The Speciation and Distribution of Thallium (Tl) in the Hyperaccumulator Iberis
Optical (
A)
and XRF maps of Tl (B) and Ca (C) in the mid-leaf vein from an Iberis plant. Our observations noted that Tl was localized to veins and closely correlated to Ca.
Thallium (Tl) is a highly toxic element that EPA lists as a priority pollutant.
However, it is the fourth most expensive metal, and its low natural abundance coupled with
industrial demand makes recovery of Tl an appealing endeavor.
The toxicity of this element had not been studied until recently,
but Tl salts are understood to be highly toxic compounds-
small doses (8 mg kg
-1) can be lethal to a human.
The narrow margin between toxicity and therapeutic benefits lead to
the discontinuation of its original use as in treatments for ringworm and other skin infections.
Toxicity of Tl to terrestrial invertebrates and plants is 100
times higher than that of Cd. Thallium has two oxidation states,
monovalent - Tl(I) and trivalent - Tl(III). Thallium is
generally present in soil as Tl(I) and, as such, is an analog of K and Ca.
Therefore, Tl may be readily taken up by plants and
thereby interfere with the vital K-dependent processes (substitution of
K in the Na/K-ATPase).
Remediation of Tl-contaminated environments has not been developed. One potential implementation for terrestrial environments uses hyperaccumulator plants to extract Tl from soil. Phytoremediation of metal-contaminated soils has been limited, with the exception of Ni and As, by the low biomass and slow growth of hyperaccumulator plants. In the case of Tl, however, the large biomass plant Iberis intermedia could make phytoextraction feasible; its biomass potentially reaches 10 t ha-1. Reports show that I. intermedia can hyperaccumulate Tl > 10,000 mg kg-1 when grown in a soil containing only 16 mg kg-1. Through Steve Rock's collaboration with researchers at CSIRO in Adelaide, Australia, EPA grew I. intermedia in 20 mg kg-1 Tl-spiked soils, and examined the plants via micro-X-ray fluorescence (XRF) and micro-X-ray absorption spectroscopy to determine the speciation and distribution of Tl.
Contacts: Kirk Scheckel;
Steven Rock (EIMS#135569)
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