and Nonsolvent-Induced Phase Separation Method for Potential

The process employed a method of combined temperature- and ... As a potential application, microgranules were coated with poly(acrylic acid) (PAA) and...
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Ind. Eng. Chem. Res. 2005, 44, 825-831

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Preparing Microgranules from Waste Polystyrene through a Novel Temperature- and Nonsolvent-Induced Phase Separation Method for Potential Adsorbent Z. Wang and Renbi Bai* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

Recycling post-consumer waste polystyrene (WPS) into useful high-value products is of great environmental and engineering interest. In this study, microgranules were prepared from WPS and were used as a potential adsorbent for heavy metal removal from aqueous solution. In comparison with the conventional approach preparing polymer particles from dilute polymer solutions, a new process that can use semidilute or concentrated polymer solutions to prepare microgranules was developed. The process employed a method of combined temperature- and nonsolvent-induced phase separation to prevent the possible aggregation of the polymer particles and, thus, made it possible to obtain microgranules from semidilute or concentrated polymer solutions. Depending on the concentration of the polymer solution, irregular and more porous or spherical and less porous microgranules were prepared. As a potential application, microgranules were coated with poly(acrylic acid) (PAA) and examined as an adsorbent for lead ion removal. It was found that the coated microgranules had good adsorption performance and fast adsorption kinetics in removing lead ions from aqueous solution. The work illustrates the great potential in reducing WPS from the solid waste stream and converting WPS into high-value microgranules that might have many applications as adsorbents in water and wastewater treatment or other industrial processes. Introduction The recycling of plastic wastes into various useful products has attracted considerable research interests over the years,1-5 largely because of the problems in the disposal of plastic wastes as a solid waste stream and the demand for resource conservation. Polystyrene (PS), because of its good strength, light weight, and durability, has been widely used as a material for manufacturing products such as cutlery, cups, food containers, agriculture trays, electrical equipment frames or cases, and the material to package electronics or other fragile items for shipment. The PS market has experienced an average growth rate of 3% over recent years, and the world consumption is expected to reach 15.5 million tonnes by 2006.6 The consumption of PS results in the generation of a large quantity of PS waste or waste polystyrene (WPS). For example, it was reported that, in the U.S. in 2001, about 9% of the total amount of waste plastics, or 2290 thousand tonnes, in the municipal solid waste stream was WPS.7 Although the postconsumer WPS still preserves its valuable properties and the recycling of WPS can reduce the quantity of solid waste that needs to be treated or ultimately disposed, most of the municipal post-consumer WPS was discarded without recycling, because the conventional practice of recycling WPS back to its original products or other low-grade products is not usually economically sustainable. One of the solutions to this problem can be to recycle WPS into other highly valuable products. In various industries, there is considerable interest in polymer particles or granules that are useful in the * To whom correspondence should be addressed. Fax: (65) 6779 1936. E-mail: [email protected].

production of polymer blends or alloys,8 functional coatings,9 adsorbents,10 catalyst supports, drug delivery carriers,11 etc. The objective of this research is to develop technologies or methods that can recycle WPS into polymeric microgranules that can be used as potential adsorbents. To convert WPS into polymeric microgranules, one needs to develop granulation methods that work with polymer solutions. The preparation of polymeric microgranules from polymer solutions can be based on the theory of liquid-liquid phase separation followed by phase inversion.12 For polymers in dilute solutions with a concentration below a critical value C*, there is no overlapping between or among the polymer macromolecules. When treated by rapid quenching or drying, the polymer macromolecules can be solidified into separate microparticles.13-15 This conventional technique is impracticable, however, when dealing with a large amount of polymeric materials, such as recycling WPS, because the value of C* in the process is generally far below 0.1 wt %. It would be of great practical interest to develop granulation methods that can use semidilute or concentrated polymer solutions to reduce the consumption for polymer solvent. The major problem with using semidilute or concentrated polymer solutions is that, when phase separation takes place, bulk polymer gels instead of individual microparticles or -granules are formed, because of the entanglement or aggregation of the polymer macromolecules. This is especially the case for amorphous polymers, such as PS, because of the strong gelation and aggregation properties of these polymers during the phase separation process. In this paper, we report a new development for the preparation of polymeric microgranules from WPS in semidilute and concentrated PS polymer solutions

