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Removal of antimony(III) from aqueous solution by graphene as an adsorbent
Chemical Engineering Journal 211–212 (2012) 406–411
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Removal of antimony(III) from aqueous solution by graphene as an adsorbent Yanqiu Leng, Weilin Guo ⇑, Shengnan Su, Chunliang Yi, Liting Xing ⇑ College of Resources and Environment, University of Jinan, Jinan 250022, China
h i g h l i g h t s " Graphene exhibited better adsorption capacity for antimony(III) from aqueous solutions. " The adsorption kinetic shows good compliance with the pseudo-second-order kinetic model. " Graphene showed excellent reusability during five cycles adsorption–desorption.
a r t i c l e
i n f o
Article history: Received 21 July 2012 Received in revised form 17 September 2012 Accepted 18 September 2012 Available online 5 October 2012 Keywords: Graphene Antimony(III) Adsorption Reusability
a b s t r a c t In this work, graphene is suggested as an adsorbent to remove Sb(III) from aqueous solution. Graphene was obtained using a modified Hummers’ method and then investigated its ability to remove Sb(III) in solutions. The graphene was characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area and Zeta potential measurement. The adsorption of Sb(III) onto graphene was carried out under various conditions, that is, the initial concentration, the contact time, the solution pH and temperature. The adsorption data were successfully modeled using Langmuir (R = 0.977) and Freundlich (R = 0.985) isotherms. The kinetics of adsorption was also investigated. The experimental data showed a good compliance with the pseudo-second-order kinetic model, indicating the process was controlled by the chemical process. The calculated adsorption capacity qe (8.056 mg/g) is in accordance with the experimental data (7.463 mg/g). In addition, graphene showed excellent reusability with 0.1 mol/L of EDTA solution as desorbing agent and could be used as a potential adsorbent in wastewater treatment. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Antimony (Sb), a hazardous substance, ubiquitously exists in environment due to natural processes and human activities. In the present day, it is widely used in a variety of industrial products, such as fire retardants, batteries, cable covering, pigments, ceramics, and glass [1]. Due to its broad applications, a large amount of Sb-containing compounds is released annually into the environment. Antimony is increasingly considered to be a toxic heavy metal with possible carcinogenic effect on human health [2]. It is known that prolonged exposure to Sb compounds can cause irritation to the respiratory tract and may lead to pneumoconiosis [3]. Environmental concerns have been aroused and research efforts about source origins, biogeochemical behavior, and health effects of Sb have increased recently [4]. Antimony and its compounds are considered as pollutants of priority interest by United States Environmental Protection Agency (USEPA) and the council of the European Communities, and the maximum admissible Sb concentration in drinking water regulated by USEPA was 6 lg/L [5].
⇑ Corresponding authors. Tel./fax: +86 531 8276 9233. E-mail address: [email protected] (W. Guo). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.09.078
Antimony can exist in four oxidation states (3, 0, +3, and +5), but it is generally found in Sb(III) and Sb(V). The trivalent inorganic forms of antimony are the most common species, and are known to be 10 times more toxic than pentavalent one [4,6]. In the present study, a large number of methods, including reverse osmosis [7], solvent extraction [8], reduction and precipitation [9], ion exchange [10] and adsorption [11] have been used for the removal of antimony from aqueous solution. Among these methods, adsorption method is one of the most effective choices for the removal of heavy metal ions from aqueous solutions because of its low cost, simplicity, rapidness and high efficiency [11]. Zhao et al. demonstrated that sodium montmorillonite has a good adsorption capacity for antimony acetate, but the adsorption temperature is 120 °C which is too high to operate [12]. Graphene, a single atomtic layer of sp2 carbon atoms, has attracted tremendous scientific attentions on account of its exceptional properties, such as extraordinary electronic and mechanical properties [13,14]. It has enormous applications in sensors, batteries, nanoelectronics, hydrogen storage and nanocomposites [15]. The large theoretical surface area (theoretical surface area of 2630 m2/g) provided the excellent adsorption capacity of graphene [14]. Due to this property, many investigations have been carried on utilizing graphene as an adsorbent to remove contaminants from
Y. Leng et al. / Chemical Engineering Journal 211–212 (2012) 406–411
aqueous solution. Huang et al. demonstrated that graphene has a high initial adsorption rate for lead(II) [16]. Zhu et al. demonstrated that magnetic graphene has a better Cr(VI) removal efficiency in solutions within only 5 min [17]. Liu et al. found that graphene can adsorb a large mount of methylene blue, indicating graphene is a good adsorbent [18]. However, few adsorption studies of antimony(III) on graphene have been reported to date. In this work, the graphene was synthesized and characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and Zeta potential. The objective of this study was to better understand the ability of graphene to immobilize antimony(III) in aqueous solution. To meet this objective, a batch of adsorption experiments were conducted.
