Photocatalytic Removal of Pesticide Dichlorvos from Indoor Air: A

Nowadays, people generally spend more than 80% of their time in an indoor environment such as home, office, school, shopping centers, etc. where they ...
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Environ. Sci. Technol. 2008, 42, 3018–3024

Photocatalytic Removal of Pesticide Dichlorvos from Indoor Air: A Study of Reaction Parameters, Intermediates and Mineralization MOHAMAD SLEIMAN,* CORINNE FERRONATO, AND JEAN-MARC CHOVELON Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON), UMR 5256 CNRS/ Université de Lyon 1, 2 Avenue Albert Einstein, F-69626 Villeurbanne cedex, France

Received September 27, 2007. Revised manuscript received December 7, 2007. Accepted January 22, 2008.

This paper presents for the first time the investigation of TiO2 photocatalysis for the removal of pesticides in gas phase. Dichlorvos was used as a model pesticide, and experiments were carried out using both static and dynamic reaction systems to explore the different aspects of the process. Thus, adsorption, reaction kinetics, and the influence of several operational parameters such as relative humidity (RH), inlet concentration, flow rate, and association of TiO2 with activated carbon (AC) were all examined in detail. Furthermore, a special attention was devoted to the analysis of reaction products by means of various analytical techniques such as Fourier transform infrared spectroscopy, automated thermal desorption technique coupled to gas chromatography–mass spectrometry instrument, gas chromatography equipped with a pulse discharge helium photoionization detector, and ion chromatography. The results showed an immediate and total removal of dichlorvos at ppbv levels (50–350 ppbv) along with a high mineralization extent (50–85%) into harmless final products (CO2, HCl, PO43-). Moreover, RH was found to significantly affect the mineralization extent and the formation of reaction intermediates. On the basis of identification data, direct charge transfer and chlorine radical (Cl•) attack were shown to play a key role in the reaction mechanism at low RH, whereas at high RH, HO• radicals were the predominant active species.

1. Introduction Over the last few years, indoor air pollution has become an issue of worldwide concern. Nowadays, people generally spend more than 80% of their time in an indoor environment such as home, office, school, shopping centers, etc. where they can be exposed to a wide variety of air pollutants such as NOx, CO, volatile organic compounds (VOCs), pesticides, particulates, etc (1, 2). Among these indoor pollutants, pesticides represent serious risks for human health due to their high toxicity. According to a recent survey done by the U.S. Environmental Protection Agency (EPA), 75% of U.S. households used at least one pesticide product indoors, and 80% of most people’s exposure to pesticides is estimated to occur in an indoor environment (3). * Corresponding author phone: (33) 4 72 43 11 50; fax: (33) 4 72 44 84 38; e-mail: [email protected]. 3018

