Contaminants FAte is ConTrolled by colloidal ORganic matter speciation – C-FACTOR

A large variety of contaminants (e.g. heavy metals, metalloids, radionuclides, pesticides, hydrocarbons, drugs) are currently found in surface waters, groundwaters, sediments and soils. These contaminants are of special concern given their extensive use and their impact on human health, aquatic life and the environment. Their mobility and environmental transfer are largely controlled by their interaction with colloids in response to their extremely reactive surface towards contaminants. Colloids are ubiquitous environmental (nano)particles, which can be transported by waters over large distances. Natural colloids are often complex assemblages between natural organic matter (NOM) and minerals, namely organo-mineral colloids (OMCs). Regarding their small size and heterogeneity, their characterization and the quantification of processes occurring at their surface are extremely challenging. Although many experimental and modeling studies were conducted during the past 30 years, current numerical models fail to accurately predict contaminants-OMCs binding. The C-FACTOR project, which aims at developing the first numerical contaminants-OMCs binding model, makes the hypothesis that a relationship between composition/size/structure/morphology and surface reactivity of OMCs exists. To verify this hypothesis and reach this ambitious goal, C-FACTOR faces four major challenges. If many studies provided evidence that NOM fractionation occurs upon its interaction with a mineral, (1) very little is known about the environmental factors and mechanisms responsible for NOM fractionation, and (2) the reactivity of NOM in OMCs towards contaminants was rarely investigated. Furthermore, (3) although NOM is now conceptualized as supramolecular assemblages (various small and macromolecules that can dissociate and fractionate), current contaminants-OMCs binding models consider NOM only as indivisible macromolecules. Finally, (4) the term “surface reactivity” is broad because OMCs surface is highly heterogeneous: various contaminants binding sites are available, which depend on the contaminant itself. To tackle these challenges, deep characterization of OMCs formed under various environmental factors, hence showing various composition, size, structure and morphology, will be conducted with a special focus on NOM speciation. A new method to probe the surface reactivity of OMCs towards contaminants will be developed. Various contaminants (metals, metalloids, organic contaminants) will be adsorbed onto OMCs to probe the nature and the amount of the different OMCs binding sites. Using these experimental data, a new mechanistic contaminants-OMCs binding model will be developed. It will (i) explicitly calculate NOM-mineral binding, accounting for NOM supramolecular structure, and (ii) be able to predict the binding of various contaminants to OMCs, hence accounting for their surface heterogeneity. The model will be tested and validated using data for natural soil and water OMCs collected under various biogeochemical and pedoclimatic conditions. The new model will be developed in a popular geochemical speciation code for a broad dissemination of the results to both public and private sectors. It will be a powerful tool for risk assessments and for the development of remediation strategies.