Synthesis and characterization of tannin-immobilized hydrotalcite as a potential adsorbent of heavy metal ions in effluent treatments

The modification of calcined hydrotalcite (HTC) by immobilizing tannin was investigated in the present work, in order to apply the modified media (TA-HTC) as an adsorbent in the removal of heavy metal ions (Cu(II), Zn(II) and U(11)) from industrial effluents. The adsorbent was characterized with FTIR, XRD, SEM, TG/DTG, surface area analyzer and potentiometric titrations. Batch experiments were performed as a function of process parameters such as agitation time, initial metal concentration, pH, ionic strength and adsorbent dose. The maximum adsorption was found at pH 6.0. The mechanism for the removal of metal ions by TA-HTC was based on ion exchange process. Experimental results showed that the adsorption of these metal ions was selective to be in the order of Cu(II)>Zn(II)>Cd(II). The process was very fast initially and maximum adsorption was observed with 3 h of agitation. The adsorption kinetics were investigated and kinetic parameters such as rate constant, external mass transfer diffusion and intraparticular mass transfer diffusion coefficients were evaluated. The adsorption process obeyed the intraparticular diffusion model. An increase of ionic strength of the medium caused a decrease in metal ion adsorption. Equilibrium isotherm data were analyzed by the Langmuir, Freundlich and Dubinin-Radushkevich equations using non-linear regression analysis. The best interpretation for the equilibrium data was given by the Langmuir isotherm and the maximum adsorption capacities were 81.47 mg g(-1) for Cu(II), 78.91 mg g(-1) for Zn(II) and 74.97 mg g(-1) for Cd(II). The adsorption efficiency towards heavy metal ion removal was tested using different industry wastewaters. The adsorption efficiency, regenerative and reuse capacities of this adsorbent were also assessed for four consecutive adsorption/desorption cycles and were found to retain the adsorption capacity. (C) 2008 Elsevier B.V. All rights reserved.