Journal of Environmental Management 147 (2015) 271e277

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Photocatalytic degradation of p-chlorophenol by hybrid H2O2 and TiO2 in aqueous suspensions under UV irradiation Anh Thu Nguyen a, Ruey-Shin Juang b, * a b

Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 32003, Taiwan Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan, Taoyuan 33302, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 April 2014 Received in revised form 31 July 2014 Accepted 27 August 2014 Available online 16 September 2014

In this study, TiO2 particles were used as photocatalysts for the degradation of aqueous p-chlorophenol (p-CP) under UV irradiation. The effect of TiO2 dose (0e3 g/L), initial p-CP concentration, H2O2 concentration (2e45 mM), solution pH (4.6e9.5), and UV light intensity on the degradation of p-CP were examined. Four oxidative degradation processes, which utilized UV alone (direct photolysis), H2O2/UV, TiO2/UV, and H2O2/TiO2/UV, were compared in a batch photoreactor with a 100-W high-pressure mercury lamp. The photodegradation of p-CP could be described by the pseudo-first-order kinetics according to the LangmuireHinshelwood model. Moreover, the apparent degradation rate constants increased considerably from 3.5  103 min1 (direct photolysis) to 19.9  103 min1 (H2O2/TiO2/UV system). © 2014 Elsevier Ltd. All rights reserved.

Keywords: Photocatalytic degradation p-Chlorophenol Titanium dioxide Hydrogen peroxide UV light LangmuireHinshelwood model

1. Introduction In general, chlorophenols are toxic and hazardous compounds that are normally present in wastewater as persistent pollutants because they are non-biodegradable. Among the different chlorophenols, p-chlorophenol (p-CP) is commonly found in wastewater from the paper, pharmaceutical, and dyestuff industries. p-CP is toxic, posing a serious risk to the environment (Ai et al., 2005; Hugul et al., 1999); in particular, this compound is directly relevant for water remediation due to its solubility and the severity of its threat to terrestrial and aquatic life (Benitez et al., 2000; Gaya et al., 2009; Liao et al., 2007). Many wastewater treatment methods have been used to safely remove chlorophenols. The conventional techniques used for p-CP removal include biological, chemical, and thermal treatments (Jardim et al., 1997). The biological methods require long reaction times because the activities of the microorganisms used to degrade the pollutant are impaired by p-CP. Chemical treatments, including flocculation, precipitation, activated carbon adsorption, and reverse

* Corresponding author. Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa First Road, Kwei-Shan, Taoyuan 33302, Taiwan. Tel.: þ886 3 2118800x5702; fax: þ886 3 2118668. E-mail addresses: [email protected], [email protected] (R.-S. Juang). http://dx.doi.org/10.1016/j.jenvman.2014.08.023 0301-4797/© 2014 Elsevier Ltd. All rights reserved.

osmosis, require post-treatments to remove the pollutants from the contaminated environment. Moreover, thermal treatments might emit other hazardous compounds (Titus et al., 2004). Recently, heterogeneous photocatalysis has been recognized as one of the best types of advanced oxidation processes (AOPs) for efficient wastewater treatment. The major advantages of heterogeneous AOPs include the complete and rapid degradation of organics to form CO2, water and mineral acids without producing polycyclic byproducts, and the use of highly active catalysts that can be adapted to specially designed reactor systems (Neppolian et al., 2007; Wang et al., 1999). Semiconductor photocatalysis has become increasingly promising during environment remediation due to the optical and electronic properties, low cost, availability, high stability, and non-toxicity of the photocatalysts (Chen et al., 2007; Gomez et al., 2012). Photocatalytic treatment involves the irradiation of suitable semiconductor particles with light energy that exceeds the bandgap, producing charge carriers (electrons and holes) that can recombine and migrate to the particle surface. Various redox reactions occur on the particle surface with different species, such as dissolved molecules, adsorbed substrates, oxygen, and hydroxyl groups. These reactions can increase the production of highly reactive oxidizing or reducing products that may react with the organics. In this case, hydroxyl radicals are critical oxidants that destroy organics (Hoffmann et al., 1995). For chlorophenols,

