Trace-element siting in iron sulfides in Ohio coals determined by secondary ion mass spectrometry (SIMS)

Abstract

Ion images of site-specific concentrations of various minor and trace elements at the surface of intact samples of iron sulfides in coals have been obtained using secondary ion mass spectrometry (SIMS). Problems related to intefeering images of complex secondary molecular ions have been largely overcome by combining SIMS imaging with ion probe step scanning. Although SIMS, when used in this way, is qualitative, it is sensitive in the parts-per-million (ppm) range and has the capability of micron-scale resolution for ion mapping. Iron sulfides in samples of Ohio coal bneds contain Mn, Co, Cu, As and Pb. The elements Si, Al, K, V, Cr, Ni and Co occur outside the iron-sulfide grains and appear to be associated with clay minerals in the coal. Where Si, Al and K occur within sulfide grains they are associated with clay minerals contained in porous pyrite. Arsenic also may be associated with clays present in porous pyrite. Of the elements found within the iron sulfides, non appear to generally favor pyrite or marcasite, but in some individual samples there can be elemental preference for one or the other sulfide phase. SIMS images clearly indicate that the occurrences of some trace elements are related to the growth of the iron-sulfide grains, and as a result, their concentrations are considered to have been controlled, at least in part, by the microenvironments that existed during the formation of the iron sulfides. Site-specific determination of the occurrences of trace elements in intact coal-sulfide samples is important with regard to development of methods of environmental management of the wastes derived from the uses of coal, and also should be useful in determining the relative organic affinity versus inorganic affinity of minor elements in coals.