A powerful tool for assessing distribution and fate of potentially toxic metals (PTMs) in soils: integration of laser ablation spectrometry (LA-ICP-MS) in thin sections with soil micromorphology and geochemistry

The dynamic behavior and inherent spatial heterogeneity,at different hierarchic levels, of the soil system oftenmake the spatial distribution of potentially toxic metals(PTMs) quite complex and difficult to assess correctly. Thiswork demonstrates that the application of laser ablation spectrometry(LA-ICP-MS) to soil thin sections constitutes an ancillarypowerful tool to well-established analytical methodsfor tracing the behavior and fate of potential soil contaminantsat the microsite level. It allowed to discriminate the contributionof PTMs in distinct soil sub-components, such as parentrock fragments, neoformed, clay-enriched or humified matrix,and specific pedogenetic features of illuvial origin (unstainedor iron-stained clay coatings) even at very low contents. PTMswere analyzed in three soil profiles located in the Muraveraarea (Sardinia, Italy), where several, now abandoned mineswere exploited. Recurrent trends of increase of many PTMsfrom rock to pedogenic matrix and to illuvial clay coatings,traced by LA-ICP-MS compositional data, revealed a pedogeneticcontrol on metal fractionation and distribution, based onadsorption properties of clay minerals, iron oxyhydroxides ororganic matter, and downprofile illuviation processes. Themain PTMs patterns coupled with SEM-EDS analyses suggestthat heavy metal-bearing mineral grains were sourced from the mine plants, in addition to the natural sedimentary input.The interplay between soil-forming processes and geomorphicdynamics significantly contributed to the PTMs spatialdistribution detected in the different pedogenetic horizons andsoil features.