Research Article  Open Access
Xiaojun Wang, Lianshui Zhang, "SO_{2} Gas Physicochemical Removal through Pulse Streamer Discharge Technique Assisted by Vapor Additive", Journal of Chemistry, vol. 2015, Article ID 872964, 10 pages, 2015. https://doi.org/10.1155/2015/872964
SO_{2} Gas Physicochemical Removal through Pulse Streamer Discharge Technique Assisted by Vapor Additive
Abstract
SO_{2} removal has drawn extensive attentions for air pollution treatment. In this paper, the pulse streamer discharge technique is investigated. Emission spectra diagnosis experimentally indicates that the SO_{2} molecule has been physically dissociated into SO and O radicals by electron collision and can be remediated through further chemical reactions during and after discharge. In order to quantitatively analyze the removal physical chemistry kinetics, a zerodimensional physicochemical reaction model is established. Without H_{2}O vapor additive, the SO_{2} removal efficiency is leanly low and only 0.296% has been achieved under pulse discharge duration of 0.5 μs. Through increasing the electrical concentration six times, the removal efficiency has been slightly heightened to 1.796% at pulse duration of 3 μs. Contrarily, vapor additive can effectively improve the removal kinetics, and removal efficiency has been remarkably heightened to 13.0195% at pulse duration of 0.5 μs with H_{2}O/SO_{2} initial concentration ratio of 0.1 : 1. OH radicals decomposed from H_{2}O through electron collision are the essential factor to achieve such improvement, which have effectively adjusted the chemical removal process to the favorite directions. The major productions have been transformed from HSO_{3} and HOSO_{2} to H_{2}SO_{4} when vapor ratio increased above 1.27 : 1.
1. Introduction
Sulfur dioxide (SO_{2}) has played important roles in acid rain formation [1]. There has been natural source of this environmentpolluting compound, such as the exhaust gas emitted from volcanoes [2]. But most of the SO_{2} ingredients produced nowadays should be ascribed to the fuel and coal combustion [3]. The exhaust gas emission from automobiles or power plants has become deteriorating social problems for generating acid rain. The acid rain can pollute the earth soil, the water, the building surface, and the metal coatings and has drawn extensive attentions from the viewpoint of government, law legislation, and the power plants, as well as the internalcombustion engine designers [4, 5]. Severe regulations on SO_{2} emissions have propelled the advancements of SO_{2} removal techniques, including spraydry or wet scrubbing and catalyst [6, 7]. Jin et al. reported that chlorine dioxide (ClO_{2}) gas could be utilized to clean up SO_{2}, and about 100% removal efficiency was achieved under optimal conditions of passing sufficient ClO_{2} gas into the scrubbing solution [8]. Wang et al. found that the ozone injection plus a glass made alkaline washing tower could efficiently achieve desulfurization [9]. Rayonbased activated carbon fibers (ACFs) at temperatures of 313–348 K had exhibited high SO_{2} removal activity [10]. Mnbased activated carbon catalysts were prepared, with MnO and Mn_{3}O_{4} coexisting in catalysts, and had exhibited SO_{2} removal ability [11]. Pt/CeO_{2} catalysts prepared on Cu (111) had been applied to assist the transformation of SO_{2} into atomic sulfur on its surface at the temperature above 300 K [12]. The Mo and Co doped V_{2}O_{5}/AC catalystsorbents were also used as catalyst for SO_{2} removal [13].
The wet scrubbing method is effective and has the utilizing prospect for flue gas desulfurization. But it should be noticed that the wet scrubbing process should be operated in relatively large reactors and some complex chemical reactions should be precisely controlled to generate gasphase oxidant such as ClO_{2} and O_{3}, exampled by the chloratechloride process as
The ClO_{2} scrubber gas is usually generated onsite since ClO_{2} can rapidly decompose through photo dissociation [14]. Despite possessing high efficiency, such wet scrubbing method has relatively high cost and should carefully dispose the end liquid waste. In addition, the design of wet scrubbing technology is highly dependent on the characteristics of the treated flue gas. Furthermore, the catalyst removing SO_{2} reaction is operated usually under relatively high temperature.
