Belayneh Bekele

Laura Lundehøj

Daugaard Jensen

Ulla Gro Nielsen

Claude Forano

Sequestration of orthophosphate by Ca2Al-NO3 Layered Double Hydroxide -Insight into reactivity and mechanism

Keywords: Layered Double Hydroxides, Ca2Al-NO3 hydrocalumite, orthophosphate, adsorption, hydroxyapatite, brushite

This study reports the mechanism of reactivity of phosphate anions (HPO4 2-) with Ca2Al-NO3 LDH. Sequestration of PO4 by Ca2Al-NO3 was quantified by fitting the adsorption isotherm with a Langmuir model. Adsorption capacity (65.2 mgP.g -1 of LDH) is greater than the theoretical anion exchange capacity indicating further mechanism of sequestration. Partial dissolution of CaAl-NO3 results in precipitation of multiple Ca-PO4 phases. Combined characterizations by XRD, 27 Al and 31 P NMR, FTIR/Raman and SEM highlight the competitive precipitation of hydroxyapatite and brushite within the LDH structure. Multiple environment of PO4 anions offer promising perspective for CaAl-NO3/PO4 as slow release fertilizer.

Introduction

Inorganic orthophosphate (PO4) is an essential resource for agriculture and animal feed additives. Indeed, 82% and 7% of their production are used respectively as fertilizers and food production (Schroeder et al., 2009). Unfortunately, PO4 is a non-renewable resource and the impoverishment of world stocks threaten the food supply to the world population (EU Report, 2013). With a mean production of 160 Mt per year over 16 000 Mt estimated reserves, in 100 years PO4 scarcity will be a severe problem. Added to these human and economic dangers, a major environmental concern arises from the dissemination of PO4 in the environment and its accumulation in surface waters through land farming and waters discarded from wastewater treatment plants (WWTP) causing harmful eutrophication of inland and coastal waters [START_REF] Oliveira | The role of phosphorus on eutrophication: a historical review and future perspectives[END_REF][START_REF] Farley | Eutrophication in Fresh Waters: An International Review[END_REF]. Consequently, the development of sustainable PO4 management processes is of high societal concern.

Recovery of PO4 from WWTP and processing as recycled PO4 slow release fertilizers (SRF), are under intensive investigations to both limit phosphate wastes and improve circular economic model of this critical raw material (Cieslik et al., 2017;[START_REF] Desmidt | Global Phosphorus Scarcity and Full-Scale P-Recovery Techniques: A Review[END_REF]. Most sustainable water-cleaning processes in WWTPs target the precipitation of phosphate as struvite (NH4MgPO4.6H2O) [START_REF] Ye | Phosphorus recovery from wastewater by struvite crystallization: Property of aggregates[END_REF] or hydroxyapatite (Ca5(PO4)3(OH), HAP) [START_REF] Chen | Phosphate removal and recovery through crystallization of hydroxyapatite using xonotlite as seed crystal[END_REF], for subsequent reuse as fertilizers. However struvite production is limited to large scale WWTPs and has so far failed to reach a sustainable economic model [START_REF] Talboys | Struvite: a slow-release fertiliser for sustainable phosphorus management?[END_REF].

Moreover, the low solubility of struvite and hydroxyapatite in water [START_REF]Layered double hydroxides applications as sorbents for environmental remediation[END_REF] limit their use as efficient SRF. Selective adsorption of PO4 from wastewaters by inorganic adsorbents that can be recycle once loaded with PO4 as SRF appears as an alternative green technology.

Layered double hydroxides (LDH) are natural and synthetic inorganic solids with a layered structure and general chemical formula [M 2+ 1-xM 3+ x(OH)2] x+ [A n-x/n.mH2O] x-(M 2+ , M 3+ and A n-being divalent and trivalent metals and anionic species). They display unique anion exchange properties largely investigated for the removal of oxoanions (NO3 -, PO4 3-, CrO4 2-, AsO4 3-,…) [START_REF] Lazaridis | Sorption Removal of Anions and Cations in Single Batch Systems by Uncalcined and Calcined Mg-Al-CO3 Hydrotalcite[END_REF] as well as they rank amongst the most selective and efficient adsorbents for PO4 anions [START_REF] Xu | Removal efficiency of arsenate and phosphate from aqueous solution using layered double hydroxide materials: intercalation vs. precipitation[END_REF][START_REF] Zhou | Novel hollow microspheres of hierarchical zinc-aluminum layered double hydroxides and their enhanced adsorption capacity for phosphate in water[END_REF][START_REF] Bernardo | Synthesis and characterization of eco-friendly Ca-Al-LDH loaded with phosphate for agricultural applications[END_REF][START_REF] Everaert | The isotopic exchangeability of phosphate in Mg-Al layered double hydroxides[END_REF]. Among the LDH, hydrocalumite-like LDH, a Ca2Al-LDH display a very high chemical reactivity with PO4 leading to great quantity of PO4 removed from water, i.e. as high as 400 mg PO4 per gram of LDH exceeding the theoretical anion exchange capacities [START_REF] Zhou | Novel hollow microspheres of hierarchical zinc-aluminum layered double hydroxides and their enhanced adsorption capacity for phosphate in water[END_REF]. This behavior makes Ca2Al-LDH a very promising PO4 sorbents and potential SRF.

Few studies have investigated PO4 release from Ca2Al-PO4 phases prepared by PO4 loading on Ca2Al LDH and their use as slow-release fertilizers. Interestingly, PO4-loaded hydrocalumite has demonstrated synergistic performances for plant growth and development of bacteria (Bradyrhizobium elkanii) responsible for N2 fixation [START_REF] Jia | Kinetics, isotherms and multiple mechanisms of the removal for phosphate by Cl-hydrocalumite[END_REF].

