High temperature dielectric properties of spent adsorbent with zinc sulfate by cavity perturbation technique

Accepted Manuscript Title: High temperature dielectric properties of spent adsorbent with zinc sulfate by cavity perturbation technique Authors: Guo Lin, Chenhui Liu, Libo Zhang, Tu Hu, Jinhui Peng, Jing Li, Shixing Wang PII: DOI: Reference:

S0304-3894(17)30091-2 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.02.010 HAZMAT 18365

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

10-11-2016 6-2-2017 7-2-2017

Please cite this article as: Guo Lin, Chenhui Liu, Libo Zhang, Tu Hu, Jinhui Peng, Jing Li, Shixing Wang, High temperature dielectric properties of spent adsorbent with zinc sulfate by cavity perturbation technique, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2017.02.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High temperature dielectric properties of spent adsorbent with zinc sulfate by cavity perturbation technique Guo Lin a, b, c, d, Chenhui Liu a, c, d, e, Libo Zhang a, b, c, d*, Tu Hu a, b, c, d, Jinhui Peng a, b, c, d, Jing Li a, b, c, d, Shixing Wang a, b, c, d a

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming

University of Science and Technology, Kunming, Yunnan 650093, China b

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and

Technology, Kunming, Yunnan 650093, China c

Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming, Yunnan

650093, China d

National Local Joint Laboratory of Engineering Application of Microwave Energy and

Equipment Technology, Kunming, Yunnan 650093, China e

Faculty of Chemistry and Environment, Yunnan Minzu University, Kunming, Yunnan 650093,

China Corresponding address: Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China, Tel.: +86-871-65138997, E-mail address: [email protected]

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Highlights 1. Cavity perturbation technique is employed to measure the dielectric properties. 2. Microwave absorption capability of ZnO is poor from 20 ºC to 850 ºC. 3. Dielectric properties of spent absorbent and zinc sulfate are influenced by temperature especially in high temperature stage. 4. Penetration depths and heating curve indicate spent adsorbent and ZnO·2ZnSO4, ZnSO4 are excellent microwave absorber. 5. The pore structures of spent adsorbent are improved significantly by microwave-regeneration directly.

Abstract: Dielectric properties of spent adsorbent with zinc sulfate are investigated by cavity perturbation technique at 2450 MHz from 20 ºC to approximately 1000 ºC. Two weight loss stages are observed for spent adsorbent by thermogravimetric-differential scanning calorimeter (TG-DSC) analysis, and zinc sulfate is decomposed to ZnO·2ZnSO4 and ZnO at about 750 ºC and 860 ºC. Microwave absorption capability of ZnSO4 increases with increasing temperature and declines after ZnO generation on account of the poor dielectric properties. Dielectric properties of spent adsorbent are dependent on apparent density and noticed an interestingly linearly relationship at room temperature. The three parameters increase gently from 20 ºC to 400 ºC, but a sharp increase both in real part and imaginary part are found subsequently due to the volatiles release and regeneration of carbon. And material conductivity is improved, which contributes to the -electron conduction appearance. Relationship between penetration depth and temperature further elaborate spent adsorbent is an excellent microwave absorber and the microwave absorption capability order of zinc compounds is ZnO·2ZnSO4, ZnSO4 and ZnO. Heating characteristics suggest that heating rate is related with dielectric properties of materials. The pore structures of spent adsorbent are 2

improved significantly and the surface is smoother after microwave-regeneration. Keywords: Microwave heating, Dielectric properties, Perturbation technique, Spent adsorbent, Zinc sulfate 1. Introduction Activated carbon (AC) is widely employed in wastewater treatment and purified air as an adsorbent due to the advantages of developed pore structures, larger specific surface area and stronger adsorption capacity [1]. However, the adsorption capacity is exhausted after a period of time, and usually the spent activated carbon (also known as spent adsorbent) is incinerated or placed in a designated area [2]. The main bottleneck for further widespread application of activated carbon is the high costs [3], meanwhile, spent adsorbent can induce a great burden for social economy and environment. Therefore, the regeneration and recycle of spent adsorbent are particularly important and sensible option [4]. Conventional regeneration techniques in industrial are implemented by thermal and chemical methods [2, 5]. However, the regeneration methods are time-consuming and energy consumption intensive, which cause a higher economic costs [6]. In recent years, microwave heating has caused widely attention as a promising regeneration method due to its characteristics of instantaneous and homogeneous heating, which resulting from rotation of dipoles in microwave field [7, 8]. Microwave regeneration activated carbon has the advantages of rapid, energy saving, precise temperature control and better adsorption capacity properties [9-11]. Moreover, microwave heating technology has been widely employed on regeneration of spent adsorbent and presents very promising results [8, 12, 13]. Qu et al. [14] investigated the regeneration of spent activated carbon containing zinc acetate by microwave-assisted, and the results of the regenerated AC-ZnO showed a larger surface area, well