As alternative method, the high energy ebeam (EB, electron beams) technology has also been utilized in power plants based on the mechanism of high energy electron collision on the O_{2}, H_{2}O, and so on, to generate the radical agents such as O, OH, and HO_{2}, for gasphase oxidizing SO_{2} in the exhaust gas [15]. There have been no or fewer amounts of wet end products, benign gas emission or easily captured aerogel dusts. The 70–98% removal efficiency had been reported through such EB method, though its disadvantage is the requirement of large space and high energy consumption, for which the injected electrons should be accelerated to several MeV magnitudes (the input power of the electron accelerator usually in the range of 10^{2}~10^{3} kV, and the accelerator is large in space). The Xray exposure due to the emissions sourced from the deep excited radicals and molecules is another environmental risk. Based on its inherent characters, the EB technique had been successfully applied in the thermal power plants in many countries [16].
Compared to the wet scrubbing, catalyzing, or ebeam technique, the pulsed corona discharges, pulsed streamer discharges, or dielectric barrier discharges (DBD) demonstrate the advantage of low cost, for which these pulsed discharges are generated under lower voltages (~10^{1} kV) through simpler power supply, and the discharge instruments could be miniaturized. Such pulsed discharge removing SO_{2}, , or other volatile organic compounds (VOC) has attracted the interests from the academic to industrial community, and successful application has been obtained in China and other countries [17, 18].
As important candidate for highefficient SO_{2} remediation, the pulse discharging technique can inject high energy electrons to physically dissociate the SO_{2} molecules and further chemically transform the SO_{2} molecules into benign or easily captured species [19, 20]. Gas additive mixed with SO_{2} can sometimes present improvement effect. Ma et al. reported that SO_{2} removal was improved by adding NH_{3} into the air stream through the DBD discharge nonthermal plasma technique [21]. But the (NH_{4})_{2}SO_{3} or NH_{4}HSO_{3} production after discharge is not thermally stable enough and can further decompose into SO_{2}. Since NH_{3} additive for discharging removal of SO_{2} is unstable, the NH_{3} injection is usually utilized into the terminal of the pulse discharging instrument to collect the H_{2}SO_{4} aerogel dust, and the cost of injected NH_{3} is also expensive [22–24]. The catalyst combined plasma technique is also noticed. For example, TiO_{2}coated glass beads had been applied for SO_{2} removal. The SO_{2} removal efficiency was improved by the radicals generated from plasma reactions and TiO_{2} photocatalyst [25].
Usually, hydroxyl (OH) radicals are highly active and can be derived from the H_{2}O decomposition [26]. The hydroxyl radical is often referred to as the “detergent” because it can react with many pollutants [27–29]. In this paper, the SO_{2} removal physical chemistry kinetics without and with vapor additive are analyzed, and the OH improvement effect on SO_{2} remediation is focused on. The pulse streamer discharge technique for SO_{2} removal is introduced in Section 2. The emission spectra are detected and diagnosed for analyzing the SO_{2} removal mechanism, and a zerodimensional physicochemical reaction model is established in Section 3. Numerical simulation is quantitatively achieved. Section 4 announces the conclusions.
2. Experimental Section
The SO_{2} removal system is diagramed in Figure 1. The SO_{2} is experimentally generated through the reaction between H_{2}SO_{4} and Na_{2}SO_{3}. N_{2} acts as carrier gas to deliver SO_{2} gas to the discharge zone. After discharge, the residual SO_{2} and other gaseous productions are neutralized by NaOH solution.
The pulse streamer discharge reactor is consisting of two electrodes, which are oppositely placed and encapsulated in a glass tube. High energy electrons are injected from one electrode driven by the pulse electric field and then streamed to the other electrode. During the electron streaming process, the SO_{2} molecules can be physically collided.
The discharge voltage is 9.5 kV, with the pulse duration of 0.5 μs. The discharge frequency is 50 Hz, which is the power frequency of China. Gas pressure in the tube is controlled at 1 atm.
In order to monitor the SO_{2} removal process by untouched technique, the emission spectra are collected through a quartz window on the surface of the discharge tube by monochromator (ACR, AM566). The collected photons are transformed into electrical signal by multiplier phototube (PMT, HAMAMATSU, and CR184) and denoised and amplified by Boxcar (SRS, SRS 280/255).
3. Results and Discussion
The emission spectra are collected and diagnosed to evaluate the species categories that appeared during discharge. In order to clarify the physical chemistry reaction kinetics, a zerodimensional physicochemical reaction model is established and numerically simulated.
