branched polyethylenimine-polyethylene glycol-β ...hosting03.snu.ac.kr/~eco/file/148.pdf ·...

Branched polyethylenimine-polyethylene glycol-β-cyclodextrin polymersfor efficient removal of bisphenol A and copper from wastewater

Ji Hwan Lee,1 Seung-Yeop Kwak 1,2

1Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea2Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South KoreaCorrespondence to: S.-Y. Kwak (E-mail: [email protected])

ABSTRACT: The present study reports on the development of water-insoluble polymers including β-cyclodextrin (β-CD), polyethyleneglycol (PEG), and branched polyethylenimine (b-PEI) denoted here as X-CD, for efficient removal of bisphenol A (BPA) and copper ions(Cu(II)) from wastewater. Fourier-transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, and differential scan-ning calorimetry results indicate that β-CD was successfully linked to b-PEI and PEG. The adsorption kinetics of BPA and Cu(II) werefound to follow a pseudo-second-order model. Adsorption isotherm data showed good correlation of BPA and Cu(II) with the Langmuirisotherm models. In regeneration tests, using ethanol and DI water (pH 2) washing, X-CD exhibited sufficient BPA and Cu(II) recoveryefficiency even after the fourth cycle. Furthermore, the adsorption performance of X-CD was unaffected by co-existing substances. Theresults demonstrate simple and environmentally friendly crosslinking without toxic/carcinogenic byproducts, and the efficient removal ofendocrine disrupting chemicals and heavy metals from aquatic environments. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48475.

KEYWORDS: adsorption; hydrogels; water purification; β-Cyclodextrin

Received 4 April 2019; accepted 20 August 2019DOI: 10.1002/app.48475

INTRODUCTION

Endocrine disrupting chemicals (EDCs) and heavy metals (HMs)are present as contaminants in aquatic environments as a resultof their widespread industrial use. Bisphenol A (BPA) is one suchEDC that is widely used as a key monomer for the synthesis ofepoxy resins and polycarbonate plastics.1 BPA can disrupt theendocrine system, resulting in reproductive disorders, neurologi-cal disease, and cancers in humans.2 Copper (Cu(II)) is a widelyused metal in industries such as plating, mining, and smelting,brass manufacturing, and petroleum refining.3 As an HM, it cancause abdominal pain or diarrhea, and in serious cases, it canresult in liver necrosis and even death.4 In recent years, the coex-istence of BPA and Cu(II) has been frequently detected in soils,surface waters, and groundwater, with levels exceeding safetystandard limits.5 However, most remediation technologies arefocused on removal of a single contaminant in solution, due tothe different physical and chemical properties of the two contam-inant classes. These approaches include adsorption, membranefiltration, microbial degradation, and chemical precipitation.4,6–10

Therefore, there is an urgent need for technologies that facilitatethe simultaneous removal of these contaminants.11,12 Onesuggested approach is to modify an existing successful method

for single contaminant removal and apply it to the simultaneousremoval of different classes of contaminants.

Adsorption is regarded as an efficient and simple method forremoving EDCs or HMs individually.13,14 Cyclodextrins (CDs)are composed of D(+)-glucopyranose units with α-(1,4)-linkagesand have a toroidal structure comprising a hydrophilic surfaceand hydrophobic cavity.15 CDs have the ability to bind size-matched organic compounds via host−guest interactions such asvan der Waals forces, hydrophobic interactions, hydrogen bond-ing, and steric effects.16,17 Among the three types of natural CDs(i.e., α-CD, β-CD, and γ-CD), β-CD is the cheapest and excellentat removing phenol from wastewater, given that its cavity size iswell-matched to the size of phenol.18 However, native β-CD iswater soluble and must be processed into water-insoluble poly-mers by crosslinking before it can be used for contaminantremoval.19,20 Although epichlorohydrin (EPI) has been the mostextensively utilized crosslinker in the production of β-CD poly-mer, it has a major drawback in that it produces toxic and carci-nogenic byproducts in the β-CD crosslinking process.16 It wouldbe desirable to use a nontoxic crosslinker for β-CD polymeriza-tion. Kono et al.20 used an epoxy-functionalized polyethylene gly-col (polyethylene glycol diglycidyl ether [PEGDE]) as a nontoxic

Additional Supporting Information may be found in the online version of this article.

