Photoassisted Fenton Degradation of Polystyrene - American

Dec 15, 2010 - Chemistry, City University of Hong Kong, 83 Tat Chee Avenue,. Kowloon, Hong Kong SAR, China, and Department of. Chemistry, University o...
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Environ. Sci. Technol. 2011, 45, 744–750

Photoassisted Fenton Degradation of Polystyrene H U I - M I N F E N G , †,‡,§ J I A - C H U A N Z H E N G , †,‡,§ N G A I - Y U L E I , ‡,§ L E I Y U , †,‡,§ K A R E N H O I - K U A N K O N G , § H A N - Q I N G Y U , * ,†,§ T A I - C H U L A U , †,‡ A N D M I C H A E L H . W . L A M * ,†,‡ Advanced Laboratory for Environmental Research & Technology, USTC-CityU, Suzhou 215123, China, State Key Laboratory in Marine Pollution, Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China, and Department of Chemistry, University of Science & Technology of China, Hefei 230026, China

Received July 5, 2010. Revised manuscript received November 29, 2010. Accepted November 30, 2010.

Fenton and photoassisted Fenton degradation of ordinary hydrophobiccross-linkedpolystyrenemicrospheresandsulfonated polystyrene beads (DOWEX 50WX8) have been attempted. While the Fenton process was not able to degrade these polystyrene materials, photoassisted Fenton reaction (mediated by broad-band UV irradiation from a 250 W Hg(Xe) light source) was found to be efficient in mineralizing cross-linked sulfonated polystyrene materials. The optimal loadings of the Fe(III) catalyst and the H2O2 oxidant for such a photoassisted Fenton degradation were found to be 42 µmol-Fe(III) and 14.1 mmolH2O2 per gram of the sulfonated polystyrene material. The initial pH for the degradation was set at pH 2.0. This photoassisted Fenton degradation process was also able to mineralize commonly encountered polystyrene wastes. After a simple sulfonation pretreatment, a mineralization efficiency of >99% (by net polymer weight) was achieved within 250 min. The mechanism of this advanced oxidative degradation process was investigated. Sulfonate groups introduced to the surface of the treated polystyrene polymer chains were capable of rapidly binding the cationic Fe(III) catalyst, probably via a cationexchange mechanism. Such a sorption of the photoassisted Fenton catalyst was crucial to the heterogeneous degradation process.

Introduction Practicable and cost-effective treatment of municipal plastic wastes has been a long-standing challenge to our modern society (1). Since after World War II, synthetic organic polymer materials have been widely used in the mass production of consumer products. Even up to now, most of the plastic * Corresponding author phone: +852 2788 7329; fax: +852 2788 7406; e-mail: [email protected]. Corresponding author address: State Key Laboratory in Marine Pollution, Department of Biology and Chemistry, City University of Hong Kong (M.H.W.L.). Corresponding author phone: 86 551 3607592; fax: 86 551 3607592; e-mail: [email protected]. Corresponding author address: Department of Chemistry, University of Science & Technology of China (H.-Q.Y.). † USTC-CityU. ‡ City University of Hong Kong. § University of Science & Technology of China. 744

