Cadmium Distribution and Speciation by Synchrotron Radiation Induced X-Ray Microprobe Techniques Application to Municipal Solid Waste and Biomass Fly Ashes
During the last few years, alternative fuels, such as wood and Municipal Solid Waste (MSW), are increasingly being used in Sweden for heat and power production.
The positive environmental impact in terms of low sulphur emissions, no net emission of CO2 to the atmosphere, and local availability are main driving forces in this development. The sustainable society concept requires the recycling or deposition of inorganic substances as agricultural nutrients (energy crops) or harmless land fills (MSW).
However, utilisation of alternative fuels also introduces new environmental hazards, resulting from the enrichment of heavy metals, such as Cd, in the combustion residues. Rather than their net concentrations alone, availability and solubility together determine the amounts of ash which may be deposited in land fills or used in agriculture. Consequently, long term risk assessment must include knowledge of the chemical speciation of the heavy metals, by which is meant their coordination numbers, oxidation states, and the local chemical properties of the host compounds.
In this study, MSW and biomass fly ash particles are being investigated (i) in the bulk, to determine whether or not the Cd concentration exhibits dependence on particle size, and (ii) within single particles, to determine the Cd distribution and speciation. Based on this, possible chemical transformation routes of Cd are proposed.
For the first time, a combination of synchrotron radiation x-ray microprobe techniques is used here to investigate the Cd distribution and in situ speciation within single biomass and MSW fly ash particles. Cd distribution within single particles and any inter-elemental correlations are investigated by Synchrotron Radiation induced micro-X-ray Fluorescence (μ-SRXRF) spectrometry; Cd speciation at single spots on individual ash particles is studied by Synchrotron Radiation induced micro-X-ray Absorption Spectroscopy (μ-SRXAS); the crystalline structures of the Cd-containing matrices are identified from Synchrotron Radiation induced micro-X-ray diffraction (μ-SRXRD) patterns, while Cd internal elemental distribution is established by Synchrotron Radiation induced micro X-ray Fluorescence Tomography (μ-SRXRFT) experiments.
Analysis of MSW fly ash particles indicates Cd to be preferentially concentrated in small areas with local concentrations up to 200 ppm, so called hot-spots. In contrast, a more homogeneous Cd distribution is found in case of biomass fly ashes. μ-XRT results show the Cd concentration to be higher in the cores of the fly ash particles than on the particles surfaces. In both ashes, Ca-containing matrices are found to be the main Cd-bearing phases. In both kinds of ash, comparisons of XAS spectra of fly ashes with reference compounds show Cd to be present exclusively in 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, both bulk and single particle investigations presented in this study tell of similar Cd concentrations throughout all the particle sizes investigated. Bulk analysis of representative MSW fly ash samples give a 134 mg/kg dry ash Cd concentration, as compared to 34 mg/kg dry ash of fly ash from biomass combustion. In particular the latter exceeds the Swedish National Forestry Board recommendations of 30 mg/kg dry ash Cd concentration for maximum trace metal levels in recycled wood ash.
Based on the results obtained, possible reaction mechanisms involving Cd are suggested, stressing the necessity to include information regarding the different mineral matrices constituting the ash particles.
synchrotron radiation based micro-probe techniques