Synthesis of High-Crystallinity Mg-Al Hydrotalcite with a Nanoflake Morphology and Its Adsorption Properties for Cu²⁺ from an Aqueous Solution

Abstract

A magnesium–aluminum-layered double hydroxide (Mg-Al LDH) with a nano-lamellar morphology was prepared by using a homogeneous precipitation and hydrothermal method, and a calcination product (Mg-Al LDO) of the Mg-Al LDH was also obtained in this work. The XRD, TEM, SEM, FTIR, N₂ ad/desorption, and TG-DTG techniques were employed to characterize the microstructures, morphologies, and thermostability levels of these two materials in detail. The results showed that both the Mg-Al LDH and Mg-Al LDO had mesoporous structures and nanoplate morphologies, with diameters of 50~200 nm. The Mg-Al LDH was transformed into Mg-Al LDO at 773 K in an air atmosphere. The adsorption properties of the Mg-Al LDH were investigated systematically with a copper chloride solution as a simulated waste. The experimental results demonstrated that the pH value of the solution had an obvious influence on its Cu²⁺ adsorption capacity, and the optimal pH value was approximately 5.0. The adsorption kinetics results showed that the Mg-Al LDH had a rapid adsorption rate, and the equilibrium adsorption capacity was 62.11 mg/g. Additionally, the Cu²⁺ adsorption could be commendably described using a pseudo-second-order model, demonstrating that the adsorption behavior is regulated by chemical sorption. The adsorption thermodynamic results indicated that the adsorption process was spontaneous at temperatures above 318 K. Moreover, the ΔG⁰ values decreased as the temperature was raised, which indicated that a higher temperature can cause a greater impetus for Cu²⁺adsorption. In addition, the positive values of the ΔH⁰ indicated that the Cu²⁺ adsorption was endothermic, and the positive ΔS⁰ values revealed an increase in the confusion at the solid–liquid interface of the adsorbent.

1. Introduction

With the development of social economies and industrial development, toxic heavy metal contamination in various types of water is becoming more and more serious, and it has become a serious threat to human and environmental health [1,2]. It has become one of the most important environmental problems from an ecological and health perspective. Heavy metals are highly toxic and have long-term sustainability, and they will not degrade by themselves in the environment but can only migrate and transform to other substances [3,4,5,6]. Additionally, some heavy metals may combine with organic matter to convert into more toxic organometallic compounds [7]. The accumulation of heavy metals through the biological chain will eventually enter the human body and cause serious harm to human health, even leading to death when its concentration reaches a certain degree.

There are many treatment methods for heavy metal ions in wastewater according to reports in the literature. These mainly include chemical precipitation [8], chemical reduction [9], ion exchange [10], membrane separation [11], adsorption [12], and biological flocculation [13]. Among these, the adsorption method is a type of wastewater treatment technology with high efficiency, a low cost, and wide application. Therefore, the development of efficient, cheap, and reproducible adsorbents has always been a main research hotspot in this field. The commonly used sorbents for the removal of heavy metal ions from aqueous solutions are fertilizer industry waste [14], polymers [15], modified chitosans [16], iron oxides [17], Mg-Al LDHs [18], activated alumina [19], and charcoal [20]. Among these adsorbents, LDHs and their derivates have attracted great attention due to being inexpensive, having a large adsorption capacity, and possessing special pore structure properties [21].

Hydrotalcite materials have excellent ion exchange and adsorption properties due to their special sandwich structure, along with the controllability of their layer intervals and anionic exchange capabilities and their high specific surface areas and adequate channel structures [22]. Meanwhile, LDHs have ample hydroxyl functional groups and super-large surface areas for effectively eliminating heavy metals from aqueous solutions. For example, Tran et al. [23] prepared a Mg-Al LDH that was intercalated with the organic acid anions of citrate and malate for the effective removal of Pb²⁺ and Cu²⁺ from an aqueous solution. Huang et al. [24] investigated Cr(VI) removal using a biochar embellished with a Mg-Al LDH inserted with ethylenediaminetetraacetic acid. Soltani et al. [25] prepared a hierarchic LDH/MOF nanomaterial as a prospective adsorbent for the concurrent elimination of toxic dyes and inorganic heavy metal ions from wastewater. In terms of the material structure, the higher the degree of crystallinity, the more stable the properties of the substance, and the better the physical and chemical properties in general. So increasing the degree of crystallinity can make the physical and chemical properties of a substance more stable [26,27]. However, according to the literature, the crystallinity of most Mg-Al LDHs is not very high [28,29], which has an adverse effect on the mechanical properties and stability of the material. In particular, contact in the solid–liquid phase interface in a solution will weaken the stability of a material, to a certain extent.

