Bisphenol A Adsorption Properties of Mesoporous CaSiO3@SiO2

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Bisphenol A adsorption properties of mesoporous CaSiO3@SiO2 grafted non-woven polypropylene fiber Kongyin Zhao, Xiaohui Wang, Tian Chen, Hui Wu, Jingang Li, Bingxiang Yang, Dongying Li, and Junfu Wei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03015 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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Industrial & Engineering Chemistry Research

Bisphenol A adsorption properties of mesoporous CaSiO3@SiO2 grafted non-woven polypropylene fiber Kongyin Zhao1, 2*, Xiaohui Wang2, Tian Chen2, Hui Wu2, Jingang Li1, Bingxiang Yang1*, Dongying Li2, Junfu Wei 1 1

. State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

Polytechnic University, Tianjin 300387, China 2

. School of Material Science and Engineering, Tianjin Polytechnic University,

Tianjin, 300387, China *

Corresponding author: [email protected], Tel.: +086-02283955362, Fax, +086 -0228395055;

ABSTRACT: Calcium silicate particles containing mesoporous SiO2 (CaSiO3@SiO2) on the surface were grafted onto polypropylene non-woven. The PP non-woven grafted CaSiO3 containing mesoporous SiO2 (PP-g-CaSiO3@SiO2) was characterized by TEM and TG. The adsorption behaviors of bisphenol A (BPA) on PP-g-CaSiO3@SiO2 were investigated. The results indicated that the large surface area and the Si-OH groups of the material improved the adsorption capacities and affinity for BPA. The adsorption rate was fast and the adsorption capacity was high, even with the background organic compound alginate sodium. The adsorption mechanism of BPA by PP-g-CaSiO3@SiO2 was investigated. Isothermal titration calorimetry (ITC) was used to illustrate the driving force for the adsorption of BPA. PP-g-CaSiO3@SiO2 was prepared simply without using any organic solvent, and could be recycled and reused. Keywords: Bisphenol A adsorption; Mesoporous CaSiO3@SiO2; Polypropylene non-woven; Graft; Isothermal titration calorimetry

1. Introduction Bisphenol A (BPA) has been widely used as an important monomer in the production of epoxy resins plastics, polycarbonate and flame retardants. BPA can be highly stable and remain in the environment for years

1-3

, incurring long-lasting

ecological harm and causing serious social concerns 4, 5. Up to now, several treatment techniques for the removal of BPA have been investigated 6, including biodegradation 7

, chemical oxidation 8, enzymatic polymerization 9 and adsorption 10. Among these

methods, adsorption offers a high-efficiency and reasonable technology for the 1

ACS Paragon Plus Environment


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removal of BPA. Many developed particle materials were selected as adsorbents such as activated carbon, biological materials and advanced oxides 11, 12. Mesoporous silica is an excellent adsorbent due to its high adsorption capacity and large surface area 13,14. The morphology and size of silica can be controlled

