Sorption of Four Hydrophobic Organic Compounds by Three

Jun 5, 2012 - Dorothea Gilbert , Gesine Witt , Foppe Smedes , and Philipp Mayer. Analytical Chemistry 2016 88 (11), 5818-5826. Abstract | Full Text HT...
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Sorption of Four Hydrophobic Organic Compounds by Three Chemically Distinct Polymers: Role of Chemical and Physical Composition Xiaoying Guo,† Xilong Wang,*,† Xinzhe Zhou,† Xiangzhen Kong,† Shu Tao,† and Baoshan Xing‡ †

Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, United States

‡

S Supporting Information *

ABSTRACT: The sorption behavior of four hydrophobic organic contaminants (HOCs) (i.e., phenanthrene, naphthalene, lindane, and 1-naphthol) by three types of polymers namely polyethylene (PE), polystyrene (PS), and polyphenyleneoxide (PPO) was examined in this work. The organic carbon content-normalized sorption coefficients (Koc) of phenanthrene, lindane, and naphthalene by PEs of same composition but distinct physical makeup of domains increased with their crystallinity reduction (from 58.7 to 25.5%), suggesting that mobility and abundance of rubbery domains in polymers regulated HOC sorption. Cross-linking in styrene−divinylbenzene copolymer (PS2) created substantial surface area and porosity, thus, Koc values of phenanthrene, lindane, naphthalene, and 1-naphthol by PS2 were as high as 274.8, 212.3, 27.4, and 1.5 times of those by the linear polystyrene (PS1). The Koc values of lindane, naphthalene, and 1-naphthol by polar PPO were approximately 1−3 orders of magnitude higher than those by PS1, and PPO had comparable sorption for phenanthrene but higher sorption for naphthalene and 1-naphthol than PS2. This can be a result that a portion of O-containing moieties in PPO were masked in the interior part, while leaving the hydrophobic domains exposed outside, therefore demonstrating the great influence of the spatial arrangement of domains in polymers on HOC sorption.

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INTRODUCTION Soil organic matter (SOM) is ubiquitously present in the environment and it is composed of a mixture of molecules with varying molecular weights. The extremely complicated chemical composition and physical conformation of SOM depend on the origin and age of the source materials.1,2 It was reported that SOM is the key sorption medium for organic chemicals in soils when its mass fraction exceeds 0.1%.3 Therefore, a better understanding of the interaction mechanisms between hydrophobic organic compounds (HOCs) and SOMs has been one of major focuses in the field of environmental science.4 Much work has been done to examine the roles of chemical composition and polarity of SOMs in their sorption for HOCs. Kang and Xing suggested that polarity instead of structure of SOMs dominantly controlled their sorption intensity for phenanthrene and there was a negative correlation between the polarity of sorbent and Koc.5 Wang et al.6 and Tanaka et al.7 also reported similar results. Whether the aliphatic carbon8−10 or aromatic carbon11−13 domains in SOMs regulate their sorption for HOCs has drawn great research attention from environmental scientists in the past few years. However, until now a widely acceptable consensus on this issue has not been established. The increasing evidence further showed that chemical composition of SOMs could not solely be used to predict their sorption behavior for HOCs. © 2012 American Chemical Society

Previous studies have demonstrated that physical makeup of SOMs had great influence on accessibility of their sorption sites, thereby affecting HOC sorption intensity and isotherm nonlinearity. It was reported that the rubbery domains had higher sorption for HOCs relative to the glassy ones,14,15 whereas other investigators concluded that the humins with more condensed and complex structure had higher sorption for HOCs (i.e., naphthalene and phenanthrene) over humic acids with relatively loose construction.16−18 These divergent findings noted that additional work is required to further address the role of the physical makeup of domains in SOMs in their sorption for HOCs. It was observed in our previous study that the spatial arrangement of domains in biopolymers greatly influenced HOC sorption.19 However, the mechanisms on how this factor affects HOC sorption by SOMs still need to be further elucidated. Due to highly heterogeneous composition and structure of SOMs, a legible molecular structural description is not yet available. Chemical composition of SOMs and physical makeup of their domains are always found to jointly regulate HOC Received: Revised: Accepted: Published: 7252

