Influence of Oxyanions on the Structural Memory Effect of Layered Double Hydroxides Under Aqueous Condition

Abstract

The structural memory effect is normally considered one of the most important properties of LDHs. However, certain anions can have adverse effects on it. In this study, three types of CLDHs (Mg2Al1-CLDH, Mg2Al0.5Fe0.5-CLDH, Mg2Fe1-CLDH) were obtained and used to observe their regeneration behaviors in the presence of sulfate, silicate, and phosphate, respectively, at initial pH values of 10 and 13. The samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA-DTG), scanning electron microscope (SEM), and N2 adsorption–desorption isotherm (BET). The results suggested that silicate and phosphate have significant impacts on the regeneration of CLDHs, while sulfate does not. Specifically, phosphate and silicate reacted with MgO to generate magnesium silicate and magnesium phosphate dibasic, which were covered on the surface of particles and hindered the hydroxylation of metal oxides. However, a higher pH can suppress the formation of new substances and promote the regeneration of LDHs. Moreover, the CLDHs with high specific surface area had a stronger anti-interference performance regarding the effects of phosphate and silicate.

1. Introduction

Layered double hydroxides (LDHs) have received considerable attention due to their high sorption capacities, low economic cost, and unique structural properties [1,2]. The general chemical formula of LDHs can be expressed as ${\left[{\mathrm{M}\left(\mathrm{II}\right)}_{1-\mathrm{x}}^{2+}{\mathrm{M}\left(\mathrm{III}\right)}_{\mathrm{x}}^{3+}{\left(\mathrm{OH}\right)}_2\right]}^{\mathrm{x}+}{[{\left({\mathrm{A}}^{\mathrm{n}-}\right)}_{{\displaystyle \frac{\mathrm{x}}{\mathrm{n}}}}·\mathrm{m}{\mathrm{H}}_2\mathrm{O}]}^{\mathrm{x}-}$, where M²⁺ represents the divalent cations (Mg²⁺, Ni²⁺, etc.), M³⁺ represents trivalent cations (Al³⁺, Fe³⁺, etc.), and An- represents the interlayer anions (CO₃²⁻, SO₄²⁻, etc.). The symbol of x is the molar fraction of the trivalent cation within the layer: x = M(II)/[M(II) + M(III)], commonly ranging from 0.2 to 0.33 [3]. Due to their high anion exchange capacity and large surface area, LDHs are considered a promising adsorbent in water treatment. In addition to anion exchange, LDHs also exhibit a unique property called a memory effect. Being calcined at the temperature from 400 to 800 °C, LDHs would be transformed into metal oxides, which can be reconstructed into its layered structure again via rehydration and incorporating anions into the interlayer from aqueous solution [4,5]. In general, the calcined LDHs (CLDHs) exhibit superior anion sorption capacity in comparison to LDHs [4]. Thus, the study of the structural memory effect of LDHs is of great significance for their extensive use in environmental remediation.

The regeneration of LDHs is normally considered as the rehydration process from the mixture of metal oxides to bimetallic hydroxide, including the dissolution of oxides and reprecipitation as hydroxides [6,7]. Some anions (CO₃²⁻, SO₃²⁻, PO₃⁴⁻, etc.) in the reconstruction medium might affect this process, which depends on the reaction between solutes and metal oxides species produced in rehydration [4]. LDHs, as an excellent absorbent of phosphate, have been studied by many researchers, including the effect of phosphate on the memory effect of LDHs. Ashekuzzaman and Jiang [8] investigated the regeneration performance of calcined Ca-based and Mg-based layered double hydroxides (LDHs) for phosphate removal from aqueous solutions containing phosphate concentrations of 3.4–10.4 mg-P/L. The results demonstrated that Mg-based LDHs exhibited effective regeneration following phosphate sorption, indicating their potential for reuse. However, Ca-based LDHs lost their layered structure after the first sorption operation, predominantly caused by the formation of precipitation of dissolved Ca²⁺ with the phosphate. Xu et al. [9] reached a comparable conclusion regarding phosphate sorption using MgAl-CLDHs and CaFe-CLDHs. However, it is worth noting that few researchers explored the effect of high-concentration phosphate on memory effects. As for sulfate, most research focused on the adsorption characteristics of sulfate on CLDHs and competition adsorption with other anions [10,11,12,13]. Few workers paid attention to the influence of sulfate on the memory effect. In addition to the previous study [14], few researchers explored the influence of silicate on memory effect. On the whole, there are rarely studies on the effect and mechanism of high-concentration oxyanions on the structural memory effect of LDHs, let alone the comparison of the effects of these oxyanions.

