One-step hydrothermal synthesis of CTAB-modified SiO₂ for removal of bisphenol A

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.

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).

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.

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).

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.

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.

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.

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).

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.

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/q_t 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(q_e−q_t) 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 q_e value (86.58 mg·g⁻¹) and the experimental value (q_e = 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).

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 q_m 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 q_e 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.

The linear plots of lnK_c 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).

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.

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.

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.

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).