In Situ Modification of Activated Carbon Made from Camellia oleifera Shell with Na₂EDTA for Enhanced La³⁺ Recovery

Abstract

It is important to recover La³⁺ from metallurgical solutions or wastewater. However, the recovery rate of La³⁺ is usually less than 1% and the recovery methods are not environmentally friendly or user-friendly. Therefore, a straightforward, efficient, clean, and economically friendly method is needed. In this investigation, a modified adsorbent, COSAC-Na₂EDTA-15, which was made from the Camellia oleifera shell (COS) and disodium ethylenediaminetetraacetic acid (Na₂EDTA), was invented. In addition, characterization of the COSAC-Na₂EDTA-15 adsorbent was conducted using SEM and XPS, and the principle of adsorption was revealed. The adsorption kinetics followed P-S-O KM, while the isotherm of COS-activated carbon (COSAC) aligned more closely with the Langmuir model. Compared to COSAC, the maximum La³⁺ adsorption capacity of COSAC-Na₂EDTA-15 increased from 50 to 162.43 mg/g, and the content of O and N changed from 7.31% and 1.48% to 12.64% and 4.15%, respectively. The surface of the COSAC-Na₂EDTA-15 exhibited abundant C, N, and O elements, and La³⁺ was detected on the sample surface after adsorption. The test and analysis results fully indicate that La³⁺ can be successfully adsorbed on the surface of COSAC-Na₂EDTA-15. Because of its easy preparation, low cost, and superior performance, activated carbon made from COS finds extensive applications in the adsorption and recovery of rare earth elements.

1. Introduction

Rare earth elements exhibit distinctive characteristics, both physical and chemical, and play a crucial role in high-tech fields and the development of advanced materials [1,2]. They are widely used in electrochemical sensors, supercapacitors, biomedical nanocomposites, luminescent materials, superconducting materials, permanent magnetic materials, and so on [3,4]. Lanthanum (La), a significant rare earth element, holds a pivotal position in strategic industries. Its primary applications encompass superconductors, ceramics, electrochemistry, catalysts, magnetism, luminescence, and additive materials [5]. According to reports, the worldwide annual consumption of lanthanum (La) is approximately 200 metric tons [6,7]. The release of La³⁺ into the environment and food chain has markedly escalated due to the widespread utilization of products containing La [8]. Lanthanum ions (La³⁺) exhibit toxicity to humans, leading to cell membrane damage and influencing the reproductive and nervous systems [9,10]. However, Hunt et al. reported that the recovery rate of La³⁺ is less than 1% [7]. Hence, to safeguard the environment and promote the sustainable utilization of limited resources, the recovery of La³⁺ from wastewater systems becomes imperative [11,12]. Currently, the main techniques used for metal ion recovery are precipitation, ion exchange, solvent extraction, and adsorption [13,14]. The adsorption method has been widely regarded because of its easy operation, high efficiency, low cost, and little pollution [15]. Activated carbon stands out as an economical and efficient adsorbent [16]. Coal, plants, and animal bones are the main sources used to produce activated carbon. However, given the sustainability and renewability of the materials, researchers have recently examined and used a variety of agricultural wastes and biomass to create activated carbon [17].

Camellia oleifera, an economically important oil crop with extensive cultivation [18], is recognized as an important woody oil tree species globally [19]. The main output obtained from Camellia oleifera is oil, while the by-product, Camellia oleifera shell (COS) [20], makes up about 50%–60% of the total weight of the fresh fruit [21]. In 2020, China produced approximately 3.68 million tons of COS [22]. The utilization rate of COS is low, and it is mainly used as a solid fuel [22] or dumped improperly [23,24], which has the potential to impact the environment. COS primarily consists of cellulose, lignin, and hemicellulose [25,26], and can serve as the perfect raw material for obtaining activated carbon [27]. COS manufactured biochar can adsorb toxic chemicals [28,29] and heavy metals [30,31] in water [32]. Nevertheless, it has not yet been employed for the recovery of rare earth ions. The application of COS-derived activated carbon to remove La³⁺ can realize the recycling of agricultural wastes, thus reducing resource waste and environmental pollution at the same time [33].

