A green method for the extraction and determination of La3+ and Ce3+ ions from water and copper solution based on Calix[4]resorcinarene chelation

Jing-Long Liu; Bin Xu; Shou-Dong Chen; Jie Li; Qian-Feng Zhang

doi:10.25259/AJC_265_2025

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Original Article

2025

:18;

2652025

doi:

10.25259/AJC_265_2025

A green method for the extraction and determination of La³⁺ and Ce³⁺ ions from water and copper solution based on Calix[4]resorcinarene chelation

Jing-Long Liu^a,*, Bin Xu^a, Shou-Dong Chen^a, Jie Li^a,, Qian-Feng Zhang^b

aSchool of Mechanical Engineering, Tongling University, No.1335 Cuihu 4th Road, Tongling, 244000, Anhui, China

bInstitute of Molecular Engineering and Applied Chemistry, Anhui University of Technology, No. 59 Hudong Road, Ma’anshan, 243002, Anhui, China

Corresponding author: Email address: 2024106@tlu.edu.cn (J.L. Liu)

Received: 2025-03-13, Accepted: 2025-06-10, Epub ahead of print: 2025-08-29, Published: 2025-09-20

Licence

This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

Green extraction and recovery of rare earth metals is an important and urgent problem for the rare earth industry. To explore a method capable of extracting and enriching rare earth elements in both aqueous and high-copper matrix solutions, a calix[4]resorcinarene chelation-cloud point extraction (CPE)-inductively coupled plasma optical emission spectrometry (ICP-OES) approach was established to achieve the extraction, enrichment, separation, and determination of rare earth elements. In this study, a methylsulfonated derivative of calix[4]resorcinarene (C4RS) was prepared, which is a polydentate ligand with four flexible methylene sulfonate groups. Through UV-vis spectroscopy, it was demonstrated that C4RS exhibits a high complexation ability towards rare earth (RE) ions. During the development of the analytical method, Triton X-114 (TX-114), a nonionic surfactant, was employed as the extractant, while water-soluble C4RS acted as the chelating agent for CPE of La³⁺ and Ce³⁺ ions. The CPE process, including experimental conditions and influencing factors for enriching La³⁺ and Ce³⁺ ions, was systematically investigated and optimized. This study found that the CPE procedure could extract La³⁺ and Ce³⁺ ions with high efficiency in water and even from copper solutions with a certain concentration. Then, the La³⁺ and Ce³⁺ ions extracted and enriched by CPE are determined by ICP-OES. The detection limits were as low as 0.0002 and 0.0003 for La³⁺ and Ce³⁺ ions, respectively, with excellent standard recovery rates and precision.

Keywords

Calix[4]resorcinarene

Chelating agent

CPE

Extraction

La3+ and Ce3+

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1. Introduction

Rare earth (RE) elements possess unique optical, electrical, and magnetic properties, earning them the moniker “critical metals” due to their essential role in the advancement of complex electrical and electronic devices, including displays, mobile phones, printed circuit boards, photovoltaic modules, and wind turbines [1-3]. As the demand for REs escalates, so does the need for their detection and inspection. Moreover, there is an increasing necessity to recycle REs from waste liquids that contain them [4-9]. Moreover, the necessity to recover renewable energy from waste streams containing such resources is becoming increasingly critical. Concurrently, diverse methodologies for extracting reusable resources from various types of waste are being actively explored and developed [10-15]. To establish a green, efficient method for extracting and enriching rare earth elements in both aqueous and high-copper matrix solutions, the selection of an appropriate extraction method, a compatible metal chelating agent, and a reliable detection technique will be the crucial determinants of success.

Liquid-liquid extraction is extensively utilized within the RE industry, with approximately 90% of the global RE products being procured via this method of extraction [4]. The conventional REs extraction agent is acid phosphate ester, known for its high extraction efficiency [16]. However, this process necessitates saponification and results in the generation of substantial amounts of saline wastewater. Moreover, phosphorus-containing extractants are toxic and present a risk to biological health.

Cloud point extraction (CPE), a micelle-mediated approach, has recently attracted widespread interest for its alignment with the fundamental principles of “green chemistry” [17,18]. CPE has emerged as a highly promising, cost-efficient, and eco-friendly method for recovering REs from various feedstocks under optimized conditions [19,20]. In this procedure, the chelating agent and extractant initially form a homogeneous mixed-phase solution. Upon heating, the solution undergoes phase separation, resulting in the formation of a surfactant-enriched phase. This phase efficiently transports the REs, enabling their separation and concentration. This distribution is attributed to the formation of surfactant micelles in the colloidal solution, as the hydrophobic termini of the ligand molecules are situated on the exterior of the ligand-REs complex.

