Effect of biochar and humic acid on the copper, lead, and cadmium passivation during composting

In this study, two different biochars (sawdust charcoal (SDC) and wheat straw charcoal (WSC)) and biological humic acid (BHA) were used with different addition rates in pig manure composting to illustrate the effect on heavy metals passivation. And the composts were applied to rape (Brassica campestris L.) growth to illustrate the stability of the passivation. Results showed the concentration of Cu, Cd, and Pb increased after composting, whereas the passivation rates of Cu, Pb, and Cd reached a maximum of 94.98%, 65.55%, and 68.78%, respectively. When the composts were applied to rape growth, the exchangeable fraction of Cu, Pb, and Cd in the soil further decreased and reduced the accumulation of heavy metals in the rape plant. The rape yield increased by 19.39%-34.35%. The optimal addition ratios of the three passivators were SDC 5%, WSC 7.5% and BHA 2.5% to reduce the health risk of heavy metals in rape products.