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Adsorption of 17α-ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite
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Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite Zhoufei Luo, Haipu Li∗, Yuan Yang, Huiju Lin, Zhaoguang Yang∗ Center for Environment and Water Resources, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 1 April 2017 Revised 27 July 2017 Accepted 19 September 2017 Available online xxx Keywords: Reduced graphene oxide Magnetic nanocomposite 17α -ethinylestradiol Adsorption
a b s t r a c t A reduced graphene oxide (rGO) magnetic nanocomposite rGO/Fe3 O4 was prepared and used to adsorb 17α -ethinylestradiol from aqueous solutions. The magnetic nanocomposite was characterized using X-ray powder diffraction, Raman spectroscopy, vibrating sample magnetometer measurements, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. It was shown that the Fe3 O4 nanoparticles are uniformly deposited on the rGO sheets. Experiments were performed to elucidate the mechanism by which 17α -ethinylestradiol adsorbs onto rGO/Fe3 O4 . The kinetic adsorption data for 17α -ethinylestradiol fitted well with a pseudo-second-order model, while the equilibrium data followed Langmuir isotherms. Thermodynamic analysis indicated that the adsorption is a spontaneous and endothermic process. The effects of water chemistry (pH, coexisting ions, humic acid level, and water matrix) on 17α -ethinylestradiol adsorption were also investigated. It was found that pH and humic acid level have noticeable effects on the adsorption of 17α -ethinylestradiol, while coexisting ions and water matrix do not. Desorption experiments performed using methanol as eluent demonstrated that the adsorbent could be reused for five cycles without significant deterioration in performance. The results show that rGO/Fe3 O4 exhibits high adsorption capacity and may be conveniently recovered, making it a promising adsorbent for 17α -ethinylestradiol removal from real-world water resources. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The synthetic estrogenic steroid 17α -ethinylestradiol (EE2) has higher estrogenic potency than other common endocrine disrupting compounds and is thus used in almost all contraceptive pills and hormone replacement therapies currently available [1]. However, EE2 presents an environmental risk owing to its endocrinedisrupting properties that can affect the endocrine systems of exposed biota by altering sex determination, delaying sexual maturity, and disturbing the synthesis of specific hormone receptors [2]. EE2 tends to accumulate in organisms and hence has attracted increasing concern regarding contamination of water resources [3]. Several studies have shown that even at concentrations of 0.1–
Abbreviation: EE2, 17α -ethinylestradiol; rGO, reduced graphene oxide; BSTFA, N,O-Bis (trimethylsilyl) trifluoroacetamide; TMCS, trimethylsilyl chloride; XRD, Xray diffraction, VSM, vibrating sample magnetometer; SEM, scanning electron microscopy; TEM, transmission electron microscopy; XPS, X-ray photoelectron spectroscopy, FT-IR, Fourier-transform infrared spectroscopy; BET, Brunauer–Emmett– Teller; GC–MS, gas chromatography-mass spectrometry. ∗ Corresponding authors. E-mail addresses: [email protected] (Z. Luo), [email protected] (H. Li), [email protected] (Y. Yang), [email protected] (H. Lin), [email protected] (Z. Yang).
10 ng l−1 , EE2 presents a potential danger to fish and other aquatic beings and can have negative effects on ecological systems [4]. EE2 is detected worldwide in effluents from municipal treatment plants, hospitals, and livestock farming [5]. Specific US National Park Service concerns include the fluvial transport of EE2 through rivers to larger water bodies from contaminated domestic wastewater and domesticated-animal waste in backcountry areas [6]. In the United States, EE2 was detected at 242 ng l−1 in an advanced wastewater reclamation plant [7]. EE2 was also detected at a concentration of 421 ng l−1 in an influent sample in a Brazilian wastewater treatment plant [8]. Furthermore, the EE2 concentration in influents in Beijing is high up to 873.8 ng l−1 . This may be due to both the large population and China’s birth control policy, which has led to the increased usage of EE2-based oral contraception pills by women [9]. These results reveal the widespread threat of EE2 pollution in aquatic environments. Therefore, the development of efficient technologies to remove EE2 from water is imperative. Adsorption is regarded as an environmentally friendly technology for water treatment. The appeal of this technology can be attributed to its effectiveness, efficiency, and economy, and different adsorbents can be designed for this purpose [10]. Owing to the high resistance of EE2 to degradation, there is a tendency to resort to physical treatment using adsorbent materials for the
https://doi.org/10.1016/j.jtice.2017.09.028 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028
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removal of EE2 from water [4]. Several researchers have demonstrated that activated carbon, carbon nanomaterials, biochar, and montmorillonite are effective and economical materials for the adsorption of EE2 [11–14]. However, these materials are difficult to separate from solution and hence are discarded after use, leading to secondary pollution. To solve this problem, magnetic carbon adsorbents have been explored [15]. Both graphene and reduced graphene oxide (rGO) have been adopted as platforms for loading specific magnetic nanoparticles, but rGO is generally more suitable because it contains a greater number of polar moieties and is easily dispersed in water [16]. Recently, magnetic rGO composites have been extensively applied to the adsorption of pollutants such as metals, steroid hormones, alkylphenols, and dyestuff from water [17–20]. It has been shown that magnetic rGO exhibits excellent adsorption capability and allows for facile operation under mild conditions. Furthermore, the magnetic properties of this composite adsorbent allow its separation from solution under an external magnetic field. Nevertheless, the adsorption of EE2 from water using magnetic graphene-based adsorbents has not yet been reported, prompting us to investigate the liquid-phase adsorption of EE2 by the rGO/Fe3 O4 composite. In this study, the magnetic nanocomposite rGO/Fe3 O4 was prepared, characterized, and used as an absorbent for the removal of EE2 from aqueous solution. The adsorption mechanism was explored by kinetic and thermodynamic analysis, and the influence of water chemistry, i.e., pH, coexisting ions, humic acid level, and water matrix. Moreover, the regeneration and reuse of the adsorbent were evaluated. 2. Materials and methods 2.1. Materials Natural graphene (≥ 99.95%) was obtained from XFNANO, China. EE2 (≥ 98%) was purchased from Sigma, and its stock solution (10 0 0 mg l−1 ) was prepared with methanol. N,OBis(trimethylsilyl)trifluoroacetamide (BSTFA) with 1% trimethylsilyl chloride (TMCS, > 99%) was purchased from Regis (USA). Methanol (≥ 99.9%, Sigma-Aldrich, USA), humic acid (fulvic acid > 90%, Sigma-Aldrich, USA), acetonitrile (Sigma-Aldrich, USA), acetone (≥ 99.9%, Tedia, USA), and n-hexane (Sigma-Aldrich, United States) were all of at least analytical grade and used as received. An SPE C18 column (Waters, USA) was used for chromatographic separation. Ultrapure water (resistivity ≥ 18.25 M cm−1 ) obtained from WaterPro water system (ULUPURE, China) was used in all experiments. 2.2. Preparation of rGO/Fe3 O4 Graphene oxide was synthesized from natural graphene according to the modified Hummer’s method [16]. rGO was then obtained by hydrazine hydrate reduction under water-cooled condenser conditions [21]. Then, 40 mg rGO was dispersed in ultrapure water under sonication for 30 min. The suspension liquid was transferred into a round-bottom flask under stirring and nitrogen purge, and 800 mg FeCl3 •6H2 O and 300 mg FeCl2 •4H2 O were added to the flask with 50 ml ultrapure water. The temperature was increased to 80 °C, 10 ml ammonia was added to the mixture, and the resultant solution was held under stirring for 30 min. The final product was washed with distilled water and placed in a vacuum oven (YGM-DZF, China) for 2 h and finally gathered using a magnetic field. A detailed description of the X-ray diffraction (XRD), Raman spectroscopy, vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier-transform
