Comparison of H3PO4 and ZnCl2 Activated Filtered Coffee Waste Carbon-Based Adsorbents in Methylene Blue Removal by Using Ultrasonic-Assisted Adsorption

In sustainable waste management, reusing coffee waste such as husks or spent coffee grounds helps convert a huge amount of waste into a new resource. In this study, H₃PO₄-activated filter coffee waste (H₃PO₄-FCW) and ZnCl₂-activated filter coffee waste (ZnCl₂-FCW) were synthesised, and the characterisation of them was revealed by SEM, EDAX, FTIR and XRD analyses. Adsorption isotherms (Langmuir, Freundlich, Temkin and Dubinin–Radushkevich), adsorption kinetics (Elovich, pseudo-second-order kinetics, pseudo-first-order kinetics including error functions) and adsorption thermodynamics were determined and the effect of parameters such as pH, adsorbent dose, initial methylene blue (MB) concentration, adsorption time and temperature on MB removal was evaluated by the ultrasonic-assisted adsorption. It was observed that ultrasonic-assisted adsorption was more suitable for the Langmuir isotherm for both adsorbents and that the adsorption kinetics was better suited to the pseudo-second-order kinetics. In addition, the ultrasonic-assisted adsorption using H₃PO₄-FCW and ZnCl₂-FCW occurred endothermically and the adsorption mechanism was physisorption. MB removal with ZnCl₂-FCW could reach over 90% when pH is between 3 and 7 in the ultrasonic-assisted adsorption process. The maximum adsorption capacity was obtained as 42.7 mg/g and 106.4 mg/g for H₃PO₄-FCW and ZnCl₂-FCW, respectively. It was seen that the q_max value of ZnCl₂-FCW was 2.5 times higher than the q_max of H₃PO₄-FCW. Moreover, reusability studies showed that MB removal of over 90% could be achieved in the 2^nd use of H₃PO₄-FCW and ZnCl₂-FCW. In conclusion, high dye removal could be achieved by synthesising low-cost adsorbents. The use of filtered coffee wastes in the adsorption of basic dyes such as MB is, therefore, important in terms of both waste reduction and management.