Valorization of spent coffee grounds as soil amendment and valuable bio-additives

Spent coffee grounds (SCGs), a primary by-product of coffee processing, hold potential as a soil amendment material due to their high organic matter and nutrient content. However, direct soil application can result in detrimental ecotoxicological effects caused by the presence of phytotoxic compounds (e.g., caffeine and polyphenols). Although these bioactive substances need to be removed for agricultural purposes, they can serve as valuable additives across diverse industrial sectors (e.g., food & beverages, pharmaceuticals, and cosmetics). In pursuit of circular bio-economy, this study investigates a comprehensive valorization approach, (i.e., a rinsing process followed by concentration step via membrane distillation) to enhance SCGs reutilization as soil amendment while simultaneously obtaining a valuable SCGs extract enriched in bio-additives. Specifically, after being collected from a local cafeteria and dried at 25°C, SCGs underwent four rinsing cycles, following the procedure outlined in a previous study [1]. This resulted in two fractions: the rinsed solid SCGs, i.e., the SCGs amendment, and a diluted SCGs extract. Subsequently, the extract was further concentrated through membrane distillation step in a cross-flow lab-scale system using a PTFE commercial membrane (active area of 140 cm2). In this concentration step, the feed and the permeate temperature were kept at 60°C and 25°C, respectively. Evaluation of the process effectiveness and concentrate quality was performed by monitoring caffeine concentration with HPLC-UV/Vis analyses and total polyphenol content (TPC) though Folin-Ciocalteu spectrophotometric assay. Results showed the possibility to reach high concentration factors of up to 8 (equivalent to almost 90% of water recovery), while maintaining high fluxes (above 15-10 L m−2 h−1). Notably, the concentrate quality was preserved and no transfer of organic matter to the distillate side was observed. On the other hand, SCGs amendment was thoroughly characterized by mixing it with quartz sand at mass ratios ranging from 1% to 20%, revealing decreased bulk density, significantly higher electrical conductivity, enhanced water holding capacity, and a moderate decrease in pH. Additionally, the adsorption capacity of the SCGs amendment was evaluated through both batch tests with copper sulfate solutions of increasing concentrations (125-625 mg/L), and dynamic leaching tests in unsaturated soil-amendment packed columns, showing good affinity for copper sulfate in both scenarios. Finally, absence of caffeine and polyphenols release during leaching tests proved the effective removal of monitored compounds, further attesting to the feasibility of the proposed valorization approach.