Removal of hexavalent chromium from aqueous solution using activated carbon prepared from walnut shell biomass through alkali impregnation processes

Walnut (Juglans regia) is a commonly used nutrient industrial crop but the shell of the walnut has no economic value. Hence to revamp the waste walnut shell biomass to useful product, activated carbon (AC) was prepared from J. regia shells by impregnating with NaOH. Different ACs were prepared by varying the impregnation ratio of char:NaOH as 1:1 (AC1), 1:3 (AC2), and 1:5 (AC3). The effect of impregnation ratios on the adsorptive properties of ACs for the adsorption of hexavalent chromium Cr(VI)] was studied. The ACs were characterized by SEM, surface functionality, and zero point charge. Langmuir, Freundlich, Temkin, and Dubinin–Radushkevitch isotherm were used to interpret the batch equilibrium data. The adsorption of Cr(VI) onto ACs followed Langmuir isotherm model. Kinetic data followed pseudo second-order rate equation. Intraparticle diffusion model and Boyd plot were used to study the mechanism of the adsorption reaction. The adsorption was both by film diffusion and intraparticle diffusion. The rate-controlling step was predicted as external mass transfer. Thermodynamic parameters were also estimated. Overall, AC with higher impregnation ratio (AC3) possessed better adsorption properties compared to AC2 and AC1.