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.

Transport and distribution of residual nitrogen in ion-adsorption rare earth tailings.

A large amount of nitrogen remains in ion-absorption rare earth tailings with in-situ leaching technology, and it continually ends up in groundwater sources. However, the distribution and transport of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) across tailings with both depth and hill slopes is still unknown. In this study, the amount of NH4+-N and nitrate nitrogen (NO3--N) was determined in tailings, and a soil column leaching experiment, served to assess the transport and distribution following mine closure. Firstly, a high concentration of NH4+-N in the leachate at the initial leaching stage was detected, up to 2000 mg L-1, and the concentration of NH4+-N clearly diminished as time passed. Meanwhile, the NH4+-N contents remained relatively high in soil. Secondly, both the content of NH4+-N and NO3--N varied greatly according to vertical distribution after leaching lasting several years. The amounts of NH4+-N and NO3--N in surface soil were much smaller than those in deep soil, with 3-4 orders of magnitude variation with depth. Thirdly, when disturbed by NH4+-N, the pH not only diminished but also changed irregularly as depth increased. Fourthly, although the amount of NO3--N was smaller than that of NH4+-N, both their distribution trend was similar with depth. In fact, NH4+-N and NO3--N were significantly correlated but this declined from the knap to the piedmont. Based on these results, it is suggested that mining activity could cause nitrogen to be dominated by NH4+-N and acidification in a tailing even if leaching occurs over several years. NO3--N derived from NH4+-N transports easily and it becomes the main nitrogen pollutant with the potential to be a long-lasting threat to the environment around a mine.

Redistribution and chemical speciation of rare earth elements in an ion-adsorption rare earth tailing, Southern China.

Mining is an activity that will change the distribution and chemical speciation of rare earth elements (REEs), thus posing a serious threat to the natural environment. However, the distribution and chemical speciation of REEs in ion-adsorption rare earth tailings remain poorly understood. In this study, we investigated the contents and forms of REEs and associated geochemical behavior in rare earth tailings in southeast China. Total rare earth elements (TREEs) contents were lower while the ratios of light REEs (LREEs) to heavy REEs (HREEs) were higher in tailings than in an unmined area. In the unmined area, the distribution characteristics of TREEs and LREEs remained consistent, whereas HREEs differed with increasing depth. However, in the tailing area, the distribution characteristics of TREEs, LREEs and HREEs tended to be consistent, reflecting the outcomes of mining activities on vertical distribution characteristics of REEs. The REEs were dominated by residual and exchangeable forms in the unmined area, while residual and exchangeable REEs accounted for 80% and 20% of the TREEs, respectively, in the three tailings. Additionally, the exchangeable and carbonate-bound REEs increased but Fe/Mn oxide-bound and organic-bound REEs declined in the unmined area, whereas their distribution characteristics were irregular in the tailings. These results suggest that mining activity could curtail REEs contents and redistribute their chemical speciation, further altering geochemical behaviors in the tailings and posing serious risks to adjacent environments.