Synthesis of zeolites Na-A, Na-X, and analcime from crushed stone waste and their applications in heavy metal removal in aqueous media

Abstract

Crushed stone waste produced by the stone crushing in the production of aggregate for the construction industry is currently discharged in large stockpiles that facilitate the dispersion of the particles by wind currents and rainwater. Such dispersion leads to environmental issues due to suspended particles in the air and natural bodies of water, as well as dust deposited on vegetation, soil, and urban settlements neighboring the quarry. To upgrade these solids to the status of feedstock for the synthesis of materials, this work developed a conversion process for the crushed stone waste into pure zeolites Na-A, Na-X, and analcime. The high concentration of silicon in this waste encouraged this application. Mild synthesis conditions were established from the analysis of the effect of the SiO2/Al2O3 mass ratio and the crystallization temperature and time on the type of produced zeolite. Zeolites Na-X showed BET surface areas between 552 and 620 m2/g. Pb2+ and Cd2+ removal from aqueous solutions by zeolites Na-A and Na-X was investigated at different pHs through adsorption isotherms. The adsorption capacities were 555.6 and 454.5 mg Pb2+/g zeolite for zeolites Na-A and Na-X, respectively. 158.7 and 113.6 mg Cd2+/g zeolite were the corresponding capacities in the case of Cd2+.