 8.8, TiO− was the predominant group on the TiO2 particles and ReOH was primarily in the molecular form. Both of them were combined by hydrogen bonding simply, thus increasing the amount of adsorption and enhancing the degradation efficiency. At pH > 9.8 as the ReOH molecules were negatively charged in alkaline media, their adsorption was expected to be affected by an increase in the repulsion effect with the TiO− groups on the semiconductor surface. Hence, the optimal values for TiO2 concentration and pH estimated by the Design Expert 8.0 software were 6.0 g/L and 8.8, respectively. 3.1.2. Optimization via ZnO When the solution was performed in the presence of ZnO, the response factor was chosen after 40 min of UV irradiation. Due to the solubility of ZnO in suspensions at pH < 6.5, it is necessary to work at neutral or alkaline pH [16]. The potential solubility is an important concern for ZnO photocatalysts owing to the possible results of catalyst inactivation and secondary pollution from free Zn2+ . Several studies have given evidence of ZnO inactivation and suggested that this should be caused by the incongruous dissolution yielding Zn(OH)2 on the ZnO surface [17,18]. The multivariate approach was carried out by studying, simultaneously, the catalyst concentration that was varied between 2.0 and 6.0 g/L and the pH that ranged from 7.0 to 11.0. Table 3 shows the experimental and codified values for the ReOH degradation under ZnO suspensions. The response factor showed that high degradation levels, over 90%, were attained for a determined set of variables. The consistency of the mathematical model was confirmed by the calculated values shown in the last column of Table 3, where there is high agreement with the experimental values. The polynomial expression describing the
Table 3. Codified variable levels from the CCD for ReOH degradation by ZnO photocatalysis. Level of variables and codes Run no. 1 2 3 4 5 6 7 8 9 10 11 12 13
ZnO (g/L)
pH
2.0 (+1) 7.0 (+1) 6.0 (−1) 7.0 (+1) 2.0 (+1) 10.0 (−1) 6.0 (−1) 10.0 (−1) 1.17 (+1.41) 8.5 (0) 6.83 (−1.41) 8.5 (0) 4.0 (0) 6.38 (−1.41) 4.0 (0) 10.62 (+1.41) 4.0 (0) 8.5 (0) 4.0 (0) 8.5 (0) 4.0 (0) 8.5 (0) 4.0 (0) 8.5 (0) 4.0 (0) 9.0 (0)
Response after 40 min, Yexp (%) Measured Calculated 82.2 87.4 94.4 95.2 83.4 89.9 88.5 100.0 91.9 91.2 93.2 92.3 91.9
82.6 88.6 93.9 95.5 83.6 89.0 87.5 100.3 92.1 92.1 92.1 92.1 92.1
ReOH degradation is presented in Equation (2), where Y is the percentage of compound degraded, Xz is the ZnO concentration in g/L and XpH is the solution pH. Y (%) = 92.10 + 1.90Xz + 4.53XpH − 2.89Xz2 2 + 0.91XpH − 1.10Xz XpH
(2)
Table 4 shows the results of statistical analysis for the degradation of ReOH by the ZnO photocatalytic process. It was observed from the table that all the linear, quadratic and interactive terms were statistically significant (p < 0.05). The regression coefficients and the ANOVA results further proposed that both the ZnO concentration and solution pH played a decisive role in the extent of ReOH degradation. The quadratic term of the ZnO concentration (Z 2 ) and the interactive term of the ZnO concentration and pH (Z∗ pH) have a negative significant relationship with the
Environmental Technology Table 4. The ANOVA results for degradation of ReOH by ZnO photocatalytic process. Variable Xz XpH Xz2 2 XpH
Xz XpH
Coefficient
DF
Standard error
F-value
p-value
1.90 4.53
1 1
0.33 0.33
33.64 191.64
0.0007 0.0001
−2.89
1
0.35
67.62
0.0001
0.91
1
0.35
6.75
0.0355
−1.10
1
0.46
5.64
0.0492
ReOH degradation. Figure 1(B) shows that the best condition for ReOH degradation was at a medium value of 4.0 g/L ZnO and a high pH value of 11.0. It is important to point out that ZnO has a pH at zpc of 9.3 [19]. Although at pH 11.0 either the surface of ZnO or ReOH was negatively charged, the degradation efficiency was the highest. This phenomenon was slightly different than the ReOH degradation mechanism using TiO2 as discussed above. One possible explanation given for this was that in alkaline media, pH 11.0, • OH radicals are easier to form by oxidizing more hydroxide anions available on the ZnO surface and thus the efficiency of the degradation was enhanced. Similar results have been reported in the photocatalytic degradation of phenols and dyes under ZnO suspensions [10,20,21]. In conclusion, ZnO is a better catalyst than TiO2 in the degradation of ReOH and its use is limited only by solution pH. Moreover, several reports have demonstrated that ZnO could extend its spectral absorption and higher photon utilization that might elucidate the higher efficiency of ZnO compared with TiO2 [22,23]. The ZnO dissolution data collected from atomic absorption spectrophotometer revealed a little loss of ZnO under UV irradiation. The solution pH during the test was averaged at 11.01 with mean deviation
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of ±0.15. The dissolution was found mostly low (0.01%) and did not increase appreciably with irradiation time.