10.1021/ie049258e CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005

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Figure 1. Schematics showing the polymer solution droplet emulsion.

through a novel temperature- and nonsolvent-induced phase separation method. The microgranules were surface-modified and were examined as an adsorbent for lead ion removal from aqueous solution. Theoretical Basis of the New Preparation Method The procedures for preparing microgranules from WPS through the novel temperature- and nonsolventinduced phase separation method can be summarized as follows: (a) Dissolve the polymeric material in a solvent to obtain the polymer solution. (b) Prepare an emulsion by mixing the polymer solution in a polymer nonsolvent that is also immiscible with the polymer solvent (see Figure 1). (c) Precipitate the polymer solution droplets in the emulsion into microgranules via phase separation induced by adding the emulsion into another polymer nonsolvent (denoted as precipitator) and lowering the temperature. (The precipitator is so chosen that it is well miscible with both the polymer solvent and the polymer nonsolvent used to prepare the emulsion.) (d) Separate the microgranules from the mixture. In this method, the bulk polymer solution is first converted into separate tiny droplets by the emulsification process, and each droplet in the emulsion remains a small polymer solution system. No matter whether a dilute, semidilute, or concentrated polymer solution is used, the phase separation takes place in terms of the tiny droplets, but the morphologies of the final products will depend on the concentration of the polymer solution. The mechanism can be explained as shown in Figure 2. The diagram on the left-hand side of Figure 2 shows the classical theory of the phase states of a polymer

solution system under various system temperatures and polymer solution concentrations. Tg is the glass transition temperature, and Tp is the phase separation temperature. Region A represents an isotropic polymer solution (one phase), B is a spinodal decomposition region (two phases, liquid and liquid), C and D are metastable regions (two phases, liquid and liquid), and E is the phase separation region (two phases, solid and liquid). The letters a-d shown on the right-hand side of Figure 2 indicate the states of the polymer solution droplets under various conditions in the emulsion during thermal treatment. a represents a stable polymer solution droplet in region A existing in the emulsion. If the polymer solution is dilute, the droplet a turns to a small system c with polymer nucleus dispersed in the polymer solvent when the emulsion system temperature is cooled to region C. Individual microparticles can be obtained from phase separation when the emulsion system temperature is further lowered to below Tp. This is the technique used in the conventional approach by other researchers. If the polymer solution is semidilute or concentrated, the tiny droplet system might fall into region B or D, respectively, when the system temperature is decreased. If in region B, the droplet a turns into the state of b, a small system with continuous penetrating polymer networks (black represents the entangled polymer molecules). After complete phase separation at a temperature below Tp, amorphous or irregular microgranules are the product obtained. If in region D, however, the droplet a exists as d, a small system with a solvent nucleus dispersed in the concentrated polymers. The complete phase separation at temperature below Tp gives the product with regular spherical morphology. Hence, the preparation of polymeric microgranules from semidilute or concentrated polymer solutions can be realized, as long as polymer entanglement or aggregation among the polymer solution droplets or tiny particles formed during the phase separation process can be properly controlled in the preparation process. For amorphous polymers such as PS, vitrification at the beginning of the phase separation stage can rapidly freeze the tiny droplet system, and as a consequence, the polymer solvent in the droplets cannot be removed instantly. This would result in aggregation among the tiny droplets or particles formed. To solve this problem, a polymer nonsolvent- or precipitator-induced phase separation can be introduced to speed up the exclusion of the polymer solvent from the droplets and thus

Figure 2. Schematic diagram showing the temperature-induced phase change and phase separation.