2. Experimental 2.1. Preparation and characterization of graphene All chemicals and reagents used for experiments and analysis were of analytical grade and used without further purification, and double distilled water was used throughout. Graphene Oxide (GO) was synthesized from graphite powder bought from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China) according to the method reported by Marcano [19]. A 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was added to a mixture of graphite powder (3.0 g) and KMnO4 (18.0 g), producing a slight exotherm to 35–40 °C. The reaction was then heated to 50 °C and stirred for 12 h. The reaction was cooled to room temperature and poured onto ice (400 mL) with 30% H2O2 (3 mL) and stirred for 1 h. The mixture was centrifuged (4000 rpm for 4 h), and the supernatant was decanted away. The remaining solid material was then washed in succession with 200 mL of water, 200 mL of 30% HCl, and 200 mL of ethanol. The material remaining after this extended, multiple-wash process was coagulated with 200 mL of ether. The solid obtained on the filter was vacuum-dried overnight at room temperature. Graphene was synthesized by hydrazine hydrate (N2H4) reduction of GO [20]. Typically, GO (100 mg) was loaded in a 250-mL round-bottom flask and water (100 mL) was then added. This dispersion was sonicated for a while until it became clear with no visible particulate matter. N2H4 (1.00 mL) was then added and the solution heated in an oil bath at 100 °C for 24 h over which the reduced GO gradually precipitated out as a black solid. The mixture was washed thoroughly with deionized water, and dried in vacuum at 60 °C. The XRD pattern of graphene was reduced on a (Philips X’Pert Pro Super X-ray) diffractometer with a Cu Ka source (k = 1.54178 A). Zeta potential was measured by a Malvern zetameter (Zetasizer 2000). The Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore diameter of graphene were determined from the N2 adsorption–desorption at 196 °C using a Micrometric ASAP 2020 system.
2.2. Batch adsorption experiments Stock solution of 200 mg/L Sb(III) solution was prepared by dissolving 0.274 g potassium antimonyl tartrate in 500 mL of water. The pH value was adjusted by adding negligible amounts of 1.0 mol/L H2SO4 or KOH. An aqueous solution (25 mL) with 10 mg graphene and suitable concentration of Sb(III) was shaken by a thermostatic reciprocating shaker at 200 rpm and 30 °C. After removal of the graphene by filtration through 0.22 um hybrid membranes, the metal concentrations in the supernatant were determined by Atomic Absorption Spectrometry(AAS) [21].
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The adsorption capacity (qt, mg/g) and the removal percentage (%) were calculated from the following equation:
qt ¼
co ct m V
ð1Þ
%¼
c0 ct 100% c0
ð2Þ
where c0 and ct are the initial and final concentrations of Sb(III) in the solution (mg/L), respectively, V is the volume of the solution (mL), and m is the mass of graphene (mg). 2.3. Adsorption isotherm experiments Batch isotherm adsorption experiments were conducted with 10 mg graphene and various concentrations of Sb(III). Subsequently, the mixtures were ultrasonicated for a while to guarantee a good dispersion of graphene. The initial Sb(III) concentration in solution ranged from 1 to 10 mg/L. The pH of the solution was adjusted to 11.0 with 0.1 mol/L KOH. The solution was shaken by a thermostatic reciprocating shaker at 200 rpm and 30 °C. After 4 h equilibration, the suspensions were filtered to determine the metal concentrations as before. 2.4. Adsorption kinetic experiments The kinetic experiments were conducted to examine the influence of reaction time on the adsorption of Sb(III) onto graphene. Using a set of 250 mL conical flasks, 10 mg graphene was added to 25 mL of 3 mg/L Sb(III) solution (pH around 11). The mixtures were ultrasonicated for a while and shaken by a thermostatic reciprocating shaker at 200 rpm and 30 °C. Flasks were taken at appropriate time intervals, and the suspensions were filtered to determine the metal concentrations as before. 2.5. Reusability experiments Reusability is an important factor for an effective adsorbent. To investigate the adsorption–desorption reliability of graphene, reusability experiments was carried out by using 0.1 mol/L EDTA (Ethylenediaminetetraacetic acid disodium salt) as the desorbing agent. After the adsorption process described above without pH adjustment, the Sb(III)-loaded graphene was withdrawn, adding EDTA equilibrated for 4 h, and washed thoroughly with deionized water, and dried in vacuum at 60 °C. And then, the recovered adsorbent was used in the subsequent five-cycle adsorption– desorption experiments. 3. Results and discussion 3.1. Characterization of graphene Fig. 1 shows the XRD diffraction patterns of graphite, graphene oxide and graphene. The graphite is characterized by a sharp peak at 26.5° while GO shows a typical peak at 9°. This indicates that graphite is almost totally transformed into GO. It can be seen that no typical diffraction peak of graphite or GO is observed in the XRD pattern for graphene, indicating that oxygen intercalated into the interlayer spacings of GO is nearly removed by using N2H4 [9]. The surface charge distribution of graphene is observed in Fig. 2. The isoelectric point (IEP) of graphene is around 3.8 determined by the pH location where zeta potential equals zero. N2 adsorption–desorption isotherms were employed to investigate the surface area and the pore structures of graphene (Fig. 3). The BET surface area is 154.43 m2/g. The discrepancy on as-obtained data and theoretical value of surface area of graphene (2630 m2/g) is
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3.2. Effect of initial concentration of Sb(III) on the adsorption Fig. 4 shows the effect of initial concentration of Sb(III) on the removal percentage of Sb(III) using 10 mg graphene at different times. It is clear to see that the removal of Sb(III) decreased with increasing initial concentration. At low concentrations, the surface area and the effective adsorption sites were relatively high, and the Sb(III) was easily adsorbed. At higher concentrations, many ions compete the adsorption sites, resulting in a decrease in the percentage removal. As different concentrations (1, 3, 8, 10 mg/L) were used, the adsorption capacities are 1.45, 5.53, 10.67, 11.91 mg/g for 4 h, respectively. It is obviously that the adsorption of Sb(III) depends on the initial concentration of Sb(III). It is also apparent that the adsorption gradually increases with the increase in contact time. 3.3. Effect of pH on the adsorption Fig. 1. X-ray diffraction patterns of graphite, graphene oxide and graphene.
