Penglei Wangab,
Xin Zhouab,
Yagang Zhang*abc,
Lulu Wangab,
Keke Zhiab and
Yingfang Jiangab
aCenter for Green Chemistry and Organic Functional Materials Laboratory, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China. E-mail: ygzhang@ms.xjb.ac.cn; Fax: +86-991-3838957; Tel: +86-18129307169
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cDepartment of Chemical & Environmental Engineering, Xinjiang Institute of Engineering, Urumqi 830023, China
First published on 21st October 2016
Two kinds of magnetic reduced graphene oxide composites (MRGO) namely MRGO-1 and MRGO-2 with different reduction degrees are synthesized by a facile method. MRGO-2 is obtained by further reduction of MRGO-1 using hydrazine to obtain a relatively deeper reduction degree. Removal of bisphenol A (BPA) from aqueous solution using MRGO-1 and MRGO-2 is investigated. The kinetics and isotherm data of BPA absorbed by MRGO-1 and MRGO-2 are both well fitted with the pseudo-second-order kinetic model and the Langmuir isotherm, respectively. The maximum adsorption capacity of MRGO-1 and MRGO-2 for BPA obtained from the Langmuir isotherm is 92.98 mg g−1 and 71.66 mg g−1 at 288.15 K, respectively. Furthermore, the used MRGO-1 and MRGO-2 could be collected and recycled efficiently via an external magnet. The BPA adsorption capacity of the MRGO-1 still remained greater than 86% of the initial adsorption capacity after nine repeated absorption–desorption cycles. A thermodynamic study shows that the adsorption is a spontaneous and exothermic process. The maximum adsorption capacity of MRGO-1 for BPA is higher than that of MRGO-2, implying a deeper reduction process to MRGO-2 from MRGO-1 is not essential for the removal of BPA from aqueous solution. The energy is saved by omitting the deep reduction process. It is also found that if severe aggregation of graphene sheets in the deep reduction process to MRGO-2 from MRGO-1 can be alleviated, the corresponding absorption capacity of MRGO-2 for BPA may be greatly improved.
To remove BPA from polluted aqueous systems, various methods have been developed, such as adsorption,2,6–14 heterogeneous Fenton degradation,15 ozonation and catalytic ozonation,16,17 biological treatment.18 Among those, adsorption is a superior and widely used method due to the low cost, simple operation condition and high efficiency.2 A variety of carbon-based materials have been reported for the removal of BPA in aqueous solution.2,9–12 As one of the most commonly used adsorbents, activated carbon is versatile for organic pollutants removal, but it has several defects, including slow pollutant uptake19 and relatively weak removal capacity for many hydrophilic micropollutants. Furthermore, the regeneration of used activated carbon is quite energy consuming (usually requiring heating to 500–900 °C), which does not fully restore its original performance.20 As a result, other kinds of adsorbents, such as carbon nanotubes,14,21 porous carbon,22 graphene composites,2,11 have been developed recently.
Graphene is a single atomic layer of sp2-hybridized carbon arranged in a honeycomb structure. The remarkable properties of graphene, such as extremely high surface area (with a calculated value of 2630 m2 g−1),23 excellent chemical stability,24 and graphitized basal plane structure,23 leading its strong π–π interactions with the aromatic compounds. Besides, as graphene based materials, graphene oxides (GO), has also been extensively investigated to remove organic pollutants due to its large specific surface area and a variety of oxygenated functional groups. Major mechanism of organic chemicals absorbed on graphene based materials is proposed to be hydrophobic interactions, hydrogen bonds, π–π interactions, electrostatic interactions. Xu et al.2 found that the reduced graphene oxides (RGO) presented very high adsorption capacity and fast adsorption rate for BPA. Both π–π interactions and hydrogen bonds might be responsible for the adsorption of BPA on RGO.2 Cortes-Arriagada et al.10 and Zhong et al.12 also demonstrated that the π–π stacking and hydrogen bonding interaction played main role in the adsorption of BPA on GO by theoretical calculation using Density Functional Theory (DFT). GO, whose basal planes are decorated mostly with epoxide and hydroxyl groups, the edges are presumably modified by carbonyl and carboxyl groups due to the presence of oxidized graphene sheets,25,26 these oxygen-containing groups on the GO and RGO basal plate can form hydrogen bonds with hydroxyl groups of BPA.10,12,27 Whereas the graphitized basal plane structure of GO is strongly disrupted due to the presence of abundant oxygen-containing groups,26 which may reduce the π–π interactions between graphene-based materials and BPA. Although the reduction or de-oxygenation of GO result in significant restoration of the sp2 carbon sites leading to re-graphitization,28 the de-oxygenation of GO will also impair the hydrogen bonding interactions between graphene based materials and BPA. In addition, severe aggregation of graphene layers during the acute reduction process of GO will reduce its specific surface areas,2,28,29 which may decrease its absorption ability for organic pollutions. Thus, it is interesting and important to investigate the reduction process of RGO and its influence on the absorption ability for BPA. To our best of knowledge, this type of study has not been reported.
