Adsorption of Cd(II), Cu(II), Ni(II) and Pb(II) onto natural bentonite: study in mono- and multi-metal systems

Abstract

In the present study, removal of Cd2+, Cu2+, Ni2+ and Pb2+ from mono- and multi-metal ion aqueous solutions using natural Ca-bentonite was studied. Removal efficiency from synthetic aqueous solutions and real wastewater was investigated under various conditions such as variable concentration of metal ion, amount of clay, pH, and contact time. Increasing pH favors the removal of metal ions while the removal is rapid with maximum adsorption being observed within 20 min. Langmuir isotherm fits the data better in both single and multi-component systems. The monolayer adsorption capacities ( $$q_{m,i}^{o}$$ ) of bentonite for Cd2+, Cu2+, Ni2+ and Pb2+ in single metal systems were found as 31.25, 32.26, 26.32 and 85.47 mg g−1, respectively. The competitive adsorption equilibrium on a multi-component system (Cd2+/Cu2+/Ni2+/Pb2+) with different initial metal ion concentrations was also determined. For all studied metal ions, the adsorption capacities in the case of multi-component system ( $$q_{m,i}^{\text{mix}}$$ ) are lower than those obtained for the particular single metal system ( $$q_{m,i}^{o}$$ ). It was observed that the equilibrium uptake amounts of each metal Cd2+, Cu2+, Ni2+ and Pb2+ in the multi-component system onto bentonite decreased considerably with increasing concentrations of the other metals due to their antagonistic effect. Bentonite displays a high selectivity toward one metal in a multi-component system with an affinity order of Cu2+ > Ni2+ > Pb2+ > Cd2+ which is related to the first hydrolysis equilibrium constant. The values of $${{q_{m,i}^{\text{mix}} } \mathord{\left/ {\vphantom {{q_{m,i}^{\text{mix}} } {q_{m,i}^{o} < 1}}} \right. \kern-0pt} {q_{m,i}^{o} < 1}}$$ demonstrate the mutual competitive effect between heavy metals in multi-component systems.