Ind. Eng. Chem. Res. 2008, 47, 1861-1867
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Conversion of Waste Polystyrene into Porous and Functionalized Adsorbent and Its Application in Humic Acid Removal Z. Wang,† Renbi Bai,*,‡ and Y. P. Ting† Department of Chemical and Biomolecular Engineering, and DiVision of EnVironmental Science and Engineering, Faculty of Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Disposal of plastic waste has become an ever-increasing concern throughout the world. There has been considerable interest or demand for recycling plastic waste into higher-value products. In this study, an attempt has been made to convert waste polystyrene (PS) into porous and functionalized granular adsorbent for a potential application in humic acid removal. Porous PS granules were prepared via a dissolution/precipitation and etching/oxidation method, and their surfaces were modified with cross-linked polyethylenimine (PEI). The surface morphologies of PS granules, oxidized PS granules, and PEI-immobilized PS granules were examined with a scanning electron microscope (SEM), and the surface properties of these granules were investigated through Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The prepared PS granules had a more-or-less spherical shape with sizes in the range of 1.3-2.4 mm, depending on the polymer concentrations used. The etching/oxidation process resulted in highly porous granules, and their surfaces were activated with polar chemical groups, which facilitated the immobilization of PEI on the external and internal surfaces of the granules in a simple solution reaction process. The PEI-functionalized PS granules were demonstrated to be an effective adsorbent for humic acid removal. 1. Introduction Plastic waste takes up a large percentage of the municipal solid waste (MSW) stream. On average, plastic waste constitutes about 6-12% of the MSW by weight1-2 but may take up to 30% by volume.2-3 The amount of plastic consumption is also predicted to increase at an annual rate of about 4% in the future.4 Because plastics continue to play an increasingly important role in packaging and consumer products, the disposal of plastic waste poses a significant environmental challenge. Once in the waste stream, plastic waste may be dealt with in one of three ways: landfill, incineration, or material recycling. As plastics are non-biodegradable, their volume does not shrink over the time. Hence, the disposal of plastic waste via landfills will consume a large space. This has, to a large extent, eliminated landfills as a future option for plastic-waste disposal. Although the incineration of plastic waste results in the reclamation of some energy, the energy recovered is far less than that used in manufacturing the plastics. In addition, the incineration of plastics generates potentially hazardous emissions, including greenhouse gases, dioxin, and heavy metals. Therefore, material recycling, in spite of being the least practiced, has become the preferred option for solving the problems of plastic waste, especially in terms of resource conservation and environmentalpollution prevention. This has more than ever been the case in today’s global trend of pursing sustainable development.5-6 Polystyrene (PS) is widely used in the manufacture of many products due to its favorable properties such as good strength, light weight, and durability and is the material of choice for packaging various electronics and other fragile items. In general, PS accounts for about 9-10% of the plastic waste in MSW.7 However, to date, most of the PS waste is discarded without material recycling, because the conventional practice of recy* To whom correspondence should be addressed. Tel.: (65) 6516 4532. Fax: (65) 6774 4202. E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering. ‡ Division of Environmental Science and Engineering.
