# In-depth characterization of phosphate intercalated Mg Al Layered double hydroxides and study of the PO 4 release properties

Alexandra Jourdain, Christine Taviot-Gueho, Ulla Gro Nielsen, Vanessa Prévot, Claude Forano

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Alexandra Jourdain, Christine Taviot-Gueho, Ulla Gro Nielsen, Vanessa Prévot, Claude Forano. In-depth characterization of phosphate intercalated Mg Al Layered double hydroxides and study of the PO 4 release properties. Dalton Transactions, 2024, 53 (22), pp.9568-9577. 10.1039/D4DT00601A. hal-04750892

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Submitted on 23 Oct 2024

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# In-depth characterization of phosphate intercalated Mg Al Layered double hydroxides to better understand the PO4 release properties

Alexandra Jourdain,ª Christine Taviot-Gueho,ª Ulla Gro Nielsen,ᵇ Vanessa Prévotª* and Claude Foranoª*

Slow-release fertilizers (SRFs) form the core of innovative strategies in sustainable agriculture. Layered Double Hydroxides (LDH), known for their high capacity to sequester plant nutrients, especially phosphate, are emerging as promising candidates for SRF synthesis. The phosphate release properties of MgAl LDH (with a targeted Mg/Al ratio of 2.0) intercalated with $HPO_4^{2-}$ anions were assessed in various aqueous environments. A comprehensive analysis, including in-depth chemical and structural characterizations (ICP-OES, XRD, PDF, 27Al NMR, 31P NMR, FTIR, SEM) of the as-prepared phase unveiled a more intricate composition than anticipated for a pure or ideal $Mg_2Al-HPO_4$ LDH, encompassing an excess of intercalated phosphate in conjunction with $K^+$. Beyond the intercalated phosphate, $ ^{31}P $ NMR speciation identified multiple $HPO_4^{2-}$ environments, indicating a portion of the phosphate reacting with intralayer $Mg^{2+}$ to form K-struvite. Additionally, some phosphates were adsorbed onto the surface of amorphous aluminum hydroxide, a side phase formed during MgAl coprecipitation. The phosphate release demonstrated rapid kinetics, occurring within 6 days. Moreover, the released phosphate increased significantly when reducing the Solid/Liquid (S/L) ratio (58%) and further increasing in the presence of carbonate ions (90%). The released phosphate varied from 11.8% to 90% under different release conditions, transitioning from water to a 3.33 mM NaHCO3 aqueous solution at a low S/L ratio (from 20 mg LDH/mL to 0.02 mg LDH/L). The simultaneous release of $K^+$, $Mg^{2+}$, $Al^{3+}$ cations indicated the complete dissolution of the K-struvite and partial dissolution of phosphate intercalated MgAl LDH. These results enhanced our understanding of the mechanism governing phosphate release from MgAl LDH, paving the way for potential phosphate recovery by LDH or for the development of LDH-based SRFs.

## 1. Introduction

Phosphorus is an essential nutrient for plant growth and more than 80% of P extracted from phosphate rocks is used as a fertilizer combined with N and K. However, an excessive administration is necessary to ensure the minimum dose to reach the plant's roots, thus causing a major dissemination and a loss in soils through lixiviation, irreversible precipitation, complexation or adsorption to mineral and organic matter.¹, ² As a result, chemical systems that can deliver the right dose of nutrients to the plant under kinetic control, have become widely known-as slow-release fertilizers (SRFs) ³, ⁴.

Most commercially available P fertilizers are based on highly soluble phosphates, embedded or coated with a natural or synthetic polymer matrix that dissolves under a controlled regime ⁵, ⁶. However, most of these polymers are not biodegradable, thereby increasing the microplastic pollution. Alternatively, phosphate minerals with low solubility, such as struvite, have been commercialized. Struvite precipitation from wastewaters with high phosphorus content, is a way to recover orthophosphate as a recycled N-P-fertilizer ⁷, ⁸ but this method requires a supply of $Mg^{2+}$.

Loading phosphate onto a mineral matrix is a third strategy for preparing SRFs. Among the mineral matrices that have been tested Magnesium oxide ⁹ and hydroxide ¹⁰, ferrihydrite ¹¹, ¹², clay minerals ¹³⁻¹⁵), Layered Double Hydroxides (LDH) display an intrinsic structural affinity for phosphate due to their unique anion exchange properties. LDH ($M^{2+}_{1-x}M^{3+}_x(OH)_2X_x.nH_2O$, abbreviated as $M^{2+}_R M^{3+}-X$ with $R=(1-x)/x$), are layered materials based on the stacking of positive layers

$[M^{2+}_{1-x}M^{3+}_x(OH)_2]^{x+}$, made of edge-sharing octahedra of divalent and trivalent metal cations, intercalated with anions and water molecules [$X^{q-x}.nH_2O$]. Their high anion exchange capacities (a.e.c.) allow to reversibly load in between 80-130 mg $PO_4^{3-}$ per gram of LDH (i.e. 26-42 mg P/g) depending on the $M^{2+}_R M^{3+}$ chemical composition and the intercalated orthophosphate species ($H_2PO_4^-$, $HPO_4^{2-}$ and $PO_4^{3-}$) (Table S1). Some studies of orthophosphate loading on LDH reported even higher P loading values through excess surface adsorption exceeding the theoretical a.e.c. ¹⁶.

