Growth and characterization of struvite-Na crystals

Growth and characterization of struvite-Na crystals

Chetan K. Chauhan a,b,n, Mihirkumar J. Joshi a,1

a Crystal Growth Laboratory, Department of Physics, Saurashtra University, Rajkot 360005, Indiab Government Science College, Sector 15, Gandhinagar 382015, Gujarat, India

a r t i c l e i n f o

Keywords:A1. BiocrystallizationA1. CharacterizationA1. Crystal morphologyA2. Gel growthB1. StruviteB1. Struvite-Na

a b s t r a c t

Sodium magnesium phosphate heptahydrate [NaMgPO4 �7H2O], also known as struvite-Na, is the sodiumanalog to struvite. Among phosphate containing bio-minerals, struvite has attracted considerableattention, because of its common occurrence in a wide variety of environments. Struvite and familycrystals were found as urinary calculi in humans and animals. Struvite-Na crystals were grown by asingle diffusion gel growth technique in a silica hydro gel medium. Struvite-Na crystals with differentmorphologies having transparent to translucent diaphaneity were grown with different growthparameters. The phenomenon of Liesegang rings was also observed with some particular growthparameters. The powder XRD study confirmed the structural similarity of the grown struvite-Na crystalswith struvite and found that struvite-Na crystallized in the orthorhombic Pmn21 space group with unitcell parameters such as a¼ 6.893 Å, b¼6.124 Å, c¼11.150 Å, and α¼β¼γ¼901. FT-IR spectra of struvite-Na crystals revealed the presence of functional groups. The TGA, DTA and DSC were carried outsimultaneously. The kinetic and thermodynamic parameters of dehydration/decomposition process werecalculated. The variation of dielectric constant with frequency of applied field was studied in the rangefrom 400 Hz to 100 kHz.

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1. Introduction

Struvite is the mineralogical name of Ammonium MagnesiumPhosphate Hexahydrate [NH4MgPO4 �6H2O] crystals. The generalchemical formula for a struvite type compound is XþY2þPO4 �nH2O, where n¼6–8. Potassium Magnesium Phosphate Hexahydrate[KMgPO4 �6H2O], also known as struvite-K, and sodium magnesiumphosphate heptahydrate [NaMgPO4 �7H2O], or struvite-Na, are newstruvite type compounds. Struvite-Na is the sodium analog to struvitein spite of the excess of water molecule. In struvite-Na, the Naþ

cations replace the NH4þ cations. Struvite has attracted considerable

attention due to its common occurrence in a wide variety of environ-ments. Struvite occurs as one of the components of the urinary calculiin humans as well as in animals [1,2]. Struvite-K was also reported asurinary calculi in the animals such as dogs [3], goats [4] and buffalocalves [5]. Struvite and struvite-K were reported as iso-structuralcompounds [6,7]. Previously, the growth and characterization ofstruvite [1,8] and struvite-K [9] were reported by Chauhan et al. Inthe present investigation, struvite-Na crystals were grown by the gelgrowth technique and characterized by Fourier transform infrared

(FT-IR) spectroscopy, powder X-ray diffraction (XRD), and dielectricstudy. The thermal analysis of the struvite-Na was also carried out byusing thermogravimetric analysis (TGA), differential thermal analysis(DTA) and differential scanning calorimetry (DSC).

2. Materials and method

2.1. Crystal growth

Struvite-Na crystals were grown by the single diffusion gelgrowth technique. Previously, the technique was used and describedelaborately to grow struvite [1] and struvite-K [9] crystals. Analyticalreagent (AR) grade chemicals and distilled water were used to growthe struvite crystals. Borosil glass test tubes of 140 mm length and25 mm diameter were used as crystallizing vessels. All test tubesand glassware were cleaned as well as autoclaved at 120 1C for15 min before use. The silica hydro gel was prepared by usingSodium metasilicate (SMS) [Na2SiO3 �9H2O] solutions with differentspecific gravity. To prepare SMS solution, 200 g SMS powder wasadded in 1 l of double distilled water in a beaker and the mixturewas stirred vigorously for 2 h for uniform mixing up using amagnetic stirrer. Thus, a dense milky SMS solution was formed,which was then kept under an undisturbed condition for a couple ofdays to allow sedimentation. The SMS solution was filtered twice

