Adsorption of nitrate and nitrite from aqueous solution by magnetic Mg/Fe hydrotalcite

In this study, magnetic Mg/Fe hydrotalcite calcined material (M-CHT) was synthesized through co-precipitation and calcination method, and was used to effectively remove nitrate and nitrite from water. M-CHT can restore its original layered structure after the adsorption of nitrate or nitrite, and can be easily separated by the applied magnetic ﬁ eld. The ﬁ rst-order and pseudo-second-order kinetic models (R 2 (cid:1) 0.97) can better describe the adsorption kinetic process. The equilibrium isotherm showed that the Langmuir model provided a better ﬁ t to the experimental data than the Freundlich model for nitrates and nitrites. With temperature increased from 298 to 308 K, the maximum adsorption capacity obtained by the Langmuir model increased from 10.60 to 16.90 mg-N/g for nitrate and 7.89 to 14.28 mg-N/g for nitrite, respectively. The adverse effect of coexisting anions ranked in the order of ClO 4 (cid:3) . Cl (cid:3) . SO 42 (cid:3) . F (cid:3) . CO 32 (cid:3) . PO 43 (cid:3) . The actual Fe 2 þ /Fe 3 þ value of M-CHT (0.56) is nearly consistent with the theoretical value of 0.5, and the saturation magnetic strength value of M-CHT is 9.15 emu/g, greatly contributing to the solid-liquid separation. Overall, M-CHT with features of magnetic properties and satisfactory adsorption capacity exhibits the greatly promising for application in wastewater puri ﬁ cation. Magnetic hydrotalcite was synthesized by co-precipitation method. Magnetic Mg/Fe hydrotalcite subjected to calcination at 500 °C (M-CHT) recovered its original double layer after the adsorption of nitrate and nitrite. The adsorption capacity was 16.90 mg-N/g for nitrate and 14.28 mg-N/g for nitrite at 35 °C.

reduction to nitrites result in the adverse effect on the environment, and could potentially cause human health problems such as blue baby syndrome in infants and stomach cancer in adults (Majumdar & Gupta 2000;Kim-Shapiro et al. 2005;Boehm 2019). Excess of nitrate in drinking water can also lead to various types of cancer in humans (Aliaskari & Schfer 2020).
Several methods for removal of nitrate or nitrite from water have been applied, such as the reverse osmosis (Berkani et al. 2019), catalytic reduction (Wcea et al.;Han et al. 2019), biological denitrification (Mulholland et al. 2008;Liu et al. 2021a), ion exchange and electrodialysis (Wiercik et al. 2020;Zeng et al. 2020). However, biological processes are easily affected by temperature, and the effluent needs further treatment, such as disinfection (Chen et al. 2009;Zeng et al. 2020). The reverse osmosis and electrodialysis are relatively expensive, and merely displace nitrate or nitrite into the concentrated waste brine, causing the disposal problems (Samatya et al. 2006). Ion exchange needs high energy or a high or expensive dose of the reagent (Zeng et al. 2020). In recent decades, the adsorption methods have received great attention in removal of nitrate or nitrite from water, due to its simplicity, sludge free operation, easiness in handing and availability of various adsorbents (Song et al. 2016;Nassar et al. 2020). So far, the clay materials such as hydrotalcites with advantages of low cost, abundant source and easy preparation, have gained great concern from researchers. Hydrotalcite comprises up and down parallel layers and a large internal space for exchange of anions from water (Jung et al. 2020). Carbonate ions and crystal water are present in the interlayer (Saifullah & Hussein 2015). Upper and lower surfaces typically include metal oxide and metal hydroxide (Xia et al. 2020). The layer structure exhibits positive charge, and the internal anion exhibits negative charge, eventually rendering the electrical neutrality of hydrotalcite . Hydrotalcite possesses a unique microporous structure, tunable denaturation, memory effect of calcination, interlayer anion-exchange ability, and a high degree of order (Ogata et al. 2018;Cheng et al. 2021). In a study reported previously by our group, the calcined Mg/Al hydrotalcite possessed high adsorption capacity for nitrate (34.36 mg N/g) and nitrite (37.17 mg N/g) (Wan et al. 2012). However, issues related to the effective separation and recovery of hydrotalcite from solution still need to be resolved. The preparation of magnetic hydrotalcite is developed to resolve the above-mentioned problem, and it has been applied to adsorb toxic anions or compounds such as methyl orange (Lin et al. 2016), phosphate (Sun et al. 2013) and arsenic (Toledo et al. 2010). However, for all we know, the researches on adsorption properties of magnetic hydrotalcite for nitrate and nitrite are still limited.
Considering that Al can damage human body and has negative effect on health, in this study, magnetic Mg/Fe hydrotalcite calcined material (M-CHT) was synthesized through the co-precipitation and calcination method, and then was used to remove nitrate and nitrite from water. The adsorption properties (including kinetics and isotherm) for nitrate and nitrite over M-CHT under batch conditions were investigated. Moreover, the adsorption mechanism was also analyzed based on the characterization (XRD, XPS, FTIR and VSM).

