Preparation and Evaluation of Self-Assembled Porous Microspheres

Article pubs.acs.org/IECR

Preparation and Evaluation of Self-Assembled Porous Microspheres− Fibers for Removal of Bisphenol A from Aqueous Solution Li Cui,†,‡ Junfu Wei,*,†,‡ Xiao Du,†,‡ and Xiangyu Zhou†,‡ †

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin, 300387, China School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, China

‡

S Supporting Information *

ABSTRACT: A novel design of PP-g-DMAEMA/PM composite fiber as an efficient adsorbent was demonstrated by combining graft polymerization of dimethylaminoethyl methacrylate (DMAEMA) with self-assembled modification of porous microspheres (PMs) on the surface of polypropylene (PP) fiber. The structure and composition of the adsorbent was characterized by BET, XPS, FTIR, DSC, FESEM, and water angle. The kinetics and isotherm data indicated that the adsorption of bisphenol A (BPA) could be well-fitted by a pseudo-second-order kinetic model and the Langmuir isotherm, respectively. The thermodynamic studies indicated that the adsorption reaction was a spontaneous and exothermic process. Because of the π−π interactions and hydrogen bonds between BPA and PP-g-DMAEMA/PM, the resulting fiber obtained a higher adsorption amount (44.43 mg/g) of BPA. The presence of NaCl in the solution could facilitate the adsorption process, whereas the strong acid or strong alkali conditions and higher temperature of the solution were unfavorable. Besides, the obtained fiber reusability without obvious deterioration in performance was demonstrated by at least seven repeated cycles. effective removal of bisphenol A in water. Xiao and Li16 prepared a novel hyper-cross-linked polymeric adsorbent modified with acetylaniline as the cross-linked bridge for the enhanced removal of bisphenol A from aqueous solution. Zhou et al. 1 7 used fibric peat which is modified with hexadecyltrimethylammonium bromide (HTAB) to remove BPA from wastewater. Although granular adsorbents have been widely used in the treatment of BPA pollution, several shortcomings such as being unwieldy, inflexible, and hard to handle limited their further application. It is important to seek new adsorbent materials to remove BPA from water. In recent years, there had been an increasing interest in the removal of BPA by modified fibers.18−20 However, few studies have been carried out to combine excellent adsorption performance of porous microspheres with the braced structures of fiber in the field of BPA abatement. Actually, loading microspheres on the surface of fiber is an attractive way to enhance the adsorption capacity of fibrous adsorbent for BPA, due to their simplicity, versatility, and advantages for controlling the shape, composition, and surface roughness. To introduce the microspheres onto the surface of fibers, several functional groups should be added by graft polymerization at first. Dimethylaminoethyl methacrylate (DMAEMA) is considered as a promising adsorption and hydrogen acceptor material for the adsorption of organic compounds and the formation of hydrogen-bonding interaction, in whose molecule tertiary amine side group is contained, and this functional group can be readily converted to positively charged quaternary ammonium group having ion exchange and electron accepting

1. INTRODUCTION The contamination of wastewater by endocrine-disruption chemicals (EDCs) is a worldwide environmental problem. Since EDCs can imitate the biological activity of natural hormones, occupy the hormone receptors, or interfere with the transport and metabolic process of natural hormones, they pose a risk to animals and humans.1−3 Bisphenol A (BPA) is regarded as a representative materials among EDCs because it is mainly used as a monomer in the production of polycarbonate, epoxy resins, and other plastics, which are widely used in industry and households and hence result in the release of BPA into the surrounding environment.4,5 In addition, BPA pollution has been reportedly detected in industrial wastewater, groundwater, surface water, and even drinking water.6 For the reasons described above we selected BPA as a model substance and target molecule for the removal of water among the EDCs herein. The current technologies to remove BPA from water/ wastewater include membrane separation,7,8 biological treatment,9,10 photocatalytic degradation,11,12 adsorption,13 and other process. Among these methods, adsorption has been considered to be superior to other techniques in view of its comparatively low costs, wide range of applications, simplicity of design, and fewer secondary products. Regarding the adsorption technique, the application of effective adsorptions is critical to guaranteeing the efficiency of water treatment. Many solid materials as adsorbents have been investigated, including active carbons, polymeric adsorbents, and natural bioadsorbents. Liu et al.14 compared the sorption of BPA onto two commercial carbons (W 20 and F 20) which had been selectively modified with nitric acid and thermal treatment under a flow of N2. The study from Wang et al.15 suggested a combined adsorption and Fenton oxidation using an acidtreated Fe-amended granular activated carbon (Fe-GAC) for © XXXX American Chemical Society

