RESUMO
More efficient soil remediation technologies are highly anticipated to treat large quantities of heavy metal-polluted urban sites nowadays. Herein, a novel hydrothermal technology of converting heavy metal-polluted soils into zeolites for in-situ immobilizing heavy metals was proposed. The zeolites (analcime and cancrinite) could be synthesized hydrothermally with certain Na/Si and Al/Si ratios. The formed zeolites could manage to change their species and structure during zeolitization to accommodate different heavy metals in soil according to their size and charge. Since smaller-size Cu2+ was introduced, analcime and some cancrinite possessing small cages could be formed adaptively to immobilize the Cu2+ by replacing Na+ and forming Cu2+-OH and Cu2+-O. Whereas, cancrinite with large channels managed to form to immobilize the larger-size Cd2+ by forming Cd2+-O. Interplanar spacing variation of zeolites also corresponded to their structural change for accommodating different heavy metals. Leaching results showed the amounts of Cu and Cd leached from the synthesized zeolites were reduced to 0.005% and 0.05% respectively, reflecting a more stable immobilization of smaller heavy metals by small cages, in agreement with the results of distribution coefficient (Kd). Negligible effect of pH environment on the leaching rates further confirmed the stable structural immobilization of heavy metals by zeolites.
