Ecological risk of typical coal gangue-based molecular sieves in cadmium-contaminated soil remediation

To investigate the application of using typical gangue-based molecular sieves as stabilizers for the remediation of soil contaminated with heavy metal cadmium (Cd), we used high iron content X-type molecular sieves (Fe-NaX) and amino-modified X-type gangue-based molecular sieves (NH₂-NaX) as stabilizers, and simulated the conditions of acid rain by using a leaching solution with an initial pH value of 5.0 conducted column leaching experiments on the soil containing heavy metal Cd to study the release behavior and migration/diffusion patterns of Cd in the soil. The results showed that after adding Fe-NaX and NH₂-NaX molecular sieves, the pH, electrical conductivity (EC) and Cd concentration in the leachate initially increased rapid, then slowly decreased and stabilized. As the number of leaching cycles increased, the cumulative amount of Cd leached generally exhibited an overall upward trend, characterized by a rapid increase in the initial phase, followed by a slower rise. The parabolic equation, Elovich modified equation and Freundlich modified equation were used to fit the cumulative release of Cd. The correlation coefficients (<italic>R</italic>²) for all these models were greater than 0.98, indicating a good fit. Compared with the control group, the exchangeable Cd content in the leachate decreased with addition of stabilizers as some Cd was converted into stable residual forms of Cd. This result indicates that both Fe-NaX and NH₂-NaX type molecular sieves can effectively inhibit the leaching of heavy metal Cd from soil under acid rain conditions, thereby reducing its ecological risk. This study not only reveals the potential value of typical gangue-based molecular sieves as stabilizers for remediation of contaminated soil under acid rain conditions, but also provides a scientific basis for their wide application in the treatment of heavy metal-contaminated soils.