Layered Double Hydroxides as Hosting Matrices for Storage and Slow Release of Phosphate Analyzed by Stirred-Flow Method

Abstract

The objective of this work was to synthesize and characterize Layered Double Hydroxides (MgAl-LDHs, MgFe-LDHs, and MgAlFe-LDHs) containing phosphate anions and study P release kinetics was conducted with the prepared materials. The release kinetics tests were performed in water by the stirred-flow method to verify the P release profile from LDHs materials. Results showed LDHs have similar characteristics to those of reactive sources of P, such as thermophosphates. The LDHs presented gradual P release and the LDH prepared with the molar ratio of cations Mg:Al = 2:1 showed a more sustained profile of P release compared to other produced materials. Thus, the chemical and structural characteristics of the LDHs make them potential materials for the use and storage of slow P release fertilizer.

Keywords:
fertilizers; nanotechnology; P kinetic release; stirred-flow method

1. Introduction

Layered Double Hydroxides (LDHs), also known as hydrotalcite like compounds, are materials with a bidimensional (2D) structure¹. LDHs can be represented by the chemical formula [M²⁺ _(1-x) M³⁺ _x(OH)₂](A^n-)_x/n ·zH₂O, where M²⁺ refers to bivalent metallic cations, such as Mg²⁺, Cr²⁺, Ca²⁺, Fe²⁺, Mn²⁺, Co²⁺; M³⁺ refers to trivalent metallic cations, such as Al³⁺, Co³⁺, Fe³⁺, Cr³⁺, and A^n- refers to interlayer anions, commonly nitrate, phosphate, carbonate or chloride. In these materials, divalent cations are partially replaced by trivalent cations, thus generating positive charges that are neutralized by anions and water molecules^2,3.

These materials are applied in catalysis, ion exchange, adsorption, photochemistry, electrochemistry, etc^4,5. Recent works have reported the potential of these compounds in agriculture as matrices for storage and slow release of agrochemicals, such as nutrients, herbicides and plant growth regulators^6-13.

Cardoso et al.⁶ intercalated 2,4-D; MCPA, and Picloram molecules in release tests performed in leaching columns and bioassays. When LDHs were compared with pure herbicides, leaching test results showed that the release is slow when the molecules are intercalated. For the bioassays, results showed that herbicide molecules intercalated in LDH presented the same pre-emergence control efficiency on Lepidium sativum as the pure herbicide molecules.

Hafez et al.⁷ intercalated gibberellic acid (GA) in MgAl-LDH (GA-LDH). The study aimed to verify the slow release of GA in different solutions (pH = 3 and pH = 7) and the degradation of the substance in two soils (sandy and medium texture). The GA-LDH results were compared to pure GA. The authors showed that GA-LDH is more effective than pure GA, since pure GA crystallizes rapidly. After 24 hours of reaction, only 4.5% and 10.4% of GA was released in solutions of pH = 3 and pH = 7. After this period, the hormone becomes insoluble and no longer reacts in the medium. After 24 hours of reaction, the GA-LDH released 80% of GA into the solution at pH = 3 and 60% at pH = 7. In addition, it maintained a slow release until 6 days after the start of the reaction. Soil degradation tests showed that the intercalation increased the GA lifetime in the soil compared to pure GA. The pure GA remained for 6 days in the medium texture soil and 10 days in the sandy soil. After the intercalation, the period of GA in the soil increased to 20 days for the medium texture soil and 28 days for the sandy texture soil.

Silva et al.¹⁴ and Silva et al.¹⁵ synthesized LDHs intercalated with NO₃ ^- anions, varying the molar ratios of Mg²⁺:Al³⁺ and Mg²⁺:Fe³⁺. In both works, the authors concluded LDHs presented protection for intercalated NO₃ ^- anions with potential uses as matrices for slow release of fertilizers.

Phosphorus (P) is an essential element for food production. P is a limited resource that has been inappropriately exploited, causing high pressure on worldwide reservese^16,17. In recent years, problems related to P have been addressed in several works, which have proposed strategies for dealing with the consequences of the uncontrolled use of this element in agriculture^18-23. P scarcity is one of the biggest challenges of global sustainability in the 21^st century, which directly affects food production and safety systems in the world²⁴. Therefore, studies on improving the use of P have increased in recent years.

In many countries, mainly those in tropical regions, soils present low P availability because of the adsorption and fixation of this element by iron and aluminum oxides²⁵. In these regions, high P content is usually applied for soil correction rather than plant nutrition^24-26.

The reduced contact between the phosphate ions and the soil mineral phase reduces the adsorption reactions/fixation of P by the soil^27,28. Thus, an alternative for improving P performance in cultures is the development of sources to provide a physical protection for phosphate ions, in order to slow the release of these ions to plants. This physical protection can be created by the intercalation of phosphate ions in LDHs^29-33.