Fig. 2. Zeta potential curve of graphite suspension versus pH.
The pH value of the solution is one of the more important factors affecting adsorption process. It is related to speciation of the metal ions in solution and the surface functional groups on the adsorbent [23]. The effect of pH on the removal percentage is presented in Fig. 5. It is clear to see that Sb(III) adsorption by graphene is sensitive to pH variations. For the use of antimony removal from water, we synthesized GO via modified Hummer’s method, and then GO was reduced to graphene with hydrazine hydrate at 100 °C for 24 h. Hydroxyl and epoxy functional groups on GO are reduced completely in this reduction process. Even though the charge density is reduced on graphene as compared to GO, it still possesses carboxyl functional groups. The IEP of graphene is about 3.8 (Fig. 2). This indicates that at pH < 3.8 graphene has positive surface charge and can act as anion exchanger, while at pH > 3.8 the surface charge of graphene is negative, which benefits for adsorbing cations. In this study, the highest adsorption (99.5%) was achieved at pH 11.0. In acidic and neutral pH, the removal percentage is relatively low. The adsorption of Sb(III) increases with decreasing pH at pH < 3.8, and about 99.5% Sb(III) is adsorbed on graphene at pH > 11.0. At pH < 3.8, the surface charge of graphene is positive because of the protonation reaction. With decreasing pH, the surface charge is more positive and the electrostatic attraction between the negatively charged antimony tartrate ion and the positively charged surface of graphene becomes stronger, and thereby results in the increase of Sb(III) adsorption. At pH > 3.8, the removal percentage of Sb(III) sharply increased and about 99.5% Sb(III) is
Fig. 3. Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of graphene.
attributed to the incomplete exfoliation and aggregation during reduction process because of the unavoidable van der Waals force between each single layer of graphene [22]. Pore Volume and Pore Size are 0.182221 cm3/g and 5.0937 nm, respectively, which are calculated by the Barret–Joyner–Halenda (BJH) analysis.
Fig. 4. Effect of initial concentration on the adsorption of Sb(III) onto graphene (graphene, 10 mg; temperature, 30 °C; pH, 6.2).
Y. Leng et al. / Chemical Engineering Journal 211–212 (2012) 406–411
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isotherm models are used for describing equilibrium studies and comparing the adsorption capacities of the adsorbents for pollutants. In this study, data from the adsorption isotherm were modeled using the Langmuir and Freundlich isotherm equations. The Langmuir model assumes uniform energies of adsorption on the surface and no interaction between adsorbates. It is a valid monolayer adsorption on a surface. The equation of the Langmuir isotherm is as follows:
1 1 1 ¼ þ qe qm kl ce qm
Fig. 5. Effect of pH on the adsorption of Sb(III) onto graphene (graphene, 10 mg; initial concentration of Sb(III), 3 mg/L; temperature, 30 °C; contact time, 4 h).
