# Chloride Adsorption on EDTA Intercalated MnFe-LDHs: Characterization, Performance, and Application

Authors: Xue Han
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Ethylenediamine tetraacetate-intercalated Fe-Mn layered double hydroxides (MnFe-EDTA-LDHs) were prepared by coprecipitation. The influence of different factors on Cl− adsorption by MnFe-EDTA-LDHs was investigated. Additionally, MnFe-EDTA-LDHs were incorporated into the concrete to study the Cl− erosion properties. The result showed that the Cl− adsorption on MnFe-EDTA-LDHs was affected by the initial pH, dosage, and coexisting anions. The Cl− adsorption process by MnFe-EDTA-LDHs accorded with the pseudo-second-order model and the Redlich–Peterson model. The maximum adsorption capacity of Cl− on MnFe-EDTA-LDHs reached 129.50 mg/g. The adsorption mechanism involved the complexation of O-containing groups, ion exchange, and electrostatic interaction. Moderate additions of MnFe-EDTA-LDHs improved the resistance of the concrete to Cl− erosion, while excessive additions reduced the Cl− erosion resistance. This work indicated that MnFe-EDTA-LDHs effectively retarded the Cl− erosion on concrete.


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## Body

## 1. Introduction

Concretes are widely used in marine construction (e.g., terminals, ports, cross-sea bridges, and offshore exploration platforms). However, high concentrations of chloride ions reduce the durability of concrete structures [1, 2]. Additionally, the chlorides lead to concrete fragmentation and deterioration [3]. To reduce the permeability of chloride ions, it is necessary to delay the transport of chloride ions in concrete. Surface coatings and corrosion inhibitors are the common methods to prevent concrete corrosion [4, 5]. Organic coatings have poor-wear resistance and low-impact resistance, while inorganic coatings are less adherent and flexible. Corrosion inhibitors can reduce the concrete corrosion, but there are difficulties in dispersion [6, 7]. The development of methods for effective chloride adsorption in concrete is essential.In recent years, the addition of layered double hydroxides ( LHDs ) to concrete can delay the transfer of Cl− [8]. The chemical formula for LDHs is as follows: [M1−x2+Mx3+(OH)2]x+[Ax/nn−∙H2O]x−, where M2+ and M3+ are divalent and trivalent metal cations [9–11]. Therefore, LDHs become a promising material for adsorbing chloride ions [8]. LDHs with inserted inhibitory anions can act as chloride ion adsorbents. On the one hand, LDHs can reduce the number of chloride ions in the aqueous solution [4, 12]. On the other hand, LDHs can release the inhibitory anions to prevent the Cl− into the concrete structure [13, 14]. Wang et al. [15] showed that MgAl-NO2-LDH effectively inhibited the Cl− and SO42− in cement mortar. The presence of SO42− affected the amount of Cl− uptake, and there was a coupling effect of Cl− and SO42− on their penetration into the mortar specimens. Xu et al. [16] found that the Cl− adsorption of CaMnFe-LDHs with NO−3 intercalation was chemisorption, and the NO−3 exchanged with Cl−. Additionally, Yang et al. [14] reported that the MgAl-LDHs prepared by the supergravity rotating bed method had a more regular and homogeneous structure. The adsorption capacity of Cl− was 3 times higher after calcination of the MgAl-LDHs. The insertion of inhibitory anions into LDHs can enhance the adsorption of Cl−. The inhibitory anions released into the aqueous solution can prevent the Cl− into the concrete structure. Wei et al. [4] prepared aminobenzoate-intercalated MgAl-LDHs to adsorb Cl− from aqueous solutions, and the adsorption process was spontaneous and exothermic. The corrosion inhibition of concrete was due to the Cl− adsorption by 2-mercaptobenzothiazole-intercalated ZnAl-LDHs [17]. Inhibitory anionic intercalation of LDHs provides good inhibition of concrete against Cl− corrosion. As mentioned above, the Cl− adsorption performance of LDHs is a key factor in the ability to inhibit Cl− corrosion. The adsorption performance of LDHs with EDTA intercalation on Cl− has rarely been reported.In this work, EDTA intercalated MnFe-LDHs (MnFe-EDTA-LDHs) were prepared. Different influencing factors on the Cl− removal by MnFe-EDTA-LDHs were investigated. Meanwhile, the resistance of LDHs to Cl− erosion on the concrete was explored. This work provided a theoretical basis for the resistance of concrete to Cl− erosion.


## 2. Materials and Methods

### 2.1. Reagents

Mn(NO3)2·4H2O, Fe(NO3)3·9H2O, EDTA-Na, and NaCl were analytical reagents. All reagents were purchased from Sinopharm Reagent Group (Shanghai, China). The water used in the experiments was deionized water (DW). The cement was conventional P·O 42.5 cement with a density of 3.06 g/cm3. The main substances of cement were SiO2 (27.68%), CaO (49.73%), Al2O3 (10.09%), Fe2O3 (3.18%), MgO (1.97%), and others (7.35%).


### 2.2. Preparation of Materials

Preparation of FeMn-EDTA-LDHs: 4.569 g Mn(NO3)2·4H2O and 3.617 g Fe(NO3)3·9H2O were dissolved in 50 mL DW to obtain Fe-Mn hybrid solution. 9.6 g NaOH and 4.653 g EDTA-Na were dissolved in 100 mL DW, and the solution was stirred to get the alkali mixed solution. Then, two mixtures were mixed in a triple flask, and the solution pH was adjusted to 12.0 by 0.1 mol NaOH. The flask was continuously filled with N2 and transferred to a water bath at 60°C for 6 h. The reaction product was cooled to room temperature and washed three times by DW. The product was dried in a vacuum freeze dryer for 24 h, which was FeMn-EDTA-LDHs.