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Figure 3. Phase change and phase separation diagram, indicating the effect of the precipitator (L, liquid; S, solid).

prevent aggregation from occurring. The mechanism is explained in Figure 3. Instead of phase separation taking place in the conventional way in the direction from “polymer solvent” toward “polymer” in the triangle (which forms bulk solid), the new method with the addition of the emulsion into a special polymer nonsolvent (precipitator) will transfer the state of the tiny polymer solution droplets from region 1 to region 2 in the diagram. Because the special polymer nonsolvent is miscible with the polymer solvent, the solvent in the droplets is instantly extracted. With the help of a temperature-induced phase separation, the formation of the microgranules can be rapidly obtained and enhanced in region 2 without the occurrence of particle aggregation. Experimental Section Materials. WPS was obtained from ECO-RRC (Resource Recovery Centre) Singapore as food containers and product-packaging material in the preparation of PS microgranules. Cyclohexane (GC, 99.5%) from Merck or p-xylene (GC, 99%) from Aldrich was used as the polymer solvent for WPS. Deionized (DI) water was used as the emulsion medium (polymer nonsolvent). Absolute ethanol or propanol (99.9%) from Hayman Limited (Essex, U.K.) was used as the special polymer nonsolvent (precipitator). Acrylic acid (AAc) (GC, 99%, Aldrich), purified by distillation before use, was used in surface modification of the PS microgranules to prepare the adsorbent. Lead(II) solution standard (1000 mg/L) was supplied by Merck and was used in the heavy metal adsorption tests for the evaluation of the adsorbent’s performance. Preparation of Microgranules from WPS. WPS was dissolved in cyclohexane solvent at 50-60 °C (or in p-xylene at 23-25 °C) to prepare the PS polymer solutions, and the concentration range of the PS polymer solutions in the experiment was in 2-10 wt %. To prepare the emulsion, 0.5 g of surfactant was first added into 200 mL of DI water in a beaker, and the mixture was heated to 50-60 °C (or 23-25 °C if p-xylene was used). Then, about 80 mL of the polymer solution was added into the beaker, and the contents in the beaker were vigorously stirred at 500 rpm on a hot-plate stirrer with the temperature controlled at 50-60 °C (or 2325 °C) to form an emulsion. After the emulsion was formed, it was transferred with a buret to the cold ethanol or propanol kept in a refrigerator at 0 °C. PS microgranules (denoted as PS-Ps) were formed and

precipitated to the bottom of the beaker. They were then separated from the liquid, washed with DI water, dried in an oven, and finally stored in a vacuum desiccator prior to use for further study. Adsorbent Preparation. Surface modification of the PS-Ps was done with poly(acrylic acid) (PAA). PAA was synthesized by radical polymerization of AAc in a 500 mL flask at room temperature and atmosphere pressure. DI water (300 mL) and AAc (35 mL, 0.5 mol) were added into the flask, and the contents in the flask were mixed by magnetic stirring quickly. After nitrogen bubbling for 15 min, 1.35 g of potassium peroxodisulfate (K2S2O8, 99%; Merck) and 0.4 g of sodium sulfite (Na2SO3, 97%; Nacalai Tesque, Kyoto, Japan) as the polymerization initiators were added into the flask with stirring. The reaction in the flask was allowed to proceed under N2 for 5 h to produce the PAA solution. Then, about 5 g of the PS-Ps was added into the PAA solution, and the mixture was shaken at room temperature and atmosphere pressure on a shaker for 2 h to allow the attachment of PAA to the surface of the PS-Ps. Finally, the PAA-immobilized PS-Ps were separated from the PAA solution by filtration, dried and cured in an oven at 85-90 °C for 8 h, and stored in a desiccator for further study. (The PAA-modified or coated polystyrene microgranules will be referred to as PS-AAPs in the discussion hereafter.) Batch Lead Adsorption Tests. Batch adsorption experiments were conducted to evaluate the performance of the PS-AAPs as an adsorbent for lead ion removal from aqueous solution. To study the effect of solution pH on lead adsorption, lead solutions with an initial concentration of 25 mg/L were prepared by diluting the 1000 mg/L standard lead solution with DI water, and then, the pH values of the solutions were adjusted to a value in the range of 1-7 with 0.1 M NaOH or 0.1 M HNO3. About 15-mg samples of the PSAAPs were weighed and added into a number of 25-mL vials, each containing 15 mL of a lead solution of one of the different solution pH values mentioned above. The contents in the vials were stirred in an orbital shaker at 170 rpm and 25 °C for adsorption to proceed. The pH of the solution in each vial was not controlled during the adsorption process. After 5 h (well above the adsorption equilibrium time of 30-60 min), the PSAAPs were separated by filtration with a 0.45-µm membrane, and the liquid was collected for the determination of the lead ion concentration. Both the initial and final lead ion concentrations in the solution in each vial were analyzed with an inductively coupled plasma emission spectrometer (ICP-ES) (Perkin-Elmer Optima 3000, Wellesley, MA). The amount of lead ions adsorbed per unit weight of PS-AAPs, q (mg/g), was calculated from the following mass balance equation

