Influence of Ligneous Bulking Agents on the Complexation of Heavy Metals to Humic Substances Formed in Pig Manure Composting

In order to investigate the influence of different lignin sources on humic substance formation during composting, this study selected two lignin sources, including wood sawdust and maize straw, to be co-composted with pig manure. Humic substances (HS) were characterized based on their fluorescence characteristics and complexing behaviors with heavy metals. The results showed that lignin sources, especially wood sawdust, were more conducive in promoting the formation of humic acids (HAs) than inorganic matter. The fluorescence excitation–emission matrix spectra also proved the positive effects of lignin on the formation of HAs during the humification process. The binding capacities of HAs isolated from mature composts for Cu and Cd followed the order of WS-90 > MS-90 > I-90, indicating that organic bulking agents are superior at increasing the complexing capacity of HAs. This finding suggests that the co-composting of pig manure with ligneous bulking agents is more advantageous at reducing the environmental risk of heavy metals.