Because of the need to develop new technologies to increase the efficiency of phosphate fertilizers, the objectives of this work were as follows: (i) synthesis and characterization of MgAl-LDHs, MgFe-LDHs and MgAlFe-LDHs containing phosphate ions; and (ii) evaluation of P release kinetics in the prepared materials. These LDHs were prepared with different molar ratios of M²⁺ and M³⁺ cations. The produced materials were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy with accessory of attenuated total reflectance (FTIR-ATR), Raman spectroscopy analyses and specific surface area by adsorption of N₂ (BET). The contents of C, N and H in the LDHs were determined by elementar analysis. For the prepared LDHs, P release kinetics were evaluated in an adaptation method stirred-flow.

2. Materials and Methods

2.1 Experimental Part

All reagents used in this work presented analytical degree of purity: Mg(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, Al(NO₃)₃·9H₂O, KNO₃, K₂HPO₄, and NaOH. The water used in all reactions was distilled, deionized and decarbonated in a Milli-Q^® Integral Water Purification System.

2.2 Synthesis of Layered Double Hydroxides

Initially, five pristine LDHs were synthesized with molar ratios ranging between M²⁺ and M³⁺ cations. All pristine LDHs were intercalated with NO₃ ^- anions. These materials were named Mg₂Al-LDH, Mg₃Al-LDH, Mg₂Fe-LDH, Mg₃Fe-LDH, and Mg₂Fe_0.5Al_0.5-LDH. The pristine LDHs were synthesized by the coprecipitation method at pH constant⁴. To prepare the material, 500 mL of a solution containing M²⁺ and M³⁺ cations in different molar ratios (Table 1) were slowly added drop by drop to 290 mL of a solution containing 1.48 mol L^-1 of KNO₃. During this time, the suspension formed was kept under agitation in an N₂ atmosphere. A solution containing 2.00 mol L^-1 of NaOH was added simultaneously to maintain a constant pH (± 0.5). The cations, molar ratio, number of mols and anions used in the synthesis of the pristine LDHs are shown (Table 1).

Table 1
List of cations, anions, molar ratio of cations and number of mols used for pristine LDHs

LDHs	Cations	*Anions	Molar Ratio M2+/M3+	**Mols of M2+	Mols of M3+
Mg2Al-LDH	Mg2+ and Al3+	NO3-	2:1	0.250	0.125
Mg3Al-LDH	Mg2+ and Al3+	NO3-	3:1	0.375	0.125
Mg2Fe-LDH	Mg2+ and Fe3+	NO3-	2:1	0.250	0.125
Mg3Fe-LDH	Mg2+ and Fe3+	NO3-	3:1	0.375	0.125
Mg2Fe0.5Al0.5-LDH	Mg2+, Fe3+ and Al3+	NO3-	2:0.5:0.5	0.250	0.0625***

• ^*
  Solution prepared from 0.428 mols of NO₃- in 290 mL of H₂O.
• ^**
  Solution prepared in 250 mL of H₂O.
• ^***
  0.0625 mols of each trivalent cation were added (Al³⁺ and Fe³⁺).

After the synthesis, the suspension was hydrothermally treated at 65°C during 24 h. After the hydrothermal treatment, the precipitate was filtered and washed with H₂O. The solid material obtained was freeze-dried for 120 h. After drying, the material was triturated and passed through an 80-mesh sieve to homogenize the particle sizes.

After synthesizing the pristine LDHs, the materials were subjected to an ion exchange reaction in solution for the incorporation of phosphate anions. The anion exchange in solution method was used for the incorporation of phosphate anions. In this method, 2.00 g of LDH precursor was suspended in 200 mL of a solution containing 1.00 mol L^-1 of K₂HPO₄. The formed suspension was kept under agitation and atmosphere of N₂ for 24 h at 65 ^oC. At the end of the exchange reaction, the material was filtered again, washed and vacuum-dried in the presence of silica gel at room temperature. After the anion exchange reaction (Table 1), the names of pristine LDHs were changed to Mg₂Al-LDH-P, Mg₃Al-LDH-P, Mg₂Fe-LDH-P, Mg₃Fe-LDH-P, and Mg₂Fe_0.5Al_0.5-LDH-P.

2.3 Characterization of the LDHs

The quantities of C, N, and H in LDHs were determined by elemental analysis. Chemical analyses were done to quantify the total levels of LDH constituent elements. Metals were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES). P quantities contained in LDHs and in the solution after the kinetics experiments were determined by molecular absorption spectrometry in the UV-Vis region, as described by Braga and Defelipo³⁴.

In Brazil, phosphate fertilizers must contain minimum guarantees of P₂O₅ quantities as determined in H₂O, 2% citric acid and neutral ammonium citrate (NAC). To compare the various LDHs synthesized and the commercial sources of P, P₂O₅ in H₂O, 2% citric acid and NAC+H₂O were analyzed in accordance with the methodology described in AOAC³⁵.