adsorbed on graphene at pH > 11.0. The increase in the adsorption of Sb(III) in alkaline solutions suggests that the electrostatic factors do not control the adsorption process onto graphene. Adsorption is likely to be based on van der Waals interactions that probably dominate the electrostatic interactions. Similar observations on pH variations have been reported by Ramesha et al. for the adsorption of anionic dye (Orange G) on graphene [24]. 3.4. Effect of contact time and temperature on the adsorption Contact time and temperature are essential parameters in practical applications. The effect of contact time and temperature on the removal percentage of Sb(III) is shown in Fig. 6. It can be seen that the removal percentage increased with the increase in time for three temperatures. The equilibrium time is 2 h, 2 h and 4 h for 60 °C, 45 °C and 30 °C, respectively. After equilibrium time, the significant increase in the adsorption percentage did not occur, so 4 htime was chosen as optimum time at 30 °C. It is also apparent that the removal percentage increases as the temperature increases. The experimental results demonstrate that the process of adsorption of Sb(III) onto graphene is endothermic. 3.5. Adsorption isotherm models
ð3Þ
where ce is the equilibrium concentration of Sb(III) in solution, qe is the amount of Sb(III) adsorbed on graphene, qm is the maximum amount of Sb(III) adsorbed on graphene, and kl is the Langmuir constant. As seen from the Fig. 7, the coefficient of determination (R) was found to be 0.977 and the maximum amount of adsorbed Sb(III) is 10.919 mg/g. The Freundlich isotherm is an empirical expression assumed a heterogeneous adsorption surface and active sites with different energy. The equation is commonly expressed as follows:
ln qe ¼ ln K F þ
1 ln ce n
ð4Þ
where KF is a Freundlich constant related to adsorption capacity and 1/n is an empirical parameter giving an indication of the favorability of adsorption. As seen from the Fig. 8, the coefficient of determination (R) was found to be 0.985 and 1/n is 0.449. It is conducted that the adsorbate is easily adsorbed when 1/n is between 0.1 and 0.5, while it is difficult to adsorb when 1/n is larger than 2.0. According to this conduction, the adsorbate is easily adsorbed in this study. In conclusion, the Freundlich isotherm fits better with the experimental data. According to the constant, 1/n, it is concluded that the adsorbate is easily adsorbed. 3.6. Adsorption kinetics In order to clarify the adsorption kinetics of Sb(III) onto graphene, the pseudo-first-order kinetic and pseudo-second-order kinetic were studied. The liner form of the pseudo-first-order equation is expressed as follows:
ln ðqe qt Þ ¼ ln qe k1 t
ð5Þ
The adsorption of Sb(III) onto graphene was conducted with five initial concentrations, 1, 3, 5, 7 and 10 mg/L at 30 °C. Adsorption
where qe and qt are the amounts of Sb(III) adsorbed onto graphene at equilibrium and at time t, respectively, and k1 is the rate constant, determined by plotting ln(qe qt) versus t.
Fig. 6. Effect of contact time and temperature on the adsorption of Sb(III) onto graphene (graphene, 10 mg; initial concentration of Sb(III), 3 mg/L; pH, 6.2).
Fig. 7. Langmuir isotherm plots obtained for the adsorption of Sb(III) onto graphene (graphene, 10 mg; contact time 4 h; pH, 11.0).
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Fig. 8. Freundlich isotherm plots obtained for the adsorption of Sb(III) onto graphene (graphene, 10 mg; contact time 4 h; pH, 11.0).
Fig. 10. The pseudo-second-order kinetic for the adsorption of Sb(III) onto graphene (graphene, 10 mg; antimony concentration, 3 mg/L; pH, 11.0).
The experimental data were also analyzed by the pseudosecond-order. The equation is expressed as follows:
t 1 t ¼ þ qt k2 q2e qe
ð6Þ
where qe and qt are the amounts of Sb(III) adsorbed onto graphene at equilibrium and at time t, respectively, and k2 is the rate constant. As seen from Figs. 9 and 10, the coefficients of determination R show that the pseudo-second-order kinetic model was able to satisfactorily describe the adsorption. This means that the adsorption process was controlled by the chemical process. Moreover, the experimental qe (7.463 mg/g) is in accordance with the calculated qe (8.056 mg/g). 3.7. Reusability studies Reusability was an important factor for an effective absorbent. EDTA is known as a very strong chelating agent for many heavy metals. Recently, EDTA was chosen as the best desorbent in many literatures [25,26]. In our experiments, the adsorbed Sb(III) can be eluted with the treatment of 0.1 mol/L EDTA. When EDTA was used as an eluent, the interaction between graphene and Sb(III) was dis-
Fig. 11. Adsorption–desorption cycle of graphene (graphene, 10 mg; antimony concentration, 3 mg/L; adsorption time, 4 h;desorption time, 4 h; pH, 6.2).
rupted, and subsequently Sb(III) was released into the eluent. In order to show the reusability of the graphene, the adsorption– desorption cycle was repeated five times using the same sample adsorbent. As the spent adsorbent was recovered, the acquired graphene was reused for the adsorption of Sb(III) and the results were shown in Fig. 11. For five consecutive cycles of adsorption– desorption, the removal percentage was still reach 60% and less than 20% decrease in adsorption efficiency was occurred. The results clearly showed that graphene could be used repeatedly without significantly losing its adsorption ability.
4. Conclusions
Fig. 9. The pseudo-first-order kinetic for the adsorption of Sb(III) onto graphene (graphene, 10 mg; antimony concentration, 3 mg/L; pH, 11.0).
This study demonstrates for the first time that Sb(III) can be effectively adsorbed by graphene in the solution. Batch adsorption experiments were carried out under various conditions, that is, the initial concentration, the contact time, the solution pH and the temperature. The adsorption isotherms of Sb(III) onto graphene could be described well by the Freundlich isotherm better than the Langmuir isotherm. The pseudo-first-order kinetic and pseudo-second-order kinetic were applied to study the kinetic of the adsorption. The pseudo-second-order kinetic provides the best correlation for the adsorption process, indicating the process was
Y. Leng et al. / Chemical Engineering Journal 211–212 (2012) 406–411
controlled by the chemical process. Finally, graphene showed excellent reusability with 0.1 mol/L of EDTA solution as desorbing agent. For five consecutive cycles of adsorption–desorption, the removal percentage was still reach 60%. On the basis of all results, the graphene exhibits a high initial application in environment pollution cleanup.
