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Article

Experimental Study of the Activation Effect of Oxalic Acid on the Dissolution of Rare Earth Elements in the Typical Diagenetic Minerals of Coal Seams

by
Mengya Ma
1,2,
Wenfeng Wang
1,2,*,
Jian Li
3,
Kun Zhang
4 and
Xin He
2,5
1
Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, China University of Mining & Technology, Ministry of Education, Xuzhou 221116, China
2
School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
3
Shanxi Institute of Geology and Mineral Resources Company Limited, Taiyuan 030001, China
4
State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines, Anhui University of Science & Technology, Huainan 232001, China
5
Key Laboratory of Coal Processing & Efficient Utilization, School of Chemical Engineering and Technology, China University of Mining & Technology, Ministry of Education, Xuzhou 221116, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(4), 525; https://doi.org/10.3390/min13040525
Submission received: 14 February 2023 / Revised: 16 March 2023 / Accepted: 4 April 2023 / Published: 7 April 2023
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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Abstract

:
Rare earth elements (REEs) are considered to be one of the most important metal raw materials, and coal seams are one of the potential sources of REEs. As a low-molecular-weight organic acid, oxalic acid has a strong ability to migrate and dissolve minerals. The coal seam is formed by herbaceous plants and contains more oxalic acid, which may affect the enrichment and transport of REEs during coal formation. Based on the provenance minerals and enrichment carriers of REEs in the coal seam, granite and its weathered minerals (plagioclase, kaolinite, montmorillonite, and quartz) were selected for oxalic acid leaching experiments, to clarify the activation ability of oxalic acid for REEs in coal seams. Experimental results have shown that oxalic acid dissolves minerals and leads to the dissolution and migration of REEs. The higher the concentration of oxalic acid, the stronger the dissolution ability. Each element has a similar dissolution ability in high-concentration oxalic acid solutions, while the ionic radius and electronegativity can cause abnormal distribution of individual elements. The REE dissolution ability in different minerals is controlled by the crystal structure, and the re-adsorption of minerals inhibits the dissolution of REEs in low-concentration oxalic acid solutions. In addition, comparative analysis of REE distribution characteristics in natural water shows that the dissolution and migration of REEs are complexly affected by many factors in addition to pH and fluid environment. Therefore, the activation effect of oxalic acid on REEs in coal seams needs to be further studied.

1. Introduction

Rare earth elements (REEs) have been progressively applied in many high technologies and futuristic industries like electronics, aerospace, renewable energy technologies, etc. [1,2,3]. The consumption of REEs has grown exponentially, due to both the soaring prices of REEs and their vulnerability to supply shortages. It is particularly important to obtain economically viable REEs from low-grade resources. Coal seams are one of the potential sources of REEs; Dai et al. (2012) found that the total content of REEs (∑REEs) in Chinese coal can be up to 100 μg/g [4]. The REEs in coal seams are mainly enriched in the weathering products of granite, including clay minerals (such as montmorillonite and kaolinite) and plagioclase [5,6,7,8]. The ore-forming materials of granites are mainly derived from igneous rocks, and their compositions are complex, mainly including plagioclase (76%–79%), quartz (20%–23%), potassium feldspar (1%–3%), biotite (1%–3%), and other accessory minerals [9,10].
Generally, the REEs are mainly adsorbed on the surface or interior of minerals by ion exchange, including physical adsorption and electrostatic adsorption [11,12]. This process is determined by the surface structure, composition, and surface charge of minerals, resulting in significant differences in REE enrichment and fractionation behavior in different minerals. For example, montmorillonite and kaolinite prefer to adsorb heavy rare earth elements (HREEs, from Tb to Lu) [13,14,15]. Moreover, affected by “lanthanide contraction”, the content of REEs adsorbed on the mineral surface decreases with the increase in ion radius. The scouring of minerals by natural fluids can lead to the dissolution and migration of REEs [7].
Oxalic acid is a low-molecular-weight organic acid and is produced naturally by plant roots, bacteria, fungi, and the decomposition of organic matter [16], which is virtually ubiquitous in soils [17]. The concentrations of oxalic acid in natural waters range from 2.5 × 10−5 to 4.0 × 10−3 mol/L [18]. Coal seams are formed by coalification after the deposition of herbaceous plants and contain more oxalic acid. Previous studies have shown that oxalic acid can promote the dissolution of Fe oxides and alter the environmental behavior of co-occurring As and other trace metalloids [19]. It can also bind strongly to metal ions, affecting their mobility and transportation via aqueous metal–oxalate complexation and competitive adsorption, and form ternary complexes with the trace metals present on mineral surfaces [20,21]. Therefore, oxalic acid in the coal seam can also dissolve REEs from the original minerals and their weathering products [22]. After that, the oxalic acid solutions containing REEs may migrate with the groundwater flow and precipitation in the narrow cracks or pores. Moreover, as a low-molecular-weight organic acid, oxalic acid is more easily polymerized into macromolecules and disappeared with the coalification during the organic matter evolution [23], resulting in the migration and precipitation of the associated REEs. The leaching and migration of REEs from minerals by oxalic acid is a complex process, which is called the activation effect in this paper.
The enrichment and distribution of REEs in coal seams are mainly controlled by the provenance characteristics of the original mineral and the deposit environment and the scouring and transportation of natural fluid, especially acidic fluids. Scholars mainly focus on the source, enrichment, and extraction method of REEs in coal seams [24,25,26] but ignore the activation effect of oxalic acid on REEs during coal formation. Based on the provenance minerals and main enrichment carriers of REEs in the coal seam, the granite and its weathered minerals (plagioclase, kaolinite, montmorillonite, and quartz) are selected in this study to carry out oxalic acid leaching experiments with different concentrations. This work aims to identify the activation capacity of oxalic acid to REEs in coal seams, which can help clarify the enrichment and migration of REEs and facilitate the full exploitation and utilization of the associated mineral resources in coal seams.

