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 e xtends the Simple Metal Sorption (SiMS) model previously presented for meta l and proton binding to humic substances (Ganguly et al., 1999). A surface complexation modeling approach is presented, where metal ion binding is con ceptualized as occurring at a single (diprotic) binding site with variable reactivity. The overall sorption constant (K-ads) is represented as the pro duct 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 coul ombic and/or heterogeneity effects, and are approximated by empirical power functions of Hf and metal to site concentration ratio (M-T/S-T), respectiv ely. Because of the difficulty in separating electrostatic and heterogeneit y effects for field sorbents, the model is applied first to synthetic data where these effects can be examined separately and cumulatively. The diffus e double layer model (DDLM) is used as a basis of comparison for single and multiple sorbents; a discrete affinity distribution model is used to explo re heterogeneity effects in the absence of electrostatics, The variable rea ctivity model then is applied to the experimental data reported by Wen et n l. (1998) for proton and metal binding to river sediment. These application s illustrate the underlying physical and chemical properties embodied in mo del parameters as well as the ability of the model to simulate sorption dat a for an environmental sorbent. Model attributes and limitations are discus sed.