ARTICLE pubs.acs.org/JPCC
Porous Hierarchically Micro-/Nanostructured MgO: Morphology Control and Their Excellent Performance in As(III) and As(V) Removal Xin-Yao Yu, Tao Luo, Yong Jia, Yong-Xing Zhang, Jin-Huai Liu,* and Xing-Jiu Huang* Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, People's Republic of China
bS Supporting Information ABSTRACT: Porous micro-/nanostructured MgO were successfully synthesized through a facile method. Flower-like and nest-like micro-/nanostructured MgO were obtained by adjusting the concentration of precipitant. When tested as adsorbent in arsenic removal, the as-prepared micro-/nanostructured MgO were effective for both As(III) and As(V) removal, particularly the As(III). The adsorption capacities of these micro-/nanostructured MgO for As(III)/As(V) are much higher than those reported for other micro-/nanostructured metal oxides. The adsorption kinetics and adsorption mechanism for As(III) and As(V) onto micro-/nanostructured MgO were also investigated. The high uptake capability of the as-prepared micro-/ nanostructured MgO make it a potentially attractive adsorbent for the removal of both As(III) and As(V) from water.
1. INTRODUCTION In recent years, three-dimensional (3D) hierarchically micro-/ nanostructured metal oxides have been received great research interest. The synergistic effect of their nanometer-sized building blocks and overall micrometer-sized structure may be desirable for a variety of applications especially in water purification.18 Their nanometer-sized building blocks provide a high surface area, a high surface-to-bulk ratio, and surface active sites which can interact with micropollutants, e.g., heavy metal ions and organic micropollutants and their overall micrometer-sized structure provides desirable mechanical strength, facile transportation, and easy recovery.1 Arsenic has been ranked as a high priority top 20 hazardous substances by the Agency for Toxic Substances and Disease Registry.9 Long-term drinking water exposure causes skin, lung, bladder, and kidney cancers.10 Arsenic in natural waters is a worldwide problem. Arsenic pollution has been reported recently in China, Chile, India, Taiwan, USA, Argentina, Poland, Canada, Japan, New Zealand, Hungary, and Mexico.10 A strict guideline limit of 10 ppb provided by the World Health Organization has been adopted as the drinking water standard by many countries.11 The predominant forms of As in groundwater and surface water are the inorganic species arsenate As(V) and arsenite As(III).9 The As(V) species exists as oxyanions (H2AsO4 and HAsO42).9 As(III) is a neutral, uncharged molecule H3AsO3 at the pH of most natural waters and is more mobile as it is less strongly adsorbed on most mineral surfaces than the negatively charged As(V) species.12 It has been recognized that As(III) is more prevalent in groundwater than was previously understood, which is of concern as it is more toxic than As(V).12 Many different methods such as r 2011 American Chemical Society
ion-exchange, precipitation, coprecipitation, adsorption, ultrafiltration, and reverse osmosis have been used for arsenic removal.10 Due to its simplicity and cost effectiveness, adsorption has been widely used to remove arsenic from water. Hierarchically structured Fe2O3, Fe3O4, CuO, and CeO2 have been used for adsorption of arsenic from water.58 However, most of these studies focused on the removal of As(V) which is less toxic and relatively easier to remove. In addition, the adsorption capacities for arsenic are too low (lower than 10 mg/g) in these studies. The pH values in their studies all are below 5, while the pH value of real water is close to neutral. In real applications of these micro-/nanostructures, some pretreatment processes including the oxidation of As(III) to As(V) and adjustment of the pH value are needed. However, the pretreatment results not only in higher run cost but also in more complex operation. This may limit their application in