The Comparison Of Different Activation Techniques To Prepare Activated Carbon Materials From Waste Cotton Fabric

Abstract In this paper, we investigate on the preparation of waste cotton fabric-based activated carbons by different methods. Two different kinds of carbon materials are prepared from waste cotton fabric, the structure and properties were characterized using instrumental analyses such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). It is revealed that the products prepared using one-step process are composed of macroporous carbon network , which looks like sponge-type morphology, and exhibit the high values of qiodine (1,198 mg/g) and qmb (235.6 ml/g), showing their potential usage as adsorbent.


Introduction
Activated carbon (AC) has been widely used as a versatile adsorbent for separation of gases, removal of organic pollutants, and so on because of its large specific surface area, porous structure, and good adsorption properties [1][2][3][4][5]. Numerous researchers have studied on the preparation of carbon adsorbents with high surface area for adsorption and separation of gas. There are two methods for manufacturing ACs: physical and chemical activation [6]. Physical activation involves carbonization of the raw material in an inert atmosphere followed by mild oxidation of the raw material. Chemical activation involves impregnation of the raw material with dehydrating chemical agents including phosphoric acid, sulfuric acid, potassium hydroxide, and zinc chloride in an inert atmosphere [7,8]. The common feature of these impregnations leads to an increase in carbonization ability and, therefore, the development of a desired pore structure.
As waste cotton fiber is composed of cellulose-which have only three element (C, H, and O) and a higher carbon content (about 44%), it may be a potential substitute material for precursor to produce carbon materials. For example, Mustafa Özdemir [9] and Djordjevic [10] prepared ACs from cotton stalks or fibers by chemical activation with ZnCl 2 and H 2 SO 4 and physical activation using CO 2 . But H 2 SO 4 is not environmentally sustainable solutions, and the two activation agents (ZnCl 2 and CO 2 ) used were more complex and expensive.
In this paper, we focus on the utilization of waste cotton fabric as raw materials for the preparation of ACs by applying one-step or two-step process of pyrolysis that use ZnCl 2 as activation agent. We study the difference between the two methods used for the preparation of ACs and the structure and properties of the waste cotton fiber-based carbon materials.

Preparation of the activated carbons (AC-WF)
In the one-step process, the waste cotton fabric was first washed for several times using tap water to remove impurities and dried in an oven at 80°C for 12 h. Then it was cut into small pieces (0.5 cm 2 ). About 5 g of cotton fabric (on a dry basis) was impregnated with 50 mL of ZnCl 2 solution (55%) for 24 h. Then, the mixture was dried at 105°C for 12 h in an hot air oven. The impregnated sample was placed in a stainless steel horizontal reactor (7 cm in diameter and 100 cm in length) and then heated to the activation temperature of 700°C for 1 h under nitrogen atmosphere (99.99%) flow (100 mL min −1 ) at a heating rate of 10°C min −1 . It was cooled down to room temperature under nitrogen atmosphere and then 50 mL of 0.1M hydrochloric acid solution was added to the carbonized product and stirred for 30 min. This mixture was filtered and washed with hot distilled water for several times to remove residual chemicals and chlorine until the pH of the filtrate was around 7. The obtained product was dried at 105°C for 10 h, stored in glass bottle for characterization and adsorption processes, and named as AC-WF-I.
The chemical activation with ZnCl 2 is a two-step process: precarbonization and activation-carbonization process. The first process is pre-carbonization. In this process, 5 g of cotton fabric (on a dry basis) was placed in a stainless steel horizontal reactor and then heated to the activation temperature of 450°C for 1 h under nitrogen atmosphere. This is followed by the carbonization process in which 50 mL of ZnCl 2 solution (55%) was added to 5 g of the pre-carbonized fabric materials and kept for 24 h. After that, the samples are activated at 700°C for 1 h under nitrogen atmosphere and then allowed to cool to room temperature and named as AC-WF-II.

Characterization of the activated carbons
The morphologies of the samples are obtained with a scanning electron microscopy (SEM) (TESCAN, MIRA3 LMH). The composition and structure are analyzed using powder X-ray diffraction (XRD, Rigaku D/max-2500 diffractometer with Cu Ka radiation) and Fourier transform infrared spectroscopy (FTIR) (Bruker Tensor 27 spectrometer in the range of 500-4,000 cm −1 on sample pellets made with KBr).

