# An insight into REEs recovery from spent fluorescent lamps: Evaluation of the affinity of an NH4-13X zeolite towards Ce, La, Eu and Y

Francesco Colombo, Riccardo Fantini, Francesco Di Renzo, Gianluca

Malavasi, Daniele Malferrari, Rossella Arletti

► To cite this version:

Francesco Colombo, Riccardo Fantini, Francesco Di Renzo, Gianluca Malavasi, Daniele Malferrari, et al.. An insight into REEs recovery from spent fluorescent lamps: Evaluation of the affinity of an NH4-13X zeolite towards Ce, La, Eu and Y. Waste Management, 2024, 175, pp.339-347. 10.1016/j.wasman.2024.01.023. hal-04533587

HAL Id: hal-04533587

https://hal.umontpellier.fr/hal-04533587v1

Submitted on 11 Apr 2025

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Research Paper

# An insight into REEs recovery from spent fluorescent lamps: Evaluation of the affinity of an NH₄-13X zeolite towards Ce, La, Eu and Y

Francesco Colombo<sup>a, *</sup>, Riccardo Fantini<sup>a</sup>, Francesco Di Renzo<sup>b</sup>, Gianluca Malavasi<sup>a</sup>, Daniele Malferrari<sup>a</sup>, Rossella Arletti<sup>a, *</sup>

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## ABSTRACT

The constantly increasing demand of Rare Earth Elements (REEs) made them to be part of the so-called “critical elements” indispensable for the energy transition. The monopoly of only a few countries, the so-called balance problem between demand and natural abundance, and the need to limit the environmental costs of their mining, stress the necessity of a recycling policy of these elements. Different methods have been tested for REEs recovery. Despite the well-known ion-exchange properties of zeolites, just few preliminary works investigated their application for REEs separation and recycle. In this work we present a double ion exchange experiment on a NH₄-13X zeolite, aimed at the recovery of different REEs from solutions mimicking the composition of liquors obtained from the leaching of spent fluorescent lamps.

The results showed that the zeolite was able to exchange all the REEs tested, but the exchange capacity was different: despite Y being the more concentrated REE in the solutions, the cation exchange was lower than less concentrated ones (16 atoms p.u.c. vs 21 atoms for Ce and La solutions), suggesting a possible selectivity. In order to recover REEs from the zeolite, a second exchange with an ammonium solution was performed. The analyses of the zeolites show that almost all of Ce and Eu remain in the zeolite, while nearly half of La and Y are released. This, once again, suggests a possible selective release of REEs and open the possibility for a recovery process in which Rare Earths can be effectively separated.

## 1. Introduction

Rare Earth Elements (REEs) are a group of seventeen elements, fifteen of which are the Lanthanides with the addition of scandium and yttrium. Their demand is constantly increasing due to their extensive use in many technological applications such as computer memory, DVDs, rechargeable batteries, autocatalytic converters, super magnets, mobile phones, LED lighting, superconductors, glass additives, fluorescent materials, phosphate binding agents, solar panels and magnetic resonance imaging (MRI) agents (Balaram, 2019).

A European foresight study on Critical Raw Materials (Bobba et al., 2020) shows how REEs are part of a group of raw materials fundamental for the energy transition. The report displays how the use of some REEs for the mentioned technologies will more than double from 2030 to 2050. The risk in the supply of these elements is higher than others because they have a very high demand rate against the number of known ores. Moreover, as reported in the study, the REEs market is mastered by China which is the world's largest producer, consumer, and exporter of REEs (e.g., the 98% of the EU's supply of REEs is provided by China). To achieve resource security, it is crucial to diversify the supply from

primary and secondary sources enabling reduced dependency on a single country and, most importantly, improving resource efficiency and circularity (European Commission, 2020). REEs waste management demands a nuanced approach due to their intricate properties and environmental sensitivity. Principles guiding their management underscore the imperative of recycling to mitigate depletion and reduce environmental impact. Practical implications necessitate sophisticated extraction techniques, stringent recycling methodologies, and robust disposal strategies strongly dependent on the waste in which REEs are contained. Implementing efficient recovery methods and stringent waste disposal protocols are critical steps to curbing environmental degradation while sustaining the supply chain for these crucial elements.

A lot of work has been done to find and characterize REEs-rich waste materials from which to recover these elements; among them, phosphogypsum, red muds (bauxite processing residue), mine tailings, coal and incinerator ashes, metallurgical slags, wastewater and different electronic and electrical equipment wastes (WEEE) have been identified as suitable candidates (Binnemans et al., 2015). Phosphogypsum, the calcium sulphate hydrated waste from the production of phosphate fertilizers, occurs in large quantities and can contain a lot of different

<footnote><sup>a</sup> Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia, Modena, Italy</footnote>

<footnote><sup>b</sup> ICGM, University of Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: francesco.colombo@unimore.it (F. Colombo), rossella.arletti@unimore.it (R. Arletti).</footnote>
REEs, mainly depending on the source rock (Cánovas et al., 2019); moreover, it may also contain radionuclides, making it a material of environmental and health concern. The concentration of REEs in red muds depend strongly on the lithologies of the bauxite host-rock (Borra et al., 2016) which can be of three types: Tikhvin-type bauxites (0.5 %) lying on aluminosilicate rocks, the “classical” lateritic bauxites (88 %) and karst bauxites (11.5 %), deposits lying on carbonates which may contain the highest amounts of REEs. Waste materials like acid mine drainage (AMD) (Ayora et al., 2016; González et al., 2020; Hedin et al., 2019; Li and Wu, 2017), mine tailings (Zhang et al., 2014) and ashes (Blissett et al., 2014; Funari et al., 2016) have a REEs concentration strongly related to the primary material, thus each case study requires a proper evaluation.

