Nanoscale biochar outperforms conventional biochar in enhancing soil health and reducing risks from polycyclic aromatic hydrocarbon-contaminated soil

Soil contamination with polycyclic aromatic hydrocarbons (PAHs) poses significant environmental and health risks due to their persistence, toxicity, and bioaccumulation. While conventional bulk biochar (b-BC) has been used for PAHs immobilization, limited studies have evaluated nano-biochar (n-BC) for this purpose. This study investigated the potential of n-BC to immobilize PAHs in industrially contaminated soil, focusing on its impact on PAHs bioavailability. B-BC were produced from palm bunch (PB), rice husk (RH), and sewage sludge (SSL) at 550 °C under N2 atmosphere and processed using a ball-milling method into nanoscale particles. Compared to b-BC, n-BC exhibited higher specific surface area, porosity, and abundance of oxygen-containing functional groups, resulting in higher immobilization efficiency immediately after soil application. Notably, n-BC exhibited high immobilization efficiency immediately after mixing with soil, underscoring its rapid remediation potential. A 6-week pot experiment showed that n-BC amendments reduced both total (Ctotal Σ16 PAHs) and freely dissolved (Cfree Σ16 PAHs) PAHs contents. In n-BC-amended soils, Ctotal Σ16 PAHs and Cfree Σ16 PAHs were 50.7–55.0 % and 76.1–88.2 % lower, respectively, compared to control soil, and significantly exceeding the values observed for b-BC-amendments. PB- and RH-derived b-BC and n-BC were most effective in PAHs immobilization. Additionally, n-BC amendments showed lower ecological risk based on the Nemerow Pollution Index (NIPI). This work represents the first comparative study directly evaluating n-BC and b-BC for PAHs immobilization, demonstrating that the rapid sorption and enhanced remediation efficiency of n-BC are strongly linked to its nanoscale properties. These findings highlight the superior remediation potential of n-BC for PAHs-contaminated soils, supporting its application as an effective and sustainable amendment. The enhanced quick and long-term immobilization effectiveness of n-BC compared to b-BC suggests its promising role in mitigating PAHs pollution and reducing related environmental risks.