The chelating agent is a critical factor in the success of CPE, while the effectiveness of calixarenes as metal ion extractants in solvent extraction techniques has been well-documented [21-25]. Mustafina and co-workers investigated the use of p-sulfonato thiacalixarene, sulfonatomethylated calix[4]resorcinarene, and calix[4]resorcinarene phosphonic acid as chelating agents to enhance the separation efficiency of lanthanides (La³⁺, Gd³⁺, and Yb³⁺) via Triton X-100-mediated CPE. Their study holds significant importance for developing highly efficient and environmentally friendly metal ion separation methods, particularly in achieving high-concentration metal ion recovery while minimizing organic solvent usage [22]. In our work, a water-soluble methylene sulfonated derivative of calix[4]resorcinarene (C4RS) has been successfully synthesized [26]. C4RS contains four flexible methylene sulfonic acid groups, and its molecules take on a bowl-shape conformation in crystal structure and solution [27], which provides a high complexation ability towards RE ions. Using C4RS as a chelating agent and Trition X-114 (TX-114) as an extractant agent, respectively, the CPE procedure could efficiently extract and enrich La³⁺ and Ce³⁺ ions. The method combining CPE with ICP-OES exhibited an ideal detection limit, an excellent standard recovery rate, and good precision.

2. Materials and Methods

2.1. Chemicals

The reagents employed in the study were commercially sourced, guaranteed reagents, and used directly after purchase without further purification. Compound 1 was synthesized using previously reported procedures from the literature [28]. C4RS can be obtained through sulfonation of compound 1 using Na₂SO₃ in a water-ethanol mixture, with an excess of CH₂O (Figure 1). This process yields a high percentage (ranging from 90% to 98%), as previously published in our work [21,29]. TX-114 (Laboratory grade) and tributyl phosphate (TBP, ≥99%) were purchased from Sigma-Aldrich. The rare earth metal stock solution (1000 μg mL^-1) was obtained from the General Research Institute for Nonferrous Metals, China. The working solutions of La³⁺ and Ce³⁺ were serially diluted with 1% (v/v) HNO₃. Britton-Robinson (B-R) buffer solution was obtained by mixing different concentrations of high-purity phosphoric acid, boric acid, and acetic acid (H₃PO₄-HAc-H₃BO₃) in ultrapure water. All trace analysis glassware was immersed in 5% (v/v) HNO₃ for a minimum of 48 h, followed by triple rinsing with ultrapure water. No rare earth ions were detected in the water or reagents.

2.2. CPE

Specific volumes of rare earth standard solution, C4RS solution, TX-114 solution, and B-R buffer solution were combined in a 50 mL graduated centrifuge tube, and the mixture was diluted to 50 mL with ultrapure water. In this context, C4RS functions as a rare-earth metal chelating agent, TX-114 serves as the extractant, and the B-R buffer solution maintains the extraction system at a stable pH. The mixed solution was heated in a 50°C water bath for 10 min, centrifuged at 3000 rpm for 10 min, and subsequently cooled in an ice bath for a few minutes. Heating was performed to induce micelle formation in the surfactant, triggering the cloud point phenomenon. Subsequent centrifugation separates the metal-chelator complexes entrapped within the micelles. The denser surfactant-enriched phase separated and deposited at the bottom of the centrifuge tube following centrifugation, while the clear aqueous phase in the upper portion of the centrifuge tube was carefully removed using a peristaltic pump or a pipette. After adding 0.3 mL of TBP solution as a defoaming agent, the remaining enriched phase was diluted to 5.0 mL with (10%, v/v) HNO₃ solution, followed by determination using ICP-OES [21,30-32]. The procedure of CPE has been shown in Figure 2.

2.3. ICP-OES and UV-vis analysis

The concentrations of Lanthanum (La I 408.672 nm) and Cerium (Ce I 413.764 nm) were determined using inductively coupled plasma optical emission spectrometry (ICP-OES) (iCAP 7400 Duo, Thermo Scientific, USA). The operating parameters for ICP-OES determination of La³⁺ and Ce³⁺ have been given in Table 1.

Table 1. ICP-AES operating parameters.

Parameter	Setting
Wavelength (nm)	408.672 (La) 413.764 (Ce)
Incident power (kW)	1.3
Auxiliary gas (L min⁻¹)	0.3
Plasma gas (L min^-1)	15.0
Nebulizer gas (L min^-1)	0.55
Sample flow rate (mL min^-1)	1.50
View distance (mm)	15
Plasma view mode	Axial
Read delay/Integration time (s)	10/5
Number of replicates	3

The absorption spectra of C4RS and its rare earth ion chelates in the UV range were recorded using a spectrophotometer (TU-1902, Beijing Purkinje General Instrument, China) fitted with a 10 mm quartz cell. The molecular absorption spectra were scanned at a medium speed with a 1.0 mm scanning step interval within the wavelength range of 220 nm to 260 nm.

