- Email: [email protected]
Synthesis of sulfonated polystyrene sphere based magnesium silicate and its selective removal for bisphenol A
Surfaces and Interfaces 14 (2019) 9–14
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Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Synthesis of sulfonated polystyrene sphere based magnesium silicate and its selective removal for bisphenol A
T
⁎
Wang Yonghaoa, Chi Biaoa, Li Mingyanga, Wei Wenqia, Wang Yongjinga, , Chen Daguib a b
College of Environment and Resources, Fuzhou University, Fuzhou 350108, China, College of Electronics and Information Science, Fujian Jiangxia University, Fuzhou 350108, China,
A R T I C LE I N FO
A B S T R A C T
Keywords: BPA removal Magnesium silicate Nanocomposites
In this work, a sulfonated polystyrene sphere (SPS) based magnesium silicate nanocomposite (SPS@MgSix) with superior adsorption capability has been prepared for the removal of BPA, a kind of emerging pollution existed in various wastewaters. It was obtained by in situ growth of the Talc-phase Mg3Si4O9(OH)10 (MgSi) on the surface of SPS, in which the SPS substrate can effectively avoid the agglomeration of MgSi. Scanning electron microscope (SEM) has shown that interwoven nano-sized MgSi uniformly distributes on the surface of SPS, leading to a high adsorption capacity (Q0: 357.14 mg L−1) for removing bisphenol A (BPA). It is worth noting that these composite materials also exhibit relative good adsorption capability even under high-concentration salt conditions, for example, the adsorption capacity still keeps at about 331.14 mg g−1 in the presence of high concentration Na+ or Ca2+. The Zeta potential tests indicate that the high adsorption capacity may be related to an electrostatic interaction between SPS@MgSix and BPA. The excellent adsorption behaviors of SPS@MgSix show that it is a highly promising candidate for removing BPA in aqueous solution.
1. Introduction Bisphenol A (BPA), a kind of emerging pollution, exists in industrial wastewater, municipal wastewater and sludge, urban landfill leachate, surface river water, sediment, and soil [1,2]. Since BPA is a typical endocrine disruptor with the character of neurotoxicity, reproductive toxicity, teratogenicity and carcinogenicity, many methods have been adopted to remove BPA from wastewater, including photocatalysis [3–5], molecular imprinting [6], bio-degradation [7–9], membrane treatment [10,11] and adsorption [12,13]. Among these methods, adsorption method is generally attractive because of its economic efficiency and facile operation. That being the case, an adsorbent with efficient adsorption capability is significant for adsorption methods [4,14–18]. Nevertheless, the environmental friendliness of adsorbents is also a factor to be considered with the increasing attention paid to the environmental safety of new materials. Silicates, a kind of traditional material, have been used widely to remove pollutions from the waste water [19–22] or extract valuable substances [23–25] because of its environmental friendliness, stable chemical properties and high thermal stability as well as lamellar structure. Recently, a review indicates that the research on silicates used to remove BPA is mainly focused on modified clays and zeolites [19,26–28], with an overall adsorption capacity is low and the highest ⁎
is only 200 mg g−1. Generally speaking, the high adsorption capacity is associated with particle size, surface charge, surface group and bonding energy. Increasing specific surface area by decreasing the particle size of adsorbent is one of the common methods. However, particles with small size can block the filter column during actual application process, and then make it difficult to achieve solid-liquid separation. Therefore, high adsorption capacity and above shortcomings must be considered simultaneously when designing adsorbent. Some studies indicated that composite materials with micro-nanostructure can effectively overcome above shortcomings [29–31]. The most common approach is to load nanomaterials onto micro-sized substrates, such as activated carbon granule, silicon dioxide, cellulose and polymers [32–35]. Among a large of substrate materials, polymers have received much more attention because of superior surface chemical modifiability and mechanical property. It is worth mentioning that polymer based nanocomposites would present excellent mechanical properties resulted from polymers substrate and high reactive activity derived from loaded nanomaterials. Polystyrene microsphere (PS) is a common commodity polymer that offers high mechanical stability, low cost and facile synthesis, the controllable particle size and easily functional surface. Moreover, functional polystyrene ion-exchange resins are often used to remove organic pollutants from water [36,37]. However, polymerized polystyrene tends to have a low specific surface area and a small amount of
Corresponding authors. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.surfin.2018.10.011 Received 15 September 2018; Received in revised form 21 October 2018; Accepted 31 October 2018 Available online 02 November 2018 2468-0230/ © 2018 Elsevier B.V. All rights reserved.
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ultrasound action to form solution C. 50.8 mg MgCl2•6H2O and 530.5 mg NH4Cl were dissolved into 30 ml deionized water and 1 ml NH3•H2O under ultrasound action for 15 min, forming solution D. Then solution C and solution D were mixed and transferred into an autoclave for hydrothermal synthesis. The hydrothermal reaction takes 8 h at 140 °C. Centrifugation, rinsing and drying procedure were identical to the above ones. The resultant product was labeled as SPS@MgSix, X represents the mass ratio of MgSi to SPS.
interspace, which results in a low removal efficiency of contaminants. A study [38] indicated that sulfonated polystyrene spheres based porous silicon dioxide assume superior catalytic activity of esterification and acylation because of its large specific surface area and abundant hollows. Inspired by the above study, we synthesized sulfonated polystyrene spheres (SPS) as substrates to load silicon dioxide by in-situ hydrolysis, followed by a hydrothermal treatment to transfer silicon dioxide to magnesium silicate. Finally, magnesium silicate (MgSi) was uniformly distributed on the surface of SPS, forming sulfonated polystyrene sphere based magnesium silicate composite materials, marked as SPS@ MgSix. BPA was used as a typical pollution to evaluate the adsorption capacity of SPS@MgSix. The common inorganic ions, such as Na+, Ca2+, Cl− and CO32−, were added into simulated wastewater including BPA to investigate the adsorption capacity under high-concentration salt conditions. As a result, SPS@MgSix displays a high adsorption capacity, as high as 357.14 mg L−1, and superior selectivity in presence of inorganic ions. The exceptionally improved adsorption capacity benefits from the uniform distribution of MgSi on the SPS and surface electrostatic adsorption.
