Equilibrium, kinetic and thermodynamic modeling for the adsorption of heavy metals onto chemically modified hydrotalcite

Abstract

The adsorption of heavy metals (Cu(II), Zn(II) and Cd(II)) from aqueous solutions with tannin-immobilized calcined hydrotalcite (TA-HTC) was studied using batch experiment technique. The surface characterizations of the adsorbent were investigated. Kinetic modeling of removal of metals was done using a pseudo-second-order rate expression. The intraparticle mass transfer diffussion of metals on adsorbent represented the rate-limiting step, with the activation energy of 27.05, 24.22 and 21.96 kJ/mol for Cu(II), Zn(II) and Cd(II), respectively. Experimental results showed that the adsorption of metals was selective to be in the order of Cu(II)>Zn(II)>Cd(II). Equilibrium isotherm data for metal ions were analyzed by the Langmuir, Freundlich, Dubinin-Radushkevich and Scatchard equations. The Langmuir adsorption isotherm described the adsorption data very well at all studied temperatures. Thermodynamic parameters such as standard enthalpy (Δ<i>H</i>^◦), standard entropy (Δ<i>S</i>^◦), standard free energy (Δ<i>G</i>^◦), activation energy and isosteric enthalpy and entropy were calculated and the results are discussed in detail.