Investigating the deposition of fibrous zeolite particles on leaf surfaces: A novel low-cost method for detecting the presence of airborne hazardous mineral fibers

Naturally occurring fibrous minerals, such as erionite, can pose a significant threat to human health when disturbed and subsequently respired. Understanding the spatial abundance and characteristics of these hazardous fibrous minerals in ambient air is crucial for minimizing human exposure and assessing risk. Conventional detection methods for airborne hazardous mineral fibers, such as those developed for asbestos, are of limited utility in environmental settings where fiber concentrations are low and different fiber types may be present and can be costly especially when monitoring large areas over long periods of time. This study presents an innovative methodology for detecting and identifying the presence of airborne naturally occurring fibrous zeolites, using leaf surface deposition sampling, SEM-EDX analysis for the detection and assessment of elemental composition, and TEM-SAED with continuous rotation diffraction (MicroED) to determine their crystallographic unit cell parameters. In total, 309 fibrous zeolite particles (FZPs) were identified on a range of tree leaf surfaces across 80 % of the sampling sites located close to both active and disused zeolite quarries in the Taupo Volcanic Region, New Zealand. The FZPs displayed various morphologies including aggregates, bundles, and fibril-like structures. Of the FZPs detected, 92.2 % were < 5 µm in length. Tetrahedral Si:(Si+Al) ratio results indicated that 40 % of the FZPs were in the reference range for zeolite mordenite. TEM-SAED plus MicroED analysis resulted in 61 % of tested FZPs indexed to unit cell parameters that matched with mordenite. This research demonstrates the potential of leaf sampling as a cost-effective method for detecting airborne FZPs while the MicroED data can be utilized for distinguishing between different types of airborne fibrous zeolites in ambient air.