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Pesticides can be present in indoor air via indoor use (spraying operations) and storage, through volatilization of pesticide residues deposited on the surface of carpets, floors, etc., or tracked in by shoes, clothes, or pets from outdoor areas (lawns, gardens, greenhouses, etc), as well as brought in by outdoor air and dust (4–6). Contrary to outdoor environment, the residues of pesticides in indoor air are away from sun, wind, rain and thus are less likely subject to degradation processes and dispersion. Several studies have showed that some of pesticides indoors can be more persistent and their concentrations levels could be higher than those found outdoors (7, 8). At present, the methods suggested to minimize the exposure to pesticides indoors are, namely, source control, increase in ventilation, and air cleaning. Source control is a good solution but is often ungovernable because pesticides can be brought in from outdoors. Increased ventilation might even transport other toxic pollutants from outdoor air. Thus, air cleaning remains the most feasible option. However, traditional methods such as filtration or adsorption using sorbent materials (e.g., activated carbon) only transfer pesticides to another phase rather than eliminating them, and additional disposal or handling steps are subsequently required. Consequently, the investigation of viable treatment methods for efficient removal of pesticides in indoor air is of great environmental interest. One of the most promising technologies for organic pollutant treatment in air is TiO2 photocatalysis. It is based upon the use of UV-irradiated TiO2 to produce strongly oxidizing species (HO•, Cl•, O2•-, h+, etc.) that are able to destroy a broad range of toxic organic pollutants (9). Some of the advantages of this method are the complete mineralization of organic pollutants into innocuous CO2, H2O and mineral acids; the use of TiO2, which is widely available, stable, inexpensive, and harmless to humans; and the reaction is carried out at room temperature, which is energetically interesting. Much work has been done in recent years on photocatalysis for air treatment (10). Nevertheless, to the best of our knowledge, no study has been carried out to investigate the removal of pesticides in air by photocatalysis. In the present paper, we report the first study on the photocatalytic treatment of pesticides in indoor air. Dichlorvos (2,2-dichlorovinyl phosphate, DDVP, see Chart 1), was selected as a model pesticide because it is one of the organophosphorus pesticides widely used indoors and in farms and greenhouses for controlling insects and parasites (11), has a relatively high vapor pressure (7.0 Pa at 20 °C), and has high acute toxicity. Furthermore, it is considered by the WHO as a “highly hazardous agent” and several cases of indoor air contamination by dichlorvos were recently reported (12, 13). In-depth study was performed to investigate the different aspects of the photocatalytic process, such as adsorption, disappearance kinetics, the effect of important practical parameters (humidity, flow rate, inlet dichlorvos concentration), as well as the analysis of reaction intermediates and final products using different advanced analytical techniques such as Fourier transform infrared spectroscopy (FTIR), automated thermal desorption technique coupled to gas chromatography–mass spectrometry instrument (ATD-GCMS), gas chromatography equipped with a pulse discharge helium photoionization detector (PDPID), and ion chromatography (IC). The obtained data allowed us to discuss the ability of photocatalysis for practical pesticide treatment in indoor air. 10.1021/es702425q CCC: $40.75

 2008 American Chemical Society

Published on Web 03/12/2008

CHART 1. Chemical structure of dichlorvos

2. Experimental Section In this work, two experimental set-ups were used to study the photocatalytic degradation of dichlorvos; the first one is a home-built glass reaction cell, connected to a vacuum pump and equipped with a furnace and a FTIR spectrometer (Bruker IFS-28). A detailed description of this system was provided in our previous study (14). The second system is a continuous flow reactor equipped with a permeation system, ATD-GCMS, and GC-PDPID analytical techniques. A schematic diagram of this experimental setup is presented in Supporting Information Figure S1. 2.1. Static Reaction Experiments Using FTIR In Situ. This system allowed in situ monitoring of the gas–solid interface during the adsorption and photocatalytic degradation experiments. The experiments were carried out in the following static mode: a specified amount of TiO2 PC500 powder from Millenium (pure anatase, specific area ) 350 m2 g-1) is pressed into a self-supporting pellet (80–100 g m-2) that is subsequently treated at high temperature (in O2 at 6 torr and 673 K for 3 h) to remove organic contamination from the TiO2 surface. After cooling the pellet to ambient temperature, the entire system is held under modest vacuum, and a blank TiO2 spectrum is recorded. Then, known amounts of dichlorvos and water in air are introduced into the cell to conduct the dark adsorption experiments. Once adsorption equilibrium is reached, the irradiation is started using a HPK 125 W lamp from Phillips, which principally emits at 365 nm with a light intensity of 3.2 mW cm-2. During the experiment, IR spectra of both gaseous and adsorbed species are regularly collected by moving up and down the TiO2 sample holder using an appropriate magnetic system. In all IR measurements, spectra were recorded in the region 1000–4000 cm-1 with a spectral resolution of 4 cm-1 and signal averaging of 24 scans. The FTIR bands assignments were based on a comparison with those already reported in the literature (15–17). Furthermore, FTIR spectra of an authentic standard of dichloroacetic acid (DCAA) adsorbed onto TiO2 were recorded and compared with those obtained during the dichlorvos degradation. 2.2. Continous Gas-flow Reaction Experiments Using ATD-GC-MS and GC-PDPID. This system was developed to study the photocatalytic reaction of dichlorvos at ppbv levels. Experiments were conducted in dynamic mode using a continuous flow-through reactor of about 85 mL, made of stainless steel and equipped with an optical Pyrex glass window (transmittance: wavelength >290 nm), a water cell to avoid heating, and the same type of UV lamp used in the first setup. The photocatalytic medium used was nonwoven fibrous papers, coated with TiO2 PC500 (Ahlstrom, France). The gaseous dichlorvos stream generated using the permeation tube (Calibrage, France) was diluted with pure helium and was subsequently mixed with oxygen and water vapor at gas flow rates corresponding to target dichlorvos concentrations ranging from 50 to 350 ppbv. Various mass flow controllers (Brooks, 5850S series) in the 0–200 mL min-1 range were used to precisely control the gases flow rates. The final gas stream humidity and temperature were measured using a thermohygrometer (Rotonic Hygropalm 1, France). Before the introduction of the gas stream containing dichlorvos, the photocatalytic medium was pretreated by UV irradiation in an oxygen/He gas flow for 12-24 h to ensure an efficient elimination of adsorbed contaminants on the surface. The standard operating conditions used for

