NH4F Dissolution Process

Ma et al., [20] 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 Approach

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. [23]

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