The use of assemblage models to describe trace element partitioning, speciation, and fate: a review

The fate of trace elements in soils, sediments and surface waters is largely determined by their binding to reactive components of which organic matter, metal oxides and clays are considered most important. Assemblage models, combining separate mechanistic complexation models for each of the reactive components, can be used to predict the solid-solution partitioning and speciation of trace elements in natural environments. In this review we provide a short overview of advanced ion binding models for organic matter and oxides and their application to artificial and natural assemblages. Modeling of artificial assemblages of mineral components and organic matter indicate that the interactions between organic and mineral components are important for trace element binding, particularly for oxyanions. The modelling of solid-solution partitioning in natural systems is generally adequate for metal cations, but less so for oxyanions, probably due to the neglect of organic matteroxide interactions in most assemblage models. The characterization of natural assemblages in terms of their components (active organic matter, reactive oxide surface) is key to successful model applications. Improved methods for characterization of reactive components in situ will enhance the applicability of assemblage models. Collection of compositional data for soil and water archetypes, or the development of relationships to estimate compositions from geospatially-available data, will further facilitate assemblage model use for predictive purposes.