In-situ dehydration study of the Sr-, Cd- and Pb-exchanged natrolite

Abstract

The removal of Sr, Cd, and Pb from nuclear and industrial waste is important as these are harmful to living organisms and the environment. Immobilization of these ions in a zeolite framework is a simple and suitable method. However, zeolites are easily dehydrated at high temperatures. Therefore, the environmental changes around these adsorbed cations and water molecules in the zeolite framework must be explored for effective immobilization and waste removal. In this study, we investigated the structural changes in fully Sr-, Cd-, and Pb-exchanged natrolites (NAT) from room temperature to 350 °C using in situ synchrotron X-ray powder diffraction and Rietveld analysis. In the thermogravimetric analysis, Sr-NAT showed a gradual weight loss up to 210 °C, whereas Cd- and Pb-NAT showed a two-step weight loss in the ranges 90–280 °C and 100–180 °C, respectively. Sr-, Pb-, and Cd-NAT exhibited low thermal expansions with the thermal expansion coefficients of −3(1) × 10−6, −1.0(7) × 10−6, and 1(2) × 10−6 K−1, respectively, at the initial stage of increasing the temperature. During the dehydration process, the coefficients of Sr- and Cd-NAT were −2.7(7) × 10−4 K−1 up to 300 °C with a 2.9% volume contraction and −5.3 × 10−4 K−1 up to 150 °C with 2.7% volume contraction, respectively. At high temperatures, structurally, the Sr2+ and Cd2+ cations had six- and seven-coordinated bonding with framework oxygens and extra-framework species, whereas Pb2+ cations had three- and five-coordinated bonding. In contrast, the extra-framework water molecules in Sr-NAT had three to five bonds, Cd-NAT had five, and Pb-NAT had six. The chain rotation angle of the secondary building units (T5O10) increased in all cases, indicating that the channel shape becomes more elliptical during dehydration. Sr- and Pb-NAT were amorphized at 350 °C and 150 °C, whereas Cd-NAT remained intact. We concluded that Sr- and Pb-NAT were not thermally stable owing to the order-disorder transition of Sr2+ and high-disorder distribution of Pb2+, respectively. Our findings provide a fundamental understanding of the structural changes and mechanism of thermal stability in natrolites containing hazardous elements.