q ) (C0 - Ce)V/m

(1)

where C0 and Ce (mg/L) are the initial lead ion concentration and the lead ion concentration at adsorption equilibrium, respectively; V (L) is the volume of the lead ion solution; and m (g) is the amount of the PS-AAPs added to each vial. For the kinetic study experiment, 15 mg of the PS-AAPs was added to 100 mL of a lead ion solution with an initial lead ion concentration of 25 mg/L and an initial solution pH value of 5.0. The mixture was shaken in an orbital shaker at 170 rpm and 25 °C, and the history of the lead ion concentration in the solution was determined by taking and analyzing

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solution samples at different time intervals. The adsorbed amount of lead ions per unit weight of the PSAAPs at time ti, q(ti) (mg/g), was calculated by an equation similar to eq 1, but considering the changes in the concentrations and volumes during the test process. Analysis. The surface morphologies of the PS-Ps and PS-AAPs were examined with a scanning electron microscope (SEM, JEOL JSM-6400). For the analysis of the PS-AAPs after lead adsorption, the PS-AAP sample was obtained from the solution through centrifugal separation at 5000 rpm for 10 min. FT-IR reflectance spectra of samples of WPS, PS-P, PS-AAP, and PS-AAP with adsorbed Pb2+ were obtained from a microscope-FT-IR spectrometer (Bio-Rad FTS3500 ARX FT-IR spectrometer and Bio-Rad UMA-500 Microscope). Each spectrum was obtained by cumulating 64 scans at a resolution of four wavenumbers. The thermal properties of the WPS used in this study and the microgranules of PS-P produced in this study were measured with DSC (differential scanning calorimeter). The polymer samples were heated from 25 to 200 °C at a heating rate of 10 °C/min under a dry nitrogen atmosphere (25 mL/min) using a Mettler Toledo DSC 822e Thermal Analyst system. Results and Discussion Production of PS-Ps. As mentioned earlier, success in producing microgranules from WPS from semidilute or concentrated polymer solutions largely depends on the selection of the proper solvent for the dissolution of the polymer, the proper nonsolvent for the formation of the emulsion of the polymer solution and another proper nonsolvent (i.e., precipitator that is different from the nonsolvent forming the emulsion) for the precipitation of the polymer solution droplets into individual particles or granules. The selection was done on the basis of the classical Hildebrand equation