purification of real water. Therefore, there is a urgent demand for economical, effective, and reliable hierarchically micro-/nanostructured metal oxides that are capable of removing both As(III) and As(V) simultaneously with high adsorption capacities from contaminated drinking water. As a nontoxic and environmentally friendly material, MgO/ Mg(OH)2 has already been widely used to remove dyes and soluble toxic ions from water.1317 As the pH of zero point of charge of MgO is 12.4, it is a suitable adsorbent for adsorption of anions due to its favorable electrostatic attraction mechanism.13 Recently, Li et al. demonstrated that MgO nanoflakes have exceptional As(III) adsorption capacity.17 However, the MgO Received: August 7, 2011 Published: October 10, 2011 22242
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The Journal of Physical Chemistry C nanoflakes used in that study were synthesized by a hydrothermal method which could not satisfy the mass production of adsorbents. Also the MgO nanoflakes are apt to aggregation in real water purification applications. In addition, the adsorption performance of the MgO nanoflakes for As(V) was not investigated. It is therefore necessary to develop hierarchically micro-/ nanostructured MgO which are simply synthesized and effective for both As(III) and As(V) removal. In this study, porous hierarchical flower-like and nest-like micro-/nanostructured MgO were synthesized by a facile method. The prepared MgO were used to remove both As(III) and As(V) from water. Both flower-like and nest-like MgO showed excellent performance for As(III)/As(V) removal compared with other reported hierarchical metal oxide micro-/ nanostructures.
2. EXPERIMENTAL SECTION Materials. Magnesium nitrate hexahydrate (Mg(NO3)2 3 6H2O), potassium carbonate (K2CO3), nitric acid (HNO3), sodium hydroxide (NaOH), and sodium arsenate dodecahydrate (Na3AsO4 3 12H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. As(V) stock solution with a concentration of 1000 mg/L was prepared by dissolving Na3AsO4 3 12H2O in double deionized water. As(III) stock solution with a concentration of 1000 mg/L in HNO3 medium was obtained from CANSPEC Co., Ltd., Shanghai, China. All chemicals were of analytical grade and used without further purification. Synthesis of Flower-like and Nest-like MgO Precursor and MgO. For synthesis of a flower-like MgO precursor, 1 M Mg(NO3)2 3 6H2O was heated to 393 K in an oil bath. Then 1 M K2CO3 solution which was heated to boiling was rapidly added into the vigorously stirred (ca. 1000 rpm) Mg(NO3)2 solution within 5 s. The mixture was further stirred for 1 min and then maintained at 393 K aging for 2 h under static conditions. After that, a white precipitate was collected and washed 3 times with double deionized water and ethanol, respectively. While for the synthesis of nest-like MgO precursor 0.5 M K2CO3 solution was used without changing other experimental conditions. To obtain MgO samples, these as-prepared MgO precursors were then calcined in a muffle furnace at 973 K for 4 h with the temperature increase rate of 5 °C/min and furnace cooling. Adsorption Experiments. For the comparisons of arsenic adsorption performance between MgO precursors and MgO, the initial As(III) and As(V) concentrations were 4.639 and 7.189 mg/L, respectively. The adsorbent dose was 0.3 g/L in a comparison study. In the kinetic study of arsenic adsorption on MgO samples, the initial As(III) and As(V) concentrations were 1.226 and 1.359 mg/L, respectively. To investigate the effect of adsorbent dose on adsorption kinetics, different loadings of MgO samples were used. For adsorption of As(III), loadings of MgO samples ranged from 0.1 to 0.5 g/L. For adsorption of As(V), loadings of MgO samples ranged from 0.1 to 0.3 g/L. The pH values of all the arsenic solutions were adjusted to 7.0 ( 0.2 by using HNO3 and NaOH. These samples were placed on a shaker for stirring. At predetermined time intervals, stirring was interrupted while 6 mL of supernatant solutions were pipetted and centrifuged for the determination of the remaining arsenic concentrations. For the equilibrium adsorption isotherm study, MgO samples with a loading of 0.3 g/L were added to arsenic solutions with different initial concentrations. The pH values of