Received: November 14, 2015 Revised: January 7, 2016 Accepted: January 26, 2016

A

DOI: 10.1021/acs.iecr.5b04306 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Preparation Procedure of PP-g-DMAEMA/PM Fibers

ability.21,22 It has been reported that DMAEMA was grafted onto many matrix materials such as polyethylene (PE), polypropylene (PP), and polyethylene coated polypropylene (PE/PP) fibers to prepare functional materials with specific surface properties. Bucio and Burillo23 studied the reaction conditions of radiation-induced graft polymerization of DMAEMA onto PE film by the preirradiation method in the presence of air. Meanwhile, PE fiber was also modified by introducing the PDMAEMA and PEGMEMA to obtain a kind of stimuli-responsive and thermoresponsive film.24 Kavakli et al.25,26 prepared a new adsorbent by radiation-induced graft polymerization of DMAEMA onto PE/PP nonwoven fabric for removal of phosphate. Kong et al.27 prepared a kind of quaternary ammonium fiber by radiation-induced grafting of DMAEMA onto PP fiber and modifying with 1-bromoalkanes, and the obtained fiber could remove Cr(VI) ions rapidly. As a conventional material, polypropylene (PP) fiber is widely used in adsorption and filtration process. It is an attractive material because of its low cost, good mechanical strength, short transit distance, and chemical/thermal resistance.28,29 Thus, PP fiber has become a potential supporting material for the uptake or separation of organic liquids, organic vapors, metal ions, and water. In this study, a new fibrous adsorbent called PP-gDMAEMA/PM was prepared by three steps: (i) The poly(vinylpyridine-co-acrylic acid) (poly(DVB-co-AA)) microspheres were prepared by distillation−precipitation polymerization. (ii) The dimethylaminoethyl methacrylate (DMAEMA) as a hydrogen acceptor was introduced onto the surface of polypropylene (PP) fiber by electron beam induced graft polymerization. (iii) Poly(DVB-co-AA) microspheres were introduced onto the surface of PP-g-DMAEMA (polypropylene grafted DMAEMA) fiber through hydrogen-bonding interaction between the carboxylic acid group and amino group. The structure and composition of the modified fibers were characterized by BET, FTIR, DSC, XPS, and FESEM. Further, the hydrophilic and hydrophobic property of the fibers was characterized by the contact angle. In addition, the adsorption properties of PP-g-DMAEMA/PM fiber for the removal of BPA from aqueous solution were investigated under different experimental conditions such as the contact time, BPA concentration, temperature, pH, and ionic strength. To evaluate

the adsorption capacity further and understand the mechanism of removing BPA, the adsorption characteristics including isotherm, kinetics, thermodynamics were studied.

2. MATERIALS AND METHODS 2.1. Chemicals. PP fiber was provided by Shijiazhuang Tobacco Center (Shijiazhuang, China). The fiber was washed by acetone and distilled water and dried at 323 K before use. DMAEMA employed for the grafting reaction was purchased from J&K Scientific Ltd. Acrylic acid and divinylbenzene (DVB, 80% grade) were purchased from Aldrich and distilled under vacuum. Acetonitrile and toluene (analytical grade, Tianjin Chemical Reagent II Co.) were dried with calcium hydride and purified by distillation before use. 2,2′-Azobisisobutyronitrile (AIBN, Chemical Factory of Nankai University) was recrystallized from methanol. Ethanol, methanol, and BPA were purchased from Tianjin Guangfu Fine Chemical Research Institute and used without further purification. 2.2. Preparation of PP-g-DMAEMA/PM Fibers. A new porous adsorbent fiber (PP-g-DMAEMA/PM) was synthesized, as described by the following steps and shown in Scheme 1. The PP-g-DMAEMA/PM fiber preparation procedure could be divided into three stages. The first stage was to modify PP fiber with amino group by high energy electron as described in our previous paper.27 At the second stage, the porous poly(DVB-coAA) microspheres were prepared by distillation precipitation copolymerization30 of divinylbenzene (DVB) and acrylic acid (AA) with toluene as the porogen. At the third stage, selfassembled reaction of the modified fibers and porous microspheres was performed as follows: the grafted fibers (PP-g-DMAEMA) and porous microspheres (PM) with the equal mass ratio (0.5 g:0.5 g) were soaked in 50 mL of acetonitrile, followed by sealing in a tube on an SHA-B shaker with gentle agitation (about 150 rpm rolling rate) for 24 h. The resulting PP-g-DMAEMA/PM fiber was washed with acetonitrile and acetone three times and dried in a vacuum oven at room temperature overnight. 2.3. Characterization. The field emission scanning electron microscopy (FESEM) was used to analyze the surface morphologies of fibers. The measurements were performed using a Hitachi S-4800 FESEM (Hitachi, Japan) with 10 kV accelerating voltage. Fourier transform infrared (FTIR) specB