The LDHs were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy with accessory of attenuated total reflectance (FTIR-ATR), analyses of Raman spectroscopy and specific surface area through adsorption of gaseous N₂ (BET).

PXRD analyses were performed with a Shimadzu X-ray diffractometer, XRD 6000, with a copper cathode lamp (λ = 0.154 nm) and a graffiti monochromer to select the copper emission region, with potential source of 40 kV and current of 40 mA. The scanning rate of 0.01° every 10 s was used to determine the parameters of unitary cells of LDHs. The sweeping 2θ ranged from 4 to 70º.

FTIR-ATR analyses were conducted with a Jasco spectrophotometer, FTIR 4100. The spectra were obtained with 216 scans, in a wavenumber range from 4000 to 400 cm^-1 and resolution of 4.0 cm^-1.

Raman measurements were performed in a Renishaw inVia confocal Raman microscope. The spectra were obtained through excitation with a near infrared laser line (λ = 785 nm) at 1.5 mW using a 50X standard objective lens. Fluorescence correction (background) was performed by removing the baseline using the baseline correction and smoothing filter functions in the software Origin 8.0.

The determination of the specific surface area (BET) was performed by the adsorption of N₂ gas in the sample using a Quantachrome analyzer, model Nova 2200e. The samples were degassed under vacuum at high temperature to remove H₂O and other volatile compounds. After degassing, the sample was transferred to the analysis module, where N₂ gas was injected into the interior of the cell to provide adsorption measures of N₂.

2.4 Kinetics of P Release

An adapted stirred-flow method, described by Strawn and Sparks³⁶, was used for the kinetics of P release experiments. The analyses system has a reaction chamber containing two filters. One of them has a 0.45 µm cellulose membrane and the other has Teflon with 25 mm pores. Both are used to retain the sample within the chamber. The chamber was placed on a magnetic stirrer (100 rpm) and coupled to a peristaltic pump, generating a flow of 1 mL min^-1. A fraction collector was coupled to the chamber outlet. Ultrapure H₂O was used as extractor. Approximately 2.00 mg of LDH was put into the agitation chamber and the pumped extraction solution was collected in the outlet of the system every 2 min up to 30 min, after that, every 6 min up to 110 min, and then each 10 min up to 150 min after the beginning of the flow. The P extracted from the samples, which was in the fractions, was determined as described by Braga and Defelipo³⁴. For this determination, 0.5 mL of the sample collected in the stirred-flow was pipetted into 25 mL plastic cups and added to 4.5 mL of deionized H₂O. Immediately afterward, 5 mL of a working reagent composed of ammonium molibidate 0.01 mol L^-1, bismuth subcarbonate 0.01 mol L^-1 and 1.6 g ascorbic acid were added. After 30 min, following the total blue color development, the samples were read on a UV-VIS spectrometer at a wavelength of 725 nm. The P contents P collected at different times in the stirred-flow assay were used to elaborate the mathematical model that best fitted the P release. The software CurveExpert version 1.4 was used to build the models.

3. Results and Discussion

3.1 Chemical Characterization

The values of molar ratio of M²⁺/M³⁺ cations present in the layers were close to those added during the synthesis: Mg₂Al-LDH (Mg/Al = 2.2); Mg₃Al-LDH (Mg/Al = 3.3); Mg₂Fe-LDH (Mg/Fe = 1.9); Mg₃Fe-LDH (Mg/Fe = 2.8), and Mg₂Fe_0.5Al_0.5-LDH (Mg/FeAl) = 2.1). These values show that there was no lixiviation of cations during the synthesis process.

The amounts of C, H, N and P for pristine LDHs and LDHs-P are shown in Table 2. The LDHs of the Mg-Al system presented higher amounts of anions, both NO₃ ^- and phosphate. It was also possible to verify, for the MgAl-LDHs and MgFe-LDHs systems, that the molar ratio M⁽²⁺⁾/M⁽³⁺⁾ increased from 2:1 to 3:1, reducing the amount of intercalated NO₃ ^- and phosphate.

