Green activated carbon from biochar waste: linking activation pathways, surface chemistry, and adsorption performance for water purification.

Biochar-derived activated carbon (BAC) is a promising sustainable adsorbent for water purification due to its tunable pore structure and adaptable surface chemistry. This review examines the relationships among feedstock composition, activation pathways, structural properties, adsorption mechanisms, and sustainability in BAC production. Pyrolysis, hydrothermal carbonization, and various activation methods are discussed in relation to pore development, adsorption performance, and environmental impact. Adsorption behavior depends not only on the BET surface area but also on pore accessibility, surface functionality, crystallinity, and transport behavior. Micropores favor the adsorption of small molecules and metal ions, whereas mesopores improve the diffusion of larger contaminants. Emerging pollutants such as PFAS, including PFOA and PFOS, are highlighted, with adsorption strongly influenced by hydrophobic and fluorophilic interactions. Sustainability challenges, including energy demand, chemical consumption, wastewater generation, and scalability, are also addressed. Finally, hybrid activation, low-chemical processing, and AI-assisted optimization are proposed as strategies for developing scalable next-generation BAC adsorbents for advanced water purification.