WEEEs comprehend, among others, NdFeB magnets, Ni-MH batteries, and phosphors from fluorescent lamps. NdFeB magnets contain about 31–32 wt% of REEs (Yang et al., 2017). The content of REEs in spent NiMH batteries is approximately 90 mg/kg for La, 27 mg/kg for Ce and 26 mg/kg for Nd (Lie and Liu, 2021), and they also contain fair amounts of Mn, Co, Zn, Cd and Al. Fluorescent lamps are based on different phosphor types: white phosphor (HALO), red phosphor (YOX), green phosphors (LAP, CAT and CBT) and blue phosphor. HALO does not contain REEs, while in the others Y, Eu, La, Ce, Tb and Gd occur (Gijsemans et al., 2018). A major requirement of WEEEs recycling is to previously optimize their physical dismantling and concentration in diversified scraps (Jowitt et al., 2018). As the separation of different phosphor types is rarely achieved in this process, the recovery of REEs requires their selective separation from a common leaching solution, generally obtained after a series of physicochemical processes aimed at the beneficiation of REEs (Ambaye et al., 2020; Xie et al., 2023; Yuksekdag et al., 2022). An effective separation can be achieved through various methods. For example, a selective precipitation can be done through different reagents favouring the precipitation of double salts, hydroxides, oxalates and carbonates (Mao et al., 2022; Pavón et al., 2018; Porvali et al., 2018; Silva et al., 2019). Solvent extraction, or liquid–liquid extraction is highly used and is commercially exploited to produce high purity single REEs; however, it is an inefficient, labour-intensive and time-consuming method (Opare et al., 2021). Adsorption of REEs can also be carried out with either organic or inorganic compounds, depending on the element considered; the effectiveness of this method is strongly related to contact time, adsorbent dose, initial concentration, solution pH and temperature (Anastopoulos et al., 2016; Iannicelli-Zubiani et al., 2015; Pinto et al., 2023). Ion exchange is another widely exploited method that is generally carried out exploiting different resins (Chen et al., 2017; Hérès et al., 2018) and MOFs.

Zeolites are a family of microporous materials, natural and synthetic, which are classified according to their structure (framework types) and chemistry. Zeolites are usually exploited for their cation exchange property which mainly depend on the Al³⁺ for Si⁴⁺ substitutions in tetrahedra. The REEs adsorption on zeolite faujasite (framework type FAU) (Baerlocher et al., 2007) from concentrated solutions has been extensively studied, notably but not exclusively for the preparation of fluid catalytic cracking (FCC) (Chen et al., 1990; Guzzinati et al., 2018). Despite this, only few studies evaluated the possible exploitation of zeolites for the recovery of REEs (Barros et al., 2019; Confalonieri et al., 2022; Eremin et al., 2017; Kocasoy and Şahin, 2007; Mosai et al., 2019; Mosai and Tutu, 2021; Motsi et al., 2009).

The use of zeolites as cation exchangers in the acidic conditions typical of mineral extraction processes can be limited by the acid hydrolysis of the Si-O-Al bonds, easier for Al-rich zeolites. This has prompted experimentation on natural (clinoptilolite) or synthetic (zeolite-L (LTL)) zeolites, with a relatively high Si/Al ratio. The cheapest synthetic zeolites, like zeolite-A (LTA) and faujasite (FAU), are produced with a low Si/Al ratio, slightly above unity, which gives a very high ion exchange capacity; nevertheless, these materials are more prone to dealumination and amorphization in acidic solution than those with higher Si/Al ratios.

The aim of this work is to evaluate the extent to which instability in acid solution is limiting for the use of low Si/Al zeolites in REE recovery. The cation exchange capacity of a synthetic NH₄ exchanged 13X zeolite (FAU framework type) (Baerlocher et al., 2007) was studied against four different rare earth elements (Ce, La, Eu and Y) and the recovery of the exchanged REEs was also evaluated. Solution concentrations were chosen by mimicking those resulting from a two-step leaching on spent fluorescent lamps with hydrochloric acid (Eduafo et al., 2015). This work aims to be the first step into the possible description of a methodology to recover REEs from wastes. In particular we want to assess the exchange capacity of the zeolite and its efficiency in both the recovery and then the further release of the REE. The mono-elemental exchanges test is fundamental and will give useful preliminary information on the possible selectivity of the material exploited towards different REEs. The separation of REEs is one of the main bottlenecks in the separation process and searching for materials that can selectively separate them is cutting edge topic in the critical raw materials research (Chen et al., 2022). Zeolites have the potential to be a selective and reusable material for the recovery of REEs, thus being a suitable study candidate.

## 2. Materials and methods

### 2.1. NH₄-13X zeolite

As will be better detailed below, an NH₄ saturated zeolite was prepared to test the counter-exchange of NH₄ for the recovery of the REEs from a solution containing only REEs and readily removable ammonium.

Molecular Sieve 13X is the sodium form of synthetic zeolite X, and it is characterized by a low Si/Al ratio, high porosity, and high cation exchange capacity of 300 meq/100 g (Mondale et al., 1995; Rees, 1970). One of the main properties of this material is its great ability to exchange cations. This zeolite is characterized by the primary building block SOD (i.e. sodalite cage) (Supplementary Fig. 1) and cages are connected by double six-rings to form accessible larger cavities (Baerlocher et al., 2007; Zhu and Seff, 1999).

Starting from the 13X zeolite of BDH Chemicals Ltd Poole England, about 30 g of NH₄ exchanged 13X form was prepared by putting 4 g of zeolite in contact with 1 L of a 0.5 M NH₄Cl solution (Carlo Erba analytical grade reagent), under stirring, for 24 h at room temperature; zeolite was then separated from the solution via filtration (Filter: Cat No 1005125, Whatman), washed three times with MilliQ and dried at 60 °C.

Major elements composition was determined by WDS-XRF (Wavelength-Dispersive Spectrometer X-ray Fluorescence) Philips PW1480, while ammonium and water content were measured by TGA (Thermogravimetric Analyses) with a Seiko SSC/5200 thermal analyser in air flow, with heating range and gradient 25–1050 °C and 10 °C/min, respectively. The chemical formula of the obtained material is (NH₄)₅₅.₅ Na₂₄.₇ Mg₅.₈ Ca₀.₀₆ Ti₀.₀₂ Fe₀.₀₄ Al₉₀.₆ Si₁₀₃.₅ O₃₈₄ x 205 H₂O.

