Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy

Abstract

While there are a variety of methods to determine the concentration of trace elements in coal and ash, there have been few attempts to determine the speciation of these elements. In this paper, it is demonstrated that X-ray absorption fine structure (XAFS) spectroscopy is capable of providing speciation information at realistic concentration levels of 10–100 ppm, provided a solid-state multielement germanium detector is used. The initial studies have concentrated on arsenic and chromium. For arsenic, two principal forms of occurrence are observed in coal: As contained in pyrite and As(V) in arsenate (AsO 4 −3). In one coal (Pittsburgh #8, DECS-12), the As is present as arsenopyrite. The As in pyrite is readily oxidized under ambient conditions to the arsenate form. In combustion ashes, all arsenic is in the form of arsenate, with at least two forms of arsenate present; As in aluminosilicate slag and calcium orthoarsenate are possibilities. Chromium in coal and in ash is observed to be present predominantly (> 95%) in the Cr +3 state. Chromium oxyhydroxide is the standard compound whose XAFS spectrum most closely resembles that of the chromium in coal, while the chromium in ash may be incorporated into the aluminosilicate slag phase.