Removal of sulfate from aqueous solution using Mg–Al nano-layered double hydroxides synthesized under different dual solvent systems

Because of its priority to remove anions, nanolayered double hydroxide (LDH) was incorporated to improve the sulfate attack corrosion resistance of cementbased materials. Herein, the synthesis of high-efficiency LDH for removal of SO4 is necessary. In this study, LDH with different Mg/Al ratios was synthesized under different dual solvent systems (water and ethylene glycol/ ethanol/tetrapropylammonium hydroxide). Based on the adsorption experimental results, the LDH synthesized with n(Mg:Al) = 2:1 under water and ethanol solvent systems (ET2.0) exhibits the best adsorption capacity. The d(003) of LDH synthesized with n(Mg:Al) = 2:1 under different dual solvent systems are 0.7844, 0.7830, and 0.7946 nm, respectively. Three LDH belong to LDH-NO3 −. The results indicated that their surface charges show obvious difference synthesized under different dual solvent systems, which leads to differences in adsorption performance. The adsorption experimental results show that ET2.0 followed pseudo second-order kinetics and Langmuir model. The ET2.0 removed SO4 through anion substitution and electrostatic interaction and exhibited excellent adsorption rate with the maximum adsorption capacity of 95.639mg/g. The effects of pore solution anion (OH, Cl, and CO3) on the removal of SO4 by the ET2.0 are limited.