15

. Although functional

mesoporous silicas have high adsorption capacities, they are not effective in practical application because these particle adsorbents are difficult to be recycled and reused 16. Some nanoparticles were mixed with polymers to obtain composite materials for the adsorption of contaminants. But the adsorption efficiency will be much compromised because the nanoparticles are entrapped in the polymer matrix 17-19. Polypropylene (PP) non-woven shows excellent thermal and chemical stability, good flexibility 20. A kind of amphiphilic polypropylene nonwoven with hydrophilic and hydrophobic microdomain was prepared through electron beam induced graft polymerization and subsequent ring opening reaction and then utilized in the adsorption of phthalic acid ester 21. In our previous work, calcium silicate particles containing mesoporous SiO2 on the surface (CaSiO3@SiO2) were grafted onto PP non-woven in the absence of any toxic surfactant or organic solvent 22. In this study, the adsorption performance of bisphenol A (BPA) by the material was extensively investigated. The adsorption rate was fast and the adsorption capacity was high, even with the background organic compound alginate sodium. Isothermal titration calorimetry (ITC) was used to illustrate the driving force for the adsorption of BPA and the adsorption mechanism was investigated. 2. Experimental 2.1. Materials Bisphenol A (BPA, Log Kow 3.32, kPa 1.73, molecular weight 244.38) was obtained from Lianxing Biotechnology Company. PP non-woven was obtained from China Non-woven Co. Ltd. Calcium chloride (CaCl2) was obtained from Tianjin Huazhen special chemical reagent factory. Acrylic acid (AA), hydrochloric acid (HCl), alcohols and sodium silicate were from Tianjin Jiangtian Chemical Co., Ltd. Sodium hydroxide (NaOH) was purchased from Tianjin Second Chemical Reagent Factory. 2.2. Preparation of PP-g-CaSiO3@SiO3 Sodium polyacrylate grafted PP non-woven (PP-g-COONa) was prepared by grafting polymerization of polyacrylic acid on PP and NaOH neutralizing

22

.

PP-g-COONa (2.0 g) was dipped into100 ml 15 wt. % Na2SiO3 aqueous solution for 0.5 hour. Then it was taken out and put into 100 ml 5 wt. % CaCl2 aqueous solution 2


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for 3 hours to synthesize CaSiO3 grafted PP non-woven (PP-g-CaSiO3). The PP-g-CaSiO3@SiO2 was produced by acid treatment the PP-g-CaSiO3 using HCl solution (pH=3) 22. The grafting degree (G) was calculated as follow: G (%) =

× 100%

(1),

where W0 and W1 is defined as the weight of the PP non-woven fiber and PP-g-CaSiO3@SiO2, respectively. PP-g-CaSiO3 and PP-g-CaSiO3@SiO2 with the grafting degree of 30±5% were selected for the adsorption of BPA. 2.3 Preparation of CaSiO3@SiO2 CaSiO3@SiO2 was produced according to the reference 23. CaSiO3 suspension was synthesized by a chemical precipitation method using the mixed solution of NaSiO3 and CaCl2 with uniform molar concentration at room temperature. The CaSiO3@SiO2 suspension was prepared by acid treatment using HCl (pH=3). The CaSiO3@SiO2 was washed with deionized water to remove the soluble ions. 2.3. Characterizations Transmission electron microscope (TEM, FEI Tecnai G2 F30) was carried out to survey the PP-g-CaSiO3@SiO2. The sample was prepared by epoxy resin embedding and ultrathin sectioning on a Leica Ultracut UCT ultramicrotome. TG and DTG of the samples were measured using a DSC analyzer (NETZSCH 200 F3, Germany) at a heating rate of 10 oC/min from 25 oC to 600 oC under continuous flow of dry nitrogen. 2.4. BPA Adsorption Approximately 0.10 g PP-g-CaSiO3@SiO2, PP-g-CaSiO3 or PP was put into conical flasks containing 100 ml 50 mg/L BPA solution

23

shaking incubator and the temperature was 25

. The flasks were agitated in a o

C.

At different times the

concentrations of BPA were determined at 278 nm by a UV spectrophotometer (UV-1100). The adsorption capacity of BPA (Qt) was calculated as follow 22: Qt= (C0-Ct) V/m

(2),

where C0 and Ct are the initial concentration of BPA and the concentration of BPA at time t, respectively. V is the volume of BPA solutions and m is the mass of adsorbent.

