Adsorption of 17β-Estradiol (E 2 ) and Pb(II) on Fe 3 O 4 /Graphene Oxide (Fe 3 O 4 /GO) Nanocomposites

Abstract

A novel solvothermal method for the deposition of Fe₃O₄ nanoparticles on the surface of graphene oxide (GO) with different ratios was developed. Synthesized Fe₃O₄/GO nanocomposites were characterized using a transmission electron microscope, X-ray diffraction analyzer, X-ray photoelectron spectroscope, vibrating sample magnetometer, and nitrogen adsorption/desorption test. Results of characterization showed that Fe₃O₄ nanoparticles were homogeneously deposited onto the surface of GO, and the diameters of Fe₃O₄ nanoparticles of 1:1 Fe₃O₄/GO nanocomposites and 2:1 Fe₃O₄/GO nanocomposites were ∼4 and ∼6 nm, respectively. Saturation magnetizations reached 9.58 and 15.14 emu/g for 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO, respectively. Adsorption experiments were carried out subsequently to investigate the adsorption kinetics and isotherms of 17β-estradiol (E₂) and Pb(II) onto Fe₃O₄/GO. In addition, effects of pH on the adsorption of E₂ and Pb(II) were also studied. Most interestingly, synergistic adsorption between E₂ and Pb(II) was found when different ratios of E₂ and Pb(II) were used. In the mixture, adsorption capacity of 10 mg/L of Pb(II) was increased from 14.8 to 20.8 mg/g with the increasing concentration of E₂. The possible reason for explanation was discussed.

Introduction

Pollution of water resources, especially the contaminations with endocrine disrupting chemicals (EDCs) and heavy metals, is a global problem and has drawn worldwide attention (Zhang and Zhou, 2008; Zhang et al., 2011). 17β-Estradiol (E₂), a sex hormone, is frequently found in wastewater and effluent of wastewater treatment plant (Zhang and Zhou, 2008). E₂ can be thousands of times more potent than other EDCs, such as nonylphenol (Zhao et al., 2008). Pb(II), one of the most representative and commonly found heavy metals, is widespread in the wastewater from industries of battery making, dying, and metallurgy (Zhao et al., 2011). It can cause serious damages to the kidney, nervous or mental system (Deng et al., 2010).

Approaches to remove these two types of pollutants separately in water, such as ion exchange, chemical precipitation, photodegradation, adsorption, oxidation, and chelation, have been extensively investigated (Zhang and Zhou, 2005, 2008; Zhao et al., 2008; Fu and Wang, 2011; Stalter et al., 2011; Madadrang et al., 2012; Zhang et al., 2012a). However, industrial wastewater is complex, and when multiple pollutants are present, adsorption shows great advantage owing to its easy use and low cost. To our knowledge, heavy metals and organic pollutants could coexist extensively in wastewater. As in here, we chose E₂ and Pb (II) to represent EDCs and heavy metals because of their worldwide existence and serious damage. Some studies investigated the interactive adsorption mechanisms among different kinds of heavy metal ions, and others studied the co-existence effects of different types of EDCs (Pan and Xing, 2010; Machida et al., 2012). However, the simultaneous removal of heavy metals and EDCs through adsorption is rarely studied (Chen et al., 2008).

Graphene oxide (GO) is a novel and versatile carbon nanomaterial. It has strong mechanical strength, excellent thermal conductivity, and extremely high specific surface area (Loh et al., 2010; Novoselov et al., 2012). In addition to the high adsorption capacity and chemical stability, the abundance of carboxyl/carbonyl, hydroxyl, and epoxy groups on GO can also promote the adsorption through functionalization (Dreyer et al., 2010; Zhao et al., 2011). However, it is difficult to collect GO by centrifugation, precipitation, or filtration because of its small particle size. With the introduction of magnetic separation technology, the use of GO as a promising adsorbent in aqueous environment is revitalized (Dreyer et al., 2010; Sun et al., 2011). Magnetic separation technology also makes the recycle of GO a reality, which cuts cost of adsorbents (Sun et al., 2011). Many researchers have studied and optimized the adsorption property of GO materials (Chen et al., 2012). He et al. combined Fe₃O₄ nanoparticles with GO by covalent bonding, and the adsorption capacity for methylene blue and neutral red cationic dyes reached 190.14 and 140.79 mg/g, respectively (He et al., 2010). By using facile thermodecomposition method, Zhu et al. (2011) synthesized a type of magnetic graphene nanocomposites decorated with core/double-shell nanoparticles. These nanocomposites showed fast and strong adsorption of Cr (VI). In spite of the progress, there is still a need to develop a simpler, milder, and environment friendly method for the preparation and modification of magnetic grapheme oxide (MGO).

