Structural properties of zeolites and their application in the removal of selected heavy metals from wastewater

Abstract

The increasing industrialization in Uganda has resulted into discharge of partially treated effluents into the environment. These effluents are characterized with a number of pollutants including but not limited to toxic metals such as lead, cadmium, and others. These toxic metals have serious negative impacts onto the environmental ecosystems not to mention the significant health risks that they pose to human health. Strategies have been proposed for metal removal but most of the available techniques are costly and technologically intensive for the small scale industries to setup. The adsorption technique is a promising approach in the removal pollutants from wastewater, due to its high versatility and ease of operation. However, the efficiency of this technique is low, and thus efficient adsorbents are still being sought. This study was aimed at designing zeolite based adsorbents for application in the removal of toxic metals from effluents.

Natural zeolites samples were collected from Eastern Uganda with the help of a geologist. Some of the natural zeolites were modified in the laboratory to enhance their adsorption properties. The structure of the natural, modified and used zeolites were characterized using scanning electron microscopy, energy dispersive spectroscopy and powder X-ray diffraction (PXRD). ImageJ was used to determine the particle size distribution of zeolites. SEM results showed that the modified zeolites possessed clear grain boundaries compared to the natural and used zeolites. In addition, ImageJ analysis showed that the modified zeolites exhibited a broad particle size distribution (2 µm – 158µm) compared to the natural zeolites (0.2µm – 4.1µm). EDX results confirmed presence of the expected cations in the natural zeolites alongside other trace metals which were not detected in the modified zeolites. Moreover, EDX showed presence of Pb2+, Fe2+ and Zn2+metals in the used modified zeolite implying that the metals were removed from the wastewater and adsorbed onto the adsorbent. PXRD results showed stellerite as the major phase (86 %) in the natural zeolites.

Under the optimal conditions, the efficiency of metal removal from spiked aqueous solutions containing lead, iron and zinc was found to be 84.0 %, 98.2 % and 74.8 %, respectively. When the modified zeolites were tested in the removal of metals from selected real environmental samples, the removal efficiency for Pb, Fe and Zn were 73.8 %, 95.5 % and 64.5 %, respectively. Results obtained using regenerated zeolites showed a slight reduction in performance of 1-4%.