This study represents an effort to extract rare earth elements from industrial production wastewater using camellia shells, a type of agricultural waste. In our study, Camellia oleifera shell activated carbon (COSAC) was synthesized through one-step pyrolysis with phosphoric acid activation, using COS as the precursor and adding a disodium ethylenediaminetetraacetic acid (Na₂EDTA) modifier. The possibility of using COSAC-Na₂EDTA to recover La³⁺ from aqueous solutions was studied. The effects of impregnation ratio, impregnation time, activation temperature, and the modifier amount on the properties of the activated carbon were studied.

2. Experimental

2.1. Materials and Reagents

The COS employed here was sourced from Ganzhou City, Jiangxi Province, China. The COS underwent a washing process with deionized water, followed by oven drying at 100 °C for 12 h, and subsequent crushing and sieving (0.106 mm) before utilization. The phosphoric acid was 85 wt%. Na₂EDTA was purchased from Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China). All chemical reagents utilized here were of analytical grade and were employed without further purification. The water used in the experiment was deionized water.

2.2. Synthesis of COSAC-Na₂EDTA

Figure 1 shows the synthesis of COSAC-Na₂EDTA. The COS powder, having 5.0 g in weight, was mixed with varying amounts of Na₂EDTA (0, 10, 15, and 20 mmol) and subsequently soaked in phosphoric acid for a duration. The samples were heated to a certain temperature (500 °C, 600 °C, and 700 °C, respectively). The heating rate was 5 °C/min. When the temperature reached the target value, the samples were kept in N₂ for 1 h, and cooled to room temperature. Following that, the samples underwent rinsing with deionized water until reaching a stable pH, were then dried in a vacuum oven at 90 °C overnight, and finally finely ground into particles smaller than 100 meshes. This process yielded Na₂EDTA-modified camellia shell activated carbon, designated as COSAC-Na₂EDTA-X, where X represents the amount of Na₂EDTA.

2.3. Characterization

A nitrogen adsorption–desorption test was performed using a Micromeritics APSP 2460 4-station full-automatic specific surface area analyzer. The surface element composition, chemical bonding environment, and adsorption sites of the activated carbon were scrutinized through XPS analysis. The surface morphology and energy spectrum of activated carbon were investigated using SEM (ZEISS Sigma 300 from Oberkochen, Germany) [33]. The concentration of La³⁺ was measured, employing the instrument parameters recommended by the manufacturer.

2.4. Adsorption and Recovery Experiment

La(NO₃)₃·6H₂O was used to prepare La³⁺ solutions of different concentrations for adsorption experiments. The La³⁺ solution, after preparation, was placed into a 100 mL conical flask. Subsequently, the adsorbent was added for the adsorption experiment, which took place in a constant temperature oscillator at 150 rpm/min for various durations. After the experiment, a membrane filter was used for filtration, then the concentration of La³⁺ was measured. When the adsorption test was finished, the desorption tests were carried out using 0.5 M HCl. The adsorption capacity, $q_{\mathrm{e}}$ (mg/g), was computed using Equation (1):

$$q_{\mathrm{e}}=\frac{\left(C_0-C_{\mathrm{e}}\right)\times V}{m}$$

(1)

where C₀ (mg/L) and C_e (mg/L) represent the initial and equilibrium concentrations of La³⁺ in the solution, respectively, V is the volume of solution (L), and m is the weight (g) of the adsorbent.

2.5. Kinetic and Isothermal Models

The fitting and analysis of the data included employing both the pseudo-first-order kinetic model (P-F-O KM) and the pseudo-second-order kinetic model (P-S-O KM). These models were utilized to identify the primary rate-controlling steps in the adsorption of La³⁺ on carbon samples, as described in Equations (2) and (3) [34,35]:

$$\ln \left(q_{\mathrm{e}}-q_{\mathrm{t}}\right)=\mathrm{l}\mathrm{n}{q}_{\mathrm{e}}-{\mathrm{k}}_{1}t$$

(2)

$$\frac{t}{q_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{q}_{\mathrm{e}}^2}+\frac{1}{q_{\mathrm{e}}}t$$

(3)

where q_e and q_t represents the amount of La³⁺ adsorbed per mass of sorbent (mg/g) at equilibrium and any time, and k₁ and k₂ are rate constants of pseudo-first-order and pseudo-second-order, respectively.