∆HM/V h φ1φ2 ) (δ1 - δ2)2

(2)

h is the where ∆HM is the enthalpy change of mixing, V total volume of the mixture, and φi represents the volume ratios and δi represents the solubility parameters of components i ) 1 and 2. Particularly, the lower the difference between δ1 and δ2, the more soluble the two components 1 and 2 will be. The value of δ for PS is known to be about 9.1.16 From the DSC analysis, the glass transition temperature of the selected postconsumer WPS in this study was found to be about Tg ) 99 °C; see Figure 4. Consequently, our study confirmed that cyclohexane or p-xylene can be used as the polymer solvent, water as the nonsolvent for the emulsion, and ethanol or propanol as the precipitator in the successful production of the PS-Ps microgranules. The corresponding δ values and other related information for these chemical components are given in Table 1. The other experimental conditions used to make the PS-Ps in the study are summarized in Table 2. As can be seen, the production process in this study used semidilute or concentrated polymer solution (2-10 wt %), in contrast with the conventional approaches that require dilute polymer solutions with polymer concentration of less than 0.1 wt %.13-15 This indicates a significant reduction in the amount of polymer solvent necessary to prepare the required polymer solution in the new process.

Figure 4. Glass transition temperature of the WPS used in this study, as determined from DSC analysis (onset, 96.11 °C; midpoint, 99.45 °C; and end point, 108.16 °C). Table 1. Properties of the PS Polymer, Polymer Solvent, Polymer Nonsolvent, and Precipitator Used in This Study δ chemical

(ka0.5/cm1.5)

polystyrene cyclohexane p-xylene water ethanol 2-propanol

8.7-9.1 8.2 8.75 23.2 12.7 11.9

boiling melting point point density water (°C) (°C) (g/mL) solubility 81 138 100 78 82

240 7 12-13 0 -130 -89

1.05 0.78 0.866 1.0 0.789 0.79

insoluble insoluble insoluble miscible miscible

Figure 5 shows typical SEM images of the PS-Ps microgranules prepared from the highly concentrated polymer solution (8 wt % WPS) and those prepared from the less concentrated or semidilute polymer solution (2 wt % WPS). The results clearly illustrate that individual microgranules without aggregation were successfully prepared from the process. It is also clear that the PSPs granules prepared from the concentrated polymer solution indeed had a spherical morphology and those from the semidilute polymer solution retained an amorphous or irregular morphology. In addition, from the SEM images at higher magnifications in Figure 6, it is observed that the surface of the PS-Ps granules prepared from the concentrated polymer solution was much less porous than that of the PS-Ps granules from the semidilute polymer solution. As analyzed earlier, the tiny droplets from the semidilute polymer solution can have continuous penetrating networks formed by the entangled polymer macromolecules. During cooling, the glassy polymer froze the tiny droplet system, and in the phase separation process, the solvent in the droplets was extracted by the precipitator. Thus, highly porous structures were formed for the PS-Ps produced from the semidilute polymer solution. On the other hand, the tiny droplets from the concentrated polymer solution had a structure with a solvent nucleus dispersed in a continuous polymer phase. The cooling and phase separation process hence produced PS-Ps that were less porous and had relatively smoother surfaces. The precipitator played an important role in the fast extraction of the polymer solvent from the droplets in a short time and thus eliminated the possible aggregation of the PS-Ps particles generated in the process. The PS-Ps microgranules produced in this study usually had a size distribution of about 20-100 µm. (The size range can be altered by changing the stirring strength in the preparation of the polymer solution emulsion if necessary.) Surface-Modified PS-Ps. Surface modification of the PS-Ps was done by immobilization or coating of PAA

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 829 Table 2. Experimental Conditions Studied in Preparing the Microgranules (PS-Ps)a

no.

solvent

polymer solution concentration (wt %)

1 2 3 4 5 6

p-xylene p-xylene p-xylene cyclohexane cyclohexane cyclohexane

2 5 10 2 5 8

temperature (°C) 23-25 23-25 23-25 50-60 50-60 50-60

emulsion concentrationb temperature (wt %) (°C) 50 40 30 50 40 30

23-25 23-25 23-25 50-60 50-60 50-60

ratio of emulsion to precipitator 1:3 1:4 1:6 1:3 1:4 1:6

a In all cases, the polymer used was post-consumer polystyrene plastic, the mechanical stirring rate was 550 rpm, the precipitator was ethanol or propanol, and the precipitator temperature (before being added to the emulsion) was 0 °C. b Emulsion concentration ) polymer solution/water.

