A stable SiO2 material marked as CTAB-Ms(x) was synthesized by a novel sol-gel method. It was modified with hexadecyl trimethyl ammonium bromide (CTAB), which resulted in high adsorption capacity. Its microstructure and surface functional groups were characterized by scanning electron microscope, transmission electron microscope and Fourier transform infrared. The results showed that CTAB-Ms(x) had a core/shell structure in which the core was a CTAB micelle and the shell was SiO2. The prepared material was applied to adsorb bisphenol A (BPA). Pseudo-first-order kinetics equation, pseudo-second-order kinetics equation, Langmuir adsorption isotherm model, Temkin adsorption isotherm model, and thermodynamic equations were used to fit and analyze the experiment results. The theoretical maximum adsorption capacities calculated according to linear and non-linear forms of the Langmuir isotherm were 370.37 mg·g−1 and 198.80 mg·g−1, and the adsorption equilibrium time was 120 min. A mechanism study showed that the high adsorption capacity was attributed to the solubilization effect of the CTAB micelle.

## INTRODUCTION

Endocrine-disrupting chemicals (EDCs) have attracted increasing attention in the last few decades, owing to their potential impacts on aquatic environments. EDCs can cause abnormalities in the functions of endocrine systems of wildlife and humans (Rochester 2013). EDCs represent a broad class of natural and synthetic chemicals, such as bisphenol A (BPA), 17β-estradiol, and 17α-ethynyl estradiol. They have estrogenic activity and they can alter the normal endocrine functions and affect the physiological status of animals and human beings. Owing to its extensive usage in industry as an intermediate for the production of polycarbonate plastics and as a major component of epoxy (Chen et al. 2016), BPA has been widely distributed in the water environment during the manufacturing and application processes, and it has been detected in different water resources, soils, aquatic animals, food and human beings in China and abroad. BPA is stable in the environment, hardly degraded and tends to bio-accumulate, which makes it very urgent and important for us to develop a sustainable, effective and economical method to remove BPA in water (Manfo et al. 2014).

So far, various technologies have been studied to remove BPA from water systems, such as biodegradation (Balest et al. 2008), photochemical catalysis (Zhang et al. 2014), and adsorption (Alsbaiee et al. 2016). Among those methods, adsorption is a superior and promising method for removing contaminants from the water system in terms of low cost, ease of operation, and lack of harmful secondary products. Several researchers synthesized and used various kinds of adsorbents to remove BPA, such as porous β-cyclodextrin polymer (Alsbaiee et al. 2016), hydrophobic Y-type zeolite (Tsai et al. 2006), organic–inorganic hybrid mesoporous material (Ph-MS) (Kim et al. 2011), clay minerals and zeolites (Dong et al. 2010). One trait they all have in common is their adsorption capacity for BPA due to the Brunauer–Emmett–Teller (BET) surface area. So, there will be a sharp decline of the absorption capacity if the BET surface area is reduced. Mesoporous SiO2 could be conveniently achieved by sol-gel processes (Zhao et al. 2000) and could be modified with various functional groups, such as vinyl alcohol (Wu et al. 2010) and P123 (Teng et al. 2011), and proved to be effective adsorbents for Cu2+ and Hg2+, respectively. The main adsorption mechanism was electrostatic interaction. These research studies showed that there was a sharp rise of the absorption capacity after modification. SiO2 could be modified by cationic surfactants. It is an ideal adsorbent for removing BPA because of its special character, which has been rarely reported.

In this work, hexadecyl trimethyl ammonium bromide (CTAB)-modified silica absorbents with core/shell structure have been prepared by a novel sol-gel method, and marked as CTAB-Ms(x). The obtained CTAB-Ms(x) was characterized and used for removal of BPA. Meanwhile, the effects of experimental conditions on the BPA adsorption capacity of CTAB-Ms(x) were investigated. A possible adsorption mechanism is proposed for the removal of BPA over CTAB-Ms(x) on the basis of the above experimental results.