Introduction
During the service period of concrete, temperature, load, electric field, ion erosion, etc. all have adverse effects on the durability of concrete, which greatly shorten the service period of concrete and cause huge safety problem [1]. In recent years, with the development of the marine construction industry, marine concrete has been widely used [2]. However, marine concrete is facing serious ion erosion problems, which has attracted widespread attention by researchers [3]. Among the erosion anions, the SO 4 2− does the greatest damage to concrete [4]. Incorporating mineral admixtures such as slag, fly ash, and silica fume can effectively improve the durability of cementbased materials [5]. The finer particle mineral admixtures have a certain filling effect, thereby improving the pore structure of cement-based materials [6]. Moreover, these active mineral admixtures can react with the hydration products and hinder the interaction reaction with sulfate [7]. However, the dosage of mineral admixtures is relatively large and cannot effectively solve the free SO 4 2− in the pore solution, which will pose a huge threat to cement-based materials. When sulfate is precipitated in the form of sodium sulfate decahydrate, it undergoes three-fold volume expansion [8]. Therefore, reducing the content of SO 4 2− in the pore solution is a meaningful study.
Nano , where M 2+ and M 3+ are divalent and trivalent metal cations, respectively; A n− is the interlayer anion [9]. [M(OH) 6 ] (M 2+ or M 3+ ) octahedrons with common edges make up the positively charged layer [10]. The interlayer anions of LDH can be replaced by other anions, so LDH has been widely used to remove anions such as nitrate, phosphate, and sulfate [11]. Because of its priority to remove anions, the researchers have focused on LDH and studied the effect of LDH on the sulfate attack corrosion resistance of cement-based materials [12]. On one hand, the sulfate is solidified by adsorption, which prevents the reaction between LDH and cement hydration products [13]. On the other hand, LDH as a nano-layer material also played a role in filling and improved the pore structure of cement-based materials [14]. The existing research has carried out many studies; however, there are still some shortcomings [15]. To be specific, the synthesis of high-efficiency LDH for removal of SO 4 2− is rarely considered and the effect of anions that may exist in the pore solution of cementbased materials on the adsorption of SO 4 2− by the LDH was rarely explored. Both factors affect the ability of LDH to adsorb sulfate, which is closely related to the sulfate corrosion resistance of cement-based materials mixed with LDH.
LDH has received wide consideration in wastewater treatment because of its priority to remove anions and low cost [16]. To improve the adsorption capacity of LDH for anions, several preparation methods have been explored as follows. Puzyrnaya et al. explored the effect of different ratios of Mg 2+ to Fe 3+ on the removal of phosphate by LDH [17]; Islam and Patel studied the adsorption kinetics and thermodynamics of Zn-Al LDH to remove nitrate [18]; and Halajnia et al. investigated the adsorption characteristics of nitrate on Mg-Fe and Mg-Al LDH [19]. The results of these studies indicate that different cation types and the ratio of M 2+ to M 3+ all affect the charge of LDH, which is closely related to the anion adsorption capacity of LDH [20]. LDH removes anions through not only electrostatic interaction but also anion substitution. According to previous research results, the order of stability of interlayer inorganic anions is CO 3 2 . For example, LDH-NO 3 − can effectively remove CO 3 2− ; however, LDH-CO 3 2− is difficult to remove NO 3 − . Therefore, based on the type of anion actually adsorbed, the type of interlayer anion of LDH synthesized must be designed [22]. In addition to anions, organic matter can be intercalated with LDH to further improve its adsorption capacity. The research results of Wei et al. show that compared with LDH-CO 3 2− , LDH intercalated with aminobenzoate can more effectively remove Cl − [23].
The morphology of LDH can also be adjusted, and LDH with a suitable morphology can expose more active sites, which is closely related to the adsorption capacity of LDH. For example, Yang et al. synthesized 2D nanosheet LDH by co-precipitation method, and CaFeAl LDH shows excellent adsorption capacity with the maximum chloride adsorption capacity 3.18 mmol/g [24]; Ji et al. prepared mesoporous MgAl LDH with the maximum bromate adsorption capacity 59.34 mg/g [25]; Hu et al. synthesized hierarchical CuAl LDH, which also shows excellent phosphate adsorption capacity [26]. Furthermore, because of the excellent properties of carbon materials, various composites containing carbon materials have been prepared [27,28]. Biochar-MgAl LDH nanocomposites synthesized by the co-precipitation method also show excellent adsorption capacity, and the maximum adsorption capacities of PO 4 3− and NO 3 − are 177.97 and 28.06 mg/g, respectively [29].
As mentioned above, compared with pristine LDH, organic cross-linked LDH and LDH composites have more active sites [30]. These active sites mainly provided by organic matter or other adsorption materials are fixed and difficult to be improved, so improving the pristine LDH adsorption capacity is important. The influence of different cation types and interlayer anions on the adsorption capacity has been extensively studied [31]. However, the maximum adsorption capacity of LDH is still limited for SO 4 2− , which is important for sulfate corrosion resistance of cement-based materials [32]. Herein, the synthesis of high-efficiency LDH for removal of SO 4 2− is necessary. In addition, the effect of anions that may exist in the pore solution of cement-based materials on the adsorption of SO 4 2− by the LDH is rarely considered and worthy of further exploration [33].
LDH removes anions through electrostatic interaction and anion substitution [34]. Therefore, in this study, LDH-NO 3 − was synthesized to remove sulfate based on the order of stability of interlayer inorganic anions. LDH with different Mg/Al ratios were synthesized under different dual solvent systems to adjust LDH surface charge and improve its adsorption efficiency for SO 4 2− . Based on adsorption test results, the best synthesis method is determined. X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR), and zeta potential were used to analyze the structure and surface charge of LDH synthesized under different dual solvent systems. Furthermore, the adsorption kinetics, isothermal adsorption, and adsorption mechanism of LDH with the best sulfate adsorption capacity were studied by adsorption experimental results, XRD and FT-IR. The effects of pH, Cl − , and CO 3 2− on the adsorption of SO 4 2− were also explored.

Synthesis process of LDH
The LDH was synthesized by the urea hydrolysis method. All chemical reagents used in this study were in accordance with the analytical grade, and the deionized water was used throughout the research. The molar ratio of n (CO(NH 2 ) 2 ):n(Mg 2+ + Al 3+ ) is set to be 10:1. The molar ratio of Mg:Al was 3.5:1, 3:1, 2.5:1, 2:1, and 1.5:1, respectively. To be more specific, first, 21 g CO(NH 2 ) 2 , 50 mL water, and 50 mL ethanol (ET) were mixed together and corresponding mass of Mg(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 ·9H 2 O was dissolved in 100 mL deionized water. Second, two solutions were mixed and stirred at 50°C for 20 min. After stirring, the mixed solution was heated at 150°C for 12 h. Then, the precipitate was separated and washed with deionized water. Finally, the LDH-ET was dried at 100°C for 6 h. LDH-EG and LDH-TPAOH were synthesized by replacing 50 mL ethanol with 50 mL ethylene glycol (EG) or 5.3 g tetrapropylammonium hydroxide, respectively. In the dual solvent systems containing ET, the LDH with the n(Mg:Al) ratio of 2:1 was named ET2.0. The naming method for other LDHs is the same.