infrared spectroscopy (FT-IR), and Brunauer–Emmett–Teller (BET) characterizations are provided in the Supplementary Material. 2.3. Adsorption experiments In each experiment, a certain amount of rGO/Fe3 O4 (0.5, 1.0, 3.0, 5.0, 7.0, or 10 mg) was added to a glass conical flask loaded with 50 ml aqueous EE2 solution of a certain concentration (10, 50, 100, 200, 500, 1000, 2000, and 3000 μg l−1 ). The pH of the solution was adjusted to a designated value (3–11) with 0.1 M NaOH or 0.1 M HCl solution and monitored using a pH meter (pHSJ-3F, JK, China). The flask was kept in a thermostat-controlled ( ± 1 °C) water bath shaker at a constant temperature (288, 298, 308, or 318 K) for a set period of time (0–7 h). At the end of the experiment, the solid and liquid phases were separated using an external magnet. The amount of EE2 adsorbed onto the rGO/Fe3 O4 was calculated using
q = (Co − Ce )
V m
(1)
where q is the amount of EE2 adsorbed onto rGO/Fe3 O4 (mg g−1 ); Co and Ce represent the initial and end concentrations of EE2 (mg l−1 ), respectively; V is the volume of the solution (l), and m is the mass of rGO/Fe3 O4 (g). All adsorption experiments were performed in triplicate. 2.4. Analytical methods The concentration of EE2 was determined by gas chromatography–mass spectrometry (GC–MS) and the sample pretreatment was using solid-phase extraction (SPE) procedure [22]. After derivatization, the sample was injected into the GC–MS system. GC–MS analyses were carried out with an Agilent 7890 series GC apparatus coupled with a 5975 series MS instrument. An HP-5MS fused-silica capillary column (30 m × 0.25 mm × 0.25 μm, Agilent, USA) was used. The limit of detection was 0.0052 μg l−1 , as inferred by the lowest standard solution detected. The recovery of this method was within the range 83.5–96.2%. More detailed descriptions of the analytical methods are given in the Supplementary Material. 2.5. Desorption and regeneration The adsorbed EE2 was desorbed with methanol, ethanol, cyclohexane, acetone, toluene, or chloroform. The adsorbents collected from the suspension by magnetic separation were re-dispersed into 5 ml of the solvent. After shaking for 30 min at room temperature, the concentration of EE2 in the supernatant was measured. Then, the adsorbents were separated and washed with ultrapure water three times. The desorption efficiency of EE2 was calculated as the ratio of the amount of desorbed EE2 to the amount of EE2 initially adsorbed. To test the reusability of the absorbent, the adsorption–desorption process was repeated six times. 3. Result and discussion 3.1. Characterization of adsorbent material The phase structure of the final product was investigated using XRD. As shown in Fig. 1a, the characteristic diffraction peaks for Fe3 O4 match well with the standard XRD data for magnetite (PDF card, NO. 75-0033). In the Raman spectrum of the synthesized adsorbent (Fig. 1b), the two strong peaks at 1327.9 cm−1 (D band) and 1590.46 cm−1 (G band) are due to the graphitic sheets, and the ID: IG (1.65) ratio can be used to estimate the
Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028
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Fig. 1. XRD spectrum (a) Raman spectrum (b) magnetization hysteresis loop (c), X-ray photoelectron spectrum (d) FT-IR pattern (e) BET analysis (f) of rGO/Fe3 O4 .
thickness of the rGO layer [23,24]. The field-dependent magnetization is measured by a vibrating sample magnetometer at room temperature, as shown in Fig. 1c, where the significant hysteresis loops in the S-shaped curves indicate the ferromagnetic behaviors of the materials. The saturation magnetization is calculated to be 58.6 emu g−1 . The chemical state of the elements in composite material is further investigated by XPS. The wide scan X-ray photoelectron spectrum (Fig. 1d) of composite material shows photoelectron lines at binding energies of ∼285, 530, and 711 eV, which are attributed to C1s, O1s, and Fe2p, respectively. The peaks for Fe2p3/2 and Fe2p1/2 are located at 711.27 and 724.74 eV, which is in accordance with the reported data for the Fe3 O4 phase in the composites [17]. The FT-IR of the composite material detected the relatively strong peaks of 3374 cm−1 , 1560 cm−1 , 1201 cm−1 and 585 cm−1 (as shown in Fig. 1e), which should be from the stretching vibration of O–H, C=O, and C–O of rGO, and Fe–O of Fe3 O4 , respectively [18], indicating the abundant functional groups and the co-existence of rGO and Fe3 O4 . N2 adsorption–desorption isotherms of composite material (Fig. 1f) referred to the type IV hysteresis hoop (IUPAC). The specific surface area of composite material was calculated to be 95.34 m2 g−1 by BET analysis. The morphology of the rGO/Fe3 O4 composite was characterized using SEM and TEM. It can be observed from the SEM images (Fig. 2a and b) that the particles are roughly anchored on the folded sheet structure. The TEM images further confirmed that
nanoparticles are dispersed on the crumpled wave-like graphene (Fig. 2c and d), and these nanoparticles were demonstrated to be iron oxides by energy-dispersive X-ray analysis (inset in Fig. 2c). Taking these data into account, we can conclude the coexistence of rGO and Fe3 O4 nanoparticles in all composites. High-resolution TEM (HRTEM) revealed that Fe3 O4 nanoparticles are aligned almost parallel to the regular lattice fringes with a d-spacing of 0.25 nm (Fig. 2e and f), corresponding to the (311) planes of a cubic crystal structured iron oxide [24]. The particle size distribution of the nanocomposite material was shown in Fig. S1, and the average particle size was calculated to be 381.19 nm. These results indicate that the product is the expected magnetic nanocomposite rGO/Fe3 O4 with a uniform distribution of Fe3 O4 nanoparticles on the rGO sheets. 3.2. Adsorption kinetics The contact time between the adsorbent and adsorbate is a vital parameter for evaluating adsorption efficiency. The absorption of EE2 from the liquid phase versus contact time is shown in Fig. 3. The absorption increases quickly at the beginning, and increasing contact time is beneficial for the adsorption of EE2 on the rGO/Fe3 O4 . The adsorption process reaches equilibrium at 6 h. Two empirical kinetic models, i.e., pseudo-first-order and pseudo second-order kinetics, were used to analyze the adsorption
Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028
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Fig. 2. SEM (a, b), TEM (c, d), and HRTEM (e, f) images of rGO/Fe3 O4 .
