Study on Regeneration of Spent Activated Carbon by Using a Clean

Green Process Synth 2017; 6: 499–510

Aiyuan Ma, Xuemei Zheng, Chenhui Liu, Jinhui Peng, Shiwei Li, Libo Zhang* and Chao Liu Study on regeneration of spent activated carbon by using a clean technology DOI 10.1515/gps-2016-0110 1 Introduction Received June 22, 2016; accepted November 21, 2016; previously published online March 2, 2017 Activated carbons (AC) have a wide range of applica- Abstract: In this paper, microwave regeneration of spent tions for liquid stream purification [1, 2] and treatment activated carbon saturated with organic compounds was of pollutants present in liquid gaseous effluents [3], investigated. It has been observed from the present exper- and removal of heavy metal ions from water [4] due to iments that the microwave regeneration temperature their excellent adsorbent properties and large surface and time have significant influences on iodine adsorp- areas [5]. However, pollutants are continuously accu- tion value and yield of the regenerated activated carbon mulated on the activated carbon surface during these (RAC). The characteristics of the RAC were examined by treatments. As a result, the adsorption capacity of the Brunauer–Emmett–Teller (BET). The RAC has a greatly activated carbon is progressively reduced until the acti- higher surface area (743.6 ~ 264.1 m2/g), total pore vol- vated carbon lose efficacy finally, and these spent ACs ume (0.54 ~ 0.22 cm3/g), and a relatively smaller average are burnt, dumped, or disposed of in landfills [6, 7]. pore width (28.83 ~ 33.58 nm) compared to the spent cata- However, these hazardous natures not only bring about lyst. The separation mechanism for activated carbon and a series of social problems, such as additional pollu- organic impurities was determined by X-ray photoelectron tion to the environment, but also waste a large amount spectroscopy (XPS) and scanning electron microscopy of reusable resources [8]. Furthermore, the price of AC (SEM) equipped with an energy-dispersive spectrometer keeps on rising. Therefore, how to resolve these prob- (EDS). It was a process in which the organic impurities lems and reuse these reusable resources have become were aggregated from the pore internal migration to the crucial issues in the world. surface at low temperatures, and the organic impuri- A wide variety of regeneration techniques of spent ties were completely decomposed as the temperature ACs have been widely studied, such as thermal regenera- increased to 900°C for 40 min. Simultaneously, a hexago- tion [9, 10], chemical methods [11, 12], electrochemical [13, nal crystal structure material of ZnO was obtained. 14], microbiological regeneration [7], microwave-assisted regeneration [15–17], and so on. Keywords: microwave regeneration; regenerated activated The report of Salvador et al. [5] showed that thermal carbon; spent catalyst. regeneration basically consists of heating the saturated AC to provide the amount of energy necessary to decom- *Corresponding author: Libo Zhang, Yunnan Provincial Key pose the retained adsorbate. However, the time and Laboratory of Intensification Metallurgy, Kunming 650093, Yunnan, energy in consuming thermal regeneration are relatively China; Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, Kunming longer and higher. At the same time, the regeneration 650093, China; and Faculty of Metallurgy and Energy Engineering, time and temperature have an important influence in the Kunming University of Science and Technology, Kunming 650093, activated carbon’s pore structure. Significantly, time and China, e-mail: [email protected] temperature bring more and more significant deteriora- Aiyuan Ma, Xuemei Zheng, Jinhui Peng, Shiwei Li and Chao Liu: tion in the adsorbent’s pore structure, thereby, reducing Yunnan Provincial Key Laboratory of Intensification Metallurgy, the final adsorption capacity and the efficiency of the Kunming 650093, Yunnan, China; Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science regeneration [18]. and Technology, Kunming 650093, China; and Faculty of Metallurgy Regeneration of the activated carbons generates and Energy Engineering, Kunming University of Science and organic impurities by chemical methods, and these Technology, Kunming 650093, China compounds consume a large number of chemistry rea- Chenhui Liu: Yunnan Provincial Key Laboratory of Intensification gents and dispose of these chemistry reagents, which Metallurgy, Kunming 650093, Yunnan, China; and Key Laboratory of Resource Clean Conversion in Ethnic Regions, Education Department is an important fact. Therefore, chemical regeneration of Yunnan Province, Yunnan Minzu University, Kunming 650500, is of no practical use in disposing the complex com- China pounds from spent catalysts, whereas such methods as 500 A. Ma et al.: Regeneration of spent activated carbon by a clean technology extractive regeneration, electrochemical and microbial Table 1: Components of the spent catalyst (%). processes basically do not have any commercial application [19]. Chemical element C Zn P Si Ca Al Content (wt%) 82.3 7.98 0.6 0.17 0.021 0.027 As an efficient and clean form of energy, microwave regeneration offers possible advantages over con- ventional treatment [20–23] such as selective heating, higher heating rates, and easy automatic control, which C implies that the microwave heating technique can be 1500 performed in a relatively short period of time, saving energy and reducing pollution [24]. The spent activated 1000 carbon in the application of microwave regeneration C technology for regeneration has indicated promising results [22, 25]. 500