Therefore, the objectives of this work are as follows: (1) to investigate and compare the effect of three high-concentration oxyanions (sulfate, silicate, and phosphate) on the structural memory effect of Mg-based LDHs (Mg₂Al1-LDH, Mg₂Al_0.5Fe_0.5-LDH and Mg₂Fe-LDH), and (2) clarify the reaction mechanism through XRD, TGA-DTG, SEM and BET. To simulate natural conditions, the CO₂ in initial solutions was not removed.

2. Experimental Section

2.1. Materials

The chemicals used in this study include MgCl₂·6H₂O (98%, SCRS Northriding, Johannesburg, South Africa), AlCl₃·6H₂O (97%, Aladdin, Riverside, CA, USA), FeCl₃·6H₂O (99%, Aladdin), Na₂CO₃ (99.8%, Macklin, Shanghai, China), Na₂SO₄ (99%, Macklin), Na₂SiO₃·9H₂O (98%, Macklin), Na₃PO₄·12H₂O (98%, Macklin), HNO₃ (65~68%, SCRC) and NaOH (98%, Macklin). The ultrapure deionized water (18 MΩ·cm) was adopted for all of experiments. Three types of LDHs were synthesized using a co-precipitation method. Firstly, the metal cation mixed solutions were prepared by dissolving corresponding chemicals, and the molar ratio of divalent ion (Mg²⁺)/trivalent ion(s) (Al³⁺, Fe³⁺) were controlled to 2:1 for the synthesis of the binary compounds (Mg₂Al-LDH, Mg₂Fe-LDH) and ternary compounds (Mg₂Al_0.5Fe_0.5-LDH). Then, the mixed solutions were simultaneously dropped into 0.5 M Na₂CO₃ solution with a pH of 10.5 by adding 10 M NaOH. Thereafter, the slurries were transferred into 100 mL autoclaves, aged at 100 °C for 20 h. Afterwards, the solid residues were acquired after filtering, washing and freeze-drying, named as LDH-1 (Mg₂Al-LDH), LDH-2 (Mg₂Al_0.5Fe_0.5-LDH) and LDH-3 (MgFe-LDH), respectively. Lastly, the synthesized LDHs were calcined at 500 °C for 3 h, named as CLDH-1, CLDH-2 and CLDH-3, respectively.

The XRD patterns of the synthesized LDHs and CLDHs are illustrated in Figure 1. The LDHs possessed the same characteristic peaks as previously reported by Tian et al. [14]. Two broad peaks (approximately 43°, 62°/2θ) in the patterns of CLDHs were attributed to magnesium oxide. The peaks (approximately 24°, 33°, 35°, 49°, 54°, 64°/2θ) appeared in the patterns of CLDH-2 are assigned to maghemite (Fe₂O₃) [15]. The peaks of spinel-type compounds were presented in the patterns of CLDH-3 (approximately 35°/2θ) [16]. The elemental compositions of CLDHs were determined by Inductively Coupled Plasma Optical Emission Spectrometer (PerkinElmer, Optima 8300, Waltham, MA, USA) after dissolving solid residues in a nitric acid solution. The results in

Table 1. Elemental compositions of calcined products (determined by ICP-OES).

Sample	Elemental Concentration (mmol/g)			Elemental Molar Ratio
	Mg	Al	Fe	Mg/(Al + Fe)	Al/Fe
CLDH-1	7.90	3.81	-	2.07	-
CLDH-2	6.58	1.50	1.51	2.19	0.99
CLDH-3	9.76	-	5.08	1.92	-

indicate that each of the Mg/(Al + Fe) molar ratios is close to the theoretical values.

2.2. Sorption Experiments

A series of solutions containing sulfate, silicate, and phosphate were prepared in the glove box to avoid CO₂ contamination, and then used for the sorption tests of CLDHs. It should be emphasized that the deoxygenated water used for the adsorption tests was prepared by simultaneously boiling and nitrogen bubbling the deionized water. Then, a certain amount of each CLDH was added into the solutions containing 3.56 mM specific oxyanion as described above, and the solid/liquid ratio was fixed to 1 g/L. The initial pH of these solutions was adjusted to 10 and 13 by adding NaOH or HNO₃. After that, the mixtures were stirred at 200 rpm for 24 h to reach equilibrium at room temperature. The solid residues were obtained by filtration with 0.22 μm membrane and stored for further analysis after drying in a vacuum dryer. In addition, the supernatants were collected to determine the pH and oxyanion concentrations.

