(539g) Biosorption of Heavy Metals Using An Acid Activated Walnut Shell

In this research the removal of heavy metals [Pb (II), Cd (II), Zn (II)] ions from aqueous solutions by chemically modified walnut shell was studied. A response surface design analysis was performed to screen the variables affecting heavy metals removal efficiency. The effects of citric acid concentration, temperature and time of modification on the adsorption capacity of metals were examined in batch reactors. The walnut shell was characterized before and after modification by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA) and acid/base titration. Also, charge distribution of the walnut shell was determined by equilibrium. Plus pH and temperature effects on the adsorption capacity of Pb, Cd and Zn were investigated by adsorption isotherms that were conducted in batch adsorbers. The results of the surface design showed that metal ions adsorption is highly dependent on the modification time and less dependent on the modification temperature and citric acid concentration, accordingly. On the other hand, SEM micrographs revealed no significant changes in the surface of the walnut shell after modification. This indicates that the degree of modification does not impair the physical properties of the shell. In addition, total acid sites content of modified walnut shell decreased from 2.6 to 2.4 mmolg^-1 which was reflected in a less amount of carboxylic sites (from 0.78 to 0.56 mmolg^-1). These last findings were supported by FTIR and charge distribution; treated material showed less accentuated energy absorption peaks at wavelengths between 1750 and 1700 cm^-1 and the point of cero charge (PCC) changed 1 unit to more basic values of pH, which is attributed to lower concentration of carboxylic acids (COOH). TGA analysis showed a small shoulder at around 380 ºC and an intensive peak at around 350 ºC. These minor and major reactions can be attributed to the hemicelluloses and cellulose decomposition, respectively (Acikalin, 2011). In the other side, the sorption experiments reported, that, the heavy metals removal increased as pH rise from 3 to 7; resulting a maximum sorption capacity of 18, 12, 4 mgg^-1 of Pb (II), Cd (II) and Zn (II), respectively, for untreated walnut shell. The sorption capacity of modified walnut shell was 2% higher than the untreated material. Finally, it was found a slight increase onto the sorption capacity as the temperature rise from 15 to 35 ºC. These findings indicate that the untreated and modified walnut shell studied herein have a potential application in the removal of contaminants presents in aqueous phase.