Environ. Sci. Technol. 2008, 42, 4540–4545
AgNO3-Induced Photocatalytic Degradation of Odorous Methyl Mercaptan in Gaseous Phase: Mechanism of Chemisorption and Photocatalytic Reaction T O N G - X U L I U , †,‡ X I A N G - Z H O N G L I , * ,† AND FANG-BAI LI‡ Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, and Guangdong Key Laboratory of Agricultural Environment Pollution Integrated Control, Guangdong Institute of Eco-Environment and Soil Science, Guangzhou 510650, People’s Republic of China
Received December 13, 2007. Revised manuscript received February 04, 2008. Accepted March 31, 2008.
In this study, AgNO3 films prepared by a simple dip-coating method were used to remove gaseous methyl mercaptan (CH3SH) for odor control. The AgNO3 films were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy/energydispersiveX-rayspectrometry(SEM/EDX),andX-rayphotoelectron spectroscopy (XPS) before and after the reaction, and asobtained products were identified by means of gas chromatography/mass spectrometry (GC/MS) and ion chromatography. The experiments demonstrated that the AgNO3 film can induce a quick chemisorption of gaseous CH3SH to form AgSCH3 and other intermediate products such as R-Ag2S, Ag4S2, and AgSH on its surface. Under UVA illumination, these sulfur products can be photocatalytically oxidized to AgSO3CH3 and Ag2SO4. Then AgSO3CH3 and Ag2SO4 will continue the chemisorption of gaseous CH3SH, similar to AgNO3, to form AgSCH3 again and release two final products, HSO3CH3 and H2SO4. Hence it is a AgNO3-induced photocatalytic reaction for odorous CH3SH degradation in gaseous phase. This fundamental research about the mechanism of chemisorption and photocatalytic reaction provides essential knowledge with potential to further develop a new process for gaseous CH3SH degradation in odor control.
Introduction Mercaptans are highly toxic and corrosive as a group of offensive odorous compounds with low odor detection thresholds. Among them, methyl mercaptan (CH3SH) may be a representative member with a very low odor threshold of around 0.4 ppb (1). Several techniques have been studied for removing CH3SH from a gaseous phase such as catalytic oxidation, adsorption, catalytic incineration, radiolytic decomposition, and biological degradation (2, 3). Recently, Satokawa et al. (4) reported that Ag-exchanged Y zeolite * To whom correspondence should be addressed. Tel: +85227666016; fax: +852-23346389; e-mail:
[email protected]. † The Hong Kong Polytechnic University. ‡ Guangdong Institute of Eco-Environment and Soil Science. 4540
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
(Ag-Y) was a favorable adsorbent to remove sulfur compounds from pipeline natural gas fuel under ambient conditions even in the presence of water vapor, which can be a new desulfurization process for polymer electrolyte fuel cells (PEFC). A few reports also demonstrated that some Agexchanged zeolites are effective for removing sulfur compounds such as dimethylsulfide, tetrahydrothiophene, and thiophenes (5). Shimizu et al. (6) studied the removal of tertbutanethiol under ambient conditions with silver nitrate supported on silica and silica-alumina, including its reaction mechanism. Unfortunately, silver nitrate, as an adsorbent, can be easily saturated and disabled. Alternatively, it has been confirmed that mercaptans can be strongly adsorbed by some metals such as gold or silver to form self-assembled monolayers (SAMs) and undergo quick degradation upon exposure to UV light irradiation in air to form sulfonate or sulfate groups eventually (7). However, such a feature attracted intensive research interests to study some fundamental interfacial interactions among adhesion, wetting, molecular recognition, and other biological interactions and also the fundamental research of novel microstructured organic materials and devices (8). To the best of our knowledge, there is a lack of research on the photocatalytic oxidation of thiolates using silver for odor treatment based on the above-mentioned characters. In this study, a novel approach to photocatalytic oxidation of the thiolates with silver under UV illumination was proposed to remove CH3SH for odor control. The possible mechanism of chemisorption and photocatalytic reaction in this new process was studied in detail.
