Article pubs.acs.org/jced
Adsorption of Trivalent Antimony from Aqueous Solution Using Graphene Oxide: Kinetic and Thermodynamic Studies Xiuzhen Yang, Zhou Shi,* Mingyang Yuan, and Lishan Liu Department of Water Engineering and Science, College of Civil Engineering, Hunan University, Changsha, Hunan 410082, China ABSTRACT: Graphene oxide (GO) used as adsorbent to remove trivalent antimony (Sb(III)) from aqueous solution was studied. Xray diffraction, Fourier transform infrared, scanning electron microscopy, X-ray photoelectron spectroscopy, and energy dispersive X-ray analysis were adopted to characterize the GO. Batch experiments were conducted to systematically investigate adsorption efficiency and influence of various parameters, such as initial pH, contact time, initial Sb(III) concentration, and temperature. It is found that GO was particularly effective for the adsorption of Sb(III) within a wide pH range of 2.0 to 10.0 under test conditions. Kinetics of the adsorption process was also studied and the pseudofirst-order rate model was found to fit experimental data well with a highest correlation coefficient (R2 > 0.99). The adsorption isotherm data were better modeled with Langmuir isotherms than with Freundlich isotherms. The thermodynamic parameters (ΔH, ΔS, and ΔG) were evaluated, and the results indicated that the adsorption process of Sb(III) on the GO was spontaneous and endothermic. In addition, a remarkable Sb(III) desorption efficiency was achieved when 0.1 mol·L−1 EDTA solution was applied to the GO absorbingly exhausted with Sb(III). Further successful experiments of consecutive adsorption−desorption cycles suggest that GO could be a promising adsorbent for removal of Sb from the contaminated solutions.
1. INTRODUCTION Antimony, a toxic heavy metal generated by natural processes and human activities, has widely existed in the environment as well as wastewaters of a variety of industrial applications such as in antimony mining, bearing metal, cable sheathing, ammunition, battery grids, and so on.1,2 Antimony and its compounds which can easily combine with sulfhydryls in the human body, disturbing enzyme activity and destroying ion balance in cells, and thus causing cellular hypoxia,3 do great harm to a human’s health. Extensive release of Sb and its compounds to the environment has led to global concern because of the increased awareness of its risk to the environment and human health. The U.S. Environmental Protection Agency and the German Research Community listed antimony as a priority pollutant.4 There are some antimony pollution areas in China, especially in the Xikuangshan antimony mine area, the world largest antimony mine in Hunan province, in the central part of China. According to recent surveys, Sb in the surface water in this antimony mine area reached 7 mg·L−1,5 and Sb in the soil samples in this area was reported to range from (100.6 to 5045) mg·kg−1.6 To control the Sb pollution and protect residents, China recently strengthened the national environmental standards on Sb, in which the Sb concentration in drinking water is limited to 5 μg·L−1, which is identical with the Sb limit in the standards of the World Health Organization (WHO).7 In order to satisfy the Sb control requirements, many methods have been evaluated to remove the Sb from water, such as ion exchange,8 solvent extraction,9 reverse osmosis,10 coagulation−sedimentation,11 and adsorption.12 Among these potential separation technologies mentioned, adsorption has been universally accepted as one of the most promising and © XXXX American Chemical Society
widely used methods due to its low operation costs, high efficiency, flexibility and simplicity of design, and ease of operation and maintenance.1 In recent years, graphene oxide (GO) with a huge theoretical surface area of 2,630 m2.g−1, has attracted more and more interest to use it as an efficient adsorbent for the removal of heavy metal ions from water.13,14 However, to the best of our knowledge, the applications of GO on the adsorption of Sb are very rare. It will be significant to study the interactions between Sb and GO. Among different valence species, Sb(III) is chosen as the target because it is 10 times more toxic than Sb(V).13 Study has been done in order to find out the influence of various factors on the adsorption behavior of Sb(III) onto GO. Meanwhile, characteristics of Sb(III) adsorption were comprehensively explored through adsorption kinetics, isotherm modeling, and thermodynamic analysis. Ultimately, the preliminary desorption study was conducted to evaluate the possibility of recycling GO and recovering Sb(III).
