Extractability and bioavailability of heavy metals and sulfur in bottom ash and fly ash from an industrial power plant

A five-stage sequential extraction 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 fly ash from a fluidized bed boiler at the industrial power plant of Laanilan Voima Oy in Oulu, Northern Finland, into the following fractions: (1) water-soluble fraction (H2O), (2) exchangeable fraction (CH3COOH), (3) easily reduced fraction (NH2OH-HCl), (4) oxidizable fraction (H2O2 + CH3COONH4), and (5) residual fraction (HF + HNO3 + HCl). Although the metals were extractable in all fractions, the highest concentrations of most of the metals occurred in the residual fraction. This fraction is the non-mobile fraction and is potentially the least harmful. The concentration of easily soluble Ca (24.5 g kg-1 (d.w.) in the fly ash was ca. 15 times higher than the typical value of 1.6 g kg-1 (d.w.) in arable land in Central Finland indicating that the fly ash is a potential agent for soil remediation and improving soil fertility. Both types of ash were alkaline (pH ∼ 9.4-9.7), which means that they have a pH - buffering capacity. According to the Finnish legislation, both types of ash can be used as a fertilizer in the forest. This paper covers also examples how ashes are utilized in some Finnish pulp and paper mills.