Environ Sci Pollut Res DOI 10.1007/s11356-016-6040-7

RESEARCH ARTICLE

Structural benefits of bisphenol S and its analogs resulting in their high sorption on carbon nanotubes and graphite Huiying Guo 1 & Hao Li 1 & Ni Liang 1 & Fangyuan Chen 1 & Shaohua Liao 1 & Di Zhang 1 & Min Wu 1 & Bo Pan 1

Received: 17 October 2015 / Accepted: 4 January 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Bisphenol S (BPS), a new bisphenol analog, is considered to be a potential replacement for bisphenol A (BPA), which has gained concern because of its potentially adverse health impacts. Therefore, studies are needed to investigate the environmental fate and risks of this compound. In this study, the adsorption of BPS and four structural analogs on multi-walled carbon nanotubes (MWCNTs) and graphite (GP) were investigated. When solid-phase concentrations were normalized by the surface areas, oxygen-containing functional groups on the absorbents showed a positive impact on phenol sorption but inhibited the sorption of chemicals with two benzene rings. Among BPS analogs, diphenyl sulfone showed the lowest sorption when hydrophobic effects were ruled out. Chemicals with a butterfly structure, formed between the two benzene rings, showed consistently high sorption on MWCNTs, independent of the substituted electron-donating or accepting functional groups. This study emphasizes the importance of chemical conformation on organic, contaminant sorption on engineered, carbonaceous materials. Keywords Bisphenol compounds . Chemical conformation . Engineered carbonaceous materials . Environmental fate . Surface accessibility Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-016-6040-7) contains supplementary material, which is available to authorized users. * Min Wu [email protected]

1

Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China

Introduction Due to the significant adverse health effects (especially endocrine-disrupting effects) of bisphenol A (BPA), many countries have restricted its use in certain products, such as plastic bottles for infants (Naderi et al. 2014). Because of its rigid double bonds of the O = S = O group, bisphenol S (BPS) is believed to have a higher thermal stability than BPA (Rwei et al. 2003), which is an advantageous property for plastic materials during their processing and application. BPS is now widely used as a monomer in the production of epoxy resins, cyclic carbonates, and sulfonated poly(ether ketone ether sulfone) (S-PEKES) and as a chemical additive in pesticides, dyestuffs, color-fast agents, leather tanning agents, flame retardants, and fiber improvers (Changkhamchom and Sirivat 2010, Molina-Molina et al. 2013). Thus, BPS is expected to be present in almost all of the products in which BPA was initially used and has been substituted for BPA in industrial applications (Liao et al. 2012c). Owing to its widespread application in various products, BPS has been detected in food products in concentration ranges of 0.005–0.609 ng/g fresh weight (fw) in the USA (Liao & Kannan 2013) and up to 36.1 ± 4.4 ng/g fw in canned foods from Spain (Viñas et al. 2010). Sub-nanogram per gram levels of BPS were detected in personal care products in both China and the USA (Liao & Kannan 2014). BPS concentrations were 0.002–1970 ng/g dry weight (dw) in sediments collected from the USA, Japan, and Korea (Liao et al. 2012d). In a municipal sewage sludge collected in 20 provinces of China, BPS was 0.17–110 ng/g dw (Song et al. 2014). BPS concentrations in water samples from Pearl River in China (135 ng/L) and Adyar River in India (6840 ng/L) were considerably higher than that of BPA (73 and 359 ng/L for the two rivers, respectively) (Yamazaki et al. 2015). BPS was also detected in indoor dust in the concentration range of