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all these arsenic solutions were adjusted to 7.0 ( 0.2. These samples were mounted on the shaker for stirring 24 h. The adsorption capacity of the adsorbents for arsenic was calculated according to the equation qe ¼
ðC0 Ce ÞV m
ð1Þ
where C0 and Ce represent the initial and equilibrium arsenic concentrations (mg/L), respectively. V is the volume of the arsenic solution (mL), and m is the amount of adsorbent (mg). All the adsorption experiments were carried out at room temperature (298 ( 2 K). All the experimental data were the average of triplicate determinations. The relative errors of the data were about 5%. Characterization. The scanning electron microscopy (SEM) images were taken by using a field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F, 10 kV). The transmission electron microscopy (TEM) investigations were carried out in a JEOL JEM-2000EX operating at 100 keV. X-ray diffraction (XRD) was performed on a D/MaxIIIA X-ray diffractometer (Rigaku Co., Japan), using Cu Kα (λKα1 = 1.5418 Å) as the radiation source. Thermogravimetric/derivative thermogravimetric (TG/DTG) analysis was performed on a SDT-Q600 DTG-TGA instrument. The nitrogen adsorption and desorption isotherms at 77 K were measured with a Micromeritics ASAP 2020 M analyzer. The Brunauer, Emmett, and Teller (BET) equation was used to obtain the specific surface areas (SBET) and the adsorption average pore width (WP). The amount of N2 adsorbed at relative pressures near unity (P/P0 = 0.99) was employed to determine the total pore volume (Vt). The density functional theory (DFT) was employed to analyze the successive pore size distribution (PSD) curves. The Fourier transform infrared (FT-IR) spectra of the obtained samples were recorded with a NEXUS-870 FT-IR spectrometer in the range of 4000400 cm1. X-ray photoelectron spectroscopy (XPS) analyses of the samples were conducted on a VG ESCALAB MKII spectrometer using an Mg Kα X-ray source (1253.6 eV, 120 W) at a constant analyzer. The arsenic concentration was determined in the liquid phase using inductively coupled plasma atomic emission spectrometry (ICP-AES, Jarrell-Ash model ICAP 9000).
3. RESULTS AND DISCUSSION Mg(NO3)2 3 6H2O and K2CO3 have been used as sources to synthesize MgO microstructures.18,19 However, most of these studies need a step of adjusting the pH value of the solution. There is no investigation into the concentration effect of K2CO3 on the morphology of MgO microstructures. Figure 1 shows the morphologies of the synthesized MgO precursors. The insets of panels a/b and c/d of Figure 1 are the SEM/TEM images of individual MgO precursor synthesized at two different K2CO3 concentrations, respectively. From Figure 1, we can see that the concentration of K2CO3 has an obvious effect on the morphology of MgO precursors. A uniform flower-like morphology in the size range of 23 μm is observed when 1 M K2CO3 was used (Figure 1a,b). When 0.5 M K2CO3 was used, a nest-like morphology with the same size as that of flower-like MgO precursor is obtained. Although the morphology of these two MgO precursors is different from each other, both of them are assembled with nanoflakes structures as building blocks indicating that the crystal nuclei tend to assemble into a flake-like 22243
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Figure 1. (a, b) SEM images of F-hydromagnesite (flower-like). (c, d) SEM images of N-hydromagnesite (nest-like). The insets of (a) and (b) are SEM images of a single F-hydromagnesite and N-hydromagnesite, respectively. The insets of (c) and (d) are TEM images of a single F-hydromagnesite and N-hydromagnesite, respectively.