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samples was freeze-dried and stored in a desiccator for characterization and sorption experiments. Two polycyclic aromatic hydrocarbons (phenanthrene and naphthalene), one phenolic compound (1-naphthol), and one aliphatic compound (lindane) were used as sorbates. The 14C-labeled and nonlabeled chemicals were also obtained from Sigma-Aldrich Chemical Co. Selected properties of the chemicals used in the present work are summarized in Table S1 in the Supporting Information. Sorbent Characterization. The C and H contents of polymers were determined with a Vario EL CHN Elemental Analyzer (Germany), and O content of PPO was derived by mass difference. Aside from the bulk elemental composition of the polymers, surface elemental composition and the abundance of carbon-based functionalities at the surfaces of PE2, PE3, PE4, PS1, PS2, and PPO were also obtained using an AXIS-Ultra X-ray Imaging Photoelectron Spectrometer (Kratos Analytical Ltd., UK) with a monochromatic Al Kα radiation source operated at 225 W, 15 mA, and 15 kV. The penetration depth for XPS scanning was below 10 nm. It is possible that the surface properties of polymers in aqueous phase are different from those in dry solid state. The static contact angles of polymers were measured with a contact angle measuring system (OCA20, Dataphysics, Germany) using the sessile drop method, and they were used to characterize their external surface hydrophobicity in aqueous phase of the sorption systems. Briefly, to obtain the contact angle of a given polymer, a droplet of 2 μL of background solution containing 0.01 M CaCl2 and 200 mg/L of NaN3 was dripped right onto the smooth surface of the polymer layer, which was prepared by spreading the polymer particles on a glass slide with the aid of double-faced adhesive tape to form a thin and uniform layer. With the drop image captured, contact angle of polymer was calculated employing the ellipsoid method. The contact angle of individual polymers was measured ten times, and the averaged values were used for discussion. Since surface area (SA) and porosity of sorbents are important factors that may affect their sorption for HOCs, sorption−desorption isotherms of N2 by all polymers at 77 K were obtained to get these two parameters using an Autosorb1-MP Surface Area Analyzer (Quantachrome Instruments, USA) after outgassing at 105 °C for 16 h, with an exception of E3 and E4 at 80 °C whose melting point are 90 and 92 °C, respectively. The SAs of polymers were derived from N2 sorption isotherms using multipoint BET method with relative pressure P/P0 from 0.05 to 0.3. The micropore volumes of polymers were obtained using the Dubinin−Radushkevich (DR) model with P/P0 ≤ 0.05, and their meso- and macropore volumes were calculated from desorption isotherms employing the Barrett−Joyner−Halenda (BJH) model. It has to be noted that surface area and porosity of polymers could be slightly underestimated, based upon the fact that a portion of pores were most likely inaccessible to N2 molecules at 77 K due mainly to the capillary condensation effect. Another possibility was that N2 molecules were unable to access a fraction of pores within reasonable equilibration time because of the diffusion limitation at such a low temperature.28 Thermal Gravimetric and Differential Scanning Calorimetric Analysis. For thermal gravimetric analysis (TGA), 5 mg of dried polymer sample was scanned with a Thermo Gravimetric Analyzer (TGA Q50, TA Instruments) at a heating rate of 10 °C/min from 0 to 900 °C. The glass transition temperature (Tg) of all sorbents was derived using a modulated

