The suitability of Robinia pseudoacacia L. (black locust) leaflets as biomonitors of airborne microplastics
(1) University of Siena, Italy, (2) Istituto Nazionale di Geofisica e Vulcanologia, Italy, (3) University of Trieste, Italy, (4) Bioscience Research Center, Italy, (5) Trent University, Canada
Here we investigated the suitability of Robinia pseudoacacia L. (black locust) leaflets for monitoring the deposition of airborne microplastics (MPs, i.e., plastic particles <5 mm), including tire wear particles (TWPs). Leaflets of R. pseudoacacia were collected at rural asphalt roadside locations (RO, n = 5), far from residential and industrial areas, and urban parks (UP, n = 5) far from traffic roads (Siena, Italy). MPs were removed by washing the leaflets with deionized water and, after filtration, were examined and identified under a stereomicroscope based on standard criteria. Daily MP deposition was estimated from leaf mass per area. MPs were qualitatively analyzed for their polymers using micro-FTIR (Fourier Transform Infrared Spectroscopy). The mass magnetic susceptibility of samples, as well as the bioaccumulation of traffic-related potentially toxic elements (PTEs, namely Fe, Al, Cu, Zn, Ba, Cr, Sb) were also analyzed. There was a significant difference in the number of MPs accumulated at RO and UP, with a total amount of 2962 MPs at RO, mainly due to TWPs, and 193 MPs in UP, where TWPs were not found. However, the number of microfibres was higher in UP compared to RO (185 vs. 86). Daily MP deposition was estimated to range from 4.2–5.1 MPs/m2/d across UP and 29.9–457.6 MPs/m2/d across RO. The polymer types at RO were dominated by a mixture of natural and synthetic gum (80%) from TWPs, followed by 15% polyamide (PA) and 5% polysulfone (PES), while in UP the proportion of PES (44%) was higher than PA (22%) and polyacrylonitrile (11%). The mass magnetic susceptibility was very modest across all samples, but with much higher values (in the range of -0.55 to 2.80 10–8 m3/kg) at RO than in UP (-0.23 to -0.643 10–8 m3/kg). The content of PTEs was similar across sites, with the remarkable exception of higher concentrations of Sb, a well-known tracer of non-exhaust (brake wearing) vehicle pollution, at RO (0.308 ± 0.008 µgg/g dw) compared to UP (0.054 ± 0.006 µg/g dw). It is concluded that the waxy leaflets and easy determination of surface area make Robinia an effective biomonitor for airborne MPs, including TWPs.
Keywords: atmosphere; biomonitoring; black locust; microplastics; tire wear particles
