A new tool for in situ monitoring of Fe-mobilization in soils

Mohamad Fakih (1) , Mélanie Davranche (1) , Aline Dia (1) , B. Nowack , Xavier Chatellier (1) , Patrice Petitjean (1) , Gérard Gruau (1)

1 GR - Géosciences Rennes

The aim of this study was to design and test a new tool for (i) the quantitative in situ monitoring of Fe(III) reduction in soils and (ii) the tracking of the potential mineralogical changes of Fe-oxides. The tool consists of small (2 — 2 — 0.2 cm) striated polymer plates coated with synthetic pure ferrihydrite or As-doped ferrihydrite (Fh­As). These slides were then inserted within two different horizons (organo-mineral and albic) located in a wetland soil with alternating redox conditions. Dissolution was quantified by X-ray fluorescence (XRF) analyses of total metal contents before and after insertion into the soil. The crystallographic evolution of Fe-oxides was characterized by scanning electron microscope equipped with an energy-dispersive spectrometer (SEM­EDS). Over the months, the ferrihydrite progressively disappeared, at rates comparable to those previously measured in laboratory studies, i.e. in the 1­10 — 10-12 mol Fe m-2 s-1 range. SEM observations indicate that the supports were highly colonized by bacteria and biofilms in the organo-mineral horizon, suggesting a biological-mediated process, while the albic horizon appeared to be characterized by a mostly chemical-mediated process. In the albic horizon, Fe-sulphide and other micro-precipitates were formed after 7 months of incubation in balance with a quasi dissolution of initial Fe-oxides.

Domaines: Géochimie

Mohamad Fakih, Mélanie Davranche, Aline Dia, B. Nowack, Xavier Chatellier, et al.. A new tool for in situ monitoring of Fe-mobilization in soils. Applied Geochemistry, 2008, 23 (12), pp.3372-3383. ⟨10.1016/j.apgeochem.2008.07.016⟩. ⟨insu-00356234⟩