behaviors. For the pseudo-first-order kinetic equation, the amount of adsorbed EE2 is given by [12]:
qt = qe 1 − e−k1 t
(2) g−1 )
where qe and qt (mg are the amounts of the adsorbed EE2 at equilibrium and after a certain period of time (h), respectively. k1 (h−1 ) is the rate constant of the pseudo-first-order adsorption. The pseudo-second-order kinetic model can be expressed as:
qt =
Fig. 3. Pseudo-first-order and pseudo-second-order kinetics for the adsorption of EE2 by rGO/Fe3 O4 (n = 3) with different initial concentrations. Adsorbent dosage: 5 mg rGO/Fe3 O4 , volume of solvent: 50 ml, temperature: 298 K, pH: 6.
q2e k2 t 1 + qe k2 t
(3)
where qe and qt (mg g−1 ) have the same definitions as those in Eq. (2), and k2 is the pseudo-second-order rate constant at equilibrium (g mg−1 h−1 ). As shown in Fig. 3 the adsorption kinetics were investigated by fitting the pseudo-first-order kinetic model (dashed lines) and the pseudo-second-order kinetic model (solid lines). The determination coefficient (r2 ) derived from pseudo-second-order fitting is better than that for the pseudo-first-order fitting, as shown in Table S1. The experimental qe(exp) values are close to the theoretical qe(cal) values in the case of the pseudo-second-order model. Therefore, the adsorption kinetics of EE2 by rGO/Fe3 O4 are better described by a pseudo-second-order kinetic model, which implies that the
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Table 1 Comparison of the EE2 adsorption capacity of rGO/Fe3 O4 with that of other adsorbents. Adsorbent
pH
Adsorption capacity (mg g−1 )
Reference
Activated carbon Single-walled carbon nanotubes Multi-walled carbon nanotubes Polyamide 612 rGO/Fe3 O4
7.0 4.7–8.8
9.6 35.6–35.7
[27] [28]
6.3 ± 0.2
0.472
[29]
4.8–9.1 6.0
25.4 37.33
[30] Present work
Fig. 4. Langmuir adsorption isotherm and Freundlich adsorption isotherm for EE2 on rGO/Fe3 O4 (n = 3), with initial solution pH 6, temperature 288–318 K, and contact time 6 h.
adsorption of EE2 on rGO/Fe3 O4 may be dependent on the amount of the solute adsorbed on the surface of the adsorbent and as well as the amount adsorbed at equilibrium [25,26].
3.3. Adsorption isotherm To further understand the adsorption behavior, the equilibrium data were also evaluated according to the Langmuir and Freundlich isotherm models. The Langmuir model is based on the assumption of monomolecular layer adsorption and can be expressed as:
ce 1 ce = + qe kl qm qm
(4)
where ce represents the equilibrium concentration of EE2 in solution (mg l−1 ), qe is the adsorption capacity at equilibrium (mg g−1 ), and qm is the theoretical maximum adsorption capacity (mg g−1 ). kl is the Langmuir isotherm constant, which is related to the affinity of binding sites (l mg−1 ). The Freundlich equilibrium isotherm is an empirical equation (Eq. 5) that is used for the description of multilayer adsorption with interactions between the adsorbed molecules:
l nqe = ln k f +
1 l n ce n
(5)
where kf and 1/n are the Freundlich constants related to the adsorption capacity and intensity, respectively. Isotherms for EE2 adsorption on rGO/Fe3 O4 are illustrated in Fig. 4. According to the fitted data in Table S2, the adsorption behavior can be described by both Langmuir and Freundlich equations, but the former is better owing to its higher determination coefficient, especially in view of the adsorption of hydrophobic compounds [26]. The Freundlich constant (1/n) that reflects the adsorption intensity of adsorbent is between 0 and 1, suggesting a favorable adsorption process [27]. The adsorption capacity of rGO/Fe3 O4 was compared with other adsorbents reported previously, as shown in Table 1, and it can be seen that the sorption capacity of rGO/Fe3 O4 is higher than those of other adsorbents. Moreover, rGO/Fe3 O4 shows good magnetic properties and thus shows promise as a novel adsorbent for EE2 removal from water.
Fig. 5. The fitted line for lnkd versus 1/T (n = 3), with an initial EE2 concentration of 10 0 0 μg l−1 , initial solution pH of 6, 5 mg rGO/Fe3 O4 , and contact time of 6 h.
3.4. Effect of temperature and adsorption thermodynamics It is known that temperature has a significant influence on adsorption processes [30]. Therefore, the effect of temperature on the adsorption of EE2 on rGO/Fe3 O4 was investigated at 288, 298, 308, and 318 K. The specific distribution coefficient kd (l g−1 ) is defined as the ratio of equilibrium concentrations of a dissolved test substance in a two phase system, and can be calculated with:
kd =
qe ce
(6)
The thermodynamic parameters related to the adsorption process can be determined by the classical Van’t Hoff equation.
ln kd =
S o R
−
H o RT
(7)
where R is the universal gas constant (8.314 × 10−3 J mol−1 K−1 ), T is the absolute temperature, S° is the change in entropy (kJ mol−1 K−1 ), and H° is the heat of adsorption (kJ mol−1 ) at a constant temperature. The values of S° and H° were computed from the slope and intercept of the plot between lnkd versus 1/T, as shown in Fig. 5. It can be seen that the adsorption of EE2 is favored by an increase in temperature. The Gibbs free-energy change (G°) for adsorption can be determined by:
Go = −RT ln kd
(8)
The calculated values of S°, H°, and G° are shown in Table S3. The positive S° may indicate an increased randomness at the solid/liquid interface during the adsorption of EE2 on rGO/Fe3 O4 [12]. The negative G° and positive H° suggest that the adsorption process is spontaneous and endothermic. With an increase in
Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028
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ing for 6 h. The removal efficiency of EE2 increased with an increase in the solid-to-liquid ratio from 0.01 to 0.1 (see Fig. 6). This increase in adsorption capacity could be attributed to the availability of a greater number of adsorption sites. Only a slight increase in adsorption was observed when the ratio exceeded 0.1. Thus, a solid-to-liquid ratio of 0.1 was selected for all subsequent experiments.