3.1. Emission Spectra Diagnosis
For the pulse discharging plasma, the emission spectrum is sourced from the mechanism that the SO_{2} gas molecules are excited through inelastic collision by the high energy electron. Since the kinetic energy of the electrons is ruled by statistical distribution principle, the SO_{2} molecules are excited to energy states in a wide range. Furthermore, the more important effect of such collision is that the SO_{2} would be decomposed into radicals. Such radicals also can be excited [30]. Then, the irradiation emitted from the widerange energy upstates of the excited molecules and radicals can be observed and collected. The emission spectra are presented in Figure 2. There have been complicated emission bands at the wavelength range from 200 to 500 nm.
The emission bands are evaluated. There appears the emission sequence at 337.13, 358.36, 376.94, 423.84, 440.48, and 469.24 nm, which is discriminated as N_{2} transition from its excited state to ground state [31]. The N_{2} appeared at the discharge zone as carrier gas as shown in Figure 1.
For the slowvarying peaks around 333.89, 373.55, and 440.12 nm, which are superposed onto the N_{2} emission sequence, they are evaluated as the continuous emission band of SO_{2} molecule and are related to the SO_{2} transition paths of , , and , respectively [32–34]. It means that the SO_{2} has been excited to the B^{1}B_{1} excited state through the inelastic collision by the high energy electrons. Then, the excited SO_{2} relaxes to its X^{1}A_{1} ground state through radiation transition. For the A^{1}A_{2} or a^{3}B_{1} excited state of SO_{2}, it is transferred from B^{1}B_{1} state through nonradiative transition process and then relaxed to the X^{1}A_{1} state by radiative transition. The electron collision onto SO_{2} molecule has induced complex excitation and energy transition processes.
There also has been an unattached emission peak around 237.17 nm in Figure 2, which is evaluated as the characteristic emission of sulfur monoxide (SO) from its excited A^{3}Π state to the X^{3}Σ state [35, 36]. SO possesses poor stability and can only be generated by dissociation of SO_{2} during the electron collision process. It indicates that some part of the SO_{2} molecules has been successfully removed through the pulse streamer discharge technique.
The possible SO_{2} removal routines are deduced based on the emission spectra and the evaluated transition paths as
In (2), the SO_{2} in ground state of X^{1}A_{1} state can be physically collided and excited by the electrons injected from the electrode in Figure 1 and dissociated into SO in A^{3}Π excited state and O in ^{3}P ground state. Such dissociative threshold energy is about 10.36 eV [37]. The excited SO compounds further transfer to the ground state of X^{3}Σ through radiation.
There also have been other possible routines such as
3.2. Establishment of SO_{2} Physicochemical Removal Dynamic Model
Due to many complex physical chemistry reactions involved, it is difficult to quantitatively analyze the SO_{2} removal process by experimental method. In this section, the removal process is investigated through establishing a zerodimensional reaction model. In order to improve the removal efficiency, the H_{2}O vapor additive is considered.
There have been two procedures for SO_{2} removal.
3.2.1. Physical Decomposition of SO_{2} and H_{2}O through Inelastic Collision by the Electrons
The electron collision dissociative cross sections are presented in Figure 3. It should be noticed that the dissociative energy of H_{2}O is smaller than that of SO_{2}, and cross sections of the former are higher at about 10^{1} cm^{2} magnitude order than that of SO_{2}. H_{2}O molecule is easier to be decomposed.
For the electron collision onto SO_{2} or H_{2}O, the physical reaction kinetics are ruled by the reaction rate coefficient, denoted as the symbol of . Such rate coefficients can be calculated by solving the Boltzmann Equation of electron collision dissociative cross sections [38]. According to the cross sections in Figure 3, the rate coefficients are calculated in this paper as
In pulse streamer discharging plasma, the SO_{2} or H_{2}O molecules can be physically decomposed. The new byproduct “fragments” are SO, O, OH, H, and so forth.
3.2.2. Further Chemical Reactions between the Byproducts and SO_{2}
The produced SO, O, OH, and H are active radicals and can further chemically react with SO_{2} or H_{2}O. There also have been other reactions. The main reaction paths are analyzed and outlined in Table 1.