The purpose of this paper was to investigate the Intensity (cps) regeneration of the spent catalyst of vinyl acetate syn- 0 C49H66O33 thesis by microwave irradiation. The efficiency of regen- C4H6Zn·2H2O C H O eration was evaluated with iodine adsorption value and 19 22 6 yield. To identify the interaction of microwave energy 10 20 30 40 50 60 70 80 90 and spent catalyst, the dielectric properties and the 2θ (°) temperature-rising characteristics of the spent cata- Figure 1: XRD pattern of the spent catalyst. lyst in the microwave field were studied. The structures and properties of the spent catalyst and the regenerated activated carbon (RAC) were characterized by nitrogen element distribution characteristics in the spent catalyst, X-ray EDS adsorption isotherm, cumulative pore volume distri- line scanning was characterized, as shown in Figure 2B and C. The bution, and pore size distribution. Scanning electron EDS line scanning from A to B shows that the major ingredients of microscopy (SEM)-energy-dispersive spectrometer (EDS) the filler material in the pore are C, O, and Zn, and it seemed that C, O, and Zn formed the C H O Zn·2H O and the other organic impurities and X-ray photoelectron spectroscopy (XPS) were used 4 6 4 2 combined in the XRD analysis. to determine the separation mechanism of the activated carbon and organic impurities. The obtained results are also extremely interesting from an economic point of 2.2 Experimental setup and methods view in saving resources and alleviating environmental pollution. 2.2.1 Test device for dielectric parameters: The dielectric parameter-measuring device scheme is shown in Figure 3. The dielectric parameter tester (Dielectric kit for Vials) is supplied by the German Püschner company. The device consists of a microwave power source, a directional coupler, a microwave receiver, and a 2 Materials and methods cavity resonator. The microwave signal receiver of AD-8320 integrated circuit can detect the signal amplitude and phase. The resonator 2.1 Experimental materials was used to hold in the analyzing cavity. The test control unit was via a USB data cable connected to the computer that calculates the The spent catalyst of vinyl acetate synthesis was obtained from a dielectric parameters. chemical plant in Yunnan province, China, and its composition is presented in Table 1. 2.2.2 Microwave equipment: A 3-kW box-type microwave reactor The X-ray diffraction (XRD, Rigaku Company, Japan) pattern of developed by the Key Laboratory of Unconventional Metallurgy the spent catalyst is displayed in Figure 1, which shows the samples in the Ministry of Education of Kunming University of Science and used in this work, mainly composed of C49H66O33, C4H6O4Zn·2H2O, and Technology was utilized for experimentation. The experimental

C19H22O6. device connection diagram is shown in Figure 4. SEM (XL30ESEM-TMP, Philips Company, Holland) equipped The microwave heating frequency is 2450 MHz, while the with an EDS (GENESIS, EDAX Company, USA) was used in analyzing power can be varied from 0 kW to 3 kW, continuously adjustable, the internal morphology and the element distribution of the spent and a thermocouple was used to measure the temperature. A mul- catalyst, shown in Figure 2. The microstructure of the spent catalyst lite crucible, with an inner diameter of 90 mm, height of 120 mm, is shown in Figure 2A. It is also shown that two substances exist in having good wave-transparent and heat shock properties, was the waste zinc acetate-activated carbon catalyst, the carbon material used. The smoke soot absorption system was composed of a suc- and the filler material in pore. To obtain more information on the tion bottle, two water bottles, a surge flask, and an aspirator pump. A. Ma et al.: Regeneration of spent activated carbon by a clean technology 501

(a) (b)

B C 20,000 Figure 3: Dielectric constant measurement device scheme.