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Journal of Crystal Growth

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n Corresponding author. Mobile: +91 9825765981.E-mail addresses: [email protected] (C.K. Chauhan),

[email protected] (M.J. Joshi).1 Mobile: þ91 9825397305.

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with a Whatman qualitative filter paper of 11 mm pore size for high-purity filtration. The SMS solution of desired specific gravity (SG)was prepared by adding double distilled water of appropriatevolume. The SG of each of the SMS solutions was determined by apycnometer, i.e. specific gravity bottle using following formula:

Specif ic Gravity ðSGÞ ¼ MS�M0

MW �M0ð1Þ

where M0¼mass of the empty dry bottle, MS¼mass of the bottlefilled with SMS solution and MW ¼mass of the bottle filled withdistilled water. In the present study, the SG of SMS solutions wasselected in the range of 1.04–1.06 for the different sets of struvite-Nacrystal growth. Aqueous solutions of sodium dihydrogen orthopho-sphate (SDO) [NaH2PO4 �2H2O] with different concentrations weremixed with the SMS solutions in appropriate amount to set the gelat the specific pH values of the mixture. The gel solutions of 20 mLvolume were carefully transferred into the respective test tubes.The test tubes were placed vertically in wooden stands and keptundisturbed for gelation. In order to check the effect of the gel pH onthe struvite-Na crystal growth, experiments with different specificinitial constant pHs were carried out. In the present study, the pHvalue of the mixture used for the gel solution was kept in the rangeof 5.5–8.0 for the different sets of struvite-Na crystal growth. As apolymerization process in the silica hydro gel is pH sensitive, thepore size distribution and hence the gel density varies with pH.Since the time necessary for gelation is very sensitive to pH, the timerequired to set the gel firmly was different for each set of mixture.Within 2 days, good quality gels were set in the test tubes. After thegelation took place, 20 mL supernatant solutions (SS) of magnesiumacetate [Mg(CH3COO)2 �4H2O] with different concentrations weregently poured along the walls of the test tube over the set gels byusing a pipette without damaging gel surface. All the test tubes werecapped with airtight stopples and kept undisturbed at 27 1C tem-perature. As shown in Table 1, different growth parameters wereused to grow struvite-Na crystals. Here, the silica gel was chosen sothat it remains stable and does not react with the diffusing solutionsor with the product crystal formed. The following reaction isexpected to occur in the gel between the two reactants.

NaH2PO4 �2H2OþMg(CH3COO)2 �4H2OþH2O-NaMgPO4

�7H2Oþ2CH3COOH (2)

The struvite-Na crystal growth experiment using the singlediffusion gel growth technique was completed within 20 days,which included the duration of the preparation of SMS solutions,gelation time and the time taken for the growth of crystals. It wasfound that the nucleation of the struvite-Na crystals was started inthe gel column below the gel–liquid interface within a day andgrowth completed within 12 days after the pouring of supernatantsolution over the gel. The grown struvite-Na crystals were gentlyremoved from the gel medium using the forceps, quickly rinsed indistilled water and then air dried on a filter paper. The growncrystals were used for further investigation.