The Langmuir and Freundlich models were employed for fitting and analyzing the data to investigate the interaction between activated carbon and the adsorbate. Both of these models have the capability to depict the adsorption process and ascertain the maximum adsorption capacity [7,36]:

$$q_{\mathrm{e}}=\frac{q_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{C}_{\mathrm{e}}}$$

(4)

$$\mathrm{l}\mathrm{n}{q}_{\mathrm{e}}=\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{F}}+\frac{1}{n}\mathrm{l}\mathrm{n}{C}_{\mathrm{e}}$$

(5)

where $q_{\mathrm{e}}$ represents the adsorption capacity of La³⁺ at equilibrium, $q_{\mathrm{m}}$ denotes the maximum adsorption capacity (mg/g), C_e denotes the concentration of adsorbate in solution at the equilibrium point (mg/L), K_L stands for the Langmuir constant (L/mg), K_F stands for the Freundlich constant, and 1/n stands for the Freundlich exponent.

3. Results and Discussion

3.1. Properties of COSAC at Different Preparation Conditions

The phosphoric acid soaking time, soaking ratio (mL H₃PO₄/g COS), activation temperature, and Na₂EDTA dosage are the key parameters in the process of preparing camellia shell-activated carbon that will affect the La³⁺ adsorption performance. To prepare highly efficient activated carbon from COS, the influence of preparation conditions has been optimized for its adsorption performance. The La³⁺ adsorption performance of COSAC following different preparation conditions is shown in Figure 2.

Firstly, the influence of COS immersion time in H₃PO₄ on the adsorption of La³⁺ was studied by fixing the activation time at 60 min, and the experimental conditions were set with an impregnation ratio of H₃PO₄ to COS of 15 mL/5 g, an activation temperature of 500 °C, an Na₂EDTA dosage of 10 mmol, a solution pH of 5.5, and an initial La³⁺ concentration of 100 mg/L. The results of the adsorption performance are given in Figure 2a. When the soaking time increased from 1 to 12 h, the adsorption efficiency of the activated carbon for La³⁺ initially rose and then declined. With an extension of the immersion time from 1 to 3 h, the La³⁺ adsorption capacity of the activated carbon experienced a rapid increase, reaching 76.5 mg/g. When the time further increased to 12 h, the La³⁺ adsorption capacity of the activated carbon slowly declined. The cause for this may be attributed to the hydrolysis of lignin in COS by phosphoric acid, leading to an enhancement in the pore size of the activated carbon as the immersion time is prolonged. However, further extension of the time may lead to excessive erosion of the carbon skeleton, thus reducing the performance of the activated carbon. Therefore, the best soaking time of 3 h was used to study the influence of the soaking ratio 10/5, 12.5/5, and 15/5 during the adsorption test. The resulting adsorption capacities of COSAC for La³⁺ were observed to be 67.85, 80.58, and 76.5 mg/g, respectively, as shown in Figure 2b. Consequently, the optimal impregnation ratio was determined to be 12.5/5. This specific impregnation ratio was subsequently employed in the subsequent experiments.

The activated carbon prepared at different temperatures (500, 600, and 700 °C) was tested in batches to study the effect of pyrolysis temperature. The adsorption capacity for La³⁺ demonstrated an increase with the rising activation temperature, as shown in Figure 2c. With the elevation of the activation temperature from 500 to 700 °C, the adsorption capacity for La³⁺ exhibited an increase from 80.58 to 135.1 mg/g. At 800 °C, the presence of phosphoric acid caused adhesion between the quartz boat and the quartz tube in the tubular furnace, which led to the failure of the experiment. Therefore, 700 °C was selected to prepare COSAC.

To assess the influence of the Na₂EDTA dosage on the La³⁺ adsorption capacity, the soaking time was kept at 3 h, the soaking ratio at 12.5/5, and the activation temperature at 700 °C, with variations in Na₂EDTA dosage (0, 10, 15, and 20 mmol). The adsorption capacity exhibited an increase with the rise in Na₂EDTA dosage from 0 to 15 mmol, as shown in Figure 2d. When the amount of Na₂EDTA was 0 mmol, the activated carbon without an Na₂EDTA modification had an La³⁺ adsorption capacity of 49.33 mg/g. When the amount of Na₂EDTA was 15 mmol, the La³⁺ adsorption capacity reached 162.43 mg/g, indicating that the Na₂EDTA modifier can greatly enhance the La³⁺ adsorption effect of activated carbon from COS. However, when the amount of Na₂EDTA exceeded 15 mmol, the adsorption capacity decreased. The reason is that the added Na₂EDTA will generate a large amount of ammonia through thermal decomposition, which is conducive to increasing the amide and amino group of the surface ring of the activated carbon [37]. This could improve the adsorption sites for La³⁺, thereby increasing the La³⁺ adsorption capacity. However, a further increase in dosage may disrupt the original active sites on the surface of COSAC, leading to a decrease in the La³⁺ adsorption capacity. Therefore, the optimal dosage is 15 mmol.