Graphene-based materials are difficult to separate from water because of their small particle size and excellent dispersibility in water,30 which makes it difficult to recycle absorbents. In the past few years, magnetic separation has emerged as a useful tool which simply involves applying an external magnetic field to collect the used adsorbents from solution. Compared with traditional methods, such as filtration, centrifugation or gravitational separation, magnetic separation is less energy consuming and can achieve better separation efficiency especially for adsorbents with small particle size. In this way, used magnetic nano adsorbents are easy to be retrieved from the reaction system under a magnetic field, and then retrieved adsorbents are regenerated for reuse. These advantages of magnetic graphene and magnetic reduced graphene oxides (MRGO) are potentially beneficial to the real applications.
In this work, MRGO were synthesized and applied to remove BPA from aqueous solutions. The objectives of this paper are (1) synthesize and characterize the MRGO (2) investigate the adsorption behaviour of MRGO with different reduction degree for the removal of BPA in aqueous solutions by controlling different experimental conditions such as the absorption time, BPA concentration, temperature, pH (3) compare and evaluate the adsorption capacity of MRGO with different reduction degree for the removal of BPA in aqueous solution (4) propose possible adsorption mechanism of BPA on MRGO through adsorption kinetics, isotherms, and thermodynamic studies.
In order to obtain MRGO with deeper reduction degree, the above described process was slightly modified, the aqueous solution of 5.4 g FeCl3·6H2O and 2.8 g FeSO4·7H2O was added dropwise to GO solution, the other part of the process was all the same. At last, 10 mL of hydrazine hydrate (80%) was added under constant stirring for 4 h at 90 °C. Other protocols were the same as the above methods. The products obtained were labelled as MRGO-2.
![]() | (1) |
Adsorption kinetic study was carried out with an initial BPA concentration of 50 mg L−1 at 288.15 K, pH 6.0 to determine the minimum time required to reach equilibrium concentration of BPA in uptake experiment. The concentrations of BPA were measured at different time intervals from 5 min to 300 min. Adsorption isotherms of different adsorbents (MRGO-1 and MRGO-2) were obtained at 288.15 K with initial BPA concentrations varied from 10 to 120 mg L−1 and initial pH of 6.0. To evaluate the thermodynamic properties of BPA by MRGO-1, absorption isotherms of MRGO-1 for BPA were also obtained at 313.15 K and 323.15 K (at pH = 6.0), respectively. The effect of pH on the adsorption of BPA using MRGO was investigated at different pH values varied from 3 to 10 with an initial BPA concentration of 50 mg L−1 at 288.15 K.
