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Ind. Eng. Chem. Res. 2007, 46, 4740-4746
KINETICS, CATALYSIS, AND REACTION ENGINEERING Semiconductor-Catalyzed Photodegradation of o-Chloroaniline: Products Study and Iodate Effect W. K. Choy and W. Chu* Department of CiVil and Structural Engineering, Research Centre for EnVironmental Technology and Management, The Hong Kong Polytechnic UniVersity, Hung Hom, Hong Kong
The photodegradation efficiency of o-chloroaniline (o-ClA) was studied in a dispersion of TiO2 with and without the use of iodate. Phosphor-coated mercury lamps that were emitting a wavelength of 300 nm, generating a total photon intensity of 5.6 × 10-6 einstein L-1 s-1, were used throughout the reaction. Decayed compounds o-chlorophenol and p-benzoquinone were identified as the major intermediates in the gas chromatography/mass spectroscopy (GC/MS) detection. The analysis was extended to total organic carbon (TOC) measurement and identification of end products, which have proven that UV/TiO2 is a clean process to mineralize aromatic organics into nontoxic CO2 and H2O. Further rate enhancement was examined by the selected oxyanion (IO3-) in the UV/TiO2. The use of IO3- generally improved the degradation; however, the performance was very much dependent on the system pH. The o-ClA decay was more favorable in acidic solution than in alkaline solution in UV/TiO2/IO3-. Such an effect could be justified by the IO3- reduction process and the generation of different radical species. Introduction The degradation of organic pollutants in water by titanium dioxide (TiO2) has become an emerging treatment technology. Because of the large-scale development of the agriculture industry, the variety and quantities of agrochemicals have been dramatically increased over a few decades. Most of the pesticides are resistant to chemical and/or photochemical degradation, under typical conditions.1 There is high potential in the applications of advanced oxidation processes (AOPs) for the remediation of contaminated waters.2 Recent research has shown that a large variety of organic pollutants have been tested for achieving complete mineralization via these processes.3,4 When the TiO2 suspension is irradiated with light energy greater than its band gap energy (∼3.2 eV), the photoinduced electrons (e-) and positive holes (h+) are produced.
TiO2 + hV f TiO2 (e- + h+)
(1)
The positive holes (h+) are considered to be the major oxidizing species during the mineralization process in TiO2 photocatalysis. In addition, the photoinduced electrons (e-) can also react with electron acceptors such as O2 dissolved in water to produce a superoxide radical anion (•O2-).
e- + O2 f •O2-
(2)
Furthermore, the positive holes (h+) can also react with OHor H2O to produce •OH radicals. * To whom all correspondence should be addressed. Tel.: (852)2766-6076. Fax: (852)2334-6389. E-mail:
[email protected].
h+ + OH- f •OH
(3)
h+ + H2O f •OH + H+
(4)
All these oxidant species are reported to be responsible for the photodecomposition of the organic substances. Recent research has also suggested the addition of external acceptors such as hydrogen peroxide (H2O2) to inhibit the recombination of electron-hole pairs and to increase the hydroxyl radical concentration.5 Previous research on UV/TiO2 has shown that the efficiency of photocatalytic degradation is dependent on various parameters. First, the oxidation efficiency is dependent on the adsorption rates of organics onto the TiO2 particles. The charge effect would influence the adsorption on nonpolar organic molecules. Yang et al.6 and Prevot et al.7 have indicated that the point of zero charge for TiO2 is observed at pH ∼6.5: TiO2 particles are charged positively at pH 6.5. Therefore, the adsorption by TiO2 is a pH-dependent process that operates on the belief that lower adsorption can slow the photodecay. Researchers have suggested various technologies to improve the performance of a TiO2catalyzed photoreaction, including surface modification of TiO2 by varying the surface area of TiO2, varying the UV irradiation sources selection, and introducing additional external electron acceptors.8 Researchers have demonstrated that the use of inorganic oxidants such as IO4-, S2O82-, BrO3-, and H2O2 can help to enhance the photodegradation rates of organic substrates by deferring the conduction band electrons and formatting reactive radical intermediates.9,10 Our previous study on BrO3- and H2O2 has shown that the photodecay rate could be increased within an appropriate range of dosage.11 Other researchers have also reported rate enhancement through the use of IO4- and S2O82during photoreaction.12,13 Before the real application, the influ-
10.1021/ie061455l CCC: $37.00 © 2007 American Chemical Society Published on Web 06/01/2007
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Figure 1. Molecular structure for (a) 2-chloroaniline and (b) 4,4′-methylene bis-(2-chloroaniline) (MBOCA).
