Sequential leaching of heavy metals and sulfur in bottom ash and fly ash from the co-combustion of wood and peat at a municipal district heating plant.

A five-stage sequential leaching procedure was used to determine the distribution of 11 metals (Cd, Cu, Pb, Cr, Mo, Zn, As, Co, V, Ni, Ba), and sulfur (S) in bottom ash and in fly ash from a fluidized bed cocombustion ( i.e. wood and peat) boiler of a municipal district heating plant at Oulu, Northern Finland, into the following fractions: (1) water-soluble fraction (H2O), (2) exchangeable fraction (CH3COOH), (3) easily reduced fraction (NH2OH-HCI), (4) oxidizable fraction (H2O2+CH3COONH4), and (5) residual fraction (HF+HNO3+HCI). Although the metals were leachable in all fractions, the highest concentrations of most of the metals occurred in the residual fraction. This kind of strong mineral acid mixture can never be found in nature, and therefore the residual fraction is not likely to be dissolved easily in the environment. The ashes were strongly alkaline (pH 9. 1) and had a liming effect ( i.e. neutralizing value) of 3. 7% (bottom ash) and 7. 1% (fly ash) expressed as Ca equivalents (dry weight), which indicate that the ashes have a pH buffering capacity. According to the enrichment factors of the elements, the concentration of sulfur in the fly ash was ca 24 times and for other elements 2. 3- 6. 3 times to those in the bottom ash. From the utilization point of view, it was notable that the total heavy metal concentrations in the ashes did not exceed the maximal allowable heavy metal concentrations for ash used as a fertilizer in agriculture. Therefore, according to the Finnish legislation, both ashes can be used as a fertilizer in agriculture as well as in forestry, because the heavy metal concentrations in both types of ash were also well below the proposed limit values for heavy metals in ash used as a forest fertilizer. [ABSTRACT FROM AUTHOR]