The morphology of MRGO-1 and MRGO-2 was characterized by TEM. Fig. 3A, B, D and E show TEM images of MRGO-1 and MRGO-2. As can be seen from the TEM images of MRGO-1 and MRGO-2 Fe3O4 nanoparticles are uniformly distributed on the surface of reduced graphene oxides (RGO), and the graphene sheets show folding nature which is clearly visible. High resolution TEM (HRTEM) was also used to characterize MRGO-1 and MRGO-2. In Fig. 3C and F, the lattice fringes are 0.252 nm and 0.252 nm in MRGO-1 and MRGO-2, which correspond to the (3 1 1) lattice planes of Fe3O4. Besides, the energy dispersive X-ray spectroscopy (EDX) clearly shows the existence of iron (Fe) in the MRGO-1 and MRGO-2 composites in Fig. S1.† The EDX results also show the relative differences in element composition of MRGO-1 and MRGO-2, especially for carbon (C) and oxygen (O). The C/O ratio of MRGO-1 is less than that of MRGO-2. In summary, TEM, HRTEM and EDX indicate coexistence of Fe3O4 nanoparticles and RGOs in MRGO-1 and MRGO-2, which is well consistent with the XRD results.
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Fig. 3 TEM images (A and B) and HRTEM images (C) of MRGO-1, TEM images (D and E) and HRTEM images (F) of MRGO-2; EDX element composition data of MRGO-1 and MRGO-2 below the TEM images. |
The chemical structures of GO, MRGO-1 and MRGO-2 were investigated by FTIR and the resulting spectra is shown in Fig. 4. The FTIR spectrum of GO exhibits the characteristic peaks for O–H (3410 cm−1), CO (1729 cm−1), aromatic C
C (1631 cm−1), carboxy C–O (1405 cm−1), epoxy C–O (1217 cm−1) and alkoxy C–O (1053 cm−1).31,33,34 The FTIR spectrum of MRGO-1 exhibits the characteristic peaks for O–H (3410 cm−1), COO− (1597 cm−1 and 1392 cm−1), C
C (1552 cm−1), epoxy C–O (1181 cm−1) and alkoxy C–O (1053 cm−1). Obvious decrease in the intensity of the characteristic absorption bands of oxygen functionalities compared to GO, suggesting that GO has been reduced in the preparation process of MRGO-1. Fan et al.35 also observed that exfoliated GO can be reduced under strongly alkaline conditions at moderate temperatures (50–90 °C). The FTIR spectrum of MRGO-2 shows three peaks at 3410 cm−1, 1556 cm−1 and 1181 cm−1, which correspond to the O–H stretching vibration, aromatic C
C stretch and C–O stretch. The band at 584 cm−1 is attributed to Fe–O.36 There is an obvious decrease for absorption bands of oxygen functionalities compared to MRGO-1, indicating the MRGO-1 is further reduced to MRGO-2.
TGA curves for MRGO-1 and MRGO-2 were measured from 25 to 800 °C in air with a heating rate of 10 °C min−1 as showed in Fig. 5. Based on the weight loss upon graphene combustion and the assumption that Fe3O4 was fully oxidized to Fe2O3 at 400 °C, the content of Fe3O4 in nanocomposites is calculated to be 62.83 wt% and 67.66 wt% for MRGO-1 and MRGO-2, respectively. Apparently, the content of reduced graphene oxides (RGO) (37.17%) in MRGO-1 is slightly higher than that of RGO (32.34%) in MRGO-2, which is due to more oxygen containing functional groups presented in MRGO-1.
Significant structural changes occurring during the reduction processing from GO to MRGO-1, and then to the MRGO-2, are also reflected in Raman spectra. The Raman spectrums show two characteristic peaks in both GO, MRGO-1 and MRGO-2 (Fig. 6). One is G band at ∼1588 cm−1, which signifies the sp2 carbon atoms in a graphitic 2D hexagonal lattice. The D band at 1341 cm−1 indicates the sp3 carbon atoms of defects and disorder.37 The intensity ratio of D/G bands (ID/IG) is generally accepted to reflect the graphitization degree of carbonaceous materials and the defect density.38 The value of ID/IG of MRGO-2 (1.21) is higher than that of MRGO-1 (1.02) and GOs (0.72), which is ascribed to the increase in the number of polyaromatic domains in reduction process.39 It also indicates decrease in the average size of the sp2 domains upon reduction of GO and MRGO-1. The possible explanation is that a certain amount of new graphitic domains are created that are smaller in size compared to the ones present in GO before reduction.28 Therefore, MRGO-1 and MRGO-2 obtained through reduction treatment result in the partially restoration of the sp2 carbon sites, which leading to the increase in degree of graphitization.
