Chemical sequential extraction of heavy metals and sulphur in bottom ash and in fly ash from a pulp and paper mill complex

Abstract

A five-stage sequential extraction procedure was used to determine the distribution of 11 metals (Cd, Cr, Cu, Mo, Pb, Zn, As, Co, V, Ni, Ba), and sulphur (S) in bottom ash and in fly ash from a fluidized bed co-combustion (i.e. wood and peat) boiler of Stora Enso Oyj Oulu Mill at 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 + CH₃COONH ₄); and (5) residual fraction (HF + HNO₃ + HCl). Although metals were extractable in all fractions, the highest concentrations of most of the metals occurred in the residual fraction. From the environmental point of view, this fraction is the non-mobile fraction and is potentially the least harmful. The Ca concentrations of 29.3 g kg^—1 (dry weight) in bottom ash and of 68.5 g kg^{— 1} (dry weight) in fly ash were correspondingly approximately 18 and 43 times higher than the average value of 1.6 g kg^{— 1} (dry weight) in arable land in Central Finland. The ashes were strongly alkaline pH (∼ 12) and had a liming effects of 9.3% (bottom ash) and 13% (fly ash) expressed as Ca equivalents (dry weight). The elevated Ca concentrations indicate that the ashes are potential agents for soil remediation and for improving soil fertility. The pH and liming effect values indicate that the ashes also have a pH buffering capacity. From the environmental point of view, it is notable that the heavy metal concentrations in both types of ash were lower than the Finnish criteria for ash utilization.