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particularly p-CP, the abstraction of hydrogen and an electron  might be significant; OH reacts primarily by adding to the ring (Fabbri et al., 2005; Mills and Le Hunte, 1997). To avoid further adverse environmental impact and risks, effective methods for p-CP removal have become increasingly important in recent years. In this study, photodegradation of aqueous p-CP was carried out using hybrid TiO2 and H2O2 under UV irradiation. The degradation of p-CP was compared among different systems, such as direct photolysis, TiO2/UV, H2O2/UV, and H2O2/ TiO2/UV. This work also aimed to study the effects of some key parameters on the photodegradation in details: TiO2 dose, solution pH, H2O2 concentration, initial p-CP concentration, and UV light intensity. The photodegradation kinetics of aqueous p-CP was also examined. 2. Materials and methods 2.1. Materials and reagents TiO2 (P-25, ca. 70% anatase and 30% rutile) acquired from Degussa, Japan, has a BET specific surface area of 53.3 m2/g and an average particle size of 50 nm, as verified by FESEM (JSM-6701F, UK). p-CP (99%) was supplied by Alfa Aesar, Britain. 4Chlorocatechol (99%), hydroquinone (99.5%), and p-benzoquinone (98%) were offered from SigmaeAldrich Co., USA. The NaOH and HCl were purchased from J.T. Backer, USA, the methanol was purchased from Merck, Germany, and the hydrogen peroxide solution (30% w/v) was purchased from SigmaeAldrich, Germany. The UV light sources were a water-cooled 20-W low-pressure UV lamp (UVL20PH-6, SEN Japan) and a 100-W high-pressure mercury lamp (HL100CH-5, SEN Japan).

 ðC  CÞ removal % ¼ 100  0 C0

(1)

where C0 is initial concentration of the reactant and C is concentration of the reactant after photo-irradiation. 3. Results and discussion 3.1. Effect of various oxidation processes on p-CP degradation Solutions of p-CP were irradiated under four different conditions: direct photolysis (UV alone), H2O2/UV, TiO2/UV, and H2O2/ TiO2/UV systems. Fig. 1 shows the changes in the degree of degradation of aqueous p-CP versus irradiation time. The direct photolysis of p-CP with a 100-W UV lamp leads to a 38% removal within 180 min. The removal increases up to 59% when the solution is treated with 1.9 mM H2O2 under UV irradiation. Approximately 70% of p-CP is degraded within 180 min when 1 g/L TiO2 is added. In addition, combining these two processes (H2O2/TiO2/UV) improves p-CP degradation significantly (up to 98% in the shorter time of 150 min). These experiments provide results similar to those of previous studies (Aceituno et al., 2002; Bertelli and Selli, 2006; Burns et al., 2002; Chen et al., 1998; Chiou et al., 2008a; Chu and Wong, 2003). The reaction mechanisms during the H2O2/UV process are summarized as follows (Barakat et al., 2005): hn

H2 O2 !2$OH

(2)

OH þ hþ /$OH

(3)

2.2. Photodegradation experiments

O2 þ e /$O 2

(4)

