Cd(II) and Pb(II) absorbed on humic acid-iron-pillared bentonite: Kinetics, thermodynamics and mechanism of adsorption

Abstract

In this study, humic acid-iron-columnar bentonite (HA-Fe-PILC) obtained after HA adsorption on Fe-PILC was characterized by SEM, BET, XRD, zeta potential, FTIR, and XPS and reused for Cd(II) and Pb(II) removal from aqueous solution and mining wastewater through intermittent experiments. The characteristics results proved that the HA-Fe-PILC was successfully synthesized with a specific surface area and isoelectric point of 168.3 m2/g and 2.6, respectively. It contained HA of 31.26 mg/g and HA was stably combined with Fe-PILC under the solution pH value of 2−6. It mainly encompassed bonds of CO, Al-O, Si-O-Fe, and HOH. The adsorption of Cd(II) and Pb(II) ions increased with the increase of pH from 2 to 6 but decreased with the increasing ionic strength of NaCl in the solution. The adsorption process was endothermic and spontaneous in nature and fitted by the pseudo-second-order kinetic model and Langmuir model very well. The maximum adsorption capacities of Cd(II) and Pb(II) obtained by Langmuir model were 31.34 and 95.23 mg/g at the temperature of 25℃, respectively. The Cd(II) and Pb(II) concentrations in actual mine wastewater after treatment by HA-Fe-PILC could meet the wastewater discharge standard of China. The spent HA-Fe-PILC could be effectively regenerated by 1.0 mol/L HCl; after four regeneration, the Cd(II) and Pb(II) removal by HA-Fe-PILC remained at 60.2 % and 81.6 %. The Cd(II) and Pb(II) adsorption on HA-Fe-PILC was carried out by means of physical adsorption, ion exchange, chemical bonds, and surface complexation. Based on these results, the HA-Fe-PILC was an efficient and reusable adsorbent for Cd(II) and Pb(II) removal from wastewater. Overall, this study is a promising demonstration for the synthesis of a novel and reusable adsorbent for environmental applications, especially for the treatment of heavy metal in wastewater effluents.