Simultaneous preparation of silica and activated carbon from rice husk ash

Abstract

Silica and activated carbon were simultaneously produced from rice husk ash with K₂CO₃. The surface area and average pore size of the activated carbon were 1713 m²/g and 4 nm. The maximum adsorption capacity of activated carbon was 210 mg/g for methylene blue and the capacitance value reached 190 F/g. The yield of silica reached 96.84% and the particle size was 40–50 nm. Potassium carbonate could be recycled. The entire synthetic procedure was simple, environmental-friendly and economical-effectively.

Introduction

In recent years, renewable energies prepared from biomass have received considerable attention because of the high price of fossil fuels and their environmental pollution problems (Lay et al., 2012). Over the last two decades, a special attention has been paid to the conversion of biomass into biofuels (Zheng et al., 2008, Wang et al., 2009, Ikura et al., 2003, Silalertruksa et al., 2011, Börjesson and Tufvesson, 2011). During the converting progress, large amount of solid residue (biomass ash) is generated. However, rarely researches are concerning about its utilization. If the biomass ash is not utilized properly, it will become tremendous waste, causing energy loss and environmental pollution.

Rice husk is a kind of agricultural by-product. It is reported that the annual outputs of rice husk worldwide and in China are about 800,000 and 400,000 t, respectively (Li and Wang, 2008, Zhang et al., 2010), and the rice husk was utilized as fuel to prepare bio-oil (Zheng, 2007). The plentiful solid residue produced after the above process is called rice husk ash. The main components of rice husk ash are carbon (45%) and silica (52%).

Activated carbon is a well-known material used in both gas and liquid phases, including medicinal use, capacitors, pollutant and odor removal, gas separation, and catalysis (Nabais et al., 2008, Lillo-Ródenas et al., 2006). Agricultural wastes are the ones, which are considered as feed stocks for activated carbon preparation (Tongpoothorn et al., 2011). Therefore, with many earlier studies, activated carbon has been prepared from biomass (Yang et al., 2010, Bagheri and Abedi, 2009, Sumathi et al., 2009, Arami-Niya et al., 2010, Kalderis et al., 2008, Sathishkumar et al., 2012). But only a few research of activated carbon prepared from rice husk ash were reported (An et al., 2011, Liu et al., 2011). On the other hand, rice husk ash not only consists of carbon, but also an important kind of environmental-friendly silicon source. Silica, which is a basic raw material in electronics, ceramic, and polymer material industries, also can be prepared from rice husk ash (An et al., 2010a, An et al., 2010b). In earlier researches, activated carbon and silica were prepared separately, and the activator recycling has not been mentioned. While in present studies, rice husk ash utilizations are focused on harmful substances adsorption (Manique et al., 2012, Chen et al., 2012), catalysis preparation (Ali et al., 2011, Rafiee et al., 2011) and additive area (Ramasamy, 2012). The carbon source and silicon source could not be sufficiently utilized, and the procedure was complex. In particular, the activator was not recycled. Therefore, it is very important to find a simple method to fully utilize rice husk ash to prepared functional material SiO₂ and activated carbon.

In our work, the aim is to prepared silica and activated carbon from rice husk ash simultaneously, and to recycle the activator. Green and sustainable chemistry is now regarded as a major scientific discipline and the studies in this area have led to the development of cleaner and relatively less toxic chemical processes with many new technologies each year (Solhy et al., 2010). Based on those methods, a green producing process was built, where wastewater and the carbon dioxide have been collected and reutilized at the same time.