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porous structure and adsorption performance compared with the spent catalyst. The level and method of activated carbon regeneration are determined by the quantity, position of the exchange cations, types of adsorbates. In addition, microwave heating is based on the dielectric properties of materials, which are described by Maxwell-Wagner effect to evaluate the microwave absorption capability of materials [15, 16]. Bradshaw et al. [17] introduced the importance of dielectric properties on the application of the granular activated carbon regeneration. Zhou et al. [18] investigated the complex permittivity of Indonesian lignite with activated carbon and graphite as microwave absorbers and found that imaginary part of activated carbon and graphite was higher than Indonesian lignite. Generally, before carrying out the regeneration experiments, it is necessary to determine the absorption capability by investigating the dielectric properties of materials at different temperature. The dielectric properties of materials are closely related with microwave frequency, temperature, types and apparent density of material. Usually, the commercial frequencies of microwave are 915 and 2450 MHz. In microwave field, the real part and imaginary part are also employed to evaluate the material penetration depth (Dp, in Eq. (1)). Microwave penetration depth is continuous changed with increasing temperature, may be result in a non-uniform temperature distribution. Several studies have attempted to investigate the dielectric properties of coal at room temperature [19, 20]. Recently, the dielectric properties and penetration depth of coals and biodegradable during pyrolysis are attracted by researchers [21, 22]. Regeneration of spent adsorbent with addition absorption material as a microwave receptor has also been researched in literature [21, 23, 24]. However, the detailed dielectric properties of spent adsorbent regeneration during different temperature are few reported [25]. This contains the variations characteristics of dielectric

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properties such as the permittivity, loss factor and dielectric loss tangent at high temperature. A detailed research on the dielectric properties of spent adsorbent in a broad temperature range needs to be performed for selecting the optimal regeneration method, and comprehending the transition regularity of adsorption materials in spent adsorbent. In the present study, the dielectric properties of spent adsorbent and zinc sulfate are explored at different temperature with a frequency of 2450 MHz. The relationship between dielectric properties and apparent density of spent adsorbent are also investigated. In addition, the dielectric loss tangent and penetration depth for both are calculated. The microscopic structures of the spent adsorbent and regeneratured adsorbent are compared. These data may be useful in spent absorbent regeneration, database perfection of dielectric properties and further understanding the microwave mechanism of heating. 2. Materials and methods 2.1 Materials In the paper, the adsorbent is used to absorb the organic matter during the purification process of zinc hydrometallurgy (during the adsorption process, 0.5-3 g/L activated carbon is added into the zinc sulfate solution, the temperature and the contact time are about 50-80 ºC and 30-90 min respectively), parts of the zinc sulfate are adsorbed by the adsorbent and the zinc content is about 4.27% in spent adsorbent. The regeneration methods and effects of spent adsorbent can be influenced by zinc sulfate as described above. Therefore, the dielectric properties of both spent adsorbent and zinc sulfate are investigated. The zinc sulfate and ZnO are supplied by Tianjin Zhiyuan Chemical Reagent Co., Ltd. And chemical reagents are analytical grade and employed as received. To minimize interference from crystal water of zinc sulfate, the sample is dried at 100 ºC

