Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
Municipal sludge composting is a widely adopted strategy for waste valorization, yet the presence of heavy metals (Cd, Cu, Ni, Zn, Pb) poses significant environmental and health risks. Conventional composting often fails to adequately immobilize these metals, leaving bioavailable fractions that can leach into soil and groundwater. Previous amendments, such as single biochar applications, have shown limited efficacy in reducing metal bioavailability, particularly for cadmium and lead, due to insufficient binding sites or suboptimal physicochemical conditions. The challenge lies in developing a cost-effective and efficient method to enhance metal passivation during composting.
This study introduces a novel synergistic approach by combining earthworm mucus—a biogenic surfactant rich in organic ligands—with two types of biochar (rice husk and straw). Earthworm mucus is known to chelate metals and alter sludge pH and EC, while biochar provides porous structures and functional groups for metal sorption. The hypothesis is that their combined action will not only reduce total metal concentrations but also shift metal speciation from labile to residual forms, thereby lowering bioavailability. This experimental protocol directly addresses the bottleneck of insufficient metal immobilization in conventional composting by leveraging the complementary mechanisms of biogenic chelation and pyrolytic sorption, offering a promising pathway for safer sludge disposal and agricultural reuse.
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WANG Xingming, LU Haopeng, SHEN Lu, CHU Zhaoxia (2026). Synergistic Effects of Earthworm Mucus and Different Biochars on Heavy Metal Bioavailability in Sludge Composting. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607013
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Frequently Asked Questions
What are the underlying mechanisms by which earthworm mucus and biochar synergistically reduce heavy metal bioavailability in sludge?
Earthworm mucus contains organic acids and amino acids that can chelate metals, increasing their solubility and mobility, while biochar provides high surface area and functional groups (e.g., carboxyl, hydroxyl) that adsorb metals. The combination likely enhances metal complexation and subsequent adsorption onto biochar surfaces, promoting transformation from exchangeable and carbonate-bound fractions to more stable Fe-Mn oxide and residual fractions. Additionally, mucus increases pH and EC, which can precipitate metals as hydroxides or carbonates, while biochar's alkalinity further stabilizes them.
How does the type and dosage of biochar influence the effectiveness of heavy metal immobilization?
Rice husk biochar was more effective for Cd removal, while straw biochar showed superior performance for Ni, Zn, and Pb. This is attributed to differences in surface chemistry and mineral content. Straw biochar typically has higher ash content and more oxygen-containing functional groups, enhancing its affinity for these metals. Dosage also matters: 10% rice husk biochar was optimal for overall sludge properties, but higher dosages of straw biochar (up to 10%) yielded greater reductions in metal bioavailability. The study suggests that biochar type should be selected based on target metals.
What are the implications of increased Cu bioavailability observed in some treatments?
While total Cu and other metals decreased, the bioavailable fraction of Cu increased in certain synergistic treatments. This may be due to the formation of soluble organic-Cu complexes with mucus-derived organic matter, which can enhance Cu mobility. This finding highlights a potential trade-off: while most metals are immobilized, Cu may become more leachable. Therefore, careful consideration is needed when applying this method to Cu-contaminated sludge, and further optimization may be required to mitigate Cu mobilization.
How do the changes in sludge physicochemical properties (pH, EC, OM) correlate with heavy metal immobilization?
Increased pH and EC generally favor metal precipitation and adsorption, reducing bioavailability. Higher OM content can provide additional binding sites for metals, but may also increase soluble metal-organic complexes. In this study, mucus and biochar additions raised pH and EC, which likely contributed to the transformation of metals to residual forms. The reduction in TN and TP suggests that nutrient dynamics are altered, but these changes did not negatively impact metal immobilization. The optimal treatment (mucus + 10% rice husk biochar) achieved the highest OM, indicating improved organic matter stabilization, which may enhance metal binding.
What are the practical implications for scaling up this composting approach in industrial settings?
The use of earthworm mucus and biochar is cost-effective and environmentally friendly. Biochar can be sourced from agricultural waste, and earthworm mucus can be obtained from vermicomposting systems. The optimal dosages (e.g., 10% biochar) are feasible for large-scale operations. However, the increased Cu bioavailability in some treatments must be addressed, possibly by adjusting biochar type or dosage. Overall, this method offers a promising solution for reducing heavy metal risk in sludge before land application, aligning with circular economy principles.
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