Calcns. based on Poisson-Boltzmann theory are used to investigate the equil. properties of an electrolyte contg. TcO4- and SO42- ions near the surface of amorphous silica. The calcns. show that the concn. of TcO4- is greater than SO42- at distances less than 1 nm from the surface due to the neg. charge d. caused by deprotonation of the amorphous silica silanol groups. At lower pH, the surface becomes protonated and the magnitude of this effect is reduced. These results have implications for the potential use of oxyanion-SAMMS for the environmental remediation of water contaminated with 99Tc.