Introduction

The presence of phosphate and nitrates in reservoirs or lakes can trigger algal blooms, leading to eutrophication (Duan and Evans 2006; Havens 2003; Walker Jr and Havens, 1995). Upon entering the human body, nitrates can be converted into nitrosamines or nitrites, which pose carcinogenic risks. Several methods have been devised to reduce the influx of phosphate and nitrate into aquatic environments, including chemical precipitation, biological techniques, ion exchange, and adsorption (Blaney et al. 2007; Delgadillo-Mirquez et al. 2016; Gustafsson et al. 2008; İrdemez et al. 2006). Biological treatment of phosphate and nitrate is mostly applicable to specific types of wastewater, such as domestic sewage. Chemical precipitation is often used to remove phosphate but tends to leave low phosphate concentrations in the effluent, and nitrate removal via this method is difficult. The use of ion-exchange resins for nitrate removal involves high operational costs. Most adsorbents are ineffective in achieving high removal rates for anionic phosphates and nitrates (Loganathan et al. 2013, 2014). To address these challenges, we developed a low-cost adsorbent and utilized a simple method to remove phosphate and nitrate from wastewater efficiently.

Layered double hydroxides (LDHs) are hydrotalcite-like minerals with a lamellar structure. The general molecular formula of LDHs is [M(II)₁−ₓM(III)ₓ(OH)₂]ₓ⁺(Aⁿ⁻ₓ/ₙ)·mH₂O (Cavani et al. 1991), where M(II) represents divalent metal cations such as Mg2⁺ and Ca2⁺ (Chen et al. 2016; Theiss et al. 2016); M(III) denotes trivalent metal cations such as Al3⁺ and Fe3⁺ (Bernardo et al. 2017; Duan and Evans 2006); Aⁿ⁻ represents appropriate n-valence anions such as CO₃2⁻ and Cl⁻ in the interlayer region of LDHs (Chitrakar et al. 2011); and x ranges from 0.22 to 0.33 (Tonelli et al. 2021). Due to the presence of exchangeable anions in the interlayer region, LDHs exhibit a high anion exchange capacity. They have been employed to adsorb various anions such as AsO₄3⁻, Cr₂O₇2⁻, SeO₃2⁻, BrO₃⁻, PO₄3⁻, NO₃⁻, and dyes in aqueous environments (Chubar et al. 2013; Das et al. 2004; Goh et al. 2008; Lei et al. 2017; Pourfaraj et al. 2017; Yu et al. 2012; Zaghouane-Boudiaf et al. 2012). Additionally, LDHs intercalated with organic acids can adsorb both cationic heavy metals and oxyanions (Lin et al. 2018; Tran et al. 2018).

Various methods have been developed for synthesizing LDHs, including co-precipitation, hydrothermal precipitation, in situ synthesis, structural memory effect, and ion exchange (Chubar et al. 2013; Duan and Evans 2006; Kaneko and Ogawa 2013). Notably, LDH samples possess unique properties such as in situ synthesis and memory effects (Duan and Evans 2006; Rocha et al. 1999; Xu et al. 2020). In the in situ synthesis process, appropriate chemicals for LDH synthesis are added to a solution containing anionic contaminants under mixing conditions, directly producing LDHs. This method effectively removes anionic contaminants from wastewater (Chao et al. 2018). Moreover, LDHs can alter their properties through calcination. When calcined LDH samples are added into an aqueous solution, they can reconstruct their chemical structures, a phenomenon known as the memory effect, which enhances the adsorption of anionic contaminants (Chao et al. 2018).

This study applied the unique characteristics of layered double hydroxides (LDHs), such as in situ synthesis and memory effects, to remove phosphate and nitrate from wastewater. Various concentrations of chemicals were selected for the in situ synthesis to investigate their effects on pollutant removal rates. Adsorption experiments utilizing the memory effect were performed in solutions with three different pH levels to estimate the maximum adsorption capacities of nitrate and phosphate. The results indicate that LDHs, as low-cost adsorbents, can achieve significant removal rates of these contaminants. Therefore, these findings can be effectively applied to realistic wastewater treatment.