Environ Sci Pollut Res

0.00083 to 26.6 (μg/g) (Liao et al. 2012b). Levels as high as 22 mg/g of BPS were reported in thermal receipt papers and recycled paper products (Liao et al. 2012c). With the presence of BPS in all of these products and in the environment, BPS accumulation, not surprisingly, has also been documented in the human body. It has been detected in the urine of US and Asian residents in concentrations ranging from 0.02–21.0 ng/ mL (Liao et al. 2012a). Unfortunately, recent studies have reported that BPS has similar estrogenic activity as BPA. For example, Grignard et al. reported that BPS and BPA displayed comparable estrogenic activity to MELN cells (Grignard et al. 2012). In other studies, BPS disrupted multiple nuclear receptors of MELN cells and interfered with the endocrine system (MolinaMolina et al. 2013), whereas low concentrations (≤100 μg/L) of BPS led to developmental toxicity, impaired normal reproduction, and disturbed the balance of steroids in zebrafish (Naderi et al. 2014). Furthermore, Ji et al. reported that ≥0.5 μg/L of BPS significantly decreased egg production and the gonadosomatic index in female zebrafish, which affected the feedback regulatory circuits of hypothalamicpituitary-gonad (HPG) axis and impaired the development of offspring (Ji et al. 2013). Finally, low concentrations (femtomolar to nanomolar) of BPS also induced cell proliferation of rat pituitary cells (Viñas and Watson 2013). Accordingly, BPS may not be a safe substitute for BPA. Considering the toxicity and the high pollution potential of BPS, studies are urgently needed to investigate the environmental fate and risks of this compound. However, few studies on the environmental behavior and fate of BPS have been performed to date. This current work is thus designed to examine the sorption mechanisms of BPS on solid particles. Natural adsorbents were avoided in this initial study because of the complicated properties of these particles. Instead, carbonaceous adsorbents, such as carbon nanotubes (CNTs) and graphite, were used as model adsorbents because of their unique and well-organized structures. Three types of functionalized multi-walled CNTs, namely hydroxylized (MH), carboxylized (MC), and graphitized (MG) CNTs, were used in this study. Graphite (GP) was selected as a model adsorbent for plane graphene surfaces. Four BPS structural analogs, namely phenol (PH), bisphenol A (BPA), dapsone (DA), and diphenyl sulfone (DS), were selected for comparison to BPS to better understand the adsorption mechanisms of BPS.

Materials and methods Materials MH, MC, and MG were purchased from Chengdu Organic Chemistry Co., Chinese Academy of Sciences. These multiwalled carbon nanotubes (MWCNTs) were synthesized in the

CH4/H2 mixture at 700 °C (MG at 2800 °C) by the chemical vapor deposition method and were purified by mixed HNO3 and H2SO4 solutions to reduce the contents of metal catalyst and amorphous carbon. GP (>99 %) was purchased from Alfa Aesar. BPS (AR, 99.0 %). PH (AR, 99.0 %), DA (AR, 97.0 %), and DS (AR, 99.0 %) were obtained from Aladdin Reagent Co. Ltd., China. BPA (AR, 99.0 %) was obtained from Sinopharm Chemical Reagent Co., Ltd, China. All adsorbates were selected because of their analogous structures. The sulfonyl group for BPS, DA, and DS was a strong electron-withdrawing group (Huang et al. 2011). The surface areas of the adsorbents were determined using a multipoint Brunauer-Emmett-Teller (BET) method (Autosorb-1C, Quantachrome, USA). Their elemental compositions, including C, H, N, and O contents, were determined using an elemental analyzer (MicroCube, Elementar, Germany). Points of zero charges (pHzpc) of the CNTs were determined by plotting zeta potentials vs. solution pH with a ZetaPlus (Brookhaven, USA). The properties of adsorbents and adsorbates are listed in Table S1 and Table 1, respectively. Measurement of adsorbate solubilities The solubilities (Cs) of BPS, PH, BPA, DA, and DS were measured in background solution (0.02 M NaCl and 200 mg/L NaN3). Aliquots of adsorbates (0.5–150 mg) were separately added into 4-mL amber glass vials with Teflonlined screw caps. Three milliliter of background solution was injected into each vial. All vials were stored in a dark area and shaken in an air-bath shaker at 25 °C for 2 days. Subsequent amounts of adsorbates were added to the vials until solid residuals were observed. These vials were shaken for another 5 days. All vials were centrifuged at 1000 g for 10 min. The supernatants were subjected to determination of solute concentrations by HPLC-UV. Batch adsorption experiments BPS, PH, BPA, DA (100 mg/L), and DS (15 mg/L) were dissolved in background solution (0.02 M NaCl and 200 mg/L NaN3) separately as stock solutions. The stock solution was diluted by the background solution to a series of concentrations. The initial concentration ranges were 1– 64 mg/L for BPS, PH, BPA, and DA and 0.5–15 mg/L for DS. Batch adsorption experiments were separately conducted for BPS, PH, BPA, DA, and DS in 8-mL amber glass vials with Teflon-lined screw caps. The aqueous:solid ratios were 8000:1 (w:w) for all adsorbates on CNTs and 60:1–1000:1 on GP. Solutions of the adsorbates solution without solid particles were referred to as the initial concentration references. The pH values of the adsorption systems were in the range of 6.8–7.2. All vials were kept in the dark and were continuously shaken in an air-bath shaker (80 rpm) at 25 °C for 7 days. According