In order to further improve the crystallinity of a Mg-Al LDH and perfect its physical and chemical properties, we combined homogeneous precipitation and hydrothermal technology to prepare a Mg-Al LDH material with a high degree of crystallinity in this work. XRD, SEM, TEM, N₂ ad/desorption, FTIR, and TG-DTG technologies were used to systematically characterize the microarchitectures, morphologies, and thermostability levels of the Mg-Al LDH. In addition, the adsorption properties of Cu²⁺ from an aqueous solution were investigated by using the solution’s pH, adsorption kinetics, thermodynamics, and dose adsorption experiments.

2. Materials and Methods

2.1. Materials

The experimental water was deionized water. CuCl₂·2H₂O was dissolved into the deionized water to prepare Cu²⁺ stock solutions of different concentrations. Magnesium chloride (MgCl₂·6H₂O), sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), aluminum nitrate (Al(NO₃)₃·9H₂O), copper chloride (CuCl₂·2H₂O), and hydrochloric acid (HCl) were obtained from the Shanghai McLean Chemical Reagent Company Limited, and all of them were analytical-grade reagents.

2.2. Adsorbent Preparation

Certain amounts of MgCl₂·6H₂O and Al(NO₃)₃·9H₂O were dissolved in 80 mL of deionized water to obtain a mixture which was denoted as solution A. Then, 6.4 g of NaOH and 4.24 g of Na₂CO₃ were diluted into 80 mL deionized water, successively, to obtain solution B. Solutions A and B were stirred separately for a period of time, and then solution A was added to solution B, drop by drop, to obtain a uniform reaction system that was stirred persistently. After dripping, the solution pH values were controlled between 9 and 10, and then the reactant continued to be fully stirred for another 30 min. Then, the mixture was shifted into a Teflon-lined stainless-steel vessel after being stirred constantly, and it was autoclaved at 393 K for 24 h. After cooling to room temperature, the mixture was filtered, washed alternately with deionized water and absolute ethyl alcohol to become neutral, and dried in an oven at 338 K for 12 h to obtain a white sample which was denoted as Mg-Al LDH. A certain number of synthesized Mg-Al LDH samples were roasted in a muffle furnace at 726 K for 4 h to obtain a white powdery substance which was denoted as Mg-Al LDO.

2.3. Adsorption Experiment Method

The working Cu²⁺ solution of 100 mg/L was prepared by dissolving a certain amount of CuCl₂·2H₂O into deionized water and reserving it in a 1000 mL measuring flask.

To detect the influence of solution pH value on Cu²⁺ adsorption, 50 mL of 100 mg/L Cu²⁺ was mingled with 50 mg of Mg-Al LDH in a 100 mL polyethylene bottle that was closed tightly and placed in a self-shaking shaker under vibration at 298 K for 6 h. The solution pH values were controlled by using 0.1 mol/L of NaOH or 0.1 mol/L of HCl, and the pH value was surveyed by a pH meter.

For the adsorption kinetic test, 50 mg of synthesized Mg-Al LDH adsorbent was added to 50 mL of 100 mg/L Cu²⁺ solution (the pH value was not adjusted) under stirring at 298 K for 5, 10, 20, 30, 60, 90, 120, 180, 240, and 360 min, respectively. Then, the same volume of suspension was taken out at each time interval and immediately centrifuged at 8000 rpm to isolate the solution from the mixture. The residual Cu²⁺ concentration in the aqueous solution was measured using an atomic absorption spectrophotometer.

