Sample(300 mg) was treated with 3 mL of 72% (w/w) H2SO4 in a pressure
tube and incubated in a water bath at 30 ◦C for 1 h. Afterwards,
84 mL of purified water was added into the tube to achieve a acid
concentration of 4.0% (w/w), and the sample was autoclaved at
121 ◦C for 1 h.