X-ray diffraction patterns of silica produced by various acids are shown in Fig. 5(a). The broad diffraction peak at 2θ angle of 22.5° is a typical silica characteristic, attributed to the absence of an ordered crystalline structure [23]. This result reveals that sodium silicate samples treated with various types of acid do not affect the crystalline structure of products. Peak intensity of silica obtained from rice husk ash is higher than that of silica produced from the alkali extraction process. This is because heating silica at high temperature will convert a less ordered structure to a more highly ordered structure [17]. The literature observed a similar pattern for silica obtained from rice husk combustion [35]. Notably, when we wash the aquagel once with water after silica formation, a small peak presents at 2θ angle from 31° to 33° in the sample, due to the formation of NaCl residue. However, the salt peak disappears when the aquagel is water-washed three times, indicating a much lower sodium concentration. This study thus concludes that water washing the aquagel at least three times was more effective in removing sodium impurities than that of a single aquagel washing with water.