2.3. Adsorption experiments 2.3.1. Screening of the ratio of MgSi to SPS A certain amount of SPS@MgSix (dosage: 125 mg L−1) was added into 50 ml conical flask containing 20 ml BPA solution (50 mg L−1). After being stirred for 3 h, the BPA concentration in the supernatant solution was tested, aiming to confirm the optimum ratio of MgSi to SPS.
2.1. Materials
2.3.2. Adsorption kinetics A certain amount of SPS, MgSi and SPS@MgSix (dosage: 125 mg L−1) were respectively added into 50 mg L−1 BPA solution, followed by stirring for 3 h to achieve adsorption equilibrium. 3 ml solution was taken at regular intervals to test the concentration of BPA in the supernatant solution, and the removal efficiency was calculated.
Styrene (CP), ammonium hydroxide (25%, AR) and polyvinylpyrrolidone (PVP, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tetraethylorthosilicate (TEOS, AR), 2,2′-Azobis (2methylpropionitrile) (AIBN, AR), Magnesium chloride (MgCl2•6H2O, 98%) and ammonium chloride (NH4Cl, 99.5%) were purchased from Tianjin Fuchen Chemical Reagent Plant.
2.3.3. Adsorption isotherm Adsorption isotherms experiments were performed by mixing different BPA concentration (range: 25–50 mg L−1) with a constant dosage of adsorbent (125 mg L−1) for 3 h. The separation process was similar with that of kinetic experiments. The adsorption capacity under adsorption equilibrium was calculated.
2.2. Synthesis of SPS@MgSix
2.3.4. Effect of pH on the BPA removal This experiments were performed by mixing 3 mg SPS@MgSix and 25 ml BPA solution under different pH values adjusted by 0.1 mol L−1 NaOH and 0.1 mol L−1 HCl. The relative experiment steps are similar with the adsorption kinetic.
2. Experiments
2.2.1. Synthesis of SPS Firstly, PS was prepared by dispersion copolymerization method and the typical procedures is listed as following: 40 mg AIBN was added into 5 ml styrene to form solution A. Then 50 ml ethanol, 10 ml distilled water and 0.7 g PVP were added into 250 ml three-neck round-bottom flask and stirred vigorously to dissolve PVP completely. Then, solution A was rapidly poured into the above solution under magnetic stirring and N2 inert atmosphere at room temperature, forming solution B. Solution B was heated to 40°C and dwelled for 5 min, then raised to 70°C for 24 h for further reaction. The white products (PS) were obtained by centrifugation and rinsed with ethanol and distilled water for three times, followed by drying at 40°C for 1 h. For typical sulfonating process, 1 g PS was added into 20 g concentrated sulfuric acid and sonicated for 30 min to obtain uniform solution, then the solution was stirred for 12 h at 40°C, followed by slow adding of 50 ml distilled water. Finally, the solution was cooled to room temperature and SPS was collected by centrifugation. The ethanol-distilled water solution (volume ration 1:1) was adopted to remove the residual concentrated sulfuric acid. The as-prepared samples were dried at 40°C for 2 h.
2.3.5. Selective adsorption experiments A constant dosage of adsorbent (125 mg L−1) was mixed with 50 mg L−1 BPA solution including different interfering ions, such as Na+ (26 mg L−1 or 130 mg L−1), Ca2+ (29 mg L−1 or 130 mg L−1), SO42− (54 mg L−1 or 270 mg L−1), Cl− (86 mg L−1 or 441 mg L−1) and CO32− (37 mg L−1 or 191.68 mg L−1). BPA solution for each selective experiment contains only BPA and one kind of interfering ion with a constant concentration. The separation process was similar with that of kinetic experiments. 2.3.6. Recycling experiments 0.1 M NaOH was used as the elution to regenerate the adsorbent. In a typical case, the used adsorbent was centrifuged and washed with ethanol for three times. The dried product was immersed in 4 ml NaOH solution for 1 h, and then was rinsed by deionized water to neutral pH. The regenerated nanocomposites were used for adsorption experiments in the subsequent cycles.
2.2.2. Synthesis of SPS@SiO2 60 mg SPS was dispersed into 120 ml ethanol and 12 ml distilled water and sonicated for 2 h, followed by adding of 2 ml NH3•H2O. After sonication for 1 h, a certain amount of TEOS was added and the ultrasound continued for 3 h. The centrifugation was used to collect precipitation and the samples were rinsed by ethanol for three times. The samples were obtained by drying at 40°C for 2 h, labeled as SPS@ SiO2. During experiments, the dosage of TEOS was adjusted to obtain SPS@SiO2 composite materials with different SiO2 loading amounts.