Materials and methods

All chemicals used in this study, with purities exceeding 95%, were obtained from Merck or Sigma-Aldrich and used without further purification. To synthesize layered double hydroxides (LDHs) for phosphate removal in the in situ experiment, magnesium nitrate [Mg(NO₃)₂·6H₂O], aluminum nitrate [Al(NO₃)₃·9H₂O], sodium hydroxide (NaOH), and sodium carbonate (Na₂CO₃) were prepared. For the elimination of nitrate in the in situ experiment, magnesium chloride (MgCl₂·6H₂O) and aluminum chloride (AlCl₃·9H₂O) were utilized (Sui et al. 2012).

Synthesis and calcination of layered double hydroxides

To produce layered double hydroxides (LDHs), a molar ratio of Mg2⁺ to Al3⁺ at 3:1 was selected. For the metal salt mixture, 0.12 mol of magnesium nitrate [Mg(NO₃)₂·6H₂O] and 0.04 mol of aluminum nitrate [Al(NO₃)₃·9H₂O] were dissolved in 60 mL of water. Then, 0.022 mol of sodium carbonate (Na₂CO₃) and 0.336 mol of sodium hydroxide (NaOH) were dissolved in 40 mL of water. The two solutions were mixed and stirred at room temperature (approximately 30 °C) for 1 h. The mixture was then transferred to a stainless steel autoclave with a Teflon liner and heated in an oven at 150 °C for 24 h. After cooling to room temperature, the solid–liquid mixture was separated by filtration and washed repeatedly with deionized water until the pH of the filtrate reached approximately 7.0. The resulting adsorbent, CO₃–LDHs, was dried at 70 °C for 24 h. To prepare Cl–LDH, the CO₃–LDHs were added to a 1 M NaCl solution, stirred for 24 h at room temperature, and filtered through a 0.45-μm filter. The solid was then washed with deionized water and dried in an oven at 70 °C for 24 h. To produce calcined LDHs, the CO₃–LDHs or Cl–LDHs were placed in a crucible and heated at a rate of 5 °C per minute to final temperatures of 300 °C, 400 °C, and 500 °C, maintaining each temperature for 3 h. The calcined samples were labeled as CO₃–LDHs–C300, CO₃–LDHs–C400, and CO₃–LDHs–C500 for CO₃–LDHs calcined at 300 °C, 400 °C, and 500 °C, respectively. Similarly, Cl–LDHs calcined at 300 °C, 400 °C, and 500 °C were labeled as Cl–LDHs–C300, Cl–LDHs–C400, and Cl–LDHs–C500, respectively.

Equilibrium adsorption experiments

Equilibrium adsorption experiments were performed using CO₃–LDHs and Cl–LDHs to adsorb phosphate and nitrate, respectively. Approximately 0.02 g of the LDH sample was added to a 50-mL Corex centrifuge tube containing the contaminant solution, with concentrations ranging from 10 to 300 mg/L. The pH of the solution was adjusted to 5, 7, and 9 using 0.05 M NaOH and H₂SO₄. The centrifuge tubes were placed in a reciprocating shaker at 150 rpm and 25 °C for 24 h to reach equilibrium. Afterward, the solutions were filtered through a 0.45-µm filter. The supernatants were collected to determine the adsorbate concentration using UV–VIS spectrophotometry (Genesys, 10 UV–Vis; Thermo Fisher Scientific, Waltham, MA, USA).

The amount of contaminant adsorbed by the LDHs at a given concentration at equilibrium, expressed as qe ​(mg/g), was calculated using the mass balance equation.

$$ q_{e} = \frac{{\left( {C_{o} - C_{e} } \right)}}{m} V $$
(1)

where C0 ​(mg/L) and Ce (mg/L) are the initial and equilibrium concentrations of the adsorbates, respectively, m (g) is the mass of the adsorbent, and V (L) is the volume of the adsorbate solution, which in this study is 0.05 L.