Table 2
Contents of C, H, N and P of different LDHs

LDHs	Elements -------------------------- (mg/g) ---------------------------
	C	H	N	P
Pristine LDHs
Mg2Al-LDH	6.70	30.50	48.20	-
Mg3Al-LDH	5.10	33.00	40.40	-
Mg2Fe-LDH	8.20	28.80	20.20	-
Mg3Fe-LDH	7.60	31.70	23.50	-
Mg2Fe0.5Al0.5-LDH	5.60	26.40	42.26	-
LDHs-P
Mg2Al-LDH-P	8.90	36.90	0.30	64.00
Mg3Al-LDH-P	8.80	36.70	1.20	63.00
Mg2Fe-LDH-P	10.60	29.50	2.50	43.20
Mg3Fe-LDH-P	12.00	31.10	3.60	32.60
Mg2Fe0.5Al0.5-LDH-P	11.00	31.50	1.00	41.80
LDHs-P Formulas
Mg2Al-LDH-P	Mg0.69Al0.31(OH)2(HPO42-)0.309(NO3-)0.001·1.18H2O
Mg3Al-LDH-P	Mg0.77Al0.23(OH)2(HPO42-)0.226(NO3-)0.004·1.13H2O
Mg2Fe-LDH-P	Mg0.65Fe0.35(OH)2(HPO42-)0.330(NO3-)0.020·1.30H2O
Mg3Fe-LDH-P	Mg0.74Fe0.26(OH)2(HPO42-)0.234(NO3-)0.026·1.19H2O
Mg2Fe0.5Al0.5-LDH-P	Mg0.68Al0.15Fe0.17(OH)2(HPO42-)0.311(NO3-)0.007·1.20H2O

This reduction is related with the charge density of the materials; the lower the molar ratio, the higher the load density of the layer, which increases the number of intercalated anions between inorganic layers³⁷.

Results showed that the ionic exchange occurred in the solution and phosphate anions were able to replace part of intercalated NO₃ ^- anions. The P values obtained after the exchange reaction are close to those reported in the literature. Gillman et al.³⁸ used MgAl-LDH and different concentrations of phosphate in the exchange reactions and obtained materials with up to 62 mg g^-1 of P. Triantafyllidis et al.³⁹ used MgAl, MgFe, and MgAlFe-LDHs and obtained materials with approximately 80 mg g^-1 of P.

For Crepaldi and Valim³⁷, one of the main hindrances to ionic exchange reaction in solution is the precursor anion content that remains after the reaction. In agriculture however, the residual N content in the synthesized LDHs after exchange reaction by phosphate is a desirable characteristic. Nitrogen is an essential element for plants; thus, LDH-P can be a fertilizer and a source of N and P for plants.

The contents of total P₂O₅, in H₂O, 2% citric acid and NAC extractors are shown (Table 3). In relation to the P fraction extracted from different extractors (H₂O, 2% citric acid and NAC), MgAl-LDHs presented the lowest extracted amount for all three extractors. The use of H₂O as an extractor can determine only the most soluble forms of P. The 2% citric acid solution extracts low solubility forms, such as P-Ca, in addition to the soluble forms. The NAC solution is able to extract low solubility forms linked with Fe and Al⁴⁰.

Table 3
Contents of total P₂O₅, in water, 2% citric acid and neutral ammonium citrate (NAC)

Material	P2O5 Total	*Soluble P2O5 in H2O	*P2O5 in 2% citric acid	* P2O5 in NAC
	-------------------------------- (%) --------------------------------------
Mg2Al-LDH-P	7.32	12.80	85.31	17.50
Mg3Al-LDH-P	7.21	16.04	72.06	18.41
Mg2Fe-LDH-P	4.94	33.70	96.29	35.18
Mg3Fe-LDH-P	3.73	46.50	78.52	48.46
Mg2Fe0.5Al0.5-LDH-P	4.78	26.00	85.26	32.05

• ^*
  In relation to Total P.

As expected, all LDHs presented no solubility in H₂O and from average to high solubility in 2% citric acid and NAC. Such characteristics are very similar to reactive sources such as thermophosphate⁴¹. Phosphate sources of high H₂O solubility, such as triple superphosphate, simple superphosphate and mono ammonium phosphate, suffer huge losses of P due to adsorption abilities of tropical soils. Sources with reactive characteristics are a good alternative under these conditions because of the slow-release effect of these materials⁴². It is worth highlighting however that source efficiency is affected by not only reactivity, but also by soil and plant factors⁴³. This suggests that LDHs can become a potential source of slow-release P in agriculture, once reactive sources show good agronomic efficiency^44,45.

3.2 Powder X-Ray Diffraction (PXRD)

The diffractograms of pristine LDHs are shown in Figure 1. These diffractograms show basal peaks (00l), (003) and (006), which indicate the presence of lamellar materials, such as hydrotalcite. Using Bragg’s equation (n·λ = 2·d·sin θ) and the average values of 2θ of the basal peaks (00l) of each LDH, it was possible to calculate the basal spacing d (spacing between two adjacent layers) of these materials. The obtained d values were 0.89; 0.88; 0.79; 0.79, and 0.81 nm for Mg₂Al-LDH, Mg₃Al-LDH, Mg₂Fe-LDH, Mg₃Fe-LDH, and Mg₂Fe_0.5Al_0.5-LDH, respectively. These values concur with those reported in the literature for intercalation of NO₃ ^- in LDHs^46,47. All synthesized materials presented good structural organization and phase purity. LDH-5, with three different cations in the layers, presented poor crystallinity compared to the others.