Materials
FeCl 3 ·6H 2 O, MgCl 2 ·6H 2 O, FeCl 2 ·4H 2 O, NaNO 2 , NaNO 3 , NaOH and Na 2 CO 3 were all of analytical grade and purchased from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). The solution used in all experiments were prepared using ultrapure water of 18.25 MΩ. The 20% ammonia used was in the form of ammonium solution.

Synthesis of M-HT and M-CHT
M-HT was synthesized by the co-precipitation method. First, a magnetic matrix solution was prepared by dissolution of FeCl 2 ·4H 2 O (0.24 mol/L Fe 2þ ) and FeCl 3 ·6H 2 O (0.48 mol/L Fe 3þ ) in 100 mL deionized (DI) water. Under the conditions of the controlled temperature of 45 + 1°C and vigorously stirring, 20% ammonia solution was added dropwise into the above solution to adjust the pH at 11 + 1. The resulting precipitate was aged at 45 + 1°C for 30 min. The as-obtained oily black precipitate was centrifuged and washed with the deionized water for several times until the solution pH was neutral. The obtained substance was stored in a 500-mL conical flask containing 100 mL of deionized water for further use.
Next, MgCl 2 ·6H 2 O (1.2 mol/L) and FeCl 3 ·6H 2 O (0.4 mol/L) were dissolved in 200 mL of deionized water (solution A). Then, solution B containing a mixture of 25.60 g of NaOH (3.2 mol/L) and 4.24 g of Na 2 CO 3 (0.2 mol/L) was prepared. The two solutions (A and B) were simultaneously added dropwise into 100 mL of the as-prepared magnetic matrix water under vigorously stirring. The temperature and pH were maintained constant at 40 + 1°C and 10 + 1, respectively. The resulting slurry was stirred for 2 h and added into a thermostatic water bath at 65 + 1°C for ∼18 h. The resulting product was centrifuged and washed with the deionized water for several times until the electrical conductivity of the supernatant was less than 300 μs/cm. Then, the product was dried at 70°C and sieved with 100 mesh to obtain the powder, which was marked as M-HT. M-HT was subjected to calcination at 500°C for 5 h, and sieved with 100 mesh to obtain the final product, which was marked as M-CHT.

Adsorption kinetics study
The adsorption capacity studies for nitrate or nitrite by M-CHT were conducted in a 500 mL flask in the batch mode. The initial nitrate or nitrite concentration maintained constant at 20 mg N/L. The effect of different temperatures (298, 303 and 308 K) on the adsorption of nitrate or nitrite by M-CHT was investigated. The solution volume and adsorbent dosage were 500 mL and 2 g/L, respectively. The adsorption capacity of M-CHT was calculated by the following equation: (1) In this equation, q t is the adsorbent capacity of the adsorbent at time t, C 0 and C t (mg N/L) are the initial concentration of nitrate or nitrite and that at time t, respectively, and m is the mass of adsorbent (g).

Adsorption equilibrium study
Adsorption equilibrium studies were carried out by utilizing a constant mass (0.10 g) of M-CHT with 100 mL of the nitrate or nitrite solution. Nitrate or nitrite concentrations were 5, 10, 15, 20, 30, 45, and 60 mg N/L. M-CHT with the nitrate or nitrite solution was placed in a temperature-controlled orbital shaker with the stirring speed of 150 rpm. The pH of mixture was not adjusted for avoiding the effect of other anions. After shaking the flasks for 24 h, the solution sample was filtered by 0.45-μm membrane. The adsorption capacity of M-CHT toward nitrate or nitrite at equilibrium was denoted as q e (mg N/g).

Analysis methods
Nitrate and nitrite concentrations were measured by Hitachi U-3010 spectrophotometer. XRD data were recorded in a 2θ range of 5°to 80°on a D8 Advance diffractometer using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was measured on a Thermo Scientific Escalab 250SXi. Transmission electron microscopy (TEM) was measured on a Tecnai G2F30 microscope. Fourier transform infrared (FTIR) spectra were measured through a WQF-510 spectrometer.

