Advanced Oxidation Processes for Wastewater TreatmentView this Special Issue
Effect of Oxygen and Hydrogen Peroxide on the Photocatalytic Degradation of Monochlorobenzene in Aqueous Suspension
The influences of oxygen and hydrogen peroxide () on the degradation and mineralization of monochlorobenzene (MCB) during UV/ process were investigated. Experimental results indicated that oxygen was a determining parameter for promoting the photocatalytic degradation. The presence of oxygen reduced the illumination time needed for the complete decay of MCB from 240 to 120 min. The photocatalytic degradation of MCB in UV// photocatalysis followed a simplified two-step consecutive kinetics. The rate constants of degradation () and mineralization () were increased from 0.016 to 0.046 min−1 and from 0.001 to 0.006 min−1, respectively, as the initial concentration of dissolved oxygen (DO) was increased from 1.6 to 28.3 mg L−1. Owing to the fact that acted as an electron and hydroxyl radicals () scavenger, the addition of should in a proper dosage range to enhance the degradation and mineralization of MCB. The optimal dosage for MCB degradation was 22.5 mg L−1, whereas the most efficient dosage for MCB mineralization was 45.0 mg L−1. In order to minimize the adverse effects of higher dosage, including the capture of radicals and competitive adsorption, and to improve the photocatalytic degradation of MCB, the sequential replenishment of was suggested. For the stepwise addition of a total dosage of 45.0 mg L−1, a complete destruction of MCB was observed within 120 min of irradiation. Additionally, the mineralization efficiency was about 87.4% after 240 min of illumination time.
During last several decades, the control of organic pollutants has received much attention for its considerable amount and variety. However, the conventional biological, physical, and chemical technologies are ineffective and limitative toward the destruction of toxic and recalcitrant organic pollutants. Heterogeneous photocatalysis, one of the so-called Advanced Oxidation Processes (AOPs), offers an advanced oxidation capable of pollutant abatement [1, 2]. Many researchers have concluded that the UV/TiO2 process is a promising technology for the degradation and mineralization of organic substrates to harmless final products in air and water media [3–6].
The mechanism of the UV/TiO2 process has been discussed extensively in the literature [7–9]. When TiO2 is irradiated with light energy equal to or higher than its band-gap, an electron () can be excited from the valence band to the conduction band and leaving a hole () in the valence band If charge separation is maintained, the paired may migrate to the surface of the photocatalyst. In aqueous phase, the photoinduced h+ is apparently able to oxidize surface hydroxyl groups or surface-bond water molecules to produce highly reactive and nonselective hydroxyl radicals () (2). The radicals are considered to be the dominant oxidizing species contributing to the photocatalytic degradation of organic substrates [10–12]. Nevertheless, the can recombine with the (3), causing a decrease in the availability of the photoinduced [13, 14]. Without electron acceptors, the limitation attributed to the recombination of pairs would reduce the photocatalytic efficiency and cause radiation energy loss. Therefore, suppressing the recombination of pairs is an important consideration in enhancing the performance of photocatalytic degradation.
In this study, both oxygen and hydrogen peroxide () were selected as electron acceptors. Monochlorobenzene (MCB), one of the hydrophobic and volatile organic compounds (VOCs), was chosen as a model compound. The objectives of the present work were to evaluate the effect of oxygen and on the photocatalytic degradation of MCB. The change in the concentration of MCB with various oxygen concentrations and dosages was examined. Additionally, the extent of mineralization was also estimated by measuring the concentration of total organic carbon (TOC) in MCB solution.
2. Materials and Methods
Anatase TiO2 powder with a specific BET surface area of 9.3 m2 g−1 and a primary particle size of 150 nm was purchased from Acros Organics and used as received. Reagent grade MCB (C6H5Cl, MW = 112.56, density = 1.106 g mL−1, vapor pressure = 12 mm Hg at 25°C, and water solubility = 494 mg L−1 at 25°C) with a purity over 99% was obtained from Merck. (30%) was obtained from Fluka Chemical. Other chemicals used for analysis, including acetonitrile, acetic acid, potassium hydrogen phthalate, phosphoric acid, and sodium peroxydisulfate were of analytical grade. Pure and gases and air were used to adjust the initial dissolved oxygen (DO) concentration to the desired levels. Deionized water was employed for solution preparation.
