Passivation effect of Cd by sepiolite in anaerobic fermentation products with pig manure and straw

Abstract: Livestock and poultry wastes are important sources of agricultural non-point source pollution. Although anaerobic fermentation is the most harmless, reduced and resourced use method for livestock waste, however, due to the use of feed additives, the content of heavy metals in livestock manure is increased, therefore, how to reduce the risk of heavy metal pollution in the biogas residue during anaerobic fermentation is still less studied. Therefore, the mixed raw materials of pig manure and corn stover were used as fermentation raw materials, and heavy metal Cd was used as research object. By adding 2.5% of sepiolite as a passivating agent, a fermentation test was carried out for 60 days at 35 ℃. The distribution of organic carbon and heavy metal Cd in organic matter were analyzed by H2O, KCl, Na4P2O7, NaOH and HNO3 fractionation methods. The results showed that the organic matter content in the biogas residue decreased rapidly in the start and middle stages of the fermentation progressed, then slow reduction in the later stage. The content of organic matter in the biogas residue decreased after the addition of passivating agent; the total organic carbon content extracted by H2O, KCl, Na4P2O7, NaOH and HNO3 were between 10%-20% during fermentation. The proportion of HA increased significantly, the proportion of FA decreased significantly, and HA/FA also increased, indicating that the addition of sepiolite increased the degree of humification of humus. The amount of Cd extracted from biogas residue reached 80% of the total amount. Most of the heavy metal Cd was presented in the humus (Na4P2O7 extraction state and NaOH extraction state), and their proportions were 20.69%-29.85% and 21.78%-29.9%, respectively. The proportion of water-soluble and exchangeable heavy metal Cd gradually decreased with the progress of fermentation process, the mineral state and residual state gradually increased. Most of Cd in the biogas residue before fermentation was primarily associated with FA of the humus. As the fermentation progressed, the HA content increased, the FA content decreased, and the HA/FA also increased gradually, which enhanced the stability of Cd in the humus, thereby reducing the toxic effect of heavy metal Cd in the biogas residue. Therefore, the mixed anaerobic fermentation of pig manure and straw with the addition of sepiolite as a passivating agent in this process can reduce the risk of heavy metal contamination.