pH-dependent release of elements from hardened and non-hardened wood ash

Bioenergy systems that utilize clean wood feedstocks are becoming more common in western Canada to produce heat and electricity. But, wood combustion leads to ash residuals that need to be managed. Although there is growing interest in value-added uses for these wood ashes, large quantities of wood ashes are currently stockpiled or landfilled. Wood ash may be self-hardened as a pretreatment strategy to improve handling and reduce reactiveness prior to land application. This study determined aqueous concentrations of constituents released from wood bottom ash (BA) and hardened wood bottom ash (HBA) when subjected to increasing levels of acidity (pH 10 to 4). Such acidic conditions are not common but may exist during some storage, landfilling or land reclamation scenarios. Acidification of BA and HBA increased aqueous concentrations of B, Ca, Cd, Co, Cu, Mg, Mn, Ni, Sr, and Zn, whereas Cr, Hg and Mo showed decreased concentrations. Hardening reduced aqueous concentrations of As, Ca, Co, Fe, Ni, P, and Pb, in HBA compared to BA over a pH range. When properly managed, hardened and non-hardened bottom ashes generated from the combustion of clean wood should pose minimal risk to the environment.