According to the chemical affinity of LDH/anions series, $CO_3^{2-} > HPO_4^{2-} > SO_4^{2-} > OH^- > F^- > Cl^- > NO_3^{-} ¹⁶$, orthophosphates display stronger interactions than most of the inorganic anions.

Phosphate release can be obtained by back-exchange of phosphate under favorable physico-chemical conditions such as high concentration of competing [$A^{x+}$] anion. While a great number of studies reported the adsorption or removal of orthophosphate by LDH for recovery or environmental depollution purposes ¹⁷, few studies investigated the release of phosphate by LDH ¹⁵, ¹⁸⁻²² and evaluated their efficiency as P fertilizers ²³⁻³⁰.

Some of these studies investigated phosphate release kinetics under model conditions, using water or carbonate, nitrate, chloride or hydroxide aqueous solutions, often far from environmental conditions ³¹. Both anion exchange reaction and LDH dissolution ²² contribute to the mechanism of release. Interestingly, P release time was ten times slower with $Mg_2Al-HPO_4^{2-}$ compared with $KH_2PO_4$ or monoammonium phosphate (MAP) ²⁵.
In most plant growth trials using LDH as P fertilizers, LDH-PO₄ behave as slow-release fertilizers, optimizing the P bioavailability according to the plant needs ²⁰, ²⁹. The agronomic efficiency of Mg₂Al-HPO₄²⁻ was also confirmed for wheat (Triticum aestivum) ²⁵ for which biomass production after 30 days of growth in tropical oxisoil was similar to that for wheat growth under KH₂PO₄ and MAP. Agrosystems treated with LDH revealed a higher phosphate content in the soil for a longer time, persisting even after rainfall ²⁵, ³². These recent studies highlight the interest of LDH-PO₄ as slow release fertilizers. However, a better understanding of their controlled-release properties based on Phosphate/LDH interactions could help to increase their efficiency. More specifically, phosphorus release by LDH-PO₄ appears to be highly dependent on pristine LDH materials and on physico-chemical conditions used. In this study, various release conditions in solution (such as pH, solid/liquid ratio, release solution) were explored in order to kinetically control the leaching of P by Mg₂Al-HPO₄²⁻ and link PO₄ release to in-deep characterization of the structure of pristine LDH.

## 2. Experimental methods

### 2.1 Chemicals

MgCl₂·6H₂O, AlCl₃·6H₂O, K₂HPO₄ and NaOH used for the material synthesis were of reagent grade and purchased from Sigma-Aldrich.

### 2.2 Samples preparation

**Mg₂AlCl-LDH synthesis.** A Mg₂AlCl LDH with a targeted Mg(II)/Al(III) molar ratio of 2:1, was synthesized by coprecipitation at a constant pH=10.0 following the procedure described in a previous work ³³, except that synthesis was scaled up to prepare about 500 g of LDH in one batch (supporting information S1). This large quantity was produced for crops trials.

**Preparation of phosphate-loaded Mg₂Al LDH by anion exchange reaction.** As described in a previous work, ³⁴. 800 mL of K₂HPO₄ solution were added to a suspension of 200 g of powdered Mg₂AlCl LDH in 2.2 L deionized water (molar ratio HPO₄²⁻/Al³⁺ = 1.0, pH = 9.0) and vigorously stirred during 6 h under N₂ atmosphere. The solid phases was centrifuged and washed thrice with deionized water. The solids were dried at 40°C overnight—and milled to obtain a fine powder. The exchanged Mg₂AlCl LDH will be referred as Mg₂AlP.

### 2.3 Characterization techniques

XRD data were collected on a Philips X'Pert pro diffractometer equipped with a X'celerator 1D detector (2.122° active length), using CuKα1/ Kα2 source in Bragg Brentano θ-θ geometry from 5 to 90°(2θ) with a scan step of 0.016°. DIFFAX software was used to simulate the Mg₂AlP XRD pattern for the identification of XRD data. The simulations of XRD patterns based on the unit cell parameters and the space group were performed using the Le Bail method ³⁵ with FullProf suite package ³⁶. The atomic Pair Distribution Function (PDF) were obtained from X-ray total scattering data collected on a PANalytical Empyrean

diffractometer equipped with a solid state GaliPIX3D detector, a focusing X-ray multilayer mirror, and an Ag anticathode (Kα₁ = 0.5594214 Å, Kα₂ = 0.5638120 Å). PDF analysis protocol and results are described in Supporting Information.

