Kinetic and Equilibrium Studies of Cr(VI), Cu(II) and Pb(II) Removal from Aqueous Solution Using Red Mud, a Low-Cost Adsorbent

The present work utilized raw red mud and hydrochloric acid-modified red mud (RMA) as adsorbents for Cr(VI), Cu(II) and Pb(II) removal from aqueous solution. pH, X-ray fluorescence, infrared spectrometry and porosity characteristics using gas adsorption were performed for characterization of adsorbents. Batch experiments were conducted to determine the influence of solution pH, temperature, initial metal ion concentration and adsorbent dose on the kinetics (0-120 min) of the metal ions removal. Equilibrium data were analyzed using Langmuir and Freundlich isotherms. Monolayer adsorption capacities of raw and acid-modified adsorbents were found to be 9.615 and 9.542 for chromium, 78.125 and 77.519 for copper and 52.083 and for lead, respectively. Acid activation has only marginal influence on the monolayer capacity of copper and chromium adsorption, but influences lead adsorption. Pseudo-second-order kinetic model suited well than other models like pseudo-first-order and intraparticle diffusion models examined. Equilibrium was attained in 10 min for chromium and 20 and 40 min on average for copper and lead, respectively, at all the conditions used in the study. The order of metal ions interaction with the surface as well as the rate of uptake was chromium > copper > lead. These findings revealed that raw and RMA serve as effective adsorbents for the removal of Cr(VI), Cu(II) and Pb(II) from aqueous solutions.