Spectroscopic and Microscopic Identification of the Reaction Products

Research Article pubs.acs.org/journal/ascecg

Spectroscopic and Microscopic Identification of the Reaction Products and Intermediates during the Struvite (MgNH4PO4·6H2O) Formation from Magnesium Oxide (MgO) and Magnesium Carbonate (MgCO3) Microparticles Erica Kirinovic,† Amanda R. Leichtfuss,† Criztel Navizaga,‡ Hanyu Zhang,‡ Jennifer D. Schuttlefield Christus,*,† and Jonas Baltrusaitis*,‡ Downloaded via TUFTS UNIV on July 6, 2018 at 21:14:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemistry, University of Wisconsin Oshkosh, 800 Algoma Boulevard, Oshkosh, Wisconsin 54901, United States Department of Chemical and Biomolecular Engineering, Lehigh University, 111 Research Drive, Bethlehem, Pennsylvania 18015, United States

‡

ABSTRACT: One of the key global challenges forthcoming will be maintaining a clean, useable natural water supply. Anthropogenic wastewater is an unavoidable result of population growth and societal development; therefore, the treatment of wastewater is of the utmost importance. The mineral struvite (magnesium ammonium phosphate hexahydrate, MgNH4PO4·6H2O) is a crystalline material that occurs naturally in decomposing organic materials and been observed in sludge derived from the anaerobic digestion of animal farming liquid wastes and treated wastewater sludge. The accumulation of struvite on pipe walls and equipment surfaces has plagued the wastewater treatment industry though the formation of struvite prior to the treatment process could potentially provide a pathway for the sustainable recovery of the major nutrients nitrogen (N) and phosphorus (P). Current methods of nutrient removal from wastewater are mostly based on insoluble Fe, Al, and Ca salt formation followed by landfill disposal without returning them to the environment. Struvite is one of the most promising chemical platforms for recovering nutrients, which previously was done using expensive water-soluble magnesium salts. Our objectives were to examine the potential of low solubility, naturally abundant magnesium inorganic materials (MgO and MgCO3) for the utilization of nutrient recovery from wastewater via time-resolved ex situ XRD, ATR-FTIR and Raman analyses, and SEM measurements to identify reactive intermediates and use spectroscopic data for kinetics analysis. Our data suggest that a common reactive intermediate between homo- and heterogeneously nucleated struvite exists that is due to the amorphous magnesium hydroxide structural units. The presence of low coordination O4C−H, O3C−H, and O1C−H surface hydroxyl groups, associated with the surface steps, edges, and kinks, is proposed to enhance struvite formation, thus an increase in their abundance and stabilization are suggested for the preparation of MgO and MgCO3 before the struvite recovery. Two different crystal morphologies (needle and rhomboidal) were observed for reactions with 600 and 4000 ppm of (NH4)2HPO4, which were proposed to form due to the kinetic control of the reaction at higher concentrations. Finally, Raman spectroscopy was used to measure the relative kinetics of struvite formation utilizing the relative populations of the magnesium carbonate and struvite, as obtained from the area under the 950 and 1125 cm−1, respectively, peaks providing a spectroscopic method to monitor reactive solid magnesium source conversion into struvite. KEYWORDS: Struvite, Nutrient recovery, Wastewater, Spectroscopy, MgO, MgCO3, Hydroxyl groups

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INTRODUCTION

this wastewater can enter watersheds causing undesirable effects called eutrophication.7,9,10 A significant amount of effort has thus been directed toward recovering these nutrients from wastewater in a form of low solubility, slow-release fertilizers, such as struvite, MgNH4PO4·6H2O.5,7,9−15 A clear advantage of struvite use as fertilizer is the presence of three nutrients, N, P,

The world is experiencing unprecedented economic growth and increase in human population, thereby requiring more sustainable utilization of natural resources.1 Plant nutrient recovery from anthropogenic waste streams, such as municipal or agricultural wastewater,2−4 is of major importance since they contain considerable amounts of nitrogen (N) and phosphorus (P) in the form of aqueous ammonia (200 to 1000 mg/L NH4+)5,6 or phosphate (200 to 1000 mg/L PO43−)5,7 ions, with exact speciation dependent on the solution pH.5,8 If untreated, © 2017 American Chemical Society

Received: September 26, 2016 Revised: December 30, 2016 Published: January 3, 2017 1567