## MATERIALS AND METHODS

### Materials and reagents

All chemicals used in experiments were analytical grade, and solutions were prepared with deionized water. The reagents and materials used in this work included CTAB (99%, Aladdin), tetraethyl orthosilicate (Chengdu Kelong Chemical Reagent Co., Ltd), ethanol (Yunnan Shandian Pharmacy Co., Ltd), NaOH (96%, Tianjin Yongda Chemical Reagent Co., Ltd), HCl (36–38%, Chongqing Dongchuan Chemical Co., Ltd), BPA (99%, Sigma-Aldrich) and filter membrane (pore size 0.45 μm, Tianjin Hengao Technology Development Co., Ltd).

### Preparation of CTAB-Ms(x)

CTAB-Ms(x) could be simply obtained by a novel sol-gel method. First, 1.65 g of CTAB was dissolved with 70.5 mL of deionized water under stirring at 55 °C. Then, 20 mL of NaOH solution (1 mol·L−1) and 7.3 mL of tetraethyl orthosilicate were added to the above CTAB solution. The reaction mixture was stirred for 48 h to obtain a white gel. The mother liquor was decanted, and the products of white gel were washed alternately with ethanol and deionized water several times until the filtrate became foamless. The product was dried under vacuum at 100 °C for 12 h.

### Preparation of Ms (ds)

CTAB-Ms(x) was heated at 550 °C in a mufﬂe furnace for 8 h with a temperature increasing rate of 2 °C·min−1. Then, the white powder was obtained and marked as Ms (ds).

### Characterization

Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet iS10 FT-IR spectrometer (Thermo Scientific, Germany). The spectra were obtained using KBr pellets over the wavenumber range of 4,000–400 cm−1 with a resolution of 2 cm−1.

The crystal structures of the sample powders were characterized by a TTRIII X-ray diffractometer (XRD, Rigaku, Japan) with Cu Kα radiation in the 2θ range from 10° to 80°. The scanning rate was 10°/min and the step size was 0.02°/s; the accelerating voltage and the applied current were 40 kV and 200 mA, respectively.

Morphology of the powder materials was examined by scanning electron microscope (SEM) (FEI QUANTA 200) observation at 20 kV. The particle morphology was observed using a transmission electron microscope (TEM) (JEM-2100) operated at 200 kV.

Batch experiments were carried out in a shaker incubator (SUKUN SKY-200B). The effects of various adsorption parameters such as contact time, initial concentration of BPA solution, solution temperature and the pH of the solutions on the adsorption process were studied and optimized. In general, sorption experiments were performed by equilibrating 0.01 g of sorbent with 50 mL of the BPA solution in 50 mL conical flasks on a shaker at 200 rpm and 298 K for 6 h. The BPA solution concentration was 20 mg·L−1; the initial pH values, which were measured using a pH meter (DENVER instrument UB-7), were adjusted to 5.8 unless otherwise stated, by adding minimum amounts of HCl solution. At the end of the adsorption, all the suspensions were filtered using cellulose acetate membrane filters with pore diameter of 0.45 μm, and the filtrates were analyzed for total BPA. The concentration of the BPA solution was determined using a UV/visible spectrophotometer (UV-2401PC). All experiments were performed in duplicate and the average values were recorded. The amounts of the BPA at equilibrium qe (mg·g−1) on the adsorbent were calculated by Equation (1):
1
where C0 (mg·L−1) is the initial concentrations of aqueous solution, Ce (mg·L−1) is the equilibrium concentration, V (L) is the volume of the solution, and m (g) is the mass of the adsorbent.

Adsorption experiments were carried out by mixing 0.01 g of the CTAB-Ms(x) and 50 mL 20 mg·L−1 BPA solution at 298 K, on a shaker with rotation speed of 200 rpm. After different time intervals, as adsorption time, of 5, 10, 20, 30, 60, 120 and 360 min, the solids and liquids were separated by filter membrane, and the exact concentration of BPA remaining in the solution was measured by a UV/visible spectrophotometer (UV-2401PC).

Adsorption experiments were carried out by mixing 0.01 g of the CTAB-Ms(x) (or Ms (ds)) and BPA solution with initial concentrations of 5, 20, 30, 40 and 80 mg·L−1 at 298 K, on a shaker with rotation speed of 200 rpm. In order to reach the saturated adsorption, the adsorption was run for 6 h. When the adsorption finished, the concentrations of the BPA were analyzed.