Fig. 6. Effect of solid-to-liquid ratio on the adsorption of EE2 by rGO/Fe3 O4 at 298 K (n = 3) with an initial EE2 concentration of 10 0 0 μg l−1 , solution volume of 50 ml, and contact time of 6 h.
temperature, G° becomes more negative, implying an increase of equilibrium capacity. From the above results, EE2 adsorbtion on the as-prepared adsorbent is more favorable at higher temperatures. 3.5. Factors affecting adsorption 3.5.1. Solid-to-liquid ratio The effect of solid-to-liquid ratio was investigated by adding different amounts of rGO/Fe3 O4 to 50 ml EE2 solutions and shak-
3.5.2. Solution pH pH has a profound influence on the efficiency of adsorptionbased water treatment processes, especially those for ionizable organic chemicals [31]. Thus, the effect of initial pH on adsorption was investigated. As shown in Fig. 7a, the adsorption capacity increases from 68.03% to the peak value (90.45%) with an increase in pH from 3 to 6, followed by a slow decrease with further increase in pH. In view of the presence of aromatic rings and phenolic hydroxyl groups in EE2 and the functional groups on the rGO surface, it could be speculated that hydrophobic forces and hydrogen bonds would be the main driving force during adsorption, and that the former would be predominant [32]. However, the hydrophobic force can be weakened for ionized compounds or charged adsorbents [33]. Thus, the adsorption capacity would decrease as the solution pH moves away from the point of zero charge (pHzpc ) of the nanocomposite material [34]. The pHzpc of rGO/Fe3 O4 is around 4.78 (Fig. S2). Furthermore, when the pH is higher than 10.5, i.e., the pKa value of EE2, the EE2 and adsorbent are both negatively charged and electrostatic repulsion was expected to reduce the adsorption capacity [35]. Thus, the adsorption is favored under moderate pH conditions, and all further experiments were conducted at an initial pH of 6.
Fig. 7. Effects of water chemistry parameters (pH, coexisting ions, humic acid level, and water matrix) on the adsorption of EE2 by rGO/Fe3 O4 at 298 K (n = 3), with an initial EE2 concentration of 10 0 0 μg l−1 , adsorbent dosage of 5 mg rGO/Fe3 O4 , solution volume of 50 ml, and contact time of 6 h.
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Fig. 8. Effect of eluting solvent on the desorption efficiency (a) (n = 3) and recyclable adsorption of EE2 by rGO/Fe3 O4 using methanol as eluent (b) (n = 3). Adsorbent 5 mg rGO/Fe3 O4 , initial EE2 concentration 10 0 0 μg l−1 , volume of solvent 50 ml, temperature 298 K, pH 6, and contact time 6 h.
3.5.3. Coexisting ions The effects of four types of cations (Na+ , K+ , Ca2+ , and Mg2+ ) and three types of anions (Cl− , NO3 − , and SO4 2− ) on the adsorption were investigated. The concentration of these ions was close to that in real environmental water, as shown in Table S4. The presence of selected ions has a positive but minor effect on the adsorption capacity, as shown in Fig. 7b (88.2–94.7%). A similar situation was reported for EE2 adsorbed onto multi-walled carbon nanotubes [36]. This could be attributed to the “salting-out” effect, which is caused by the reduced solubility of organic compounds in aqueous solution [37]. 3.5.4. Humic acid Humic acid is a ubiquitous and representative natural organic matter (NOM) in aquatic environments. Thus, the effect of humic acid addition on adsorption was explored. As seen in Fig. 7c, the accumulated presence of humic acid significantly inhibits the adsorption of EE2 on rGO/Fe3 O4 . This negative effect may be due to direct competitive adsorption or physical pore blockage [38]. 3.5.5. Water matrix interferences Four different natural water samples spiked with EE2 were used to investigate the actual application of rGO/Fe3 O4 to real-world samples. Drinking water, spring water, lake water, and river water were collected from the laboratory tap, Yuelu spring, Dongting Lake, and Xiangjiang River, respectively. The basic information and chemical parameters of the natural water samples are given in Table S5. The prepared rGO/Fe3 O4 was used as an absorbent to remove EE2 from spiked environmental water samples (50 ml each). As show in Fig. 7d, the adsorption capacities of EE2 are comparable for all water samples, even in the presence of humic acid. These results demonstrate that the adsorption ability of rGO/Fe3 O4 is not affected dramatically by natural water matrices. 3.6. Desorption and regeneration The reusability of an adsorbent is considered to have a great cost benefit for practical applications. Six different solvents including methanol, ethanol, cyclohexane, acetone, toluene, and chloroform were used as eluents for desorption of EE2 from rGO/Fe3 O4 . The quantitative desorption efficiency by methanol is 95.4% (Fig. 8a), followed by that of ethanol (88.3%). The regeneration and reuse of rGO/Fe3 O4 were evaluated and the results are presented in Fig. 8b. The adsorption capacity remains above 80%, even after five cycles.
4. Conclusion The magnetic nanocomposite rGO/Fe3 O4 featuring a uniform distribution of Fe3 O4 nanoparticles on rGO sheets was prepared and characterized by XRD, Raman spectroscopy, VSM, XPS, FT-IR, BET, SEM, and TEM. The adsorption of EE2 onto rGO/Fe3 O4 followed pseudo-second-order kinetics, while the adsorption isotherm was well described by the Langmuir model. In addition, thermodynamic studies showed that the adsorption is spontaneous and endothermic, thus allowing for higher removal efficiency even at elevated temperatures. Water chemistry, including solution pH and humic acid level, had a profound influence on the adsorption, while, coexisting ions and the water matrix had little impact. The prepared rGO/Fe3 O4 showed good absorption efficiency even after five regeneration and reuse cycles, indicating its promise for practical applications in environmental remediation. Acknowledgment This study was supported by the Special Fund for Agro-scientific Research in the Public Interest (No. 201503108). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.09.028. References [1] Schroeder LM, Blackwell B, Klein D, Morse AN. Rate Uptake of three common pharmaceuticals in celery, apium graveolens. Water Air Soil Pollut 2015;226:123. [2] Dzieweczynski TL, Hebert OL. The effects of short-term exposure to an endocrine disrupter on behavioral consistency in male juvenile and adult Siamese fighting fish. Arch Environ Contam Toxicol 2013;64:316–26. [3] Hamid H, Eskicioglu C. Fate of estrogenic hormones in wastewater and sludge treatment: a review of properties and analytical detection techniques in sludge matrix. Water Res 2012;46:5813–33. [4] Aris AZ, Shamsuddin AS, Praveena SM. Occurrence of 17α -ethinylestradiol (EE2) in the environment and effect on exposed biota: a review. Environ Int 2014;69:104–19. [5] Ying G, Kookana RS, Ru Y. Occurrence and fate of hormone steroids in the environment. Environ Int 2002;28:545–51. [6] Bradley PM, Battaglin WA, Iwanowicz LR, Clark JM, Journey CA. Aerobic biodegradation potential of endocrine-disrupting chemicals in surface-water sediment at Rocky Mountain National Park, USA. Environ Toxicol Chem 2016;35:1087–96. [7] Yang X, Flowers RC, Weinberg HS, Singer PC. Occurrence and removal of pharmaceuticals and personal care products (PPCPs) in an advanced wastewater reclamation plant. Water Res 2011;45:5218–28. [8] Pessoa GP, de Souza NC, Vidal CB, Alves JA, Firmino PI, Nascimento RF, et al. Occurrence and removal of estrogens in Brazilian wastewater treatment plants. Sci Total Environ 2014;490:288–95.
