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Article

A Study of the Recovery of Phosphorus from Aqueous Solutions Using Zr-Doped MgMn-Layered Double Hydroxide (MgMnZr-LDH)

by
Haixin Zhang
1,
Ayako Chōnan
1,
Ming Zou
1,
Munkhpurev Bat-Amgalan
1,
Naoto Miyamoto
2,
Naoki Kano
2,* and
Shuang Zhang
3,4
1
Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-Nocho, Nishi-ku, Niigata 950-2181, Japan
2
Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-Nocho, Nishi-ku, Niigata 950-2181, Japan
3
State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China
4
Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, Nanchang 330013, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(19), 3420; https://doi.org/10.3390/w15193420
Submission received: 31 July 2023 / Revised: 12 September 2023 / Accepted: 27 September 2023 / Published: 28 September 2023
Browse Figures
Versions Notes

Abstract

:
A new type of adsorbent, Zr-doped three metal element adsorbent MgMnZr-LDH(3), was synthesized using Mg(NO3)2·6H2O, Mn(NO3)2·6H2O, and ZrCl2O·8H2O and employed to adsorb phosphate ions from an aqueous solution. The materials were characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), an X-ray diffractometer (XRD), thermogravimetric–differential thermal analysis (TG−DTA), Fourier-transform infrared spectroscopy (FT-IR), and nitrogen adsorption/desorption experiments (N2 Brunauer–Emmett–Teller (BET)). The influences of the contact time (0~48 h), pH value (3, 5, 7, and 9), initial concentration (0~50 μg/dm3), and temperature (15, 25, 35, and 45 °C) on the adsorption of phosphate (P) ions were studied to investigate P adsorption from wastewater. The results showed that the Zr-modified three metal element adsorbent MgMnZr-LDH(3) had a superior adsorption effect on phosphate anions, which was about 2.18 times greater than that of the two metal element adsorbent MgMn-LDH(3). The MgMnZr-LDH(3) adsorption process conformed to the pseudo-second-order model. The adsorption isotherm can generally be described more satisfactorily for P using the Langmuir isotherm, and the maximum adsorption capacity of P was shown to be 30.8 mg/g. Under optimum experimental conditions (contact time: 24 h; pH 8; and temperature: 25 °C), the effect of competing anions (Cl−, SO42−, and NO3−) on the adsorption of phosphate ions was also investigated, and only the phosphate ions showed high selectivity. The good adsorption performance of MgMnZr-LDH(3) towards phosphate (P) ions was attributed to the anion exchange and complex reaction.