Magnetization spectrums (Fig. 7) of the MRGO-1 and MRGO-2 were measured at room temperature over the range of −20 to 20 kOe. The saturation magnetization of MRGO-1 and MRGO-2 are calculated to be 15.6 and 14.7 emu g−1, respectively, indicating a high magnetism. The magnetic hysteresis loops are S-like curves. The magnetic remanence of the MRGO-1 and MRGO-2 are 0.61863 and 0.54761 emu g−1, indicating there is almost no remaining magnetization when the external magnetic field was removed. Thus MRGO-1 and MRGO-2 exhibit superparamagnetic behavior. A simple laboratory set-up was established and conducted to evaluate the magnetic separation effect of MRGO-1, as shown in Fig. 7 (bottom inset). After adsorption, MRGO-1 was completely separated from the aqueous solution within 6 s in the presence of an external magnet.
XPS measurements were conducted to further analyze surface element composition and chemical states. XPS survey spectrums of the MRGO-1 and MRGO-2 in Fig. 8A show photoelectron lines at binding energy of 285, 530 and 711 eV which are attributed to C 1s, O 1s and Fe 2p, respectively. In the spectrum of Fe 2p (Fig. 8B), the peaks of Fe 2p3/2 and Fe 2p1/2 are showed up at 711.2 and 724.9 eV rather than at 710.35 and 724.0 eV which is evidence for γ-Fe2O3.40 This result implies successful formation of Fe3O4 phase in the RGO. The C 1s peaks of the MRGO-1 and MRGO-2 were shown in Fig. 8C and D, the peaks at 284.7, 286.2, 287.9 eV and 289 eV correspond to aromatic rings (C–C/CC), the epoxy/hydroxyls carbon (C–O/C–OH), the carbonyl carbon (C
O), and the carboxylate carbon (O–C
O),36 respectively. The quantitative analysis shows that the content of oxygen-containing carbon in the MRGO-2 is lower than that of MRGO-1, which also supports that MRGO-2 is further reduced compared to MRGO-1. These results are well consistent with FT-IR and Raman studies.
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Fig. 8 XPS spectra of GO, MRGO-1 and MRGO-2. (A) Survey spectra, high resolution (B) Fe 2p peaks of MRGO, (C) C 1s peaks of MRGO-1, (D) C 1s peaks of MRGO-2. |
The nitrogen adsorption–desorption curve was obtained to evaluate the specific surface area and pore size distribution. Fig. 9A shows the SBET of the MRGO-1 and MRGO-2 are measured as 310.04 m2 g−1 and 145.74 m2 g−1. MRGO-1 and MRGO-2 both show type IV isotherms classified by IUPAC with the distinct hysteresis loops close to H3 type feature. It further suggests that the MRGO-1 and MRGO-2 have a characteristic of typical lamellar stacking. The specific surface area of MRGO-1 and MRGO-2 are significantly lower than the theoretical surface area for isolated graphene sheets (2620 m2 g−1) is probably due to the aggregation of the GOs upon reduction.41 Compared to MRGO-1, the lower surface area of MRGO-2 may be caused by the deeper reduction process. Noticeably, such a high surface area (310.04 m2 g−1) for MRGO-1 is significantly superior to most of Fe3O4–graphene composites reported in the literature.12,29,42,43 Moreover, Fig. 9B also shows the pore size distribution of MRGO-1 and MRGO-2. The total pore volumes of MRGO-1 and MRGO-2, calculated from the nitrogen adsorption isotherm, are 0.2968 and 0.2850 cm3 g−1, respectively. The average pore diameter of MRGO-1 and MRGO-2 are 3.829 and 7.822 nm, respectively.
