Equilibrium Surface Complexation Modeling with Metastable Natural Colloids: the Key to Predict the Oxidation State Distribution of Trace Elements?

Predicting the behavior and fate of redox-sensitive trace elements (TEs; e.g., As, U, Cu, Cr) in natural systems is challenging. Colloids have been reported to control TEs speciation and catalyze TEs redox reactions in many environments. We hypothesize that the lack of accurate thermodynamic models that account for the role of colloids in TEs speciation explains our inability to predict their redox state distribution in the environment. The slow evolution of the colloidal compartment in response to prevailing bio/hydro/pedo/climatological conditions needs to be decoupled from the fast TEs redox reactions promoted by colloidal surfaces. Further progress is hampered by experimental and theoretical challenges associated with capturing the extreme physical and chemical heterogeneity of colloids, their metastable structures, and their dynamic transformation behavior.