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Photocatalytic UV-degradation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the presence of silver doped zeolite
⁎Corresponding author. Tel.: +971 6 515 2409; fax: +971 6 515 2450. skanan@aus.edu (Sofian M. Kanan)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Silver incorporated into the Y zeolite framework was prepared and characterized. Low temperature luminescence analysis indicates the formation of silver-silver excimers (excited state dimers) and exciplexes (excited state trimers) that were found to be activated at 250 nm and 300 nm, respectively. The catalytic activity of the modified material was tested toward the photodecomposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The site selective activity was evaluated using two sources of UV irradiation at 254 and 302 nm. The catalyst was found to effectively decompose TCDD in a methanol/tetrahydrofuran solution to reach 86% after 5 h. In the presence of the catalyst, a hydroxyl derivative was identified as an intermediate for the 302 nm catalytic pathway. This was not observed for the 254 nm process. Furthermore, irradiating of TCDD under 254 nm UV source for 6 h resulted in the formation of three products with molecular ion peaks of 113, 128, and 158 amu. On the other hand, the 302 nm gives two major products with molecular ion peaks of 220 and 252 amu. The study also indicates that the hydroxyl derivative could be potentially more toxic than the parent TCDD while all other isolated products were found to be less toxic than TCDD.
Keywords
Silver
Zeolite
Catalyst
Photodegradation
UV light
TCDD
1 Introduction
Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are widely distributed combustion products found in air, water, and sediments/soil. The physical and chemical properties of these congeners tend to vary according to the degree and position of the chlorine substitution in the structure (Kim and Keefe, 2000; Kulkarni et al., 2008; Lohmann and Jones, 1998). PCDDs/Fs in the environment occur as a result of combustion processes of landfills, agricultural, household, sludge, and medical wastes (Kim and Keefe, 2000; Kulkarni et al., 2008; Lohmann and Jones, 1998). They are often formed as trace amounts of undesired impurities in the synthesis of organochlorine chemicals such as the polychlorinated biphenyls and chlorinated phenols, or any of their relative derivatives. Their distribution in the environment is mainly due to their persistency and bioaccumulation in the food chain. Several studies have reported that 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), which is the most toxic of all PCDDs/Fs, causes chloracne in humans, skeletal deformities, kidney defects, and weakened immune responses (ATSDR, 1998).
Furthermore, due to the toxicity of these compounds and their high availability in the environment it is important to study methods for the environmental decontamination of PCDDs/Fs. Several studies have discussed the degradation of PCDDs/Fs on a laboratory scale, including UV-light, sunlight and γ-ray irradiation (Ukisu and Miyadera, 2002; Kieatiwong et al., 1990; McPeters and Overcash, 1993; Hilarides et al., 1994). The photolysis process permits the transformation of PCDDs/Fs to lower chlorinated compounds or degradation products, causing a decrease in their toxicity. Photolysis experiments of TCDD using water are extremely challenging due to their low water solubility, which is reported in the range of 0.2–0.008 μg/L. For this reason, such experiments are typically carried out using organic aqueous solutions (Kim and Keefe, 2000).
Materials such as the mixed-phase TiO2 electrode, highly ordered TiO2 and Co3O4/TiO2 nanotube arrays, have shown to photocatalytically oxidize a wide spectrum of organic compounds in an effective manner (Dai et al., 2013; Zhang et al., 2011; Jiang et al., 2007; Mills et al., 2005). In addition, zeolites are aluminosilicates with well-defined pore and channel structures commonly used as commercial adsorbents for water purification and as catalysts in the petroleum industry. Incorporating transition metals in the synthesis of zeolites was found to increase their catalytic activity toward the decomposition of toxic agents in environmental systems (Kanan et al., 2003, 2006, 2007, 2010, 2011; Kanan and Nusri, 2014; Ahern et al., 2014).
The objective of this paper was to examine the effect of silver nanoclusters doped into zeolites on the photodecomposition of 2,3,7,8-TCDD using high and mid-energy UV lights. In addition, enzyme linked immune sorbent assay (ELISA) was used to estimate optical density (OD), in order to predict possible toxicity changes of the 2,3,7,8 TCDD due to the exposure to UV irradiation. ELISA measures the ability of an analyte molecule to bind to the anti-dioxin antibody in comparison with a standard molecule. In this specific ELISA, the test response is a competitive inhibition of a polyclonal antibody specific to 2,3,7,8-tetrachlorodibenzo-p-dioxin and is strongest for analyte structures which are most similar to the target. Moreover, gas chromatography/mass spectrometry (GC/MS) analysis was performed to predict the products formed as a result of the irradiated 2,3,7,8-TCDD.