### 2.3. Adsorption Experiments

All adsorption experiments were conducted with a 150 mL conical flask oscillated in a shaker of 150 r/min at 25°C. The pH was adjusted by infinitesimal NaOH or HNO3 solution at 0.1 mol/L. The effect of initial solution pH was investigated between 2.0 and 11.0 at 50 mg/L Cl−. Different dosage (0.1∼3 g/L) of FeMn-EDTA-LDHs was added to the Cl− solution to investigate the dosage effects. The mixed solutions of 50 mg/L Cl− and coexist ions (Na+, K+, Ca2+, Mg2+, Fe3+, SO4−, NO3−, CO32−, PO43−, and SiO32−) were configured. The concentration of coexisting ions was set to 0.001, 0.01, and 0.1 mol/L. The effect of coexisting ions for Cl− adsorption was investigated. The effect of different temperatures (15°C∼50°C) on the Cl− removal by FeMn-EDTA-LDHs was investigated. For adsorption kinetics experiments, the reaction time was set to form 10 min∼600 min. For isotherm experiments, the Cl− concentration was set from 50 to 1000 mg/L. All experiments were repeated three times. Additionally, adsorption-desorption experiments were carried out on FeMn-EDTA-LDHs using NaOH as a desorbent.


### 2.4. Data Analysis

After the reaction was completed, 10 mL solution was centrifuged for 10 min and the Cl− concentration of the solution was measured using a chloride ion content measuring instrument (NJCL-B, Meiyu, Shanghai, China). The removal efficiency (Equation (1)) and the adsorption capacity (Equation (2)) were calculated.(1)R=C0−CeC0×100%,(2)Q=C0−Ce×Vm,where R (%) is the removal efficiency; C0 (mg/L) and Ce (mg/L) are the initial concentration of Cl− and the remaining concentration of Cl− in the solution after adsorption equilibrium, respectively; qe (mg/g) is the Cl− adsorption capacity of FeMn-EDTA-LDHs, mg/g; V (mL) is the volume of the solution; and m (mg) is the dosage of the FeMn-EDTA-LDHs.The effect of adsorption time (10∼600 min) on the Cl− removal was investigated at 25°C, dosage 1 g/L, Cl− concentration 50 mg/L, and pH 7.0. The experimental data were fitted and analysed using the pseudo-first-order model (3), the pseudo-second-order model (4), the Elovich model (5), the intraparticle diffusion model (6),and the Boyd kinetic model (7).(3)qt=qe1−e−k1t,(4)qt=qe2k2t1+qek2t,(5)qt=1βln1+αβt,(6)qt=Kdt1/2+Ci,(7)Bt=−0.4977−ln1−FF=qtqα,where qe and qt are the adsorption capacity at adsorption equilibrium and at a time “t,” respectively, mg/g; k1 (min−1) and k2 (g/mg·min) are adsorption rate constant of the pseudo-first-order and the pseudo-second-order model, respectively; α (mg/g·min) and β are the initial absorbance and desorption constant, respectively; Kd (mg/m·min1/2) is the intraparticle diffusion constant; Ci is the boundary layer constant; qɑ represents the amount sorbed (mg/g) at the infinite time; and Bt is a mathematical function of F. The Bt values at different contact times were calculated. The calculated Bt values were plotted against time t (min).The effect of the initial concentration (50∼1000 mg/L) on the Cl− removal was investigated at pH 7.0, a dosage of 1 g/L, and 600 min. The experimental data were fitted and analysed by Langmuir (8), Freundlich (9), Temkin (10), Sips (11), Redlich–Peterson (12), Toth (13),and Hill (14)isotherm models.(8)qe=qmaxKLCe1+KLCe,(9)qe=KfCe1/n,(10)qe=RTbTlnATCe,(11)qe=KsCeβH1+asCeβH,(12)qe=KRCe1+aRCeβ,(13)qe=KTCeaT+KTCe1/n,(14)qe=QsHCenHKD+CenH,where qe (mg/g) is the adsorption capacity at equilibrium, mg/g; Ce (mg/L) is the Cl− concentration at adsorption equilibrium; qmax (mg/g) and KL (L/mg) are the theoretical maximum adsorption capacity and the Langmuir equilibrium constant, respectively; Kf (mg1−n·Ln/g) and n are the Freundlich equilibrium constant and dimensionless number, respectively; AT (1/g) and bT (kJ/mol) are Temkin constants; R is the global constant of gases (8.314 kJ·mol−1), and T is the absolute temperature (K). Ks (mg/g), KT (mg/g), and QSH (mg/g) are the maximum adsorption capacity. As (L/mg) and βs are the Sips isotherm constants. KR (L/g), aR (L/mg), and β are the Redlich–Peterson isotherm constants. aT (L/mg) is the Toth isotherm constant. KD is the Hill constant. nH is the cooperativity coefficient of binding interaction obtained from the slope of a Hill plot.


### 2.5. Immobilisation Experiments

The concrete samples (size: 60 mm × 60 mm × 60 mm) were prepared by mixing a certain amount of FeMn-EDTA-LDHs as an admixture. The concrete specimens are prepared as shown in Table1. The concrete samples were placed in 5% chloride solution after 28 d curing. The concrete samples were immersed at room temperature for 30 and 60 d.Table 1
Proportioning of cement slurries.
Cement (g)DW (mL)FeMn-EDTA-LDHs (%)C04001600C14001601C24001602C34001603C44001604After soaking, the concrete samples were dried in a drying oven at 60°C for 24 h. The concrete samples were cut into small pieces at 2 mm intervals and broken into powder. Subsequently, the powder was mixed with 100 mL of DW and soaked for 24 h. The mixture was filtered, and the filtrate was collected. Finally, the Cl− concentration in the filtrate was measured using a chloride ion content meter. The mass fraction of Cl− at different depths of the concrete samples was calculated.


### 2.6. Structural Characterisation of the Materials

The surface morphology structure was analysed by the scanning electron microscope (SEM, JSM-7500F, Electronics, Japan). Surface elemental analysis is done by energy dispersive spectroscopy (EDS, JSM-2100F, Electronics, Japan). Zeta potential analyser (ZS-90, Malvern, UK) analyses the surface potential of FeMn-Cl-LDHs at different pH and calculates zero potential point. The crystalline structure was analysed by an X-ray Diffraction (XRD, D8, Bruker, Germany). The functional groups of FeMn-EDTA-LDHs before and after adsorption were investigated by Fourier transform interferometric radiometer (FTIR, Nicolet460, Thermo Fisher Scientific, USA).