2.3. Characterizations

ICP-OES was used to determine the concentrations of silicate in the supernatant. The concentrations of sulfate and phosphate were measured using the Dionex Aquion Ion Chromatography System (Thermo Fisher Scientific, Mexico City, Mexico). The XRD patterns (scanning speed: 5° min⁻¹) of solid residues were collected on a D8 Advance X-ray diffraction (Bruker, Billerica, Germany) using Cu Kα radiation operating with 40 kV and 40 mA. TGA-DTG curves were obtained in the temperature range from 30 to 800 °C, and the heating rate was 10 °C/min using a Thermogravimetric Analyzer (TGAQ500, TA Instruments, New Castle, DE, USA) under a flowing nitrogen atmosphere (60 cm³/min). The BET curves of CLDHs were collected on an automatic specific surface area analyzer (Micromeritics, TriStar II 3020, Norcross, GA, USA) at 77.3 K with the pretreatment under vacuum at 120 °C for 7 h. The SEM images were observed on Regulus 8100 (Hitachi, Tokyo, Japan) with the acceleration voltages of 5 kV.

3. Results and Discussion

3.1. Sorption Results

The sorption amounts of sulfate, silicate and phosphate by CLDHs at initial pH values of 10 and 13 are shown in Figure 2. Sulfate sorption by CLDH-1 exceeded that of CLDH-2 and CLDH-3 at both pH levels, while the capacities of CLDH-2 and CLDH-3 were comparable. In contrast, CLDH-2 exhibited the highest silicate adsorption capacity at both pH 10 and 13. The sorption amount of silicate by CLDH-1 was almost same as that by CLDH-3. These results indicate that CLDH composition significantly influences oxyanion affinity. For phosphate adsorption at pH 10, the capacity followed the sequence CLDH-1 > CLDH-2 > CLDH-3. This sequence differed at pH 13, where CLDH-2 > CLDH-1 > CLDH-3. Furthermore, sorption capacities for all oxyanions were consistently higher at pH 10 than at pH 13. This difference is attributed to two primary factors: (i) higher pH results in more fierce competition between oxyanions and OH groups for adsorption sites and (ii) higher pH causes the adsorbent surface to carry more negative charges, correspondingly leading to an increase in the repulsive interaction between the oxyanions and adsorbent surface [8]. Collectively, both solution pH and CLDH composition exert pronounced influences on oxyanion sorption. Figure 2d displays the measured supernatant pH after reaction. At an initial pH of 10, the final pH increased (approximately 10.5 to 11.3) for all systems, primarily due to hydroxyl ion release during the hydroxylation of metal oxides [17]. Conversely, at an initial pH of 13, the final pH remained relatively stable for silicate adsorption but decreased slightly following sulfate and phosphate adsorption by CLDHs.

3.2. Solid Residues Analysis

The XRD patterns of CLDHs following reactions under various conditions are presented in Figure 3. At an initial pH of 10, LDHs regenerated from CLDHs in the absence of competing oxyanions (Figure 3a). However, no LDH phases were observed after reaction with silicate at pH 10 (Figure 3e), indicating that silicate significantly inhibits the rehydroxylation of metal oxides. CLDH-2 and CLDH-3 failed to reconstruct their original layered structure after phosphate sorption at pH 10. In contrast, CLDH-1 regenerated successfully under these conditions, albeit with residual unreacted material present (Figure 3g). This indicates that phosphate also has effects on the memory effect of LDHs, and it seems that silicate has greater influences on the memory effect than phosphate. Noticeably, the regeneration of each LDH occurred at pH 13 in the presence of silicate or phosphate (Figure 3f,h), which seems to demonstrate that pH and silicate or phosphate ions have synergistic influences on the regeneration of LDHs. Sulfate did not hinder the rehydroxylation process, as evidenced by the regeneration of all LDHs at both pH 10 and pH 13 (Figure 3c,d). However, the crystallinity of LDHs regenerated at pH 13 in sulfate-containing systems was higher than that at pH 10. This trend was also observed for silicate and phosphate systems. The enhanced crystallinity at high pH is primarily attributed to the greater ease of achieving high Mg²⁺ supersaturation, which promotes nucleation [18]. In addition, the d003 values of raw LDHs are between 7.58 and 7.70 Å, which corresponds to carbonate-type LDHs [19,20,21]. As for generated LDHs, the interlayer spacings was marginally increased compared to raw LDHs. In particular, the d003 values growth of regenerated LDHs in the presence of sulfate at pH of 10 is relatively larger, which should be caused by the intercalation of sulfate. Because the sulfate possesses tetrahedral structure, and carbonate has a structure of plane regular triangle, and the ionic radius of SO₄²⁻ (258 pm) is bigger than that of CO₃²⁻ (178 pm) [22].

