Abstract

The concentration of As, Cr and Ni and their speciation (As^3+;5+, Cr^3+;6+ and Ni^0;2+) in milled coal, bottom ash and ash collected by electrostatic precipitator (ESP) from a coal fired-power plant in western Canada were determined using HGAAS, ICP-AES and XANES. The chemical fractionation of these elements was also determined by a sequential leaching procedure, using deionized water, NH₄OAC and HCl as extracting agents. The leachate was analyzed by ICP-AES. Arsenic in the milled coal is mostly associated with organic matter, and 67% of this arsenic is removed by ammonium acetate. This element is totally removed from milled coal after extraction with HCl. Arsenic occurs in both the As³⁺ and the As⁵⁺ oxidation states in the milled coal, while virtually all (>90%) of the arsenic in bottom ash and fly ash appears to be in the less toxic arsenate (As⁵⁺) form. Both Ni and Cr in the milled coal are extracted by HCl, indicating that water can mobilize Ni and Cr in an acidic environment. The chromium is leached by water from fly ash as a result of the high pH of the water, which is induced during the leaching. Ammonium acetate removes Ni from bottom ash through an ion exchange process. Chromium in milled coal is present entirely as Cr³⁺, which is an essential human trace nutrient. The Cr speciation in bottom ash is a more accentuated version of the milled coal and consists mostly of the Cr³⁺ species. Chromium in fly ash is mostly Cr³⁺, with significant contamination by stainless-steel from the installation itself.