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The continuous development of biogas engineering in recent years has been accompanied by an increasing number of biogas-residue types in the environment. The existence of various heavy metal elements in biogas residues, particularly excessive amounts of zinc and copper in pig farm biogas residues, hinders their safe utilisation. Several studies have focused on an effective composition for biogas residues or on their effects on soil and plants. However, few investigations regarding the removal of harmful substances from biogas residues have been conducted. The characteristics of heavy metals leached from pig farm biogas residues using eight types of leaching agents were analysed in this study. It is expected that, based on this study, different uses for biogas residues can be developed under the premise of security. The results showed that the most suitable extractant of heavy metals from pig farm biogas residues was EDTA2Na. The leaching rates increased over time. The leaching rates of copper and zinc after 2 h were 21.91% and 48.66%, respectively. After being leached for 2 h by EDTA-2Na, the concentration of zinc met the requirements for moderate alkaline soil, according to the Control Limits against Harmful Substances in Composts for Agricultural Use (DB44/T 361-2006) standard.

Characteristics of leaching agents for heavy metal extraction and safe utilisation of pig farm biogas residues