Figure 2. XRD patterns of hydromagnesite. The inset is the XRD pattern of MgO.
structure in the MgOCO2H2O system.20 Both surfaces and edges of these two MgO precursor samples are smooth, and the nanoflakes are dense. The size of these nanoflakes ranges from tens of nanometers to hundreds of nanometers, and the thickness of these nanoflakes which is in the range of tens of nanometers could be roughly estimated from the SEM/TEM images. XRD was used to identify the phase structure of the MgO precursors. As shown in Figure 2, all diffraction peaks in the XRD pattern of these two MgO precursors can be readily indexed to be monoclinic hydromagnesite Mg5(CO3)4(OH)2 3 4H2O as confirmed from the reported data (JCPDS file no. 25-0513).21 Therefore, the two MgO precursors fabricated at two different
K2CO3 concentrations are assigned to flower-like hydromagnesite (F-hydromagnesite) and nest-like hydromagnesite (N-hydromagnesite), respectively. The TGADTA technique was used to analyze the thermal behavior of the hydromagnesite samples and thus could provide the necessary data for the following decomposition process. The TGADTA curves for the pyrolysis of as-prepared hydromagnesite are shown in Figure 3. It can be seen that for both F-hydromagnesite and N-hydromagnesite three mass-loss steps decomposition took place. The first mass loss corresponds to the removal of the water of crystallization followed by the removal of CO2 and further decomposition of the hydroxide.21 The complete decomposition temperatures of the hydromagnesite is about 973 and 873 K for F-hydromagnesite and N-hydromagnesite, respectively. For comparison, both F-hydromagnesite and N-hydromagnesite were calcined at 973 K for 4 h. A typical XRD powder pattern of the MgO sample prepared by the decomposition of hydromagnesite is shown in the inset of Figure 1. All of the reflections in the inset of Figure 1 can be indexed to be a cubic phase of MgO in agreement with the values reported in the literature (JCPDS file no. 79-0612).21 No remaining Mg5(CO3)4(OH)2 3 4H2O peaks could be observed suggesting the completely conversion of Mg5(CO3)4(OH)2 3 4H2O to MgO after calcination. The SEM images of MgO samples after calcination of Mg5(CO3)4(OH)2 3 4H2O are shown in Figure 4. Panels a/b and c/d of Figure 4 correspond to F-hydromagnesite and N-hydromagnesite, respectively. We can see that the morphology of 22244
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The Journal of Physical Chemistry C Mg5(CO3)4(OH)2 3 4H2O did not change a lot after calcination. Compared to Mg5(CO3)4(OH)2 3 4H2O, the surface of MgO samples becomes rough. TEM observations (Figure 5) were conducted in order to investigate the surface of the MgO samples in detail. It can be seen that the MgO samples are highly porous and some of the pores are via holes. During the calcination
Figure 3. TGDTG curves of F-hydromagnesite (a) and N-hydromagnesite (b).
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process of Mg5(CO3)4(OH)2 3 4H2O, water vapor and CO2 were produced, and pores and holes could be created which could increase the specific surface area of MgO samples. The flowerand nest-like MgO samples are assigned to F-MgO and N-MgO, respectively. To further analyze and quantify the pore structure of MgO samples, the nitrogen adsorptiondesorption isotherms for the MgO precursors and MgO are shown in panels a and b of Figure 6. Panels c and d of Figure 6 show the pore size distribution of F-MgO and N-MgO, respectively. The pore structure parameters of these samples, such as specific surface area, pore volume, and average pore size are listed in Table S1 in the Supporting Information. The SBET values of F-hydromagnesite and N-hydromagnesite are 21.2755 and 18.0631 m2/g, respectively. After calcination, the SBET of MgO exceeds that of MgO precursor and the SBET of N-MgO (32.9650 m2/g) exceeds that of F-MgO (24.6622 m2/g). From Figure 6c we can see that the pore size distribution of F-MgO is quite broad and multimodal with small mesopores (ca. 25 nm) and larger ones (ca. 1050 nm). While the pore size distribution of N-MgO is very narrow with an average Wp of 21.7847 nm which exceeds that of F-MgO (12.2960 nm). In addition, the Vt of N-MgO (0.1795 cm3/g) is higher than that of F-MgO (0.0758 cm3/g). The difference in specific surface area and pore volume between F-MgO and N-MgO is most likely due to the change in pore structure at higher temperature. To investigate the potential application of the synthesized porous MgO for arsenic removal, the adsorption performance of MgO precursors and MgO samples for As(III) and As(V) were tested. Figure 7 shows the As(III)/As(V) adsorption
Figure 4. (a, b) SEM images of F-MgO. (c, d) SEM images of N-MgO. 22245
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Figure 5. (a, b) TEM images of F-MgO. (c, d) TEM images of N-MgO.