3



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When the concentration of BPA did not change with the adsorption time t, the equilibrium concentration of BPA (Ce) was recorded and the equilibrium adsorption capacity was reached and noted as Qe. In order to investigate the adsorption isotherm of BPA, 0.10g samples were put into conical flasks containing 100 ml BPA solutions (the concentration was 10-150 mg/L). After shaking the mixed solution for 5 h at 25 oC, the equilibrium adsorption capacity (Qe) was determined. To investigate the effect of temperature on the adsorption capacity, the flask was placed in an incubator for 5 h at predetermined temperature (288K, 298 K, 308 K, 318 K and 328 K) to ensure adsorption equilibrium. To study the effect of pH on adsorption capacity, the pH of BPA solution was controlled to 2.0-11.0 by adding hydrochloric acid or sodium hydroxide solution. To investigate the effect of background organic compounds on the adsorption capacity, sodium alginate (NaAlg) at lower concentration (0.03wt. %) was added into the BPA solution when the adsorption experiments were conducted. 2.4. Isothermal titration calorimetry (ITC) experiments An isothermal calorimeter (VP-ITC, MicroCal Inc., Northampton, MA) was used to evaluate the interactions between BPA and CaSiO3@SiO2. Then 5µl BPA solution (0.219 mM) filled into a 283 µL syringe were used to titrate a suspension of CaSiO3@SiO2 at 9.65 × 10-3 mg/ml into the calorimetric cell accurately thermostated at 25 ◦C. The intervals between injections were 270 s and the agitation speed was 307 rpm. An exothermic heat pulse was detected following every injection. The data analysis was performed using the ORIGIN 7.5 software and the thermodynamic parameters (K, ∆H, ∆S) were calculated by the Gibbs free energy (∆G) equation. 2.5. Regeneration and reuse of PP-g-CaSiO3@SiO2 The PP-g-CaSiO3@SiO2 after adsorbing BPA was washed with 100 mL 0.1 mol/L NaOH aqueous solution to remove BPA. The sample was washed with deionized water, dried at 60 °C in vacuum for 24 hours, and reused. The PP-g-CaSiO3@SiO2 was used repeatedly for 8 times and the adsorption capacities were recorded. 3. Results and discussion 3.1 Characterizations 3.1.1. The morphology of PP-g-CaSiO3@SiO2 Fig. 1 shows the TEM images of PP-g-CaSiO3@SiO2 and it is found that the particle size of mesoporous CaSiO3 was 30-100 nm. The size was consistent with the 4


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size of CaSiO3@SiO2 reported in the literature 24. As was shown in the TEM images, some CaSiO3@SiO2 particles accumulated because of the reaction of hydroxyl groups on

the

surface

of

the

particles.

The

high-resolution

TEM

image

of

PP-g-CaSiO3@SiO2 was obtained in Fig. 1 (b). It is found that, after acid modification, mesoporous structure appeared on these particles. Some stripes were well distinguished with the width of 0.30 nm. This demonstrated the successful preparation of mesoporous structure in PP-g-CaSiO3@SiO2 by the treatment of dilute hydrochloric acid.

(a)

(b)

Fig. 1 - TEM images of PP-g-CaSiO3@SiO2. (a) scale bar was 200 nm (b) scale bar was 5 nm.

3.1.2. Thermo gravimetric (TG) Fig. 2 shows the TG and DTG of three different samples (PP-g-CaSiO3@SiO2, PP-g-CaSiO3 and PP). PP fiber had only one pyrolysis stages due to the decomposition of PP. The thermal degradation process of PP-g-CaSiO3 occurred around 106 oC. The weight loss in this stage resulted from the evaporation of water. PP-g-CaSiO3 lost weight of 21.52% at 276 oC, showing a sharp peak on the corresponding DTG curve for the dehydration of oxy acid. This suggested that Si-OH groups accounted for a large proportion. The loss of weight around 333 oC was resulted from the dehydration of Si-OH. The last thermal degradation process occurred around 405 oC, which was the sharpest among all the peaks, due to the thermal decomposition of PP fiber. Finally, this sample remained 38.85% of the original fiber with only CaSiO3 left after heating. The thermal degradation of PP-g-CaSiO3@SiO2 was similar with PP-g-CaSiO3, but almost every peak appeared earlier than PP-g-CaSiO3. The PP-g-CaSiO3 before acid modification had better thermal stability. However, the rate of loss-weight was much 5



different. The loss-weight rate of PP-g-CaSiO3@SiO2 around 220 oC was 31.67%, which was 10% more than the loss-weight rate of PP-g-CaSiO3. The increase was owing to the more Si-OH on the surface of PP-g-CaSiO3@SiO2 after treatment by hydrochloric acid. Ultimately, the residual rate of PP-g-CaSiO3@SiO2 was only 15.75%, which decreased significantly compared with PP-g-CaSiO3 (38.85%).