The effect of solution temperature on the adsorption process was studied at different temperature of 288, 298, 308 and 318 K with contact time of 6 h by adjusting a temperature-controlled mechanical shaker (SUKUN SKY-200B). The dosage of CTAB-Ms(x) was 0.01 g. In this experiment, the thermodynamic parameters of the adsorption were determined.

### Effect of solution pH

In order to study the influence of pH on adsorption, the initial pH of the solutions was varied from 2 to 10. The pH was adjusted by adding 0.1 mol·L−1 HCl solutions or 0.1 mol·L−1 NaOH solutions and was measured using a pH meter (Denver instrument UB-7). Adsorption was carried out by adding 0.01 g of CTAB-Ms(x) (or Ms (ds)) into 50 mL of 20 mg·L−1 BPA solution at 298 K, on a shaker at rotation speed of 200 rpm, and the solid–liquid contact time was 6 h.

### Error analysis

The non-linear regression has been an important tool to determine the best isotherm model compared to the experimental data. Due to the inherent bias resulting from linearization, four non-linear error functions were applied to evaluate the best fit into the isotherm models of the experimental equilibrium data. The error equations employed were as follows (Shayesteh et al. 2016).

Non-linear chi-square test (χ2):
2
The average relative error (ARE):
3
A derivative of Marquardt's percent standard deviation (MPSD):
4
The average percentage errors (APE):
5

In the above equations, the subscripts ‘exp’ and ‘calc’ indicate the experimental and calculated values of adsorption capacities, respectively, and N is the number of observations in the experimental data.

## RESULTS AND DISCUSSION

#### FT-IR spectrograms analysis

In order to investigate the chemical compositions of the prepared composites, FT-IR spectroscopy measurements of commercially purchased CTAB powder, synthesized CTAB-Ms(x) and Ms (ds) were taken as shown in Figure 1(a). For the purchased CTAB powder, the absorption bands at 2,838, 2,923 and 2,993 cm−1 were due to -CH2- bands existing in the CTAB. The absorption bands at 1,080 and 450 cm−1 were due to Si-O bands existing in Ms (ds). For the synthesized CTAB-Ms(x) composites shown in Figure 1(a), we found that the -CH2- bands shifted from 2,838 cm−1 to 2,856 cm−1 and the peaks of 2,993 cm−1 disappeared because of the formation of the bonding between CTAB and part of SiO2. It was noted that the absorption bands at 1,080 and 450 cm−1 were also caused by Si-O bands existing in CTAB-Ms(x). From the above comparison, it may be concluded that CTAB was successfully introduced into SiO2 for CTAB-Ms(x), and CTAB in Ms (ds) could be completely removed after calcination at 550 °C for 8 h. Figure 1(b) shows the XRD patterns of CTAB-Ms(x) and Ms (ds); the broad peak around 25° is attributed to the amorphous silica (Ge et al. 2017).
Figure 1

FT-IR spectra of CTAB-Ms(x), Ms (ds) and CTAB (a); XRD patterns of CTAB-Ms(x), Ms (ds) (b).

Figure 1

FT-IR spectra of CTAB-Ms(x), Ms (ds) and CTAB (a); XRD patterns of CTAB-Ms(x), Ms (ds) (b).

#### Morphology

The morphologies of CTAB-Ms(x) and Ms (ds) composites were observed with SEM and TEM as shown in Figure 2. From the SEM images, it can be clearly seen that there was no significant change in the morphology of Ms (ds) powder (Figure 2(b)) compared with CTAB-Ms(x) powder (Figure 2(a)). In order to further investigate the microstructure of prepared particles, CTAB-Ms(x) and Ms (ds) were characterized by TEM as shown in Figure 2(c) and 2(d). It is clear that particles of CTAB-Ms(x) had a core–shell structure. There are many CTAB-cores distributed in the particles of CTAB-Ms(x). The core size was relatively uniform, and the diameter of the CTAB-core was in the range from 10 nm to 50 nm (Figure 2(c)). However, the core–shell structure was completely destroyed after calcination for Ms (ds) as shown in Figure 2(d). The results indicated that the core–shell structure with a silica-shell and CTAB-core had been successfully fabricated in CTAB-Ms(x).
Figure 2

SEM images of CTAB-Ms(x) (a) and Ms (ds) (b) and TEM images of CTAB-Ms(x) (c) and Ms (ds) (d).