Figure 6. N2 adsorption/desorption isotherms of F-hydromagnesite/N-hydromagnesite (a) and F-MgO/N-MgO (b). Pore size distributions of F-MgO (c) and N-MgO (d).
performance of MgO precursors and MgO samples at an initial As(III)/As(V) concentration of 4.639/7.189 ppm. For both
As(III) and As(V) adsorption, F-hydromagnesite has higher adsorption capacity than N-hydromagnesite while N-MgO has 22246
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The Journal of Physical Chemistry C higher adsorption capacity than F-MgO. This can be explained from the SBET of MgO precursors and MgO samples. We have shown that the SBET of F-hydromagnesite is higher than that of N-hydromagnesite while N-MgO has higher SBET than F-MgO. As the MgO precursors do not show higher performance for As(III)/As(V) removal, only MgO samples were tested and compared in the following adsorption kinetics and isotherm studies. The kinetics of adsorption which describes the solute uptake rate governing the residence time of the adsorption reaction is one of the most important characteristics that define the efficiency of adsorption. The kinetics of As(III) and As(V) adsorption at different F-MgO/N-MgO loadings in the lab-prepared water samples are shown in Figure 8. For both As(III) and As(V),
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with the increase of F-MgO/N-MgO loadings, the removal rate constant increases and the final equilibrium As(III)/As(V) concentration in the treated water decreases. The adsorption of As(III) and As(V) was rapid at first and then slowed considerably. Arsenic was initially adsorbed by the exterior surface of the MgO nanoflakes. When the adsorption at the exterior surface reached the saturation level, the arsenic began to enter the nanoflakes via the pores within the flakes and was adsorbed by the interior surface of the flakes. When arsenic ion diffused into the pores of the nanoflakes, the diffusion resistance was increased, which in turn led to a decrease in diffusion rate. Figure 8a shows the variation of As(III) concentration with time at an initial As(III) concentration of 1.226 ppm. It demonstrates that the removal of As(III) by N-MgO is faster and more efficient than F-MgO. Figure 8b shows the variation of As(V) concentration with time at an initial As(V) concentration of 1.359 ppm. It can be seen that N-MgO also shows higher performance than F-MgO in As(V) removal. The higher SBET of N-MgO contributes to their better adsorption kinetics. The above adsorption kinetic experimental data can be best fitted into a pseudo-secondorder rate kinetic model. The pseudo-second-order models is presented as t 1 1 ¼ þ t qt k2 qe 2 qe
Figure 7. Comparison of the As(III)/As(V) adsorption performance of MgO precursors and MgO samples at an initial As(III)/As(V) concentration of 4.639/7.189 ppm.
ð2Þ
where qe and qt are the amount of As(III)/As(V) adsorbed at equilibrium and at time t, respectively. k2 is the rate constant of the pseudo-second-order model of adsorption (g/mg/min). For the pseudo-second-order model, the values of k2 and qe can be obtained by a plot of (t)/(qt) against t. The pseudo-second-order kinetics plots for the adsorption of As(III) and As(V) onto MgO samples are shown in panels c and d of Figure 8, respectively. Panels c and d of Figure 8 demonstrate that the experimental data
Figure 8. Adsorption kinetics of As(III) (a) and As(V) (b) onto MgO samples at an initial As(III)/As(V) concentration of 1.226/1.359 ppm. Pseudosecond-order kinetics plots for the adsorption of As(III) (c) and As(V) (d) onto MgO samples. 22247
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Figure 9. Adsorption isotherms of As(III) (a) and As(V) (b) onto MgO samples. Linearized Freundlich isotherm for As(III) adsorption (c) and linearized Langmuir isotherm for As(V) adsorption (d) by MgO samples.
