Recovery of rare earth elements by nanometric CeO2 embedded into electrospun PVA nanofibres

Rare earth elements (REEs) are critical raw materials with a wide range of industrial applications. As a result, the recovery of REEs via adsorption from REE-rich matrices, such as water streams from processed electric and electronic waste, has gained increased attention for its simplicity, cost-effectiveness and high efficacy. In this work, the potential of nanometric cerium oxide-based materials as adsorbents for selected REEs is investigated. Ultra-small cerium oxide nanoparticles (CNPs, mean size diameter ≈ 3 nm) were produced via a precipitation-hydrothermal procedure and incorporated into woven–non-woven polyvinyl alcohol (PVA) nanofibres (d ≈ 280 nm) via electrospinning, to a final loading of ≈34 wt%. CNPs, CNP–PVA and the benchmark material CeO2 NM-212 (JRCNM02102, mean size diameter ≈ 28 nm) were tested as adsorbents for aqueous solutions of the REEs Eu3+, Gd3+ and Yb3+ at pH 5.8. Equilibrium adsorption data were interpreted by means of Langmuir and Freundlich data models. The maximum adsorption capacities ranged between 16 and 322 mgREE gCeO2−1, with the larger value found for the adsorption of Yb3+ by CNP. The trend of maximum adsorption capacity was CNPs > NM-212 > CNP–PVA, which was ascribed to different agglomeration and surface area available for adsorption. Langmuir equilibrium constants KL were substantially larger for CNP–PVA, suggesting a potential higher affinity of REEs for CNPs due to a synergistic effect of PVA on adsorption. CNP–PVA were effectively used in repeated adsorption cycles under static and dynamic configurations and retained the vast majority of adsorptive material (>98% of CeO2 retained after 10 adsorption cycles). The small loss was attributed to partial solubilisation of fibre components with change in membrane morphology. The findings of this study pave the way for the application of CNP–PVA nanocomposites in the recovery of strategically important REEs from electrical and electronic waste.


= 2.303 10
Eq. (S2) Where A is the absorbance, ρ is CeO2 (= 7.28 g cm -3 ), l is the optical path length (= 1 cm) and c is the concentration of ceria suspension (g mL -1 ). The intersection of the extrapolated linear part of of Equation S1 (Tauc plot) gives an estimation of the optical band gap ( Figure S1b).
Determination of optical particle size. When the particle size becomes smaller than the exciton radius, i.e. ≈ 8 nm for CeO2 (Ramasamy, Mohana and Rajendran 2018), quantum confinement leads to sizedependent enlargement of the band gap and results in a blue-shift in the absorption edge. The particle size can be determined by the following equation (Goharshadi, Samiee and Nancarrow 2011) Where Eg,b is the bulk band gap (3.19 eV for ceria), r is the particle radius (m), is the reduced plank constant, δ is the bulk dielectric constant and me and mh are the effective masses of electron and hole, respectively. They can both be expressed based on the mass of free electron m0, as me = mh = 0.4m0. When r is very small (comparable to Bohr radius), the third factor in the equation can be neglected. As all variables of Eq. S3 are known, the equation can be solved for r, yeilding r = 2.3 nm. This value is considered to be the median of a number-average particle size (Dieckmann et al. 2009).

S3. Physico-chemical characterisation of NM-212
The material NM-212 has been the subject of extensive characterisation efforts (Singh et al. 2014, Gosens et al. 2014, Römer et al. 2019. A summary of NM-212 physico-chemical properties is given in Table S1.  (Singh et al. 2014), 2 measured in this study Figure S3 shows the elugram determined with AF-FFF by UV-Vis detection at λ = 300 nm and the calculated z-average hydrodynamic diameter dH determined via DLS. The measured dH ranged between 100 and 350 nm, with an average dH = 174 nm and a PDI = 0.02. The average dH was calculated by averaging the dH values determined by the full width at half maximum FWHM of the UV-Vis elugram (Caputo et al. 2019). The PDI was calculated as (σ/dH,mean) 2 where dH,mean is the mean dH value and σ is its standard error.

S4. Evaluation of TEM micrographs
CNPs. Particle identification was carried out with the ImageJ software using the Particle Sizer plugin and setting ellipsoid as fitting shape. Selected statistical parameters of the size distribution (mean, median, PDI) are shown in the table below. The particles' specific surface area was found by applying Eq. S4 (Baldim et al. 2018 Where s is the dispersity of the distribution, D is the median Feret min diameter and ρ is the CeO2 density (≈ 7.28 g cm -3 ).
CNP-PVA. Unlike for CNP, the short and long axis length of nanoceria agglomerates in PVA fibres were determined manually. Figure S4b reports a selection of TEM micrographs showing nanoceria agglomerates. Figure S4b. TEM micrographs of CNP-PVA nanofibres with nanoceria agglomerates.

S6. Morphological change of CNP_PVA upon water immersion
SEM micrographs of as spun CNP-PVA and CNP-PVA_w (after 4-hour immersion in water) are reported in Figure S6a. In water, PVA electrospun membranes underwent a morphological reorganization from solid fibers to a highly hydrated gel, which collapsed once they were left drying. However, CNP agglomerates remained enclosed in the newly formed PVA hydrogel. Figure S6a. SEM micrographs of CNP-PVA: as spun CNP-PVA nanofibers (left), and after water immersion imaged using secondary electrons (center) and backscattered electrons (right) with inverted colours (PVA: white) PVA electrospun membranes have a very high specific surface area and swell extensively in water. Due to the loss of soluble components, the mass of the membrane decreases ( Figure S6b). After 4 hours of water exposure, pure PVA membranes have lost 35% of the original mass, while CNP-PVA membranes have lost only 18%. This finding suggests that the mass loss can be mostly ascribed to PVA and not to CNPs.