2 Experimental
2.1 Reagents and solvents
A standard solid powder of 2,3,7,8-TCDD was purchased from Accustandard (New Haven, CT, USA) and dissolved in a solution of 1:1 Methanol/Tetrahydrofuran. All solvents were HPLC grade purchased from Aldrich Chemical Company.
2.2 Irradiation of 2,3,7,8-TCDD
Photodecomposition of 2,3,7,8-TCDD was monitored using a Varian fluorescence spectrometer. A 10-ppm 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) stock solution was prepared in a 1:1 (v:v) tetrahydrofuran (THF)/methanol mixture. All irradiations were performed with a UV lamp (model UVS-28) from VWR Scientific, Inc. at 254 nm and 302 nm. The relative intensity of each lamp is 1300 W/cm2 at 3 in. Each sample was irradiated in quartz test tubes with an internal diameter of 12.5 mm, a length of 10 mm, and 1 mm wall thickness with only one test tube being irradiated at a time. The 2,3,7,8-TCDD solutions were prepared and exposed to UV light at a distance of 3.0 in., where a maximum output of the lamp was reached.
2.3 Silver-zeolite preparation
The AgY catalyst was prepared using the ion-exchange method previously reported (Kanan et al., 2003, 2010; Ahern et al., 2014) In brief, 1.0 g of the sodium salt of zeolite Y was mixed with a solution containing 10 mmol of Ag(NH3)2+ at 70 °C for about 48 h. Finally, the AgY zeolite sample was filtered, washed three times with distilled water and then dried in an oven at 100 °C for 24 h. The sample was then treated at 300 °C for 2 h and stored in the dark. The amount of silver loaded on Y-zeolite was determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The analysis shows a silver content of 5.5%.
2.4 Low temperature luminescence measurements
Solid state luminescence measurements of the AgY samples were recorded using QM-1 luminescence spectrometer from Photon Technologies International (PTI) at 77 K. The instrument is equipped with an emission monochromator and double excitation monochromators, a 75 W xenon arc lamp, and a photomultiplier tube serving as the detector. For the luminescence measurements, the samples were made into a pressed pellet and attached to a copper surface. The copper plate was then attached to the transfer tube for the low temperature experiments.
2.5 GC/MS experiments
Gas chromatography/mass spectrometry (GC/MS) was used to identify the products of irradiated 2,3,7,8-TCDD. Measurements were made on a Varian CP-3800 Gas Chromatograph with a Varian Saturn 2000 GC/MS/MS serving as the detector. A 30 m × 0.25 mm ID Factor Four VF-5 MS column from Varian was used. Components of various samples were separated using the following parameters: injector temperature set at 150 °C, and detector temperature set at 320 °C. The initial oven temperature of 150 °C was held for 3 min. Then the temperature was ramped to 150 °C at a rate of 20 °C/min and held constant for 3 min. Finally, the temperature was ramped to 300 °C at the rate of 20 °C/min and held constant for 3 min. Helium was used as the carrier gas with a flow rate of 1.0 mL/min.
2.6 Enzyme Linked Immunosorbent Assay (ELISA) experiments
ELISA kit was purchased from Cape Technologies (South Portland, ME, USA). Experiments were carried out by evaporating irradiated TCDD samples under a stream of nitrogen in 2 mL amber glass vials, using a specific keeper solution (100 ppm Triton X-100 in 80:20 methanol:tetraethylene glycol (PEG)) provided in the kit, with subsequent sample reconstitutions in methanol. The PCDD/F ELISA which uses a rabbit polyclonal antibody has been previously described (Cape Technologies, 2006; Carlson and Harrison, 1998; Samara et al., 2010). Standards in the methanol/keeper solution are transferred into the antibody-coated tubes to which an aqueous sample diluent has already been added. This mixture is incubated overnight to allow capture of the analyte by the immobilized antibody. After washing the tubes with a 0.01% v/v Triton X-100 in water solution, a conjugate of a dioxin-like competitor coupled to the enzyme horseradish peroxidase is introduced to compete for the available binding sites on the antibody (those not occupied by the analyte). The amount of horseradish peroxidase-competitor conjugate bound is inversely proportional to the logarithm of the TCDD concentration in the sample incubation step. After 15 min of incubation time, tubes are washed with water, enzyme substrate is added to the tubes, and color is generated by the captured horseradish peroxidase-competitor conjugate in direct proportion to the amount captured. After 30 min of incubation time, the stop solution is added to arrest color development. Finally, optical density (OD) readings are obtained using a tube reader or spectrophotometer at 450 nm. OD results were compared to a 2,3,7,8-TCDD standard curve run in parallel.
