An innovative tool for in-situ monitoring of Fe and associated trace metal mobilization in soils
Résumé
Iron (III) oxides are ubiquitous components of soils. Soil Fe (III) oxides often contain trace metals, either adsorbed onto their surfaces or coprecipitated within their crystal lattices. Thus, the biogeochemical cycles of Fe and trace metals in soils are closely linked together, and many studies have been dedicated to the redox chemistry and stability of Fe(III) oxides in soils as this stability controls to a large extent the ability of trace metals to contaminate ground- and surface water systems. However, such studies are faced with one severe drawback, namely the intimate mixing of Fe(III) oxides with other mineral and/or organic phases. Because of this intimate mixing, it is often difficult to recover pure samples of soil Fe(III) oxides for chemical and mineralogical analyses, and even more difficult to follow the mineralogical changes and trace metal releases that take place during the reduction of theses oxides The aim of this study was to design and test a new tool for the in situ monitoring of Fe(III) reduction and associated trace metal mobilization in soils and for tracking the potential changes in the mineralogy of Fe solid phases occurring during this process. The tested tool consists of small size (4 cm2) striated polymer plates covered by Fe oxides of various mineralogy (ferrihydrite, lepidocrocite and goethite), and enriched in trace metals (Cd or As). The plates are prepared under laboratory conditions, with the Fe oxides being disposed on them through coprecipitation of synthesized Fe-oxides. To test the ability of the system, plate specimens were inserted both in laboratory soil columns and in the different soil horizons (organic-rich, albic and redoxic) of a field soil profile (Mercy wetland, France). Reducing conditions were induced in the laboratory column by mean of a N2 flux, while reducing conditions were developed in the soil profile naturally, by water saturation in response to water table rise. Results show that the polymer plates are unaltered through time, and can be removed easily without alteration of the Fe-oxides disposed on them, suggesting that this technique can be safely used to study for Fe oxide monitoring in soils. Fe amounts were quantified by XRF before and after plate incubation. Fe oxide evolution was imaged by SEM and characterized mineralogically by XRD. After three months of incubation in the natural soil profile, 69% of the ferrihydrite and 15% of the lepidocrocite were dissolved, while the more crystallized goethite remained unaltered. SEM observation coupled with XRD analysis of the plates revealed precipitation of new iron sulfide and oxide phases, while SEM data evidenced intensive colonization of the plates by bacteria and biofilms suggesting that microorganisms played a key role in the occurring mineralogical changes. Studies dedicated to the identification of the nature of the bacteria consortium involved in the reduction process and of the nature of the newly crystallized secondary mineralogical phases are under progress, as are kinetic studies of the trace metal release process that accompanied Fe(III) reduction.