Synergistic Effects of Earthworm Mucus and Different Biochars on Heavy Metal Bioavailability in Sludge Composting
This study investigated the synergistic effects of earthworm mucus and two biochar types (rice husk biochar and straw biochar) on heavy metal bioavailability during sludge composting. Sludge was amended with earthworm mucus alone or combined with biochars at varying proportions, and the impacts on physicochemical properties, total heavy metal concentrations, bioavailable fractions, and chemical speciation were analyzed. Results showed that mucus addition increased sludge pH and electrical conductivity (EC) but decreased total nitrogen (TN) and total phosphorus (TP) contents. Synergistic mucus-biochar composting further elevated pH and EC while reducing TN, with the optimal treatment being mucus plus 10% rice husk biochar. Mucus-only composting reduced total concentrations and bioavailability of Cd, Cu, Ni, Zn, and Pb. Adding biochars significantly enhanced these reductions. Specifically, mucus with rice husk biochar achieved the best Cd removal, with total and bioavailable Cd decreasing by 27.03%–55.68% and 9.52%–28.57% (P<0.05), respectively, compared to controls. Mucus with straw biochar was most effective for Ni, Zn, and Pb, reducing total contents by 3.81%–5.72%, 7.93%–34.62%, and 42.61%–79.46%, and bioavailable fractions by 2.90%–26.31%, 15.58%–24.14%, and 32.30%–36.88% (P<0.05), respectively. Speciation analysis revealed that Cd, Ni, and Pb carbonate-bound fractions transformed into residual forms, and exchangeable fractions shifted to Fe-Mn oxide-bound forms. Straw biochar addition resulted in the highest residual fractions for Cd, Cu, Ni, Zn, and Pb, increasing by 0.35%–7.02%, 8.61%–12.90%, 16.62%–23.02%, 17.33%–26.10%, and 16.12%–27.20% (P<0.05), respectively. These findings demonstrate that earthworm mucus combined with rice husk or straw biochar effectively reduces heavy metal concentrations and bioavailability in sludge, offering a promising strategy for sludge composting.