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All chemicals and reagents used in this study are of analytical grade or better and used without further purification. Double distilled water was used throughout. GO was synthesized with the modified Hummers’ Received: October 7, 2014 Accepted: January 6, 2015
A
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Figure 1. (a) XRD pattern of graphite and GO; (b) FTIR spectrum of GO;(c) XPS spectrum of C 1s.
method15 using graphite powder purchased from Baichuan Graphite Co., Ltd., Qingdao, China. 2.2. Instrumentation. The Sb concentration was determined using an AA-7000 atomic absorption spectrophotometer (Shimadzu, Kyoto, Japan). The X-ray diffraction (XRD) patterns were recorded by an X-ray diffractometer (D8Advance, Bruker) using Cu Kα radiation. Fourier transform infrared (FTIR) spectroscopy measurements were conducted by using a Nicolet 5700 spectrometer with KBr pellet at room temperature. Scanning electron microscopy (SEM, JSM-6700F, JEOL Corp.) was used to characterize the surface morphology of the GO and Sb(III)-absorbed GO samples. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Fisher Scientific (K-Alpha 1063) probe using monochromatic Al Kα radiation (hv = 1486.6 eV). Spectrum analysis was done by performing a Shirley background subtraction, followed by decomposition into several Gaussian peaks. The fitting method was set up so that peak positions and widths were the same for all spectra, and only the amplitude was fitted independently for each spectra and each Gaussian subpeak. 2.3. Batch Adsorption/Desorption Procedure. A stock solution of Sb(III) (100 mg·L−1) was prepared by dissolving 0.27424 g of Sb potassium tartrate (K(SbO)C4H4O8·(1/ 2)H2O, Bodi Chemical Holding Co., Ltd., TianJin, China) in 1000 mL of double distilled water. The required solutions of different concentrations were produced by using the stock solution with successive dilution. The adsorption experiments were performed at batch scale. GO was added to 120 mL polyethylene bottles containing 50 mL of Sb(III) solution with known concentrations and pH value. The solution pH was adjusted with 0.1 M HCl or 0.1 M NaOH to the desired values. The bottles were then shaken over a desired time period on a
rotary shaker at 120 rpm. After agitation, the supernatants were filtered using syringe filters (mixture membrane, 25 mm diameter, 0.45 μm pore size), and the remaining Sb(III) concentration in the solution was determined using the atomic absorption spectrophotometer (AAS). To ensure the accuracy and reliability of the collected data, all experimental results were corrected by blank tests in which no adsorbent was added into the Sb(III) solution, and all adsorption experiments were performed in duplicate. The Sb(III) adsorbed by GO was calculated as the difference between the initial and equilibrium mass of Sb(III) in the aquatic phase using the following equation: qt =
(C0 − Ct )V /1000 W
(1)
−1
where qt (mg·g ) is the amount of Sb(III) adsorbed at time t, C 0 and C t (mg·L −1 ) are the initial and equilibrium concentrations of Sb(III) in the solution, respectively, V (mL) is the volume of the Sb(III) solution, and W (g) is the amount of adsorbent. The Sb(III) removal efficiency E (%) is calculated by the following equation: E /% =
C0 − Ct × 100 C0
(2)
2.4. Batch Desorption Procedure. The Sb(III) desorbed GO was prepared by placing 20 mg of GO into Sb(III) solution (10 mg·L−1) and shaking on an shaker at 120 rpm until the GO was adsorptively saturated. The Sb(III) desorbed GO was separated from the solution through a membrane ((mixture membrane, 25 mm diameter, 0.45 μm pore size)), washed using double distilled water, dried at oven temperature 70 °C, and then 20 mg was placed in 0.1 mol·L−1 ethylenediaminetetraB