Environ Sci Pollut Res Table 1

Selected properties of PH, BPS, BPA, DA, and DS

MW molar weight, pKa the negative log-transformed acid dissociation constant, Kow octanol-water partition coefficient a

Data from Lin and Xing (2008)

b

Data from http://toxnet.nlm.nih.gov/

c

Data from Hou et al. (2010)

d

Data from http://www.drugbank.ca/drugs/DB00250

e

Data from Wick and Gschwend (1998)

f

Solubility was measured in background solution at room temperature

to our preliminary study, 7 days was enough to reach apparent sorption equilibrium. All vials were then centrifuged at 1000 g for 10 min, and the supernatants were sampled for adsorbate quantification. For the pH-dependent adsorption study, the initial adsorbate concentrations were fixed at 10.0 mg/L. Each suspension was adjusted to a pH of 2–11 with HCl or NaOH, and the final pH was also measured after equilibrium. The other experimental procedure was the same as described above. All experiments were run in duplicate.

acetonitrile:deionized water, and the wavelength for the UV detector was 257 nm. The mobile phase for BPA, DA, and DS analysis consisted of a 40:60 (v:v) ratio of acetonitrile:deionized water, and the wavelength for the UV detector for each analyte was 280, 290, and 234 nm, respectively. The flow rate of the mobile phases was 1 mL/min and all the sample injection volumes were 10 μL.

Data analysis Quantification of adsorbates All of the aqueous-phase equilibrium concentrations of adsorbates in the adsorption experiment were analyzed on an HPLC instrument (Agilent Technologies 1200) equipped with a reversed-phase C18 column (5 μm, 4.6 × 150 mm, Acclaim 120, Dionex). For PH analysis, the mobile phase consisted of a 20:78:2 (v:v:v) ratio of acetonitrile:deionized water:acetic acid. The wavelength for the UV detector was 268 nm. The mobile phase for BPS consisted of a 35:65 (v:v) ratio of

The following logarithmic forms of the Freundlich model (FM) and Polanyi-Manes model (PMM) were used for data processing: FM : logS e ¼ logK F þ n logC e PMM : logS e ¼ logS þ ZðRTlnðC s =C e ÞÞ 0

ð1Þ a

ð2Þ

In these equations, Se (mg/kg) and Ce (mg/L) represent the equilibrium solid-phase and aqueous-phase concentrations, respectively. KF [(mg/kg)/(mg/L)n] is the Freundlich sorption

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coefficient, and n is the Freundlich nonlinearity factor. S0 (mg/ kg) is the PMM adsorption capacity. Z and a are fitting parameters of PMM. RTln(Cs/Ce) (kJ/mol) is the effective adsorption potential, in which R is the universal gas constant [8.314 × 10−3 kJ/(mol K)], and T is the absolution temperature (298.15 K in this study). Cs (mg/L) is the solubility of adsorbates at 25 °C. The angles formed between two benzene rings of the molecules were calculated using ChemBioOffice 2010. The molecular structure was optimized using molecular mechanics theory (MM2) (Trubiano et al. 2004).

Results and discussion Fitting of adsorption isotherms The adsorption isotherms of BPS and the four structural analogs, BPA, DA, DS, and PH, on MWCNTs and GP are shown in Figure S1. After initial comparison of the fitting performance using different models, the adsorption isotherms of BPS, BPA, DA, and DS were fitted with PMM, and the adsorption isotherms of PH were fitted with FM. The fitted parameters are summarized in Table 2. The r2adjs PMM values Table 2

BPA

DA

DS

PH

Accessibility of difference surfaces The surface areas of the adsorbents varied from 3.4 m2/g for GP to 228.0 m2/g for MH (Table S1). The lower surface area of GP may have been responsible for its lower adsorption than MWCNTs. When solid-phase concentrations (Se) were normalized by surface area (SA), PH showed higher sorption (Se/SA) on MH and MC than on MG and GP. On the contrary, the other chemicals showed higher sorption on MG and GP than on MH and MC. This difference indicates that the dominant adsorption mechanism of PH may be different from other chemicals. MH and MC are both surface modified with oxygencontaining functional groups. The hydroxyl and carboxyl functional groups on particular surfaces may provide sites for hydrogen bonds with adsorbates. However, at the same

