Adsorption experiments of REE onto kaolinite at 0.025 and 0.5 M NaCl

The rare earth elements (REE, La-Lu, Y) are currently considered critical metals as a result of their widespread use in modern technology, especially in the green energy sector, and because their supply chain is dominated by China (Goodenough et al. 2017, and references therein). Ion adsorption REE deposits are the main worldwide source of the heavy REE (HREE: Tb, Dy, Ho, Er, Tm, Yb, Lu) and Y (Sanematsu and Watanabe 2016). They form after the weathering of granitic bedrocks that contain igneous and/or hydrothermal REE-bearing minerals (Sanematsu and Watanabe 2016). The acidic solutions percolating through the profile release the REE from these minerals, which can be complexed with the ions present in these solutions. Later, the REE are incorporated into phosphates, oxides, or are adsorbed onto the 1:1 layer silicates kaolinite and halloysite (Sanematsu and Watanabe 2016). We carried out a series of experiments with the aim of gaining further insight on the processes that facilitate the adsorption of REE onto kaolinite in ion adsorption REE deposits.

The experiments consisted of introducing kaolinite powder in a solution with low (0.025 M) and high (0.5 M) ionic strength (I) NaCl, following a solid/ratio of 2.5 g/L (Coppin et al. 2002), and 100 ppb REE. The starting material was purified kaolinite powder (‘China clay BP light kaolin’ provided by Imerys^®), with a 2 µm average particle size, a loss on ignition (LOI) value of 11.16 wt.%, and a ζ-potential of −36.6 mV. The solution was prepared using Fisher Laboratory® reagent grade NaCl (pH∼6) and the multi-element calibration standard 8500–6944 from Agilent^® (10 ppm Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sc, Sm, Tb, Th, Tm, Y, Yb). Two experiments were performed, one lasted 5 consecutive days, with one sampling per day, and the other lasted one day, with one sampling after 2, 6 and 24 h. In addition, to study the behaviour of the kaolinite in the solution, a series of blank experiments without REE was also carried out. The samples were centrifuged at 2500 rev min^–1 for 40 min, the supernatant was extracted and diluted with 4% HNO₃, and was then analysed by inductive coupled plasma-mass spectrometry (ICP-MS). Finally, the tubes were then introduced into a desiccator at 40°C to allow the solution to evaporate and ultimately analyse the kaolinite powder.

The REE composition of the solutions was relatively steady with time, with no significant increase or decrease in REE concentrations, indicating rapid adsorption consistent with the results of research by Coppin and coauthors (Coppin et al. 2002). In all experiments, the REE from La to Lu and Y behaved similarly. At low I concentrations of 25–40 ppb of the individual REE were obtained, whereas at high I no adsorption or even mild desorption was observed (more than 100 ppb was analysed in the solution). In contrast, Sc and Th were always found in low concentrations in the solution (<5 ppb at high and low I).

These preliminary experiments show that even at low concentrations in the solution (100 ppb instead of 1 ppm in (Coppin et al. 2002)) REE can be adsorbed onto kaolinite. In addition, sorption is more favourable in low I solutions, while high I solutions may lead to desorption (as reported by Moldoveanu and Papangelakis 2012).