Sustainable removal of methylene blue from water using three coffee waste types

Since approximately 90% of the total weight of coffee fruit is not utilized during its commercial cycle, the coffee industry generates a massive amount of residues that affect environmental dynamics. This study explores an alternative use for three types of coffee waste (coffee husk CH, spent coffee SC and expired coffee EC) as efficient biosorbent materials for treating water contaminated with industrial dyes such as methylene blue (MB). Characterization was performed by analyzing their crystalline structures (XRD), thermal stability (TGA) and functional groups (FTIR), revealing that fat removal aligned crystalline domains enhancing mechanical integrity, while deprotonated carboxyl and hydroxyl groups drove electrostatic MB binding. Furthermore, the inherent content of lipids and proteins delayed the material decomposition. Raw and defatted coffee residues were exposed to two main treatments consisting of three stages with sequential exposure to alkaline and acid conditions, ethanol and H₂O₂, followed by a multi-phase processing for comparative analysis. The highest removal percentages were recorded for pretreated CH (98.14%) at pH 10 and 50 ppm of MB, no-pretreated SC (T1) at pH 5 and 50 ppm of MB (96.76%), and pretreated EC at pH 10 and 100 ppm of MB (98.48%). Adsorption kinetics indicated a pseudo-second-order mechanism and Sips isothermal model behavior, involving not only physisorption but also chemisorption on a heterogeneous surface with energetically distinct adsorption sites for all the coffee residues. Finally, a regeneration study using HCl as the desorption agent demonstrated a stable adsorption performance for up to 14 cycles at 50 ppm, particularly for coffee husk samples.