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

Abstract

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 (ε²⁰⁵Tl_avg = +2.5) than in plant elements formed later (ε²⁰⁵Tl_avg = -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.

Figure 1.

[Tl] of plant parts for various Tl-amended substrates. Numbers next to individual plant parts denote the average [Tl] for that plant part in μg/kg. A.-B. depicts Tl added as a NIST-997 standard solution at two concentrations. C.-E. depicts Tl added as three various mineral substrates. Each mineral substrate was added to a total concentration of 20 mg/kg per pot. Control plants were below detection (not shown here). Note the two scales to more accurately compare high-concentration (A.-B.) and low-concentration (C.-E.) plants.

Figure 2.

Distribution of total Tl contents (Table 1) within Brassica juncea from five unique Tl-amendments. Amendments shown here include two concentrations of NIST 997 Tl standard solution, amazonite (feldspar) from Amelia Courthouse, VA, hendricksite (mica) from Sterling Hill, NJ, and manganese nodule, NOD-A-1, a USGS reference material.

Figure 3.

ε²⁰⁵Tl values of plant parts for two NIST-997 Tl-amended substrates. NIST-997 ε²⁰⁵Tl = +0.0. Numbers next to individual plant parts denote the average ε²⁰⁵Tl values for that plant part.

Figure 4.

ε²⁰⁵Tl values measured for various parts of B. juncea after 12 weeks of growth in a controlled greenhouse. Tl addition was added incrementally utilizing a NIST 997 Tl standard solution with a known isotopic value of ε²⁰⁵Tl = +0.0. Shown here are the results of both the 20 μg/kg NIST 997 solution and 100 μg/kg NIST 997 solution trials.