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Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite Zhoufei Luo, Haipu Li∗, Yuan Yang, Huiju Lin, Zhaoguang Yang∗ Center for Environment and Water Resources, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 1 April 2017 Revised 27 July 2017 Accepted 19 September 2017 Available online xxx Keywords: Reduced graphene oxide Magnetic nanocomposite 17α -ethinylestradiol Adsorption
a b s t r a c t A reduced graphene oxide (rGO) magnetic nanocomposite rGO/Fe3 O4 was prepared and used to adsorb 17α -ethinylestradiol from aqueous solutions. The magnetic nanocomposite was characterized using X-ray powder diffraction, Raman spectroscopy, vibrating sample magnetometer measurements, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. It was shown that the Fe3 O4 nanoparticles are uniformly deposited on the rGO sheets. Experiments were performed to elucidate the mechanism by which 17α -ethinylestradiol adsorbs onto rGO/Fe3 O4 . The kinetic adsorption data for 17α -ethinylestradiol fitted well with a pseudo-second-order model, while the equilibrium data followed Langmuir isotherms. Thermodynamic analysis indicated that the adsorption is a spontaneous and endothermic process. The effects of water chemistry (pH, coexisting ions, humic acid level, and water matrix) on 17α -ethinylestradiol adsorption were also investigated. It was found that pH and humic acid level have noticeable effects on the adsorption of 17α -ethinylestradiol, while coexisting ions and water matrix do not. Desorption experiments performed using methanol as eluent demonstrated that the adsorbent could be reused for five cycles without significant deterioration in performance. The results show that rGO/Fe3 O4 exhibits high adsorption capacity and may be conveniently recovered, making it a promising adsorbent for 17α -ethinylestradiol removal from real-world water resources. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The synthetic estrogenic steroid 17α -ethinylestradiol (EE2) has higher estrogenic potency than other common endocrine disrupting compounds and is thus used in almost all contraceptive pills and hormone replacement therapies currently available [1]. However, EE2 presents an environmental risk owing to its endocrinedisrupting properties that can affect the endocrine systems of exposed biota by altering sex determination, delaying sexual maturity, and disturbing the synthesis of specific hormone receptors [2]. EE2 tends to accumulate in organisms and hence has attracted increasing concern regarding contamination of water resources [3]. Several studies have shown that even at concentrations of 0.1–
Abbreviation: EE2, 17α -ethinylestradiol; rGO, reduced graphene oxide; BSTFA, N,O-Bis (trimethylsilyl) trifluoroacetamide; TMCS, trimethylsilyl chloride; XRD, Xray diffraction, VSM, vibrating sample magnetometer; SEM, scanning electron microscopy; TEM, transmission electron microscopy; XPS, X-ray photoelectron spectroscopy, FT-IR, Fourier-transform infrared spectroscopy; BET, Brunauer–Emmett– Teller; GC–MS, gas chromatography-mass spectrometry. ∗ Corresponding authors. E-mail addresses: [email protected] (Z. Luo), [email protected] (H. Li), [email protected] (Y. Yang), [email protected] (H. Lin), [email protected] (Z. Yang).
10 ng l−1 , EE2 presents a potential danger to fish and other aquatic beings and can have negative effects on ecological systems [4]. EE2 is detected worldwide in effluents from municipal treatment plants, hospitals, and livestock farming [5]. Specific US National Park Service concerns include the fluvial transport of EE2 through rivers to larger water bodies from contaminated domestic wastewater and domesticated-animal waste in backcountry areas [6]. In the United States, EE2 was detected at 242 ng l−1 in an advanced wastewater reclamation plant [7]. EE2 was also detected at a concentration of 421 ng l−1 in an influent sample in a Brazilian wastewater treatment plant [8]. Furthermore, the EE2 concentration in influents in Beijing is high up to 873.8 ng l−1 . This may be due to both the large population and China’s birth control policy, which has led to the increased usage of EE2-based oral contraception pills by women [9]. These results reveal the widespread threat of EE2 pollution in aquatic environments. Therefore, the development of efficient technologies to remove EE2 from water is imperative. Adsorption is regarded as an environmentally friendly technology for water treatment. The appeal of this technology can be attributed to its effectiveness, efficiency, and economy, and different adsorbents can be designed for this purpose [10]. Owing to the high resistance of EE2 to degradation, there is a tendency to resort to physical treatment using adsorbent materials for the
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Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028
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removal of EE2 from water [4]. Several researchers have demonstrated that activated carbon, carbon nanomaterials, biochar, and montmorillonite are effective and economical materials for the adsorption of EE2 [11–14]. However, these materials are difficult to separate from solution and hence are discarded after use, leading to secondary pollution. To solve this problem, magnetic carbon adsorbents have been explored [15]. Both graphene and reduced graphene oxide (rGO) have been adopted as platforms for loading specific magnetic nanoparticles, but rGO is generally more suitable because it contains a greater number of polar moieties and is easily dispersed in water [16]. Recently, magnetic rGO composites have been extensively applied to the adsorption of pollutants such as metals, steroid hormones, alkylphenols, and dyestuff from water [17–20]. It has been shown that magnetic rGO exhibits excellent adsorption capability and allows for facile operation under mild conditions. Furthermore, the magnetic properties of this composite adsorbent allow its separation from solution under an external magnetic field. Nevertheless, the adsorption of EE2 from water using magnetic graphene-based adsorbents has not yet been reported, prompting us to investigate the liquid-phase adsorption of EE2 by the rGO/Fe3 O4 composite. In this study, the magnetic nanocomposite rGO/Fe3 O4 was prepared, characterized, and used as an absorbent for the removal of EE2 from aqueous solution. The adsorption mechanism was explored by kinetic and thermodynamic analysis, and the influence of water chemistry, i.e., pH, coexisting ions, humic acid level, and water matrix. Moreover, the regeneration and reuse of the adsorbent were evaluated. 2. Materials and methods 2.1. Materials Natural graphene (≥ 99.95%) was obtained from XFNANO, China. EE2 (≥ 98%) was purchased from Sigma, and its stock solution (10 0 0 mg l−1 ) was prepared with methanol. N,OBis(trimethylsilyl)trifluoroacetamide (BSTFA) with 1% trimethylsilyl chloride (TMCS, > 99%) was purchased from Regis (USA). Methanol (≥ 99.9%, Sigma-Aldrich, USA), humic acid (fulvic acid > 90%, Sigma-Aldrich, USA), acetonitrile (Sigma-Aldrich, USA), acetone (≥ 99.9%, Tedia, USA), and n-hexane (Sigma-Aldrich, United States) were all of at least analytical grade and used as received. An SPE C18 column (Waters, USA) was used for chromatographic separation. Ultrapure water (resistivity ≥ 18.25 M cm−1 ) obtained from WaterPro water system (ULUPURE, China) was used in all experiments. 2.2. Preparation of rGO/Fe3 O4 Graphene oxide was synthesized from natural graphene according to the modified Hummer’s method [16]. rGO was then obtained by hydrazine hydrate reduction under water-cooled condenser conditions [21]. Then, 40 mg rGO was dispersed in ultrapure water under sonication for 30 min. The suspension liquid was transferred into a round-bottom flask under stirring and nitrogen purge, and 800 mg FeCl3 •6H2 O and 300 mg FeCl2 •4H2 O were added to the flask with 50 ml ultrapure water. The temperature was increased to 80 °C, 10 ml ammonia was added to the mixture, and the resultant solution was held under stirring for 30 min. The final product was washed with distilled water and placed in a vacuum oven (YGM-DZF, China) for 2 h and finally gathered using a magnetic field. A detailed description of the X-ray diffraction (XRD), Raman spectroscopy, vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier-transform