Kinetics study
As seen in Figure 1, in the first 500 min, M-CHT rapidly adsorbed nitrate and nitrite; then, the adsorption rate became sluggish, and adsorption saturation time was 750 min. A majority of active adsorption sites were available for nitrite or nitrate in the first 500 min, while after that, the active adsorption sites on M-CHT were gradually saturated. With the contact time increasing, the amount of adsorbed nitrate and nitrite increased, and at308 K, almost 80% of the nitrate and nitrite was removed in 750 min. With the temperature increasing from 298 to 308 K, the adsorption rates of nitrate and nitrite increased. Three common kinetics models including First-order, pseudo-second-order and intraparticle diffusion kinetics models were used to fit the experimental data, which are expressed by the following equations (Allen et al. 1989;Ho & Mckay 1998;Ogata et al. 2018): k 3 (mg/g min 0.5 ) are the rate constants of the pseudo-second-order and intraparticle diffusion kinetics models, respectively. Table 1 shows the adsorption kinetics results which were fitted by the first-order, pseudo-second-order, and intraparticle diffusion models. According to the values of the correlation coefficient with nitrate and nitrite, higher R 2 values (!0.97) were fitted by the first-order and pseudo-second-order models. Specifically, the firstorder model is well fitted, indicating that nitrate and nitrite adsorption rates are controlled by diffusion. In addition, the adsorption data were fitted with the pseudo-second-order model, and adsorption was concluded to be chemical adsorption (Hu et al. 2016). In addition, the initial adsorption rate can be calculated by v 0 ¼ k 2 Â q e 2 from pseudo-second-order model. The adsorption rate v 0 ( Â 10 À4 ) of nitrate increased from

Equilibrium study
Adsorption isotherm is an important manner that describes maximum adsorption capacity of M-CHT for nitrate or nitrite. To discuss the effect of different temperatures, the adsorption equilibrium of nitrate and nitrite on M-CHT was investigated at 298, 303 and 308 K. As showed in Figure 2, the equilibrium concentration of nitrate or nitrite increased, and the equilibrium adsorption capacity was also increased. With the temperature increasing from 298 to 308 K, adsorption equilibrium was achieved more rapidly. The adsorption capacities of nitrate and nitrite were high at high temperatures.   Equilibrium data was fitted by two isotherm models: Langmuir and Freundlich models. These isotherm models were expressed by the following Equations (5) and (6), respectively (Langmuir 1916;Freundlich et al. 1998).
where C e is the nitrate or nitrite concentration of the solution at equilibrium (mg/L), Q 0 is the monolayer capacity of adsorbent (mg/g), K L is the Langmuir constant (L/mg), and K F (mg/g) (L/mg) and n are the Freundlich temperature-dependent constants. Table 2 summarizes the adsorption isotherm parameters for nitrate or nitrite. The Langmuir isotherms model afforded the better fitting results with R 2 values (!0.97), which was greater than those obtained by the Freundlich model, indicating that nitrate and nitrite are uniform on the adsorbent surface and adsorption may be the monolayer adsorption (Wan et al. 2012;Rodrigues et al. 2019). Meanwhile, at 298, 303 and 308 K, the maximum adsorption capacities of M-CHT for nitrate were 7.89, 13.96, and 19.89 mg N/g, respectively. The corresponding values for nitrite were 10.60, 12.40 and 16.90 mg N/g. The adsorption capacities of nitrate and nitrite at different temperatures increased, ranking in the order of 298 K , 303 K , 308 K. High temperature was favorable for adsorption; this tendency was in agreement with that reported by Rodrigues (Rodrigues et al. 2019). At 308 K, M-CHT exhibited highest adsorption capacity, and Langmuir parameters for nitrate adsorption were Q 0 ¼ 14.28 mg N/g, and K L ¼ 1.39 L/mg; the corresponding values for nitrite were Q 0 ¼ 16.90 mg N/g, and K L ¼ 0.59 L/mg.
The thermodynamic parameters of adsorption process include the standard free energy change (ΔG°, kJ/mol), the standard enthalpy change (ΔH°, kJ/mol) and the standard entropy change (ΔS°, kJ/mol), respectively, which can be calculated by the following equations (Aksu 2002;Debnath & Ghosh 2008): where R (8.314 J/mol K) is the ideal gas constant, T (K) is the temperature, C ad, e is the concentration of nitrate or nitrite on M-CHT at equilibrium (mg-N/L) and K c ' is the apparent equilibrium constant. The value of K c ' in the lowest experimental nitrate or nitrite concentration can be obtained (Chudoba 2020). It is known that adsorption reaction is a spontaneous process when ΔG°are negative values (Shahwan 2021). Herein, the thermodynamic parameters for the adsorption of nitrate and nitrite by M-CHT are given in Table 3 Figure 3 and Table 4 present the Arrhenius equations, considering the satisfactory correlation coefficient values of 0.9934 and 0.9298. The adsorption processes have an adsorption activation energy value of 35.92 kJ/mol for nitrate and that of 67.12 kJ/mol for nitrite onto M-CHT. When the Ea value is lower than 40 kJ/mol, the adsorption type can be considered as a physical adsorption process; When the Ea value is greater than 40 kJ/mol, it suggests chemical adsorption (Bagheri et al. 2015). Herein, the Ea value of nitrite adsorption was greater than 40 kJ/mol, indicating the feasibility of the adsorption process being predominantly chemical in nature. The Ea value of nitrate adsorption was lower than 40 kJ/mol, which might be the physical adsorption.