The ATR-FTIR spectra were recorded with a Thermo Nicolet 5700 spectrometer (Thermo Fisher Scientific, USA). Thermogravimetric analysis (TGA) were obtained with a Setaram 92-16.16 instruments (France), at a heating rate of 5°C·min⁻¹ under air atmosphere. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) images were recorded using respectively a JSM-7500F Field Emission Scanning Electron Microscope operating at an acceleration voltage of 3 kV, and a JEOL JEM2100Plus microscope operating at 200 kV. C, H, N, S elemental analyses were realized using a CHNS/O Thermo Scientific FLASH SMART (Thermo Fisher Scientific, Germany). Solid state ²⁷Al and ³¹P MAS NMR spectra were recorded at 11.7 T on a JEOL ECZ500CR 500 MHz NMR spectrometer (Japan) using a 3.2 mm double resonance MAS NMR probe with 14 kHz spinning speed. The quantitative ²⁷Al and ³¹P and NMR spectra were referenced to a 1.0 M solution of AlCl₃ (δ<sub>iso</sub>(²⁷Al) = 0 ppm) and concentrated phosphoric acid (85% H₃PO₄; δ<sub>iso</sub>(³¹P) = 0 ppm). Analysis of the spectra were performed using ssNake software ³⁷.

LDH matrices composition and supernatants concentration of Mg, Al, P and K were determined by ICP-OES using ICP-OES 5800 Agilent (USA). The solid samples were solubilized using the mineralization protocol described in Supporting information. For release experiments, quantification of phosphate in solution was performed by colorimetry dosage with the phosphoantimonyl-molybdenum blue complex method (λ = 880 nm) using a TECAN-Magellan robot (EVO® Tecan (Switzerland) platform with robotic arm, 4 needles (capacity 5 to 950 µL)), microplate shaker, cryostat) with a Safire II® Tecan 96-wells microplate reader. All the experiments were performed in triplicate.

### 2.4 Experiments and modelling of release kinetics

Phosphate release kinetic experiments—were carried using 20 mg of Mg₂AlP. Each of the tubes was filled with deionized water (pH = 8.0 ; V = 2.0 mL or 50.0 mL). The tubes were shaken for 9 different periods of time (t = 1, 2, 3, 6, 10, 15, 20, 25 and 30 days) under air atmosphere then solids and supernatants were separated by centrifugation. The solid residues were dried at 40 °C for further characterizations. The release percentage is defined as the ratio of the desorbed P relative to the P content loaded in the parent LDH. Release experiments were repeated 3 times. For each phosphate release curve, the data were modelled using the common kinetic models (Table S2).

*Release as a function of the solid/liquid ratio [S/L].* The release amount was measured as a function of the solid (mg LDH)/liquid (volume of deionized water) ratio ([S/L] = 0,02 to 20 mg/mL). Mg₂AlP suspensions were shaken at 21°C on a planetary orbital shaker during 6 days. Supernatants and solids were recovered and analyzed.
**Release as a function of the pH.** In 50 mL tubes, 20 mg of P-LDH were introduced followed by 50 mL of aqueous solution at different pH (2.0, 4.0, 6.0, 8.0, 10.0 and 12.0) adjusted with diluted HCl or NaOH solution. Samples were shaken at 21°C on a planetary orbital shaker during 6 days. Supernatants and solids were recovered and analyzed.

**Release as a function of the carbonated environment and soil juice.** The experiment on L/S ratio effect was repeated in a 3.33 mM NaHCO₃ solution (pH = 8.4). These conditions were previously used to simulate carbonate content in soil.²⁵ Another similar experiment was carried out in soil juice. Soil juice was extracted following a reported procedure (see Supporting information) and the chemical composition is reported in Table S3.

# 3. Results and discussion

## 3.1 Deep insight of Mg₂AlP structure.

Orthophosphate loading was carried out using an anion exchange reaction at pH = 9.0. At this pH, HPO₄²⁻ is the predominant orthophosphate species in solution (>98%). Analyses performed on Mg₂AlCl by ICP-OES (Table 1) confirms the precipitation of Mg²⁺ and Al³⁺ metal cations from the precursor solution in the expected ratio. The Mg/Al ratio is preserved after the anion exchange reaction with phosphate as expected for a pure topochemical reaction.

<table><caption>Table 1. Chemical analyses of Mg₂AlCl and Mg₂AlP LDH.</caption><thead><tr><th>(Mg/Al)<br>expected</th><th>Molar<br>ratio</th><th>(Mg/Al)<sub>ICP</sub></th><th>(P/Al)<br>expected</th><th>(P/Al)<sub>ICP</sub></th><th>K/<br>Al</th><th>(H<sub>2</sub>O)<sub>TGA</sub><br>(245°C)</th></tr></thead><tbody><tr><td>2.00</td><td>Mg<sub>2</sub>AlCl</td><td>2.14</td><td>-</td><td>-</td><td>-</td><td>14.6 %</td></tr><tr><td>2.00</td><td>Mg<sub>2</sub>AlP</td><td>2.13</td><td>0.50</td><td>0.64</td><td>0.1<br>7</td><td>19.8 %</td></tr></tbody></table>

Despite a thorough washing of Mg₂AlP, an excess of phosphorus (28 %) compared with the expected amount from a.e.c. was measured in Mg₂AlP, jointly with K⁺ cation (K/Al = 0.17). On the other hand, the hydration content measured by TGA at T=245°C (adsorbed and intercalated water molecules) (Table 1) was higher for Mg₂AlP than for Mg₂AlCl. These results indicate that the Mg₂AlP sample contains not only phosphate ions but also hydrated potassium ions, as described in the case of motukoreaite-type phase containing sodium/sulfate ions (Mg₆Al₃(OH)₁₈[Na(H₂O)₆][SO₄]₂·6H₂O)³⁸. Consistently with the formation of a layered matrix, SEM images showed a comparable platelet-like morphology before and after phosphate intercalation. The preservation of particles morphology during anion exchange is also in good agreement with a topochemical reaction (Figure S1).