sorption, which makes it difficult to evaluate the significance of one single aspect. The analogy of SOMs to polymers in terms of sorption was put forward in the 1990s, and some methodologies used for polymer science have been used for studies on SOMs.2,4,20−23 The experimental observations revealed that the synthetic polymers could be used as model SOMs to better understand the interaction mechanisms between HOCs and SOMs of complicated chemical composition and diverse physical forms of sorption domains. With polymers as a simplified model for SOMs, impact of the physical properties of SOMs on HOC−SOM interaction can be better clarified using polymers of the same chemical composition but distinct physical forms of sorption domains. Likewise, influence of the chemical properties of SOMs on HOC sorption can be investigated using model polymers with different chemical composition. For the studies on HOC sorption to polymers, Saquing et al.24 examined the influence of various plastics on transport and fate of HOCs in landfills, and they observed that plastics were potential sinks for HOCs in landfills because of their much higher sorption for the tested compounds relative to lignocellulosic materials and slow desorption rate from glassy plastics. A previous study was conducted to obtain the partitioning coefficient of organic solutes by polydimethylsiloxane (PDMS), with an aim to determine the time needed for their extraction in solid-phase microextration.25 Also, Endo et al.26,27 determined the sorption capacity of diverse compounds by polyoxymethylene (POM) and polyacrylate (PA), and they reported that POM and PA had higher sorption for H-bond donating compounds than PDMS and PE due to their H-bond accepting properties. This noted the importance of the electronic properties of polymers in HOC sorption. However, to date, the exact mechanisms controlling sorption of HOCs to polymers are still poorly understood. Based upon the above discussion, the key objectives of this work were to (1) probe the correlation between physical makeup of sorption domains in model SOMs and their sorption for HOCs using polymers of same chemical composition; (2) examine the dependence of HOC sorption to polymers on their chemical composition; and (3) further test the roles of the spatial arrangement of polymer sorption domains in HOC sorption.

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MATERIALS AND METHODS Sorbents and Sorbates. To achieve our research aim, seven kinds of polyethylene with distinct molecular weight and density (PE1−PE7), two kinds of polystyrene with different molecular structure (i.e., one being linear (PS1) and the other being 20% cross-linked (PS2)), and an O-containing polymer poly (2,6-dimethyl-1,4- phenyleneoxide) (PPO) were chosen as model SOMs. The polymers in each class (e.g., PEs and PSs) possessed almost the same chemical composition. They were thus used to examine the influence of physical makeup of domains in sorbents on HOC sorption. Comparison of HOC sorption by three categories of polymers will help understand the dependence of HOC sorption on their chemical composition and spatial arrangement of sorption domains. PS2 and PPO were purchased from J&K Chemical Co., Ltd., USA. PE1−PE7 and PS1 were from Sigma-Aldrich Chemical Co. Polymers originally in pellet form were ground to fine powers (diameter PE > PPO. Surfaces of PEs were solely composed of C−C and C−H moieties, in agreement with their XPS data and high abundance of alkyl chains in composition. Different from PEs, PSs and PPO were rich in aromatic components in their molecular structure. PPO contained oxygen in its molecular structure, thus being a polar polymer. Effect of the Physical Makeup of Domains. Polymers are classified into crystalline, semicrystalline, and amorphous states according to the degree of regularity of their molecular chain arrangement. Due to distinct thermodynamic and segment motion properties, the domains in amorphous state include glassy and rubbery subfractions. Glassy polymers have rigid and condensed structure, while the rubbery ones have relatively expanded, flexible structures. The PE is a typical example of semicrystalline polymer,29 which contains both amorphous and crystalline sorption domains. The glass transition temperature (Tg) of a noncrystalline or semicrystalline material is a critical temperature point at which the material changes its physical form from glassy to rubbery state. Since the Tg value of PE was reported to be around 201 K,30 pure PE was viewed as a rubbery polymer with abundant amorphous methylene chains at room temperature.24,31 No Tg values of PEs were detected via thermal analysis in the present study, confirming that amorphous fraction of the tested PEs with distinct molecular weight and density all behaved as rubbery domains. As nonpolar aliphatic polymers with no specific functional groups, PEs were only able to interact with all tested compounds through van der Waals interactions instead of other mechanisms (e.g., π−π bond, hydrogen bonding). Sorption isotherms of phenanthrene, lindane, naphthalene, and 1-naphthol by PE1 and PE3−PE7 were highly linear with n

log Q = log K f + n log Ce

where Kf is sorption coefficient ((mg/kg)/(mg/L)n), and n (dimensionless) is the linearity index of sorption isotherms. Q 7254

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Figure 1. Organic carbon content ( foc)-normalized sorption isotherms of phenanthrene, naphthalene, lindane, and 1-naphthol by PE1−PE7 (top), PS1, PS2, and PPO (bottom). PE1 (◊), PE2 (○), PE3 (△), PE4 (×), PE5 (□), PE6 (−), PE7 (+), PS1 (*), PS2 (●), and PPO (▲).