Figure 2

SEM images of CTAB-Ms(x) (a) and Ms (ds) (b) and TEM images of CTAB-Ms(x) (c) and Ms (ds) (d).

### Adsorption of BPA by CTAB-Ms(x)

The adsorption amount of BPA along the contact time is shown in Figure 3(a). From Figure 3(a), it can be seen that the adsorption balance can be established in about 120 min for removal of BPA. The adsorbing capacity was 85.58 mg·g−1 at the equilibrium.
Figure 3

Experimental variation of adsorbed amounts of BPA on CTAB-Ms(x) versus time (a) and the linear fit of experimental data using pseudo-first-order kinetic model (b) and pseudo-second-order kinetic model (c) (T = 298 K; adsorbent dose = 0.2 g·L−1; the initial concentration = 20 mg·L−1; pH value 5.8).

Figure 3

Experimental variation of adsorbed amounts of BPA on CTAB-Ms(x) versus time (a) and the linear fit of experimental data using pseudo-first-order kinetic model (b) and pseudo-second-order kinetic model (c) (T = 298 K; adsorbent dose = 0.2 g·L−1; the initial concentration = 20 mg·L−1; pH value 5.8).

The pseudo-first-order equation and pseudo-second-order equation were used for modeling the kinetics of BPA adsorption. The pseudo-first-order equation (Singh & Tiwari 1997) is generally expressed as Equation (6):
6
where qe (mg·g−1) and qt (mg·g−1) are the amounts of BPA adsorbed at the equilibrium and at time t (min), respectively, and K1 (min−1) is the rate constant of first-order adsorption.
The pseudo-second-order adsorption kinetic rate equation (Ho & McKay 1999) is expressed as Equation (7):
7
where qe (mg·g−1) and qt (mg·g−1) are the sorption capacity at equilibrium and time t (min), respectively. K2 (g·mg−1·min−1) is the rate constant of the pseudo-second-order sorption.

Figure 3(b) and 3(c) show the result of fitting the experimental data with the linear form of the pseudo-first-order and pseudo-second-order kinetic equations, respectively. In Figure 3(c), a good linear plot of t/qt versus t was presented and the regression coefficient of it was 0.9998, while the regression coefficient was 0.97005 for the linear plot of ln(qe−qt) versus t (Figure 3(b)). The result confirmed that the pseudo-second-order kinetic model was suitable to describe the adsorption process of BPA on CTAB-Ms(x). Meanwhile, as is shown in Table 1, the difference between the calculated qe value (86.58 mg·g−1) and the experimental value (qe = 85.58 mg·g−1) was very small, further showing that the adsorption process of BPA on CTAB-Ms(x) could be fitted well with the pseudo-second-order kinetic model, which suggested that the rate-limiting step may be chemisorption (Özacar & Şengil 2003; Coleman et al. 2006). In a word, the adsorption of BPA over CTAB-Ms(x) was mainly due to electrostatic interactions between the oxygen atoms of BPA and the CTAB-core (Dong et al. 2010).

Table 1

Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
K1R2qe.cal (mg·g−1)qe.exp (mg·g−1)K2R2qe.cal (mg·g−1)qe.exp (mg·g−1)
0.0347 0.97005 74.098 85.58 0.0012 0.9998 86.58 85.58
Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
K1R2qe.cal (mg·g−1)qe.exp (mg·g−1)K2R2qe.cal (mg·g−1)qe.exp (mg·g−1)
0.0347 0.97005 74.098 85.58 0.0012 0.9998 86.58 85.58

The influence of initial concentration of BPA solution on the adsorbing capacity is shown in Figure 4(a). While the initial concentration of BPA solution increased from 5 to 80 mg·L−1, the adsorbing capacity of CTAB-Ms(x) increased from 22 to 185 mg·g−1, while the adsorbing capacity of Ms (ds) did not increase obviously. When the initial concentration of BPA was less than or equal to 5 mg·L−1, the BPA in the aqueous solution was completely adsorbed by CTAB-Ms(x). The considerable adsorbing capacity could be attributed to the solubilization effect of the CTAB-core. An adsorption isotherm is usually used to describe the adsorption behavior of BPA. Langmuir and Temkin adsorption models are two of the most common types of adsorption isotherm. The Langmuir model has been applied to many sorption processes and used to explain the monolayer adsorption of dyes over a homogeneous surface (Mall et al. 2006). The non-linear and linear forms of this model are given by Equations (8-1) and (8-2), respectively:
8-1
Figure 4

Effect of the initial concentration on adsorbed BPA (a); linear fitting of experimental data using Langmuir (b) and Temkin (c) sorption isotherms; adsorption isotherms non-linear correlation of BPA adsorption onto the CTAB-Ms(x) using Langmuir and Temkin (d) (T = 298 K; adsorbent dose = 0.2 g·L−1; pH value 5.8; time = 360 min).