From structural point of view, the diffractogram of the precursor Mg₂AlCl (Figure S2) can be indexed in a three-layer rhombohedral 3R₁ polytype (01/ reflection with l = 2,5,8) typical of stacked [Mg₁ₓAlₓ(OH)₂]⁺ LDH layers ³⁹—although

broad character of 01/ reflections indicates intergrowth with 2H₁ polytypes ⁴⁰.

After anion exchange with phosphate, the basal spacing deduced from the position of the first reflection on the XRD patterns expands from 7.66 Å (Mg₂AlCl) to 9.30 Å (Mg₂AlP) while the position of the 110 reflection related to the distance between cations within [Mg₁ₓAlₓ(OH)₂]⁺ layer remained unaffected (Figure 1 A). These values are similar to those reported by Benicio et al.²⁹

It should be emphasized that various basal distances ranging from ~8.9 Å to ~11.0 Å have been reported in the literature for phosphate-intercalated LDH based on XRD data. As can be seen in figure 1A, in the present case, it was possible to explain all the reflections observed in particular those located at low angles ( 2θ< 30°) by considering the formation of 1H polytype with a large unit cell of a= 9.138 Å (3*a₀*) and c= 9.31 Å in the space group P-3. The formation of 1H polytype is not surprising since polytypes with octahedral interlayer sites are commonly observed with non-planar oxyanions.⁴¹ The strong similarities between phosphate and sulfate anions in terms of size and geometry (thermochemical radii r(SO₄²⁻)=258 pm and r(PO₄³⁻)=238 pm)⁴² leads us to compare our results to that reported for LDH phases intercalated by sulfate SO₄²⁻, widely studied in the literature. In one hand, LDH-SO₄ phases exhibit different basal spacings and space groups. Mg₂Al-SO₄ LDH reported by V. Constantino et al.⁴³ and E. Alvarez-Ayuso et al.⁴⁴ display basal spacings respectively equal to 8.80 Å and 8.62 Å, similar to the zincowoodwardite Zn₂Al(OH)₆(SO₄)₁/₂·nH₂O mineral, with space groups P-3 and R-3m with a *d*<sub>(003)</sub> distance between 8.5 and 8.9 Å. On the other hand, the mineral K-motukoreaite with chemical composition [Mg₆Al₃(OH)₁₈][SO₄)₂K].12H₂O displays a basal spacing of 10.87 Å.³⁸,⁴⁵ In this phase, the interlayer domains are composed of [K(SO₄)₂(H₂O)₁₂]³⁻ with the intercalated alkali metal cations coordinated by six water molecules in an octahedral arrangement leading to a greater basal spacing (10.87Å).

Recently, the same authors have reported the formation of shigaite-like LDH [Mn₀.₆₆₇Al₀.₃₃₃(OH)₂][HPO₄)₀.₂₂₂K₀.₁₁₁].⁴⁶ In the latter example, excess of phosphate anions are intercalated along with potassium cations in the manner of ion pair resulting in a strong positional and orientational ordering of interlayer species and the formation of superstructure in the xy plane. Structural analogy with Mg₂AlP clearly indicates that we have formed a similar phase i.e. co-intercalated with phosphate anions and K⁺ cations, and with an ordering of the interlayer species introducing a tripling of a parameter (3*a₀*). From the value of a₀ (3.046 Å) which refers to the parameter of the sub-cell, one can estimate the Mg/Al ratio in the crystalline part of the sample which is close to 2.8.⁴⁷,⁴⁸

This result differs from the bulk ratio value determined by ICP-OES (Table 1) and suggests the presence of aluminum-rich secondary phase, and lower theoretical a.e.c..
<figure><img src="image_2.png" /><figcaption>Fig. 1. Powder X-ray diffraction pattern (experimental data (red cross), simulated (solid black line) diagram based on the unit cell parameters a= 9.138 Å and c= 9.31 Å in P-3 space group, Bragg reflections (green ticks), and FTIR spectrum (A) and Mg2AlP (B) of</figcaption></figure>

To further investigate the structure of Mg₂AlP sample, the local order was examined by analyzing the pair distribution function (PDF) extracted from X-ray total scattering data. The experimental PDF at low r values (<10 Å) are given in Figure S3. The comparison with Mg₂AlCO₃ (from a previous unpublished study) and Mg₂AlCl as reference samples clearly shows that phosphate intercalation leads to significant changes on the PDF of Mg₂AlP. The distance range examined being limited to the interlayer distance, the peaks observed are mainly from distances within the hydroxide layer plus one interlayer. Most often, in the case of LDH, because of the low contribution of the interlayer species (weak X-ray scattering power and static / dynamic disorder), PDF peaks only reflect the distribution of the distance within the hydroxide layer. In the present case, we clearly observe a contribution of interlayer species which are added to M---OH/M—M pairs. The changes of intensity are observed for P2, P4 and P5 PDF peaks (Figure S3) and could be attributed to atomic pairs involving interlayer species, such as K⁺ and HPO₄²⁻/HPO₄²⁻ displaying a high scattering power and located right below Mg/Al cations as illustrated in Figure S4.

