Crystal chemistry of trace elements in natural and synthetic goethite

International audienceThe crystal chemistry of Cr (0.73%), Mn (0.88%), Co (0.166%), Ni (0.898%), Cu (0.263%), and Zn (0.026%) in natural goethite (αFeOOH) from an earthy saprolite formation in West-Africa was investigated by fluorescence-yield extended X-ray absorption fine structure (EXAFS) spectroscopy. Synthetic goethites and phyllomanganates were used as model compounds for structural determinations. The measurement of EXAFS spectra at energies higher than the Fe K-edge (Co, Ni, Cu, and Zn) is notoriously difficult because the fluorescence signal from trace elements is typically two orders of magnitude less intense than the Fe fluorescence from the matrix. This difficulty was circumvented by measuring total intensity (signal + background) with high precision on the ID26 undulator EXAFS spectrometer of the European Synchrotron Radiation Facility (ESRF) with a fast, highly linear, and low-noise diode detector. Cr, Cu, and Zn have the same local structure in natural and synthetic goethites. With the exception of the Cu polyhedron, which is distorted by the Jahn-Teller effect, Me-O and Me-Fe distances are similar to Fe-O and Fe-Fe distances in goethite. No significant steric effect was observed around Cu and Zn in spite of the ∼14% increase in ionic radii compared to Fe 3+. The compliance of the Fe site to these substitutional impurities probably is accomplished by displacement of nearest O and Fe shells (relaxation effect) and modification of interpolyhedral angles (compliance effect) owing to the corner-sharing topology of the goethite structure. X-ray absorption near-edge structure analysis reveals an average valence of ∼3.7 to 3.8 for Mn, and EXAFS indicates that manganese is present as a phyllomanganate impurity having a hexagonal layer symmetry like asbolane, lithiophorite, and hexagonal birnessite. Cobalt is trivalent and located in the phyllomanganate layer, and also possibly in the interlayer, substituting for Mn. Selective uptake of cobalt by the Mn oxide impurity results from the oxidation of soluble Co 2+ to insoluble Co 3+ by Mn 3+/Mn 4+, this mineral-surface redox reaction accounting for the well-known geochemical affinity of Co for Mn at the earth's surface. No more than ∼20% of the amounts of Mn and Co in natural goethite substitute for Fe in the structure, if present at all. About 75% of total Ni is substituted for Fe in natural goethite and ∼25% is associated with the phyllomanganate impurity as a Ni(OH) 2-MnO 2 mixed-layer phase (Ni-asbolane). The Ni site in synthetic goethite is strongly relaxed with a contraction of the goethite structure in the plane of edge-sharing double chains ( bc plane), and an expansion in the direction of corner linkages (approximately the ab plane). This anisotropic relaxation of the Ni site locally reduces the distortion of the goethite structure, and could be due to a clustering of Ni atoms

Le programme AQUAE : Rôle des aménagements d'origine anthropique (zones enherbées et fossés) dans le transfert et la rétention des produits phytosanitaires

This program is part of the common research action AQUAE (2000-2003) between Cemagref and INRA. It aims at understanding and quantifying the mechanisms of retention, degradation and transport of pesticides at the field and the catchment scales. The study relates in particular to man-made impact causing hydrological discontinuities such as ditches network and grass and wooded cover zones. Experiments were undertaken on two catchments with different geomorphological and climatic characteristics, La Jaillière (44) and Roujan (34), in order to analyse the contamination of water by pesticides. The factors determining the efficiency of these man-made hydrological discontinuities are extensively studied : hydrometeorological conditions, geomorphological position, soil characteristics; land-use, vegetation cover classes, and the oxydo-reduction conditions. A typology of these various types of discontinuities is established on a great range of catchments, in order to characterise the various experimental situations and to ensure the applicability of the results on un-gauged catchments. Finally, a distributed hydrological modelling approach (the model MHYDAS : Modélisation Hydrologique Distribuée des AgroSystèmes = Distributed Hydrological Modelling of AgroSystems) of water and pollutant transfer, integrating the principal mechanisms observed on hydrological discontinuities in farmed catchments, is conceived and is in an advanced phase of development. The first tests of the model at the catchment scale enable the simulation of hydrograph and pesticides concentrations at various locations of ditches network. Furthermore, it enables the testing of various scenarios of land management and the impact of man-made planning on water and pesticides fluxes.Ce programme fait partie de l'action structurante AQUAE (2000-2003) entre le Cemagref et l'INRA et a pour objectif de comprendre et quantifier les phénomènes de rétention, de dégradation ou de transfert subis par les produits phytosanitaires entre la parcelle agricole et le milieu aquatique. L'étude porte notamment sur les discontinuités hydrologiques que constituent les fossés et les dispositifs enherbés ou boisés. Ces structures font l'objet de suivis en continu et d'expériences spécifiques, afin d'analyser le devenir des produits phytosanitaires en leur sein, sur deux bassins versants aux caractéristiques différentes : La Jaillière (44) et Roujan (34). Les facteurs déterminant leur efficacité tampon sont étudiés, qu'il s'agisse des conditions hydrométéorologiques, de leurs positions géomorphologiques, des caractéristiques du sol ou de la végétation, et de l'alternance des conditions d'oxydo-réduction. Une typologie de ces différents types de discontinuités est établie sur une grande gamme de bassins versants sur lesquels les équipes participantes travaillent ou ont déjà travaillé. Cette typologie a permis de caractériser les principales situations existantes ou envisageables, afin d'assurer (la généralisation ou l'extrapolation) des résultats obtenus sur les bassins versants expérimentaux. Enfin, une approche de modélisation hydrologique distribuée (le modèle MHYDAS : Modélisation Hydrologique Distribuée des AgroSystèmes) des transferts d'eau et de polluants à l'échelle du versant, intégrant les principaux mécanismes observés sur les discontinuités hydrologiques, est en phase avancée de développement. Les premiers tests du modèle ont permis de simuler les hydrogrammes de crue en différents points du réseau de fossés et les ordres de grandeur des flux de pesticides à l'exutoire du bassin versant