### 2.2. 13X stability tests

To assess whether the 13X zeolite maintains the crystallinity during the REE-exchange procedure, preliminary stability tests were performed by interacting the zeolite with HCl solutions at the pH value of 4.4 for 24 h at room temperature. Treated zeolite was separated by centrifugation (8000 rpm, 5 min), washed three times with MilliQ water and then dried at 60 °C. A second stability test was then performed with the same procedure on the material recovered after the first test at pH 4.4, to evaluate the possibility of zeolite re-use for more exchange cycles. The occurrence of amorphous material was then evaluated on X-ray powder diffraction (XRPD) patterns collected with an X'Pert PRO diffractometer and analysed by applying the Rietveld - RIR method using GSAS software (Larson and Dreele, 2000) and methods described in Gualtieri (2000) and Gualtieri et al. (2019).
## 2.3. REEs exchange tests

Four single-element solutions mimicking the concentrations found in leaching solutions of fluorescent lamps (Ce 0.03 M, La 0.04 M, Eu 0.006 M and Y 0.17 M) (Eduafo et al., 2015) were prepared. For Ce and La cation exchange tests proper amounts of Ce(NO₃)₃·6H₂O (99 %, Aldrich) and La(NO₃)₃·6H₂O (99.9 %, Alfa Aesar) were dissolved in MilliQ with a final pH of 4.66 and 4.33, respectively. For Eu and Y cation exchange tests, Eu₂O₃ (99.9 %, Aldrich) and Y₂O₃ (99.9 %, Alfa Aesar) were dissolved in MilliQ with the addition of HNO₃ (Carlo Erba, analytical grade reagent), obtaining a final pH of 2.67 and 1.55, respectively. Since pH values below 4 can induce zeolites dealumination and/or amorphization, while pH values higher than 5 can lead to REE precipitation (Han, 2020), TRIS (tris(hydroxymethyl)aminomethane, 99.9 %, Aldrich) was used to buffer the pH at about 4.4 for both the Eu and Y solutions.

The REEs exchange tests were performed by putting the NH₄-13X zeolite in contact with the single-element REE solutions under stirring for 24 h at room temperature (Confalonieri et al., 2022; Fercia et al., 2009; Pérez-Botella et al., 2022) using three different liquid-to-solid (l/s) ratios: 10/1, 50/1 and 100/1 ml/g. The zeolites were then separated from the solutions by centrifugation (8000 rpm, 5 min), washed three times with MilliQ water and dried at 60 °C. The solutions, once separated from the zeolite, were filtered with a 0.45 µm filter. Elemental Analyses (FLASH 2000 CHNS Analyzer) were performed to evaluate the nitrogen content in zeolite samples in “FLASH” dynamic combustion (modified Dumas’ method), a method that allows to heat sample at 1800 °C and analyse the released elementary gases, separated through a chromatographic column, using a highly sensitive thermal conductivity (TCD) detector. SEM-EDS analysis was performed on REE exchanged zeolites powders compressed into thin discs and coated with carbon. Thermogravimetric characterization was carried out applying the same experimental conditions used for NH₄-13X zeolite.

The final exchange solutions were analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES) Perkin Elmer Optima 4200 DV to evaluate the content of Ce, La, Eu and Y (Supplementary Table 2). The pH of the solutions was measured at the beginning of the exchange test, after 1 h, 2 h, 20 h, 22 h and 24 h, data are reported in Supplementary Table 3. Starting from the data obtained by SEM-EDS, elemental analysis and thermogravimetric analysis (Supplementary Table 4) the chemical formula of each sample was determined. The water content was calculated by subtracting the ammonium from the total weight loss obtained from the TG. The ammonium content was quantified from the N content obtained from the elemental analysis. The data of water and ammonium were then integrated with the bulk chemistry obtained from SEM-EDS and weighted at 100. From the data so obtained the chemical formula was calculated. The mg/g ratio of REE in each sample was then calculated and the removal percentage of REEs extracted from the solutions was estimated knowing the amount of REEs in the samples and the amount of REEs in the starting solutions (Supplementary Table 2).

The exchange isotherms were drawn by indicating the fraction of zeolite anions compensated by REE cations vs. the fraction of REE equivalents in solution at the equilibrium with the zeolite, viz. at the end of each exchange. More in detail: In the x-axis has been reported the ratio between the charges of rare earth cations in the final solution vs. the total charge of cations in the solution. In the y-axis has been reported the ratio between the equivalents of rare earth introduced in the zeolite vs. the total anionic sites of the zeolite.

## 2.4. REEs recovery from zeolite

In order to recover REEs from the exchanged zeolites, REEs were extracted by a further exchange with a 0.8 M NH₄Cl solution, since as previously tested, resulted to be an efficient concentration (Confalonieri et al., 2022). The REEs enriched zeolites were put in contact with the solution for 24 h at room temperature under stirring, using a solid/liquid

ratio of 1/125 g/ml. The zeolite powders were then separated from the solution and characterized as described above.

The chemical formula of each sample was calculated as described in 2.3. Knowing both the chemical formula of the sample before and after the REE recovery, the percentage of REEs recovered was estimated.

# 3. Results and discussion

## 3.1. pH stability tests

Stability tests at pH 4.4 (Supplementary Table 1) showed a considerable increase in amorphous content after the first treatment (24 h), while no significant changes occurred after the second treatment. It is therefore possible to assume that the amorphization stabilised at less than 30 wt%. The amorphization could derive from zeolite dealumination and the consequent precipitation of EFAL (extra-framework aluminium species) on the surface and inside the porosity of the zeolite (Janssen et al., 2001). The phenomenon corresponds to partial dealumination of the zeolite. The hydrolysis of some bridging Si-O-Al deactivates the strong lattice anion and makes free an ammonium cation, increasing the pH of the solution. The phenomenon can be quantitatively evaluated in the experimental setting by considering that two points increase of pH from 4 to 6 corresponds to the liberation of about 1.25 E-5 mol of cations, which does not significantly affect the exchange capacity of about 5 E-3 equivalents of the zeolite sample used. The lower increase of pH in the back-exchanges corresponds to the stabilisation of the zeolite. The XRPD patterns of the sample before and after pH tests do not show other major differences (Fig. 1).

