Adsorption of Bisphenol A Based on Synergy between Hydrogen

DOI: 10.1021/la502816m. Publication Date (Web): November 3, 2014 .... Thatchaphong Phatthanakittiphong , Gyu Seo. Nanomaterials 2016 6 (7), 128 ...
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Article pubs.acs.org/Langmuir

Adsorption of Bisphenol A Based on Synergy between Hydrogen Bonding and Hydrophobic Interaction Xiangyu Zhou,†,‡,§ Junfu Wei,*,†,‡ Kai Liu,†,‡ Nana Liu,†,‡ and Bin Zhou†,§ †

State Key Laboratory of Hollow Fiber Membrane Materials and Processes, ‡School of Environmental and Chemical Engineering, and School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, P. R. China

§

S Supporting Information *

ABSTRACT: The study mainly investigated the synergetic adsorption of hydrogen bonding and hydrophobic interaction. To simplify the adsorption driving forces and binding sites, the hydrophilic and hydrophobic microdomain was introduced onto polypropylene (PP) nonwoven. The amphiphilic structure was constructed for the adsorption of bisphenol A (BPA). A solvent shielding experiment was conducted to calculate the contributions of diverse interactions. Also, a specific structure without hydrophilic microdomain was constructed as comparison to determine the adsorption rate and quantify the diffusion behaviors. On the basis of doubleexponential model, the adsorption process can be distinctly divided into three stages, namely film diffusion stage, intralayer diffusion stage, and dynamic equilibrium stage. The adsorption rate was dramatically improved due to the influence of hydrophilic microdomain and participation of hydrogen bonding adsorption. Discussions on adsorption priority were also proposed. The results of surface energy heterogeneity revealed that the hydrophilic microdomain or the hydrogen bonding site was occupied preferentially.

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INTRODUCTION In comparison to other physical methods, adsorption is an effective separation method for the removal organics from dilute aqueous solutions.1−3 Thus, adsorption has been widely used in the fields of decontamination, purification, bioengineering, etc. However, descriptions or simulations of adsorption process can be of great difficulty, which could be attributed to two major reasons. First, conventionally used adsorbents (e.g., activated carbon) have extremely complex surface chemistries, resulting in a wide variation in types of binding sites available to the solutes. The diverse functional groups onto the surface of adsorbents have tremendous impacts on adsorption behaviors.4 The second reason is the participation of diverse driving forces between adsorbents and adsorbates, such as chemisorption, hydrophobic interaction, hydrogen bonding, electrostatic interaction, complexation, etc.5 Actually, they seldom work in isolation. The adsorption process is controlled or influenced simultaneously by synergy of various interactions.6 However, present studies mostly emphasize on the importance of individual interaction but seldom propose a method to determine the relative contribution of diverse interactions. Originally, microdomain is a biochemical concept which describes functional unit embedded into biomembrane. It is a specific region of cell membrane that has distinct structures and functions.7−9 Actually, similar structures (e.g., various functional groups) exist on the surface of common adsorbents. They may have tremendous impacts on the adsorption behavior © 2014 American Chemical Society

and many studies on adsorption mechanism have discovered this phenomenon.10,11 However, because of the complexity of functional groups and diversity of adsorption driving forces, previous studies have rarely focused on this notion. In this work, to simplify adsorption interactions, the hydrophilic and hydrophobic microdomain was introduced onto polypropylene (PP) nonwoven. Polypropylene materials are generally utilized in adsorption and filtration process.12 They are attractive materials due to the random network of overlapped fibers, multiple connected pores, high thermal and chemical stability, and low cost.13,14 Furthermore, there are little available sites on the surface, and thus little interactions exist between PP matrix and specific adsorbates (such as BPA in Figure 5).15 The original PP matrix turns to be an excellent supporter of bonding sites. After amphiphilic microdomain was introduced onto PP surface (Figure 1), the driving forces can be restricted to hydrogen bonding and hydrophobic interaction only. Another key point for the determination of synergetic adsorption is selection of target adsorbate. It is noted that bisphenol A (BPA) is an organic compound with hydrophilic hydroxyl groups and hydrophobic aromatic groups (Figure 2a). The amphiphilic property makes BPA molecule an excellent Received: July 17, 2014 Revised: October 16, 2014 Published: November 3, 2014 13861

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Figure 1. Schematics of surface modification and structures of microdomain.

