In this research, three adsorbents, namely, Ephedra strobilacea char (ESC), E. strobilacea char modified using phosphoric acid (ESP) and E. strobilacea char zinc chloride (ESZ) were prepared for the adsorption of methylene blue. Adsorption was studied as a function of pH (2-11), adsorbent dose (0.01-0.1 g), time (1-120 min), initial MB concentration (30-100 mg/L), agitation speed (120-180 rpm) and temperature (298-338 K). From the results obtained it was observed that with the increase in the pH value, the percentage MB removal increases from 48.62, 68.74 and 46.68% to 92.69, 99.71 and 95.33% for ESC, ESP and ESZ adsorbents, respectively. Hence the optimized pH value for ESC, ESP and ESZ is 8, 6 and 9 respectively. Additionally the percentage MB removal increased from 55.2, 56.19 and 58.25% to 95.81, 99.5 and 95.5% for ESC, ESP and ESZ adsorbents, respectively, by increasing adsorbent dose from 0.01 to 0.1 g. Hence the optimized adsorbent dose for ESC, ESP and ESZ is 0.05, 0.07 and 0.05 g respectively. The experimental data were analyzed by the Langmuir, Freundlich, and Temkin isotherms. Results showed that the maximum monolayer adsorption capacity of ESC, ESP and ESZ adsorbents for the adsorption of MB was 31.152, 21.929 and 37.037 mg/g at 318 K, respectively. The kinetic data were fitted to the pseudo-first-order, pseudo-second-order and intraparticle diffusion models. Adsorption of these adsorbents followed Langmuir adsorption isotherm models and pseudo-second-order kinetics. Thermodynamic parameters were evaluated to predict the nature of adsorption. These results indicate the endothermic and spontaneous nature of the adsorption process.