## 2.1. Reagents

Mn(NO3)2·4H2O, Fe(NO3)3·9H2O, EDTA-Na, and NaCl were analytical reagents. All reagents were purchased from Sinopharm Reagent Group (Shanghai, China). The water used in the experiments was deionized water (DW). The cement was conventional P·O 42.5 cement with a density of 3.06 g/cm3. The main substances of cement were SiO2 (27.68%), CaO (49.73%), Al2O3 (10.09%), Fe2O3 (3.18%), MgO (1.97%), and others (7.35%).


## 2.2. Preparation of Materials

Preparation of FeMn-EDTA-LDHs: 4.569 g Mn(NO3)2·4H2O and 3.617 g Fe(NO3)3·9H2O were dissolved in 50 mL DW to obtain Fe-Mn hybrid solution. 9.6 g NaOH and 4.653 g EDTA-Na were dissolved in 100 mL DW, and the solution was stirred to get the alkali mixed solution. Then, two mixtures were mixed in a triple flask, and the solution pH was adjusted to 12.0 by 0.1 mol NaOH. The flask was continuously filled with N2 and transferred to a water bath at 60°C for 6 h. The reaction product was cooled to room temperature and washed three times by DW. The product was dried in a vacuum freeze dryer for 24 h, which was FeMn-EDTA-LDHs.


## 2.3. Adsorption Experiments

All adsorption experiments were conducted with a 150 mL conical flask oscillated in a shaker of 150 r/min at 25°C. The pH was adjusted by infinitesimal NaOH or HNO3 solution at 0.1 mol/L. The effect of initial solution pH was investigated between 2.0 and 11.0 at 50 mg/L Cl−. Different dosage (0.1∼3 g/L) of FeMn-EDTA-LDHs was added to the Cl− solution to investigate the dosage effects. The mixed solutions of 50 mg/L Cl− and coexist ions (Na+, K+, Ca2+, Mg2+, Fe3+, SO4−, NO3−, CO32−, PO43−, and SiO32−) were configured. The concentration of coexisting ions was set to 0.001, 0.01, and 0.1 mol/L. The effect of coexisting ions for Cl− adsorption was investigated. The effect of different temperatures (15°C∼50°C) on the Cl− removal by FeMn-EDTA-LDHs was investigated. For adsorption kinetics experiments, the reaction time was set to form 10 min∼600 min. For isotherm experiments, the Cl− concentration was set from 50 to 1000 mg/L. All experiments were repeated three times. Additionally, adsorption-desorption experiments were carried out on FeMn-EDTA-LDHs using NaOH as a desorbent.


## 2.4. Data Analysis

After the reaction was completed, 10 mL solution was centrifuged for 10 min and the Cl− concentration of the solution was measured using a chloride ion content measuring instrument (NJCL-B, Meiyu, Shanghai, China). The removal efficiency (Equation (1)) and the adsorption capacity (Equation (2)) were calculated.(1)R=C0−CeC0×100%,(2)Q=C0−Ce×Vm,where R (%) is the removal efficiency; C0 (mg/L) and Ce (mg/L) are the initial concentration of Cl− and the remaining concentration of Cl− in the solution after adsorption equilibrium, respectively; qe (mg/g) is the Cl− adsorption capacity of FeMn-EDTA-LDHs, mg/g; V (mL) is the volume of the solution; and m (mg) is the dosage of the FeMn-EDTA-LDHs.The effect of adsorption time (10∼600 min) on the Cl− removal was investigated at 25°C, dosage 1 g/L, Cl− concentration 50 mg/L, and pH 7.0. The experimental data were fitted and analysed using the pseudo-first-order model (3), the pseudo-second-order model (4), the Elovich model (5), the intraparticle diffusion model (6),and the Boyd kinetic model (7).(3)qt=qe1−e−k1t,(4)qt=qe2k2t1+qek2t,(5)qt=1βln1+αβt,(6)qt=Kdt1/2+Ci,(7)Bt=−0.4977−ln1−FF=qtqα,where qe and qt are the adsorption capacity at adsorption equilibrium and at a time “t,” respectively, mg/g; k1 (min−1) and k2 (g/mg·min) are adsorption rate constant of the pseudo-first-order and the pseudo-second-order model, respectively; α (mg/g·min) and β are the initial absorbance and desorption constant, respectively; Kd (mg/m·min1/2) is the intraparticle diffusion constant; Ci is the boundary layer constant; qɑ represents the amount sorbed (mg/g) at the infinite time; and Bt is a mathematical function of F. The Bt values at different contact times were calculated. The calculated Bt values were plotted against time t (min).The effect of the initial concentration (50∼1000 mg/L) on the Cl− removal was investigated at pH 7.0, a dosage of 1 g/L, and 600 min. The experimental data were fitted and analysed by Langmuir (8), Freundlich (9), Temkin (10), Sips (11), Redlich–Peterson (12), Toth (13),and Hill (14)isotherm models.(8)qe=qmaxKLCe1+KLCe,(9)qe=KfCe1/n,(10)qe=RTbTlnATCe,(11)qe=KsCeβH1+asCeβH,(12)qe=KRCe1+aRCeβ,(13)qe=KTCeaT+KTCe1/n,(14)qe=QsHCenHKD+CenH,where qe (mg/g) is the adsorption capacity at equilibrium, mg/g; Ce (mg/L) is the Cl− concentration at adsorption equilibrium; qmax (mg/g) and KL (L/mg) are the theoretical maximum adsorption capacity and the Langmuir equilibrium constant, respectively; Kf (mg1−n·Ln/g) and n are the Freundlich equilibrium constant and dimensionless number, respectively; AT (1/g) and bT (kJ/mol) are Temkin constants; R is the global constant of gases (8.314 kJ·mol−1), and T is the absolute temperature (K). Ks (mg/g), KT (mg/g), and QSH (mg/g) are the maximum adsorption capacity. As (L/mg) and βs are the Sips isotherm constants. KR (L/g), aR (L/mg), and β are the Redlich–Peterson isotherm constants. aT (L/mg) is the Toth isotherm constant. KD is the Hill constant. nH is the cooperativity coefficient of binding interaction obtained from the slope of a Hill plot.