The mechanism of adsorption of phosphate by Ca-based LDH adsorbents is still under examination [START_REF] Radha | Mechanism of the anion exchange reactions of the layered double hydroxides (LDH) of Ca and Mg with Al[END_REF]. Some authors claimed that PO4 adsorption follows a topotactic anion exchange reaction with intercalation of HPO4 2-anions in between the LDH layers [START_REF] Radha | Mechanism of the anion exchange reactions of the layered double hydroxides (LDH) of Ca and Mg with Al[END_REF]. However, most convincing results clearly show that Ca2Al LDH undergo a progressive dissolution followed by the immediate precipitation of various Ca-PO4 phases [START_REF] Lu | Enhanced removal of phosphate by hydrocalumite: synergism of AlPO4 and CaHPO4•2H2O precipitation[END_REF][START_REF] Zhou | Efficient and controllable phosphate removal on hydrocalumite by multi-step treatment based on pH-dependent precipitation[END_REF]. The dissolution/precipitation phenomenon is driven by the difference between the solubility's of CaAl-LDH and most of the common calcium phosphate phases (Table 1). Hydroxyapatite (Ca5(PO)4(OH)) and brushite (CaHPO4.2H2O) (often referred to DCPD for dicalcium phosphate dihydrate) are the most commonly reported calcium phosphate phases that can form from a reaction between Ca2Al-LDH and PO4 [START_REF] Khitous | Removal of phosphate from industrial wastewater using uncalcined MgAl-NO3 layered double hydroxide: batch study and modeling[END_REF][START_REF] Vieille | Hydrocalumite and Its Polymer Derivatives. 1. Reversible Thermal Behavior of Friedel's Salt: A Direct Observation by Means of High-Temperature in Situ Powder X-ray Diffraction[END_REF]. Brushite was obtained when PO4 adsorption was performed at acidic pH (pH 5) while in basic medium (pH 11) HAP predominantly precipitates [START_REF] Tsuji | Phosphate recovery by generating hydroxyapatite via reaction of calcium eluted from layered double hydroxides[END_REF]. Jeong-Woo Son et al. (Jeong-Woo Son et al., 2015) confirmed these results for phosphate removal by hydrocalumite at basic pH but they observed the precipitation of other phases (gibbsite, boehmite, calcite). The total conversion of hydrocalumite into one of these phases should result in more than 388 mg of adsorbed PO4 (or 127 mg P) per g of hydrocalumite, values much higher than reported in literature [START_REF] Lu | Enhanced removal of phosphate by hydrocalumite: synergism of AlPO4 and CaHPO4•2H2O precipitation[END_REF]. The structure and composition of the precipitated Ca-PO4 phase as well as the rate of Ca2Al-LDH conversion are strongly dependent on various parameters such as Ca 2+ /PO4 3-molar ratio, pH, PO4 concentration. Moreover, the mechanisms of PO4 adsorption and phase conversion are mostly ignored.

Conversion into hydroxyapatite is a limiting factor for a 100% PO4 recovery and may hinder applications of these materials as SRF because extreme pH conditions would be needed to desorb PO4 from HAP [START_REF] Vieille | Hydrocalumite and Its Polymer Derivatives. 1. Reversible Thermal Behavior of Friedel's Salt: A Direct Observation by Means of High-Temperature in Situ Powder X-ray Diffraction[END_REF].

As Ca2Al LDH have a great ability for PO4 sequestration and potential release, there is a need for further investigations in order to better understand the mechanism involved in this reaction and the role of chemical composition and morphology to control PO4 adsorption and release. Therefore, our study focused on Ca2Al-NO3 LDH phase whose reactivity toward PO4 adsorption has not been yet investigated. Combination of XRD, solid state NMR spectroscopy, and electron microscopy characterization was implemented to identify the CaPO4 phases formed and to assess the mechanism of reactivity of phosphate anions with Ca2Al-NO3 LDH materials.

Materials and methods

Materials

All the reagents were analytical grade and used without further purification. Ca(NO3)2•4H2O, Al(NO3)3•9H2O, NaOH, K2HPO4, (L)-ascorbic acid, hexaammonium heptamolybdate (NH4)6Mo7O24.4H2O), sulfuric acid (H2SO4), potassium antimony(III) oxide tartrate (K(SbO)C4H4O6.0.5H2O) were purchased from Sigma-Aldrich. Adsorption studies were performed using K2HPO4. Deionized water was used exclusively in all procedures.

Synthesis of Ca 2 Al-NO 3

Ca2Al-NO3 was synthesized by the co-precipitation method in a reactor using peristaltic pumps for simultaneous addition of Ca 2+ /Al 3+ metal salts and NaOH in order to keep the pH constant following the procedure published by [START_REF] Vieille | Hydrocalumite and Its Polymer Derivatives. 1. Reversible Thermal Behavior of Friedel's Salt: A Direct Observation by Means of High-Temperature in Situ Powder X-ray Diffraction[END_REF]. The Ca 2+ /Al 3+ molar ratio was fixed to 2 and the reagent concentrations equal to 1M and 2M respectively for

([Ca 2+ ] + [Al 3+ ]
) and NaOH. The synthesis was performed at a pH = 11.5 and under N2 flow to prevent CO3 2-contamination. The reaction was vigorously stirred and completed in 4-5 h. The resulting slurry was aged at room temperature for 24 h. The mixture was centrifuged and the solid washed thoroughly with deionized water and dried at 40 °C for 24h. 6

Adsorption isotherms and modeling

The adsorption isotherm experiments were carried out in 2-ml Eppendorf vials. 15 mg of Ca2Al-NO3 LDH was suspended in 1.5 ml of K2HPO4 aqueous solution with concentrations ranging from 10 to 1000 mg P.L -1 (0, 50,100,200,300,400,500,600,700,800,900,1000 mgP.L -1 ) (P-refers to phosphorous). Suspensions were shaken for 2h00 at 20°C under air atmosphere in order to achieve the adsorption equilibrium [START_REF] Khitous | Removal of phosphate from industrial wastewater using uncalcined MgAl-NO3 layered double hydroxide: batch study and modeling[END_REF]. The supernatants were separated using centrifugation (14000 rpm, 21475 G-force, 20 min) and pH values were measured to be in the range of 8.1±0.4 throughout the adsorption experiment.

Concentrations of phosphate in the solution was determined using a Nicolet Evolution 500 UV-Vis spectrophotometer at λ = 880 nm, following the Molybdenum blue method [START_REF] Broberg | Analytical determination of orthophosphate in water[END_REF].