2.2. Characterization techniques

The grown crystals in a fine powder form were analyzed bypowder XRD using a Philips X'Pert SW diffractometer system withmonochromatic Cu Kα radiation. The powder XRD data wereanalyzed using Powder-X software. The FT-IR spectrum of pow-dered samples of the grown crystals was recorded on a Nicolet6700 FT-IR spectrometer having optical resolution of 0.09 cm–1, inthe range from 400 cm–1 to 4000 cm–1 in a KBr disc medium. TheTGA, DTA and DSC were carried out on Linseis STA PT-1600, in theatmosphere of air from 35 1C to 900 1C at a heating rate of 10 1C/min. The dielectric study was carried out by measuring capaci-tance of the pressed pellet of sample of known dimension at roomtemperature on an Agilent 4284A precession LCR meter within thefrequency range from 400 Hz to 100 kHz.

3. Results and discussion

3.1. Crystal growth

The gel growth technique was described by Henisch [10] andHenisch et al. [11], as well as by Patel and Rao [12]. Recently,struvite [1,8] as well as struvite-K [9] crystals with differentmorphologies were grown using the gel growth technique. Someof the struvite-type compounds were grown by the gel growthtechnique [13,14]. Struvite type compounds M[Mg(H2O)6](XO4),where M¼Rb, Tl and X¼P, As, were obtained by means of thegelatine-gel diffusion technique by Weil [13]. Yang and Sunreported the formation of new struvite type compound Mg2KNa(PO4)2 �14H2O [15]. Earlier Mathew et al. [16] reported growth andstructure refinement of sodium magnesium phosphate heptahy-drate NaMgPO4 �7H2O crystals. Takagi et al. [17] synthesizedstruvite-type compound Mg2KH(PO4)2 �15H2O. Previously thestruvite structure was thought to be unable to accommodateunivalent cations smaller than Kþ ion (1.33 Å). But the naturalformation of the hazenite {KNaMg2(PO4)2 �14H2O} at Mono Lake,California, and the synthesis of struvite-Na invalidate this hypoth-esis. The smaller ionic size of Naþ in struvite-Na is effectivelycompensated by the Naþ–H2O pair. Alkemper and Fuess [18]synthesized sodium magnesium phosphate crystals with orthor-hombic structure; whereas, Mathew et al. [16] synthesized MgNa-PO4 �7H2O crystals with tetragonal structure. In the present study,struvite-Na crystals were grown by the single diffusion gel growthtechnique and the effects of various growth parameters on thegrowth and morphology of crystals were reported. Crystals withdifferent morphologies such as prismatic type, star type anddendritic type were grown in the gel. Fig. 1(a) shows the photo-graph of struvite-Na crystals grown in the gel. Crystallization ofstruvite-Na was found to be possible within the range of 6–7.5 pHof gel. The prismatic type struvite-Na crystals with differentdiaphaneity and diverse apparent size ranging from 1 to 6 mmwere found in the cases with 1.05 SG of SMS solutions withinthe pH range of 6.0–7.5. The grown crystals had transparent,

Table 1Different growth parameters used to grow struvite-Na crystals.

Set Specific gravityof SMS

Concentration ofSDO (Reactant I) (M)

pH value of the gel Concentration of supernatant solutionof magnesium acetate (Reactant II) (M)

Liesegang Rings observedhaving pH values

1 1.04 1.00 5.5–8.0 1.00 8.02 1.04 1.50 5.5–8.0 1.50 –

3 1.05 0.50 6.0–8.0 0.50 47.54 1.06 0.75 6.0–8.0 0.75 46.85 1.06 1.00 6.0–8.5 1.00 47.06 1.06 1.50 6.0–7.5 1.00 47.0

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translucent and opaque diaphaneity, depending upon the location andthe growth conditions. It was noticed that transparency of the growncrystals gradually decreased with the increasing pH value of the gel aswell as with the increasing concentration of reactants. The pH duringthe gelling has a profound effect on gel structure and gel density. Asthe pH increases, the gel structure changes from distinctly box-likenetwork to a structure of loosely bound platelets, which appear to lackcross-linkages and the cellular nature becomes less distinct. Hence, thecrystals are not well defined and transparent due to this improperformation of the cells at higher pH values. Moreover, a greater geldensity implies smaller pore size and poor communication among thepores. In such a situation the nucleation density decreases andcontamination increases, which in turn spoils the perfection, shapeand transparency of the gel grown crystal.