Figure 5. Shapes or surface morphologies of the PS-Ps produced from different polymer solution concentrations: (a) 8 and (b) 2 wt %. Figure 6. SEM images of the surfaces of PS-Ps produced from (a) 8 and (b) 2 wt % polymer solution at high magnification.

on the surface for the introduction of the carboxylic groups. A leaching test in DI water solution was performed, and the result did not show PAA leaching off the surface of the granules, indicating that the drying and curing process might have resulted in some crosslinking or branching reactions among the PAA molecules, which enabled PAA to be firmly coated on the surfaces of the PS-Ps in the study. Figure 7 shows an SEM image of the surface of the PAA-modified PS-Ps (i.e., PS-AAPs). Compared to the surface of the PS-Ps in Figure 6, it is clear that a layer of PAA was coated on the surface and the surface of PS-AAPs became nonporous. To confirm the existence of PAA on the surface, FT-IR spectra of WPS, PS-Ps, and PS-AAPs

were obtained; see Figure 8. A new peak at the wavenumber of 1720 cm-1, which is due to the carbonyl groups (CdO), is clearly seen for the surface of the PSAAPs, confirming the existence of PAA on the surface. From NaOH (0.1 M) titration tests, the amount of carboxylic groups immobilized on the PS-AAPs from PAA was determined to be about 10 mg/g. Performance of Lead Ion Adsorption. Figure 9 shows the adsorption performance of lead ions on the PS-AAPs under various initial solution pH values. At pH values below 3.5, there was little lead ion adsorption, and with an increase in pH from 3.5 to 6, lead ion adsorption increased significantly. The high adsorption

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Figure 10. FT-IR spectra of (a) PS-AAPs and (b) PS-AAPs with lead ion adsorption. Figure 7. SEM image of the surface of PS-AAPs with coated PAA (surface became nonporous).

Figure 11. Kinetics of lead ion adsorption on PS-AAPs (initial pH ) 5, C0 ) 25 mg/L, T ) 25 °C, r ) 170 rpm).

at higher solution pH, such as pH 6. The adsorption mechanism can be proposed as (where M represents Pb): Figure 8. FT-IR spectra of (a) WPS, (b) PS-Ps, and (c) PS-AAPs.

Figure 9. Effect of initial solution pH on the adsorption performence of lead ions on the PS-AAPs (C0 ) 25 mg/L, V ) 15 mL, m ) 15 mg, T ) 25 °C, r ) 170 rpm).

performance of lead ions on the PS-AAPs at pH 6 or near neutral pH can be of practical benefit as many water and wastewater sources can have pH in the neutral range (higher pH’s were not tested because of the possibility of precipitation). Because the pKa value of carboxylic groups is commonly known to be around 4.5, the R-COOH functional groups on the surfaces of the PS-AAPs would be increasingly dissociated into RCOO-‚‚‚H+ at greater solution pH values above 3.5. Hence, the attractive electrostatic interaction between the RCOO- and the Pb2+ ions to be adsorbed could promote more lead ions to be adsorbed on the PS-AAPs