The adsorption capacity of the sorbent was measured by the difference between the initial and remaining concentrations of phosphate. The equilibrium adsorption concentrations, qe (mg.g -1 ), were calculated according the following equation =

( )
, where co is the initial concentration of (mg P. L -1 ); ce is the equilibrium concentration (mg P.L -1 ); V is the volume of the solution used (L); and W is the weight of the adsorbents (g). Sorption isotherms were obtained by plotting experimental qe versus ce. The adsorption isotherms were analyzed using the linearized equations of Langmuir and Freundlich, sorption models, expressed as:

Langmuir: = + (1) Freundlich: = (2)
where ce (mg.L -1 ) is the concentration of phosphate at equilibrium and qe (mg.g -1 ) is the sorption capacity of the solid at equilibrium. K (L/mg) is Langmuir constant, qm (mg.g -1 ) is the saturated sorption capacity of sorbent. KF and n are Freundlich parameters. KDR (mol 2 /J 2 ) is the activity coefficient. R is the universal gas constant (8.314 J/mol K), and T (K) is temperature.

Phosphate Exchange with LDH

The phosphate exchange experiments were carried out in 30 ml adsorption vials with 90 mg of LDH and 9 ml of a K2HPO4 aqueous solution at 20°C under air atmosphere. The K2HPO4 concentrations equal to 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 g HPO4 2-.L -1 . The mixtures were stirred for 5h. Solids were separated by centrifugation (14000 rpm, 21475 G-force, 20 min) and washed, repeatedly. Precipitates were dried at 40 °C for 24h. pH values of supernatants were measured to be in the range of 8.2±0.3 throughout the exchange experiment. The concentration range was selected in order to vary the initial HPO4 2-/Ca2Al-NO3 mass ratio from 32.6 to 97.9 mgP.g -1 of CaAl-NO3. Considering that such concentration correspond to mass ratio from the early stage of adsorption to saturation we then prepared a series of PO4 loaded samples representative of all the steps of reaction.

Characterization techniques

Chemical analyses of the LDH were conducted using the ICP-OES after dissolving the samples in 1 M HNO3 solution. X-ray diffraction (XRD) measurements were performed on a Philips X'Pert Pro diffractometer using CuKα radiation (λ = 1.5405 Å). PXRD patterns were recorded over the 5-70° 2θ range at an accelerating voltage of 40 kV, a current of 30 mA, a scan speed of 2°min -1 . Fourier-transform infrared (FTIR) spectra were recorded with a Nicolet-5700 spectrophotometer in the range of 400-4000 cm -1 using the KBr pellet technique. The Raman spectra were recorded from 150 to 1500 cm -1 at room temperature using a confocal micro-Raman spectrometer (T64000 Jobin Yvon) with an excitation wavelength of 514.5 nm (argon-ion laserline). Spectra were acquired at a spectral resolution of 1 cm -1 with charge coupled device (CCD) multichannel detector cooled by liquid nitrogen to 140 K coupled with Olympus confocal microscope. SEM images were recorded using a JSM-7500F FESEM operating at 3 KV.

Samples to be imaged were mounted on conductive carbon adhesive tabs. ICP-AES analyses were realized with a HORIBA-Jobin-Yvon ULTIMA C spectrometer.

27 Al MAS and 27 Al 3QMAS NMR spectra were recorded on a Varian INOVA 600 MHz NMR spectrometer (14.1 T) equipped with a 3.2 mm triple resonance MAS probe using 15 kHz spinning speed and a 1s relaxation delay. 27 Al MAS NMR spectra were recorded using a short, selective π/18 pulse to ensure quantitative spectra. Quantitative 31 P MAS NMR MAS spectra were recorded on a JEOL 500 MHz NMR spectrometer (11.7 T) using a 3.2 mm double resonance probe, a π/4 pulse, a spinning speed of 13 kHz, and a relaxation delay (typically 480-540 s) optimized for each sample. 85 % phosphoric acid (0.0 ppm) and 1 M AlCl3 have been used as reference for 27 Al and 31 P, respectively. Deconvolutions of the 27 Al and 31 P MAS NMR spectra has been performed using QuadFit [START_REF] Kemp | QuadFit--a new cross-platform computer program for simulation of NMR line shapes from solids with distributions of interaction parameters[END_REF] and MestReNova.

Results and Discussion

Structure and chemical analysis of Ca 2 Al-NO 3

The chemical composition (Ca1.91Al(OH)5.82(NO3).1.63H2O) of the as-prepared Ca2Al-NO3 LDH phase, determined from ICP-AES and TGA data, is in good agreement with the chemical formula expected for a pure synthetic NO3-hydrocalumite. The high pH value (11.5) used for the synthesis ensured a total coprecipitation of Ca 2+ /Al 3+ metal ions. Indeed, the experimental Ca 2+ /Al 3+ ratio (1.91) is close to the ideal composition (Ca 2+ /Al 3+ = 2) expected from initial molar ratio used. The powder X-ray diffractogram (PXRD) (Fig. 1A) and the FTIR spectrum (Fig. 1B) of Ca2Al-NO3 LDH confirmed the formation of the desired material. Indeed PXRD exhibits the structural features including sharp diffraction lines of a well-crystallized Ca2Al-NO3 hydrocalumite phase (Renaudin et al., 1999). The cell parameters were refined in the trigonal space group 3 1 (Fig. 1). The main structural difference compared to the hydrotalcite structure arises from the distortion of the octahedral layer due to the heptacoordination of Ca 2+ ions (6+1) bonded to six OH groups and an intercalated H2O molecule (Fig. SI-1). This corrugated layered structure is associated to typical Ca,Al-OH, Ca,Al-O and