2. Materials and Methods

2.1. Mineral Samples

The weathered minerals of granite (plagioclase, kaolinite, montmorillonite, and quartz) were bought from the Shanxi Datong Jinyuan Kaolin Co., Ltd. (Datong, China). All these minerals showed a purity of over 98% and can be used to represent minerals in coal seams. The granite minerals were sampled from the Benxi formation in the northeastern Junggar Basin, China, which is one of the typical coal-bearing formations in the coal basin. The granite sample was grayish-white with a fine–medium-fine structure and was mainly composed of plagioclase (76%–79%), quartz (20%–23%), and potash feldspar (1%–3%). All mineral samples were crushed to 74 um and dried at 80 °C for 24 h. The sieve method was used to divide each mineral sample into four parts for oxalic acid leaching treatment to ensure that these samples were as consistent as possible.

2.2. Oxalic Acid Leaching Experiment

Oxalic acid extraction experiments were carried out on mineral samples at various concentrations. The experimental procedure is shown in Figure 1. Firstly, 6.300 g of oxalic acid dehydrate (C2H2O4·H2O, with purity over 99.8% and produced by the Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was weighed using a Sartorius BSA223S (Sartorius ag, Göttingen, Germany) analytical balance and put into a capacity bottle of 1.0 L. The ultra-pure water, prepared with the Milli-QA10 ultra-pure water system, was added into the capacity bottle to obtain an oxalic acid solution with a concentration of 0.05 mol/L. Part of the solution was then proportionally diluted to 0.01 mol/L using the same method. The lower concentration of oxalic acid of 0.01 mol/L in this study is slightly higher than the oxalic acid concentration in the natural water. The higher concentration of 0.05 mol/L was calculated based on the fact that all oxalate in plants is converted into oxalic acid; that is, the oxalate accounts for about 3% of the dry matter by weight in plants [27], and the dry matter accounts for about 20% of plants. Calcium oxalate (CaC2O4, molecular weight is 128) was used to represent the oxalate in plants to calculate the highest concentration of oxalate ions in plants, which is about 0.057 mol/L. Therefore, it can be considered that the concentrations of oxalic acid leaching solutions in this study (0.01 and 0.05 mol/L) can cover the concentration range of oxalic acid in the coal seams.
The oxalic acid solution with concentrations of 0.01 and 0.05 mol/L and the ultra-pure water was configured with 6 bottles and placed in a capacity bottle of 0.1 L. Among them, 5 bottles of each solution were used for the leaching treatment of the plagioclase, kaolinite, montmorillonite, granite, and quartz minerals of 1.000 g, and the remaining oxalic acid solution was used as a blank control. These reaction solutions were placed in a dark environment and periodically oscillated for half an hour each week. The oxalic acid leaching treatment of the minerals was continued for 3 months to allow the minerals to fully react with the oxalic acid. Finally, the leaching solution was filtered through a 0.25 μm microporous membrane before subsequent testing.

2.3. Analysis Method

2.3.1. Morphology Observation

The morphology of the mineral samples before and after oxalic acid treatment was observed using scanning electron microscopy (FEI Quanta™ 250, Thermo Fisher Scientific (China) Co., Ltd., Suzhou, China) with energy dispersive X-ray (SEM-EDX, Thermo Fisher Scientific (China) Co., Ltd., Suzhou, China) at the Advanced Analysis and Computation Center (AACC) of China University of Mining and Technology (CUMT). The images were captured using the retractable solid-state backscatter electron detector (SSB-SED, Thermo Fisher Scientific (China) Co., Ltd., Suzhou, China)). The working distance was 14–20 mm with a beam voltage of 30 kV, and the magnification could reach 1 million times.

2.3.2. Element Analysis

The contents of REEs and related trace elements (Ba, Sr) in mineral samples, oxalic acid leachates, and chemical blanks were tested using a Thermo Fisher (X Series II, Thermo Fisher Scientific (China) Co., Ltd., Suzhou, China) inductively coupled plasma mass spectrometry (ICP-MS) at AACC of CUMT. The detection limit was up to 10−3 μg/L, and the total analysis error was less than 0.1%. The testing process is detailed as follows:
(1)
Pretreatment of mineral samples and leachates
Solid minerals require wet digestion treatment before elemental testing with ICP-MS (Thermo Fisher Scientific (China) Co., Ltd., Suzhou, China). The wetting digestion treatment procedure in this study was according to the Chinese national standard (GB-W07103(GSR-1)) and Yan et al. [28], with an ultra-clave microwave high-pressure reactor (made by Milestone). The required reagents in the digestion tube for each 0.05 g rock sample were 5 mL HF (concentration ≥ 40%, produced by Huantai Huiyong Chemical Products Sales Co., Ltd., Zibo, China) and 2 mL HNO3 (the concentration was 70%, produced by Jinzhou Qilong Chemical Sales Co., Ltd., Jinzhou, China). All chemical reagents were analytical-grade products that were used without further purification. The dissolved samples were transferred and heated at 180 °C for 4 h and then cooled and diluted to 100 mL with ultrapure H2O for ICP-MS analysis. Additionally, two blank samples (100 mL, diluted from 2% (v/v) HNO3 (70%)) were tested between each sample to minimize test errors.
It is worth noting that the mineral digestion products and oxalic acid leaching solutions were not pre-concentrated in this study to prevent possible element loss due to the hydrolysis of iron and aluminum ions. However, the calculated concentrations of Ba2+ in the digestion mineral solutions, except for plagioclase, were abnormally high (more than 200 μg/L), which may have lead to the high measured value of Eu element during the ICP-MS analysis. Therefore, the barium was separated by the precipitation with H2SO4, referring to the method proposed by Rojano et al. [29].
(2)
Quality control and limit of detection
The ICP-MS instrument was calibrated before the element content test. The REE calibration curves were constructed in the same acidic media as the samples, and the REE multi-element standard solution with a concentration range of 0.01 μg/L to 20 μg/L (GNM-M15173-2013, purchased from Beijing Yihua Tongbiao Technology Co., Ltd. Beijing, China). Each set of calibration samples was tested 3 times to calibrate the instrument. Additionally, a concentration of 2–8 μg/L of the Ba2+ solution was added into the standard solution for the Eu concentration testing to evaluate the effect of Ba2+ on the accuracy of instrument testing. The test results show a non-obvious abnormality in the Eu concentration, indicating that the ICP-MS instrument can automatically eliminate low-concentration ionic interference.
In addition, to correct for matrix differences across the entire mass range of the ICP-MS instrument and to analyze experimental errors due to the digestion treatment method, standard reference materials with certified elemental concentrations were selected for digestion and element content testing. As shown in Table 1, the content of the post-digested REEs did not change significantly compared to their original values. Except for Yb, Lu, and Eu, the analytical precision of the other REEs was within 10% of the relative standard deviation. It can be considered that the test results of the digested solid mineral samples are accurate and available.
(3)
Normalized patterns
Generally, the test results for element contents in the solution are normalized for further analysis, and the widely used standardized patterns are NASC-normalized (data from Haskin et al. [30]), UCC- normalized (data from Taylor and Mclennan [31]) and chondrite-normalized patterns (data from Sun and McDonough [32]). Since this work focuses primarily on the dissolution of REEs from minerals under oxalic acid leaching experiments, the chondrite normalization pattern was used for element-test date analysis of oxalic acid leaching and mineral digestion products.