All of the degradation experiments were carried out in a batch photoreactor (Chiou et al., 2008a,b). This system consisted of a cylindrical 1.0-L Pyrex-glass cell (10 cm I.D. and 10 cm height). A 100-W high-pressure mercury lamp was located in the center of the reactor within a 5-cm diameter double-walled cooling tube. The lamp and tube were immersed in the reactor cell, and the light path was 80 mm. The photoreactor was filled with 0.8 L of an aqueous solution containing p-CP, and P-25 TiO2 powder was added (1 g/L). The pH of the solution was adjusted by adding 0.1 M NaOH or HCl using a digital pH meter (Horiba F-23, Japan). The temperature of the photoreactor was maintained at 25 ± 1  C by a water-cooling jacket (Eyela, NCB-2600, Japan). The suspension was kept uniform through agitation with a magnetic stirrer (100 rpm). The time at which the UV lamp was turned on was considered time zero or the beginning of the experiment. The experiments were performed for 180 min. Liquid samples (1 mL) were collected at 30-min intervals. The samples were filtered through a 0.45 mm syringe filter (Millipore) to remove any TiO2 particles; the samples were stored in brown glass bottles until the concentrations of the contents were analyzed by HPLC (Jasco, Japan). The column was a Merck LiChroCART® 250-4.6 (250 mm length, 4.6 mm diameter), packed with Purospher® STAR RP-18 end-capped (5 mm). A mixture of methanol (60%, v/v) and deionized water was used as the mobile phase at 1.0 mL/min. An aliquot of the sample (10 mL) was injected into the HPLC (Jasco, Japan) for analysis at 280 nm. The retention time (Rt) of p-CP was observed between 8.2 and 9.5 min (1.3 min of appearance). All solutions were prepared using deionized water throughout this study. Each experiment was performed in triplicate under identical conditions. The removal efficiency of p-CP by photodegradation is calculated by

H2 O2 þ e /$OH þ OH

(5)

 H2 O2 þ $O 2 /$OH þ OH þ O2

(6)

RH þ $OH/$R þ H2 O/further oxidation

(7)

where RH refers to the phenolic compound. The H2O2/UV system can generate $OH radicals (Eq. (2)), enhancing the rate of photocatalytic reaction. H2O2 is a better

Fig. 1. Effect of various oxidation processes on p-CP removal via 100-W UV light at 25  C (C0 ¼ 0.50 mM, 1 g/L TiO2, 1.9 mM H2O2, initial pH 6.7).

A.T. Nguyen, R.-S. Juang / Journal of Environmental Management 147 (2015) 271e277

electron acceptor than oxygen in other mechanisms. Therefore, the amount of electron recombination will decrease. However, the hydroxyl radicals formed during the photolysis with H2O2 may react with the holes generated during excitation, reducing the capacity of the electronehole recombination (Eq. (5)). Consequently,  the superoxide radical ( O 2 ) (Eqs. (4) and (6)) affects the rate of photocatalysis less than hydroxyl radicals, according to previously proposed mechanisms (Barakat et al., 2005; Chiou et al., 2008b). The LangmuireHinshelwood (LeH) model describes gasesolid or fluidesolid reactions (Kutty and Ahuja, 1995). Specifically, this model is used to describe the kinetics of photocatalytic reactions incorporating aqueous organics (Beltran et al., 2005; Lathasree et al., 2004; Petukhov, 1997; Yonar et al., 2006). The development of the Langmuir model to obtain a kinetic equation depends on the limiting step (adsorption, surface reaction, or desorption). Thus, the constants appearing in the model group many others, both kinetic and equilibrium. The LeH model relates the degradation rate (r) to the concentration of the organic reactant (C), as shown in Eq. (8):

r¼

dC kr Kad C ¼ dt 1 þ Kad C



C C0

Fig. 2. Effect of the TiO2 catalyst loading on p-CP removal via 100-W UV light at 25  C (C0 ¼ 0.50 mM, initial pH 6.7).

