Effects of Simulated Lung Fluid (SLF) on fibrous zeolites through real-time SC-XRD investigations

Zeolites are minerals widely distributed in many geological contexts and, due to their unique properties, are currently used in a multitude of processes. Among them, the fibrous zeolite erionite is carcinogenic to humans via inhalation, and its interaction with biological systems has been extensively investigated over the past decades. However, the reasons for its toxicity are still unclear, and other fibrous zeolites are currently being studied by the scientific community. This study investigated the chemical and structural modifications of three natural fibrous zeolites (offretite, mordenite, and erionite) in contact with simulated lung fluid (SLF) as a function of interaction time. The results show that all tested zeolites immersed in SLF behave similarly, exchanging the extra-framework cation Ca2+ with the Na+ from the solution. Thus, similarly to erionite, mordenite and offretite fibres can also release calcium in the lung environment. The refined chemical composition of offretite pointed to an unbalanced formula, with a significant lack of positive charges due to Ca2+-Na+ substitution. Such a lack could be balanced by protons attached to framework oxygen atoms, originating Brønsted acid sites. This finding is of paramount importance since surface protons and OH groups could modify the acidity of the zeolites, potentially increasing their chemical reactivity and, consequently, enhancing the carcinogenic response.