390.4℃ DTG

80

60

90

-4

80

DTG/(%/min)

317.4℃

70

-2

221.5℃

-6

TG

-8

50 40

30 PP-g-CaSiO @SiO 3 2 20 100

200

-10 369.4℃

300 400 Temperature /℃

-12 500

DTG

106.3℃ TG

120

-1

100 80

-3 -4 -5

276.0℃

60

-6

50

100

600

200 300 400 Temperature /℃

(a)

-7

405.2℃

PP-g-CaSiO3

500

100 DTG

-2

333.3℃

70

40

0

80

60

60

40

40

20

-8

0

-9 600

-20

TG

PP 458.7℃

100

200

(b)

300 400 Temperature/℃

500

DTG/(%/min)

82℃

100

TG/%

0

DTG/(%/min)

90

TG/%

100

TG /%

20 0 600

(c)

Fig. 2 - TG and DTG curves of (a) PP-g-CaSiO3@SiO2, (b) PP-g-CaSiO3 and (c) PP.

3.2 Adsorption isotherm and adsorption mechanism of BPA Fig. 3 shows the adsorption isotherms of BPA on three different samples. An increased equilibrium adsorption capacity (Qe) was observed until saturation was attained. The Qe of PP-g-CaSiO3@SiO2, PP-g-CaSiO3 and PP were 53.8, 30.2 and 15.1 mg/g, respectively. The Qe of PP-g-CaSiO3@SiO2 was larger than the other two fibers because of the high specific surface area and the Si-OH groups on the surface of PP-g-CaSiO3@SiO2 fiber. The specific surface area of PP-g-CaSiO3@SiO2 was enhanced significantly due to the grafted mesoporous SiO2 on the PP non-woven.

80 PP-g-CaSiO3 PP-g-CaSiO3@SiO2

70 60 Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PP

50 40 30 20 10 0

0

10

20

30

40 50 60 Ce (mg/L)

70

80

90 100

Fig. 3 - The isothermal adsorption curves of BPA byPP-g-CaSiO3@SiO2, PP-g-CaSiO3 and PP. pH = 7.0, adsorption temperature 298 K. 6


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Fig.

4

demonstrates

the

schematic

of

hydrogen

bonding

between

PP-g-CaSiO3@SiO2 surface and BPA from the perspective of chemical adsorption. There are no obvious interactions between PP fiber and BPA molecule. So the Qe of PP fiber was low. However, after grafting of CaSiO3 and acid treatment, a great many of Si-OH groups were created on the fiber surface. The hydrogen bonds played an important role in chemical adsorption. In order to prove that hydrogen bonds were formed between Si-OH and -OH at both ends of BPA, the FTIR spectra of PP-g-CaSiO3@SiO2 was performed before and after BPA adsorption. Fig. 5 shows the FTIR spectra of PP-g-CaSiO3@SiO2 before and after BPA adsorption. The broad band at 3383 cm−1 representing the stretching vibration of O−H groups shifted to 3364 cm−1. The shift indicated that hydrogen bonds were formed in the adsorption process.

Fig. 4 - The schematic of hydrogen bonding between PP-g-CaSiO3@SiO2 and BPA.

PP-g-CaSiO3@SiO2

Transmittance %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PP-g-CaSiO3@SiO2 After adsorptioin

O-H

4000

3500

Si-O-Si

C-H

3000

C=O

2500

2000

C-H

1500 -1

O-Si-O

1000

500

Wavenumber(cm )

Fig. 5 - FTIR spectra of PP-g-CaSiO3@SiO2 before and after BPA adsorption.

