Hydrothermal conversion of model polluted soil into zeolite P to in-situ immobilize heavy metals via zeolitization and its microstructure behavior

Abstract To develop a new technology to remediate pollution sites stably and effectively, hydrothermal conversion of polluted soil (model soil) into zeolite P for in situ immobilization of Heavy Metals (HM) was carried out. Zeolite P could be synthesized from soil hydrothermally with a large range of Al/Si ratio (0.33–1.0) in 48 h at 200 °C. With immobilizing heavy metals (Cr, Zn and Cd), the crystallization of Zeolite P (Cr-ZP, Zn-ZP and Cd-ZP) was delayed with the order of Cr >Zn >Cd because the stronger the ligand effect of heavy metals have, the more heavily the nucleation and growth were influenced. During immobilization via zeolitization, a self-adaptive structuring behavior was confirmed by EDAX, BET and Rietveld refinement analyses: zeolite P structured more [AlO₄]⁻ to immobilize HM because the flexible 8-member-Ring channels (8 mRs) with higher Al/Si ratio could complex with more HM readily. The encapsulated HM occupied the original Na positions (Na1 and Na2) of zeolite P competitively because of their different cation size, e.g., the smallest Cr³⁺ could substitute all Na1 because Na1 is closer to the Framework Oxygen (FO); the largest Cd²⁺, however, substituted Na1 (9%) and Na2 (14%) evenly. The shrunken 8 mRs due to heavy metal immobilization also caused different chemical environments for different heavy metals immobilized because the smaller heavy metal cation could get closer to the framework, and also coordinated with more FO.