infrared spectroscopy (FT-IR), and Brunauer–Emmett–Teller (BET) characterizations are provided in the Supplementary Material. 2.3. Adsorption experiments In each experiment, a certain amount of rGO/Fe3 O4 (0.5, 1.0, 3.0, 5.0, 7.0, or 10 mg) was added to a glass conical flask loaded with 50 ml aqueous EE2 solution of a certain concentration (10, 50, 100, 200, 500, 1000, 2000, and 3000 μg l−1 ). The pH of the solution was adjusted to a designated value (3–11) with 0.1 M NaOH or 0.1 M HCl solution and monitored using a pH meter (pHSJ-3F, JK, China). The flask was kept in a thermostat-controlled ( ± 1 °C) water bath shaker at a constant temperature (288, 298, 308, or 318 K) for a set period of time (0–7 h). At the end of the experiment, the solid and liquid phases were separated using an external magnet. The amount of EE2 adsorbed onto the rGO/Fe3 O4 was calculated using
q = (Co − Ce )
V m
(1)
where q is the amount of EE2 adsorbed onto rGO/Fe3 O4 (mg g−1 ); Co and Ce represent the initial and end concentrations of EE2 (mg l−1 ), respectively; V is the volume of the solution (l), and m is the mass of rGO/Fe3 O4 (g). All adsorption experiments were performed in triplicate. 2.4. Analytical methods The concentration of EE2 was determined by gas chromatography–mass spectrometry (GC–MS) and the sample pretreatment was using solid-phase extraction (SPE) procedure [22]. After derivatization, the sample was injected into the GC–MS system. GC–MS analyses were carried out with an Agilent 7890 series GC apparatus coupled with a 5975 series MS instrument. An HP-5MS fused-silica capillary column (30 m × 0.25 mm × 0.25 μm, Agilent, USA) was used. The limit of detection was 0.0052 μg l−1 , as inferred by the lowest standard solution detected. The recovery of this method was within the range 83.5–96.2%. More detailed descriptions of the analytical methods are given in the Supplementary Material. 2.5. Desorption and regeneration The adsorbed EE2 was desorbed with methanol, ethanol, cyclohexane, acetone, toluene, or chloroform. The adsorbents collected from the suspension by magnetic separation were re-dispersed into 5 ml of the solvent. After shaking for 30 min at room temperature, the concentration of EE2 in the supernatant was measured. Then, the adsorbents were separated and washed with ultrapure water three times. The desorption efficiency of EE2 was calculated as the ratio of the amount of desorbed EE2 to the amount of EE2 initially adsorbed. To test the reusability of the absorbent, the adsorption–desorption process was repeated six times. 3. Result and discussion 3.1. Characterization of adsorbent material The phase structure of the final product was investigated using XRD. As shown in Fig. 1a, the characteristic diffraction peaks for Fe3 O4 match well with the standard XRD data for magnetite (PDF card, NO. 75-0033). In the Raman spectrum of the synthesized adsorbent (Fig. 1b), the two strong peaks at 1327.9 cm−1 (D band) and 1590.46 cm−1 (G band) are due to the graphitic sheets, and the ID: IG (1.65) ratio can be used to estimate the
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3
Fig. 1. XRD spectrum (a) Raman spectrum (b) magnetization hysteresis loop (c), X-ray photoelectron spectrum (d) FT-IR pattern (e) BET analysis (f) of rGO/Fe3 O4 .
thickness of the rGO layer [23,24]. The field-dependent magnetization is measured by a vibrating sample magnetometer at room temperature, as shown in Fig. 1c, where the significant hysteresis loops in the S-shaped curves indicate the ferromagnetic behaviors of the materials. The saturation magnetization is calculated to be 58.6 emu g−1 . The chemical state of the elements in composite material is further investigated by XPS. The wide scan X-ray photoelectron spectrum (Fig. 1d) of composite material shows photoelectron lines at binding energies of ∼285, 530, and 711 eV, which are attributed to C1s, O1s, and Fe2p, respectively. The peaks for Fe2p3/2 and Fe2p1/2 are located at 711.27 and 724.74 eV, which is in accordance with the reported data for the Fe3 O4 phase in the composites [17]. The FT-IR of the composite material detected the relatively strong peaks of 3374 cm−1 , 1560 cm−1 , 1201 cm−1 and 585 cm−1 (as shown in Fig. 1e), which should be from the stretching vibration of O–H, C=O, and C–O of rGO, and Fe–O of Fe3 O4 , respectively [18], indicating the abundant functional groups and the co-existence of rGO and Fe3 O4 . N2 adsorption–desorption isotherms of composite material (Fig. 1f) referred to the type IV hysteresis hoop (IUPAC). The specific surface area of composite material was calculated to be 95.34 m2 g−1 by BET analysis. The morphology of the rGO/Fe3 O4 composite was characterized using SEM and TEM. It can be observed from the SEM images (Fig. 2a and b) that the particles are roughly anchored on the folded sheet structure. The TEM images further confirmed that
nanoparticles are dispersed on the crumpled wave-like graphene (Fig. 2c and d), and these nanoparticles were demonstrated to be iron oxides by energy-dispersive X-ray analysis (inset in Fig. 2c). Taking these data into account, we can conclude the coexistence of rGO and Fe3 O4 nanoparticles in all composites. High-resolution TEM (HRTEM) revealed that Fe3 O4 nanoparticles are aligned almost parallel to the regular lattice fringes with a d-spacing of 0.25 nm (Fig. 2e and f), corresponding to the (311) planes of a cubic crystal structured iron oxide [24]. The particle size distribution of the nanocomposite material was shown in Fig. S1, and the average particle size was calculated to be 381.19 nm. These results indicate that the product is the expected magnetic nanocomposite rGO/Fe3 O4 with a uniform distribution of Fe3 O4 nanoparticles on the rGO sheets. 3.2. Adsorption kinetics The contact time between the adsorbent and adsorbate is a vital parameter for evaluating adsorption efficiency. The absorption of EE2 from the liquid phase versus contact time is shown in Fig. 3. The absorption increases quickly at the beginning, and increasing contact time is beneficial for the adsorption of EE2 on the rGO/Fe3 O4 . The adsorption process reaches equilibrium at 6 h. Two empirical kinetic models, i.e., pseudo-first-order and pseudo second-order kinetics, were used to analyze the adsorption
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Fig. 2. SEM (a, b), TEM (c, d), and HRTEM (e, f) images of rGO/Fe3 O4 .