1. Introduction

Phosphorus (P) is required as a fertilizer resource for the cultivation of crops, as a renewable biomass, and is anticipated to be used as a biofuel in a low-carbon society [1]. It is also used in large quantities in other fields, such as metal processing, chemical products, and food additives. However, the amount of high-quality phosphate ore reserves that can be acquired with the current mining costs is reported to last for only another 40 to 50 years on a global scale, and it is predicted that the reserves will be depleted in about 100 years even if underground resources, which cost several times more to mine, are included [2].
Meanwhile, P is also an essential macronutrient for all life forms and is a factor limiting biological productivity in many terrestrial and marine environments [3]. It is released into environmental waters as a result of various human activities, such as the weathering of rocks and industrial, agricultural, and domestic processes. Typical garbage effluents have a total phosphorus concentration of about 10 mg/L, with orthophosphate as the major form of phosphate. The World Health Organization (WHO) has set the maximum P discharge limit at 0.5~1 mg/L as a guideline. P is often present in wastewater at low concentrations mostly as P ions, but also as organic and inorganic oligophosphates and polyphosphates (particulate P) [4,5]. Although P is an essential nutrient, excess levels promote the eutrophication of surface waters, causing algal overgrowth, oxygen depletion, and aquatic mortality [6]. In addition, when the human body ingests too much phosphate, it will have an impact on the person’s health. Phosphate can bind to calcium so that calcium ions are excreted from the body, leading to bone loss, and thereby, causing osteoporosis. At the same time, phosphate can also interfere with the normal functions of the respiratory tract, digestive tract, liver, and heart, leading to the occurrence and aggravation of corresponding diseases. Therefore, it can be said that, at present, there is a heightened demand for the removal of phosphorus/phosphoric acid from water environments.
As described above, the early development of phosphorus removal and recovery technology is strongly desired from both the resource security and water environment conservation and improvement points of view.
Although various physical methods are available, such as crystallization, electrodialysis, and reverse osmosis, the removal of phosphate from contaminated wastewater is primarily accomplished via either chemical precipitation or adsorption. Chemical precipitation is an effective method for removing high concentrations of phosphate, but a sophisticated control system is required. The adsorption method is more effective because of its lower cost, higher adsorption capacity, greater selectivity, faster regeneration rate, reduced sludge formation, and easier operation [7].
At present, the mainstream method for removal and recovery involves the treatment of water with ion exchangers using organic polymers; however, the utilization of the ion exchangers has a limitation in that they are dependent on petroleum. There is also a concern about the depletion of raw materials because they are difficult to decompose, and there are waste treatment and reuse problems after aging. On the other hand, inorganic ion exchangers have advantages because many are composed of elements that exist within the crust of the Earth (abundant elements with a high Clark number). Ion exchangers also have the following advantages: good resistance to weather and chemicals, low deterioration and deterioration by microorganisms, the possibility of regeneration and resource recycling via a relatively safe method, they are highly safe for the human body, and they have a low-level environmental effect upon disposal. Of the inorganic ion exchangers, cation and zwitterionic exchangers have been studied extensively, and some of them have been put to practical use [8]. However, there has been a little research conducted regarding the types, modification, and utilization of anion exchangers with regard to the phosphate ion, and a few have been put to practical use. There is growing interest in the use of hydrotalcite, an inorganic ion exchanger, as a phosphate adsorbent. Hydrotalcite is a promising material because it may have a very high adsorption capability [9,10].
The general formula for LDH is [M1−x2+Mx3+(OH)2]x+[An−]x/n·yH2O, where M2+ and M3+ are divalent and trivalent metal cations, respectively, and An− is an anion incorporated in the interlayer space, with water molecules for charge neutrality and structural stability. As shown in Figure 1, LDH is a layered material with a hydroxide sheet in which trivalent and divalent cations are partially and isomorphically substituted. Then, a net positive charge is generated on the LDH layer via the balance of water molecules with exchangeable anions intercalated in the interlayer space between the two brucite-like layers. Interlayer anions can be exchanged with other anions with higher selectivity. Because of the high charge density, wide interlayer area, good thermal stability, flexible interlayer region to accommodate various anion species, and high anion exchange capacity of the interlayer anions of the sheet, LDH has been used in several studies to remove various anion species, such as fluoride, selenite, arsenate, perchlorate, chromate, phosphate, vanadate, and antimony [11,12].
Up to now, there have been many studies on LDH composites as adsorbents. For example, SONG et al. prepared a polyethylene glycol-modified MgAl-LDH to adsorb Cu(II), Pb(II), and Cd(II) [13]. NAZIR et al. grew ZIF-67 in situ on LDH to form porous composites to develop materials that remove methylene blue and methyl orange [14]. NAZIR et al. synthesized trimetallic layered double hydroxide (NiZnAl-LDH) nanosheets using the hydrothermal method and used them for the purification of Rhodamine B and methyl orange [15]. Although in earlier studies, there were many cases of LDH which consisted of divalent and trivalent metallic elements; however, there are a few examples in which tetravalent metallic elements were introduced. Therefore, in this study, the co-precipitation method was used to prepare LDH composed of two metal elements (Mg and Mn), and LDH was also prepared from three metallic elements including Zr. (The operation process is simple, minimal equipment is required, the preparation process is easy to control, and the application range is wide.) The introduction of metallic elements such as zirconium (Zr) into the layer structure increases the formal positive charge and adsorption [16].
The objective of this work was to evaluate the efficiency of LDH for the removal of P from an aqueous solution. The material was then characterized using XPS, SEM, XRD, TG–DTA, FT-IR, and BET analysis, and the effects of important parameters, such as the pH, contact time, initial concentration, and temperature, on the adsorption of P were investigated.

2. Experimental Section

2.1. Materials and Reagents

Phosphate ion standard solutions were prepared by diluting a standard solution (1000 mg/dm3 H3PO4 solution) purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). All chemicals and reagents, such as Mg(NO3)2·6H2O (99.0%, Kanto Chemical Co., Ltd.), Mn(NO3)2·6H2O (98.0%, Kanto Chemical Co., Ltd.), ZrCl2O·8H2O (99.0%, Wako Pure Co., Ltd. Tokyo, Japan), HNO3 (60.0%), NaOH (97.0%), and Na2CO3 (99.8%), used in this study were of analytical grade and used without further purification. Throughout the process, water (>18.2 MΩ) treated using the ultrapure water system (RFU 424TA, Advantech Aquarius, Suite A Dublin, CA, USA) was used. The pH value was measured using a pH meter (HORIBA F-72, Tokyo, Japan).

2.2. Synthesis of Adsorbent

2.2.1. Synthesis of MgMn-LDH

Quantities of 0.6 mol/L Mg(NO3)2·6H2O and 0.1 mol/L Mn(NO3)2·6H2O were mixed according to the molar ratio Mg/Mn = 3 in 100 mL of the solution and added dropwise to 250 mL of ultrapure water while stirring with a magnetic stirrer. The pH was maintained at about 10 by adding 150 mL of a mixed solution of 0.2 mol/L NaOH and 0.1 mol/L Na2CO3. Then, the suspension was left to stand for 24 h before being centrifuged. The supernatant after centrifugation was discarded, the precipitate was washed with ultrapure water, and the centrifugation process was repeated until the pH of the precipitate reached near-neutrality. Next, the pellets were removed, placed into Petri dishes, and dried at room temperature for 48 h. The dried precipitate was pulverized and used as MgMn-LDH (this LDH is, hereafter, referred to as MgMn-LDH(3)).

