The Adsorption-Desorption of Cations at the Silica-Water Interface and its Implication in Wafer-Cleaning Efficacy
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metals with silica gel. Silica gel can adsorb a host of chemical species because of its high surface area and large functional sites. The adsorption phenomena occur both in natural processes and in many water systems. After years of investigation and development, the adsorption reactions are best described by combining the electrical double layer with the surface complexation models. As with the thermodynamic data for dissolved species in water chemistry, Schindler et al. [4] and Gisler [5] collected experimental data for adsorption reactions onto silica gel from which the adsorptions of Fe(III), Cu(II), Cd(II), Pb(II), and Mg(II) onto silica surface were demonstrated. This paper will describe the adsorption of Fe, Cu, and Mg onto silica gel using the STABCAL computer program [6] and compare the modeling results to the silicon wafer surface. THEORETICAL Theoretical models of silica-water interface The electrical double layer theory was developed by Gouy and Chapman [7] which describes the concentration (activity) of the species in the bulk phase and near the surface. Take adsorbed H' for instance, (H+}, = {H+ }b exp(NF/RT)
(1) 57
Mat. Res. Soc. Symp. Proc. Vol. 477 01997 Materials Research Society
where iy = surface potential, F = Faraday constant, R = gas constant, and T = absolute temperature. { }represents activity, and subscripts s and b denote the surface and bulk phase species, respectively. Surface potential V of (hydr)oxide depends on the nature of oxide, pH, ionic strength, and combined surface charge from all adsorbed species. For example, the PZC (point of zero charge) of silica is about 2. The silica surface is positively charged at pH below the PZC and is negatively charged at pH above the PZC. The surface complexation theory was pioneered by Stumm, who emphasizes chemical reactions among adsorbed species as well as surface sites [8]. The reactions involved on the surface are considered analogous to the formation of soluble complexes in the bulk water governed by the equilibrium constants and mass balance equations [9]. Equilibrium between two surface species XaOH and XaOH 2+can be represented: XaOH + HI = XaOH 2+
KI = {XaOH 2+ }./ {XaOH} {HHlb
(2)
where K1 is the equilibrium constant at the surface. Acid and base of silica surface Silica can behave as acid or base depending on the pH of the system. In a system that involves only the water, three surface species are SiOH (neutral), SiOH2+(positive), and SiO- (negative). Those equilibrium equations and constants are [10]: SiO-H = SiO- + H' SiO-H + H+ = Si-OH2+
logK11 s = -7.5 logK1,2' = (not available)
(3)
(4)
Adsorp tion of metal ions on silica surface The important parameters for the modeling of adsorption of metal ions on silica are surface area, surface site, and dG (free energy) of the adsorption, which are extracted from the experimental results [4-5]. Every mole of silica contains 9613 m2 of surface area and 0.0871 moles of surface site. The adsorption species and the estimated equilibrium constants of Fe(III), Cu(II), and Mg(II) on si
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