behaviors. For the pseudo-first-order kinetic equation, the amount of adsorbed EE2 is given by [12]:
qt = qe 1 − e−k1 t
(2) g−1 )
where qe and qt (mg are the amounts of the adsorbed EE2 at equilibrium and after a certain period of time (h), respectively. k1 (h−1 ) is the rate constant of the pseudo-first-order adsorption. The pseudo-second-order kinetic model can be expressed as:
qt =
Fig. 3. Pseudo-first-order and pseudo-second-order kinetics for the adsorption of EE2 by rGO/Fe3 O4 (n = 3) with different initial concentrations. Adsorbent dosage: 5 mg rGO/Fe3 O4 , volume of solvent: 50 ml, temperature: 298 K, pH: 6.
q2e k2 t 1 + qe k2 t
(3)
where qe and qt (mg g−1 ) have the same definitions as those in Eq. (2), and k2 is the pseudo-second-order rate constant at equilibrium (g mg−1 h−1 ). As shown in Fig. 3 the adsorption kinetics were investigated by fitting the pseudo-first-order kinetic model (dashed lines) and the pseudo-second-order kinetic model (solid lines). The determination coefficient (r2 ) derived from pseudo-second-order fitting is better than that for the pseudo-first-order fitting, as shown in Table S1. The experimental qe(exp) values are close to the theoretical qe(cal) values in the case of the pseudo-second-order model. Therefore, the adsorption kinetics of EE2 by rGO/Fe3 O4 are better described by a pseudo-second-order kinetic model, which implies that the
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Table 1 Comparison of the EE2 adsorption capacity of rGO/Fe3 O4 with that of other adsorbents. Adsorbent
pH
Adsorption capacity (mg g−1 )
Reference
Activated carbon Single-walled carbon nanotubes Multi-walled carbon nanotubes Polyamide 612 rGO/Fe3 O4
7.0 4.7–8.8
9.6 35.6–35.7
[27] [28]
6.3 ± 0.2
0.472
[29]
4.8–9.1 6.0
25.4 37.33
[30] Present work
Fig. 4. Langmuir adsorption isotherm and Freundlich adsorption isotherm for EE2 on rGO/Fe3 O4 (n = 3), with initial solution pH 6, temperature 288–318 K, and contact time 6 h.
adsorption of EE2 on rGO/Fe3 O4 may be dependent on the amount of the solute adsorbed on the surface of the adsorbent and as well as the amount adsorbed at equilibrium [25,26].
3.3. Adsorption isotherm To further understand the adsorption behavior, the equilibrium data were also evaluated according to the Langmuir and Freundlich isotherm models. The Langmuir model is based on the assumption of monomolecular layer adsorption and can be expressed as:
ce 1 ce = + qe kl qm qm
(4)
where ce represents the equilibrium concentration of EE2 in solution (mg l−1 ), qe is the adsorption capacity at equilibrium (mg g−1 ), and qm is the theoretical maximum adsorption capacity (mg g−1 ). kl is the Langmuir isotherm constant, which is related to the affinity of binding sites (l mg−1 ). The Freundlich equilibrium isotherm is an empirical equation (Eq. 5) that is used for the description of multilayer adsorption with interactions between the adsorbed molecules:
l nqe = ln k f +
1 l n ce n
(5)
where kf and 1/n are the Freundlich constants related to the adsorption capacity and intensity, respectively. Isotherms for EE2 adsorption on rGO/Fe3 O4 are illustrated in Fig. 4. According to the fitted data in Table S2, the adsorption behavior can be described by both Langmuir and Freundlich equations, but the former is better owing to its higher determination coefficient, especially in view of the adsorption of hydrophobic compounds [26]. The Freundlich constant (1/n) that reflects the adsorption intensity of adsorbent is between 0 and 1, suggesting a favorable adsorption process [27]. The adsorption capacity of rGO/Fe3 O4 was compared with other adsorbents reported previously, as shown in Table 1, and it can be seen that the sorption capacity of rGO/Fe3 O4 is higher than those of other adsorbents. Moreover, rGO/Fe3 O4 shows good magnetic properties and thus shows promise as a novel adsorbent for EE2 removal from water.
Fig. 5. The fitted line for lnkd versus 1/T (n = 3), with an initial EE2 concentration of 10 0 0 μg l−1 , initial solution pH of 6, 5 mg rGO/Fe3 O4 , and contact time of 6 h.
3.4. Effect of temperature and adsorption thermodynamics It is known that temperature has a significant influence on adsorption processes [30]. Therefore, the effect of temperature on the adsorption of EE2 on rGO/Fe3 O4 was investigated at 288, 298, 308, and 318 K. The specific distribution coefficient kd (l g−1 ) is defined as the ratio of equilibrium concentrations of a dissolved test substance in a two phase system, and can be calculated with:
kd =
qe ce
(6)
The thermodynamic parameters related to the adsorption process can be determined by the classical Van’t Hoff equation.
ln kd =
S o R
−
H o RT
(7)
where R is the universal gas constant (8.314 × 10−3 J mol−1 K−1 ), T is the absolute temperature, S° is the change in entropy (kJ mol−1 K−1 ), and H° is the heat of adsorption (kJ mol−1 ) at a constant temperature. The values of S° and H° were computed from the slope and intercept of the plot between lnkd versus 1/T, as shown in Fig. 5. It can be seen that the adsorption of EE2 is favored by an increase in temperature. The Gibbs free-energy change (G°) for adsorption can be determined by:
Go = −RT ln kd
(8)
The calculated values of S°, H°, and G° are shown in Table S3. The positive S° may indicate an increased randomness at the solid/liquid interface during the adsorption of EE2 on rGO/Fe3 O4 [12]. The negative G° and positive H° suggest that the adsorption process is spontaneous and endothermic. With an increase in
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ing for 6 h. The removal efficiency of EE2 increased with an increase in the solid-to-liquid ratio from 0.01 to 0.1 (see Fig. 6). This increase in adsorption capacity could be attributed to the availability of a greater number of adsorption sites. Only a slight increase in adsorption was observed when the ratio exceeded 0.1. Thus, a solid-to-liquid ratio of 0.1 was selected for all subsequent experiments.