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materials in use are of very poor degradability and tend to accumulate in our environment. Besides creating huge solid waste management problems on land, plastic marine debris is also becoming a significant environmental problem to our oceans and coastal waters (2-4). Polystyrene is one of the major components of municipal plastic wastes as it is widely being used as packaging materials for food and consumer products. In fact, the Asia-Pacific region has shown the most rapidly growing consumption of polystyrene in the recent decade. The global production of polystyrene in 2008 has reached 15.4 million metric tons and around 20% of this was consumed in China. This translated to some 10 billion of polystyrene containers being utilized and, subsequently, disposed of. Recovery and recycling of polystyrene may not be economically viable because of the volatile market price of the virgin plastic (5-7). Waste-to-energy conversion via incineration and liquid fuel production via catalytic cracking (8-12) have been proposed as alternative waste polystyrene treatment strategies to recycling and landfill disposal, although they are generally costly and prompt to generate secondary pollution, such as the emission of polycyclic aromatic hydrocarbons and dioxins (13-15). Innovative and environmentally feasible treatment technologies for polystyrene wastes are urgently needed. Advanced oxidation for environmental treatment refers to those oxidative processes that generate highly oxidizing species, such as hydroxyl radicals ( · OH), under moderate conditions, for the destruction and ultimate mineralization of targeted contaminants (16, 17). Despite the fact that numerous advanced oxidative processes have been developed for the treatment of organic contaminants in natural waters and wastewaters (17-20), there were only a few attempts on polystyrene. Pioneering works on the heterogeneous photocatalytic degradation of polystyrene by titanium dioxide (TiO2) have demonstrated the solid-phase photodegradation of polystyrene by blending selected photocatalysts with the polymer to form composite materials (12, 21-23). Nevertheless, this approach may be feasible in the development of photodegradable plastics for future applications; it cannot deal with those ordinary polystyrene wastes that have already been disposed of. The Fenton and photoassisted Fenton processes generate highly reactive oxidizing hydroxyl radicals in situ from the environmental friendly oxidant H2O2 in the presence of iron salt catalysts (24-26). These Fenton-type reactions have been widely utilized in the advanced oxidative environmental treatment of waters, wastewaters, and soils (27-31). However, to the best of our knowledge, their applications to the degradation of solid plastic wastes have not been explored. In this work, we study the feasibility of polystyrene mineralization by Fenton and photoassisted Fenton reactions. Our results showed that photoassisted Fenton degradation of polystyrene can be brought about by the grafting of cation-exchanging functionalities onto the polymer substrate.

Experimental Section Materials and Methods. Cross-linked polystyrene (PS) microspheres were synthesized according to literature method (32). Sulfonated cross-linked polystyrene beads (DOWEX 50WX8) (CS-PS) was purchased from Dow. Ferric chloride (Riedel), 30% hydrogen peroxide (UNI-CHEM), hydrochloric acid, and sulfuric acid (Riedel) were used without further purified. Broad-band UV irradiation was provide by a 250 W Hg(Xe) light source (Model 66485) with a digital power supply system (Model 69911) by Newport Inc. 10.1021/es102182g

 2011 American Chemical Society

Published on Web 12/15/2010

Surface morphology of the PS and CS-PS particles was characterized by scanning electron microscopy using a Philips XL30 ESEM-FEG and a Hitachi S-4800 scanning electron microscope. Dissolved organic carbon content of degradation mixtures was determined by filtering the mixtures through 0.45 µm pore-size membrane filters (Membrane Solutions LLC) followed by the determination of their total organic carbon contents by a Shimadzu TOC-5000A total carbon analyzer. Infrared spectroscopy of the PS materials and degradation mixtures were measured by a Nicolet 360 Fourier transform infrared spectrophotometer. Sample pellets were prepared by mixing the polymer materials or freeze-dried residues of the degradation mixtures with KBr followed by mortaring and pressing with a pressure of 10 tonnes. An Applied Biosystems SCIEX QSTAR Elite hybrid quadrupole/ time-of-flight (Q-TOF) tandem high-resolution mass spectrometer (Applied Biosystems, Darmstadt, Germany) with electrospray ionization (negative ToFMS and product ion modes) was used to characterize the water-soluble intermediates generated during the course of the heterogeneous photoassisted Fenton degradation. The ToF mass range monitored was 50.0000 to 2000.0000 Da. High-resolution molecular mass data were analyzed by the ABI/SCIEX LCMS software ANALYT QS Ver. 2.0. Dissolved iron concentration in the degradation systems was determined by a PerkinElmer Optima 2100DV inductively coupled plasma atomic emission spectrophotometer (ICP-AES). Sulfonation of Polystyrene Wastes. Polystyrene foams obtained from commercially available polystyrene “lunchboxes” (1 g) were cut into small pieces by a pair of scissors, swelled in 10 mL of 1,2-dichloroethane, and heated to 80 °C. Concentrated sulfuric acid (6 mL) was added dropwise over 30 min, and the resulting mixture was further reacted for 2 h. Then 50 mL of 3 M sulfuric acid was added, followed by dilution with 250 mL of deionized water. The sulfonated polystyrene wastes were filtered and washed thoroughly with deionized water until neutral pH. Fenton and Photoassisted Fenton Degradation of Polystyrene Materials. In a typical degradation run, polystyrene particles (PS and CS-PS) or sulfonated polystyrene wastes were suspended in an aqueous solution of FeCl3 and hydrogen peroxide under constant magnetic stirring. Hydrochloric acid was added to the reaction mixture to adjust pH. The final volume of the reaction mixture was maintained at 25 mL. To optimize the concentrations of Fe(III) and H2O2 for the degradation, [FeCl3] and [H2O2] in the range 0 to 0.63 mM and 0 to 176 mM, respectively, have been explored. Broadband UV irradiation for photoassisted Fenton degradation was administered by a 250 W Hg(Xe) light source placed 5 cm from the reaction mixture.

