Adsorption of Zinc(II) and Chromium(III) Ions by Modified Zeolites

Abstract

The results of a study of the adsorption of zinc(II) and chromium(III) ions by zeolites modified by HCl are presented. According to the data of elemental analysis and electron spectroscopy, the zeolites contain at least 75% heulandite in their composition. The removal of aluminum ions from the tetrahedral framework of the sorbent formed by AlO4 and SiO4 occurs upon modifying the zeolites by HCl. This leads to the destruction of the main component, heulandite, and compaction of the silicate layer. The time of establishment of adsorption equilibrium in the zeolite–aqueous solution system is 90 min. The isotherms of adsorption of zinc(II) ions are obtained in a temperature range 298–318 K at pH 5.6–5.8. It is shown that the adsorption of zinc(II) ions increases with growing temperature. The isotherms of adsorption of chromium(III) ions are obtained in a temperature range 298–318 K at pH 7.6–7.9. It is found that the adsorption of chromium(III) ions decreases with increasing temperature. The differential heats of adsorption are calculated based on the isosteres of adsorption. The increase in the adsorption of chromium(III) ions is accompanied by a decrease in the differential heat of adsorption from 36.9 to 26.8 kJ/mol. This may be associated with the nonuniformity of the surface of the adsorbent, due to what the metal ions are adsorbed on less active regions of the surface as far as the growth in the saturation of more active sites. The change in the differential heat of adsorption of zinc(II) ions with the growth in the degree of filling of the surface has a nonmonotone character. In the region of low degrees of filling of the surface, the differential heat of adsorption decreases, and then increases, which is probably determined by polymolecular adsorption presuming the presence of an adsorbate–adsorbate interaction. The efficiency of treatment of industrial wastewaters of the electroplating industry in the case of the use of the modified zeolites is over 97%.