2.4. Characterization A Rigaku MiniFlex600 diffractometer with Cu Kα radiation (40 kV, 15 mA) in the continuous scanning mode was used to collect Powder Xray diffraction (PXRD) data. The morphology of the samples was investigated by a JSM-6700F scanning electron microscopy (SEM). The concentration of BPA was determined by UV–Vis spectrophotometer (UV-1700, SHIMADZU). Malvern Zetasizer Nano ZS90 was adopted to test the sample's surface charges. Fourier infrared spectrometer (Nicolet iS10, Thermo Fisher Scientific) was used to obtain the information on
2.2.3. Synthesis of SPS@MgSix 50 mg SPS@SiO2 was dispersed into 20 ml deionized water under 10
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Fig. 1. Schematic illustration of the formation of SPS@MgSix nanocomposites.
the functional group of samples. 3. Results and discussion SPS@MgSix nanocomposite materials were obtained by liquid phase reaction combined with hydrothermal treatment, indicated in Fig. 1. SPS@SiO2 was firstly prepared by in-situ hydrolysis of TEOS. Subsequently, SPS@MgSix nanocomposite materials were obtained by hydrothermal method in the presence of SPS@SiO2, Mg2+ and NH3•H2O. Sulfonation of PS enables the combination of PS and SiO2 convenient. Fig. 2 shows the infrared adsorption spectra (IR) and XRD patterns of PS and SPS. As shown in Fig. 2a, the peak located in 500–1018 cm−1 and 3000–3100 cm−1 can be assigned to CeH adsorption peak of benzene ring, and the peak at 1400–1700 cm−1 is the C ] C stretch absorption peak of benzene ring along with the peak at 2863 cm−1 and 2923 cm−1 belong to CeH stretch absorption peak of methylene. However, a new peak at 1175 cm−1 can be assigned to S = O (-SO3H) for SPS samples, demonstrating sulfonic acid group have been decorated onto the surface of PS samples. From Fig. 2b, the peak at 19.8°, 28.2°, 34.6° and 60.9° can be indexed to (020), (−115), (200) and (−332) diffraction peaks of the layered silicate material of Talc phase, Mg3Si4O9(OH)4 (JCPDS, 03–0174), respectively. For SPS samples, the strong peak appears at 20.5°, belonging to the characteristic diffraction peak of SPS, which almost overlaps with the peak of MgSi at 19.8°. Therefore, the diffraction peak of MgSi@SPSx samples is similar with that of SPS, but the characteristic diffraction peaks of MgSi appear with increasing load amount of MgSi. Fig. 3 indicates the SEM images of SPS, [email protected], [email protected], [email protected]. It reveals that SPS is the monodisperse microsphere with a smooth surface and a diameter at about 600 nm. When being loaded MgSi, the surface becomes rough, while the morphology of microsphere still maintains unchanged, independent on the loading amount. As shown in Fig. 3b, 3c and 3d, among three samples, only [email protected] samples appear relatively uniform MgSi distribution on the surface of
Fig. 3. SEM images of (a) SPS, (b) SPS@MgSi [email protected].
0.3,
(c) SPS@MgSi
0.5
and (d)
the microsphere, and MgSi scattered on the SPS surface for SPS@ MgSi0.3 as well as MgSi agglomerates inducing to the reunion of microsphere for [email protected] samples. Based on the results originated from UV–Vis adsorption spectrum, [email protected] exhibited superior adsorption capacity for BPA. In this case, [email protected] was adopted to conduct further experiments. Fig. 4a shows the adsorption kinetic behavior of BPA untaken by [email protected] at a pH of 6.0 and room temperature, with MgSi and SPS for comparison. Obviously, the adsorption rate of BPA onto [email protected] is faster than those of MgSi and SPS. Among three samples, [email protected] achieves adsorption equilibrium within 40 min with a removal efficiency 98%, while MgSi and SPS reach adsorption equilibrium after 80 min and the removal efficiency is only 70% and 46%, respectively. The pseudo-first-order and pseudo-second-order models were adopted to simulate the kinetic data and the fitted parameters are listed in Table 1. Obviously, the pseudo-second-order well fits the adsorption kinetics, indicating the rate-determining step of the adsorption of BPA on [email protected] could be a chemical adsorption process. Fig. 5 depicts isothermal adsorption fitting curve of BPA on SPS@ MgSi0.5. The related fitted parameters of Langmuir and Freundlich
Fig. 2. (a) FTIR adsorption spectra of PS, SPS and [email protected], (b) XRD patterns of MgSi, SPS and SPS@MgSix. 11
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Table 3 Comparison of adsorption capacity for various adsorbents. Adsorbent
Maximum adsorption capacity (mg g−1)
Ref.
[email protected]
357.14
β-Cyclodextrin-carboxymethyl cellulose based hydrogel Palygorskite Oxide graphene Active carbon Organic-montmorillonite Carbon nano tube Metal-organic frameworks Zeolite
167
This work [39]
219.3 80 307 256.41 16.05 252.5 98
[40] [41] [42] [43] [44] [45] [46]
Fig. 4. Removal efficiencies of BPA on [email protected], MgSi and SPS as a function of contact time. Table 1 Pseudo-first-order and pseudo-second-order adsorption kinetic constants for BPA adsorption. Pseudo-first-order model Adsorbent [email protected]
−1
K1 (min 0.0278
)
2
R 0.8489
Pseudo-second-order model K2 (g mg min−1) 0.0022
R2 0.9976
Fig. 6. Effect of solution pH on equilibrium adsorption capacities of SPS@ MgSi0.5.