Results and Discussion Degradation of Polystyrene by Fenton Processes. In the field of advanced oxidation treatment of environmental contaminants, heterogeneous Fenton processes generally refer to the mediation of Fenton degradation of dissolved organic pollutants by iron-containing solid catalysts or Fe(II)/ Fe(III) ions supported by porous, inert solid substrates (33). On the other hand, literature report on the degradation of solid matrices by Fenton-type reactions catalyzed by dissolved Fe(II)/Fe(III) salts was scarce. This latter approach of heterogeneous Fenton-type degradation of both virgin polystyrene microspheres (PS), fabricated by emulsion polymerization, and commercially available DOWEX 50WX8 cross-linked polystyrene beads with surface sulfonate functionality (CS-PS) is the main theme of this study. The fabricated virgin PS microspheres were characterized by scanning electron microscopy and FTIR spectroscopy (Figure S1 in the Supporting Information). The particle size of the virgin PS particles was in the range 12-15 µm and that of the

FIGURE 1. Photoassisted Fenton treatment of PS and CS-PS beads: (a) changes in dissolved organic carbon (DOC) content of the degradation mixtures during the course of the treatment; (b) - (e) SEM images of the CS-PS beads at different time intervals (t from 0 min to 60 min) of the photoassisted Fenton degradation. The initial media pH was 2.0. Each data point was the mean value of three independent degradation runs (with error bar showing (1 S.D.). CS-PS beads was 250-350 µm. These PS microspheres represented the commonly found hydrophobic polystyrene material in municipal plastic wastes. Conversely, the surface of the CS-PS beads, because of their sulfonate decorations, was more hydrophilic and possessed cation-exchange properties. We envisioned that a comparison of the behavior of these two polystyrene materials in the Fenton and related reactions might reveal the correlation between surface properties of plastic materials and their degradability by advanced oxidation processes. Fenton-type advanced oxidations can be brought about by both Fe(II) and Fe(III) species. In our present study, Fe(III) salt (FeCl3) was adopted as the precursor for the generation of the active Fe(II) to mediate the Fenton processes. This practice was based on the consideration that Fe(III) was more stable toward air oxidation and easier to handle. SEM photomicrography, FTIR spectroscopy, dissolved organic carbon (DOC) determination, and tandem high-resolution mass spectrometry were employed to evaluate the various evidence of polymer degradation by observing morphological changes in the polystyrene particles, monitoring the increase in dissolved organic carbon content in the degradation mixture, and characterizing the chemical nature of the soluble degradation fragments. Besides these examinations, dried weight of the suspended polymer materials before and after the Fenton and related processes was also measured to check for material loss due to degradation. With suspensions of 250 mg of the polystyrene particles and a Fe(III) and H2O2 loading up to 0.42 mM and 141 mM, respectively, at pH 2.0, no evidence of any degradation of both kinds of polystyrene materials was observed after 5 h of treatment at room temperature. Heating the mixtures to g75 °C did not induce any degradation. This suggested that hydroxyl radicals ( · OH) generated by the Fenton process alone cannot bring about oxidative degradation of polystyrene. Increasing the surface wettability of the polymer material by sulfonation did not lead to any improvement. UV irradiation is known to enhance the rate of Fenton processes (34-37). As the mere Fenton process has been demonstrated inefficient in degrading polystyrene particles, degradation experiments were repeated with the application of broad-band UV irradiation. Figure 1a shows the change in the dissolved organic carbon content of the degradation mixtures during the course of the photoassisted Fenton reaction (300 min). In the case of virgin PS microspheres, no significant change in the degradation mixture was