Table 2. Physicochemical Properties of the Sorbents Used in this Studya pore volume (cm3/g) sorbent

average molecular weight

Vmic

Vmac + Vmes

surface area (m2/g)

PE1 PE2 PE3 PE4 PE5 PE6 PE7 PS1 PS2 PPO

high density 3,000,000−6,000,000 35,000 4,000 high density low density linear low 280,000

0 0.009 0.004 0 0.001 0 0 0 0.022 0.025

0.007 0.017 0.007 0.004 0.013 0.003 0.003 0.004 0.492 0.094

6.1 1.4 5.1 2.2 10.7 1.6 0.2 0.2 70.2 64.6

contact angle (°C) 135.6 146.4 130.4 132.9 136.8 138.2 136.5 133.0 130.9 143.0

± ± ± ± ± ± ± ± ± ±

2.9 2.0 2.1 2.6 5.0 3.7 1.6 4.3 2.6 1.9

DT (°C) 461 461 442 436 459 451 459 397 399 434

Tg (°C)

fusion enthalpy (J/g)

crystallinity (%)

168.4 140.8 73.4 86.0 169.1 118.3 97.0

58.5 48.9 25.5 29.9 58.7 41.1 33.7

100.2 216.2

a

DT: decomposition temperature of polymers (°C) derived from TGA analysis; Vmic: micropore volume; Vmac + Vmes: sum of macro- and mesopore volume.

Figure 2. Differential scanning calorimeter (DSC) spectra of PEs, PSs, and PPO.

abundance of crystalline domains. It was reported that crystallinity of sorption domains could influence partitioning of HOCs to carbohydrates and other semicrystalline (bio) polymers, because the crystalline subdomains had low sorption capacity and accessibility for HOCs and their alignment could affect the availability of amorphous domains for HOC sorption.32 PE is capable of forming a portion of crystalline domains due to its regular alkyl chain structure. In the crystalline zone, segments of molecules are regularly arranged in a crystal lattice. In contrast, the randomly arranged molecular segments in rubbery domains often exhibit an expanded and flexible physical form. The crystallinity of domains in all PEs

values of 0.944−1, (Figure 1) suggesting a partitioningdominant sorption process. Compared to other PEs, PE2 exhibited slightly nonlinear sorption for phenanthrene, lindane, and naphthalene with n values ranging from 0.821 to 0.867 (Table 3). This could be a result of its ultra high molecular weight, which greatly increased the winding degree of its alkyl chains thereby giving a larger micro- and overall porosity in contrast to other PEs (Table 2). Microporosity of SOM was also reported to be positively correlated with the degree of isotherm nonlinearity for chlorinated benzene sorption.31 The crystallinity of domains in polymers derived from DSC data has widely been accepted as an index for describing their 7255

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was derived from the heat flow curve of DSC (Figure 2, Table 2), showing the abundance of crystalline domains. The Koc values of phenanthrene, lindane and naphthalene by PEs derived from equilibrium concentration at 0.4 times of their water solubility (Ce = 0.4 Sw) were negatively correlated with the crystallinity of their sorption domains (Figure 3, Tables 2

Table 3. Parameters of Freundlich Model-Based Sorption Isotherm Fitting for Polymersa sorbent

log Kf

phenanthrene PE1 3.942 PE2 4.041 PE3 4.076 PE4 4.093 PE5 3.911 PE6 3.945 PE7 4.036 PS1 2.859 PS2 5.290 PPO 5.145 lindane PE1 2.334 PE2 2.469 PE3 2.757 PE4 2.990 PE5 2.277 PE6 2.544 PE7 2.632 PS1 1.444 PS2 3.948 PPO 3.275 naphthalene PE1 2.674 PE2 2.827 PE3 2.903 PE4 2.876 PE5 2.696 PE6 2.757 PE7 2.853 PS1 2.417 PS2 4.129 PPO 4.438 1-naphthol PE1 1.901 PE2 1.825 PE3 1.811 PE4 1.941 PE5 1.760 PE6 1.741 PE7 1.757 PS1 1.886 PS2 2.649 PPO 2.835