3. Experimental Results

3.1. REE Occurrences in Minerals

Based on the provenance minerals and enrichment carriers of REEs in the coal seam, the REE contents of granite, plagioclase, clay minerals (kaolinite and montmorillonite), and quartz were tested, as shown in Table 2. It can be seen that the contents of total rare earth elements (∑REEs) in mineral samples of granite (263.595 μg/g), montmorillonite (160.946 μg/g), kaolinite (97.208 μg/g), plagioclase (2.986 μg/g), and quartz (0.515 μg/g) decrease successively. Among them, the clay minerals (kaolinite and montmorillonite) contain much more REEs than feldspar minerals (plagioclase) and quartz. In addition, except for the HREE values of quartz, which are lower than the detection limit (0.001 μg/L), the ratio of light rare earth elements to heavy rare earth elements (LREE/HREE) of plagioclase (4.972), kaolinite (9.510), granite (16.051), and montmorillonite (16.578) increase sequentially. This means that the montmorillonite is more likely to adsorb LREEs during the weathering process of granite.
The REE contents of different minerals are similarly characterized by exhibiting chondrite-normalized patterns, as shown in Figure 2. The REE normalized pattern curves of these mineral samples show a right-leaning distributional feature of LREE enrichment. Except for plagioclase, other minerals show negative anomalies for the Eu element. It means that these weathered mineral samples (montmorillonite, kaolinite, and quartz) inherit the rare earth element distribution characteristics from the original granites. It also proves that these purchased weathered mineral samples can be used to represent the minerals in the coal seam. The higher Eu assignment coefficient in the plagioclase during its formation results in a slight positive anomaly in the Eu content.

3.2. REE Distributions in Oxalic Acid Leaching Solution

Since quartz mineral has low REE contents, the granite, montmorillonite, kaolinite, and plagioclase minerals were selected for REE extraction experiments with pure water and oxalic acid solutions of different concentrations (0.01, 0.05 mol/L) for three months. The REE content in the leached solution was tested using ICP-MS, as shown in Table 2. There is little REE dissolution in the pure water of all mineral samples, and the content of ∑REEs increases rapidly with the increase in the oxalic acid concentration, indicating that oxalic acid can promote the migration and dissolution of REEs in minerals. The concentrations of LREEs and HREEs in different oxalic acid leaching solutions are shown in Figure 3, and the sum of them can represent the dissolved ∑REEs. It can be seen that the kaolinite mineral had the highest dissolved ∑REEs concentrations, followed by granite, montmorillonite, and plagioclase. At the same time, the dissolution concentration of LREEs is higher than that of HREEs in all of these mineral samples. The value of LREEs/HREEs increases with the concentration of oxalic acid (Table 3), indicating that oxalic acid promotes the dissolution of LREEs better than HREEs.
In addition, the chondrite-normalized dissolved ∑REEs concentrations in the oxalic acid leaching solution is shown in Figure 4. The REE curves for each mineral sample in the oxalic acid leaching solution of 0.05 mol/L have a more pronounced regularity. The contents of dissolved REEs decrease with increasing atomic number, with positive anomalies for the elements Eu and Yb.