Such a greater degree of interlayer ordering is attributed to the partial co-intercalation of K⁺ and phosphate leading to strong electrostatic interactions with the hydroxide layers. This explains the interlayer distance and the appearance of supercell reflections in the XRD pattern. Thus, relying on chemical analysis results and XRD characterization and considering the presence of both HPO₄²⁻ and H₂PO₄⁻ species, it becomes possible to precisely compensate for the positive charge of the layers: [Mg₂.₈Al₁(OH)₇.₆] [(HPO₄)₀.₄₅(H₂PO₄)₀.₁₉K₀.₁₇,3.₂H₂O]. The strong interactions between the interlayer species and the hydroxylated [Mg₁ₓAlₓ(OH)₂]ₓ⁺ layers enhance the rigidity of the layers. Consequently, the degree of order of the (Mg, Al) (OH)₆ octahedra, is increased as shown by the highly resolved FTIR vibrational bands in the 400-1350 cm⁻¹ range (Figure 1B and Figure S5). Additionally, hydrogen bond network whose vibrations extend down to 2600 cm⁻¹, is more pronounced compared to Mg₂AlCl. Bands at 1078 cm⁻¹ (v₃) and 987 cm⁻¹ (v₁) correspond to the stretching vibration of the HPO₄²⁻ anion ⁴⁹. The intercalation of hydrogenophosphate is confirmed by the presence of two ¹H NMR signals (Figure S6) pointed at 1.1 and 3.6 ppm, in addition to the signal of intercalated and adsorbed H₂O and MOH (4.6 ppm) ⁵⁰. This broad signal is

attributed to fast proton exchange facilitated by hydrogen bonding between (Mg, Al)O-H and H-O-H.

The ²⁷Al NMR spectra of the chloride precursor (data not shown) and the exchanged phase (Figure 2A) were resolved with two contributions, Alᵢ and Alᵢᵢ. The Alᵢ resonance (78.6%, δᵢ{so} = 11.31 ppm, CQ = 1.6, η = 0) arises from Al octahedral sites in the LDH ¹⁶, ³⁴. The second resonance Alᵢᵢ (21.4%, δᵢ{so} = 9.54 ppm, CQ = 2.9, η = 0.5) is assigned to Al environment site in AlOH, amorphous aluminum hydroxide side phase, often formed during LDH fast coprecipitation ³⁴, ⁵¹.

The ³¹P solid state NMR provides additional details on the local environment of phosphate species loaded on Mg₂Al LDH. In data reported in the literature surface adsorption often predominates over intercalation, and when intercalation does occur the structural signature remains ambiguous. This complexity is reflected in the ³¹P NMR data whose spectra show the presence of several phosphate environments, surface adsorbed, exchangeable intercalated species or precipitated as poorly soluble mineral phases (struvite, bobierrite, cattiite, newberyite, hydroxyapatite).

<figure><img src="image_3.png" /><figcaption>Fig. 2: Experimental ²⁷Al SSNMR (A) and ³¹P SSNMR spectra (B) of Mg₂AlP along with simulated spectra showing the different components.</figcaption></figure>
<table><caption>Table 3: Lorentzian/Gaussian deconvolution of ³¹P NMR spectra of Mg₂AlP</caption><thead><tr><th>Site</th><th>Mg<sub>2</sub>AlP<br>δ<sub>iso</sub> (ppm)<br>(Intensity<br>%)</th><th>Lorentzian</th><th>Gaussian</th><th>Assignment</th></tr></thead><tbody><tr><td>A</td><td>8.2 (13)</td><td>20</td><td>566</td><td>Intercalated<br>HPO<sub>4</sub><sup>2-</sup> in<br>Mg<sub>2</sub>Al</td></tr><tr><td>B</td><td>5.8 (9)</td><td>500</td><td>0</td><td>HPO<sub>4</sub><sup>2-</sup> K-<br>Struvite</td></tr><tr><td>C</td><td>2.2 (35)</td><td>1000</td><td>300</td><td>HPO<sub>4</sub><sup>2-</sup><br>adsorbed<br>on Mg<sub>2</sub>Al</td></tr><tr><td>D</td><td>-3.3 (43)</td><td>2100</td><td>0</td><td>HPO<sub>4</sub><sup>2-</sup><br>adsorbed<br>on Al<sub>2</sub>OH</td></tr></tbody></table>

### 3.2 Phosphate release properties of Mg₂AlP.

**Effect of various solid/liquid ratio (S/L in mg/mL).** Variable solid/liquid ratios were used in deionized water (from S/L = 0.02 to 20 mg/mL), as 20 mg of Mg₂AlP in volume from 1 mL to 1000 mL. For S/L = 10.0 mg/mL (Figure 3). The desorption kinetic was recorded over a period of 30 days. After 30 days, 12.3 % of the phosphate was released by Mg₂AlP (i.e. 22.6 mgHPO₄²⁻/gMg₂AlP), 11.8 % during the first 6 days, and 0.5% more during the following 24 days. With a lower solid concentration (S/L = 0.4 mg/mL) (Figure 3) the delivery percentage increased to 35.5 % of total P (i.e. 65.2 mgHPO₄²⁻/gMg₂AlP).