Figure 3. Relationship between Koc values of phenanthrene (◊), lindane (□), naphthalene (△), and 1-naphthol (○) by PEs and the crystallinity of sorption domains in these polymers. Here, Koc values were derived from equilibrium concentrations of sorbates at 0.4 Sw.

and 3), indicating that sorption of the tested compounds by polymers of same composition was dependent on their abundance of rubbery domains. This is because the rubbery domains have high mobility thus high accessibility for HOC sorption.9,14,15 It was evident that among all tested compounds, crystallinity of sorption domains in PEs had the strongest effect on Koc values of lindane, moderate and comparable effect on those of phenanthrene and naphthalene, and the lowest influence on Koc values of 1-naphthol. It seems that effect of the crystallinity of sorption domains in polymers of same composition on Koc values of HOCs differed with their chemical structure. Saquing et al.24 also observed the similar correlation between crystallinity of sorption domains in several plastic materials with different densities and their sorption for toluene. Endo et al.27 compared sorption of 56 organic chemicals including several classes of polar compounds and organochlorine pesticides to polymeric materials, and reported that their partitioning coefficients between polyoxymethylene (POM) and water (KPOM/W) were consistently lower than those between polyacrylate (PA) and water (KPA/W) (by a factor of 4 on average). The authors explained that the crystalline regions in POM were unavailable for partitioning, while PA was much more amorphous and such domains were highly available for partitioning of the tested compounds. Different from PEs, two PS samples contained a large amount of aromatic components and they practically had the same composition as indicated by the elemental composition and XPS data (Table 1). However, these two PS samples exhibited a huge sorption difference for various compounds (Figure 1, Table 3), further indicating the importance of the physical makeup of sorption domains in polymers of similar chemical composition for HOC sorption.6,33−35 As glassy polymers, PSs and PPO possessed heterogeneous components such as methylene chains and benzene rings, and additionally PPO had ether moieties. The sites in sorption domains with heterogeneous compositions had dissimilar sorption affinity for a given sorbate.8,11,12 This could be one reason for nonlinear sorption behaviors of all tested compounds by PSs and PPO.36 Pore surface-adsorption

n

R2

Koc (0.4 Sw)

Rsolid/solution

0.986 0.821 0.993 0.994 0.944 0.997 0.983 0.812 0.795 0.707

0.998 0.997 0.999 0.999 0.998 0.993 0.999 0.996 0.980 0.984

10400 14800 14000 14600 9930 10300 12800 908 249500 220000

1/40 1/40 1/40 1.2/40 1.5/40 2/40 1.5/40 6/15 0.4/40 0.4/40

0.974 0.867 1.000 0.994 0.989 0.995 0.987 0.890 0.594 0.628

0.994 0.995 0.999 0.999 0.996 0.997 0.999 0.983 0.991 0.993

244 286 668 1140 218 406 490 26 5520 1410

12/8 11/8 12/8 13/8 10/8 16/8 14/8 25/4 4.5/40 2.5/8

1.000 0.832 1.000 1.000 0.997 1.000 1.000 0.812 0.557 0.567

0.997 0.996 0.997 0.991 0.997 0.992 0.994 0.993 0.993 0.992

553 514 935 881 575 667 830 176 4820 11600

6.5/8 6.5/8 6.5/8 7/8 6/8 5/8 4/8 14/8 1/8 0.8/15

0.989 0.999 0.988 0.997 1.000 1.000 0.999 0.963 0.731 0.769

0.989 0.991 0.988 0.985 0.981 0.981 0.992 0.988 0.983 0.988

88 78 70 101 67 64 66 67 101 223

20/4 15/4 15/4 15/4 15/4 15/4 15/4 20/4 15/4 10/4

a

Sw: water solubility (mg/L); Koc: organic carbon content-normalized sorption coefficient (L/kg); Rsolid/solution: solid-to-solution ratio (mg/ mL).