model compound for this study. Furthermore, BPA is known as one of endocrine disrupting chemicals (EDCs).16−18 Its toxicological features make this research have more practical significance. The modified PP nonwoven (PP-g-GMA-OA) with hydrophilic and hydrophobic microdomain could be further utilized in separation or decontamination. In this study, the amphiphilic structures were constructed and the hydrophilic and hydrophobic microdomain was introduced onto surface of PP nonwoven. Afterward, the amphiphilic PP nonwoven was tested and evaluated as simplifier of adsorbent materials. To elucidate the synergy between hydrogen bonding and hydrophobic interaction and clarify the contributions of different interactions, a solvent shielding experiment was designed. Besides, a unique structure without hydrophilic microdomain was constructed as comparison to quantify the diffusion process and adsorption rate. The adsorption process could be divided into three steps according to the double-exponential kinetics model. In addition, the contribution of hydrophobic interaction and hydrogen bonding could be quantitatively calculated according to comparative structure and shielding experiment. A series of speculations, verifications, and discussions on synergetic adsorption, diffusion process, and adsorption priority were proposed.

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MATERIALS AND METHODS

Adsorbent Preparation. The amphiphilic PP-g-GMA-OA with the hydrophilic and hydrophobic microdomain was prepared by grafting of glycidyl methacrylate (GMA) onto PP nonwoven through electron beam (EB) radiation induced graft polymerization and subsequently converted the epoxy group of GMA to hydrophilic hydroxyl and hydrophobic long-chain alkylamine through reaction with n-octylamine.19 The schematics of surface modification are shown in Figure 1. Characterization. To analyze the functional groups on the surface of PP samples, the FTIR spectra were recorded on Fourier transform infrared spectrometer (Necolet 6700) in the wavenumber range of 600−4000 cm−1 under ambient condition (Figure S1). The surface chemical composition was characterized by X-ray photoelectron

Figure 2. Schematic of hydrogen bonding and hydrophobic interaction (a) and FTIR spectra (b) of BPA, original PP nonwoven, and PP-g-GMA-OA before and after BPA adsorption.

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spectroscopy (XPS), and the analysis was carried out on an AEM PHI 5300X spectrometer with an Al Kα X-ray source (1486.71 eV of photons) to determine the C, N, and O (Figure S2). The software package Thermo Avantage 3.9.3 was used to fit the spectra peaks. PP samples were coated with gold−palladium alloy prior to observation, and the surface morphologies of PP samples were observed using the scanning electron microscopy (SEM) by an S-2500C (Hitachi, Japan) microscope (Figure S3). Adsorption Experiments. The batch adsorption experiments were carried out to determine the adsorption behaviors for BPA onto modified PP nonwoven. BPA solution was dissolved in ethanol as stock solution (100 g/L) and then diluted sequentially to a series of concentrations (ranging from 5 to 100 mg/L). The volume ratio of ethanol to water was below 0.001 to avoid cosolvent effect. A certain amount of adsorbent was placed in sealed 250 mL conical flask with 100 mL of BPA solution. The bottles were placed in an incubator shaker at 150 rpm for 24 h at predetermined temperature (298, 308, and 318 K) to ensure the adsorption equilibrium. The initial (C0) and equilibrium (Ce) concentrations were determined by HPLC analysis (Waters 2695, reversed phase C18, 250 mm × 4.6 mm). The adsorption capacity was evaluated by the formula

q=

(C0 − Ce) × V m

bond of PP substrate had no obvious variation. This confirmed that there are no obvious interactions between PP substrate and BPA molecule. The variations of FTIR spectra are consistent with chemical structures and previous speculations. Besides, it should be noted that hydrophobic interaction had less influence on the FTIR spectra compared with hydrogen bonding interaction. This could be because: (1) The forces involved in formation of hydrogen bond include electrostatic interaction. The intensity of hydrogen bond is much stronger than hydrophobic interaction.23 (2) The structure of hydrophobic microdomain is similar to PP substrate (Figure 2a), the peaks overlapped with each other, and most variations in the spectra were concealed. Solvent Shielding Experiment. To evaluate the synergetic adsorption between hydrogen bonding and hydrophobic interaction and determine the relative contributions of driving forces, a solvent shielding experiment was conducted. Previous studies predicted that a comparison of adsorption from organic solvents with different polarities could directly derive the relative contributions of different mechanisms.11 Herein, we altered the polarity and structure of solvent and attempted to hinder one of the interactions. In this section, adsorption isotherm experiment was conducted in water, n-hexane, and chloroform solvent to illustrate the contributions of different adsorption sites. The isotherm data in the experiment was fitted to two different isotherm models:

(1)

where C0 and Ce are the initial and the final concentrations (mg/L), respectively; V is the volume of the solution (L); m is the mass of adsorbent (g). To analyze the adsorption kinetics, the adsorbents were added with an initial BPA concentration of 100 mg/L at room temperature. Experiments were conducted at various time intervals to determine the kinetic parameters.