Fig. 5. The Langmuir adsorption and Freundlich adsorption isotherm of BPA on the [email protected] adsorbent. Table 2 Langmuir and Freundlich parameters of BPA adsorption on [email protected]. Adsorption model
Constant
Value
Langmuir
Q0 b R2 logK n R2
357.14 mg g−1 0.011 L mg−1 0.9978 1.0371 0.7830 0.9232
Freundlich
Fig. 7. The variation of adsorption capacities of [email protected] for BPA under various inorganic ions.
In addition, pH value of solution is a key factor to affect the surface charges of adsorbent. As shown in Fig. 6, the isoelectric point of SPS@ MgSi0.5 is pH 2, thus when a pH value is higher than 2, the surface of [email protected] carries negative charges and the charge amount increases with pH value. In the other hand, BPA would combine H+ in the solution to form ion state with positive charges, causing the electrostatic adsorption between BPA and [email protected] when the pH is between 2 and 6. However, when the pH is greater than 6, BPA becomes electronegative because of dissociation of two hydroxyl groups from its structure, forming electrostatic repulsion between BPA and SPS@ MgSi0.5, which decrease the adsorption capacity of [email protected]. In the
models are listed in Table 2. As shown in Fig. 5 and Table 2, the correlation coefficient values imply that the Langmuir model better fits the experimental data. The maximum adsorption capacity (Q0) of SPS@ MgSi0.5 for BPA fitted by the Langmuir model is 357.14 mg g−1, which is higher than those of most of adsorbents for BPA in the wastewater, summarized in Table 3. 12
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Science Foundation of Fujian Province(2016J01223, 2015J01654) and Key Laboratory of Eco-materials Advanced Technology (Fuzhou University). References [1] Q. Sun, et al., Fate and mass balance of bisphenol analogues in wastewater treatment plants in Xiamen City, China, Environ. Pollut. 225 (2017) 542–549. [2] N. Morin, H.P. Arp, S.E. Hale, Bisphenol A in solid waste materials, leachate water, and air particles from Norwegian waste-handling facilities: presence and partitioning behavior, Environ. Sci. Technol. 49 (2015) 7675–7683. [3] X. Tan, et al., Three-dimensional MnO2 porous hollow microspheres for enhanced activity as ozonation catalysts in degradation of bisphenol A, J. Hazard. Mater. 321 (2017) 162–172. [4] C.Y. Wang, et al., Novel Bi12O15Cl6 photocatalyst for the degradation of bisphenol A under visible-light irradiation, ACS Appl. Mater. Interfaces 8 (2016) 5320–5326. [5] J. 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Fig. 8. The recycle stability of [email protected] for BPA.
literature [47], BPA adsorption on the reduced GO presents a similar pH dependency. To evaluate the practicability of [email protected], we investigate its selective adsorption behavior in the presence of inorganic ions. As indicated in Fig. 7, K+ and CO32− have negative effect on the BPA uptaken on the [email protected] regardless of a high or low concentration. The high-concentration K+ or CO32− induces nearly 50% reduction of adsorption capacity of [email protected]. The above phenomenon can be enhanced by adding more [email protected] because of its low price. Under both low and high concentration, Cl− and SO42− almost have no effect on the adsorption behavior of BPA on the [email protected], but the influence from Ca2+ and Na+ depends on the concentration. For example, under a low concentration the influence from Na+, enabling the adsorption capacity reduce to 331.14 mg g−1, is stronger than that from Ca2+, but this comparison is reversed under high concentration. Noteworthy, the adsorption capacity of [email protected] for BPA still be maintained at 200 mg L−1 even if high-concentration K+ or CO32− is present in waste water, indicating that [email protected] has superior ability of anti-interference from common inorganic ions. The stability is important for evaluating the feasibility of this adsorbent. The desorption and re-adsorption performance of [email protected] were conducted in four sequential cycles as shown in Fig. 8. It displays that the adsorption capacity decreases slightly with increasing cycle numbers. But it still keeps a decent capacity for BPA uptaken after four cycles, and the adsorption capacity still reach 290 mg g−1, only 67 mg g−1 lower than the maximum adsorption capacity. This value is still higher than those of some other adsorbents [39–41,43–46]. After four cycle's treatments, the final BPA solution can be concentrated several times and the concentrated BPA solution can be incinerated to achieve the goal of harmlessness. 4. Conclusion In brief, [email protected] nanocomposites have been prepared for the removal of BPA under mildly acidic conditions for the first time. The interwoven MgSi dispersedly distributes on the surface of SPS microsphere, which enables [email protected] nanocomposites show unprecedentedly high adsorption performance for BPA with a adsorption capacity of 357.14 mg L−1. In addition, the nanocomposites exhibits superior stability, selectivity and reusability. We anticipate that it may be a promising candidate for the removal of the BPA from acidic brine wastewater upon further optimization. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21577018, 21477128, 61376002), Natural 13
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14
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Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Synthesis of sulfonated polystyrene sphere based magnesium silicate and its selective removal for bisphenol A
T
⁎
Wang Yonghaoa, Chi Biaoa, Li Mingyanga, Wei Wenqia, Wang Yongjinga, , Chen Daguib a b
College of Environment and Resources, Fuzhou University, Fuzhou 350108, China, College of Electronics and Information Science, Fujian Jiangxia University, Fuzhou 350108, China,
A R T I C LE I N FO
A B S T R A C T
Keywords: BPA removal Magnesium silicate Nanocomposites
In this work, a sulfonated polystyrene sphere (SPS) based magnesium silicate nanocomposite (SPS@MgSix) with superior adsorption capability has been prepared for the removal of BPA, a kind of emerging pollution existed in various wastewaters. It was obtained by in situ growth of the Talc-phase Mg3Si4O9(OH)10 (MgSi) on the surface of SPS, in which the SPS substrate can effectively avoid the agglomeration of MgSi. Scanning electron microscope (SEM) has shown that interwoven nano-sized MgSi uniformly distributes on the surface of SPS, leading to a high adsorption capacity (Q0: 357.14 mg L−1) for removing bisphenol A (BPA). It is worth noting that these composite materials also exhibit relative good adsorption capability even under high-concentration salt conditions, for example, the adsorption capacity still keeps at about 331.14 mg g−1 in the presence of high concentration Na+ or Ca2+. The Zeta potential tests indicate that the high adsorption capacity may be related to an electrostatic interaction between SPS@MgSix and BPA. The excellent adsorption behaviors of SPS@MgSix show that it is a highly promising candidate for removing BPA in aqueous solution.