The S/L ratio the percentage of phosphate release was multiplied by 2.86 from S/L 10 to 0.4 mg/mL. Whatever the S/L ratio used, kinetics of release are fast, more than 96% of the released phosphate are desorbed in 6 days. The shapes of the kinetic plots are similar. Modelling the desorption kinetic curves using various desorption or diffusion models (zero order, pseudo-first order, pseudo-second order, Elovich, liquid

film diffusion, intraparticle diffusion) has been widely used in literature to determine the rate constants and to discuss the mechanisms of desorption. Few attempts were done to model the kinetic plots of phosphate release by LDH matrices. Most papers concluded that either the pseudo-second order (Mg₃Fe-HPO₄²⁻) ³¹ or the modified Freundlich (Ca₃Fe-HPO₄²⁻) ²² models gave the best fits whatever the sorbent/sorbate system. These approaches appear often purely mathematical and physical significance of the results may be far from reality as mentioned in some critical papers ⁶⁰⁻⁶². Indeed, attempts to fit our data (S/L 10.0 and 0.4 mg/mL) with various models, employing both linear (Figure S7) and non-linear (Figure 3B) equations, led to similar conclusions. Rate constants were determined for pseudo-second order model to be equal to 0.18 days⁻¹ and 0.23 days⁻¹ for respectively S/L 10.0 and 0.4 mg/mL.

The kinetic study was extended to various S/L ratio (Figure 4A). Up to 58% of phosphate was released for the lowest S/L value 0.02 mg/mL (11.8% for the highest S/L=20 mg/mL). A significant variation that points out the major impact of S/L parameter on the phosphate release behaviour of LDH.

**Effect of release solutions.** P release was also evaluated for Mg₂AlP in contact with NaHCO₃ solution ([HCO₃⁻] = 3.33 mM, pH = 8.4) and soil juice in order to get closer to environmental conditions (Figure 4A). For each S/L ratio, P release amount decreases following the series HCO₃⁻ > H₂O > Soil juice, reaching maximum values at highly dilute concentration (S/L = 0.02 mg/mL), 89%, 58% and 49% respectively. As expected, carbonate anions force the release phosphate. An investigation of the role of carbonate speciation on the P release has highlighted the effect of the anion competition. P release was increased when using CO₃²⁻ solution was involved instead of HCO₃⁻ (71 % vs. 47 % and 61 % vs. 31 % at charge (HₓCO₃⁽²⁻⁾)/charge (HPO₄²⁻)) respectively equal to 25 and 5, with S/L=10mg/mL). These results highlight that exchange of HPO₄²⁻ with carbonate contributes as a part of the overall P release.

<figure><img src="image_4.png" /><figcaption>Fig. 3: A) Kinetic of phosphate release by Mg₂AlP at ■ S/L = 10.0 mg/mL ● S/L = 0.4 mg/mL and B) kinetic fitting using pseudo-second order (....), intraparticle diffusion (- - -), Elovich (....) models.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4: (A) Kinetic of phosphate release (%) by Mg₂AlP for various S/L ratio (21°C, 6 days contact time) in water solution (blue line), NaHCO₃ solution (red line) and soil solution (green line). (B) The percentage of K⁺, Mg²⁺, and Al³⁺ released from the Mg₂AlP-LDH in H₂O as a function of the S/L ratio after 6 days.</figcaption></figure>

**Mechanism of P release.** The concentrations of K, Mg and Al were determined by ICP-OES and solid residues were characterized by XRD and FTIR. K⁺ cations originally in Mg₂AlP interlayer galleries and K-struvite, were totally released for S/L ratio < 1 mg/mL (Figure 4B); 85% already at the highest S/L ratio = 20.0 mg/mL. Chemical stability of MgAl LDH is pH dependent ⁶³, the lower the pH the higher the dissolution.

Interestingly, the decrease in the S/L ratio also favors the solubilization of Mg₂AlP, associated with a progressive release of Mg²⁺ and Al³⁺ cations in solution (Figure 4B) up to 35 % or 20 % respectively for S/L = 0.02 mg/mL, while 0.6 % of Mg²⁺ and 0.05 % of Al³⁺ contents were dissolved, whereas for S/L=20.0 mg/mL. Mg²⁺ and Al³⁺ releases followed the same tendency that phosphorus release when Mg₂AlP was immersed in carbonate and soil solutions: HCO₃⁻ > H₂O > soil juice (Figure S8). Dissolution in soil juice is limited by the presence of non-negligible quantities of Mg²⁺, HₓPO₄(3-x)⁻ and K⁺ ions (Table S3). All these results highlight that solubilization of solid phases (Mg₂AlP, K-struvite) contributes also to the global release of phosphate.