© 2019 Wiley Periodicals, Inc.

48475 (1 of 9) J. APPL. POLYM. SCI. 2020, DOI: 10.1002/APP.48475

https://orcid.org/0000-0002-8903-4287

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crosslinker for β-CD polymerization and showed that crosslinkedβ-CD efficiently removed BPA from an aqueous solution. Thismaterial has important advantages, such as green synthesis andefficient organic contaminant removal, but it also poses a needfor lots of crosslinkers to yield a water-insoluble adsorbent, dueto the by-product of water-soluble β-CD oligomers. This isbecause the hydroxyl groups of β-CD have a relatively low reac-tivity to epoxy groups, compared with amines, which are widelyused as the most common crosslinkers for epoxy groups.21

Therefore, we hypothesize that the addition of amine reduces theamount of crosslinkers such as PEGDE. This lower degree ofcrosslinking results in less spatial blocking of the β-CD cavitiesby the PEG chains and thus leads to more efficient inclusioncomplexing of organic contaminants.

On the other hand, a few studies have recently reported simulta-neous removal of HMs and organic contaminants such as EDCs,dyes, and drugs from multicomponent solutions by using β-CDcrosslinked with non-toxic acids such as citric acid (CA) andethylenediaminetetraacetic acid (EDTA).12,22,23 These crosslinkedβ-CD cannot remove HMs, but also remove alkali/alkaline-earthmetals from aqueous solution because CA and EDTA are power-ful chelating agents for all divalent metals. Moreover, syntheticmethods need catalyst, high reaction temperature (over 140�C),and/or tedious and multistep reaction.

Branched polyethylenimine (b-PEI) is a well-known hydrophilicpolymer with a high content of amine groups and is widely usedin various applications. It has been frequently conjugated withPEG or β-CD in the application of drug delivery, owing to its lowtoxicity.24,25 Moreover, b-PEI has an ability to remove HMs selec-tively from aqueous environments because amine groups can che-late HMs, not alkali/alkaline-earth metals.26,27 The introduction ofb-PEI to PEG-β-CD-based adsorbents could lower the degree ofcrosslinking and also impose the selective HM removal property.

Herein, we report on the environmentally friendly one-pot syn-thesis of a water-insoluble b-PEI-PEG-β-CD polymer, denoted asX-CD, for the simultaneous removal of EDCs and HMs from anaqueous solution. Each component of X-CD plays a crucial rolein its function. The hydrophobic cavities of β-CDs are expectedto capture the EDC molecules, forming inclusion complexes. Theepoxy groups of PEGDE are responsible for the crosslinking. b-PEI is considered the backbone of this crosslinked polymer andthe introduction of b-PEI is expected to lower the usage of cros-slinkers for the yield of the water-insoluble adsorbent. Moreover,as a widely used material for HM removal, b-PEI also controlsthe HM adsorption performance. The X-CD makes it possible todevelop efficient and nontoxic processes for the purification ofwastewater containing organic and ionic contaminants.

MATERIALS AND METHODS

Materialsβ-CD, b-PEI (average Mw �25 000, average Mn �10 000),PEGDE (average Mn 500), BPA, copper nitrate trihydrate (Cu(NO3)2�3H2O), Pluronic F127 (EO108PO70EO108), cetyl trimethylammonium bromide (CTAB), sodium nitrate (NaNO3),hydrochloric acid (HCl), and sodium hydroxide (NaOH) werepurchased from Sigma-Aldrich and were used without further

purification. The water used in all syntheses and analysis wasdeionized with resistivity exceeding 18.0 MΩ cm.

Preparation of b-PEI-PEG-β-CD PolymersX-CDs were prepared by modifying a process that has beendescribed previously.20 Briefly, 5.7 g of β-CD was dissolved in20 mL of a 1.5 M aqueous NaOH solution. A total of 3.9 g ofPEGDE and 0.78 g of b-PEI were subsequently added to theβ-CD solution. The resulting solution was stirred at 60 �C for 5 hto form water-insoluble X-CD, and the resulting products weresequentially washed with HCl solution and DI water. Finally, thehydrogel particles were dried in a vacuum oven at 90 �C for 12 h,providing the form of xerogels. The effect of b-PEI and PEGDEconcentration on the adsorption property of X-CD wereinvestigated.