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Fig. 9 Nitrogen adsorption and desorption isotherms (A) and pore size distribution profile (B) of MRGO-1 and MRGO-2. |
ln(qe − qt) = ln![]() | (2) |
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Fig. 10 Effect of time on the adsorption of BPA by MRGO-1 and MRGO-2 (20 mg of adsorbent and 100 mL of 50 mg L−1 BPA at 288.15 K). |
The pseudo-second-order model includes all the steps of adsorption including external film diffusion, adsorption, and internal particle diffusion which is summarized in eqn (3).46
![]() | (3) |
Pseudo-first-order | Pseudo-second-order | |||||
---|---|---|---|---|---|---|
Adsorbents | qe (mg g−1) | k1 (1/min) | R2 | qe (mg g−1) | k2 (mg g−1 min−1) | R2 |
MRGO-1 | 35.47 | 0.06419 | 0.8736 | 60.94 | 0.00576 | 0.9997 |
MRGO-2 | 14.99 | 0.01787 | 0.8816 | 45.25 | 0.00355 | 0.9991 |
Absorbents | BPA Co (mg L−1) | Amount of absorbent (g L−1) | k2a (mg g−1 min−1) | Reference |
---|---|---|---|---|
a The adsorption rate constant from pseudo-second-order model and the unit is normalized to mg g−1 min−1. | ||||
As-grown CNTs | 10.0 | 0.125 | 0.008167 | 9 |
rGOs | 1.0 | 0.02 | 0.002183 | 12 |
Magnetic rGOs | 1.0 | 0.02 | 0.002383 | 12 |
1.0 CEC-HDTMA | 100 | NA | 0.005483 | 49 |
Activated carbon-PCB | 60 | 0.25 | 0.003030 | 50 |
Fe3O4/GO hybrids | 50 | 0.5 | 0.000941 | 47 |
MRGO-1 | 50 | 0.2 | 0.005761 | This work |
MRGO-2 | 50 | 0.2 | 0.003551 | This work |
![]() | (4) |
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Fig. 11 Adsorption isotherms of BPA by MRGO-1 and MRGO-2 at 288.15 K. The solid lines are the Langmuir model simulation; the dotted lines are the Freundlich model simulation. |
In eqn (4), qe represents the adsorbed BPA per gram of absorbent (mg g−1) at equilibrium, Ce represents the equilibrium concentration of BPA in solution (mg L−1), KL is Langmuir constant (L mg−1), which is related to the affinity of the binding sites, and qm represents the theoretical maximum adsorption capacity of the adsorbent (mg g−1).
The Freundlich model is an empirical expression, which assumes that a multilayer adsorption occurs on the heterogeneous surface or surface supporting sites of various affinities. The equation is commonly described as shown in eqn (5).52
qe = KFCe1/n | (5) |
Langmuir model | Freundlich model | |||||
---|---|---|---|---|---|---|
Absorbents | qm (mg g−1) | Ka (L mg−1) | R2 | n | KF (mg g−1) | R2 |
MRGO-1 | 98.98 | 0.05566 | 0.9951 | 2.732 | 15.29 | 0.9773 |
MRGO-2 | 71.66 | 0.05681 | 0.9913 | 12.82 | 12.29 | 0.9759 |
According to the correlation coefficient (R2) values, adsorption of BPA by MRGO-1 and MRGO-2 fitted better by the Langmuir isotherm than the Freundlich isotherm. In other words, the adsorption of BPA by MRGO-1 and MRGO-2 take place in a monolayer adsorption manner. In addition, the maximum adsorption amount of MRGO-1 and MRGO-2 for BPA at 288.15 K is 92.98 mg g−1 and 71.66 mg g−1, respectively. Clearly, the maximum adsorption amount of MRGO-1 is higher than that of MRGO-2 for BPA, which is also decent and even superior values compared to the ones using other adsorbents previously reported in the literatures summarized in Table 4. Previous literature has reported that Fe3O4 has almost no adsorption for BPA.14 It is reasonable and understandable that adsorption capacity of MRGO-1 and MRGO-2 for BPA is lower than that of graphene and SWCNTs because of the high Fe3O4 content in MRGO-1 and MRGO-2. The specific adsorption capacity of RGO in MRGO is actually higher. According to the TGA data, the content of RGO in MRGO-1 is 37.17%, the maximum adsorption amount for BPA by RGO in MRGO-1 is calculated to 250.15 mg g−1. For MRGO-2, the maximum adsorption amount of RGO in MRGO-2 is calculated to 221.58 mg g−1. The relatively higher adsorption amount of BPA on RGO in MRGO than that of reported graphene is ascribed to the more adsorption site of RGO in MRGO, because the presence of Fe3O4 is able to prevent the aggregation of graphene sheet.
