A comparative study of Cu and Zn passivation by humic acid, fly ash, biochar, and zeolite during pig manure anaerobic fermentation process

This study established an anaerobic fermentation system using pig manure/corn straw as substrate to compare the influence on biogas production and Cu and Zn passivation by humic acid (HA), biochar (BC), fly ash (FA), and zeolite (ZS). Results showed that the cumulative gas production was promoted, and the availability of Cu and Zn was decreased substantially with the introduction of passivators. Multiple comparison analysis (LSD) indicated that the passivation rate of Cu in the presence of HA was 41.43%, which was better than 33.86%, 31.65%, and 29.63% for BC, FA, and ZS. The passivation ratios of Zn were 12.59% and 12.33% corresponding to HA and FA and were higher than BC and ZS. The passivation effect of Cu was better than Zn with the introduction of four passivators. Additionally, this study also proved that organic–inorganic composite passivators exhibited more excellent immobilization effect on Cu and Zn than single passivators. The optimum mixing ratios of HA, FA, and BC for Cu and Zn passivation were 7.5% : 7.5% : 7.5% and 5.0% : 7.5% : 7.5%. This study compares the passivation effect of common used passivators on Cu and Zn during anaerobic fermentation of pig manure and suggests to adopt organic–inorganic composite passivators for reducing the bioavailability and the environmental risk of heavy metals.