SEQUESTRATION OF METAL(LOID)S BY MINERAL-ORGANIC MATTER ASSOCIATIONS

Soil organic matter is complex and can be seen as a continuum of progressively decomposing organic compounds containing variable amounts of inorganic constituents such as minerals and adsorbed metal(loid)s. The structure and composition of organic matter changes during the interaction with minerals, either through the adsorption of soluble organic species on minerals or through the formation of minerals within pores spaces of organic matter. Studies at the bulk scale showed that mineral-organic matter associations often depicted a greater ability to sequester metal(loid)s than minerals or organic matter alone. However, the mode of occurrence of metal(loid)s in these complex mixtures of organic matter and minerals has not been addressed so far. Here, we characterized the sequestration of the metal(loid)s Cu, Ag, As, and Pb by mineral-organic matter associations in particulate organic matter and organic colloids. Combinations of the Focused Ion Beam and Ion mill technology, Transmission Electron Microscopy and Atom Probe Tomography allowed the identification and characterization of Cu-, Ag-, As- and Pb-bearing nanomaterials and adsorbed or incorporated species within organic matrices and mineral inclusions. They specifically showed (a) the close association of Cu-sulfides and Cu-oxides nanoparticles with silicates, (b) the nucleation of Ag- and Cu- nanoparticles in organic matter and on the surface of Fe-(hydr)oxides, (c) the formation of acanthite (Ag₂S) nanoparticles and aggregates on the surface of S-bearing Fe-hydroxides, (d) the occurrence of Pb-bearing oxysalt minerals and Zn-spinel nanoparticles in pores spaces of the organic matrix; (e) the close association of Pb-bearing species with polar functional groups in the organic-matrix and (f) the formation of nano-thick Fe-arsenate layers on the surface of Fe-hydroxides. These observations show that the sequestration of metal(loid)s in organic matter is highly diverse and depends often on the occurrence mineral-organic matter associations. From the analytical standpoint, our studies also suggest that Atom Probe Tomography may be used to visualize the three-dimensional distribution of metal(loid)s in organic matter and their spatial relationship to organic functional groups.