Synergistic Effects of Earthworm Mucus and Different Biochars on Heavy Metal Bioavailability in Sludge Composting

• • Mucus plus 10% rice husk biochar optimally increased sludge pH and EC while reducing TN, with OM content highest, indicating improved nutrient retention and stabilization. • • Mucus with rice husk biochar reduced total Cd by 27.03%–55.68% and bioavailable Cd by 9.52%–28.57% (P<0.05), outperforming other treatments for Cd immobilization. • • Mucus with straw biochar achieved the greatest reductions in total Ni (3.81%–5.72%), Zn (7.93%–34.62%), and Pb (42.61%–79.46%), and bioavailable fractions (Ni: 2.90%–26.31%, Zn: 15.58%–24.14%, Pb: 32.30%–36.88%) (P<0.05), demonstrating superior passivation. • • Straw biochar addition increased residual fractions of Cd, Cu, Ni, Zn, and Pb by up to 7.02%, 12.90%, 23.02%, 26.10%, and 27.20% (P<0.05), respectively, indicating enhanced transformation to stable forms and reduced environmental risk.