Exploring the role of biochar and Fe₂O₃ in mitigating copper and zinc bioavailability in co-composting of pig manure and wine grape pomace

This research aimed to investigate the distinct roles of biochar and Fe2O3 in the passivation of Cu and Zn during the co-composting of pig manure (PM) and wine grape pomace (WGP). The findings revealed that Fe2O3 and biochar achieved Cu and Zn passivation rates of 39.8% and 14.7%, and 52.0% and 20.4% respectively. However, combining biochar and Fe2O3 resulted in lower passivation rates of 31.1% for Cu and 17.6% for Zn, attributed to 36% to 39% inhibition of heavy metal-resistant core colonies, Pseudoxanthomonas and Chelatococcus. Simultaneously, biochar-enriched compost demonstrated a higher content of carboxylic and phenolic hydroxyl groups in humic acid than Fe2O3. Furthermore, dominant genera like Pseudomonas and Corynebacterium, along with certain microbial metabolic activities, were found to positively influence Zn passivation. These results could guide the use of the biochar and Fe2O3 and contribute to a deeper understanding of the Cu/Zn passivation pathways in phenol-rich waste composting.