Thanks for the interesting question! Since SiO2 is practically not soluble in acids, you can treat the silica powder with 70% H2SO4. Consequently, TiO2 will be converted to the soluble [TiO]SO4*2H2O, which is easily soluble in water.
Fe2O3 can be removed upon using a reductive acid, e.g. ascorbic or citric acid, while chelating or macrocyclic ligands can improve the solubility due to the formation of Fe2+/Fe3+ complexes. Plants take up iron via their roots by 3,4-Dihydroxy cinnamic acid or phyto siderophors, however, the solubility of Fe from the soil will be enhanced by reduction of Fe3+ to Fe2+ and/or complexation by chelating ligands, such as organic hydroxy carbolxylic acid.
Hope this helps a bit,
Thomas
PS: You can also check my bioinorganic chemistry lecture, which you can download from my homepage...
Hydrofluoric acid (HF) is commonly used to remove titanium oxide (TiO2) from silica in the acid washing process. HF is highly effective at dissolving and removing various metal oxides, including titanium oxide, due to its strong ability to form stable fluorides with these elements. The hydrofluoric acid reacts with the titanium oxide in the silica to form soluble titanium fluoride compounds. These compounds can be easily removed from the mixture, leaving behind purified silica.