3.2. Reactions under optimized conditions 3.2.1. ReOH degradation ReOH degradation was performed in suspensions under optimized conditions, TiO2 6.0 g/L and pH 8.8 as shown in Figure 2(A). For comparison, the results obtained from dark adsorption and direct photolysis are also included in the figure. The presence of TiO2 notably accelerated the ReOH degradation with virtually 82% of ReOH degradation being achieved after 2 h irradiation. On the other hand, the dark adsorption was found to have a low removal (
ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20
Photocatalytic degradation of resorcinol, an endocrine disrupter, by TiO2 and ZnO suspensions Sze-mun Lam , Jin-chung Sin , Ahmad Zuhairi Abdullah & Abdul Rahman Mohamed To cite this article: Sze-mun Lam , Jin-chung Sin , Ahmad Zuhairi Abdullah & Abdul Rahman Mohamed (2013) Photocatalytic degradation of resorcinol, an endocrine disrupter, by TiO2 and ZnO suspensions, Environmental Technology, 34:9, 1097-1106, DOI: 10.1080/09593330.2012.736538 To link to this article: https://doi.org/10.1080/09593330.2012.736538
Accepted author version posted online: 10 Oct 2012. Published online: 31 Oct 2012. Submit your article to this journal
Article views: 209
View related articles
Citing articles: 10 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tent20
Environmental Technology, 2013 Vol. 34, No. 9, 1097–1106, http://dx.doi.org/10.1080/09593330.2012.736538
Photocatalytic degradation of resorcinol, an endocrine disrupter, by TiO2 and ZnO suspensions Sze-mun Lam, Jin-chung Sin, Ahmad Zuhairi Abdullah and Abdul Rahman Mohamed∗ School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Pulau Pinang, Malaysia (Received 5 January 2012; final version received 24 September 2012 ) In the work presented here, photocatalytic systems using TiO2 and ZnO suspensions were utilized to evaluate the degradation of resorcinol (ReOH). The effects of catalyst concentration and solution pH were investigated and optimized using multivariate analysis based on response surface methodology. The results indicated that ZnO showed greater degradation and mineralization activities compared to TiO2 under optimized conditions. Using certain radical scavengers, a positive hole, together with the participation of hydroxyl radicals, were the oxidative species responsible for ReOH degradation on TiO2 whereas, the ZnO photocatalysis occurred principally via hydroxyl radicals. Some hitherto unreported pathway intermediates of ReOH degradation were identified using gas chromatography-mass spectrometry. A tentative reaction mechanism for the formation of these intermediates was proposed. Moreover, the figure-of-merit electrical energy per order was employed to estimate the electrical energy consumption. Keywords: photocatalysis; resorcinol; degradation intermediate; electrical energy consumption; response surface methodology
1. Introduction There is increasing evidence that endocrine disruption is impacting wildlife and humans adversely on a global scale. The endocrine disruption is defined as a hormonal imbalance initiated by exposure to a pollutant, which leads to alterations in development, growth, and/or reproduction in an organism or its progeny [1–3]. The causative chemicals of endocrine disruption in wildlife populations are wide ranging and include natural and synthetic steroids, pesticides, and a plethora of industrial chemicals. Resorcinol (ReOH) is one of the endocrine disrupting chemicals (EDCs), which used in many industrial processes including adhesives, dyes, tanneries, food processing, agricultural research, pharmaceuticals and cosmetics. The exposure to ReOH can occur at its production sites, in the effluent stream, through its use in pharmaceutical applications and in cigarettes [4]. It was also found as a pollutant in surface water and filtered ground water. In fact, numerous in vivo tests of rats and epidemiological studies of humans have demonstrated that ReOH can interfere with triiodothyronine (T3) and thyroxine (T4) metabolism, causing disruptions to thyroid activity [5]. Various chemical, physical, and biological treatment processes are currently proposed for removing and destructing the organic pollutants in effluents. However, the conventional water and wastewater treatment plants using activated sludge and/or charcoal adsorption systems are ineffective and non-destructive for many EDCs [6]. In recent years, an alternative to the conventional methods ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