irradiation experiments are as follow: inlet dichlorvos concentration, 50–350 ppbv; relative humidity, RH ) 0–60%; total flow rate, 50–300 mL min-1; reactor temperature, 298 ( 2 K; TiO2 deposited amount, 5 g m-2; UV intensity, 4.3 mW cm-2; oxygen content, 2% in He; and irradiated surface, 10 cm2. It should be pointed out that He was used instead of N2 and only 2% oxygen in He was employed in order to avoid the overlapping of chromatographic peaks of N2/O2 with those of CO/CO2 during the GC-PDPID analysis. During the irradiation experiment, the gas phase reaction intermediates were collected using multibed solid adsorbent tubes (Carbotrap C, Carbotrap B, and Carbosieve SIII) with a total sampling volume of 150 at 30 mL min-1. Analysis was carried out using a GC-MS (Clarus 500, Perkin-Elmer,) equipped with a thermal desorption unit (Turbomatrix, Perkin-Elmer). MS identification was conducted by comparing the retention times and mass spectra to available authentic standards, using the NIST library with a fit higher than 90%. An effort was also made to quantify the major reaction intermediates present in gas phase in order to determine the main reaction routes and to better understand the reaction mechanisms. The quantification was based on ATD-GC-MS external calibration curves of pure standards of dichlorvos, dichloroacetaldehyde (DCA), and trichloroacetaldehyde (TCA). For the nonavailable products, a qualitative comparison of their concentrations observed at two RH levels (0 and 40%) was carried out on the basis of their IR and/or MS peaks intensities. The formation of CO/CO2 during the reaction was also monitored regularly (each 5 min) using a gas chromatograph equipped with (i) two packed chromatographic columns: a Porapack Q (L: 5 m, i.d.: 1/8”) and a Molecular sieve 13-X (L: 2 m, i.d.: 1/16”) from RESTEK, and (ii) a pulse discharge helium photoionization detector (PDPID) model D-3-I-HP (Valco instruments Inc.). Calibration of CO/CO2 was carried out using a standard mixture of CO/CO2 (5 ppmv) in He. The adsorbed amounts of chlorine and phosphates ions were quantified using ion chromatography (Dionex DX-120) after a solid–liquid extraction of the photocatalyst medium using deioinized water (adjusted at pH 8 with NaOH). The conditions of IC analyses were described elsewhere (18).