2.2.2. Synthesis of MgMnZr-LDH

In addition, 0.6 mol/L Mg(NO3)2·6H2O, 0.1 mol/L Mn(NO3)2·6H2O, and 0.1 mol/L ZrCl2O·8H2O was mixed according to the molar ratio Mg/(Mn + Zr) = 2~5 and Mn/Zr = 1 in 100 mL of the solution. Then, 250 mL of ultrapure water was added dropwise to the solution while stirring with a magnetic stirrer. The subsequent synthesis scheme was the same as described above (this LDH is, hereinafter, referred to as MgMnZr-LDH).

2.3. Adsorption Experiment of P

In this experiment, 100 mL of a suitable amount of the LDH synthesized in Section 2.2 and a suitable concentration of P solution were put into a 200 mL conical flask, and adsorption was carried out using an isothermal agitator in order to examine the effects of conditions such as the molar ratio, pH, contact time, initial P concentration, and temperature of the various LDH metals on the adsorption behavior. After adsorption, the solution in the flask was filtered, and the concentration of P was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES, SEIKO, Chiba, Japan: SPS1500).
P uptake by the adsorbent was calculated using Equation (1):
q e = C 0 − C e V 1000 × M
where qe is the adsorption capacity of P at equilibrium (mg/g), C0 and Ce are, respectively, the initial and equilibrium concentrations of P in a batch system (mg/dm), V is the volume of the solution (dm3), and M is the dry weight of each adsorbent (g) [17,18,19,20].

2.4. Langmuir and Freundlich Isotherm Model

Based on Dahiya et al. [21,22], two common adsorption models, the Langmuir and Freundlich isotherm models, were applied to evaluate the adsorption data obtained for P.
The Langmuir model assumes monolayer adsorption onto a surface and is given by Equation (2):
C e q e = C e q max + 1 K L q max
where Ce is the equilibrium concentration of P in the aqueous phase (mg/dm), KL is the Langmuir adsorption constant (dm3/m), and qe and qmax are, respectively, the amount of adsorption of P at equilibrium and the maximum adsorption capacity on the surface of adsorbent (mg/g).
The linearized Freundlich isotherm model is represented by Equation (3):
lnqe = lnKF + (1/n) lnCe
where KF and 1/n indicate the adsorption capacity and intensity of the system, respectively. The plots of qe versus Ce on a log scale can be plotted to determine values of 1/n and KF, depicting the constants of the Freundlich model [23].

2.5. Kinetic Studies

Kinetic models have been proposed to determine the mechanism of the adsorption process and provide useful data to improve adsorption efficiency and the feasibility of the scaled-up process [24,25]. In our investigation, the mechanism of the adsorption process was studied by fitting pseudo-first-order and pseudo-second-order models to the experimental data.
The pseudo-first-order model is given by Equation (4):
ln ( q e − q t ) = ln q e − k 1 t
where qe and qt are the adsorption capacities of P at equilibrium and time t, respectively, (mol⋅g−1) and k1 is the rate constant of pseudo-first-order adsorption (h−1).
The linear form of the pseudo-second-order model is given by Equation (5):
t q t = 1 k q e 2 + t q e
where k is the rate constant of pseudo-second-order adsorption (g/mol·h−1).

2.6. Reusability and Regeneration Studies

Desorption studies were conducted to recover the spent adsorbate from the aqueous solutions using 0.1 mol/L NaOH solution to recover and recycle P. After the adsorption experiment (each adsorption condition was as follows: contact time: 24 h; pH 8; temperature: 25 °C; sorbent dose: 50 mg; initial concentration: 50 μg/dm3), LDH was filtered, and the LDH remaining on the filter was washed with ultrapure water and dried at room temperature for 48 h. To perform P desorption, LDH was placed in a 200 mL flask with 100 mL of 0.1 mol/L NaOH solution. In order to investigate the effect of the contact time on the desorption behavior, the experiment was carried out by varying the contact time between 0 and 48 h.
To investigate the reusability of LDH, adsorption/desorption cycling experiments were performed. The adsorption conditions were as follows: contact time: 24 h; pH 8; temperature: 25 °C; sorbent dose: 500 mg; initial concentration: 100 μg/dm3. LDH was recovered by following the same procedure described above after adsorption. Then, P was desorbed in a 200 mL flask with 100 mL of 0.1 mol/L NaOH solution, with a contact time of 24 h. This was defined as one cycle, and this cycle was performed three times in total [26]. In each cycle, the filtrate was collected, and the concentration of P (the amount adsorbed and desorbed) was determined using ICP-AES.

2.7. Characterization

The material characterization methods used in this study include X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Yokohama, Kanagawa, Japan: K-Alpha), scanning electron microscopy (SEM, JEOL, Akishima, Tokyo, Japan: JCM-6000), X-ray diffractometer (XRD, Bruker, Yokohama, Kanagawa, Japan: D2 Phaser), Thermogravimetric-differential thermal analysis (TG–DTA, Rigaku, Akishima, Tokyo, Japan: Thermo plus TG 8120), Fourier-transform infrared spectroscopy (FT-IR, JASCO, Tokyo, Japan: FTIR-4200), and N2 Brunauer–Emmett–Telle (N2-BET, Micromeritics, Norcross, GA, USA: TriStar 3020).

