Uptake and transport mechanisms of rare earth hyperaccumulators: A review

Due to their use in a number of advanced electronic technologies, Rare earth elements (REEs) have recently emerged as a key strategic resource for many nations worldwide. The significant increase in demand for REEs has thus greatly increased the mining of these substances, but this industrial-scale expansion of mining activities also poses potential risks to the surrounding environment, flora, fauna, and humans. Hence efficient REE remediation is one potential remediation process involving in situ clean-up of contaminated soil which has gained much attention in recent years, due to its low cost and lack of secondary pollution. However, some crucial aspects of phytoremediation, such as the precise-mechanisms of absorption, transport, and tolerance of REEs by hyperaccumulators -are poorly understood. This review briefly discusses the environmental risks associated with excess REEs, the efficacy of phytoremediation technologies coupled with, appropriate hyperaccumulator species to migrate REEs exposure. While REEs hyperaccumulator species should ideally be large-biomass trees and shrubs suitable for cropping in subtropical regions areas, such species have not yet been found. Specifically, this review focuses on the factors affecting the bioavailability of REEs in plants, where organic acids are critical ligands promoting efficient transport and uptake. Thus the uptake, transport, and binding forms of REEs in the above-ground parts of hyperaccumulators, especially the transporters isolated from the heavy metal transporter families, are discussed in detail. Finally, having summarized the current state of research in this area, this review proceeds to discuss current knowledge gaps and research directions. With a focus on hyperaccumulators, this review serves as a basis for future phytoremediation strategies of rare earth mining-impacted environments and addresses ecosystem/environmental degradation issues resulting from such mining activity.