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for 12 h, and then stored in a desiccator. The spent adsorbent which contains zinc sulfate and organic matter, is obtained from an enterprise in Yunnan province and the particle size of spent adsorbent is less than 0.15 mm. The results of proximate and ultimate analyses of spent adsorbent are presented in Table 1. Table1 here 2.2 Measurement of dielectric properties Generally, the network parameters (such as short-circuit transmission line method, reflectance and transmittance in free space, open-ended coaxial probe) and resonant cavity or cavity perturbation method are employed to measure the dielectric properties of samples [26-30]. Cavity perturbation method has been successfully adopted to evaluate the relative complex permittivity of solid samples at high temperature because of the superiorities of more reliable accuracies and operation simple [31-33]. During experiments, the sample is placed in a cylindrical resonant cavity (TM0n0). The inner dimensions of the cavity are 190 mm height and 200 mm diameter, respectively. The scattering parameters of samples are measured by a vector network analyzer (VNA) (E5071C Agilent), and for the equipment, the maximum measured value of dielectric loss is about 3.0. High temperature is obtained by an eddy current heating system, which was placed inside the holder cavity. Each sample is employed and compacted uniformly in a quartz tube (inner dimensions 4.1 mm, height 52 mm). The permittivity measurement experiments are carried out from room temperature to a special temperature at an interval of 50 ºC with a microwave frequency of 2450 MHz under air atmosphere. Before measuring, the device is adjusted to minimize the errors and unloaded quality factor (QF) of the resonant cavity is about 10000. Heating system and dielectric properties

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measurement are automated. The results are recorded and stored for off-line analysis and all samples are tested three times under the same conditions. This measuring continues until the entire temperature data is obtained. 2.3 Penetration depth calculation Penetration depth which also known as skin depth (Dp), is defined as the depth from the surface into the materials at which the microwave wave power declines to 1/e from surface value. The penetration depth can be calculated as follow [24, 34]:



(1)

√

Where 

is the microwave wavelength in free space (cm),  = 12.24 cm at microwave

frequency of 2450 MHz, ' and '' are the dielectric constant and loss factor, respectively. Dp is a vital parameter in describing and evaluating materials temperature distribution and absorption capability during the microwave heating process (cm).   2.4 Microwave thermal processing equipment In the present research, microwave reactor is designed and made by the Key Laboratory of Unconventional Metallurgy. Microwave heating experiments are conducted in a lab-made microwave muffle furnace, and the microwave equipment is made up of four sections: including two magnetrons as microwave sources with a frequency of 2.45 GHz and power of 1.5 kW for each magnetron. The magnetrons are cooled by a circulating cooling water system; two waveguides, which are used to transport microwaves; a resonance cavity to manipulate microwaves for a specific purpose; and a control system to adjust the temperature and microwave power. The inner dimensions of the microwave cavity are 420 mm, 260 mm and 260 mm for in

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length, width and height, respectively. In order to resolve the continuous temperature measurement during the heating process, the thermocouple (connected to the computer system) with a thin layer of aluminum shielding is employed to measure temperature, and placed at the closest proximity to the material. 3. Results and discussion 3.1 TG-DSC analysis of zinc sulfate and spent adsorbent Thermal analyses of zinc sulfate and spent adsorbent are presented in Fig. 1. The thermal-thermogravimetric experiments are carried out by a STA449F3 analyzer (NETZSCH, Germany). It can be found from Fig. 1 (a), the weight loss is about 20% at temperatures ranging from 85 ºC to 110 ºC. This observation indicates that the water is evaporated. Up to 300 ºC, the slight weight loss is caused by the removal of organic matter and an endothermic peak is found at 290 ºC in DSC curve. Then, a sharp decline in TG curve and a substantial increase in DSC curve are observed in range of 300-690 ºC. In the stage, parts of the spent adsorbent have been activated by steam, and some volatiles and organic matter are decomposed. With a continuous increase in temperature, the carbon is combusted under the air and releasing heat. In addition, the DSC curve declines sharply at 670 ºC, which indicates that the carbon has been almost combusted completely. The TG-DSC curves of zinc sulfate are shown in Fig. 1 (b). As can be seen that there is a strong endothermic peak at temperatures ranging from 200 ºC to 400 ºC and the weight loss is about 10.5%, which is close to the theoretical value of water content for ZnSO4·H2O (about 10.06%). It can be deduced that the final crystal water is evaporated. During 400 ºC to 650 ºC, the TG curve keeps stable, however, a gradually decline of the DSC curve is noticed due to appear the H phase transform to N phase of ZnSO4 [35]. From 650 ºC to 1000 ºC, the ZnSO4 starts to decompose and

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two endothermic peaks are observed at 750 ºC and 970 ºC, respectively. However, the TG and DSC curves show a continuous decline, and an upward peak is presented at 866 ºC, which indicates a decomposition of intermediate product. The decomposition process of ZnSO4 can be described as follow [35, 36]: 3ZnSO4 = ZnO·2ZnSO4 + SO3 

