Effect of Zeolitization of CFBC Fly Ash on Immobilization of Cu2+, Pb2+, and Cr3+

Abstract

Two zeolitization processes, hydrothermal and molten-salt, were used to examine the effect of zeolitization of circulating fluidized-bed combustion (CFBC) fly ash on the immobilization of Cu2+, Pb2+, and Cr3+ in this study. As a consequence of zeolitization, most of the dissolved Cu2+, Pb2+, and Cr3+ had been immobilized in the synthetic aluminosilicate-aggregates-enriched matrixes under hydrothermal conditions, reaching maximum immobilization efficiencies of ∼89.1%, ∼82.4%, and ∼99.1%, respectively; whereas under molten-salt conditions, the synthetic cancrinite-enriched matrixes were most effective to immobilize Cu2+, Pb2+, and Cr3+ with maximum immobilization efficiencies of ∼94.5%, ∼50.7%, and ∼38.3%, respectively. Examinations of the synthetic matrixes by X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDS), and sequential EDTA extractions suggest that physicochemical reactions responsible for the immobilization of heavy metal ions were ascribed to the speciation of insoluble species such as carbonates/silicates, hydroxides, and oxides and especially to the encapsulation of such species in aluminosilicates aggregates or cancrinite-enriched agglomerates. Furthermore, it is also proved that the speciation of soluble chromate in molten-salt processes contributed to the unexpected low immobilization efficiency of Cr3+ and that the Ca-enriched CFBC fly ash is an appropriate candidate for immobilization of Cu2+ and Pb2+. Both zeolitization processes could be employed in solidification/stabilization of soils and sediments contaminated by heavy metals.