This report presents a simpler and milder solvothermal method for the synthesis of Fe₃O₄/GO nanocomposites, and the adsorption of E₂ and Pb(II) separately or as a mixture was carefully investigated.

Experimental

Reagents

GO was purchased from Nanjing XFNANO Materials Tech Co., Ltd. (China). Octanol, octylamine, and Pb(NO₃)₂ were obtained from Sichuan Chemical Co., Ltd. Fe(acac)₃ (99.9% purity) was purchased from Sigma-Aldrich. E₂ was obtained from Alfa Aesar. Methanol was obtained from Tedia. All reagents were of analytical grade and were used without further purification.

Synthesis of Fe₃O₄/GO nanocomposites

Fe(acac)₃ (0.3532 g) and GO (0.1766 g) were dissolved in a mixture of octylamine (10.0 mL) and octanol (24.0 mL). The solution was transferred into a Teflon-lined autoclave. After that, the autoclave was sealed and heated to 110°C for 1 h to remove the trace of oxygen and moisture, and then maintained at 240°C for 2 h in an oven. After reaction, the autoclave was cooled to room temperature and 30 mL ethanol was added to the black-colored mixture. The black precipitates were separated by a commercial magnet. After washing with ethanol for several times and drying, 2:1 Fe₃O₄/GO nanocomposites were prepared. Through the same process, 1:1 Fe₃O₄/GO nanocomposites were obtained by changing the mass ratio of Fe(acac)₃ and GO to 1:1.

Characterization of Fe₃O₄/GO nanocomposites

A transmission electron microscope (TEM) from Hitachi (model H-800) was used to observe the morphological characteristics of Fe₃O₄/GO nanocomposites.

X-ray diffraction (XRD) analysis was performed using a Rigaku D/max-2400 X-ray Spectrometer with Ni-filtered Cu Kα radiation. The XRD analyzer was operated at 40 kV and 100 mA. The scan speed was set at 2° min⁻¹, and the range of 2 theta was set from 20 to 80.

A PHI Quantera X-ray photoelectron spectroscope (XPS) using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) was used to further analyze the chemical composition of these nanocomposites.

The magnetic properties of Fe₃O₄/GO nanocomposites were tested by using a LakeShore-7304 vibrating sample magnetometer (VSM) at room temperature.

An ASAP2010 gas sorption analyzer (Micromeritics) was hired to conduct nitrogen adsorption/desorption test. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) equation.

General procedure of adsorption study

A stock solution of 100 mg/L of E₂ or Pb(II) was prepared with methanol and diluted to different concentrations with deionized water when necessary (Zaib et al., 2012; Lin et al., 2013). Experiments were conducted in an air-bath shaker, which was stirred at 30°C with a speed of 180 rpm. After adsorption, Fe₃O₄/GO nanocomposites were separated by a magnet. All parallel (in duplicate) and control experiments were conducted under the same conditions.

Concentration of E₂ was detected with high-performance liquid chromatography (HPLC; Agilent LC1200). The mobile phase used for HPLC was a mixture of methanol and water (70:30, v/v) with a flow rate of 1.0 mL/min. The UV detector was operated at 200 nm. The concentration of Pb(II) was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, IRIS-advantages; Thermo Fisher Scientific Co., Ltd.).

Adsorption experiments were carried out to investigate the adsorption kinetics and isotherms of the adsorption of E₂ and Pb(II) onto Fe₃O₄/GO. The effect of pH on adsorption was also investigated. Furthermore, the effect of different concentrations of E₂ and Pb(II) on the adsorption of Pb(II) and E₂ was studied, respectively. Except for the adsorption kinetics and equilibrium time determination, all experiments were conducted with the same dosage of 0.5 g/L of 2:1 Fe₃O₄/GO and sampled at equilibrium time. The experimental procedure and models used for the determination of equilibrium time, adsorption kinetics, and isotherms are listed as follows.

Equilibrium time determination

To determine the equilibrium time of adsorption, 0.1–1 g/L 1:1 Fe₃O₄/GO adsorbents and 2:1 Fe₃O₄/GO adsorbents were added into 10 mg/L E₂ solution followed by sampling at a time of 0.5, 1, 2, 4, 6, 8, 10, and 12 h. With the same conditions and processes, 0.5 g/L 1:1 and 2:1 Fe₃O₄/GO adsorbents were added into 10 mg/L Pb(II) solution. The adsorption amount Q_t of E₂ or Pb(II) was calculated by Equation (1) as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm Q } _ { \rm t } = \frac { ( C_0 - C_ { \rm t } ) V } { m } \tag { 1 } \end{align*} \end{document}

where C₀ and C_t are, respectively, initial concentration and concentration at time t (mg/L), V is the solution volume of E₂ or Pb(II) (L), and m is the adsorbent dosage (g).