2.2. Experimental Apparatus and Procedure
A hollow cylindrical photoreactor with a working capacity of 2.5 l and equipped with a water jacket was used in this study. Cooling water from a thermostatic bath (TUNGTEC BL-20) was circulated through the photoreactor jacket to keep the temperature at 30°C. Irradiation was performed using a 15 W blacklight lamp (F15T8 BLB, UVP) with a maximum emission at 365 nm. The UV lamp was vertically immersed in a quartz tube placed in the center of the photoreactor. The light intensity inside the photoreactor, as measured by potassium ferrioxalate actinometry, was 5.68 μ Einstein . The reaction mixture was continuously agitated by a magnetic stirrer to keep the TiO2 particles suspended.
Two series of experiments were carried out to examine the effects of oxygen and on the photocatalytic degradation of MCB. Prior to MCB solution preparation, all deionized water was first purged by gas for 30 min to minimize the amount of DO. Subsequently, the deionized water was purged with either air or gas to obtain the predetermined DO concentration. The initial concentration of MCB was fixed at 0.1 mM and the dosage was 1.0 g L−1 unless otherwise stated. The initial pH of solution was adjusted to 7 by adding an appropriate volume of dilute NaOH or solutions. After 30 min of premixing in the dark, the UV lamp was switched on to initiate the photocatalytic degradation.
The experimental procedure for the -assisted batch was similar to the oxygen-assisted batch. In this case, the MCB solution was prepared using deionized water that had been deoxygenated by gas. After powder was added into the solution, the solution pH was adjusted to 7 with dilute NaOH or HNO3 solutions. Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min. After the addition of into the suspension, the UV lamp was turned on.
At given irradiation time intervals, samples were withdrawn from the irradiated suspension. These collected samples were immediately centrifuged and filtered through a 0.22 μm Millipore filter for further analysis.
2.3. Analytical Methods
The residual MCB measurement was carried out by HPLC (Biotronik HPLC BT 7900) equipped with a Linear UVIS 200 UV detector and an ODS2 C18 column (length 25 cm, inner diameter 4.6 mm). The mobile phase was composed of acetonitrile (70%), water (29%), and acetic acid (1%) and the flow rate was kept at 1.5 mL min−1. The detection wavelength selected for detecting MCB was 265 nm. A TOC analyzer (O. I. Analytical Model 700) equipped with a nondispersive infrared detector (NDIR) was employed to monitor the concentration of TOC. The DO concentration was quantified by an oxygen membrane electrode (Oxi 320, WTW).
3. Results and Discussion
3.1. Direct Photolysis and -Assisted Photolysis
Two preliminary experiments were carried out to estimate the contribution of direct photolysis and -assisted photolysis for the removal of MCB. The variation in the MCB concentration during irradiation can be seen in Figure 1. For the direct photolysis, there was about 42.8% degradation of MCB after 240 min of illumination time. This result implied that the UV light had evident contribution on the degradation of MCB which was in agreement with the observation reported by other researcher . The change in the MCB concentration fitted an exponential decay curve, suggesting the direct photolysis of MCB followed the pattern of pseudo first-order kinetics. A linear regression was obtained with natural logarithmic normalized concentration against illumination time. The degradation rate constant was calculated to be 0.002 min−1.
In order to evaluate the degradation ability of -assisted photolysis, an irradiation experiment was conducted under the condition of dosage equal to 22.5 mg L−1. Corresponding to the obtained experimental result recorded in Figure 1, it was found that the concentration of MCB decreased gradually during the period of irradiation. The extent of MCB degradation in the -assisted photolysis within 240 min of irradiation was approximately 60.2% and the pseudo first-order degradation rate constant was 0.004 min−1. The typical time-dependent MCB concentration in the presence of both UV light and was similar to that under the UV irradiation only. It was stated that has an extremely low absorption at UV light of 365 nm , therefore, the generation of radicals due to the photolysis of would be insignificant. As a result, the observable MCB decay can mainly be ascribed to the direct photolysis. Moreover, the additional MCB degradation can be explained by the oxidative ability of .
3.2. UV/TiO2/O2 Photocatalysis
3.2.1. Degradation of MCB
The dependency of the photocatalytic degradation of MCB on the initial DO concentration was studied in the range from 1.6 to 28.3 mg L−1. Typical time-dependent MCB concentration during photocatalytic degradation is illustrated in Figure 2(a). As can be seen, when TiO2 suspension was exposed to UV light, the MCB concentration decreased markedly with illumination time in comparison with the same experiment performed in the absence of TiO2. The difference between direct photolysis and UV/TiO2/ photocatalysis revealed that UV light and TiO2 photocatalyst together had a significant effect on the degradation of MCB. Since radicals were the key feature of UV/TiO2 process, the degradation of MCB was primarily related to the generated radicals.
