Modes of occurrence of trace elements in coal from XAFS spectroscopy

Experimental aspects of X-ray absorption fine structure (XAFS) spectroscopy are described for determining the mode of occurrence of selected trace elements in coal. For elements between calcium and molybdenum in the periodic table, information relating to the mode of occurrence can be deduced from the XAFS spectrum provided the element's concentration exceeds about 5 ppm. This spectroscopic method of determining elemental modes of occurrence complements electron microscope or microprobe methods because it provides information on element forms dispersed in the organic fraction of coal as well as on the mineralogical forms of the element. XAFS spectra for the lithophile elements, Ti, V, Cr and possibly Mn, indicate that these elements can be associated with both the minerals (principally illite) and the macerals in coals of rank up to high volatile bituminous. XAFS data also confirm that Mn, Zn, As and Br, can be largely organically associated in certain coals. XAFS spectra for As, and to a lesser extent, Se show that these elements will oxidize over time, once the coal has been exposed to air.