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Langmuir model:

RESULTS AND DISCUSSION Adsorption Driving Forces. Function can be regarded as an embodiment of structure. Given the amphiphilic structure of PP-g-GMA-OA adsorbent and BPA molecule (Figure 2a), hydrophobic interaction and hydrogen bonding may be involved in the adsorption process. It is noted that weak interactions could change the bond lengths or energies and then affect vibration and rotation of molecules. In theory, the X−H (such as O−H and N−H) bond could lengthen the hydrogen bond, leading to a red-shift of X−H stretching frequency and an increase in the infrared absorption cross section for the X−H stretching vibration.20 The influence of hydrophobic interaction and proximity of BPA molecules restricted the vibration of hydrophobic microdomain, and the infrared peaks may be shifted to a certain extent.21,22 Consequently, FTIR spectra can be a useful tool for identifying the interactions between molecules and providing informations of functional groups. The FTIR spectra of BPA, original PP nonwoven, and PP-gGMA-OA before and after adsorption of BPA are shown in Figure 2b (detailed data are shown in Figure S4). The wide peak at 3412 cm−1 corresponding to the stretching vibration of N−H and O−H groups of PP-g-GMA-OA shifted to 3360 cm−1. The shift indicated that the hydroxyl and amino groups take part in the adsorption as hydrogen bonding sites. Also, the peak of carbonyl shifted from 1730 to 1734 cm−1 after adsorption, indicating that carbonyl is a proton acceptor as well. The peaks at 1245 and 1150 cm−1 which is assigned to the stretching vibration of C−N and C−C bonds had a small shift toward larger wavenumber (from 1251 to 1259 cm−1 for C−N bonds, from 1160 to 1168 cm−1 for C−C bonds). This could be deemed to be the influence of hydrophobic interactions. The peaks responding to stretching vibration (3000−2800 cm−1) and deformation vibration (1455 and 1370 cm−1) of the C−H

Freundlich model:

qe = (qmKLCe)/(1 + KLCe)

qe = KFCe1/ n

(2) (3)

where qe is the amount of adsorbate adsorbed at equilibrium (mg/g), Ce is the equilibrium solution phase concentration (mg/L), KF is the Freundlich affinity coefficient [(mg/g)/(mg/ L)(1/n)], n is the dimensionless number related to surface heterogeneity, qm is the maximum adsorption capacity (mg/g), and KL is the Langmuir constant related to the energy of adsorption (L/mg). Equilibrium data of Langmuir and Freundlich for adsorption of BPA in water, n-hexane, and chloroform solvent are displayed in Figure 3. The relative parameters obtained from nonlinear analysis are presented in Table 1. According to regression coefficients (R2), it could be observed that the Langmuir model is a little more suitable than the Freundlich

Figure 3. Adsorption isotherm of BPA on PP-g-GMA-OA in different solvents. 13863

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Table 1. Isotherm Parameters for the Adsorption of BPA in Different Solvents Langmuir

Freundlich

solvent

KL (L/mg)

qm (mg/g)

R2

KF [(mg/g)/(mg/L)(1/n)]