Figure 1
Diffractograms of pristine LDHs: Mg₂Al-LDH, Mg₃Al-LDH, Mg₂Fe-LDH, Mg₃Fe-LDH, Mg₂Fe_0.5Al_0.5-LDH

For Mg₃Al-LDH and Mg₃Fe-LDH, the materials with the cation ratio of M²⁺:M³⁺ = 3:1 showed more intense basal peaks (00l), indicating higher crystallinity. The lower proportion of trivalent cation in the layers increased the crystallinity of the materials. The lower the number of cations M²⁺ replaced by M³⁺, the lower the distortion of the layers and consequently the better the arrangement of stacking for the structure^15,48.

The synthesized Mg₂Fe_0.5Al_0.5-LDH showed lower crystallinity when compared with the LDHs synthesized with two different cations. This result is highlighted by the lower intensity of the basal peaks in the diffractogram of Mg₂Fe_0.5Al_0.5-LDH. The presence of three different types of cations in the layers results in lower structural organization when compared with LDHs synthesized with two different types of cations.

Figure 2 presents the diffractograms of pristine LDHs subjected to anionic exchange reaction in solution. Mg₂Al-LDH-P and Mg₃Al-LDH-P presented significant changes in the diffraction patterns, compared to pristine Mg₂Al-LDH and Mg₃Al-LDH. Two phases are found for these materials. In the first phase, the basal peaks are described in the diffractogram as ○(003) and ○(006) and the basal spacing values found for this phase were 0.62 nm for Mg₂Al-LDH-P and 0.64 nm for Mg₃Al-LDH-P. In the second phase, the basal peaks are marked in the diffractogram as ●(003), ●(006), and ●(009). The basal spacing values found for this phase were 0.92 nm for Mg₂Al-LDH-P and 0.81 nm for Mg₃Al-LDH-P. The obtained results for C, N, and P quantifications are presented in Table 2. After the ionic exchange reaction, the LDHs presented significant amounts of N and C. As expected, the presence of N suggests the exchange reaction between nitrate by phosphate anions was incomplete. The presence of CO₃ ^2- is due to the contamination by atmospheric CO₂. Even when all reactions (coprecipitation and exchange) are conducted in an inert environment (N₂), handling during washing, drying and analyses exposes LDHs to the air. Due to the high basicity presented by these materials, contact with atmospheric CO₂ causes them to be absorbed and incorporated to LDH in the CO₃ ^2- form⁴⁹. Thus, there may be co-intercalation of NO₃ ^-, phosphate, and CO₃ ^2- anions in LDHs subjected to anionic exchange reactions.

Figure 2
Diffractograms of LDHs after exchange reactions: Mg₂Al-LDH-P, Mg₃Al-LDH-P, Mg₂Fe-LDH-P, Mg₃Fe-LDH-P, Mg₂Fe_0.5Al_0.5-LDH-P. (○) Phase one^-; (●) Phase two; ★ K₂HPO₄

In the literature, the basal spacing values found for LDHs intercalated with NO₃ ^- are 0.87 nm^49,50 and 0.89 nm⁵¹. For LDHs intercalated with CO₃ ^2-, the values are 0.76 nm^49,52 and 0.75 nm⁵³. LDHs intercalated with phosphate have very different basal space values: 0.84 nm⁵⁴, and 0.78 nm, 0.80 nm, 1.19 nm⁵⁵ and 1.11 nm¹². These different basal space values for the intercalation of phosphate are attributed to the different positions adopted by these anions between the inorganic layers (perpendicular, inclined and planar). Factors such as the charge density of the layers, hydration state and synthesis methods used can affect the position of the anions between the layers.

For Mg₂Fe-LDH-P and Mg₃Fe-LDH-P, the calculated basal spacing values were 0.79 nm and 0.78 nm. For these LDHs, the amount of P found in relation to nitrogen (Table 2) is significant, which suggests phosphate ions can also be intercalated. For the MgAl-HPO₄ system, Frost et al.⁵⁶ found basal spacing values between 0.78 nm and 1.19 nm. For the Mg-Fe system, Triantafyllidis et al.³⁹ found basal spacing values around 0.77 nm.

3.3 Fourier transform infrared spectroscopy with accessory of attenuated total reflectance (FTIR-ATR)

FTIR-ATR spectra for pristine LDHs are presented in Figure 3. All spectra have a wide band in the 3400 cm^-1 region. The presence of these bands is attributed to -OH stretching of H₂O molecules and to hydroxyl groups present in the layers. The spectrum also presents a strong band in the 1763 cm^-1 region, referring to C=O bonds, which allows inferring on the presence of CO₃ ^2- in LDHs.

