Direct determination of cadmium speciation in municipal solid waste fly ashes by synchrotron radiation induced mu-X-ray fluorescence and mu-X-ray absorption spectroscopy.

Abstract

Cadmium is a toxic metal that causes environmental concern in connection with utilization and land filling of ash from combustion of municipal solid waste (MSW). Collecting information about the chemical associations of Cd in ash is fundamental since this affects its solubility and leachability from the ash material. In the work presented here, the content, distribution, and chemical forms of toxic metals especially of Cd on/in individual Municipal Solid Waste (MSW) fly ash particles have been investigated in situ by synchrotron radiation induced mu-X-ray fluorescence and absorption spectrometry. The use of an excitation energy of 27 keV made it possible to detect trace metals, such as Cd, present at ppm levels routinely. Changing the excitation energy in the vicinity of the absorption edge of Cd (26.71 keV), the absorption spectra of this element were measured for the first time in this high energy range in micron-sized spots of individual fly ash particles. The measurements indicated Cd to be preferably concentrated in some small areas ("hot-spots") with high concentration (up to 200 ppm) rather than in a homogeneous distribution or as a thin coating on the whole particle surface, making the surface-reaction the most probable mechanism of Cd enrichment during MSW combustion processes. Comparisons of XAS spectra of fly ashes and reference compounds showed that in the particles studied Cd is present in the oxidation state +2. Analyses of linear combinations of standard spectra allowed estimating the Cd presence within fly ash particles as an admixture of primarily CdSO4, CdO, and CdCl2 as well as an unidentified compound not included as a standard.