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Industrial & Engineering Chemistry Research

calculated as the ratio of the amount of the desorbed BPA to the amount of the BPA initially adsorbed, and then the regenerated PP-g-DMAEMA/PM fibers were reused for the next adsorbent. To test the reusability of the fibrous adsorbent, this adsorption−desorption process was repeated seven times under the same conditions.

troscopy was carried out with Necolet 6700 (Thermo Necolet, USA), and X-ray photoelectron spectroscopy (XPS) analysis was carried out on an AEM PHI 5300X spectrometer with an Al Kα X-ray source to confirm the chemical structure of modified fibers. Measurements of differential scanning calorimentry (DSC) for PP, PP-g-DMAEMA, and PP-gDMAEMA/PM were performed on Q200 (TA Instruments) from 298 to 573 K with a heating rate of 10 K/min. The weights of the samples were in the range of 7−8 mg, and the DSC measurements were performed under nitrogen atmosphere. The specific surface area of adsorbent was measured with a Quantachrome ASAP2010 surface area measurement instrument following the BET method. The wettability and hydrophilic−hydrophobic property regulation of the samples were measured by a contact angle analyzer (KRUSS DSA100, Germany). 2.4. Adsorption Experiment. The batch adsorption experiments were carried out to determine the adsorption behaviors for BPA onto modified PP fibers. BPA solution was dissolved in ethanol as stock solution (1000 mg/L) and then diluted sequentially to a series of concentrations (ranging from 10 to 100 mg/L). The volume ratio of ethanol to water was below 0.001 to avoid the effect of cosolvent. A certain amount of adsorbent was placed in sealed 250 mL glass 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 (273, 298, and 323 K) to ensure the adsorption equilibrium. The adsorption capacity was evaluated by the following formula:

q=

(C0 − Ce)V m

3. RESULTS AND DISCUSSION 3.1. Characterization of PP-g-DMAEMA/PM Fibers. The FESEM images of original PP, PP-g-DMAEMA, and PP-gDMAEMA/PM fibers are shown in Figure 1. It is shown that

(1)

where C0 and Ce are the initial and the final concentrations (mg/L), respectively, V is the volume of the solution (L), and m is the mass of adsorbent (g). The adsorption kinetic study was carried out with an initial BPA concentration of 50 mg/L at 298 K, pH 6.0, to determine the minimum time required for adsorption to reach equilibrium and the temperature for adsorption to reach the maximum adsorption capacity. The concentration of BPA was measured at different time intervals ranging from 0 min to 4 h. The effect of pH on the adsorption capacity of PP-gDMAEMA/PM fiber toward BPA was studied with an initial BPA concentration of 50 mg/L in a pH range of 1.0−9.0 at 298 K. The solution pH was adjusted with a 0.1 M HCl or NaOH solution. The effect of the ionic strength on the adsorption of BPA was studied by adding NaCl to 50 mg/L BPA solutions with concentrations ranging from 0.005 to 0.5 M at 298 K and pH 6.0. After adsorption, the concentration of BPA was determined by a high-performance liquid chromatography (HPLC) system (Waters 2695) equipped with a reversed phase C18 column (250 mm × 4.6 mm) and a UV detector. Detection wavelength for BPA was set to 276 nm. The mobile phase contains 60% acetonitrile and 40% water, delivered at a constant flow rate of 1 mL/min. A typical quantification limit for BPA under these conditions was approximately 10 μg/L. The desorption and regeneration of PP-g-DMAEMA/PM fibers were investigated. After adsorption, the PP-g-DMAEMA/ PM fibers saturated with BPA were added into 500 mL of methanol/water (v/v, 4/1) solutions. The mixture solutions were shaken for 2 h at 150 rpm and 298 K to reach desorption equilibrium. Furthermore, the desorption efficiency of BPA was