To explore the influence of temperature on Cu²⁺ adsorption, 50 mL of 100 mg/L Cu²⁺ (the solution pH was not adjusted) was mingled with 50 mg of Mg-Al LDH in a polyethylene bottle that was closed tightly and placed in a self-shaking shaker at 298 K, 308 K, and 318 K, respectively.

In the experiments that investigated the effect of dose on adsorption performance, 50 mL of 100 mg/L Cu²⁺ (the solution pH was not adjusted) was mixed with different-quality Mg-Al LDH samples in polyethylene bottles that were closed tightly and placed in a self-shaking shaker at 298 K for 6 h.

After the adsorption experiments were finished, the mixtures were percolated and the concentrations of Cu²⁺ were analyzed by an atomic absorption spectrophotometer. The adsorption capacities (q_e, mg/g) and removal rates (η%) were calculated using the following two formulas, respectively:

$$q_{\mathrm{e}}=\frac{(c_0-c_e)}{m}\times V \mathrm{and}$$

(1)

$$\mathsf{\eta }=\frac{(c_0-c_e)}{c_0}\times 100$$

(2)

In the above formulas, q_e (mg/g) is the Cu²⁺ adsorption capacity, c₀ and c_e are the original and equilibrium concentrations of Cu²⁺ (mg/L), V (L) is the solution volume, and m (g) is the adsorbents mass.

2.4. Analytical and Characterization Methods

X-ray diffraction (XRD) diagrams of the products were tested on an X’pert Pro X-ray diffractometer (Philips, Eindhoven, The Netherlands) to determine their crystal structures. Scanning electron microscopy (SEM) experiments were conducted on a JSM-5610LV/INCA electron microscope (JEOL, Tokyo, Japan). Transmission electron microscope (TEM) images were acquired using a JEM-2100 microscope (JEOL, Japan) that was handled at 200 kV. In order to clearly define the pore geometries and pore diameters, the Mg-Al hydrotalcite samples were measured via N₂-sorption, and a BJH analysis of the individual desorption isotherm experiments was conducted. Brunauer–Emmett–Teller (BET) specific surface areas were obtained from the adsorption information in the relative pressures between 0.05 and 0.30, and the pore diameters were determined using the Barrett–Joyner–Halenda (BJH) model while the total pore volumes were determined from the amount adsorbed at a relative pressure of 0.995. The pore geometries, such as “ink bottle”, “slit-like”, and “cylindrical” shapes, were obtained according to the types of adsorption and desorption isotherms and the hysteresis loops. Nitrogen ad/desorption was performed on Quantachrome Autosorb-iQ equipment (Quantachrome, Boynton Beach, FL, USA) at −469 K. Fourier-transform infrared spectroscopy (FTIR) spectra of the samples were obtained by using a spectrometer (NICOLET Nexus 470, Nicolet, Ramsey, MN, USA) and effecting limits of 4000~400 cm⁻¹ with the specimens prepared by the KBr method. The TG-DTG experiment was carried out on a PerkinElmer STA6000 instrument in dry-air ambience with a 10 °C/min heating rate. The concentrations of Cu²⁺ were determined by an atomic absorption spectrophotometer (TAS-990, Puyang general, Beijing, China).

3. Results and Discussion

3.1. Sample Characterization

The XRD patterns of the Mg-Al LDH and Mg-Al LDO are shown in Figure 1. It can be observed in Figure 1a that the Mg-Al LDH showed sharp and symmetric diffraction peaks at 2θ of 11.6°, 23.42°, and 34.84°, which corresponded to the (003), (006), and (009) crystal faces of the hydrotalcite (JCPDS 98-000-6296), suggesting that hydrotalcite characteristics with a layered structure and a high degree of crystallinity were obtained [30]. In addition, it can be clearly seen in Figure 1b that the characteristic diffraction peaks ((003) and (006)) of the Mg-Al LDH disappeared after high-temperature calcination, indicating that the structure of the Mg-Al LDH was totally damaged, and there was chaos in the residual layers [31]. Moreover, the image shows that the calcined products of the Mg-Al LDH were referred to as Mg-Al LDO while the outcome of the wide peaks was attributed to the generation of periclase [31].

