Removal of dyes from aqueous solutions with untreated coffee residues as potential low-cost adsorbents: Equilibrium, reuse and thermodynamic approach

Industrial wastes of coffee (untreated coffee residues, UCR) were used as low-cost adsorbents for the removal of dyes (reactive and basic) from single-component aqueous solutions. The cost potential is high given for the non-further treatment of the coffee residues (just only washing with distilled water to remove dirt/color, and drying in oven). The characterization of adsorbents was carried out with SEM micrographs, titrations for point of zero charge (PZC), BET analysis for surface area, titrations (Boehm method) for surface functional groups, and energy dispersive X-ray microanalysis (EDX) for elemental analysis/composition. The optimum pH found after adsorption experiments was pH = 2 for reactive and pH = 10 for basic dyes. The equilibrium experimental data were fitted to Langmuir, Freundlich, and Langmuir–Freundlich model (best correlation: R2 > 0.997). The calculated maximum adsorption capacities (Qmax) for the reactive dye at 25 °C were 179 mg/g (pH = 2) and 295 mg/g for the basic one (pH = 10). Pseudo-first, -second, and -third order kinetic equations were used to fit the kinetic data (pseudo-second order equation presented the most sufficient correlation, R2 > 0.992). Some other adsorption parameters, as agitation rate, initial dye concentration and temperature (25, 45 and 65 °C) were also determined. Also, a full thermodynamic evaluation was realized, calculating the parameters of enthalpy, free energy and entropy (ΔH0, ΔG0, and ΔS0). The desorption was evaluated with experiments for the optimum desorption pH and desorption kinetics, while the ability of reuse was determined with 10 cycles of adsorption–desorption (the reduction in adsorption percentages from the 1st to 10th cycle was approximately 7% for both dyes). Additionally, experiments in dyeing mixtures were realized in dyeing mixtures composed of (i) separately reactive or basic dyes, and (ii) simultaneously reactive and basic dyes. After the dosage of 3 g/L of adsorbent, a very slight change was observed in equilibrium for all types of dyeing mixtures studied.

► Untreated coffee residues for reactive and basic removal.
► Equilibrium data were fitted to: Lanmguir, Freundlich, L–F model (Qmax = 240 mg/g for basic and 175 mg/g for reactive dye).
► Kinetic data were fitted to the pseudo-first, -second and -third order model.
► Thermodynamic parameters were calculated.
► After 10 cycles of adsorption–desorption, the reduction in adsorption percentages from the 1st to 10th cycle was approximately 7% for both dyes.