After being dissociated by electron collision, the produced O radical can chemically participate in reaction for SO_{2} removal by forming SO_{3}, or forming SO and O_{2}. The OH radicals have played important roles in the removal process, and new molecules, such as HSO_{3}, HOSO_{2}, and H_{2}SO_{4}, are synthesized. There also have been reverse reactions to transform the new products into SO_{2} pollutant molecules. The main reaction routines are graphoutlined in Figure 4.
Based on the reaction graph, the reaction kinetics are numerically modeled as timevarying differential equation set. Every differential equation in the set is proposed based on the Arrhenius principle that the concentration of a given th species (one species selected from the reacting ingredients in the model, such as SO_{2}, SO, SO_{3}, O, O_{2}, H_{2}O, OH, HO_{2}, H, HSO_{3}, HOSO_{2}, and H_{2}SO_{4} in Table 1 and Figure 4) is changing according to the law of conservation of matter [48]. Among the reactions, there has been losing process of th species caused by the reaction between th and th species; then, the decreased concentration in unit time, or the losing rate of concentration , is described as , in which the symbols of , denoted the respective concentration of th or th species and as the rate coefficient of the reaction between th and th species.
All the concentration decreasing processes of th species in unit time should be abstracted from the reactions about th species losing processes and linear superimposed together as
Similarly, the concentration generating processes of th species in unit time, which are abstracted from all the reactions related to the th species generating processes based on the reaction between species th and th, are denoted as
Then, the concentration varying process of th species in unit time is decided by the losing and generating process and denoted as
Through the same procedures, every kind of species in the model is corresponding to a given differential equation. Consequently, an equation set including 13 equations is established in this paper to describe the varying concentration of 13 kinds of different species. The timeresolved concentration evolutions of all species are obtained by solving this differential equation set by RungeKutta algorithm [49].
It should be noticed that there are no spatial variables in (12). This means that the concentrations of all the species are uniformly hypothesized. The diffusion of electrons, SO_{2} molecules, and the byproducts has been ignored. Since there only has been concentration evolution of every species in time scale, a zerodimensional physicochemical reaction model is established in this paper. During the simulation based on Table 1 and Figure 4, the discharge energy is set as 120 Td. The plasma temperature is 5000 K. The gas pressure is 1 atm, and the gaseous reactions are carried out at room temperature.
3.3. SO_{2} Removal Kinetic Simulation
3.3.1. SO_{2} Removal Physicochemical Kinetics without H_{2}O Vapor Additive
According to the reaction model without vapor additive, SO_{2} can be dissociated by electron collision during discharge. To clarify the removal kinetics, timeresolved concentration evolution of SO_{2}, O, and SO and further oxidized species such as O_{2} and SO_{3} are presented in Figure 5.
(a)
(b)
In Figure 5(a), the SO_{2} concentration is varied at a monotonic decreasing trend when discharge time increased. The SO_{2} removal has obviously been achieved through the pulse streamer discharge technique. After discharge lasted for 0.5 μs, the removal efficiency is about 0.296%, which is leanly low. Most of the removed SO_{2} has been transformed to SO and O_{2} as shown in Figure 5(b), with the former concentration accumulating to 7.163 × 10^{16} cm^{−3} and the latter to 3.458 × 10^{16} cm^{−3}. For the SO_{3}, its final concentration is about 1.082 × 10^{15} cm^{−3}. When it comes to the O radicals, there appears an accumulating trend during discharge and concentration of 2.506 × 10^{15} cm^{−3} has been accumulated. After discharge, the O species have been fast consumed out to be zero to form SO_{3}, SO, and O_{2}.
The removal process of the SO_{2} is deduced as two procedures. The first is the decomposition of SO_{2} into SO and O. The second is the oxidation process, during which the O_{2} is easier to be generated through the reaction between O and SO_{2} with a higher reaction rate coefficient of 1.17 × 10^{−12} cm^{3}s^{−1} than that for forming SO_{3} of 3.52 × 10^{−14} cm^{3}s^{−1}. The O radical decomposed from SO_{2} during discharge has played the key roles in the SO_{2} removal process under the hypothesis without H_{2}O vapor additive.
The injected electrical energy is essential to influence the SO_{2} removal efficiency. With the discharge pulse duration widened, the inputted electron concentration is increased. Under such a variance, the removal efficiency of SO_{2} is presented in Figure 6. There appears an increasing trend of the removal efficiency with the pulse duration heightened. In the same reaction model, more electrons injection induces more SO_{2} to be physically decomposed. The further chemical reactions for forming O_{2}, SO_{3}, and so on are then accelerated.