Isotherm fitting results for BPS, BPA, DA, DS, and PH Adsorbents

BPS

were in the range of 0.967 to 0.997, and those for the FM model were 0.954 to 0.986, indicating satisfactory fitting performance. Single point adsorption coefficients, Kds, were calculated at Ce = 1 and 10 mg/L based on the fitting result of PMM or FM and are also listed in Table 2. The chemicals exhibited lower adsorption on GP than on MWCNTs, and PH showed the lowest adsorption on both MWCNTs and GP (Table 2).

r2adj

SEE

logS0

SE

Z

SEc

a

SE

Kd (L/kg) Ce = 1 mg/L

Ce = 10 mg/L

MG

0.994

0.030

4.850

0.025

−2.00E-04

1.00E-04

2.92

0.43

12,330

4185

MH MC GP

0.990 0.996 0.984

0.044 0.024 0.037

4.990 4.768 3.858

0.041 0.020 0.201

−3.00E-04 −2.00E-04 −4.28E-02

3.00E-04 7.75E-05 4.59E-02

2.77 2.99 1.20

0.37 0.16 1.53

17,080 6635 400.4

5605 3111 123.3

MG

0.981

0.061

5.011

0.063

−5.00E-04

5.00E-04

2.82

0.19

14,180

6524

MH MC GP MG MH MC GP MG MH MC GP

0.967 0.997 0.978 0.995 0.993 0.991 0.991 0.974 0.997 0.997 0.978

0.076 0.025 0.055 0.032 0.035 0.035 0.038 0.073 0.026 0.026 0.069

1.70E-03 4.09E-05 1.00E-04 6.70E-03 2.40E-03 2.20E-03 1.40E-02 1.03E-01 1.96E-02 2.25E-02 1.27E-02 SEc 0.047 0.040 0.027 0.054

16,300 8335 322.3 10,040 18,290 11,560 381.1 8121 9769 5880 331.9

6434 4298 161.0 4704 6148 4095 171.8 9107 5548 3853 202.5

0.091 0.078 0.053 0.794

−1.50E-03 −8.58E-05 −6.62E-05 −2.72E-02 −7.40E-03 −5.50E-03 −4.55E-02 −1.72E-01 −1.05E-01 −1.29E-01 −3.02E-02 n 1.08 0.80 0.84 0.85

0.25 0.15 0.40 0.34 0.41 0.17 1.22 0.09 0.12 0.22 0.11

0.976 0.968 0.986 0.954

0.093 0.020 0.078 0.032 0.025 0.028 0.044 0.209 0.044 0.044 0.052 SEc 0.054 0.046 0.031 0.063

2.39 3.49 3.57 1.49 1.86 1.99 1.30 1.03 1.10 1.04 1.68

MG MH MC GRA

5.047 4.770 3.334 5.081 5.010 4.818 3.723 5.246 4.926 4.802 3.376 log KF 2.042 2.755 2.635 0.653

110.1 569.1 431.3 4.5

133.7 358.5 299.5 3.2

r2 adj adjusted coefficient of determination, SEE standard error of estimation, SE standard error, Kd Se/Ce (calculated from the fitted model equations)

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time, these functional groups may also interact with water molecules through hydrogen bonds to form water clusters (Peng et al. 2003, Wu et al. 2012, Yang and Xing 2010). The water clusters formed on the surface of MH and MC also make the surface sites less accessible to organic chemicals (Peng et al. 2003), especially for molecules with larger molecular volumes, because of steric hindrance. For this reason, the accessibility of the hydrophobic surface is higher for MG and GP than MH and MC, as observed in a previous study (Zhang et al. 2012). This feature is probably why the surface area normalized sorption (Se/SA) of BPS, BPA, DA, and DS on MH and MC was lower than that on MG and GP (Figs. 1b–e). Among the five investigated chemicals, PH possesses the smallest molecular size, which may have less surface accessibility restrictions. PH could be adsorbed in the hydrophobic pores or functional groups, which may not be easily accessible to large molecules. This difference will result in PH higher surface occupation on MH and MC.