(2)

ZnO·2ZnSO4 =3ZnO + 2SO3 

(3)

As temperature increases, it can be deduced that ZnSO4 is decomposed to ZnO completely after 1000 ºC. Figure 1 here 3.2 Dielectric properties for zinc sulfate The materials dielectric response properties with the effects of an electric field are regarded as the relative complex dielectric constant, *, which is one of the widely studied materials dielectric parameter. It represents the charge storing capacity regardless of dimension for materials [37]. The complex dielectric constant and materials absorbed power for per unit volume are defined as follow [38, 39]: ∗

(4)

P = 2πfε0 ε'' |E|2 (5) Where j=√ 1 as can be seen from Eq. (4), the relative complex permittivity is composed by two components, the dielectric constant (') and the loss factor (''). The dielectric constant is also called real part, which reflects how much electrical energy is reflected and absorbed. The behavior of materials within microwave heating is determined by real part. The loss factor is also known as

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imaginary part, which represents the extent of the electrical energy dissipation to generate heat within the materials [16], and it also can be confirmed from Eq. (5). P is the absorbed power of materials (W), f is the microwave frequency (Hz), 0 is the free space permittivity (8.854 10-12 F/m), and E is the electric field magnitude (N/C). Dielectric loss tangent (tan) is another important dielectric parameter. It directly reflects the ability of dielectric materials converts the stored electromagnetic energy into heat at a given frequency and temperature. The loss tangent is calculated as follow: tanδ

(6)

To evaluate the absorption capability of zinc sulfate at different temperature, the dielectric properties for zinc sulfate are measured. As analysis in Fig. 1 (b), the zinc sulfate is observed stable before 650 ºC (regardless of crystal water) and starts to decompose after 650 ºC. Consequently, the relationship between temperature and dielectric properties of ZnSO4 are shown in Fig. 2 (before decomposition) and Fig. 3 (after decomposition), respectively. The real part is observed dependent on temperature greatly in Fig. 2. And the imaginary part and dielectric loss tangent gently increase from room temperature to 350 ºC due to the crystal water evaporation. The three parameters increase substantially at temperatures ranging from 400 ºC to 600 ºC. The slope between 400-600 ºC for real part, imaginary part and dielectric loss tangent is much larger than 20-350 ºC. The results indicate the dielectric properties of ZnSO4 are enormously influenced by temperature and the microwave absorption capability of ZnSO4 increases with increasing temperature. With temperature achieves 650 ºC and ZnSO4 starts to decompose, even the real part increases within 650-750 ºC, however, the imaginary part and dielectric loss tangent remain steady and even

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decreased. Compared with Fig. 3 and Fig. 2, it can be found that the intermediate product (ZnO·2ZnSO4) has a more positive microwave absorption capability in contrast with ZnSO4. With further increases in temperature, a large amount of ZnO is generated. The dielectric properties of ZnO at different temperature are listed in Table 2. As can be seen, the variations of imaginary part and dielectric loss tangent are smaller and the values are lower than ZnSO4, which indicate that the absorption capability of ZnO is poor. Therefore, the imaginary part and dielectric loss tangent are observed a sharp decline in Fig. 3. It can be deduced that the values of ', '' and tan could be remained constant and even presented continuous decrease with increasing temperature. Figure 2 here Figure 3 here Table 2 here 3.3 Dielectric properties for spent adsorbent Apparent density is also considered as an important factor for spent adsorbent regeneration. The variations of spent adsorbent on dielectric properties with different apparent density at room temperature are presented in Fig. 4. It can be observed that the real part increases with increasing apparent density. It seems that the real part is greatly dependent on the apparent density and noticed a linear relationship. Almost similar profiles for imaginary part and dielectric loss tangent can be concluded from Fig. 4. The ', '' and tan increase approximately by 45.05%, 116.45% and 49.53% respectively when the apparent density varied from about 369 kg/m3 to 625 kg/m3. As can be found in Table 1, the water is presented in spent adsorbent and it is considered to be a good microwave absorber agent due to its polarization and dielectric properties [40]. It can positively contribute to the dielectric properties. The unit volume water increases with increasing apparent