CharacterizationFourier-transform infrared (FT-IR) spectra were acquired forsamples mixed with KBr pellets using a Nicolet iS10 IR spectropho-tometer (Thermo Fisher Scientific, Waltham, MA, USA). The com-position and crosslinking degree of X-CDs were determined byelemental analysis (EA). EA was performed using a Flash EA 1112analyzer (Thermo Fisher Scientific, Waltham, MA, USA). The CDcontent and crosslinking degree of X-CD were then calculated usingthe composition of X-CD.28,29 Thermogravimetric analysis (TGA)was performed on a Q500 system (TA Instruments, New Castle,DE, USA) at a scan rate of 10 �C min−1 in a nitrogen atmosphere.Differential scanning calorimetry (DSC) measurements were per-formed under a nitrogen atmosphere, over a temperature range of−80 to 100 �C at a heating rate of 10 �C min−1 using a DiscoveryDSC (TA Instruments, New Castle, DE, USA). Field-emission scan-ning electron microscopy (FE-SEM) was performed using a SUPRA55VP microscope (Zeiss, Oberkochen, Germany) with an appliedvoltage of 2.0 kV. X-ray diffraction (XRD) pattern was acquiredwith a New D8 Advance X-ray diffractometer (Bruker, Billerica,MA, USA) using monochromatized Cu Kα X-rays (λ = 1.541 Å).The zeta potentials were measured using a Photal ELS-8000analyzer (Otsuka Electronics, Osaka, Japan). The pH of the solutionwas adjusted to the desired value in the range 2–12 using 0.1 MHCl and NaOH. N2 adsorption–desorption isotherms wererecorded at 77 K using a Micromeritics ASAP 2000 system(Micromeritics, Norcross, GA, USA). The multipoint Brunauer–Emmett–Teller (BET) method was used to calculate the specific sur-face areas. The specific surface area was then obtained using theMicromeritics Density Functional Theory Plus software.

Batch Adsorption ExperimentsEach adsorption experiment was performed with magnetic stir-ring at 200 rpm. Adsorbent (5 mg) was added to a BPA orCu(II) solution (20 mL) and then stirred at room temperature fordiffering durations. After the adsorption process, adsorbents werefiltered through a membrane filter of pore size 0.2 μm. The resid-ual concentrations of BPA or Cu(II) in the filtrate were deter-mined by a Lambda 25 UV–vis. Spectrometer at wavelength of276 and 548 nm, respectively. In particular, Cu(II) concentrationsin the filtrate were determined colorimetrically by complexingCu(II) with 1,2-diaminoethane and measuring the absorbance ofthe negative complex ion.8 The adsorption capacity of the adsor-bent was calculated using eq. (1):

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qe =C0−Ceð ÞV

Mð1Þ

where qe (mg g−1) is the adsorption capacity of adsorbents at equi-librium, C0 and Ce (mg L−1) are the initial and equilibrium BPAor Cu(II) concentrations in the solution, respectively, V (mL) isthe volume of the solution, and M (mg) is the mass of adsorbents.

The adsorption kinetics were evaluated using the pseudo-first-order and pseudo-second-order kinetic models, which can beexpressed as shown in eqs. (2) and (3)30:

qt = qe 1−e−k1t� � ð2Þ

qt =q2ek2t

1 + k2qetð3Þ

where qt (mg g−1) is the adsorption capacity of adsorbents attime t, k1 (min−1), and k2 (g mg−1 min−1) are the pseudo-first-order and pseudo-second-order rate constant, respectively.

The equilibrium adsorption isotherm was determined using theFreundlich and Langmuir isotherm models, which can beexpressed as shown in eqs. (4) and (5)31:

qe =KFC1=ne ð4Þ

qe =qLKLCe

1 +KLCeð5Þ

where KF ((mg g−1) (L mg−1)n) and n (unitless) are Freundlich con-stants related to the capacity and intensity of adsorption, respec-tively, and KL (L mg−1) and qL (mg g−1) are Langmuir constantsrelated to the adsorption energy and the maximum adsorptioncapacity, respectively. All experiments were performed in triplicate.