[12]
[13]
Acknowledgments
[14]
The authors would like to thank the financial support from the National Nature Science Foundation of China (Grant No. 41172222).
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Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Removal of antimony(III) from aqueous solution by graphene as an adsorbent Yanqiu Leng, Weilin Guo ⇑, Shengnan Su, Chunliang Yi, Liting Xing ⇑ College of Resources and Environment, University of Jinan, Jinan 250022, China
h i g h l i g h t s " Graphene exhibited better adsorption capacity for antimony(III) from aqueous solutions. " The adsorption kinetic shows good compliance with the pseudo-second-order kinetic model. " Graphene showed excellent reusability during five cycles adsorption–desorption.
a r t i c l e
i n f o
Article history: Received 21 July 2012 Received in revised form 17 September 2012 Accepted 18 September 2012 Available online 5 October 2012 Keywords: Graphene Antimony(III) Adsorption Reusability
a b s t r a c t In this work, graphene is suggested as an adsorbent to remove Sb(III) from aqueous solution. Graphene was obtained using a modified Hummers’ method and then investigated its ability to remove Sb(III) in solutions. The graphene was characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area and Zeta potential measurement. The adsorption of Sb(III) onto graphene was carried out under various conditions, that is, the initial concentration, the contact time, the solution pH and temperature. The adsorption data were successfully modeled using Langmuir (R = 0.977) and Freundlich (R = 0.985) isotherms. The kinetics of adsorption was also investigated. The experimental data showed a good compliance with the pseudo-second-order kinetic model, indicating the process was controlled by the chemical process. The calculated adsorption capacity qe (8.056 mg/g) is in accordance with the experimental data (7.463 mg/g). In addition, graphene showed excellent reusability with 0.1 mol/L of EDTA solution as desorbing agent and could be used as a potential adsorbent in wastewater treatment. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Antimony (Sb), a hazardous substance, ubiquitously exists in environment due to natural processes and human activities. In the present day, it is widely used in a variety of industrial products, such as fire retardants, batteries, cable covering, pigments, ceramics, and glass [1]. Due to its broad applications, a large amount of Sb-containing compounds is released annually into the environment. Antimony is increasingly considered to be a toxic heavy metal with possible carcinogenic effect on human health [2]. It is known that prolonged exposure to Sb compounds can cause irritation to the respiratory tract and may lead to pneumoconiosis [3]. Environmental concerns have been aroused and research efforts about source origins, biogeochemical behavior, and health effects of Sb have increased recently [4]. Antimony and its compounds are considered as pollutants of priority interest by United States Environmental Protection Agency (USEPA) and the council of the European Communities, and the maximum admissible Sb concentration in drinking water regulated by USEPA was 6 lg/L [5].
⇑ Corresponding authors. Tel./fax: +86 531 8276 9233. E-mail address: [email protected] (W. Guo). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.09.078
Antimony can exist in four oxidation states (3, 0, +3, and +5), but it is generally found in Sb(III) and Sb(V). The trivalent inorganic forms of antimony are the most common species, and are known to be 10 times more toxic than pentavalent one [4,6]. In the present study, a large number of methods, including reverse osmosis [7], solvent extraction [8], reduction and precipitation [9], ion exchange [10] and adsorption [11] have been used for the removal of antimony from aqueous solution. Among these methods, adsorption method is one of the most effective choices for the removal of heavy metal ions from aqueous solutions because of its low cost, simplicity, rapidness and high efficiency [11]. Zhao et al. demonstrated that sodium montmorillonite has a good adsorption capacity for antimony acetate, but the adsorption temperature is 120 °C which is too high to operate [12]. Graphene, a single atomtic layer of sp2 carbon atoms, has attracted tremendous scientific attentions on account of its exceptional properties, such as extraordinary electronic and mechanical properties [13,14]. It has enormous applications in sensors, batteries, nanoelectronics, hydrogen storage and nanocomposites [15]. The large theoretical surface area (theoretical surface area of 2630 m2/g) provided the excellent adsorption capacity of graphene [14]. Due to this property, many investigations have been carried on utilizing graphene as an adsorbent to remove contaminants from
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aqueous solution. Huang et al. demonstrated that graphene has a high initial adsorption rate for lead(II) [16]. Zhu et al. demonstrated that magnetic graphene has a better Cr(VI) removal efficiency in solutions within only 5 min [17]. Liu et al. found that graphene can adsorb a large mount of methylene blue, indicating graphene is a good adsorbent [18]. However, few adsorption studies of antimony(III) on graphene have been reported to date. In this work, the graphene was synthesized and characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and Zeta potential. The objective of this study was to better understand the ability of graphene to immobilize antimony(III) in aqueous solution. To meet this objective, a batch of adsorption experiments were conducted.