15,000

Zn 3 Results and discussion 10,000 cps (eV)

5000 O 3.1 Dielectric property test results and analysis P Zn 0 Interaction of microwaves with materials depends on their 012345 678 9 10 11 12 dielectric properties, which were determined by heating a Energy (keV) material subjected to electromagnetic fields [26]. Dielec- C tric properties [27] consisted of dielectric constant (ε′), dielectric loss (ε″), and loss tangent (tan δ). The dielectric constant is a measure of the ability of a material to store electromagnetic energy, and dielectric loss is a measure of the ability of a material to convert electromagnetic energy to heat, while loss tangent used to describe how well a material absorbs microwave energy, is the ratio of dielectric loss factor and the dielectric constant (tan δ = ε″/ε′) [28]. The heating rate of a material under a microwave field is closely related to loss tangent; a material with a higher loss tangent will heat faster compared to a lower loss tangent.

The dielectric parameters (ε′, ε″, and tan δ) of H2O, Figure 2: SEM/EDS pattern of the spent catalyst. polytetrafluoroethylene, C4H6O4Zn, ZnO, and spent catalyst were measured at room temperature at 2.45 GHz by the cavity perturbation method, and the results are presented The flue dust and the exhaust were collected and absorbed in the in Table 2. experimental process. As the results shown in Table 2, the system error of the dielectric coefficient was estimated to be 3%–5% during 2.2.3 Experimental methods: Three hundred grams of spent the perturbation method tests. The dielectric constants catalysts was accurately quantified and placed in a microwave of deionized water (80.4 F/m) [29] and polytetrafluoro- reactor box. The effects of regeneration temperatures (400°C, ethylene (2.08 F/m) [30] were used as standards, result- 500°C, 600°C, 700°C, 800°C, 900°C) and regeneration times (20, 30, 40, 50, 60 min) on the iodine adsorption value and the yield of ing in 79.79 and 2.04 F/m, respectively, 0.76% and 1.92% the regenerated activated carbon were studied in the atmosphere of differences. Thus, the results of this evaluation system nitrogen. were credible, which had certain valuable feasibility and 502 A. Ma et al.: Regeneration of spent activated carbon by a clean technology

Thermocouple and display Flow control dampers

SV PV

Nitrogen

Suction bottle Gas bottle Surge flask Aspirator pump

Microwave irradition store

The control system of microwave Sample Thermal insulation material

Figure 4: Experiment equipment for microwave regeneration.

Table 2: Dielectric parameters of different substances.