1. Introduction Bisphenol A (BPA), a kind of emerging pollution, exists in industrial wastewater, municipal wastewater and sludge, urban landfill leachate, surface river water, sediment, and soil [1,2]. Since BPA is a typical endocrine disruptor with the character of neurotoxicity, reproductive toxicity, teratogenicity and carcinogenicity, many methods have been adopted to remove BPA from wastewater, including photocatalysis [3–5], molecular imprinting [6], bio-degradation [7–9], membrane treatment [10,11] and adsorption [12,13]. Among these methods, adsorption method is generally attractive because of its economic efficiency and facile operation. That being the case, an adsorbent with efficient adsorption capability is significant for adsorption methods [4,14–18]. Nevertheless, the environmental friendliness of adsorbents is also a factor to be considered with the increasing attention paid to the environmental safety of new materials. Silicates, a kind of traditional material, have been used widely to remove pollutions from the waste water [19–22] or extract valuable substances [23–25] because of its environmental friendliness, stable chemical properties and high thermal stability as well as lamellar structure. Recently, a review indicates that the research on silicates used to remove BPA is mainly focused on modified clays and zeolites [19,26–28], with an overall adsorption capacity is low and the highest ⁎
is only 200 mg g−1. Generally speaking, the high adsorption capacity is associated with particle size, surface charge, surface group and bonding energy. Increasing specific surface area by decreasing the particle size of adsorbent is one of the common methods. However, particles with small size can block the filter column during actual application process, and then make it difficult to achieve solid-liquid separation. Therefore, high adsorption capacity and above shortcomings must be considered simultaneously when designing adsorbent. Some studies indicated that composite materials with micro-nanostructure can effectively overcome above shortcomings [29–31]. The most common approach is to load nanomaterials onto micro-sized substrates, such as activated carbon granule, silicon dioxide, cellulose and polymers [32–35]. Among a large of substrate materials, polymers have received much more attention because of superior surface chemical modifiability and mechanical property. It is worth mentioning that polymer based nanocomposites would present excellent mechanical properties resulted from polymers substrate and high reactive activity derived from loaded nanomaterials. Polystyrene microsphere (PS) is a common commodity polymer that offers high mechanical stability, low cost and facile synthesis, the controllable particle size and easily functional surface. Moreover, functional polystyrene ion-exchange resins are often used to remove organic pollutants from water [36,37]. However, polymerized polystyrene tends to have a low specific surface area and a small amount of
Corresponding authors. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.surfin.2018.10.011 Received 15 September 2018; Received in revised form 21 October 2018; Accepted 31 October 2018 Available online 02 November 2018 2468-0230/ © 2018 Elsevier B.V. All rights reserved.
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ultrasound action to form solution C. 50.8 mg MgCl2•6H2O and 530.5 mg NH4Cl were dissolved into 30 ml deionized water and 1 ml NH3•H2O under ultrasound action for 15 min, forming solution D. Then solution C and solution D were mixed and transferred into an autoclave for hydrothermal synthesis. The hydrothermal reaction takes 8 h at 140 °C. Centrifugation, rinsing and drying procedure were identical to the above ones. The resultant product was labeled as SPS@MgSix, X represents the mass ratio of MgSi to SPS.
interspace, which results in a low removal efficiency of contaminants. A study [38] indicated that sulfonated polystyrene spheres based porous silicon dioxide assume superior catalytic activity of esterification and acylation because of its large specific surface area and abundant hollows. Inspired by the above study, we synthesized sulfonated polystyrene spheres (SPS) as substrates to load silicon dioxide by in-situ hydrolysis, followed by a hydrothermal treatment to transfer silicon dioxide to magnesium silicate. Finally, magnesium silicate (MgSi) was uniformly distributed on the surface of SPS, forming sulfonated polystyrene sphere based magnesium silicate composite materials, marked as SPS@ MgSix. BPA was used as a typical pollution to evaluate the adsorption capacity of SPS@MgSix. The common inorganic ions, such as Na+, Ca2+, Cl− and CO32−, were added into simulated wastewater including BPA to investigate the adsorption capacity under high-concentration salt conditions. As a result, SPS@MgSix displays a high adsorption capacity, as high as 357.14 mg L−1, and superior selectivity in presence of inorganic ions. The exceptionally improved adsorption capacity benefits from the uniform distribution of MgSi on the SPS and surface electrostatic adsorption.
