Synthesis and Characterization of NaX Zeolite from Coal Gangue and Its Efficacy in Cd and Pb Remediation in Water and Soil.

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