DOI: 10.1021/acssuschemeng.6b02327 ACS Sustainable Chem. Eng. 2017, 5, 1567−1577

Research Article

ACS Sustainable Chemistry & Engineering

yield locally supersaturated Mg2+ ions or direct surface mediated struvite growth on hydroxylated magnesium surface) is yet to be determined. In this study we utilized X-ray diffraction (XRD) in conjunction with time-resolved ex situ attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR) and Raman spectroscopy measurements of two insoluble magnesium source − MgO and MgCO3 − transformations into struvite in the presence of simulated NH4+ and PO43− containing wastewater solutions. XRD was utilized to monitor the speciation changes of the bulk materials, while the light spectroscopy was used to elucidate the nature and speciation of the products formed during these reactions on the surface of the particles and compared them with those obtained using homogeneous MgCl2 initiated struvite nucleation and growth. Finally, scanning electron microscopy (SEM) images were obtained on the synthesized particles to perform a qualitative comparison between the precursor and product morphology.

and magnesium (Mg), in one crystalline unit. This is due to the fact that unbalanced use of nutrients, such as N, P, potassium (K), calcium (Ca), Mg, and sulfur (S),16 can often preclude their full utilization leading to the pollution of soil and groundwater.17 While N and P are major nutrients, magnesium is the fifth major plant nutrient18 and a constituent of chlorophylls a and b. Recent studies have shown that twothirds of people surveyed in developed countries received less than their minimum daily Mg requirement.19 Although Mg is a common constituent of many minerals, comprising 2% of Earth’s crust, most soil Mg (∼98%) is incorporated in the crystal lattice structure of minerals and thus not directly available for plant uptake.20 Therefore, current state-of-the art methods focus on obtaining H2O soluble fertilizers via reaction with strong acids21 or recovering struvite using nucleation of aqueous Mg2+ ions in the presence of stoichiometric amounts of NH4+ and PO43− under alkaline conditions. Predominantly soluble Mg salts, such as MgSO4 or MgCl2, are used5 which comprise only a small portion of the natural magnesium containing minerals. Other Mg sources, such as periclase (magnesium oxide, MgO), dolomite (CaMg(CO3)2), magnesite (magnesium carbonate, MgCO3), and brucite (Mg(OH)2), are more naturally abundant but possess very low solubility. They present an excellent opportunity for struvite formation but are rarely used, and the mechanistic understanding of struvite formation reactions and the corresponding intermediates on low solubility Mg containing minerals is low.5,7,13,22,23 This is due to a conceptually different nucleation approach in this case involving two-dimensional solid particle reactions. For example, Stolzenburg et al. proposed surface Mg(OH)+ formation via reaction of solid MgO and liquid H2O as a first step to obtain solid Mg(OH)2.22 Mg(OH)2 then rapidly dissolved yielding Mg2+ causing supersaturation and inducing rapid (∼2−10 min) struvite precipitation resulting in dendritic growth and small struvite crystal sizes. Growth results obtained from MgO mimicked those obtained via homogeneous MgCl2 solutions.22 The formed particle crystal type has been observed to vary significantly in other works with the crystal type obtained from MgCl2 differing from MgO precursors.5 For example, irregular shaped crystals were obtained using some MgCl2 precursor reactions,15 while rod24 and hexagonal shapes25 were observed in other experiments. The ex situ Raman spectroscopy study of small (100 μm) MgO microparticles at circumroom temperatures showed the presence of struvite, as well as brucite in small particles, while struvite was the predominant material detected on large particles deposited from complex simulated swine manure solutions.7 A series of similar compounds (dittmarite − MgNH4PO4·H2O, bobierrite − Mg3(PO4)2·8H2O, newberyite − Mg(HPO4)·3H2O, hannayite − Mg(NH 4 ) 2 (HPO 4 ) 4 ·6H 2 O, schertelite − Mg(NH4)2H2(PO4)4·4H2O), however, can exist exhibiting vibrational peaks close to 942 cm−1 due to the PO43− stretching band.26 Struvite has been shown to transform into newberyite at room temperature in excess Mg and circumneutral pH, while at elevated temperatures it tended to dehydrate into dittmarite and bobierrite.27 MgCO3 showed lower propensity toward phosphate removal from cattle manure wastewater than MgO,28 and often it was solubilized by strong acids, such as H3PO4, first to directly obtain soluble Mg2+29 or reacted to form Mg(OH)2.30 Solid magnesite, however, also showed a propensity toward struvite formation directly,30 but the overall reactive mechanism (solution mediated solid dissolution to

