Removal of toxic elements from aqueous solution using bentonite modified with l-histidine

Abstract

This study proposes the use of bentonite modified with l-histidine for the removal of Cu, Co, Cr, Fe, Hg, Ni, U and Zn from aqueous solutions such as those impacted by acidic drainage. The surface areas of natural bentonite and bentonite–histidine were 73.8 and 61.2 m2 g−1, respectively. Elemental analysis showed an increase in the amount of carbon (0.258%) and nitrogen (0.066%) for the bentonite–histidine. At a fixed solid/solution ratio, the operating variables affecting the adsorption of metal ions from aqueous solution such as pH, initial concentration, contact time and temperature were studied in batch mode. The Freundlich isotherm model yielded a better fit than the Langmuir for the adsorption of Cu, Co, Ni and Zn, implying adsorption on a heterogeneous surface. Adsorption kinetics followed a pseudo-second-order model, suggesting chemisorption as the rate-limiting step. The apparent activation energy was greater than 40 kJ mol−1 for Cu, Zn, Ni, Co and U, which is characteristic of a chemically controlled reaction. Thermodynamic constants ΔG and ΔH showed that the adsorption of metals was endothermic and spontaneous. Adsorption of heavy metals onto bentonite–histidine was efficient at low pH values, meaning that the adsorbent could be useful for remediating acid mine water.