3. Results and Discussion

3.1. UV spectrophotometry

The UV spectra of C4RS and its chelates with varying concentrations of rare earth ions (La³⁺ and Ce³⁺) were analyzed to assess the quantitative binding capacity of C4RS with rare earth ions. A sequence of UV spectra have been depicted in Figures 3 and 4, using ultrapure water and C4RS solution as reference solutions, respectively. The introduction of La³⁺ or Ce³⁺ ions results in a notable increase in absorption intensity, as well as a pronounced bathochromic shift in the maximum peak absorption wavelength. When employing the C4RS solution as the reference, the maximum characteristic absorption of the C4RS-La was observed at 297 nm, whereas the C4RS-Ce exhibited its maximum absorption wavelength at 299 nm, as illustrated in Figures 3(b) and 4(b).

(a) UV-vis spectra of CSRS in aqueous solutions of La3+ with different concentrations, (a) Ultrapure water as reference solution; (b) C4RS aqueous solution as reference solution.

UV-vis spectra of CSRS in aqueous solutions of Ce3+ with different concentrations, (a) Ultrapure water as reference solution; (b) C4RS aqueous solution as reference solution.

The relative calibrated absorbance of the C4RS-Ln was obtained using C4RS as the reference solution. Additionally, the relative absorbance of C4RS-La at 297 nm exhibits a well-defined linear dependence on La³⁺ ion concentrations over the range of 2.0–20.0 mg L^-1, and a similar situation was found for C4RS-Ce at 299 nm. The linear regression equations of La³⁺ and Ce³⁺ were shown in the following two formulas Eqs. (1) and Eqs. (2).

(1)

A_{297 nm} - A_{(C4RS,  297 nm)} = 0.00 50 [{La}^{3 +}] - 0.00 15, r^{2} = 0. 999

(2)

A_{299 nm} - A_{(C4RS,  299 nm)} = 0.00 26 [{Ce}^{3 +}] + 0.00 15, r^{2} = 0. 997

Where [La³⁺] and [Ce³⁺] represent the mass concentrations (mg L^-1) of La³⁺ and Ce³⁺, respectively, A_297nm and A_299nm denote the absorbances of C4RS-La at 297 nm and C4RS-Ce at 299 nm, while A_{(C4RS,297 nm)} and A_{(C4RS,299 nm)} correspond to the absorbances of C4RS at 297 nm and 299 nm, respectively.

The above analysis demonstrates that water-soluble C4RS serves as an excellent chelating agent for La³⁺ and Ce³⁺ ions. Furthermore, spectroscopic analysis and quantitative experiments confirm the relatively stable formation of C4RS-La/C4RS-Ce complexes, which explains the well-established linear relationship between complex formation and rare earth ion concentrations.

3.2. Optimum extraction conditions

To identify the optimal extraction conditions for La³⁺ and Ce³⁺ ions recovery using C4RS, critical parameters including pH, C4RS concentration, Triton X-114 (TX-114) dosage, equilibrium temperature, and equilibrium time were systematically studied, as shown in Table 2. A mixed standard solution of La³⁺ and Ce³⁺ at a concentration of 1.0 mg L^-1 was transferred into a 50 mL centrifuge tube and diluted to the final volume of 50 mL with water. The CPE procedure was then applied to the mixed solution, and the resulting enriched phase containing La³⁺ and Ce³⁺ was quantitatively analyzed using the instrument of ICP-OES. In the subsequent discussion of the results, the extraction efficiency was evaluated and calculated based on spiked recovery measurements.

Table 2. Optimizing experimental parameters for CPE.

Parameters	Range of experimental parameters
pH	1	2	3	4	5	6	7	8	9	10
0.5% (v/v) TX-114, mL	0.2	0.5	0.8	1.0	1.5	1.8	2.0
1.0 mmol L⁻¹ C4RS, mL	0.1	0.2	0.5	1.0	1.5	2.0	2.5	3.0
Equilibrium temperature, °C	20	30	40	50	60	70
Equilibrium time, min	0	5	10	20	25	30

3.2.1. Effect of pH

pH plays as one of the most pivotal factors influencing the formation of metal chelates in the CPE procedure, serving as a fundamental prerequisite for the success of the experiment [17]. C4RS functions as a chelating agent, capable of binding with rare earth metals in solution, with its efficacy depending on the extent of deprotonation present. In other words, changes in pH levels affect the binding affinity of C4RS for rare earth ions. However, metal ions will form hydroxide precipitates under alkaline conditions, which are also difficult to be combined with chelating agents. Previous studies have reported that the neutral form of chelate compounds demonstrates a significantly stronger interaction with micellar aggregates compared to their ionic counterparts [33,34].

The effect of pH on the CPE procedure for extracting La³⁺ and Ce³⁺ was examined within the pH range of 1 to 10. It is evident from the original image that the color of the extraction phase varies at different pH levels (Figure 5). This phenomenon is primarily attributed to the protonation/deprotonation of phenolic hydroxyl (−OH) and sulfonic acid (−SO₃H) groups in C4RS under varying pH conditions, which subsequently modulates its chelation efficiency with metal ions.