## 2.5. Immobilisation Experiments

The concrete samples (size: 60 mm × 60 mm × 60 mm) were prepared by mixing a certain amount of FeMn-EDTA-LDHs as an admixture. The concrete specimens are prepared as shown in Table1. The concrete samples were placed in 5% chloride solution after 28 d curing. The concrete samples were immersed at room temperature for 30 and 60 d.Table 1
Proportioning of cement slurries.
Cement (g)DW (mL)FeMn-EDTA-LDHs (%)C04001600C14001601C24001602C34001603C44001604After soaking, the concrete samples were dried in a drying oven at 60°C for 24 h. The concrete samples were cut into small pieces at 2 mm intervals and broken into powder. Subsequently, the powder was mixed with 100 mL of DW and soaked for 24 h. The mixture was filtered, and the filtrate was collected. Finally, the Cl− concentration in the filtrate was measured using a chloride ion content meter. The mass fraction of Cl− at different depths of the concrete samples was calculated.


## 2.6. Structural Characterisation of the Materials

The surface morphology structure was analysed by the scanning electron microscope (SEM, JSM-7500F, Electronics, Japan). Surface elemental analysis is done by energy dispersive spectroscopy (EDS, JSM-2100F, Electronics, Japan). Zeta potential analyser (ZS-90, Malvern, UK) analyses the surface potential of FeMn-Cl-LDHs at different pH and calculates zero potential point. The crystalline structure was analysed by an X-ray Diffraction (XRD, D8, Bruker, Germany). The functional groups of FeMn-EDTA-LDHs before and after adsorption were investigated by Fourier transform interferometric radiometer (FTIR, Nicolet460, Thermo Fisher Scientific, USA).


## 3. Results and Discussion

### 3.1. Characterization Analysis

Figure1 shows that FeMn-EDTA-LDHs morphological features are observed by SEM. Before the reaction (Figure 1(a)), the surface was rough and loose, showing an irregular lamellar structure with sharp angles [18]. After Cl− adsorption (Figure 1(b)), the structure of FeMn-EDTA-LDHs was stacked, agglomerated, and collapsed. By EDS analysis, the elements of C (31.89%), O (26.10%), Mn (27.68%), and Fe (14.33%) were present in the FeMn-EDTA-LDHs before adsorption. Additionally, the ratio of Mn to Fe was exactly 2 : 1, which was consistent with the ratio used in the preparation. After adsorption (Figure 1(b)), the element of chlorine appeared in the EDS, which indicated that the FeMn-EDTA-LDHs adsorbed Cl− from the aqueous solution.Figure 1
SEM-EDS images of Cl− before (a) and after (b) adsorption by FeMn-EDTA-LDHs.
(a)(b)The N2 adsorption-desorption isotherm of FeMn-EDTA-LDHs is displayed in Figure 2(a). The adsorption-desorption isotherms were classified as type IV isotherms [19, 20]. The pore size distribution (Figure 2(b)) of FeMn-EDTA-LDHs was 2∼50 nm, which demonstrated that FeMn-EDTA-LDHs were mesoporous materials [7]. The specific surface area and pore size of FeMn-EDTA-LDHs were 115.38 m2/g and 17.24 nm, respectively.Figure 2
N2 adsorption-desorption curves (a), pore size distribution (b), FTIR (c), and XRD (d) of MnFe-EDTA-LDHs.
(a)(b)(c)(d)The function group of FeMn-EDTA-LDHs before and after adsorption is shown in Figure2(c). The strong and broad diffraction peak at 3420 cm−1 was identified as the −OH [14, 21]. FeMn-EDTA-LDHs held an absorption band near 1600 cm−1, which was a superposition of C=C/C=O functional groups in the carboxyl group [4, 21]. The band at 1411 cm−1 corresponded to the asymmetric vibration peak of −COOH, which indicated the successful intercalation of EDTA. After the reaction, the −OH diffraction peak remained at 3435 cm−1. However, the intensity and wavenumber of the −OH peaks were weakened and shifted, respectively. This phenomenon may attribute to a complex between the −OH and Cl− [3, 20]. Furthermore, the characteristic peak at 545 cm−1 was the Mn-O and Fe-O functional groups [1, 16]. After adsorption, the Fe-O and Mn-O peaks shifted lower wavenumbers, which attributed to the complexation of Fe-O and Mn-O groups with Cl− [13].The XRD analysis of the FeMn-EDTA-LDHs is shown in Figure2(d). At 2θ of 11.54°, 23.32°, 33.03°, 57.16°, and 58.66° were typical diffraction peaks of layered double hydroxides [7, 16, 22]. This peak was corresponded to the (003), (006), (009), (110), and (113) diffraction crystallographic planes, respectively [16]. The intensity of the peaks after adsorption was reduced. This may be due to the ion exchange caused by chloride ion adsorption, which changed the charge density [23]. This indicated that the Cl− entered the crystals’ interior during the adsorption process, causing changes in the lattice structure [9, 23]. The weakening of characteristic peaks was due to the collapse and plugging of the material structure after adsorption, which was consistent with the SEM result [4].