Batch adsorption experiments
To determine the optimal synthesis method, the removal of SO 4 2− by LDH with various n(Mg:Al) ratios under different dual solvent systems was studied by the batch adsorption experiments. Na 2 SO 4 was used to prepare SO 4 2− solution in this research. The acquiescent parameters of batch adsorption experiments consist of volume (50 mL), temperature (298 K), adsorbent mass (4 g/L), initial concentration (5 mmol/L), and rotational speed (150 rpm). The sulfate concentration was tested by ion chromatography. The sulfate removal rate is determined by the following formula: where C 0 and C e represent the initial and equilibrium concentration of SO 4 2− , respectively.
According to the batch adsorption experiment results, the LDH with the optimal adsorption capacity was systematically tested in subsequent adsorption experiments. First, different concentrations of Na 2 SO 4 solution and LDH were mixed to investigate the adsorption isotherm of LDH on SO 4 2− . Then, at regular intervals (15, 30, and 60 min, etc.), the concentrations of Na 2 SO 4 solution were tested to study adsorption kinetics of LDH on SO 4 2− . Finally, the influence of the presence of OH − (pH 2-13), CO 3 2− (0-30 mmol/L), or Cl − (0-30 mmol/L) on the adsorption capacity of LDH was explored.

Characterization
Bruker D8 Advance XRD was used to characterize the crystal structure of LDH. The tube pressure is 40 kV, the tube flow is 40 mA, the scanning range is 10-80°, and the step length is 0.02°. FTIR-650 was applied to record the spectra within 4,000-400 cm −1 of LDH using the potassium bromide tablet method. The zeta potential of LDH was determined by Zeta Potential Analyzer. The FT-IR spectrum of the EG2.0, ET2.0, and TPAOH2.0 samples are presented in Figure 3. The absorption peak at around 3,465 cm −1 is assigned to the stretching vibrations of -OH groups [35,36]. The band at around 1,639 cm −1 is related to the angular deformation of water molecules [37]. The absorption peak at around 476 cm −1 corresponds to the M-O band, and the M represents the existence of Mg or Al [38]. The absorption peak at around 1,380 cm −1 is associated with the NO 3 − of NO 3 -LDH [39]. Therefore, it can be concluded that the three LDHs prepared are all NO 3 -LDH. Moreover, there is little difference in crystal structures and functional groups of three LDHs from the results of FT-IR analysis, which is consistent with the XRD analysis results as above.
The positive and negative zeta potential corresponds to the positive and negative surface charge of LDH, which is closely related to the adsorption capacity of LDH to        However, their surface charges and system stability have certain differences, indicating that the zeta potential of LDH is positively correlated with the adsorption properties. Among EG2.0, ET2.0, and TPAOH2.0, ET2.0 has the best adsorption performance. Therefore, we have launched a systematic research on ET2.0.