Kinetic adsorption experiment

The CO3–LDHs–500 and Cl–LDHs–500 were selected for the kinetic adsorption experiments on phosphate and nitrate, respectively. Solutions containing phosphate or nitrate at 20, 100, and 200 mg/L of 50 mL were prepared. A 0.02 g sample was added to the solution at 150 rpm using a magnetic stirrer. The samples in the pH 7 solutions at 25℃ were collected for periods of 0, 5, 10, 15, 30, 45, 60, 90, 120, 180, 240, and 300 min. The samples were filtered and measured using UV/VIS. The data were evaluated using pseudo-first- and pseudo-second-order models.

In situ adsorption experiment

The in situ adsorption experiments were performed using three different molar concentrations. The molar ratios of Mg2⁺ to Al3⁺ (3:1) were set to 0.006:0.002 mol/L (Group 1), 0.012:0.004 mol/L (Group 2), and 0.024:0.008 mol/L (Group 3). The concentrations of phosphate and nitrate were maintained at 200 mg/L. Since the pH values of the solutions during the experimental process were controlled to approximately 12, the initial pH value was not a crucial parameter for the in situ adsorption experiment.

To simulate phosphate removal efficiency in realistic wastewater, Mg (NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O, based on the selected molar ratios, were added to a solution containing 200 mg/L of phosphate. Subsequently, Na₂CO₃, with a Mg2⁺/CO₃2⁻ molar ratio of 0.05, was added to the solution. The pH of the solution was adjusted to 12 using 0.1 N NaOH and H₂SO₄. The solution was stirred at 150 rpm, and the phosphate concentration was measured at given intervals. The amount of phosphate adsorbed (​qt; mg/g) on the LDHs at any time t was calculated using Eq. (2).

$$ q_{t} = \frac{{\left( {C_{o} - C_{t} } \right)}}{m} V $$
(2)

where Co and Ct are the adsorbate concentrations in the solution at the beginning and at any time, respectively; m (g) is the mass of the adsorbent; and V is the volume of the solution. In this study, V was 0.5 L.

For the removal of nitrate from water, MgCl₂·6H₂O and AlCl₃·9H₂O were used. The molar concentrations of Mg2⁺ and Al3⁺ were maintained at the same levels as those used in the phosphate removal experiments. Following the addition of MgCl₂·6H₂O and AlCl₃·9H₂O to the solution, 0.01 mol of Na₂CO₃ was added. The pH was adjusted to 12, and the amount of nitrate adsorbed was calculated using Eq. (2).

Characterization of layered double hydroxides

The morphology of the LDHs was determined using field-emission scanning electron microscopy (SEM; Hitachi S-4800; Hitachi Ltd., Tokyo, Japan). The crystalline structure of the LDH samples was analyzed using X-ray diffraction (XRD) patterns obtained from a PANalytical PW3040/60 X'Pert Pro with Cu Kα radiation (λ = 0.154 nm). The 2θ range was set from 5° to 60°, with a scanning rate of 3°/min. Fourier transform infrared (FTIR) spectra were measured using a PerkinElmer 1600 FTIR spectrophotometer. The surface charges of the LDH samples in various pH solutions were measured using a zeta potential analyzer (Shimadzu, Zetasizer Nano-Z). The main elements in the LDHs were identified using X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientific, Wilmington, DE, USA).

Equilibrium and kinetic adsorption models

To elucidate the characteristics of phosphate and nitrate adsorption onto the LDHs, the Langmuir and Freundlich models were applied. These models are expressed as Eqs. (3) and (4), respectively.

$$ q_{e} = \frac{{Q_{\max } K_{L} C_{e} }}{{1 + K_{L} C_{e} }} $$
(3)
$$ q_{e} = K_{F } C_{e}^{1/n} $$
(4)

where qe and Ce are defined in Eq. 1, Qmax (mg/g) is the maximum adsorption capacity of the adsorbate on the adsorbent, KL (L/mg) is the Langmuir constant, and KF [(mg/g)/(mg/L)ⁿ] and n (dimensionless) are Freundlich constants that lie between 1 and 10.

For adsorption kinetics, the nonlinear forms of the pseudo-first-order and pseudo-second-order models are expressed as Eqs. (5) and (6), respectively.

$$ q_{t} = q_{e} \left( {1 - e^{{ - k_{1} t}} } \right) $$
(5)
$$ q_{t} = \frac{{q_{e}^{2} k_{2} t}}{{1 + k_{2} q_{e} t}} $$
(6)

where k1 (1/min) is the rate constant in the pseudo-first-order equation and k2 (g/mg min) is the rate constant in the pseudo-second-order equation.