On one side, at high suspension concentration (S/L = 10 mg/mL), the Mg₂Al LDH structure appeared stable over a period of 30 days under release conditions, as supported by XRD patterns (Figure 5A) and FTIR spectra (Figure 5B). The solid residues kept a similar FTIR feature with a constant vibration bands intensities ratio $I(v_{3(PO4)} 1070 cm^{-1})/I(\delta_{OMO} 450 cm^{-1})$ equal to 0.60 and an unmodified XRD pattern distances as described above. On the other side, the release experiment had a stronger effect on the and particle size and shape (Figure 5C & 5D). The material evolved from a morphology of hexagonal platelets with narrow particle size distribution (Ø ~ 200 ± 50 nm) to smaller platelets of heterogeneous shape and size, due to a partial dissolution starting from the particle edges. Immersion of Mg₂AlP in a lower S/L ratio has a stronger effect on the Mg₂Al structure and particularly on the interlayer composition. After 6 days of experiment, with 0.04 mg Mg₂AlP/mL, basal spacing of Mg₂AlP residue decreased to 8.6 Å (Figure S9) with an increase of carbon content to 1.70 % (vs. 9.3 Å and 0.86 % with S/L = 20 mg/mL).

This highlights the intercalation of carbonate and the anion exchange. The lamellar morphology and the shape of the platelets were maintained over the release experiments (Figure S10). From the XRD data no secondary phase can be identified, however the motukoreaite-like feature at low angles is lost (Figure S9).

In conclusion, the mechanism of phosphate release involves simultaneously the anion exchange of HPO₄²⁻ intercalated in Mg₂AlP by carbonate anion arising from dissolution of atmospheric CO₂(g) or present in aqueous solution (eq. 1) and a partial dissolution of mineral matrices, both LDH (eq. 2) and K-struvite (eq. 3). In such basic occurring conditions released Al³⁺ should partly precipitate as amorphous aluminum hydroxide (eq. 4).

$$ [\text{Mg}_{2.8}\text{Al(OH)}_{7.6}] [(\text{HPO}_{4})_{0.45}(\text{H}_{2}\text{PO}_{4})_{0.19}\text{K}_{0.17}] + 3.2\text{H}_{2}\text{O}] + \text{CO}_{3}^{2-}_{\text{aq.}} \rightleftharpoons [\text{Mg}_{2.8}\text{Al(OH)}_{7.6}] (\text{CO}_{3})_{0.5} \cdot \text{nH}_{2}\text{O} + 0.64\text{HPO}_{4}^{2-}_{\text{aq.}} + 0.17\text{K}^{+} \quad (\text{eq. 1}) $$

$$ [\text{Mg}_{2.8}\text{Al(OH)}_{7.6}] [(\text{HPO}_{4})_{0.45}(\text{H}_{2}\text{PO}_{4})_{0.19}\text{K}_{0.17}] + 3.2\text{H}_{2}\text{O}] \rightleftharpoons 2.8\text{Mg}^{2+}_{\text{aq.}} + \text{Al}^{3+}_{\text{aq.}} + 7.6\text{OH}^{-}_{\text{aq.}} + 0.64\text{HPO}_{4}^{2-}_{\text{aq.}} + 0.17\text{K}^{+} + 3.2\text{H}_{2}\text{O} \quad (\text{eq. 2}) $$

$$ \text{KMg}(\text{PO}_4) \cdot 6\text{H}_2\text{O} \rightleftharpoons \text{K}^+_{\text{aq.}} + \text{Mg}^{2+}_{\text{aq.}} + \text{PO}_4^{3-}_{\text{aq.}} + 6\text{H}_2\text{O} \quad (\text{eq. 3}) $$

$$ \text{Al}^{3+}_{\text{aq.}} + 3\text{OH}^{-}_{\text{aq.}} \rightleftharpoons \text{Al(OH)}_{3} \quad (\text{eq. 4}) $$

<figure><img src="image_6.png" /></figure>
4)

Fig. 5: XRD (A) and FTIR (B) of Mg₂AlP after release in H₂O at 0, 2, 15, 30 days and S/L = 10 mg/mL. SEM images of Mg₂AlP (C) before and (D) after release in H₂O at 30 days and S/L = 10 mg/mL.

Contributions of anion exchange and dissolution were calculated from Mg/Al/P/K release data (Figure 6) in regard to the global chemical composition and stoichiometry of the inorganic phases for the different S/L ratio tested. At high S/L ratio, almost pure anion exchange mechanism was involved, since dissolution was limited by the concentration. For the lowest S/L ratio (60% of P released), the portion of phosphate released by exchange corresponds to 28 % of P in matrix, 10 % of P sourced from K-struvite (KMgPO₄) and the rest of P released

(22%) are due to LDH matrix dissolution.

A comparison of the phosphate release performances by Mg₂AlP with other MgAl-PO₄ LDH phases reported in the literature is challenging especially due to the limited number of studies and significant variation in experimental conditions, e.g., the choice of LDH, the loading rate, surface adsorption vs intercalation, phase impurities. Indeed, the rate of P release strongly depends on both the LDH/Phosphate interaction and the physicochemical conditions of release. This work demonstrated that total release can be achieved under harsh conditions (high CO₃²⁻ concentrations, low solid/liquid ratio). This is of great interest for phosphate recovery after sequestration by LDH. However, for fertilization purposes, such extreme conditions are irrelevant, and LDH display limitation in phosphate release as shown on Figure 3 & 6. Similarly, Everaert et al.²⁰ work underlined the same limitations, since using 2mM HCO₃⁻ solution, Mg₂Al-PO₄ did not release more than 51%.