Sulfate adsorption kinetics and isotherms
As shown in Figure 5, the adsorption kinetic experimental results show that SO 4 2− adsorption capacity by ET2.0 increased rapidly with time. Within 1 h, the adsorption capacity increased rapidly and reached 93.650 mg/g, which is very close to the maximum adsorption capacity. Kinetic simulation of the adsorption process is used to characterize the variation of adsorption capacity with time under certain temperature conditions. Commonly used adsorption kinetic models are pseudo first-order kinetics and pseudo second-order kinetics. Two equations were expressed as follows: where k 1 (min −1 ) and k 2 [g/(mg min)] are the adsorption rate constant, respectively; q e and q t (mg/g) represent the adsorption capacity at equilibrium and time t, respectively. The correlation coefficient (R 2 ) of pseudo first-order kinetics and pseudo second-order kinetics is 0.8920 and 0.9936, respectively. It can be seen from Figure 5 that pseudo second-order kinetics equation can better fit the experimental data. The adsorption capacity (q e = 95.639 mg/g) calculated by pseudo second-order kinetics equation is close to the experimental results. This could be explained that the adsorption rate is controlled by the chemisorption mechanism, which involves chemical reactions, electron gain and loss, or electron sharing [41,42]. This indicates that LDH removes anions mainly through anion substitution.
When the adsorption reaches equilibrium at a constant temperature, the concentration of adsorbate on the surface of the adsorbent and in the solution is distributed according to a certain rule. There is a certain functional relationship, and this relationship is called the adsorption isotherm. Two common adsorption models Langmuir and Freundlich are as follows: where q e (mg/g) is the adsorption capacity at equilibrium, q m (mg/g) is the saturation absorptive capacity, C e (mg/L) is the concentration of adsorbate at equilibrium, K L (L/mg) is the Langmuir adsorption constant, and K F and n represent Freundlich adsorption coefficient and adsorption intensity characteristic constant, respectively. The fitting results of the two models are shown in Figure 6. The correlation coefficient (R 2 ) of Langmuir and Freundlich model is 0.9992 and 0.9778, respectively. Therefore, the Langmuir model is more suitable to describe the adsorption process of ET2.0. The basic assumption of the Langmuir adsorption isotherm model is a monolayer adsorption [43]. The number of adsorption sites is fixed, the energy of each adsorption site is the same, each adsorption site adsorbs only one molecule, and there is no interaction between them [44][45][46].

Effect of the solution pH
To explore adsorption mechanism and the possibility of ET2.0 applied to cement-based materials, the effect of pH on the adsorption of SO 4 2− by the ET2.0 was investigated. As shown in Figure 7, with the decrease in pH value, the adsorption capacity of ET2.0 to sulfate and zeta potential increases, which indicates ET2.0 can remove SO 4 2− by electrostatic interaction. However, when the pH value changed from 4 to 2, the adsorption capacity of ET2.0 suddenly dropped from 100.899 to 73.955 mg/g. This may be because of the crystal structure of ET2.0 changed when the pH was too low. When the pH is 12, the maximum adsorption capacity of ET2.0 to sulfate is 84.08 mg/g. It can be concluded that ET2.0 still has a good ability to remove sulfate ions under alkaline conditions and shows great potential to be applied to cement-based materials.

Adsorption mechanism
XRD and FT-IR of ET2.0 and ET2.0-SO 4 2− (before and after the adsorption of SO 4 2− ) were tested to figure out the adsorption mechanism of ET2.0. As shown in Figure 8a, the crystal interplanar spacing of (003), (006), (012), (019), etc. of ET2.0 was not changed after removing SO 4 2− , indicating that the crystal structure of ET2.0 was not changed. However, there are some differences 2− through electrostatic interaction. Combined with the above analysis, the schematic diagram of the adsorption mechanism is shown in Figure 9, it has clearly demonstrated that ET2.0 removes SO 4 2− through anion substitution and electrostatic interaction.  Figure 10a, and it can be seen that the effect of Cl − is limited. Even when the concentration of Cl − is six times that of SO 4 2− , the adsorption capacity of ET2.0 to sulfate is still above 90 mg/g. However, the CO 3 2− has an obvious effect on the adsorption of SO 4 2− by the ET2.0. As shown in Figure 10b, with the increase in concentration of CO 3 2− , the adsorption capacity of ET2.0 to SO 4 2− decreases significantly. When the CO 3 2− Figure 9: The schematical mechanism of the adsorption effect.  concentration is less than 2.5 mmol/L. Therefore, the effect of CO 3 2− on the adsorption of SO 4 2− by the ET2.0 is limited in pore solution. The above conclusions indicate that ET2.0 can be used to improve the sulfate corrosion resistance of cement-based materials.

Conclusion
In this study, high-efficiency LDH was synthesis by urea hydrolysis method under different dual solvent systems. According to the experimental results, the following conclusions can be drawn: indicating that ET2.0 can be used to improve the sulfate corrosion resistance of cement-based materials. Related research will be carried out in the future.
Funding information: This research was funded by the "National Natural Science Foundation of China" No. 51872064. This research was also supported by "State Key Laboratory of Solid Waste Reuse for Building Materials" SWR-2020-005.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.