Regeneration ExperimentsThe regeneration of the adsorbents after BPA or Cu(II) adsorptionwas conducted by washing adsorbents in ethanol or DI water(pH 2), respectively, under agitation for 10 h. The regeneratedadsorbents were separated by decanting ethanol or DI water(pH 2) and then reused for subsequent adsorption cycles.

The recovery efficiency of the adsorbent was calculatedusing eq. (6):

R%=qrqi

× 100 ð6Þ

where qi and qr are the adsorption capacities of the adsorbentbefore and after regeneration, respectively. All experiments wereperformed in triplicates.

Simultaneous Adsorption ExperimentsThe selective adsorption performance of X-CD toward targetcontaminants were evaluated by dispersing 10 mg of adsorbentsinto 20 mL of the aqueous solutions with concentration of100 mg L−1 BPA, 100 mg L−1 Cu(II), 100 mg L−1 F127,100 mg L−1 CTAB, and 100 mg L−1 Na(I).

The effect of other contaminants in BPA or Cu(II) adsorption onX-CD was determined by eq. (7)22:

Rq, i =qm, iqs, i

ð7Þ

where Rq,i (unitless) is the ratio of adsorption capacities for con-taminant i at equilibrium, qm,i and qs,i (mg g−1) are the adsorp-tion capacity for contaminant i in the multicomponent systemand single component system with the same initial concentration,respectively.

RESULTS AND DISCUSSION

Optimization of Crosslinking ParametersAs shown Figure 1, the β-CD and b-PEI were anchored with PEGmolecules by reacting hydroxyl groups of β-CD and amine groupsof b-PEI with epoxy moiety of PEGDE through a ring-openingreaction, producing X-CD. It was necessary to find the appropriateconcentration range of b-PEI and PEGDE to yield water-insolubleX-CDs by adjusting their concentrations. The results of this processare shown in Supporting Information Figure S1 (X axis = theamount of PEGDE; Y axis = the amount of b-PEI) and can be clas-sified into three categories: water-soluble (liquid-like), intermediate(suspension-like), and water-insoluble (gel-like), as shown inSupporting Information Figure S2. The results show that the addi-tion of b-PEI reduced the amount of PEGDE required for the prep-aration of water-insoluble X-CD, and the conceptual non-linearboundary that includes intermediate samples could divide thewater-soluble and water-insoluble regions.

Figure 2 shows the FT-IR spectra of b-PEI, β-CD, and X-CDs. Inthe spectrum of b-PEI, IR bands attribute to N–H stretching vibra-tions (around 3400 cm−1) and aliphatic C─H stretching vibrations(around 2900 cm−1). Two strong bands at 1570 and 1460 cm−1 areresulted from the N─H vibrations of 1� and 2� amino groups,respectively. The band at 1460 cm−1 also ascribes to the ─CH2─moiety. The C─N stretching vibrations result in the IR bands

Figure 1. Representative scheme for overall experiment. [Color figure can be viewed at wileyonlinelibrary.com]



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around 1300 cm−1 and in the range 1040–1120 cm−1.32 The β-CDhave IR bands around 3400, 2900, 1630, 1157, 1030, and 945 cm−1

due to the O─H stretching vibration, the ─CH2─ stretching vibra-tion, the glucopyranose ring stretching vibration band, the coupledν (C–C/C–O) stretching vibration, antisymmetric glycosidic νa

(C─O─C) vibrations, and the R-1,4-bond skeleton vibration ofβ-CD.33,34

X-CDs also contain IR bands at around 3400, 2900, 1630, and1570 cm−1, corresponding to the superposition of the O─Hstretching vibration of β-CD and the N─H stretching vibration ofb-PEI, the ─CH2─ stretching vibration, the glucopyranose ringstretching vibration band for β-CD, and the N─H bending vibra-tion band for b-PEI, respectively.32,33 However, X-CD_11.2_0.3 andX-CD_15.7_0 did not exhibit an N─H bending vibration band forb-PEI because they contained little or no b-PEI. Furthermore, theIR bands near 1460 and 1360 cm−1, corresponding to the ─CH2─scissoring bending vibration and the C─H bending vibration, wereobserved in the all X-CDs. These absorption bands indicate thatPEGDE reacted with the CD hydroxyl groups to form ethercrosslinkages.20 X-CD also exhibited characteristic IR bands at1157, 1030, and 945 cm−1, which correspond to the coupled ν(C─C/C─O) stretching vibration, antisymmetric glycosidic νa(C─O─C) vibrations, and the R-1,4-bond skeleton vibration ofβ-CD, respectively.34 The FT-IR results reveal that X-CDs werewell-synthesized by crosslinking β-CD with PEG and/or b-PEI.

