Characterization of Coffee Residues and Derived Biochar via Fourier-Transform Infrared Spectroscopy: Current Status and Outlook

Coffee processing generates large amounts of solid residues, including spent coffee grounds, husks, parchment, and silverskin, which represent an abundant biomass resource with significant potential for valorization. Thermal conversion of these residues into biochar has attracted increasing attention due to its applications in soil amendment, adsorption, energy production, and environmental remediation. Fourier-transform infrared (FTIR) spectroscopy is one of the most widely used techniques for characterizing properties of biomass and biochar because of its rapid analysis, minimal sample preparation, and ability to identify functional groups. This review summarizes the current state of FTIR applications in the characterization of coffee residues and derived biochar. Spectral features across feedstock types, thermal processes, and certain application scenarios are discussed, along with emerging areas such as semi-quantitative analysis, dynamical inspection, and predictive modeling. Particular attention is given to the interpretation of functional group assignments and transformations, the use of spectral indices, and the limitations of FTIR in complex biomass-biochar systems. Finally, future perspectives highlight the need for standardized spectral metrics, integration with complementary analytical techniques, and development of chemometric and modeling approaches to enable more quantitative and mechanistic interpretation of FTIR data in coffee residue and biochar research.