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Removal efficiencies of Sb(III) on the GO under various initial solution pH values ranging from 1.0 to 12.0 were presented in Figure 3a. It is founded that the removal efficiency was sharply raised from 80 % to almost 100 % when the pH of the initial solution increased from 1.5 to 2.0, followed by the removal efficiency remaining constant at 100 % in the pH range of 2.0 to 10.0, and then the efficiency rapidly decreasing to below 68 % as the pH of the initial solution changed from 10.0 to 12.0. According to the literature,18 species of Sb(III) are pH dependent: at the strong acidic condition (pH < 2), positive species of Sb(III), SbO+, and Sb(OH)2+, are predominant; when the pH is between 2.0 and 10.04, neutral species, Sb(OH)3, and HSbO2, are the main species, and when the solution is strongly basic (pH > 10), only SbO2− exists. Since there existed mainly positive and negative species of the Sb(III) under the strong acidic condition (pH < 2) and the strong basic condition (pH > 10), respectively, these positive and negative species would compete with the H+ and OH− for occupying the adsorption binding sites on the GO, respectively, resulting in the removal percentage of Sb(III) being lower than that at pH 2.0 to 10.0. Since pH value 7 is close to that at which pollutants are usually found in the environment, thus, this value is selected for further studies. The adsorbent dosage is also one of the crucial parameters in the adsorption processes, as it determines the adsorbent capacity for a given initial concentration. Figure 3b shows the effect of the GO dosage on the adsorption of Sb(III) from the solution under the condition of initial solution pH 7.0, contact time 120 min, solution temperature 298 K, and initial Sb(III) concentration 10 mg·L−1. It is clear from the figure that the amount of Sb(III) adsorbed increased as the GO dosage increased, until it reached 100 % when 30 mg of GO was used. The amount of Sb(III) adsorbed increased along with the GO dosage mainly because the higher dosage provided more available surface areas of GO, and thus more active sites available for adsorption. In the following experiments, a dosage of 20 mg of GO was selected, which corresponded to 88 % removal efficiency. 3.3. Effect of Contact Time and Temperature on Adsorption. The influence of the contact time and temperature are the most essential parameters in practical applications. The result of the effect of contact time and temperature on the removal percentage of Sb(III) are shown in Figure 4. The results indicated that the removal percentage increased with the increase of time for three temperatures. The equilibrium time is about 10, 10, and 15 min for 318, 308, and 298 K, respectively, the short equilibrium time is very important for the adsorption. After equilibrium time, there is no significant increase in the adsorption percentage that occurred, so 20 min was chosen as the optimum time at 298 K. It is also apparent that the absorption process is more favorable when the temperature increased; especially when temperature increased from (298 to 308) K, the removal efficiency of Sb(III) increased from about 88 % to 96 %. This is mainly because the increase of the temperature strengthened the surface activity of GO. As a result, the attractive forces between GO surface and Sb(III) are strengthened and then the sorption increased. Leng et al.1 has obtained a similar result using graphene. 3.4. SEM and EDAX Analysis. The SEM images of the untreated and Sb(III)-loaded adsorbent are shown in Figure 5. Energy dispersive X-ray analysis (EDAX) was applied to investigate the change in the elemental contents of GO before
acetic acid (EDTA; 50 mL) at the temperatures of 298 and 333 K, shaking the solution until the desorption reached equilibrium.