(Ca,Al)-O-(Ca,Al) lattice vibrations located at 782, 592, 527 and 420 cm -1 , respectively [START_REF] Zhang | Near-infrared and mid-infrared investigations of Na-dodecylbenzenesulfate intercalated into hydrocalumite chloride (CaAl-LDH-Cl)[END_REF]. Intercalation of NO3 -anions was confirmed by the strong infrared absorption at 1380 cm -1 and 854 cm -1 relative respectively to the antisymmetric stretching ν3 and out-ofplane bending ν2 vibrations of the D3h NO3 -anions. The broad vibrations between 3400 cm -1 and 3650 cm -1 are characteristics of all LDH and arises from the symmetric and asymmetric stretching (ν(OH)) of the hydroxide layers and intercalated water molecules (Fig. 1B), 3640 cm -1 and 34493 cm -1 , respectively for free (Ca2Al)-O-H groups and H-bonded interlayered water molecules. The bending mode of interlayer water molecules appears between 1622-1637 cm -1 . Ca2Al-NO3 was precipitated as large hexagonal platelets with sizes between 1-3 µm and thickness ranging between 25-110 nm leading to a high aspect ratio (Fig. 7A) and a low BET specific surface area of 11 m 2 .g -1 . 27 Al MAS NMR experiment was employed to assess the purity of the parent Ca2Al-NO3 LDH (Fig. SI-2). The spectrum was simulated to determine the purity of the Ca2Al-LDH. The isotropic chemical shift δiso( 27 Al) = 11.0(4) ppm is in agreement with the value reported by Qiu et al. [START_REF] Qiu | Mechanism of boron uptake by hydrocalumite calcined at different temperatures[END_REF] and Jensen et al. [START_REF] Jensen | A solid state NMR study of layered double hydroxides intercalated with para-amino salicylate, a tuberculosis drug[END_REF] for Ca2Al-LDH intercalated respectively with NO3 -or para-aminosalicylate, respectively. From 90-95 % of the intensity originates from Ca2Al-LDH, when the contribution from the 27 Al satellite transitions (ca 5 %) to the center band is included (Table SI-1 andFig 4) ppm reported for Ca2Al-Cl by Andersen et al. [START_REF] Andersen | Characterization of the α-β Phase Transition in Friedels Salt (Ca2Al(OH)6Cl•2H2O) by Variable-Temperature 27Al MAS NMR Spectroscopy[END_REF]. This small variation is due to a change of anion, as also seen for MgAl-and ZnAl-LDH (Pushparaj et al, 2015;[START_REF] Sideris | Identification of Cation Clustering in Mg-Al Layered Double Hydroxides Using Multinuclear Solid State Nuclear Magnetic Resonance Spectroscopy[END_REF][START_REF] Sideris | Mg/Al Ordering in Layered Double Hydroxides Revealed by Multinuclear NMR Spectroscopy[END_REF].

Adsorption isotherm of PO4 by Ca 2 Al-NO 3

In order to monitor the reactivity of Ca2Al-NO3 material with phosphate and to quantify the amount of sequestrated PO4, Ca2Al-NO3 was suspended at 20°C and for 2h00 with solutions containing increasing amounts of HPO4 2-anions (section 2.2) and the PO4 content adsorbed per gram of sorbent versus PO4 equilibrium concentration (qe vs ce) was plotted (Fig. 2A). The isotherm plot is typical of an H-type adsorption isotherm curve according Giles's classification [START_REF] Giles | A general treatment and classification of the solute adsorption isotherm[END_REF]. This adsorption behavior clearly indicates a high affinity of PO4 anions toward the Ca2Al-LDH matrix. Both Langmuir and Freundlich models have been used to fit phosphate adsorption by LDH [START_REF] Hatami | Comparison of adsorption and desorption of phosphate on synthesized Zn-Al LDH by two methods in a simulated soil solution[END_REF][START_REF] Wang | Functionalizing biochar with Mg-Al and Mg-Fe layered double hydroxides for removal of phosphate from aqueous solutions[END_REF]. The Langmuir model assumes monolayer adsorption on a homogeneous surface with no interactions between the adsorbed molecules; while the Freundlich model is often used to describe chemisorption on heterogeneous surface [START_REF] Chen | Modeling of Experimental Adsorption Isotherm Data[END_REF]. Both models were used to fit our experimental data.

The Langmuir model give the best linear fit (with R 2 >0.9980) adsorption constants were determined from the slope and intercept of the plots (Fig. 2B and Table SI-2). The experimental PO4 adsorption capacity of Ca2Al-NO3 LDH is equal to 65.20 mgP.g -1 , a value higher than the one expected for a pure anion exchange reaction of HPO4 2-(52.75 mgP.g -1 ), 1.23 time higher, and much higher if we consider PO4 3-exchange (35.17 mgP.g -1 , 1.85 time higher). Such a high PO4 adsorption capacity was already reported in the literature [START_REF] Jia | Kinetics, isotherms and multiple mechanisms of the removal for phosphate by Cl-hydrocalumite[END_REF][START_REF] Lu | Enhanced removal of phosphate by hydrocalumite: synergism of AlPO4 and CaHPO4•2H2O precipitation[END_REF] for Ca2Al-Cl LDH (59.55 mg.g -1 ). Nature of the intercalated anion (NO3 -vs Cl -) does not affect the total PO4 sequestration but seems to play a role on the mechanism of interaction (Langmuir vs Freundlich)..

It must be mentioned that in our case PO4 adsorption experiments were performed at kinetic equilibrium and that no Ca 2+ nor Al 3+ ions were detected in the supernatants. Indeed Ca 2+

release percentage compared to total Ca 2+ amount was negligible, less than 2.82% at 0 mgP/L, and decreased to zero till 0.6 mgP/L (Fig. SI-3). Moreover, the amount of phosphate uptake by the Ca2Al-NO3 LDH are much lower than the values expected for a total conversion of Ca2Al-NO3 into hydroxyapatite (126.6 mg P.g -1 ) or brushite (211.0 mgP.g -1 ). Dissolution/precipitation is probably not total and the mechanism of transformation of CaAl LDH appears much more complex than described in the literature. Consequently, we performed XRD and 27 Al SSNMR on samples loaded with increasing amounts of PO4 in order to follow structural change. Ex-situ PXRD patterns (Fig. 3-A) were recorded for Ca2Al-NO3 LDH contacted with HPO4 2-solutions under batch reaction conditions. HPO4 2-anions react with the LDH structure from a very low PO4 loading (32.6 mg P.g -1 LDH; Ca-0.3P). At 43.5 mg P.g -1 LDH (Ca0.4P), ( 002) and ( 004 Interestingly, the two intense reflections at 7.564 Å and 3.782 Å with a spacing ratio of 2.0 are observed for all samples. They cannot account for any known calcium phosphate phases but can be explained by a shift of the Ca2Al-NO3 ( 002) and ( 004 Ca2Al-CO3 LDH that display a shortest basal spacing (Renaudin et al., 1999;[START_REF] François | A Cementitious Compound with Composition 3CaO-Al2O3-CaCO3-11H2O[END_REF]. Contamination by CO2 and conversion into CO3 2-is favored by the basicity of the CaAl matrix. This reactivity can be explained by the high stability of the monocarboaluminate phase against both dissolution and phosphate reaction. Obviously phosphate anions cannot be exchanged and stabilized in the Ca2Al galleries.