3.2. Kinetics and Isotherms

The best Na₂EDTA-modified activated carbon from COS (COSAC-Na₂EDTA-15) was prepared with a soaking time of 3 h, a soaking ratio of 12.5/5, an activation temperature of 700 °C, and an Na₂EDTA dosage of 15 mmol. Figure 3 illustrates the adsorption performance of COSAC-Na₂EDTA-15 at different contact times and initial La³⁺ concentrations. The adsorption capacity of lanthanum ions exhibits a rapid increase over time, reaching 90% of the maximum adsorption capacity within thirty minutes, as shown in Figure 3a. With a continued increase in time, the adsorption progressively decelerated and eventually reached equilibrium after 2 h. At this time, the adsorption capacity was 162.43 mg/g.

In Figure 3b, the adsorption capacity at different initial La³⁺ concentrations is illustrated. As the initial concentration increases, the adsorption capacity reaches its peak value when the concentration exceeds 100 mg/g.

In order to examine the adsorption mechanism of COSAC-Na₂EDTA-15 for La³⁺, the P-F-O KM and P-S-O KM were applied for nonlinear regression fitting of the adsorption data. The corresponding kinetic fitting curves are given in Figure 4a and Figure 4b, respectively. In comparison to the P-F-O KM, the P-S-O KM exhibited a stronger correlation with the adsorption data of La³⁺ on COSAC-Na₂EDTA-15, and its correlation coefficient was 0.99. This result suggests that the process of the adsorption of La³⁺ onto COSAC-Na₂EDTA-15 mainly involved chemical adsorption [38].

According to the results of the Langmuir and Freundlich adsorption isotherm models, the correlation coefficient for the Langmuir model (R² = 0.99) surpasses that of the Freundlich model (R² = 0.91), suggesting that the adsorption of La³⁺ by COSAC-Na₂EDTA-15 aligns more closely with the Langmuir model, as shown in Figure 4c,d. Therefore, the adsorption of La³⁺ on Na₂EDTA-modified camellia shell activated carbon was monolayer adsorption.

3.3. Adsorption Selectivity and Recycling Characteristics

There are a lot of coexisting ions such as sodium, magnesium, copper, and calcium ions in waste water that may affect the adsorption of lanthanum ions. Hence, it is imperative to conduct an analysis of the selectivity of La³⁺ over Na⁺, Mg²⁺, and Cu²⁺ ions. The adsorption selectivity and recycling characteristics of tea tree shell activated carbon are shown in Figure 5. Figure 5a illustrates that after four cycles COSAC-Na₂EDTA-15 still exhibits good recovery performance for lanthanum ions. In Figure 5b, it is evident that Na⁺, Mg²⁺, and Cu²⁺ exert minimal impact on the adsorption capacity of lanthanum ions. This observation suggests that COSAC-Na₂EDTA-15 exhibits excellent ion selectivity.

To assess the practical applicability of COSAC-Na₂EDTA-15 for the recovery of La³⁺,

Table 1. Lanthanum (La³⁺) adsorption capacities of various adsorbents.

Absorbent	Max. Adsorption Capacity (mg/g)	Equilibrium Time (h)	Refs.
Amine modified activated carbon	107	1.5	[15]
Iron oxide-titanium oxide nanoparticles	89.63	1	[5]
MSFs-1	77.74	3	[11]
PVA/SHMP HENF	181.82	2	[39]
ADSORBSIA™ As500	24.77	1	[40]
La-IIP	62.8	0.5	[41]
Valorised Ulva Sp. biomass	119.297	2	[2]
ZnO clay nanocomposite hydrogel	58.8	2	[42]
Sargassum fluitans	73.62	24	[43]
Amberlite IRC86	40.60	--	[44]
COSAC	50	2	This study
COSAC-Na2EDTA-15	162.43	2	This study

compiles the maximum adsorption capacities (expressed in mg/g) of various adsorbents for La³⁺. In comparison with the adsorbents documented in the literature, both COSAC and COSAC-Na₂EDTA-15 have a higher lanthanum adsorption performance and great potential for La³⁺ recovery.

