How does the structural organization of iron-organic matter nano-aggregates control their reactivity?

In natural systems, organo-mineral iron-organic matter (Fe-OM) heteroaggregates are a key factor in the dynamics of metallic pollutants. Their capacity to adsorb metals and metalloids depends on their structural organization, which is itself controlled by the physico-chemical conditions prevailing in the environment. Calcium (Ca) and aluminum (Al) are major cations commonly encountered in natural waters and can interact with OM and/or Fe. Their impact on the structure of Fe-OM aggregates and therefore on their reactivity could thus be major. With Ca and Al, Fe-OM associations structural organization moves from a colloidal behavior to a micrometric network whose connections are ensured either by Ca dimers or by Al monomers, oligomers or amorphous hydroxides. Iron(III) is organized as oligomers and ferrihydrite-like nanoparticles (Fh-Nps). The size and proportion of Nps-Fh increase with the increase of Ca or Al concentration in response to the partial screening of Fe-OM interactions by the binding of Ca and Al with OM. The presence of Ca which decreases the Fe and OM interactions, increases the availability of Fe binding sites, thus improving the adsorption capacities of Fe-MO aggregates with respect to As. These results shed new light for the understanding of the dynamics of Fe, MO and associated elements which mobility could more limited as expected regarding the possible formation of an interconnected micrometric network. (author)