Production of activated carbon from bagasse and rice husk by a single-stage chemical activation method at low retention times

Abstract

The production of activated carbon from bagasse and rice husk by a single-stage chemical activation method in short retention times (30–60 min) was examined in this study. The raw materials were subjected to a chemical pretreatment and were fed to the reactor in the form of a paste (75% moisture). Chemicals examined were ZnCl₂, NaOH and H₃PO₄, for temperatures of 600, 700 and 800 °C. Of the three chemical reagents under evaluation only ZnCl₂ produced activated carbons with high surface areas. BET surface areas for rice husk were up to 750 m²/g for 1:1 ZnCl₂:rice husk ratio. BET surface areas for bagasse were up to 674 m²/g for 0.75:1 ZnCl₂:bagasse ratio. Results were compared to regular two-stage physical activation methods.

Introduction

Activated carbon is a well known material used in ever increasing numbers of environmental applications, in environment protection, in water and wastewater treatment, in gas filters, etc. Activated carbon can be produced theoretically from any carbonaceous material rich in elemental carbon. In the recent years, there is growing interest in the production of activated carbons from agricultural by-products and residual wastes. Agricultural by-products available in large quantities are bagasse-the fibrous by-product resulting from the milling of sugarcane- and rice husks. The annual global production of 800 million tonnes of sugarcane results in 240 million tonnes of bagasse while the estimated annual world rice production is about 571 million tonnes resulting in approximately 140 million tonnes of rice husk available annually for utilization. Despite the wide consumption of bagasse and rice husks as a fuel for the mill boilers, for electricity and steam generation, animal feed or as a raw material for the manufacture of paper and board, the residues still remain as a surplus which poses a disposal problem for mill owners.

The processing and transformation of agricultural residues into activated carbon with good adsorption properties would alleviate problems of disposal and management of these waste by-products, while providing a high quality end product for water and wastewater treatment that could potentially expand the carbon market.

Production of activated carbon from bagasse and rice husk is achieved through pyrolysis and activation with chemical or physical means. Chemical impregnation with KOH and NaOH of pyrolysed rice husk followed by activation at 650–800 °C resulted in activated carbons with extremely high surface areas (1413–3014 m²/g) (Ahmedna et al., 2004, Guo et al., 2003). Pyrolysis of rice husk followed by H₃PO4 impregnation and activation at high temperatures (700–900 °C) produced activated carbon with a surface area of ∼450 m²/g (Kennedy et al., 2004). Activated carbon produced by chemical activation with KOH or H₃PO4 achieved high yield and removal efficiencies comparable to those of commercial products (Lozano-Castello et al., 2002, Nakagawa et al., 2004, Rahman et al., 2005). Other chemical agents studied include ZnCl₂, FeCl₃ · 6H₂O, KCl, CaCl₂ · 7H₂O & FeSO₄ · 7H₂O. Use of these agents and activation at 600 °C resulted in activated carbons with surface areas ranging from 168 to 480 m²/g. The optimal chemical mixture was with ZnCl₂ 10% (w/w) (Yalcin and Sevinc, 2000).

Physical activation of dry bagasse in a two-stage carbonization/activation process within a temperature range of 750–840 °C produced activated carbon with significant surface areas (404–607 m²/g), the surface area increasing with increasing temperature (Juang et al., 2002). Two-stage steam activation of sugarcane bagasse resulted in activated carbons with 565 m²/g surface area (Ng et al., 2002). Chemical carbonization of bagasse with concentrated sulfuric acid at a 4:3 ratio and subsequent CO₂ activation at 900 °C produced activated carbons with very high surface areas (403–1433 m²/g) for long retention times (Valix et al., 2004). Chemical impregnation of bagasse with concentrated sulphuric acid and heating at 150 °C for 24 h, produced activated carbons with 312 m²/g surface area, which effectively removed 90% of Pb from samples within 2 h contact time (Ayyappan et al., 2005). Low temperature carbonization of H₂SO₄ impregnated bagasse, followed by subsequent activation with CO₂ at high temperatures (900 °C), resulted in activated carbons with surface areas ranging from 403 to 1433 m²/g, the lowest referring to the carbonized only matter (Valix et al., 2004). Zinc chloride up to 1:1 (w/w) was used as the chemical agent for bagasse impregnation in the work of Tsai et al. (2001). The mixture was heated in N₂ for 0.5 h and the resulting surface areas were up to 790 m²/g for 1:1 (w/w) biomass to chemical ratio.

In all previously cited studies, the produced carbons had significant surface areas and adsorption capabilities, in many cases comparable to the commercially available carbons. However, their production method involved several stages of preparation, long retention times for pyrolysis and activation and often elevated temperatures.

This study aimed to produce activated carbon from bagasse and rice husk in a single activation process in relatively short retention times. For this purpose, the precursor was impregnated with a chemical agent before being fed to the reactor. Results were compared to those of conventional two-stage physical processes.