Substances Parameters

Dielectric constant ε′ (F/m) Loss factor ε″ (F/m) Loss tangent tan δ

H2O 79.79 2 0.0251 Polytetrafluoroethylene 2.04 0.013 0.0064

C4H6O4Zn 2.18 0.018 0.0083 ZnO 1.40 0.04 0.0286 Spent catalyst 9.34 0.68 0.0728

=+ −+232 = applicability. Also, it was found that the loss tangent of Ttm 12.8352.17 2.19tt0.04 (R 0.9993) (3) the spent catalyst was higher than those of C H O Zn and 4 6 4 The results showed that the spent catalyst average heating ZnO. Therefore, the spent catalyst could be easily heated rates were 40°C, 36.5°C, 26°C/min for different masses of under a microwave field. 50 g, 100 g, and 150 g, respectively, and the temperature reached 920°C, 840°C, and 600°C in 23 min. Thus, the smaller the quality of the spent catalyst, the faster is the 3.2 Temperature rising of spent catalyst apparent heating rate. The greater spent catalyst mass has a smaller heating rate, which is consistent with the In the microwave field, the temperature-rising charac- experimental results. On one hand, the material quality teristics of materials are closely related to the amount of increased, when the unit mass microwave power density materials and the microwave power. ratio was decreased. Meanwhile, the materials increase The spent catalyst quality affecting heating behavior the contact area and heat the external environment; on is shown in Figure 5A under the microwave output power the other hand, when the microwave power is constant, of 900 W. The relationship between the temperature of the the greater the amount of soot material, the harder is the spent catalyst sample and the time with different masses microwave penetrating uniformly into the inside of the of 50 g, 100 g, and 150 g, respectively, and the empirical material. Therefore, during the experimental range, with equations are shown in Eqs. (1)–(3): the increasing smoke quality, the microwave ability of the spent catalyst under microwave heating is weakened, and =+ −+232 = Ttm 50.16 84.29 3.02tt0.04 (R 0.9965) (1) the heating rate is decreased. The heating curve of 100 g of spent catalyst at

232 microwave powers of 700 W, 900 W, and 1100 W, respec- Tt=− 5.97 +−93.864.150tt+=.070 ()R .9983 (2) m tively are shown in Figure 5B. The empirical formula of A. Ma et al.: Regeneration of spent activated carbon by a clean technology 503

A 1000 In addition, the unit volume of the spent catalyst absorbed the microwave power, or the microwave energy 800 dissipated power in smoke and could be expressed as the following equation [31],

600 P2=πƒEε′′ 2

C) (7) °

T ( 400 In Eq. (7), P is the microwave energy of the material 50 g absorption, ƒ is the microwave frequency, ε″ is the dielec- 200 100 g 150 g tric loss factor, which is a function of temperature, and E 0 is the electric field strength. Equation (7) showed that to improve the power of the 0510 15 20 25 microwave heating means to increase the electric field t (min) intensity in the case wherein other conditions remained B 1000 unchanged. With the electric field strength (E) increasing, the microwave energy could be better and uniformly pene- 800 trates into the interior of the material. With constant smoke penetrating the microwave deeper, smoke absorbed more 600 microwave power and temperature increased. Therefore,

C) increasing the microwave heating power properly short- °

T ( 400 ened the heating time and improved the smoke apparent average heating rate. 700 W 200 900 W 1100 W 3.3 Regeneration of spent activated carbon 0

0510 15 20 25 In order to evaluate the reactivation efficiency of the RAC, t (min) the effects of the reactivation temperature and time on iodine adsorption capacity and the yield of the RAC were Figure 5: (A) Spent catalyst heating rate (900 W) curve at a different quality; (B) spent catalyst heating rate (100 g) curve at a different investigated. The iodine adsorption value of the activated microwave power. carbon had been tested according to the National Stand- ard Testing Methods of P. R. China (GB/T12496.8-1999), and the yield was calculated by Eq. (8), the sample temperature and time could be seen from =× Eqs. (4) to (6), Yield (%)M/M0 100 (8)

232 =+ −+ = where M0 is the weight (g) of the spent catalyst activated 1Ttm 4.03 48.76 1.37tt0.02 (9R 0.9 60) (4) carbon, and M is the weight (g) of the RAC. The experimental research results are shown in Tt=− 5.97 +−93.864.150tt23+=.070 ()R2 .9963 m (5) Figure 6A and B. As can be seen in Figure 6A, the iodine adsorption value of the RAC increased (from 368.55 mg/g =+ −+232 = Ttm 12.7690.63 3.37tt0.05 (R 0.9965) (6) to 930.55 mg/g) when the temperature increases from 25°C to 1000°C, and the percentage yield of the RAC decreases It can be seen from Figure 5B that the main influence continuously from 100% to 65.8%. of the microwave power on the temperature of the materi- The effects of regeneration time on the iodine adsorp- als was that the microwave power increased, the apparent tion values and the yield of the regenerated activated average heating rate of the spent catalyst increased, and carbons are shown in Figure 6B. As can be seen, the regen- the time to attain the same temperature was shortened. eration time had great influence on the development of The conclusion is as follows: within a certain range, there the iodine adsorption values and the yield. At the initial is an increase in the material temperature with increasing stage, as the hold time increased from 0 to 40 min, the microwave output power. iodine adsorption values increased significantly from 504 A. Ma et al.: Regeneration of spent activated carbon by a clean technology