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density. Thereby, it may be explained due to the role of water in spent adsorbent. Another important reason may be the complex chemical components within spent adsorbent, which have a good microwave absorption capability at room temperature (such as ZnSO4). It can be deduced that the temperature characteristics are largely dependent on amount and thickness of materials. Figure 4 here The trends of ', '' and tan for spent adsorbent from room temperature to 1000 ºC are shown in Fig. 5. It also can be seen that the three parameters strongly depend on temperature, especially at high temperature. As be found in Fig.5 (a) that the real part remains increase slowly from room temperature to 600 ºC. Such observation has been reported by Peng et al. [24] due to the moisture evaporation. The result is different from the biomass and bituminous coal, which dielectric properties decrease with moisture releasing due to the relatively poor microwave responses of the materials at low temperatures [24, 38, 41]. At temperatures ranging from 600 ºC to 800 ºC, the real part is observed increase by 185.67%. As temperatures continue to increase, the real part keeps nearly unchanged between 800 ºC and 900 ºC and then a substantial increase is found beyond 900 ºC. It is caused by the release of organic matter and volatiles from spent adsorbent, breaking of chemical bonds and the polarization effects occurred in the sample [42]. Polarization capability of material is related with real part at a certain frequency, the larger real part and the greater polarization capability [43]. The increase in real part with temperature may also due to the increase of dipole change or movement in high temperature electromagnetic field [20, 44]. It is significant to research the characterization of ', '' and tan as a function of increasing temperature. In Fig. 5 (b) and (c), a gradual increase is observed between room temperature and 300 ºC, and then a nearly constant values are presented from 300 ºC to 450 ºC. A sharp increase in

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imaginary part and dielectric loss tangent are found at temperatures ranging from 450 ºC to 600 ºC. The rapid increase in imaginary part is due to the release of volatiles and some carbon generated from spent adsorbent. The larger imaginary part and the greater conductivity of material, it can contribute to the -electron conduction appearance [18, 19]. In addition, when temperature increases to above 450 ºC, along with spent absorbent is regenerated and more free charges become available, which result in the responsiveness of substances to microwave [45]. This will in turn promote the electronic conduction. Moreover, the dielectric polarization could also benefit to imaginary part for some extent at high temperature [32]. The '' shows unchanged with temperatures continue increasing, however, the  tan sharp decreases from 0.115 to 0.023. The results indicate that the microwave absorption capability increases with increasing temperature and then keeps nearly a constant, the capability of electromagnetic converting to heat increases and then decreases with temperature. Atwater J. E. et al. [46] investigated the dielectric properties of activated carbons at different microwave frequencies and found that the dielectric losses increased to the maximum values (more than 10) in range of 2.3 to 2.8 GHz and the microwave can be absorbed sufficiently. Liu et al. [39] researched the dielectric properties of chars at high temperature and different microwave frequencies. The results indicated that the chars were strong absorber materials at high temperature and 2-4 GHz (the dielectric loss exceeds 30). The above results indicate that the carbon materials have excellent microwave absorption capability. The dielectric loss of clean adsorbent that we employed is more than the measurement range and cannot be measured at room temperature. However, the dielectric loss is less than 0.05 after adsorption of organic matter and zinc sulfate. It can be found that both of zinc sulfate and organic matter are have observably

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negatively impact on the dielectric properties of adsorbent. Figure 5 here 3.4 Effects of apparent density and temperature on penetration depth The penetration depth of spent adsorbent and zinc sulfate is illustrated in Fig. 6. Temperature dependent on penetration depth of ZnSO4 before and after decomposition are depicted in Fig. 6 (a) and (b). Combined with TG-DSC curves of ZnSO4, it can be found that the Dp is observed moderate decline due to the water evaporation at temperatures ranging from 20 ºC to 350 ºC, which is the spill temperature of crystal water. Penetration depth declines sharply, which decreases by 86.17% from 126.17 cm to 16.57 cm at 400-600 ºC and further indicates the microwave absorption capability of ZnSO4 is more positive. In addition, it appears an ascendant peak from 350 ºC to 400 ºC and the Dp values of ZnSO4 are found larger at initial temperature stage (400-500 ºC). The results indicate the Dp of ZnSO4 is affected immensely by temperature and the absorption capability at high temperature is much better than low temperature. With increasing temperature, parts of ZnSO4 are decomposed to ZnO·2ZnSO4 and ZnO. Then with large numbers of ZnO are generated which dielectric properties are much poor (in Table 2), the penetration depth increases substantial. The effects of apparent density on penetration depth for spent adsorbent at room temperature are shown in Fig. 6 (c). It is noticed that the penetration depth decreases approximately linear with increasing temperature. It can be deduced that the spent adsorbent is a good microwave absorbent at room temperature. When the apparent density is greater and the penetration depth is smaller, it can result in a non-uniform heating due to the actual depth is larger than the theoretical calculation depth [21]. Therefore, it is an important function for the relationship between apparent density and

