Activated carbons have been prepared from rice husk (RH) and coffee husk (CH) using ZnCl2 as activating agent. These materials were characterized by thermogravimetric and elemental analysis, infrared spectroscopy (FTIR), the point of zero charge (PZC), scanning electron microscopy (SEM), and nitrogen adsorption. The removal efficiency of the obtained adsorbents was tested using indigo carmine (IC) at different pH, as a model dye, in both distilled water and textile wastewater. The results showed that the dye adsorption on the natural adsorbents and the activated carbons was favorable at acidic pH (3.0). Moreover, the best IC removal in both matrices was reached by the material prepared from CH and activated with ZnCl2 (CH-ZnCl2). This result correlated well with the higher value PZC (7.1), large specific surface area, and porosity characteristics. Additionally, the dye adsorption on the CH-ZnCl2 surface was improved using the smaller particle size and the optimal dose of adsorbent; the adsorptions obtained were much better than those of a commercial activated carbon. Furthermore, the isotherms study showed the adsorption to be monolayer type according to Langmuir and Redlich-Peterson equations. The adsorption kinetics followed a pseudo-second-order kinetic model, and intra-particle diffusion showed that IC adsorption on the surface is faster than inside CH-ZnCl2. The thermodynamic parameters suggest that the IC adsorption is a physical process, which is spontaneous and endothermic. According to density functional theory calculations and experimental FTIR analysis, oxygenated groups and aromatic rings are relevant during the IC adsorption through hydrogen bonds and π-πinteractions. © 2021 Elsevier Ltd.