THE ROLE OF MICROANALYSIS IN DETERMINING TRACE ELEMENT DISTRIBUTIONS IN ENERGY MATERIALS, AND THEIR IMPLICATIONS (Invited Presentation)

The distribution of trace elements in coal has profound environmental, human health, and economic considerations, and coal scientists have long argued over their mode(s) of occurrence. Historically, trace elements in coal have been classified as organic, inorganic, or having mixed affinities. These classifications are based on statistical correlation, analysis of float/sink separates, or selective leaching. Each approach requires some degree of interpretation and at best is semiquantitative. In the 1970s, Finkelman [1] was among the first to apply scanning electron microscopy, together with newly available energy dispersive analysis, to identify inorganic constituents of coal. This application provided breakthroughs such as direct evidence for the occurrence of arsenic in pyrite, and recognition that monazite and xenotime are widely occurring hosts for rare earth elements (REEs) in coal. Finkelman also showed a lack of arsenopyrite and chromite in most coals, refuting common occurrence of these phases as hosts for arsenic and chromium, respectively. His study revealed that lead selenide, absent in detrital sediments, is an important host for both lead and selenium, especially in Appalachian Basin coals.

Finkelman’s observations inspired others to address key unknowns such as the occurrence of mercury and halogens in coal, arsenic and REEs in coal fly ash, and speciation of chromium in fly ash. The EPA Mercury and Air Toxics Standards [2], were implemented in recognition that mercury and trace element emissions from coal combustion are harmful to human health and the environment. Current interest by DOE in coal and coal ash as sources of REEs [3] may help reduce our dependence on international sources for this commodity. In each of these cases, microanalysis of energy materials, pioneered by Finkelman, provides a starting point for understanding the occurrence of trace elements and their implications.

[1] Finkelman, R.B., 1981, U.S. Geological Survey Open File Report OF-81-99, 301 p.

[2] U.S. Environmental Protection Agency [EPA], 2011, Mercury and air toxics standards (MATS), https://www.epa.gov/mats.

[3] U.S. Department of Energy [DOE], National Energy Technology Laboratory, 2017, Rare earth elements from coal and coal by-products, https://www.netl.doe.gov/research/coal/rare-earth-elements.