Effects of rice husk biochar-aided vermicomposting of sewage sludge on heavy metals distribution in earthworm subcellular fractions

This study explored rice husk biochar's impact on heavy metal distribution in earthworm subcellular fractions during sludge vermicomposting to optimize municipal sludge utilization. Indoor experiments with 2%, 4%, 6%, 8%, and 10% biochar in sludge analyzed metal distribution in cellular solutes, debris, and solid particles. The results showed biochar increased pH and total nitrogen (TN), reduced electrical conductivity (EC), total organic carbon (TOC), total phosphorus (TP), and significantly decreased zinc (Zn), copper (Cu), chromium (Cr), and lead (Pb), with no effect on total potassium (TK). Increasing biochar addition caused Cr, Cu, Pb, and Zn in earthworm subcellular fractions to first enrich, then decrease. Cu, Pb, Zn are distributed as cellular solutes > solid particles > debris. 4% biochar raised Pb/Cu in solutes and Cr/Zn in solid particles; 8% increased Zn/Cu/Cr in debris. Correlation analysis linked reduced sludge heavy metals to earthworm bioaccumulation, with earthworm metal increase from subcellular accumulation. Sludge pH correlated most significantly with the three subcellular fractions. Overall, rice husk biochar reduces sludge heavy metal activity via pH modulation and adsorption, driving selective subcellular metal enrichment in earthworms, achieving "adsorption-precipitation segregation" and providing strategies for sludge resource utilization.