cling waste PS to its original products or other low grade products has usually not been economically sustainable.8 A possible solution to this problem may be therefore to recycle or convert waste PS into some higher-value products. In various industries, polymer supports (in the form of a polymer network in which reactive species are chemically immobilized) have found a wide range of applications and therefore have shown great commercial or economic value. In particular, polymer supports with immobilized functional groups are increasingly used as polymeric adsorbents in advanced separation technologies. For example, polymer particles immobilized with various surface ligands have been studied for the removal and recovery of heavy metal ions in wastewater treatment, separation/purification of proteins in bioengineering, and minimization of organic pollutants in water supply.9-10 The immobilized reactive or functional groups on the polymer supports may include ligands such as amine, pyridine and imidazole, carboxylic acid, and polybenzimidazoles, etc.11-14 Polymer supports in the form of micro or macro beads or granules may be prepared from various materials, such as synthetic polymers, polysaccharides, and polymer composites through various processes such as suspension polymerization and suspension gelation or chelation.15 Active ligands or functional groups are then immobilized on the surfaces of the beads or granules through surface modification. Porous beads or granules are usually preferred as they provide a greater specific surface area for ligand immobilization as well as for adsorption separation. In this article, we report the development of a process to convert waste PS into porous and functionalized granular adsorbent. The PS granules were first prepared from waste PS through a dissolution/precipitation method. Then, the granules were treated in a controlled chromic acid process to enhance the porous structure and to activate the surface. Finally, the porous PS granules were immobilized with cross-linked PEI (polyethylenimine) to obtain a functionalized adsorbent. As an environmental application, the prepared adsorbent was examined
10.1021/ie070583m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/19/2008
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for humic acid removal from aqueous solutions in a series of batch adsorption experiments. The interest in humic acid removal in this study was based on the fact that humic substances are ubiquitous in various surface waters and groundwater. One of the major problems with the presence of humic substances in water is their reaction with chlorine in water treatment to produce trihalomethanes that are known human carcinogens.16 There have been considerable challenges to minimize the presence of humic substances in drinking water supply.17 Although adsorption has been considered as a preferred technology for this purpose, the commonly used or commercially available adsorbents, such as activated carbon, do not usually perform well in humic acid removal. Various analytical methods have been used to characterize the materials and interactions involved in the process, including differential scanning calorimetry (DSC) for the PS granules before and after the acid treatment, Fourier transform infrared (FTIR) spectroscopy for the interaction of PEI with the oxidized PS granules, and scanning electron microscopy (SEM) for the surface morphologies of the PS granules before and after surface modification and with or without humic acid adsorption. 2. Experimental
Figure 1. Major steps in the process to prepare PS-G-PEI adsorbent from waste PS.
2.1. Materials. Waste PS was obtained from the ECO Resource Recovery Centre, Singapore, in the form of food containers and product-packaging materials for the preparation of PS granules. Xylene (GC grade, 99%) from Aldrich was used, without further purification, as the polymer solvent for dissolving waste PS. Absolute ethanol (99.9%) from Hayman Ltd (England) was used as the polymer nonsolvent to precipitate the PS granules. Chromium (VI) oxide (99+%) and acetic acid (glacial, 100%) supplied by Aldrich and Merck, respectively, were used to prepare the etchant and oxidation solution for the pretreatment of the PS granules. Polyethylenimine (PEI, 50 wt % solution in water) from Aldrich was used in the surface modification of the porous PS granules. Glutaraldehyde solution (1.2% in water) from Fluka (Japan) was used as the cross-linking agent for PEI. Humic acid (in the form of sodium salt) from Aldrich was used in the adsorption experiment to evaluate the adsorption performance of the adsorbent. 