Industrial sludge, a byproduct of wastewater treatment from heavily polluting industries like electroplating and chemicals, poses a growing environmental challenge due to its high content of heavy metals such as copper (Cu), zinc (Zn), lead (Pb), and nickel (Ni). Traditional disposal methods like landfilling and incineration are costly and contribute to pollution. A recent study published in the Journal of Environmental Engineering Technology by Wang Xing-ming, Liu Xin, Shen Lu, and Chu Zhao-xia explores a more sustainable and effective solution: using different types of biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More in combination with earthworm composting to passivate these heavy metals and reduce environmental risks.

The researchers investigated the effects of adding three types of biochar—rice husk charcoalCharcoal is a black, brittle, and porous material produced by heating wood or other organic substances in a low-oxygen environment. It is primarily used as a fuel source for cooking and heating.   More, bamboo charcoal, and straw charcoal—along with earthworms to industrial sludge for composting. They analyzed changes in the sludge’s physicochemical properties, total heavy metal content, bioavailable forms, and morphological distribution, as well as the associated ecological and health risks.

The results showed that earthworm-only composting, compared to sludge composting alone, lowered the sludge’s pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More and organic matter (OM) content while increasing total nitrogen (TN), total phosphorus (TP), and electrical conductivity (EC). The addition of biochar further enhanced these effects. Specifically, 4% bamboo charcoal proved most effective in improving physicochemical properties, bringing the sludge pH closer to neutral and significantly reducing OM (19.16%, a 26.89% decrease from the control). This is attributed to bamboo charcoal’s higher acidity and porous structure, which promotes organic matter decomposition.

Crucially, the combined treatment of biochar and earthworms significantly reduced both the total content and bioavailability of heavy metals. For Ni, Cu, and Zn, 4% rice husk charcoal, when combined with earthworm composting, showed the best removal effects, reducing total concentrations by 31.71%, 13.51%, and 25.89% respectively, compared to the control group (T0). The bioavailability (effective state) of these metals also saw significant reductions: 42.80% for Ni, 23.27% for Cu, and 11.06% for Zn. This enhanced removal is partly due to rice husk charcoal’s unique silicate structure, which strongly passivates Ni, Cu, and Zn.

Morphological analysis revealed that the biochar-earthworm co-composting transformed heavy metals into less mobile and less bioavailable forms. Cu was largely converted into organic bound and residual states, while Pb, Ni, and Zn transitioned into iron-manganese oxide bound and residual states. The 4% bamboo charcoal treatment achieved the highest conversion rate and best passivation effect for these transformations.

The study also assessed the environmental risks associated with the treated sludge. The potential ecological risks (RI) for all three biochar types combined with earthworms were categorized as low (RI < 150). Rice husk charcoal yielded the lowest overall ecological risk (RI of 95.9), while straw charcoal had a slightly higher risk (RI of 97.04). Health risk assessments for non-carcinogenic substances (Cu, Zn, Ni, Pb) showed that the total non-carcinogenic risk index (HI) for both adults and children across all treatment groups was less than 1, indicating acceptable levels for human exposure. Straw charcoal, in particular, resulted in the lowest total health risk (HI = 0.092). Oral intake was identified as the primary exposure pathway, followed by skin contact and respiratory intake.

In conclusion, this research demonstrates that the synergistic application of biochar and earthworm composting is an effective strategy for reducing heavy metal content and ecological risks in industrial sludge. While bamboo charcoal excels in improving sludge physicochemical properties and passivating heavy metals, rice husk charcoal is superior in terms of overall heavy metal removal and safety. Straw charcoal shows strong performance in controlling overall health risks. All three biochar types, in conjunction with earthworms, present promising potential for sustainable industrial sludge management and resource utilization.

Source: Xing-ming, W. A. N. G., Xin, L. I. U., Lu, S. H. E. N., & Zhao-xia, C. H. U. (2025). Research on the passivation of heavy metals in industrial sludge and environmental risks using multi-type biochar in collaboration with earthworms. Journal of Environmental Engineering Technology.