Potential of spent Turkish coffee as a low-cost biosorbent for the removal of anionic methyl red and cationic nile blue: a comparative adsorption study

Dye-containing wastewater poses significant environmental threats due to its toxicity, persistence, and adverse effects on aquatic ecosystems. This study explored the potential of spent Turkish coffee (STC), a widely consumed beverage in Türkiye, as a low-cost biosorbent for the removal of anionic methyl red (MR) and cationic nile blue (NB) from aqueous solutions. The physicochemical properties of STC were characterized using SEM-EDX, FTIR, XRD, and BET analyses to assess surface morphology, elemental composition, functional groups, crystallinity, and surface area. Adsorption experiments examined the effects of contact time, pH, STC dosage, initial dye concentration, and temperature. Maximum removal efficiencies were 72.75% for MR at 45 min and 90.54% for NB at 30 min. Adsorption data fitted the Langmuir isotherm and pseudo-second-order kinetic models, indicating monolayer adsorption and chemisorption. Thermodynamic analysis showed that the adsorption process was spontaneous and exothermic. Functional groups on STC, including – OH, –CH, C = O, C – O, and C – H, played a significant role in dye binding, and maximum adsorption capacities (qm) were 51.82 mg/g for MR and 105.83 mg/g for NB. These findings confirm that STC is an effective, sustainable, and environmentally friendly adsorbent with significant potential to valorize food waste and promote circular economy practices in sustainable wastewater treatment.