The micromorphologies and dispersion characteristics of the Mg-Al LDH and Mg-Al LDO are shown in Figure 2. It can be seen in Figure 2a–c that the synthesized Mg-Al LDH had a typical lamellar structure [32]. However, the lamellar sizes and thicknesses were different and the overlap phenomenon between the sheets was prominent. In addition, the degree of dispersion of the samples was not uniform, indicating the samples had high surface energy levels. Figure 2d–f shows the morphology characteristics of the Mg-Al LDO. It can be seen that the shape of the Mg-Al LDO was also that of a laminar microstructure, with a uniform size. The results showed that the morphologies of the synthesized samples did not change significantly after calcination.

Figure 3 shows the TEM images of the Mg-Al LDH and Mg-Al LDO samples. It can be observed in Figure 3a that the morphology of the Mg-Al LDH, with diameters ranging from tens to hundreds of nanometers, was mainly lamellar, which was consistent with the typical plate morphology of hydrotalcite [33]. In addition, a small number of rod-like substances also appeared in the Mg-Al LDH product, and the reason for this phenomenon was that some individual rod-like morphologies were not completely transformed to lamellar structures during the reaction process (Figure 3b). Figure 3c shows a complete lattice fringe, corresponding to the high-resolution transmission electron microscope image of the Mg-Al LDH, which indicated that the crystal growth was relatively orderly. Figure 3e,f shows the transmission electron microscopy images of the Mg-Al LDO. It can be observed that the morphology of the Mg-Al LDO was basically the same as that of the Mg-Al LDH. The images show a nanoplate morphology with a certain degree of agglomeration, which indicated that the surface energy of the sample was relatively high.

Figure 4 shows the N₂ ad/desorption isotherms and the pore-size distribution curves of the Mg-Al LDH and Mg-Al LDO nanomaterials. It can be seen in the curve features in Figure 4a that both the Mg-Al LDH and Mg-Al LDO were type IV isotherms with H3 shape hysteresis loops according to the IUPAC classification [34,35], indicating the existence of mesoporosity and slit-like shapes in the two samples. The pore-size distribution curves of the samples are plotted in Figure 4b. It can be observed that a majority of the pore sizes were found to be in the range of 0~30 nm, suggesting that a large degree of mesoporosity existed in both the Mg-Al LDH and Mg-Al LDO materials. In addition, the pore volume change rate of the Mg-Al LDH at 63.03 nm was the largest, and that of the Mg-Al LDO, at 42.25 nm, was larger. These results showed that the number of pores with pore sizes ranging from 25 nm to 70 nm was reduced after the Mg-Al LDH calcination. As summarized in

Table 1. Structural property parameters of the Mg-Al LDH and Mg-Al LDO samples.

Parameters	Mg-Al LDH	Mg-Al LDO
BET surface area (m2/g)	50.21	115.71
Pore volume (cm3/g)	0.39	0.31
Average pore diameter (nm)	63.03	42.25

, the BET specific surface areas of the Mg-Al LDH and Mg-Al LDO were 50.21 and 115.71 m²/g, respectively. The pore volumes of the Mg-Al LDH and Mg-Al LDO were 0.39 and 0.31 cm³/g, respectively. In comparing the Mg-Al-LDO samples after calcination to the Mg-Al-LDH nanosheets, it was obvious that the BET surface areas of the Mg-Al LDO were significantly increased. However, the pore volumes decreased slightly. This may have been because the pore structures of the Mg-Al LDH collapsed during calcination, thus reducing the pore volumes of the material, and at this time, more nanoparticles were obtained, which increased the specific surface areas of the nanomaterials. The result showed that more pores were formed in the Mg-Al LDO, which resulted in a large increase in its specific surface area.