Results and discussion

The SEM images of the calcined and uncalcined Cl–LDHs are shown in Fig. 1 and the SEM images of the calcined and uncalcined CO3–LDHs are shown in Figure S1. The calcined and uncalcined LDHs exhibited a hexagonal platelet-like shape typical of these materials, with diameters ranging from approximately 0.1 to 1.0 µm. The results indicated that calcination temperature did not significantly alter their external appearance. The CO₃–LDHs and Cl–LDHs showed similar shapes, indicating that replacing carbonate in the interlayer of LDHs with chloride did not affect their external morphology. When carbonate in the interlayer of LDHs is replaced by chloride, the external appearance is retained. Carbonate in LDHs can form carbon dioxide, which is released into the atmosphere or water when the calcination temperature exceeds 200 °C. At 600 °C, the chemical structure of the LDHs is destroyed, generating MgO and Al₂O₃. Consequently, the external appearance of Cl–LDHs and CO₃–LDHs remains unchanged when calcined below 550 °C.

Figure1
figure 1

SEM image of calcined and uncalcined Cl–LDHs a uncalcined LDHs, b 300 °C, c 400 °C, d 500 °C

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The FTIR spectra of the calcined and uncalcined LDHs are shown in Fig. 2. Both calcined and uncalcined LDHs exhibited identical functional groups. The band near 3490 cm⁻1 was assigned to the O–H group, a typical feature of LDHs. The bands observed near 1643 cm⁻1 and 1330 cm⁻1 were ascribed to the C = O and C–O groups, respectively, which primarily originate from carbonates. Although chloride can partially replace carbonate in the interlayer to form Cl–LDHs, a small amount of carbonate remains to maintain a stable chemical structure. Additionally, the peak intensities of functional groups such as OH, C = O, and C–O decreased with increase in calcination temperature. The peak intensities of the C = O and C–O groups in the CO₃–LDHs were higher than those in the Cl–LDHs, due to the partial replacement of carbonate by chloride.

Fig. 2
figure 2

FTIR spectra of a uncalcined LDHs, b 300 °C, c 400 °C, and d 500 °C

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The elemental compositions of CO3–LDHs measured by XPS are shown in Figure S2. The main elements of CO3–LDHs involved oxygen, magnesium, aluminum, and carbon. As expected, O, Mg, Al, and C were the primary elements comprising the composites. The element contents of CO3–LDHs and Cl–LDHs are shown in Table 1. The main elementary compositions of CO3–LDHs are similar to those of Cl–LDHs. Besides the above-mentioned elements, chlorine is a main element in the Cl–LDHs. The decrease in the C and O contents of the composites with increase in calcination temperature was attributed to the reaction of carbonate to form CO2.

Table 1 Elemental composition of CO3–LDHs and Cl–LDHs
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The adsorption–desorption isotherms of the CO₃–LDHs are illustrated in Figure S3 to further elucidate the pore characteristics of the synthesized LDHs. Since the pore characteristics of CO₃–LDHs are similar to those of Cl–LDHs, only the data for CO₃–LDHs are presented. The adsorption/desorption isotherms did not exhibit a significant hysteresis loop. According to the IUPAC classification, these curves comply with the type II isotherm, indicating that LDHs without micropores are not porous materials. The BET specific surface area (SBET), average pore size, and pore volume are presented in Table 2. The SBET values ranged from 20 to 35 m2/g, consistent with the literature (Chao et al. 2018; Duan and Evans 2006; Lin et al. 2018). The average pore size increased with rise in calcination temperatures, while the pore volume decreased. The calcination process likely caused the carbonate in the interlayer to form carbon dioxide, enhancing pore size (Chao et al. 2018; Ji et al. 2017). Additionally, replacing carbonate with chloride caused a slight increase in the pore size of the LDHs.