2.2. Preparation of PS Granules. Dependent on the desired sizes of the granules, a certain amount (10-30 g) of the waste PS was dissolved in 100 mL of the xylene solution at room temperature (23-25 °C) and with stirring for 1 h to obtain a uniform PS polymer solution. Then, the polymer solution was slowly added in droplets, through a needle pump nozzle, into an ethanol solution that had a concentration in the range of 7462.5% in a beaker. The polymer solution droplets precipitated in the ethanol solution and formed solid granules with a moreor-less spherical shape. The granules were left in the ethanol solution for 24 h for hardening. Subsequently, the PS granules, with an average size in the range of 1.37-2.34 mm, were separated by filtration, dried in a vacuum dryer at room temperature, and then stored in a desiccator for further use. The PS granules prepared from this process are denoted as PS-G in the article. (In the following sections, unless otherwise indicated, the granules were prepared by dissolving 10 g of PS in 100 mL of xylene and then precipitated in a 74% ethanol solution). 2.3. Surface Etching, Oxidation, and Grafting of PS-G. Chromic acetic solution was used as an etching and oxidation chemical to treat the PS-G granules to enhance their apparent porous structure and to activate their surfaces for immobilization of PEI. A 10 g amount of chromium(VI) oxide was first dissolved in 15 mL deionized (DI) water and then mixed with
50 mL of the acetic acid for 10 min. A 10 g amount of PS-G was subsequently added into the solution, and the mixture was refluxed at 100 °C for 8 h. During the process, the original white color of the granules was clearly observed to turn green, indicating the attachment of the reduced chromium ions on the surfaces of the PS-G granules. Holes were visible on the surfaces of the granules even to the naked eye. The adsorbed chromium ions on the porous and oxidized PS-G were removed by washing the granules in a 6 M HCl solution under vigorous stirring, followed by washing with DI water (20 min for each). The color of the granules was observed to revert back to white. Finally, the porous and oxidized PS-G granules were dried in an oven at 70 °C for 8 h and then at room temperature to a constant weight. The surfaces of the porous and oxidized PS-G were then modified with PEI (Mw ) 240 000 g/mol) to convert the granules into an effective adsorbent. A 5 wt % PEI solution was first prepared by diluting a 50 wt % PEI solution with DI water, and then the porous and oxidized PS-G granules were added into the solution in a flask. The content of the flask was shaken in a water-bath shaker at 100 rpm and at room temperature for 12 h. The PEI-grafted granules were finally separated from the solution by filtration and dried in an oven at 90 °C for 24 h. The color of the granules was observed to turn from white to buff. The PEI-grafted granules were further treated in 100 mL of 1 wt % glutaraldehyde solution with stirring at 60 °C to cross-link the PEI molecules grafted on the surfaces. Finally, the granules were washed with sufficient DI water to remove any remaining glutaraldehyde and uncross-linked or physically adsorbed PEI, and were dried in a vacuum dryer at room temperature to a constant weight. The porous PS granules with immobilized PEI are referred to as PS-G-PEI in the article hereafter. The major steps in the process to obtain the adsorbent, that is, from PS-G to PS-G-PEI, are schematically shown in Figure 1. 2.4. Characterizations. The surface morphologies of the PSG, oxidized PS-G, and PS-G-PEI granules were examined with a scanning electron microscope (SEM, JSM-6400, JEOL, Peabody, MA) at ∼10-20 kV. The granules were mounted on a metal sample substrate by double-sided adhesive tape, coated
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with platinum, degassed, and then scanned with the microscope to obtain the images. To examine the surface properties and surface interactions, the FTIR reflectance spectra for the PS-G, oxidized PS-G, and PS-G-PEI granules were obtained from a Microscope-FTIR spectrometer (Bio-Rad FTS-3500 ARX FTIR and Bio-Rad UMA-500 Microscope) over the frequency range of 500-2000 cm-1, with each spectrum being composed from cumulating 64 scans at a resolution of four wave numbers. Thermal properties of the PS-G, oxidized PS-G, and PS-GPEI granules were analyzed using a differential scanning calorimeter. The polymer samples were heated from 25 °C to 200 °C at a heating rate of 10 °C/min under a dry nitrogen atmosphere (25 mL/min) in a Mettler Toledo DSC 822e Thermal Analyst system. 2.5. Batch Experiments for Humic Acid Adsorption. Batch adsorption experiments were conducted to evaluate the performance of the PS-G-PEI granules as an adsorbent for humic acid removal from aqueous solutions. In the kinetic adsorption experiments, a 0.025 g amount of PS-G-PEI was added into each of two 100 mL flasks containing 40 mL humic acid solution at an initial concentration (C0) of 10 or 30 mg/L, respectively. The initial pH value of the solutions was adjusted to about 6.5. The contents of the flasks were shaken in a waterbath shaker at room temperature and 150 rpm for adsorption to take place. The concentrations of humic acid in the solutions were determined from the absorbance readings measured every minute at 254 nm with a UV-vis spectrometer (Agilent 8453, Germany) equipped with a flow-through cell. The conversion from the absorbance readings to the weight concentrations was done through the calibration curves prepared under similar conditions, as described in details elsewhere.18-19 The adsorbed amount of humic acid per unit weight of granules at time t, q(t) (mg g-1), was calculated from the mass balance equation as
