Utilization of Crofton weed for preparation of activated carbon by microwave induced CO2 activation

https://doi.org/10.1016/j.cep.2014.05.001Get rights and content

Highlights

  • Crofton weed, a harmful biomass is utilized for preparing the value-added activated carbon.

  • Influences of the three vital process parameters were investigated using analysis of variance.

  • Microwave heating has been used which allows the reactions proceed faster resulting in lower processing time and energy consumption.

  • The proximity in experiment results and the predicted values validate the success of the optimization process.

  • The prepared activated carbon exhibits high adsorption capacity and well develops pore structure.

Abstract

Crofton weed was converted into a high-quality activated carbon (CWAC) via microwave-induced CO2 physical activation. The operational variables including activation temperature, activation duration and CO2 flow rate on the adsorption capability and activated carbon yield were identified. Additionally the surface characteristics of CWAC were characterized by nitrogen adsorption isotherms, FTIR and SEM. The operating variables were optimized utilizing the response surface methodology and were identified to be an activation temperature of 980 °C, an activation duration of 90 min and a CO2 flow rate of 300 ml/min with a iodine adsorption capacity of 972 mg/g and yield of 18.03%. The key parameters that characterize quality of the porous carbon such as the BET surface area, total pore volume and average pore diameter were estimated to be 1036 m2/g, 0.71 ml/g and 2.75 nm, respectively. The findings strongly support the feasibility of microwave heating for preparation of high surface area porous carbon from Crofton weed via CO2 activation.

Introduction

Activated carbons (AC) are important kinds of porous carbon material with abundantly developed pore structure, strong adsorption ability, high surface area and thermo stability, they are widely used in many different industries, such as separation and purification processes, including both gas and aqueous media [1], [2], [3], catalyst supports [4] and removal of organic dyes and pollutants from industrial wastewater [5] and from other aqueous media [6]. The selection of an appropriate precursor plays an important role deciding the characteristics of the AC as well as the economics of the manufacturing plant. To identify new precursors that are cheap, accessible and available in large quantity has been a perennial challenge in commercial manufacture for economic benefits [7]. Toward which, different biomass based feedstock such as rice bran, coconut shell and waste materials were used as the raw materials since they are sustainable sources having high fixed carbon content [8], [9], [10].

Croton weed, a kind of global exotic weeds originated from Mexico, and which has spread extensively in many countries around the world such as America, Australia and the countries in Southeast Asia due to its strong ability to adapt to different environmental conditions [11]. Since 1940s, Croton weed has spread extensively in south and western of China. Lots of the farm lands, pasture fields and forests have been destroyed causing huge economic losses. This has drawn the attention of the society and many methods have been developed to control it, such as manual, chemical and biological control, no obvious progress is made. In 2003, the Chinese ministry of environmental protection released a list of “The First Batch of Exotic Invasive Species” and croton weed was rated the first [12], [13]. According to the published literatures, croton weed can be used as bio-pesticide [14], organic fertilizer and feedstuff, feedstock for production of marsh gas [15]. Although, croton weed can be utilized as a biomass resource to prepare the AC, the relevant literature is very limited. The attempts pertaining to preparation of CWAC has been limited to Xia et al. and Wu et al. [16], [17].

Activated carbons have been traditionally produced by the partial gasification of the char either with steam or CO2 or a combination of both. The gasification reaction results in removal of most reactive carbon atoms and in the process simultaneously produce a wide range of pores (predominantly micropores), resulting in porous activated carbon. In general, the methods for AC production are divided into two classes: physical activation and chemical activation. Physical activation is essentially a two-step process, where the carbonization of a carbonaceous material forms the first step, while the second step involves the activation of the resulting char at elevated temperature in the presence of suitable oxidizing gases such as carbon dioxide, steam, air or their mixtures. Chemical activation involves the impregnation of a carbonaceous material with an activation agent and heat treatment of the impregnated material under inert atmosphere. Physical activation is widely adopted industrially for commercial production owing to the simplicity of process and the ability to produce AC with well developed micro porosity and desirable physical characteristics such as the good physical strength.

Referring to the heating methods for the preparing of AC, interests are growing in the application of microwave (MW) heating. The conventional heating methods do not ensure a uniform temperature of the precursor owing to their variation in the size and shape as the mode of heating is through conduction and convection. This conventional heating mode generates a temperature gradient from the hot surface of the sample particle to its interior and impedes the effective removal of gaseous products to its surroundings, demanding higher processing time and energy consumption. Recently, microwave heating is being increasingly utilized for variety of applications, as heating is uniform, where the absorbed microwave readily transforms into heat inside the particles by dipole rotation and ionic conduction [18], [19], [20]. Recently MW heating has been widely used to produces as well as to regenerate AC, the relevant literature is very limited [21], [22], [23]. There is no study regarding AC prepared from the Crofton weed with MW heating in the presence of physical activation.

This urged research toward upgrading and utilization of the harmful biomass Crofton weed. In this regard, the objective of this work is to evaluate the operational conditions for improving the porosity and adsorption capacity of CWAC using MW heating. Effects of the activation temperature, activation duration and CO2 rate on the adsorption capacity and yield of CWAC were investigated systematically. The resultant products were characterized using the nitrogen adsorption isotherm, FTIR and SEM analysis.

Section snippets

Materials

Crofton weed were collected from Kunming, Yunnan Province of China. The raw materials were crushed, sieved into a uniform size of 5–7 mm, then were washed thoroughly with distilled water to remove foreign material and then oven-dried at 105 °C overnight and stored in a moisture free environment for utilization in the experiment. The proximate analyses of the Crofton weed were as follow: volatile 76.41%, ash 1.90% and fixed carbon 21.69%.

Carbonization of Crofton weed

The carbonization of raw precursors was carried out by

Results and discussion

Table 2 shows the experimental conditions for preparation of CWAC generated by the Design Expert software covering the parameters such as activation temperature, activation duration and CO2 flow rate. The CWAC are characterized for iodine number, yield and results are listed as well in Table 2.

Conclusions

Crofton weed, a harmful biomass is utilized for preparing AC with microwave heating exhibits well developed pore structure. The effects of three vital process parameters, activation temperature, activation duration and CO2 flow rate on the adsorption capacity and yield of AC were investigated systematically. The process parameters were optimized utilizing the Design Expert software and were identified to be an activation duration of 90 min, an activation temperature of 980 °C and a CO2 flow rate

Acknowledgements

The authors would like to express their gratitude to the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20115314120014) and the Kunming University of Science and Technology Personnel Training Fund (No. KKSY201252077) for financial support.

References (34)

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