Leachability of Metals in Grate-Fired Boiler Wood Ash from a Small Municipal District Heating Plant (6 MW)

摘要

A five-stage sequential leaching procedure was used to determine the distribution of 13 metals (Cd, Cu, Pb, Cr, Fe, Mn, Zn, Ni, Co, As, V, Ba, Ti), phosphorous (P) and sulphur (S) in grate-fired boiler wood ash from a small municipal district heating plant (6 MW) at Keminmaa, Northern Finland, into the following fractions: (1) water-soluble fraction (H2O), (2) exchangeable fraction (CH_3COOH), (3) easily reduced fraction (HONH3CI), (4) oxidizable fraction (H_2O_2 + CH3COONH4), and (5) residual fraction (HF + HNO_3 + HCI). Although metals were leachable 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, and is not likely to be dissolved under the conditions normally found in nature. The Ca (84.2 g kg-1; dry weight) and Mg (12.4 g kg~-1; dry weight) concentrations were ca. 53 and 62 times higher in the ash than the typical values of 1.6 and 0.2 g kg~-1 (dry weight), respectively, in arable land in Central Finland. The ash was strongly alkaline (pH - 12.0) and had a liming effect of 30.6% expressed as Ca equivalents (dry weight). The strongly elevated Ca and Mg concentrations indicate that the grate-fired boiler wood ash from the municipal district heating plant of Keminmaan Energia Oy is also a potential agentfor soil remediation and improving soil fertility. The pH and liming effect value indicate that wood ash has a pH buffering capacity.