Geochemical insights into plant uptake of Technology-critical elements: A case study on lettuce from European soils

While vegetable uptake of traditional metal contaminants is a well-studied pathway to human exposure and risk, a paucity of information exists on the uptake of emerging metal contaminants. This study evaluated the uptake of the Technology-critical elements (TCEs) gallium (Ga), germanium (Ge), niobium (Nb), tantalum (Ta), thallium (Tl), and rare earth elements (REEs) into lettuce cultivated in 21 European urban soils. For comparison, the uptake of cadmium (Cd) was also analysed. First, the uptake was predicted by multiplying soil concentrations with previously established bioconcentration factors (BCFs). Subsequently, multiple regression models incorporating geochemical variables as predictors were used to determine whether prediction accuracy could be improved. A "3-predictor model" incorporated soil TCE concentration, pH, and organic matter (OM), and a "7-predictor model" added data on clay content and the soil concentrations of Fe, Al, and Mn as well. With the exception of Cd, Ge, and Tl, the BCF approach provided unsatisfactory predictions (R² < 0.5), while the 7-predictor models yielded the best predictions, even when accounting for the greater number of predictors. While the most important predictors of uptake varied somewhat between the TCEs, the concentrations of TCEs in the soil generally explained the largest proportion of the variation. The least influential predictors in our dataset were [Mn_soil], [Fe_soil], and soil OM. Incorporating geochemical data generally improved the predictions of uptake by lettuce, and these findings underscore the need for more detailed characterisations of the uptake potential of TCEs by food plants and subsequent consequences for human health.