3. Results and Discussion

3.1. Characterization of LDH

3.1.1. XPS Analysis

The XPS patterns of MgMnZr-LDH before and after P adsorption are shown in Figure 2a,b. Firstly, as shown in Figure 2a, before the adsorption of P, Zr3d is divided into two types: 182.90 eV and 185.27 eV. The peak of 182.90 eV is ZrO2, and 185.27 eV is the peak from Zr(OH)4. The peaks of 285.54 eV and 290.26 eV of C1s were derived from C-C binding and CO32− (Na2CO3), respectively. The peak values of N1s can be divided into 399.85 eV, 404.35 eV, and 407.82 eV. These peaks may be attributed to the following: 339.85 eV is the peak of N-O, 404.35 eV is the peak of NaNO2, and 407.82 eV is the peak of Mg(NO3)2, Mn(NO3)2, and ZrO(NO3)2. The peak values of O1s can be separated into 530.41 eV and 532.39 eV. The peak of 530.41 eV is close to the location of the metal oxide, and that of 532.39 eV is the peak from the metal hydroxide. The Mn2p peaks can be separated into 642.56 eV and 646.10 eV. The peak of 642.56 eV represent MnO2, but that of 646.10 eV cannot be attributed definitively. The peak value of Na1s may be 1072.35 eV, near the location of NaOH, and the Mg1s peak of 1304.81eV is MgO.
Then, the results after P adsorption were investigated, as follows. As shown in Figure 2b, the peak of the phosphoric acid compound was captured and appears at the P2p peak of 134.08 eV. The results show that MgMnZr-LDH is feasible as an adsorbent for P adsorption. At the same time, the peak of Zr3d can be divided into two types: 182.67 eV and 185.02 eV. The peak of 182.67 eV is ZrO2, and 185.02 eV is the peak from Zr(OH)4. The peaks of 285.62 eV and 290.25 eV of C1s come from C-C binding and CO32− (Na2CO3), respectively. The peak values of O1s can be separated into 530.08 eV and 532.13 eV, of which 530.08 eV is located near the metal oxide position, and 532.13 eV is the peak from the metal hydroxide. The Mn2p peaks can be separated into 642.44 eV and 645.57 eV. The peak of 642.44 eV should be MnO2, although that of 645.57 eV cannot be attributed clearly. The Mg1s peak of 1304.77 eV is MgO [27].

3.1.2. SEM Analysis

Large rectangular particles were observed in the SEM images of MgMn-LDH(3) and MgMnZr-LDH(3), as shown in Figure 3a,b, and their surfaces were covered with some irregular chorionic particles [28]. Figure 3b demonstrates the surface topography of MgMnZr-LDH(3). Compared with that of MgMn-LDH(3), its cubic structure is more angular and crystalline.

3.1.3. XRD Analysis

The XRD patterns of MgMn-LDH(3) and MgMnZr-LDH(3) are shown in Figure 4. As exhibited in Figure 4, the planes of (003), (006), (009), (015), and (113) of MgMn-LDH are located at 2θ of 11.5°, 24.3°, 31.5°, 37.6°, and 60.2°, respectively. All these characteristic peaks are in accordance with the typical patterns of MgMn-LDH [29]. Two peaks at 2θ of 18.2°and 41.5° could be assigned to the characteristic peaks of Mg2MnO4 [30]. The peak at 51.8° is MnCO3. Moreover, the peak that appeared at 2θ of 63.8° indicates the presence of MgO [31].
For MgMnZr-LDH(3) with Zr, the typical XRD peak pattern of LDH with a sharp peak is shown, suggesting that LDH layered structures with high crystallinity can be obtained [32,33,34]. Compared to MgMn-LDH(3), MgMnZr-LDH(3) produces spikes at 2θ of 34.5° (012) and 58.7° (110), and it can be assumed that the introduction of Zr leads to LDH having a layered structure with higher crystallinity [35].

3.1.4. TG−DTA Analysis

TG−DTA curves are shown in Figure 5a,b. It can be seen from Figure 5a,b that in the range of 20 °C~180 °C, the TG curve of MgMn-LDH(3) has a weight loss of −10.0%, and in the range of 120 °C, there is an obvious endothermic peak in the DTA curve. However, the TG curve of MgMnZr-LDH(3) has a weight loss of −24.8%, and an endothermic peak is observed on the DTA curve near 90 °C. It can be considered that this is due to weight loss through the evaporation of water both at the LDH surface and that interwoven between the layers [36]. The TG curve of MgMn-LDH(3) has a weight loss of −27.8% in the range of 180 °C~770 °C, while the DTA curve shows an endothermic peak in the range of 600 °C. The thermogravimetric curve of MgMnZr-LDH(3) loses −40.0% in the high temperature range of 180 °C~770 °C, while the DTA curve shows an endothermic peak around 590 °C. This can be attributed to the weight loss of anions such as OH bases present in the layered structure of LDH and Cl− between the layers [37].
For MgMnZr-LDH(3), the dehydration temperature is lower than that of MgMn-LDH(3), indicating that its structure has changed. This phenomenon may be due to the successful intercalation of Zr molecules into the interlayer of MgMnZr-LDH(3), making the interlayer surface. The hydrophobicity of the layer is enhanced, and the water absorption capacity between the layers is reduced.