2.3. Adsorption experiments 2.3.1. Screening of the ratio of MgSi to SPS A certain amount of SPS@MgSix (dosage: 125 mg L−1) was added into 50 ml conical flask containing 20 ml BPA solution (50 mg L−1). After being stirred for 3 h, the BPA concentration in the supernatant solution was tested, aiming to confirm the optimum ratio of MgSi to SPS.
2.1. Materials
2.3.2. Adsorption kinetics A certain amount of SPS, MgSi and SPS@MgSix (dosage: 125 mg L−1) were respectively added into 50 mg L−1 BPA solution, followed by stirring for 3 h to achieve adsorption equilibrium. 3 ml solution was taken at regular intervals to test the concentration of BPA in the supernatant solution, and the removal efficiency was calculated.
Styrene (CP), ammonium hydroxide (25%, AR) and polyvinylpyrrolidone (PVP, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tetraethylorthosilicate (TEOS, AR), 2,2′-Azobis (2methylpropionitrile) (AIBN, AR), Magnesium chloride (MgCl2•6H2O, 98%) and ammonium chloride (NH4Cl, 99.5%) were purchased from Tianjin Fuchen Chemical Reagent Plant.
2.3.3. Adsorption isotherm Adsorption isotherms experiments were performed by mixing different BPA concentration (range: 25–50 mg L−1) with a constant dosage of adsorbent (125 mg L−1) for 3 h. The separation process was similar with that of kinetic experiments. The adsorption capacity under adsorption equilibrium was calculated.
2.2. Synthesis of SPS@MgSix
2.3.4. Effect of pH on the BPA removal This experiments were performed by mixing 3 mg SPS@MgSix and 25 ml BPA solution under different pH values adjusted by 0.1 mol L−1 NaOH and 0.1 mol L−1 HCl. The relative experiment steps are similar with the adsorption kinetic.
2. Experiments
2.2.1. Synthesis of SPS Firstly, PS was prepared by dispersion copolymerization method and the typical procedures is listed as following: 40 mg AIBN was added into 5 ml styrene to form solution A. Then 50 ml ethanol, 10 ml distilled water and 0.7 g PVP were added into 250 ml three-neck round-bottom flask and stirred vigorously to dissolve PVP completely. Then, solution A was rapidly poured into the above solution under magnetic stirring and N2 inert atmosphere at room temperature, forming solution B. Solution B was heated to 40°C and dwelled for 5 min, then raised to 70°C for 24 h for further reaction. The white products (PS) were obtained by centrifugation and rinsed with ethanol and distilled water for three times, followed by drying at 40°C for 1 h. For typical sulfonating process, 1 g PS was added into 20 g concentrated sulfuric acid and sonicated for 30 min to obtain uniform solution, then the solution was stirred for 12 h at 40°C, followed by slow adding of 50 ml distilled water. Finally, the solution was cooled to room temperature and SPS was collected by centrifugation. The ethanol-distilled water solution (volume ration 1:1) was adopted to remove the residual concentrated sulfuric acid. The as-prepared samples were dried at 40°C for 2 h.
2.3.5. Selective adsorption experiments A constant dosage of adsorbent (125 mg L−1) was mixed with 50 mg L−1 BPA solution including different interfering ions, such as Na+ (26 mg L−1 or 130 mg L−1), Ca2+ (29 mg L−1 or 130 mg L−1), SO42− (54 mg L−1 or 270 mg L−1), Cl− (86 mg L−1 or 441 mg L−1) and CO32− (37 mg L−1 or 191.68 mg L−1). BPA solution for each selective experiment contains only BPA and one kind of interfering ion with a constant concentration. The separation process was similar with that of kinetic experiments. 2.3.6. Recycling experiments 0.1 M NaOH was used as the elution to regenerate the adsorbent. In a typical case, the used adsorbent was centrifuged and washed with ethanol for three times. The dried product was immersed in 4 ml NaOH solution for 1 h, and then was rinsed by deionized water to neutral pH. The regenerated nanocomposites were used for adsorption experiments in the subsequent cycles.
2.2.2. Synthesis of SPS@SiO2 60 mg SPS was dispersed into 120 ml ethanol and 12 ml distilled water and sonicated for 2 h, followed by adding of 2 ml NH3•H2O. After sonication for 1 h, a certain amount of TEOS was added and the ultrasound continued for 3 h. The centrifugation was used to collect precipitation and the samples were rinsed by ethanol for three times. The samples were obtained by drying at 40°C for 2 h, labeled as SPS@ SiO2. During experiments, the dosage of TEOS was adjusted to obtain SPS@SiO2 composite materials with different SiO2 loading amounts.