The original image after the CPE procedure under different pH conditions.

As depicted in Figure 6, the extraction efficiency for both La³⁺ and Ce³⁺ ions in water and copper solutions remains low across the pH range of 1–3, indicating that neither metal chelates nor micelles phase was occurring. It is also evident that the extraction system under weak acidic conditions favors the binding of La³⁺ and Ce³⁺ ions by the chelating agent C4RS. Although La³⁺ and Ce³⁺ ions appear to have higher extraction rates in copper solutions under alkaline conditions, this is likely due to false-positive results due to spectral interference generated by copper ions. In our previously published work, it was indicated that the chelation ability of C4RS for copper ions is relatively weak and has a comparatively low efficiency, less than 20%, in the CPE process [21]. Thus, the optimal pH for the proposed extraction system was set at 6, and the subsequent experiments were carried out at a pH of 6.

Effect of pH on the extraction of La3+ and Ce3+ ions in water and Copper solution represented as dot plots. pH 1, 2, 3, 4, 5, 6, 7, 8, 9, 10; 1.0 mmol L-1 C4RS, 1.0 mL; TX-114: 0.5% (v/v), 1.0 mL

3.2.2. Effect of TX-114

The surfactant concentration employed in the CPE process is another crucial parameter. In this experiment, TX-114 was chosen as the extractant, while C4RS served as the metal chelating agent, respectively. TX-114 and C4RS demonstrate strong mutual solubility in water, resulting in an elevated cloud point temperature and potentially the disappearance of the cloud point phenomenon. The extraction efficiency of La³⁺ and Ce³⁺ ions was examined using TX-114 in volumes ranging from 0.2 to 2.0 mL (Figure 7).

Effect of TX114 on the extraction of La3+ and Ce3+ ions represented as bar graphs; pH 7; 1.0 mmol L-1 C4RS, 1.0 mL; 5% (%, v/v) TX-114 solution, 0.2, 0.5, 0.8, 1.0, 1.5, 1.8, 2.0 mL. The error bars are standard for “± one standard deviation of three trials.”

The results indicate that the concentration of TX-114 is insufficient to achieve complete extraction of metal chelates; however, it is noteworthy that higher concentrations do not necessarily yield better outcomes. This can be attributed to the fact that excessive surfactant concentrations increase the viscosity of the extraction solution, thereby reducing the atomization efficiency in ICP-OES analysis. When the volume of TX-114 solution is maintained within the range of 1.5–2.0 mL, the extraction efficiencies for La³⁺ and Ce³⁺ ions reach 92.5%–95.5% and 89.5%–93.0%, respectively. Based on these findings, further investigations were conducted using 1.5 mL of TX-114 solution.

In addition, the present study confirms that CPE achieves efficient separation using substantially reduced quantities of extractant (1.5 mL of 5% TX-114 per 50 mL aqueous sample). This green methodology minimizes organic reagent usage by >90% and reduces hazardous waste production compared to conventional liquid-liquid extraction techniques, demonstrating remarkable environmental benefits.

3.2.3. Effect of C4RS dosage.

In the CPE procedure employed in this study, chelating agents and extractants engage in a synergistic interaction, influencing one another. This phenomenon can be primarily attributed to the unique molecular structure of C4RS, which features hydrophilic sulfonic acid groups at the upper rim, hydrophobic isobutyl groups at the lower rim, and a central phenyl ring cavity. Essentially, C4RS functions as a surface-active agent, leading to inevitable intermolecular interactions with TX-114. The influence of C4RS dosage on the extraction efficiency of La³⁺ and Ce³⁺ ions in CPE was thoroughly investigated across a concentration range of 0.1-3.0 mL, as illustrated in Figure 8. With increasing concentrations of C4RS, the cloud point phenomenon of TX-114 becomes less pronounced or may not occur during the extraction process, as previously mentioned. The extraction efficiency of La³⁺ and Ce³⁺ increases up to a C4RS concentration of 0.01 mmol L⁻¹ and reaches 95.0% and 93.0%, respectively. In this case, the optimum C4RS dosage used for further experiments was 0.5 mL.

Effect of C4RS on the extraction of La3+ and Ce3+ ions represented as bar graphs; pH 7; 5% (%, v/v) TX-114 solution, 1.5 mL; 1.0 mmol L-1 C4RS: 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 mL. The error bars are standard for “± one standard deviation of three trials.”

3.2.4. Equilibrium temperature and time

To optimize the extraction process by minimizing both equilibrium temperature and time, the effects of these parameters on the extraction efficiency of La³⁺ and Ce³⁺ were systematically explored across temperatures ranging from 20–70°C and times ranging 0-30 min, as illustrated in Figure 9. The findings indicated that extraction efficiency was enhanced when the equilibrium temperature exceeded 50°C; however, it did not continue to rise. A similar trend was observed for the equilibrium time. It can be seen that an equilibration temperature of 50°C and an equilibrium time of 10 min were sufficient to achieve optimal extraction efficiency.

