NH4F Dissolution Process
Ma et al.,  proposed an innovative and recyclable method to prepare silica from RHA. In this approach silica was firstly dissolved into NH4F solution producing (NH4)2SiF6 and NH3. Ammonia was recycled by a receiver to produce NH3 H2O which was used in the precipitation silica from (NH4)2SiF6. The process can be resumed as following reactions:
Silica dissolution and precipitation procedure are described in reactions (1) e (2), respectively. Silica obtained by this procedure was spherical with a diameter of 50-60 nm and a recovery yield up to 94.6%. The authors claim this ecological procedure since no residues was created and both reactants NH4F and NH3 returned to the process.
Biological route is also a green alternative procedure to obtain nano sized silica from RH/RHA at room temperature. Fusarium oxysporum is a fungus capable of convert metallic salts into metals or metal oxide form. This fungus was applied to convert silica present in RH to quasi-spherical and nano-sized silica particle [21, 22]. Fusarium Oxysporum was capable to biotransformated amorphous silica to a crystalline state with spherical shape and particle size of 2-6 nm.