Abstract
Alkaline fusion is a pivotal process influencing the cost of synthesizing zeolite from coal gangue. This study examined the effects of alkaline fusion temperature (X (1)), treatment duration (X (2)) and the NaOH/coal gangue weight ratio (X (3)) on the composition and properties of the products, as well as their adsorption capacities for Cd(2+) (q (e,Cd)) and Pb(2+) (q (e,Pb)). Response surface methodology (RSM) was employed to analyze the interactions among these factors, and the adsorption mechanisms for Cd(2+) and Pb(2+) were investigated using X-ray diffraction, scanning electron microscopy-EDS, Fourier transform infrared, X-ray photoelectron spectroscopy, and N(2) adsorption-desorption techniques. The results reveal that (1) under optimized conditions-X (1) of approximately 800 °C, X (2) of around 2.8 h and X (3) of 1.2-the maximum q (e,Cd) and q (e,Pb) for the synthesized NaX zeolite can reach 181.3 and 419.9 mg/g, respectively. (2) The RSM models indicate that increasing the X (3) value can lower the required X (1). For q (e,Cd) and q (e,Pb) of 150 and 350 mg/g, respectively, with X (2) fixed at 2 h, increasing X (3) from 0.976 to 1.134 and from 0.900 to 1.289 enables a reduction in X (1) from 800 to 600 °C. (3) NaX zeolite primarily adsorbs Cd(2+) and Pb(2+) through ion exchange, allowing these ions to enter the zeolite's cage structure. Pb(2+) can also precipitate as hydrocerussite (Pb(3)(CO(3))(2)(OH)(2)) within the zeolite channels, while Cd(2+) has a more significant impact on the [SiO(4)] and [AlO(4)] tetrahedra. (4) The synthesized NaX zeolite effectively reduces the exchangeable Cd content in contaminated soil from 3.51 to below 1.5 mg/kg. The remediation performance of the NaX zeolite for Cd and Pb in water and soil can be further enhanced by optimizing its Si/Al ratio and pore structure.