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Uptake and Fractionation of Thallium by Brassica juncea in a Geogenic Thallium-Amended Substrate

Rader, Shelby T., Maier, Raina M., Barton, Mark D., Mazdab, Frank K.
Environmental science & technology 2019 v.53 no.5 pp. 2441-2449
Brassica juncea, bioavailability, feldspar, heavy metals, isotope fractionation, isotopes, leaves, manganese, mica, planning, plant growth, pods, stems, thallium
This study shows thallium (Tl) concentrations in Brassica juncea (Indian mustard) tissue are more than an order of magnitude higher (3830 μg/kg) than that of the substrate (100 μg/kg) and are strongly influenced by the underlying mineralogy; i.e., Tl bioaccessibility depends on the mineral structure: K-feldspar > Mn nodule > hendricksite mica. The majority of Tl for all substrates is contained in edible parts of the plant, i.e., leaves (41% of total Tl, on average) ≥ flower stems (34%) > seed pods (11%) ≈ stems (10%) > flowers (3%). We also show that Tl isotope fractionation induced by B. juncea is substantial, at nearly 10 ε²⁰⁵Tl units, and generates systematic plant-specific patterns. Progressive plant growth strongly fractionates Tl isotopes, discriminating against ²⁰⁵Tl as the plant matures. Thus, ²⁰⁵Tl values are systematically higher in the early formed stem (ε²⁰⁵Tlₐᵥg = +2.5) than in plant elements formed later (ε²⁰⁵Tlₐᵥg = −2.5 to +0.1), which demonstrates the large degree of translocation and the associated effects during plant growth. This study establishes the potential of Tl isotopes as a new tool for understanding heavy metal (re)distribution during anthropogenic and geologic processes and the utility of such information in environmental and health-related planning and in phytomining or bioprospecting.