Equations 3a and 3b also indicate that lead adsorption on the PS-AAPs can lower the solution pH value. This prediction was indeed confirmed in experiments where the solution pH at adsorption equilibrium was found to be lower than the initial solution pH value for initial pH values greater than 4; see Figure 9. The mechanism in eqs 3 also suggests that lead adsorption on the PSAAPs might be effected through the formation of surface complexes with the carboxylic groups on the PS-AAPs. The FT-IR spectra in Figure 10 show that peak changes occurred at 1419 and 1559 cm-1 after lead ion adsorption on the PS-AAPs. These peaks are characteristic of the antisymmetric and symmetric stretching vibrations of the carboxylic groups, respectively, suggesting that lead ion adsorption on the PS-AAPs indeed involved in the formation of surface complexes with the carboxylic groups. The typical results of lead adsorption kinetics studies are shown in Figure 11. It can be observed that the adsorption of lead ions on the PS-AAPs was relatively fast and the adsorption equilibrium was reached within 30-60 min. This phenomenon can be attributed to attractive electrostatic interactions, possibly the high

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density of adsorption sites (i.e., -COOH) on the PSAAPs, and the nonporous surface features of the PSAAPs. Therefore, lead adsorption can be mainly controlled by the diffusion of lead ions from the bulk solution to the surface of the PS-AAPs. The adsorption of lead ions from aqueous solution onto conventional granular adsorbents, such as activated carbon, has been reported to be a long process (adsorption equilibrium reached in several hours or much longer),17 because the adsorption kinetics of metal ions onto such porous adsorbents is mainly controlled by an inner-surface diffusion process that is generally much slower than the bulk diffusion of metal ions in solutions. Again, the change of the solution pH with the adsorption of lead ions on the PS-AAPs can be observed in the figure. The conventional adsorbent of activated carbon is generally known to be expensive and has often been reported to have a low adsorption capacity for metals such as lead at less than 10 mg/g.17-19 This probably justifies the many studies in recent years to develop new or novel adsorbents for the removal of metals, including lead, from aqueous solution.20-26 The results in Figures 9 and 11 suggest that the PS-AAP granules can be a better adsorbent than the conventional adsorbent of activated carbon for lead removal, because of their higher adsorption capacity (up to 25 mg/g), faster adsorption kinetics, and possibly lower-cost raw material (WPS). Conclusions Microgranules can be successfully prepared from WPS through a novel temperature- and nonsolvent-induced separation method. This method enables semidilute or concentrated polymer solutions to be used in the preparation of the microgranules. Microgranules from WPS can be converted into an effective adsorbent for lead removal from aqueous solution, with benefits of high adsorption capacity and fast adsorption kinetics. This study demonstrates the great potential in recycling WPS into high-value products, such as adsorbents, to minimize post-consumer plastic wastes that have presented a great environmental challenge for their disposal. The method reported in this work can also be extended to other waste plastics. Acknowledgment The financial support of the Academic Research Funds, National University of Singapore, is acknowledged. Literature Cited (1) Yoshioka, T.; Ota, M.; Okuwaki, A. Conversion of a used poly(ethylene terephthalate) bottle into oxalic acid and terephthalic acid by oxygen oxidation in alkaline solutions at elevated temperatures. Ind. Eng. Chem. Res. 2003, 42, 675-679. (2) Yamaye, M.; Hashime, T.; Yamamoto, K.; Kosugi, Y.; Cho, N.; Ichiki, T.; Kito, T. Chemical recycling of poly(ethylene terephthalate). 2. Preparation of terephthalohydroxamic acid and terephthalohydrazide. Ind. Eng. Chem. Res. 2002, 41, 3993-3998. (3) De la Puente, G.; Arandes, J. M.; Sedran, U. A. Recycled plastics in FCC feedstocks: Specific contributions. Ind. Eng. Chem. Res. 1997, 36, 4530-4534. (4) Paszun, D.; Spychaj, T. Chemical recycling of poly(ethylene terephthalate). Ind. Eng. Chem. Res. 1997, 36, 1373-1383.