1000 105 microwave regeneration method (MRM) were higher than A Iodine adsorption value 100 those prepared with thermal regeneration method (TRM) 900 Yield by muffle furnace. At the same time, under the constant 95 800 activation temperature, the microwave regeneration time 90 was significantly shorter, and the iodine adsorption value 700 85 and yield of RAC by MRM were significantly improved. alue (mg/ g)

600 80 ield (% ) ption v Y 500 75 3.4 Characterization of the reactivated 70 400 activated carbon Iodine adsor 65 300 3.4.1 Pore structures analysis 0 200 400 600800 1000 T (°C) For the structural and chemical characteristics of the spent B 900 100 catalyst, the RAC prepared by MRM and TMR was characterized by nitrogen adsorption isotherm, cumulative pore 800 95 volume distribution, and pore size distribution, as shown ) in Figure 7A–C, respectively. Nitrogen isotherm analysis 90 700 showed that the RAC prepared by MRM has great higher alue (mg/g Iodine adsorption value 85 surface area, total pore volume, and a relatively smaller 600 Yield ield (% ) ption v average pore width compared to that of the RAC prepared 80 Y 500 by TRM and the spent catalyst as shown in Figure 7 and 75 Table 4. This result clearly indicates that the pore structure Iodine adsor 400 can be significantly improved with microwave regeneration 70 treatment. 300 0102030 40 50 60 t (min) Figure 6: Effects of the temperature (A) and time (B) on iodine 3.4.2 XPS analysis adsorption value and yield. The surface chemical structures of the spent catalyst and the RAC were also determined with XPS (Thermo ESCALAB 368.55 mg/g to 880.62 mg/g, and the yield of the RAC 250Xi, Thermo Fisher Scientific Company, USA). XPS was decreased from 100% to 70.55% with the regeneration applied to further investigate the state of zinc, and the time increasing from 0 to 1 h, indicating that the pores spectrums of the spent catalyst and the RAC by MRM at were formed and enlarged simultaneously in this stage. 900°C are shown in Figure 8. Significant intensities of the With the activation time going on, the iodine adsorption C1s (284.0 eV), O1s (531.0 eV), and the F1s (684 eV) levels values and the yield of the RAC gradually became slow for the spent catalyst are shown in Figure 8A, while the and remained constant. observed peaks at 1044.0, 1021.0, and 88.0 eV correspond The iodine adsorption value and yield under different to the Zn2p1/2, Zn2p3/2, and Zn3p levels, respectively. The regeneration methods are shown in Table 3. The experi- relation was reported by Mekhalif et al. [32]. However, the ment results in Table 3 shows that the iodine adsorption peaks of F1s, Zn2p, and Zn3p line for the RAC in Figure 8C value, the yield of the spent catalyst, and the RAC by were not found.

Table 3: The iodine adsorption value and yield of the spent catalyst and the RAC by different regeneration methods.

Regeneration methods T (°C) t (min) Iodine adsorption value (mg/g) Yield (%)

Spent catalyst – – 368.55 100 TRM 900 30 572.59 78.15 TRM 900 60 680.59 72.7 TRM 900 120 816.61 65.9 MRM 900 40 880.62 70.55 A. Ma et al.: Regeneration of spent activated carbon by a clean technology 505

A 400 Table 4: Textural parameters of spent catalyst and regenerated Spent catalyst activated carbon. 350 RAC-MRM RAC-TRM 300 Material BET surface areas Pore volume Average pore 2 3 SBET (m /g) (cm /g) size (nm) 250 Spent catalyst 264.1 0.22 33.58

olume (ml/g) 200 MRM (40 min) 743.6 0.54 28.83 TRM (40 min) 628.5 0.49 30.88 150 pition v BET, Brunauer–Emmett–Teller. 100 Adsor