(8)

where kr is the rate constant and Kad is the adsorption equilibrium constant. When the adsorption is relatively weak or the concentration of the reactant is low, Eq. (8) can be simplified to be the firstorder kinetics with an apparent rate constant kapp (Eq. (9)):

ln

273

 ¼ kr Kad t ¼ kapp t

(9)

Plotting ln(C/C0) vs. reaction time (t) generates a straight line with a slope equal to kapp (Chiou et al., 2008b). Table 1 shows the kinetic parameters for p-CP degradation in four oxidative processes. The initial reaction rate in the H2O2/TiO2/ UV system (9.57  103 mM/min) is nearly double that of the TiO2/ UV system (3.98  103 mM/min). This difference is attributed to the contributions of the hydroxyl radicals produced by H2O2 during the H2O2/TiO2/UV process. Additionally, 70% removal of the p-CP under UV irradiation (Fig. 1) indicates that the degradation is primarily due to the direct activation of TiO2 (Chiou et al., 2008b). 3.2. Effect of TiO2 dose on p-CP degradation

also indicated that excess catalyst will decrease the light penetration via the shielding effect of the suspended particles (Burns et al., 2002; Chiou et al., 2008b; Dixit et al., 2010; Neppolian et al., 2007; Sobczynski et al., 2004). However, compared to the other doses of TiO2, 1.0 g/L promotes a high removal of p-CP (79%). Therefore, to conserve the catalyst, 1.0 g/L TiO2 was used to perform all experiments in this study. 3.3. Effect of solution pH on p-CP degradation The chain reactions for photodegradation in the presence of TiO2 can be summarized as follows (Okamoto et al., 1985; Wang et al., 1998):

TiO2 !hþ þ e

(10)

H2 O þ hþ /$OH þ Hþ

(11)

hn

þ $O 2 þ H /$HO2

To evaluate the effect of TiO2 dose on the degradation of p-CP, different amounts of P-25 TiO2 (0e3.0 g/L) were used with a 0.50 mM solution of p-CP. Fig. 2 shows the effect of TiO2 dose on p-CP removal under 100-W UV light at 25  C (C0 ¼ 0.50 mM, pH 6.7). Direct photocatalysis (0 g/L TiO2) removes lowest amount (41%). The degradation of p-CP increases from 68% to 83% when the TiO2 dose increases from 0.5 to 3.0 g/L. This observation reveals that less p-CP is degraded by UV alone than the system with the TiO2 catalyst. Consequently, TiO2 has exhibited an enormous capacity for promoting the interactions between hydroxyl radicals and p-CP. No significant difference is found between the degradations using 2.0 g/L (81%) and 3.0 g/L (83%) of TiO2. Previous studies have



 pKa ¼ 4:88

(12)

$HO2 þ $HO2 /H2 O2 þ O2

(13)

 $O 2 þ $HO2 /HO2 þ O2

(14)

þ HO 2 þ H /H2 O2

(15)

Eqs. (3), (4) and (10))e(15) show that the hydroxyl radicals are formed under light excitation excited when the positive holes react with H2O and OH on the TiO2 surface. The reaction proceeds in the reverse direction when the pH exceeds the pKa (4.88) for the HO2 radical. The lack of HO2 radicals inhibits Eqs. (13)e(15). Hence, the formation of OH radicals occurs through the reactions of positive

Table 1 The apparent rate constants (kapp), correlation coefficients (R2), and initial degradation rates (r0) for p-CP while using various oxidation methods under irradiation from a 100W UV (C0 ¼ 0.50 mM, 1 g/L TiO2, 1.9 mM H2O2). Process

kapp (min1)

Direct photolysis H2O2/UV TiO2/UV H2O2/TiO2/UV

3.5 4.9 6.1 19.9

   

103 103 103 103

R2

r0 (mM/min)

0.987 0.998 0.993 0.999

1.77 2.60 3.98 9.57

   

103 103 103 103

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Fig. 3. Effect of the initial solution pH on p-CP removal via TiO2/UV at 25  C (100-W UV, C0 ¼ 0.50 mM, 1 g/L TiO2).