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penetration depth to guarantee the uniform heating of materials in microwave field. As can be seen from Fig. 6 (d), it can be separate into two distinct stages, a continuous decrease is found at temperature up to 600 ºC. Then, a gradual increase is presented at temperatures ranging from 600 ºC to 1000 ºC. An initial increase of penetration depth with temperature (room temperature to 100 ºC) was reported [30, 44] due to the removal of moisture. However, the result is opposite in this research, it may be explained by the complex chemical compositions including some organic matter and inorganic elements such as Zn, Si and Ca different from the previous study. Another reason could be the activation role of moisture, which is always employed as an activator in spent activated carbon regeneration [47, 48]. The continuous decrement of penetration depth may benefit from the liberation in volatiles and formation of activated carbon, which may due to the combined action of moisture, chemical composition (such as ZnSO4 within spent absorbent) and microwave energy. Once the carbon form, the absorption capability of sample increases sharply. Hence, the microwave absorption capability is found to be the highest at 600 ºC. As temperature exceeds to 600 ºC, some of the activated carbon which have been regenerated are oxidized, burned and transformed to ash [49]. And then the penetration depth is observed increase gradually, in addition, some inorganic compounds are also decomposition with increasing temperature. Figure 6 here 3.5 Temperature characteristics of spent adsorbent and zinc sulfate Heating cures of zinc sulfate and spent adsorbent with microwave power of 700 W, sample mass of 40 g are illustrated in Fig. 7 (a) and (b), respectively. As shown in Fig. 7 (a), the variation of zinc sulfate temperature with time can be roughly separate into three distinct stages. The first

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stage (I) is from room temperature to 350 ºC, the second stage (II) covers from 372 ºC to 793 ºC and the third (III) extending to about 900 ºC. To present the relationship between time and temperature more clarity, the temperature is fitted to the linear function as follow: y = ax + b

(7)

Where y is the temperature (ºC), x denotes the time (min), a (ºC/min) and b (ºC) represent the slop and intercept, respectively. The fitting results for temperature are listed in Table 3. It can be found that the function fits well with the experimental data due to the high values of correlation coefficient R2 (>0.9). Moreover, the slop value of I, II and III indicates that the heating rate of stage II is much larger than I and III, i.e. the microwave absorption capability of sample is positive when the sample is composed by ZnSO4 or ZnSO4 with ZnO·2ZnSO4, and the microwave absorption capability is poor when ZnO is generated. This is consistent with the dielectric properties results of zinc sulfate at different temperature. As show in Fig. 7 (b), the temperature increases rapidly with time and it just takes 14 min from room temperature to approximately 1000 ºC, which indicates the microwave absorption capability of spent adsorbent is much good. From room temperature to about 200 ºC, the temperature increases slowly due to the release of moisture and volatiles which may have relative low dielectric properties than carbon material. Meanwhile, the moisture has a role of activation during spent adsorbent regeneration. As increasing temperature, parts of carbon is regenerated and the temperature increasing fiercely with time. It is also can be confirmed from the results of spent adsorbent dielectric properties. Figure 7 here Table 3 here

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3.6 SEM analysis Regenerated adsorbent is obtained under microwave heating at temperature of 800 ºC, microwave power of 700 W, sample mass of 40 g and holding time of 30 min. The microscopic structures of the spent adsorbent and regeneratured adsorbent are observed by Scanning Electron Microscope (SEM) (TESCAN VEGA3 SBH) and shown in Fig. 8. It can be seen in Fig. 8 (a), the spent adsorbent is wrapped by impurities. The surface of spent absorbent is rough and it is almost impossible to detect the pore structures. However, the abundant pore structures are visible in Fig. 8 (b) and impurities are removed at high temperature. The surface of regenerated adsorbent is more smooth than spent adsorbent. The phenomenon further confirms that it is feasible to regenerate the spent adsorbent by microwave heating directly. Figure 8 here 4. Conclusion The real part, imaginary part, and dielectric loss tangent of spent adsorbent with zinc sulfate are measured at a microwave frequency of 2450 MHz under air atmosphere from room temperature to about 1000 ºC using cavity perturbation technique. The following conclusions are drawn from the measure and experimental results: (1) Dielectric properties of spent adsorbent are greatly dependent on apparent density and a linearly function is noticed at room temperature. ' remains almost constant, '' and tan increase gently from room temperature to 400 ºC, but a sharp increase both in ' and '' are observed due to the release of volatiles, regeneration of carbon in spent adsorbent. The increase of dielectric loss factor is beneficial to the conductivity of material, which contributes to the -electron conduction appearance.