(5) Zhang, Z. B.; Hirose, T.; Nishio, S.; Morioka, Y.; Azuma, N.; Ueno, A.; Ohkita, H.; Okada, M. Chemical recycling of waste polystyrene into styrene ever solid acids and bases. Ind. Eng. Chem. Res. 1995, 34, 4514-4519. (6) Polystyrene-The International Market 2003; Gobi International, UK: 2003. (7) Municipal Solid Waste in The United States: 2001 Facts and Figures; Report EPA530-R-03-011; Office of Solid Waste and Emergency Response (5305W), U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 2003. (8) Narkis, M.; Rosenzweig, N. Polymer Powder Technology; John Wiley: New York, 1995. (9) Tosko Aleksandar Misev. Powder Coatings: Chemistry and Technology; John Wiley: New York, 1991. (10) Alexandratos, S. D.; Crick, D. W. Polymer-supported reagents: Application to separation science. Ind. Eng. Chem. Res. 1996, 35, 635-644. (11) Arshady, R. Microspheres for biomedical applicationss Preparation of reactive and labeled microspheres. Biomaterials 1993, 14, 5-15. (12) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (13) Yarovoy, Y. K.; Baran, G.; Wunder, S. L.; Wang, R. W. Submicron-size particles of ultrahigh molecular weight polyethylene produced via nonsolvent and temperature-induced crystallization. J. Biomed. Mater. Res. 2000, 53, 152-160. (14) Brown, H. R.; Wignall, G. D. A SANS study of the dimensions of polystyrene formed by freeze-drying from dilute solution. Macromolecules 1990, 23, 683-685. (15) Matsuyama, H.; Teramoto, M.; Kuwana, M.; Kitamura, Y. Formation of polypropylene particles via thermally induced phase separation. Polymer 2000, 41, 8673-8679. (16) Coleman, M. M.; Graf, J. F.; Painter, P. C. Specific Interactions and the Miscibility of Polymer Blends; Technomic: Lancaster, PA, 1991. (17) Machida, M.; Kikuchi, Y.; Aikawa, M.; Tatsumoto, H. Kinetics of adsorption and desorption of Pb(II) in aqueous solution on activated carbon by two-site adsorption model. Colloids Surf. A 2004, 240, 179-186. (18) Abdel-Halim, S. H.; Shehata, A. M. A.; El-Shahat, M. F. Removal of lead ions from industrial wastewater by different types of natural materials. Water Res. 2003, 37, 1678-83. (19) Reed, B. E.; Robertson, J.; Jamil, M. Regeneration of granular activated carbon (GAC) columns used for removal of lead. J. Environ. Eng.: ASCE 1995, 121, 653-62. (20) Jin, L.; Bai, R. B. Mechanisms of lead adsorption on chitosan/PVA hydrogel beads. Langmuir 2002, 18, 9765-9770. (21) Deng, S. B.; Bai, R. B.; Chen, J. P. Aminated polyacrylonitrile fibers for lead and copper removal. Langmuir 2003, 19, 5058-5064. (22) Deng, S. B.; Bai, R. B.; Chen, J. P. Behaviors and mechanisms of copper adsorption to hydrolyzed polyacrylonitrile fibers. J. Colloid Interface Sci. 2003, 260, 265-272. (23) Deng, S. B.; Bai, R. B. Removal of trivalent and hexavalent chromium with aminated polyacrylonitrile fibers: performance and mechanisms. Water Res. 2004, 38, 2423-2431. (24) Demirbas, A. Adsorption of lead and cadmium ions in aqueous solutions onto modified lignin from alkali glycerol delignication. J. Hazard. Mater. 2004, 109, 221-226. (25) Babel, S.; Kurniawan, T. A. Low-cost adsorbents for heavy metals uptake from contaminated water: A review. J. Hazard. Mater. B 2003, 97, 219-43. (26) Selatnia, A.; Boukazoula, A.; Kechid, N.; Bakhti, M. Z.; Chergui, A.; Kerchich, Y. Biosorption of lead (II) from aqueous solution by a bacterial dead Streptomyces rimosus biomass. Biochem. Eng. J. 2004, 19, 127-135.

Received for review August 16, 2004 Revised manuscript received November 28, 2004 Accepted December 3, 2004 IE049258E