Table 2 The pore characteristics of the synthesized LDHs samples
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The XRD patterns of the calcined and uncalcined CO₃–LDHs are shown in Fig. 3. Since the crystal structure of Cl–LDHs is similar to that of CO₃–LDHs, the results for Cl–LDHs are not presented here. The structures of the LDHs were identified by the corresponding peaks in the XRD pattern. The uncalcined CO₃–LDHs exhibited a typically well-crystallized nature. According to JCPDS No. 89–0460, the positions of 2θ at 11.4° and 23.9° are defined as D (003) and D (006), respectively. The basal spacing of D (003) for the CO₃–LDHs, estimated from Bragg's law, was 7.68 Å. This result is consistent with previous reports (Tran et al. 2018; Zhang et al. 2016). When the CO₃–LDHs were calcined at 300 °C, 400 °C, and 500 °C, the D (003) and D (006) peaks in the 2θ position disappeared. The basal spacing of the CO₃–LDHs changed, and carbonate in the interlayer formed carbon dioxide. However, the other peaks were retained. These results demonstrate that the calcination process can alter the partial structure of LDHs while retaining their main characteristics. This finding corresponds with the hypothesis regarding the memory effect.

Fig. 3
figure 3

XRD patterns of a CO3–LDHs, b CO3–LDH–C300, c CO3–LDH–C400, d CO3–LDH–C500

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The adsorption isotherms of phosphate on calcined and uncalcined CO₃–LDHs at pH 5, 7, and 9 are shown in Figure S4. Similarly, the adsorption isotherms of nitrate on calcined and uncalcined Cl–LDHs at pH 5, 7, and 9 are illustrated in Figure S5. All isotherms exhibited concave downward curves, indicating effective adsorption of phosphate by CO₃–LDHs and nitrate by Cl–LDHs. The amounts of phosphate and nitrate adsorbed decreased with increase in solution pH. This phenomenon can be attributed to the competition between OH⁻ ions and phosphate or nitrate ions for adsorption sites. The results suggest that ion exchange is the primary mechanism for adsorption. Furthermore, an increase in calcination temperature was found to enhance the adsorption capacities for both phosphate and nitrate. These findings also demonstrate that the memory effect of LDHs can significantly enhance the adsorption of anionic contaminants.

The data fitting the Langmuir and Freundlich models for the adsorption of phosphate on the CO₃–LDHs and nitrate on the Cl–LDHs are presented in Tables 3 and 4, respectively. According to the R-squared values, the data fit the Langmuir model well, indicating that the maximum adsorption capacities of phosphate and nitrate can be accurately estimated using the Langmuir model. The n values in the Freundlich model exceeded 1.0, demonstrating that the adsorption isotherms exhibited concave downward curves. As expected, the adsorption amounts of phosphate and nitrate decreased in high-pH solutions. Because LDHs have rich exchangeable anions in the interlayer, the ion exchange is a critical mechanism for the anion removal. Moreover, the adsorptive amounts of anions increase with the decrease in solution pH values. The ion exchange is regarded as the primary adsorption mechanism. Although phosphate might adsorb on the LDHs through surface adsorption, the adsorptive amounts by the surface adsorption are far lower than those by the ion exchange. Furthermore, the amounts of phosphate and nitrate adsorbed increased with increase in calcination temperature. The pore volume and SBET values of the adsorbents also increased with calcination temperature. A higher pore volume and specific surface area enhance the contact between contaminants and the surface of the adsorbent, improving adsorption efficiency.

Table 3 Adsorption isotherm parameters of phosphate on the adsorbents
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Table 4 Adsorption isotherm parameters of nitrate on the adsorbents
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The maximum adsorption amounts of phosphate for all CO₃–LDHs samples at pH 5–9 exceeded 100 mg/g. These results indicate that LDHs can maintain high adsorption capacities for phosphate, despite minor pH changes in the solution, making them suitable for phosphate removal from wastewater. The maximum nitrate adsorption amounts for all Cl–LDHs ranged from 45 to 70 mg/g. Similarly, the maximum adsorption amounts did not vary significantly with the pH of the solutions. The adsorption amounts of phosphate were higher than those of nitrate, which can be attributed to the weak ion-exchange ability of monovalent nitrate anions. However, the cost of synthesizing LDHs is significantly lower than that of ion-exchange resins. Additionally, nitrate is challenging to remove via precipitation, and other nitrate removal processes need high operational costs. Therefore, Cl–LDHs represent low-cost adsorbents for removing nitrate from wastewater.