Adsorption kinetics

With pseudo-first-order and pseudo-second-order reaction model, as shown in Equation (2) and (3), adsorption kinetics of E₂ and Pb(II) onto Fe₃O₄/GO were calculated. The kinetics models are written as follows (Deng et al., 2010; Feng et al., 2010; Koç et al., 2011): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\log ( { \rm Q}_{ \rm e} - { \rm Q}_{ \rm t} ) = \log \ { \rm Q}_{ \rm e} - k_1 t / 2.303 \tag{2}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}t / { \rm Q}_{ \rm t} = 1 / k_2 { \rm Q}_{ \rm e}^2 + t / { \rm Q}_{ \rm e} \tag{3}\end{align*} \end{document}

where Q_e and Q_t are equilibrium adsorption capacity and adsorption amount at time t (mg/g), k₁ and k₂ are adsorption rate constants (h⁻¹ and g/[mg·h]), and t is adsorption time (h).

Adsorption isotherms

To determine adsorption isotherms, initial concentrations of E₂ and Pb(II) solution were, respectively, set at 5–10 mg/L and 10–60 mg/L (Huang et al., 2011; Zhang et al., 2012b). The model of Langmuir and Freundlich adsorption isotherms (Huang et al., 2011; Joseph et al., 2011; Zhang et al., 2012a), as shown in Equation (4) and (5), was used to investigate the adsorption mechanism of E₂ and Pb(II) on Fe₃O₄/GO, respectively. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}1 / { \rm Q}_{ \rm e} = 1 / { \rm Q}_{ \max} + 1 / ( K_{ \rm L} \ { \rm Q}_{ \max} \ C_{ \rm e} ) \tag{4}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\log \ { \rm Q}_{ \rm e} = 1 / n \log \ C_{ \rm e} + \log \ K_{ \rm F} \tag{5}\end{align*} \end{document}

where Q_e and Q_max are equilibrium adsorption capacity and monolayer adsorption saturation capacity (mg/g), C_e is equilibrium concentration (mg/L), K_L is Langmuir adsorption constant (L/mg), K_F is Freundlich adsorption constant (mg^1–1/n L^1/n/g), and 1/n is the heterogeneity factor.

Investigation of pH effect and synergistic effect

To investigate the effect of pH on the adsorption by Fe₃O₄/GO adsorbents, the adsorption at various pHs was determined. By titrating with 0.1 M HCl or 0.1 M NaOH, initial pHs were, respectively, set at 3, 5, 7, 9, and 11. Initial concentrations of E₂ and Pb(II) were 10 mg/L.

To investigate the effect of Pb(II) concentration on the adsorption of E₂, mixture of 10 mg/L of E₂ and 10–60 mg/L of Pb(II) were prepared with an equal volume, respectively. All these experiments were conducted at neutral pH. Under the same conditions, the effect of 5–10 mg/L of E₂ on the adsorption of 10 mg/L of Pb(II) was also investigated.

Desorption and regeneration

After batch adsorption, desorption and regeneration experiments were conducted to investigate the reusability of Fe₃O₄/GO adsorbents. The same adsorption and desorption process was repeated five times with the same Fe₃O₄/GO nanocomposites. Desorption process was started by washing adsorbents with ethanol and deionized water for several times. Then, 50 mL of 0.1 M NaOH was sequentially mixed with adsorbents and transferred into an air-bath shaker. The mixture was shaken at a speed of 150 rpm for 10 h at 30°C to thoroughly complete the desorption process.

Results and Discussion

Characterization of Fe₃O₄/GO nanocomposites

Morphology of Fe₃O₄ nanoparticles on GO was examined by TEM under different magnifications. The Fe₃O₄ nanoparticles are dispersed homogeneously on the surface of GO, as shown in Fig. 1a and c. According to Fig. 1b and d, the diameters of Fe₃O₄ nanoparticles of 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO are 3.98 and 6.11 nm, respectively. The insert photographs in Fig. 1b and d are selected-area electron diffraction of Fe₃O₄ nanoparticles of both nanocomposites. The distinct rings caused by Fe₃O₄ nanoparticles in both nanocomposites indicate their polycrystal nature (Huiqun et al., 2006; Shen et al., 2010).