## 3.2. REEs exchange of NH₄-13X

NH₄-13X samples were put in contact with different volumes of solutions having concentration of each REE chosen as typical of the effluent of WEEE waste acid leaching (Eduafo et al., 2015). Liquid-to-solid ratios of 10, 50 and 100 ml/g were used, and the concentrations of the solutions were 0.029 M, 0.037 M, 0.005 M and 0.13 M for Ce, La, Eu and Y, respectively (Supplementary Table 2). The composition of the initial 13X and of the REE-exchanged zeolites are reported in Supplementary Table 4, while the calculated chemical formulas are given in Table 1.

The fraction of REE extracted from solution is reported in Fig. 2A as a function of the liquid-to-solid ratio. At a liquid-to-solid ratio of 10 ml/g, virtually all Ce, La, and Eu passes from the solutions to the zeolite, while, in the same conditions, only nearly half of Y is extracted from the solution. At increasing liquid-to-solid ratio (up to 100 ml/g), Eu is still completely extracted while the fractions extracted from Ce, La and Y solutions decreases with the increasing of the solution volume, reaching extraction levels of 40, 29 and 7 %, respectively for Ce, La and Y, at liquid-to-solid ratio of 100 ml/g. The REE adsorbed by zeolite increases with the volume of contacting solution (Fig. 2B) with a trend strongly dependent on the initial concentration of the REE solution. The amounts of REEs exchanged are reported in mg REE/g zeolite, instead of meq, to give a clearer vision of the applicative potential of this recovery process. In the case of the very diluted Eu solution, the amount adsorbed is linearly dependent on the volume of solution, reaching a value of 63 mg REE/g zeolite at a liquid-to-solid ratio 100 ml/g. The observed linearity suggests that, in these experimental conditions, the affinity of Eu for the solution does not depend on the adsorbed amount; this trend usually indicates that the adsorbent capacity is far from being saturated. For the more concentrated solutions of Ce, La and Y, the adsorbed amount is in the 40–60 mg/g range already for 10 ml solution per g of zeolite. At higher liquid-to-solid ratios, the adsorbed amount of Ce and La increases nonlinearly with solution volume, reaching values of 135–160 mg/g at liquid-to-solid ratio of 50 ml/g. At a liquid-to-solid ratio 100 ml/g the amount of REE incorporated remain almost in the same range (150–160 mg/g), indicating the saturation of the zeolite is approaching. This
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** XRPD pattern of the samples before and after the pH test. 10 wt% of corundum was added to the samples as internal standard (std in labels) to calculate the amount of amorphous material.</figcaption></figure>

<caption>**Table 1**</caption>

Atomic composition of the REE-exchanged zeolites. The charge unbalance (C.U. %) was calculated with the formula below C.U.% = Alcations - (Σmonovalentcations) - 2*(Σbivalentcations) ... - n*(Σnivalentcations) * 100.