Adsorbent | pH | T (K) | SBET (m2 g−1) | Max Qea (mg g−1) | Reference |
---|---|---|---|---|---|
a Maximum adsorption capacity obtained from the Langmuir model or from the adsorption capacity at the highest initial concentration.b Data not available. | |||||
Graphene | 6 | 302.15 | 327 | 181.6 | 2 |
Porous carbon produced at 700 °C | NAb | 296.15 | 251 | 11.4 | 22 |
Porous carbon produced at 1000 °C | NAb | 296.15 | 300 | 41.8 | 22 |
SWCNTs | 6.5 | NAb | 520 | 199 | 11 |
rGO-1 | 6.5 | NAb | 330 | 152 | 11 |
MWCNTs | 6.5 | NAb | 120 | 59.4 | 11 |
Graphite | 6.5 | NAb | NAb | 4.14 | 11 |
Commercial graphene | 6.5 | NAb | NAb | 26.0 | 11 |
Magnetic rGOs | 6.5 | 293 | 128 | 48.74 | 12 |
ZFAF prepared from coal fly ash | 11.2 | 288.15 | 2.8 | 3.5 | 13 |
SMZFA L prepared from coal fly ash | 9.6 | 288.15 | 50.6 | 56.8 | 13 |
Surfactant-modified montmorillonite | NAb | 288.15 | NAb | 20.7–32.0 | 53 |
HDTMA-modified SAz-2-MMT | 7 | 297.15 | 76 | 151.52 | 49 |
Fe3O4/GO | 6 | 288.15 | 127 | 84.75 | 47 |
10% CNTs/Fe3O4 | 6.2 | 288.15 | 17.9 | 38 | 14 |
20% CNTs/Fe3O4 | 6.2 | 288.15 | 34.3 | 40 | 14 |
50% CNTs/Fe3O4 | 6.2 | 288.15 | 50.6 | 48 | 14 |
Modified CNTs | 6 | 280.15 | 95 | 70 | 9 |
P-CDP | 7 | 288.15 | 263 | 88 | 54 |
MRGO-1 | 6 | 288.15 | 310 | 92.98 | This work |
MRGO-2 | 6 | 288.15 | 146 | 71.66 | This work |
ΔG° = −RT![]() ![]() | (6) |
![]() | (7) |
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Fig. 12 Adsorption isotherms of BPA by MRGO-1 at three different temperatures (288.15 K, 313.15 K and 318.15 K). |
The adsorption equilibrium constant, K°, can be calculated by plotting lnK1 versus Ce and extrapolating Ce to zero. The value of the intercept is ln