observed. VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Dissolved organic carbon (DOC) content in the degradation mixture was below detection throughout the course of the irradiation. No loss in the net dried weight of the PS material within experimental error was observed after several degradation runs. On the other hand, the aqueous media of the degradation mixtures containing CS-PS beads turned from clear and transparent to dark brown in color in the initial 120 min of irradiation. Then, this dark brown color gradually faded, and the mixture turned back into a clear and transparent solution with the complete disappearance of the suspended CS-PS beads. DOC content of the degradation mixtures increased rapidly in the first 50 min of irradiation and gradually dropped afterward. The rise in DOC content at the onset of the photoassisted Fenton process was consistent with the oxidative cleavage of the polystyrene polymer chains to produce more water-soluble, lighter molecular weight organic fragments in the degradation mixture. These more hydrophilic fragments were further degraded by the photoassisted Fenton reaction, eventually to carbon dioxide. This explains the drop in DOC content at the later stage of the degradation. In terms of the bulk polystyrene solid matrix, the photoassisted Fenton process has achieved 100% removal within 300 min of UV irradiation. The level of DOC left in the degradation mixture was ca. 550 mg L-1. This is equivalent to a mineralization efficiency of >89% (based on the amount of carbon in the polymer, assuming that the polystyrene material in CS-PS is fully sulfonated). This rate of mineralization is considerably faster than those reported heterogeneous photocatalytic degradation processes mediated by polymer-embedded TiO2 (12, 21-23). Figure 1b-1e shows the scanning electron photomicroscopy images of the CS-PS beads isolated at different time intervals during the photoassisted Fenton degradation. Prior to degradation, the surface of these beads was smooth (Figure 1b). As the photoassisted Fenton process proceeded, small chasms were observed on the surface of the particles (Figure 1c). These chasms further spread over the surface and extended to the bulk of the beads (Figure 1d). Diameter of the beads has also expanded to 350-420 µm with the spreading of the chasms. This is probably due to the lowering of the degree of cross-linking in the polystyrene polymer by oxidative bond cleavage mediated by the photoassisted Fenton reaction. The drastic increase in surface area of the degrading CS-PS beads may further enhance the degradation. After 60 min of irradiation, there were very few disorganized solid particles left in the degradation mixture (Figure 1e). Characterization of Degradation Intermediates. FTIR spectroscopy was adopted as a functional approach to characterize the chemical functionality of those dissolved organic species produced in the course of the photoassisted Fenton degradation process (Figure S2 in the Supporting Information). The freeze-dried degradation mixture displayed growing IR peaks at 1637, 1200, 1128, and 1043 cm-1, in the initial 120 min of treatment, which were corresponded to the aromatic CdC and the alcoholic/phenolic C-O stretching. The appearance of dissolve organic species bearing phenyl and alcoholic/phenolic functionalities in the degradation mixture indicates the oxidative cleavage and dissolution of the polymer chains of the polystyrene material during the photoassisted Fenton process. Previous studies on the stability and degradation of polystyrene have suggested that polymer chain breakage can be brought about by attacks from exogenic oxidative radicals, such as superoxyl, peroxyl and hydroxyl radicals, or endogenic phenyl radicals generated from the UV excitation of the phenyl rings on the polystyrene chains (12, 38, 39). In this study, the lack of polymer degradation in the virgin PS microspheres by the photoassisted Fenton reaction indicates that polymer chain breakage caused by UV irradiation on the polystyrene material under our experimental conditions was not significant. 746

