Adsorptive removal of anionic azo dye from aqueous solution using activated carbon derived from extracted coffee residues

This study reports the synthesis of activated carbon derived from extracted coffee residues (AC-ECRs) via wet impregnation using different ratios of potassium hydroxide as an activating agent. The influence of the impregnation ratio on physicochemical properties such as surface characteristics, structural properties, and energetic heterogeneity were evaluated using nitrogen adsorption isotherms and adsorption energy distributions (AEDs). The overall porosity and energetic heterogeneity were improved with increasing impregnation ratio, resulting in a positive impact towards the adsorption capacity of substrate. Particularly, the correlation between AEDs and adsorption capacity clearly indicated that mesopore distribution plays an essential role on the adsorption capacity of acid orange 7 as a wider pore size offers more favorable conditions for adsorbing the substrate. This is the first report documenting the effect of impregnation ratio on surface heterogeneity and the correlation between AEDs and adsorption capacity of an azo dye. Adsorption kinetics and equilibrium isotherm studies insinuate that the overall process follows the pseudo-second-order and Sips isotherm models, respectively. Thermodynamic and isosteric heat of adsorption analyses revealed that the adsorption process was exothermic and is primarily governed via physisorption.