To further characterize the solid residues, a thermogravimetric analysis (TGA) of CLDH-1 and CLDH-3 was performed (Figure 4). The TGA-DTG curves reveal three distinct stages of weight loss for the post-reaction samples. The initial stage (<230 °C) corresponds to the elimination of water, encompassing weakly adsorbed water (below 100 °C) and interlayer water (100–230 °C) [16]. The DTG curve for CLDH-1 exhibits two endothermic peaks: the first (~330 °C) signifies partial dehydroxylation, while the second (~360 °C) corresponds to further dehydroxylation concurrent with interlayer anion removal [23]. In contrast, CLDH-3 displays only a single endothermic peak (~350 °C), indicative of overlapping dehydroxylation and anion removal processes [24]. The third stage of mass loss arises from the further dehydroxylation of the host layers, culminating in the transformation to a spinel phase [25,26]. Overall, CLDH-3 exhibited a lower total mass loss than CLDH-1. In general, the total mass loss of the CLDH-3 is less than CLDH-1. In addition, the increasing decomposition temperature in the second stage can be clearly observed in Figure 4b,d, corresponding to the DTG curves of CLDH-1 after the reaction with sulfate and phosphate at an initial pH of 10, respectively. Furthermore, Figure 4b,d (DTG curves for CLDH-1 after sulfate and phosphate sorption at pH 10, respectively) show a noticeable increase in decomposition temperature during the second stage. Minimal total mass loss occurred for CLDH-1 that reacted with silicate and for CLDH-3 that reacted with silicate or phosphate at pH 10 (Figure 4c,g,h), consistent with these samples being predominantly composed of undecomposable metal oxides, as corroborated by XRD analysis. In addition, the endothermic peaks at 800 °C are observed in the DTG curves of CLDH-1 and CLDH-3 at initial pH of 10 (Figure 4b,f), assigned to the elimination of sulfate [27].

In addition, SEM images of CLDH-3 after reaction with sulfate, silicate, and phosphate, are shown in Figure 5. It can be observed that there are no obvious differences between pH values of 10 and 13 in the presence of silicate or phosphate, and the morphologies of them almost presented a sheet structure. However, the solid residues after reaction with sulfate showed irregular lumps, different from other samples. FTIR spectra of the CLDHs after reaction were collected to identify the chemical bonds present in the products, as shown in Figure 6. Peaks at approximately 3470 cm⁻¹ and 1670 cm⁻¹ are assigned to the stretching and bending vibrations of OH groups, respectively [14]. A peak at approximately 1440 cm⁻¹ corresponds to the vibration mode of CO₃²⁻. Absorbance bands at 1100, 1080, and 1040 cm⁻¹ are attributed to the vibrations of sulfate, phosphate, and silicate, respectively [14,28,29]. Due to varying sorption densities of the CLDHs under different conditions, these bands are only clearly visible in the spectra of samples exhibiting high sorption amounts of SO₄²⁻, SiO₃²⁻, and PO₄³⁻.

3.3. Reaction Mechanism

The transformation process of LDHs from CLDHs has been extensively investigated, and the reaction mechanism has been proposed. Take the regeneration process of MgAl-LDH for example [30], there are some reactions including hydrolysis of MgO and Al₂O₃ and dissociation of Mg(OH)₂ and Al(OH)₃ occurring during the regeneration process of LDHs from CLDHs, which can be simplified as follows:

$$\mathrm{MgO}+{\mathrm{H}}_2\mathrm{O} \leftrightharpoons { \mathrm{Mg}\left(\mathrm{OH}\right)}_2\leftrightharpoons {\mathrm{Mg}}^{2+}+{2\mathrm{OH}}^{-}$$