Effects of initial pH
Hydrotalcite-like material is an alkali compound, and the solution pH profoundly affects its adsorption performance. Figure 4 shows the results of adsorption capacities at equilibrium (q e ) under different initial pH values. M-

Uncorrected Proof
CHT exhibited a high nitrate adsorption capacity at initial pH range of 3.36-8.45, as well as high nitrite adsorption capacity at initial pH range of 3.57-9.4, indicating that M-CHT has high adsorption capacity toward nitrate or nitrite in a wide range of solution pH. The maximum nitrate and nitrite adsorption capacities were 9.91 mg N/g at pH 6.33 and 14.24 mg N/g at pH 6.38, respectively. When initial pH is .4.0, the final pH after adsorption exceeded 10.5, suggesting that M-CHT is a strongly alkaline material. After adsorption, pH increased possibly due to the release of OH À from hydrotalcite. At the same time, at initial pH values of 4-10, these trends were not significant, indicating that hydrotalcite exhibits a certain buffering effect on the change of the solution pH; hence, within a certain range of pH, the effect of pH on adsorption capacity of nitrate and nitrite by CHT is not extremely significant, and the adoption range is wide (Ahmed et al. 2020). With the decrease in the solution pH to 2.50, the nitrate and nitrite adsorption capacities decreased to 6.41 mg N/g and 10.12 mg N/g, respectively, with the corresponding decrease in the final solution pH to 10.09 and 10.04. Thus, a strong acidic environment reduces stability of the laminate structure of materials (Ferreira et al. 2006), thus decreasing adsorption capacity for anions. With the increase in the pH, the competitive adsorption of nitrate or nitrite by a high number of OH À in the solution increased, leading to the decreased adsorption of the nitrate or nitrite.

Effects of coexisting anions
Typically, the anions such as F À , Cl À , ClO 4 À , SO 4 2À , CO 3 2À , and PO 4 3À are present in nitrate-and nitrite-contaminated water, which can compete with nitrate or nitrite for adsorption sites on materials (Gierak & Lazarska 2017). As showed in Figure 5, in the control group (no coexisting ions), the removal efficiencies of nitrate and nitrite by M-CHT were 46.34% and 68.56%, respectively. From the general trend observed in the figure, the adsorption capacity of nitrate or nitrite significantly decreased in the presence of coexisting anions. The order of influence is PO 4 3À . CO 3 2À . F À . SO 4 2À . Cl À . ClO 4 À . The adsorption ability of M-CHT for nitrite or nitrate from the solution was mainly dependent on the electrical affinity of its positive surface. In the presence of PO 4 3À in the solution, the adsorption capacity was significantly decreased. After the calcination of M-CHT, the interlayer water or interlayer anions were lost, and the material surface exhibited a positive charge. The higher the coexisting anion valence, the poorer the adsorption of nitrite or nitrate by M-CHT (Li et al. 2016). Thus, PO 4 3À is the most competitive anion. In addition, the anion radius affected the adsorption capacity. Compared to Cl À , the anion F À with smaller radius has more adverse effect on adsorption of nitrate or nitrate.

XRD analysis
XRD was employed to investigate the sample structure. Figure 6 shows the XRD patterns of M-HT, M-CHT, and M-CHT-A. M-CHT-A-NO 3 À and M-CHT-A-NO 2 À represent the calcined hydrotalcite with adsorbed nitrate and nitrite, respectively. As showed in Figure 6, before calcination, M-HT exhibited a typical HT-CO 3 2À structure with the sharp, symmetric (003), (006), (110) and (113)  The interlayer spacing was calculated by using the basal spacing (d003) minus the width of the brucite-like layer (Wan et al. 2012). Herein, the internal spaces of CHT and M-CHT were 0.293 nm and 0.299 nm, respectively, indicating that the addition of magnetic matrix does not affect the internal space of M-HT.