2.4. Characterization A Rigaku MiniFlex600 diffractometer with Cu Kα radiation (40 kV, 15 mA) in the continuous scanning mode was used to collect Powder Xray diffraction (PXRD) data. The morphology of the samples was investigated by a JSM-6700F scanning electron microscopy (SEM). The concentration of BPA was determined by UV–Vis spectrophotometer (UV-1700, SHIMADZU). Malvern Zetasizer Nano ZS90 was adopted to test the sample's surface charges. Fourier infrared spectrometer (Nicolet iS10, Thermo Fisher Scientific) was used to obtain the information on
2.2.3. Synthesis of SPS@MgSix 50 mg SPS@SiO2 was dispersed into 20 ml deionized water under 10
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Fig. 1. Schematic illustration of the formation of SPS@MgSix nanocomposites.
the functional group of samples. 3. Results and discussion SPS@MgSix nanocomposite materials were obtained by liquid phase reaction combined with hydrothermal treatment, indicated in Fig. 1. SPS@SiO2 was firstly prepared by in-situ hydrolysis of TEOS. Subsequently, SPS@MgSix nanocomposite materials were obtained by hydrothermal method in the presence of SPS@SiO2, Mg2+ and NH3•H2O. Sulfonation of PS enables the combination of PS and SiO2 convenient. Fig. 2 shows the infrared adsorption spectra (IR) and XRD patterns of PS and SPS. As shown in Fig. 2a, the peak located in 500–1018 cm−1 and 3000–3100 cm−1 can be assigned to CeH adsorption peak of benzene ring, and the peak at 1400–1700 cm−1 is the C ] C stretch absorption peak of benzene ring along with the peak at 2863 cm−1 and 2923 cm−1 belong to CeH stretch absorption peak of methylene. However, a new peak at 1175 cm−1 can be assigned to S = O (-SO3H) for SPS samples, demonstrating sulfonic acid group have been decorated onto the surface of PS samples. From Fig. 2b, the peak at 19.8°, 28.2°, 34.6° and 60.9° can be indexed to (020), (−115), (200) and (−332) diffraction peaks of the layered silicate material of Talc phase, Mg3Si4O9(OH)4 (JCPDS, 03–0174), respectively. For SPS samples, the strong peak appears at 20.5°, belonging to the characteristic diffraction peak of SPS, which almost overlaps with the peak of MgSi at 19.8°. Therefore, the diffraction peak of MgSi@SPSx samples is similar with that of SPS, but the characteristic diffraction peaks of MgSi appear with increasing load amount of MgSi. Fig. 3 indicates the SEM images of SPS, [email protected], [email protected], [email protected]. It reveals that SPS is the monodisperse microsphere with a smooth surface and a diameter at about 600 nm. When being loaded MgSi, the surface becomes rough, while the morphology of microsphere still maintains unchanged, independent on the loading amount. As shown in Fig. 3b, 3c and 3d, among three samples, only [email protected] samples appear relatively uniform MgSi distribution on the surface of
Fig. 3. SEM images of (a) SPS, (b) SPS@MgSi [email protected].
0.3,
(c) SPS@MgSi
0.5
and (d)
the microsphere, and MgSi scattered on the SPS surface for SPS@ MgSi0.3 as well as MgSi agglomerates inducing to the reunion of microsphere for [email protected] samples. Based on the results originated from UV–Vis adsorption spectrum, [email protected] exhibited superior adsorption capacity for BPA. In this case, [email protected] was adopted to conduct further experiments. Fig. 4a shows the adsorption kinetic behavior of BPA untaken by [email protected] at a pH of 6.0 and room temperature, with MgSi and SPS for comparison. Obviously, the adsorption rate of BPA onto [email protected] is faster than those of MgSi and SPS. Among three samples, [email protected] achieves adsorption equilibrium within 40 min with a removal efficiency 98%, while MgSi and SPS reach adsorption equilibrium after 80 min and the removal efficiency is only 70% and 46%, respectively. The pseudo-first-order and pseudo-second-order models were adopted to simulate the kinetic data and the fitted parameters are listed in Table 1. Obviously, the pseudo-second-order well fits the adsorption kinetics, indicating the rate-determining step of the adsorption of BPA on [email protected] could be a chemical adsorption process. Fig. 5 depicts isothermal adsorption fitting curve of BPA on SPS@ MgSi0.5. The related fitted parameters of Langmuir and Freundlich
Fig. 2. (a) FTIR adsorption spectra of PS, SPS and [email protected], (b) XRD patterns of MgSi, SPS and SPS@MgSix. 11
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Table 3 Comparison of adsorption capacity for various adsorbents. Adsorbent
Maximum adsorption capacity (mg g−1)
Ref.
[email protected]
357.14
β-Cyclodextrin-carboxymethyl cellulose based hydrogel Palygorskite Oxide graphene Active carbon Organic-montmorillonite Carbon nano tube Metal-organic frameworks Zeolite
167
This work [39]
219.3 80 307 256.41 16.05 252.5 98
[40] [41] [42] [43] [44] [45] [46]
Fig. 4. Removal efficiencies of BPA on [email protected], MgSi and SPS as a function of contact time. Table 1 Pseudo-first-order and pseudo-second-order adsorption kinetic constants for BPA adsorption. Pseudo-first-order model Adsorbent [email protected]
−1
K1 (min 0.0278
)
2
R 0.8489
Pseudo-second-order model K2 (g mg min−1) 0.0022
R2 0.9976
Fig. 6. Effect of solution pH on equilibrium adsorption capacities of SPS@ MgSi0.5.
Fig. 5. The Langmuir adsorption and Freundlich adsorption isotherm of BPA on the [email protected] adsorbent. Table 2 Langmuir and Freundlich parameters of BPA adsorption on [email protected]. Adsorption model
Constant
Value
Langmuir
Q0 b R2 logK n R2
357.14 mg g−1 0.011 L mg−1 0.9978 1.0371 0.7830 0.9232
Freundlich
Fig. 7. The variation of adsorption capacities of [email protected] for BPA under various inorganic ions.
