Real-Time Observation of Fibrous Zeolites Reactivity in Contact with Simulated Lung Fluids (SLFs) Obtained by Atomic Force Microscope (AFM)

Abstract

Inhalation of fibrous erionite particles has been linked to malignant mesothelioma. Accordingly, erionite is considered the most carcinogenic mineral. The reactivity and the nature of erionite biotoxicity has been the subject of intensive research. Despite very close chemical and structural relationships between erionite and offretite, the reactivity of offretite in lung fluids remains unknown. In this paper, the interaction of erionite and offretite surfaces with simulated lung fluids was investigated by means of in situ atomic force microscope (AFM). To simulate different environments in the lungs, artificial lysosomal fluid (ALF) and Gamble's solution were used. In ALF (4.15 < pH < 4.31) the dissolution of erionite and offretite surfaces was detected, as well as an evident removal of particles (mainly attributed to impurities) from the crystal faces. Instead, the growth of a layer of a yet unknown phase on the surface of both zeolites was observed during the interaction with Gamble's solution (7.4 < pH < 8.48). The thickness of this layer reached a few tens of nanometers and covered all the observed areas. The understanding of the observed processes is of paramount importance, since they could be potentially involved in the mechanisms triggering the toxicological effects of erionite fibres.

Surface and Bulk Modifications of Fibrous Erionite in Mimicked Gamble’s Solution at Acidic pH

“…The structural modifications observed in the present work are in reasonable agreement with reference data [43,44] albeit differences are apparent. The combined uptake of Na and release of Ca has been observed consistently, regardless of pH and simulated lung fluid formulation in the case of prismatic erionite-Ca [44]. However, no structural data of the incubated samples were reported to date for describing the on-going process at the atomic level.…”

“…On this basis, the preferential release of Si observed in our experiments for incubation times longer than one hour might be due to previous de-alumination of the erionite surface by weathering processes that weakened the silicate framework. In addition, for zeolites like erionite, where Si/Al ratio is higher than 3.0, it was reported that the selective Al removal leads to the formation of un-dissolvable amorphous silicate framework [44,45,67]. Accordingly, our HR-TEM investigation revealed the occurrence of an amorphous rim on the fibre surface of pristine erionite (see below), similarly to what observed in many regulated asbestos amphiboles [56,68].…”

“…It must be pointed out that Fe could potentially arise from leaching of Fe-bearing nontronite particles adhering at the fibre surface [35] (Figure 2d,e) and might eventually be incorporated into the neo-formed hydrated sulphates detected by XRPD. In addition, previous work showed that the structure of this clay is very stable under acidic condition [65], even more than the zeolite framework [44,66]. The leaching of Si and Al framework cations increased with incubation time (up to 1170 (311) mg/kg and 130 (1) mg/kg, respectively).…”

“…Recent dissolution experiments, performed on erionite incubated in artificial simulated lung fluids (SLF), revealed the occurrence of structural modifications consisting of complex EF cations exchange processes with the SLF [43,44] and their relocation within the erionite cage [43]. Moreover, a partial amorphization of the fibre surface due to depletion in aluminium and extra-framework cations [45] and the growth of a layer of an unknown phase on the surface of prismatic erionite [44] were also reported. This neo-formed phase was supposed to hinder the surface reactivity of the mineral.…”

Simulated biological fluids – a systematic review of their biological relevance and use in relation to inhalation toxicology of particles and fibres

“…Frequently, ALF is used in parallel studies with other SLFs to allow comparisons to be made. It has been used to assess a wide range of materials including the solubility of airborne particulate matter (Wiseman and Zereini 2014;Polezer et al 2019), fly ash (Bourliva et al 2020), bioaccessibility of PAHs (Gao et al 2019) and rare earth elements (Gao et al 2020), mobility of Pt group elements (Zereini et al 2012), the solubility of chrysotile asbestos contained within fragments of break-debris (Boyles et al 2019), zeolite surface behaviour (Giordani et al 2019), NP aggregation (Fashina et al 2013;, drug delivery systems (Mulla et al 2017), and biotransformations of graphene oxide (Liu et al 2018;Qi et al 2018).…”