50 in Figure 8B and D, respectively. Figure 8B and D shows 0 a common main peak at 284.6 eV, which is a characteris- 0.0 0.2 0.4 0.6 0.81.0 tic of aliphatic carbons [32]. The rather important feature

Relative pressure (P/P0) of Figure 8B is the concentrate at 288.0 eV, and it com- prises contributions of oxidized carbon species (O-C=O) B [32, 33]. This is corroborated by the presence of oxi- 0.5 dized carbon species (C4H6O4Zn·2H2O and other organic impurities) noted for the spent catalyst; however, these 0.4 oxidized carbon species have been completely decom- Spent catalyst posed by microwave regeneration treatment, as shown RAC-MRM 0.3 RAC-TRM in Figure 8D. olume (ml/g) 0.2 re v 3.4.3 XRD analysis Po

0.1 The characterization of the RAC prepared by MRM was performed after microwave roasting. The XRD pattern 0.0 of the RAC by MRM at 900°C was shown in Figure 9B. It 050 100 150 200 250 300 350 400 Pore width (Å) was matched with that of the spent catalyst XRD pattern C (Figure 9A). The results showed that the peak for carbon 0.06 (C) is enhanced significantly, and the peaks for both

Spent catalyst C49H66O33, C4H6O4Zn·2H2O, and C19H22O6 disappeared. At the 0.05 RAC-MRM same time, the ZnO formants were found. RAC-TRM 0.04 3.4.4 SEM-EDS analysis 0.03 In order to determine the separation mechanism that is 0.02 dV(d) (ml/g·Å) associated with activated carbon and impurities, the

0.01 structural characteristics of the spent catalyst and the RAC at different temperatures were studied with SEM-EDS, and 0.00 the results are shown in Figure 10. Figure 10A and B shows that the organic impurities 0255075 100 125 150 Pore width (Å) were observed at the surface of the activated carbon after activation. The patterns of RAC at 400°C were character- Figure 7: (A) Nitrogen adsorption isotherms on spent catalyst and ized by SEM-EDS, as shown in Figure 11. The EDS line RAC; (B) total pore volume distribution for spent catalyst and RAC; scanning from point a to point b shows that the major (C) pore size distributions for spent catalyst and RAC. substances of the surface materials in activated carbon are organic compounds that contain C, O, and Zn. It is a To better understand the characteristics of the Zn process wherein the organic impurities were aggregated substrates, The C1s spectra of the Zn substrates for the from the pore internal migration to the surface under spent catalyst and the RAC were studied and are shown microwave irradiation (compared with Figure 2). 506 A. Ma et al.: Regeneration of spent activated carbon by a clean technology

A B Raw curve

Zn2p3/2 Sum curve Baseline C1s C-C Zn2p1/2 F1s O-C=O O1s

C-C Zn3p O-C=O

0 200 400 600800 1000 280 285 290 295 Binding energy (eV) C D Raw curve RAC Baseline Cls C-C O1s

0 200 400 600800 1000 280285 290295 Binding energy (eV)

Figure 8: XPS spectra of the spent catalyst (A, B) and the RAC (C, D).

. of the spent catalysts were still saturated with some by- 1-C4H6O4Zn 2H2O 2-C H O products at 800°C, which may have remained inside the C 49 66 33 3-C19H22O6 micropore network of the spent activated carbon and will 4-ZnO not decompose until reaching a high temperature. However, 4 more macropores were obtained after activation for 40 min 4 C 4 B at 900°C (Figure 10G). The comparison of Figure 10F and G 4 4 4 4 indicates clearly that a regular macroporosity and a rather C 2 2 3 homogeneous surface are obtained by activation at 900°C 2 1 Intensity (cps) 1 1 3 2 for 40 min, and the organic impurities were completely 3 C decomposed. In addition, when the activated carbon and the A organic impurities were separated, simultaneously, some bright white by-products were obtained. These bright white particles were analyzed by SEM-EDS, and the results are

20 40 60 80 shown in Figure 12. The results show that these by-products 2θ (°) are ZnO, and their structure is a hexagonal crystal.

