Direct Determination of Cadmium Speciation in MSW and Biomass Single Fly Ash Particles Using SR Based µ-XR Spectroscopy Techniques

Cadmium is a toxic metal causing environmental concern in connection with utilization and land filling of ash. Knowledge about the chemical associations of Cd in ash is fundamental in the understanding of its solubility and leachability from the ash. In the work presented here, the content, distribution and chemical forms of Cd on/in individual Municipal Solid Waste (MSW) and biomass fly ash particles have been investigated in situ by Synchrotron Radiation induced μ-X-ray fluorescence, absorption spectrometry (XAS) and fluorescence tomography. These X-ray micro-beam techniques can be used to reveal spatially resolved information about structural and chemical properties of particles. The use of an excitation energy of 27 keV made it possible to detect trace metals, such as Cd at ppm levels, but also other elements, like Ca, K, Cl and Br. Absorption spectra of Cd in the energy range around the absorption edge of Cd (26.71 keV) were recorded for micron-sized spots of ash particles. Fluorescence tomography was then use to establish if Cd compounds are situated on the particle surface (i.e. most prone to leaching) or at some depth within the particle, where they might be more shielded from chemical attack by water. The measurements indicated Cd to be preferably concentrated in some small areas (“hot-spots”) with high concentration (up to 200 ppm) in MSW fly ash particles and in a homogeneous distribution over the whole particle surface in the case of biomass. Comparisons of XAS spectra of fly ashes and reference compounds showed Cd to be present in the oxidation state +2 and mainly as CdSO₄, CdO and CdCl₂. Although previous studies have indicated Cd to be enriched in the smaller-size ash particles, in this study Cd was found to be reasonably well distributed throughout all the particle sizes investigated. Based on the results obtained, possible reaction mechanisms involving Cd are suggested.