holes with water and OH on the TiO2 surface (Chiou et al., 2008a; Wang et al., 1999). Fig. 3 shows the changes in the residual concentrations of p-CP at different pH values. The performance of the TiO2/UV system exerts no remarkable pH effect, removing 82%, 88% and 94% at pH 4.6, 7.3 and 9.5, respectively. The best results were obtained at neutral pH (Alimoradzadeh et al., 2012), where p-CP is degraded approximately 93% within 180-min irradiation. Such an effect decreased as follows: pH 9.5 > pH 7.3 > pH 4.6. 3.4. Effect of the initial p-CP concentration on p-CP degradation To understand the concentration effect, different initial p-CP concentrations from 0.15 to 1.50 mM were studied using TiO2/UV at 25  C (1 g/L TiO2, initial pH 6.7), as shown in Fig. 4. At initial concentrations of 0.15, 0.25, and 0.45 mM, approximately 96%, 98%, and 99% of the p-CP was removed within 30, 90, and 150 min, respectively. The total amount of p-CP degraded increases under the studied conditions. However, the fraction of unreacted reactant (C/ C0) increases with increasing reactant concentration (C0). For C0 being from 0.75 to 1.50 mM, only 84%e57% of the p-CP, respectively, is removed within 180 min. These experiments revealed that the

Fig. 4. Effect of the initial p-CP concentration on its removal via 100-W UV light at 25  C (1 g/L TiO2, initial pH 6.7).

photocatalytic oxidation is promising at low organic reactant concentrations (Chiou et al., 2008b). Specifically, the active sites may be covered by p-CP and its intermediates, reducing the generation of eehþ and thus the photodegradation efficiency at high p-CP concentrations (Konstantinou and Albanis, 2004). While the initial p-CP concentration increases, the light intensity, illumination time, and the mass of catalyst remain constant; hence, the $OH and O2 2 species formed on the surface of catalyst remain constant, and the relative concentrations of $OH and O2 2 available for reactions with p-CP decrease, slowing the degradation (Lathasree et al., 2004). Another factor may involve the competition between the adsorbed reactants and the H2O molecules for the photodegraded hþ, decreasing the degradation rate (Abdollahi, 2012; Gaya and Abdullah, 2008). In addition, higher initial p-CP concentration could make the shielding effect as presented in section 3.2. This could reduce the light penetration and provide some turbidity in the solution and decrease the degradation efficiency (Juang et al., 2010). Besides, the intermediates or byproducts formed during the degradation of pCP may strongly affect the reaction rate in solution. This hypothesis is consistent with the results of previous studies. Neppolian et al. (2007) have reported that the intermediates formed during the photodegradation of p-CP are p-benzoquinone, 4-chlorocatechol, and hydroquinone, in addition to many other minor intermediates. Similar results were also obtained by Czaplicka (2005) and Cheng et al. (2007). In this work, HPLC tests showed some other peaks besides a peak of p-CP degradation at 8.2 min, as shown in Fig. 5. By comparison of retention time with the standard, the detected 3 peaks after 30 min of irradiation time were p-benzoquinone, hydroquinone, and 4-chlorocatechol with a retention time of Rt ¼ 2.7, 2.9, and 4.0 min, respectively. 3.5. Effect of the H2O2 concentration on p-CP degradation 

To gain a deeper insight into the effective reactivity of the OH radicals and to limit the detrimental light scattering effect due to the presence of TiO2, the p-CP degradation was studied in aqueous solutions containing different amounts of hydrogen peroxide (H2O2/UV system). Fig. 6 shows the effect of added H2O2 (2.0e45.0 mM) on p-CP degradation at pH 6.7 using the H2O2/UV system. The p-CP degradation was very fast under the H2O2/UV conditions. Using 2.0e45.0 mM H2O2 significantly increases the removal efficiency. These results are similar to previous reports (Barakat et al., 2005; Dixit et al., 2010; Ghaly et al., 2001; Shen et al., 1995). When the solution contains 2.0 mM H2O2, the degradation is

Fig. 5. HPLC chromatogram showing the peaks of p-CP and intermediates of p-benzoquinone, hydroquinone and 4-chlorocatechol.