ence of the inorganic additives must be accurately examined, because some of them may affect the degradation adversely under specific conditions. The organic substance studied in this paper is o-chloroaniline (abbreviated hereafter as o-ClA) (see Figure 1a), which can be produced in the existence of aniline during chlorination (disinfection) processes. Usually, 4,4′-methylene bis-(2-chloroaniline) (MBOCA, C13H12Cl2N2) is formed (see Figure 1b). Although information about the effects of long-term exposure to MBOCA was limited, studies in animals have shown that MBOCA can be harmful to the liver of exposed dogs and rats. MBOCA also causes cancer of the lung, liver, breast, and bladder in animals. The International Agency for Research on Cancer has determined that MBOCA is probably carcinogenic to humans.14 Therefore, the treatment of o-ClA could minimize its adverse effect onto human and the environment. The family of chloroaniline (p-ClA) has been analyzed in the photodecay of TiO2 and its degradation pathway was proposed.15 In this study, parameters including the TiO2 and o-ClA dosages, the generation of intermediates and end products, and the pH effect of IO3- were investigated under the photodegradation of o-ClA. Experimental Section Chemicals Used. o-ClA (C6H5NH2Cl, 99.5+% purity) was purchased from Fluka Chemika GmbH. Initially, o-ClA at 0.095 mM in doubly distilled water was adopted, unless specified otherwise. The photocatalyst TiO2 powder was P25 grade (containing 70% anatase and 30% rutile), with an average particle size of 30 nm and a Brunauer-Emmett-Teller (BET) surface area of 50 m2/g.16 Potassium iodate (KIO3) (analytical reagent (AR) grade, 99.7+% purity) was purchased from International Laboratory USA. All the graduated flasks and glass droppers used for temporary sample storage and transfer were washed with acetone that was purchased from Tedia. Other chemicals provided by Tedia include acetonitrile, methyl alcohol, and ethyl acetate. The organic intermediates were identified from the pure standard of p-benzoquinone and o-chlorophenol, which were purchased from Acro and Riedelde Haen, respectively. Standards of the ionic products sodium chloride and soldium nitrite were obtained from Fisher Chemical and AnalaR, respectively. Hydrochloric acid and sodium hydroxide were used to adjust the initial pH of the solutions. Photocatalytic Degradation. The photocatalytically degraded reaction for the experiments was conducted in a RPR-200 RAYONET photochemical reactor, which was purchased from the Southern New England Ultraviolet Company (as depicted in Figure 2). During the experiment, all the sample solutions were illuminated by eight monochromatic UV lamps at a wavelength of 300 nm. A magnetic stirrer was installed at the
Figure 2. Schematic diagrams showing the top view (left) and sectional view (right) of the photoreactor.
base of the reactor to obtain a homogeneous TiO2 suspension during the reaction. A ventilation fan that was installed within the reactor was used when conducting the reaction, such that the reacting temperature was maintained at ∼23 °C. Nevertheless, the temperature effect is minimal in this study. Only a variation of 5% was observed for the samples in the temperature range of 10-40 °C (data not shown). After TiO2 powder was added into the mixtures of reactants, a quartz column that contained the reactants was immediately placed into the preheated photochemical reactor. The reaction time was recorded and samples were taken from the quartz column at a predetermined time until the total degradation of o-ClA reached 80% or higher. The collected samples were filtered by a gas-tight syringe that was installed with a 0.45µm membrane filter. The samples were stored in 4-mL glass vials with Telfon caps before high-performance liquid chromatography (HPLC) analysis. Instrumental Analysis. The remaining o-ClA in the collected samples was quantified by the HPLC system. The HPLC system used in the experiment comprised of a LINEAR UVIS200 HPLC Detector, a Water SYMMETRY 5 µm C18 (3.9 mm × 150 mm) column, and an Alltech HPLC pump with an injector port. The maximum absorption wavelength (λmax) for o-ClA detection was selected at 285 nm. The mobile phase of o-ClA in this HPLC system consisted of 60% acetonitrile and 40% HPLC water. The flow rate was adjusted to 1.0 mL/min. Full loop injection at 20 µL was performed, and the retention of o-ClA was 4.95 min. The syringe was cleaned by acetone and methyl alcohol after each injection and washed by the next sample solution before injection. The presence of chloride and nitrite was determined by the ion chromatography equipment (Dionex Series 4500i), which was composed of an anion column (Dionex Ionpac AS11 (4 mm × 250 mm) and AG11 (4 mm × 50 mm), and a suppressor (ASRS-II) in auto-suppression recycling mode. A mixture of 0.8 mM of sodium bicarbonate (NaHCO3) and 0.8 mM of sodium carbonate (NaCO3) was used as the mobile phase, eluting at a rate of 1 mL/min. The ammonium ion concentration was measured by the ammonia gas-sensing electrode (Cole-Parmer). The total organic carbon (TOC) was determined by a Shimadzu model TOC 5000 analyzer that was equipped with an ASI automatic sample injector. The identification of intermediates was conducted via gas chromatography/mass spectroscopy (GC/MS), using a HewlettPackard system that was composed of a model HP 3800 gas chromatograph connected with Varian Factor Four capillary column (model VF-5ms, 30 m × 0.25 mm), coupled with a Varian model 1200L quadrupole MS/MS operating in electron ionization (EI) mode at 900 V. Helium gas in a constant flow of 1 mL/min was used as the carrier gas. An autosampler was installed and 1 µL of sample was injected from the injector port (280 °C) in the splitless mode. The extraction solvent was
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Figure 3. Photokinetic profile at various o-chloroaniline (o-ClA) and TiO2 dosages.