Figure 4

Effect of the initial concentration on adsorbed BPA (a); linear fitting of experimental data using Langmuir (b) and Temkin (c) sorption isotherms; adsorption isotherms non-linear correlation of BPA adsorption onto the CTAB-Ms(x) using Langmuir and Temkin (d) (T = 298 K; adsorbent dose = 0.2 g·L−1; pH value 5.8; time = 360 min).

8-2
where kL (L·mg−1) is the Langmuir adsorption constant related to energy of adsorption, qm (mg·g−1) signifies maximum adsorption capacity, qe (mg·g−1) is the amount adsorbed at equilibrium concentration Ce (mg·L−1). The constants kL and qm were calculated from the slope and intercept of the plot of 1/qe versus 1/Ce. The values of the constants obtained for the Langmuir isotherm are shown in Table 2.
Table 2

Adsorption isotherm constants and values of different error analyses for isotherm models (linear and non-linear methods)

Langmuir equation
Temkin equation
LinearNon-linearLinearNon-linear
qm (mg·g −1370.37 198.80 bT (J·mol−164.0951 64.0951
KL (L·mg−10.107 0.354 KT (L·g−14.10549 4.10549
R2 0.9320 0.9481 R2 0.9697 0.9597
χ2 86.97 10.74 χ2 9.8595 9.8596
ARE −0.7916 −0.5155 ARE −0.4776 −0.4776
MPSD 112.1 13.53 MPSD 12.927 12.928
APE −15.83 −10.31 APE −9.5518 −9.5519
Langmuir equation
Temkin equation
LinearNon-linearLinearNon-linear
qm (mg·g −1370.37 198.80 bT (J·mol−164.0951 64.0951
KL (L·mg−10.107 0.354 KT (L·g−14.10549 4.10549
R2 0.9320 0.9481 R2 0.9697 0.9597
χ2 86.97 10.74 χ2 9.8595 9.8596
ARE −0.7916 −0.5155 ARE −0.4776 −0.4776
MPSD 112.1 13.53 MPSD 12.927 12.928
APE −15.83 −10.31 APE −9.5518 −9.5519
The Temkin isotherm contains a factor which takes into account the adsorbent–adsorbate interactions. Thus, the equation can be used to describe adsorption on heterogeneous surfaces. By neglecting the lowest and highest concentration values, the model assumes that heat of adsorption (function of temperature) of all molecules in a layer would decrease linearly rather than logarithmically with the increasing surface coverage (Huang et al. 2010). The non-linear and linear forms of this model are given by Equations (9-1) and (9-2) respectively:
9-1

9-2
where KT denotes the equilibrium binding constant (L·g−1), bT is the constant related to the heat of adsorption (J·mol−1), R is the universal gas constant (8.314 J·mol−1·K−1), and T is the temperature (K). The values of the constants obtained for the Temkin isotherm are also shown in Table 2.

The experimental data were fitted using Langmuir and Temkin isotherm models and the linear and non-linear relationships are shown in Figure 4(b)4(d), respectively. The values of the Langmuir and Temkin isotherm parameters calculated from adsorption equilibrium data are listed in Table 2. The high value of the linear and non-linear regression coefficient (R2) for the Langmuir and Temkin isotherms for CTAB-Ms(x), respectively, shows that these models give good fit to the adsorption isotherm. Meanwhile, the maximum adsorption capacity qm for the adsorption of BPA on CTAB-Ms(x) were 370.37 mg·g−1 (linear) and 198.80 mg g−1 (non-linear), respectively. However, the adsorption capacity qe was only 1.2–16.7 mg·g−1 for Ms (ds) (Figure 4(a)). The results indicated that the adsorption of CTAB-Ms(x) depended strongly on its CTAB-core. The adsorption capability of CTAB-Ms(x) and other adsorbents is listed in Table 3. From Table 3, the efficiency for CTAB-Ms(x) removing BPA from aqueous solution was significantly high comparing to other reported absorbents. Unlike most of the other reported adsorbents, the extremely high adsorption capability of CTAB-Ms(x) was attributed to the solubilization of the CTAB-core rather than the high BET surface area. Actually, the BET surface area of CTAB-Ms(x) was too low to be measured.