Effect of equilibrium temperature and time on the extraction of La3+ and Ce3+ ions.

3.2.5. Antifoam agent

Due to the unique chemical properties, the solution of the surfactant will have a large surface tension, resulting in heavy foams. Therefore, it is essential to introduce a small amount of anti-foaming agent to the surfactant-enriched phase to prevent foam formation before analysis by ICP-OES. In this case, the TBP/ethanol solution was used as an antifoam agent. To assess the efficacy of the defoamer, six groups of samples were analyzed in comparative tests, as presented in Figure 10. The results showed that the test group with defoamer has higher stability, while the group without defoamer has greater deviation and poor stability. Further investigation demonstrated that adding 0.4 mL of a 50% (v/v) TBP/ethanol mixed solution effectively minimized surfactant foaming.

Effect of antifoam agent on the result of instrument measurement.

3.3. Extraction of rare earth ions from copper solution

In order to assess the capability of C4RS in extracting La³⁺ and Ce³⁺ ions from high-concentration copper(Ⅱ) solutions, further experiments were conducted. The solutions with the ratio of copper ions to rare earth ions (La³⁺, Ce³⁺) of 100, 500, 1000, 2000, 3000, 4000, 5000, and 10000 times were prepared, respectively. The extraction efficiency was investigated by measuring the recovery rate with added standard, and the findings have been illustrated in Figure 11.

Extraction efficiency of La3+ and Ce3+ ions in copper solutions with different concentrations.

The results indicate that C4RS exhibits a slightly higher extraction efficiency for La³⁺ ions compared to Ce³⁺ ions across varying concentrations of copper ion solutions. The primary extraction rate of La³⁺ and Ce³⁺ can reach more than 85% when the mass concentration ratio of Cu²⁺/Ln²⁺ is <1000, and the extraction efficiency remains close to 70% even when the mass concentration ratio of Cu²⁺/Ln²⁺ exceeds 4000.

3.4. Method evaluation

Under optimized conditions, using a 50 mL sample volume, the analytical performance of the method was thoroughly evaluated, with detection limit, precision, and accuracy serving as the primary evaluation criteria. Under the predetermined ICP-OES parameters, the concentrations of La³⁺ and Ce³⁺ exhibited a linear relationship with emission intensity values ranging from 0.1 to 1.0 mg L^-1, with correlation coefficients exceeding 0.999.

3.4.1. Limit of detection (LOD) and quantification

Since neither La³⁺ nor Ce³⁺ was detected in the blank samples, the LOD was established using the spiked samples. Six spiked samples containing La³⁺ and Ce³⁺ at a concentration of 0.01 mg L^-1 each were prepared. Based on the previously established CPE procedure, optimized amounts of C4RS and TX-114 solutions were sequentially introduced to the system. Following the CPE, the six samples were subjected to ICP-OES analysis, with the results detailed in Table 3. The LODs were calculated using the formula LOD = 3σ, and the LOQs were determined by LOQ = 10σ, where σ represents the standard deviation (SD) obtained from six replicate measurements. The LODs for La³⁺ and Ce³⁺ were 0.0002 mg L^-1 and 0.0003 mg L^-1, respectively, while their corresponding LOQs reached 0.0006 mg L^-1 and 0.001 mg L^-1.

Table 3. Analytical results of the spiked sample (n=6).

Rare earth ions	Concentration (mg L^-1)			Mean (mg L^-1)	σ (mg L^-1)	LOD (mg L^-1)
La³⁺	0.0095	0.0096	0.0097	0.0096	0.00006	0.0002
La³⁺	0.0096	0.0096	0.0096	0.0096	0.00006	0.0002
Ce³⁺	0.0098	0.0097	0.0099	0.0098	0.00010	0.0003
Ce³⁺	0.0098	0.0098	0.0100	0.0098	0.00010	0.0003

3.4.2. Precision and accuracy of method

Following the CPE protocol, the spiked samples containing La³⁺ and Ce³⁺ at different concentrations were prepared and subsequently analyzed using ICP-OES. The relative standard deviations (RSDs) for the four groups of spiked samples at different concentration levels (0.02, 0.03, 0.10, and 0.20 mg L^-1) were 3.0%, 2.7%, 6.3% and 1.3% (n= 6) for La³⁺ and 4.4%, 3.0%, 5.5% and 1.5% (n= 6) for Ce³⁺ respectively. The excellent spiked recoveries of 95.1%, 93.8%, 91.3%, and 94.7% for La³⁺ and 96.4%, 94.6%, 93.0%, and 96.8% for Ce³⁺, respectively, as shown in Table 4. All experimental values were analyzed using Dixon’s Q test. No outliers were detected in the dataset (n=6) at the 95% confidence level (α=0.05). It is evident that the method demonstrates a considerable degree of precision and accuracy.

