Cadmium | EPFL Graph Search

Le cadmium est l'élément chimique de numéro atomique 48, de symbole Cd. Le corps simple cadmium est un métal. Un alliage contenant du cadmium ou un objet recouvert d'une fine couche de cadmium (métal) est dit cadmié. Généralités et histoire de la découverte du cadmium Le cadmium est un élément du groupe 12 et de la période 5. Stricto sensu, c'est un métal pauvre, qui ne répond pas à la définition des éléments de transition par l'IUPAC ; en pratique cependant, il est très souvent assimilé aux métaux de transition dans les manuels et de très nombreux ouvrages. Il fait partie du « groupe du zinc », ou groupe B, qui comprend, par numéro atomique croissant, Zn, Cd et Hg, éléments caractérisés par deux électrons sur la sous-couche s au-delà d'une sous-couche d complète. La configuration électronique du cadmium est . Zinc et cadmium sont des métaux électropositifs assez semblables. vignette|Pièce de cadmium centimétrique.|gauche|158x158px L'élément a été découvert en pionnie

Etude du cycle biogéochimique du cuivre et du cadmium dans deux écosystèmes forestiers

Catherine Keller

The biogeochemical cycle of copper and cadmium has been studied in two forested ecosystems for two years. These two sites are: a) a Norway spruce forest (Picea abies) on an acid brown soil (FAQ: Dystric cambisol) and b) a mixed coniferous forest (Picea abies, Pinus cembra, Larix decidua and Abies alba) on a podzol. Both of them are located on low heavy metal polluted sites: the Swiss Plateau (850 m a.s.l.) for the first one and the oriental edge of the Mont-Blanc Massif (1650 m a.s.l.) for the second one. The Cu and Cd stocks in the vegetation and in the soil are evaluated. Heavy metal fluxes through the ecosystems are determined by analysis of concentrations and quantities of rain, throughfalls, stemflows, lysimetric soil solution and litterfall. In addition, the forms of Cu and Cd immobilization in soils are studied by means of sequential extraction, whereas immobilization forms in solution are studied by means of ion exchange chromatography and selective electrode. The results obtained allowed us to draw conclusions concerning the general pollution level in the experimental sites and the annual and/or detailed behaviour of Cu and Cd in the ecosystems. The conclusions are the following: As expected, the sites are not polluted since concentrations and fluxes are low. The chemical composition of solutions is modified during transition through the ecosystem, especially the copper concentrations. The acid brown soil humic layer is more efficient than the podzol humic layer in modifying this composition. There is no significant seasonal variation of concentrations and fluxes. However, the metal diffusion through the ecosystem is almost immediate. The soil plays an essential role in the immobilization of the heavy metals. The organic and organo-mineral layers are particularly active in this process. Exportations out of the ecosystems are limited. They are relatively more important for the podzol than for the acid brown soil and higher for Cd than for Cu. Transfer and stocking forms, and the ways of input to the soil depend on the metal considered: Copper input to the soil is mainly due to litterfall. In the soil, the metal has a preference to be associated with organic matter. In solution, it forms complexes with dissolved organic matter (mainly fulvic acid of MW < 1000 dalton). So, it is strongly retained within the soil and it cannot be easily drained out of the soil profile. At the opposite, cadmium is mostly transported by liquid means (throughfalls). In solution, it is present as free cadmium ions. In the soil, it is less strongly fixed than Cu since Cd is mostly associated with Fe and Mn oxides. For both of the metals, the free ion concentration in solution depends on pH and the saturation rate of organic matter. The latter has a heterogenous and polyfunctional character which explains a similar complexing capacity for all the samples studied. The differences observed in copper and cadmium behaviour can explain the variable diffusion of a polluting Cu and Cd flux through a given ecosystem and to the aquifer. The soil intervenes as a discriminating factor when the two experimental sites are compared.

EPFL1991

Do Exudates Mitigate Cd Biouptake by Rhizobacterium Sinorhizobium melilotii?

To better understand the role of exudates in Cd speciation and biouptake by bacterium Sinorhizobium meliloti, the content of different exudate components including siderophores, proteins and polysaccharides were quantified in absence and presence of 10M Cd at pH of 5.0 and 7.0. Obtained results demonstrated that the release of exudates by S. meliloti is constitutive process rather than induced by the presence of cadmium. Nonetheless, exudates complex cadmium and reduced significantly cadmium free ion concentration. Cd bioavailability to S. meliloti was characterized by the amount of the adsorbed and intracellular Cd. Adsorbed Cd at pH 5.0 was higher then that at pH of 7.0, in qualitative agreement to the higher free cadmium concentration and lower amounts of exudates released at that pH. The observed reduction of intracellular cadmium at pH of 5.0 with respect to pH of 7.0 was attributed to the prevailing competition between proton for cadmium for transport sites.

2006

Etude du cycle biogéochimique du cuivre et du cadmium dans deux écosystèmes forestiers

Catherine Keller

EPFL1991