In situ Mössbauer spectroscopy and soil solution monitoring to follow spatial and temporal iron dynamics

Green-blue colours characteristic of hydromorphic soils have been recognized to be due to green rust minerals, for which the name "Fougerite" has been proposed. In reducing conditions, iron activity in soil solution is controlled by the dissolution and precipitation of iron oxides and especially of these transient but well crystallised green rust minerals. Recent study by EXAFS has suggested the partial substitution of Fe2+ by Mg2+. Consequently, the general formula is: [Fe(II)(1-x) Mg(II)y Fe(III)x (OH) (2+2y)]x+ [x/n An- (1-x+y) H2O]x- where An- is the interlayer anion and x is the mole ratio Fe3+ / Fetotal (Refait et al., 2001). The aim of this study is to follow the spatial and temporal iron dynamics by using in situ instruments. For the first time, a field Mössbauer spectrometer, adapted to the Earth's conditions from the instrument developed for Mars missions has been used to obtain spectra directly in a soil. MIMOS-II operates in back-scattering geometry measuring the 14.41 keV Mössbauer radiation and the 6.4 keV Fe X-rays. The sensor head has been installed on an Archimedes screw into a PVC tube equipped with plexiglass windows through which spectra can be acquired at a given depth (± 0.4 mm). A printed circuit board carries the digital electronics for instrument control, communication, data acquisition and storage. No sample preparation is needed, the measurement is non-destructive and the volume probed is about 1-2 cm diameter and 100 to 200 µm depth. A spectrum is acquired in about 2-3 days. The tube is placed in hydromorphic soils in Fougères (Brittany, France). For the analysis of the soil solution, we have installed a multiparametric probe with a measurement cell at 80 cm depth, isolated from the atmosphere. In this cell, several parameters (pH, Eh, dissolved oxygen, temperature, conductivity) are directly measured and samples are collected with tubing. Dissolved Fe(II) and S(-II) are immediatly measured in the field with a portable spectrometer after filtration. The chemical analysis is completed in the laboratory. The first results show both spatial and temporal mineralogical variations. The first evidence is the spatial variation of the x ratio of the green rust mineral at a given time. In the deepest horizon (approximately 100 cm), the green rusts are more reduced (x = 0.33) than at 50 cm (x = 0.65). The second evidence, obtained by resetting the instrument exactly at the same vertical position, is that the x ratio changes with time. This shows that the green rust minerals are highly reactive solid phases. These mineralogical variations can be correlated with fluctuations of the water table and the development of oxidoreduction processes in the soil solution. (Résumé d'auteur