3. Results and Discussion 3.1. Adsorption of Dichlorvos on the TiO2 Surface. In heterogeneous photocatalysis, the reactant adsorption is a critical step of the reaction, and it is of primary importance for the comprehension of the reaction processes. Therefore, the adsorption of dichlorvos onto the TiO2 surface was investigated in static mode using FTIR spectroscopy. Figure 1 shows the FTIR spectra of adsorbed dichlorvos for increasing concentration, at 298 K. The IR spectra have been divided in two regions: the high frequency region (approximately 3200–2700 cm-1), which contains the methyl group stretching vibrations, and the low frequency region (approximately 1700–1000 cm-1), which contains the C-O and PdO stretching vibrations and the methyl bending vibrations. Incident radiations below 1000 cm-1 are strongly absorbed by the TiO2. In the upper part of the left panel, the inset shows the region 3800–3200 cm-1, which corresponds to the hydroxyl groups on the TiO2 surface. As it can be seen, by increasing coverage of dichlorvos on the TiO2 surface (spectra a-e), the intensity of isolated hydroxyl groups (bands at 3693, 3677, 3661, 3631 cm-1) decreases significantly, whereas the associated hydroxyl groups (broadband 3500–3200) increase in intensity. Simultaneously, in the region of 1200–1100 cm-1 (right panel), the stretching vibration of PdO (∼1180 cm-1) develops and shifts to lower frequencies (∼1150 cm-1), whereas the stretching modes of C-O remains unchanged by increasing the dichlorvos coverage. VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. FTIR spectra of adsorbed dichlorvos on the TiO2 surface for different inlet concentrations. The inset in the left panel (high frequency region) shows the interaction of OH groups with dichlorvos. The inset in the right panel shows dichlorvos coverage (band at 2960 cm-1) during desorption at 573 K (experimental points, open circles; model, dashed line). The frequency decrease of PdO band can be explained by a strong interaction with the catalyst surface, which implies a loss of the PdO moiety of some of its double bond character (16). These observations indicate that dichlorvos adsorption mainly involves the PdO bond and surface hydroxyl groups. Thus, dichlorvos is likely adsorbed via hydrogen bond between the oxygen of the phosphoryl group and the Ti-OH surface groups. A second adsorption mode could also exists, in which an adduct is formed between the electron-rich phosphoryl oxygen and the Lewis sites (Ti4+, Ti3+) at TiO2 surface. However, the present FTIR data could not confirm or give direct evidence on the existence of this adsorption mode. On the other hand, to evaluate the affinity of dichlorvos to TiO2 surface, a controlled thermal desorption was performed at different temperatures (14). The results showed that dichlorvos does not desorb from the surface even at relatively high temperatures (100–200 °C). Moreover, assuming a first-order desorption kinetic model and a pre-exponential factor of 10-13 s-1, the dichlorvos activation energy of desorption (Ed) was estimated to 170 kJ mol-1 (see inset in the right panel of figure 1). This value indicates that dichlorvos is very strongly adsorbed on the TiO2 surface, and it can displace adsorbed water molecules, which have lower Ed values (60–120 kJ mole-1) as reported in the literature (19). 3.2. Control Experiments. Using the second setup, control experiments were carried out to check the effect of photolysis, flow rate, and oxygen content on the conversion of dichlorvos. Experiments were carried out with a dichlorvos concentration of 180 ppbv, RH) 0%, UV intensity at 365 nm ) 4.3 mW cm-2. Conversion of dichlorvos was calculated at steady state conditions using: % conversion )

[DDVP]in - [DDVP]out [DDVP]in

× 100

(1)

[DDVP]in and [DDVP]out are the inlet and outlet dichlorvos concentrations, respectively. The obtained results (see Supporting Information Figure S2) showed that photolysis causes only 4% of dichlorvos conversion, whereas using photocatalysis, a quasitotal conversion was observed (95 ( 2%). Consequently, it can be considered that the reaction occurs in a pure photocatalytic regime where photolysis is negligible. Moreover, the photocatalytic efficiency was found to be independent of the variation of oxygen content in the gas 3020

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FIGURE 2. Effect of dichlorvos inlet concentration on the oxidation rate. stream over the range 1–20% indicating that even at low % of O2 in gas phase, an efficient removal of dichlorvos can be achieved. On the other hand, the increase of flow rate in the range 100–300 mL min-1 exerted an insignificant impact on the removal efficiency of dichlorvos. This can be justified by the high activity of the photocatalyst and the absence of mass transfer limitations. Thus, in our conditions, the reaction kinetic process is limited only by the chemical step. 3.3. Reaction Kinetic. In an attempt to better understand the reaction kinetics, the effect of inlet dichlorvos concentration on the reaction rate was investigated. Experiments were performed at different initial concentrations of dichlorvos from 50 to 350 ppbv, at 40% RH and at a total flow rate of 150 mL min-1. Figure 2 depicts the dependence of the reaction rate on the inlet dichlrovos concentration. The reaction rate (r) was defined by the following equation, r )