### 3.2. Dosage

The effect of the dosage on Cl− adsorption is shown in Figure 3(a). The unit adsorption capacity decreased rapidly when the dosage was increased from 0.1 g/L to 1 g/L. The dosage exceeded 1 g/L, and the unit adsorption capacity decreased slightly. With increasing dosage, the active site increased, andthe removal efficiency rose [24]. The removal efficiency increased very little when the dosage exceeded 1 g/L. This was considered to be the limited availability of Cl− in solution. Therefore, 1 g/L was supposed to be the optimum dosage for this work.Figure 3
The effects of dosage (a) and initial solution pH (b) on the Cl− removal by FeMn-EDTA-LDHs. Zeta potential of FeMn-EDTA-LDHs at different pH (c). The effects of coexisting cations (d), coexisting anions (e), and adsorption temperature (f) on the Cl− removal by FeMn-EDTA-LDHs.
(a)(b)(c)(d)(e)(f)


### 3.3. Initial Solution pH

The effect of the initial solution pH on the Cl− adsorption by FeMn-EDTA-LDHs is shown in Figure 3(b). The Cl− adsorption capacity increased in the pH of 2.0∼7.0. At pH > 7, the adsorption capacity dropped sharply to 11.55 mg/g. The zero potential point (pHpzc) of FeMn-EDTA-LDHs was 7.42 (Figure 3(c)). The solution pH was below 7.42, and the H+ occupied the O-containing groups on the FeMn-EDTA-LDHs surface. The O-containing groups on the FeMn-EDTA-LDHs surface protonated and carried positive electricity, and the Cl− adsorption capacity was enhanced. When the solution pH was higher than 7.42, the O-containing groups deprotonated, and the Cl− adsorption capacity was weakened [4, 7, 18]. Thus, the electrostatic interaction between Cl− and FeMn-EDTA-LDHs played a role in the removal process.


### 3.4. Coexisting Ions

The effect of coexisting cations on Cl− adsorption was investigated (Figure 3(d)). However, the adsorption efficiency was barely affected by increasing the concentration of Na+, K+, Ca2+, Mg2+, and Fe3+ from 0 to 0.1 mol/L. This indicated that the cations had almost no effect on the Cl− removal from aqueous solutions by FeMn-EDTA-LDHs. The effect of coexisting anions on Cl− adsorption was researched (Figure 3(e)). Nitrate had no perceptible impact on the Cl− removal. Sulphate, carbonate, silicate, and phosphate had an obvious inhibitory effect on the Cl− removal [16]. The order of impact was carbonate > sulphate > silicate > phosphate.


### 3.5. Adsorption Temperature

This work examined the effect of adsorption temperature (15°C∼50°C), and the results are shown in Figure3(f). The removal efficiency decreased with increasing temperature. This result indicated that the adsorption process was exothermic and that high temperature reduced the removal efficiency. Previous literature showed that high temperatures decreased the Cl− adsorption efficiency [4].


### 3.6. Adsorption Kinetics

Figure4(a) shows that the Cl− adsorption capacity by FeMn-EDTA-LDHs increased rapidly from 21.85 mg/g to 33.69 mg/g within 10∼30 min. Then, the Cl− adsorption capacity increased slowly to 47.97 mg/g within 30∼600 min. The increase in adsorption between 10 and 30 min was attributed to the presence of abundant unused adsorption sites for FeMn-EDTA-LDHs [4, 5, 22].Table2 shows that the correlation coefficient (R2) of the pseudo-second-order kinetic model (R2 = 0.923) was higher than those of the pseudo-first-order kinetic model (R2 = 0.784) and the Elovich model (R2 = 0.903), indicating that the pseudo-second-order model can more accurately describe the Cl− adsorption process. This result suggested that the Cl− adsorption process was a chemical reaction (e.g., ion exchange or complexation) [3]. Previous studies demonstrated that O-containing groups on LDHs were capable of complexing with Cl− [16]. The data were analysed using an intraparticle diffusion model (Figure 4(a)). The fitted curve did not pass through the origin, suggesting that the adsorption process was influenced by various factors [16]. Therefore, the adsorption rate was affected by membrane diffusion and intraparticle diffusion. [14]. Additionally, the adsorption rate was influenced by the dosage, boundary layer resistance, and electrostatic interaction. Xu et al. [16] considered that membrane diffusion played a dominant role in the Cl− adsorption by LDHs. This was also consistent with the results of the present study. The Boyd model (Figure 4(b)) indicated that boundary layer/film diffusion predominantly determines the Cl− adsorption process.Figure 4
Adsorption kinetics model, (a) and Boyd kinetic model, (b) of Cl- removal by FeMn-EDTA-LDHs.
(a)(b)Table 2
The kinetic model fitting parameters.
Pseudo-first-order modelPseudo-second-order modelElovich modelIntraparticle diffusion modelBoyd kinetic modelqek1R2qek2R2αβR2k1C1R2k2C2R246.440.0290.78449.980.0010.92316.0920.1390.9031.17424.810.7450.0160.260.857Figure 5
The adsorption-desorption experiments of FeMn-EDTA-LDHs.


### 3.7. Adsorption Isotherms

Figure5 shows that the Cl− adsorption by FeMn-EDTA-LDHs first increased and reached stability. This was that LDHs had adequate active sites capable of reacting with chloride ions at low concentrations. At high concentration, the active site on FeMn-EDTA-LDHs could not remove all the Cl− from the aqueous solution.Figure 6
Isothermal adsorption model.Table3 shows that the correlation coefficients (R2) for Freundlich and Hill were less than 0.8, indicating that these two models could not accurately account Cl− adsorption process. The R2 of the other models were all above 0.9. The parameter of qmax (129.50 mg/g), Ks (55.77 mg/g), and KT (94.27 mg/g) indicated the maximum adsorption capacity. Then, Ks and KT differed from the actual adsorption capacity (132.90 mg/g). So, neither the Sips nor the Toth model could better describe the Cl− adsorption process. The Langmuir model indicated that the Cl− adsorption process was monolayer adsorption [16, 24]. The KL value from 0 to 1 indicated that the adsorption process was favourable [24]. Based on the KL values, the adsorption of FeMn-EDTA-LDHs was favourable for Cl− [7, 18]. The maximum adsorption capacity was 129.50 mg/g. Notably, the maximum adsorption capacity (129.50 mg/g) was very close to the actual adsorption capacity (132.90 mg/g), which also indicated that the Langmuir model could better account for the removal process. Additionally, the Temkin model (R2 > 0.932) described the experimental results well (Table 3). This result indicated that strong intermolecular forces existed between the Cl− and absorbent [16]. The Redlich–Peterson model indicated that the adsorption process was monolayer and multilayer adsorption [14, 16]. This suggested that the Cl− adsorption process was subject to multiple mechanisms (e.g., electrostatic interaction and complexation) [5]. Additionally, the Cl− adsorption capacity of different adsorbents was compared (Table 4). The Cl− adsorption capacity of FeMn-EDTA-LDHs was much higher than that of other adsorbents. This indicated that FeMn-EDTA-LDHs had an excellent Cl− adsorption capacity.Table 3
Fitting parameters of the adsorption isotherm.
ModelParameterActual adsorption capacity132.90Langmuirqmax129.50KL0.276R20.958FreundlichKf61.74n8.090R20.860TemkinbT46.33AT13.31R20.932SipsKs55.756βs0.615as0.402R20.997Redlich–PetersonKR56.243aR0.587β0.948R20.986TothKT94.274aT1.842n1.057R20.989HillQsH259.53nH0.124KD4.272R20.825Table 4
Comparison of the Cl− adsorption capacity of different adsorbents.
AdsorbentQmax (mg/g)ReferencesCaAl-Cl-LDHApproximately 25 mg/g[5]MgAl-LDHs intercalated with aminobenzoate ions32.16 mg/g[4]CNTs/MgAl-LDHs75.58 mg/g[3]MgAl-LDHs34.82 mg/g[14]CaMnFe-LDHs144.01 mg/g[16]MnFe-EDTA-LDHs129.50 mg/gThis work


