Composition, mineralogy, and surface area of the ion-adsorption clay and sieved fractionsTable 1 presents the chemical composition and the BET surface area of the as-is ion-adsorption clay sample and each size fraction. As shown, the total REEs (TREE) content is 3,204 mg/kg or 0.32 wt%. It can also be seen that the REE content decreases with increasing particle size. The same trend is observed for all other elements except for silicon that has an opposite trend. In terms of size fraction weigh percentages, S1 (< 0.25 mm) accounts for 48% of the total mass, followed by S2 (0.25–0.5) 15% and then S3 (0.5–2 mm) that accounts for 37% of the total mass. Based on the compositional analysis, Y, Ce, and La are the major REEs in this ion-adsorption clay sample. It also contains high amount of Nd, but it also contains good amount of Dy (which is a critical HREE), which makes this ionic clay a valuable source of HREEs.Table 1 Elemental composition of the ion-adsorption clay and its size fractions: S1 (< 0.25 mm), S2 (0.25–0.5), S3 (0.5–2 mm).In terms of bulk elements, the ion-adsorption clay contains high amount of Fe (8.3 wt% which is almost the same as the Al content (8.4 wt%)). The presence of Al indicates the presence of clay minerals such as kaolinite, halloysite, or other clay minerals as well as other minerals containing Al. The presence of high amount of Fe indicates that the ion-adsorption clay is rich in other minerals beside the clay minerals. This indicates a major difference between this ion-adsorption clay deposit sample and the ones from southern China which mainly contain clay minerals and very low amount of Fe22,23,24,25,26.In terms of surface area, S1 (< 0.25 mm) has the largest surface area of 32.5 m2/g followed by S2 (0.25–0.5 mm) and S3 (0.5–2 mm) which have very similar surface areas (24.7 and 24.9 m2/g). The nonlinear relationship between the surface area and the total amount of REEs indicates that higher amount of REEs within the smaller size fractions is not simply a function of the surface area of the material.The mineralogy of the ionic clay fractions (S1–S3) were determined by X-ray diffraction and Rietveld analysis was used to quantify the amount of each mineral. More details about these methods are provided in the Experimental section. The XRD diffractograms are presented in Fig. 1a-c. The clay mineral was found to be kaolinite (Al2Si2O5(OH)4), and other minerals include quartz (SiO2), Cronstedtite-1T (Fe3.66Al0.02 Si1.32O5(OH)4), and goethite (Fe0.83Al 0.17O(OH)). The goethite in the XRD diffractograms have Fe substitution by Al which is a natural occurrence reported in the literature27. The elemental composition in Table 1 shows that with increasing particle size, the Al content decreases and the Si content increases; this trend is consistent with the mineral phase composition presented in Fig. 1a-c as larger size fraction contains more quartz and less kaolinite. The detection of P (Table 1) can be an indication of monazite or xenotime that contains mineralized REEs, but because of its low content, it was not detected by XRD.Figure 1XRD diffractograms and Rietveld analysis of the ion-adsorption clay size fractions: S1 (< 0.25 mm), S2 (0.25–0.5), S3 (0.5–2 mm).Table 2 summarizes the Rietveld analysis results along with the composition of select elements from Table 1. The smallest size fraction S1 (< 0.25 mm) has the highest content of TREEs (4604 ppm), which is associated with its highest content of clay minerals (kaolinite, 79.6 wt%) as shown in Fig. 1a. The REE content decreases with increasing the particle size in line with the decrease of kaolinite content with increasing particle size. It is obvious that REEs in this ion-adsorption clay sample are associated with kaolinite (the clays mineral) as adsorbed ions, and later, the amount of desorbable REEs is quantified in this study.Table 2 Rietveld analysis results indicating the mineral phase quantification along with the composition of Al, Fe, Si, and TREE in each ion-adsorption clay size fraction: S1 (< 0.25 mm), S2 (0.25–0.5), S3 (0.5–2 mm).Figures 2, 3 and 4 display the results obtained from Electron Probe Microanalysis (EPMA), including backscattered electron (BSE) images and elemental mapping for the three different size fractions of the ion-adsorption clay sample (S1 to S3), respectively. Additionally, for comparison, the elemental composition of the analyzed elements as measured by ICP-OES and ICP-MS is presented in yellow boxes.Figure 2EPMA back scattered electron image and elemental mapping results for the target elements: Al, Fe, Si, P, Ce, La, Nd, Y, and Dy in S1 (< 0.25 mm) size fraction of the ion-adsorption clay. For comparison, the compositional analysis obtained from ICP-OES and ICP-MS are presented in yellow boxes on each panel.Figure 3EPMA back scattered electron image and elemental mapping results for the target elements: Al, Fe, Si, P, Ce, La, Nd, Y, and Dy in S2 (0.25–0.5 