Under the discharge pulse with duration of 3 μs, which bears six times energy compared to the pulse with duration of 0.5 μs, the removal efficiency has only heightened to 1.796%. From the viewpoint of energy consumption, such SO_{2} removal through direct decomposition by electron inelastic collision has high cost and low efficiency.
3.3.2. Vapor Additive Effect on SO_{2} Physicochemical Removal Kinetics
Without H_{2}O vapor added, the SO_{2} removal efficiency is very low. To improve the removal process, the H_{2}O vapor is considered, which is usually mixed in the SO_{2} exhaust gases and the outinjecting H_{2}O vapor is also very easy and cheap. According to the reaction model in Table 1 and Figure 4, the OH and H radicals, decomposed from H_{2}O molecules by electron collision, can participate in many chemical reactions related to SO_{2} or the radicals. Even the H_{2}O itself can transform SO_{3} into H_{2}SO_{4}. More effective removal is expected. But the attenuation effect of the OH radical should be noticed, by which the SO can be reversely transformed into SO_{2}.
In Figure 7, the timeresolved concentration variance of SO_{2} and all other byproducts is presented under the initial concentration ratio between H_{2}O and SO_{2} of 0.1 : 1. The discharge pulse duration is the same as that in Figure 5 of 0.5 μs. Compared to the 0.296% removal efficiency in Figure 5, the removal efficiency is remarkably improved by H_{2}O vapor additive, and higher removal efficiency of 13.0195% has been finally achieved in Figure 7(a). Such variance is ascribed to the reason that the injected electrons are effectively utilized by H_{2}O, and the produced H and OH radicals have efficiently accelerated the SO_{2} removal kinetics, which can be verified from the byproduct concentration variance in Figures 7(b), 7(c), and 7(d).
(a)
(b)
(c)
(d)
In Figure 7(b), the SO_{3} formation is affected by vapor additive. Its final concentration is about 3.90 × 10^{15} cm^{−3}, which is at the same magnitude order as that without vapor additive. But a monotonic increasing trend for the SO_{3} concentration appeared, which is due to the HO_{2} oxidizing SO_{2} and the reaction between HOSO_{2} and O_{2}, though O radicals have been consumed out after discharge.
The obvious increment occurred for SO concentration in Figure 7(b), which has accumulated to 1.036 × 10^{18} cm^{−3} after 0.5 μs, and is higher than that without vapor additive of 7.163 × 10^{16} cm^{−3} at 10^{2} cm^{−3} magnitude order. Such a remarkable increment is decided by the H radicals, which are directly decomposed from H_{2}O molecules. The H radical can react with SO_{2} to produce SO and OH and is formulated in (13). Vapor additive has accelerated the SO generation efficiency:
More OH production is beneficial to the O_{2} generation according to
But the O_{2} concentration of only 3.049 × 10^{16} cm^{−3} has been obtained after 0.5 μs in Figure 7(b), which is slightly lower than 3.458 × 10^{16} cm^{−3} without H_{2}O vapor added. The decrement is ascribed to the consumption of OH not only by O to produce O_{2} as ruled by (14), but also by other reaction paths to produce HSO_{3}, HOSO_{2}, H_{2}SO_{4}, or even SO_{2} as shown in Table 1. And the consumption of O_{2} by HOSO_{2} and SO is another important reason for the decrement of O_{2} concentration.
The concentrations of HSO_{3} and HOSO_{2} in Figure 7(c) have accumulated to 1.736 × 10^{18} and 0.724 × 10^{18} cm^{−3}, respectively. For the former, it has become the major production due to its highest final concentration. There also has been 0.093 × 10^{18} cm^{−3} H_{2}SO_{4} produced through the reaction between SO_{3} and H_{2}O or between HSO_{3} and OH. And the concentration of HO_{2} is about 0.084 × 10^{18} cm^{−3} in Figure 7(c). Such low concentrations imply that both H_{2}SO_{4} and HO_{2} are not the main final productions under the initial H_{2}O/SO_{2} ratio of 0.1 : 1.