[DDVP]in - [DDVP]out ×F S

(2)

where dichlorvos concentrations are in mol m-3, F is the total flow rate (m3 min-1), and S is the irradiated surface (m2). The observed linear dependence between the reaction rate and the inlet dichlorvos concentration reveals a pseudofirst order reaction kinetic. As generally described, this behavior is in agreement with a Langmuir–Hinshelwood

FIGURE 3. Influence of relative humidity on the conversion and mineralization of dichlorvos. (L-H) kinetic model when the reactant concentration is low enough and no catalyst saturation occurs (3). r )-

d[DDVP] k[DDVP] ) = k[DDVP] dt 1 + k[DDVP]

(3)

3.4. Effect of Humidity. Humidity represents one of the most important parameters for photocatalysis applications in the gas phase. Many authors have reported a significant impact of humidity on the degradation efficiency of air contaminants (20, 21). In most cases, the studies revealed a dual function of water vapor: a competitive adsorption with the reactant on the available surface sites and changes in hydroxyl radical population levels. In this study, special attention was given to the comprehension of the role of this parameter in the reaction processes. Figure 3 illustrates the variation of both conversion and mineralization of dichlorvos (180 ppbv) as a function of RH. Mineralization (%) was determined using the following expression. % mineralization )

[CO] + [CO2] × 100 (4) 4 × ([DDVP]in - [DDVP]out)

As can be observed, the effects of RH on the conversion and mineralization of dichlorvos are different; the conversion appears to be insensitive to the variation of RH, whereas the mineralization drops drastically with the increase of RH and reaches a plateau at a RH value of ca. 50%. A closer look at the evolution profile of conversion vs RH shows that conversion increases very slightly at low RH (0–5%RH). This can be related to the increase of HO• formed by oxidation of water molecules by holes (h+). However, the quasitotal conversion found at dry conditions (0% RH) let us suppose that other oxidizing species such as holes (h+), superoxide (O2•-), or Cl• radicals might be also responsible for the degradation of dichlorvos. Moreover, the fact that conversion was not inhibited by the increase of RH suggests that adsorption competition does not play a key role in the dichlorvos degradation. This is consistent with the high affinity of dichlorvos for TiO2 surface. On the other hand, despite the high dichlorvos conversion observed, the mineralization, in contrast, was significantly reduced with the increase of RH. Two probable reasons could be inferred to explain this surprising result: (i) the adsorption competition between water molecules and weakly adsorbed reaction intermediates and (ii) the formation, at high RH, of strongly bound-surface species that resist degradation and deactivate the catalyst as already reported (22). To experimentally check these assumptions, experiments were carried out at two different RH values (0 and 40%), and analysis of reaction intermediates and final products was