n

R2

chloroform water n-hexane

0.0737 0.0467 0.0314

47.95 45.59 18.66

0.9917 0.9919 0.9906

8.32 5.36 1.63

2.58 2.16 2.06

0.9635 0.9848 0.9622

model. It is known that the Langmuir isotherm model is derived to simulate the monolayer adsorption on homogeneous surface. The Freundlich model is an empirical equation, which is usually employed to heterogeneous systems and can be applied to multilayer adsorption.24 It is inappropriate to draw a conclusion that the adsorption took place in a monolayer adsorption manner only based on the regression coefficients (R2) because the regression coefficients of Langmuir (>0.99) and Freundlich (>0.96) isotherm models are quite similar. It is considered that the “butterfly” structure of BPA molecule may have great influence on the formation of multilayer and increase the dimensional restrictions of multilayer adsorption.25 Thus, the monolayer adsorption may be observed, and the Langmuir isotherm model is seemed to be more suitable. As depicted in Figure 3, solvent type has great influence on BPA adsorption. The adsorption capacities order is chloroform > water > n-hexane. The regularity is mainly caused by competitive adsorption of solvent, which is a common phenomenon in solid−liquid adsorption system. In chloroform, there is little interaction between the polar chloroform solvent and PP-g-GMA-OA. Thus, both hydrogen bonding and hydrophobic interaction are involved in adsorption. The chloroform solvent molecules would compete less with BPA molecules, resulting in a higher adsorption capacity (qm). In aqueous solution, the hydrogen bonding sites or the hydrophilic microdomain is hindered by water molecules. The hydrophobic interaction becomes a predominant mechanism, and the hydrogen bonding is actually isolated by aqueous environments to some extent, leading to a lower adsorption capacity (qm). Similarly, n-hexane molecules can be adsorbed onto hydrophobic microdomain and majority of hydrophobic interaction is excluded. According to adsorption capacity (qm) in different solution, it could be preliminarily deemed that the hydrophobic interaction may be the predominant driving force. Adsorption Process. To further illustrate the synergy of hydrogen bonding and hydrophobic interaction in adsorption rate and diffusion behaviors, a specific structure was constructed as comparison. It is noticed that the structure of lauryl methacrylate (LMA) (Figure 4b) is similar to the grafted monomer in this study (Figure 4a) but lacks the hydrophilic parts (hydroxyl and amino groups). Therefore, PP-g-LMA was constructed as comparison. Previously, several mathematical methods have been proposed to describe adsorption kinetics, which can be generally classified as adsorption reaction model (such as pseudo-first-order kinetic model and pseudo-second-order model) and adsorption diffusion model (such as liquid film diffusion model and intraparticle diffusion model). Both models are applied to describe the kinetic process of adsorption successfully. However, they are quite different in nature and belong to different system.28 Initially, pseudo-first-order and the pseudo-second-order models were used to fit the experimental data. The pseudofirst-order kinetic model is expressed as

Figure 4. Structure of PP-g-GMA-OA (a) and PP-g-LMA (b) and schematic of adsorption process.

qt = qe(1 − e−k1t )

(4)

where k1 (min−1) is the rate constant of pseudo-first-order adsorption and qe (mg/g) and qt (mg/g) are the amount of BPA adsorbed onto PP nonwoven at equilibrium and time t (min), respectively. The pseudo-second-order kinetic model has the form qt = qe 2k 2t /(1 + qek 2t )

(5)

where k2 (g/(mg min)) is the rate constant of pseudo-secondorder adsorption. It is noteworthy that the adsorption reaction models originating from chemical reaction kinetics are based on the whole process of adsorption without considering the diffusion process. Hence, double-exponential model was used herein. It is generally utilized in the adsorption of heavy metal ion onto porous materials and presented as follows:29,30 13864

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D1 D exp( −K D1t ) − 2 exp( −K D2t ) ma ma

In this stage, the solute molecules transfer across the liquid film according to the mass balance law. The second portion describes a gradual adsorption stage (Figure 5b) where diffusion is rate limiting. In this stage, the BPA molecules penetrate into the inner grafted layer and the external film resistance can be neglected (KD2 ≪ KD1). The kinetics behavior of this stage is similar to intraparticle diffusion process. The third portion is dynamic equilibrium stage (Figure 5c). All the kinetic parameters and nonlinear regression correlation coefficients are listed in Tables 2 and 3. The pseudo-second-order model (R2 > 0.99) and double-exponential model (R2 > 0.99) were fitted better than the pseudo-firstorder model (R2 > 0.89). In addition, the experimental qe,exp values were quite close to adsorption capacity calculated from the pseudo-second-order kinetic model (q2e,cal) and doubleexponential model (qDe,cal). To quantify the initial adsorption rate ν (mg/(g min)), the double-exponential equation was derived as follows:

(6)

where D1 and D2 are adsorption rate parameters (mg/L) of rapid and slow step, respectively. KD1 and KD2 are diffusion parameters (min−1) which control the adsorption process of rapid and slow step, respectively, and ma is the mass of adsorbent. Based on double-exponential theory, the adsorption process can be divided into two steps, namely a rapid step involving external and internal diffusion, followed by a slow step controlled by intraparticle diffusion. 29−31 Although the morphology of PP adsorbent is different from porous materials, however, it can be deduced from adsorption kinetics that the adsorption and diffusion behaviors are similar. Figure 5 shows the adsorption kinetics of PP-g-GMA-OA, PP-g-LMA (densities of GMA and LMA are 22 mmol/g), and

v=

dqt dt

=

D1 D K D1 exp( −K D1t ) + 2 K D2 exp( −K D2t ) ma ma (7)