q(t) )
(C0 - Ct)V m
(1)
where C0 and Ct (mg L-1) are the initial humic acid concentration and the humic acid concentrations at any time t, respectively; V is the volume of the humic acid solution, and m is the weight of the granules added. During the adsorption process, the solution pH values were not controlled because the addition of an acid or a base would change the ionic conditions in the solutions. To compare the adsorption performance, similar kinetic adsorption experiments were also conducted for PS-G. The adsorption isotherm experiments were carried out for PSG-PEI at 25 °C under the conditions of initial pH ) 6.5 and C0 in the range of 5 to 30 mg/L. 3. Results and Discussion 3.1. Production of PS-G Granules. The mechanism of the dissolution/precipitation method to prepare PS granules is based on phase separation. The first requirement for using this method is that the polymer would dissolve in a certain solvent but precipitate in another nonsolvent. The other requirement is that the solvent and the nonsolvent should be highly miscible. When the polymer is dissolved in the solvent, and the polymer solution is added in droplets into the nonsolvent, the nonsolvent extracts the solvent and mix together to form a single liquid phase, whereas the polymer in the droplets precipitates as a solid phase. In the present study, xylene was used as the polymer solvent to dissolve waste PS, and ethanol solution was used as the polymer nonsolvent to obtain the PS-G granules. The selection
Table 1. Concentration of PS Polymer Solution, Composition of the Non-Solvent, and the Corresponding Diameter of PS-G Granules Nonsolvent
#
concentration of PS in xylene solution (g/100 mL)
weight ratio of water/ethanol
1 2 3 4
10 16 20 30
35/100 45/100 50/100 60/100
ethanol (%) 74 69 67 62.5
diameter of PS-G (mm) 1.37 1.51 1.89 2.34
of the solvent and nonsolvent is based on the excellent solubility of PS in xylene (taking place at room temperature and in less than 10 min) and the complete miscibility of xylene with ethanol in which PS is insoluble. However, it was found that when absolute ethanol was used as the nonsolvent, the polymer droplets sank rapidly to the bottom of the beaker before they were solidified and attained a regular shape or structure, owning to the low density of the nonsolvent. To solve this problem, an ethanol-water mixture solution was successfully used as the nonsolvent. Experiments showed that the ideal water/ethanol ratio of the nonsolvent was dependent on the polymer solution concentration (i.e., the PS/xylene ratio). With the increase in polymer solution concentrations, a greater water/ethanol ratio would be needed for the nonsolvent. This consequently led to granules of greater diameters being obtained as well. Table 1 summarizes the experimental conditions (polymer solution concentrations versus water/ethanol ratios) that produced relatively spherical PS-G granules and the corresponding average sizes of the granules. Adding water in ethanol to form the nonsolvent increased the density of the nonsolvent as well as the surface tension of the polymer solution droplets in the nonsolvent. Part (a) of Figure 2 shows a typical SEM image of the PS-G granule prepared using the process. Although the surface of the PS-G appeared to be smooth, the granule, when broken, displayed a very porous internal structure (part (b) of Figure 2). The porous structure may be attributed to the exclusion of the solvent from the polymer solution droplets. At the beginning of the phase separation process, the continual metastable polymer phase in the solution droplets vitrified and thus froze the polymer solution droplet system. Subsequently, when the solvent in the droplets was extracted by the nonsolvent in the precipitation process and was completely excluded in the final drying stage, the space originally occupied by the solvent in the droplets turned into pores in the granules. Because of the immiscibility of water in the nonsolvent solution with xylene in the polymer solution and the greater surface tension of water than ethanol, a thin but dense outer surface of the granules was therefore produced. 3.2 Etching/Oxidation of PS-G Granules with Chromic Acetic Acid. Prior to the surface modification with PEI, the PS-G granules were treated with chromic acetic acid solution to etch the outer dense surface and thereby expose the internal pores. The treatment was also to activate the surfaces of the PS-G granules (by introducing carboxyl groups onto the surfaces) for PEI grafting. From the typical SEM image of the oxidized PS-G in Figure 3, it is evident that the oxidized PS-G granules were highly porous, indicating that the etching process with chromic acetic acid was indeed effective in unmasking the internal pores of the granules. The DSC spectra in Figure 4 shows that the glass transition temperature (Tg) of PS-G was about 105 °C, similar to that of PS, but increased to 110 °C for the oxidized PS-G. This phenomenon can be attributed to the introduction of polar groups
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Figure 4. DSC spectra of (a) PS (Tg ) 104 °C), (b) PS-G (Tg ) 105 °C), and (c) oxidized PS-G (Tg ) 110 °C).