Referring to Figure 2(a), the presence of oxygen was contributed to the photocatalytic degradation. Complete destruction of MCB was observed after 240 min of irradiation when the experiment was performed under initial DO concentration equal to 1.6 mg L−1. In addition, for the initial DO concentration of 28.3 mg L−1, the illumination time required for complete decay of MCB was reduced to 120 min. The depletion of the DO concentration during irradiation, as recorded in Figure 2(b), confirmed that oxygen was involved in the photocatalytic degradation. Accordingly, the improvement of the degradation of MCB, which is related to the increasing initial DO concentration, can be attributed to the fact that oxygen acted as electron acceptor to trap the photoinduced [17, 18] Through the reduction of oxygen with , reactive superoxide radical anions () was produced (4). Simultaneously, the pair recombination was restrained. Stabilizing the primary carrier led to promote the generation of radicals. Moreover, other oxidizing species such as and were also formed (5)–(7). It was believed that additional radicals would be generated through sequential reactions [11, 19–21]. Consequently, an increase in the DO concentration would accelerate the degradation of MCB.
3.2.2. Mineralization of MCB
The decay of TOC during photocatalytic degradation was monitored to evaluate the mineralization of MCB. From the plot shown in Figure 2(c), the improvement of the mineralization of MCB in the presence of oxygen was observed. The mineralization efficiency increased from 42.6 to 93.1% within 240 min of irradiation as initial DO concentration varied from 1.6 to 28.3 mg L−1. Similar observations have been reported in the literature for other model compounds [22–24].
For the initial DO concentrations of 17.9 and 28.3 mg L−1, there were still measurable DO concentrations after 240 min of illumination time. Since there was sufficient oxygen in the photocatalytic system, the TOC decreased abidingly during photocatalytic degradation. Moreover, besides the electron acceptor function, oxygen can also participate in the oxidative reaction to promote the mineralization of organic substrates and intermediates [25, 26]. Consequently, the depression of TOC decay was ascribed to the lack of oxygen involved in the photocatalytic degradation afterward in the case of lower initial DO concentration.
A comparison of Figures 2(a) and 2(c) indicated that the TOC decayed with illumination time in parallel with the MCB degradation. However, it should be noted that complete disappearance of MCB occurred within 120 to 240 min of irradiation under various initial DO concentrations, whereas residual TOC was still observed. This phenomenon implied that transient organic intermediates were likely to present in the photocatalytic system.
3.2.3. Two-Step Consecutive Kinetics
Based on the total organic carbon concentration in the photocatalytic system, the mass balance can be expressed as [27, 28] where is the total organic carbon concentration in the system, is the carbon concentration in the MCB at time , and is the carbon concentration in the intermediates at time . Correspondingly, the carbon concentration in the intermediates can be determined from the concentrations of MCB and TOC. Figure 2(d) illustrates the variation of carbon concentration in the intermediates for different initial DO concentration during the photocatalytic degradation of MCB. As can be seen, the intermediates accumulated first and decomposed thereafter.
Since the mineralization of MCB occurs through the intermediates, a simplified two-step consecutive kinetics can be used to describe the photocatalytic degradation [29, 30] where is the degradation rate constant of MCB and is the mineralization rate constant of the intermediates. The ultimate product of the photocatalytic degradation can be CO2, , and relevant inorganic ions. Each step in (9) is assumed to be a first-order and irreversible reaction. Applying the two-step consecutive kinetics, the carbon concentrations in the MCB and the intermediates can be expressed by Accordingly, the two rate constants and can be derived from the disappearance of MCB and the intermediates.
As listed in Table 1, the increase in the value of was proportional to the initial DO concentration. The enhancement of was about 3 times higher when the initial DO concentration was 28.3 mg L−1 rather than 1.6 mg L−1. Take the calculated and the determined into (11), the value of mineralization rate constant was obtained. It was evident that the mineralization rate constant depended on the initial DO concentration. The value of increased with increasing initial DO concentration to a certain level and thereafter remained almost constant. The increased 8 times when the initial DO concentration was increased to 17.9 mg L−1 rather than 1.6 mg L−1. Interestingly, the values of were lower than those of , this revealed that the mineralization may be the rate-limiting step in the photocatalytic degradation of MCB. Therefore, prolonged illumination time would be reasonable for complete mineralization.