### 3.8. Adsorption-Desorption Experiments

In this experiment, 0.1 mol/L NaOH solution was used as the desorption solution and regenerated by shaking in a shaker at 30°C for 4 h at 150 rpm. The adsorption-desorption experiments were carried out 5 times (Figure6). The Cl− removal efficiency by FeMn-EDTA-LDHs decreased slightly with an increasing number of cycles. The Cl− removal efficiency still maintained at over 90% after 5 regeneration cycles. The removal efficiency decreased was due to the reduction of functional groups and the blockage of the pore. This indicated that FeMn-EDTA-LDHs had excellent reproducible utility for Cl−.


### 3.9. Effect of FeMn-EDTA-LDHs on the Cl− Erosion Resistance of Concrete

The Cl− concentration curve at different depths after the concrete was immersed in 5% NaCl solution is shown in Figure 7. The Cl− content in the concrete gradually decreased with depth. Notably, the experimental group C0 had a higher Cl− content than others. This result indicated that FeMn-EDTA-LDHs could enhance the ability of the concrete to resist Cl− erosion. Furthermore, the proportion of FeMn-EDTA-LDHs increased, and the Cl− content increased then decreased. The Cl− content in the concrete was lowest when the incorporation amount of FeMn-EDTA-LDHs was 3%. In addition, the Cl−content of the C4 group was higher than that of the C3 group. This phenomenon was contributed to the expansion of concrete pore structure by the excess LDHs, which weakened the resistance of concrete to Cl− infiltration [16]. Previous studies were also consistent with the results of this work [15, 16].Figure 7
The distribution of Cl− concentration in concrete at 30 d (a) and 60 d (b).
(a)(b)With the increase of erosion time, the concentration of Cl−in concrete is higher. This suggested that the water-soluble Cl− gradually diffused into the concrete. The Cl− content in the concrete at 60 d was higher than that at 30 d. Therefore, the Cl− continued to erode and transport into the concrete, and the erosion deepened over time until the equilibrium dynamic [1, 6, 25]. This work showed that moderate of FeMn-EDTA-LDHs could effectively enhance the resistance of concrete to Cl− erosion.


## 3.1. Characterization Analysis

Figure1 shows that FeMn-EDTA-LDHs morphological features are observed by SEM. Before the reaction (Figure 1(a)), the surface was rough and loose, showing an irregular lamellar structure with sharp angles [18]. After Cl− adsorption (Figure 1(b)), the structure of FeMn-EDTA-LDHs was stacked, agglomerated, and collapsed. By EDS analysis, the elements of C (31.89%), O (26.10%), Mn (27.68%), and Fe (14.33%) were present in the FeMn-EDTA-LDHs before adsorption. Additionally, the ratio of Mn to Fe was exactly 2 : 1, which was consistent with the ratio used in the preparation. After adsorption (Figure 1(b)), the element of chlorine appeared in the EDS, which indicated that the FeMn-EDTA-LDHs adsorbed Cl− from the aqueous solution.Figure 1
SEM-EDS images of Cl− before (a) and after (b) adsorption by FeMn-EDTA-LDHs.
(a)(b)The N2 adsorption-desorption isotherm of FeMn-EDTA-LDHs is displayed in Figure 2(a). The adsorption-desorption isotherms were classified as type IV isotherms [19, 20]. The pore size distribution (Figure 2(b)) of FeMn-EDTA-LDHs was 2∼50 nm, which demonstrated that FeMn-EDTA-LDHs were mesoporous materials [7]. The specific surface area and pore size of FeMn-EDTA-LDHs were 115.38 m2/g and 17.24 nm, respectively.Figure 2
N2 adsorption-desorption curves (a), pore size distribution (b), FTIR (c), and XRD (d) of MnFe-EDTA-LDHs.
(a)(b)(c)(d)The function group of FeMn-EDTA-LDHs before and after adsorption is shown in Figure2(c). The strong and broad diffraction peak at 3420 cm−1 was identified as the −OH [14, 21]. FeMn-EDTA-LDHs held an absorption band near 1600 cm−1, which was a superposition of C=C/C=O functional groups in the carboxyl group [4, 21]. The band at 1411 cm−1 corresponded to the asymmetric vibration peak of −COOH, which indicated the successful intercalation of EDTA. After the reaction, the −OH diffraction peak remained at 3435 cm−1. However, the intensity and wavenumber of the −OH peaks were weakened and shifted, respectively. This phenomenon may attribute to a complex between the −OH and Cl− [3, 20]. Furthermore, the characteristic peak at 545 cm−1 was the Mn-O and Fe-O functional groups [1, 16]. After adsorption, the Fe-O and Mn-O peaks shifted lower wavenumbers, which attributed to the complexation of Fe-O and Mn-O groups with Cl− [13].The XRD analysis of the FeMn-EDTA-LDHs is shown in Figure2(d). At 2θ of 11.54°, 23.32°, 33.03°, 57.16°, and 58.66° were typical diffraction peaks of layered double hydroxides [7, 16, 22]. This peak was corresponded to the (003), (006), (009), (110), and (113) diffraction crystallographic planes, respectively [16]. The intensity of the peaks after adsorption was reduced. This may be due to the ion exchange caused by chloride ion adsorption, which changed the charge density [23]. This indicated that the Cl− entered the crystals’ interior during the adsorption process, causing changes in the lattice structure [9, 23]. The weakening of characteristic peaks was due to the collapse and plugging of the material structure after adsorption, which was consistent with the SEM result [4].