The methylene blue (MB) and iodine value
The methylene blue (MB) capacity and iodine value are determined by China National Standards (GB/T12496.8-1999 and GB/T12496.10-1999) to examine the adsorption capability of the prepared ACs.

Orthogonal experiments
Chemical activation is mainly directed toward the preparation of ACs. As the precursor is finely divided, the homogeneity of the mixture with the reagent is ensured. So, if the objective is to prepare granular AC, the impregnation step has to be carried out with special care to ensure the intimate contact between the precursor and the reagent. From many references, we have found that there are two kind of techniques to chemical activation. One of the techniques is the two-step process, that is, the precursor was pre-carbonized and then the carbonized materials are put into the activating agent solution. After that, the activating agent-impregnated carbonized materials were activated at different temperature for certain activation time to prepare ACs. The other technique is the one-step process, the activating reagent was dissolved in water and mixed with the precursor, and then the heat treatment (carbonization) was carried out in a flow of nitrogen at 500°C [11,12].
In this paper, in order to optimize the reaction conditions, we mainly investigate the effect of carbonization temperature/time or activation temperature/time, impregnation time, and the ratio of ZnCl 2 to fabric on the characterization of the prepared ACs from waste cotton fabric through orthogonal experimental design. Taking ACs carbons prepared by the one-step and twostep technologies as model, orthogonal experiments with four factors of three levels are designed. A series of ACs preparation experiments are carried out with different reaction conditions.
The methylene blue and iodine values at every level of every factor are shown in Tables S1 and S2. The range analysis is aimed to clarify the significance levels of different influencing factors on the iodine and methylene blue adsorption characterization of the prepared ACs. Based on the result of range analysis, the most significant factor could be disclosed. Tables S1 and S2 summarize the statistics analysis of the effect of different factors on the iodine and methylene blue values of prepared ACs. The k is the average of the sum of each level of factor values, and the range value (R) for each average is the difference between the maximal and minimal values of the three levels.
Based on the results of range analysis, the significance sequence of all the investigated influencing factors is lined. In the one-step process, for the iodine value, the order of significance levels is shown as follows: ratio of ZnCl 2 /fabric, impregnation time, carbonization temperature, and carbonization time. In the meanwhile, for methylene blue value, the order is: ratio of ZnCl 2 /Fabric, carbonization time, impregnation time, and carbonization temperature. Based on the factors, the optimum conditions for the one-step process are shown in Table 1.
In the two-step process, for the iodine value, the order of significance levels is shown as follows: ratio of ZnCl 2 /carbonized fabric, carbonized fabric impregnation time, activation time, and activation temperature. In the meanwhile, for the methylene blue value, the order is: ratio of ZnCl 2 /carbonized fabric, activation temperature, activation time, and carbonized fabric impregnation time. Based on the factors, the optimum conditions for the two-step process are shown in Table 1.
Under the optimum conditions, the methylene blue and iodine values of AC prepared by the one-step process are 235.6 mL/g and 1,198.6 mg/g, respectively. For the two-step process, these values are 65.9 mL/g and 465.8 mg/g, respectively. The one-step process exhibits better adsorption capability than the steam physical activation method. Figure 1 shows the SEM images of the prepared carbon materials by the two kinds of methods. SEM images of the precarbonation and activation-carbonation of two-step method are shown in Fig. 1a and b, respectively. The pre-carbonation samples (Fig. 1a) are composed of many irregular mass that exhibit rough surfaces with many small irregular fragments over the surface. The morphology of activation-carbonation samples (AC-WF-II, Fig.1b) is similar to that of pre-carbonation samples, expect a few holes on the surface. When activating with one-step process, in the SEM images of the resulting sample c and d, many pores of various sizes in a honeycomb can be observed on the sample surface. Figure 1d is the crosssectional SEM image of AC-WF-I and shows many uniformly distributed pores and some pores are link up with each other. The SEM images shown in Fig. 1(c,d) clearly reveal that the AC-WF-I prepared by one-step process is composed of macroporous network of interconnected porous carbon, which looks like sponge-type morphology. Hence, the AC prepared by the one-step activation method is expected to have a better adsorption capacity.