As shown in Fig. 1(c), transparent prismatic crystals, with apparentsize of 6 mm were noticed at higher depth from the gel–liquidinterface. Star type poly-crystals were noticed in the cases of 1.04and 1.06 SG of gel near the gel–liquid interface. Since the diffusion ofthe magnesium acetate from the supernatant solution is more nearthe gel–liquid interface, higher concentration gradient develops in thegel near the gel–liquid interface, which produces local increases insupersaturation and this results in the growth of poly-crystals.

As shown in Fig. 1(b), Liesegang rings were also observed for6.8 and higher pH values of the gel. Table 1 illustrates the cases forwhich Liesegang rings were observed. During the diffusion of reactantI present in the SS into another reactant contained in the gel withhigher pH values, typical periodic structures of definite rings ofprecipitants were produced. These rings are known as Liesegang ringsand are separated by clear spaces in the gel. Henisch and Grasia-Ruiz[19–20] had explained that the Liesegang rings patterns arise from theinterplay between the reaction kinetics and the diffusion of thereactants used. It was observed that the number of Liesegang ringsincreased with the increasing pH of the gel. Prismatic struvite-Nacrystals were also observed along with Liesegang rings.

3.2. Powder XRD

Fig. 2 exhibits the powder XRD pattern of struvite-Na crystals.POWDER-X software was used to determine the unit cell para-meters of the grown struvite-Na crystal. The crystal structure of

struvite-Na was found to be orthorhombic with cell parameterssuch as a¼6.893 Å, b¼6.124 Å, c¼11.150 Å and α¼β¼γ¼901.

3.3. FT-IR study

The FT-IR spectrum of the struvite-Na crystals is presented inFig. 3. The vibrational band assignments according to literatureand experimental data are summarized in Table 2.

FT-IR spectroscopy provides a sensitive tool for the detection ofwater of crystallization in minerals and their binding states in acrystal. As shown in Table 2, there are four regions found in theFT-IR spectrum depicting the absorptions due to water of crystal-lization, which closely matched to the previously reported peaksin several inorganic hydrated compounds [21–28]. Intense bandsappearing at 3275.6 cm–1, 3389.5 cm–1 and 3521.5 cm–1 indicateH–O–H stretching vibrations of water of crystallization. Here, theposition of relatively broad band peak near 3275.6 cm–1 suggeststhat the water is strongly hydrogen bonded to the Mg cations.The weak bands appearing at 2401.1–2478 cm–1 in the spectrumcan be assigned due to H–O–H stretching vibrations of cluster ofwater molecules of crystallization. The medium intense bandsappearing at 1655.5–1704.4 cm–1 in the spectrum indicate theH–O–H bending modes of vibrations suggesting the presence ofwater [29]. A medium absorption band at 893.2 cm–1 indicates thewagging modes of vibration of the coordinated water and themetal–oxygen bond in the complex.

Vibrational modes of tetrahedral XY4 molecules are well known[30]. Julien et al. [31] had described the vibrational modes of thematerials containing PO4

3– anions. In the FT-IR spectrum, the ν1symmetric stretching vibration of tetrahedral PO4

3– anions units wasfound to be at a medium band at 1022.8 cm–1 which corresponds tothe previously reported values for different phosphate minerals[32–34]. The position of the symmetric stretching vibrations ismainly dependent on the type of mineral, the cation present andcrystal structure. The positions of the asymmetric stretching vibra-tions ν3 of phosphate PO4

3– anion units in struvite were found at thestrongest peaks 1065.9 cm–1, 1168.4 cm–1and 1239.5 cm–1, whichare in accordance with the values reported earlier [29,32,34,35].The symmetric bending vibrations ν2 of PO4

3– units were observednearly at the starting points of spectrum which corresponds to the

Fig. 1. (a) Struvite-Na crystals grown in the gel medium. (b) Formation of Liesegang rings in the gel medium. (c) Prismatic type struvite-Na crystals.

