Adsorption of chromium (VI) on low-cost adsorbents derived from agricultural waste material: a comparative study and experimental design

In this study, the Medlar Activated Carbon (MAC) and Cucumis Melo Activated Carbon (CMAC) were used as adsorbents for Cr (VI) removal by adsorption in batch mode from aqueous solutions. The experimental results obtained showed that the percentage of metal elimination increases with the increase of adsorbent dose, contact time and temperature but decreases with increase of initial pH and metal initial concentration. A 2(4) full factorial experimental design was employed to determine the optimum values and degree of importance of parameters. The process of Cr (VI) adsorption by studied adsorbents is strongly influenced by initial concentration of Cr (VI) and initial pH of the solution. The maximum values of the adsorption yield are obtained at low pH 1.5 (acid medium). The adsorption kinetics follow well the pseudo-second order model, and the external diffusion seems to be the decisive step in the process of adsorption of Cr (VI) ions by the adsorbents studied. The modelling of the isotherms showed that the Langmuir model appears to be the most consistent with the experimental data of Cr (VI) adsorption by the different adsorbents. The maximum adsorption capacity, determined from the Langmuir model at pH 1.5 and 25 degrees C, is 34.12 mg/g for MAC and 54.28 mg/g for CMAC. The thermodynamic parameters of the Cr (VI) adsorption on the biomasses indicate that the process is spontaneous, favourable and endothermic. The values of the activation energy (<7 kJ/mol) indicate that the adsorption is physical in nature.