performed. The results are discussed below in the next paragraphs. 3.5. Reaction Products. Using the two experimental setups described in the Experimental Section, a number of reaction intermediates and final products were identified and quantified during the photocatalytic degradation of dichlorvos. Table 1 summarizes the identified reaction products, their MS and IR data, as well as the comparison of concentrations of some products measured at 0 and 40% RH. As it can be seen in Table 1, different reaction intermediates were detected in the gas phase according to the RH level: at 0% RH, the reaction intermediates observed were trichloroacetaldehyde (TCA), dichloroacetaldehyde (DCA), phosgene (COCl2), carbon monoxide (CO), and chlorinated products such as CHCl3 and CCl4, whereas at high RH (40%) the main reaction intermediate was DCA, and only traces of COCl2 and CO were detected. On the other hand, higher amount of adsorbed species such as dichloroacetic acid (DCAA), phosphate (PO43-), and formate (HCOO-) were found at humid conditions (RH 40%) after the same irradiation time. These findings indicate that water strongly affects the formation of reaction products and that reaction pathways are likely to be different depending on the RH level. The production of TCA, CHCl3, and CCl4 at dry conditions suggests that, in the absence of water, chlorine radicals (Cl•) are involved in the photocatalytic degradation of dichlorvos. The formation of Cl• could be ascribed to the oxidation of chlorine ions adsorbed on TiO2 but also to the UV photolysis process. This hypothesis was corroborated by detection, using IC, of a small amount of chlorine ions initially adsorbed on the fresh photocatalyst surface and during the photolysis of dichlorvos. In contrast, at high RH, water appears to out-compete Cl• initiated reactions (H2O + Cl• f HCl + HO•) and inhibits the formation of the chlorinated products. Similar results were reported by others for the photocatalytic degradation of chlorinated compounds such as trichloroethylene (TCE) (23–25). We note also that the presence of water substantially reduces the formation of COCl2, CO, and CO2. The drop of COCl2 concentration might be explained by two reasons: first, the hydrolysis of COCl2 in the presence of water to produce CO2 and HCl (16, 17); and second, the suppression of CHCl3, which can be oxidized to COCl2 (25, 26). Similarly, CO is recognized to be a reaction product of phosgene; thus, its concentration decrease at high RH can be rationalized as a result of the significant elimination of COCl2. On the other hand, the CO2 concentration was greatly reduced at 40% RH, indicating a less efficient oxidation of reaction intermediates to CO2. Two assumptions were postulated in the previous paragraph to justify this observation. The first one associated to the adsorption competition with reaction intermediates, seems to be inadequate as comparable concentrations of reaction intermediates in gas phase were found at both 0 and 40% RH (see Table 1). Moreover, the concentrations of reaction intermediates found in gas account for less than 5%, approximately, of the overall carbon mass balance (inlet dichlorvos concentration, 180 ppbv). The second supposition dealing with the formation of strongly bound surface species resistant to mineralization appears to be more appropriate because of the detection of a higher amount of adsorbed dichloroacetate and formate ions (HCOO-) at high RH. This means that, at high RH, a significant fraction of carbon mass balance remains strongly adsorbed onto the TiO2 surface and is very slowly converted to CO2 in the gas phase. 3.6. Mechanisms. On the basis of the identification results and literature data, a tentative reaction scheme for the photocatalytic degradation of dichlorvos in gas phase was VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. MS and FTIR Data for the Photocatalytic Degradation Products of Dichlorvos, Obtained with Different Relative Humidity (0 and 40%)a Conc. product Gas phase

CO CO2 HCl CHCl3 CCl4 DCA COCl2 TCA

Adsorbed on TiO2

DCAA DMP phosphate formate

IR data (frequencies, cm-1)

MS data (fragment, m/z)

2144 2360, 2340 2885 118, 83, 85 117, 119, 121, 82, 84 112, 114, 116, 84, 86, 49 (sdc) 1833, 1820 146, 111, 113, 84, 82 (sd) 1603, 1406, 1226 (sd) 126, 110, 94, 47 (sd) 1120, 1052 1357, 1376, 1582

0% RH

40% RH

85 ppbv 520 ppbv + + + 3 ppbv + 11 ppbv

30 ppbv 310 ppbv + ndb nd 18 ppbv traces 95%) even after 2 days of irradiation. Simultaneously, the mineralization dropped slightly from 50 to 42%. The decrease in the mineralization efficiency could be due to the adsorption of strongly bound surfaces and, especially, phosphate ions that incorporate into the TiO2 surface and lead to its deactivation. This was corroborated by the detection of significant amounts of phosphates adsorbed on the TiO2 surface at the end of experiment using ion chromatography. 3.8. Performance of TiO2/Activated Carbon. In an attempt to improve the treatment efficiency, an associated TiO2/AC paper (Ahlstrom, France) was tested for the degradation of dichlorvos (180 ppbv) at 40% RH. The first results were encouraging, showing no detectable reaction products in gas phase and a slight increase of mineralization from 50 to about 60%. Furthermore, no sign of catalyst deactivation was observed, even after one week of irradiation. On the other hand, the analysis of TiO2/AC surface after the irradiation experiment showed that the nature of the reaction intermediates and products is the same as for TiO2, indicating that reaction the mechanism has not been altered nor changed by the addition of AC. Moreover, a large accumulation of reaction intermediates such as DCA, DMP, and DCAA was found on the AC surface. Neverthless, a significant decrease of the quantities of intermediates adsorbed on AC was observed during the irradiation of TiO2/AC, in static regime. Consequently, the photoactivity enhancement seems to be related to two main processes: (a) the large adsorption capacity of AC, which retain some of reaction products and permits it to overcome the competition effect of water as already reported by Ao et al (30); and (b) the presence of intimate contact surface permitting the diffusion of a fraction of reaction intermediates adsorbed on the AC to the TiO2