K. The standard enthalpy change (ΔH°) and the standard entropy change (ΔS°) are calculated from eqn (8).
![]() | (8) |
The slope and intercept of lnK° versus 1/T are −ΔH°/R and ΔS°/R. The linear fit obtained is displayed in Fig. S3.† The thermodynamic parameters calculated at three different temperatures are summarized in Table 5. The negative standard free energy change (ΔG°) indicates that the adsorption is spontaneous. The negative ΔH° value implies the exothermic character of adsorption, which is also supported by the results that the adsorption capacity of BPA decreased when temperature increased. The negative standard entropy change (ΔS°) reflects decreased randomness at the solid liquid interface during the adsorption of BPA on MRGO-1.9
Thermodynamic constant | T (K) | ||
---|---|---|---|
288.15 | 313.15 | 328.15 | |
ln![]() |
1.202 | 0.8992 | 0.4684 |
ΔG° (kJ mol−1) | −2.979 | −2.341 | −1.278 |
ΔH° (kJ mol−1) | — | −19.82 | — |
ΔS° (J mol−1 K−1) | — | −52.27 | — |
Generally, physical absorption has characteristic ΔG° value in the range of 20 to 0 kJ mol−1, while for chemical absorption, ΔG° value is in the range of 400 to 80 kJ mol−1.55 As can be seen in Table 5, ΔG values varied from −2.979 to −1.278 kJ mol−1 within measured temperature range, indicating MRGO-1 and BPA form weak interaction associated to physical absorption process. In addition, for chemical absorption process, the ΔH value ranges from 83 to 830 kJ mol−1 while for physical absorption, the ΔH value ranges from 8 and 25 kJ mol−1.55 The ΔH value for BPA adsorption by MRGO-1 is 19.82 kJ mol−1, also indicating physical absorption occurs in BPA uptake by MRGO.
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Fig. 13 The effect of pH on adsorption of BPA onto MRGO-1 (A) and MRGO-2 (B) (20 mg of adsorbent and 100 mL of 50 mg L−1 BPA at 288.15 K). |
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Fig. 14 Adsorption of BPA by recycled MRGO-1 (20 mg of MRGO and 100 mL of 50 mg L−1 BPA at 288.15 K). |
Electrostatic interaction is often used to interpret the ionic organic chemicals adsorption onto oxygen-containing carbonaceous adsorbents, while based on the results of pH, the electrostatic interactions could not be the dominate force in adsorption process.
The intensity of hydrophobic interaction can be predicted by the octanol–water partition coefficient (Kow) of organic chemicals. The Kow (158.48) and low water solubility (120–300 mg L−1)12 of BPA possibly support the absorption of BPA on absorbents through hydrophobic interaction. Considering the fact that the surface area normalized adsorption capacity of MRGO-1 is much lower than that of MRGO-2 due to the higher oxygen contents in MRGO-1 than that in MRGO-2, hydrophobic interactions may play a role in adsorption of BPA by MRGO-1 and MRGO-2. According to the previous report, for BPA adsorption on carbonaceous adsorbents, the hydrophobic interaction does not play a dominate role.12 Instead, the π–π interactions and hydrogen bonding are mainly responsible for the adsorption of BPA on graphene based materials. Xu et al.2 found that RGO presented very high adsorption capacity and fast adsorption rate for BPA. Both π–π interactions and hydrogen bonds may be responsible for the adsorption of BPA on RGO. Cortes-Arriagada et al.10 also demonstrated that the π–π stacking and hydrogen bonding were main forces in the adsorption of BPA on GO by DFT calculations. In this work, we find that hydrogen bonding interaction is not the dominant factor for BPA adsorption by MRGO-1 and MRGO-2. Considering that oxygen-containing functional groups are in favour of the formation of hydrogen bonds between BPA molecules and MRGO, the surface area normalized adsorption capacity of MRGO-1 should be higher than that of MRGO-2 due to the fact that MRGO-1 has more oxygen-containing functional groups than MRGO-2 when hydrogen bond is the dominant adsorption force of BPA. However, the experimental result is completely opposite. MRGO-2 has more graphitized structure than MRGO-1 which could enhance π–π interactions between BPA and MRGO. The surface area normalized adsorption capacity of MRGO-2 is higher than that of MRGO-1, which supports strongly that π–π interaction between BPA and the graphene sheets of MRGO should be the dominant mechanism for the adsorption of BPA on MRGO. These results also imply that for RGO, if severe aggregation in the reduction process can be alleviated, deeper reduction of GO may be beneficial for its enhancement of absorption capacity of BPA due to strengthened π–π interactions.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23542e |
This journal is © The Royal Society of Chemistry 2016 |