a 1-mL mixture of 50% ethyl acetate and 50% hexane with 0.2 g of sodium chloride. One milliliter of sample was added to the eluent, and the mixture was then shaken by the mixer (Barnstead Thermolyne Type 37600) for 1 min. Extracts were collected for analysis. Separation of intermediates were ramped by a temperature program with an initial temperature at 50 °C for 1 min, rising to 200 °C at a rate of 5 °C/min for another 3 min, then it finally reached to 260 °C at the same rate and was stable for 2 min. The peaks observed for the reaction intermediates were referenced with the appropriate external standards. Results and Discussion Dosage Effect on o-ClA and TiO2. The decay kinetic rates of o-ClA (k, expressed in units of min-1) in a TiO2 suspension followed the pseudo-first-order decay model:17
ln
()
Ct ) -kt C0
(5)
where C0 and Ct are the concentration of o-ClA unexposed and at time t, respectively. The effects of both the initial o-ClA concentration ([o-ClA]0) and TiO2 dosage were examined and the corresponding o-ClA decay rates are summarized in Figure 3. For a fixed o-ClA concentration, it is generally observed that the decay was enhanced significantly by low doses of TiO2. However, as the TiO2 dosage increased to ∼0.1 g/L, further additions could only result in an insignificant increment to the decay. Taking the lowest [o-ClA]0 (0.095 mM) as an example, the decay rate was increased by a factor of 3 at a TiO2 dosage of 0.1 g/L, compared to that of direct photolysis (no TiO2 addition). Further increases in the TiO2 amount did not give a linear rate enhancement, whereas the decay of o-ClA was only increased by 10% when the TiO2 dosage was increased from 0.1 g/L to 1.4 g/L. This trend was generally applicable to other lower [o-ClA]0 concentrations; however, at higher [o-ClA]0 (e.g. 0.95 mM), retardation at high TiO2 dosages was observed. Under these circumstances, the decay of o-ClA was reduced by 10% as the TiO2 dosage increased from 0.1 g/L to 1 g/L, indicating that the dosage of TiO2 was in excess in the solution. Carp et al. have suggested that TiO2 under UV irradiation could assist
Figure 4. Graph showing the generation of intermediates of o-ClA in the UV/TiO2 process. (Note: [o-ClA]0 ) 0.95 mM; [TiO2] ) 0.1 g/L; initial pH ) 7.)
the photoreaction by promoting the generation of electron-hole pairs, which increases the production of •OH to oxidize aromatic organics into less-toxic substances.18 The increase may be due to the larger total surface area that is available for the photocatalytic reaction on the TiO2 particles as the TiO2 dosage increases. However, the excessive TiO2 suspended in the solution could attenuate the available UV intensity by decreasing the light penetration, which results in an adverse effect to the process. Such an effect is less significant in an optically dilute solution (i.e., lower [o-ClA]0), because the ratio of UV photon/ [o-ClA]0 is relatively higher than that in an optically dense solution (i.e., higher [o-ClA]0). Moreover, the o-ClA concentrations across our tested ranges were determined to be inversely proportional to the reaction rate at any fixed TiO2 concentration [TiO2]. The organicdependent phenomenon has been observed by Wei and Wan in the investigation of photodegradability of phenol.19 Mils et al.15 and Fox et al.20 have reported that a pre-adsorption process by TiO2 is feasible before the degradation proceeds. The adsorption is not limited to the target organics; Leng et al. have claimed that the photogenerated intermediates and OH- are also the adsorbates onto TiO2.21 At high [o-ClA]0 concentrations, the amount of intermediates formed from the degradation is relatively high, thereby reducing the number of available active sites on TiO2 for the adsorption of H2O/OH- and o-ClA, causing a decrease in the reaction rate constant of o-ClA itself. Photoproduct Identification. 1. Organic Intermediates. The investigation of degraded intermediates is valuable to verify that the photochemical process does not generate more toxic substances in the effluent. The initial concentration of o-ClA has been increased from 10-5 M (in the previous kinetic study) to 10-3 M. This is to ensure that the concentrations of major intermediates are high enough to be detected in the GC/MS analysis. The extract of the solvent after reaction was analyzed by GC/MS. In the UV/TiO2 process, two major intermediates (o-chlorophenol and p-benzoquinone) were identified and their peak areas were compared with the original standards. Other intermediates should be involved in the o-ClA decay, but the levels are possibly too low to be detected by GC/MS. Therefore, the full degradation pathway is reserved until other intermediates are identified by other instruments. Their profiles in the process are displayed with the decay of o-ClA in Figure 4. Ap-
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Figure 6. Graph showing the degradation rate k (denoted by the cross symbol, ×) and distribution of species C/C0 (represented by the solid line) under different pH conditions. (Note: The dashed line is the degradation rate generated from the model.)
