A critical evaluation of conventional kinetic and isotherm modeling for adsorptive removal of hexavalent chromium and methylene blue by natural rubber sludge-derived activated carbon and commercial activated carbon

The adsorptive removal of Cr(VI) and methylene blue (MB) was studied in a batch reactor using activated carbon (RAC), prepared from natural rubber waste, along with the commercial activated carbon (CAC). Maximum uptake of Cr(VI) and MB by the RAC was 21 and 30 mg g-1, respectively, whereas the corresponding uptake by CAC was 145 and 224 mg g-1. The kinetics of adsorption, however, was found to be faster in RAC than CAC. Both adsorbents were characterized by XRD, FT-IR, and FESEM-EDS. The predictability of various kinetic models, including the Weber-Morris model, was adversely affected by linearization. A multi-linear plot of adsorbed concentration versus square root of time failed to justify the multi-resistance hypothesis of mass transfer. Experimental kinetic data matched well with four surface reactions and an intraparticle diffusion model but showed substantial deviation from the numerical solution of another Fickian model incorporating mass balance and Langmuir isotherm.