3. RESULTS AND DISCUSSION 3.1. Characterization of GO. XRD patterns of graphite and GO are exhibited in Figure 1a. A sharp diffraction peak at 2θ = 26.5° appeared in the XRD pattern of graphite, while a broad and relatively weak diffraction peak is observed at 2θ = 10.9° for GO, corresponding to the typical diffraction peak of GO nanosheets,16 which indicates graphite was almost oxided into GO. The GO could also be characterized by FTIR spectroscopy; the functional groups are shown in Figure 1b, such as the O−H peak at 3433 cm−1, the CO group at 1630 cm−1, the C−OH group at 1388 cm−1, and the C−O−C group at 1098 cm−1. The detailed deconvolution of XPS spectra has been carried out using multipack software, and it is plotted in Figure 1c. The peaks at 284.9, 287.0, 288.3, and 289.3 eV (Figure 1c) are assigned to carbon atoms in CC/C−C, C− O, CO, and O−CO, respectively, which suggested that graphite have been successfully oxided to GO.17 The isoelectric point (IEP) of GO, which is determined as the pH location where the ζ potential equals zero, is around 1.0. The surface area of the GO is determined as 315.59 m2·g−1 by the N2 adsorption−desorption method (Figure 2). This
Figure 2. Nitrogen adsorption−desorption isotherm and pore size distribution (inset) of GO.
value is different from the theoretical surface area which is reported as 2630 m2·g−1 in the literature. The discrepancy of the surface area between the one obtained in this study and the theoretically reported one in the literature is attributed to the incomplete exfoliation with ultrasonication in the study since the ultrasonication might not overcome van der Waals force between each single layer of GO.1 3.2. Influence of pH and Adsorbent Dosages. The pH value of the solution is one of the most important environmental factors that characterize the metal ion’s adsorption because it influences both the metal binding sites on the adsorbent and the metal ion chemistry in the aqueous solution. The effect of pH on Sb(III) removal was investigated under the conditions of pH range of 1.0 to 12.0, initial Sb(III) concentration of 10 mg·L−1, and GO dosage of 0.6 g·L−1. The results are shown in Figure 3a. C
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Figure 3. (a) Effect of initial solution pH on the Sb(III) adsorption removal (C0 = 10 mg·L−1, dosage = 30 mg/(50 mL), T = 298 K, contact time = 120 min, and agitation speed = 120 rpm); (b) effect of GO dosage on the Sb(III) adsorption removal (C0 = 10 mg·L−1, volume = 50 mL, T = 298 K; contact time = 120 min, agitation speed = 120 rpm, and initial solution pH = 7.).
between the ions and the GO. Ions of Cl−, NO3−, and SO42− could only have weak bonding interaction with the functional groups on GO and form an outer-sphere complex with GO, which is the same as they complex with ferric (hydr)oxides.19−23 Ions of PO43−, AsO33−, and CO32− have stronger bonding interaction with the GO and could form a relatively stronger inner-sphere complex,24,25 resulting in stronger negative influence on the adsorptive removal of Sb(III). These results are consistent with the previous studies.26 3.6. Adsorption Kinetics. Many mathematical models are used to describe the interactions between the pollutant molecules or ions and the solid adsorbent. To evaluate kinetics of the adsorption of Sb(III) anions onto the solid surface of GO, two kinetic models, namely, the pseudo-first-order and pseudo-second-order models, were used as given in following text. The linear form of pseudo-first-order kinetics is considered one of the most commonly used equations to describe adsorption from an aqueous environment by a solid adsorbent, and it can be given as
Figure 4. Effect of contact time and temperature on the adsorption of Sb(III) onto GO (pH = 7.0 ± 0.1, T = 298 K, C0 = 10 mg·L−1, and adsorbent dosage = 20 mg/(50 mL)).