Experimental Section Preparation of AgNO3 Film Sheets. AgNO3 (analytical grade) was purchased from Johnson Matthey Chemicals Ltd. and used to prepare AgNO3 films on Whatman glass fiber sheets with an area of 18 cm × 26 cm, in which the AgNO3 solution (0.01 g of AgNO3 in 25 mL of distilled water) was uniformly dripped on the glass fiber sheet with a catalyst loading of 0.21 g m-2. Then the prepared AgNO3 films were dried in an oven at 70 °C for 24 h in the dark. Experimental Setup. All experiments of adsorption and photocatalytic oxidation of CH3SH in a gaseous phase were conducted in a batch photoreactor system equipped with three UVA lamps (light intensity ) 1.24 mW cm-2) and a CH3SH analyzer (Detcon DM-100-CH3SH) with a detection limit of 0.1 ppm. The synthetic odorous gas was prepared by mixing CH3SH and zero air gases in a gas mixing chamber and its humidity was controlled at 52% ( 2%, before it was introduced into the photoreactor. A detailed description of the experimental setup is present in the Supporting Information as Figure S1. Experimental Procedure. (1) In the experiments of chemisorption and photocatalytic reaction, the CH3SH gas with an initial concentration of about 40 ppm (about 55 µmol) was purged into the photoreactor and was kept in the dark until it reached a gas-solid adsorption/desorption equilibrium. Then UVA light was turned on and photoreaction took place. After the CH3SH concentration was reduced to below 0.1 ppm, the light was turned off and the fresh CH3SH gas was injected into the photoreactor to make the same initial concentration again. The above experimental procedure was repeated for five cycles to monitor the variation of CH3SH concentration inside the photoreactor (2). To characterize the AgNO3 films affected by the chemisorption and photocatalytic reaction, four AgNO3 films were prepared by equally coating glass sheets with 0.15 g of AgNO3 and then drying the 10.1021/es7031345 CCC: $40.75
2008 American Chemical Society
Published on Web 05/14/2008
sheets at 70 °C for 12 h. One of the AgNO3 films was used as a blank sample, named “AgNO3”. The other three AgNO3 films were exposed to CH3SH for 4 h in the dark to reach a gas/solid adsorption equilibrium. One of them was named “Ag-MM-dark” and the other two, which reacted with CH3SH under UVA illumination for 5 and 165 h, respectively, were named “Ag-MM-UVA5” and “Ag-MM-UVA165” (3). Since the adsorbed CH3SH on the AgNO3 film can be oxidized to some ionic species such as CH3SO3- and SO42- under UVA illumination, an experiment to separate chemisorption and photocatalytic reactions was conducted with the following procedure: (i) A AgNO3 film sheet consisting of seven small pieces was placed in the photoreactor, in which each piece of AgNO3 films had a size of 4 cm2 and 0.05 g of AgNO3 equally and could be individually taken out during the photocatalytic reaction as required. (ii) The synthetic CH3SH gas was purged into the photoreactor to make up an initial CH3SH concentration of 100 ppm, and the CH3SH was chemically adsorbed by the AgNO3 film in the dark. This step was repeated 5 times until the surface of AgNO3 film was fully covered by adsorbed CH3SH. According to the CH3SH concentrations before and after each chemisorption in the photoreactor, the amount of adsorbed CH3SH on the AgNO3 film was determined to be 96 µmol/piece. (iii) After chemisorption, the photoreactor was refilled with fresh air to expel all residual CH3SH gas and then UVA lamps were turned on to conduct the photocatalytic reaction for up to 165 h. The AgNO3 film samples were taken out piece by piece at different time intervals for further analysis. The experiment was repeated twice, and the film samples were extracted in 20 mL of 1 M nitrate solution at 25 °C for 48 h and then filtered through a 0.22-µm Millipore filter. The dissolved ions in the nitrate solution were analyzed by ion chromatography (IC). Analytical Methods. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectrometry (EDX), and X-ray photoelectron spectroscopy (XPS) were employed to fully characterize the AgNO3 films. Gas chromatography/mass spectrometry (GC/MS) and IC analyses were used to identify the intermediate products. Details of the analytical conditions are included in the Supporting Information.
Results and Discussion Chemisorption and Photocatalytic Reaction of CH3SH on AgNO3 Film. Experimental results about the reduction of CH3SH in gaseous phase versus time in five cycles, including the first cycle in the dark and the following four cycles under UVA illumination, are shown in Figure 1a. The results showed that CH3SH after the first injection disappeared quickly in the dark due to chemisorption, but its adsorption rate during the dark period gradually reduced in the following four cycles significantly. The results also revealed that the reduction of CH3SH under UVA illumination was well maintained in the following four cycles with a similar pattern. To distinguish the CH3SH reduction on the AgNO3 film by chemisorption alone from the chemisorption/photoreaction, the experimental data were further calculated to determine the firstorder rate constants (k) of CH3SH reduction in the dark and under UVA illumination. The k values for chemisorption alone under dark conditions varied from k ) 0.3202 min-1 in the first cycle and k ) 0.0347 min-1 in the second cycle down to k < 0.01 min-1 in the following cycles. These results indicate that the chemisorption of CH3SH on the AgNO3 film was quickly saturated after the first cycle. However, consistent k values under UVA illumination in the four cycles were determined to be 0.0567, 0.0588, 0.0615, and 0.0573 min-1, respectively, which indicated that the saturated AgNO3 film could be refreshed under UVA illumination due to the photocatalytic reaction. Hence, a combination of chemisorption and photocatalytic reaction on the AgNO3 film would
FIGURE 1. (a) Time course of the decomposition of CH3SH with silver in the photoreactor. (b) Odor removal efficiency with initial CH3SH concentration of 3 ppm for up to 60 min. be a good approach to reduce CH3SH from gaseous phase continuously. To evaluate the efficiency of odor removal at a lower strength, one more experiment was conducted under the same experimental conditions with an initial CH3SH concentration of 3 ppm for 60 min. Since the applied CH3SH analyzer has a detection limit of 0.1 ppm, an olfactometry analysis using a forced-choice dynamic olfactometer (Olfactomat-n2) with a detection limit of 10 ou/m3 in accordance with the European Standard Method (EN13725) was employed to determine odor concentration when CH3SH concentration was below 0.1 ppm. The experimental results as shown in Figure 1b revealed that the initial odor concentration of 8228 ou/m3 ([CH3SH]0 ) 3 ppm) was quickly reduced to 402 ou/m3 at 15 min, 116 ou/m3 at 30 min, and