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Characterization of the chemical nature of the dissolved organic species in the degradation mixture was attempted by tandem high-resolution mass spectrometry. Samples of the CS-PS degradation mixture (0.5 mL) were taken at regular time intervals throughout the course of the photoassisted Fenton process, filtered through 0.45 µm pore size membrane filters, and freeze-dried. Residues were reconstituted in methanol and analyzed by tandem high-resolution mass spectrometry via direct infusion. Mass spectra (in negative ion mode) obtained at 30, 60, 90, 120, 180, 240, and 300 min are shown in Figures S3-S9 in the Supporting Information. At all time points, numerous mono-, di-, and multianionic species were consistently observed in the degradation mixture. They were originated from the cleavage of the polymer chains of the degrading polystyrene beads. The number of mono- and dianionic species increased with the progress of the degradation. As the photoassisted Fenton reaction proceeded, more and more polymeric fragments of polystyrene in the degradation mixture were further broken down into lighter-weight molecular species by the advanced oxidative process. Among the various molecular degradation intermediates, species with ca. m/z 201.0 and 173.0 (z ) 1), and that ca. m/z 153.0 (z ) 2) were consistently revealed by mass spectrometry under the negative ionization mode. Figure 2 shows the highresolution MS/MS spectra of these commonly observed molecular ions together with their product ions after MS/ MS fragmentation. The exact mass of the monoanionic molecular ion at m/z 201.0049 corresponds to the molecular structure {HO-CH2CH2-[C6H4(SO3)]}- (Figure 2a). These hydroxylated ethylene phenyl sulfonate fragments were likely to be derived from the oxidative cleavage of the polystyrene chains in the CS-PS beads. Its product ion at m/z 157.0086 is attributable to the phenyl sulfonate ion, {[C6H5(SO3)]}-, from the loss of the hydroxyl ethylene fragment of its parent ion. The exact mass of the monoanionic molecular ion at m/z 173.0264 corresponds to the molecular structure of sulfonated phenolate, {[HO-C6H4(SO3)]}-, probably arose from the oxidative cleavage of the phenyl sulfonate pendants from the polyethylene backbone (Figure 2b). The product ions at m/z 93.0488 and 79.9691 are attributable to the phenolate, {[C6H5O]}-, and sulfonate, {[SO3]}-, ions, respectively. The dianionic molecular ion at m/z 153.0118 can be related to the molecular structure {HO-CH[C6H4(SO3)]CH2-CH2[C6H5O]}2- arose from the propylene fragments containing a phenyl and a phenyl sulfonate moiety with oxidative hydroxylation on one end of the propylene chain as well as on the phenyl pendant (Figure 2c). The prominent product ion at m/z 120.9791 is a monoanionic species which corresponds to the fragment {CH3CH2[C6H5O]}- split from the parent ion. From the high-resolution MS/MS analysis, polymer chains in the polystyrene material of CS-PS were rapidly cleaved upon the onset of the photoassisted Fenton process. The small hydroxylated ethylene and propylene fragments with phenolate and phenyl sulfonate pendants in the degradation mixture are direct evidence to the breaking up of the bulk polymer of CS-PS by oxidative scissoring of the polystyrene chains. These hydroxylated and phenolated species also explain the origins of the alcoholic/phenolic C-O stretching peaks in our FTIR studies. Optimization of Conditions of the Photoassisted Fenton Degradation. Effects of the various photoassisted Fenton reaction conditions, namely: initial pH and the loading of Fe(III) and H2O2, on the efficiency of CS-PS degradation, expressed as the amount of dried weight loss of the CS-PS beads within 1 h, were investigated (Figure S10 of the Supporting Information). Similar to other Fenton-type processes, the photoassisted Fenton degradation of polystyrene was enhanced at low pH (40). Degradation efficiency

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FIGURE 2. Tandem MS/MS spectra of the commonly observed degradation intermediates. The most probable molecular and structural formulas of the major molecular ion fragments, together with their corresponding exact mass-to-charge ratios are shown next to those fragments. Figures quoted in brackets were the m/z differences (in parts-per-million) between m/z of the actual and empirical ion fragments.

