A mixture of rice husk ash (10 g) calcined at 650 °C for 6 h and 1 M
NaOH (320 cm3) was heated under reflux for 3, 5 and 9 h. The reflux
process was considerably modified in comparison with previous
studies [8,9]. In our study, longer time periods were used for the reflux
process and the washing process was systematically repeated. The
following procedures were carefully performed to obtain a final
product having the necessary high purity. The reaction mixture was
filtered via a Whatman filter paper using a vacuum pump to assist.