## 3.2. Dosage

The effect of the dosage on Cl− adsorption is shown in Figure 3(a). The unit adsorption capacity decreased rapidly when the dosage was increased from 0.1 g/L to 1 g/L. The dosage exceeded 1 g/L, and the unit adsorption capacity decreased slightly. With increasing dosage, the active site increased, andthe removal efficiency rose [24]. The removal efficiency increased very little when the dosage exceeded 1 g/L. This was considered to be the limited availability of Cl− in solution. Therefore, 1 g/L was supposed to be the optimum dosage for this work.Figure 3
The effects of dosage (a) and initial solution pH (b) on the Cl− removal by FeMn-EDTA-LDHs. Zeta potential of FeMn-EDTA-LDHs at different pH (c). The effects of coexisting cations (d), coexisting anions (e), and adsorption temperature (f) on the Cl− removal by FeMn-EDTA-LDHs.
(a)(b)(c)(d)(e)(f)


## 3.3. Initial Solution pH

The effect of the initial solution pH on the Cl− adsorption by FeMn-EDTA-LDHs is shown in Figure 3(b). The Cl− adsorption capacity increased in the pH of 2.0∼7.0. At pH > 7, the adsorption capacity dropped sharply to 11.55 mg/g. The zero potential point (pHpzc) of FeMn-EDTA-LDHs was 7.42 (Figure 3(c)). The solution pH was below 7.42, and the H+ occupied the O-containing groups on the FeMn-EDTA-LDHs surface. The O-containing groups on the FeMn-EDTA-LDHs surface protonated and carried positive electricity, and the Cl− adsorption capacity was enhanced. When the solution pH was higher than 7.42, the O-containing groups deprotonated, and the Cl− adsorption capacity was weakened [4, 7, 18]. Thus, the electrostatic interaction between Cl− and FeMn-EDTA-LDHs played a role in the removal process.


## 3.4. Coexisting Ions

The effect of coexisting cations on Cl− adsorption was investigated (Figure 3(d)). However, the adsorption efficiency was barely affected by increasing the concentration of Na+, K+, Ca2+, Mg2+, and Fe3+ from 0 to 0.1 mol/L. This indicated that the cations had almost no effect on the Cl− removal from aqueous solutions by FeMn-EDTA-LDHs. The effect of coexisting anions on Cl− adsorption was researched (Figure 3(e)). Nitrate had no perceptible impact on the Cl− removal. Sulphate, carbonate, silicate, and phosphate had an obvious inhibitory effect on the Cl− removal [16]. The order of impact was carbonate > sulphate > silicate > phosphate.


## 3.5. Adsorption Temperature

This work examined the effect of adsorption temperature (15°C∼50°C), and the results are shown in Figure3(f). The removal efficiency decreased with increasing temperature. This result indicated that the adsorption process was exothermic and that high temperature reduced the removal efficiency. Previous literature showed that high temperatures decreased the Cl− adsorption efficiency [4].


## 3.6. Adsorption Kinetics

Figure4(a) shows that the Cl− adsorption capacity by FeMn-EDTA-LDHs increased rapidly from 21.85 mg/g to 33.69 mg/g within 10∼30 min. Then, the Cl− adsorption capacity increased slowly to 47.97 mg/g within 30∼600 min. The increase in adsorption between 10 and 30 min was attributed to the presence of abundant unused adsorption sites for FeMn-EDTA-LDHs [4, 5, 22].Table2 shows that the correlation coefficient (R2) of the pseudo-second-order kinetic model (R2 = 0.923) was higher than those of the pseudo-first-order kinetic model (R2 = 0.784) and the Elovich model (R2 = 0.903), indicating that the pseudo-second-order model can more accurately describe the Cl− adsorption process. This result suggested that the Cl− adsorption process was a chemical reaction (e.g., ion exchange or complexation) [3]. Previous studies demonstrated that O-containing groups on LDHs were capable of complexing with Cl− [16]. The data were analysed using an intraparticle diffusion model (Figure 4(a)). The fitted curve did not pass through the origin, suggesting that the adsorption process was influenced by various factors [16]. Therefore, the adsorption rate was affected by membrane diffusion and intraparticle diffusion. [14]. Additionally, the adsorption rate was influenced by the dosage, boundary layer resistance, and electrostatic interaction. Xu et al. [16] considered that membrane diffusion played a dominant role in the Cl− adsorption by LDHs. This was also consistent with the results of the present study. The Boyd model (Figure 4(b)) indicated that boundary layer/film diffusion predominantly determines the Cl− adsorption process.Figure 4
Adsorption kinetics model, (a) and Boyd kinetic model, (b) of Cl- removal by FeMn-EDTA-LDHs.
(a)(b)Table 2
The kinetic model fitting parameters.
Pseudo-first-order modelPseudo-second-order modelElovich modelIntraparticle diffusion modelBoyd kinetic modelqek1R2qek2R2αβR2k1C1R2k2C2R246.440.0290.78449.980.0010.92316.0920.1390.9031.17424.810.7450.0160.260.857Figure 5
The adsorption-desorption experiments of FeMn-EDTA-LDHs.


