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

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 X 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 similar to10 to similar to11 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 (similar to10). 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, (C) 2002 Elsevier Science B.V. All rights reserved.