The adsorption capacities of the X-CDs (10 mg) were measuredin a solution with an initial BPA concentration of 100 mg L−1 for120 min, in order to identify the X-CD sample with optimizedadsorption ability. As shown in Table I, the only water-insolublesample without b-PEI was X-CD_15.7_0, which exhibited a BPAadsorption capacity of 21.7 mg g−1. With the addition of b-PEI,the amount of PEGDE could be reduced to 0.78 g. The X-CD_3.9_0.78 showed the highest BPA adsorption capacity(26.9 mg g−1) among the X-CDs and was therefore determined asthe optimal adsorbent and used for further investigations.

EA was also conducted to evaluate the composition andcrosslinking degree of X-CD_15.7_0 and X-CD_3.9_0.78(Supporting Information Table S1). The b-PEI content in X-CD_3.9_0.78 could be calculated based on its N content becauseall of the N within X-CDs comes from b-PEI molecules (there isno N in β-CD and PEG). Furthermore, the β-CD and PEGDEcontent can be calculated by solving simultaneous linear equa-tions (Supporting Information Text S1). Thus, the compositionof X-CD_15.7_0 was defined as 65.15 wt% of β-CD and 34.85 wt% of PEGDE, and the composition of X-CD_3.9_0.78 wasdefined as 72.85 wt% of β-CD, 19.35 wt% of b-PEI, and 7.80 wt%of PEGDE, as shown in Table II. The crosslinking degree of X-CD_3.9_0.78 is five times smaller than that of X-CD_15.7_0.These results are consistent with the hypothesis that the additionof b-PEI reduces the amount of PEGDE required for preparationof a water-insoluble X-CD and also enhances the adsorption ofEDCs by X-CD.

Figure 2. FT-IR spectra for X-CDs. [Color figure can be viewed atwileyonlinelibrary.com]

Table I. Experimental Design and BPA Adsorption Capacity for X-CDSamples

Sample PEGDE (g) b-PEI (g)

Adsorptioncapacity(mg g−1)

X-CD_0.78_2.3 0.78 2.3 5.5

X-CD_1.57_1.57 1.57 1.57 24.5

X-CD_3.9_0.78 3.9 0.78 26.9

X-CD_3.9_3.9 3.9 3.9 6.8

X-CD_7.8_0.78 7.8 0.78 18.9

X-CD_7.8_7.8 7.8 7.8 8.2

X-CD_11.2_0.3 11.2 0.3 11.9

X-CD_15.7_0 15.7 0 21.7

Table II. Composition and Crosslinking Degree of X-CDs

Sample

Composition

Crosslinking degree (%)β-CD (wt%) b-PEI (wt%) PEGDE (wt%)

X-CD_15.7_0 65.15 0 34.85 125.01

X-CD_3.9_0.78 72.85 19.35 7.80 25.02





Characterization of X-CDThe surface morphology of X-CD_3.9_0.78 was investigatedusing FE-SEM, the results of which are shown in Figure 3. TheFE-SEM images show that X-CD_3.9_0.78 has a smooth and

non-porous surface, which is in accordance with the result of theBET analysis, which indicates that specific surface area for the X-CD_3.9_0.78 was not detected (shown in Supporting InformationFigure S3).

Figure 3. FE-SEM images for X-CD. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4. TGA result of X-CD (a), DSC curve for X-CD (b), XRD pattern of X-CD (c), zeta potential of X-CD as a function of pH (d). [Color figure can beviewed at wileyonlinelibrary.com]






The thermal behavior of X-CD_3.9_0.78 was also studied usingTGA and DSC. As shown in Figure 4(a), the degradative behavior ofone of X-CDs in an inert atmosphere (N2) can be observed. Theweight losses below 250 �C were caused by the elimination of thephysically adsorbed water from X-CD_3.9_0.78. It is clear that X-CD_3.9_0.78 exhibits a high thermal resistance, as appreciable deg-radation did not take place until the temperature reached nearly