could be well fitted with the linear form of the pseudo-secondorder model. The correlation coefficient values for pseudosecond-order model are all above 0.97 (Tables S2 and S3, Supporting Information), suggesting that the pseudo-secondorder model best represents the adsorption kinetics in our adsorbent systems. With the increase of MgO loading, the removal efficiency of As(III)/As(V) increases and the rate constant (k2) also improves indicating a faster adsorption of arsenic. Tables S2 and S3 (Supporting Information) also show that the rate constant of As(V) on F-MgO/N-MgO is higher than that of As(III) under the similar experimental conditions which indicates that the MgO samples remove As(V) faster than As(III). In order to evaluate the adsorption capacities of F-MgO/ N-MgO for As(III) and As(V) near the neutral pH environment, the equilibrium adsorption isotherm was investigated by varying the initial arsenic concentrations. Panels a and b of Figure 9 show the adsorption isotherms of As(III)/As(V) on F-MgO/N-MgO at room temperature. Two empirical equations, Langmuir and Freundlich isotherm models, were used to analyze the experimental data. The mathematical expressions of the Langmuir isotherm and the Freundlich isotherm models are Ce 1 Ce ¼ þ q m KL qe qm ln qe ¼
1 ln Ce þ ln KF n
ð3Þ
ð4Þ
where qm and KL are Langmuir constants, representing the maximum adsorption capacity of adsorbents (mg/g) and the energy of adsorption, respectively. KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively.
Table 1. Equilibrium Adsorption Isotherm Fitting Parameters for As(III)/As(V) onto F-MgO/N-MgO F-MgO As(III) adsorption
Freundlich
11/n
KF ((mg/g)
) 22.76
N-MgO 40.74
isotherm
As(V) adsorption
Langmuir
n
0.5775
0.7280
R2 qmax (mg/g)
0.99627 343.64
0.98155 378.79
isotherm KL (L/mg)
0.071
0.075
R2
0.99202
0.98341
For the Langmuir isotherm model, the values of qm and KL can be calculated from the slope and intercept of plots of (Ce)/(qe) versus Ce. For the Freundlich isotherm model, the values of n and KF can be obtained by a plot of ln qe against ln Ce. The parameters of the Langmuir and Freundlich models were calculated and are listed in Table 1. From Figure 9a we can see that the adsorption capacity of the F-MgO/N-MgO for As(III) did not reach the adsorption saturation and it could increase further with the increase of the equilibrium As(III) concentration. The adsorption capacity of N-MgO is higher than that of F-MgO. The adsorption capacities of N-MgO and F-MgO reach 643.8 and 252.3 mg/g at very low equilibrium As(III) concentrations of 7.8 and 4.5 mg/L, respectively. From the correlation coefficients, it can be seen that the adsorption data for As(III) fit the Freundlich isotherm model better than the Langmuir isotherm model. The Freundlich model curve fitting result is shown in Figure 9c. The As(V) adsorption data for both F-MgO and N-MgO could be best fitted with the Langmuir isotherm (Table 1 and Figure 9d). The maximum capacities of F-MgO and N-MgO for As(V) were 22248
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Table 2. The Maximal As(III)/As(V) Adsorption Capacities of F-MgO/N-MgO and Other Hierarchical Metal Oxide Micro-/Nanostructures adsorption capacity (mg/g) adsorbents