**Effect of pH on phosphate release.** Acidity is a major parameter that influences a large number of chemical (phosphate desorption, mineral dissolution...) and physical processes (ion diffusion,...) in soils whose pH range 6.0 – 8.0. These pH conditions may also affect the chemical stability of the Mg2Al LDH matrices and their phosphate release behavior. For example, Benicio et al.⁶⁴ reported that release capacities for MgₓFe and MgₓAl phases in 2 % citric acid were doubled and quadrupled respectively, compared to release in deionized water. herein the present work (Figure 7A) the P release percentages are reported after suspensions of Mg2AlP in various-pH solutions (contact time 6 days, S/L = 0.4 mg/mL). At intermediate pH values from 4.0 to 8.0, the buffering effect of the LDH neutralizes the excess of acidity or alkalinity and leads to equilibrated final pH values around pH 8.0 (Figure 7B).

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Fig. 6: Quantification of contributions in Mg₂AlP phosphate release mechanism in deionized water (20 mg Mg₂AlP in V (mL)).

Fig. 7: A) P release percentages of Mg₂AlP at various initial pH (S/L = 0.4 mg/L). B)

<figure><img src="image_9.png" /><figcaption>Evolution of pH under P release.</figcaption></figure>

The highest P release content (60%) is obtained at neutral pH. In such conditions the anion exchange may be considered as the major process as confirmed by the decrease of the basal spacing from 9.4 Å to 7.7 Å (Figure S11A) and the increase in the I(v₃(CO₃))/I(v₃(PO₄)) FTIR intensity ratio (Figure S11B).

At pH = 2.0, about 85 % of P are released due to a nearly complete dissolution, while at pH = 4 less carbonate is trapped in water compared to higher pH conditions, resulting in a lower portion of dissolved matrix.

Extreme basic pH (12.0) does not affect much the release, confirming that anion exchange with carbonate was driving the release and not the increase of pH (between HCO₃⁻ and CO₃²⁻ medium) for the experiment in NaHCO₃ 3.33 mM.

## Conclusion

This ability to immobilize higher amounts of phosphate in LDH than expected, attributed to the presence of K⁺ in the interlayer domain, offers significant advantages for the design of K/P SRF formulations. Phosphate-loaded MgAl LDH (69 mg P/g LDH) exhibits tunable and reproducible phosphate release properties that strongly depend on different physicochemical parameters (solid/liquid ratio, ionic strength, carbonate concentration, pH). The P-release rate can vary between 11.8% to 60% in water and may even reach 89% in presence of HCO₃⁻. In such conditions, total release is obtained after 6 days. Characterization of the as prepared material and residues collected after release using XRD, SSNMR and FTIR reveals the structural complexity of LDH intercalated by tetrahedral HPO₄²⁻ oxoanions. In addition to the intercalated hydrogenophosphate MgAl-LDH, three other phosphate species have been identified by ³¹P NMR, in agreement with other analytical data: 1) formation of K-struvite due to a minor hydrolysis of Mg₂Al LDH by HPO₄²⁻, 2) adsorption of HPO₄²⁻ by amorphous aluminum hydroxide generated during the initial coprecipitation and partial hydrolysis of LDH, 3) a HPO₄²⁻ species intercalated in excess with K⁺ cation similarly to the motukoreaite mineral. The kinetics and rate of phosphate release in this LDH are highly dependent on the physicochemical conditions of release, including the
solid/liquid ratio, the presence of interfering ions in the medium, and the pH. The interstratification/contamination by carbonate anions occurring during the release is not reversible and the loss of K⁺ reduces the recyclability of the phase under simple conditions. It was noted that, whatever the S/L ratio, the kinetics of release remained quite rapid, and the equilibrium release rate Qₑ is reached before the sixth day. Mg₂Al LDH materials, thanks to their high phosphate loading capacity, show great potentiality for a controlled release of phosphate, as demonstrated in a forthcoming paper highlighting their efficiency for wheat growth.

## Author Contributions

**Alexandra Jourdain** : Investigation, Methodology, Writing original draft, **Christine Taviot-Gueho** : Investigation, Validation, Writing - Review & Editing **Ulla Gro Nielsen** : Validation, Review **Vanessa Prévot** : Funding acquisition, Conceptualization, Supervision, Writing - Review & Editing and **Claude Forano** : Conceptualization Project administration, Supervision, Writing - Review & Editing.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

The authors acknowledge the European Regional Development Fund for the funding pf the FERTI+ project and Dr. A. Jourdain salary and the French government IDEX-ISITE initiative 16- IDEX-0001 (CAP 20-25) for supporting the international mobility of Dr. A. Jourdain. Prof. Ulla Gro Nielsen acknowledge The Danish Research Foundation (DFF Grøn Omstilling; grant: 95-305-23601-01130).

The authors acknowledge UCA-Partner for the access to XRD, TEM, TECAN, ICP-OES instruments.

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