The nature of thallium crystals in Brassica oleracea (kale): a synchrotron multi-technique investigation

Brassica cultivars have the ability to hyperaccumulate thallium when growing on soils contaminated with this element. Earlier research identified the presence of crystalline thallium deposits in the leaves of Brassica oleracea var. acephala. The aim of this study was to investigate the nature of these thallium crystals. A combination of synchrotron-based methods was used involving micro-X-ray fluorescence (µXRF) for the elemental distribution in the leaves and micro-X-ray diffraction mapping (µXDM) for the identification of the crystals. Thallium concentrates along the foliar margins, especially near vascular bundles, and dense congregations of minute thallium-rich crystals are observed in these areas. The thallium speciation is revealed to be nearly exclusively monovalent thallium, whilst the crystals are largely cubic thallium chloride, TlCl(s). The formation of thallium-rich crystals in the form of thallium chloride appears to be a tolerance mechanism similar to that of halophytes, and possibly a way in which excess thallium is expelled from leaves.