Biochar, a sustainable soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More derived from organic materials, has been lauded for its ability to immobilize heavy metals (HMs) in contaminated soils. However, the effectiveness and environmental safety of biochar can diminish over time due to a process known as biochar aging. Recent studies have highlighted significant gaps in our understanding of how biochar ages and the subsequent risks it may pose.

A systematic review of recent literature reveals that most research on biochar aging employs artificial methods such as physical, chemical, and biological aging, rather than examining natural field conditions. Chemical aging, in particular, significantly alters biochar’s characteristics more drastically than other methods, affecting properties like surface area, pore volume, and elemental composition.

These changes can potentially lead to the remobilization of heavy metals and the formation of new pollutants like polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs), and colloidal or nano-sized biochar particles. Such transformations introduce secondary risks to the environment, challenging the perceived stability and safety of biochar as a soil amendment.

Given these findings, there’s a critical need for more accurate aging models and assessment methods that consider both the physicochemical transformations of biochar over time and its long-term ecological impacts. Future research should focus on developing these models and conducting field-based aging studies to ensure biochar remains a safe and effective solution for managing soil contamination. This holistic approach will better inform the deployment of biochar in real-world applications, ensuring its benefits are realized without unforeseen environmental costs.