Heavy metal adsorption on iron oxide and iron oxide-coated silica ...

585.2.5 Surface Charge DistributionPotentiometric titrations were conducted to verify the PZC of Alfa Aesar and US silica.Usually titrations can be performed with either acid or base. In acid titrations, the silicasuspension needs to be adjusted to a higher pH and is then titrated with an acid. Thisprocess is reversed when the titration is conducted with base.In this study, titration of silica was initially performed with acid (HNO3). Theresulting PZC of Alfa Aesar and US silica was about 5.0 and 4.9, respectively, and arehigher than that of other studies listed in Table 5.4. These differences which may becaused by the dissolution of silica. Yates (1975) studied the influence of oxide solubilityduring a titration in determining oxide surface charge. He found that the dissolution doesnot affect titrations below pH 7, because the neutral species Si(OH)4 ° predominates.However, the hydrolysis of Si(OH)4° occurs above this pH consuming Off (has a buffereffect around its pKa1, 9.46, Yates, 1975) and thus affecting the titration procedure.When the effect of dissolution is not corrected in the calculation, the surface chargeincreases. Therefore, the consumption of O1-1- should be considered when performingtitrations with acid, especially when the silica concentration in the suspension is low andsurface dissolution is potentially a significant fraction of the total silica present (2 g L -1 inthis study).Ultimately, base titrations were used to determine the surface charge distributionof silica samples. The PZC value of silica samples in this study is 2.85 (Figure 5.10) andis consistent with other studies. The acid-washed US silica sample has less than 1.5%(mass) impurities as seen in Table 5.3; nevertheless, these impurities are likely on thesurface affecting the surface charge.