A.T. Nguyen, R.-S. Juang / Journal of Environmental Management 147 (2015) 271e277

275

Fig. 6. Effect of the initial H2O2 concentration on p-CP removal via H2O2/UV process at 25  C (100-W UV, C0 ¼ 0.50 mM, initial pH 6.7).

Fig. 7. First-order plots of a p-CP degradation conducted at 25  C under different UV light intensities (C0 ¼ 0.50 mM, 1 g/L TiO2, 2.0 mM H2O2, initial pH 6.7).

only 59% within 180 min. After adding 20.0 mM H2O2, the removal increases to 99% within 150 min. When adding 45.0 mM H2O2, the degradation is also 99% within 120 min and this is the optimal result for this parameter. However, previous studies have revealed that further increase in H2O2 concentration would decrease the degradation rate. According to Dixit et al. (2010) and Ghaly et al. (2001), this phenomenon occurs when the excess H2O2 reacts with the hydroxyl radicals formed previously; therefore, H2O2 acts as an inhibitor consuming the hydroxyl radicals responsible for degrading the pollutant. In addition, the surface reaction mechanisms are also possible as follows (Miller and Valentine, 1999):

change of the solution in the UV/H2O2 degradation of p-CP. Other previous studies also mentioned the color change of solution in degradation but with TiO2 catalyst. For examples, Chiou et al. (2008b) showed that the color change of solution was due to the contact of white P-25 TiO2 with colorless solution of catechol or pyrogallol in the UV/TiO2 systems degradation of phenolic compounds. In this study, the reactant concentrations are so low that it is harder to see the color change from possible cause of the intermediates. In addition, the 1 g/L TiO2 is also one of the reasons cover the observation of the possible color change of solution during the photodegradation.

Sþ þ H2 O2 /S þ Hþ þ $HO2

 Sþ : oxidized catalytic surface site (16)

S þ H2 O2 /OH þ $OH

S : reduced catalytic surface site



(17)

S þ þ O 2 /S þ O2

(18)

S þ $OH2 /Sþ þ HO 2

(19)

S þ $OH/Sþ þ OH

(20)

In this model, a single rate-limiting step controlling H2O2 loss and degradation of contaminant was not combined. The generation of superoxide and hydroxyl radicals, scavenging reactions of oxygen radicals with the catalyst surface and degradation of contaminant with hydroxyl radicals which are formed from both surface and solution of reactions is established. Eqs. (16) and (17) show H2O2 reacts with both oxidized and reduced surface sites. Eqs. (18) and (19) show superoxide and perhydroxyl reactions with the catalytic surface. However, some escape scavenging is a possible hypothesis. Eq. (20) shows the possibility of the hydroxyl radical scavenged by the catalytic surface. The color of p-CP solution changed from colorless to light yellow after 30 min of degradation. After 180 min, the color of solution was lemon yellow. No previous studies have mentioned the change of solution color during or after the UV/H2O2 process of p-CP without TiO2. However, some intermediates appeared during the degradation process, as shown in section 3.4, possibly explaining this discrepancy. In this case, after doing some experiments, it is observed that p-benzoquinone is the main reason causing the color

3.6. Effect of UV light intensity on p-CP degradation The effect of UV light intensity (20e100 W) on the p-CP degradation is studied while using 1.0 g/L TiO2 with or without H2O2. Fig. 7 shows the results obtained at pH 6.7 when the initial concentration of p-CP was 0.50 mM. All of the reactions followed the pseudo-first-order kinetics (Chiou et al., 2008b). The degradation rate constants are 2.2  103 and 17.6  103 min1 when the light intensity was 20 and 100 W, respectively, during the UV/TiO2 process, which are lower than the corresponding constants obtained in the UV/H2O2/TiO2 system (7.1  103 and 30.7  103 min1, respectively) as shown in Table 2. A light intensity of 100 W significantly affects p-CP degradation rate, producing 8-fold (UV/TiO2 system) and 4.3-fold (UV/H2O2/ TiO2 system) relative to that of 20 W, respectively. Specifically, the degradation rate was improved when increasing the light intensity (Alimoradzadeh et al., 2012; Gomez et al., 2009; Wang et al., 2009). 3.7. Photodegradation kinetics of p-CP The kinetics of p-CP degradation was studied using the UV/TiO2 system. Fig. 8 shows the concentration versus time curves,