FIG. 1.

TEM images of 1:1 Fe₃O₄/GO (a, b) and 2:1 Fe₃O₄/GO (c, d) under different magnifications. Insert photographs are selected-area electron diffraction of accompanying Fe₃O₄ nanoparticles.

XRD and XPS were employed to identify the specific nanoparticles anchored on the surface of GO. Figure 2a–c show the structure information of 2:1 Fe₃O₄/GO, 1:1 Fe₃O₄/GO and pure Fe₃O₄ nanoparticles, respectively. Compared with the pure Fe₃O₄ nanoparticles, reflections of (220), (311), (400), (511), and (440) caused by these two as-prepared Fe₃O₄/GO nanocomposites are exactly the same as those caused by pure Fe₃O₄ nanoparticles (JCPDS card No. 19-0629). It was confirmed that Fe₃O₄ nanoparticles were successfully deposited onto the surface of GO. Moreover, the graphite structure in GO was identified by the intensity peak of (002) at 23.0^o (Deng et al., 2010; Fan et al., 2010). By using Debye–Scherrer formula, the calculated crystal particle sizes of Fe₃O₄ nanoparticles of 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO were 4.15 and 5.91 nm, respectively, which were remarkably consistent with the sizes determined by TEM. XPS was used to further verify the structure and composition of these two nanocomposites. Figure 3a and b show full scan spectra of 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO, in which the intensity peaks of C 1s, O 1s, and Fe 2p are acquired at binding energies of approximately 284.7, 530.3, and 711.2 eV, respectively. The peaks of Fe 2p 1/2 and Fe 2p 3/2 of both as-prepared nanocomposites at 724.7 and 711.1 eV, as shown in the insert plots of Fig. 3, are consistent with the peaks of Fe 2p of Fe₃O₄ (Tian et al., 2011). The deconvolution of O 1s, as shown in Fig. 3c and d, reveals three peaks at 533.3, 531.2, and 530.2 eV, which correspond to the carboxyl group and carbonyl group in GO and oxygen in Fe₃O₄, respectively (Wu and Xu, 2005; Yang et al., 2009). Thus, it could be further proved that the sites of epoxy/hydroxy groups in GO were mostly replaced by Fe₃O₄ nanoparticles, and these nanoparticles were deposited on the surface of GO.

FIG. 2.

XRD patterns of (a) 2:1 Fe₃O₄/GO, (b) 1:1 Fe₃O₄/GO, and (c) pure Fe₃O₄ nanoparticles.

FIG. 3.

XPS spectra of 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO. (a, b) are full scan spectra of 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO; (c, d) are O 1s spectra of 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO; the insert plots are partially enlarged drawings of Fe 2p spectra.

Magnetic hysteresis curves from VSM in Supplementary Fig. S1 show that the saturation magnetizations of 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO are 9.58 and 15.14 emu/g, respectively. With superparamagnetic property, both nanocomposites also presented no coercivity and remanence at room temperature. Furthermore, an easy separation of nanocomposites from wastewater can be realized by using a commercial magnet in a few seconds. Thus, both nanocomposites can potentially be used in wastewater treatment.

BET surface area of GO, 1:1 Fe₃O₄/GO, and 2:1 Fe₃O₄/GO was calculated to be 171.41, 238.35, and 203.72 m²/g, respectively.

Equilibrium time of adsorption

Time courses of adsorption of E₂ by 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO are presented in Supplementary Fig. S2a and b. Supplementary Figure S2c shows the time course of adsorption of Pb(II) by 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO adsorbents. The adsorption rate is extremely high during the first 2 h, because of the abundance of adsorbates and active sites on Fe₃O₄/GO. With increased incubation time, the adsorption rate becomes constant after 10 h. Therefore, it can be considered that the adsorption reached equilibrium after 10 h. Compared with the adsorption of E₂ by granular activated carbon, which showed an equilibrium time of 125 h, the adsorption by Fe₃O₄/GO nanocomposites was much faster (Zhang and Zhou, 2005). Moreover, other researchers have reported that adsorption of Pb(II) and E₂ by functionalized graphene and molecularly imprinted polymers could reach equilibrium within 30–40 min (Deng et al., 2010; Wang et al., 2011).

