Polyfunctional ionogenic compound sorption: challenges and new approaches to advance predictive models

Polyfunctional ionogenic compounds are unique in that they sorb to environmental solids at multiple receptor sites via multiple interaction mechanisms. However, existing sorption models fail to accommodate: (i) sorption via a single mechanism (e.g., cation exchange) at one sorbent receptor site type (e.g., exchange site) distributed across multiple soil components (e.g., organic matter and aluminosilicates); and (ii) sorption at a specific sorbent receptor site (e.g., exchange site) involving distinct sorbate structural moieties (e.g., -NH(3)(+) and -COOH) and distinct interaction mechanisms (e.g., cation exchange and cation bridging). In response, this study offers a mechanism-based framework for conceptualizing the equilibrium solid-water sorption coefficient, K(d), with particular emphasis on the mechanisms of cation exchange and surface complexation/cation bridging. The unique mapping of sorbate structural moieties, sorbent receptor sites, and sorption mechanisms is used to advance mechanism-specific probe compounds for cation exchange and surface complexation/cation bridging for quantifying the relevant site abundance and baseline sorption free energy. Existing literature studies point to the feasibility of developing mechanism-specific structural corrections to "adjust" mechanism-specific probe sorption measures to estimate the magnitude of sorption for any polyfunctional ionogenic compound of interest. Advancement of our conceptual framework to a quantitative K(d) model requires more extensive evaluation of ionogenic compound sorption under consistent experimental conditions.