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EXPERIMENTAL SECTION

Synthesis of Struvite. Struvite synthesis from simulated wastewater was performed using magnesium oxide (MgO, Baker Analyzed Reagent), magnesium carbonate (MgCO3, Matheson, Coleman & Bell, CB486), and magnesium chloride (MgCl2·6H2O, further in the text referred to as MgCl2, Fisher Scientific, Certified A.C.S.). Magnesium chloride was used as the conventional homogeneous nucleation reagent, whereas MgO and MgCO3 were used as heterogeneous struvite nucleation proxies. Simulated NH4+ and PO43− containing wastewater was prepared by adding various concentrations (600 to 4000 ppm) of dibasic ammonium phosphate ((NH4)2HPO4, Fisher Scientific, Certified A.C.S.) that were temperature controlled between 30 and 32 °C with constant stirring at 350 rpm. HPO42− is a predominant ion in weakly basic aqueous solutions (pH of 9.3 to 10.5, depending on the experiment) with pKa of 7.2131 that is necessary for struvite to form.5,12,30,32 Solutions ranging in concentrations between 600 to 1000 ppm MgO, MgCO3, or MgCl2 were added to the wastewater and stirred for up to 10 min. The products were filtered, dried using vacuum filtration, and kept in a desiccator prior to characterization. Kinetic experiments were performed using the same method with the experiment stopped at the various time points. Methods of Characterization. Many traditional surface analysis techniques, such as XRD, ATR-FTIR and Raman spectroscopy, and SEM, were used to analyze the precipitated products from the reaction of the magnesium containing precursors and simulated wastewater. X-ray Diffraction. XRD experiments were performed using a Rigaku D/Max-2000 T powder X-ray diffractometer operating at the voltage of 40 kV and current of 40 mA. The step size used was 0.02°. The Cu anode (0.15406 nm) was used. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. Ex situ ATR-FTIR spectroscopy was performed using a Thermo-Nicolet Nexus 670 FTIR equipped with a liquid-nitrogencooled narrow-band mercury cadmium telluride (MCT) detector. Typically 500 scans were acquired at an instrument resolution of 4 cm−1 over the spectra range between 650 and 4000 cm−1. The ATRFTIR internal reflection element (IRE, Pike Technologies) that was used was ZnSe. Spectra were obtained by creating films of the synthesized materials, which were pressed onto the IRE to increase contact with the solids for improved resolution. Raman Spectroscopy. Raman spectra were acquired using a WITec alpha300R confocal Raman microscope using 532 nm laser and ×20 objective. Laser intensity at the sample was ∼54 mW. Scanning Electron Microscopy. Solid particles were dispersed onto carbon tape that was attached to an aluminum stub. SEM images were acquired using a Hitachi 2460N environmental SEM at an accelerating voltage of 20 kV. Images were acquired after a reaction time of 10 min or at every sampling point for each magnesium source (vide inf ra). 1568


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struvite (JCPDS file No. 15-0762).33 The peak intensity slightly varies between each precursor showing slightly different crystalline planes being predominant. Specifically, the (012) and (211) planes as well as (022) and (221) planes change relative intensities as well as that of the predominant (020) mirrored by the (040) plane indicating the preferential orientation in the [010] direction on MgO derived struvite samples.35 These data show that struvite can be formed via aqueous conversion of insoluble magnesium precursors − either MgO or MgCO3 − in the presence of ammonium and phosphate source to yield solid particles structurally very similar to those of natural struvite. Slight differences in the XRD spectra, specifically in the prevalent crystalline planes, potentially suggest that growth conditions (and precursors) will affect morphological product distribution. A similar ratio of (001) and (110) XRD peak intensities for MgO grown at different temperatures was attributable to distinct morphological growth of the particles.36 The apparent differences in morphology of the struvite formed from MgCl2 when compared to those formed from MgO and MgCO3 will later be investigated using electron microscopy. ATR-FTIR spectroscopy was used to acquire spectral data of the struvite synthesized from the reaction of each of the magnesium precursors with (NH4)2HPO4. Figure 2a shows a reference struvite spectrum obtained from The RRUFF Project33 which was used to compare to the synthesized materials. The remainder of Figure 2 shows the spectral analysis of struvite synthesized from the reaction of 4000 ppm (NH4)2HPO4 and 1000 ppm of (b) MgO, (c) MgCl2, and (d) MgCO3. The peak assignments are shown in Table 1. ATRFTIR spectra were acquired using ZnSe internal reflection element and represent surface sensitive measurements providing chemical functionality information, as opposed to bulk structural information obtained by XRD. Due to the lower limit of ZnSe crystal range, measurements were obtained from 650 to 4000 cm−1 region omitting far-infrared spectral vibrations due to the crystal lattice vibrations. The far-infrared spectral vibrations will be revisited via Raman analysis (vide inf ra).