3.3. REE Dissolution Capacity

Due to the different units of rare earth elements in the leaching solution (μg/L) and the mineral (μg/L), the REE dissolution rates (D) were used in this study to represent the dissolution capacity of REEs, defined as the ratio of the total weight of each mineral in the oxalic acid solutions to the minerals (Equation (1)).
D = C L × M L C S × M S × 100 %
where D is the dissolution rate of REE, %; CL is the dissolution concentration of elements; ML is leachate quality; CS is the element concentrations in minerals; and MS is the mineral quality.
The REE dissolution rates (D) of each mineral in different concentrations of oxalic acid leach solution were calculated, as shown in Figure 5. The relatively small test error means that the test results are available. The element dissolution rates (D) of kaolinite and plagioclase are higher than that of granite and montmorillonite, even though they have relatively lower REE content (Table 2), indicating that the dissolution abilities of REEs in kaolinite and plagioclase minerals are relatively stronger. In addition, the dissolution rate of ∑REE increases with the increase in oxalic acid concentration from 0.01 to 0.05 mol/L, and the increase in element dissolution rates of kaolinite and plagioclase are much higher than that of granite and montmorillonite.
The dissolution rates of LREEs and HREEs in different concentrations of leaching solutions also increase as the concentration of oxalic acid increases, as shown in Figure 5b. In the oxalic acid leaching solutions of 0.01 mol/L, the dissolution rate of LREEs is lower than that of HREEs for all minerals, which is contrary to the dissolution amount of LREEs in the leaching solution is higher than that of HREEs. This is because the LREE content is relatively higher in the original mineral samples, and REE dissolution may be affected by other factors, such as the re-absorption of minerals [15].
As the concentration of oxalic acid increases, the difference between the dissolution rates of LREEs and HREEs gradually decreases. The dissolution rate of LREEs is even greater than that of HREEs in granite minerals, indicating that the promoting effect of oxalic acid on the dissolution of LREEs is more obvious than HREEs. It also can be seen in Figure 6 that the dissolution rate of each REE in the same mineral sample is similar in high-concentration oxalic acid leaching solution compared to the low concentration.

4. Discussion

4.1. REE Dissolution

4.1.1. Occurrence and Dissolution of REEs

Previous studies have shown that oxalic acid has a strong ability to dissolve minerals, even more than citric acid, because there are more H+ and ligands at the same concentration [33]. The decomposition of silicate minerals is mainly a hydrolysis process. Firstly, the H+ in solution reacts with Si-O-Si or Al-O-Al on the surface to hydrolyze the minerals, resulting in the release of Al and Si in the mineral structure. After that, the ligands in the oxalic acid solution can form complexes with Al or Si on the surface of minerals to reduce the activation energy of mineral hydrolysis reactions, thereby promoting mineral dissolution.
REEs mainly occur in granite and its weathering minerals in the form of isomorphic substitutions and adsorption states. During the mineral dissolution in the oxalic acid soaking solution, the REE metal cations (REE3+) existing on the mineral surface can exchange ions with the hydrogen ions formed by electrolysis of oxalic acid and dissolve into the oxalic acid solution to quickly form hydrated cations (REE(H2O)n3+) [34,35]. The smaller the radius of the REE3+, the larger its surface charge density and hydration number [36]. A scanning electron microscope (SEM) was used to observe the surface morphology changes of the mineral samples before and after oxalic acid leaching treatment for three months, as shown in Figure 7. It can be seen that the flaky structure of the kaolinite mineral crystals has been eroded, and the crystalline forms of the montmorillonite and plagioclase have become agglomerated, while the appearance characteristics of granite did not change significantly.
Oxalic acid molecules in aqueous solutions can be ionized twice to form HC2O4− and C2O42−, as shown in Equations (2) and (3), and their relative content changes with the pH value of the solution [26]. Based on the ionization constants of the oxalic acid molecules, the pH values of the oxalic acid solution were 2.06 and 1.49 for concentrations of 0.01 and 0.05 mol/L, respectively. At this point, oxalic acid in the solution mainly exists in the form of H2C2O4 and HC2O4−, with little C2O42−, and the oxalic acid solution with higher concentration has more HC2O4−.
Primary   ionization : H C O O - C O O H ⇄ H C O O - C O O − + H +   K 1   = 5.60   ×   10 − 5
Sec ondary   ionization : H C O O - C O O − ⇄ C O O - C O O 2 − + H +   K 2   = 5.42   ×   10 − 5
Note: K1 and K2 are the ionization equilibrium constants.
The REEs in minerals continuously exchange ions with H+ in oxalic acid solutions and promote the secondary ionization of oxalic acid to form C2O42−. Dissolved REE3+ can bind to HC2O4− and C2O42− to form oxalates, then coordinate with water molecules to form oxalate complexes. The rare earth oxalate has low solubility, between 0.172 and 0.472 g/L at 20 °C [37], which is usually used in industry to prepare rare earth oxides by precipitation [38]. In this study, the mineral samples have low REE dissolution. Among them, the kaolinite minerals in 0.05 mol/L oxalic acid leaching solution have the highest amount of ∑REEs dissolution (36.54 μg/L, Table 2), far lower than the H+ content in oxalic acid solutions of primary ionization (8700 μg/L of 0.01 mol/L and 32,000 μg/L of 0.05 mol/L). Therefore, it is difficult for the REEs dissolved in mineral samples to form oxalate precipitates. The REEs are dissolved from mineral samples in oxalic acid leaching solutions in the form of metal cations (REE3+) and rapidly form aqueous cation ions (REE(H2O)n3+). After that, the REE(H2O)n3+ combines with the hydrogen oxalate ion (HC2O4−) formed by oxalic acid ionization to form hydrogen oxalate and dissolves it in solution. Among them, the ability of complex formation of HREE-oxalates is higher than that of LREE-oxalates.

