Leachability of Cu and Ni in wood ash-amended soil as impacted by humic and fulvic acid

Abstract

Land application is a cost-effective way of disposing of wood ash. However, high volume wood ash application tends to increase soil pH, solubilizing soil organic matter (SOM), and enhancing metal mobility. Column studies (1.5×20 cm) were conducted to determine the leachability of Cu and Ni in an ash-amended soil. In the first study, deionized water was used to leach three columns: ash, ash mixed with topsoil, and ash mixed with topsoil overlying a spodic horizon. In the second study, humic acid (HA), fulvic acid (FA), and a mixture of humic and fulvic acids (HAFA) were used to leach ash columns. Leaching was conducted, one pore volume (PV) at a time, under unsaturated conditions. Leachate concentrations of dissolved organic carbon (DOC), Cl, Cu, and Ni, as well as pH were determined. Leachate pH ranged from ∼10 to ∼11 in the water- and acid-leached columns, respectively. In the ash-only columns, Cu leaching was in the form of soluble complexes with Cl. When wood ash was mixed with soil, Cu leached as soluble complexes with DOC at high pH (∼10). Columns with a Bh horizon had 30% less leaching of Cu, possibly due to increased metal retention. Nickel leachability was not affected to the same extent as Cu. Organic acid-leached ash columns showed lower DOC, Cu, and Ni leaching, suggesting that these two metals and DOC were precipitating out with DOC at the high ionic strengths of these solutions (>0.3 mol dm −3). Precipitation was greater in the more reactive FA-leached columns. This study shows the need to understand the system to which wood ash is applied to minimize potential harmful effects.