For initial conditions t = 0, v = v0 =

D1 D KD + 2 KD ma 1 ma 2

(8)

It could be observed from Table 3 that the initial adsorption rate improved significantly due to synergetic adsorption of hydrophilic and hydrophobic microdomain. Similarly, the diffusion parameters (KD1 and KD2) of PP-g-GMA-OA increased obviously. This could be the effect of the hydrophilic hydroxyl and amino groups which effectively reduced the diffusion resistance and thus accelerated the adsorption and diffusion process. Besides, the adsorption capacities dramatically improved which was ascribed to the influence of hydrophilic microdomain and participation of hydrogen bonding adsorption. Generally, the double-exponential function is usually used to describe a two-step adsorption mechanism controlled by diffusion. However, it should be noted that there are two types of adsorption sites with different energy, namely hydrogen bonding and hydrophobic site. It should be considered whether the BPA molecules occupied the adsorption sites with high energy preferentially. The concentration-dependent thermodynamics may be the key point to verify the hypothesis.11 In this section, Do’s model was selected and it is generally used to illustrate the surface energy heterogeneity of adsorbent, which takes the isosteric adsorption enthalpy change as a function of the adsorbate fractional loading:32

Figure 5. Adsorption kinetics of BPA onto PP-g-GAM-OA, PP-gLMA, and original PP nonwoven and schematics of diffusion process.

original PP nonwoven. The adsorption of BPA increased quickly with time and then gradually reached equilibrium. The removal rate of BPA onto PP-g-GMA-OA can reach more than 80% in the initial 200 min, and apparent equilibrium is reached within 600 min. In addition, it could be observed that the original PP nonwoven has no obvious interactions, which coincides with FTIR spectra. Based on adsorption kinetics, the plot may present multilinearity and can be distinctly divided into three steps. The first, sharper portion is attributed to the diffusion of BPA molecules through the solution to the external surface of adsorbent or the film diffusion of solute molecules (Figure 5a).

Table 2. Parameters for Pseudo-First-Order and Pseudo-Second-Order Kinetic Model pseudo-first-order kinetic model

pseudo-second-order kinetic model

adsorbent

qe,exp (mg/g)

k1 (min−1)

q1e,cal (mg/g)

R2

k2 (g/(mg min))

q2e,cal (mg/g)

R2

PP-g-GMA-OA PP-g-LMA original PP

38.33 27.32 2.94

0.0164 0.0082 0.0054

35.14 24.52 2.67

0.8834 0.8937 0.9784

4.47 × 10−4 3.43 × 10−4 1.99 × 10−4

38.74 27.46 3.01

0.9963 0.9972 0.9820

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Table 3. Parameters for Double-Exponential Model rapid step qDe,cal

adsorbent PP-g-GMA-OA PP-g-LMA original PP

D1 (mg)

38.26 27.48 2.91

slow step −1

KD1 (min )

1.86 0.72 0.13

D2 (mg)

0.0408 0.0394 4.66 × 10−3

⎫ ⎧ αβθ ⎬ + μθ ΔH(θ ) = ΔH0⎨1 − [1 + (β − 1)θ ] ⎭ ⎩

1.80 1.85 0.12

KD2 (min−1) −3

3.10 × 10 2.86 × 10−3 4.65 × 10−3

R2

ν0 (mg/(g min))

0.9978 0.9977 0.9865

0.8091 0.3426 0.0116

(9)

where ΔH(θ) is the isosteric adsorption enthalpy change at the loading of θ, ΔH0 is the enthalpy change at zero loading, β is the pattern parameters of surface heterogeneity, μ is the interaction energy of the adsorbate adsorbed onto adsorbent, and α is defined as α = δH /ΔH0