mm) size fraction of the ion-adsorption clay. For comparison, the compositional analysis obtained from ICP-OES and ICP-MS are presented in yellow boxes on each panel.Figure 4EPMA back scattered electron image and elemental mapping results for the target elements: Al, Fe, Si, P, Ce, La, Nd, Y, and Dy in S3 (0.5–2 mm) size fraction of the ion-adsorption clay. For comparison, the compositional analysis obtained from ICP-OES and ICP-MS are presented in yellow boxes on each panel.Observations from the BSE images reveal that the particle size increases from S1 to S3. There are significant decreases in Al content, while the Fe content decreases to some extent from S1 to S3. Conversely, the Si content increases from S1 to S3, which is consistent with the increasing presence of SiO2 minerals in the samples as shown in the XRD diffractograms in Fig. 1.The detected presence of Nd, La and Ce (as proxies for LREEs) in the EPMA elemental maps is attributed to possibly monazite mineral, as they are co-located with phosphorus (P). It is important to note that EPMA could not detect the ion adsorbed REEs on the surface of kaolinite. The values for La, Nd and Ce presented in the yellow boxes represent the total La, Nd, and Ce, originating from both the ion adsorbed REEs and mineralized forms in possibly monazite. The elemental mapping of Dy and Y (as proxies for HREEs) was also detected. The results indicate that Y is not present in these samples, confirming that no xenotime is present in this ion-adsorption clay sample.Since the concentration of ion-adsorbed REEs decreases with increasing particle size from S1 to S3, the values presented in the yellow boxes also show a decreasing trend from S1 to S3.Figure 5 provides a comprehensive overview of the phase identification of different minerals in the S1 clay size fraction, utilizing elemental mapping. For comparison, the figure also includes results from Rietveld analysis and compositional analysis. This multifaceted figure offers valuable insights into the distribution of various minerals within the ion-adsorption clay sample, incorporating elemental composition, Rietveld analysis of mineral phases, particle morphology, mineral distribution, and elemental mapping, all within a single image. Notably, the fraction with the smallest particles (S1) contains the highest concentration of kaolinite, which aligns with the observed higher content of TREEs in this specific fraction.Figure 5EPMA back scattered electron image along with mineral phase identification and Rietveld analysis results. Elemental mapping results for the target elements: Al, Fe, Si, P, Dy, Y, La, Ce, and Nd in S1 ion-adsorption clay size fraction (< 0.25 mm) are also presented. For comparison, the compositional analysis obtained from ICP-OES and ICP-MS are presented in yellow boxes on each panel.Desorption of desorbable REEsTo determine the equilibrium time, desorption kinetic experiments were performed with 0.15 mol/L (NH4)2SO4, L/S of 3 mL/g at pH 3. These conditions have shown to results in maximum extraction of desorbable REEs28. As shown in Fig. 6a, the reaction kinetics is very fast and TREE extraction reaches equilibrium within 30 min. On the contrary to REEs, extraction of Al was 58 \(\frac{{mg}_{Al}}{{kg}_{sample}}\) at 30 min and 92 \(\frac{{mg}_{Al}}{{kg}_{sample}}\) at 120 min, which was low and indicated that the studied conditions cannot extract mineralized bulk elements; however, very small amount of Al can be desorbed from the clay mineral, consistent with a previous report in the literature 28. It is also observed that Al extraction does not reach equilibrium and it keeps increasing with increasing time; therefore, it is desirable to keep the desorption time as small as possible, to minimize the extraction of Al (Fig. 6b). Extraction of Fe was very low, below 5 \(\frac{{mg}_{Fe}}{{kg}_{sample}}\) which indicates there are no desorbable Fe in the ion-adsorption clay sample and mineralized Fe cannot be extracted under the mild conditions used for extraction process.Figure 6Desorption Kinetics for: a) TREE and b) Al using 0.15 mol/L ammonium sulfate, L/S of 3 mL/g, at pH 3 and ambient temperature. The error bars represent three replicates.To determine the distribution of different REE adsorption forms (physiosorbed and chemisorbed) within the as-is ion-adsorption clay sample and its three size fractions (S1–S3), two sets of tests were conducted. One set involved the presence of 0.15 mol/L (NH4)2SO4, while the other did not include (NH4)2SO4. In both sets of experiments, sulfuric acid was added to the slurry containing the clay and lixiviant, reducing the slurry’s pH to 3. The same experiments were repeated at the clay’s native pH (~ 5) without the addition of sulfuric acid. By comparing the desorption efficiencies \(\frac{{mg}_{REE}}{{kg}_{sample}}\) under these two conditions, the contributions from the different REE adsorption forms were calculated. The calculation