Iron speciation in iron–organic matter nanoaggregates: a kinetic approach coupling Quick-EXAFS and MCR-ALS chemometrics

Abstract : Iron–organic matter (Fe–OM) nanoaggregates are highly abundant in wetlands. Iron oxyhydroxides and natural OM are strong sorbents for metals and metalloids due to their high density of binding sites. They are thus considered as a major vector for the transport of metallic pollutants in this type of aquatic system. However, their structural organization is complex and varies under physico-chemical environmental conditions. The goal of this present study is to characterize these various iron phases and their growth processes. The formation of Fe-species was followed by Quick-EXAFS during oxidation/hydrolysis kinetics of pre-formed Fe(II)–humic acid complexes. Data were then processed using the MCR-ALS chemometric method. It demonstrated that, in the presence of OM, Fe(II) oxidation/hydrolysis leads to the synthesis of Fe-oligomers and ferrihydrite-like nanoparticles, both being bound to OM. The formation of the oligomers is the result of the inhibition of ferrihydrite polymerization by OM. Ferrihydrite and oligomers grow concomitantly during Fe(II) oxidation. When Fe(II) is completely oxidized, ferrihydrite forms at the expense of oligomers. For a given Fe/OM ratio, the ferrihydrite/oligomer ratio depends on the oxidation/hydrolysis kinetics which is strongly influenced by O2 and OH− availability. The organization of these structures constrains their binding site density and availability, which induces dramatic environmental implications regarding their capacity to trap metallic pollutants.