In this research, adsorption capacity of E₂ by 1:1 and 2:1 Fe₃O₄/GO with a dosage of 0.1–1.0 g/L was 8.08–19.48 mg/g and 7.57–16.05 mg/g, respectively. It indicated that the 1:1 Fe₃O₄/GO nanocomposites showed a slight advantage in adsorption capacity. When the dosage was increased, the advantage was retained. However, the adsorption capacity of Pb(II) by 2:1 Fe₃O₄/GO adsorbents was higher than that of 1:1 Fe₃O₄/GO. In addition, the 2:1 Fe₃O₄/GO adsorbents had much higher saturation magnetization so that they could be separated by a magnet more conveniently. Therefore, 2:1 Fe₃O₄/GO adsorbents were a better choice and were used for investigation in the following experiments, excluding adsorption kinetics.

Adsorption kinetics

According to the models and experimental results of Q_t at different time, the plots of (a–c) and (A–C) in Supplementary Fig. S3 are pseudo-first order and pseudo-second-order models describing adsorption kinetics for E₂ and Pb(II) by Fe₃O₄/GO nanocomposites, respectively. The linear regression coefficient (R²), Q_e, k₁ and k₂ were obtained from the linear correlation, slope, and intercept of each plot. Specific outcomes are listed in Supplementary Tables S1 and S2.

The R² of pseudo-first-order model (0.9419–0.9951) and R² of pseudo-second-order model (0.9796–0.9999) are high, indicating the models are fitting to describe the adsorption kinetics. Moreover, the calculated Q_e from pseudo-second-order model was in accordance with the experimental equilibrium adsorption capacity (Q_exp) in most cases. In contrast, Q_e obtained from pseudo-first-order model failed to agree with Q_exp, except for the adsorption of E₂ by 2:1 Fe₃O₄/GO at the dosage of 0.1 g/L. Therefore, the pseudo-second-order model is better used to describe the adsorption kinetics, which is also supported by other reports on the better fitting of pseudo-second-order model (Yu et al., 2011; Ji et al., 2012; Zaib et al., 2012).

Adsorption isotherms

With Q_e and C_e representing different initial concentrations of E₂ or Pb(II) solution, plots of 1/Q_e versus 1/C_e and logQ_e versus logC_e are shown in Supplementary Fig. S4, respectively. R², Q_max, K_L, K_F, and 1/n were obtained from the linear correlation, slope, and intercept of each plot. Specific information is listed in Supplementary Table S3.

Both adsorption isotherms can be fitted with high R² (0.9777–0.9994). However, fitting with Freundlich adsorption, isotherms showed higher R² (>0.9939) than that with Langmuir adsorption isotherms (R²>0.9777), indicating better fitting with Freundlich adsorption isotherms. Moreover, the calculated parameters of 1/n (lower than 1) further revealed a favorable and normal process fitting with Freundlich adsorption isotherms model (Mi et al., 2012).

Based on the experiment results and Langmuir adsorption isotherms model, the experimental and maximum adsorption capacity of E₂ and Pb(II) determined in this study was compared with other adsorbents from previous research (Table 1).

Table 1.

Adsorption Capacity of E₂ and Pb(II) on Various Matrixes

Pollutants	Adsorbents	Q_exp (mg/g)	Q_max (mg/g)	Ref.
E₂	SWNTs	26.7 (NA)		Zaib et al. (2012)
	t-SWNTs	26.9 (NA)		Zaib et al. (2012)
	Molecularly imprinted polymer		96.75 (0.28 mg/m²)	Grace et al. (2010)
	Nonimprinted polymer		19.98 (0.11 mg/m²)	Grace et al. (2010)
	Powdered activated carbon		44.1 (0.10 mg/m²)	Grace et al. (2010)
	Fe₃O₄/GO (2:1)	19.48 (0.10 mg/m²)	86.96 (0.43 mg/m²)	Present work
	GO	8.08 (0.05 mg/m²)		Present work
Pb(II)	SiO₂/graphene composite		113.6 (0.45 mg/m²)	Hao et al. (2012)
	Functionalized graphene		406.6 (100 mg/m²)	Deng et al. (2010)
	Magnetic chitosan/graphene oxide		76.94 (0.08 mg/m²)	Huang et al. (2011)
	Modified graphene nanosheets		35.46 (NA)	Wang et al. (2013)
	Fe₃O₄/GO (2:1)		46.95 (0.23 mg/m²)	Present work
	GO	9.72 (0.06 mg/m²)		Present work

NA, not available.