Table 4. Analytical results of precision and accuracy (n=6).

La³⁺	Concentration after CPE (mg L^-1)			Mean (mg L^-1)	Spiked (mg L^-1)	RSD (%)	Recovery (%)
Spiked sample 1	0.194	0.189	0.186	0.190	0.20	3.0	95.1
Spiked sample 1	0.200	0.186	0.186	0.190	0.20	3.0	95.1
Spiked sample 2	0.280	0.287	0.294	0.282	0.30	2.7	93.8
Spiked sample 2	0.275	0.277	0.276	0.282	0.30	2.7	93.8
Spiked sample 3	0.890	0.859	0.891	0.913	1.00	6.3	91.3
Spiked sample 3	0.985	0.867	0.987	0.913	1.00	6.3	91.3
Spiked sample4	1.857	1.878	1.904	1.894	2.00	1.3	94.7
Spiked sample4	1.912	1.926	1.888	1.894	2.00	1.3	94.7
Ce³⁺	Concentration after CPE (mg L^-1)			Mean (mg L^-1)	Spiked (mg L^-1)	RSD (%)	Recovery (%)
Spiked sample 1	0.197	0.191	0.186	0.193	0.20	4.4	96.4
Spiked sample 1	0.207	0.193	0.183	0.193	0.20	4.4	96.4
Spiked sample 2	0.283	0.289	0.298	0.284	0.30	3.0	94.6
Spiked sample 2	0.277	0.277	0.278	0.284	0.30	3.0	94.6
Spiked sample 3	0.900	0.910	0.872	0.930	1.00	5.5	93.0
Spiked sample 3	0.988	0.911	0.997	0.930	1.00	5.5	93.0
Spiked sample 4	1.896	1.910	1.948	1.935	2.00	1.5	96.8
Spiked sample 4	1.959	1.972	1.925	1.935	2.00	1.5	96.8

4. Conclusions

The CPE of La³⁺ and Ce³⁺ from water and copper solutions is highly efficient with sulfonatomethylated calix[4]resorcinarene, which demonstrates greater complexation affinity for Ln³⁺ compared to Cu²⁺. It is mainly attributed to C4RS possessing four unique, flexible methylene-linked sulfonic acid groups. The flexible conformation of the methylene groups enables optimal spatial adaptability for multidentate ligands, while the sulfonic acid groups ensure C4RS has excellent water solubility, achieving effective phase transfer during the extraction process. The quantitative correlation between C4RS and La³⁺ or Ce³⁺ ions was measured using UV spectroscopy, showing that the intensity of the absorption of C4RS-Ln complexes increases linearly with La³⁺ or Ce³⁺ ion concentrations.

The CPE conditions and enrichment parameters for La³⁺ and Ce³⁺ ions have been optimized. The CPE procedure has proven effective for extracting and separating trace La³⁺ and Ce³⁺ from a copper solution with a 1000:1 mass ratio, achieving a primary extraction efficiency of up to 90%. The results demonstrate that the CPE method based on C4RS chelation can effectively achieve the separation and enrichment of rare earth ions, such as La³⁺ and Ce³⁺, from copper-based solutions.

In this study, the CPE procedure system could be coupled with ICP-OES for the trace analysis of La³⁺ and Ce³⁺ ions, with detection limits of 0.0002 mg L^-1 and 0.0003 mg L^-1 for La³⁺ and Ce³⁺, respectively. The relative standard deviations ranged from 1.3% to 6.3%, and the spiked recoveries were between 91.3% and 96.8%. The method of CPE-ICP-OES based on C4RS could be applied to the separation, extraction, enrichment, and quantitative analysis of La³⁺ and Ce³⁺ in water and copper solution.

However, this study only investigated the extraction of lanthanum and cerium ions by C4RS in aqueous and copper-containing solutions, which presents limitations regarding the broader range of rare earth elements. Future research will focus on developing and optimizing extraction protocols for multiple rare earth elements in different matrices. The proposed methodology is expected to demonstrate robust applicability for rare earth recovery and quantification in complex environmental samples, particularly wastewater streams and soil systems.

Acknowledgment

This project was supported by the Tongling University Scientific Research Project, China (2024tlxyptZD06) and the Higher Education Institutions Education and Teaching of Anhui Province, China (2023jyxm0723).

CRediT authorship contribution statement

Jing-Long Liu: Design, Manuscript preparation, Manuscript editing and review, Visualization, Methodology, Experimental studies, Data curation, Funding acquisition, Conceptualization, Project administration. Bin Xu: Experimental operation, Manuscript preparation, Investigation, Data curation. Shou-Dong Chen: Statistical analysis, Supervision, Formal analysis, Data curation, Conceptualization, Funding acquisition. Jie Li: Manuscript preparation, Investigation, Validation, Project administration, Data curation, Conceptualization. Qian-Feng Zhang: Supervision, Statistical analysis, Project administration, Validation, Supervision, Data curation.