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previously reported values for various phosphate minerals [32].Moreover, the asymmetric bending vibrations ν4 of PO4

3– units instruvite were observed around 507 cm–1. Here, ν1 and ν3 involvethe symmetric and asymmetric stretching modes of the P–O bonds,whereas ν2 and ν4 involve mainly O–P–O symmetric and asym-metric bending modes with a small contribution of P vibration. Theabsorption peaks near 687.8 cm–1 ascribe the presence of oxygen–metal bond. Thus, the FT-IR spectrum of struvite-Na proves thepresence of water of hydration, P–O vibration and PO4

3– ion andmetal–oxygen vibrations.

3.4. Thermal study

Recently, thermal studies of struvite [1] and struvite-K [9] werereported. In the present study, the TGA, DTA and DSC were carriedout simultaneously. Fig. 4 shows the TGA, DTA and DSC profiles bythe curves (a), (b) and (c), respectively. From the TGA curve, it wasfound that the struvite-Na started dehydrating just above theroom temperature and finally at 600 1C it became 63.9% of theoriginal weight and remained almost constant up to the end ofanalysis. Mass loss at temperatures above 100 1C indicates associa-tion of water molecules. From TGA, the number of water moleculesassociated with the crystal was estimated to be 5. Total mass loss isfound to be 36.1%, which may be due to the loss of water ofcrystallization. This weight loss corresponds to the followingreaction for struvite-Na.

NaMgPO4 �7H2O-NaMgPO4þ7H2O (3)

The number of water molecules associated with struvite-Na isfound to be 5 instead of 7 from TGA, which may be due to thereasons such as (i) struvite-Na is thermodynamically unstable and

Fig. 2. Powder XRD pattern of struvite-Na crystals.

Fig. 3. FT-IR spectrum of gel grown struvite-Na.

Fig. 4. TGA, DTA and DSC profiles for the gel grown struvite-Na crystal.

Table 2Assignments of absorption bands in the FT-IR spectrum of struvite-Na.

Assignments Reported IR frequencies from otherminerals wavenumbers (cm-1)

Observed IR frequencies for struvite-Na wavenumbers (cm-1)

Absorption peaks due to waterof crystallization

H–O–H stretching vibrations of watercrystallization

3280–3550 3275.6, 3389.5, 3521.5

H–O–H stretching vibrations of cluster of watermolecules of crystallization

2060–2460 2401.1, 2478

H–O–H bending modes of vibrations 1590–1650 1655.5, 1704.4Wagging modes of vibration of coordinated water 808 893.2

Absorption peaks due to PO43–

unitsν1 symmetric stretching vibration of PO4

3– units 930–995 1022.8ν2 symmetric bending vibration of PO4

3– units 404–470 407ν3 asymmetric stretching vibration of PO4

3– units 1017–1163 1065.9, 1168.4, 1239.5ν4 asymmetric bending modes 509–554 507.4

Metal–oxygen bonds Metal–oxygen bonds 400–650 687.8Deformation of OH linked to Mg2þ 847 893.2

Table 3Thermodynamic parameters of dehydration/decomposition of struvite-Na.

Thermodynamic parameters Symbol Value

Standard entropy of activation Δ‡S1 13.76 J Mol–1 K–1

Standard enthalpy of activation Δ‡H1 96.28 kJ Mol–1

Standard Gibbs energy of activation Δ‡G1 91.02 kJ Mol–1

Standard internal energy of activation Δ‡U1 99.47 kJ Mol–1

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starts dehydrating just above the room temperature. (ii) For TGAstudy powder sample is used. Struvite-Na may lose some of thewater molecules during the sample preparation in the powderform, i.e., during the crushing of struvite-Na crystals.