Figure 1. FESEM images of the original and modified PP fibers: (a, b) original PP; (c, d) PP-g-DMAEMA; (e, f) PP-g-DMAEMA/PM fibers.

the average diameter of the original PP (Figure 1a and Figure 1b) fiber was about 24.47 μm. By contract, for PP-g-DMAEMA fiber, the diameter was increased to 28.85 μm because the heterogeneous grafting layer was formed on the surface of the PP-g-DMAEMA fiber, and the fiber became coarse. These findings indicated that DMAEMA was grafted onto the fiber during irradiation. After self-assembled reaction, the surface of PP-g-DMAEMA fiber was covered by the particles, and the diameter of this fiber increased to 29.25 μm. In general, particles on the fibers had spherical shape with smooth surface and the diameters of the particles ranged from 0.248 to 0.604 μm. Specifically, these particles had a lot of nanosize pores, which could provide the channels for BPA to access the inner binding sites in the adsorption process. The nitrogen adsorption and desorption isotherms of fibers at 77K are shown in Figure 2a. The shape of adsorption isotherms for PP-g-DMAEMA/PM fiber can be considered as a combination of types I and IV according to the IUPAC classification.31 The steep increase of adsorbed amount at low relative pressure (P/P0 < 0.1) indicated the presence of micropores on these materials. At intermediate P/P0, a capillary condensation step, and an H4-type hysteresis loop were observed, suggesting the formation of slit-shaped mesopores. The pore size distributions of the modified PP fibers were estimated by the BJH method, and the results are shown in C


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Figure 2. (a) N2 adsorption−desorption isotherms of original PP, PP-g-DMAEMA, and PP-g-DMAEMA/PM fibers. (b) Pore size distributions from adsorption branches for PP samples calculated by BJH method.

Figure 2b. The obtained fiber exhibited a typically porous structure with a primary pore size distribution (PSD) in the range of 1−2 nm. Besides, the values of well-developed peaks of the modified PP fiber greatly increased toward increasing PM content, revealing the enhancement of micropore structure. Meanwhile, with increasing mass fraction of PM, the microporosity of fibers increased while the total pore volume increased as well (see Supporting Information, Table S1). The nitrogen adsorption and desorption results provided evidence for the existence of both micropores and mesopores in the PPg-DMAEMA/PM fiber. To obtain more information about the surface grafting polymerization and self-assembled of PP fibers, XPS analysis was employed to quantitatively determine the surface chemical composition of the original and modified PP fibers. The XPS survey spectra results are given in Figure 3, and the surface

[−COO−] groups.34 The appearance of these peaks suggested that the DMAEMA was successfully grafted on the surface of PP fibers.35,36 After the self-assembled process, the porous microspheres were determined by comparing the O 1s spectrum of the PP-g-DMAEMA/PM fiber with the PP-gDMAEMA fiber. Actually, the successful attachment of porous microspheres was indicated by an increase in oxygen concentration (from 14.43% to 21.01%) and atomic ratio of O/C (from 0.18% to 0.28%). These changes confirmed that the porous microspheres containing oxygen were introduced onto the surface of PP-g-DMAEMA fiber successfully. The FTIR spectra of original PP, PP-g-DMAEMA, and PP-gDMAEMA/PM are shown in Figure 4. Compared with the

Figure 4. FTIR spectra of original PP, PP-g-DMAEMA, and PP-gDMAEMA/PM fibers.

Figure 3. XPS survey spectra of the original PP, PP-g-DMAEMA, and PP-g-DMAEMA/PM fibers.

original PP fiber, the appearance of a new band at 1725 cm−1 in PP-g-DMAEMA fiber was due to the stretching vibrations of CO, and the bands at 1147 cm−1 reflected the stretching vibrations of C−O−H and C−N. These findings indicated that DMAEMA had been grafted onto the PP fiber. After selfassembly, a broad band ranging from 3100 to 3700 cm−1 corresponding to the PM of the stretching vibration of C−O− H group appeared. In addition, the new peaks at 1624 cm−1 are assigned to benzene ring. The effects of DMAEMA and PM on the thermal properties of the composite fibers were examined by DSC analysis. Typical DSC curves of different fibrous samples are shown in Figure S1

elemental composition is listed in Table S2 (Supporting Information). It can be seen from the XPS survey spectrum of original PP fibers that only one characteristic peak was observed at 284.08 eV corresponding to C 1s. This strong emission peak was mainly caused by carbon atoms in the methylene chain of PP.32 After radiation-induced graft polymerization onto PP fiber with DMAEMA, two new peaks were observed. One appeared at 400.3 eV which corresponds to N 1s (4.55%) binding energy of tertiary amine [−N(CH3)2] groups.33 The other new peak having higher binding energy appearing at 533.2 eV in the O 1s (14.43%) spectrum corresponds to ester D


