Sorption of hexavalent chromium ion onto natural swelling clay and acidified clay as a nano adsorbent: Kinetic and thermodynamic study

Natural clay (NC) from Makabaye (Far North Cameroon) was pre-treated according to the stocks method and the resulting powder was subjected to acid treatment (HCl at 0.5 M) to obtain acidified clay (AC) material. These materials (NC and AC) were subjected to morphological and structural characterization and then to successive adsorption (in batch mode) and desorption studies for hexavalent chromium until AC lost its adsorptive properties. These analyses showed the exfoliation of the clay after acidification. The influence of experimental parameters such as the variation in contact time, the mass of adsorbent, the pH of the solutions, the initial concentration of Cr (VI) ions were studied showed that each of the parameters influences the adsorption process. The results showed that adsorption equilibrium was reached after 20 min with both adsorbents. The optimum quantities of Cr (VI) ions adsorbed by the adsorbent were obtained for masses of 0.100 g. Maximum adsorption of Cr (VI) ions was obtained at pH = 3 with both adsorbents. This work also showed that by keeping the optimum parameters fixed, the quantity of chromium ions adsorbed increased significantly before stabilising at 15 mg/L with the two clay samples. Under these same optimum conditions, the maximum quantity adsorbed was obtained with the acidified clay and the natural clay, i.e. 29.41 mg/g and 27.78 mg/g respectively. The application of adsorption isotherms has shown that the Langmuir model (R² = 0.980 with AC and R² = 0.971 with NC) and Dubinin-Radushkevich model (R² = 0.917 with AC and R² = 0.991 with NC) better describes the adsorption mechanism of Cr (VI) ions than those of Freundlich and Temkin model. The kinetic study has shown that the pseudo-second order equation satisfactorily describes the adsorption mechanism (R² = 0.998 with AC and R² = 0.995 with NC). The thermodynamic study showed that the adsorption of Cr (VI) on the two adsorbents is spontaneous, exothermic with NC and endothermic with AC. The desorption study showed that the adsorbent (AC) could be regenerated to 98.32 % after 70 min under agitation at 150 rpm for a concentration of 1.5 M EDTA. The reused adsorbent showed desorption percentages ranging from 98.88 % (after the 1st recycling) to 85.56 % (after the 6th recycling). A very rapid decrease in the desorption of chromium by AC was observed from the 7th cycle of reuse (57.68 %) until the complete loss of this desorption at the 10th cycle (0.12 %).