The mesoporous structure on the surface of PP-g-CaSiO3@SiO2 also played an 7



important role in the physical adsorption of BPA. The impact of a carbon-based backbone (polypropylene) was the supposed effect of Si-OH presented convincingly. In order to explore the adsorption isotherm, the experimental data were fitted into the Langmuir and Freundlich isotherm models. Langmuir models equation are as follow:

(3), where Qe is amount of adsorption at equilibrium (mg/g), Qm is maximum capacity =

+

of adsorbent (mg/g), Ce is the equilibrium concentration of BPA (mg/L), and KL is the Langmuir constant related to the affinity of binding sites (L/mg). Freundlich equation can be expressed as:

lg =lg +

lg

(4),

where KF is the Freundlich constant (L/g) and n is the Freundlich exponent obtained from the liquid phase adsorption isotherm. The Langmuir and Freundlich isotherm constants were obtained from the plots of Ce/Qe versus Ce, and lgQe versus lgCe, respectively. And the results are shown in Fig. 6. The adsorption isotherm parameters evaluated from the linear plots are displayed in Table S1. The Qm of PP-g-CaSiO3@SiO2 reached 80.13 mg/g, while the Qm of PP was only 21.32 mg/g. It was the combination of the hydrogen bond interaction and the mesoporous structure on the surface of PP-g-CaSiO3@SiO2 that made the difference. According to the coefficients (R2), the Langmuir model was more suitable than the Freundlich model for the adsorption of BPA. It is probably ascribed to the “butterfly” structure of BPA which has great impact on the formation of multilayer and increases the dimensional restrictions of multilayer adsorption 25. 4.5

8 PP-g-CaSiO3 PP-g-CaSiO3@SiO2

6

PP

3.0 In(Qe)

Ce/Qe(g/ml)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

1.5

PP-g-CaSiO3 PP-g-CaSiO3@SiO2

2

PP

0

0.0

0

20

40 60 Ce (mg/L)

80

1

100

2

3 In(Ce)

(a)

4

5

(b)

Fig. 6 - Fitting curves of Langmuir model (a) and Freundlich model (b) of BPA. 8


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3.4 The interaction between BPA and PP-g-CaSiO3@SiO2 It is difficult to test the interaction between PP-g-CaSiO3@SiO2 and BPA directly. So CaSiO3 containing mesoporous SiO2 (CaSiO3@SiO2) was prepared to investigate the interaction between PP-g-CaSiO3@SiO2 and BPA. Fig. 7 shows the experimental curve for the titration of BPA into CaSiO3@SiO2 suspensions in the same solvent at 298 K. The results could illustrate the driving force for the adsorption of BPA to CaSiO3@SiO2. Fig 8(a) is a typical thermogram of BPA adsorption to CaSiO3@SiO2. The peaks corresponded to the individual aliquots of added BPA solution, and the current applied for the compensation of the reaction heat was plotted in energy units against time. Fig 8(b) shows the titration curve, resulting from the integration of the individual peaks and plotted as ∆H in kcal/mol against the molar ratio BPA/CaSiO3@SiO2. The released heat was detected after each titration of BPA. The binding affinity (K), binding enthalpy (∆H) and binding entropy (∆S) between BPA and CaSiO3@SiO2 were obtained to be 1.52 × 102 L/mol, -4.74 kJ/mol and -15.85 J/(K*mol), respectively. The Gibbs’ energy change during the binding process was calculated to be -12.44 kJ/mol from the equation ∆G = −RT lnK. This indicated that the adsorption process was thermodynamically favorable, and the BPA was adopted to the CaSiO3@SiO2 via H-bonds between -OH at both ends of BPA and HO-Si of CaSiO3@SiO2. The reaction is exothermic and high temperatures would not in favor of forming the hydrogen bonds.