340 �C. The thermal degradation of X-CD_3.9_0.78 might be acomplex process involving overlapped degradation of its compo-nents such as b-PEI, PEG, and β-CD. It is worth mentioning that anincrease of the thermal resistance of X-CD_3.9_0.78 occurred withregard to the b-PEI, PEG, and β-CD, which is typical of crosslinkedpolymers.35–37 Figure 4(b) indicates that X-CD_3.9_0.78 is a mate-rial with glass transitions below room temperature, such that it hasan elastomeric behavior and an amorphous nature suitable for theadsorption process.38 The glass transition temperature (Tg) ofX-CD_3.9_0.78 was determined to be 3.84 �C, which is higher thanpreviously reported glass transition temperatures for a β-CD/PEGpolymer.39 The amorphous nature of X-CD_3.9_0.78 was also con-firmed by diminish of characteristic peaks of β-CD in Figure 4(c).These are because the introduction of b-PEI increased the rigidity inthe system. As displayed in Figure 4(d), the zeta potentials ofX-CD_3.9_0.78 in DI water were determined in a pH range of2.0–12.0, showing that the surface charge is positive at pH < 11.6and negative at pH > 11.6. Therefore, the point of zero charge(pHPZC) value was evaluated at 11.6. The zeta potential showed ahighest value near pH 5.0, which was in consonance with the previ-ously reported charge behavior of b-PEI resulted from the proton-ation of amine groups in different level with pH.40

BPA and Cu(II) Adsorption by X-CDThe pH of a solution is one of the most important variables thataffects adsorption capacity because it significantly influences theprotonation/deprotonation of the functional groups of adsorbentsand impacts the speciation of the adsorbate. BPA is a neutral mole-cule below pH 7.0 and starts deprotonation above pH 7.0.41 BPAadsorption experiments should be conducted below pH 7.0 in orderto maximize the amount of neutral BPA available for capture withinthe hydrophobic cavity of β-CD. In the case of Cu(II) adsorption,the protonation of the amine groups of X-CDs reduces the numberof active adsorption sites for the Cu(II) at lower pH values. Byincreasing pH values, the number of active sites available to captureCu(II) will increase because the positive charge density on theadsorbent surface decreases. However, the efficiency with whichX-CDs remove Cu(II) will be reduced again at pH > 6.0 because theCu(II) will precipitate as Cu(OH)2.

42 We therefore performed theBPA and Cu(II) adsorption experiments using the X-CDs at pH 6.0to obtain the maximum adsorption capacity.

Figure 5(a) shows the adsorption kinetics of BPA and Cu(II) byX-CD at C0 (100 mg L−1) from 0 to 1140 min. BPA andCu(II) adsorption gradually increased and achieved equilibriumat �1140 min. Compared to BPA, the adsorption of Cu(II) wasfound to be slower because X-CD showed a lower rate constantand initial adsorption rate (h) for Cu(II) (h= k2 × q2e ) than forBPA. Adsorption kinetics of BPA and Cu(II) onto X-CD wereinvestigated using pseudo-first-order and pseudo-second-orderkinetic models. All of the kinetic parameters are listed inTable III. The resulting correlation coefficients indicate thatBPA and Cu(II) adsorption can be approximated with a pseudo-second-order kinetic model.

The BPA and Cu(II) adsorption isotherms of X-CD were mea-sured for various C0 (6.25–100 mg L−1), as shown in Figure 5(b),

Figure 5. Adsorption kinetics of X-CD for BPA and Cu(II) (a), Adsorptionisotherms of X-CD for BPA and Cu(II) (b), Recovery efficiencies of X-CDover four adsorption–regeneration cycles (c). [Color figure can be viewed atwileyonlinelibrary.com]





and investigated by applying the Freundlich and Langmuir iso-therm models to the equilibrium adsorption data. The isothermmodel parameters are summarized in Table IV. The resultingcorrelation coefficients indicate that the Langmuir model fit theadsorption of BPA and Cu(II) by X-CD. The type of adsorption(i.e., irreversible, favorable, linear, and unfavorable) can thus bepredicted using the separation factor RL, defined using eq. (8)43:

RL =1

1 +KLC0ð8Þ

where KL (L mg−1) is the Langmuir constant and C0 (mg L−1) isthe initial adsorbate concentration.