3.4. Mechanism Analysis

Through XPS characterization of COSAC and COSAC-Na₂EDTA-15, we are able to comprehend how the Na₂EDTA alteration affects the quantity of O- and N-containing functional groups. The N1s and O1s peaks in the Na₂EDTA-modified activated carbon are significantly enhanced compared with the unmodified activated carbon, as shown in Figure 6 and

Table 2. Proportion of elements C, N, P, and O in the sample.

Sample	C (%)	N (%)	P (%)	O (%)
COSAC	89.51	1.48	1.7	7.31
COSAC-Na2EDTA-15	80.98	4.15	2.23	12.64

. The unmodified COSAC also contains O and a small amount of N. After modification, there is an increase in the percentage of O from 7.31% to 12.63%, and the percentage of N rises from 1.48% to 4.15%. XPS analysis indicates that Na₂EDTA effectively enhances the content of N- and O-containing functional groups. This contributes to the enhanced adsorption capacity for La³⁺.

For a more comprehensive understanding of the La³⁺ adsorption mechanism on activated carbon surfaces, XPS was used to analyze the peak shift after La³⁺ adsorption on activated carbon. The C 1s peak was smoothed by a Gaussian function. Figure 7 is the C 1s peaks before and after the activated carbon adsorption of lanthanum ions. The C 1s spectrum of COSAC-Na₂EDTA-15 prior to adsorption reveals four distinct peaks with binding energies at 284.8 eV (C-C), 285.99 eV (C-O/C-N), 288.13 eV (COOR), and 290.97 eV (π-π*). When La³⁺ was adsorbed, the binding energies of C-C, C-O/C-N in COSAC-Na₂EDTA-15 were transferred to higher regions. Figure 7 shows that the energy of the C-O/C-N peak has elevated from 285.99 to 286.03 eV. These observations suggest that the C-C and C-O/C-N functional groups present on the surface of COSAC-Na₂EDTA-15 establish coordination bonds with La³⁺, with the stable La³⁺ adsorption on the activated carbon surface.

The adsorption–desorption isotherms and pore size distribution of COSAC and COSAC-Na₂EDTA-15 are given in Figure 8. The BET specific surface areas for these samples were determined to be 1443.45 m²/g for COSAC and 1068.68 m²/g for COSAC-Na₂EDTA-15. Figure 8 illustrates that the majority of pore sizes in COSAC and COSAC-Na₂EDTA-15 are below 20 nm, suggesting that the pores in the tested materials are predominantly mesoporous.

To validate the successful adsorption of La³⁺ by the activated carbon derived from tea camellia shell modified with Na₂EDTA, the morphological changes and elemental distributions of COSAC-Na₂EDTA-15 were analyzed using a scanning electron microscope and an energy dispersive spectrometer (SEM-EDS). As depicted in Figure 9, COSAC-Na₂EDTA-15 exhibits a porous structure, characterized by a notable presence of C and O. The detection of lanthanum on the surface of the sample validates the XPS findings and indicates the uniform adsorption of La³⁺ on the surface of COSAC-Na₂EDTA-15.

4. Conclusions

The agricultural waste camellia shell was used as the precursor, and Na₂EDTA was added to the phosphoric acid activation process to prepare COSAC. This enhancement effectively improves the adsorption capacity for La³⁺. The adsorption of La³⁺ on modified camellia shell activated carbon is more focused on monolayer adsorption, which is more consistent with the Langmuir model. In contrast to the P-F-O KM, the P-S-O KM exhibits a superior level of linear fitting, suggesting that the predominant mechanism is chemical adsorption during the adsorption. The peak value of the adsorption capacity for La³⁺ is 162.43 mg/g. XPS analysis revealed that the proportion of oxygen (O) in COSAC-Na₂EDTA-15 rose from 7.31% to 12.64%, while the proportion of nitrogen (N) increased from 1.48% to 4.15%. SEM-EDS analysis confirmed the presence of abundant carbon (C), nitrogen (N), and oxygen (O) elements on the surface of COSAC-Na₂EDTA-15. Additionally, La³⁺ elements were identified on the sample’s surface, indicating the stable adsorption of La³⁺ on the surface of COSAC-Na₂EDTA-15. The COSAC has the advantages of easy preparation, low price, and excellent performance, and is expected to become a carbon material for recovering rare earth elements from wastewater.