Adsorptive removal of Cr(VI) from aqueous solution using rice husk and rice husk ash

This study investigated the removal of hexavalent chromium (Cr(VI)) by rice husk and rice husk ash from aqueous solution using different doses of adsorbent (5-25 g/L), initial Cr(VI) concentration (5-100 mg/L), pH (2-8), and contact time (5-150 min). Maximum Cr(VI) removal was observed at pH 2. The results revealed that Cr(VI) removal efficiency increases with an increase of the adsorbent dose, but the removal efficiency decreases with an increase of initial Cr(VI) concentration, while keeping all the other parameters constant. The efficiencies of rice husk and rice husk ash for Cr(VI) removal were 94.0% and 88.4%, respectively, from solutions containing 5.0 mg/L of Cr(VI) at a pH of 2 and adsorbent concentration of 20 g/L after 150 min of equilibration. The pseudo-second-order model was superior to pseudo-first-order model in adsorption and was controlled by chemisorptions and intraparticle diffusion process. Among Freundlich and Langmuir isotherms, the latter had a better fit with the experimental data. The maximum Langmuir adsorption capacities were 3.78 and 2.27 mg/g for rice husk and rice husk ash, respectively. Overall, the results suggest that both rice husk and rice husk ash are efficient low-or no-cost adsorbent, useful for the removal of Cr(VI) from aqueous solution.