(10)

where α represents the extent of surface energy heterogeneity of adsorbent and δH reflects the enthalpy change with loading fraction from 0 to 1. As for the adsorption of BPA onto PP-g-GMA-OA and PP-gLMA in chloroform solvent, ΔH(θ) can be obtained directly from the adsorption isoster from Figures S5 and S6 of the Supporting Information. The calculated isosteric enthalpy change was pretty well fitted by eq 10, and the four parameters, ΔH0, α, β, and μ are listed in Table 4. The correlation coefficient (R2) indicates that the fitted results are reliable, and Do’s model can describe the surface energy heterogeneity efficiently. Table 4. Nonlinear Fitting Results of Isosteric Adsorption Enthalpy and Parameters adsorbent

−ΔH0 (kJ/mol)

α

β

M

R2

PP-g-GMA-OA PP-g-LMA

49.18 32.07

0.798 0.760

5.21 5.34

3.97 4.12

0.9985 0.9941

It is known that energy can be a reflection of process. It can be obtained from Table 4 and Figure 6a that the extrapolated initial adsorption enthalpy changes (−ΔH0) of PP-g-GMA-OA is much larger than that of PP-g-LMA. The larger enthalpy changes could be the influence of hydrogen bond. In this system, hydroxyl and alkylamine acted as proton donator or acceptor. The hydrogen bonding energy is usually in the range of 8−50 kJ/mol, which is much higher than hydrophobic interaction and the participation of hydrogen bond improved the adsorption enthalpy changes markedly.33 On the other hand, it can be concluded that adsorption enthalpy changes decreased significantly with the increase of fractional loading. The hydrogen bonding has greater influence on the initial stage (θ < 0.4). In other words, the high-energy hydrophilic microdomain/hydrogen bonding site was occupied preferentially. From the point of energy, the hydrogen bond offered more energy to overcome the energy barrier of adsorption and diffusion process, thus increasing the adsorption rate. As a summary, the adsorption process of PP-g-GMA-OA can be described as follows: (1) The entire adsorption process in this system can be divided into three steps. In film diffusion stage, BPA molecules transfer across the liquid film toward PP surface. In the intralayer diffusion stage, the BPA molecules transferred from the surface into the inner grafted layer and the

Figure 6. Do’s model of BPA adsorption onto PP-g-GMA-OA and PPg-LMA (a) and schematic of adsorption priority (b).

external film resistance can be neglected. The last stage is dynamic equilibrium stage. (2) The hydrogen bonding sites effectively reduced the diffusion resistance and accelerated adsorption. (3) During each step, BPA molecules may occupy the high-energy hydrogen bonding sites first and then spread to low-energy hydrophobic sites (Figure 6b). Quantification of Driving Forces. Based on the comparative structure and isotherm data in solvent shielding experiment, the contributions of hydrophobic interaction and hydrogen bonding could be quantitatively calculated. In chloroform solvent, it is assumed that both hydrogen bonding and hydrophobic interaction are involved in adsorption. Thus, the adsorption capacity can be considered as an addition of both interactions: aqhydrophobicity + bqH‐bond = qe,exp(chloroform,PP‐g‐GMA‐OA) = 41.37 mg/g 13866

(11)

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where qhydrophobicity and qH‑bond are adsorption capacity of hydrophobic interaction and hydrogen bonding (mg/g), respectively. a and b are participation factors of hydrophobic interaction and hydrogen bonding in chloroform. In theory, chloroform solvent has inevitable influence on adsorption process; thus, the values of a and b are defined as

a ≤ 1,

b≤1

* Supporting Information Experimental details and characterizations of original and modified PP nonwoven. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail [email protected]; Tel +86-022-8395-5898 (J.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (No. 41301542), National High Technology Research and Development Program of China (2013AA065601), and Key Technologies R&D Program of Tianjin (13ZCZDGX00500).

(13)

dqhydrophobicity + qH‐bond = qe,exp(n‐hexane,PP‐g‐GMA‐OA)

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(14)

where c and d are participation factors which determine the contribution of hydrogen bonding in aqueous solution and hydrophobic interaction in n-hexane, respectively. In addition, the PP-g-LMA has similar structure to amphiphilic PP-g-GMA-OA but lacks hydrophilic parts; thus, the PP-g-LMA adsorption can be considered as the effect of hydrophobic interaction only: qhydrophobicity = qe,exp(water,PP‐g‐LMA) = 27.32 mg/g

AUTHOR INFORMATION

Corresponding Author

qhydrophobicity + cqH‐bond = qe,exp(water,PP‐g‐GMA‐OA)

