Biomonitoring lanthanum in tree bark samples from urban and rural areas across Leicestershire (UK)

Background: The presence of lanthanum (La) in Leicestershire's (UK) topsoils and wild edible mushrooms exceeded the established soil screening level and could contribute to the maximum level of intake of lanthanides for vegetables. Tree bark was used to gain a better understanding of the environmental presence of this metal.

Methods: Bark samples (thickness of 2–6 mm) were collected from 96 trees from Leicester (n=55) and surrounding rural/suburban areas (41) at 1.50–1.80 metres from the ground to limit contamination from topsoil/dust. La was monitored by ICP-MS in cleaned/ground/homogenised samples mineralised with HNO₃/H₂O₂ [LoD=0.00068 ng/g dry weight (dw)]. The results were compared with previous studies performed on 106 mushrooms and 850 topsoils collected in the same areas.

Results: Higher levels were found in bark samples collected in the urban area (median and ranges, in ng/g dw): 9.679 (2.128-150.769) vs. 8.344 (1.815-59.801), which is in line with our previous observations performed on wild mushrooms and topsoils. Thus, our results could suggest a higher presence of sources of La in urban areas, attributed to the technological uses of this metal, for example, in catalytic converters and phosphors. The content of La varied between bark samples collected across the four cardinal subareas in which Leicester city was divided (median values, in ng/g): 38.664 (SE) > 17.104 (SW) > 9.128 (NE) > 6.707 (NW). A similar pattern was detected in mushrooms and topsoils, although higher levels were found in the southwest quadrant for both samples, respectively (1.286 and 19.621; both in µg/g).

Conclusions: The presence of La in Leicestershire could be related to traffic volumes, which are much higher in the city. In general, this presence is estimated to be similar or lower to those described in similar studies, e.g., in oak tree bark from Genoa, Italy (0.1-1.7 µg/g), which might indicate a lower airborne contamination by La in Leicester.