Table 3

Comparison of adsorption capacity for BPA with other reported absorbents

CTAB-Ms(x) NAb 298 – 370.37 (Linear)
198.80 (Non-linear)
Our study
Vinyl–SiO2 NAb 298 – 136.97 Zhou et al. (2013)
Modified CNTs 6.0 280.15 95 70 Kuo (2009)
SMZFA F prepared from coal fly ash 10.4 298 91.5 114.9 Dong et al. (2010)
SMZFA L prepared from coal fly ash 9.6 298 50.6 56.8 Dong et al. (2010)
Hydrophobic Y-type zeolite 7.0 298 504 111.1 Tsai et al. (2006)
Ph-MS NAb 298 750 337 Kim et al. (2011)
CTAB-Ms(x) NAb 298 – 370.37 (Linear)
198.80 (Non-linear)
Our study
Vinyl–SiO2 NAb 298 – 136.97 Zhou et al. (2013)
Modified CNTs 6.0 280.15 95 70 Kuo (2009)
SMZFA F prepared from coal fly ash 10.4 298 91.5 114.9 Dong et al. (2010)
SMZFA L prepared from coal fly ash 9.6 298 50.6 56.8 Dong et al. (2010)
Hydrophobic Y-type zeolite 7.0 298 504 111.1 Tsai et al. (2006)
Ph-MS NAb 298 750 337 Kim et al. (2011)

bThe pH value is close to natural conditions.

The effect of temperature on the adsorption of BPA was studied. It was found that the adsorption amount qe (mg·g−1) decreased while increasing the solution temperature from 298 to 318 K (Figure 5(a)). Kc is the standard thermodynamic equilibrium constant. Thermodynamic parameters, including standard Gibbs free energy change (ΔG0), standard enthalpy change (ΔH0), and standard entropy change (ΔS0) (Yurtsever & Sengil 2009), are calculated using the following Equations (10)–(13):
10

11

12

13
where R (8.314 J·mol−1·K−1) is the gas constant, T (K) is absolute temperature, Ca (mg·L−1) is the change in the concentration of BPA before and after adsorption and, Ce (mg·L−1) is the equilibrium concentration of BPA in the solution.
Figure 5

Effect of the temperature on adsorbed BPA (a) and sorption isotherm (b) (adsorbent dose = 0.2 g·L−1; initial concentration = 20 mg·L−1; pH value 5.8; time = 360 min).

Figure 5

Effect of the temperature on adsorbed BPA (a) and sorption isotherm (b) (adsorbent dose = 0.2 g·L−1; initial concentration = 20 mg·L−1; pH value 5.8; time = 360 min).

The linear plots of lnKc versus 1,000/T are shown in Figure 5(b). From the slope and intercept of this linear plot, the thermodynamic parameters are calculated and listed in Table 4. The positive value of ΔS0 (384.6 J·mol−1·K−1) suggested the increased randomness of the solution interface during the adsorption of BPA on the adsorbent. The values of ΔG0 were negative from 288 K to 318 K (−2.015, −3.575, −8.250 and −13.279 KJ·mol−1 at 288, 298, 308 and 318 K, respectively). It indicated the process of BPA adsorption by the CTAB-Ms(x) was non-spontaneous from 288 K to 318 K. The positive ΔH0 value (109.8 KJ·mol−1) indicated that the adsorption of BPA molecules onto CTAB-Ms(x) adsorbent was an endothermic process. The results further explained the fact that the adsorption quantity increased with temperature rising. Moreover, the absolute value of ΔH0 greater than 60 KJ·mol−1 demonstrated that the adsorption process of BPA over CTAB-Ms(x) possess the characteristic of chemisorption since the absolute values of the adsorption heat are Van der Waals force 4–10 KJ·mol−1, hydrophobic interaction 5 KJ·mol−1, hydrogen bond 2–40 KJ·mol−1 and chemisorption 60 KJ·mol−1 (Sun et al. 2010).