3.1.5. FT-IR Analysis

The FT-IR spectra of MgMn-LDH(3), MgMnZr-LDH(3), and MgMnZr-LDH(3) (after adsorption) are shown in Figure 6. It can be seen from Figure 6 that the waveforms of MgMn-LDH(3) and MgMnZr-LDH(3) are basically the same. The peak at 632cm−1 is the in-plane bending vibration absorption peak of C-O bonds. The band at 841 cm−1 results from the M-O bending (M = metal) [38]. The asymmetric stretching vibration absorption peak of C-O in interlayer CO32− is at 1373 cm−1. The band at 1545 cm−1 is the vibration of bicarbonate (HCO3−) ions [39]. The adsorption bands of stretching and the bending vibration of the interlayer water molecules are located at 1630 cm−1 and 3450 cm−1, respectively [40].

3.1.6. BET Surface Area, Pore Volume, and Pore Size Analyses

In order to better understand LDH prepared from different quantities of metals (two-metal-element or three-metal-element LDH) as an adsorbent, the specific surface area, pore volume, and pore size of MgMn-LDH(3) and MgMnZr-LDH(3) were determined using the N2 adsorption method. The pore characteristics of the LDHs determined using the BET method are summarized in Table 1. The results show that MgMnZr-LDH(3) has a larger specific surface area and pore volume than those of MgMn-LDH(3). This is consistent with the SEM results. It also shows that MgMnZr-LDH(3) is more suitable as an adsorbent for phosphorus.

3.2. P Adsorption Experiments on LDH

3.2.1. Effect of Different LDH Metal Molar Ratios on Adsorption of P

LDH was synthesized by varying the molar ratio conditions of various metals (Mg, Mn, and Zr). Under the conditions of a 24 h contact time, a pH of 8, a temperature of 25 °C, and an LDH dosage of 100 mg with an initial concentration of 5 μg/dm3, the adsorption experiments were carried out to compare the adsorption capacity of the different molar ratios of various metals. The results are shown in Figure 7. From Figure 7, it can be seen that MgMnZr-LDH synthesized with a molar ratio of three had the highest adsorption capacity in the range of Mg/(Mn + Zr) = 2–5 under our experimental conditions. The optimal LDH in our study is denoted as MgMnZr-LDH(3) hereafter.

3.2.2. Effect of Contact Time on Adsorption of P

Adsorption experiments were carried out with the contact time of MgMnZr-LDH(3) and P solution ranging from 0 to 48 h, and the other conditions were pH 8; temperature: 25 °C; LDH dosage: 20 mg; and an initial concentration of 20 μg/dm3. The adsorption amount as a function of contact time is shown in Figure 8a. The rate of adsorption increased significantly from the beginning to 12 h, and there was no significant change after 24 h. Hence, the optimized contact time was taken as 24 h for the rest of the experimental work.
In addition, kinetic analysis was carried out by applying pseudo-first-order and pseudo-second-order models to the obtained data. The analytical results are summarized in Figure 8b–d, and the kinetic parameters obtained from these results are summarized in Table 2. From this table, it can be seen that MgMnZr-LDH(3) is better fitted to the pseudo-second-order model.

3.2.3. Effect of pH Value on Adsorption of P

The effect of pH on the adsorption of phosphate ions was studied for pH levels of three, five, seven, and nine (under the conditions of a contact time of 24 h, a temperature of 25 °C, an LDH dosage of 20 mg, and an initial concentration of 20 μg/dm3). The results are shown in Figure 9. It can be seen from Figure 9 that the adsorption capacity tends to decrease as the pH value increases. One of the main reasons for this trend may be the competition between OH groups and phosphates for the LDH adsorption sites. As the pH increases, the OH− content also increases and may lead to a reduction of adsorption capacity because of competition with PO43−. Another possible cause is a change in the solubility of lactate dehydrogenase. In some studies [41,42,43,44,45], the solubility of LDH has been shown to increase with an increase in pH. Large amounts of Mg2+, Mn3+, and Zr4+ are dissolved in the P solution with an increase in pH, and then the adsorption capacity of the phosphate ions tends to decrease with an increase in pH.

