Characterization of Neodymium Speciation in the Presence of Fulvic Acid by Ion Exchange Technique and Single Particle ICP-MS.

It has been well known that the free ion concentration of metals plays a vital role in metal bioavailability. However, measurement of this fraction is still not easy over years of development. Nowadays, rare earth elements (REEs) are drawing more attentions as an emerging contaminant due to their wide applications in our daily life. To analyze the free ion concentration of neodymium (Nd), we adopted ion-exchange technique (IET) to investigate the changes on Nd free ion concentration in the presence of fulvic acid (FA). With the dynamic mode of IET analysis, the concentrations of Nd free ion were in the range of 0.85-36.8 × 10-8 M at the total Nd concentration of 5 × 10-7 M when FA varied from 0.4 to 10 M. However, these concentrations were 3-58 times higher than the one calculated by WHAM 7.0, which may be due to the particulate Nd spontaneously formed in solution. With single particle ICP-MS analysis, we found 0.25%-2.36% of Nd was in the form of colloids when the total Nd concentrations varied from 8.5 × 10-9 to 4.7 × 10-7 M, with the average particle sizes in the range of 26.5-39.2 nm. The presence of FA significantly decreased the number of Nd colloids, but increased the average particle size. Under the TEM, we found that Nd colloids were amorphous, with the size less than 200 nm. The present study provided a relatively new perspective on REE speciation in water. The natural organic matters not only affect the free ion concentration of Nd, but also influenced the size and numbers of Nd colloids in solution.

Controls on rare-earth element transport in a river impacted by ion-adsorption rare-earth mining.

Rare-earth elements (REEs) are known to be a group of emerging pollutants, but the geochemistry of REEs in river waters in ion-adsorption rare-earth mining areas has attracted little attention. In this study, samples of the <0.45 µm and 0.22-0.45 µm (large colloids) water fractions and acid-soluble particles (ASPs) were collected from a river impacted by ion-adsorption rare-earth mining activities. The roles of ligand complexation, colloid binding, and particle adsorption in REE transport and distribution were also investigated. Results showed higher concentrations of REEs in the <0.45 µm fraction of all sampling sites (3.30 × 10-2-9.42 µM) compared with that in the control site (1.21 × 10-3 µM); this fraction was also characterized by middle REE enrichment at upstream sites, where REEs are mainly controlled by the <0.22 µm fraction (55%-94% of the species found in the <0.45 µm fraction) and ligand complexation (REE3+, REE(SO4)+, and REE(CO3)+). At downstream sites, heavy REE enrichment was observed, which was largely determined by binding to large colloids (68%-83% of the species found in the <0.45 µm fraction) and adsorption to particles (>90% of the acidified bulk water). Furthermore, REE patterns indicated that the REE-associated large colloids were mineral or mixed mineral-organic matter (OM) at upstream sites and OM-dominated or functionalized at downstream sites. The particles were mainly coated by inorganic matter substances (e.g., Fe/Al oxyhydroxides). In summary, our results reveal that REE patterns provide a useful tool to study the fate of REEs in ion-adsorption rare-earth mining catchments.