is the ‘advanced oxidation processes’ (AOPs) that have shown promising techniques for the purpose of removing EDCs from effluents of biological treatment plants and surface water [1,7,8]. In particular, heterogeneous photocatalytic oxidation (HPO) has garnered much attention as an efficient method for degrading the persistent pollutants in water due to the organic compounds can be nearly mineralized by irradiation energy in the near-ultraviolet (UV) range. The HPO is a process in which the destruction of recalcitrant pollutants is governed by the combined actions of an energetic irradiation source, an oxidizing agent, and a semiconductor photocatalyst. When semiconductor particles are illuminated with near-UV irradiation, electron–hole pairs are generated within the semiconductor photocatalyst. Dissolved oxygen in the solution can scavenge excited electrons restraining the electron-hole recombination. Organic compounds may undergo oxidation directly at the hole. The hole can also undergo charge transfer with adsorbed water molecules or hydroxide anions forming • OH radicals. Previously, the photocatalytic oxidation of ReOH was investigated mostly in titanium dioxide (TiO2 ) dispersions [4,9]. Even though zinc oxide (ZnO) is less studied, it presented promising outcomes due to its higher photocatalytic performance in comparison with TiO2 [10]. With respect to the photocatalytic degradation of ReOH by TiO2 and ZnO, few studies reported in the literature have placed emphasis on the influence of operating variables in their suspension reaction systems [5,11]. Nevertheless, studies
1098
S.-m. Lam et al.
focusing on the theory behind the operational parameters and their interactions are insufficient and present intricate tasks for process optimization. In addition, the photocatalytic degradation pathway of ReOH has not been entirely elucidated. Earlier work by Duczmal and Sobczynski [12] obtained three oxidation intermediates in the TiO2 photocatalytic degradation of ReOH and explained the role of the oxidizing species in the process. The efforts to identify degradation intermediates of ReOH over ZnO and S-doped ZnO catalysts using high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) were reported only to have two oxidation intermediates [11,13]. Hence, the present work aims to optimize the operational parameters for the degradation of ReOH using TiO2 and ZnO suspensions using a multivariate approach based upon a response surface methodology (RSM). Important parameters such as catalyst concentration and solution pH were investigated in the experimental design. To understand better the role of the participation of the oxidizing species in the reaction mechanism, radical scavengers were used. The study also investigated the reaction pathway and disclosed new stable ReOH intermediates. Finally, the electrical energy consumption and related treatment costs were calculated. 2.
Materials and methods
2.1. Materials Titania P25 having 80% anatase and 20% rutile (mean diameter 30 nm and surface area 50 m2 /g) was obtained from Degussa. ZnO having a hexagonal phase (mean diameter 180 nm and surface area 5 m2 /g) and ReOH were purchased from ACROS Organics. All other chemicals used for analytical methods were reagent grade. Chemical standards include ethanol and acetonitrile from Fischer Chemicals, p-benzoquinone from Sigma-Aldrich and sodium iodide from Hayashi Pure Chemical. 2.2. Photocatalytic experiments All experiments were carried out in a batch-mode immersion well photoreactor. The photoreactor was made of Pyrex glass with dimensions of 200 × 100 × 60 mm (height × outer diameter × inner diameter). In the centre of the cylindrical photoreactor, a 15 W UV Pen-ray (UVP, Inc.) lamp with a maximum emission at about 365 nm was applied as the UV source. The average UV output intensity at 10 mm away from UV light, measured by radiometer (Cole Parmer, Series 9811) was 1.060 mW/cm2 . The temperature of the system was maintained at 26 ± 2◦ C by cooling the water jacket. In a typical experiment, the desired amount of photocatalyst was added to the photoreactor containing 350 mL of a 20 mg/L ReOH solution. Where required, pH adjustment was done using equimolar NaOH and HCl solutions. During all experiments, the heterogeneous mixture was magnetic stirred and bubbled with air at a fixed flow rate of 4 mL/min.