3 Results and discussion
Silver nanoclusters incorporated into zeolite Y were prepared via an ion exchange process and characterized using ICP-AES, SEM–EDX, and low temperature steady state luminescence spectroscopy (Ahern et al., 2014). Three-dimensional excitation emission matrix (EEM) spectra composed of sets of emission scans at consecutive excitation wavelengths were collected at 77 K. These matrices form a 3D spectra with excitation on the x axis, emission on the y axis, and signal intensity on the z axis. Excitation data were collected between the range of 530–200 nm at an increment of 3 nm while, the emissions were collected for each excitation between 230 nm and 970 nm at an increment of 1 nm. Distinct high energy (HE) and low energy (LE) emission bands strongly dependent on the excitation wavelength were observed, as shown in Fig. 1, a top-down view of an EEM scan of the AgY zeolite. Since the Ag+ ion is of a d10 configuration where the transition is spin forbidden, this result indicates the formation of a site selective excitation that is solely dependent on the silver-silver aggregation (Kanan et al., 2011). The formation of silver-silver aggregate is responsible for the observed luminescence bands in the prepared sample. Therefore, we expected to have various catalytic properties upon varying the excitation wavelength since each Ag-Ag oligomer can be activated at certain wavelength. This catalytic activity was tested at two excitation wavelengths namely, 254 nm and 302 nm. Exciting the catalyst at 254 nm was found to pronounce the HE emission band which is assigned to the excited state dimers (excimers) whereas; exciting the catalyst at 302 nm enhanced the LE emission band that is assigned for the excited state trimers (exciplexes).
To test the catalytic properties of the silver modified sample, 2,3,7,8-TCDD was selected due to its known toxicity and environmental spread. In specific, the photodegradation of TCDD was studied in the presence and the absence of the catalyst upon irradiation at 254 nm and 302 nm. A 10 ppm solution of 2,3,7,8-TCDD shows a strong emission band at 320 nm and a weak band at 345 nm upon excitation at 280 nm as shown in Fig. 2. Both emissions correspond to the same excitation that was observed at 275–280 nm and assigned to the intense π to π∗ transition. Synchronous scan luminescence spectra (SSLS) involve scanning the excitation and emission wavelength monochromators simultaneously with a constant wavelength difference between them. This technique was selected because it is more selective than ordinary fluorescence methods for the analysis of fluorescing mixtures. In addition, the observed bands in the SSLS are well defined compared to the emission bands observed in the ordinary luminescence, hence; it will provide clear and accurate kinetic monitoring. Fig. 3 represents the SSLS of the TCDD solution at Δλ = 30 nm before and after being irradiated with 254 nm UV light at various times. The spectra show clear luminescence bands at approximately 275–280 nm and 310 nm and also results in the presence of two isosbestic points at 295 nm and 340 nm, indicating the formation of two new compounds. The luminescence intensity of 2,3,7,8-TCDD’s peak (observed at 275–280 nm) decreases gradually during the irradiation for 90 min where a max decomposition of 72% was reached. However, irradiating the sample for longer times tend to enhance the peak intensity that corresponds to the starting material. This observation indicates that long time exposure to the UV-light’s energy enhances the propagation process of the generated radicals and thus regenerates the starting material. In addition, upon exciting the 2,3,7,8-TCDD solution at 302 nm, similar effects were observed but with slower degradation process. GC–MS analysis showed two major GC bands with molecular ion peaks at 322 and 252 amu assigned for TCDD and dichlorodibenzo-p-dioxin (DCDD) derivatives, respectively.