3.2.4. Effect of Initial Concentration on Adsorption of P

Experiments on the adsorption of phosphorus (P) by MgMnZr-LDH(3) were conducted by varying the initial concentrations from 0 μg/dm3 to 50 μg/dm3 under the following conditions: pH 8; contact time: 48 h; sorbent dosage: 20 mg; and temperature: 25 °C. As shown in Figure 10a, there was a continuous increase in the uptake of P up to a concentration of 60 μg/dm3. To evaluate the relevant parameters mentioned in Section 2.4, Langmuir and Freundlich isotherms were applied to the data obtained from the variation of initial concentrations.
Adsorption isotherms are commonly used to reflect the performance of adsorbents in the adsorption processes. The adsorption data obtained for P using LDH were analyzed using Langmuir (Figure 10b) and Freundlich (Figure 10c) equations. The correlation coefficients (R2) of these isotherms for P on LDH are shown in Table 2 together with other relevant parameters.
From Table 3, it can be seen that the R2 value is comparatively large, and there was favorable adsorption of P by MgMnZr-LDH(3). In particular, the R2 values in the Langmuir isotherm were large (0.975). This result suggests that the adsorption of P on LDH mainly occurred as a result of a monolayer reaction. The amount of P adsorbed on MgMnZr-LDH(3) was 32.7 P-mg/g. The Freundlich equation showed high linearity with a correlation coefficient of 0.944. The value of 1/n fell in the range 0 < 1/n < 1, indicating that a relatively strong bond between the adsorbent and the target material was formed.

3.2.5. Effect of Temperature on Adsorption of P

We investigated the effect of temperature on the adsorption capacity of P by MgMnZr-LDH(3) at different temperatures (15, 25, 35, and 45 °C) under the following conditions: contact time: 48 h; pH 8; LDH dosage: 20 mg; and initial concentration: 20 μg/dm3. The results in Figure 11 show that the adsorption capacity did not vary significantly in this temperature range.

3.2.6. Effect of Competing Ions on Adsorption of P

The effect of competitive anions on the adsorption of phosphate ions was studied. In this experiment, the initial concentration of phosphate ions (PO43−) was taken as 10 mg/dm3 in the presence of the same concentration of common ions (Cl−, SO42−, and NO3−). Adsorption experiments were performed by administering 100 mg LDH to a 100 mL solution. The other experimental conditions (contact time, pH, and temperature) were basically the same as those set out in Section 3.2.1 above. The results are shown in Figure 12. From Figure 12, it can be seen that only PO43− was selectively adsorbed. Based on this result, it is considered that the MgMnZr-LDH(3) synthesized in this work has a very high selectivity for PO43−.

3.2.7. Adsorption Experiments Using Seawater

Adsorption experiments using seawater were carried out to examine the practicality of the adsorbent. A phosphoric acid standard solution (5 μg/dm3) was added to seawater and 100 mg LDH into the 100 mL solution. The other experimental conditions (contact time, pH, and temperature) were basically the same as those set out in Section 3.2.1 above. The experimental results are shown in Table 4. MgMnZr-LDH(3) showed high selectivity only for PO43− in the presence of high concentrations of Cl− and SO42−, which are the main components of seawater. The adsorption rate of phosphate under our experimental condition was 53.5%.

3.2.8. Comparison between MgMn-LDH and MgMnZr-LDH with Regard to Phosphate Ion Adsorption Amounts

In order to investigate the effect of the presence or absence of Zr with the adsorbents on the adsorption amount, adsorption experiments (contact time: 24 h; pH 8; temperature: 25 °C; sorbent dose: 500 mg; and initial concentration: 100 μg/dm3) were performed using MgMn-LDH(3) and MgMnZr-LDH(3). The results are shown in Figure 13. This shows that the Zr-modified three-element type of adsorbent MgMnZr-LDH(3) adsorbed about 2.18 times more phosphate ions than the two-element type of adsorbent MgMn-LDH(3) did. In addition to the ion exchange between the phosphate ions and the anions in the LDH layer, the direct complex formation of the Zr (IV) center between the phosphate ions and the LDH layer can contribute to an increase in phosphate uptake through the addition of Zr [46,47]. Therefore, it is considered that the adsorption amount of phosphate by MgMnZr-LDH(3) was higher than that achieved by MgMn-LDH(3) through ion exchange alone.

3.3. Desorption Experiments of P

The desorption behavior of P was investigated using 0.1 M NaOH solution from LDH after P adsorption, with the aim of recovering and recycling P. In order to investigate the effect of contact time on the desorption behavior, a desorption experiment was carried out by varying the shaking time between 1 and 48 h. The experimental results are shown in Figure 14. It took about 24 h for the desorption to reach equilibrium. After 24 h of desorption, about 80% of the adsorbed amount was desorbed. Based on this result, the desorption time was fixed at 24 h for the next reuse experiment.

3.4. Reusability and Regeneration Studies of LDH

To investigate the reusability of LDH, adsorption/desorption cycling experiments were performed using MgMnZr-LDH(3). Figure 15 shows that the adsorption rate was about 97% in the first, second, and third cycles. The amounts of desorption relative to the amounts adsorbed were about 31%, 63%, and 83% for the first, second, and third cycles, respectively. The amount of adsorption did not change significantly from the first cycle, even when the number of cycles was multiplied. The amount of desorption increased as the number of cycles increased.