and after Sb(III) adsorption. Before adsorption, irregular surface structure was observed in the surface. The mass transfer of Sb(III) onto GO was beneficial. After the reaction with Sb(III), we can clearly see that many crystals were absorbed onto GO. The EDAX spectrum shows that GO is mainly composed of carbon, oxygen, sulfur, potassium, and manganese, while the existence of sulfur, potassium, and manganese was induced by the use of sulfuric acid and potassium permanganate oxidized graphite in preparing GO. Through the comparison of EDAX spectrum before and after adsorption, the adsorption of Sb(III) onto GO was further confirmed. The increase in O content was due to the adsorption of antimony(III) oxide. 3.5. Effect of Coexisting Anions. Since the Sb(III) exits as anions,18 coexisting anions in wastewater are an important factor that should be considered during the Sb(III) adsorption process. In this study, the influence of six coexisting anions (Cl−, NO3−, SO42−, CO32−, PO43−, and AsO43−) on Sb(III) removal by GO was examined, and the results are shown in Figure 6. From Figure 6, it is found that the ions of Cl−, NO3−, and SO42− have negligible influence on the removal of Sb(III) while the ions of PO43−, AsO33−, and CO32− have slightly decreased the removal efficiency from 100 % to (98.5 to 99) %. This phenomenon could be attributed to the bonding interaction
qt = qe(1 − e−k1t )
(3) −1
where qe and qt (mg·g ) are the amount of Sb(III) adsorbed per unit weight of the adsorbent at equilibrium and time t and k1 (g·mg−1·min−1) is the pseudo-first-order rate constant.27 The adsorption kinetics can also be represented by the pseudo-second-order model, which is the expressed linear form in the following: ⎛ ⎞−1 1 1 1 − qt = ⎜⎜ + t ⎟ k 2qe2 ⎟⎠ ⎝ qe −1
(4)
−1
where k2 (g·mg ·min ) is the pseudo-second-order rate constant. The constants can be determined experimentally by plotting t/qt.28 Figure 7 and Table1 showed the linear plot of the pseudofirst-order and pseudo-second-order models of Sb(III). In contrast to the pseudo-second-order model, the exceptionally high correlation coefficient (R2 > 0.999) and closer values of qe,cal (qe obtained by calculation) and qe,exp (qe obtained by experiment) suggested that the adsorption data of Sb(III) fit well with the pseudo-first-order model.29 3.7. Adsorption Isotherm. Adsorption isotherm is usually used to express the surface properties and affinity of the D
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Figure 5. SEM images and EDAX spectrum of GO before and after Sb(III) adsorption: a and c, before Sb(III) adsorption; b and d, after Sb(III) adsorption.
Figure 7. Comparison of different models in fitting of kinetics data for Sb(III) adsorption on GO (pH = 7.0 ± 0.1, T = 298 K, C0 = 10 mg· L−1, and adsorbent dosage = 20 mg/(50 mL)). Figure 6. Effect of coexisting anions on the Sb(III) adsorption on the GO (pH = 7.0 ± 0.1, T = 298 K, C0 = 10 mg·L−1, and adsorbent dosage = 30 mg/(50 mL)).
where Q0 (mg·g−1) is a constant related to the maximum amount of adsorbed solute, b (L·mg−1) is a constant related to the energy of adsorption, and Ce (mg·L−1) is the equilibrium Sb(III) concentration in solution. The values of constants Q0 and b can be determined experimentally by plotting 1/qe versus Ce. The Freundlich isotherm is particularly suitable as an empirical isotherm for heterogeneous surfaces by multilayer adsorption with interaction between adsorbed molecules. The Freundlich isotherm is commonly expressed by 1 log qe = log K f + log Ce (6) n
sorbent. Furthermore, it can be applied to compare the adsorptive capacities of the absorbent for different pollutants in aqueous solution. In this research, the Langmuir and Freundlich equations were used to fit the experimental data. The Langmuir isotherm, originally developed to describe the sorption of gases onto the solid surface, is the simplest adsorption that has been used extensively by many researchers to model the solute adsorption in various adsorbents from a liquid solution. The linear form of the Langmuir equation is written as Ce C 1 = 0 + e0 qe Qb Q
where Kf (mg·g−1) and 1/n (L·g−1) are the constants that are characteristics of the system, which indicate the adsorption capacity of the absorbent and the intensity of the adsorption, respectively. The Freundlich coefficients can be determined from the plot of log qe versus log Ce. Figure 8 shows the
(5) E
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Table 1. Comparison Constants of the Pseudo-First-Order and Pseudo-Second-Order Adsorption Kinetic Models pseudo-first order
pseudo-second order
C0
qe,exp
qe,cal
(mg·L−1)
(mg·g−1)
(mg·g−1)
k1
R2
(mg·g−1)
k2
R2
10
21.98
21.99
0.1688
0.999
20.29
0.009
0.951
qe,cal
(ΔG), and entropy (ΔS) were determined from the experimental data at different temperatures using the following equations:36
Langmuir and Freundlich isotherm plots for the adsorption of Sb(III) onto GO.