(pore-filling) mechanism has widely been used to interpret the nonlinear sorption behavior of HOCs to SOMs with abundant condensed sorption domains, and it was applicable for the sorbents that were composed mainly of fused benzene rings.16,37 Such a mechanism should be operating for PSs and PPO because they contained substantial benzene rings and a large portion of condensed domains as indicated by their high Tg values (i.e., 100.2 °C for PS1 and 216.2 °C for PPO), large SA and high porosity (i.e., PS2 and PPO) (Table 2). However very low SA and porosity were detected for PS1 despite its Tg value at 100.2 °C (Table 2). This could be a result that a 7256

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which in turn exerted substantial influence on sorption intensity for HOCs. Effect of the Spatial Arrangement of Domains in Sorbents. It was reported that polarity of SOMs in humic substances was inversely correlated with the Koc values of HOCs.5,7 As a polar polymer, PPO had substantial Ocontaining functional groups (ether moieties) as indicated by its high oxygen content (13.68%) (Table 1). If these Ocontaining functionalities were exposed outside and adsorption is a dominant sorption mechanism, they may act as H bond acceptor and interact with water molecules (H bond donor), thus forming water clusters at its surfaces through Hbonding.26,27,43 The water clusters would reduce the accessibility of HOCs to the sorption domains and compete with them for sorption sites, thereby reducing their sorption.44 However, it was interesting to note that the observed Koc values of phenanthrene, naphthalene, lindane, and 1-naphthol by PPO were 1−3 orders of magnitude higher than those by PS1, and the Koc value of phenanthrene by PPO was comparable to that by PS2 and higher than that of naphthalene and 1-naphthol by this polymer, although these sorbates had quite different composition. This indicates that the O-containing functionalities did not significantly suppress HOC sorption to PPO. The XPS and elemental composition data showed that the surface polarity (O/C) of PPO was lower than its corresponding bulk value (0.09 and 0.13, Table 1), further supporting that a portion of O-containing moieties were masked in the interior, while leaving the sorption domains of benzene rings and methylene chains exposed outside to water. The PPO had a contact angle of 142.97°, and this value was even higher than that of PS1 and PS2, again showing the very hydrophobic nature at its interface with water (Table 2). Such a spatial arrangement of the sorption domains in PPO favored its surface adsorption for the tested compounds. It has been falsely assumed that nonpolar sorbents, including polymers, always have higher sorption for HOCs than the polar ones. Sorption intensity comparison of the tested compounds by PPO, PS1, and PS2 suggested that in many cases, the “like-dissolves-like” concept is not mechanistically operative for sorption by solid materials.43 This point was also noted by Endo et al.26 The reason was that HOC sorption to the sorbents, including polymers, was dependent on the relative strength of the attractive interactions between water and water molecules and those between water and HOC molecules, as well as the cavity formation energy of HOCs in water phase.43 Results of this study further revealed that the spatial arrangement of sorption domains in polymers also had great influence on their sorption for HOCs especially for those with abundant polar functionalities. Environmental Implications. This study demonstrated that sorption of HOCs by polymers of identical chemical composition was highly dependent on their abundance of rubbery domains. The spatial arrangement of domains in polymers was identified to be an important factor affecting their sorption for HOCs. The polymer-based observations are valuable for elucidating the underlying interaction mechanisms between HOCs and SOMs of complicated chemical composition and diverse physical forms of sorption domains. This can be achieved by using polymers of simple structure and composition as model SOMs, thus we are able to control some of their characteristics relatively constant but aim to examine the effect of the variations in other specific properties of SOMs (e.g., physical makeup of sorption domains and