method is shown in the Experimental section.Figure 7 illustrates the desorption results for the three size fractions (S1–S3) at two different pH levels: the native (~ 5) pH and pH 3. Desorption results at the native pH (~ 5) are associated with physiosorbed REEs, while those at pH 3 are linked to chemisorbed REEs.Figure 7Desorption in \(\frac{{mg}_{REE}}{{kg}_{sample}}\) for the three size fractions S1 (< 0.25 mm), S2 (0.25–0.5), S3 (0.5–2 mm) using 0.15 mol/L ammonium sulfate at L/S of 3 mL/g and two pH: a) native pH (~ 5), b) pH 3.Figure 7a demonstrates that smaller particles contain more physiosorbed REEs compared with larger particles, which aligns with the higher kaolinite content in smaller particles. Additionally, it is observed that more heavy rare earth elements (HREEs) are desorbed compared with light rare earth elements (LREEs) despite the fact that the concentration of LREEs is higher than HREEs in this sample. The reason is that most of the LREEs in this sample are in the mineralized form and cannot be desorbed under the mild conditions of the desorption process and they require high temperature and highly acidic conditions to be extracted. On the contrary, the HREEs are adsorbed on the ion-adsorbed clay and they can be desorbed during the desorption process. Table 3 presents the occurrence modes of LREEs and HREEs in each size fraction which confirms LREEs are mostly in the mineralized form and HREEs are mostly physiosorbed and chemisorbed.Table 3 Distribution of REE occurrence modes of LREEs and HREEs in the three size fractions S1 (< 0.25 mm), S2 (0.25–0.5), S3 (0.5–2 mm).Based on the compositional data presented in Table 1, the percentage of LREEs and HREEs in the as-is clay, as well as S1–S3 size fractions, are 63% and 38%, respectively (except for S2, which is 57% LREEs and 43% HREEs). According to the desorption data in Fig. 7a,b, the desorbed amount of LREEs in S1–S3 is 22%, while that of HREEs is 78%. This result suggests that different particle size fractions exhibit a similar preference for adsorbing both LREEs and HREEs.Table 3 highlights the differences in occurrence modes between light rare earth elements (LREEs) and heavy rare earth elements (HREEs). It indicates that while LREEs are more abundant, they are predominantly mineralized and require more intensive conditions for extraction. In contrast, most HREEs are adsorbed on the surface. Additionally, both LREEs and HREEs tend to favor physical adsorption over chemisorption.Table 4 provides an overview of the TREE content in each size fraction and the percentage of physiosorbed, chemisorbed, and mineralized REEs within each size fraction. It is evident that this ion-adsorption clay contains a significant amount of mineralized REEs that cannot be desorbed under typical conditions and necessitate more aggressive treatments such as acid baking followed by water leaching for extraction. Moreover, the table highlights that the majority of the desorbable REEs in this clay are physiosorbed, accounting for over 80% of the total. The chemisorbed REEs can be adsorbed onto the clay minerals like kaolinite, as well as associated minerals such as quartz and goethite. Further investigation into this chemisorption mechanism will be explored in a future study.Table 4 Distribution of REE occurrence modes in the three size fractions S1 (< 0.25 mm), S2 (0.25–0.5), S3 (0.5–2 mm).Comparison between the studied ion-adsorption clay and those from other regionsThe ion-adsorption clay in this study differs from traditional ones in a few respects, including mineral compositions and REE occurrence modes. Traditional ion-adsorption deposits typically contain less than 2 wt% iron22,23,24,25,26. However, in this South American sample, two primary iron minerals, Cronstedtite-1T and goethite, are identified, contributing 8.3% of the sample’s weight in iron. These minerals are associated with 20.5%, 11.1%, and 17.2% of chemisorbed REEs in size fractions S1, S2, and S3, respectively, suggesting that using conventional desorption methods may lead to significant REE losses. Consequently, it is essential to reduce the pH from native pH to 3 to efficiently desorb REEs from this deposit. Adjusting pH to 3 could enhance REE recovery by 25.8%, 12.5%, and 20.8% for each respective size fraction compared with native pH.Additionally, monazite, identified as the possible mineralized form of REEs and not reported in conventional deposits, requires aggressive extraction conditions, such as treatment with 98 wt% H2SO4 or 70 wt% NaOH. In this South American ion-adsorption clay, over half of the REEs are mineralized, predominantly as LREEs. Given the co-existence of adsorbed and mineralized REEs, and the marked tendency for LREEs to be mineralized and HREEs to be adsorbed, a two-step recovery process can be utilized if 100% recovery of all REEs is targeted: 1) Desorption to extract most HREEs, and 2) Intense leaching to recover most LREEs.