5

0.0

0

-0.5

kcal/mole of injectant

-5 -10 -15 µ cal/sec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-20 -25 -30 -35

-1.5 -2.0 -2.5 -3.0 -3.5

-40 -45

-1.0

0

20

40

60 80 Time (min)

100

120

-4.0

140

0.0

0.2

(a)

0.4 0.6 Molar Ratio

0.8

1.0

(b)

Fig. 7. Experimental curve for the titration of a 0.219 mM solution of BPA into a dispersion of CaSiO3@SiO2 (9.65 × 10-3 mg/ml) in the same solvent at 298 K;

In order to investigate the interaction between BPA in of different concentrations 9



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and CaSiO3@SiO2, batch experiments with different concentrations but with the same mole ratio have been done. As is shown in Table S2, the ∆G indicated that the adsorption processes of all the BPA concentrations were thermodynamically favorable and the negative value of the enthalpy change suggested that these processes were exothermic. Since |∆H| > |T∆S|, this means that the binding reaction is driven mainly by the entropy change

26

. This is primarily attributed to the hydrogen bond between

both ends of BPA and the Si-OH of CaSiO3@SiO2. The experiment results indicated that the adsorption behavior of BPA on CaSiO3@SiO2 is good at low concentrations. 3.5 Adsorption kinetics The adsorption kinetics of BPA on PP, PP-g-CaSiO3 and PP-g-CaSiO3@SiO2 fibers and the schematics of diffusion process are revealed in Fig. 8. The adsorption capacity (Qt) of BPA increased with the time and ultimately approached adsorption equilibrium within 2.5 h. According to Fig. 8 (b), the adsorption process can be distinctly divided into three steps. The first is the interface diffusion stage when BPA molecules in solution diffuse to the external surface of non-woven fibers covered with Si-OH groups. The second stage describes that plenty of BPA molecules penetrate into the mesopores of CaSiO3@SiO2. Finally, a dynamic equilibrium of diffusion is achieved. PP-g-CaSiO3@SiO2 adsorbed more BPA than the other two fibers due to its larger specific surface area and more Si-OH on the fiber. The pseudo-first-order and pseudo-second-order models were used to describe the adsorption kinetics data and Fig. 9 showed the fitting curves. The kinetic parameters for adsorption data obtained from the fitting curve were listed in Table S3. The pseudo-first-order kinetic model is as follow 27:

ln(Qe-Qt) = lnQe -k1t

(5)

The pseudo-second-order kinetic model has the following form 28:

=

+

(6),

where Qe and Qt are the amount of BPA adsorbed on the fibers at equilibrium and at time t, respectively. k1 and k2 are the rate constants of the first-order and second-order adsorption obtained from the plots of ln(Qe-Qt) against t and t/Qt versus t, respectively.

10


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Qt (mg/g)

60 50


40

PP

30 20 10 0

0

1

2

3

4

5

t (h)

(a)

(b) Fig. 8 - The kinetic adsorption curves of BPA by PP, PP-g-CaSiO3 and PP-g-CaSiO3@SiO2 (a) and the schematics of diffusion process (b). pH = 7.0, C0 = 50 mg/L, adsorption temperature 298 K. 25

4.2 3.6


3.0

PP


20

PP t/Qt (h g/mg)

ln(Qe-Qt)/(mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.4 1.8 1.2

15 10 5

0.6 0

1

2

3

4

0

5

t /h

0

1

2

3

4

5

t /h

Fig. 9 - Fitting curves of pseudo-first order kinetic model (a) and pseudo-second order kinetic model (b).