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groups such as carboxyl groups and amino groups remaining in PP-g-DMAEMA/PM during the preparation of PP-g-DMAEMA by DMAEMA. Thus, we can expect that two kinds of adsorbent−adsorbate interactions might be responsible for the adsorption of BPA on PP-g-DMAEMA/PM.3 One main interaction was the π−π interaction between the benzene rings of BPA and the PP-g-DMAEMA/PM. The other was hydrogen bonding between the oxygen/nitrogen-containing groups contained in both BPA and PP-g-DMAEMA/PM. Meanwhile, the high surface area and porous structure of PP-gDMAEMA/PM increased the adsorption sites, which were beneficial to BPA adsorption.29 3.3. Adsorption Kinetic Assay. To elucidate the superiority of the self-assembled PP fiber, an experiment was designed and conducted to study the adsorption capacities of the composite fibers. Figure 6 shows the adsorption kinetics

(Supporting Information), in which the melting temperature (Tm) of PP (169.0 °C) was slightly higher than those of PP-gDMAEMA (161.6 °C), and a new melting peak appeared. It was probably due to the grafted branches, which disrupted the regularity of the chain structures in PP and increased the spacing between the chains: consequently, the percent crystallization, and therefore heat of fusion, decreased. In addition, compared with the PP-g-DMAEMA, a new melting peak appeared on the curve of PP-g-DMAEMA/PM fibers at 97 °C, which was because of the PM on the surface of PP fiber. The contact angle is a parameter of wettability and a reflection of surface energy and diffusion resistance. As shown in Figure 5, the contact angles of PP, PP-g-DMAEMA, and PP-

Figure 5. Contact angles of (a) PP, (b) PP-g-DMAEMA, and (c) PPg-DMAEMA/PM fibers.

g-DMAEMA/PM are 130°, 69°, and 30°, respectively. The values of the static contact angle indicated that PP fiber was a hydrophobic adsorbent, while PP-g-DMAEMA and PP-gDMAEMA/PM were hydrophilic adsorbents. This could be explained as follows: (i) The PP fiber structures contained alkane chains, and these groups were hydrophobic. Thus, the PP fiber was hydrophobic. (ii) PP-g-DMAEMA was modified with hydrophilic groups (amino/ester groups), which improved the hydrophilicity of PP-g-DMAEMA. (iii) The PM which was on the surface of PP-g-DMAEMA/PM fiber had a lot of polar functional groups (carboxy group), and its hydrophilicity was higher than PP-g-DMAEMA. Consequently, the hydrophilic surface of PP-g-DMAEMA/PM could be beneficial to increasing the adsorption capacity for contaminants. 3.2. Adsorption Mechanism. The adsorption mechanism of BPA on PP-g-DMAEMA/PM might be explained as shown in Scheme 2. Because BPA has benzene rings, it could be speculated that the main intermolecular force between BPA and the polydivinylbenzene of PM should be the π−π interaction. Besides, there were some residual oxygen/nitrogen-containing

Figure 6. Effect of contact time on the adsorption of BPA by PP, PP-gDMAEMA, and PP-g-DMAEMA/PM fibers (50 mg of samples in 50 mL of 50 mg L−1 BPA solution at 298 K). The dotted line is the pseudo-first-order model simulation; the solid line is the pseudosecond-order model simulation.

curves of original PP, PP-g-DMAEMA, and PP-g-DMAEMA/ PM fibers. The results showed that the original PP fiber had

Scheme 2. Adsorption Mechanism of BPA on PP-g-DMAEMA/PM

E


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Industrial & Engineering Chemistry Research Table 1. Kinetic Parameters for the Adsorption of BPA by PP, PP-g-DMAEMA, and PP-g-DMAEMA/PM Fibers pseudo-first-order

pseudo-second-order

sample

qe,exp (mg g−1)

k1 (min−1)

qe,cal (mg g−1)