Fe₃O₄/GO nanocomposites showed much higher adsorption capacity for E₂, with Q_max of 86.96 mg/g and Q_exp of 19.48 mg/g, compared with other adsorbents. GO surface offers strong π-π interactions and is highly negatively charged, which contributes to the strong ability to adsorb E₂ and Pb(II) (Fan et al., 2013). To better explain the effect of Fe₃O₄ nanoparticles from Fe₃O₄/GO nanocomposites, the comparison batch adsorption experiment was conducted using 0.5 g/L of pure GO adsorbents under the same conditions. After 10 h, the adsorption capacity of E₂ and Pb(II) reached 8.08 and 9.72 mg/g, respectively. It could be seen that the adsorption capacity of E₂ was decreased in order of 1:1 Fe₃O₄/GO>2:1 Fe₃O₄/GO>GO. For Fe₃O₄/GO nanocomposites, it seemed that the π-π conjugation of GO with E₂ was not affected by coating with Fe₃O₄ nanoparticles. The slight increase in E₂ adsorption may be due to the increased specific area caused by Fe₃O₄ nanoparticles compared with pure GO.

Meanwhile, the adsorption capacity of Pb(II) was decreased in order of 2:1 Fe₃O₄/GO>1:1 Fe₃O₄/GO>GO, which was negatively related to the specific surface area of these adsorbents. Moreover, the comparison among zeta potentials of GO, 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO (Supplementary Fig. S5) showed little difference at neutral condition. Therefore, the increasing of adsorption capacity had to be caused by other reasons. Compared with GO and 1:1 Fe₃O₄/GO, the inorganic interactions between Fe₃O₄ nanoparticles and Pb(II) were considered to contribute to the increase of the adsorption capacity of Pb(II) by 2:1 Fe₃O₄/GO owing to the higher loading of Fe₃O₄ nanoparticles on the surface of 2:1 Fe₃O₄/GO. Because of that, the Fe₃O₄ nanoparticles were negatively charged and Pb(II) was positively charged, electrostatic attraction was most likely to occur. In comparison, more negative charges were carried by more Fe₃O₄ nanoparticles in 2:1 Fe₃O₄/GO than 1:1 Fe₃O₄/GO. Thus, stronger electrostatic attraction occurred, which was responsible for the higher adsorption capacity of Pb(II) by 2:1 Fe₃O₄/GO. Therefore, it demonstrated that the addition of Fe₃O₄ nanoparticles onto the surface of GO was favorable for the adsorption of Pb(II). However, GO nanosheets and functionalized graphene from other studies show higher adsorption for Pb(II) than the Fe₃O₄/GO nanocomposites in Table 1. This may be due to the fact that the adsorption affinity between Fe₃O₄ nanoparticles and Pb(II) was not as strong as the chelation formed by Pb(II) and organic compounds on the surface of chitosan-gelatin/GO, EDTA/GO, and chitosan/GO (Huang et al., 2011; Madadrang et al., 2012; Zhang et al., 2011).

Effect of pH on adsorption

Supplementary Figure S6 shows the effect of pH on the adsorption of E₂ and Pb(II). Supplementary Fig. S5 shows the zeta potential of GO, 1:1 Fe₃O₄/GO, and 2:1 Fe₃O₄/GO. As we can see, the pH_zpc occurred around pH 9. The most favorable pH for the adsorption of E₂ was 7, and acidic or alkaline condition inhibited the adsorption process, which agreed with the fact that the adsorption between E₂ and Fe₃O₄/GO was mainly through π-π conjugation (Pan and Xing, 2008; Pan et al., 2008). When solution pH<pH_zpc<pKa of E₂ (about 10.4), the −COOH groups of Fe₃O₄/GO and −OH of E₂ were mutually repulsive, thus inhibiting the π-π conjugation. Furthermore, when solution pH>pKa value of E₂>pH_zpc, enormous restraint on the adsorption of E₂ would occur. Because of that, more protons given by E₂ competed with E₂ to get adsorbed (de Mes et al., 2005; Chen and Hu, 2010).

Neutral and alkaline conditions were more beneficial for the adsorption of Pb(II), which agreed with the fact that one of the most dominant adsorption mechanisms for Pb(II) was through electrostatic interaction between the negatively charged Fe₃O₄/GO and the positively charged Pb(II). Moreover, the adsorption capacity was almost the same (22.6 mg/g) at neutral and alkaline conditions. When pH<7.7 (pKa of Pb(II)), the negative charges on Fe₃O₄/GO decreased along with increasing pH, resulting in the decrease of adsorption capacity of Pb(II) (Naiya et al., 2009a, 2009b; Mi et al., 2012). When the solution was alkaline, electrostatic attraction increased the adsorption capacity of Pb(II) while precipitation might occur and decreased the concentration of the solution (Kovačević et al., 2000). As a result, little difference was observed for the adsorption capacity between neutral and alkaline conditions.