In the DTA curve an endothermic peak was observed at 183.4 1C,which might be due to release of crystalline water. Processes involvinga loss of mass usually give rise to endothermic nature in DTA tracebecause of the work of expansion. During this endothermic process,the amount of heat change was found to be 403.10 μVs/mg. On furtherheating at higher temperatures, anhydrate struvite-Na was obtained.In the DTA curve two exothermic peaks were observed: one mediumpeak at 100 1C and the second strong one at 674 1C. Second exothermicpeak might be due to high temperature phase transition. During thisexothermic process at 674 1C, the amount of heat change was foundto be –28.40 μVs/mg. The DSC curve exhibited peaks at the sametemperatures as peaks were obtained in the DTA curve. In the TGAcurve, no remarkable change was observed for the peak noticed at674 1C in DTA and DSC curves, since no change took place in the massof the specimen.

Kinetic and thermodynamic parameters were calculated byapplying the Coats and Redfern relation [36] to the thermogram.The values of activation energy E, frequency factor A and the orderof reaction n are 102.66 kJ Mol–1, 4.17�1013 and 2, respectively.Table 3 gives the values of thermodynamic parameters of dehy-dration and decomposition for struvite-Na, which were calculatedby applying well known formulas, as described by Laidler [37].Here, positive value of Δ‡S1 depicts that the process is sponta-neous. Positive value of Δ‡H1 shows that the enthalpy increasesduring the process and such a process is an endothermic process.Positive value of Δ‡G1 demonstrates that struvite-Na is thermo-dynamically unstable.

3.5. Dielectric study

The dielectric constants and dielectric loss of struvite-Na wereevaluated with the frequency of applied field at room temperature.For struvite-Na the value of dielectric constant was 22.09 at400 Hz and was reduced to 12.31 at 100 kHz frequency of appliedfield. The value of dielectric constant was the maximum at lowerfrequencies, since all mechanisms such as space charge, orienta-tion, ionic and electronic polarizations are operative at lowerfrequencies. But at higher frequencies, these mechanisms cannotfollow the frequency of applied electric field and hence the valuesof dielectric constant are low. The value of dielectric loss alsodecreased with the increasing value of frequency.

4. Conclusion

Crystals of struvite-Na can be grown by using the singlediffusion gel growth technique and growth conditions play animportant role in the growth morphology of the grown crystals.The grown struvite-Na crystals had transparent, translucent andopaque diaphaneity, depending upon the location and the growthconditions. The phenomenon of the formation of Liesegang ringswas observed for 6.8 and higher pH values of the gel. The powderXRD studies confirmed the structural similarity of the grownstruvite-Na crystals with struvite. It was found that struvite-Nacrystallized in the orthorhombic Pmn21 space group with unit cellparameters such as a¼6.893 Å, b¼6.124 Å, c¼11.150 Å, andα¼β¼γ¼901. The absorption peaks obtained in the FT-IR spec-trum confirmed the water of crystallization, symmetric as well asasymmetric stretching and bending vibration of PO4 units andmetal–oxygen bonds in struvite-Na. The presence of water mole-cules was also confirmed by TGA. Two remarkable peaks wereobserved in the DTA and DSC curves. A very strong endothermic

peak observed at 183.4 1C attributed to release of crystalline water.A medium exothermic peak observed at 674 1C attributed to hightemperature phase transition. It was found that the dehydrationand decomposition process was endothermic, spontaneous andthe substance was thermally unstable. Dielectric study showedthat the dielectric constant and dielectric loss decreased with theincreasing value of the applied frequency.

Acknowledgments

The author (CKC) is thankful to Shri A. M. Tiwari, PrincipalSecretary and Commissioner of Higher Education, Department ofEducation, Government of Gujarat; and Dr. Pragna Vadher, Principal,Government Science College, Gandhinagar, for their encouragements.

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Please cite this article as: C.K. Chauhan, M.J. Joshi, Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.052i