The mixture was equilibrated for 5 min in the dark and subsequent by the UV irradiation. 2.3. Analytical methods The solution pH of all test samples was measured using a calibrated Mettler Toledo 320 pH meter. An atomic absorption spectrophotometer (Shimadzu, AA-6650) was used to estimate the dissolution of ZnO in the course of photoreaction. Standard solutions bracketing the Zn2+ content of samples were prepared and ran at the same time as the test samples. The concentrations of the ReOH in the reaction mixture at different reaction times were monitored by HPLC (Perkin Elmer, Series 200) equipped with an isocratic gradient pump, a 20 L injection circuit and a variable wavelength UV detector. The detection wavelength was set at 238 nm. A C18 column (length 150 mm × inner diameter 4.6 mm × particle size 5 μm) was used in the sample analysis with acetonitrile–water mobile phase in the ratio of 30:70 (v/v) at a flow rate of 1 mL/min. A sample of about 5 mL was drawn at given time interval of irradiation. The liquid samples were centrifuged at 5600 rpm for 10 min and then filtered through 0.20 μm millipore filters to separate the particles. GC-MS was employed to identify the potential intermediates during the degradation of ReOH. The GC-MS equipment used for the analysis was a Perkin Elmer Clarus 600 gas chromatograph. Prior to instrumental analysis, photoproducts were acidified with diluted HCl and extracted three times from 20 mL catalyst free test samples with dichloromethane (CH2 Cl2 ). The extract was then dried with anhydrous Na2 SO4 and then evaporated nearly to dryness using rotary evaporator. Separation was carried out in an Elite-5MS column (length 30 m × inner diameter 0.25 mm × particle size 0.25 μm). The injector temperature was 280◦ C and the sample was injected in the split less mode for 1 min. The column temperature was held at 65◦ C for 1 min and then increased at 20◦ C/min to 280◦ C, where it was held for 10 min. Helium was used as the carrier gas. For the MS detection, standard electrospray ionization conditions (70 eV) were used with a source temperature of 280◦ C and the mass range from 20 to 300 m/z units. To monitor the extent of mineralization during the ReOH degradation, changes in total organic carbon (TOC) were measured using a TOC analyser (Shimadzu, TOC-VCPH ). In order to determine the reproducibility of the results, at least duplicated runs were carried out for each condition for averaging the results and the experimental error was found to be within ±4%. 2.4.
Investigation of photocatalytic reaction mechanism To elucidate the roles of the photogenerated hole, • OH radical and superoxide radical in the photocatalytic
Environmental Technology reactions with the TiO2 and ZnO under UV light irradiation, the ReOH degradation with the interference of 1 mM scavenging species such as ethanol, sodium iodide and p-benzoquinone were investigated. In a separate experiment, acetonitrile was used instead of water in order to confirm the roles of • OH radicals in both photocatalytic systems. 2.5.
Experimental design
The multivariate approach was performed using a rotatable central composite design (CCD). Analysis of the experimental data was supported by statistical graphics software system Design Expert version 8.0.6 (STAT-EASE Inc., Minneapolis, USA).