2. Experimental 2.1. Preparation and characterization of graphene All chemicals and reagents used for experiments and analysis were of analytical grade and used without further purification, and double distilled water was used throughout. Graphene Oxide (GO) was synthesized from graphite powder bought from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China) according to the method reported by Marcano [19]. A 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was added to a mixture of graphite powder (3.0 g) and KMnO4 (18.0 g), producing a slight exotherm to 35–40 °C. The reaction was then heated to 50 °C and stirred for 12 h. The reaction was cooled to room temperature and poured onto ice (400 mL) with 30% H2O2 (3 mL) and stirred for 1 h. The mixture was centrifuged (4000 rpm for 4 h), and the supernatant was decanted away. The remaining solid material was then washed in succession with 200 mL of water, 200 mL of 30% HCl, and 200 mL of ethanol. The material remaining after this extended, multiple-wash process was coagulated with 200 mL of ether. The solid obtained on the filter was vacuum-dried overnight at room temperature. Graphene was synthesized by hydrazine hydrate (N2H4) reduction of GO [20]. Typically, GO (100 mg) was loaded in a 250-mL round-bottom flask and water (100 mL) was then added. This dispersion was sonicated for a while until it became clear with no visible particulate matter. N2H4 (1.00 mL) was then added and the solution heated in an oil bath at 100 °C for 24 h over which the reduced GO gradually precipitated out as a black solid. The mixture was washed thoroughly with deionized water, and dried in vacuum at 60 °C. The XRD pattern of graphene was reduced on a (Philips X’Pert Pro Super X-ray) diffractometer with a Cu Ka source (k = 1.54178 A). Zeta potential was measured by a Malvern zetameter (Zetasizer 2000). The Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore diameter of graphene were determined from the N2 adsorption–desorption at 196 °C using a Micrometric ASAP 2020 system.
2.2. Batch adsorption experiments Stock solution of 200 mg/L Sb(III) solution was prepared by dissolving 0.274 g potassium antimonyl tartrate in 500 mL of water. The pH value was adjusted by adding negligible amounts of 1.0 mol/L H2SO4 or KOH. An aqueous solution (25 mL) with 10 mg graphene and suitable concentration of Sb(III) was shaken by a thermostatic reciprocating shaker at 200 rpm and 30 °C. After removal of the graphene by filtration through 0.22 um hybrid membranes, the metal concentrations in the supernatant were determined by Atomic Absorption Spectrometry(AAS) [21].
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The adsorption capacity (qt, mg/g) and the removal percentage (%) were calculated from the following equation:
qt ¼
co ct m V
ð1Þ
%¼
c0 ct 100% c0
ð2Þ
where c0 and ct are the initial and final concentrations of Sb(III) in the solution (mg/L), respectively, V is the volume of the solution (mL), and m is the mass of graphene (mg). 2.3. Adsorption isotherm experiments Batch isotherm adsorption experiments were conducted with 10 mg graphene and various concentrations of Sb(III). Subsequently, the mixtures were ultrasonicated for a while to guarantee a good dispersion of graphene. The initial Sb(III) concentration in solution ranged from 1 to 10 mg/L. The pH of the solution was adjusted to 11.0 with 0.1 mol/L KOH. The solution was shaken by a thermostatic reciprocating shaker at 200 rpm and 30 °C. After 4 h equilibration, the suspensions were filtered to determine the metal concentrations as before. 2.4. Adsorption kinetic experiments The kinetic experiments were conducted to examine the influence of reaction time on the adsorption of Sb(III) onto graphene. Using a set of 250 mL conical flasks, 10 mg graphene was added to 25 mL of 3 mg/L Sb(III) solution (pH around 11). The mixtures were ultrasonicated for a while and shaken by a thermostatic reciprocating shaker at 200 rpm and 30 °C. Flasks were taken at appropriate time intervals, and the suspensions were filtered to determine the metal concentrations as before. 2.5. Reusability experiments Reusability is an important factor for an effective adsorbent. To investigate the adsorption–desorption reliability of graphene, reusability experiments was carried out by using 0.1 mol/L EDTA (Ethylenediaminetetraacetic acid disodium salt) as the desorbing agent. After the adsorption process described above without pH adjustment, the Sb(III)-loaded graphene was withdrawn, adding EDTA equilibrated for 4 h, and washed thoroughly with deionized water, and dried in vacuum at 60 °C. And then, the recovered adsorbent was used in the subsequent five-cycle adsorption– desorption experiments. 3. Results and discussion 3.1. Characterization of graphene Fig. 1 shows the XRD diffraction patterns of graphite, graphene oxide and graphene. The graphite is characterized by a sharp peak at 26.5° while GO shows a typical peak at 9°. This indicates that graphite is almost totally transformed into GO. It can be seen that no typical diffraction peak of graphite or GO is observed in the XRD pattern for graphene, indicating that oxygen intercalated into the interlayer spacings of GO is nearly removed by using N2H4 [9]. The surface charge distribution of graphene is observed in Fig. 2. The isoelectric point (IEP) of graphene is around 3.8 determined by the pH location where zeta potential equals zero. N2 adsorption–desorption isotherms were employed to investigate the surface area and the pore structures of graphene (Fig. 3). The BET surface area is 154.43 m2/g. The discrepancy on as-obtained data and theoretical value of surface area of graphene (2630 m2/g) is
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3.2. Effect of initial concentration of Sb(III) on the adsorption Fig. 4 shows the effect of initial concentration of Sb(III) on the removal percentage of Sb(III) using 10 mg graphene at different times. It is clear to see that the removal of Sb(III) decreased with increasing initial concentration. At low concentrations, the surface area and the effective adsorption sites were relatively high, and the Sb(III) was easily adsorbed. At higher concentrations, many ions compete the adsorption sites, resulting in a decrease in the percentage removal. As different concentrations (1, 3, 8, 10 mg/L) were used, the adsorption capacities are 1.45, 5.53, 10.67, 11.91 mg/g for 4 h, respectively. It is obviously that the adsorption of Sb(III) depends on the initial concentration of Sb(III). It is also apparent that the adsorption gradually increases with the increase in contact time. 3.3. Effect of pH on the adsorption Fig. 1. X-ray diffraction patterns of graphite, graphene oxide and graphene.