Fig. 6. Effect of solid-to-liquid ratio on the adsorption of EE2 by rGO/Fe3 O4 at 298 K (n = 3) with an initial EE2 concentration of 10 0 0 μg l−1 , solution volume of 50 ml, and contact time of 6 h.
temperature, G° becomes more negative, implying an increase of equilibrium capacity. From the above results, EE2 adsorbtion on the as-prepared adsorbent is more favorable at higher temperatures. 3.5. Factors affecting adsorption 3.5.1. Solid-to-liquid ratio The effect of solid-to-liquid ratio was investigated by adding different amounts of rGO/Fe3 O4 to 50 ml EE2 solutions and shak-
3.5.2. Solution pH pH has a profound influence on the efficiency of adsorptionbased water treatment processes, especially those for ionizable organic chemicals [31]. Thus, the effect of initial pH on adsorption was investigated. As shown in Fig. 7a, the adsorption capacity increases from 68.03% to the peak value (90.45%) with an increase in pH from 3 to 6, followed by a slow decrease with further increase in pH. In view of the presence of aromatic rings and phenolic hydroxyl groups in EE2 and the functional groups on the rGO surface, it could be speculated that hydrophobic forces and hydrogen bonds would be the main driving force during adsorption, and that the former would be predominant [32]. However, the hydrophobic force can be weakened for ionized compounds or charged adsorbents [33]. Thus, the adsorption capacity would decrease as the solution pH moves away from the point of zero charge (pHzpc ) of the nanocomposite material [34]. The pHzpc of rGO/Fe3 O4 is around 4.78 (Fig. S2). Furthermore, when the pH is higher than 10.5, i.e., the pKa value of EE2, the EE2 and adsorbent are both negatively charged and electrostatic repulsion was expected to reduce the adsorption capacity [35]. Thus, the adsorption is favored under moderate pH conditions, and all further experiments were conducted at an initial pH of 6.
Fig. 7. Effects of water chemistry parameters (pH, coexisting ions, humic acid level, and water matrix) on the adsorption of EE2 by rGO/Fe3 O4 at 298 K (n = 3), with an initial EE2 concentration of 10 0 0 μg l−1 , adsorbent dosage of 5 mg rGO/Fe3 O4 , solution volume of 50 ml, and contact time of 6 h.
Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028
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Fig. 8. Effect of eluting solvent on the desorption efficiency (a) (n = 3) and recyclable adsorption of EE2 by rGO/Fe3 O4 using methanol as eluent (b) (n = 3). Adsorbent 5 mg rGO/Fe3 O4 , initial EE2 concentration 10 0 0 μg l−1 , volume of solvent 50 ml, temperature 298 K, pH 6, and contact time 6 h.
3.5.3. Coexisting ions The effects of four types of cations (Na+ , K+ , Ca2+ , and Mg2+ ) and three types of anions (Cl− , NO3 − , and SO4 2− ) on the adsorption were investigated. The concentration of these ions was close to that in real environmental water, as shown in Table S4. The presence of selected ions has a positive but minor effect on the adsorption capacity, as shown in Fig. 7b (88.2–94.7%). A similar situation was reported for EE2 adsorbed onto multi-walled carbon nanotubes [36]. This could be attributed to the “salting-out” effect, which is caused by the reduced solubility of organic compounds in aqueous solution [37]. 3.5.4. Humic acid Humic acid is a ubiquitous and representative natural organic matter (NOM) in aquatic environments. Thus, the effect of humic acid addition on adsorption was explored. As seen in Fig. 7c, the accumulated presence of humic acid significantly inhibits the adsorption of EE2 on rGO/Fe3 O4 . This negative effect may be due to direct competitive adsorption or physical pore blockage [38]. 3.5.5. Water matrix interferences Four different natural water samples spiked with EE2 were used to investigate the actual application of rGO/Fe3 O4 to real-world samples. Drinking water, spring water, lake water, and river water were collected from the laboratory tap, Yuelu spring, Dongting Lake, and Xiangjiang River, respectively. The basic information and chemical parameters of the natural water samples are given in Table S5. The prepared rGO/Fe3 O4 was used as an absorbent to remove EE2 from spiked environmental water samples (50 ml each). As show in Fig. 7d, the adsorption capacities of EE2 are comparable for all water samples, even in the presence of humic acid. These results demonstrate that the adsorption ability of rGO/Fe3 O4 is not affected dramatically by natural water matrices. 3.6. Desorption and regeneration The reusability of an adsorbent is considered to have a great cost benefit for practical applications. Six different solvents including methanol, ethanol, cyclohexane, acetone, toluene, and chloroform were used as eluents for desorption of EE2 from rGO/Fe3 O4 . The quantitative desorption efficiency by methanol is 95.4% (Fig. 8a), followed by that of ethanol (88.3%). The regeneration and reuse of rGO/Fe3 O4 were evaluated and the results are presented in Fig. 8b. The adsorption capacity remains above 80%, even after five cycles.
4. Conclusion The magnetic nanocomposite rGO/Fe3 O4 featuring a uniform distribution of Fe3 O4 nanoparticles on rGO sheets was prepared and characterized by XRD, Raman spectroscopy, VSM, XPS, FT-IR, BET, SEM, and TEM. The adsorption of EE2 onto rGO/Fe3 O4 followed pseudo-second-order kinetics, while the adsorption isotherm was well described by the Langmuir model. In addition, thermodynamic studies showed that the adsorption is spontaneous and endothermic, thus allowing for higher removal efficiency even at elevated temperatures. Water chemistry, including solution pH and humic acid level, had a profound influence on the adsorption, while, coexisting ions and the water matrix had little impact. The prepared rGO/Fe3 O4 showed good absorption efficiency even after five regeneration and reuse cycles, indicating its promise for practical applications in environmental remediation. Acknowledgment This study was supported by the Special Fund for Agro-scientific Research in the Public Interest (No. 201503108). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.09.028. References [1] Schroeder LM, Blackwell B, Klein D, Morse AN. Rate Uptake of three common pharmaceuticals in celery, apium graveolens. Water Air Soil Pollut 2015;226:123. [2] Dzieweczynski TL, Hebert OL. The effects of short-term exposure to an endocrine disrupter on behavioral consistency in male juvenile and adult Siamese fighting fish. Arch Environ Contam Toxicol 2013;64:316–26. [3] Hamid H, Eskicioglu C. Fate of estrogenic hormones in wastewater and sludge treatment: a review of properties and analytical detection techniques in sludge matrix. Water Res 2012;46:5813–33. [4] Aris AZ, Shamsuddin AS, Praveena SM. Occurrence of 17α -ethinylestradiol (EE2) in the environment and effect on exposed biota: a review. Environ Int 2014;69:104–19. [5] Ying G, Kookana RS, Ru Y. Occurrence and fate of hormone steroids in the environment. Environ Int 2002;28:545–51. [6] Bradley PM, Battaglin WA, Iwanowicz LR, Clark JM, Journey CA. Aerobic biodegradation potential of endocrine-disrupting chemicals in surface-water sediment at Rocky Mountain National Park, USA. Environ Toxicol Chem 2016;35:1087–96. [7] Yang X, Flowers RC, Weinberg HS, Singer PC. Occurrence and removal of pharmaceuticals and personal care products (PPCPs) in an advanced wastewater reclamation plant. Water Res 2011;45:5218–28. [8] Pessoa GP, de Souza NC, Vidal CB, Alves JA, Firmino PI, Nascimento RF, et al. Occurrence and removal of estrogens in Brazilian wastewater treatment plants. Sci Total Environ 2014;490:288–95.
Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028
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Please cite this article as: Z. Luo et al., Adsorption of 17α -ethinylestradiol from aqueous solution onto a reduced graphene oxide-magnetic composite, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.028