<table><thead><tr><th>Sample</th><th>Chemical Formula</th><th>C. U. [%]</th></tr></thead><tbody><tr><td>NH<sub>4</sub>-13X</td><td>(NH<sub>4</sub>)<sub>55.48</sub> Na<sub>24.73</sub> Mg<sub>0.83</sub> Ca<sub>0.06</sub> Ti<sub>0.02</sub> Fe<sub>0.04</sub> Al<sub>90.65</sub> Si<sub>103.47</sub> O<sub>384</sub> x 206 H<sub>2</sub>O</td><td>11.53</td></tr><tr><td>NH<sub>4</sub>-13X_Ce<sub>0.03M_1/10</sub></td><td>Ca<sub>5.61</sub> (NH<sub>4</sub>)<sub>40.98</sub> Na<sub>17.57</sub> Al<sub>85.95</sub> Si<sub>108.69</sub> O<sub>384</sub> x 250.27 H<sub>2</sub>O</td><td>12.29</td></tr><tr><td>NH<sub>4</sub>-13X_Ce<sub>0.03M_1/50</sub></td><td>Ca<sub>21.02</sub> (NH<sub>4</sub>)<sub>12.59</sub> Na<sub>9.6</sub> Al<sub>86.81</sub> Si<sub>105.57</sub> O<sub>384</sub> x 276.87 H<sub>2</sub>O</td><td>1.79</td></tr><tr><td>NH<sub>4</sub>-13X_Ce<sub>0.03M_1/100</sub></td><td>Ca<sub>22.88</sub> (NH<sub>4</sub>)<sub>10.2</sub> Na<sub>9.29</sub> Al<sub>85.08</sub> Si<sub>106.20</sub> O<sub>384</sub> x 280.19 H<sub>2</sub>O</td><td>-3.39</td></tr><tr><td>NH<sub>4</sub>-13X_La<sub>0.04M_1/10</sub></td><td>La<sub>6.93</sub> (NH<sub>4</sub>)<sub>36.51</sub> Na<sub>17.06</sub> Al<sub>85.75</sub> Si<sub>109.10</sub> O<sub>384</sub> x 252.61 H<sub>2</sub>O</td><td>13.30</td></tr><tr><td>NH<sub>4</sub>-13X_La<sub>0.04M_1/50</sub></td><td>La<sub>21.93</sub> (NH<sub>4</sub>)<sub>11.35</sub> Na<sub>8.29</sub> Al<sub>85.65</sub> Si<sub>106.04</sub> O<sub>384</sub> x 277.62 H<sub>2</sub>O</td><td>0.26</td></tr><tr><td>NH<sub>4</sub>-13X_La<sub>0.04M_1/100</sub></td><td>La<sub>20.42</sub> (NH<sub>4</sub>)<sub>9.52</sub> Na<sub>7.87</sub> Al<sub>85.69</sub> Si<sub>108.07</sub> O<sub>384</sub> x 274.16 H<sub>2</sub>O</td><td>8.22</td></tr><tr><td>NH<sub>4</sub>-13X_Eu<sub>0.006M_1/10</sub></td><td>Eu<sub>0.78</sub> (NH<sub>4</sub>)<sub>52.02</sub> Na<sub>19.18</sub> Mg<sub>0.30</sub> P<sub>0.04</sub> Ca<sub>0.12</sub> Al<sub>87.10</sub> Si<sub>107.97</sub> O<sub>384</sub> x 242.49 H<sub>2</sub>O</td><td>15.57</td></tr><tr><td>NH<sub>4</sub>-13X_Eu<sub>0.006M_1/50</sub></td><td>Eu<sub>3.82</sub> (NH<sub>4</sub>)<sub>42.46</sub> Na<sub>17.87</sub> Mg<sub>0.40</sub> P<sub>0.05</sub> Ca<sub>0.08</sub> Al<sub>87.09</sub> Si<sub>108.36</sub> O<sub>384</sub> x 247.56 H<sub>2</sub>O</td><td>17.57</td></tr><tr><td>NH<sub>4</sub>-13X_Eu<sub>0.006M_1/100</sub></td><td>Eu<sub>7.64</sub> (NH<sub>4</sub>)<sub>32.60</sub> Na<sub>17</sub> Mg<sub>0.89</sub> P<sub>0.03</sub> Ca<sub>0.10</sub> Al<sub>87</sub> Si<sub>108.05</sub> O<sub>384</sub> x 254.68 H<sub>2</sub>O</td><td>16.64</td></tr><tr><td>NH<sub>4</sub>-13X_YO<sub>17M_1/10</sub></td><td>Y<sub>14.11</sub> (NH<sub>4</sub>)<sub>21.31</sub> Na<sub>17.46</sub> Mg<sub>0.18</sub> P<sub>0.42</sub> Ca<sub>0.11</sub> Al<sub>86.95</sub> Si<sub>105.32</sub> O<sub>384</sub> x 266.51 H<sub>2</sub>O</td><td>6.74</td></tr><tr><td>NH<sub>4</sub>-13X_YO<sub>17M_1/50</sub></td><td>Y<sub>15.97</sub> (NH<sub>4</sub>)<sub>16.45</sub> Na<sub>17.51</sub> Mg<sub>0.17</sub> P<sub>0.45</sub> Ca<sub>0.15</sub> Al<sub>87.03</sub> Si<sub>104.98</sub> O<sub>384</sub> x 269.07 H<sub>2</sub>O</td><td>5.94</td></tr><tr><td>NH<sub>4</sub>-13X_YO<sub>17M_1/100</sub></td><td>Y<sub>16.40</sub> (NH<sub>4</sub>)<sub>14.25</sub> Na<sub>17.01</sub> Mg<sub>0.47</sub> P<sub>0.61</sub> Ca<sub>0.17</sub> Al<sub>87.17</sub> Si<sub>104.66</sub> O<sub>384</sub> x 271.59 H<sub>2</sub>O</td><td>7.70</td></tr></tbody></table>

behaviour suggests that the affinity of the zeolites for REE cations is decreasing when the amount of adsorbed REE is near the saturation capacity of the exchanger. A different response can be identified for Y. At higher liquid-to-solid ratios, the amount of Y incorporated in the zeolite remains almost the same, indicating that the saturation conditions for Y are reached at the liquid-to-solid ratio of 10 ml/g. All the samples analysed proved to be homogeneous, different point analyses were done for each sample with SEM-EDS, only one of them resulting with a standard deviation of 0.9, while all the others were below 0.5.

Taking into consideration the amount of each REE needed to saturate the zeolite (i.e. about 150 mg/g for Ce and La, and 75 mg/g for Y) it is possible to compare the efficiency of the zeolite considered for this work to different other materials that have been exploited with the same purpose. A lot of different bio/adsorbent materials have been tested with very different results.

Lanthanum bio/adsorption parameters showed a very low affinity in the case of activated carbon modified with potassium permanganate, that reached a value of 0.071 mg/g and carbamoylmethylphosphine

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** REE fraction drawn from solution (A) and amount of REE incorporated in the zeolite (B) as a function of the solid/liquid ratio. Initial concentrations (mg/L): Ce 0.029, La 0.037, Eu 0.005 and Y 0.13. When not reported, the error bars are smaller than the symbols. The lines are guides for the eye.</figcaption></figure>
oxide functionalized with (N-(phosphonomethyl)iminodiacetic acid) (37.4 mg/g); a lower adsorption compared to the one obtained in our work was found using cellulose intercalated zinc-aluminium layered double hydroxide nanocomposite (92.51 mg/g). Hybridized silica-chitosan adsorption is similar to the one of our zeolite, reaching a value of 120.7 mg/g; other materials, such as kenaf cellulose and Co₀.₂Mn₀.₈Fe₂O₄ nanoparticles reached values significantly higher, respectively of 259 mg/g and 750 mg/g. In the same way cerium adsorption results change a lot for each different material considered. In the case of carbamoylmethylphosphine oxide functionalized with (N-(phosphonomethyl)iminodiacetic acid) Ce adsorption reached values of 49 mg/g; values of 96.25 mg/g were obtained with cellulose intercalated zinc-aluminium layered double hydroxide nanocomposite; significantly higher values compared to the one obtained in our work where obtained with kenaf cellulose (244 mg/g) and Co₀.₂Mn₀.₈Fe₂O₄ nanoparticles (748 mg/g), but both methods involved strong acidic solutions for the recovery of REEs and thus the possible reuse of the material. Yttrium, as in the case of La, reached very low adsorption values with activated carbon modified with potassium permanganate (0.0895 mg/g); higher adsorption values compared to the one in our work were obtained with cellulose intercalated zinc-aluminium layered double hydroxide nanocomposite (102.25 mg/g), graphene oxide nanosheets (135.7 mg/g) and hybridized silica-chitosan (158.8 mg/g) (Costa et al., 2020). Lanthanum uptake was also evaluated for two different natural smectites (Iannicelli-Zubiani et al., 2015) and significantly lower values, respectively of 34.73 and 31.95 mg/g, were obtained compared to the 150 mg/g of our work. Ion exchange resins have been largely exploited for the possible recovery of REEs and the results obtained are generally worse than the one obtained in our work. Lewatit TP-260 and Amberlite IRC747 reached value of lanthanum adsorption slightly higher than 0.05 mg/g and 0.1 mg/g for Y, with the total amount of REEs summing up to about 7 mg/g (Hérès et al., 2018). Resorcinol-formaldehyde resin reached saturation values of 45.2 mg/g for La and 60.5 for Ce (Shady et al., 2011). In a 2020 report (Kegl et al., 2020) highlighting the REEs adsorption efficiency of different nanomaterials it is reported that just a few of them showed an adsorption value for Ce and La higher than the one obtained in our work, while none of them showed a value higher than the one we obtained for Y. Also, different zeolites have been tested in recent years as possible exchanging material for the recovery of REEs. NH₄ exchanged LTL and natural clinoptilolite for example proved to be efficient exchangers, but both of them had a significantly lower exchange capacity than our material (Confalonieri et al., 2022; Mosai and Tutu, 2021). Zeolite X exchanged with Y for different purposes than the