Figure 2. Typical SEM images showing the surface morphology and internal structure of the PS-G granules: (a) surface morphology, and (b) internal structure.
Figure 3. Typical SEM images showing the surface morphology and internal structure of the porous and oxidized PS-G granules: (a) overall morphology, (b) external surface, and (c) internal pore structure.
(i.e., carboxyl groups) onto the PS chains of the oxidized PS-G granules during the etching and oxidation process. These polar groups hindered the rotation of the polymer chains and thus
gave rise to the formation of the interchain hydrogen bonds, leading to increased rigidity of the oxidized PS chains and thus a higher Tg for the oxidized PS-G granules. The hydrogen bonds between the polymer chains restricted the chain mobility, and therefore they needed more thermal energy in the DSC heating cycle for phase transition. The FTIR spectra also confirmed the presence of the carboxyl groups on the oxidized PS-G granules (section 3.4, latter). The attachment of the polar groups on the oxidized PS-G granules would facilitate the grafting of PEI on the surfaces. 3.3. Immobilization of PEI on Oxidized PS-G. PEI is a polymer containing primary, secondary, and tertiary amines and has been used as a functional polymer in many studies on surface modification. The different types of the amine groups in PEI could protonate to various extents at different solution pH values, resulting in the polymer with positive surface charges in solutions of a wide pH range.19 This property implies that PEI can readily react with the surface functional groups of negative charges on a solid to effectively achieve its surface modification under a wider range of solution pH conditions. The reaction mechanisms of PEI with the oxidized PS-G granules may be proposed as shown in eq 2(a), 2(b), and 2(c),
where R, R1, R2, and R3 represent the various alkyl groups that are associated with the primary, secondary, or tertiary amines in PEI, and R′ denotes the PS-G granule. To improve the stability of PEI immobilized on the granules in an aqueous solution, they were treated in a dilute glutaraldehyde solution. This resulted in the PEI molecules being partially cross-linked by glutaraldehyde. Tests with exhaustive washing of the cross-linked granules did not reveal noticeable dissolution of PEI from the granules. The cross-linking reaction of PEI with glutaraldehyde may be illustrated in eq 3, which shows the reaction of an aldehyde with a secondary amine to form a network structure. 3.4. Spectroscopic Evidence. FTIR analysis was employed to examine the chemical structure of the granules to confirm the effectiveness of each step during the oxidation and PEI grafting process in the preparation of the PS-G-PEI adsorbent.
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Chemical bonds within a material can absorb energy and vibrate at certain characteristic frequencies. As each moiety is capable of a number of different modes of vibration, such as stretching and bending, a number of peaks can be attributed to the presence of a particular chemical structure in the FTIR spectra. Figure 5 shows the FTIR spectra of PS-G, oxidized PS-G and PS-GPEI, respectively. In part (a) of Figure 5, the strong absorption peak at 1601 cm-1 (aromatic CdC ring stretching vibration), the weak peaks between ∼1600-2000 cm-1 (aromatic C-H out-of-plane bends), and the peaks at 757 and 699 cm-1 are the characteristic peaks of polystyrene. The spectrum of the oxidized PS-G in part (b) of Figure 5 contains a new absorption peak for CdO (hydrogen-bonded) stretching at 1707 cm-1, which is attributed to the carboxylic groups. This result confirms that the chromic acetic acid etching/oxidation process introduced carboxylic acid groups on the surfaces of the PS-G granules. After PEI grafting, the spectrum of PS-G-PEI in part (c) of Figure 5 exhibits a new strong absorption peak at 1653 cm-1, which is ascribed to the bending vibration of the NH2 groups in PEI. In addition, the new peaks at 1560 cm-1 (Amine salts, >NH2+ or -NH3+) and 1404 cm-1 (carboxylate salts, -COO-) are associated with the -COO-‚‚‚-H2+N