R2

k2 (g mg−1min−1)

qe,cal (mg g−1)

R2

PP PP-g-DMAEMA PP-g-DMAEMA/PM

0.56 28.02 44.43

0.06649 0.12667 0.16815

0.31 26.18 42.09

0.9837 0.9861 0.9754

0.33637 0.00767 0.00697

0.34 27.89 44.35

0.9627 0.9919 0.9991

almost no adsorption for BPA due to the few effective adsorption sites on the surface of PP fiber, and the hydrophobic adsorption force could not work for its high crystallinity. However, the PP-g-DMAEMA/PM fiber obtained the largest adsorption amount (44.43 mg/g), which was 127 times greater than that of original PP. These results further suggested that effective porous microsphere coatings were introduced onto PP fibers. The adsorption kinetic curve for BPA on modified PP fibers showed that the adsorption process reached equilibrium after 40 min and remained constant until the end of the experiment. Meanwhile, the adsorption curves were fit by two simplified kinetic models: pseudo-first-order equation and pseudo-second-order equation. These models are expressed as shown in eqs 2 and 3, respectively.37−39 ln(qe − qt) = ln qe − k1t t 1 t = + qt qe k 2qe 2

(2) Figure 7. Adsorption isotherms of BPA by PP-g-DMAEMA/PM fiber at three different temperatures (50 mg of adsorbent in 50 mL of BPA solution reacted for 3 h). The dotted line is Langmuir model simulation; the solid line is Freundlich model.

(3)

where qt is the amount of BPA removed (mg g−1) at time t, qe is the equilibrium adsorption capacity (mg g−1), and k1 (min−1) and k2 (g mg−1 min−1) are the rate constants of, respectively, pseudo-first-order and pseudo-second-order adsorption. All the kinetic parameters for adsorption of BPA are calculated and given in Table 1. Compared with pseudo-first-order kinetic model, pseudo-second-order kinetic model fitted the experimental data better and the correlation coefficients (R2) were both beyond 0.99 (except for original PP). Furthermore, the theoretical values of equilibrium adsorption amount (qe,cal), calculated from pseudo-second-order kinetic model, were in good agreement with the experimental value (qe,exp). Therefore, the pseudo-second-order kinetic model was suitable to describe the adsorption behaviors of BPA on tested fibers, suggesting the intraparticle diffusion process as the rate-limiting step of the adsorption in solution. It also confirmed that the adsorption rate was related to the content of active adsorption site on the matrix of fibrous adsorption. Thus, the BPA adsorption rate of PP-g-DMAEMA/PM was much higher than that of PP fiber, and it might be used as an efficient emergency adsorbent for the fast removal of BPA from polluted water in the future. 3.4. Equilibrium Adsorption Assay. Adsorption isotherm models are efficient avenues in revealing the interaction between the adsorbent and adsorbate when the adsorption process reaches equilibrium. Figure 7 shows adsorption isotherms of BPA on PP-g-DMAEMA/PM fiber at three different temperatures. It could be observed that the adsorption capacity of PP-g-DMAEMA/PM fiber increased with the increasing equilibrium concentration of BPA. This can be attributed to the increasing driving force of the concentration gradient because the increase in BPA concentration can accelerate the diffusion BPA molecules onto PP-g-DMAEMA/PM fiber. The equilibrium adsorption data are fitted by the Langmuir eq 4 and the Freundlich eq 5:40,41

Ce 1 1 = Ce + qe qm qmKL

ln qe =

(4)

1 ln Ce + ln KF n

(5) −1

where qe is the equilibrium adsorption capacity (mg g ), Ce is the equilibrium concentration (mg L−1), and KF and n are characteristic constants. The monolayer saturation adsorption capacity of adsorbate (qm) and the other constants that are evaluated by application of the isotherm equation are also shown in Table 2. For the two studied models, the Langmuir Table 2. Isotherm Parameters for the Adsorption of BPA by PP-g-DAMEMA/PM Fiber Langmuir

Freundlich

temp (K)

qm (mg g−1)

KL (L mg−1)