Figure 5. Mineralization of o-ClA: (a) reduction of total organic carbon (TOC) and (b) detection of end-products.
proximately 80% of o-ClA quickly disappeared within 120 min of reaction time; however, such an effective rate was retarded afterward, likely because of the competition of active sites between o-ClA and intermediates on the TiO2 surface.3 In Figure 4, it can also be observed that o-chlorophenol at ∼10-5 mM was generated as the primary product, reaching its maximum concentration after 60 min of reaction time. When it was gradually decayed, the secondary intermediate, p-benzoquinone, was formed. The amount of p-benzoquinone was approximately an order of magnitude less than that of ochlorophenol, and its degradation was also observed after 420 min. The level of intermediates detected was far less than that of o-ClA applied, implying that the organic intermediates could undergo further decay and would not accumulate in the system. 2. End-Products and Mineralization. The identification of fast-decaying intermediates has already proven the UV/TiO2 process to be successful in converting o-ClA to low level organics in the aqueous phase. However, the measurement of TOC and the generation of end-products are of great importance to verify that the treatment process is eventually beneficial to the environment. The TOC decay illustrated in Figure 5a showed a reducing trend, indicating that this process could mineralize most of o-ClA and the photoproducts to nontoxic substances (CO2 and H2O). The tailing is due to the lower reaction rates of mineralizing low-molecular-weight organic acids, as indicated by Santos et al.22 However, they reported a complete mineralization in UV/TiO2. They examined the final products and determined that their formation involved the breaking of C-C bonds and decarboxylation of intermediates such as malic, acetic, oxalic, or formic acid by further reaction with •OH radicals. Each bond breaking is accompanied by the formation of an acid with a shorter chain and a CO2 molecule. An increase of ionic products was quantified during the mineralization process, simultaneously. Figure 5b shows the accumulation profiles of chloride (Cl-), nitrite (NO2-), and
ammonium (NH4+) ions in the solution. At the initial stage of reaction, Cl- and NH4+ ions were detected and their levels continued to increase during the reaction. From the results of intermediate formation/decay, as discussed previously, the generation of Cl- ions is due to •OH attack onto o-ClA. Similar observations were also reported by Winarno and Getoff.23 In addition, the formation of NH4+ and/or NH3 is a pH-dependent process and proceeds as follows:
NH3 + H2O h NH4+ + OH-
(6)
The pKa value of eq 6 was reported to be 9.24;24 therefore, an inadequate amount of OH- in a low-pH solution would favor the formation of NH4+ ions. Under our experimental conditions, a decrease in the initial pH from 7 to 3.7 (data not shown) was observed after 540 min of reaction. As in our previous study, the decrease was likely due to the generation of low-molecularweight organic acids (such as formic acid) in the oxidation process as the end-products during o-ClA degradation.25 Thus, NH4+ is the dominant species in the solution. Meanwhile, a very low level of nitrite (∼1/10 of the NH4+ concentration) was identified at a later stage of the process. According to Stumm and Morgan,26 the NH4+ ion can be eventually oxidized to nitrate (via the nitrite) in an oxidizing environment:
NH4+ + 1.5O2 h NO2- + H2O + 2H+
(7)
NO2- + 0.5O2 h NO3-
(8)
3. Effect on Solution pH. To examine the pH effect, the decay of o-ClA at various pH levels was adjusted, using either diluted H2SO4 or NaOH. The comparison presented in Figure 6 shows that the initial decay rates were observed to increase generally from a low pH to a high pH, but the trend leveled off to ∼0.1 min-1 when the pH was greater than 7. This feature is partially due to the surface charge of TiO2, as mentioned previously. Positive charges on the surface (pH 6.5) enhance the transportation of holes to the surface, which react with OH- and H2O and generate •OH radicals; hence, the rate of photo-oxidation increases. In addition, the
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role of pKa must be taken into consideration. The pKa of o-ClA was reported by Winarno and Getoff23 to be 2.6, which indicates that cationic o-ClA could dissociate one proton into its molecular form in the acidic medium, as shown in eq 9.
Cationic o-ClA is the dominant species at pH
