In addition to hydrated silica,27,28 RHs are mainly composed of
cellulose, hemicellulose, and lignin,32,33 and low concentrations
of various metal cations.27 Although the organic components
can be removed by thermal decomposition, care must be
exercised since some metal cations, especially K+, were reported
to be able to catalyze the melting of silica.34−39 Therefore, RHs
were first boiled in 10 wt % HCl solution for 2 h to remove
most of the metal cations. Silica nanoparticles were successfully
obtained by pyrolyzing the HCl treated RHs at 700 °C for 2 h.