Adsorption of Methylene Blue, Methyl Orange, and Crystal Violet on Microporous Coconut Shell Activated Carbon and Its Composite with Chitosan: Isotherms and Kinetics

In this work, two adsorbents, namely coconut shell activated carbon (CSAC) and a composite of CSAC with chitosan termed as CHS-CSAC, were prepared. The two adsorbents were tested for their adsorption capacity for methylene blue (MB), methyl orange (MO), and crystal violet (CV) from their aqueous solutions under batch adsorption studies. The adsorbents were characterized by Brunauer–Emmett–Teller (BET), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The adsorbents were found to be microporous with specific active surface areas of 542.06 m2/g and 498.67 m2/g for CSAC and CHS-CSAC, respectively. The effect of pH, adsorbent dosage, and contact time was tested in sequential order for each adsorbate-adsorbent combination. For both adsorbents, maximum adsorption capacity values for the two cationic dyes (MB and CV) were obtained at a pH of 10. For MO, which is an anionic dye, maximum adsorption capacity was seen at a pH of 3 for both adsorbents. The optimal initial concentration for MB, MO, and CV were 45 mg/L, 40 mg/L, and 70 mg/L, respectively at the optimal values of dosage (10 mg) and contact time (2 h), for both the adsorbents. The batch adsorption data obtained at the optimal conditions were best fitted to Langmuir isotherm and pseudo-second-order and Elovich model kinetics for all the combinations. Superior performances of the adsorbents were observed in terms of maximum achievable adsorption capacity for each dye. The maximum loading capacity of MB, MO and CV were 217.35 mg/g (CSAC), 105.2 mg/g (CHS-CSAC) and 320.5 mg/g (CSAC), respectively. For the cationic dyes, the maximum adsorption capacity values were almost equal. The presence of rate-limiting steps other than intra-particle diffusion was envisaged in the adsorption mechanism study for the cationic dyes (MB and CV). Intra-particle diffusion was found to be the major rate-limiting step in the case of the anionic dye (MO) adsorption.