4.1.2. Distribution of REEs in the Leaching Solution

The test results of REE dissolution in oxalic acid leaching solutions (Figure 3 and Figure 5) showed that the increase in oxalic acid concentration (from 0.01 to 0.05 mol/L) is conducive to REE dissolution from minerals, and the promotion effect on the dissolution of LREEs is stronger than in HREEs. In general, each REE has similar chemical properties and dissolution capabilities, and the amount of dissolution elements is mainly controlled by their original distribution properties in minerals. Chakhmouradian and Wall [39] believed that the desorption of rare earth ions from minerals is governed by the attractive forces between the outer complexes and minerals, and the enrichment of exchangeable REE3+ is supported by the adsorption and desorption of minerals [40]. Therefore, the concentration difference of REEs in different concentrations of the oxalic acid solution may be due to the re-adsorption by the mineral after dissolution.
There are many non-saturated charges on the mineral surface. Based on the principle of electrical neutrality, the mineral surface will adsorb equal amounts of opposite ions to reach electrical equilibrium. When the rare earth ions in the oxalic acid leaching solution have the same valence and similar concentration, the REEs with relatively smaller ionic radius have larger covalent bonds and electrostatic interactions to form more stable complexes with water molecules, which have a larger hydrated radius and weaker re-adsorption ability by minerals [41]. As shown in Figure 8, the ionic radius of rare earth elements gradually decreases with increasing atomic number. Therefore, HREEs with larger surface charge density and hydration number (n) [36] are hard for minerals to re-adsorb, resulting in higher content in the low-concentration oxalic acid leach solutions.
In addition, the adsorption of minerals to the REEs in solution includes physical adsorption (intermolecular force) and electrostatic adsorption (chemical bond force between ions) [35]. As mentioned earlier, REEs in oxalic acid leaching solutions are mainly found in the forms of dissolved hydrated cations and oxalate complexes. In particular, the electrostatic adsorption of minerals to hydrated cations is stronger than the physical adsorption of oxalate complexes. Compared with HREE, LREE-oxalate has a weaker complexation ability [41] and a higher proportion of hydrated cations, which led to an easier re-adsorption of LREEs by minerals in low-concentration oxalic acid leaching solutions.
The higher oxalic acid concentration increases the concentration of hydrogen ions in the leach solution, promotes the ion exchange between REEs and hydrogen ions on the mineral surface, and greatly increases the REE content in the leach solution (Figure 4). At this point, the re-adsorption of minerals has a limited effect on the REE dissolution, resulting in a higher increase in the LREE content in the leach solution. In high-concentration oxalic acid leaching solution, each element has a similar dissolution ability (Figure 6b), and its content distribution (Figure 4b) is close to the original occurrence in minerals (Figure 2), macroscopically showing that concentration increase in oxalic acid can promote the dissolution of LREEs more so than HREEs.
It is worth mentioning that the positive concentration anomalies of Eu and Yb elements (Figure 4b) in the high-concentration oxalic acid leaching solution (0.05 mol/L) are related to electronegativity. As shown in Figure 8, the Eu and Yb elements have lower electronegativity than their neighbors, which means that they are more reactive and more prone to exchange ions with hydrogen ions. As a result, both of the elements are dissolved easily and have a higher content in the leach solutions. Among them, the element Eu has a negative concentration anomaly in the original mineral and a relatively weak positive concentration anomaly in the leach solution, while Yb has a distinct positive concentration anomaly.

4.2. Dissolution Capacity of REEs in Minerals

4.2.1. Crystal Structure of Minerals

The dissolution of REEs from minerals is mainly affected by the erosive ability of oxalic acid on minerals and by the re-adsorption of minerals to hydrated cations and complexes of REEs. The crystal structure and properties of minerals will inevitably affect the dissolution capability of REEs. The VESTA software was used to fit the crystal of these minerals, and the crystal models (CIF date) of these minerals were originally from the American Mineralogist Crystal Structure Database, as shown in Figure 9.
The crystal of kaolinite belongs to the layered structure of the triclinic crystal system. Its small interlayer spacing has a weak capacity to accommodate hydrated cations, and the ion exchange with hydrogen ions in solution mainly occurs at the edge of mineral particles [42]. The dissolution of ∑REEs in low-concentration oxalic acid solution is relatively lesser. As the concentration of oxalic acid increases, the layered crystal structure of kaolinite is easily destroyed, resulting in a rapid increase in the dissolution amount and dissolution rate of REEs (Figure 5 and Figure 6).
The crystal of plagioclase belongs to the framework structure of the triclinic system. The crystal structure is relatively stable and the dissolution of REEs in low-concentration oxalic acid leach solutions is relatively minor. When the concentration of oxalic acid increases and destroys the crystal structure, the dissolution rate of REEs in minerals increases rapidly and is even slightly higher than that of kaolinite minerals (Figure 5a). This is due to the fact that kaolinite has a larger specific surface area and higher cation exchange capacity compared with plagioclase, which is beneficial to the adsorption of REE ions [35,39].
Montmorillonite crystals belong to the hierarchical structure of monoclinic systems and have strong ionic adsorption and exchange capabilities. The REE ions released from minerals undergoing chemical erosion can migrate with rainwater or groundwater and accumulate on clay minerals mainly through exosphere complexation adsorption, which leads to a very high REE content of original montmorillonite minerals [43,44]. Some scholars even regard montmorillonite as the main adsorbent for metal ions [45]. Therefore, the dissolution amount and dissolution rate of REEs are very low in montmorillonite minerals (Figure 3 and Figure 5a). Coppin et al. (2002) found that montmorillonite has a much higher adsorption capacity for REEs than kaolinite in lower pH environments [13], and the adsorption capacity of montmorillonite to ions gradually increases as the concentration of ions decreases [45].
Granite minerals can be considered as the aggregates of kaolinite, plagioclase, and montmorillonite, and their REE dissolution characteristics indicate the combined action of these minerals. The appearance characteristics changed slightly after oxalic acid treatment (Figure 6), and the REE dissolution amount and dissolution capacity of granite minerals were higher than that of montmorillonite minerals but lower than kaolinite and plagioclase in different concentrations of oxalic acid leaching solution (Figure 3 and Figure 5).

4.2.2. Environmental Conditions

In addition, the dissolution of the REE in minerals is controlled by environmental conditions. On the one hand, the higher concentrations of oxalic acid leach solutions have lower pH values and more H+ for ion exchange with REEs. Generally, the dissolved REE mainly forms an anionic phase (REE3+) in neutral and acidic solutions, and hydroxyl hydrated ionic phase (RE(OH)n(3-n)+ (n = 1–3)) in alkaline solutions [42,46,47,48]. The phase states of the REEs can be transformed when the pH changes in the solution [42], which will also affect their content distributions.
On the other hand, environmental conditions can also affect the adsorption capacity of minerals for REE ions. Bradbury and Baeyens (2002) believed that the adsorption capacity of minerals to ions increases with the increase in pH value [48], which is due to the fact that the formation of outer-sphere complexes at the ion-exchange sites in a lower pH environment favors rapid ion exchange, while the reversibility of ion exchange at higher pH is hampered by the formation of inner-sphere complexes [49]. As a result, REEs are more soluble and are less affected by mineral reabsorption in high-concentration oxalic acid leaching solutions, which have similar elemental distribution characteristics to the original occurrence in minerals.