Dynamic and Reactivity of PO4 anions with

In conclusion, the XRD patterns of all samples clearly show a mixture of one or more following phase Ca2Al-NO3, Ca2Al-CO3, Al(OH)3, Ca5(PO4)3(OH) and Ca(HPO4).2H2O. These results indicate a different reactivity toward HPO4 2-for Ca2Al-NO3 compared to Ca2Al-Cl.

Indeed, Zhou et al. [START_REF] Zhou | Efficient and controllable phosphate removal on hydrocalumite by multi-step treatment based on pH-dependent precipitation[END_REF] reported that under PO4 adsorption and for a similar range of pH Ca2Al-Cl undergoes a complete transformation into a mixture of gibbsite, hydroxyapatite and tricalciumphosphate (Ca3(PO4)2). When adsorption is performed at lower pH values using H2PO4 -aqueous solution again transformation of Ca2Al-Cl is total. However, the resulting calcium phosphate formed deviate between the reported studies. For example, brushite is the main calcium phosphate phase and gibbsite is converted into AlPO4 for Jia et al. [START_REF] Jia | Kinetics, isotherms and multiple mechanisms of the removal for phosphate by Cl-hydrocalumite[END_REF]) and Lu et al. [START_REF] Lu | Enhanced removal of phosphate by hydrocalumite: synergism of AlPO4 and CaHPO4•2H2O precipitation[END_REF]

FTIR and Raman analysis

The strong structural changes undergo by the Ca2Al-NO3 phase upon reaction with phosphate, evidenced by XRD analysis, are clearly confirmed by analysis of the Raman (Fig. 4) and FTIR (Fig. SI-5) spectra. First of all, PO4 the vibration bands (Table 2) related to PO4 groups increase during PO4 adsorption as shown on both FTIR and Raman spectra. Loading of HPO4 2-leads to the removal of NO3 -anions which is linked to the decrease of the ν3(NO3) stretching band intensity (1055 cm -1 ). The formation of Ca2Al-CO3 LDH, as suggested by the XRD data (section 3.3.1), is confirmed by the appearance of a broad and intense ν3(CO3) multiplet band (1350-1550 cm -1 ) (Fig. SI-5). The large feature of the CO3 vibration bands, unusual for LDH material, must account for the presence of other carbonated phase such as carbonated substituted HAP [START_REF] Gibson | Novel synthesis and characterization of an AB-type carbonatesubstituted hydroxyapatite[END_REF]. At low frequencies, the symmetric (ν2) (433 and 450 cm -1 ) and asymmetric (ν4) (582, 593, 609 cm -1 ) bending modes of the PO4 3-anions (Fig. SI-5) overlap with the lattice vibrations νAl(OH)6 ofromgibbsite (534 cm -1 ) and hydrocalumite-like phases (580, 528, 425 cm -1 ).

The splitting of the ν4 PO4 vibration band into 3 bands accounts for the multiplicity of environment in the HAP, brushite and substituted-CO3 HAP structure as reported by the literature [START_REF] Swamiappan | Synthesis of carbonate substituted hydroxyapatite by Pechini method[END_REF]. Resolution of several MO-H vibration feature increases from Ca-03P to Ca-09P, due to a clear observation of the νas and νs vibrations of Al(OH)3

Gibbsite (3470, 3527 and 3619 cm -1 ) superimposed on the HAP bands (3527 and 3619 cm -1 , 3100 -3400 cm -1 ).

27 Al NMR and 31 P MAS NMR characterizations

reaction with HPO4 2-and to quantify the relative concentration of the different phosphate phases formed. The27 Al MAS NMR spectra (Fig. 5) all show one resonance in the octahedral range in agreement with the coordination of Al in the LDH structure. In addition, a small amount of tetrahedral Al is seen at δiso( 27 Al) ≈ 73 ppm, constituting less than 5% in all the 27 Al NMR spectra except for the Ca2Al-NO3 with 0 and 0.9P loading. Upon phosphate exposure of 0.3P an upfield shoulder to the LDH resonance assigned to AOH appears [START_REF] Jensen | A solid state NMR study of layered double hydroxides intercalated with para-amino salicylate, a tuberculosis drug[END_REF]Pushparaj et al., 2015). Different models for simulation of the 0.3P 27 Al MAS NMR spectrum has been applied but an unambiguous model could not be identified. However, it was estimated that 15-30 % of the Al belonged to the LDH. Furthermore, a slight shift in δiso( 27 Al) in the LDH resonance from 11.0(4) to 11.3(4) ppm was observed indicating a change in the intercalated anion, e.g., carbonate as seen by PXRD (Fig. 3-A). At higher phosphate loadings the AOH phase is clearly the dominating phase and consequently the center of the signal is located below 10 ppm.

Moreover, the total width of the spinning side band manifold (not shown) changes from ≈400 kHz to ≈600 kHz implying a 50 % increase of the quadrupole interaction of the predominant 27 Al resonance. Thus, at high P loading the predominant Al species is AOH, which has a stronger quadrupole interaction (2.5 MHz) than Ca2Al-LDH (1.6 MHz) [START_REF] Jensen | A solid state NMR study of layered double hydroxides intercalated with para-amino salicylate, a tuberculosis drug[END_REF], c.f., Table SI-2. However, Ca2Al-LDH might still be present with relative Al concentration up to 10-15 % but not distinguished due to the overlap between the LDH and AOH resonances. Hence, the 27 Al MAS NMR confirms dissolution of the LDH and precipitation of aluminum hydroxide (AOH) upon phosphate exposure.

