In Situ Sizing and Speciation of Trace Rare Earth Element-bearing Nanoparticles via Online Coupling of Hollow Fiber Flow Field-Flow Fractionation and Multiple Detectors

Abstract

The occurrence and mobility of REE-bearing nanoparticles (NPs) play a crucial role in REE enrichment processes, yet their characterization at trace concentrations (ng/mL-ppb level) remains challenging due to their heterogeneity and low abundance. Here, we present a high-resolution method combining hollow fiber flow field-flow fractionation (HF5) with UV-vis spectrometry (UV) and inductively coupled plasma mass spectrometry (ICPMS) for in situ size separation and speciation of REE-bearing nanoparticles in complex matrices. In order to evaluate and optimize to achieve high-resolution nanoparticle fractionation, the variables of carrier solution composition, focusing time, radial flow rate, and axial flow rate were investigated in detail. Applied to weathered horizons from the Nan’an HREE deposit, this method revealed distinct size distributions and concentrations of REE-bearing nanoparticles via HF5-ICPMS. Nanoparticles smaller than 80 nm dominated, accounting for 70.0% in the fully weathered horizon (FWH) and 87.8% in the semi-weathered horizon (SWH), with significantly higher concentrations in SWH than in FWH. Vertical distribution analysis showed that smaller nanoparticles correlate more strongly with REE enrichment. Correlations between REEs and Al, Fe, Mn peaks within the 20-80 nm size range indicated associations with clay minerals and Fe-Mn (oxyhydr)oxides. Additionally, HREE-bearing nanoparticles occurred at higher concentrations than LREE-bearing ones, consistent with deposit characteristics, confirming nanoparticles as a major occurrence form of REEs. HF5-ICPMS results agreed well with transmission electron microscopy (TEM) size data, demonstrating the method's reliability for nanoparticle size characterization in weathered horizons. This HF5-based approach provides a robust and applicable method for characterizing REE-bearing nanoparticles in complex regolith samples. It offers valuable insights into REE enrichment mechanisms and occurrence forms in IARDs, with broad implications for understanding nanoparticle-mediated REE mobility and enrichment in surface environments.