4.3. The Inspiration from REE Dissolution in Acidic Natural Waters

It is worth noting that the REE dissolutions in natural water are more complicated than the oxalic acid leaching conditions in this study. The REE distributions in the groundwater of granite weathering crust and granite fissure water in south China are shown in Figure 10 (date referenced from [50]); and comparing the REE dissolution characteristics in the oxalic acid leaching solution in this study (Figure 4), it can be seen that the REE content in oxalic acid leaching solution is higher than that in groundwater, and the distribution of elements in these solutions is generally similar, showing a rightward trend of each element.
However, there are also some differences, such as the negative anomalies of Ce in natural water. This is because the groundwater and fissure water are in weak acidic conditions, with a pH value between 4.3 and 6.8, which is higher than the oxalic acid leachate (pH = 1.49 and 2.06) in this study. Ce3+ is easily oxidized to Ce4+ in low acidic, neutral, and alkaline solutions, and then forms the Ce(OH)4 precipitation or fixes on clay minerals [51], resulting in an obvious negative anomaly in natural water. Therefore, the pH value of the solution will affect the distribution of REEs.
In addition, Figure 11 shows the dissolved REEs in Lusatia Lake, Germany (date referenced from [52]). It can be seen that the dissolution amount of REEs decreases with the increase in pH value in the groundwater of the Quaternary aquifer, while there is no obverse relationship between dissolution of REEs and pH in the groundwater of the dump aquifer.
It means that the fluid environment will also affect the REE distributions [53,54,55]. At the same time, as previously mentioned, Eu element has a relatively strong dissolution ability, with positive content anomalies in the oxalic acid leaching solutions (Figure 4) and fissure water (Figure 10b) after long immersion, while scouring of groundwater can continuously dissolve the Eu element and result in a negative content anomaly shown in Figure 10a and Figure 11.
An interesting phenomenon is that the Yb element is a negative anomaly in the groundwater of the Quaternary aquifer and dump aquifer with pH values between 4.0 and 5.4 and the acidic mine lake (Figure 11), while it is not an anomaly in granite weathering crust groundwater and granite fissure water (Figure 10). This implies that the distribution of REEs in natural water is very complex and possibly controlled by more environmental conditions than pH and fluid conditions [54,55]. In summary, the action of oxalic acid is one of the factors affecting the migration of REEs in coal seams; the enrichment and distribution of REEs in coal still need further study.

5. Conclusions

In this study, plagioclase, kaolinite, montmorillonite, quartz, and granite minerals were selected for oxalic acid leaching experiments to analyze the dissolution of REEs from the minerals. The conclusions were drawn as follows:
(1) Oxalic acid dissolves minerals and leads to the migration of REEs from the minerals in the coal seams, and the higher the concentration of oxalic acid, the stronger the dissolution ability. The dissolution capability of each element is similar in the high-concentration oxalic acid solutions, but the ionic radius and electronegativity can cause abnormal distribution of individual elements (Eu and Yb);
(2) The REE dissolution ability in different minerals is controlled by the crystal structure and properties. The element dissolution abilities of kaolinite and plagioclase are higher than that of granite and montmorillonite. Moreover, the re-absorption of minerals affects the REE dissolutions in the oxalic acid solution of low concentration, while it has a limited effect in the high concentration;
(3) The dissolution and migration of REEs are complexly affected by many factors in addition to the original occurrence in minerals, the properties of elements (such as ion radius and electronegativity), and environmental conditions (pH value, flue conditions, and other factors). Therefore, the activation effect of oxalic acid on REEs in coal seams needs to be further studied.

Author Contributions

Data curation, J.L.; Funding acquisition, W.W., K.Z. and X.H.; Methodology, M.M. and W.W.; Validation, X.H.; Writing—original draft, M.M.; Writing—review and editing, M.M. and K.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (Nos. U1903207, 41972176, 42002184, 42202200), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); and the Fundamental Research Funds of China University of Mining and Technology (No. 2020CXNL11).

Data Availability Statement

Data is contained within the article. The data presented in this study are available.