R2

KF

n

R2

273 298 333

446.62 327.84 196.16

0.00252 0.00305 0.00311

0.9934 0.9922 0.9930

1.46878 1.38133 0.84993

1.11598 1.14243 1.14668

0.9883 0.9859 0.9866

equation showed a more significant correlation (R2 > 0.99) than in the case of the Freundlich equation with the experimental data, which suggested a monolayer adsorption of BPA on PP and the modified PP fibers. Moreover, it could be seen that qm calculated from Langmuir equation decreased with increasing temperature, indicating that the adsorption was unfavorable at high temperature. In addition, the maximum adsorption amount of PP-gDMAEMA/PM fiber for BPA at 298 K was 327.84 mg/g. The adsorption capacities for BPA based on other materials F


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thereby governs the adsorbent−adsorbate electrostatic interactions. The effect of pH on the adsorption capacity of the PPg-DMAEMA/PM toward BPA was examined, and the results obtained are shown in Figure 8. With pH value increasing from

previously reported in the literature are summarized in Table S3 (Supporting Information). It can be seen that the adsorption capacity by PP-g-DMAEMA/PM fiber in the present study is among the highest compared with other adsorbents excepted for few carbon materials. Meanwhile, note that the adsorption equilibrium time of the obtained fiber is shorter than most of the reported materials. One reason can be attributed to the low solubility of BPA in water, and another reason is the interaction between BPA and the functional group of the obtained fibrous sorbent. Therefore, PP-g-DMAEMA/PM fiber was an excellent BPA adsorbent in wastewater treatment. 3.5. Adsorption Thermodynamics Study. The thermodynamic parameters provide in-depth information about internal energy changes that are associated with adsorption. The standard free-energy change (ΔG0), the standard enthalpy change (ΔH0), and the standard entropy change (ΔS0) are calculated from the temperature-dependent adsorption isotherms to predict the adsorption process. The ΔG0, ΔH0, and ΔS0 are calculated from the following equations: ΔG = −RT ln K 0

ln K 0 = Kd =

(6)

ΔS 0 ΔH 0 − R RT

C0 − Ce V Ce m −1

(7)

Figure 8. Effect of pH on the removal of BPA by PP-g-DMAEMA/PM (50 mg of samples in 50 mL of 50 mg L−1 BPA reached for 3 h at 298 K).

(8)

1 to 9, the adsorption capacity of the modified fiber toward BPA initially increases and subsequently decreases. The PP-gDMAEMA/PM exhibited excellent affinity toward BPA at pH between 5 and 7. At low pH (7), the adsorption capacity decreases quickly. These phenomena can be explained by the net charge of BPA and adsorbent at different pH values. The decrease of the adsorption capacity of PP-g-DMAEMA/PM for pH < 5 can be ascribed to the protonation of the amine group of DMAEMA, resulting in the partial breakage of hydrogen bonds.5 For pH > 7, BPA molecules were mostly ionized to mono- or divalent anions after deprotonation.42 Thus, the reduction of the adsorption capacity of PP-g-DMAEMA/PM observed in the alkaline pH range might be due to the repulsive electrostatic interactions established between the negatively charged surface of adsorbent and the bisphenolate anion. 3.7. Effect of Ionic Strength on Adsorption of BPA. In order to find the effects of ionic strength, the adsorbent of BPA onto PP-g-DMAEMA/PM was studied at different NaCl concentrations, and the results are shown in Figure 9. It was obvious that the adsorption capacity of PP-g-DMAEMA/PM was improved in the presence of NaCl. When the NaCl concentration was below 0.1 M, the adsorption capacity increased rapidly. These phenomena can be explained by the positive net charge of the modified fibers and the molecular form of the BPA under the conditions of the adsorption experiments (pH = 6). Therefore, the ions from NaCl are placed between the BPA molecules and the PP-g-DMAEMA/ PM fiber surface. This produces a screening effect of the surface charge that favors adsorbate−adsorbent dispersion interactions, thereby enhancing the adsorption of BPA.43 In addition, the presence of NaCl in solution also causes a salting-out effect, decreasing the solubility of BPA and enhancing, therefore, its adsorption on the PP-g-DMAEMA/PM. However, the adsorption capacity was decreased slightly when the NaCl also competed with BPA for the adsorption sites on the PP-g-

