Rice husk (RH) is one of the abundant agricultural biomasses in nature which received an increased attention recently, due to availability and cheapness of resource. Approximately 20% of rough rice mass is husk which contains hemicelluloses, cellulose, lignin, and the other inorganic elements (Phonphuak and Chindaprasirt 2015). Utilization of RH to produce advantageous materials is of great importance in the control of air pollution. The main inorganic component of rice husk ash (RHA) is silica, and other components including Al2O3, K2O, Na2O, CaO, MgO, and Fe2O3 may subsist in a little proportion (Azat etal. 2019). RHA has multidisciplinary applications as a source of silica in the fabrication of advanced materials such as mesoporous silica (Costa and Paranhos 2018), dental nanocomposite (Noushad etal. 2014), and zeolites (Panpa and Jinawath 2008). One effective application of RH is the fabrication of silica with developed porosity by different chemical routes to apply in the removal of contaminants like heavy metal ions and dyes from wastewater (Chuah etal. 2005; Pham etal. 2020).

The non-isothermal decomposition of RH in the air atmosphere is the facile technique to produce the nano-structured silica with the high specific surface area (Liou 2004). The silica extracted from RH by mild hydrochloric acid solution contains the micro- and nano-metric particles (Carmona etal. 2013). The purity of powder fabricated in the presence of hydrochloric acid is due to deletion of impurities (Costa and Paranhos 2018).

The pre-treatment by citric acid is an efficient method for the fabrication of silica with the high purity, declining the environmental impact (Azat etal. 2019). Oxalic acid was found to be the best acid for the synthesis of silica aerogel from RH (Temel etal. 2017). The control of reaction factors such as ethanol/water ratio, and pH in the sol-gel process,