Boosting sewage sludge safety with nano-biochar for polycyclic aromatic hydrocarbons immobilization and ecotoxicity reduction

Polycyclic aromatic hydrocarbons (PAHs) are common pollutants in sewage sludge (SSL), posing environmental and health risks. Stabilizing these contaminants is crucial for improving the ecological safety and reuse potential of SSL. This study investigated the effectiveness of nano-biochars (n-BC) and bulk biochars (b-BC) derived from willow (WL) and rice husk (RH) in reducing freely dissolved (Cfree) PAHs in SSL and mitigating its ecotoxicity. RH-derived biochars demonstrated superior performance, achieving 1.5- to 4.7-fold greater reductions in Cfree PAH content compared to WL-derived variants. Notably, n-BC proved more effective than b-BC, with n-BC-RH and n-BC-WL showing up 1.2- to 2.0-fold greater reductions, respectively. The optimal BC’s dose range was 2.5–5 %, with diminishing returns observed at higher concentrations (10 %). The n-BC treatment also showed enhanced toxicity reduction, improving Aliivibrio fischeri luminescence and Lepidium sativum root growth by up to 109 % and 369 % compared to b-BC. Analysis revealed that inorganic minerals (present in ash) in both BC types played a key role in PAH immobilization. These results highlight n-BC's potential as an innovative solution for sustainable SSL management. This research addresses a critical gap in sludge management by proposing practical, scalable, and sustainable solutions for PAH contamination.