Abstract
The removal behaviour of rare earth element (REE), (Sc, Y, La-Lu), ions onto nanoscale zerovalent iron (nZVI) particles has been investigated. Batch sorption isotherms were conducted using REE-bearing acid mine drainage (AMD) and a range of different synthetic REE solutions, which were exposed to nZVI at 0.1–4.0 g/L. Maximum adsorption capacity of Yb and La was 410 and 61 mg/g respectively (1000 mg/L LaCl3 and YbCl3 starting concentration, initial pH = 4.5, T = 294 K), the highest currently reported in the literature. Aqueous REE removal to ultratrace concentrations (<1 µg/L, >99.9% removal) was also recorded after 30 min (the first sampling interval) exposure of ≥0.5 g/L nZVI to 10 mg/L aqueous REE solutions (nitrate counterion). Similar rapidity and near-total removal ability was recorded for the exposure of nZVI to the AMD, however, a greater nZVI concentration was required, with the removal of all REEs (with the exception of La, Ce, Nd and Gd) to <1 µg/L when exposed to nZVI at 4.0 g/L for 30 min. In all systems nZVI was selective for the removal of HREE ions in preference to LREE ions, with the mechanism determined using HRTEM-EDS and XPS analysis as via surface mediated precipitation. Overall the results demonstrate nZVI as exhibiting great promise as an effective and versatile agent for simultaneous REE ion recovery and fractionation.
Highlights
The rare earth elements (REEs), (Sc, Y, La-Lu), are a group of chemically similar elements that have gained increasing importance in recent years due to their unique functionality in many modern materials and systems, including electronics, optics, superconductors, R.A
Given the mean free path of Fe is equivalent to approximately 6 atomic layers, this detection of Fe0 in the X-ray photoelectron spectroscopy (XPS) analysis volume indicates that the oxide thickness of the nanoscale zerovalent iron (nZVI) is less than approximately 5 nm, which corroborates the aforementioned nZVI oxide thickness measurement using HRTEM
The following can be concluded: (1) REE removal from solution can be achieved using relatively low nZVI concentrations, with removal to ultratrace levels (i.e. < 1 μg/ L) at the first sampling time (30 min) recorded when nZVI were exposed to synthetic REE nitrate solutions (REE starting concentration 10 mg/L) and acid mine drainage (AMD) (REE starting concentrations ranging from 36 to 200 μg/L) using nZVI concentrations of 1.0 and 4.0 g/L respectively
Summary
The rare earth elements (REEs), (Sc, Y, La-Lu), are a group of chemically similar elements that have gained increasing importance in recent years due to their unique functionality in many modern materials and systems, including electronics, optics, superconductors, R.A. Annual growth in the global demand for REEs is estimated to be between 8 and 11% and as such the global importance of the REEs is becoming similar to more established commodities like oil, steel and coal [2]. Despite such demand a recent report by the Natural Environmental Research Council (NERC) has defined the REEs as the most critical chemical group in terms of global supply risk [3]. In addition the chemical similarity of the REEs dictates that even if REEs are captured their separation from each other remains a significant technical and economic challenge
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