3.3. UV/TiO2/ Photocatalysis
3.3.1. Degradation of MCB
According to its high oxidation potential and electrophilic, is a stronger electron acceptor than oxygen [31–33]. A series of experiments were conducted with deoxygenated TiO2 suspension in order to evaluate the effect of dosage on the photocatalytic degradation of MCB. The examined range of dosage was varied from 5.6 to 78.0 mg L−1. From the typical temporal file exhibited in Figure 3(a), it can be seen that the degradation of MCB was sensitive to the variation of dosage. A pseudo first-order kinetic model was provided to simulate the degradation of MCB. Table 2 summarizes the degradation rate constant derived from the gradient of the plot of the natural logarithm of the normalized MCB concentration against the illumination time. When the dosage increased from 5.6 to 22.5 mg L−1 the value of increased from 0.018 to 0.026 min−1. A further increase in the dosage from 22.5 to 78.0 mg L−1, however, led to a significant decline in the from 0.026 to 0.011 min−1. The optimum dosage for the degradation of MCB was around 22.5 mg L−1 which was similar to that observed by other researchers [20, 34–36].
The influence of dosage on the degradation of MCB can be explained in terms of the number of generated radicals and the capture of radicals [36–39]. It is well known that can trap photoinduced to stabilize the paired Additional radicals could be yielded via the reaction between and or (12) and (13). As a result, the addition of into the photocatalytic system was expected to promote the degradation of MCB. Exceeding the optimum dosage, however, the excess would trap the radicals to form weaker oxidant radicals. Accordingly, the capture of radicals was occurred through (14) and (15). The decline in the radical concentration, trigged by the higher dosage, restrained the degradation of MCB.
The variation of DO concentration during the UV/TiO2/ photocatalysis for a variety of dosage is illustrated in Figure 3(b). It was found that the DO concentration increased markedly with illumination time to reach a maximum value and decreased thereafter. The increase in the DO concentration was related to the decomposition of [39, 40] Correspondingly, the addition of seemed to act as an oxygen source. The later decrease in the DO concentration implied that the generated oxygen was involved in the mineralization.
3.3.2. Mineralization of MCB
Similar to the degradation of MCB, the mineralization of MCB also took place throughout the entire irradiation. Figure 3(c) shows the concentration of TOC as a function of illumination time under various dosages. As can be seen, the extent of TOC decay increased from 44.4 to 74.1% with dosage increased from 5.6 to 45.0 mg L−1 after 240 min of irradiation. At higher dosage, however, the mineralization efficiency decreased to 62.5%. Reviewing the result shown in Figure 3(a), there was still observable MCB throughout the entire irradiation period in the case of dosage equal to 78.0 mg L−1. This meant that the constraint on the mineralization efficiency may be ascribed to the competitive MCB.
Based on (8), the carbon concentration in the transient organic intermediates was determined as shown in Figure 3(d). Interestingly, the maximum value of intermediate concentration was decreased when the H2O2 dosage varied from 5.6 to 78.0 mg L−1. When dosage was 5.6 mg L−1, the intermediates accumulated abidingly to its maximum value of 5.6 mg L−1 as TOC and then decreased. Similarly, the maximum values were 4.7, 3.5, 3.2, and 3.0 mg L−1 as TOC in the cases of H2O2 dosage equal to 11.2, 22.5, 45.0, and 78.0 mg L−1, respectively. The decline in the maximum concentration of the intermediates revealed that some fraction of mineralization was occurred in parallel with the degradation of MCB.
Applying two-step consecutive kinetic model to the determined intermediate concentration, the mineralization rate constant was calculated as summarized in Table 2. It can be seen that the value of was decreased with increasing dosage. The highest value was 0.002 min−1 under the experimental conditions of dosage equal to 5.6, 11.2, and 22.5 mg L−1. Notably, when the dosage was 78.0 mg L−1, the determined value was lower than 0.001 min−1. According to the principle of the two-step consecutive kinetics, the calculation of value was depend on the concentration of the intermediates and the value of . Consequently, the decline of the maximum intermediates concentration as dosage was increased led to a lower value.