According to the R2 values, the adsorption of BPA on PP-g-CaSiO3 and PP-g-CaSiO3@SiO2 fibers follows the pseudo-second-order rate law. The adsorption of BPA on PP fiber follows the pseudo-first-order model rather than the pseudo-second-order model. 3.6 Influence of temperature on the adsorption of BPA 11



The Qe of BPA on PP-g-CaSiO3@SiO2 under different concentrations and at different temperatures was listed in Fig. 10. The adsorption of BPA decreased with the increasing of temperature. Evidently, high temperature was detrimental for the adsorption of BPA. The adsorption reaction of BPA is an exothermal reaction and low temperature is good for the adsorption. The result agreed well with Gibbs’ energy change tested by ITC. The material could be applied under normal temperature conditions (298 K) with high adsorption capacity. 70 60 50 Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 30

288K 298K 308K 318K 328K

20 10 0

0

10

20

30

40 50 60 Ce (mg/L)

70

80

90 100

Fig. 10 - The Qe of BPA on PP-g-CaSiO3@SiO2 fiber under different BPA concentrations and different temperatures. pH = 7.0.

In order to quantify the adsorption affinity of different sorbents, the activation energy of adsorption was estimated by the Arrhenius equation:

lnk = -

+ lnA

(7),

where Ea is the Arrhenius activation energy indicating the minimum energy that reactant must have, A is the Arrhenius factor, R is the gas constant (8.314 J/mol•K), and k is the rate constant of adsorption, which is k2 in pseudo-second order kinetic model. T is the adsorption temperature (K). Activation energy is an important factor affecting the adsorption rate. The parameters for Arrhenius equation are shown in Table S4. The activation energy value (Ea) of PP-g-CaSiO3@SiO2 was calculated to be 3.92 kJ/mol, which was lower than PP-g-CaSiO3 (4.21 kJ/mol) and PP (5.24 kJ/mol). These results implied that the mesoporous SiO2 on the surface of PP-g-CaSiO3@SiO2 could efficiently reduce the energy barrier and improve the adsorption capacity. The rate constant (k2) increased with the increasing of temperature and the Ea was below 40, indicating that the adsorption process was fast. 12


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3.7 Influence of pH values on the adsorption of BPA The pH of solution has significant influence on the adsorption capacity of adsorbent via changing the characterizations of adsorbent and the adsorbate. As a result, the polarity, hydrophobicity and interaction of them could be affected. Therefore, batch experiments of BPA adsorption onto PP-g-CaSiO3@SiO2 were conducted at different pH values and the results are shown in Fig. 11. The Qe of BPA increased with the pH value and the maximum adsorption capacity (47.67 mg/g) of BPA was observed when the pH value was 5.0. Then the Qe decreased with the increasing of pH value (8-12). When the concentration of H+ in solution increased, the non-ionic form of BPA accounted for more proportion and the BPA had high hydrophobicity. Therefore, the Qe decreased remarkably. In an alkaline conditions, the -OH in BPA dissociated and resulted in the increasing of solubility of BPA in the solution. The enhanced electronegativity also leads to the reduced Qe.

60 50 Qe (mg/g)

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40 30 20 10 0

2

4

6 8 10 pH value of the BPA solution

12

Fig. 11 - Effect of pH value on the Qe of BPA when PP-g-CaSiO3@SiO2 was used as the adsorbent. C0 = 50 mg/L, adsorption temperature 298 K.

3.8 The influence of sodium alginate on the adsorption of BPA In the daily life, BPA in water are always found along with some background organic compounds such as tannic acid, humic acid and alginate sodium. Therefore, in order to test the practical adsorption capacity of PP-g-CaSiO3@SiO2, alginate sodium with the concentration of 0.03 wt.% was utilized as the background organic compounds in the adsorption experiments. Fig. 12 shows the kinetic adsorption curves of BPA using PP-g-CaSiO3@SiO2 and PP as the adsorbents with and without sodium alginate. There was no difference between the equilibrium adsorption capacity of BPA 13



with and without alginate sodium for both PP-g-CaSiO3@SiO2 and PP fibers. It seemed that there was no influence of background organic compounds on the adsorption of BPA. 60 50

Qt (mg/g)

40

PP-g-CaSiO3@SiO2 with NaAlg PP with NaAlg PP-g-CaSiO3@SiO2 without NaAlg

30

PP without NaAlg

20 10 0

0

2

4

6

8

10

Time (h)

Fig. 12 - The kinetic adsorption curves of BPA on PP and PP-g-CaSiO3@SiO2 with and without sodium alginate. pH = 5.0, C0 = 50 mg/L, adsorption temperature 298 K.