Figure 3
FTIR-ATR spectra of the pristine LDHs: Mg₂Al-LDH, Mg₃Al-LDH, Mg₂Fe-LDH, Mg₃Fe-LDH, Mg₂Fe_0.5Al_0.5-LDH

The spectra for the pristine LDHs also show the presence of bands in the 1350 cm^-1 region, attributed to vibration modes (ν3) of NO₃ ^-. In LDH, it is possible to find two types of nitrate, with bands in the 836 cm^-1, 1380 cm^-1 and 1763 cm^-1 regions that present “free” NO₃ ^- and in the 825 cm^-1, 840 cm^-1 1050 cm^-1 and 1425 cm^-1 regions that represent “non-free” NO₃ ^{- (48,57}. Figure 4 shows the spectra for LDHs after the exchange reaction. These spectra also present wide bands in the 3400 cm^-1 region, characteristic of the presence of -OH groups. The presence of phosphate can be evidenced by the bands in the 1050 cm^-1, 870 cm^-1 and 550 cm^-1 regions, which correspond to ν3(PO₄ ^3-); ν1(PO₄ ^3-) and ν4(PO₄ ^3-) vibrations⁵⁸. The band in the 1350 cm^-1 region can be attributed to the vibration modes of NO₃ ^- or CO₃ ^2-. In the literature, several works have reported the overlap of CO₃ ^2- and NO₃ ^- bands in the region ranging from 1350 to 1400 cm^(-1) in LDHs^59-63.

Figure 4
FTIR-ATR spectra of LDHs after anionic exchange reaction: Mg₂Al-LDH-P, Mg₃Al-LDH-P, Mg₂Fe-LDH-P, Mg₃Fe-LDH-P, Mg₂Fe_0.5Al_0.5-LDH-P

3.4 Raman Spectroscopy

Figure 5 presents the Raman spectra for pristine LDHs. The spectra show bands in the 470 cm^-1 region, which are attributed to the vibration of connections Al-O-Al and Fe-O-Fe present in the layers of the MgAl-LDH, MgFe-LDH and MgFeAl-LDH systems.

Figure 5
Raman spectra of pristine LDHs : (I) 400 - 750 cm^-1 region and (II) 950 - 1150 cm^-1 region for (a) Mg₂Al-LDH, (b) Mg₃Al-LDH, (c) Mg₂Fe-LDH, (d) Mg₃Fe-LDH, (e) Mg₂Fe_0.5Al_0.5-LDH. In the band index, X refers to Fe or Al, depending on the sample. The AI bands are associated to the anion intercalated vibrations

In the 550 cm^-1 region, the bands refer to the signature octahedral structure of brucite-type layers, i.e. Al-O-Mg or Fe-O-Mg. The 710 cm^-1 band is specifically attributed to NO₃ ^- ion, while in the 1050 cm^-1 region, it corresponds to the anion intercalated (independent of the type of anion) in LDH like materials^64-69.

Figure 6 shows the Raman spectra for LDHs subjected to anionic exchange reaction. In these spectra, it is possible to see the permanence of bands in the 470 cm^-1 and 550 cm^-1 regions, corresponding to the lamellar structure of LDHs. Except for LDH-5P, the band is not present in the 710 cm^-1 region, referring to NO₃ ^-. The presence of phosphate is evidenced by the emergence of bands in the region between 940 cm^-1 and 960 cm^-167. Slight changes can also be observed in the positions of the bands of the materials subjected to the exchange reaction in relation to pristine LDHs, such as in bands located in the 1050 cm^-1 region. These results are in line with those presented by Palmer et al.⁷⁰, which show the form and position of Raman bands depending on the nature and concentration of the intercalated anions in LDHs.

Figure 6
Raman spectra of LDHs after exchange reaction: (I) 400 - 750 cm^-1 region and (II) 900 - 1150 cm^-1 region for (a) Mg₂Al-LDH-P, (b) Mg₃Al-LDH-P, (c) Mg₂Fe-LDH-P, (d) Mg₃Fe-LDH-P, (e) Mg₂Fe_0.5Al_0.5-LDH-P. In the band index, X refers to Fe or Al, depending on the sample. The AI bands are associated to the anion intercalated vibrations.

3.5 Specific Surface Area (BET method)

Table 4 presents the measurements of specific surface area, pore volume and mean pore diameter for pristine LDHs and LDHs-P. The increased molar ratio between M²⁺/M³⁺ cations present in the layers reduced the specific surface area, pore volume and mean pore diameter. For MgAl-LDHs, the reduction was 1.44 times the specific surface area and 1.50 times the pore volume. For MgFe-LDHs, both specific surface area and pore volume were reduced by 1.23.

