Stellerite seeded facile synthesis of zeolite heulandite with exceptional aqueous Cd2+ capture performance
Clinoptilolite (HEU) mineral is by far the most common natural zeolite and has been described as excellent adsorbent for heavy metal ions and radioactive 137Cs+. Heulandite, possessing the same framework topology of clinoptilolite, has lower Si/Al ratio than clinoptilolite. Thus, superior performance is expected for heulandite on harmful ion removal due to its higher cation exchange capacity. But it was proved to be a great challenge to synthesize heulandite because of the harsh synthesis conditions and the poor reproducibility of the reported synthesis methods. Herein, heulandite was facilely obtained via a hydrothermal synthesis using natural stellerite (STI) zeolite as seed. It is the first time for natural stellerite zeolite to be used as seed, which however results in a different zeolite. The present results reveal that the phase selectivity for heulandite is sensitive to the crystallization conditions including SiO2/Al2O3 and Na2O/SiO2 ratios of the initial gel, seed content, and heating temperature. The pure heulandite can be well reproduced in a narrow crystallization field. As-synthesized heulandite shows much higher Cd2+ sorption capacity compared with the natural and synthesized clinoptilolite. Compared to the Na+ form of heulandite, the H+ form of heulandite shows a much better thermalstability, which makes the applications of heulandite in catalysis and gas separation possible.