Activated carbons prepared from rice hull by one-step phosphoric acid activation

Abstract

Activated carbons were prepared from rice hull by one-step phosphoric acid activation in this work. The evolution of pore structure and surface chemistry in the activation temperature range of 170–450 °C was investigated through various characterization techniques. The results showed that the development of porosity (extent of activation) was negligible at activation temperature below 300 °C, and rapid evolution occurred in 300–400 °C. Porous activated carbon with bimodel pore structure (pore < 1 nm and pore > 1 nm) and BET surface area as high as 1295 m²/g was obtained at 450 °C. The ash contents of samples prepared in this study were in the range of 5–21%. The ash contents of carbons prepared in this study initially decreased from 21.03% to 4.89% with the change of temperature from 170 to 300 °C, then increased to 8.72% at 450 °C. Boehm titration results suggested that low activation temperature (⩽300 °C) benefits the formation of acidic surface groups. With the increase of activation temperature from 300 to 350 °C, the concentrations of strong, intermediate and weak acidic surface groups decreased from 2.23, 1.87, and 2.73 to 1.66, 1.32, and 2.16 mmol H⁺/g, respectively. Over 350 °C, the change of these groups were insignificant. FTIR results revealed the existence of carbonyl-containing, phosphorus-containing groups, and groups containing Si–O bond. The relative concentration of carbonyl-containing groups decreases with an increase in activation temperature, while that of phosphorus-containing groups follows the reverse trend. The content of Si–O decreased first, then slowly increased with the increase of activation temperature. Boehm titration and FTIR (Fourier transform infrared spectroscopy) results indicated that the surfaces of these carbons contain both temperature-sensitive and temperature-insensitive groups. The temperature-sensitive part consists mainly of carbonyl-containing groups, such as carboxylic groups, while the temperature-insensitive part is primarily phosphorus-containing groups and groups containing Si–O bond. This study demonstrated that carbon products with relative low ash content and high activation degree can be prepared from rice hull by H₃PO₄ activation at suitable temperature.

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).