Evaluation of the distribution of heavy metals and their chemical forms in ESP-fractions of fly ash

In this study we have examined the distribution of the chemical forms of Cd, Cu, Fe, Mn, Pb and Zn in the coal and coal + biomass fly ash (FA) collected in a three stage electrostatic precipitator (ESP) of a pulverized fuel fired boiler system. The concentrations of environmentally available volatile metals (Cd, Cu, Pb and Zn) increased towards finer grained ESP fractions, whereas the concentrations of hardly volatile metals (Fe and Mn) underwent a slight reduction. The concentration of Cd in all ESP fractions of FA was very low (< 1.5 mg/kg).We have proposed a new approach to determining the occurrence mode of trace metals in FAs by the modified Tessier scheme. A relatively uniform pattern of chemical fractionation was obtained for metals having various properties. The effect of co-combustion of biomass was observed for Cu and Mn distribution. Cu present in coal + biomass FA exhibits a clearly higher mobility than Cu in coal FA. The behavior of Mn is just the opposite and a difference in mobility is smaller. The characteristic feature of metal occurrence in the fly ash is a large share of the environmentally persistent fraction (40–90%) and a very low proportion of the water leaching fraction (< LOD). This has a large environmental significance, as it can be predicted that the pulverized coal combustion (PCC) FAs, even if found in extremely unfavorable environmental conditions, will not become an effective source of heavy metals.The sequential extraction results made it possible to propose an outline of the mechanism of formation of individual chemical fractions of the investigated metals.

► Distribution of Cu, Fe, Mn, Pb and Zn in fly ash collected in multistage ESP of PCC. ► Chemical fractionation pattern of Cu, Fe, Mn, Pb and Zn in ESP fractions of fly ash. ► Role of co-combustion of biomass. ► New approach to evaluating of occurrence of chemical forms of trace metals in fly ash.