All such concentration variances are decided by the H_{2}O physical decomposition into H and OH through electron inelastic collision, and the H_{2}O has been consumed with its final concentration decrement amount of about 1.2802 × 10^{18} cm^{−3} after discharge lasted for 0.5 μs in Figure 7(d). And the OH radicals have played the major roles for SO_{2} removal to transfer SO_{2} into HSO_{3}:
When the concentration ratio between H_{2}O and SO_{2} is 0.1 : 1, the major productions are HSO_{3} with a little HOSO_{2}, and the H_{2}SO_{4} concentration is lower than them with 10^{2} cm^{−3} magnitude orders. For SO_{2} removal, the main production is expected to be H_{2}SO_{4}, since H_{2}SO_{4} is chemically stable and can be easily neutralized by alkali or captured by fabric filter or electrostatic precipitator (ESP). In order to adjust the final productions, the vapor ratio is varied in Figure 8. It presents that the higher the vapor ratio is, the more the H_{2}SO_{4} molecules have been produced. The H_{2}SO_{4} concentration is even higher than that of HSO_{3} when the initial vapor/SO_{2} concentration ratio is above 1.27 : 1.
(a)
(b)
H_{2}O additive with higher ratio has generated more OH radicals and consequently accelerated the reactions between HSO_{3} and OH as
by which the HSO_{3} has been transformed into H_{2}SO_{4}. Such reaction has simultaneously decreased the HSO_{3} concentration and increased the H_{2}SO_{4} concentration, as shown in Figure 8(a). More vapor additive has effectively adjusted the SO_{2} removal physicochemical kinetics to the favorite directions, and H_{2}SO_{4} has become the major production when the initial vapor mixing ratio is above 1.27 : 1.
For other species such as HOSO_{2} and SO in Figure 8(a), the former is increased at a monotonic trend, but its highest final concentration at vapor ratio of 2 : 1 is obviously lower than that of H_{2}SO_{4}. The latter SO is increased at low vapor ratio and decreased at high vapor ratio. Such varying trends of HOSO_{2} and SO are ascribed to the more OH decomposed at higher vapor ratio. The HOSO_{2} concentration is heightened through the reaction between OH and SO_{2}, and the SO concentration is decreased by the reaction between OH and SO to reproduce SO_{2}. Such reactions are formulated as follows:
In conclusion, vapor additive has effectively improved the SO_{2} removal efficiency in Figure 8(b). In the simulation, even 89.1% removal efficiency has been achieved at the initial concentration ratio of 2 : 1 between H_{2}O and SO_{2}.
4. Conclusions
SO_{2} removal is important for air pollution treatment. In this paper, the pulse streamer discharge technique is investigated. Emission spectra diagnosis implies that the SO_{2} molecules have been physically dissociated by the injected electrons and transformed into SO and O. In order to quantitatively clarify the complex removal kinetics, a zerodimensional physicochemical simulating model is established. Simulation indicates that the SO_{2} removal without H_{2}O vapor additive is leanly achieved with the final efficiency of only 0.296%. The injected electrical energy can improve the removal efficiency, and an increment trend is presented with the pulse duration increased. But the improvement is not very notable. After six times concentration of electrons injected, the SO_{2} removal efficiency is increased from 0.296% at the pulse duration of 0.5 μs to only 1.80% at the pulse duration of 3 μs. In order to improve the removal process, the H_{2}O vapor additive is applied. Under the pulse duration of 0.5 μs and the initial concentration ratio between H_{2}O and SO_{2} at 0.1 : 1, there appears remarkable increment of the SO_{2} removal efficiency as 13.0195%. But the major productions are HSO_{3} and HOSO_{2}, and H_{2}SO_{4} concentration is lower than them with 10^{2} cm^{−3} magnitude order. More H_{2}O additive has generated more OH radicals, which effectively adjusted the SO_{2} physicochemical removal process to the favorite directions. H_{2}SO_{4} has become major production when initial vapor ratio is above 1.27 : 1. Even 89.1% removal efficiency has been achieved at the concentration ratio of 2 : 1 between H_{2}O and SO_{2}.
From the viewpoint of energy consumption and pollutant gas removal efficiency, the H_{2}O vapor additive is verified and effective enough to be considered for commercial applications in pulse streamer discharge system for SO_{2} removal.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work is financially supported by National Nature Science Foundation of China (no. 10875036) and the Fundamental Research Funds for the Central Universities (no. 12MS146).
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