Competitive adsorption of metal ions onto hydrophilic silicon surfaces from aqueous solution

Si surfaces prepared using the IMEC-clean were exposed to a dilute solution of ten metals, Ba, Ca, Co, Cr, Cu, Fe, K, Ni, Sr, and Zn, in dilute nitric acid solution. The metal deposition onto hydrophilic silicon surfaces was found to be dependent on the metal concentration, pH, and adsorption time, the power-dependence of metal surface concentrations, as expressed in [M+](n'), ranged from 0.27 for K+ to 0.73 for Cr3+, With an average value of 0.54. The [H+] power dependence of metal surface concentrations, as [H+](m'), ranged from -0.07 for K to -0.56 for Sr, with an average value of -0.30. We interpret these results to be consequences of all cations, in solution, including H+, competing for a limited number of adsorption sites. Using the Langmuir assumptions for adsorption behavior, we derive mathematical relationships for such behavior, with the key result being sigma(i) = sigma K-i[M-i(+])/l + Sigma(i)K(i)[M-i(+)].sigma(i) is the surface concentration of the metal, [M-i(+)] the solution concentration of the metal cation, and K-i is the equilibrium constant governing its attachment to the surface, for the ith species. sigma is the areal density of adsorption sites on the surface. Summation proceeds over all cations, including H+, in solution. Consequences of this relationship include the observed fractional power dependencies of H+ and the metal cations, and the reduced adsorption onto the surface of any given metal cation when in the presence of other cations, including H+. (C) 1999 The Electrochemical Society. S0013-4651(98)04-082-8. All rights reserved.