XPS analysis
To analyze the surface composition and elemental states of M-CHT, the XPS was adopted. All elements were marked in the full spectrum map (Figure 7(a)). The typical binding energies were observed at 56.92 eV, 301.38 eV, 532.80 eV, and 727.12 eV, corresponding to Mg2p, C1s, O1s, and Fe2p, respectively, which are consistent with main constituent elements of M-CHT.
As presented in Figure 7(b), the binding energies at 712.3 eV and 725.6 eV were assigned as the characteristic of Fe 3þ , and the binding energies at 710.6 eV and 723.9 eV were assigned as the characteristic of Fe 2þ (Liu et al. 2020a). In addition, the binding energy at 719.06 eV is the common satellite peak of both Fe 3þ and Fe 2þ . As calculated by the peak areas from XPS, the peak area ratios of Fe 2þ and Fe 3þ were 35.96% and 64.04%, respectively. The actual Fe 2þ /Fe 3þ value of 0.56 is basically consistent with theoretical value of 0.5, indicating that Fe 3 O 4 is doped in hydrotalcite (Yan et al. 2015;Liu et al. 2020a). Figure 8 shows FTIR spectra of M-HT, M-CHT and M-CHT-A. The wide band at 3,460 cm À1 corresponded to the -OH bending vibration from hydroxyl groups and interlayer water (Yan et al. 2015;Shi et al. 2020). A weak band at 1,649 cm À1 was assigned as the bending vibration of the interlayer water (Abdelkader et al. 2011;Liu et al. 2020a). The peak at 1,364 cm À1 is as the vibrational peak of CO 3 2À (Saiah et al. 2009). The band between 400 cm À1 and 800 cm À1 is attributed to the stretching bands of magnesium iron skeleton (Liu et al. 2020a).

FTIR analysis
The above results indicated that M-HT exhibits characteristics of hydrotalcite, comprising interlayer water and carbonate, and the introduction of magnetic substrate does not change its properties. After calcination, the characteristic peaks of hydrotalcite at 3,460 cm À1 and 1,649 cm À1 became weak or disappeared, mainly due to the collapse of the lamellar structure, disappearance of functional groups such as OH À , CO 3 2À , and H 2 O at high temperature, and conversion of sample to a mixed oxide (Wan et al. 2012). After the adsorption of nitrate or nitrite, new peaks were observed at 1,384 cm À1 and 1,271 cm À1 , corresponding to nitrate and nitrite (Ogata et al. 2018), indicating the successful adsorption of nitrate and nitrite on M-CHT. Moreover, for all samples, the peak at 582 cm À1 corresponded to the Fe-O stretching vibration (Liu et al. 2020b(Liu et al. , 2021b, which

VSM analysis
XRD, XPS and FTIR results indicated that Fe 3 O 4 is successfully loaded on the hydrotalcite matrix. Figure 9 shows the magnetic hysteresis curve of M-CHT at room temperature. M-CHT exhibited a magnetization of 9.15 emu/g. The coercive force and remanence were close to zero, indicating that M-CHT is a superparamagnetic material (Xu & Wang 2012;Shen et al. 2019). The inset image in Figure 9 showed that the result of magnetic separation of M-CHT after 5 min, which indicated that M-CHT can be easily separated and recovered.  Uncorrected Proof CONCLUSIONS In this study, M-CHT synthesized by the co-precipitation and calcination method exhibited high adsorption capacity for nitrate or nitrite from contaminated water under a wide range pH (initial pH ranged from 3 to 9). The adsorption kinetics and isotherm of nitrate and nitrite can be described with the first-order, pseudosecond-order model and Langmuir model, respectively. In the presence of coexisting anions, the removal efficiency of nitrate or nitrite over M-CHT decreased in the order of PO 4 3À . CO 3 2À . F À . SO 4 2À . Cl À . ClO 4 À .
XRD and FTIR analysis revealed that M-CHT can recover its original layered structure after the adsorption of nitrate or nitrite. Meanwhile, XRD and XPS analysis confirmed that Fe 3 O 4 was successfully loaded on hydrotalcite, and did not affect the hydrotalcite structure. M-CHT was a magnetic material and it can be easily recycled using a magnet. Thus, M-CHT exhibits great prospects for application in wastewater purification.