FIGURE 3. Change of [Fe3+] concentration before and during the course of the photoassisted Fenton degradation of the CS-PS beads (250 mg). Initial loading of FeCl3 was 0.42 mM. H2O2 (141 mM) was not added until upon the application of broad-band UV irradiation. The initial pH was 2.0. Each data point was the mean value of three independent degradation runs (with error bar showing (1 S.D.). dropped significantly as initial media pH increased beyond 2.5. Media pH > 4.5 was not attempted in order to avoid the precipitation of Fe(III)/Fe(II) species. Eventually, pH 2.0 was taken as the initial pH for the degradation. CS-PS (i.e., DOWEX 50WX8 resins) is a cation-exchange resin that can bind metal ions. The adsorption capacity of CS-PS for Fe(III) at room temperature, pH 2.0, was measured to be 335 µmol-Fe g-1CS-PS (fitted to a Langmuir isotherm, Figure S11 of the Supporting Information). Increasing the loading of Fe(III), and H2O2, increased the degradation efficiency of CS-PS. The degradation efficiency reached ca. 100% at 42 µmol-Fe g-1CS-PS and 14.1 mmol-H2O2 g-1-CS-PS. These conditions were adopted in all subsequent degradation studies in this work. Mechanism of the Heterogeneous Photoassisted Fenton Degradation. As revealed in the heterogeneous photoassisted Fenton degradation of virgin PS and sulfonated CS-PS particles, sulfonate functionalities on the CS-PS beads are crucial to the ultimate mineralization of the polystyrene material. We attempted to understand this phenomenon by determining the location of the Fe(III)/Fe(II) catalyst during the course of the degradation. Figure 3 shows the concentration of dissolved [Fe] in a mixture of CS-PS beads (250 mg) and FeCl3 (0.42 mM, equivalent to 42 µmol-Fe g-1-CSPS) at pH 2.0 at room temperature. In the first 30 min of the experiment, UV irradiation was not applied. The pale yellow color of the FeCl3 solution was rapidly decolorized upon the addition of the CS-PS beads. ICP-AES analysis of the aqueous CS-PS suspension, after filtering through a 0.45 µm membrane filter, revealed that the dissolved [Fe] concentration has dropped rapidly. After the initial 30 min of “dark” experiment, H2O2 (141 mM, equivalent to 14.1 mmol g-1-CS-PS) was then added and UV irradiation was applied. The mixture rapidly turned deep brown in color. The level of dissolved [Fe] in the aqueous media remained low until after 30 min of irradiation, when it started to rise gradually back to its initial concentration. These observations were consistent with the adsorption of the Fe(III)/Fe(II) ions onto the cation-exchanging sulfonate groups on the surface of the CS-PS beads at the early stage of the experiment, leading to the decrease in the level of dissolved [Fe] in the aqueous media. This adsorption process brought the Fenton reaction catalyst in close proximity to the polystyrene polymer chains so that the highly oxidizing species generated by the photoassisted Fenton process did not need to diffuse too long a distance before they could attack the polymer chains. Upon the addition of H2O2 and the application of UV irradiation, the photoassisted Fenton reaction on the surface of the CS-PS beads immediately 748