3. Results and discussion 3.1. Optimization of ReOH degradation The design consisted of three series of experiments: (i) a two-level rotatable CCD 22 in which the factor levels are coded to the usual low (−1) and high (+1) values; (ii) four axial or star points localized on the axis of each variable at a distance ±α = 1.41 from the designed centre and (iii) three centre points that can be replicated to give an estimation of experimental error variance. Experiments were undertaken in random order to provide protection against the effects of lurking variables. 3.1.1. Optimization via TiO2 Table 1 shows the experimental results for the ReOH degradation after 80 min UV irradiation using TiO2 suspensions. The catalyst concentration was ranged between 2.5 and 6.0 g/L and the pH ranged from 4.0 to 10.0. These values were selected according to our previous experiments Table 1. Codified variable levels from the CCD for ReOH degradation by TiO2 photocatalysis. Level of variables and codes Run no. 1 2 3 4 5 6 7 8 9 10 11 12 13
XT (g/L)
XpH
2.5 (+1) 4.0 (+1) 6.0 (−1) 4.0 (+1) 2.5 (+1) 9.0 (−1) 6.0 (−1) 9.0 (−1) 1.78 (+1.41) 6.5 (0) 6.72 (−1.41) 6.5 (0) 4.25 (0) 2.96 (−1.41) 4.25 (0) 10.04 (+1.41) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0) 4.25 (0) 6.5 (0)
Response after 80 min, Y (%) Measured Calculated 63.8 67.8 67.9 70.2 65.1 70.2 62.3 69.4 69.9 69.2 68.7 69.3 68.9
62.9 67.2 67.9 70.4 65.6 70.4 62.3 69.2 69.2 69.2 69.2 69.2 69.2
1099
and also from the data in the literatures for similar laboratory experiments [6,7,9,14]. The figures in parenthesis represented the codified values. The results reported in Table 1 revealed that a high coincidence was obtained when comparing the two last columns, demonstrating a good description between the mathematical model and experiment. The response factor of ReOH was best explained by a second degree polynomial expression as shown in Equation (1), where Y was the percentage of compound degraded, XT was the TiO2 concentration in g/L and XpH was the solution pH. Each coefficient of a variable in the equation determined the change in mean response per unit increase in the associated independent variable while the other variables were kept constant. Y (%) = 69.20 + 1.69XT + 2.07XpH − 0.61XT2 2 − 1.51XpH − 0.43XT XpH
(1)
The statistical analysis results expressed in terms of the regression coefficient, degrees of freedom (DF), standard error, F and p values are shown in Table 2. From the statistical analysis, it was evident that all the linear and quadratic terms of solution pH were statistically significant (p < 0.05, where p denotes the probability). The regression coefficients and the analysis of variance (ANOVA) results further suggested that the solution pH was the parameter exhibiting most significant effect on the degradation rate. For the regression coefficients, both the magnitude and sign are important, as the former indicates the importance or influence of the variable on the response factor, whereas the sign determines its effect direction. A positive sign of the coefficient represents a synergistic effect, while a negative sign indicates an antagonistic effect. The quadratic term of solution pH has a negative significant relationship with the ReOH degradation process, whereas the quadratic term of the TiO2 concentration (T 2 ) and the interactive term of the TiO2 concentration and the pH (T∗ pH) exhibited a negative sign but a less significant effect on the ReOH degradation. The response surface diagram in Figure 1(A) indicates that the degradation of ReOH was not favoured in acidic media (pH < 4). An increase in solution pH enhanced the degradation, which reached a maximum at pH 8.8. This may be due Table 2. The ANOVA results for degradation of ReOH by TiO2 photocatalytic process. DF
Standard error
F-value
p-value
1.69 2.07
1 1
0.25 0.25
46.68 69.95
0.0002 0.0001
XT2
−0.61
1
0.27
5.23
0.0561
2 XpH
−1.51
1
0.27
32.28
0.0007
XT XpH
−0.43
1
0.35
1.48
0.2635
Variable XT XpH
Coefficient
1100
Figure 1.
S.-m. Lam et al.
Response surface showing the percentage of ReOH degradation using TiO2 (A) and ZnO (B).
to the TiO2 has a zero point charge (zpc) closed to the neutrality (6.25), while the ReOH presented pKa values at 9.3 and 9.8 [15]. At pH 8.8, TiO− was the predominant group on the TiO2 particles and ReOH was primarily in the molecular form. Both of them were combined by hydrogen bonding simply, thus increasing the amount of adsorption and enhancing the degradation efficiency. At pH > 9.8 as the ReOH molecules were negatively charged in alkaline media, their adsorption was expected to be affected by an increase in the repulsion effect with the TiO− groups on the semiconductor surface. Hence, the optimal values for TiO2 concentration and pH estimated by the Design Expert 8.0 software were 6.0 g/L and 8.8, respectively. 