In addition, pH value of solution is a key factor to affect the surface charges of adsorbent. As shown in Fig. 6, the isoelectric point of SPS@ MgSi0.5 is pH 2, thus when a pH value is higher than 2, the surface of [email protected] carries negative charges and the charge amount increases with pH value. In the other hand, BPA would combine H+ in the solution to form ion state with positive charges, causing the electrostatic adsorption between BPA and [email protected] when the pH is between 2 and 6. However, when the pH is greater than 6, BPA becomes electronegative because of dissociation of two hydroxyl groups from its structure, forming electrostatic repulsion between BPA and SPS@ MgSi0.5, which decrease the adsorption capacity of [email protected]. In the
models are listed in Table 2. As shown in Fig. 5 and Table 2, the correlation coefficient values imply that the Langmuir model better fits the experimental data. The maximum adsorption capacity (Q0) of SPS@ MgSi0.5 for BPA fitted by the Langmuir model is 357.14 mg g−1, which is higher than those of most of adsorbents for BPA in the wastewater, summarized in Table 3. 12
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Science Foundation of Fujian Province(2016J01223, 2015J01654) and Key Laboratory of Eco-materials Advanced Technology (Fuzhou University). References [1] Q. Sun, et al., Fate and mass balance of bisphenol analogues in wastewater treatment plants in Xiamen City, China, Environ. Pollut. 225 (2017) 542–549. [2] N. Morin, H.P. Arp, S.E. Hale, Bisphenol A in solid waste materials, leachate water, and air particles from Norwegian waste-handling facilities: presence and partitioning behavior, Environ. Sci. Technol. 49 (2015) 7675–7683. [3] X. Tan, et al., Three-dimensional MnO2 porous hollow microspheres for enhanced activity as ozonation catalysts in degradation of bisphenol A, J. Hazard. Mater. 321 (2017) 162–172. [4] C.Y. Wang, et al., Novel Bi12O15Cl6 photocatalyst for the degradation of bisphenol A under visible-light irradiation, ACS Appl. Mater. Interfaces 8 (2016) 5320–5326. [5] J. 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Rathnayake, et al., Environmental applications of inorganic-organic clays for recalcitrant organic pollutants removal: bisphenol A, J. Colloid Interface Sci. 470 (2016) 183–195. [28] K. Ortiz-Martinez, et al., Single and multi-component adsorptive removal of bisphenol A and 2,4-dichlorophenol from aqueous solutions with transition metal modified inorganic-organic pillared clay composites: effect of pH and presence of humic acid, J. Hazard. Mater. 312 (2016) 262–271. [29] Q. Cao, F. Huang, Z.Y. Zhuang, Z. Lin, A study of the potential application of nanoMg(OH)2 in adsorbing low concentrations of uranyl tricarbonate from water, Nanoscale 4 (2012) 2423–2430. [30] C.R. Li, Z.Y. Zhuang, F. Huang, Z.C. Wu, Y.P. Hong, Z.* Lin, Recycling rare earth
Fig. 8. The recycle stability of [email protected] for BPA.
literature [47], BPA adsorption on the reduced GO presents a similar pH dependency. To evaluate the practicability of [email protected], we investigate its selective adsorption behavior in the presence of inorganic ions. As indicated in Fig. 7, K+ and CO32− have negative effect on the BPA uptaken on the [email protected] regardless of a high or low concentration. The high-concentration K+ or CO32− induces nearly 50% reduction of adsorption capacity of [email protected]. The above phenomenon can be enhanced by adding more [email protected] because of its low price. Under both low and high concentration, Cl− and SO42− almost have no effect on the adsorption behavior of BPA on the [email protected], but the influence from Ca2+ and Na+ depends on the concentration. For example, under a low concentration the influence from Na+, enabling the adsorption capacity reduce to 331.14 mg g−1, is stronger than that from Ca2+, but this comparison is reversed under high concentration. Noteworthy, the adsorption capacity of [email protected] for BPA still be maintained at 200 mg L−1 even if high-concentration K+ or CO32− is present in waste water, indicating that [email protected] has superior ability of anti-interference from common inorganic ions. The stability is important for evaluating the feasibility of this adsorbent. The desorption and re-adsorption performance of [email protected] were conducted in four sequential cycles as shown in Fig. 8. It displays that the adsorption capacity decreases slightly with increasing cycle numbers. But it still keeps a decent capacity for BPA uptaken after four cycles, and the adsorption capacity still reach 290 mg g−1, only 67 mg g−1 lower than the maximum adsorption capacity. This value is still higher than those of some other adsorbents [39–41,43–46]. After four cycle's treatments, the final BPA solution can be concentrated several times and the concentrated BPA solution can be incinerated to achieve the goal of harmlessness. 4. Conclusion In brief, [email protected] nanocomposites have been prepared for the removal of BPA under mildly acidic conditions for the first time. The interwoven MgSi dispersedly distributes on the surface of SPS microsphere, which enables [email protected] nanocomposites show unprecedentedly high adsorption performance for BPA with a adsorption capacity of 357.14 mg L−1. In addition, the nanocomposites exhibits superior stability, selectivity and reusability. We anticipate that it may be a promising candidate for the removal of the BPA from acidic brine wastewater upon further optimization. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21577018, 21477128, 61376002), Natural 13
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