The adsorption process can be predicted using the RL value: unfa-vorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), and irre-versible (RL = 0). The RL values for the adsorption of BPA andCu(II) by X-CD show that the X-CD adsorbed BPA andCu(II) favorably. Furthermore, the Langmuir model explains thatBPA and Cu(II) formed a monolayer in X-CD, with the adsorp-tion driving force as the only interaction toward the CD cavitiesand amine groups of b-PEI.

The maximum BPA and Cu(II) adsorption capacities of X-CDwere also estimated to be 65.3 and 52.4 mg g−1, respectively,using the Langmuir model. It is quite remarkable that X-CDreadily adsorbs BPA in aqueous environments, given that N2

adsorption–desorption isotherms confirm that X-CD has little

porosity. In some cases, xerogels, dried form of hydrogels, mayexpand their network structures, while swelling in an aqueousenvironment. The swelling property of X-CD was evaluated usingwater uptake W%, calculated by modified eq. (9)44:

W%=Wswell−Wdry

Wdry× 100 ð9Þ

where Wdry (g) is the weight of dried X-CD and Wswell (g) is theweight of swelled X-CD after soaking in DI water for 24 h.

As shown in Supporting Information Table S2, the water uptakeof X-CD was calculated to be 226.3%, indicating that thecrosslinked structure of dried X-CD could expand in an aqueousenvironment. Therefore, the high adsorption capacity of X-CDcompared to specific surface area may suggest a strong interac-tion toward BPA and Cu(II) and the expansion of crosslinkedstructure. The maximum adsorption capacity of X-CD must becompared with those of previously reported CD-based adsor-bents, because doing so would allow us to determine the potentialof X-CD as an adsorbent for the removal of EDCs and HMs. Themaximum BPA and Cu(II) adsorption capacity of X-CD in unitof mmol g−1 for comparison are sufficiently comparable withthose previously reported in the literature, as shown in Table V.

Regeneration of X-CDIn each round of the reusability tests, after adsorbing BPA orCu(II) from aqueous solutions, X-CD was extracted from thepurified water, and the BPA or Cu(II) was desorbed from the

Table III. Adsorption Kinetic Parameters for Adsorption of X-CD

Pseudo-first-order kinetic Pseudo-second-order kinetic

qe k1 R2 qe k2 h R2

BPA 54.8 0.0365 0.9378 60.1 0.0008 2.8896 0.9888

Cu(II) 48.5 0.0102 0.9669 53.2 0.0003 0.7880 0.9911

Table IV. Adsorption Isotherm Parameters for Adsorption of X-CD

Freundlich Langmuir

KF n R2 qL KL RL R2

BPA 21.7 4.0 0.8185 65.3 0.2019 0.0472–0.4421 0.9563

Cu(II) 28.5 6.6 0.6643 52.3 0.7533 0.0131–0.1752 0.9733

Table V. Maximum Adsorption Capacities and Ratios of Adsorption Capacities of CD-Based Adsorbents

Adsorbent BPA Cu(II) Rq,BPA/Rq,Cu(II) Reference

CD-poly(glycidyl methacrylate)-MNPs 0.88 μmol g−1 - - 6

POSS-PCL-β-CD/Fe3O4 micelles 0.12 mmol g−1 - - 13

β-CD-modified magnetic graphene oxide - 0.81 mmol g−1 - 4

Magnetic graphene oxide-supported β-CD - 0.49 mmol g−1 - 14

CD-poly(glycidyl methacrylate)-SiO2-MNPs 0.10 mmol g−1 0.20 mmol g−1 - 11

Citric acid-crosslinked β-CD 0.38 mmol g−1 0.92 mmol g−1 0.9 / 1.0 12

X-CD 0.29 mmol g−1 0.82 mmol g−1 1.0 / 0.9 This study




extracted X-CD by washing with ethanol or DI water (pH 2).The regenerated X-CD was then used in the next round of reus-ability tests.