The FTIR spectra of the Mg-Al LDH and Mg-Al LDO are shown in Figure 5. It can be seen in Figure 5a that there were stretching vibration peaks of the H₂O and –OH groups near 3543 cm⁻¹ and there were bending vibration peaks of the H₂O and –OH groups near 1642 cm⁻¹, indicating that there were water molecules and hydroxyl groups in the spaces between the hydrohalic-like layers [36]. The absorption peaks can be obviously seen at 1377 cm⁻¹ in the spectrum of the Mg-Al LDH, which agreed with the vibrational modes of the CO₃²⁻ anions [37]. In addition, in the spectrum of the Mg-Al LDH, the bands at approximately 667 cm⁻¹ and 428 cm⁻¹ could be assigned to the existence of the lattice translational modes (667 cm⁻¹) and the Al–O bond (428 cm⁻¹) [38,39]. These results confirmed that some functional groups, such as CO₃²⁻, Al–O, and –OH, were contained in the Mg-Al LDH. It can be seen in Figure 5b that the absorption peaks of the Mg-Al LDO were very similar to those of the Mg-Al LDH.

The TG and DTG curves of the Mg-Al LDH are shown in Figure 6. As presented, there were two weightlessness stages for the Mg-Al LDH in the continuous heating process. The first stage of weightlessness from 298 K to 518 K corresponded to the removal of the waters that were physiosorbed on the outside surface of the hydrohalic material and the waters that were intercalated into the interlayer galleries of the Mg-Al LDH [40]. The weight damage of the hydrone for the Mg-Al LDH was 13.75%, indicating the Mg-Al LDH contained a large number of water molecules. The second stage of weightlessness from 518 K to 1073 K was ascribed to the dihydroxylation of the layers and the elimination of the volatile matter arising from the interlayer CO₃²⁻ for the Mg-Al LDH, with a weight loss of 28.15%. The great weight damage in the second stage demonstrated that there were many OH⁻ groups on the surface and with CO₃²⁻ in the interlayers of the Mg-Al LDH [41]. Moreover, it can be noticed from the DTG curve that there was an obvious endothermic peak at approximately 483 K which was associated with the loss of the exposed OH⁻ present on the external surfaces of the Mg-Al LDH. In addition, there was an obvious endothermic peak at 673 K which was assumed to be due to dihydroxylation and decarbonation [22].

3.2. Batch Adsorption of Cu²⁺ onto the Mg-Al LDH

3.2.1. The Effect of Solution pH on Cu²⁺ Adsorption

Solution pH value has a significant influence on adsorption performance in the adsorption process because the charge and adsorption active sites on the surfaces of a sorbent and an adsorbate are importantly affected by it [42]. Figure 7 shows the relationship between the original pH value and the Cu²⁺ adsorption capacity. It can be observed that the Cu²⁺adsorption capacity increased first and then decreased with increasing the pH value from 4 to 9. In particular, the maximum adsorption capacity was 89.13 mg/g when the pH value was approximately 5. This was because the different charge properties of the Mg-Al LDH reacted to the different pH-value environments. The results showed that there were more negative charges on the adsorbent surface. In addition, the Mg-Al LDH would dissolve, to a certain extent, in the strong acidic medium, resulting in the collapse of the layered structures. However, the Cu²⁺ was easily combined with the OH⁻ ions to precipitate copper hydroxide in the strong alkaline environment, which weakened the process of the adsorption of Cu²⁺ on to the Mg-Al LDH. In view of the related references and our experimental results [43,44], it was known that the optimal pH condition for Cu²⁺adsorption onto the Mg-Al LDH was a weakly acidic environment. According to the above results, we did not specifically regulate the pH value of the copper chloride solution in the following adsorption experiments because the pH value of the deionized water was approximately 6.

3.2.2. Adsorption Kinetics

In order to achieve industrial production, adsorption equilibrium time is an important evaluation index in the wastewater treatment process. The effect of the contact time on Cu²⁺ adsorption onto the Mg-Al LDH was studied at the range of 5~360 min, and the experimental data are presented in Figure 8a. It can be seen that the adsorption rate was fast in the initial stage (5~120 min), and nearly 30% of the Cu²⁺ ions were adsorbed by the Mg-Al LDH in 120 min. Then, with the extension of the contact time, the adsorption capacity of the Cu²⁺ increased little by little until equilibrium was established. When the adsorption time was 360 min, 62.1% of the Cu²⁺ ions were removed. This was mainly due to the abundance of active sites that were absolutely exposed to the Cu²⁺ in the aqueous solution during the initial period, and they were easily taken up. As the adsorption reaction proceeded, the Cu²⁺ adsorbed onto the Mg-Al LDH, which prevented the residual Cu²⁺ from being adsorbed onto the adsorbent due to the electrostatic repulsion between the ions of the same charge. Then, the Cu²⁺ attempted to move into the pores of the Mg-Al LDH, which became difficult. This was a slow and stable process.