= 16.37 mg/g

ASSOCIATED CONTENT

S

In aqueous solution, the hydrogen bonding sites or the hydrophilic microdomain is hindered by water molecules. Similarly, in n-hexane solvent, n-hexane molecules can be adsorbed onto hydrophobic microdomain and thus hinder hydrophobic interaction. The participation factors were also added in equations

= 36.13 mg/g

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REFERENCES

(1) Du, Z.; Deng, S.; Bei, Y.; Huang, Q.; Wang, B.; Huang, J.; Yu, G. Adsorption behavior and mechanism of perfluorinated compounds on various adsorbentsA review. J. Hazard. Mater. 2014, 274, 443−454. (2) Liu, X.; Chen, G. R.; Lee, D. J.; Kawamoto, T.; Tanaka, H.; Chen, M. L.; Luo, Y. K. Adsorption removal of cesium from drinking waters: A mini review on use of biosorbents and other adsorbents. Bioresour. Technol. 2014, 160, 142−149. (3) Dąbrowski, A.; Podkościelny, P.; Hubicki, Z.; Barczak, M. Adsorption of phenolic compounds by activated carbona critical review. Chemosphere 2005, 58, 1049−1070. (4) Maity, N.; Payne, G. F. Adsorption from aqueous solutions based on a combination of hydrogen bonding and hydrophobic interactions. Langmuir 1991, 7, 1247−1254. (5) Gu, B.; Schmitt, J.; Chen, Z.; Liang, L.; McCarthy, J. F. Adsorption and desorption of natural organic matter on iron oxide: mechanisms and models. Environ. Sci. Technol. 1994, 28, 38−46. (6) Lin, D.; Xing, B. Adsorption of phenolic compounds by carbon nanotubes: role of aromaticity and substitution of hydroxyl groups. Environ. Sci. Technol. 2008, 42, 7254−7259. (7) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Local control of microdomain orientation in diblock copolymer thin films with electric fields. Science 1996, 273, 931−933. (8) Zhang, W.; Trible, R. P.; Samelson, L. E. LAT palmitoylation: its essential role in membrane microdomain targeting and tyrosine phosphorylation during T cell activation. Immunity 1998, 9, 239−246. (9) De, R. C.; Park, C.; Thomas, E. L.; Lotz, B. Microdomain patterns from directional eutectic solidification and epitaxy. Nature 2000, 405, 433−437. (10) Moreno-Castilla, C. Adsorption of organic molecules from aqueous solutions on carbon materials. Carbon 2004, 42, 83−94. (11) Pan, B.; Xing, B. Adsorption mechanisms of organic chemicals on carbon nanotubes. Environ. Sci. Technol. 2008, 42, 9005−9013. (12) Matsuyama, H.; Yuasa, M.; Kitamura, Y.; Teramoto, M.; Lloyd, D. R. Structure control of anisotropic and asymmetric polypropylene membrane prepared by thermally induced phase separation. J. Membr. Sci. 2000, 179, 91−100. (13) Thakur, V. K.; Vennerberg, D.; Kessler, M. R. Green aqueous surface modification of polypropylene for novel polymer nanocomposites. ACS Appl. Mater. Interfaces 2014, 6, 9349−9356. (14) Park, H. J.; Na, C. K. Preparation of anion exchanger by amination of acrylic acid grafted polypropylene nonwoven fiber and its ion-exchange property. J. Colloid Interface Sci. 2006, 301, 46−54.

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Combining all equations and inequations, it could be calculated that the value of qhydrophobicity (27.32 mg/g) is much larger than qH‑bond (ca. 14.05 mg/g). The hydrophobic interaction is the predominant driving force in this system. The value of c (≤ 0.6270) confirmed that the hydrogen bonding is hindered in aqueous. Also, the value of d (≤0.0849) indicated that the nhexane has tremendous impacts on hydrophobic interaction and the hydrophobic microdomain is mainly occupied by nhexane.

