27-5 The Influence of Heavy Metal Redox State on Plant Accumulation and Isotopic Fractionation using Thallium

Session: Environmental Geochemistry and Health (Posters)

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Naturally occurring heavy metals can be toxic to humans and the environment, even in low concentrations. This is especially true for metals that can be easily transferred into biological material, where biomass may be used for biomonitoring and remediation, phytomining, or may serve as a vector for introduction into the food chain. However, the influence of various geochemical parameters on plant biosignatures is still poorly understood, especially for some heavy metals. Thallium (Tl) is one such metal, which occurs in two oxidation states (Tl⁺ and Tl³⁺), has two stable isotopes (²⁰³Tl and ²⁰⁵Tl), and is naturally present at very low concentrations in geologic material, typically around 1 mg/kg. The ingestion of Tl can result in acute sickness in humans even in low concentrations, with roughly 15 mg/kg being lethal. The magnitude of negative health effects is partly controlled by the oxidation state of Tl, with oxidized Tl (Tl³⁺) being significantly more toxic than the reduced form (Tl⁺). Several Brassicaceae family plants, including Brassica juncea (B. juncea), are common vegetables or crops and have been shown to hyperaccumulate a variety of heavy metals, including Tl. However, it is unclear how the oxidation state of Tl may impact bioavailability, plant biosignatures, and ultimately, plant toxicity. Here we present data from greenhouse trials to evaluate the effects of Tl redox state on plant signatures within B. juncea.  

Fifteen individual plants were cultivated, with three treatment groups based on the substrate along with a control group: 1) reduced Tl (TlNO₃), 2) oxidized Tl (Tl(NO₃)₃), and 3) equal proportions of Tl⁺ and Tl³⁺. Concentrations within plant parts of individuals grown in Tl spiked soil ranged by more than an order of magnitude, from ~1.7 mg/kg to ~66 mg/kg. In general, the highest concentrations were found in the leaves and the lowest in the flowers, with plants grown in Tl⁺-amended soils having higher concentrations than those of the Tl³⁺ group, with the mixed treatment group displaying intermediate values. These data point to the potential role redox has on bioavailability of various metals, with implications for metal sourcing, remediation, and public health planning. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8687

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