one exploited in this paper reached values of 80 mg/g, very similar to the one obtained in this work (Guzzinati et al., 2018). The composition of the exchanged zeolites provides some information about their stability; in fact, the Si/Al ratio of 1.23 ± 0.03 (Fig. 3A) is almost unaffected by the amount of REE in the solid and the volume of solution with which the sample interacted. These Si/Al ratios, significantly higher than the 1.14 value of the parent 13X, derive from a partial dissolution of Al, as expected by treating an Al-rich zeolite with an acid solution. The constancy of the Si/Al when the solid is treated with large volumes of acid solution suggests that the extraction of aluminium by acid hydrolysis has reached a stable state, as a function related to the pH of the solution. The pH behaviour of the solution depends on the exchanged REE, initial pH and liquid-to-solid ratio (Supplementary Table 3). In general, when the experimental conditions limit NH₄ exchange, pH tend to increase while, high NH₄ exchange results in a slight decrease in pH. The decrease of pH may be due to an increment of NH₄⁺ concentration in the solution that promote acid hydrolysis. On the other hand, the increase in pH may be related to a smaller increase of NH₄⁺ concentration and to the possible competition between H⁺ protons and REE cations adsorbed in the NH₄-13X (Sánchez-Hernández et al., 2018), reasonable stabilizing over time at values close to neutrality.

Hydrolysis of the Al-O-Si bonds passes through the formation of extra-framework aluminium species (EFAL) which no more form lattice anions whose charge has to be compensated by exchanged cations. The unbalance between cation charge and aluminium content in the sample, reported in Fig. 3B, provides a first indication on the fraction of aluminium not forming lattice anions. The pristine 13X already has a significant charge unbalance (about 11 % of Al not requiring charge compensation), probably due to the presence of amorphous aluminium oxyhydroxides left over from zeolite synthesis. The charge unbalance of the exchanged samples is strictly related to their REE content. Samples with low REE content present a larger charge unbalance, suggesting that the dealumination of the samples has formed EFAL species not dissolved by the acid treatment. In samples characterized by the highest REE exchange, the charge unbalance systematically decreases with the REE content. Samples with 20 REE p.u.c. exhibit nearly perfect charge compensation, suggesting that virtually all aluminium is part of the framework.

This effect is probably related to the well-known stabilisation of the faujasite framework by REEs, which is the basis of the use of REE-Y catalysts in FCC (fluid catalytic cracking) (Vogt and Weckhuysen, 2015). Indeed, the highest thermal and hydrothermal stability of REE-exchanged zeolite Y allowed its use in the high-temperature steam-

<figure><img src="image_6.png" /><figcaption>Fig. 3. Modification of the zeolite with REE exchange: (A) Si/Al ratio and (B) fraction of Al not contributing to the compensation of the cation charge ((Al-cation charge)/Al) vs. REE cations per unit cell.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Na (filled symbols) and NH₄ (void symbols) content of the exchanged zeolites as a function of the REE content. Zeolites exchanged with Ce (squares), La (circles), Eu (diamond), Y (triangles). The lines are polynomial fittings for Na and NH₄ content.</figcaption></figure>

<caption>**Table 2**</caption>

Chemical formulas of the REE-exchanged zeolites after ammonium counter ex-change. The charge unbalance was calculated as described in Table 1.