The photodecomposition of TCDD, one of the most toxic molecules of environmental scientists concern, was also investigated in the presence of the AgY catalyst. It was observed that the photodecomposition of 2,3,7,8-TCDD follows an integrated pseudo first order rate law. However, the presence of the AgY catalyst was found to enhance the photodecomposition of TCDD when compared to the uncatalyzed TCDD. Fig. 4 shows the SSLS recorded at Δλ = 30 nm for TCDD irradiated at 254 nm with AgY catalyst as a function of irradiation time, where a luminescence band was observed with a maximum intensity at 275 nm. The addition of the AgY catalyst leads to a decrease in the band intensity since TCDDs tend to be adsorbed on the catalysts’ surface. The amount of the adsorbed TCDD on the catalyst is estimated to reach 18.5% of the initial concentration. Upon irradiation, the 275 nm bands’ intensity gradually decreases, while a new band at a higher wavelength was observed. Interestingly, the 275 nm band intensity gradually decreases even when the TCDD sample was irradiated for longer times. Therefore, the presence of the AgY catalyst inhibits the TCDD regeneration by adsorbing the produced derivatives or by capturing the chloride radicals by the available silver ion sites. Fig. 5 shows the SSLS of TCDD irradiated with 254 nm light source for 6 h with and without the catalyst. It clearly shows that 86% of the irradiated TCDD decomposed in the presence of the catalyst. Recall that the TCDD decomposition with UV light alone reached to 72% upon irradiation for 90 min (see Fig. 3) but this decomposition was declined to 28% after the light exposure for 5 h (see Fig. 5).
Interestingly, the photodecomposition of TCDD upon irradiation with 302 nm UV light in the presence of the AgY catalyst shows a different profile than the results obtained using the high energy UV source. Fig. 6 shows the SSLS of TCDD irradiated for 302 nm in the presence of the catalyst at different period of times, the spectra shown in this figure presented two profiles. The first trend shows a gradual and slow reduction in the TCDD bands’ intensity as a function of time (see Fig. 6b) for the first 20 min. The second profile was observed upon irradiation from 30 through 150 min (see Fig. 6c). As shown in Fig. 6, the TCDD band at 275 nm pronounced a red shift upon UV irradiation for 30 min. This shift indicates the formation of a new product, which is potentially a very strong luminophore since the intensity of this band is much larger than the initial 2,3,7,8-TCDD band intensity even without adding the catalyst under similar conditions. The new generated product tends to decompose upon further irradiation with 302 nm as shown in Fig. 6c.
The change observed in the pattern between 30 and 50 min was further analyzed using GC/MS to offer a plausible explanation for this variation. The break in trend between 0–30 min and 50–90 min could be explained due to the byproducts formed during the decomposition of the TCDDs. Yet, these products must be PCDDs/Fs as the emission intensity is well formed at 290 nm. Furthermore, this change was observed until 90 min of irradiation exposure at which the intensity dropped back to the initial exposure level. On the other hand, this change was not observed with TCDD irradiation without the catalyst presence; hence, the silver doped catalysts have a direct effect on the 2,3,7,8-TCDD decomposition patterns. These findings are further elaborated in the immunoassay toxicity equivalent part of the study.
Fig. 7a, shows the GC/MS profile of the TCDD initial solution, where the TCDD peak eluted after 9.986 min. Mixing of the TCDD solution with the AgY catalyst under the exposure to 302 UV light for 20–40 min results in a reduction of the 9.986 min peak intensity that is associated to the formation of a new peak at 16.587 min (Fig. 7b) which disappeared after 40 min of the UV exposure as indicated in Fig. 7c. The mass spectrum of the 16.587 min peak presented in Fig. 8 indicating a molecule with a major molecular ion peak of 340 amu and major ion fragments at 284, 177, 158, and 148 amu. Analysis of the presented Mass profile indicates the formation of a hydroxyl derivative (Compound (1)) as shown in Scheme 1. This peak was clearly pronounced upon irradiating the TCDD at 302 nm only because the larger size Ag-Ag exciplexes were activated and thus provide a good fit to establish the hydrolysis process of the cyclic ether moiety in TCDD. Irradiation of Compound (1) for a longer time regenerated the starting TCDD molecule, which undergoes further degradation upon exposure for longer periods of time. Scheme 1 shows a proposed mechanism that describes the transformation of 2,3,7,8-TCDD with the presence of the catalyst upon irradiation at 302 nm. Finally, GC/MS analysis of the irradiated catalyzed solution at 302 nm indicates the formation of two major products with molecular ion peaks at 220 and 252 amu. These peaks represent the formation of hydroxyl-chloro diphenyl ether (TCDD-OH) and a dichlorodibenzo-p-dioxin (DCDD) derivative that seems to be stable even when the irradiation under 302 nm was extended for 15 h (See Scheme 2). Moreover, these products were previously observed when 2,3,7,8-TCDD was irradiated for several different lengths of time as shown in Fig. 3.