Table 4
Specific surface area and pore volume of different LDHs

Pristine LDHs	S.S.A.*	P.V.**	LDHs-P	S.S.A.*	P.V.**
	(m²/g)	cm³/g		m²/g	cm³/g
Mg2Al-LDH	76.0	2.64	Mg2Al-LDH-P	19.7	7.69
Mg3Al-LDH	52.6	1.76	Mg3Al-LDH-P	10.0	3.57
Mg2Fe-LDH	189.0	76.5	Mg2Fe-LDH-P	61.2	24.4
Mg3Fe-LDH	153.0	62.0	Mg3Fe-LDH-P	58.4	23.2
Mg2Fe0.5Al0.5-LDH	47.9	1.93	Mg2Fe0.5Al0.5-LDH-P	12.5	4.89

• ^*
  S.S.A = Specific Surface Area;
• ^**
  P.V. = Pore Volume

The specific surface area values found in the literature for LDHs generally range from 50 to 80 m² g^-1. Some works however found values of 200 m² g^-171,72. The reduction of porosity and specific surface area with increased molar ratio M²⁺/M³⁺ in LDHs was previously reported by Jaynes and Vance⁷³, Di Cosimo et al.⁷⁴, You et al.⁷⁵, and Cantrell et al.⁵². The increased molar ratio reduces the charge density of the layers, and consequently, the specific surface area of these materials. For all LDHs, the sorption and desorption N₂ isotherms (presented in supplementary material) are type II, according to IUPAC (International Union of Pure Applied Chemistry), with close hysteresis, typical of mesoporous materials (pore size between 2-50 nm)⁷⁶.

3.6 Kinetics of P Release

LDHs were subjected to release tests of P in H₂O. The pH values of the suspensions before the stirred-flow experiments were Mg₂Al-LDH-P = 7.85; Mg₃Al-LDH-P = 8.15; Mg₂Fe-LDH-P = 8.37; Mg₃Fe-LDH-P = 9.01, and Mg₂Fe_0.5Al_0.5-LDH = 8.81. The cumulative rate releases and P release profiles in solution for Mg₂Al-LDH-P and Mg₃Al-LDH-P over time are presented in Figure 7. For all LDHs studied, the curves that represent the profile of P release in solution are represented by two distinct and complementary moments: (i) fast release and (ii) slow release. The initial fast release is linked with phosphate adsorbed in the surface of LDHs crystallites, which are promptly exchanged. Then, the release slows because of the exchange reaction of the intercalated phosphate with the anions present on reaction medium.

Figure 7
P release in solution of Mg₂Al-LDH-P and Mg₃Al-LDH-P in relation to the time. (a) release profile, (b) cumulative rate release.

The P profiles release and cumulative release rates for Mg₂Al-LDH-P and Mg₃Al-LDH-P are shown in Figure 7. Mg₂Al-LDH-P showed better adjustment to the Hoerl model, as did Mg₃Al-LDH-P to Vapor Pressure model. The adjustments were done using the equations obtained for each model. The turning point was also estimated, where the release rate of P becomes descendant and slower. In Mg₂Al-LDH-P, this change occurs at 22 min, while for Mg₃Al-LDH-P, this point is reached more rapidly, at 12 min. The cumulative rate of the two LDHs shows that Mg₂Al-LDH-P releases P more slowly than Mg₃Al-LDH-P. After 150 min of flow, Mg₂Al-LDH-P released approximately 60% of its total P content, while Mg₃Al-LDH-P released around 83% for the same amount of time.

Figure 8 shows the release profiles of P and cumulative release rates for Mg₂Fe-LDH-P and Mg₃Fe-LDH-P. The release profiles of P for Mg₂Fe-LDH-P and Mg₃Fe-LDH-P were adjusted for the Vapor Pressure and Hoerl modified models. The turning point was reached at 11 min for Mg₂Fe-LDH-P and 8 min for Mg₃Fe-LDH-P. Despite reaching the change point more rapidly, Mg₃Fe-LDH-P presents lower declivity than Mg₂Fe-LDH-P, with a longer decline phase and consequently a slower release. The cumulative release rate of Mg₂Fe-LDH-Pand Mg₃Fe-LDH-P corroborates that Mg₃Fe-LDH-P has a slower release. At the end of 150 min of flow, Mg₂Fe-LDH-P had released around 93% of its total content of P, while Mg₃Fe-LDH-P released approximately 84%.

Figure 8
P release in Mg₂Fe-LDH-P and Mg₃Fe-LDH-P solutions in relation to time. (a) release profile, (b) cumulative release rate.

Figure 9 presents the release profile of P and the cumulative release rate for Mg₂Fe_0.5Al_0.5-LDH-P. Mg₂Fe_0.5Al_0.5-LDH-P had the same release profile with two distinct and complementary moments, as well as the previous ones. The Vapor Pressure model best represents the release profile of this LDH. It took Mg₂Fe_0.5Al_0.5-LDH-P 16 min to reach the maximum point of release. Compared to the other materials however, Mg₂Fe_0.5Al_0.5-LDH-P has a faster P release after entering the phase of descending release. The cumulative release rate shows that only Mg₂Fe_0.5Al_0.5-LDH-Pcompletely released its full amount of P. The cumulative release reached approximately 100% after 130 min of flow.

Figure 9
P release in Mg₂Fe_0.5Al_0.5-LDH solution in relation to time. (a) release profile. (b) cumulative release rate.