surface during the photocatalytic process, due to the concentration gradient between AC and TiO2. However, to better understand the synergy effect of TiO2/AC, further experiments and different characterization techniques are required. 3.9. Implications of the Results. In summary, the promising results of this study suggest that photocatalysis could be a viable option for practical treatment of pesticides in indoor air. The only drawback lies in the production of some unwanted byproduct in the gas phase which can be potentially harmful. However, as showed in this work, these products represent less than 5% of the inlet pesticide concentration. Moreover, the association of photocatalysis and adsorption processes using a TiO2/AC filter proved to be an effective solution for improving the treatment efficiency and eliminating the gaseous byproduct. On the basis of the data provided in this work, scaling-up and optimization of an indoor air cleaner unit using a TiO2/AC filter to work at real-world conditions is currently under investigation.

Acknowledgments The authors are grateful to Joseph Dussaud (Ahsltrom) for supplying the photocatalytic papers. We also thank Dr. Daniel Bianchi for the loan of the FTIR experimental setup and Fabrice Arsac for his assistance.

Supporting Information Available Schematic diagram of the experimental setup used for the photocatalytic degradation of dichlorvos in dynamic regime at ppb levels; Effect of photolysis, flow rate, and oxygen content on the dichlorvos conversion. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Robinson, J., Nelson, W. C. National Human Activity Pattern Survey Data Base, U.S. EPA, Research Triangle Park, North Carolina, 1995. (2) Grossman, J. What’s hiding under the sink: Dangers of household pesticides. Environ. Health Perspect. 1995, 103, 550–554. (3) Environmental Protection Agency. An introduction to indoor air quality. Pesticides. http://www.epa.gov/iaq/pesticid.html. (accessed August 5, 2007). (4) Butte, W. Sources and impacts of pesticides in indoor environments. Indoor Air Pollution (Handbook of Environmental Chemistry), Pluschke P.; Springer: Berlin, Germany, 2004; pp 89–117.. (5) Nishioka, M. G.; Bukholder, H. M.; Brinkman, M. C. Distribution of 2,4-dichlorophenoxyacetic acid in floor dust throughout homes following homeowner and commercial lawn applications: quantitative effects of children, pets and shoes. Environ. Sci. Technol. 1999, 33, 1359–1365. (6) Wallace, J. C.; Brzuzy, L. P.; Simonich, S. L.; Visscher, S. M.; Hites, R. A. Case study of organochlorine pesticides in the indoor air of a home. Environ. Sci. Technol. 1996, 30, 2715–2718. (7) Simcox, N. J.; Fenske, R. A.; Wolz, S. A.; Lee, I. C.; Kalman, D. A. Pesticides in household dust and soil: exposure pathways for children of agricultural families. Environ. Health Perspect. 1995, 103, 1126–1134. (8) Anderson, D. J.; Hites, R. A. Chlorinated pesticides in indoor air. Environ. Sci. Technol. 1988, 22, 717–720. (9) Peral, J.; Domenech, X.; Ollis, D. F. Heterogeneous photocatalysis for purification, decontamination and deodorization of air. J. Chem. Technol. Biotechnol. 1997, 70, 117–140. (10) Zhao, J.; Yang, X. Photocatalytic oxidation for indoor air purification: A literature review. Build. Environ. 2003, 38, 645– 654. (11) Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Dichlorvos; Public Health Service, U.S. Department of Health and Human Services: Atlanta, Georgia, 1997. (12) Bouvier, G.; Blanchard, O.; Momas, I.; Seta, N. Pesticide exposure of non-occupationally exposed subjects compared to some occupational exposure: A French pilot study. Sci. Total Environ. 2006, 366, 74–91. VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(13) Ohura, T.; Amagai, T.; Senga, Y.; Fusaya, M. Organic air pollutants inside and outside residences in Shimizu, Japan: levels, sources and risks. Sci. Total Environ. 2006, 366, 485–499. (14) Arsac, F.; Bianchi, D.; Chovelon, J. M.; Ferronato, C.; Herrmann, J. M. Experimental microkinetic approach of the photocatalytic oxidation of isopropyl alcohol on TiO2. Part 1. Surface elementary steps involving gaseous and adsorbed C3HxO species. J. Phys. Chem. A 2006, 110, 4202–4212. (15) Tribustyn, D. A.; Voronstov, A. Experimental study of dimethyl methylphosphonate decomposition over anatase TiO2. J. Phys. Chem. B 2005, 109, 21884–21892. (16) Rusu, C. N.; Yates, J. T. Adsorption and decomposition of dimethyl methyl phosphonate on TiO2. J. Phys. Chem. B 2004, 12292–12298. (17) Fan, J.; Yates, J. T. Mechanism of photoxidation of trichloroethylene on TiO2: Detection of intermediates by infrared spectroscopy. J. Am. Chem. Soc. 1996, 118, 4686–4692. (18) Sleiman, M.; Conchon, P.; Ferronato, C.; Chovelon, J.-M. Iodosulfuron degradation by TiO2 photocatalysis: kinetic and reactional investigations. Appl. Catal. B. Environ. 2006, 71, 279–290. (19) Hugenschmidt, M. B.; Gamble, L.; Campbell, C. T. The interaction of H2O with a TiO2 (110) surface. Surf. Sci. 1994, 302, 329– 340. (20) Amama, P. B.; Itoh, K.; Murabayashi, M. Photocatalytic degradation of trichloroethylene in dry and humid atmospheres: role of gas-phase reactions. J. Mol. Catal. A: Chem. 2004, 217, 109–115. (21) Obee, T. N.; Brown, R. T. TiO2 photocatalysis for indoor air applications: effects of humidity and trace contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3butadiène. Environ. Sci. Technol. 1995, 29, 1223–1231. (22) Demeestere, K.; Dewluf, J.; Witte, B. D.; Langenhove, H. V. Titanium dioxide mediated heterogeneous photocatalytic deg-