In contrast, the catalyzed solution that underwent 254 nm UV irradiation yields similar products upon irradiation for 60 min with very weak peak intensity for Compound (1). Further irradiation of the solution to 15 h resulted in the formation of three products with major molecular ion peaks at 113, 128, and 158 amu. Molecular mass analyses of these compounds show the presence of chlorobenzene, chlorophenol, and methoxychlorophenol, respectively (see Scheme 2).
After the fluorescence results were obtained, several samples were selected for confirmation using enzyme linked immunosorbent assay ELISA. In ELISA, the test response is a competitive inhibition of a polyclonal antibody specific to 2,3,7,8-tetrachlorodibenzo-p-dioxin and is strongest for analyte structures which are most similar to the target. The specificity of the test is predominant for PCDDs/Fs that contain 3–6 chlorines, with a strong preference for the 2,3,7,8-Cl-substituted congeners (Samara et al., 2010). After each ELISA batch analysis, the % negative control (NC) value (optical density (OD) as a percent of the negative control OD) is calculated for each standard and sample. The response values are expressed as a percentage of the negative control, which is 100 ppm Triton X-100 in 80/20 methol/PEG. In this report we present the data in terms of the optical density of each sample against the 2,3,7,8-TCDD standard.
The ELISA results are based on the optical density of the sample in which a higher OD, a less toxic tested sample, is observed by a decrease in color and its intensity. Fig. 9, shows the ODs obtained for the samples in the presence of the catalyst at 254 nm and 302 nm between 0 and 60 min irradiation. The graph shows a slight decrease in optical density at 20 min, followed by a constant increase for the material irradiated using 254 nm light. The results suggest that there was an increase in toxicity at 20 min but declined thereafter. The same profile appeared for the solution irradiated with 302 nm light for 20 min. However, the OD continued to decline upon further irradiation under 302 nm UV light until 40 min. This increase in toxicity is in parallel to the increase that was observed for the GC peak that corresponds to Compound (1). Our results suggest the formation of a TCDD-OH adduct that might potentially be more toxic than its parent compound. To our knowledge, no other studies have reported on the potential toxicity of this adduct. Furthermore, most studies report on the atmospheric oxidation mechanisms of TCDD, where the formation of the TCDD-OH adduct is an intermediate step in the degradation process (Wang and Tang, 2011). Finally in each set of experiments upon irradiation for longer than 40 min, showed a gradual increase in optical density suggesting a decrease in toxicity. As shown in Scheme 2, two major products were identified namely; DCDD and chlorophenyl phenol-ether derivatives. Both derivatives were found to be less toxic than the parent TCDD molecule (Van den Berg et al., 2006).
4 Conclusions
Silver modified zeolite Y has proven capable of degrading 2,3,7,8-TCDD, the most toxic PCDD/F derivative, in a methanol/THF solution. In this study, the presence of silver loaded on the zeolite decomposes 2,3,7,8-TCDD by 86% after UV irradiation at 254 nm for 6 h. This is a noticeable increase over UV irradiation alone which reduced the TCDD concentration by 28% within the same time frame. Enzyme linked immunosorbent assay (ELISA) testing indicates the formation of potentially toxic intermediates that appeared within 40 min of the irradiation for 302 nm in the presence of the catalyst, but all resultant compounds were less toxic than the parent 2,3,7,8-TCDD. The results of this study open doors for the possibility of utilizing zeolites for decontamination of environmental sources containing PCDDs/Fs or dioxin-like contaminants.
Acknowledgments
The authors would like to thank the American University of Sharjah for the support of this work. Also we thank Professor Howard Patterson for providing an access for solid state low temperature luminescence measurements.
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