Acknowledgments

We thank Bin Liang from China University of Geosciences (Beijing) for language help. We would like to thank the anonymous reviewers for their constructive and detailed comments on this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 13 00525 g001 550
Figure 1. Oxalic acid leaching experiment for the minerals.
Figure 1. Oxalic acid leaching experiment for the minerals.
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Figure 2. Chondrite-normalized REE patterns in the mineral samples.
Figure 2. Chondrite-normalized REE patterns in the mineral samples.
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Minerals 13 00525 g003 550
Figure 3. Dissolved LREEs and HREEs from minerals in oxalic acid leachates. (a) 0.01 mol/L oxalic acid leachates; (b) 0.05 mol/L oxalic acid leachates. The red error bar is the standard deviation of HREE, and the black error bar is the standard deviation of LREE.
Figure 3. Dissolved LREEs and HREEs from minerals in oxalic acid leachates. (a) 0.01 mol/L oxalic acid leachates; (b) 0.05 mol/L oxalic acid leachates. The red error bar is the standard deviation of HREE, and the black error bar is the standard deviation of LREE.
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Minerals 13 00525 g004 550
Figure 4. Chondrite-normalized patterns of dissolved REEs from minerals in oxalic acid leachates: (a) 0.01 mol/L oxalic acid leaching solution; (b) 0.05 mol/L oxalic acid leaching solution.
Figure 4. Chondrite-normalized patterns of dissolved REEs from minerals in oxalic acid leachates: (a) 0.01 mol/L oxalic acid leaching solution; (b) 0.05 mol/L oxalic acid leaching solution.
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Minerals 13 00525 g005 550
Figure 5. REE dissolution rates of samples in oxalic acid leachates of different concentrations: (a) Dissolution rates of ∑REEs; (b) Dissolution rates of LREE and HREE. The blue error bar in (a) is the standard deviation of the ∑REE; the red and black error bars in (b) are the standard deviations of the LREE and HREE.
Figure 5. REE dissolution rates of samples in oxalic acid leachates of different concentrations: (a) Dissolution rates of ∑REEs; (b) Dissolution rates of LREE and HREE. The blue error bar in (a) is the standard deviation of the ∑REE; the red and black error bars in (b) are the standard deviations of the LREE and HREE.
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Minerals 13 00525 g006 550
Figure 6. The dissolution rate of REE in oxalic acid leachates of different concentrations: (a) 0.01 mol/L oxalic acid leaching solution; (b) 0.05 mol/L oxalic acid leaching solution.
Figure 6. The dissolution rate of REE in oxalic acid leachates of different concentrations: (a) 0.01 mol/L oxalic acid leaching solution; (b) 0.05 mol/L oxalic acid leaching solution.
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Minerals 13 00525 g007 550
Figure 7. Morphological changes of mineral samples before and after soaking in oxalic acid solutions of 0.05 mol/L.
Figure 7. Morphological changes of mineral samples before and after soaking in oxalic acid solutions of 0.05 mol/L.
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Figure 8. Electronegativity and the trivalent ionic radius distributions of REEs.
Figure 8. Electronegativity and the trivalent ionic radius distributions of REEs.
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Figure 9. Mineral crystal structure and ion exchange process.
Figure 9. Mineral crystal structure and ion exchange process.
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Figure 10. Chondrite-normalized patterns of dissolved REEs in the natural water of south China: (a) Groundwater of granite weathering crust, (b) Granite fissure water (date referenced from [50]).
Figure 10. Chondrite-normalized patterns of dissolved REEs in the natural water of south China: (a) Groundwater of granite weathering crust, (b) Granite fissure water (date referenced from [50]).
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Minerals 13 00525 g011 550
Figure 11. Chondrite-normalized patterns of dissolved REEs in an acidic mine lake (Lusatia lake, Germany) with pH values: (a) Groundwater of the Quaternary aquifer, (b) Groundwater of the dump aquifer (date referenced from [52]).
Figure 11. Chondrite-normalized patterns of dissolved REEs in an acidic mine lake (Lusatia lake, Germany) with pH values: (a) Groundwater of the Quaternary aquifer, (b) Groundwater of the dump aquifer (date referenced from [52]).
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Table
Table 1. Contents of REEs in the GBW07103(GSR-1) (μg/g).
Table 1. Contents of REEs in the GBW07103(GSR-1) (μg/g).
ElementMean (μg/g)Content (μg/g)Standard Value (μg/g)Recovery Rate (%)RSD (%)
La54.68054.475 ± 0.61854.0001.0131.1
Ce103.118103.214 ± 0.648108.0000.9550.6
Pr12.37812.372 ± 0.68712.7000.9755.6
Nd42.94642.994 ± 0.23747.0000.9140.5
Sm9.4479.449 ± 0.0139.7000.9740.1