In order to further illustrate the adsorption mechanism, pseudo-first-order and pseudo-second-order models [45,46] were adopted to fit the adsorption data. The two equations used were as follows:

$$\ln (q_e-q_t)=\ln q_e-k_{1}t \mathrm{and}$$

(3)

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e{}^2}+\frac{1}{q_e}t$$

(4)

In the above equations, k₁ is the pseudo-first-order adsorption rate constant (min⁻¹), k₂ is the pseudo-second-order adsorption rate constant (min·g⁻¹·mg⁻¹), and q_e (mg·g⁻¹) and q_t (mg·g⁻¹) are the adsorption capacities at an equilibrium state and at any time, respectively.

The fitting results are presented in Figure 8b,c, and the simulation parameters are presented in

Table 2. Parameters calculated by the pseudo-first-order and pseudo-second-order models.

Experimental	Pseudo-First-Order					Pseudo-Second-Order
qe (mg·g−1)	k1 (min−1)	qe (mg·g−1)	R2	Standard Error		k2 (g·mg−1·min−1)	qe (mg·g−1)	R2	Standard Error
62.11	0.0084	67.32	0.9041	Intercept	Slope	0.0001	78.74	0.9532	Intercept	Slope
				0.1100	0.00096				0.2407	0.0013

. As shown, the correlation coefficient (R² = 0.9532) of the pseudo-second-order model was larger than that of the pseudo-first-order model (R² = 0.9041). However, the calculated value of q_e obtained by the pseudo-first-order model (67.32 mg/g) was closer to the experimental value of q_e (62.11 mg/g). These results demonstrated that the pseudo-second-order model was more appropriate to fit the adsorption data. Furthermore, the value of k₂ obtained by the pseudo-second-order model was relatively small, suggesting that the adsorption rate decreased with extension of the reaction time. These results demonstrated that the pseudo-second-order model could better simulate the Cu²⁺ adsorption, and it was regulated by the chemical adsorption in the adsorption process [47].

3.2.3. The Effect of Dosage on Cu²⁺ Adsorption

The influence of dosage on the adsorption capacity and removal rate of the Mg-Al LDH adsorbent was investigated to determine an optimal solid-to-liquid ratio for the removal of 100 mg/L of Cu²⁺ in an aqueous solution. The mass of the adsorbent varied from 10 mg to 150 mg in 50 mL of Cu²⁺ solution, and the adsorption results are shown in Figure 9. The experimental results showed that the removal percentage of the Cu²⁺ linearly increased when the adsorbent dosage was raised to 90 mg/50 mL. After this adsorption saturation point, the curve no longer rose monotonously, and it gradually showed a stable state. As the dosage of Mg-Al LDH increased from 10 mg to 150 mg, the removal percentage of the Cu²⁺ increased from 30.02% to 66.02%. At the same time, the blue curve shows that the adsorption capacity of the Cu²⁺ showed a monotonically decreasing trend with the increase in the adsorbent mass from 10 mg to 150 mg. This was because the number adsorption sites increased sharply after the addition of adsorbent while the concentration of the Cu²⁺ in the liquid phase decreased when equilibrium was reached with a higher amount of Mg-Al LDH. As the amount of Cu²⁺ in the solid depended on the concentration in the liquid, the Cu²⁺ adsorption capacity also decreased. The results indicated that the optimal solid-to-liquid ratio was 90 mg/50 mL under this adsorption condition.

