Stratégies analytiques pour la spéciation du sélénium inorganique et organique dans les eaux naturelles

Le sélénium est aujourd'hui reconnu comme un oligo-élément, il est donc indispensable à la vie, mais il peut devenir toxique pour l'Homme et les animaux dans une fourchette très étroite de concentration. Le cycle naturel du sélénium montre son existence à différents états d'oxydation et sous plusieurs formes inorganiques et organiques, chaque espèce possédant des propriétés différentes (toxicité, propriétés organoleptiques...). La connaissance de sa spéciation est donc essentielle pour appréhender sa réactivité et son impact environnementaux. Notre laboratoire s'intéresse depuis 1991 à la chimie environnementale du sélénium, particulièrement dans les systèmes aquatiques. Cet article décrit nos développements analytiques récents permettant la spéciation du sélénium au niveau de la dizaine de ng (Se).L-1 à l'aide du couplage chromatographie liquide haute performance - spectrométrie de masse couplée à un plasma à couplage inductif (HPLC-ICPMS). Les travaux s'orientent autour de deux axes. D'un coté, l'amélioration des performances du détecteur ICPMS est réalisée grâce à l'utilisation d'instruments équipés d'une cellule de collision/réaction et à la modification du mode d'introduction de l'échantillon (nébulisation pneumatique ou génération d'hydrures). D'un autre côté, le développement de techniques de pré-concentration représente un moyen astucieux pour abaisser artificiellement les limites de détection du couplage HPLC-ICPMS. Pour conserver tout l'intérêt de spéciation directe de cette technique, les protocoles de pré-concentration proposés permettent d'atteindre simultanément l'ensemble des formes séléniées, i.e organiques et inorganiques

Cadmium speciation assessed by voltammetry, ion exchange and geochemical calculation in soil solutions collected after soil rewetting

International audienceAnalytical techniques and speciation models have been developed to characterize the speciation of Cd in soil solution. They provide an estimate of operationally defined species of Cd that need to be compared, especially for soil solutions highly concentrated in organic matter as are the solutions collected after soil rewetting. This work deals with the comparison between the speciation of Cd measured by anodic stripping voltammetry (ASV) and ion exchange and the speciation of Cd calculated using Visual MINTEQ. The aim of this study was to quantify and explain the differences in Cd speciation observed between the three approaches. Cd speciation was assessed in soil solutions collected 4, 8, 24, 48, 96 and 144 h after the rewetting of an air-dried contaminated soil. To optimize the computed speciation of Cd, other physico-chemical parameters were followed (e.g. pH, ionic strength and the concentrations of major anions, major cations and dissolved organic carbon) and a brief characterisation of dissolved organic matter (DOM) was performed. The discrepancy between model predictions and analytical measurements highlighted the need for caution in the interpretation of geochemical speciated data for Cd. The major result of this study was that a characterization of DOM based on its specific UV-absorbance at 254 nm improved the accuracy of model predictions. Another finding is that labile Cd complexes, even organic, may have been included in the electrochemically labile fraction of Cd measured by ASV

Cadmium uptake and distribution in Arabidopsis thaliana exposed to low chronic concentrations depends on plant growth

This study tested the hypothesis that Cd uptake is correlated with the shoot or root growth of Arabidopsis thaliana ecotype Columbia cultivated hydroponically at environmentally relevant Cd concentrations: 20, 100 and 350 nmol L-1. Growth of both roots and shoots were delayed at 350 nmol L-1. The rate of Cd uptake determined by spiking the nutrient solution with Cd-109 for 24 h, was significantly correlated with the root growth rate. The fraction of Cd absorbed that was allocated to shoots was constant with time but decreased with increasing exposure to Cd. Autoradiography and gamma counting showed that Cd was preferentially allocated to developing leaves. Hence, the quantity of Cd in shoots depended both on the root growth, which probably governed the uptake, and on the maturity of the leaves, which may have determined the Cd allocated to shoots through changes in the transpiration stream