Unlocking the Potential of Spent Coffee Grounds as a Source of Humic-like Substances with High Antioxidant Capacity

The environmental impacts of the coffee industry are enormous, with large quantities of solid and liquid wastes generated globally. In the last years, measures have been implemented to valorize spent coffee grounds (SCGs) biowaste as a resource. The present study assessed the unexplored potential use of SCGs as a source of valuable humic substances (HS)-like materials, and linked their antioxidant properties to their chemical composition. Humic acid-like (HAL) and fulvic acid-like (FAL) were extracted from SCGs and characterized by elemental composition, UV-vis, FTIR, NMR and EPR spectroscopies. Their antioxidant capacity was determined in vitro through the ABTS decolorization assay and voltammetric techniques. Elemental and spectroscopic characteristics were in the range of soil HS. Both HAL and FAL showed a high antioxidant capacity (2.29 ± 0.18 mmole− gHAL−1 and 3.63 ± 0.21 mmole− gFAL−1). The differences in the antioxidant and electrochemical responses of HAL and FAL are related to their chemical structure. The extraction of HS-like with remarkable antioxidant properties makes them a promising low-cost material to be upcycled in the agri-food system as natural fertilizer or novel antioxidants. For the first time, we demonstrated the potential of spent coffee grounds to act as a low-cost source of humic-like substances with enhanced antioxidant capacity.