A variable reactivity model for ion binding to environmental sorbents

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 (K_ads) is represented as the product of three terms: K_ads = K_chem K_coul K_het. The chemical contribution to metal binding is included in K_chem, while K_coul and K_het define coulombic and/or heterogeneity effects, and are approximated by empirical power functions of H⁺ and metal to site concentration ratio (M_T/S_T), 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 cumulatively. The diffuse double layer model (DDLM) is used as a basis of comparison for single and multiple sorbents; a discrete affinity distribution model is used to explore heterogeneity effects in the absence of electrostatics. The variable reactivity model then is applied to the experimental data reported by Wen et al. (1998) for proton and metal binding to river sediment. These applications illustrate the underlying physical and chemical properties embodied in model parameters as well as the ability of the model to simulate sorption data for an environmental sorbent. Model attributes and limitations are discussed.