Tree bark biomonitoring of lutetium contamination in Leicestershire, England

Tree bark was used to gain an understanding of the atmospheric presence of lutetium (Lu) in Leicestershire (UK). Bark samples were collected from 96 trees in Leicester (n=55), as well as from the surrounding rural/suburban areas (41). Lu was monitored using ICP-MS in cleaned/ground/homogenised samples mineralised with HNO₃/H₂O₂ [LoD=0.000654 ng/g dry weight (dw)]. The results were compared with those of previous studies performed on 106 mushrooms and 850 topsoils collected from the same areas. Slightly higher levels were found in bark samples collected in rural areas (median and ranges, in ng/g dw): 0.584 (0.402-1.071) vs. 0.580 (0.182-2.118), which is in line with the trend detected in the topsoils [0.123 (0.069-0.162) vs. 0.117 (0.084-0.182), mg/kg]. This may be logical as metals released into the air from pollution sources will eventually reach the soil surface. However, the content of Lu was higher in mushrooms collected in the main urban area [0.347 (0.285-293.837) vs. 0.196 (0.780-8.116), ng/g dw)], which could be explained by the small effect of topsoils on the levels of Lu detected in mushrooms, as they showed no statistical correlation (p-value=0.506). Furthermore, our previous observations showed minimal anthropic contamination in the topsoils, which could explain the similar concentrations of Lu in the bark of trees monitored in the urban and rural areas. Lu content varied between bark samples collected across the city of Leicester (median values, in ng/g): 0.892 (SE) > 0.646 (SW) > 0.585 (NE) > 0.544 (NW). Similarly, lower concentrations were found in the soil samples monitored in parks located in the northwest quadrant. The calculated enrichment factors were 2.15 and 2.58 using scandium relative to the upper continental crustal concentration, suggesting particulate phase deposition of Lu at natural background levels in both areas of Leicestershire. However, further studies are required as the atmospheric transport of Lu and other lanthanide elements is poorly understood.