Metals partitioning in soils by SEM-EDS-SDD mapping of the elements distribution: Applications for geochemical exploration and environmental sciences

English Abstract:

Soils are composed of a mixture of components that play roles in metals binding: humic substances, oxides, carbonates, charcoal, detritals etc. Deportation of trace metals between these components is still not well documented and expected to depend on variety of environmental factors. Conventional analysis methods, such as partial or sequential extractions, provide information on metals abundance related to specific components, but are likely not as selective as indicated. The current objective is to determine the metal's binding sites in anomalous soils at the microscopic scale, mostly using scanning electron microscopy (SEM) with a backscattered electron detector (BSE) and last generation energy dispersive spectrometer (EDS-SDD). The experiment was carried out on a collection of soils from sites known to show various natural or anthropogenic metal enrichments. The mapping of chemical elements distribution allowed locating different metals phases and precipitation sites, even if metals were in trace amount. High magnification imaging and microanalyses were performed to characterize the nature of the chemical signal (organic matter, oxides, clays, residual minerals, etc.). Results show, for example, that copper in humic soil dominantly precipitates on organic matter up to a saturation point, where it start precipitation as distinct authigenic sulfides, and in a lesser extent to accumulates in iron and manganese oxides. Each metal has its own distinctive behavior. For the mineral exploration industry, knowledge of the metals partitioning and dispersion mode shall improve the deconvolution of the signal in order to locate their source and discriminate true from false anomaly. In regard of environmental sciences, a better comprehension of the metals binding sites will help to anticipate the associated risk and it shall provide important information to design the appropriate decontamination or remediation methods.