3.1.2. Optimization via ZnO When the solution was performed in the presence of ZnO, the response factor was chosen after 40 min of UV irradiation. Due to the solubility of ZnO in suspensions at pH < 6.5, it is necessary to work at neutral or alkaline pH [16]. The potential solubility is an important concern for ZnO photocatalysts owing to the possible results of catalyst inactivation and secondary pollution from free Zn2+ . Several studies have given evidence of ZnO inactivation and suggested that this should be caused by the incongruous dissolution yielding Zn(OH)2 on the ZnO surface [17,18]. The multivariate approach was carried out by studying, simultaneously, the catalyst concentration that was varied between 2.0 and 6.0 g/L and the pH that ranged from 7.0 to 11.0. Table 3 shows the experimental and codified values for the ReOH degradation under ZnO suspensions. The response factor showed that high degradation levels, over 90%, were attained for a determined set of variables. The consistency of the mathematical model was confirmed by the calculated values shown in the last column of Table 3, where there is high agreement with the experimental values. The polynomial expression describing the
Table 3. Codified variable levels from the CCD for ReOH degradation by ZnO photocatalysis. Level of variables and codes Run no. 1 2 3 4 5 6 7 8 9 10 11 12 13
ZnO (g/L)
pH
2.0 (+1) 7.0 (+1) 6.0 (−1) 7.0 (+1) 2.0 (+1) 10.0 (−1) 6.0 (−1) 10.0 (−1) 1.17 (+1.41) 8.5 (0) 6.83 (−1.41) 8.5 (0) 4.0 (0) 6.38 (−1.41) 4.0 (0) 10.62 (+1.41) 4.0 (0) 8.5 (0) 4.0 (0) 8.5 (0) 4.0 (0) 8.5 (0) 4.0 (0) 8.5 (0) 4.0 (0) 9.0 (0)
Response after 40 min, Yexp (%) Measured Calculated 82.2 87.4 94.4 95.2 83.4 89.9 88.5 100.0 91.9 91.2 93.2 92.3 91.9
82.6 88.6 93.9 95.5 83.6 89.0 87.5 100.3 92.1 92.1 92.1 92.1 92.1
ReOH degradation is presented in Equation (2), where Y is the percentage of compound degraded, Xz is the ZnO concentration in g/L and XpH is the solution pH. Y (%) = 92.10 + 1.90Xz + 4.53XpH − 2.89Xz2 2 + 0.91XpH − 1.10Xz XpH
(2)
Table 4 shows the results of statistical analysis for the degradation of ReOH by the ZnO photocatalytic process. It was observed from the table that all the linear, quadratic and interactive terms were statistically significant (p < 0.05). The regression coefficients and the ANOVA results further proposed that both the ZnO concentration and solution pH played a decisive role in the extent of ReOH degradation. The quadratic term of the ZnO concentration (Z 2 ) and the interactive term of the ZnO concentration and pH (Z∗ pH) have a negative significant relationship with the
Environmental Technology Table 4. The ANOVA results for degradation of ReOH by ZnO photocatalytic process. Variable Xz XpH Xz2 2 XpH
Xz XpH
Coefficient
DF
Standard error
F-value
p-value
1.90 4.53
1 1
0.33 0.33
33.64 191.64
0.0007 0.0001
−2.89
1
0.35
67.62
0.0001
0.91
1
0.35
6.75
0.0355
−1.10
1
0.46
5.64
0.0492
ReOH degradation. Figure 1(B) shows that the best condition for ReOH degradation was at a medium value of 4.0 g/L ZnO and a high pH value of 11.0. It is important to point out that ZnO has a pH at zpc of 9.3 [19]. Although at pH 11.0 either the surface of ZnO or ReOH was negatively charged, the degradation efficiency was the highest. This phenomenon was slightly different than the ReOH degradation mechanism using TiO2 as discussed above. One possible explanation given for this was that in alkaline media, pH 11.0, • OH radicals are easier to form by oxidizing more hydroxide anions available on the ZnO surface and thus the efficiency of the degradation was enhanced. Similar results have been reported in the photocatalytic degradation of phenols and dyes under ZnO suspensions [10,20,21]. In conclusion, ZnO is a better catalyst than TiO2 in the degradation of ReOH and its use is limited only by solution pH. Moreover, several reports have demonstrated that ZnO could extend its spectral absorption and higher photon utilization that might elucidate the higher efficiency of ZnO compared with TiO2 [22,23]. The ZnO dissolution data collected from atomic absorption spectrophotometer revealed a little loss of ZnO under UV irradiation. The solution pH during the test was averaged at 11.01 with mean deviation
1101
of ±0.15. The dissolution was found mostly low (0.01%) and did not increase appreciably with irradiation time.
3.2. Reactions under optimized conditions 3.2.1. ReOH degradation ReOH degradation was performed in suspensions under optimized conditions, TiO2 6.0 g/L and pH 8.8 as shown in Figure 2(A). For comparison, the results obtained from dark adsorption and direct photolysis are also included in the figure. The presence of TiO2 notably accelerated the ReOH degradation with virtually 82% of ReOH degradation being achieved after 2 h irradiation. On the other hand, the dark adsorption was found to have a low removal (