3.4. Evaluation of Oxygen and H2O2 as Electron Acceptors
According to its characteristics as electron acceptor and oxidizing agent, the presence of oxygen in photocatalytic system was beneficial for the photocatalytic degradation of MCB. The experimental results confirmed that the photocatalytic degradation of MCB in the presence of oxygen followed a simplified two-step consecutive kinetics. For kinetic analysis, both degradation and mineralization rate constants had been demonstrated to increase with increasing DO concentration. However, this is an important consideration to bear in mind that the DO concentration in contaminated water may be limited and aeration would cause the stripping of volatile organic compound such as MCB.
It is preferential to use where there is a limited availability of oxygen. Considering its hydrophilic property, the H2O2 would come into contact with the hydroxylated TiO2 particles in aqueous solution and trap the photoinduced . Furthermore, could also serve as an oxygen source to improve the mineralization. Excess H2O2, however, could act as radical scavenger in the photocatalytic system. Moreover, the may compete with MCB for the active sites of TiO2 particle [32, 41–44] to suppress the photocatalytic degradation of MCB. To avoid the capture of radical and competitive adsorption under high H2O2 dosage, sequential replenishment of into UV/TiO2 system was suggested.
Referring to Figure 3, the most efficient degradation was observed where the dosage was 22.5 mg L−1, whereas the optimal dosage for MCB mineralization was 45.0 mg L−1. Accordingly, total dosage of equal to 45.0 mg L−1 was chosen for the evaluation of the photocatalytic degradation of MCB under various addition sequences. In the experiment of stepwise addition of , 22.5 mg L−1 of was added into the TiO2 suspension, once before irradiation and again after 40 min of illumination time. As the result illustrated in Figure 4(a), 120 min of irradiation was sufficient for complete elimination of MCB in the case when a two-step addition was made. On the contrary, 180 min of illumination time was needed for the one-time addition. The enhancement of the degradation efficiency was correlated to the higher degradation rate constant when the dosage was 22.5 mg L−1 rather than 45.0 mg L−1. Moreover, the second addition of was attributed to sustain the suppression of the pair recombination.
Monitoring the variation of DO concentration during photocatalytic degradation, it was observed that the oxygen accumulated continuously within 120 min of illumination time then decreased thereafter for both cases (Figure 4(b)). The concentration of the intermediates showed a similar tendency as depicted in Figure 4(c). Oxygen generated as a result of the decomposition of whereas the accumulation of intermediates was due to the degradation of MCB. According to the effective degradation under the two-step addition of , the concentration of intermediates increased faster than that under one-time addition of . Furthermore, since oxygen would participate in the mineralization of MCB, both the concentrations of oxygen and intermediates declined afterward. A comparison of the experimental profiles shows that depletion of the intermediates concentration was shaper for the two-step addition of than that for the one-time addition. Correspondingly, after 240 min of irradiation the mineralization efficiency of MCB in the case of two-step addition of was 87.4% higher than the value of 74.1% in the case of one-time addition. The increment in the mineralization efficiency can be explained as due to the restraint of the capture of radicals, the additional radicals related with second dose of , and the participation of oxygen in photocatalytic degradation. Accordingly, it was evident that the sequential replenishment of into UV/TiO2 system was advisable as a way to improve the efficiency of the photocatalytic degradation.
The direct photolysis and the oxidative potential of were proven to have slight contribution on the degradation of MCB. Notably, UV light and TiO2 together showed a marked effect. Increasing the DO concentration was beneficial for the photocatalytic degradation of MCB. Correspondingly, the degradation and mineralization rate constants increased with the DO concentration. Owing to the mineralization of MCB occurs through the intermediates, a simplified two-step consecutive kinetics can be used to describe the photocatalytic degradation in UV/TiO2/O2 photocatalysis. For the UV/TiO2/ photocatalysis, of lower dosage acted as electron acceptor to enhance the degradation efficiency. When the dosage was high, however, the degradation was suppressed due to the capture of radicals and the competitive adsorption of . In order to abate the disadvantages caused by using a higher dosage, sequential replenishment of into UV/TiO2 system was performed. Experimental results demonstrated that both degradation and mineralization efficiencies were enhanced by the restraint of the capture of radicals, the additional radicals caused from the second stage addition of , and the participation of oxygen in photocatalytic degradation.
The authors would like to thank the National Science Council, R.O.C. for financial support of this study under Contract no. NSC 90-2211-E-238-003.
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