ΔG = ΔH − T ΔS
(7)
ΔG = −RT ln KD
(8)
ln KD = ΔS /R − ΔH /RT
(9)
where KD is the distribution coefficient; the values of KD can be obtained by plotting ln(qe/Ce) vs qe and extrapolating to zero qe;31 T (K) is the absolute temperature; ΔH and ΔS are obtained from the slope and intercept of the van’t Hoff plot of ln KD versus 1/T, respectively, and R (8.314 × 10−3 kJ·mol−1· K−1) is the universal gas constant. According to the result of the adsorption isotherm experiments, 1/T is taken as the abscissa and ln KD as the ordinate to plot the linear fitting. The result is shown in Figure 9. The slope and intercept of the linear plot were obtained from
Figure 8. Comparison of different isotherms in fitting of isothermal data for Sb(III) adsorption on GO (pH = 7.0 ± 0.1, T = 298 K, contact time = 120 min, and adsorbent dosage = 20 mg/(50 mL)).
As shown in Figure 8, the regression coefficients of the Langmuir model are higher than those of the Freundlich model, indicating that the isotherm data fit the Langmuir model better, and the adsorption process is a monolayer adsorption in a surface with a finite number of identical sites, which are homogeneously distributed over the adsorbent surface. The saturated adsorption capacity (Q0) of Sb(III) onto GO at 298 K is 36.49 mg.g−1, which is good agreement with the experimental values (qe,exp = 34.24 mg.g−1). This result suggests that the adsorption capacity of GO is much higher than many other adsorbents such as multiwalled carbon nanotubes (MWCNTs; 0.33 mg·g−1),30 bentonite (0.56 mg·g−1),31 cyanobacteria microcystis biomass (4.88 mg·g −1 ), 32 Fe 2 O 3 -modified MWCNTs (6.23 mg·g−1),33 graphene (8.06 mg·g−1),1 hydrous oxide of Fe and Mn (12.2,17.1 mg·g−1),34 and chemically bonded adsorbent (21.9 mg·g−1).35 The best-fit Freundlich parameters 1/n at 298 K is 0.1708. The 1/n value of less than 0.5 indicates a favorable adsorption process. The parameters of (308 and 318) K are also listed in Table 2. Generally, the amount adsorbed at equilibrium (qe) increased when the solution temperature increased, indicating the endothermic nature of the adsorption process of Sb(III) by GO from the aqueous solution. 3.8. Thermodynamic Data. To study the thermodynamics of the Sb(III) adsorbed onto GO, the thermodynamic constants such as change in enthalpy (ΔH), Gibbs free energy
Figure 9. Relationship of KD and T.
