Removal of Hexavalent Chromium from Model Tannery Wastewater by Adsorption Using Mongolian Natural Zeolite

Abstract

We applied Mongolian natural zeolite to adsorptive removal of hexavalent chromium contained in tannery wastewater. At first, characterization was carried out for various kinds of natural zeolites obtained from Tsagaan Tsav and Urgon deposits in Dornogovi province, southeast part of Mongolia. From the X-ray diffractometry analysis, the zeolite samples were composed of mordenite, quartz, gismondine, clinoptilolite, and chabazite, and many of the samples contained clinoptilolite, which is the most abundant natural zeolite. The BET surface areas and the pore volumes of the samples were much lower than those of pure zeolites, and ranged from 15 to 160 m2·g–1 and from 0.03 × 10–6 to 0.1 × 10–6 m3·g–1, respectively. A part of the zeolite samples were modified by a metal cation, barium cation, in order to adsorb hexavalent chromium, which is anion of chromate in aqueous solution. The content of barium in the modified zeolite was much higher than that in the unmodified one, so that the successful zeolite modification could be confirmed. Secondly, batch equilibrium adsorptions of hexavalent chromium from aqueous solution, the model tannery wastewater, by the natural zeolites were carried out. Although the adsorption performances varied with the natural zeolite, all of the zeolites modified by barium cation could adsorb hexavalent chromium favorably and the fractional removal was over 0.9 at maximum, while the unmodified ones could not almost at all. The higher solution pH gave the higher adsorption performance and, then, main adsorbate was CrO42–, since this species is dominant in higher pH according to the speciation of hexavalent chromate anion. Consequently, we proposed to utilize Mongolian natural zeolite to treat the tannery wastewater containing hexavalent chromium.