Valorization of coffee and tea wastes into carbonaceous materials: Composition, processes, applications, and sustainability challenges

The increasing generation of agro-industrial waste, particularly spent coffee grounds (SCG) and tea waste (TW), presents both environmental challenges and valuable opportunities for sustainable resource recovery. This review provides in its first part a comprehensive overview of the general composition of coffee and tea wastes, highlighting their richness in carbonaceous compounds, polyphenols, and essential nutrients. It further explores their potential valorization into high-value carbon-based materials through various thermochemical routes. Emphasis was placed on the full processing pathway from pre-treatment to thermal treatment, followed by post-treatment techniques such as chemical and physical activation to enhance porosity and surface reactivity. In addition, the review covered the most studied applications of the derived biochar and activated carbon materials, including water treatment, soil amendment, energy storage, and CO₂ capture. Finally, the review critically addressed the practical challenges associated with biomass processing. By synthesizing the current advances, this work underscores the promise and limitations of converting tea and coffee wastes into carbonaceous materials and outlines future directions for improving circular bioeconomy goals.