Figure 9, and then ΔH and ΔS were determined according eq 9; the ΔG values corresponding each temperature were also found according to eq 7. The thermodynamic constants derived from the adsorption of Sb(III) onto GO are listed in Table 3. The magnitude and direction of the driving force of a reaction depend on the magnitude and tendency of ΔH, ΔS, and the temperature. The negative value of ΔG for Sb(III) at all temperatures indicates that the sorption process of Sb(III) on GO is spontaneous
Table 2. Langmuir and Freundlich Isotherm Parameters for Adsorption of Sb (III) on GO Langmuir isotherm
Freundlich isotherm
T
qe,exp
Q0
b
K
(mg·g−1)
(mg·g−1)
(L·mg−1)
R2
(mg·g−1)
1/n
R2
298 308 318
34.24 35.57 36.72
36.49 37.87 39.21
0.51 0.52 0.53
0.9787 0.9755 0.9824
19.32 19.77 20.25
0.1708 0.1762 0.1789
0.8822 0.8625 0.8539
Kf
F
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4. CONCLUSION This research demonstrated that the kinetics and isotherm for the adsorption of Sb(III) onto GO could be well fitted by the pseudo-first-order kinetics and Langmuir isotherm, respectively, and the adsorption was a spontaneous and endothermic process. The results presented in this work showed that GO possesses excellent adsorption ability toward Sb(III) in aqueous solution. The removal percentage of Sb(III) onto GO is very high within a wide pH range of 2.0 to 10.0. In addition, the adsorption capacity of GO is much greater than many other reported adsorbents such as MWCNTs,33 bentonite, 31 cyanobacteria microcystis biomass, 34 Fe 2 O 3 -modified MWCNTs,35 graphene,1 hydrous oxide of Fe and Mn,36 and chemically bonded adsorbent.37 Further, the consecutive adsorption−desorption cyclic experiments demonstrated that the adsorption capacity of GO are well kept. Therefore, it might be concluded that the GO could be used as an promising adsorbent for removal of Sb(III) from aqueous systems.
Table 3. Thermodynamic Parameters for Adsorption of Sb (III) onto GO T
ΔG
ΔH −1
ΔS −1
K
(kJ·mol )
(kJ·mol )
298 308 318
−4.876 −5.096 −5.316
1.68
(kJ·mol
−1
·K
−1
)
0.022
thermodynamically. The obtained positive ΔH (1.68 kJ·mol−1) values suggest that the adsorption process of Sb(III) on GO is endothermic and the adsorption process is more favorable at higher temperature.31 The positive standard entropy change (ΔS, 0.022 kJ·mol−1·K−1) could be due to the occurrence of a structure breaking process during heating, such as changes of ion hydration ratio or dissociative chemisorption. Moreover, the Langmuir constant, b, which is a temperature dependent equilibrium constant, follows a van’t Hoff equation: b = b0 exp( −ΔH /RT )
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AUTHOR INFORMATION
Corresponding Author
where R and T are defined previously, ΔH is the heat of adsorption, and b0 is the nature of a frequency factor.37 Since the adsorption of Sb(III) is endothermic (ΔH > 0), b should increase with increasing temperature. The results presented in Tables 2 and 3 showed similar trends.31 3.9. Desorption Studies. For practical application, reusability was an important parameter for the potential application of an adsorbent. In this study, desorption tests were performed to elute Sb(III) adsorbed on GO using 0.1 M EDTA solutions. When the EDTA was applied as an eluent, the interaction between GO and Sb(III) was disrupted and Sb(III) was released into the eluent subsequently. After the desorption in EDTA solution reached equilibrium, the regenerated GO was reused in the following Sb(III) adsorption experiments under the same condition as the previous adsorption process. The adsorption efficiency of Sb(III) onto the regenerated GO only decreased by 14.5 % after four consecutive adsorption− desorption cycles (shown in Figure 10). Desorption experiments under different temperatures, 298 and 333 K, have been carried out. And the result showed that the desorption velocity decreased with the increase of the temperatures.
*E-mail:
[email protected]. Funding
This work was supported by the National Science & Technology Pillar Program of Chinathe Twelfth Five-year Planunder Contract No. 2012BAJ24B03. Notes
The authors declare no competing financial interest.
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Figure 10. Adsorption−desorption cycles of GO (GO = 20 mg, C0 = 10 mg·L−1, adsorption time = 120 min, desorption time = 120 min, and pH = 7.0 ± 0.1). G
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DOI: 10.1021/je5009262 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