3.5. Comparison with Other Adsorbents

In many studies, the adsorption performance has generally been evaluated and expressed according to the maximal (or equilibrium) adsorption capacity. However, the maximal adsorption capacity is sensitive to the influence of the initial concentration of the target pollutant. If the adsorbent is exposed to a higher adsorbate concentration, it can easily show a higher adsorption capacity. Similarly, if the adsorbent is exposed to a lower adsorbate level, the adsorption capacity decreases. Therefore, in addition to the maximal adsorption capacity, it is useful to estimate adsorption performance using the concept of the partition coefficient (PC) [48,49,50,51]. Therefore, the PC (Equation (6)) was also introduced in this work to evaluate adsorption performance:
PC = Adsorption capacity/Final concentration
A comparison of the adsorption performance of phosphate on various adsorbents in previous studies is listed in Table 5.

4. Conclusions

In this work, a new type of adsorbent, a Zr-doped three-metal-element adsorbent, MgMnZr-LDH(3), was synthesized using Mg(NO3)2·6H2O, Mn(NO3)2·6H2O, and ZrCl2O·8H2O and employed to adsorb phosphate ions in an aqueous solution. MgMnZr-LDH(3) had a better adsorption effect on phosphate anions, at about 2.18 times larger than that of the two-metal-element adsorbent MgMn-LDH(3). The pseudo-second-order model and Langmuir isotherm demonstrated the adsorption of PO43− under optimal adsorption conditions. The maximal adsorption capacity of MgMnZr-LDH(3) for PO43− was estimated to be 30.8 P-mg/g. The effect of common anions (Cl−, SO42−, and NO3−) on the adsorption of PO43− was not significant, and only PO43− was selectively adsorbed. In the case of the adsorption experiment using seawater, MgMnZr-LDH(3) showed high selectivity only for PO43− in the presence of high concentrations of Cl− and SO42−. In addition, in the adsorption/desorption experiments, MgMnZr-LDH(3) exhibited a high adsorption capacity for PO43−, which proves that MgMnZr-LDH (3) can be an effective adsorbent for PO43−. This indicates good application prospects in relation to the removal or recovery of PO43− from aqueous solutions. From the viewpoint of environmental protection, this is important information that can be applied in the treatment industrial wastewater including pollutants.

Author Contributions

Experiments, data evaluation, and writing, H.Z.; experiments and data evaluation, A.C.; instruments of measurement, M.Z.; instruments of measurement, M.B.-A.; instruments of measurement and data evaluation, N.M.; supervision and writing, N.K.; data analysis and investigation of the literature, S.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Research Program (C), No. 21K12290) and JST SPRING, Grant Number JPMJSP2121.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to their originality.