The 31 P MAS NMR is a very sensitive probe of the phosphorus speciation and it is therefore possible to distinguish between e.g., different mineral phases.

The 31 P MAS NMR spectra for the Ca2Al-NO3 all show one resonance at 3 ppm with one set of weak spinning sideband which is indicative of orthophosphate in agreement with earlier studies [START_REF] Bernardo | Synthesis and characterization of eco-friendly Ca-Al-LDH loaded with phosphate for agricultural applications[END_REF]. The resonance can be adequately fitted with four resonances as seen in the deconvolutions in Fig. 6 and the obtained parameters from deconvolutions are summarized in Table 3. The dominating resonance at δiso( 31 P) = 3.0(3) ppm constitutes 71-79 % of the phosphates species and is based on the isotropic shift assigned to hydroxyapatite (Ca10(PO4)6(OH)2) [START_REF] Aue | Solid-state Phosphorus-3 1 Nuclear Magnetic Resonance Studies of Synthetic Solid Phases of Calcium Phosphate: Potential Models of Bone Mineral[END_REF][START_REF] Rothwell | High-resolution variable-temperature phosphorus-31 NMR of solid calcium phosphates[END_REF]. This is in agreement with the PXRD, where formation of Ca5(PO4)3(OH) (HAP) is clearly observed. The small resonance at δiso( 31 P) = 1.4-1.7 ppm is assigned to brushite constituting 6-9% of the sample [START_REF] Aue | Solid-state Phosphorus-3 1 Nuclear Magnetic Resonance Studies of Synthetic Solid Phases of Calcium Phosphate: Potential Models of Bone Mineral[END_REF][START_REF] Rothwell | High-resolution variable-temperature phosphorus-31 NMR of solid calcium phosphates[END_REF]. The two resonances at δiso( 31 P) ≈ 4.3-4.5 and 5.6-6.0 ppm constitute a total of 16-21 % of the samples. The latter resonance (δiso( 31 P) = 5.6-6.0 ppm) agrees well with a dehydrated oxyhydroxyapatite, Ca10(PO4)6(OH)2-2xOx, which may precipitate during phosphate adsorption experiments. Thus, that 31 P NMR confirms that the CaAl-NO3 LDH degrades during the adsorption experiments and mainly hydroxyapatite is formed, which are in agreement with earlier studies for Cl-hydrocalumite [START_REF] Gibson | Novel synthesis and characterization of an AB-type carbonatesubstituted hydroxyapatite[END_REF][START_REF] Swamiappan | Synthesis of carbonate substituted hydroxyapatite by Pechini method[END_REF]: Renaudin et al., 1999).

Morphological evolution under PO4 adsorption

Adsorption of HPO4 2-causes strong changes of the Ca2Al-NO3 morphology as shown by SEM analysis (Fig. 7). Indeed, the large hexagonal platelets of the pristine material (Fig. 7-A), with sizes in the range 1-2 µm, undergo a partial dissolution followed by a nucleation/crystallization process occurring at the surface of CaAl-NO3 particles. Fig. 7-B,C and D display these small HAP nanocrystals (size < 100nm) with needle shape homogeneously dispersed all over the solid. Moreover, at 0.6P loading (Fig 7 -C) well defined crystals are clearly observed whose parallelepiped forms look like that of the brushite crystals [START_REF] Onac | Cave Minerals of San Salvador Island, Bahamas[END_REF]. It is clear that some of the large hexagonal platelets are still present at high PO4 loading. These observations confirms the hypothesis of formation of Ca2Al-CO3, Ca5(PO4)3(OH,CO3) and Ca(HPO4).2H2O (Fig. 7D). This result is supported by the adsorption data (section 3.2) that indicates an adsorption capacity greater than expected for a pure anion exchange reaction. However the Langmuir adsorption behavior does not evidence this multiple reactivity because both brushite and HAP formations are strongly thermodynamically favorable.

Mechanism of HPO

Unlike most of the data published on hydrocalumite/phosphate reactivity that evidence the conversion of Ca2Al-Cl into hydroxyapatite, our results show that i) brushite is also formed and ii) part of Ca2Al LDH remains due to a stabilization by CO3 2-intercalation.

A phosphate concentration of only 400 P mg.L -1 is sufficient for the Ca2Al-NO3 structure to collapse, i.e. a HPO4 2-/Ca 2+ molar ratio of 0.189 while for a total conversion of hydrocalumite into brushite or hydroxyapatitie HPO4 2-/Ca 2+ molar ratio of 1. complexes are the preliminary steps of condensation of Ca-PO4 phases (Fig. 8). Costless energy surface nucleation is then favored. Competition occurs between precipitations of hydroxyapatite and brushite minerals. Even if HAP (PO4 3-/Ca 2+ =3/5) must be preferentially favored when surface PO4 3-/Ca 2+ ratio increases Brushite (PO4 3-/Ca 2+ =1/1) was also forms as a secondary phase even at such high pH ( ! " = 9.94). The presence of carbonate in the medium may quenched the formation of calcium phosphate phase, resulting in the stabilization of the CO3hydrocalumite like structure (PXRD, FTIR and Raman data). Interestingly, as shown by scanning electron microscopy, brushite and HAP crystallites with a wide distribution of particle size from nanometer to micrometer are dispersed all over the remaining CO3-hydrocalumite phase. Such composite material with embedded Ca-PO4 phases could give promising result for slow-released phosphate fertilizers. The recycling and reuse of phosphorus could therefore be achieved.

Conclusion

This study highlights the specific reactivity of Ca2Al-NO3 hydrocalumite-like LDH toward phosphate sequestration. Up to 65.20 mgP.g -1 was loaded into the resulting composite, values greater than the anion exchange capacity of Ca2Al-NO3. A mixture of brushite, HAP and Ca2Al-CO3 LDH phases was obtained allowing a distribution of PO4 structures and morphologies. As HAP [START_REF] Giroto | Controlled release from hydroxyapatite nanoparticles incorporated into biodegradable, soluble host matrixes[END_REF][START_REF] Lei | Tailoring hydroxyapatite nanoparticles to increase their efficiency as phosphorus fertilisers in soils[END_REF] and Brushite [START_REF] Hermassi | Fly ash as reactive sorbent for phosphate removal from treated waste water as a potential slow release fertilizer[END_REF]) have been investigated as potential slow release fertilisers, HAP and brushite nanoparticle embedded inside a Ca2Al matrix may offer promising PO4 slow release properties.

