contaminants. Yet, Goworek et al. (1997) found that some compounds may be hindered

by a narrow pore size due to steric forces. Therefore, the adsorption ability for that

particular compound would be higher in a silica with larger pores.

 Adsorption on silica surfaces is primarily due to ion interactions and hydrogen

bonding with the silanol groups. Therefore, the adsorption kinetics in relation to these

silanols is greatly dependent on the pH. Silica's point of zero charge ranges between 2

and 3 (Papirer, 2000; Persello, 2000). At a pH less than the PZC, the surface has a

positive charge due to the formation of Si-OH2 At a pH above the PZC, the surface of

the silica has a negative charge due to the deprotonation of the silanol group resulting in

Si-O-. As the pH approaches 7, the Si-O- sites become significant (the pKa for SiOH =

SiO- + H is 7) (Cox, 1993). These Si-O- sites readily react with cations in solution and

an increase in pH would greatly increase the adsorption of these compounds (Kinniburgh

and Jackson, 1981). For nonionic compounds in solution, adsorption can occur at the

silanol groups by hydrogen bonding or by Van der Waals forces. For anionic compounds,

there may be some adsorption at a pH above the PZC of silica, but it would be occurring

at the TiO2 surface because it is positively charged at pH < 6.0. One factor to consider

when adjusting pH in order to achieve the highest adsorption onto silica-titania

composites is the dissolution of silica at a pH greater than 9 (Iler, 1979). In order to

create a long-lasting photocatalytic composite, the dissolution of silica would need to be

avoided.