Testing thallium phytoextraction using Silene latifolia and Biscutella laevigata on natural thallium-enriched and thallium-spiked substrates

Background and aims: Thallium is a non-essential trace element occurring as a contaminant from industrial pollution, but it is also economically valuable. Phytoextraction for thallium using hyperaccumulator plants offers a potential opportunity to remediate thallium-contaminated soil while obtaining a valuable product (then called 'phytomining'). Silene latifolia is among the most promising candidates for thallium phytomining, but its performance in this context remains untested. It was compared here with the better studied thallium hyperaccumulator Biscutella laevigata. Methods: Silene latifolia seeds were collected from Allchar (North Macedonia) and Ganges (France), while for B. laevigata the Ganges accession was used. Plants were cultivated under controlled conditions on substrate from the Allchar site in North Macedonia with natural high thallium enrichment and on artificially thallium-spiked substrate to obtain key parameters required for future upscaling of thallium phytomining. Furthermore, the possible effects of Spirulina as a biostimulant on thallium phytoextraction yield were evaluated. Results: Cultivated on thallium-enriched substrate for one month, S. latifolia (Allchar accession) attained up to 12,600 μg Tl g^–1 DW in the shoot, compared to 3920 μg Tl g^–1 DW when grown on thallium-spiked substrate. In the Ganges accession of S. latifolia, and particularly also in B. laevigata, notably lower shoot thallium concentrations were found. While biostimulants promoted plant biomass in all accessions analyzed, thallium yield in the shoots increased only in B. laevigata. Conclusions: Thallium phytomining has yet to be tested at field scale, but experimental results suggest high potential for the S. latifolia (Allchar accession). Substantial variation exists in phytoextraction potential between S. latifolia accessions, which can be exploited for plant selection, while a positive response to the biostimulant in thallium yield was observed only in B. laevigata.