3.2.4. Adsorption Thermodynamics

To investigate the influence of temperature on the Cu²⁺ adsorption onto the Mg-Al LDH, the Gibbs free-energy change (ΔG^ϴ), entropy change (ΔS^ϴ), and enthalpy change (ΔH^ϴ) were obtained using the following two formulas [48,49]:

$$\mathrm{L}\mathrm{n}{K}_L=\frac{{∆S}^{\mathsf{\theta }}}{R}-\frac{{∆H}^{\mathsf{\theta }}}{RT} \mathrm{and}$$

(5)

$${∆G}^{\mathsf{\theta }}=∆H^{\mathsf{\theta }}-\mathrm{T}∆S^{\mathsf{\theta }}$$

(6)

In the above formulas, K_L refers to the Langmuir equilibrium constant (L/mol), R refers to the gas molar constant (8.314 J/mol·K), and T refers to the reaction temperature (K). In view of the relationship between ΔG^ϴ and K_L, the values of ΔH^ϴ and ΔS^ϴ were obtained using the intercept and the slope from the Van’t Hoff plots of ln K_L versus 1/T.

It can be seen in Figure 10a that the adsorption capacity of the Mg-Al LDH toward the Cu²⁺ increased with the extension of the reaction temperature, demonstrating that a high temperature was conducive to the diffusion and adsorption of Cu²⁺ onto the Mg-Al LDH.

The computational thermodynamic parameters are shown in

Table 3. Values of the thermodynamic parameters for the Cu²⁺ adsorption onto the Mg-Al LDH at various temperatures.

Sample	T (K)	ΔGϴ (kJ/mol)	ΔHϴ (kJ/mol)	ΔSϴ (J/(mol·K)	R2	Standard Error
Mg-Al LDH	298	3.97	65.24	205.49	0.9056	Intercept	Slope
	308	1.92				5.6509	1746.40
	318	−0.14

. As outlined, the ΔG⁰ values for the Mg-Al LDH at 298 K and 308 K were positive during the adsorption process, indicating that the Cu²⁺ adsorption onto the Mg-Al LDH was nonspontaneous [28]. However, when the reaction temperature was increased to 318 K, the ΔG⁰ values became negative, indicating that the Cu²⁺ adsorption onto the Mg-Al LDH was spontaneous at a higher temperature. Moreover, the ΔG⁰ values diminished with increasing reaction temperatures, demonstrating that a higher temperature can produce a greater impetus for Cu²⁺ adsorption. In addition, the positive value of ΔH⁰ indicated that the Cu²⁺ adsorption was endothermic, and hyperthermy was beneficial for improving the adsorption performance. Simultaneously, the positive ΔS⁰ values disclosed an increase in the disorder at the solid–liquid interface in the solution [50]. During the entire adsorption process, hyperthermia contributed to the Cu²⁺ dissolution and its increased affinity with the Mg-Al LDH, and it was also beneficial for the activation of the active sites on the surface of the adsorbent [51].

4. Conclusions

Mg-Al LDH samples with nano-lamellar morphologies and diameters of 50~200 mm were prepared using a homogeneous precipitation and hydrothermal method. Then, the corresponding calcined product, Mg-Al LDO, was also obtained in this work. The characterization results showed that the two samples had similar micro-morphological characteristics, but the latter had a larger BET specific surface area, indicating there were more open-framework structures and reactive sites on the adsorbent’s surface. The batch experimental results demonstrated that the Mg-Al LDH, with its nano-lamellar morphology, had a rapid adsorption speed and relatively large adsorption capacity (62.11 mg/g) for Cu²⁺ in a solution. The pH value of the solution had a vital effect on the adsorption efficiency, and the adsorption capacity was the largest when the pH value was approximately 5. The results of the adsorption kinetics indicated that the Cu²⁺ adsorption behavior could be interpreted by a pseudo-second-order model, demonstrating that the adsorption process was controlled by chemical sorption. The results of the dose experiments indicated that the optimal solid-to-liquid ratio was 90 mg/50 mL. The adsorption thermodynamic results indicated that adsorption process was spontaneous at hyperthermia (>318 K), and the ΔG⁰ values decreased with increasing temperatures, which indicated that a higher temperature could produce a greater driving force for Cu²⁺adsorption onto the Mg-Al LDH in an aqueous solution. These meaningful results could potentially stimulate further study on the synthesis of magnesium aluminum hydrotalcite material and promote its development.