As(III)
As(V)
pH
reference
F-MgO/N-MgO
>252.34/643.84
343.64/378.79
7
this study
doughnut-like CuO
4.7
4
1g
γ-Fe2O3 flower
4.75
4
1h
Fe3O4 flower
4.65
4
1h
Chestnut-like Fe2O3 CeO2 flower
137.5 6.7
4 3
1e 1f
determined at 343.64 and 378.79 mg/g, respectively. The two different adsorption isotherm models may be attributed to the different surface charge effects of As(III) and As(V) species under the neutral pH environment. It has been demonstrated that As(III) exists predominantly as noncharged H3AsO3 and the predominant As(V) species exists as negatively charged H2AsO4 and HAsO22 under the neutral pH environment. The surface of MgO/Mg(OH)2 is positively charged under the most common pH environment. The repulsive or attractive interaction between MgO samples and noncharged As(III) is little so that the adsorption of As(III) should continue to increase with the As(III) concentration. For As(V), there exists an electrostatic attraction between positively charged MgO samples and negatively charged As(V) species and the adsorbed As(V) species have a repulsive effect on As(V) species in the solution. To assess the arsenic removal performance of MgO samples, the adsorption capacity of the MgO samples for As(III)/As(V) was compared with other hierarchical metal oxide micro-/ nanostructures (Table 2). Table 2 indicates that the adsorption capacity of MgO samples is much higher than that of the other hierarchical metal oxide micro-/nanostructures for removing As(III)/As(V) from polluted water. For instance, the adsorption capacity of Fe3O4 flower for As(V) was just 4.65 mg/g, while for F-MgO/N-MgO the adsorption capacity for As(V) was 343.64/ 378.79 mg/g, which is about 74/81 times that of Fe3O4 flower. In consideration of the adsorption capacity for As(III), F-MgO/ N-MgO appears to be much superior to the doughnut-like CuO. In addition, the pH value for the adsorption of arsenic in other reported hierarchical metal oxide micro-/nanostructures is relatively low. In real applications it is needed to adjust the pH value of the contaminated water. Furthermore, most of these micro-/ nanostructures are tested for removal of As(V), which is less toxic than As(III). For F-MgO/N-MgO, a single step arsenic (As(III) and As(V)) removal process is possible without the need for pretreatment (oxidation and pH adjustment). To investigate the adsorption mechanism of As(III)/As(V) onto MgO samples, the chemical status of As(III)/As(V) species after their adsorption onto MgO samples was investigated by XPS over the adsorbent surface after arsenic adsorption (Figure 10). In Figure 10a, Mg(2p) spectra of N-MgO after adsorption of As(III) showed two peaks at binding energy of 49.35 and 51.36 eV, which should be attributed to magnesium of MgO and magnesium of Mg(OH)2, respectively.22 This means that part of the MgO interacts with water to form Mg(OH)2. This point was also supported by XPS scans of O(1s) (Figure S1, Supporting Information). The interaction of MgO with water to form Mg(OH)2 was also reported by Li et al.17 As(3d) spectra of
Figure 10. High-resolution Mg(2p)As(3d) XPS scans of N-MgO surface after As(III) (a) and As(V) (b) adsorption.
