Composition of ashes from heating plants and solubility of ash-forming elements determined by BCR sequential extraction

Wood ash from small-scale (<1 MW) wood-fired heating plants represents a valuable secondary resource containing essential plant nutrients such as Ca, K, and P, but may also harbor potentially hazardous elements. Its current utilization and disposal are typically based solely on total elemental composition, disregarding element mobility and environmental behavior. This study introduces element mobility as an additional criterion for assessing ash quality and suitability for recycling. Sequential extraction using a modified BCR (Community Bureau of Reference) protocol with four solvents (water, acetic acid, hydroxylamine hydrochloride, and ammonium acetate) was applied to evaluate the leachability of 26 elements. Results indicate high extractabilities for several potentially hazardous elements—Sr (54 %), B (48 %), V (44 %), Cd (38 %), and Tl (27 %)—suggesting potential environmental risks despite low total concentrations. In contrast, key plant nutrients, especially P (15 %) were less mobile but could be mobilized under oxidizing conditions. While dissolution of K (74 %) and Ca (12 %) in water and weak organic acid indicates partial plant-availability already in the untreated ash, 26 % resp. 88 % remained in the residual fraction, indicating only long-term plant availability through weathering. The findings demonstrate that both total concentrations and leaching behavior must be considered in environmental assessments. For fertilizer production, pretreatment to remove Tl and enhance P solubility is recommended. From a waste management perspective, controlled ash processing can promote nutrient recovery and support circular economy objectives. These insights contribute to optimizing wood ash management strategies and minimizing environmental risks associated with its reuse. • Element mobility as a new criterion for assessment of wood ash as fertilizer. • Sequential extraction reveals wide variation of element leachability from ash. • P becomes extractable via oxidation with hydrogen peroxide and ammonium acetate. • Only Tl exceeds limits among toxic elements but can be selectively removed.