The study material was comprised of 23 samples of ashes generated after the combustion of conventional and alternative fuels combined with selected fractions of municipal waste. The analyses performed involved determining the total concentration of As, Al, Cr, Fe, Ni, and their bioaccessible, ion-exchange, and carbonate-bound fractions. It was found that all of the samples analysed may display an elevated level of susceptibility to the reduction processes, which undoubtedly increases the mobility of trace elements, including the toxic ones. The predominant elements were Al and Fe, whereas considerably lower concentrations were observed for Ni, Cr, and As. The percentage share of the ion-exchange and carbonate-bound fraction ranged from 49% of the total concentration for As to as much as 0.35% in the case of Fe. The calculated Risk Assessment Code index points to a high risk related to the presence of As, medium to low risk related to the presence of Ni and Cr, and low to no risk related to the presence of Fe and Al. The calculated values of the Ecological Risk Index, associated with the combustion of selected municipal waste fractions and low-quality hard coals, combusted individually or in combination with different types of wood, point to a very high ecological risk. This is mainly related to the high concentrations and toxicity of As.
Keywords: Risk Assessment Code (RAC; chemical indicators; conventional and alternative fuels; mRAC); municipal waste; trace metals and metalloids.