RESULTS AND DISCUSSION Fundamental Characterization of Synthesized Struvite Using XRD and ATR-FTIR. Struvite was synthesized in “proof of concept” experiments using 1000 ppm (1000 mg/L) of MgO, MgCO3, and MgCl2 reacted with 4000 ppm of dibasic ammonium phosphate, (NH4)2HPO4 (1080 mg/L of NH4+ and 2880 mg/L of PO43−), to obtain high rates of reaction and high conversion efficiencies of both solid reactants (MgO and MgCO3) as well as dissolved MgCl2 into struvite. The reactions of each magnesium precursor and the (NH4)2HPO4 solutions produced high purity struvite, which was analyzed using XRD and ATR-FTIR spectroscopy, are shown in Figures 1 and 2.

Figure 1. Stacked XRD spectra of powder products compared to reference spectrum34 in (d) of struvite. XRD patterns are shown for struvite synthesized by reacting 4000 ppm (NH4)2HPO4 with each of the following 1000 ppm magnesium precursors: (a) MgO, (b) MgCO3, or (c) MgCl2. Struvite was analyzed at the 10 min time point for all reactions.

Figure 1 shows the powder XRD patterns of a reference struvite spectrum33 that was used as a comparison for the synthesized struvite. The data shows that the material obtained is highly crystalline in all three cases and agrees very well with the peak positions for the reference struvite obtained from the literature34 (not shown) and the standard XRD pattern for

Figure 2. ATR-FTIR spectra of powder products were obtained for (a) a reference struvite spectrum obtained from The RRUFF Project33 and synthesized struvite after the reaction of 4000 ppm (NH4)2HPO4 with 1000 ppm of (b) MgO, (c) MgCl2, and (d) MgCO3 are shown. Spectral absorptions due to the ZnSe internal reflection element are shown using the ‘*’ symbol. 1569


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Table 1. Infrared and Raman Peak Assignments for Struvite Synthesized from the Various Starting Materials37,38 struvite from MgCl2

struvite from MgO

101, 144, 189, 229, 297, 393, 440, 570 700, 750 702, 758 874 880 950 950 981, 1020 982, 1010 1125 1125 1429 1431 1469 1468 1593, 1675 1580,1690 2212−2482 2200−2457 2800−3430 2800−3430 3498, 3603 3659, 3720

struvite from MgCO3 699, 754 882 950 978, 1020 1125 14730 1470 1603−1742 2206−2445 2800−3430 3661

literature

FTIR and Raman band assignment

750 880 950 1010 112548,49 1430 1470 1800−1500 2200−2500 2800−3430 3500−370042,48,49

lattice vibrational modes water−water H-bonding NH4+-water H-bonding ν3(PO43−) symmetric stretch ν3(PO43−) antisymmetric stretch ν1(CO32−) symmetric stretch NH4+ ν4 asymmetric bending split by “restrictive rotation” water ν2 (H−O−H) deformation + ν2(NH4+) water-PO43− H-bonding water ν1-ν3 sym-antisym stretch, NH4+ sym-antisym stretch surface Mg−OH groups

Table 2. Molar Composition of the Low (600 ppm) and High (4000 ppm) Concentrations of (NH4)2HPO4 for Constant Loading of MgCl2, MgO, or MgCO3 of 1000 ppm molar ratio [MgCl2(MgO,MgCO3)][NH4+][PO43−], mol/L experiment

MgCl2

MgO

MgCO3

1000 ppm/600 ppm 1000 ppm/4000 ppm

[0.0049][0.0091][0.0045] [0.0049][0.060][0.030]

[0.025][0.0091][0.0045] [0.025][0.060][0.030]

[0.012][0.0091][0.0045] [0.012][0.060][0.030]

Figure 3. Time resolved Raman spectra of powder products from the reaction of 1000 ppm MgCl2 with 4000 ppm (NH4)2HPO4 acquired from t = 1 to t = 10 min. Reference spectra of pure MgCO3 and MgO are also shown. A sharp peak at 0 cm−1 is due to the elastic (Rayleigh) scattering.

