Rice husk accounts for a significant amount of agricultural waste in many rice-producing countries. In 2015, Indonesia produced about 75 million tons of paddy, and rice husk accounts for 20-22% of the weight of paddy (Saleem et al., 2014). The accumulation of rice husk waste can pose a threat to the environment if it is not properly controlled. Rice husk is separated from rice grains during the milling process because it is low in nutrients (Lee et al., 2017). Due to its poor nutrient composition, rice husk is usually used as an animal food ingredient and as a low-cost burning fuel, which is ineffective and can lead to air pollution. Nevertheless, compared to other biomass fuels, rice husk contains unusually high levels of cellulose, lignin, and ash, which are its major constituents. The actual composition varies, but the typical composition is as follows: 38% cellulose, 22% lignin, 20% ash, 18% pentosanes, and 2% other organics (Chandrasekhar et al., 2003). Rice husk ash (RHA)
contains amorphous silica in the range of 20-25 wt.%.
In recent times, rice husk has been utilized in various applications such as in the manufacture of fertilizer (due to its high lignin content), in the preparation of activated carbon, and as an industrial fuel for gasification or combustion boilers (Namdeo, 2018). Scientists are particularly attracted to the potential of rice husk as a raw material for silica-based materials, as well as pure silicon, silica nitride, silicon tetrachloride zeolite, and amorphous silica (Sun and Gong, 2001).
Controlled combustion of rice husk produces ash that contains high purity amorphous silica. Amorphous silica can be obtained from RHA when the rice husks are burned at a temperature of 700°C, and this is transformed into crystalline silica when it is burned at a temperature of over 850°C (Fernandes et al., 2016). This silica has many applications as a filler, adsorbent, catalyst support, a component of star gels, and a source for producing superior quality silicon and its compounds.
Silica is a basic raw material that is widely used in semiconductors, ceramics, polymers, and several industries, such as the rubber industry and pharmaceuticals (Fernandes et al., 2016). In particular, silica can be used for gas adsorption (Dhaneswara et al., 2019, Fatriansyah et al., 2019) and heavy metal remediation in water (Dhaneswara et al., 2018). In mesoporous form, silica can be used too in gas adsorption application (Wilson and Mahmud, 2015).
Since silica can be produced from RHA, several reports have addressed the extraction of silica from rice husk. This process not only produces valuable silica but also reduces pollution problems caused by the uncontrolled combustion of rice husk.
Many researchers have developed methods for extracting silica from rice husks. Riveros and Garza (1986) reported that silica can be recovered from rice husk by acid leaching. Later, Kalapathy et al. (2000) discovered the alkaline extraction sol-gel method for recovering silica from rice husk, which is based on the fact that silica can be dissolved in alkaline solution. Because silica has high solubility in a solution with a pH above 10, it can dissolve in alkaline solution to form sodium silicate (Qomariyah et al., 2019). Acidification is also a necessary part of the process to produce silica gel. This process has numerous advantages; it is not as costly or as damaging to the environment as the quartz fusing method (Todkar et al., 2016). It has been reported that the character and purity of silica are more affected by chemical treatment than by thermal treatment (Daifullah et al., 2004).
This study aims to synthesize amorphous silica using rice husk waste as a raw material of SiO2. A simple reflux process using an alkaline solution (sodium hydroxide) was followed by acidification to form silica gel. Then, the effects of various alkaline concentrations and the two acidification methods were investigated by examining the physical, structural, and mechanical properties of the amorphous silica obtained from rice husk waste.