Acknowledgements

This work was partially funded by the The Innovation Fund Denmark via the project "RecoverP". UGN, LL, and NDJ acknowledges financial support from the Villum Foundation via the "Villum Young Investigator Programme" (grant VKR022364) and the 600 MHz Fig. 3: A) ex-situ XRD of Ca2Al-NO3/HPO4 2-series for various HPO4 2-loading at 0; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9 g/L ( Ca2Al-NO3; Ca2Al-CO3; Al(OH)3 Gibbsite; Ca5(PO4)3(OH)

Hydroxyapatite; Ca(HPO4).2H2O Brushite) and B) in-situ XRD of Ca2Al-NO3 in contact with a 2M HPO4 2-solution.

Fig. 4: Raman spectra of the Ca2Al-NO3/HPO4 2-series with increasing HPO4 2-loading (from 0.3 to 0.9 g/L) at A) medium and B) low energy range.

Linearized form

Ce/qe = 0.01534 + 0.00354Ce R 2 = 0.99973 A)

ν4 (PO 4 ) ν2(PO 4 ) The dominating resonance at δiso( 31 P) = 3.0(3) ppm constitutes 71-79 % of the phosphates species and is based on the isotropic shift assigned to hydroxyapatite

ν 1 A' 1 (CO 3 ) ν (AlO6)

Tables and Figures

  . SI-2a). We note that a Lorentzian line broadening gave the best fit. Moreover, the spinning sidebands in the 27 Al MAS NMR spectrum had a clear indication of dynamics on the time scale of the 1 st order quadrupole interaction (MHz) (Fig.SI-2) as earlier reported[START_REF] Bitetto | Probing the Dynamics of Layered Double Hydroxides by Solid-State 27Al NMR Spectroscopy[END_REF]. It slightly differs from the value δiso( 27 Al) = 9.5(

  Ca 2

  ) the most intense reflections of Ca2Al-NO3 pristine material have vanished meanwhile some new diffraction reflections appear that accounts for the formation of new phases. These new phases were identified from comparison of PXRD diffractograms of calcium phosphate phases known to appear during PO4 reaction with hydrocalumite, Ca5(PO4)3(OH) (hydroxyapatite; 09-0432 JCPDS file), CaHPO4 (monetite; monoclinic, 09-0080 JCPDS file), Ca(HPO4).2H2O (Brushite; triclinic 09-0077 JCPDS file), octacalcium phosphate (Ca8H2(PO4)6.5H2O (OCP) and Al(OH)3 (Gibbsite; 33-0018 JCPDS file). TableSI-3 display the Bragg distances for the reflections of all Ca-xP samples and reference phases (Brushite, hydroxyapatite, gibbsite). A comparison of the PXRD patterns evidence the multiple reactivity of phosphate anions with Ca2Al-NO3 giving rise to a competition between exchange reactions into the Ca2Al structure and precipitation of Ca/PO4 phases.First, the presence of the characteristic (002)/(110) diffraction doublet from Gibbsite (4.814 Å and 4.390 Å), appear at the lowest PO4 loading (Ca-0.3P), which confirm dissolution of Ca2Al-NO3. However, from the point of view of the PXRD data, the amount of Gibbsite does not increase noticeably when increasing phosphate loading, suggesting that dissolution of the pristine layered structure is only partial. This dissolution is confirmed by the occurrence at higher PO4 loading of some reflections from the mineral hydroxyapatite including the (002), (102), and the (121)/(211)/(112)/(300)/(202) reflections at 2θ = 25.90(3), 28.19(3)° and 31-35° 2θ respectively.Reflections assigned to the HAP phase are broad with a low intensity, due to particles with small sizes and low crystallinity. For Ca2Al-Cl hydrocalumite, PO4 adsorption leads to a collapse of the structure and the formation of hydroxyapatite following a dissolution/precipitation mechanism[START_REF] Son | Analysis of phosphate removal from aqueous solutions by hydrocalumite[END_REF]. Formation of brushite is also suggested by the sharp reflection at 29.42(3)°, whose intensity increases with PO4 adsorption. This reflection is unambiguously assigned to the (-141) reflection from brushite.

  while Quian et al.[START_REF] Zhou | Efficient and controllable phosphate removal on hydrocalumite by multi-step treatment based on pH-dependent precipitation[END_REF] reported the conversion of hydrocalumite into HAP, CaCO3 and Al(OH)3 at pH = 5.0In-situ PXRD experiments were conducted on the Ca2Al-NO3 sample, previously wetted as a paste (solid:water mass ratio = 2:1), loaded in a home-made diffraction cell (Fig.SI-4). The solid was in contact with a 2M HPO4 2-solution (Fig.3-B) in order to force the exchange by HPO4 2-and minimize the dissolution reaction. Under such conditions of in-situ XRD exchange reaction, the high solid/liquid ratio must limit the dissolution of the NO3-hydrocalumite and prevent the precipitation of Ca-PO4 phase. The designed cell insures a fast PO4 diffusion to the LDH adsorption sites. A topotactic phase transformation of the pristine material occurs in less than 14 min as shown by the in-situ XRD pattern (Fig.3-B). A group of three new diffraction lines (10.64(5) Å, 5.28(2) Å, and 3.50(1) Å), with integer spacing ratio (L1/L2 = 2.02; L1/L3 : 3.04), clearly accounts for the formation of a layered phase with a basal spacing mean value of 10.57(5) Å. This 1.957 Å increase of the interlayer distance was also observed when Ca2Al-NO3 was contacted with pure water. This structural expansion is reversible under hydration/dehydration process. Thus, this well-defined structural expansion arises from the hydration of the interlayer and the intercalation of a single layer of water molecules but has only previously been observed for Ca2Al-NO3 phase. The fast hydration reaction is probably a key step in the dissolution and phosphate reaction of the NO3-hydrocalumite as H2O molecules can diffuse easily in the interlayer domains. While NO3-containing LDH are known to be the most reactive LDH phases towards anion exchange reactions and particularly with orthophosphate anions, condensation of HPO4 2-with the LDH layer occurs prior to exchange preventing the formations of CaAl-PO4 phase.