hydrophilicity (solubility of 82,100 mg/L, Table 1), among the five adsorbates. Hydrophobic effects may play an important role in the overall adsorption on the hydrophobic surfaces of MWCNTs and GP. However, although DS is more hydrophobic than BPS (solubilities of 19.4 mg/L for DS vs. 909 mg/ L for BPS), the Kds of DS on MWCNTs (such as 5880 L/kg for MC at Ce = 1 mg/L) and GP (332 L/kg at Ce = 1 mg/L) were not higher than BPS (such as 6635 L/kg for MC at Ce = 1 mg/L) (Table 2). In addition, the logKow of the chemicals poorly correlated with the adsorption coefficients (Kds at Ce = 1 mg/L and Ce = 10 mg/L) on all four adsorbents. The equilibrium aqueous-phase concentrations could be normalized by solubility (Ce/Cs in Fig. 2) to screen for hydrophobic effects. As presented in Fig. 2, the sorption differences between chemicals were enlarged compared with the isotherms before solubility normalization. The adsorption of the other two benzene ring chemicals on CNTs and GP followed the order of DS < DA < BPA < BPS. Previous studies have suggested that the π-π interaction is stronger in donor-acceptor pairs than donor-donor or acceptor-acceptor pairs (Chen et al. 2007). The electron-withdrawing group connected to the benzene rings should strengthen the π-π interaction between the

Structural benefits of BPS and its analogs for their high adsorption PH exhibited the lowest adsorption on MWCNTs and GP (Figure S1), probably because of its relatively higher 2

3

MG MH MC GP

1.5 1

2.5 2

0.5 1.5

0

a

b 1

-0.5 -0.5

0

0.5

1

1.5

-1

2

3

3

2.5

2.5

2

2

1.5

1.5

c -1

-0.5

0

0.5

1

1.5

0

-0.5

0.5

2

-1

0

-0.5

0.5

3 2.5 2 1.5

e 1 -0.5

1.5

0

2

d

logCe (mg/L)

-1

1

1

1

log(Se/SA) µg/m2

log(Se/SA) µg/m2

Fig. 1 Adsorption isotherms of PH (a), BPS (b), BPA (c), DA (d), and DS (e) on four different adsorbents. Solid-phase concentrations were normalized to the BET surface area. All experiments were run in duplicate and each data point is one measurement

0.5

logCe (mg/L)

1

1.5

1

1.5

2

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Fig. 2 Adsorption isotherms of PH, BPS, BPA, DA, and DS on MG (a), MH (b), MC (c), and GP (d). Equilibrium aqueous concentrations are normalized to solubility. All experiments were run in duplicate and each data point is one measurement

4.5

5

a

4

4

3.5 PH BPS BPA DA DS

3

logSe (mg/kg)

2.5 2 1.5 -5.5 5 4.5

b

4.5

-3.5

-1.5

3.5 3 2.5

0.5

-5.5 4

c

3.5

-3.5

-1.5

0.5

-3.5

-1.5

0.5

d

3

4

2.5

3.5

2 3

1.5

2.5

1

2 -5.5

0.5 -3.5

-1.5

0.5

-5.5

log(Ce/Cs)

benzene rings and CNTs (Chen et al. 2007). However, DS showed relatively low adsorption at pH below 7, although it also contains a sulfonyl group. This result 500

MG MH MC GP

400 300

14000

a

b

12000 10000 8000 6000

200

4000 100

2000

0

0 1

3

5

7

9

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12000

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d

10000

8000

0

0 1

3

5

7

9

1

11

3

5

pH 10000

Kd (L/kg)

Kd (L/kg)

Fig. 3 Kd values as a function of pH for PH (a), BPS (b), BPA (c), DA (d), and DS (e) on four different adsorbents. Dashed lines represent the pKa values of the respective adsorbates. The pHZPC of MH and MC were 4. All experiments were run in duplicate and each data point is one measurement. The initial adsorbate concentrations were fixed at 10.0 mg/L

may suggest that the electron-withdrawing property of the sulfonyl group could not adequately explain BPS sorption.

e

8000 6000 4000 2000 0 1

3

5

pH

7

9

11

7

9

11

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Conversely, a hydroxyl group is electron donating. Previous studies have reported that introducing this electrondonating substitution also strengthened the π-π interaction between the aromatics and CNTs (Lin and Xing 2008). If the hydroxyl group promoted BPS sorption through π-π donor-acceptor interactions, the sorption should be enhanced if electron-donating property is further increased. However, this was not the case in the present study. For example, the electron-donating ability of amino groups was stronger than hydroxyl groups (Chen et al. 2008, Sheng et al. 2010). When the hydroxyl groups in BPS were replaced with amino groups to become DA, the adsorption decreased. Thus, π-π donoracceptor interactions were not the dominant mechanism. In comparison to the other chemicals, DA displayed a much lower pKa (2.41; shown in Table 1). The pH of the solutions after equilibrium was approximately 7, which was higher than the pKa of DA, but lower than the pKa of the other chemicals. Considering that electrostatic interactions may influence adsorption, pH-dependent sorption experiments were conducted and different adsorbate-adsorbent combinations were compared. As shown in Fig. 3, altering the pH resulted in more enhanced changes of the adsorption on MH and MC than MG and GP because of the lack of surface functional groups on MG and GP. Among different adsorbates, DS sorption showed the least dependence on pH because this chemical has no dissociable functional groups. For PH and BPA, the Kd values increased