Adsorption of mixture of E₂ and Pb(II)

Figure 4 shows the effect of E₂ and Pb(II) on the adsorption of Pb(II) and E₂, respectively. The adsorption capacity of E₂ did not change with the increasing concentration of Pb(II) and varied between 6.0 and 7.3 mg/g, which was similar to the adsorption capacity (8.32 mg/g) for pure E₂, as shown in Fig. 4a. Therefore, it can be concluded that the concentration of Pb(II) has little impact on the adsorption of E₂. However, as shown in Fig. 4b, the presence of E₂ greatly promoted the adsorption of Pb(II) and the adsorption capacity of Pb(II) gradually increased from 14.8 to 20.8 mg/g with the increasing concentration of E₂. The increased capacity was higher than the adsorption capacity of Pb(II) (13.34 mg/g) in the absence of E₂. Therefore, for a mixture of 10 mg/L of E₂ and Pb(II), a higher adsorption capacity of 20.8 mg/g for Pb(II) and unaffected adsorption capacity of 7.34 mg/g for E₂ were achieved.

FIG. 4.

Effect of initial concentration of Pb(II) for (a) E₂ adsorption and the effect of initial concentration of E₂ for (b) Pb(II) adsorption on 2:1 Fe₃O₄/GO nanocomposites.

The effects of E₂ and Pb(II) on the adsorption of each other and on Fe₃O₄/GO could be explained by the different mechanisms of their adsorption. The adsorption of E₂ was mainly through π-π conjugation between Fe₃O₄/GO and E₂, and the adsorption of Pb(II) was mainly through electronic interactions. Therefore, the adsorption of E₂ was hardly affected by the presence of Pb(II). In contrast, the adsorption of E₂ promoted the adsorption of Pb(II) through either direct interaction with the adsorbed E₂ or the expanded specific surface area and active sites of adsorbents.

After adsorption, XPS analysis was carried out to clarify the previously proposed conjecture specifically. The Fe₃O₄/GO nanocomposites bearing E₂, Pb(II), and the mixture were separately tested. The XPS prectra in Fig. 5 clearly reveal the Fe 2p, O 1s, C 1s, and Pb 4f peaks after adsorption, indicating the general difference between the adsorption of E₂ and Pb(II). More specific plots of the deconvolution of O 1s and the comparison of Pb 4f are exhibited in Fig. 6. In Fig. 6a, O 1s spectra of Fe₃O₄/GO nanocomposites after the adsorption of E₂ reveal four peaks at the position of 533.30, 531.90, 531.20, and 530.2, corresponding to (1) O=C-O, (2) -OH, (3) C=O, and (4) Fe₃O₄, respectively. All the functional groups belong to Fe₃O₄/GO nanocomposites, excluding some -OH groups from E₂. It is proved that the chemical adsorption of E₂ is mainly via π-π conjugation. Figure 6b shows four O 1s peaks at the position of 533.30, 532.60, 531.20, and 530.20, in accordance with (1) O=C-O, (2) O-C-O, (3) C=O, and (4) Fe₃O₄, respectively. Except for physical adsorption, Pb(II) was proved to be adsorbed onto Fe₃O₄/GO nanocomposites by reacting with −COOH and producing O-C-O (Lin et al., 2012; Madadrang et al., 2012; Orsetti et al., 2013). As shown in Fig. 6c, five O 1s peaks at the position of 533.30, 532.60, 531.50, 531.20, and 530.20 indicate the existence of (1) O=C-O, (2) O-C-O, (3) OH⁻, (4) C=O, and (5) Fe₃O₄, respectively. The–OH groups in E₂ were considered to be reacting with Pb(II) and tending to be more negatively charged as OH⁻ (Madadrang et al., 2012). Moreover, the XPS analysis of Pb 4f spectra in Fig. 6d shows higher peaks of Pb from the mixture than that from Pb(II) alone with same concentrations, which further indicates the approval of the existence of E₂ for the adsorption of Pb(II).

FIG. 5.

XPS spectra of Fe₃O₄/GO nanocomposites after the adsorption of (A) E₂, (B) Pb(II), and (C) the mixture of E₂ and Pb(II).

FIG. 6.

XPS spectra of Fe₃O₄/GO nanocomposites after adsorption. (a–c) are O 1s spectra of Fe₃O₄/GO nanocomposites after the adsorption of E₂, Pb(II) and the mixture of E₂ and Pb(II), respectively; (d) shows the Pb 4f spectra of Fe₃O₄/GO nanocomposites after the adsorption of Pb(II) and the mixture of E₂ and Pb(II).

