Adsorption of hexa-valent chromium using treated wood charcoal--elucidation of rate-limiting process

In the present study, locally available wood charcoal was used as an adsorbent to remove Cr (VI) from water. It was found to be giving poor removal efficiency whereby only 19% of Cr (VI) was removed. Considering the fact that wood charcoal possesses a honeycomb structure, an acid treatment was tried with HCl, H2SO4 and HNO3. Treatment with concentrated hydrochloric acid has improved the removal efficiency of wood charcoal to 94%. Kinetic studies were carried out with various systemic parameters, namely initial Cr (VI) concentration (0.5, 1, 2 mg/L), adsorbent size (0.11, 0.18, 0.25, 0.36, 0.51 mm) and agitation speed (130 to 180 rpm) to understand and determine the equilibrium time, order of reaction, rate constants, diffusion coefficients, and to determine the maximum adsorption capacity and also the rate limiting process. It was found that the uptake of Cr (VI) onto wood charcoal reached equilibrium within the first 6 h of contact time. Isothermal studies explained by using the Freundlich model revealed that the maximum adsorptive capacity (Q(max)) of the treated wood charcoal is 677 microg/g, which is well within the standard/feasible value for a wood-based charcoal. The process limiting the rate of adsorption (rate limiting step) was analyzed using the kinetic data as well as using various systemic parameters such as initial Cr (VI) concentration, adsorbent size, and agitation speed was finally confirmed by the multiple interruption test. It was concluded that the adsorption process was controlled by film diffusion.