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VARIABLE BOUND-SITE CHARGING CONTRIBUTIONS TO SURFACE COMPLEXATION MASS ACTION EXPRESSIONS
Citation:
Loux, N T. VARIABLE BOUND-SITE CHARGING CONTRIBUTIONS TO SURFACE COMPLEXATION MASS ACTION EXPRESSIONS. Presented at 219th American Chemical Society National Meeting, San Francisco, CA, March 26-30, 2000.
Description:
One and two pK models of surface complexation reactions between reactive surface sites (>SOH) and the proton (H+) use mass action expressions of the form: Ka={[>SOHn-1z-1]g>SOH(0-1)aH+EXP(-xeY/kT)}/{[>SOHnz]g>SOH(n)} where Ka=the acidity constant, [ ]=reactive species concentrations, n=the initial number of protons associated with
the site (n=+2 or +1), z=the initial site charge (z=+1, +1/2 or 0), aH+=the chemical activity of the proton, g=an activity coefficient for a bound site, EXP( ) is the exponential to the base of the natural logarithm, e=the electron charge, Y=the surface potential (relative to bulk solution), k=the Boltzmann constant, T=the absolute temperature, and x is a variable commonly given values of either one (with traditional modeling) or two (with expressions derived from chemical potentials of the reacting species. An investigation of bound site charging energies within the context of the Entropic Balanced Surface Potential Model (EBSPM) suggests: 1) charging energies estimated by the expression eY are predicated on an assumed constant value of Y (i.e., creating or eliminating one additional charged site does not significantly alter Y), 2) EBSPM estimated charging energies (at near constant Y) are of the form (1-t)eY where t is the fraction of potential determining ion charge neutralized by counterions in the surface region (t ranges between zero and one) and 3) the EBSPM value for x is (2-t); therefore, EBSPM derived values of x lie in a range between the traditional value of one and the chemical
potential derived value of two. A preliminary assessment using Poisson Boltzmann theory supports these findings.