3.9 Reusability of PP-g-CaSiO3@SiO2 The repeated application of the PP-g-CaSiO3@SiO2 was investigated for 8 times and the results were shown in Fig. 13. PP-g-CaSiO3@SiO2 conserved a good activity after regeneration cycle. The slight decrease in Qe may be likely due to the drop of the mesoporous SiO2 on the fiber surface during washing process. The good recyclability of the functional non-woven fiber proved the feasibility of PP-g-CaSiO3@SiO2 applied in practical water treatment.

60 50 Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 30 20 10 0

0

1

2

3

4 5 6 Cycle times

7

8

9

Fig. 13 - Reusability of PP-g-CaSiO3@SiO2 for the adsorption BPA.

3.10 Comparison with other adsorbents 14


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Several materials were compared with PP-g-CaSiO3@SiO2 for the removal of BPA in the literature. According to Table 1, the equilibrium adsorption capacities (Qe) obtained in this work are medium of the reported works but the equilibrium time (Te) is shorter, with only 2.5 h to reach the adsorption equilibrium. The low-cost, availability and reusability of the PP-g-CaSiO3@SiO2 made it better than other materials in practical application for the removal of contaminants. Table 1 - Comparison of the published materials and the PP-g-CaSiO3@SiO2 Adsorbent

Qe (mg/g)

Te (h)

Reference

TiO2

11.52

9

29

Powdered activated carbons

0.60

3

30

Hydrophobic zeolite

39.5

4

31

Porous polyethylene vinyl acetate

5

6

32

Electrospun carbon nanofibers

0.25

8

33

Hyper-cross-linked polystyrene resins

93

7.5

34

Hollow carbon porous nanospheres

203

10

35

Fabric peat

31.40

6

36

Hybrid mesoporous silicas

1.86

2

37

PP-g-CaSiO3@SiO2

47.67

2.5

This work

Conclusions The results of adsorption isotherms and kinetics indicated that the large surface area and Si-OH groups on PP-g-CaSiO3@SiO2 could improve the adsorption capacities and affinity for BPA. Six-caned hydrogen bond was formed between Si–OH of PP-g-CaSiO3@SiO2 and BPA. The time to reach equilibrium was about 2.5 h, which was shorter than most commercial adsorbents, and the adsorption capacity reached 47.67 mg/g, even with the background organic compound such as alginate sodium. ITC was used to illustrate the driving force for the adsorption of BPA and the results indicated that the adsorption behavior of BPA on CaSiO3@SiO2 is good at low concentrations (ppb). PP-g-CaSiO3@SiO2 was an excellent adsorbent for BPA and could be reused. Low temperature and low pH value is good for the adsorption of BPA. The preparation of PP-g-CaSiO3@SiO2 is fast, low-cost and environment friendly without any organic solvent. The PP-g-CaSiO3@SiO2 can be reused and will impel the waste water treatment.

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Acknowledgments The research is supported by National Science Foundation of China (51103102, 51678409), the Science and Technology Plans of Tianjin (16JCZDJC37500, 15PTSYJC00240), and the project of Tianjin science and technology correspondent (16JCTPJC44800).

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Four tables presenting the adsorption results evaluated from the linear plots are listed in the Supporting Information, including Langmuir and Freundlich adsorption isotherm parameters, fitting parameters ∆H, K from ITC analysis, Kinetic parameters and BPA adsorption parameters for Arrhenius equation.

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Graphical Abstract (for review)


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Bisphenol A Adsorption Properties of Mesoporous CaSiO3@SiO2

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