Transformation of keratinous waste into biochar for sustainable multifunctional applications

Keratin wastes are generated more than 8 million tonnes annually, which largely end up in landfills or incineration. This represents an environmental burden and a significantly underexploited resource with high nitrogen (N) (15–18%) and sulphur (S) (2–5%) that distinguishes itself from the conventional lignocellulosic biomass. The present review consolidates the current understanding of keratin-based biochar across its full value chain. It aims to provide a comprehensive analysis of the chemical composition of the major feedstock, including the processing of wastes using the hydro-pyrolysis and carbon thermolysis methods and examines the kinetics of biochar production. The keratin-based biochar exhibits variability in the yield and quality when subjected to slow and fast-pyrolysis and differs with the technique employed namely thermal pyrolysis, hydrothermal carbonization, catalytic and engineered modification. The multifunctional keratin biochar possesses the capacity to improve soil fertility, pollutant adsorption and removal, and renewable energy retention. The diverse applications are subsequently surveyed, spanning soil-fertility enhancement, pollutant adsorption in water and gas streams, carbon sequestration and renewable-energy storage, while addressing the key challenges in standardization and contamination. Collectively, this review positions keratin waste valorisation as a technically and economically potential route for advancing circular-bioeconomy integration and clean energy transition.