Figure 5(c) shows that the recovery efficiencies of X-CD for BPAand Cu(II) were 87.3 and 79.2% after four adsorption–desorptioncycles, respectively. This high recovery efficiency of X-CD forBPA could be because β-CD adsorbs BPA in water and desorbsBPA in ethanol, owing to the affinity order of BPA (ethanol >β-CD > water) as previously reported.43 The sufficient recoveryefficiency of X-CD for Cu(II) can be ascribed to b-PEI adsorbingCu(II) at pH 6 and desorbing Cu(II) at pH 2 because the pKa ofthe primary amine groups in b-PEI is 4.5.45 Conventional adsor-bents like activated carbon require the application of cost-ineffective regeneration procedures, which constitute �75% oftheir total maintenance and operating costs.46 However, X-CDcan be regenerated simply by washing with ethanol or DIwater (pH 2).

Simultaneous Adsorption of X-CDTable V shows the results of simultaneous adsorption of BPAand Cu(II) on X-CD. According to the literature, there are threetypes of simultaneous adsorption47: (1i) if Rq > 1, synergism,where the adsorption of contaminant i is promoted by the pres-ence of coexisting contaminants; (2) if Rq < 1, antagonism, wherethe adsorption of contaminant i is impeded by the presence ofco-existing contaminants; (3) if Rq = 1, non-interaction, wherethe adsorption of contaminant i is not affected by the presence ofco-existing contaminants.

X-CD exhibits an Rq,BPA of 1, indicating that the BPA adsorptioncapacity of X-CD is not affected by the presence of co-existingorganic substances such as F127 and CTAB in multicomponentsystems. This phenomenon illustrates that coexisting organic sub-stances cannot enter into the crosslinked network of X-CD. Theresults of Rq,Cu(II) also confirm the interactive effect of BPA onCu(II) adsorption. Our group reported that amine groups of b-PEIare able to selectively complex HMs over alkali/alkaline-earthmetals by sharing a lone electron pair on the nitrogen atom withunoccupied d orbitals in HMs.26 However, X-CD containing b-PEIexhibits an Rq,Cu(II) of slightly less than 1. This could be explainedas follows: BPA and Cu(II) competitively interact with the aminegroups, while BPA rapidly penetrates into the crosslinked networkof X-CD and interacts with the amine groups earlier thanCu(II) because BPA has a higher adsorption rate than Cu(II).Then, active adsorption sites for Cu(II) are reduced, resulting insuppressed Cu(II) adsorption of X-CD in multicomponentsystems. The Rq of X-CD must also be compared with that of pre-viously reported CD-based adsorbents. Although X-CD adsorbsBPA and Cu(II) in the presence of coexisting substances such asF127, CTAB, and Na(I), the Rq,BPA and Rq,Cu(II) of X-CD arecomparable with those of previously reported CA-crosslinkedβ-CD that adsorb BPA and Cu(II) in the absence of coexistingsubstances such as surfactants and alkali/alkaline-earth metals.12

However, the CA-crosslinked β-CD would exhibit low Rq,Cu(II),especially in the presence of alkali/alkaline-earth metals similar tothe real aqueous environment, due to the lack of selective adsorp-tion property toward HMs.

CONCLUSIONS

We report on the preparation and adsorption performance of awater-insoluble b-PEI-PEG-β-CD polymer that readily removesBPA and Cu(II) from aqueous environments. A water-insolubleb-PEI-PEG-β-CD polymer was synthesized by simply reactingPEGDE with b-PEI and β-CD. FT-IR, EA, TGA, and DSC ana-lyses indicate that β-CD was successfully crosslinked with b-PEIand PEGDE. As expected, X-CD showed sufficient adsorptioncapacities for BPA and Cu(II). Furthermore, X-CD wasregenerated several times without a significant loss of adsorptioncapacity. Finally, the adsorption experiment in a multicomponentsystem revealed that the presence of coexisting substances didnot significantly affect the adsorption performance X-CD, andthat BPA and Cu(II) were successfully removed from aqueoussolutions, suggesting a selective and simultaneous adsorptionproperty. It is remarkable that X-CD efficiently removes BPAand Cu(II) in aqueous environments due to the expansion ofcrosslinked structure even though N2 adsorption–desorption iso-therms show that X-CD has little porosity. The excellent adsorp-tion performance of X-CD makes it attractive for efficient andeco-friendly purification of industrial wastewater containingEDCs and HMs.

ACKNOWLEDGMENTS

This work (Grants No. 0668-20170140) was supported by Busi-ness for Cooperative R&D between Industry, Academy, andResearch Institute funded Korea Small and Medium BusinessAdministration in 2017.

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