portion of its domains were frozen at 77 K thus decreasing their mobility. It was difficult for N2 molecules to penetrate into some inner nanoscaled micropores of PS1, and N2 molecules could mainly sorb on the external surface, thus its SA and porosity were underestimated using N2 sorption at such a low temperature as stated before.28 Same phenomenon could also occur for PS2 and PPO. Hence sorption isotherms of all tested compounds to these polymers were generally nonlinear (Figure 1, Table 3). The Tg of PS2 was not detected from DSC analysis even though the temperature was increased to 280 °C, which can be ascribed to its heterogeneous structure as induced by crosslinking (20%). The heterogeneous structure made PS2 nonuniformly absorb heat during the heating process, thus an obvious endothermic peak could not be observed from DSC analysis. In addition, it was reported that the thermal stability of PS2 increased with increasing cross-linking degree of styrene and divinylbenzene.38 It was most likely that 20% cross-linking was high enough to freeze the motion of a large portion of extensive segments in the polymer backbone, hence the glass transition was too weak to be detected.39 Consistent with our observation, Li et al.38 reported that no glass transition was observed for PS2, when the cross-linking degree was over 30%. Theoretically, sorption of the tested compounds to PSs and PPO should be dominated by a surface-adsorption (both external and pore surfaces) process, because they were glassy polymers with abundant condensed domains. The Koc values of phenanthrene, lindane, naphthalene, and 1-naphthol by PS2 were respectively 274.8, 212.3, 27.4, and 1.5 times as high as those by PS1 (Table 3), which can be attributed to its much higher SA and porosity (Table 2). The SA and overall porosity (a sum of macro-, meso-, and micropore volume) of PS2 were quite high (70.22 m2/g and 0.514 cm3/g), comparable to those of multiwalled carbon nanotubes with outer diameter >50 nm.32 In contrast, these values for PS1 were very small (0.23 m2/g and 0.004 cm3/g), even lower than those of PEs (Table 2). The vast discrepancy in SA and porosity of two polystyrenes (PS1 and PS2) could result from their physical makeup difference. The cross-linking structure that established and supported the skeleten of PS2 created abundant pores and sorption sites.40 Sorbate molecules could be sorbed to the sites in nanoscaled voids in PS2 though pore-filling (pore surface-adsorption). Large SA of PS2 with hydrophobic sites also provided substantial contact regions for HOC sorption. These two aspects jointly caused the much higher sorption intensity than PS1. Due to absence of cross-linking, PS1 molecules generally appeared to be linear polymers instead of rigid reticular structure as PS2. The π−π bonds were readily formed between benzene rings of neighboring molecules in PS1, thereby providing very small effective SA and porosity for HOC sorption. The SA of the cross-linked polymer (PS2) was mainly originated from surface of the smallest composition unit nuclei, and it was strongly dependent on the divinylbenzene content in monomer mixture.41 With increasing divinylbenzene content in PS2, a greater number of rigid nuclei would be created and intramolecularly cross-linked, which increased the number of micropores and their connectivity, thereby increasing its SA.42 Hence, Koc values of all tested compounds by PS1 were 1−3 orders of magnitude lower than PS2 (Figure 1, Table 3). It was evident that the cross-linking of molecular segments in polymers (e.g., PS2) greatly affected their SA and porosity, 7257

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chemical composition) on HOC sorption. Since HOCs are ubiquitous in the environment and PEs and PSs are major plastic materials of the discarded municipal solid waste, they may interact upon contact. Therefore, the findings on their interaction mechanisms are useful for predicting the transport, fate, and persistence of HOCs in municipal solid waste disposal sites, and their associated health risks.

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ASSOCIATED CONTENT

S Supporting Information *

Selected physicochemical properties of the chemicals used in this study (Table S1); sorption kinetics of phenanthrene by PS2 and PPO (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (86) 10-62757822; fax: (86) 10-62767921; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was in part supported by National Natural Science Foundation of China (40971246 and 40730737), National Basic Research Program (2007CB407301), the Startup Fund for the Peking University 100-Talent Program, the Scientific Research Foundation of the Ministry of China for Returned Chinese Scholars (413-152-008), Maoyugang Undergraduate Research Program, Peking University (126WO-472-184), and the USDA-AFRI (2009-35201-05819).

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