## 3.7. Adsorption Isotherms

Figure5 shows that the Cl− adsorption by FeMn-EDTA-LDHs first increased and reached stability. This was that LDHs had adequate active sites capable of reacting with chloride ions at low concentrations. At high concentration, the active site on FeMn-EDTA-LDHs could not remove all the Cl− from the aqueous solution.Figure 6
Isothermal adsorption model.Table3 shows that the correlation coefficients (R2) for Freundlich and Hill were less than 0.8, indicating that these two models could not accurately account Cl− adsorption process. The R2 of the other models were all above 0.9. The parameter of qmax (129.50 mg/g), Ks (55.77 mg/g), and KT (94.27 mg/g) indicated the maximum adsorption capacity. Then, Ks and KT differed from the actual adsorption capacity (132.90 mg/g). So, neither the Sips nor the Toth model could better describe the Cl− adsorption process. The Langmuir model indicated that the Cl− adsorption process was monolayer adsorption [16, 24]. The KL value from 0 to 1 indicated that the adsorption process was favourable [24]. Based on the KL values, the adsorption of FeMn-EDTA-LDHs was favourable for Cl− [7, 18]. The maximum adsorption capacity was 129.50 mg/g. Notably, the maximum adsorption capacity (129.50 mg/g) was very close to the actual adsorption capacity (132.90 mg/g), which also indicated that the Langmuir model could better account for the removal process. Additionally, the Temkin model (R2 > 0.932) described the experimental results well (Table 3). This result indicated that strong intermolecular forces existed between the Cl− and absorbent [16]. The Redlich–Peterson model indicated that the adsorption process was monolayer and multilayer adsorption [14, 16]. This suggested that the Cl− adsorption process was subject to multiple mechanisms (e.g., electrostatic interaction and complexation) [5]. Additionally, the Cl− adsorption capacity of different adsorbents was compared (Table 4). The Cl− adsorption capacity of FeMn-EDTA-LDHs was much higher than that of other adsorbents. This indicated that FeMn-EDTA-LDHs had an excellent Cl− adsorption capacity.Table 3
Fitting parameters of the adsorption isotherm.
ModelParameterActual adsorption capacity132.90Langmuirqmax129.50KL0.276R20.958FreundlichKf61.74n8.090R20.860TemkinbT46.33AT13.31R20.932SipsKs55.756βs0.615as0.402R20.997Redlich–PetersonKR56.243aR0.587β0.948R20.986TothKT94.274aT1.842n1.057R20.989HillQsH259.53nH0.124KD4.272R20.825Table 4
Comparison of the Cl− adsorption capacity of different adsorbents.
AdsorbentQmax (mg/g)ReferencesCaAl-Cl-LDHApproximately 25 mg/g[5]MgAl-LDHs intercalated with aminobenzoate ions32.16 mg/g[4]CNTs/MgAl-LDHs75.58 mg/g[3]MgAl-LDHs34.82 mg/g[14]CaMnFe-LDHs144.01 mg/g[16]MnFe-EDTA-LDHs129.50 mg/gThis work


## 3.8. Adsorption-Desorption Experiments

In this experiment, 0.1 mol/L NaOH solution was used as the desorption solution and regenerated by shaking in a shaker at 30°C for 4 h at 150 rpm. The adsorption-desorption experiments were carried out 5 times (Figure6). The Cl− removal efficiency by FeMn-EDTA-LDHs decreased slightly with an increasing number of cycles. The Cl− removal efficiency still maintained at over 90% after 5 regeneration cycles. The removal efficiency decreased was due to the reduction of functional groups and the blockage of the pore. This indicated that FeMn-EDTA-LDHs had excellent reproducible utility for Cl−.


## 3.9. Effect of FeMn-EDTA-LDHs on the Cl− Erosion Resistance of Concrete

The Cl− concentration curve at different depths after the concrete was immersed in 5% NaCl solution is shown in Figure 7. The Cl− content in the concrete gradually decreased with depth. Notably, the experimental group C0 had a higher Cl− content than others. This result indicated that FeMn-EDTA-LDHs could enhance the ability of the concrete to resist Cl− erosion. Furthermore, the proportion of FeMn-EDTA-LDHs increased, and the Cl− content increased then decreased. The Cl− content in the concrete was lowest when the incorporation amount of FeMn-EDTA-LDHs was 3%. In addition, the Cl−content of the C4 group was higher than that of the C3 group. This phenomenon was contributed to the expansion of concrete pore structure by the excess LDHs, which weakened the resistance of concrete to Cl− infiltration [16]. Previous studies were also consistent with the results of this work [15, 16].Figure 7
The distribution of Cl− concentration in concrete at 30 d (a) and 60 d (b).
(a)(b)With the increase of erosion time, the concentration of Cl−in concrete is higher. This suggested that the water-soluble Cl− gradually diffused into the concrete. The Cl− content in the concrete at 60 d was higher than that at 30 d. Therefore, the Cl− continued to erode and transport into the concrete, and the erosion deepened over time until the equilibrium dynamic [1, 6, 25]. This work showed that moderate of FeMn-EDTA-LDHs could effectively enhance the resistance of concrete to Cl− erosion.


## 4. Conclusions

FeMn-EDTA-LDHs were prepared by coprecipitation. The pseudo-second-order and the Redlich–Peterson models provided a better fit for the experimental data. The Cl− adsorption capacity on MnFe-EDTA-LDHs reached 129.50 mg/g. At dosage 1 g/L and initial pH 7.0, FeMn-EDTA-LDHs held better Cl− removal efficiency. The cations barely affected the Cl− removal from aqueous solutions by FeMn-EDTA-LDHs. Sulphate, carbonate, silicate, and phosphate obviously inhibited the Cl− removal. The removal mechanism is involved in the complexation of O-containing groups, ion exchange, and electrostatic interaction. Moderate of MnFe-EDTA-LDHs improved the resistance of concrete to Cl− erosion, while excessive additions led to a reduction in the Cl− erosion resistance. This work indicated that FeMn-EDTA-LDHs retarded the Cl− erosion on concrete. The mechanism by which LDHs improved the resistance of cement samples to Cl− erosion will be explained by more means in next works.



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*Source: 5375810-2022-12-09.xml*