<table><thead><tr><th>Sample</th><th>Chemical Formula</th><th>C. U. (%)</th></tr></thead><tbody><tr><td>Ce recovery_1/<br>10</td><td>Ce<sub>6.22</sub>(NH<sub>4</sub>)<sub>55.35</sub> Na<sub>5.94</sub>Mg<sub>0.60</sub>P<sub>0.03</sub>Ca<sub>0.21</sub>Al<sub>86.94</sub><br>Si<sub>106.33</sub>O<sub>384</sub>x 256.61 H<sub>2</sub>O</td><td>8.06</td></tr><tr><td>Ce recovery_1/<br>50</td><td>Ce<sub>18.23</sub>(NH<sub>4</sub>)<sub>32.64</sub> Na<sub>2.53</sub>Mg<sub>0.79</sub>P<sub>0.03</sub>Ca<sub>0.11</sub>Al<sub>86.67</sub><br>Si<sub>104</sub>O<sub>384</sub>x 277.94 H<sub>2</sub>O</td><td>-3.69</td></tr><tr><td>Ce recovery_1/<br>100</td><td>Ce<sub>18.16</sub>(NH<sub>4</sub>)<sub>31.06</sub> Na<sub>2.59</sub>Mg<sub>1.08</sub>P<sub>0.02</sub>Ca<sub>0.07</sub>Al<sub>87.21</sub><br>Si<sub>103.93</sub>O<sub>384</sub>x 277.64 H<sub>2</sub>O</td><td>-1.07</td></tr><tr><td>La recovery_1/<br>10</td><td>La<sub>4.93</sub>(NH<sub>4</sub>)<sub>48.37</sub> Na<sub>5.08</sub>P<sub>0.04</sub>Al<sub>86.24</sub>Si<sub>110.16</sub>O<sub>384</sub>x<br>258.17 H<sub>2</sub>O</td><td>20.86</td></tr><tr><td>La recovery_1/<br>50</td><td>La<sub>11.03</sub>(NH<sub>4</sub>)<sub>29.98</sub> Na<sub>2.28</sub>Mg<sub>0.45</sub>P<sub>0.08</sub>Ca<sub>0.08</sub>Al<sub>88.53</sub><br>Si<sub>108.79</sub>O<sub>384</sub>x 260.86 H<sub>2</sub>O</td><td>26.18</td></tr><tr><td>La recovery_1/<br>100</td><td>La<sub>11.38</sub>(NH<sub>4</sub>)<sub>29.77</sub> Na<sub>2.41</sub>Mg<sub>0.33</sub>P<sub>0.02</sub>Al<sub>88.21</sub>Si<sub>109.05</sub><br>O<sub>384</sub>x 260.98 H<sub>2</sub>O</td><td>24.79</td></tr><tr><td>Eu recovery_1/<br>10</td><td>Eu<sub>0.68</sub>(NH<sub>4</sub>)<sub>61.27</sub> Na<sub>8.5</sub>Mg<sub>0.41</sub>P<sub>0.01</sub>Ca<sub>0.10</sub>Al<sub>86.25</sub><br>Si<sub>109.09</sub>O<sub>384</sub>x 254.16 H<sub>2</sub>O</td><td>16.75</td></tr><tr><td>Eu recovery_1/<br>50</td><td>Eu<sub>3.84</sub>(NH<sub>4</sub>)<sub>54.83</sub> Na<sub>6.51</sub>Mg<sub>0.43</sub>Ca<sub>0.17</sub>Al<sub>84.60</sub>Si<sub>110.29</sub><br>O<sub>384</sub>x 266.02 H<sub>2</sub>O</td><td>15.14</td></tr><tr><td>Eu recovery_1/<br>100</td><td>Eu<sub>6.74</sub>(NH<sub>4</sub>)<sub>47.81</sub> Na<sub>3.93</sub>Mg<sub>0.21</sub>Ca<sub>0.19</sub>Al<sub>88.01</sub>Si<sub>107.80</sub><br>O<sub>384</sub>x 262.49 H<sub>2</sub>O</td><td>18.24</td></tr><tr><td>Y recovery_1/<br>10</td><td>Y<sub>8.62</sub>(NH<sub>4</sub>)<sub>43.90</sub> Na<sub>5.96</sub>Mg<sub>0.28</sub>P<sub>0.13</sub>Ca<sub>0.16</sub>Al<sub>86.99</sub><br>Si<sub>107.29</sub>O<sub>384</sub>x 262.07 H<sub>2</sub>O</td><td>12.98</td></tr><tr><td>Y recovery_1/<br>50</td><td>Y<sub>9.38</sub>(NH<sub>4</sub>)<sub>42.62</sub> Na<sub>5.13</sub>Mg<sub>0.29</sub>P<sub>0.10</sub>Ca<sub>0.15</sub>Al<sub>86.31</sub><br>Si<sub>107.83</sub>O<sub>384</sub>x 263.97 H<sub>2</sub>O</td><td>12.09</td></tr><tr><td>Y recovery_1/<br>100</td><td>Y<sub>10.01</sub>(NH<sub>4</sub>)<sub>41.29</sub> Na<sub>5.29</sub>Mg<sub>0.45</sub>P<sub>0.05</sub>Ca<sub>0.13</sub>Al<sub>87.90</sub><br>Si<sub>106.51</sub>O<sub>384</sub>x 263.77 H<sub>2</sub>O</td><td>12.82</td></tr></tbody></table>

ridden conditions of the FCC plan regenerator (Li and Rees, 1986). This lattice stabilisation has been variously attributed to the polarization effect of REE clusters or REE cations in specific positions in the zeolite cages (Guzman et al., 2005; Scherzer et al., 1975; Schüßler et al., 2011). This effect was mainly observed for Al-poor zeolite Y under high-temperature steaming conditions; it is now relevant to observe a similar stabilization also in the REE exchange Al-rich zeolite X in acidic environments. It can also be observed that the initial 13X zeolite was not completely in NH₄ form and a significant amount of Na cations were still present. This means that both NH₄ and Na⁺ can be exchanged by REE cations, as showed by the results reported in Table 1. The fraction of Na and NH₄ cations in the exchanged zeolites are reported in Fig. 4 as a function of the REE charge fraction. It may be noted that the NH₄ content decreases nearly linearly with the amount of REE exchanged. The

Na fraction takes a different trend, remaining almost constant until at least half zeolite anions are compensated by REE cations and decreasing only for further REE exchange. This indicates a preferential exchange of NH₄ by REE and, significantly, this trend is common to all REE. Clearly, the affinity of the zeolite for REE is much higher than for ammonium, while residual sodium is much less easily exchanged.

### 3.3. REEs recovery from zeolite

The chemical formula and composition of the zeolites after the counter-exchange test in ammonium solution are reported in Table 2 and Supplementary Table 4, respectively. The fraction of REE extracted from the zeolites by the NH₄ solution vs. the initial amount of REE in the zeolite are reported in Fig. 5. Two well-defined patterns appear. La and Y can be recovered by cation exchange with NH₄⁺ at 30–45 % while, under the same conditions, Ce and Eu are only recovered at 10–18 % suggesting a higher affinity of the zeolitic framework for the latter two elements. This behaviour could be related to the variation of charge unbalance before and after the recovery process; in fact, in the case of La and Y the charge unbalance increases after the recovery with a consequent release of great amount of charge compensating ions while, in the case of Ce and Eu, the variation of charge unbalance before and after the recovery is low, suggesting a major stabilization of 13X zeolite after the ionic exchange.

