Activated carbons prepared from rice hull by one-step phosphoric acid activation
Introduction
Rice hull is a major by-product of the rice milling industry and one of the most commonly available lignocellulosic materials with high ash content. A typical rice hull composition is listed in Table 1 [1]. The worldwide annual rice hull output is about 80 million tons [2]. Rice hull is usually used as a low-value energy resource in many countries, burned in the field, or discarded, which are unfavorable to environment. This motivates the investigation of producing value-added products from rice hull, such as silica and activated carbons, as well as solving some environmental problems. Activated carbons are a group of established, universal, and versatile adsorbents due to their high surface area, controllable pore size distribution, and amendable surface reactivity.
The manufacture processes of activated carbons include physical or chemical methods. Both methods have previously been investigated to produce activated carbons from rice hull. Activated carbons with surface areas of 197 and 273 m2/g were produced from rice hull and precarbonized rice hull by one-step steam activation [3], [4], respectively; activated carbons with surface areas as high as 3000 and 2500 m2/g were prepared from rice hull by KOH and NaOH chemical activation [1], [5]. Activated carbons with surface areas of 1100 m2/g in pores greater than 1 nm and 300 m2/g were prepared from rice hull by ZnCl2 activation [6], [7]. Activated carbons prepared from rice hull by H3PO4 activation have been investigated for the removal of different pollutants [8], [9], [10], however, limited information about the properties of these carbons was provided. Highly mesoporous carbons with relatively low surface area and total pore volume were obtained from rice hull through a two-stage process (precarbonization followed by H3PO4 activation in temperature range of 700–900 °C) [11]. Activated carbon with BET surface area and total pore volume as high as 874 m2/g and 0.713 cm3/g was also prepared from rice hull by H3PO4 activation [12].
Previous results indicated that activated carbons with desirable surface area and pore structure can be prepared from rice hull. However, high ash content of activated carbons prepared from rice hull by H3PO4 activation, which is usually in the range of 20–70% [8], [10], [12], prohibits their application and commercial production. In view of that phosphoric acid-activated carbons have been proven to be highly effective adsorbents for the removal of heavy metal ions from aqueous solutions [13] because of their remarkable high cation-exchange capacities, which is due to the existence of a large number of acidic surface groups that provide exchangeable protons [14], [15]. Considering its plentiful and renewable supply, rice hull as a starting material for the production of activated carbon by H3PO4 activation deserves more intensive investigation. The primary objective of this study is to investigate the influence of activation temperature on the development of pore structure and chemistry properties of carbons prepared from rice hull in relatively low activation temperature range. To achieve this objective, a one-step H3PO4 activation process was applied on rice hull. Activated carbons were prepared by gradually increasing the activation temperature to different values in the range of 170–450 °C while keeping other activation parameters constant. The pore structure and chemistry properties of the obtained carbons were characterized with various techniques including: nitrogen adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental and ash content analysis, Boehm titration, and Fourier transform infrared spectroscopy (FTIR).
Section snippets
Materials
Rice hull used in this study was provided by the Southern Regional Research Center of the US Department of Agriculture in New Orleans, LA. The rice hull as received was first washed with distilled and deionized water and dried in oven at 105 °C for one day. Then the washed rice hull was ground in a coffee mill, and the fraction passing through 50 mesh US Standard sieve was used in the preparation.
Preparation
Ten grams of rice hull was mixed with 50 g of 30 wt% of phosphoric acid solution resulting in an
Pore structure characterization results
The pore structure development of sample obtained at 170 °C was nearly negligible. Therefore, only the nitrogen adsorption isotherms of carbons prepared at temperatures ⩾250 °C were reported (Fig. 1). With final activation temperature <300 °C, the activation extent of rice hull was relatively low demonstrated by the low nitrogen adsorption of RH250. The nitrogen adsorption isotherms of the carbons prepared in this study were primarily of Type I, a characteristic of microporous materials. However,
Conclusion
The results of this work demonstrated that activated carbons with high surface area and strong acidic surface chemistry properties as well as low ash content can be prepared from rice hull by one-step H3PO4 activation at relatively low temperatures. A threshold activation temperature exists for the development of porous structure on carbons prepared from rice hull by H3PO4 activation. This temperature is 300 °C for the process investigated in this study. The activated carbons tend to be
Acknowledgment
This study was supported by a grant from the Waste-management Education and Research Consortium, Las Cruces NM.
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