Simultaneous Removal of Pb and Zn Heavy Metals Using Fly Ash Na-X Zeolite and Its Carbon Na-X(C) Composite

Pure zeolite (Na-X) and a zeolite-carbon composite (Na-X(C)) were investigated as adsorbents of heavy metals-Pb and Zn from an aqueous solution. These materials were synthesized from fly ash-a waste from conventional hard coal combustion. Both solids were characterized using XRD, SEM-EDS, nitrogen adsorption/desorption, particle size and elemental composition analyses. The adsorption study was performed at pH 5 in the systems containing one or two adsorbates simultaneously. The obtained results showed that the pure zeolite was characterized by a more developed surface area (728 m/g) than its carbon composite (272 m/g), and the mean pore diameters were equal to 1.73 and 2.56 nm, respectively. The pure Na-X zeolite showed better adsorption properties towards heavy metals than its Na-X(C) composite, and Zn adsorbed amounts were significantly higher than the Pb ones (the highest experimental adsorption levels were: for Zn-656 and 600 mg/g, and for Pb-575 and 314 mg/g, on the Na-X and Na-X(C) surfaces, respectively). The zinc ions are exchanged with the cations inside the zeolite materials structure more effectively than lead ions with a considerably larger size. In the mixed systems, the competition between both heavy metals for access to the active sites on the adsorbent surface leads to the noticeable reduction in their adsorbed amounts. Moreover, the hydrochloric acid was a better desorbing agent for both heavy metals, especially Pb one (desorption reached 78%), than sodium base (maximal desorption 25%).