A Variable Reactivity Model for Ion Binding to Environmental Sorbents

Abstract

The conceptual and mathematical basis for a new general-composite modeling approach for ion binding to environmental sorbents is presented. The work extends the Simple Metal Sorption (SiMS) model previously presented for metal and proton binding to humic substances (Ganguly et al., 1999). A surface complexation modeling approach is presented, where metal ion binding is conceptualized as occurring at a single (diprotic) binding site with variable reactivity. The overall sorption constant (Kads) is represented as the product of three terms: Kads = Kchem Kcoul Khet. The chemical contribution to metal binding is included in Kchem, while Kcoul and Khet define coulombic and/or heterogeneity effects, and are approximated by empirical power functions of H+ and metal to site concentration ratio (MT/ST), respectively. Because of the difficulty in separating electrostatic and heterogeneity effects for field sorbents, the model is applied first to synthetic data where these effects can be examined separately and cumu...