Acknowledgments

The authors are grateful to Yuko Amaki and Haruo Morohashi of the Industrial Research Institute of Niigata Prefecture for the XPS and TG–DTA measurements and useful advice. The authors also thank Ohizumi, M. of the Office for Environment and Safety and Iwafune, K., Nomoto, T., and Tanaka, T. of the Facility of Engineering in Niigata University for permitting the use of the ICP-AES, XRD, and SEM.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 03420 g001
Figure 1. Structure of LDH.
Figure 1. Structure of LDH.
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Water 15 03420 g002aWater 15 03420 g002b
Figure 2. (a) XPS patterns of MgMnZr-LDH before adsorption. (b) XPS patterns of MgMnZr-LDH after adsorption.
Figure 2. (a) XPS patterns of MgMnZr-LDH before adsorption. (b) XPS patterns of MgMnZr-LDH after adsorption.
Water 15 03420 g002aWater 15 03420 g002b
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Figure 3. (a) SEM image of MgMn-LDH(3). (b) SEM image of MgMnZr-LDH(3).
Figure 3. (a) SEM image of MgMn-LDH(3). (b) SEM image of MgMnZr-LDH(3).
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Figure 4. XRD patterns of MgMn-LDH(3) and MgMnZr-LDH(3).
Figure 4. XRD patterns of MgMn-LDH(3) and MgMnZr-LDH(3).
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Figure 5. (a) MgMn-LDH(3) TG−DTA curves. (b) MgMnZr-LDH(3) TG−DTA curves.
Figure 5. (a) MgMn-LDH(3) TG−DTA curves. (b) MgMnZr-LDH(3) TG−DTA curves.
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Figure 6. FTIR spectra of MgMn-LDH(3), MgMnZr-LDH(3) and MgMnZr-LDH(3) after adsorption of P.
Figure 6. FTIR spectra of MgMn-LDH(3), MgMnZr-LDH(3) and MgMnZr-LDH(3) after adsorption of P.
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Figure 7. Effect of molar ratio on the adsorption of phosphate ion.
Figure 7. Effect of molar ratio on the adsorption of phosphate ion.
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Water 15 03420 g008aWater 15 03420 g008b
Figure 8. (a) Effect of contact time on the adsorption of phosphate ions. (b) Adsorption kinetics for the adsorption of phosphate ions. (c) Pseudo-first-order plot for P adsorption by MgMnZr-LDH(3). (d) Pseudo-second-order plot for P adsorption by MgMnZr-LDH(3).
Figure 8. (a) Effect of contact time on the adsorption of phosphate ions. (b) Adsorption kinetics for the adsorption of phosphate ions. (c) Pseudo-first-order plot for P adsorption by MgMnZr-LDH(3). (d) Pseudo-second-order plot for P adsorption by MgMnZr-LDH(3).
Water 15 03420 g008aWater 15 03420 g008b
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Figure 9. Effect of pH on the adsorption of phosphate ions.
Figure 9. Effect of pH on the adsorption of phosphate ions.
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Water 15 03420 g010aWater 15 03420 g010b
Figure 10. (a) Initial concentration dependence of phosphate uptake. (b) Langmuir isotherm plots of P sorption. (c) Freundlich isotherm plots of P sorption.
Figure 10. (a) Initial concentration dependence of phosphate uptake. (b) Langmuir isotherm plots of P sorption. (c) Freundlich isotherm plots of P sorption.
Water 15 03420 g010aWater 15 03420 g010b
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Figure 11. Effect of temperature on the adsorption of phosphate ions.
Figure 11. Effect of temperature on the adsorption of phosphate ions.
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Figure 12. Comparison of phosphate uptake for several anions.
Figure 12. Comparison of phosphate uptake for several anions.
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Figure 13. Comparison of phosphate uptake between MgMn-LDH(3) and MgMnZr-LDH(3).
Figure 13. Comparison of phosphate uptake between MgMn-LDH(3) and MgMnZr-LDH(3).
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Figure 14. Time dependence of phosphate desorption from MgMnZr-LDH(3).
Figure 14. Time dependence of phosphate desorption from MgMnZr-LDH(3).
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Figure 15. Repetition of phosphate adsorption–desorption.
Figure 15. Repetition of phosphate adsorption–desorption.
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Table 1. Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size of MgMn-LDH(3) and MgMnZr-LDH(3).
Table 1. Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size of MgMn-LDH(3) and MgMnZr-LDH(3).
SampleBET Surface Area
[m2/g]		Pore Volume
[cm3/g]		Pore Size
[nm]
MgMn-LDH(3)41.350.10329.977
MgMnZr-LDH(3)192.50.43198.975
Table 2. Kinetic parameters of P adsorption by the MgMnZr-LDH(3).
Table 2. Kinetic parameters of P adsorption by the MgMnZr-LDH(3).
Experimental ValuePseudo-First-Order ModelPseudo-Second-Order Model
qe
(P-mg/g)		qe
(P-mg/g)		k1
(min−1)		R2qe
(P-mg/g)		k2
(g/mg·min−1)		R2
25.1017.100.094900.947025.400.035200.9950
Table 3. Isotherm parameters of P sorption onto MgMnZr-LDH(3).
Table 3. Isotherm parameters of P sorption onto MgMnZr-LDH(3).
Langmuir IsothermFreundlich Isotherm
qm (P-mg/g)RLR2KF (P-mg/g)1/nR2
30.800.052100.975018.900.21600.9440
Table 4. The concentration of each anion in seawater before and after adsorption.
Table 4. The concentration of each anion in seawater before and after adsorption.
Cl−SO42−PO43−
Preadsorption Concentration (mg/L)1.73 × 1042.42 × 1035.09
Postadsorption Concentration (mg/L)1.81 × 1042.43 × 1032.37
Table 5. Comparison of the adsorption properties of several adsorbents.
Table 5. Comparison of the adsorption properties of several adsorbents.
AdsorbentsFinal Concentration (mg/L)Adsorption Capacity (mg/g)Partition Coefficient (mg/g·mM−1)Reference
Zr(IV)-loaded orange waste gel7.35857.007.747[52]
Rapid cooled basic oxygen furnace slag4.5003.6200.8044[53]
Cross-linked chitosan beads3.16052.1016.49[54]
Calcined cobalt hydroxide15.00155.010.33[55]
La nanosphere-coated Mn/Fe LDH13.20346.526.25[56]
MgMnZr-LDH(3)1.13630.8027.11This study
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Zhang, H.; Chōnan, A.; Zou, M.; Bat-Amgalan, M.; Miyamoto, N.; Kano, N.; Zhang, S. A Study of the Recovery of Phosphorus from Aqueous Solutions Using Zr-Doped MgMn-Layered Double Hydroxide (MgMnZr-LDH). Water 2023, 15, 3420. https://doi.org/10.3390/w15193420

AMA Style

Zhang H, Chōnan A, Zou M, Bat-Amgalan M, Miyamoto N, Kano N, Zhang S. A Study of the Recovery of Phosphorus from Aqueous Solutions Using Zr-Doped MgMn-Layered Double Hydroxide (MgMnZr-LDH). Water. 2023; 15(19):3420. https://doi.org/10.3390/w15193420

Chicago/Turabian Style

Zhang, Haixin, Ayako Chōnan, Ming Zou, Munkhpurev Bat-Amgalan, Naoto Miyamoto, Naoki Kano, and Shuang Zhang. 2023. "A Study of the Recovery of Phosphorus from Aqueous Solutions Using Zr-Doped MgMn-Layered Double Hydroxide (MgMnZr-LDH)" Water 15, no. 19: 3420. https://doi.org/10.3390/w15193420

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