### 3.4. Exchange isotherms

The affinity of the zeolite for trivalent REE cations is much higher than the affinity for monovalent Na and NH₄. Therefore, both REE exchange and NH₄ counter-exchange can be used to plot exchange isotherms of REE in 13X. The charge fraction of REE - defined as the ratio charge of REE/the total positive charges - in the zeolite is reported for all samples vs. the relative normality in solution (Fig. 6). All exchange isotherms present a sharp rise of exchanged REE at the lowest concentration in solution, confirming the high affinity of REE for the 13X sites. The isotherms show that Ce and La present a saturation plateau at about 0.78 charge fraction, which corresponds to a capacity of about 66 charges p.u.c. In contrast, yttrium presents a less well-defined plateau at lower REE content. A lower exchange capacity of 13X for Y than for heavier REE has been already observed (Guzzinati et al., 2018). The Langmuir isotherm $q = K * q_{max} * c / (1 + K * c)$, where q is the fraction of
<figure><img src="image_8.png" /><figcaption>Fig. 5. Fraction of REE extracted by ammonium solution as a function of the initial REE content in the zeolite.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Charge fraction of REE in the zeolite vs. REE concentration fraction in equilibrium solution. The lines are Langmuir fit of the Ce and Y data.</figcaption></figure>

exchange sites in the zeolite occupied by REE cations, $q_{max}$ is the fraction of sites occupied by REE cations at the saturation of the exchanger, $K$ is the affinity constant and $c$ is the C/C° fraction between REE normality and total cation normality in solution, can be used to verify if the exchange occurs with the same chemical potential throughout the concentration field. The isotherms of Ce and Y can be satisfactorily fit by a Langmuir isotherm, suggesting a good homogeneity of their exchange sites. The $q_{max}$ values for Ce and Y are very close (0.79 and 0.60 for Ce and Y, respectively), while the affinity constants significantly differ being 45 and 8, respectively. In the case of La, the isotherm cannot be fitted by a Langmuir isotherm, suggesting a heterogeneity of exchange site affinity for REE. However, the trend of the isotherm suggests an average affinity between those of Ce and Y. In the case of Eu all the experimental points are at very low C/C° values and the fitting with an isotherm model cannot be considered reliable.

## 4. Conclusions

This research has provided indications on the possible use of zeolites

for the recovery of Ce, La and Y from simulated waste solutions highlighting the strengths and weaknesses of these materials. At first it was assessed that the acidic conditions in which the tests were performed, due to the possible precipitation of REEs at higher pH values, did not significantly affect the cation exchange properties of the zeolite exploited. Despite the initial crystallinity loss of the material, once stabilised, it proved to be an effective exchanging material for REEs. As mentioned before, the REEs exchange also entails the stabilization of the Al-rich zeolite X in acidic environments.

$NH_4$-13X zeolite revealed a good affinity towards REEs. For Ce, La and Y a maximum value of respectively 23, 22 and 16 atoms p.u.c. exchanged was found, while the starting concentration of Eu was too low to saturate the zeolite for every liquid-to-solid ratio, and the data obtained are not sufficient to evaluate the Eu affinity. The best exchange condition in terms of liquid-to-solid ratio for Ce and La is 50/1: the zeolite reaches amounts of REE p.u.c. near the saturation values, and a little REE remains in the solution. Y is concentrated enough to already saturate the zeolite also at a liquid-to-solid ratio of 10/1 yet leaving a lot of REE in the solution. With higher liquid to solid ratios, either for Ce, La
and Y the amount of REE p.u.c. exchanged slightly increase, but not enough to account for the consequent increase of REEs remaining in the solutions. Although Y is the most concentrated REE in the starting solution (0.13 M), it is less exchanged than Ce and La, which were significantly less concentrated (respectively 0.03 M and 0.04 M). While Ce and La have a similar behaviour in exchanging with NH₄ and the residual Na, Y exchanges NH₄ in the same way, but is unable to exchange Na like Ce and La. Ce and La have therefore a higher affinity for the zeolite than both NH₄ and Na, while Y is more affine than NH₄ but less than Na. A hypothesis for finding the reasons for that could be the significantly smaller ionic radius of Y with respect to that of Ce, La and Na, suggesting that zeolite NH₄-13X can be selective toward REEs with larger ionic radii. For applicative purposes, the Na remaining in the zeolite can thus play a relevant role; therefore, further experiments should be aimed at the total removal of Na from the zeolite before the REE-exchange tests, in order to evaluate the variations of the affinities in a fully NH₄-exchanged 13X.

The recovery of REEs from the exchanged zeolites is another aspect to work on. Ce and Eu recoveries were almost nil, while almost half of La and Y were recovered. With a view to the work of Eduafo et al. (2015), from which the concentrations of the starting solutions were taken, these findings are crucial. In the perspective of a future work in which the zeolite will be interacted with two different solutions, one enriched in Ce and La and the other one in Eu and Y, NH₄-13X zeolite can potentially selectively release La over Ce and Y over Eu. Further studies to increase the removal of Ce and Eu from the pores should be performed, in order to maximize the zeolite recycle and minimize the cost. Nevertheless, the fact that Ce and Eu remain trapped in the pores represent an important result in terms of zeolite selectivity.

## CRediT authorship contribution statement

**Francesco Colombo:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization, Methodology.
**Riccardo Fantini:** Writing – review & editing.
**Francesco Di Renzo:** Data curation, Methodology, Formal analysis, Writing – review & editing.
**Gianluca Malavasi:** Methodology, Writing – review & editing.
**Daniele Malferrari:** Formal analysis, Writing – review & editing.
**Rossella Arletti:** Supervision, Data curation, Writing – review & editing, Conceptualization, Methodology.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2024.01.023.

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