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started to disintegrate the polymer matrix of the polystyrene particles. After most of the polymer solid matrix has been broken down, the level of dissolved [Fe] in the degradation mixture rose and eventually returned to its initial concentration after most of the CS-PS beads have been mineralized. The identity of the reactive species involved in the heterogeneous photoassisted Fenton degradation of polystyrene is not clear at the moment. The absence of any polymer chain cleavage in the thermal Fenton reaction on virgin PS microspheres (Figure 1) implies that hydroxyl radicals ( · OH), the principal active oxidant in Fenton processes, are not capable of degrading polystyrene. At least, it appears that hydroxyl radicals may not be the only oxidant that brought about the initial disintegration of the crosslinked polystyrene materials. Numerous studies on photoassisted Fenton processes have suggested the generation of Fe(IV) and/or Fe(V) oxo species ([FeIV)O]/[FeV)O]), via the photolysis of some Fe(III) superoxide species (35, 41-43). High-valent iron-oxo catalysts are capable of inducing the oxidative cleavage of C-H and C-C bonds (44, 45). Perhaps the best known example is cytochrome P-450 in biological systems where a high-valent iron oxo center is believed to be actively involved in the catalytic oxidation, hydroxylation, and bond activation metabolic processes (46). Nevertheless, the participation of high-valent iron-oxo species in our heterogeneous photoassisted Fenton process is currently beyond elucidation. At this stage, our results were only able to suggest the possible involvement of additional powerful oxidants other than hydroxyl radicals in the initiation of polystyrene degradation. These additional oxidants have to be generated in close proximity to the polymer chains of polystyrene via the anchoring of the Fe(III)/Fe(II) catalysts onto the surface cation-exchanging sulfonate sites of the polymer. Once the polystyrene chains are broken down into shorter, more water-soluble fragments, their further degradation and mineralization can proceed. Scheme 1 shows the proposed photoassisted Fenton degradation scheme of polystyrene material. Of course, further investigations are needed to identify the nature of the oxidants responsible for the oxidative degradation and the exact mechanism involved. Photoassisted Fenton Degradation of Waste Polystyrene Foams. The grafting of cation-exchanging sulfonate groups onto polystyrene substrates can be conveniently achieved via sulfuric acid mediated sulfonation of the phenyl pendants of the polystyrene chains. This enables the heterogeneous photoassisted Fenton degradation process to be applied on ordinary polystyrene wastes. We demonstrated this by using commercially available polystyrene foam “lunch-boxes”, cut into millimeter-size pieces, as the polystyrene substrate. Despite the fact that it is impracticable to fine-tune the sulfonation reaction and generate surface sulfonate groups in the polystyrene foam with density similar to that in the DOWEX 50WX8 CS-PS beads, we still applied the previously established optimal heterogeneous photoassisted Fenton reaction conditions to the degradation of the polystyrene foams. Figure 4 shows the change in dissolved organic carbon content in the degradation mixture throughout the course of photoassisted Fenton degradation of the sulfonated polystyrene foam material. The DOC curve resembled that of the degradation of CS-PS beads (Figure 1) with the peak DOC value appeared much earlier. Over 99% mineralization was achieved within 250 min. This suggests that commercial polystyrene foams are easier to be degraded than the CS-PS beads, probably due to their lower degree of cross-linking. The fact that both microspherical CS-PS beads and the millimeter-scale polystyrene foam pieces were able to be mineralized efficiently by the heterogeneous photoassisted Fenton process indicated that the process is not significantly sensitive to the particle size of the polymer materials.

SCHEME 1. Proposed Mechanism for the Initiation of the Heterogeneous Photo-Assisted Fenton Degradation of Sulfonated Polystyrene Materials

To conclude, we have demonstrated that heterogeneous photoassisted Fenton reaction was able to mediate the mineralization of polystyrene materials that contain sulfonate decorations, with efficiency surpassing that of the heterogeneous photocatalytic polymer degradation by embedded TiO2. No incorporation of any catalyst or reagent into the

polymer matrices a priori is needed and the degradation process can be readily applied on ordinary polystyrene wastes. The binding of the cationic Fe(III)/Fe(II) Fenton catalyst to the polystyrene polymer chains, probably via the cation-exchanging surface sulfonate groups, is crucial to the initiation of the heterogeneous advanced oxidative degradation. This may facilitate the direct oxidative cleavage of the polystyrene polymer chains by additional powerful oxidants, other than hydroxyl radicals, generated by the photoassisted Fenton process. Once the polystyrene chains are broken down into shorter, more water-soluble fragments, their further degradation and mineralization by those oxidants as well as hydroxyl radicals can proceed. This strategy of heterogeneous photoassisted Fenton degradation and mineralization might also be applicable to the advanced oxidative treatments of other synthetic polymer materials. Works toward this goal are in progress.

FIGURE 4. Photoassisted Fenton degradation of polystyrene substrate (1 g) from a commercial polystyrene foam “lunch-box”. Initial loading of FeCl3 and H2O2 was 0.42 mM and 141 mM, respectively, with further addition of H2O2 oxidant (14.1 mmol) at every 60 min intervals during the course of the degradation. The initial media pH was 2.0. Data shown were averaged DOC values from three independent degradation runs (with error bar showing (1 S.D.).

Acknowledgments The work described in this paper was funded by the Collaborative Research Project (CityU 2/06C) funded by the Research Grants Council of Hong Kong SAR, China. H.-M.F. and J.-C.Z. acknowledge the University of Science & Technology of China and City University of Hong Kong Joint Ph.D. Scheme. VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information Available Figures S1-S11. This material is available free of charge via the Internet at http://pubs.acs.org.

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