On the uptake of rhenium by plants: Accumulation and recovery from plant tissue

Rhenium (Re) is one of the scarcest and more broadly dispersed elements on Earth. Due to its paucity and high demand by the aerospace and petroleum industries, it is also one of the most valuable metals in the international market. In recent years, reports about the occurrence of Re in soils surrounding copper-molybdenum mines and its uptake by the local vegetation have fueled research on the feasibility of Re phytomining. Still, the mechanisms of Re accumulation and its potential retrieval from plant tissue have not been elucidated. Hence, in this work, infrared (IR) spectrometry and scanning electron microscopy (SEM) analysis were employed for the first time to assess how rhenium is accumulated in plants. Moreover, we have proposed a simple methodology based on dry leaf tissue disruption and water rising for recovering Re. The process was tested on the glasshouse-grown alfalfa plants and field samples (Acacia, white clover, and grass) collected in the vicinity of the Assarel-Medet copper mine (Bulgaria), where soil Re levels are high. Our findings demonstrate that following root absorption and translocation, Re was stored in the vacuoles of alfalfa leaf cells as water-soluble potassium perrhenate (KReO4). The solubility of the leaf stored Re compounds was further demonstrated by the proposed aqueous extraction procedure, as mixing 1 g of dry leaf in 30 mL of distilled water during 30 min sufficed to achieve almost complete Re recovery in the ensuing leachates of alfalfa (99.82%), Acacia (97.95%), white clover (99.15%), and grass (99.05%). Although additional research is necessary to confirm the effectiveness and practicability of the proposed Re retrieval at field scale, this study may prove instrumental to increase the economic and environmental viability of Re phytomining.