(1)

$${\mathrm{Al}}_2{\mathrm{O}}_3+{3\mathrm{H}}_2\mathrm{O}\leftrightharpoons {2\mathrm{Al}\left(\mathrm{OH}\right)}_3\leftrightharpoons {2\mathrm{Al}\left(\mathrm{OH}\right)}_2^{+}+{2\mathrm{OH}}^{-}\leftrightharpoons { 2\mathrm{Al}\left(\mathrm{OH}\right)}_2^{+}+{4\mathrm{OH}}^{-}\leftrightharpoons {2\mathrm{Al}}^{3+}+{6\mathrm{OH}}^{-}$$

(2)

$${\mathrm{Al}\left(\mathrm{OH}\right)}_3+{\mathrm{OH}}^{-}\leftrightharpoons {\mathrm{Al}\left(\mathrm{OH}\right)}_4^{-}$$

(3)

It can be found that the hydroxide anions actively participate in these reactions, indicating that solution pH exerts a critical influence on the process. In alkaline solutions, the dissociation of Mg(OH)₂ is suppressed, while Al(OH)₄⁻ predominates with negligible Al³⁺ concentration. Consequently, LDH regeneration can be described as a process wherein Al(OH)₄⁻ incorporates into the Mg(OH)₂ lattice, followed by simultaneous interdiffusion of cations to achieve homogeneous distribution and form the final layered structure. According to the phenomena of present experiments, the oxyanions have significant effects on the regeneration of LDHs in addition to the solution pH. The oxyanions in the solution might react with dissolved metal ions from CLDHs, thereby leading to the formation of some other phases blocking the hydroxylation process of metal oxides. However, higher pH probably inhibits the formation of new phases and thus would promote the regeneration of LDHs. Additionally, nascent Mg(OH)₂ and Al(OH)₃ lamellae may initially shield internal oxide phases from hydrolysis. The subsequent dissolution of these hydroxide layers enables hydration to proceed.

The successful regeneration of LDHs in the presence of sulfate indicates that sulfate ions do not inhibit the rehydration of metal oxides. This is attributed to sulfate’s inability to form precipitates with Mg, Al, or Fe species that would obstruct regeneration. Sulfate adsorption by CLDHs occurs primarily through three mechanisms: intercalation, electrostatic attraction, and ligand complexation [10]. The adsorption of sulfate via ligand complexation can be proved by the soaring decomposition temperature in the second stage observed in the TGA-DTG curve of CLDH-1 at pH 10 (Figure 4b). Sulfate was grafted on the layer of regenerated LDHs through ligand exchange, forming bridge-type bidentate bonds [31,32,33]. The absence of similar thermal decomposition shifts in CLDH-1 at pH 10 (other anions) and CLDH-3 at both pH levels suggests ligand complexation does not occur during their sulfate removal processes. Intercalation is confirmed by the notably increased interlayer spacing of sulfate-regenerated LDHs, consistent with sulfate’s larger ionic radius. While electrostatic attraction likely contributes to adsorption, this mechanism cannot be directly verified with current data. Moreover, the larger specific surface area of CLDH-1 (177.6 m²/g) is one of the reasons why the sorption amount by CLDH-1 was more than that sorbed by CLDH-2 (152.6 m²/g) and CLDH-3 (112.3 m²/g), as shown in Figure 7 and Figure 8.

High-concentration silicate reacts with dissolved Mg and Al species released during the CLDH phase transition, forming surface-deposited magnesium silicate and geopolymer-like substances. These deposits inhibit the rehydration of the metal oxide mixture into bimetallic hydroxides [14]. However, at the higher pH, Mg²⁺ seems to be prone to combine with a higher concentration of hydroxyl ions, while the combination with silicate ions was suppressed. Consequently, less magnesium silicate was produced. Simultaneously, the geopolymer-like material formed under high-pH conditions facilitates LDH reconstruction. Furthermore, ternary oxides with high specific surface areas demonstrate greater resistance to silicate interference. This occurs because the larger surface area reduces the average ion concentration in the immediate vicinity of the CLDH particles at equivalent bulk concentrations. Consequently, reaction products cannot achieve complete surface coverage, enabling the hydroxylation process to proceed effectively.