Declaration of competing interest

The authors declare that there are no known financial conflicts of interest or personal relationships that could have influenced the findings presented in this study.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of AI-assisted technology for assisting in the writing of the manuscript and no images were manipulated using AI.

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1. Introduction

2. Materials and Methods

2.1. Chemicals
2.2. CPE
2.3. ICP-OES and UV-vis analysis

3. Results and Discussion

3.1. UV spectrophotometry
3.2. Optimum extraction conditions
3.3. Extraction of rare earth ions from copper solution
3.4. Method evaluation

4. Conclusions

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[5] Xie F., Zhang T.A., Dreisinger D., Doyle F.. A critical review on solvent extraction of rare earths from aqueous solutions. Minerals Engineering. 2014;56:10-28. https://doi.org/10.1016/j.mineng.2013.10.021
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[7] Gaustad G., Williams E., Leader A.. Rare earth metals from secondary sources: Review of potential supply from waste and byproducts. Resources, Conservation and Recycling. 2021;167:105213. https://doi.org/10.1016/j.resconrec.2020.105213
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[8] Cao Y., Shao P., Chen Y., Zhou X., Yang L., Shi H., Yu K., Luo X., Luo X.. A critical review of the recovery of rare earth elements from wastewater by algae for resources recycling technologies. Resources, Conservation and Recycling. 2021;169:105519. https://doi.org/10.1016/j.resconrec.2021.105519
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[10] Abdelzaher M.A., Hamouda A.S., El-Kattan I.M.. A comprehensive study on the fire resistance properties of ultra-fine ceramic waste-filled high alkaline white cement paste composites for progressing towards sustainability. Scientific Reports. 2023;13:22097. https://doi.org/10.1038/s41598-023-49229-4
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[11] Abdelzaher M.A., Farghali A.A., Hamouda A.S.. Effective impact of nano-plastic-waste incorporated with nanotitina on the physical, mechanical and microstructural properties of white cement pastes composites for progressing towards sustainability. Scientific Reports. 2024;14:12581. https://doi.org/10.1038/s41598-024-62661-4
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[12] Rao M., Xia H., Xu Y., Jiang G., Zhang Q., Yuan Y., Zhang L.. Study on ultrasonic assisted intensive leaching of germanium from germanium concentrate using HCl/NaOCl. Hydrometallurgy. 2024;230:106385. https://doi.org/10.1016/j.hydromet.2024.106385
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[13] Zhang Y., Liu Z., Wang J., Du H., Sun Q., Gao R., Xu Z.. Efficient and high-selective lithium extraction from waste LiMn₂O₄ batteries by synergetic pyrolysis with polyvinyl chloride. Waste Management (New York, N.Y.). 2025;198:95-105. https://doi.org/10.1016/j.wasman.2025.02.049
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[14] Chen X.-M., Li H.-Y., Wei C.-C., Cheng J., Diao J., Xie B., Pan F.. Selective chemical etching of vanadium slag enables highly efficient and clean extraction of vanadium. ACS Sustainable Chemistry & Engineering. 2025;13:1327-1335. https://doi.org/10.1021/acssuschemeng.4c08516
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[15] Ren H., Quan Y., Liu S., Hao J.. Effectiveness of ultrasound (US) and slightly acidic electrolyzed water (SAEW) treatments for removing Listeria monocytogenes biofilms. Ultrasonics Sonochemistry. 2025;112:107190. https://doi.org/10.1016/j.ultsonch.2024.107190
[Google Scholar]

[16] Xie F., Zhang T.A., Dreisinger D., Doyle F.. A critical review on solvent extraction of rare earths from aqueous solutions. Minerals Engineering. 2014;56:10-28. https://doi.org/10.1016/j.mineng.2013.10.021
[Google Scholar]

[17] Sun M., Liu S., Zhang Y., Liu M., Yi X., Hu J.. Insights into the saponification process of di(2-ethylhexyl) phosphoric acid extractant: Thermodynamics and structural aspects. Journal of Molecular Liquids. 2019;280:252-258. https://doi.org/10.1016/j.molliq.2019.02.025
[Google Scholar]

[18] Mortada W.I.. Recent developments and applications of cloud point extraction: A critical review. Microchemical Journal. 2020;157:105055. https://doi.org/10.1016/j.microc.2020.105055
[Google Scholar]

[19] Mandal S., Lahiri S.. A review on extraction, preconcentration and speciation of metal ions by sustainable cloud point extraction. Microchemical Journal. 2022;175:107150. https://doi.org/10.1016/j.microc.2021.107150
[Google Scholar]