Figure 9: XRD patterns of the spent catalyst (A) and the RAC (B). 4 Conclusions The pyrolysis creates gradual porosity (Figure 10B–G) as the temperature increases from 400°C to 900°, and the The spent catalyst was rapidly heated, and the dielec- porosity is more developed. Macropores were formed after tric properties and the temperature-rising character- activation for 40 min at 800°C (Figure 10F), and the pores istics of the spent catalyst in the microwave field were A. Ma et al.: Regeneration of spent activated carbon by a clean technology 507

A B

C D

E F

G H

Figure 10: SEM patterns of the spent catalyst and the RAC at different temperatures. (A) Spent catalyst, (B) 400°C, (C) 500°C, (D) 600°C, (E) 700°C, (F) 800°C, (G) 900°C, (H) bright white particles in 900°C.

studied and analyzed. The effects of the reactivation that the reactivation efficiency of the regenerated acti- temperature and time on iodine adsorption capacity and vated carbon is optimal at the microwave reactivation the yield of the RAC were studied. The results showed temperature of 900°C and reactivation time of 40 min. 508 A. Ma et al.: Regeneration of spent activated carbon by a clean technology

A (b) A

(a)

B B 17,500 C 14,000 Zn 15,000 12,000 12,500 10,000 10,000 8000 cps (eV) 7500 Zn

cps (eV) 6000 5000 4000 2500 Zn O O Zn Zn 0 2000 Zn 0 02 4 6810 12 14 Energy (keV) 0 2468101214 Energy (keV)

C Figure 12: SEM/EDS patterns of the by-products.

C O Zn organic impurities are completely decomposed as the temperature increases to 900°C for 40 min. Simultane- ously, a hexagonal crystal structure material of ZnO was obtained. In addition, the reactivation of the spent catalyst has a great practical significance in saving resources and alleviating environmental pollution.

0510 15 20 25 30 35 Acknowledgments: This work was supported by the Distance (µm) National Natural Science Foundation of China (51464024), National Basic Research Program of China (2012HB008), Figure 11: SEM/EDS patterns of the RAC at 400°C. and Yunnan Provincial Science and Technology Innovation Talents Scheme Technological Leading Talent (2013HA002).

Nitrogen isotherm analysis showed that the RAC by MRM has a greatly higher surface area (743.6 ~ 264.1 m2/g), total pore volume (0.54 ~ 0.22 cm3/g), and a relatively References smaller average pore width (28.83 ~ 33.58 nm) compared to the spent catalyst. The separation mechanism for [1] Ledesma B, Román S, Sabio E, Álvarez-Murillo A. J. Supercrit. Fluids 2015, 104, 94–103. the activated carbon and organic impurities was deter- [2] Mailler R, Gasperi J, Coquet Y, Buleté A, Vulliet E, Deshayes S, mined by XPS and SEM-EDS. It is a process wherein the Zedek S, Mirande-Bret C, Eudes V, Bressy A, Caupos E, organic impurities are aggregated from the pore internal Moilleron R, Chebbo G, Rocher V. Sci. Total Environ. 2016, 542, migration to the surface at low temperatures, and the 983–996. A. Ma et al.: Regeneration of spent activated carbon by a clean technology 509

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Jinhui Peng Libo Zhang

Jinhui Peng is a PhD supervisor at Kunming University of Science and Technology, China, and is mainly engaged in microwave heating in the application of metallurgy, chemical engineering, and materi- Libo Zhang is a PhD supervisor at Kunming University of Science and als science. He has received many awards, among which are the Technology, China, and is mainly engaged in the microwave heating State Technological Invention Award and the Natural Science Award in the application of metallurgy, chemical engineering, and materials of Kunming province. science.

Shiwei Li Chao Liu

Shiwei Li obtained his doctorate from Northeastern University in Chao Liu is pursuing his doctorate at Kunming University of Science 2013. Currently, he works at Kunming University of Science and and Technology, China, where he is currently carrying out research on Technology. His primary research interests include microwave met- microwave energy application, metallurgy, and chemical engineering allurgy, hydrometallurgy, and comprehensive recovery of the wastes under the supervision of Professor Jinhui Peng. His main research in metallurgy fields. subject is treatment of hazardous waste with microwave metallurgy.