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radation of gaseous dimethyl sufide: parameter study and reaction pathways. Appl. Catal. B: Environ. 2005, 60, 93–106. Ou, H. H.; Lo, S. L. Photocatalysis of gaseous trichloroethylene (TCE) over TiO2: The effect of oxygen and relative humidity on the generation of dichloroacetyl chloride (DCAC) and phosgene. J. Hazard. Mater. 2007, 146, 302–308. Hegedus, M.; Dombi, A. Gas-photocatalytic oxidation of chlorinated ethenes over titanium dioxide: perchloroethene. Appl. Catal. B: Environ. 2004, 53, 141–151. Hung, G. H.; Mariñas, B. J. Role of chlorine and oxygen in the photocatalytic degradation of trichloroethylene vapor on TiO2 films. Environ. Sci. Technol. 1997, 31, 562–568. Choi, W.; Hoffmann, M. R. Novel photocatalytic mechanisms for CHCl3, CHBr3, and CCl3CO2- degradation and the fate of photogenerated trihalomethyl radicals on TiO2. Environ. Sci. Technol. 1997, 31, 89–95. Feigenbrugel, V.; Le Person, A.; Le Calvé, S.; Mellouki, A.; Munoz, A.; Wirtz, K. Atmospheric fate of dichlorvos: photolysis and OHinitiated oxidation studies. Environ. Sci. Technol. 2006, 40, 850– 857. Lusaki, A. C.; Muggli, D. S. Photocatalytic oxidation of dichloroacetic acid and dichloroacetyl chloride on TiO2: Active sites, effect of H2O, and reaction pathways. Catal. Lett. 2003, 89, 732– 740. Evgenidou, E.; Konstantinou, I.; Fytianos, K.; Albanis, T. Study of the removal of dichlorvos and dimethoate in a titanium dioxide mediated photocatalytic process through the examination of intermediates and the reaction mechanism. J. Hazard. Mater. B 2006, 137, 1056–1064. Ao, C. H.; Lee, S. Indoor air purification by photocatalyst TiO2 immobilized on an activated carbon filter installed in an air cleaner. Chem. Eng. Sci. 2005, 60, 103–109.

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