The results demonstrated that both calcined and uncalcined CO₃–LDHs and Cl–LDHs effectively adsorbed phosphate and nitrate. For calcined CO₃–LDHs and Cl–LDHs, the memory effect significantly enhanced the adsorption capacities of these contaminants. To further understand the memory effect, samples were collected after adsorption and analyzed using XRD. The XRD patterns of Cl–LDHs calcined at various temperatures are shown in Figure S6. As compared with Fig. 3, notable differences before and after adsorption were observed, particularly the disappearance of the D (003) peak at 2θ = 11.4°. These results indicate that the structure of the LDHs had been reconstructed, thus confirming the hypothesis regarding the memory effect as demonstrated by the XRD patterns before and after adsorption.

The maximum adsorption capacities of phosphate and nitrate occur under calcination at 500 °C. The kinetic adsorption of phosphate on CO₃–LDHs–500 is shown in Figure S7. The results revealed the kinetic adsorption of phosphate at initial concentrations ranging from 20 mg/L to 200 mg/L, with the adsorption reaction reaching equilibrium within 60 min. Notably, the adsorptive amount of phosphate increased with the initial concentration. The amount of phosphate adsorbed in the 200 mg/L solution was significantly higher than that in the 100 mg/L solution, indicating that a continuous increase in initial concentration still increased the adsorption amount. Similarly, the kinetic adsorption of nitrate on Cl–LDHs–500 is shown in Figure S8. The adsorption reaction with initial concentrations ranging from 20 to 200 mg/L also reached equilibrium within 60 min, and the adsorptive amounts of nitrate increased with the initial concentration. However, the increase in the adsorptive amount for the 200 mg/L initial concentration was only slightly higher than that for the 100 mg/L initial concentration, revealing that nitrate removal was limited as the initial concentration increased.

The data for the kinetic adsorption of phosphate and nitrate on the LDHs fitting the pseudo-first-order and pseudo-second-order models are listed in Table 5. The rate constants increased with increase in initial concentrations. The data fit the pseudo-second-order model well, with R-squared values of more than 0.97. These results demonstrated that chemical adsorption is the primary adsorption mechanism. Moreover, the initial concentration significantly affected the adsorption amounts. The adsorption amounts of phosphate and nitrate at specific concentrations also corresponded to previous descriptions.

Table 5 Parameters of kinetic adsorption models for phosphate and nitrate on LDHs under 500 °C calcination
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In situ synthesis is critical for the application of LDHs in wastewater treatment. The concentrations of Mg2⁺ and Al3⁺ may affect the removal rate. In this study, three different concentrations with a 3:1 molar ratio of Mg2⁺ to Al3⁺ were tested. Figure 4 shows the results of the in situ synthesis experiment with a phosphate concentration of 200 mg/L without pH adjustment. Phosphate removal rates increased with reaction time, reaching 60%–80% within 120 min. PO₄3⁻ and HPO₄2⁻ were the dominant species due to the alkaline nature of the solution. The OH⁻ and CO₃2⁻ in the solution competed with phosphate for adsorption sites, resulting in a prolonged time to reach equilibrium. Nonetheless, the in situ process effectively removed phosphate from the solution. Additionally, the removal rates of phosphate followed the order: group 3 > group 2 > group 1. Although increasing the additive amounts of chemical agents improved the removal rates, the differences were small. For realistic wastewater treatment, operators can select the appropriate amounts of additives based on the desired phosphate removal rates. This method can be used following biodegradation or chemical precipitation.

