Adsorptive removal of hexavalent chromium and methylene blue from simulated solution by activated carbon synthesized from natural rubber industry biosludge

Abstract Activated carbon (AC) was prepared from biosludge (BS) of natural rubber waste to remove hexavalent chromium (Cr(VI)) and methylene blue (MB) from aqueous solution for the first time. Both BS and AC were characterized through proximate analysis, X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscope, energy dispersive X-ray spectroscopy, N 2 adsorption–desorption isotherms, and thermogravimetric analysis. Batch adsorption experiments were conducted by varying pH: 1-12, AC dosage: 2-18 g L−1, initial concentration: 50-900 mg L−1, contact time: up to 53 h, temperature: 20-50 °C. The removal efficiency of AC remained constant for a wide range of pH for both contaminants. Adsorption was found to be rapid as more than 80% of maximum capacity was exhausted within 2 h. The maximum adsorption capacity of Cr(VI) and MB was determined to be 36.01 and 31.14 mg g−1, respectively. Modeling of kinetic data confirmed pseudo-second order model as the best fit for both Cr(VI) and MB. Both external and intra-particle diffusion were found to contribute in adsorption mechanism. Electrostatic interaction plays a major role during these adsorptive mass transfer due to anionic species of Cr(VI) and cationic forms of MB. The Cr(VI) adsorption data followed Freundlich isotherm, whereas MB adsorption data fitted best with Langmuir isotherm. Thermodynamic studies revealed endothermic nature of adsorption for both pollutants enhanced adsorption capacity with an increase of temperature. Desorption of loaded AC with Cr(VI) or MB either in 0.2 (N) HCl or NaOH solutions was also studied.