  0 and 0.6 are needed. At maximum adsorption, 48.0% of Ca2Al-NO3 were converted into calcium phosphate phases, HAP (Ca5(PO4)3(OH)) and brushite (Ca(HPO4).2H2O). Obviously, when Ca2Al-NO3 is contacted with a solution of HPO4 2-salt, HPO4 2-diffuse easily inside the layered structure up to the Ca 2+ reactive sites thanks to a fast hydration of the structure (in-situ PXRD) that increases the interlayer distance and opened the structure for PO4 diffusion. The various phosphorus environments shown by 31 P NMR indicate that HPO4 2-reaction probably occurs at the Ca2Al layer surface through an initial complexation of Ca 2+ ions assisted by both the expulsion of the labile coordinated water and the hydrolysis of Ca-OH bonds. Clearly the corrugated Ca2Al layers do not stabilize enough the Ca 2+ ions inside the hepta-coordinated sites. Ca 2+ ions can easily diffuse out of the layers. Chemical extraction of Ca 2+ ions and formation of hydrated

Fig. 1 :

 1 Fig. 1: (A) XRD pattern and (B) FTIR spectrum of Ca2Al-NO3. Fig. 2: A) Experimental and calculated adsorption isotherms of Ca2Al-NO3/HPO4 2-system. B) Insert linear form of the Langmuir fit.

Figure 5 :Fig. 6 :

 56 Figure 5: The 27 Al MAS NMR spectra of the LDH exposed to the different phosphate concentrations. The dotted line shows the position of resonance for Ca2Al-NO3 LDH. Fig. 6: Deconvolutions of the 31 P MAS NMR spectra of the Ca2Al-NO3 LDH with different phosphate loadings a-g): Ca2Al-NO3 with 0.3-0.P, h): hydroxyapatite and j) brushite, two resonances from impurities are observed at 0.1 and -1.4 ppm. The top line is the experimental spectrum (Exp), the second is the simulated (Sim) and the bottom line is the residual (Dif).

Fig. 7 :

 7 Fig. 7: SEM images of A) pristine Ca2Al-NO3 material and loaded with various amounts of PO4, B) Ca0.3P, C) Ca0.6P, D) Ca0.9P.

Fig. 8 :

 8 Fig. 8: Initial step of the conversion of Ca2Al-NO3 LDH into calcium phosphate minerals.

  Fig.2

  Fig.3

  Fig. 5

  ) reflection to high 2θ values.Simultaneously intense carbonate vibration bands are present for all phases. This cannot account for the precipitation of calcium carbonate phase such as calcite reported by Jeong-Woo et al.

[START_REF] Son | Analysis of phosphate removal from aqueous solutions by hydrocalumite[END_REF]

as corresponding diffraction lines are not observed in the XRD patterns. This CaAl phase with a mean interlayer spacing or basal spacing (dbs) of 7.564 Å must arise from the topotactic transformation of the pristine Ca2Al-NO3 LDH (dbs = 8.513 Å) into a

  4 2-reactivity with Ca 2 Al-NO 3 LDH

	According results from combined PXRD, 27 Al and 31 P SSNMR, Raman, FTIR and SEM
	analysis, HPO4 2-anions react spontaneously with Ca2Al-NO3 LDH in solution. While PO4 anion
	exchange was reported into Zn2Al and Mg2Al LDH to give respectively Zn2Al-HPO4 2-(Steftel et
	al., 2018) and Mg2Al-HPO4 2-(Everaert et al., 2016), Ca2Al-HPO4 LDH was not formed,
	indicating that if HPO4 2-anions intercalates in between the hydrocalumite layers they react with
	the layer to destroy the structure. Reaction proceeds according the competing formation of HAP
	(eq. 1), brushite (eq.2) and Ca2Al-CO3 (eq.3):
	Ca2Al(OH)6.(NO3).nH2O(s) + 6/5HPO4 2-(aq.)	2/5Ca5(PO4)3(OH)(s) + Al(OH)3(s) + 7/5OH -(aq.) +
	NO3 -(aq.) + (n+6/5) H2O (eq. 1)
	Ca2Al(OH)6.(NO3).nH2O(s) + 2HPO4 2-(aq.) + 4H2O	2CaHPO4.2H2O(s) + Al(OH)3(s) + 3OH -(aq.)
	+ NO3 -(aq.) + nH2O (eq. 2)
	Ca2Al(OH)6.(NO3).nH2O(s) + 1/2CO3 2-(aq.)	Ca2Al(OH)6.(CO3)0.5.nH2O(s) + NO3 -(aq.) (eq. 3)

Table legends Table 1 :

 legends1 Solubility products of some calcium phosphate compounds.

Table 2 :

 2 Assignment of PO4 infrared and Raman vibration bands.

Table 3 :

 3 Parameters obtained from the deconvolutions of the 31 P MAS NMR spectra for Ca2Al-NO3.

	Table 1
	Calcium phosphate phases	pKsp	Ref.
	Ca5(PO)4(OH)	-58.4	(Bell et al., 1978)
	Ca3(PO4)2	-25.5	(Fowler et al., 1986)
	Ca8H2(PO4)6.5H2O	-96.6	(Tung et al., 1988)
	CaHPO4.2H2O	-6.59	(Gregory et al., 1970)
	Ca2Al(OH)6(OH).2H2O	-11.4	(Gacsi et al., 2016)

Table 2

 2

	Vibrations (cm -1 )	Infrared	Raman
	ν3 (PO4)as	1025, 1063, 1105, 1138	1070; 1081
	ν1 (PO4)s		962; 1050

Table 3 :

 3

	Sample	Assignment	δiso( 31 P)(ppm)	I(%)
	Ca2Al-NO3 0.3P

Al and 31 P SSNMR were used to investigate the structural changes of Ca2Al-NO3 during