Characterization of AC-WF
In the one-step process, the activating reagent ZnCl 2 has been dissolved in water and then mixed with the precursor. In this way, the hydration of the precursor is facilitated, and the swelling of the fibers allows for a better access of the reactant to the interior of the fibers, which forces the incorporation of the reactant to the interior of the fibers. Once the impregnation step is finished, carbonization is carried out at high temperature under a flow of nitrogen; many gas including H 2 O, CH 4 , and CO 2 were produced and carried away by N 2 air flow. And, at last, the resulting carbon is washed to eliminate the rest of the chemical. Hence, a number of pores were formed on the surface and interior of the fibers.
FTIR is carried out to analyze the composition of the samples. As shown in the FTIR spectrum (Fig. 2), the appearance of a few number of adsorption peaks in the spectrum of AC-WF-I and AC-WF-II suggested that the chemical structure was simple. The peak at 3439 cm -1 reflects the -OH stretching vibration, the bands at 1625 cm -1 and 1436 cm -1 were the characteristic of stretching vibration of C=C, while the band at 1109 cm -1 indicated the C-O stretching vibrations in alcohols, phenols, or ether or ester groups [13]. Therefore, the prepared materials are mainly composed of amorphous carbons with different oxygencontaining surface groups. These carbon-oxygen groups may exercise a profound effect on the surface properties of ACs and thus influence their adsorption characteristics, that is, these groups could be the potential active sites for the interaction with the adsorbate. The XRD curves of the AC-WF prepared by two different methods are shown in Fig. 3. For the AC-WF-I and AC-WF-II, the diffraction profiles exhibit only a broad peaks at approximately 2θ = 24°. XRD pattern reveals the amorphous state of the obtained samples. Accordingly, no pronounced graphitization occurs under the present carbonization/activation condition.

The methylene blue (MB) and iodine adsorbed values
The iodine value (mg of iodine adsorbed/g of carbon) and methylene blue value (ml of methylene blue adsorbed/g of carbon) are considered as a measure of adsorption capability of AC. Normally, q iodine denotes the amount of micropore and q mb denotes the amount of mesopore of AC. As for the two-step process, in which the cotton fabric is carbonized at the first stage and activated with ZnCl 2 at the subsequent stage, porous carbons with lower adsorption capacities for methylene blue and iodine could be obtained. Under the optimal preparation condition, the values of q iodine and q mb for the AC-WF-II samples are 465.8 mg/g and 65.9 ml/g, respectively. Particularly, by the one-step process, in which the cotton fabric is carbonized and activated at the same stage, the higher values of q iodine (1198 mg/g) and q mb (235.6 ml/g) can be obtained. Therefore, the one-step process exhibits the better adsorption capability than the two-step process with different preparation condition.
The comparison of capability of the waste cotton fabric-based AC with the National Standard is shown in Table 2. From the results, we can observe that the iodine and the methylene blue values of the AC-WF-I prepared under the optimum condition are higher than those of the first-grade product specified in the National Standard.

Preliminary adsorption studies of PC-WF
Equilibrium time is one of the most important parameters in the design of economical wastewater treatment systems. Figure 4 represents the adsorption capacity of 0.2 g of AC-WF-I versus the contact time for varying initial concentrations of dye between 250, 350, and 450 mg/L,. The adsorption is initially (contact time﹤15 min) fast and then slows; the equilibrium was attained in about 120 min. The fast adsorption at the initial stage was probably due to the great concentration gradient between the dye in solution and the dye in the AC-WF-I because there must be a number of vacant sites available at the beginning. The progressive increase in adsorption and, consequently, the attainment of equilibrium adsorption are initially due to the limited mass transfer of the dye molecules from the bulk solution to the external surface of the adsorbent and are subsequently due to the slower internal mass transfer within the adsorbent particles [11,12]. As seen in Fig. 3, the amount of dye adsorbed increased evidently from 174.5 to 319.8 mg/g for BC-GR and from 499.5 to 842.5 mg/g for CR-GL with the increase in the initial concentration from 250 to 450 mg/L.

Conclusions
Two kinds of waste cotton fiber-based ACs are prepared by chemical activation with ZnCl 2 with different carbonization and activation methods. The optimum conditions are obtained respectively. The characterization results reveal that one-step process is better than the two-step process with respect to the porosity development. Therefore, the carbon products obtained by using one-step method exhibit high values of q iodine (1,198 mg/g ) and q mb (235.6ml/g) . Therefore, they can be used as low-cost adsorbent and considered as an alternative to the commercial AC.
The carbonization method described in this work is a facile, onestep, efficient, economic, and environmentally benign synthesis strategy to produce porous carbons, which can greatly cater for the global energy, resources, and environmental problems. Thus, this method is promising for the large-scale recycling of waste cotton textiles.