From urban biosolids to biochar and nanobiochar: A scoping review of environmental applications and circular resource recovery.

Increasing volumes of urban biosolids present environmental challenges and opportunities for resource recovery and the circular bioeconomy. This scoping review synthesises evidence on the conversion of biosolids into biochar and nanobiochar, with emphasis on agronomic performance, environmental risk, techno-economic considerations, and regulatory aspects. Following PRISMA-ScR guidelines, 119 peer-reviewed publications from 2010 to 2026, together with selected grey literature, were analysed. Biochar application was generally associated with improved soil physicochemical properties, microbial activity, nutrient retention, and carbon sequestration. Nanobiochar may offer additional benefits because of its higher surface area and reactivity, but its environmental mobility, ecotoxicological behaviour, and long-term field performance remain uncertain. Key implementation challenges include PFAS fate during thermochemical conversion, contaminant redistribution across solid, liquid, and gaseous phases, feedstock variability, production costs, and the absence of harmonised regulatory frameworks for biosolids-derived biochar products. Broader adoption is likely to require long-term field validation, life cycle assessment, standardised characterisation methods, integrated environmental risk assessment, and clearer regulatory pathways.