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CONCLUSIONS In this study, hydrophilic and hydrophobic microdomain was constructed onto PP nonwoven and the synergetic adsorption of hydrophobic interaction and hydrogen bonding was illustrated in detail. There are three major conclusions from this work: 1. According to solvent shielding experiment and comparative structure, it could be calculated that hydrophobic interaction was the predominant driving force compared with hydrogen bonding. Hydrophobic microdomain played an important role in adsorption capacity. 2. Based on adsorption kinetics, the adsorption could be distinctly divided into three steps, namely film diffusion stage, intralayer diffusion stage, and dynamic equilibrium stage. The existence of hydrophilic microdomain accelerated the adsorption process. 3. The results of surface energy heterogeneity revealed that the hydrogen bonding sites with high energy could be preferentially occupied. 13867

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(15) Thakur, V. K.; Vennerberg, D.; Kessler, M. R. Green aqueous surface modification of polypropylene for novel polymer nanocomposites. ACS Appl. Mater. Interfaces 2013, 5, 1302−1309. (16) Zhang, L.; Fang, P.; Yang, L.; Zhang, J.; Wang, X. Rapid method for the separation and recovery of endocrine-disrupting compound bisphenol AP from wastewater. Langmuir 2013, 29, 3968−3975. (17) Wang, F.; Yao, J.; Sun, K.; Xing, B. Adsorption of dialkyl phthalate esters on carbon nanotubes. Environ. Sci. Technol. 2010, 44, 6985−6991. (18) Teramoto, K.; Kimata, S.; Harada, T.; Chuman, M.; Sakohara, S. Separation of bisphenol-A by unimolecular micelles formed by pHresponsive polymer grafted onto polypropylene nonwoven fabrics. Chem. Eng. J. 2014, 236, 490−497. (19) Zhou, X.; Wei, J.; Zhang, H.; Liu, K.; Wang, H. Adsorption of phthalic acid esters (PAEs) by amphiphilic polypropylene nonwoven from aqueous solution: The study of hydrophilic and hydrophobic microdomain. J. Hazard. Mater. 2014, 273, 61−69. (20) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Nesbitt, D. J. Definition of the hydrogen bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83 (8). (21) Xu, J.; Wang, L.; Zhu, Y. Decontamination of bisphenol A from aqueous solution by graphene adsorption. Langmuir 2012, 28, 8418− 8425. (22) Salim, C. J.; Liu, H.; Kennedy, J. F. Comparative study of the adsorption on chitosan beads of phthalate esters and their degradation products. Carbohyd. Polym. 2010, 81, 640−644. (23) Oepen, B. V.; Kördel, W.; Klein, W. Sorption of nonpolar and polar compounds to soils: Processes, measurements and experience with the applicability of the modified OECD-Guideline 106. Chemosphere 1991, 22, 285−304. (24) Foo, K. Y.; Hameed, B. H. Insights into the modeling of adsorption isotherm systems. Chem. Eng. J. 2010, 156, 2−10. (25) Pan, B.; Xing, B. Competitive and complementary adsorption of bisphenol A and 17α-ethinyl estradiol on carbon nanomaterials. J. Agric. Food Chem. 2010, 58, 8338−8343. (26) Myers, A. L.; Prausnitz, J. M. Thermodynamics of mixture-gas adsorption. AIChE J. 1965, 11, 121−127. (27) Fang, Q.; Chen, B.; Lin, Y.; Guan, Y. Aromatic and hydrophobic surfaces of wood-derived biochar enhance perchlorate adsorption via hydrogen bonding to oxygen-containing organic groups. Environ. Sci. Technol. 2013, 48, 279−288. (28) Qiu, H.; Lv, L.; Pan, B. C.; Zhang, Q. J.; Zhang, W. M.; Zhang, Q. X. Critical review in adsorption kinetic models. J. Zhejiang Univ. Sci., A 2009, 10, 716−724. (29) Chiron, N.; Guilet, R.; Deydier, E. Adsorption of Cu(II) and Pb(II) onto a grafted silica: isotherms and kinetic models. Water Res. 2003, 37, 3079−3086. (30) Wilczak, A.; Keinath, T. M. Kinetics of sorption and desorption of copper(II) and lead(II) on activated carbon. Water Environ. Res. 1993, 65, 238−244. (31) Cheung, W. H.; Szeto, Y. S.; McKay, G. Intraparticle diffusion processes during acid dye adsorption onto chitosan. Bioresour. Technol. 2007, 98, 2897−2904. (32) Li, H.; Xu, M.; Shi, Z.; He, B. Isotherm analysis of phenol adsorption on polymeric adsorbents from nonaqueous solution. J. Colloid Interface Sci. 2004, 271, 47−54. (33) Huang, J.; Huang, K.; Liu, S.; Luo, Q.; Xu, M. Adsorption properties of tea polyphenols onto three polymeric adsorbents with amide group. J. Colloid Interface Sci. 2007, 315, 407−414.

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dx.doi.org/10.1021/la502816m | Langmuir 2014, 30, 13861−13868