N-MgO after adsorption of As(III) also showed two peaks located at 44.33 and 46.12 eV, which should be attributed to As(III)O and As(V)O, respectively.23 It could be suggested that some of As(III) was oxidized into As(V) in or after the adsorption procedure. Figure 10b shows the Mg(2p) and As(3d) spectra of N-MgO after adsorption of As(V). The Mg(2p) is similar to which after adsorption of As(III). While the As(3d) spectra showed only one peak located at about 45 eV which is the binding energy of As(V)O.23 The coordination chemistry of adsorbed As(III)/As(V) species onto MgO samples was studied using FT-IR (Figure S2, Supporting Information). In order to investigate the effect of interaction of MgO with water, a control experiment in which no arsenic was added into water during the adsorption was performed. The FT-IR spectra of F-MgO/N-MgO, F-MgO/NMgO after reaction with water and after adsorption of As(III)/ As(V) are shown in Figure S2 in the Supporting Information. From panels a and b of Figure S2 we can see that a new, sharp, and strong peak at about 3700 cm1 appears which can be attributed to the OH stretching vibrations in the Mg(OH)2 crystal structures after the interaction of MgO samples with water. After adsorption of arsenic the intensity of this peak decreases with the increase of As(III)/As(V) concentration. This demonstrates that As(III)/As(V) reacted with the in situ formed Mg(OH)2, which can be assigned to one of the mechanisms of arsenic adsorption onto MgO samples. In addition, there appears a new peak at about 779 and 841 cm1 for both F-MgO and N-MgO after adsorption of As(III) and As(V), respectively. These two peaks both should be attributed to ν(AsO).24 The above analysis suggested that the adsorption of As(III)/ As(V) onto MgO samples is not a simple and single process. The whole adsorption procedure involves the reaction of partial MgO with water to form Mg(OH)2, the adsorption of As(III)/As(V) 22249
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The Journal of Physical Chemistry C onto MgO and Mg(OH)2, and the reaction of As(III)/As(V) with Mg(OH)2. These multiple adsorption processes contribute to the high adsorption capacity of micro-/nanostructured MgO for arsenic in our study.
4. CONCLUSIONS In summary, porous hierarchical flower-like and nest-like micro-/nanostructured MgO were successfully synthesized. These novel micro-/nanostructures were composed of selfassembled MgO nanoflakes. When tested as adsorbent for arsenic removal, these micro-/nanostructured MgO exhibit excellent As(III) and As(V) removal performance. The maximum adsorption capacities of flower-like and nest-like MgO for As(III) are higher than 252.34 and 643.84 mg/g, while for As(V) it could reach 343.64 and 378.79 mg/g, respectively. Both the As(III) and As(V) adsorption capacity of these micro-/nanostructured MgO are much higher than most reported values from other metal oxide micro-/nanomaterials. A single step arsenic (As(III) and As(V)) removal process with these micro-/nanostructured MgO samples is possible without the need of pretreatment (oxidation and pH adjustment). The whole adsorption procedure involves the reaction of partial MgO with water to form Mg(OH)2, the adsorption of As(III)/As(V) onto MgO and Mg(OH)2, and the reaction of As(III)/As(V) with Mg(OH)2. These multiple adsorption process contributes to the high adsorption capacity of micro-/nanostructured MgO for arsenic in our study. These materials may be useful in many other applications, for example, in removal of organic pollutants and as a catalyst for organic synthesis. ’ ASSOCIATED CONTENT
bS
Supporting Information. Specific surface areas and pore characteristics of the MgO precursors and MgO samples, kinetics parameters of As(III)/As(V) adsorption onto FMgO/N MgO, high-resolution O (1s) XPS scans of MgO surface after As(III)/As(V) adsorption, and IR spectra of As(III)/As(V) adsorbed on MgO samples. This material is available free of charge via the Internet at http://pubs.acs.org.
ARTICLE
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (X.J.H);
[email protected] (J.H.L).
’ ACKNOWLEDGMENT This work was supported by the One Hundred Person Project of the Chinese Academy of Sciences, China, the National Key Scientific Program—Nanoscience and Nanotechnology (No. 2011CB933700), the China Postdoctoral Science Foundation (No. 20110490386), and the National Natural Science Foundation of China (Grant Nos. 21103198 and 60801021). Tao Luo thanks the Knowledge Innovation Program of the Chinese Academy of Sciences. ’ REFERENCES (1) Hu, J.-S.; Zhong, L.-S.; Song, W.-G.; Wan, L.-J. Adv. Mater. 2008, 20, 2977. (2) Chen, X. T.; Zhong, H. M.; Ma, Y. L.; Cao, X. F.; Xue, Z. L. J. Phys. Chem. C 2009, 113, 3461. 22250
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