Passivation Mechanism of Cu and Zn with the Introduction of Composite Passivators During Anaerobic Digestion of Pig Manure

In this study, biochar, humic acid and fly ash were added to passivate Cu and Zn in biogas residue during anaerobic digestion of pig manure. The optimal composite proportion of passivators was obtained and the passivation mechanism of Cu and Zn was further examined. Results showed the content ratio of humic acid (HA) to fulvic acid (FA) in biogas residue increased. Cu and Zn in humus before anaerobic digestion mainly combined with FA, and mainly combined with HA after anaerobic digestion. The proportion of HA-Cu and HA-Zn in the treatment groups was significant higher than that in the control group. The bioavailability of heavy metals was inhibited with passivators at the early stage of anaerobic digestion. Humic substances play a major role in the passivation of heavy metals at the middle and later stages. This study can provide a theoretical basis for the safe application of biogas fertilizers.