Table 2 The apparent rate constants (kapp) for p-CP under different UV light intensities (20e100 W) during two processes: TiO2/UV and H2O2/TiO2/UV (C0 ¼ 0.50 mM, 1 g/L TiO2, 2.0 mM H2O2). Rate constant

20 W

100 W

TiO2/UV

H2O2/TiO2/UV

TiO2/UV

H2O2/TiO2/UV

kapp (min1)

2.2  103

7.1  103

17.6  103

30.7  103

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oxidation processes. The apparent first-order rate constants were 1.04  103, 0.44  103, 0.15  103, 0.11  103, and 0.049  103 min1 at an initial p-CP concentration of 0.15, 0.25, 0.50, 0.75, and 1.10 mM, respectively, under 100-W UV irradiation. This photodegradation was promising at low pollutant concentrations, and the degradation rate was improved by increasing the UV light intensity. Moreover, this work emphasized the importance of optimizing the catalyst dose before examining the effects of other variables on the photocatalytic reactions. References

Fig. 8. Effect of the initial p-CP concentration on the apparent rate constant at 25  C using TiO2/UV (100-W UV, 1 g/L TiO2).

revealing two distinct stages: the initial (sharp) stage (t < 30 min) and the final (gradual) stage (t > 30 min). The apparent rate constants of p-CP degradation were determined during these two stages while following pseudo-first-order kinetics according to the LangmuireHinshelwood model. When a threshold coverage of the TiO2 surface is reached due to the adsorption of reaction intermediates, the TiO2 surface is not yet occupied by the intermediates in the initial stage (i.e., before 30 min of degradation process), explaining the transition. After the short initial period, the intermediates to occupy the surface sites, decreasing the degradation rate (Chiou et al., 2008a). Table 3 shows the apparent rate constants kapp at different initial p-CP concentrations under 100-W UV irradiation: 1.04  103, 0.44  103, 0.15  103, 0.11  103, and 0.049  103 min1 at an initial p-CP concentration of 0.15, 0.25, 0.50, 0.75, and 1.10 mM, respectively. Moreover, Fig. 8 shows that the initial stage (t < 30 min) has 5 data points for performance. However, the final stage (t > 30 min) has only 4 data points. This discrepancy arises because the degradation at the lowest initial concentration (0.15 mM) was completed before 40 min, in contrast to the higher concentrations. Specifically, these results verified that the degradation of organic pollutants increases at low concentrations (Chiou et al., 2008a,b).

4. Conclusions Various oxidation processes have been utilized to compare the degradation of p-chlorophenol (p-CP) in aqueous solutions under UV irradiation. The combined H2O2/TiO2/UV process was the most effective, exhibiting a maximum 98% degradation of p-CP at 0.50 mM while using 1.9 mM H2O2 and 1 g/L TiO2 over 150 min. The pH effect on p-CP degradation decreased in the following order: pH 9.5 > pH 7.3 > pH 4.6. The LangmuireHinshelwood model could well describe the photodegradation of p-CP during all four

Table 3 The apparent rate constants (kapp) of p-CP at different initial concentrations under 100-W UV irradiation. Rate constant

Initial concentration C0 (mM) 0.15

0.25

0.50

0.75

1.10

kapp (min1) 1.04  103 0.44  103 0.15  103 0.11  103 0.049  103

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Photocatalytic degradation of p-chlorophenol by hybrid H₂O₂ and TiO₂ in aqueous suspensions under UV irradiation.

In this study, TiO2 particles were used as photocatalysts for the degradation of aqueous p-chlorophenol (p-CP) under UV irradiation. The effect of TiO...
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