−1

where R is the universal gas constant (8.314 J K mol ), T is the absolute temperature (K), Kd is the distribution adsorption coefficient, C0 is the initial concentration (mmol L−1), Ce is the equilibration concentration of BPA in solution (mmol L−1), V is the volume of the suspension (L), and m is the mass of the absorbent (g). The adsorption equilibrium constant, K0, can be calculated by plotting ln Kd versus Ce and extrapolating Ce to zero. The value of the intercept is that of ln K0. The thermodynamic parameters calculated from eqs 6−8 at three different temperatures are listed in Table 3. It can be seen Table 3. Thermodynamic Parameters of BPA Adsorption on PP-g-DMAEMA/PM T (K) thermodynamic constant

273

298

323

ln K0 (kJ/mol) ΔG0 (kJ/mol) ΔH0 (kJ/mol) ΔS0 (J/mol·K)

6.43 −14.59

4.47 −11.07 −52.37 138.42

2.86 −7.68

that the ΔG0 values are negative for BPA, suggesting that the reaction is spontaneous. The enthalpy ΔH0 was a measure of the energy barrier that must be overcome by reacting molecule. The negative ΔH0 suggested that the adsorption reactions onto PP-g-DMAEMA/PM are exothermic in nature, thereby meaning that increasing temperature will not favor the adsorption of BPA onto PP-g-DMAEMA/PM. The negative ΔS0 reflected a decrease in randomness at the solid−liquid interface during the adsorption process. The values of the thermodynamic parameters gave us insight into the interaction between PP-gDMAEMA/PM and BPA in water. 3.6. Effect of pH on Adsorption of BPA. It was wellknown that pH is an important parameter that can affect the adsorption properties of adsorbent in aqueous phase because it determines the charge of both adsorbent and adsorbate and G


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4. CONCLUSIONS In summary, a new porous and fibrous adsorbent (PP-gDMAEMA/PM) for the fast removal of BPA from water has been prepared by grafting DMAEMA onto PP fibers and subsequent PM was introduced on it by the self-assembled method. The influence of the experimental conditions on the adsorption performances of BPA has been elucidated. The kinetics and isotherm data can be well fitted with the pseudosecond-order kinetic model and the Langmuir isotherm, respectively. The large adsorption capacity of PP-g-DMAEMA/PM for BPA might be due to its porous structures with aromatic rings and the residual oxygen/nitrogen-containing groups, which can form π−π interactions and hydrogen bonds with the benzene rings and hydroxyl groups of BPA. Furthermore, the obtained fiber can be easily regenerated through methanol/water washing within 2 h. The sorption capacity of the adsorbent was well maintained at 93% even for seven regeneration cycles. Taken together, this novel architecture of PP-g-DMAEMA/PM composite fiber may find extensive use in environment remediation applications.

Figure 9. Effect of ionic strength on the removal of BPA by PP-gDMAEMA/PM (50 mg of samples in 50 mL of 50 mg L−1 BPA reached for 3 h at 298 K).

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DMAEMA/PM, and the binding sites of it available for BPA were now occupied by NaCl.3 3.8. Desorption and Reusability of PP-g-DMAEMA/PM Fibers. Considering cost-effective application of PP-g-DMAEMA/PM fibers in wastewater treatment, the possibility of regeneration and reusability was further investigated. In this study, methanol/water solution was used for the desorption of BPA, as it has been used to desorb guest molecules from the pores of the obtained fibers. The adsorption capacity with seven consecutive adsorption−regeneration cycles is shown in Figure 10. It can be seen that the removal efficiency of adsorbent was

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04306. DSC curves of original PP and modified PP fibers (Figure S1), BET surface areas, pore volumes, and average pore size of PP samples (Table S1), surface chemistry composition of original and modified PP fibers and the atomic ratio of O/C bonds (Table S2), and adsorption capacity of BPA on PP-g-DMAEMA/PM fiber in comparison to other literature values (Table S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-022-8395-5898. Fax: +86-022-8395-5451. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant 41301542), National High Technology Research and Development Program of China (Grant 2013AA065601), and Key Technologies R and D Program of Tianjin (Grant 13ZDSF00100).

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Figure 10. Regeneration cycles for PP-g-DMAEMA/PM fibers.

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still higher than 93% after regeneration seven times, indicating that the prepared adsorbent possessed excellent adsorption capacity and stability. The decrease in sorption capacity was mainly because there was still a small amount of BPA bound to the sorbents after methanol/water treatment. These results demonstrated that the PP-g-DMAEMA/PM fibers would be a promising adsorbent for the removal of BPA from water, especially in the emergency disposal of BPA water pollution accidents. H


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