a

slightly when the pH increased from 4 to 8 due to the electrostatic attraction occurred between PH, BPA, and MWCNTs. When the pH increased close to their pKa values (10.0 for PH and 10.1 for BPA), the electrostatic repulsion of the negatively charged adsorbents and adsorbates became dominant, which is why the Kd values decreased with increased pH. Similar results have also been reported for polar phenolics (Lin & Xing 2008). Although BPS shares similar structure and properties with BPA, the sulfonyl group may have more negative charge and thus BPS may be more significantly impacted by electrostatic repulsion. When the pH increased close to the pKa of BPS, the proportion of dissociated species (one or two dissociated –OH) of BPS increased. Overall, the high hydrophilicity of the dissociated molecules and the electrostatic repulsion between the dissociated molecules and MWCNTs likely contributed to the decreased adsorption of the chemicals in response to rising pH. The Kd values for DA slightly increased as the pH increased to levels between 2 and 4. The pKa of DA is 2.41, which means at pH 2.41, half DA molecules are neutral, while half DA molecules are protonated (positively charged). When pH increased from 2 to 4, the proportion of protonated DAwill decrease. As a result, the electrostatic repulsion between the positively charged DA and positively charged MWCNTs will decrease when pH increased from 2 to 4. When the pH was higher than the pKa of DA, the effect of pH on DA adsorption was insignificant due to the neutral molecular of DA.

c

IV III II

I

b

d

BPS

DS

Fig. 4 Schematic diagram of BPS (a, b) and DS (c, d) adsorption on MWCNTs and GP. The letters I, II, III, and IV indicated the possible adsorption areas of surface, groove, interstitial spaces, and inner pores, respectively (Pan et al. 2008). BPS molecules can wedge into the groove

MWCNTs

GP

region of MWCNTs, but this structural benefit is not available for DS molecules. GP surface sites might be quickly saturated by this planar, structural molecule

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The morphology of CNTs has also been reported to control the adsorption of organic pollutants (Pan et al. 2008). CNTs tend to aggregate in solution because of Van der Waals interactions between each particle. Thus, most CNTs would be present as aggregates or bundles in the sorption system. Organic molecules are generally too large to fit into the inner pores of CNTs, and three types of adsorption areas are available for organic contaminants on CNTs: external surface, interstitial pores, and groove areas. A BPA molecule could wedge into the groove region of CNT because of its Bbutterfly^ structure, leading to a higher adsorption than 17α-ethinyl estradiol molecules (Pan et al. 2008). This statement was confirmed in another study applying DFT calculations. Jin et al. showed that BPA molecules adsorbed on reduced graphene oxides via V-shaped structures at minimum distance to maximize π-π stacking (Jin et al. 2015). This type of butterfly structure or V-shaped structure may have structural benefit to adsorption then the flat aromatic structures (Sun et al. 2013). In this study, BPS and DA also have the butterfly structure, whereas DS is a plane molecule (Fig. 4). The intersection angle of BPS and DA was almost the same as BPA (Table 1), indicating that the BPS and DA molecules could wedge into the groove region of CNTs, similar to BPA molecules; however, DS molecules did not have this feature (Fig. 4). This difference in molecular conformation is likely one of the major reasons why DS displayed the lowest adsorption of DS after solubility normalization. The planar structure of DS may be advantageous in adsorption onto GP; however, because of the better fitness of DS on the GP surface compared with the other chemicals, the GP surface sites may be saturated quickly.

Conclusions Our experimental results demonstrate that chemicals with a butterfly structure formed between the two benzene rings show consistently high sorption on MWCNTs, independent of the substituted electron-donating or accepting functional groups. A chemical with two benzene rings presented in the same plane did not show this structural advantage for its sorption. As the first experimental attempt to investigate the adsorption mechanisms of BPS, the results of this study are expected to offer useful information towards understanding and predicting the environmental fate and risks of BPS.

Acknowledgments This research was supported by the National Natural Scientific Foundation of China (41222025, 41273138), Program for New Century Excellent Talents in University, Chinese Ministry of Education (NCET-10-0971), and Recruitment Program of Highly-Qualified Scholars in Yunnan (2010CI109).

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