Table 4

Thermodynamic parameters for the adsorption of BPA on CTAB-Ms(x)

AbsorbentT(K)ΔG0 (KJ·mol−1)ΔH0 (KJ·mol−1)ΔS0 (J·mol−1·K−1)R2
CTAB-Ms(x) 288 −2.015 109.8 384.6 0.9301
298 −3.575
308 −8.250
318 −13.279
AbsorbentT(K)ΔG0 (KJ·mol−1)ΔH0 (KJ·mol−1)ΔS0 (J·mol−1·K−1)R2
CTAB-Ms(x) 288 −2.015 109.8 384.6 0.9301
298 −3.575
308 −8.250
318 −13.279

The obtained experimental results are directly associated with the mechanism of the process. The above experiment results showed that CTAB-Ms(x) had a much higher BPA adsorption capability than Ms (ds). In TEM measurements, the completely different morphology between particles of CTAB-Ms(x) and Ms (ds) suggested that the CTAB-core was an important part of the CTAB-Ms(x) adsorbent. Meanwhile, the adsorption kinetics study and adsorption thermodynamics showed that the adsorption of BPA on CTAB-Ms(x) was a chemisorption process. This was further confirmed by the pH effect experiments.

Figure 6

Effect of pH on adsorption of BPA by CTAB-Ms(x) and Ms (ds) (T = 298 K; adsorbent dose = 0.2 g·L−1; initial concentration = 20 mg·L−1; time = 360 min).

Figure 6

Effect of pH on adsorption of BPA by CTAB-Ms(x) and Ms (ds) (T = 298 K; adsorbent dose = 0.2 g·L−1; initial concentration = 20 mg·L−1; time = 360 min).

Figure 7

Scheme of the possible reactions in the adsorption process of BPA.

Figure 7

Scheme of the possible reactions in the adsorption process of BPA.

#### The cost of BPA removal

The cost of materials and instruments as shown in Table 5. We obtained CTAB-Ms(x) adsorbent by the novel sol-gel method and the weight of CTAB-Ms(x) was 1.6 g. The cost of CTAB-Ms(x) was about US$1.8 g−1. From Figure 3(a), it can be seen that the adsorbing capacity of CTAB-Ms(x) for BPA was 85.58 mg·g−1 at the equilibrium. Consideration of costs shows that CTAB-Ms(x) adsorbent can be used to remove BPA from water at a cost of about US$21.2 g−1 without regeneration.

Table 5

Cost of materials and instruments

CTABEthanolNaOHTetraethyl orthosilicateShaker incubator
Parameter 100 g 500 mL 500 g 500 mL 580 W
Cost US$24 US$2 US$2.6 US$33 US$0.244 (6 h) CTABEthanolNaOHTetraethyl orthosilicateShaker incubator Parameter 100 g 500 mL 500 g 500 mL 580 W Cost US$24 US$2 US$2.6 US$33 US$0.244 (6 h)

## CONCLUSIONS

In this paper, a core–shell structured silica adsorbent marked as CTAB-Ms(x) was synthesized by a novel sol-gel method and was characterized by SEM, TEM and FT-IR techniques. The adsorption process of BPA over CTAB-Ms(x) was investigated in detail. The results indicated that the CTAB-core of CTAB-Ms(x) played a dominant role in enhancing the adsorption capacity of the adsorbent for the BPA removal. The adsorption process was studied with the pseudo-second-order kinetics model, Langmuir adsorption isotherm model and thermodynamic model. The results of the mechanism study illustrated the absorption of BPA on CTAB-Ms(x) was electrostatic interaction. Also, the maximum adsorption capacities calculated according to linear and non-linear forms of the Langmuir isotherm were 370.37 mg·g−1 and 198.80 mg·g−1, respectively. CTAB-Ms(x) can be a promising absorbent to remove BPA and other organic pollutants.

## ACKNOWLEDGEMENTS

This work was jointly supported by the National Natural Science Foundation of China (No. 21163023 and No. 21261026) and the Key Program of Yunnan Province Foundation (No. 2013FA005).

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