Phosphate sorption by LDH can be attributed to interlayer anion exchange, precipitation, ligand complexation, and electrostatic attraction [9,34]. The precipitation is the predominant removal mechanism at high concentrations (>100 mg P/L). In the present experiments, it is possible that Mg²⁺ released from CLDH can be combined with phosphate species to form precipitation because of the high phosphate concentration (110.25 mg P/L). The composition of phosphate precipitation is closely related to the existing species of phosphate in water. The different phosphate species are created by the following reactions at 25 °C [35,36]:

$${\mathrm{PO}}_4^{3-}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{HPO}}_4^{2-}+{\mathrm{OH}}^{-} \mathrm{pK}=2.12$$

(4)

$${\mathrm{HPO}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{OH}}^{-} \mathrm{pK}=7.21$$

(5)

$${\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{OH}}^{-} \mathrm{pK}=12.36$$

(6)

At initial pH of 10, H₂PO₄⁻ was the predominant species, accounting for 99.4%. The dissolved Mg²⁺ from CLDH can react with H₂PO₄⁻ to generate MgHPO₄.

$${\mathrm{HPO}}_4^{2-}+{\mathrm{Mg}}^{2+}={ \mathrm{MgHPO}}_4 \mathrm{pK}=5.8$$

(7)

The precipitation would cover the particle surfaces, blocking the further hydration of metal oxides. This explains both the regeneration failure of LDH-2 and LDH-3 and the presence of unreacted MgO in solids recovered after CLDH-1/phosphate reaction at initial pH 10. However, successful LDH regeneration at pH 13 likely relates to the extremely low Mg²⁺ concentration in strong alkaline solution. At such low concentrations, Mg²⁺ cannot react significantly with PO₄³⁻ (83.1%) or HPO₄²⁻ (16.9%) to form obstructive Mg₃(PO₄)₂ or Mg(HPO₄)₂ precipitates. Consequently, particle surfaces remain uncovered, allowing unimpeded metal oxide hydroxylation. This demonstrates the beneficial effect of high pH on LDH regeneration. Notably, CLDH-1 regenerated at pH 10 despite phosphate presence, unlike CLDH-2 and CLDH-3. This suggests MgHPO₄ precipitation does not completely suppress CLDH-1 hydroxylation. The distinct behavior of CLDH-1 likely stems from its physical characteristics rather than chemical composition. Specific surface area is one of the most important properties of CLDHs as adsorbents, and it can significantly affect reactions during the rehydration process. Therefore, BET surface area analysis and BJH pore size distributions are presented (Figure 7). All isotherms conform to IUPAC Type II classification. BJH analysis reveals CLDH-1 pores are predominantly below 10 nm, while CLDH-2, and especially CLDH-3, exhibit greater proportions of pores >10 nm. Correspondingly, CLDH-1 possesses the highest specific surface area (177.6 m²/g), significantly exceeding CLDH-2 (152.6 m²/g) and CLDH-3 (112.3 m²/g) (Figure 8). This suggests insufficient precipitation coverage on CLDH-1′s extensive surface, enabling sustained metal oxide hydration. Thus, CLDH specific surface area critically governs phase transformation. Furthermore, the elevated decomposition temperature in the second stage for CLDH-1 at pH 10 (Figure 4d) indicates phosphate grafting onto layers, forming inner-sphere monodentate or bidentate surface complexes [12,34,37], analogous to sulfate adsorption. The absence of this thermal shift in DTG curves of CLDH-1 and CLDH-3 at pH 13 indicates no phosphate sorption via ligand complexation under these conditions.

In summary, the inhibitory effect of inorganic anions on LDH regeneration from CLDHs hinges on two critical factors: (1) the anion’s capacity to form precipitates with metal ions released from CLDHs and (2) whether sufficient precipitate is generated to achieve complete particle surface coverage, thereby blocking metal oxide hydration. In the present study, the high concentrations of silicate and phosphate can react with the metal ions to generate precipitation, which is covered on the surface of metal oxides, blocking the regeneration of LDHs. However, the sulfate cannot react with the metal ions. Therefore, sulfate has less inhibitory effect on the regeneration of LDHs.

4. Conclusions

This study investigated the influence of sulfate, silicate, and phosphate on the memory effect of layered double hydroxides (LDHs) at pH values of 10 and 13. The findings demonstrate that silicate, phosphate, and solution pH significantly impact LDH regeneration. Specifically, silicate and phosphate ions react with MgO to form magnesium silicate and magnesium hydrogen phosphate (MgHPO₄). These precipitates deposit on the surfaces of calcined LDHs (CLDHs), inhibiting the hydroxylation of metal oxides and consequently preventing LDH regeneration. Conversely, elevated pH suppresses the formation of these obstructive precipitates, thereby promoting LDH reconstruction. Furthermore, a larger specific surface area enhances resistance to these inhibitory effects by reducing the coverage of precipitates on CLDH surfaces.