Article (refereed) -postprintGroenenberg, Jan E.; Lofts, Stephen. 2014. The use of assemblage models to describe trace element partitioning, speciation, and fate: a review. Environmental Toxicology and Chemistry, 33 (10). 2181-2196. 10.1002/etc.2642 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Running head: Review assemblage models understanding of the behaviour and speciation of trace elements in surface waters [6, 7] 9 groundwaters [8, 9] and soils [10][11][12], rare earth and radionuclide geochemistry [13, 14] and 10 interactions of potential toxic trace elements with biota [15][16][17]. Combination of ion-binding 11 models for the various components into "assemblage models" is a promising way to predict 12 partitioning and speciation of metals and metalloids in natural environments [18][19][20][21] and 13 waste materials and the dynamic behaviour of metals in soils, including transport to surface 14 water [22, 23]. Concurrent with the increase in the number of mechanistic models of ion-
15binding has been an increase in the use of such models in environmental risk and hazard 16 assessment of trace elements [24][25][26]. Increased use of assemblage models for these purposes conceptual underpinning, parameterisation and methods to assess site specific inputs.
22Furthermore, insight is needed on the consequences of choices of concepts, parameterisation,
23and site-specific inputs on the performance and utility of the models.
24Assemblage model conceptAssemblage models combine thermodynamic models for inorganic speciation and 1 mineral equilibria with separate models for ion binding to various reactive components 2 (oxides, clay, organic matter). The ion binding models are based on general thermodynamic 3 principles. However, the complexity of ion binding to heterogeneous complexants requires 4 simplifications in order to obtain practical models [27]. In this paper we will use the term 5 surface complexation for the binding of ions to mineral surfaces and humic ion binding for 6 binding to humics. In general we refer to these models as ion binding models. The models 7 distinguish explicitly between electrostatic and ion-specific binding and account for 8 competition among ions for binding. This allows the models to be applied across wide ranges binding. An alternative is the generalized composite approach, in which binding to the whole 21 assemblage is simulated using a composite binding phase with generic surface sites. The 22 generic surface sites represent average properties of the soil or sediment surface rather than 23 specific minerals [31]. Binding constants and reaction stoichiometries are fit to experimental 24 7 data for the site specific assemblage [32], and are therefore not transferable to other sites. For 1 an overview of applications of the general composite approach one is referred to [31, 33]....