Eu0.7490.749 ± 0.0050.8500.8810.7
Gd9.1939.123 ± 0.2139.3000.9892.3
Tb1.7731.769 ± 0.0371.6501.0752.1
Dy9.8979.889 ± 0.04110.2000.9700.4
Ho1.9461.931 ± 0.0482.0500.9492.5
Er6.7676.709 ± 0.2946.5001.0414.3
Tm0.9800.97 ± 0.0441.0600.9255.6
Yb6.0095.992 ± 0.0747.4000.8121.3
Lu0.9510.95 ± 0.0771.1500.8279.1
Table
Table 2. Contents of REEs in the raw mineral samples (μg/g).
Table 2. Contents of REEs in the raw mineral samples (μg/g).
ElementGraniteKaolinitePlagioclaseMontmorilloniteQuartz
MeanContentMeanContentMeanContentMeanContentMeanContent
LREELa72.07472.231 ± 1.44522.3322.494 ± 1.05150.7370.73 ± 0.02240.73140.73 ± 2.930.170.153 ± 0.017
Ce117.833117.356 ± 1.95639.40939.799 ± 3.0531.1181.116 ± 0.03668.6168.148 ± 4.2350.2330.168 ± 0.018
Pr11.9611.889 ± 0.2434.8424.809 ± 0.3540.120.12 ± 0.0048.7118.69 ± 0.4140.0220.021 ± 0.002
Nd39.4539.328 ± 0.67516.6616.525 ± 1.2970.3420.341 ± 0.01128.70329 ± 1.6070.0690.052 ± 0.004
Sm5.645.626 ± 0.1153.7843.82 ± 0.2610.1140.114 ± 0.0054.4994.5 ± 0.1740.0110.011 ± 0.001
Eu1.1791.187 ± 0.0520.9340.883 ± 0.1630.0550.055 ± 0.0010.5360.54 ± 0.0320.0020.002 ± 0.001
HREEGd5.4655.5 ± 0.1213.3363.485 ± 0.5650.1040.103 ± 0.0043.7363.7 ± 0.1780.0080.008 ± 0.001
Tb0.6830.675 ± 0.0260.5820.605 ± 0.1010.0190.019 ± 0.0010.4460.452 ± 0.033
Dy3.6423.526 ± 0.1332.9842.984 ± 0.2860.1570.156 ± 0.0071.8662.005 ± 0.098
Ho0.70.695 ± 0.0160.4250.434 ± 0.0330.020.02 ± 0.0010.3170.319 ± 0.012
Er2.1392.12 ± 0.1041.0071.008 ± 0.1520.0550.055 ± 0.0011.0581.06 ± 0.045
Tm0.3280.324 ± 0.020.120.116 ± 0.0140.0140.014 ± 0.0010.1760.175 ± 0.005
Yb2.1832.164 ± 0.0580.7090.661 ± 0.1450.1110.11 ± 0.0041.3411.334 ± 0.061
Lu0.3190.318 ± 0.0260.0860.0856 ± 0.0010.020.02 ± 0.0010.2160.22 ± 0.013
LREEs/HREEs16.0519.514.97216.578
∑REEs263.59597.2082.986160.9460.515
Table
Table 3. Contents of REEs in the oxalic acid leachates (μg/L).
Table 3. Contents of REEs in the oxalic acid leachates (μg/L).
ElementGraniteKaolinite
Oxalic Acid Concentration (mol/L)Oxalic Acid Concentration (mol/L)
00.010.0500.010.05
MeanContentMeanContentMeanContentMeanContentMeanContentMeanContent
LREELa 0.2740.276 ± 0.0064.14.125 ± 0.1880.0060.006 ± 0.0010.1070.105 ± 0.0095.9256.001 ± 0.252
Ce 0.3870.387 ± 0.0084.0674.108 ± 0.312 0.6260.613 ± 0.0515.2215.307 ± 0.628
Pr 0.030.03 ± 0.0010.2590.252 ± 0.021 0.0170.017 ± 0.0011.4931.5 ± 0.134
Nd 0.0950.094 ± 0.0020.6640.68 ± 0.056 0.10.1 ± 0.0065.5245.194 ± 0.5
Sm 0.0050.005 ± 0.0010.0940.094 ± 0.006 0.0480.049 ± 0.0021.4921.5 ± 0.159
Eu 0.0020.002 ± 0.0010.0520.051 ± 0.0050.2080.21 ± 0.0060.0630.061 ± 0.0080.8180.789 ± 0.235
HREEGd 0.0230.023 ± 0.0010.180.019 ± 0.0030.0080.008 ± 0.0010.1470.147 ± 0.0142.0012.001 ± 0.15
Tb 0.0020.0020.020.19 ± 0.04 0.0410.041 ± 0.0030.3360.336 ± 0.012
Dy 0.0140.014 ± 0.0010.0750.076 ± 0.007 0.3780.378 ± 0.0221.8811.834 ± 0.434
Ho 0.0090.009 ± 0.001 0.0760.075 ± 0.0080.3230.319 ± 0.012
Er 0.010.010.060.058 ± 0.008 0.2150.215 ± 0.020.7510.741 ± 0.051
Tm 0.0310.031 ± 0.020.0910.09 ± 0.004
Yb 0.0080.008 ± 0.0010.0520.054 ± 0.011 0.2050.203 ± 0.030.6120.602 ± 0.067
Lu 0.0010.001 0.0230.023 ± 0.0020.0710.067 ± 0.012
LREEs/HREEs 13.91223.264 0.86 5.02
∑REEs 0.859.633 2.077 36.538
ElementPlagioclaseMontmorillonite
Oxalic acid concentrationOxalic acid concentration
00.010.0500.010.05
meancontentmeancontentmeancontentmeanContentmeancontentmeancontent
LREELa 0.0040.004 ± 0.0010.3930.392 ± 0.0166 0.0090.009 ± 0.0010.0490.049 ± 0.002
Ce 0.610.612 ± 0.0190.0540.054 ± 0.001 00.098 ± 0.005
Pr 0.0660.066 ± 0.001 0.0060.006 ± 0.001
Nd 0.0050.005 ± 0.0010.1990.2 ± 0.006 0.0090.0090.0230.023 ± 0.001
Sm 0.0560.056 ± 0.002 0.0040.004 ± 0.001
Eu 0.0010.001 ± 0.0000.0240.025 ± 0.003 0.0030.003 ± 0.001
HREEGd 0.0820.082 ± 0.002 0.0120.012
Tb 0.0170.017 ± 0.001
Dy 0.0020.002 ± 0.0010.1030.103 ± 0.002 0.0030.003 ± 0.0010.0040.004 ± 0.001
Ho 0.0180.021 ± 0.001
Er 0.0550.054 ± 0.004 0.0040.0041 ± 0.003
Tm 0.0070.007
Yb 0.0030.003 ± 0.0010.080.079 ± 0.003
Lu 0.0090.009 ± 0.001
LREEs/HREEs 2 3.63 69.1
∑REEs 0.015 1.719 0.0210.202
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MDPI and ACS Style

Ma, M.; Wang, W.; Li, J.; Zhang, K.; He, X. Experimental Study of the Activation Effect of Oxalic Acid on the Dissolution of Rare Earth Elements in the Typical Diagenetic Minerals of Coal Seams. Minerals 2023, 13, 525. https://doi.org/10.3390/min13040525

AMA Style

Ma M, Wang W, Li J, Zhang K, He X. Experimental Study of the Activation Effect of Oxalic Acid on the Dissolution of Rare Earth Elements in the Typical Diagenetic Minerals of Coal Seams. Minerals. 2023; 13(4):525. https://doi.org/10.3390/min13040525

Chicago/Turabian Style

Ma, Mengya, Wenfeng Wang, Jian Li, Kun Zhang, and Xin He. 2023. "Experimental Study of the Activation Effect of Oxalic Acid on the Dissolution of Rare Earth Elements in the Typical Diagenetic Minerals of Coal Seams" Minerals 13, no. 4: 525. https://doi.org/10.3390/min13040525

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