Fig. 2. Zeta potential curve of graphite suspension versus pH.
The pH value of the solution is one of the more important factors affecting adsorption process. It is related to speciation of the metal ions in solution and the surface functional groups on the adsorbent [23]. The effect of pH on the removal percentage is presented in Fig. 5. It is clear to see that Sb(III) adsorption by graphene is sensitive to pH variations. For the use of antimony removal from water, we synthesized GO via modified Hummer’s method, and then GO was reduced to graphene with hydrazine hydrate at 100 °C for 24 h. Hydroxyl and epoxy functional groups on GO are reduced completely in this reduction process. Even though the charge density is reduced on graphene as compared to GO, it still possesses carboxyl functional groups. The IEP of graphene is about 3.8 (Fig. 2). This indicates that at pH < 3.8 graphene has positive surface charge and can act as anion exchanger, while at pH > 3.8 the surface charge of graphene is negative, which benefits for adsorbing cations. In this study, the highest adsorption (99.5%) was achieved at pH 11.0. In acidic and neutral pH, the removal percentage is relatively low. The adsorption of Sb(III) increases with decreasing pH at pH < 3.8, and about 99.5% Sb(III) is adsorbed on graphene at pH > 11.0. At pH < 3.8, the surface charge of graphene is positive because of the protonation reaction. With decreasing pH, the surface charge is more positive and the electrostatic attraction between the negatively charged antimony tartrate ion and the positively charged surface of graphene becomes stronger, and thereby results in the increase of Sb(III) adsorption. At pH > 3.8, the removal percentage of Sb(III) sharply increased and about 99.5% Sb(III) is
Fig. 3. Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of graphene.
attributed to the incomplete exfoliation and aggregation during reduction process because of the unavoidable van der Waals force between each single layer of graphene [22]. Pore Volume and Pore Size are 0.182221 cm3/g and 5.0937 nm, respectively, which are calculated by the Barret–Joyner–Halenda (BJH) analysis.
Fig. 4. Effect of initial concentration on the adsorption of Sb(III) onto graphene (graphene, 10 mg; temperature, 30 °C; pH, 6.2).
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isotherm models are used for describing equilibrium studies and comparing the adsorption capacities of the adsorbents for pollutants. In this study, data from the adsorption isotherm were modeled using the Langmuir and Freundlich isotherm equations. The Langmuir model assumes uniform energies of adsorption on the surface and no interaction between adsorbates. It is a valid monolayer adsorption on a surface. The equation of the Langmuir isotherm is as follows:
1 1 1 ¼ þ qe qm kl ce qm
Fig. 5. Effect of pH on the adsorption of Sb(III) onto graphene (graphene, 10 mg; initial concentration of Sb(III), 3 mg/L; temperature, 30 °C; contact time, 4 h).