Fig. 4
figure 4

The result of in situ synthesis experiments for phosphate

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Figure 5 shows the results of the in situ synthesis experiment with a nitrate concentration of 200 mg/L. The nitrate removal rates ranged from approximately 20 to 60%. The curves rose sharply within the first 10 min and then slightly decreased before stabilizing. The adsorption reactions reached equilibrium within 20 min. The slight decrease is attributed to the release of nitrate from the LDHs, as partial carbonate in the solution replaced nitrate on the LDHs. Nitrate removal rates were lower than those of phosphate because replacing other anions on LDHs with nitrate is challenging. As shown in Fig. 5, although the removal rates of nitrate followed the order: group 3 > group 2 > group 1, the effect of the chemical agents was relatively higher. This method, applied to nitrate removal from wastewater, requires higher amounts of chemical agents. Previously, finding a low-cost process for removing nitrate from wastewater was challenging. The in situ method for LDHs provides a selective and cost-effective process for nitrate removal.

Fig. 5
figure 5

The result of in situ synthesis experiments for nitrate

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The characterization techniques of XRD, FTIR, and BET above-mentioned also show that the microscopic mechanism of the memory effect in the calcined LDH enables to spontaneously reconstruct its original layered structure upon exposure to anionic contaminant in aqueous solution, while enhancing the anion adsorption capabilities.

During 300 to 500 °C calcination process, the layered structure of LDHs partially disintegrates by releasing its interlayer anions such as carbonate (CO), dehydrating H2O and losing -OH, then forming metal oxides like MgO and Al₂O₃. These oxides feature highly reactive surfaces, laying the groundwork for the subsequent of reconstruction reactions. When calcined LDH materials are exposed to water or anion-containing solutions, these metal oxides rapidly form hydroxides, such as Mg(OH)₂ and Al(OH)₃. These hydroxide layers exhibit capabilities to readsorb anions such as phosphate and nitrate from the solution, in accordance to reconstruct the layered structure (Ye et al. 2022).

This phenomenon is further confirmed by XRD analysis, as illustrated in Fig. 3. For instance, the XRD pattern of calcined Cl–LDHs shows the disappearance of characteristic peaks associated with the layered structure, particularly the (003) peak at 11.4°. However, after rehydration in solution, these characteristic peaks are partially restored, especially the reappearance of the (003) peak, indicating that the original layered structure can be reconstructed during pollutant removal processes.

Additionally, as shown in the FTIR analysis in Fig. 2, calcined LDHs exhibit a weakening or disappearance of the O–H stretching vibration peak at 3490 cm⁻1 and the C = O stretching vibration peak at 1643 cm⁻1. However, after reconstruction, the intensity of these functional group peaks is partially restored, indicating that the memory effect facilitates the reincorporation of interlayer anions, thereby repairing the structure. Furthermore, the BET surface area analysis in Table 2 further confirms the impact of calcination and reconstruction on the structure of LDHs materials. The results show a significant increase in the surface area of calcined LDHs, along with changes in pore characteristics. For example, after calcining CO₃–LDH at 500 °C, the surface area increased from 20.4 m2/g in the uncalcined sample to 24.1 m2/g, while the average pore diameter decreased from 23.9 nm to 16.9 nm. This suggests that the calcination process leads to the desorption of interlayer anions and partial collapse of the structure, while the memory effect allows these features to be partially restored during rehydration. Collectively, these experimental results demonstrate that the memory effect in LDHs is driven by multiple microscopic processes, including thermal decomposition, structural reconstruction, and reincorporation of anions. This effect enables calcined LDHs materials to spontaneously restore their layered structure in aqueous solutions, thereby exhibiting excellent performance in the removal of anionic pollutants from water. These findings are crucial for understanding the structural properties of LDHs materials and their applications in water treatment.

Conclusion

The objective of this study was to develop a process for removing phosphate and nitrate from wastewater using layered double hydroxides (LDHs) characterized by the memory effect and in situ synthesis. The removal rates of phosphate and nitrate were thoroughly examined. The results indicated that calcined LDHs, with their larger pore sizes, possessed higher adsorptive capacities compared to uncalcined LDHs. The memory effect effectively increased the amounts of adsorbed contaminants. The kinetic adsorption of phosphate and nitrate reached equilibrium within 60 min. In situ synthesis proved to be an effective method for removing phosphate and nitrate from wastewater. Ion exchange was identified as the primary adsorption mechanism, resulting in a higher adsorptive capacity for phosphate compared to nitrate. When using LDHs to remove nitrate from wastewater, higher amounts of chemical agents or adsorbents are necessary.