[20] Basque J., Lavoie J., Reynier N., Larivière D.. Quantitative separation of thorium from rare earth elements and uranium in a rare earth element sulfuric acid leachate using cloud point extraction. Microchemical Journal. 2023;190:108724. https://doi.org/10.1016/j.microc.2023.108724
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[21] Al-Mohaimeed A.M., Abou El-Reash Y.G., AlMarri A.H., Hashem M.A., Wahba M.G., Ismail M.A.. Selective removal of lanthanum from monazite – (Nd) ores by pH-controlled mixed micelle cloud point extraction. Microchemical Journal. 2023;193:109158. https://doi.org/10.1016/j.microc.2023.109158
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[22] Liu J.-L., Zhou X.-M., Sun M., Jia A.-Q., Shi H.-T., Zhang Q.-F.. A resorcinarene based chelating agent for selective cloud point extraction of Pb²⁺ ions in water: Synthesis, structural characterization and analytical applications. Arabian Journal of Chemistry. 2023;16:104866. https://doi.org/10.1016/j.arabjc.2023.104866
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[23] Mustafina A., Elistratova J., Burilov A., Knyazeva I., Zairov R., Amirov R., Solovieva S., Konovalov A.. Cloud point extraction of lanthanide(III) ions via use of Triton X-100 without and with water-soluble calixarenes as added chelating agents. Talanta. 2006;68:863-868. https://doi.org/10.1016/j.talanta.2005.06.011
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[24] Wei Y., Li Y., Quan X., Liao W.. Cloud point extraction and separation of copper and lanthanoids using Triton X-100 with water-soluble p-sulfonatocalix[4]arene as a chelating agent. Microchimica Acta. 2010;169:297-301. https://doi.org/10.1007/s00604-010-0353-x
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[25] Mustafina A., Zakharova L., Elistratova J., Kudryashova J., Soloveva S., Garusov A., Antipin I., Konovalov A.. Solution behavior of mixed systems based on novel amphiphilic cyclophanes and Triton X100: Aggregation, cloud point phenomenon and cloud point extraction of lanthanide ions. Journal of Colloid and Interface Science. 2010;346:405-413. https://doi.org/10.1016/j.jcis.2010.03.002
[Google Scholar]

[26] Zairov R.R., Elistratova Y.G., Mustafina A.R., Amirov R.R., Pilishkina L.M., Antipin I.S., Konovalov A.I.. Extraction of lanthanum and gadolinium(III) at the cloud point using p-sulfonatocalyx[n]arenes as chelating agents. Colloid Journal. 2009;71:69-75. https://doi.org/10.1134/s1061933x09010086
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[27] Kazakova E.K., Makarova N.A., Ziganshina A.U., Muslinkina L.A., Muslinkin A.A., Habicher W.D.. Novel water-soluble tetrasulfonatomethylcalix[4]resorcinarenes. Tetrahedron Letters. 2000;41:10111-10115. https://doi.org/10.1016/s0040-4039(00)01798-6
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[28] Sanabria E., Esteso M.Á, Pérez-Redondo A., Vargas E., Maldonado M.. Synthesis and characterization of two sulfonated resorcinarenes: A new example of a linear array of sodium centers and macrocycles. Molecules (Basel, Switzerland). 2015;20:9915-9928. https://doi.org/10.3390/molecules20069915
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[30] Liu J.L., Zhang P.Z., Jia A.Q., Shi H.T., Zhang Q.F.. Supramolecular Assemblies of Sulfonatomethylated Calix[4]resorcinarenes with Aquated Sodium(I), Cesium(I), and Aluminum(III) Ions. ChemistrySelect. 2022;7 https://doi.org/10.1002/slct.202104118
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[31] Favre-Réguillon A., Draye M., Lebuzit G., érard, Thomas S., Foos J., Cote G., érard, Guy A.. Cloud point extraction: An alternative to traditional liquid–liquid extraction for lanthanides(III) separation. Talanta. 2004;63:803-806. https://doi.org/10.1016/j.talanta.2003.12.033
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[32] Li Y., Hu B.. Cloud point extraction with/without chelating agent on-line coupled with inductively coupled plasma optical emission spectrometry for the determination of trace rare earth elements in biological samples. Journal of Hazardous Materials. 2010;174:534-540. https://doi.org/10.1016/j.jhazmat.2009.09.084
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[33] Azooz E.A., Al-Murshedi A.Y.M., Abodiame A.A.M., Shabaa G.J., El Abbadi N.K., Snigur D.. A novel green cloud point extraction-based switchable hydrophilicity solvent method for antimony separation and quantification from various bottled beverages by HGAAS. Microchemical Journal. 2024;207:111824. https://doi.org/10.1016/j.microc.2024.111824
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[34] Frankewich R.P., Hinze W.L.. Evaluation and optimization of the factors affecting nonionic surfactant-mediated phase separations. Analytical Chemistry. 1994;66:944-954. https://doi.org/10.1021/ac00079a005
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