The influence of cationic impurities in silica on its crystallization and point of zero charge.

The influence of the heat treatment on the crystal structure change of five (two laboratory prepared and three commercial) silica powders were investigated. Silicas were heat treated in the temperature range from 150 to 1300 degrees C in air. Also, the influence of the washing procedure with 0.01 mol/L HCl in order to desorb cations present as impurities in silica samples, was studied. The change of the amorphous into the crystal structure (alpha-cristobalite) takes place in the range 1050-1300 degrees C, depending on the chemical purity of silica samples. Points of zero charge, pHpzc, of the same silica samples, ranging from 3.8 to 7.1, were also determined by batch equilibration method. It is shown that the desorption of cations from silica samples, i.e., increase in their purity, leads to a shift in pHpzc toward lower values.