Immobilization effect of Pb and Cd during composting of pig manure by combination of biochar and microbial agent

Abstract: Heavy metals residual in pig manure compost causes potential risk on the soil and food security. The essence of the composting process is the process of microbial fermentation. The addition of biochar can change the living conditions of microorganisms in the addition system, and may directly or indirectly affect the population changes of functional microorganisms, thus affecting the migration of heavy metals. Therefore, this study used biochar and composite microbial bacteria agent as immobilization materials. Three kinds of biochar, which were peanut shell biochar, wood biochar, and corn stalk biochar, and composite microbial agents were added with different proportions during pig manure composting to investigate their immobilization effect of Pb and Cd. The results showed that all 9 treatments met the requirement of harmless treatment, while the temperature maintained above 50 ℃ for over 2 days, high temperature (over 50 ℃) maintain time was 20, 9, 12, 13, 8, 12, 14, 12, 17 d, and the seed germination index reached 80%-85%. Different proportion of biochar had significant influence on temperature change during composting. The immobilization effect of three factors on heavy metal Cd reached a significant level (P<0.05), and the largest contribution rate was the proportion of biochar. There are linear correlation between heavy metal Pb, Cd immobilization effect and the average pH value, the average temperature during the composting and the organic matter degradation rate of the compost. The immobilization effect of Pb showed a significant positive correlation with the average pH value and average temperature during composting (P<0.05); and the immobilization effect of Cd was significant positive correlated with the average pH value (P<0.05) and degradation rate of organic matter (P<0.01). In the course of compost, the content of protein and polysaccharide content in water-soluble organic substances in the compost sample was significantly reduced, and the content of carboxyl group and aromatic family was increased, which indicated that the composting process was a process of gradually synthesizing macromolecular substances such as organic substances and so on, which can affect the conversion of heavy metals from the highly reactive form into lower activity form. The optimal treatment of Pb immobilization effect was 24% peanut shell biochar and 1% fungus agent in this study, and the exchangeable distribution rate decreased by 16.32% compared with raw materials before compost, and the immobilization effect was 74.60%. Analysis of variance showed that the immobilization effect of biochar ratio on Pb was extremely significant, and the contribution rate was the largest. The optimal treatment for the immobilization effect of Cd was 24% sawdust charcoal and 1.5% bacteria agent. Compared with raw materials before compost, the immobilization effect of exchangeable Cd decreased by 7.96%, and the immobilization effect was 58.13%.