The release of colored compounds containing wastewaters into the environment and surface water resources is known as a serious threat to public health and aquatic ecosystems. Accordingly, in the present study, cost-effective and efficient activated carbons (ACs) were prepared employing agricultural wastes (walnut shell) over a two-stage physical activation under CO2 at various activation temperature (700–1000 °C) and time (30–120 min). The prepared ACs were characterized by various analyses of SEM, XRD, FTIR, BET, and also their adsorption performance was checked via the measure of methylene blue (MB) number in order to determine selected activated carbon. According to the reported results, the AC produced at the activation temperature and time of 900 °C and 60 min, respectively, indicated better physicochemical properties and more favorable performance compared to others. The selected AC had the highest BET surface area (SBET) of 903.9 m2/g and the greatest adsorption capacity of 307.4 mg/g as the MB number. Furthermore, the adsorption capacity as iodine number and pH of point of zero charge (pHPZC) were reported to be 1150 mg/g and 4.6, respectively, for the selected AC. In the following, the performance of the selected AC was further evaluated as adsorption of synthetic (MB) and natural dyes from biologically treated wastewaters under different conditions. Maximum absorption capacity of the MB was obtained to be 307.45 mg/g at optimum conditions (the adsorbent dosage of 1 g/L, adsorbate concentration of 400 mg/L, contact time of 3h, and agitation speed of 150 rpm). Moreover, the selected AC demonstrated appreciable performance at adsorbent dosage of 1 g/L and the contact time of 240 min using three biologically treated wastewaters. The highest adsorption capacity of 395.7 mg/g was obtained by the biologically treated baker's yeast wastewater as the adsorbate. The selected AC revealed the highest conformity with the Langmuir isotherm model (R2 = 0.9999) and the Pseudo-second-order kinetic model (R2 = 0.9994).