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 applica- tion [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, micro- wave 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 (a) (b) B C 20,000 Figure 3: Dielectric