Identification of potential plant species hyperaccumulating light rare earth elements (LREE) in a mining area in Minas Gerais, Brazil.

Phytoextraction of rare earth elements (REE) from contaminated soils has gained importance during the last few decades. The Poços de Caldas municipality in Brazil is known for its mineral richness, including large reserves of REE. In this study, we report light REE (La, Ce, Sm, Pr, and Nd) in soils and plants collected in an area. Composite soil samples and plant individuals were collected, and total concentrations of LREE in soils were determined by wavelength dispersive X-ray fluorescence (WDXRF). The plant available LREE concentrations in soils were estimated upon the acetic acid method (F1 fractions) of the stepwise sequential extraction procedure, together with plant content that was analysed by inductively coupled plasma mass spectrometry (ICP-MS). The total sum concentrations of tested LREE in soils varied from 5.6 up to 37.9 g kg-1, the bioavailable fraction was ca. 1%, and a linear relationship was found between them. The only exception was Sm, whose availability was lesser and did not show a linear relationship. The concentration of LREE in non-accumulator plants varied from 1.3-950 mg kg-1 for Ce, La 1.1-99 mg kg-1, Sm 0.04-9.31 mg kg-1, Pr 0.1-24.1 mg kg-1, and Nd 0.55-81 mg kg-1. The concentration of LREE among shoots did not show a linear relation either with the available fraction or total content. The screening also revealed Christella dentata (Forssk.) Brownsey & Jermy, Thelypteridaceae family, as a promising hyperaccumulator species. The concentrations of LREE among shoots of six individuals of this species were in the ranges from 115 to 1872 mg kg-1 for Ce, La 190-703 mg kg-1, Sm 9-48 mg kg-1, Pr 32-144 mg kg-1, and Nd 105-478 mg kg-1.

X-ray fluorescence spectroscopy (XRF) applied to plant science: challenges towards in vivo analysis of plants.

X-ray fluorescence spectroscopy (XRF) is an analytical tool used to determine the elemental composition in a myriad of sample matrices. Due to the XRF non-destructive feature, this technique may allow time-resolved plant tissue analyses under in vivo conditions, and additionally, the combination with other non-destructive techniques. In this study, we employed handheld and benchtop XRF to evaluate the elemental distribution changes in living plant tissues exposed to X-rays, as well as real-time uptake kinetics of Zn(aq) and Mn(aq) in soybean (Glycine max (L.) Merrill) stem and leaves, for 48 hours, combined with transpiration rate assessment on leaves by an infrared gas analyzer (IRGA). We found higher Zn content than Mn in stems. The latter micronutrient, in turn, presented higher concentration in leaf veins. Besides, both micronutrients were more concentrated in the first trifolium (i.e., youngest leaf) of soybean plants. Moreover, the transpiration rate was more influenced by circadian cycles than Zn and Mn uptake. Thus, XRF represents a convenient tool for in vivo nutritional studies in plants, and it can be coupled successfully to other analytical techniques.