The Hoerl and Vapor Pressure models are variations of the exponential model. These models are extremely flexible and can be used in cases where other non-linear models fail. Due to the complexity of the medium and the number of variables involved, the Hoerl model has been used in studies to evaluate plants nutrition experiments with sufficient accuracy⁷⁷.

Comparatively, among the five LDHs-P studied, the Mg₂Al-LDH-P obtained better results from its release profile and cumulative release rate. The highest amount of P release in all LDHs-P occurred by exchange reactions. Once LDHs-P are in contact with water, they dissociate part of the hydroxyls from the layers¹⁵ thus increasing the pH value of the suspensions. The pH values of the suspensions after the stirred-flow experiments were Mg₂Al-LDH-P = 8.62; Mg₃Al-LDH-P = 9.15; Mg₂Fe-LDH-P = 9.92; Mg₃Fe-LDH-P = 9.88, and Mg₂Fe_0.5Al_0.5-LDH-P = 9.03. Under these pH values, atmospheric CO₂ dissolved in the LDH suspension and reacted with H₂O, producing CO₃ ^2-37. The CO₃ ^2- formed in suspension provides the exchange with phosphate intercalated in LDHs.

For Silva et al.¹⁵, the differences in release rates are controlled by factors such as layer components, charge density of the layers, and size of crystallites. The different compositions and M²⁺/M³⁺ molar ratios presented in this work also evidenced these characteristics. Mg₂Fe_0.5Al_0.5-LDH-P with three different cations in the layers promoted the formation of a material with higher structural disorder and lower crystallinity, as evidenced by PXRD measurements. The low crystallinity presented by this material provided a fast release rate compared to the other LDHs-P with better structural organization.

MgAl-LDHs-P released less P than the compounds with Fe³⁺, which demonstrates that Al³⁺ retained phosphate in its structure with a stronger connection and thus lower possibility of displacing this anion. For the MgFe-LDHs-P, the amount of P initially released (fast moment) was higher than the amounts released in the samples containing Al³⁺. This relationship suggests LDHs synthesized with Al³⁺ ions have better P retention capacity and that the release occurs gradually.

As suggested by Silva et al. ¹⁵, in addition to the physical protection of anions provided by intercalation, LDHs also contain a chemical protection called the “interlayer ionic barrier”. Both are responsible for the gradual release of intercalated anions. These authors conducted a slow release study of NO₃ ^- intercalated in MgAl-LDHs and MgFe-LDHs. A solution of NaHCO₃ was used for the release test. According to the authors, the exchange of NO₃ ^- intercalated in LDHs by the CO₃ ^2- of the solution starts in the edges of the layers, where the newly exchanged CO₃ ^2- anions form a barrier that hinders the exit of NO₃ ^- located in the internal region of the layers and prevents its fast release.

The results obtained in this work demonstrate that LDHs are materials that can resolve problems related with P losses in tropical soils, due to the protection and gradual release of the phosphate ions they contain. With regard to P recycling, LDHs are also potential materials because they can incorporate phosphate anions from aqueous solutions and then be used in the soil as sources of P. There are several reports in the literature about the use of LDH for P removal from sewage sludge, in residual water and even in ocean water^32,78-81.

4. Conclusions

The results presented in this work showed that by using the co-precipitation method followed by the ionic exchange method in solution it was possible to synthesize LDH containing phosphate ions in different systems, with different types of cations and different molar ratios (M²⁺:M³⁺) between them (Mg₂Al, Mg₃Al, Mg₂Fe, Mg₃Fe and MgFeAl). In addition to significant amounts of P, the ionic exchange reaction produced materials with amounts of N and C. Among the LDHs obtained, Mg₂Al-LDH-P presented the highest amount of P. The contents of P₂O₅, extracted with H₂O, 2% citric acid and CNA + H₂O extractors, showed that LDHs-P have similar characteristics to reactive sources of P, such as commercial thermophosphates. In studies on the kinetics of P release, LDHs-P showed a gradual P release profile and Mg₂Al-LDH-P presented a slower release than the other materials. Likewise, LDHs presented structural and chemical characteristics that make them potential materials for use as storage matrices for slow P release. New studies and tests with plants and soils (bioassays) are being developed by our research group to try to validate this new LDH technology for use as a source of P. The development of a unique material able to solve all worldwide problems related to P in agriculture is not an easy task and may even be unfeasible. LDHs have characteristics however that make them potential materials for solving one or more aspects of this problem.

5. Acknowledgments

This study was supported by the Fundação de Amparo à Pesquisa do Estado de Minas Gerais - FAPEMIG, Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq, and Rede Mineira de Química (RQ-MG).

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Publication Dates

• Publication in this collection
  27 Aug 2018
• Date of issue
  2018

History

• Received
  11 Nov 2017
• Reviewed
  05 July 2018
• Accepted
  05 Aug 2018