Isolation of mesoporous biogenic silica from the perennial plant Equisetum hyemale

The structure-conserving isolation of biogenic silica from the perennial plant Equisetum hyemale is investigated using several chemical and thermal treatments. X-ray diffraction and small-angle X-ray scattering are employed to characterize crystalline phases and nanostructure of the resulting material, respectively, complemented by nitrogen sorption for porosity analysis. A mild, long-time treatment of the native dry plant with hydrogen peroxide leads to a fully intact silica replica of the outer epidermis of the plant, exhibiting a pronounced nanostructure with a high internal surface area. A similar material with even higher surface area is obtained by a short-time chemical treatment with hydrochloric acid followed by a calcination treatment. The highest grade silica with largest surface area (>400 m(2)/g) is obtained by calcination at 500 degrees C. Omitting the HCl treatment yields a much denser material with rather low surface area, and the total porosity decreases with increasing temperature and vanishes at 750 degrees C. Additionally, a transformation from amorphous silica to cristobalite is observed at this temperature. This unfavorable behavior is mainly attributed to the interaction of the biogenic silica with alkali ions present in the native plant, which disrupts the silica network and leads to the collapse of the pore structure.