Because of the possible adsorption mechanisms of E₂ and Pb(II) by Fe₃O₄/GO nanocomposites as discussed, the adsorbents could be fully used for the treatment of the mixture. Moreover, Pb(II) alike contaminants can potentially be better removed by using Fe₃O₄/GO nanocomposites after the adsorption of E₂ alike organic pollutants.

Desorption and regeneration

To investigate the reusability of Fe₃O₄/GO adsorbents after adsorbing single E₂ or Pb(II) and the mixture, regeneration of 2:1 Fe₃O₄/GO nanocomposites was conducted for 10 times after the adsorption of E₂ or Pb(II). Results are presented in Supplementary Fig. S7. The adsorption capacity of Fe₃O₄/GO nanocomposites for E₂ and Pb(II) was 87.0% and 88.0%, respectively, after three recycle times. After recycling for five times, the adsorption capacity of Fe₃O₄/GO nanocomposites for Pb(II) and E₂ was still 75% and 71%, respectively. And after recycling for 10 times, the recycling adsorption capacity reached 63% and 60% for Pb(II) and E₂, respectively. In the mixture, the adsorption capacity separately reached 79% and 66% for Pb(II) and E₂ after recycling for 10 times. The saturation magnetizations of 2:1 Fe₃O₄/GO were also tested to investigate its magnetic property after the recycling of adsorption-desorption of the mixture of E₂ and Pb(II). Results are presented in Supplementary Fig. S8. The data showed no obvious decrease of saturation magnetizations after 5 times (92%) and 10 times recycling (88%), indicating the excellent potential for application.

Conclusions

In summary, Fe₃O₄/GO nanocomposites at the ratios of 1:1 and 2:1 were successfully prepared by a facile solvothermal synthesis method. The Fe₃O₄ nanoparticles were homogeneously anchored on the surface of GO, and the diameters were ∼4 and ∼6 nm for 1:1 Fe₃O₄/GO and 2:1 Fe₃O₄/GO, respectively. Both nanocomposites were superparamagnetic without coercivity and remanence at room temperature. With saturation magnetizations of 9.58 emu/g for 1:1 Fe₃O₄/GO and 15.14 emu/g for 2:1 Fe₃O₄/GO, both adsorbents could be easily separated through an external magnetic field. Batch adsorption experiments showed better fitting for pseudo-second-order adsorption kinetics model and agreed better with Freundlich adsorption isotherms in a single system. Moreover, the adsorption was found to be pH dependent and the optimal pH for the adsorption of E₂ and Pb(II) was neutral. We also demonstrated that the presence of Pb(II) showed little influence on the adsorption of E₂. However, the adsorption capacity of Pb(II) was increased with increasing the concentration of E₂. The prepared Fe₃O₄/GO nanocomposites could be used in the treatment of wastewater, especially for the treatment of effluents carrying both EDCs and heavy metals such as E₂ and Pb(II). Moreover, Fe₃O₄/GO nanocomposites could potentially be excellent adsorbents for the removal of Pb(II) alike contaminants after the adsorption of E₂ alike organic pollutants.

Footnotes

Acknowledgments

The authors gratefully acknowledge the support provided by the National Natural Science Foundation of China (Grant No. 51308183), Natural Science Foundation of Jiangsu Province of China (Grant No. BK20130828), Major Science and Technology Program for Water Pollution Control and Treatment (Grant No. 2012ZX07103-005), and National Key Technologies R&D Program of China (Grant No. 2012BAB03B04).

Author Disclosure Statement

No competing financial interests exist.

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Supplementary Material

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Adsorption of 17β-Estradiol (E 2 ) and Pb(II) on Fe 3 O 4 /Graphene Oxide (Fe 3 O 4 /GO) Nanocomposites

Abstract

Abstract

Introduction

Experimental

Reagents

Synthesis of Fe3O4/GO nanocomposites

Characterization of Fe3O4/GO nanocomposites

General procedure of adsorption study

Equilibrium time determination

Adsorption kinetics

Adsorption isotherms

Investigation of pH effect and synergistic effect

Desorption and regeneration

Results and Discussion

Characterization of Fe3O4/GO nanocomposites

Equilibrium time of adsorption

Adsorption kinetics

Adsorption isotherms

Effect of pH on adsorption

Adsorption of mixture of E2 and Pb(II)

Desorption and regeneration

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Supplementary Material

Synthesis of Fe₃O₄/GO nanocomposites

Characterization of Fe₃O₄/GO nanocomposites

Characterization of Fe₃O₄/GO nanocomposites

Adsorption of mixture of E₂ and Pb(II)