Upcycling tannery sludge into micro/nano protein fibers to enhance the cellulose-based films for mechanical properties and flame retardancy

Micro/nano protein fibers have attracted increasing attention owing to their advantageous properties, including high tensile strength, biodegradability, and exceptional thermal stability, which make them suitable for applications in advanced materials. However, traditional preparation methods often suffer from high material costs and energy-intensive manufacturing processes, which hinder large-scale production. Herein, we present an innovative low-carbon approach for converting tannery sludge into micro/nano protein fibers, which converts 62.14% of the protein in sludge into protein fibers smaller than 5 μm and retains 97.61% of the chromium in the fibers. Surprisingly, the micro/nano protein fibers enhance the cellulose-based films for mechanical properties and flame retardancy. The incorporation of 10% protein fibers resulted in a 55.40% increase in the tensile strength of the cellulose-based films, along with significant improvements in Young's modulus (22.39%) and toughness (38.25%). Furthermore, the addition of micro/nano protein fibers substantially enhances the cellulose-based films for flame retardancy, as demonstrated by a 16 °C increase in the peak temperature of heat loss. Moreover, the peak heat release rate was reduced by 21.60%, while the total heat release decreased by 28.17%. This low-carbon and eco-friendly process utilizing leather tannery sludge not only provides a sustainable source of raw materials for protein fibers, but also contributes to the circular economy by repurposing industrial waste.