adsorbed on graphene at pH > 11.0. The increase in the adsorption of Sb(III) in alkaline solutions suggests that the electrostatic factors do not control the adsorption process onto graphene. Adsorption is likely to be based on van der Waals interactions that probably dominate the electrostatic interactions. Similar observations on pH variations have been reported by Ramesha et al. for the adsorption of anionic dye (Orange G) on graphene [24]. 3.4. Effect of contact time and temperature on the adsorption Contact time and temperature are essential parameters in practical applications. The effect of contact time and temperature on the removal percentage of Sb(III) is shown in Fig. 6. It can be seen that the removal percentage increased with the increase in time for three temperatures. The equilibrium time is 2 h, 2 h and 4 h for 60 °C, 45 °C and 30 °C, respectively. After equilibrium time, the significant increase in the adsorption percentage did not occur, so 4 htime was chosen as optimum time at 30 °C. It is also apparent that the removal percentage increases as the temperature increases. The experimental results demonstrate that the process of adsorption of Sb(III) onto graphene is endothermic. 3.5. Adsorption isotherm models
ð3Þ
where ce is the equilibrium concentration of Sb(III) in solution, qe is the amount of Sb(III) adsorbed on graphene, qm is the maximum amount of Sb(III) adsorbed on graphene, and kl is the Langmuir constant. As seen from the Fig. 7, the coefficient of determination (R) was found to be 0.977 and the maximum amount of adsorbed Sb(III) is 10.919 mg/g. The Freundlich isotherm is an empirical expression assumed a heterogeneous adsorption surface and active sites with different energy. The equation is commonly expressed as follows:
ln qe ¼ ln K F þ
1 ln ce n
ð4Þ
where KF is a Freundlich constant related to adsorption capacity and 1/n is an empirical parameter giving an indication of the favorability of adsorption. As seen from the Fig. 8, the coefficient of determination (R) was found to be 0.985 and 1/n is 0.449. It is conducted that the adsorbate is easily adsorbed when 1/n is between 0.1 and 0.5, while it is difficult to adsorb when 1/n is larger than 2.0. According to this conduction, the adsorbate is easily adsorbed in this study. In conclusion, the Freundlich isotherm fits better with the experimental data. According to the constant, 1/n, it is concluded that the adsorbate is easily adsorbed. 3.6. Adsorption kinetics In order to clarify the adsorption kinetics of Sb(III) onto graphene, the pseudo-first-order kinetic and pseudo-second-order kinetic were studied. The liner form of the pseudo-first-order equation is expressed as follows:
ln ðqe qt Þ ¼ ln qe k1 t
ð5Þ
The adsorption of Sb(III) onto graphene was conducted with five initial concentrations, 1, 3, 5, 7 and 10 mg/L at 30 °C. Adsorption
where qe and qt are the amounts of Sb(III) adsorbed onto graphene at equilibrium and at time t, respectively, and k1 is the rate constant, determined by plotting ln(qe qt) versus t.
Fig. 6. Effect of contact time and temperature on the adsorption of Sb(III) onto graphene (graphene, 10 mg; initial concentration of Sb(III), 3 mg/L; pH, 6.2).
Fig. 7. Langmuir isotherm plots obtained for the adsorption of Sb(III) onto graphene (graphene, 10 mg; contact time 4 h; pH, 11.0).
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Fig. 8. Freundlich isotherm plots obtained for the adsorption of Sb(III) onto graphene (graphene, 10 mg; contact time 4 h; pH, 11.0).
Fig. 10. The pseudo-second-order kinetic for the adsorption of Sb(III) onto graphene (graphene, 10 mg; antimony concentration, 3 mg/L; pH, 11.0).
The experimental data were also analyzed by the pseudosecond-order. The equation is expressed as follows:
t 1 t ¼ þ qt k2 q2e qe
ð6Þ
where qe and qt are the amounts of Sb(III) adsorbed onto graphene at equilibrium and at time t, respectively, and k2 is the rate constant. As seen from Figs. 9 and 10, the coefficients of determination R show that the pseudo-second-order kinetic model was able to satisfactorily describe the adsorption. This means that the adsorption process was controlled by the chemical process. Moreover, the experimental qe (7.463 mg/g) is in accordance with the calculated qe (8.056 mg/g). 3.7. Reusability studies Reusability was an important factor for an effective absorbent. EDTA is known as a very strong chelating agent for many heavy metals. Recently, EDTA was chosen as the best desorbent in many literatures [25,26]. In our experiments, the adsorbed Sb(III) can be eluted with the treatment of 0.1 mol/L EDTA. When EDTA was used as an eluent, the interaction between graphene and Sb(III) was dis-
Fig. 11. Adsorption–desorption cycle of graphene (graphene, 10 mg; antimony concentration, 3 mg/L; adsorption time, 4 h;desorption time, 4 h; pH, 6.2).
rupted, and subsequently Sb(III) was released into the eluent. In order to show the reusability of the graphene, the adsorption– desorption cycle was repeated five times using the same sample adsorbent. As the spent adsorbent was recovered, the acquired graphene was reused for the adsorption of Sb(III) and the results were shown in Fig. 11. For five consecutive cycles of adsorption– desorption, the removal percentage was still reach 60% and less than 20% decrease in adsorption efficiency was occurred. The results clearly showed that graphene could be used repeatedly without significantly losing its adsorption ability.
4. Conclusions
Fig. 9. The pseudo-first-order kinetic for the adsorption of Sb(III) onto graphene (graphene, 10 mg; antimony concentration, 3 mg/L; pH, 11.0).
This study demonstrates for the first time that Sb(III) can be effectively adsorbed by graphene in the solution. Batch adsorption experiments were carried out under various conditions, that is, the initial concentration, the contact time, the solution pH and the temperature. The adsorption isotherms of Sb(III) onto graphene could be described well by the Freundlich isotherm better than the Langmuir isotherm. The pseudo-first-order kinetic and pseudo-second-order kinetic were applied to study the kinetic of the adsorption. The pseudo-second-order kinetic provides the best correlation for the adsorption process, indicating the process was
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controlled by the chemical process. Finally, graphene showed excellent reusability with 0.1 mol/L of EDTA solution as desorbing agent. For five consecutive cycles of adsorption–desorption, the removal percentage was still reach 60%. On the basis of all results, the graphene exhibits a high initial application in environment pollution cleanup.
[12]
[13]
Acknowledgments
[14]
The authors would like to thank the financial support from the National Nature Science Foundation of China (Grant No. 41172222).
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