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(2) Before zinc sulfate starts to decompose (room temperature to 600 ºC), dielectric properties increase with increasing temperature. With a continuous increase in temperature, '' and tan remain steady and even decrease, which indicate the order of microwave absorption capability is ZnO·2ZnSO4, ZnSO4 and ZnO. (3) Relationship between penetration depth and temperature further elaborate the absorption capability of spent adsorbent and zinc compounds, i.e. spent adsorbent and zinc compound except zinc oxide exhibit strong microwave absorption capability, both of them are excellent microwave absorber. Heating cures of zinc sulfate and spent adsorbent suggest that the ability of absorption microwave is related with material dielectric properties. The spent adsorbent can be regenerated directly by microwave heating and developed pore structures of regeneration adsorbent are observed by SEM analysis. Acknowledgments The authors are grateful for the financial support by the National Natural Science Foundation of China (51464024) and the Yunnan Province Young Academic Technology Leader Reserve Talents (2012HB008) References [1] X. Li, F.I. Hai, D.N. Long, Simultaneous activated carbon adsorption within a membrane bioreactor for an enhanced micropollutant removal, Bioresource Technol. 102 (2011) 5319-5324. [2] K.Y. Foo, B.H. Hameed, Microwave-assisted regeneration of activated carbon, Bioresource Technol. 119 (2012) 234-240. [3] J. Wei, P. Liang, X. Cao, X. Huang, Use of inexpensive semicoke and activated carbon as biocathode in microbial fuel cells, Bioresource Technol. 102 (2011) 10431-10435.

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Figures

Fig. 1 TG-DSC curves of zinc sulfate and spent adsorbent, (a) spent adsorbent; (b) zinc sulfate

24

Fig. 2 Temperature dependences of dielectric properties of ZnSO4 before decomposition with initial apparent density of 1250 kg/m3

25

Fig. 3 Temperature dependences of dielectric properties of ZnSO4 after decomposition with initial apparent density of 1250 kg/m3

26

Fig. 4 Dielectric properties of spent adsorbent at different apparent density

27

Fig. 5 Dielectric properties of spent adsorbent at different temperature with initial apparent density of 558 kg/m3

28

Fig. 6 Penetration depth of spent adsorbent and ZnSO4; (a): before decomposition of ZnSO4, (b): after decomposition of ZnSO4, (c): apparent density of spent adsorbent versus Dp, (d): temperature of spent adsorbent versus Dp

29

Fig. 7 Heating curves of samples, (a): zinc sulfate, (b): spent adsorbent

30

(a)

(b)

Fig. 8 SEM images of spent adsorbent (a) and regenerated adsorbent (b)

31

Tables Table 1 Proximate and ultimate analyses of spent adsorbent parameter

value Proximate analysis (wt.%)

moisture

20.04

ash*

10.59

volatile

30.91

fixed carbon

34.46 Ultimate analysis (wt.%)

silicon

0.81

calcium

0.78

magnesium

0.19

zinc

4.27

* Dry basis

Table 2 Dielectric properties of ZnO parameters

20 ºC

100 ºC

250 ºC

400 ºC

550 ºC

700 ºC

850 ºC

'

1.773

1.773

1.814

1.855

2.017

2.026

2.053

''

0.032

0.029

0.027

0.026

0.037

0.043

0.031

tan

0.018

0.017

0.015

0.014

0.018

0.021

0.015

 

Table 3 Regression parameters in the linear function of temperature Stages

a (ºC/min)

b (ºC)

R2

I

33.19

28.41

0.97

II

76.68

-345.75

0.97

III

4.09

748.34

0.93

 

32