Peaks were observed at 2890 and 2357 cm−1 and have previously been assigned to NH4+ ν1-ν3 symmetric-antisymetric stretches in combination with water ν1-ν3 symmetric-antisymmetric stretches.37,38 The peak at 1430 cm−1 together with the shoulder at 1470 cm−1 is due to the NH4+ ν4 asymmetric bending motion split by the restrictive rotation.37 An interesting peak distribution can be seen from 1050 to 800 cm−1 where the PO43− group ν3 antisymmetric stretch has been reported in the literature at 1010 cm−1.37 In the struvite reference spectrum from the RRUFF Project,33 shown in Figure 2a, a very strong band appears at 980 cm−1 and is attributed in our work to the same PO 4 3− group ν 3 antisymmetric stretch. It is shifted to a lower wavelength from the literature value of 1010 cm−1.37 We propose that the peak at 980 cm−1 observed in the struvite reference spectrum as well as struvite synthesized by the magnesium precursors

(Figure 2 (b), (c), and (d)) results from the split of the ν3 antisymmetric stretch into two bands of varying intensity with the higher wavelength component situated at 1023 cm−1. This ν3 band is triply degenerate if PO43− tetrahedron is perfectly symmetrical and splits into several bands if that symmetry is distorted, which was shown here to be present for all the magnesium precursors.37 Additionally, the IR inactive symmetric PO43− stretch may become apparent weakly in the same 940 cm−1 region.39 Another explanation provided in the literature was related to the water librational band overlapping the ν3 PO43− band and contributing to the shape, intensity, and frequency of this broad feature.39 The dissimilarity in the ν3 PO43− bands obtained from reference struvite spectrum as well as those synthesized using aqueous solutions of (NH4)2HPO4 and MgO or MgCO3 suggests a local (hydrated) surface structure of these materials. The experimental spectra (b) and 1570


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Figure 4. Scanning electron microscope images of (a) MgO, (b) 1000/600 ppm MgO/(NH4)2HPO4, (c) 1000/4000 ppm MgO/(NH4)2HPO4, (d) MgCO3, (e) 1000/600 ppm MgCO3/(NH4)2HPO4, (f) 1000/4000 ppm MgCO3/(NH4)2HPO4, (g) 1000/600 ppm MgCl2/(NH4)2HPO4, and (h) 1000/4000 ppm MgCl2/(NH4)2HPO4.

In the MgCl2 precursor experiments, the high precursor concentration represents about one order of magnitude higher than the concentration of NH4+ and PO43− ions. The higher concentration in combination with a fast mixing at 350 rpm was used to ensure that no transport limitations occurred, and struvite formation was surface reaction limited. Spectra were acquired of solid dried synthesized particles separated from the MgCl2 precursor solution post reaction with (NH4)2HPO4 after 1, 2, 3, 4, 5, 7, and 10 min of reaction time. The spectra at the various time points are shown in Figure 3, with the 1 min timepoint shown as the bottom spectrum and the 10 min time-point being displayed as the top spectrum in Figure 3, together with the Raman spectra of pure, unreacted MgCO3 and MgO. For the spectra acquired at the various time points shown in Figure 3, a series of well-defined sharp bands can be observed on MgCl2 derived samples with peaks at 101, 144, 189, 229, 297, 393, 440, 570, 950, 997, 1058, 1451, 1690, 2357, 2952, 3124, 3178, 3246, 3496, 3603 cm−1. The peaks from 101 through 570 cm−1 can be attributed to the skeletal vibrations of struvite which are in agreement with previous work.26 A pronounced peak at 950 cm−1 is due to the symmetric PO43− stretch, while small peaks at 997 and 1058 cm−1 can be attributed to asymmetric PO43− stretches. A highly structured NH4+ and water-bonding region between 2200 and 4000 cm−1 can also be observed in Figure 3 for the time-resolved spectra. These peaks are due to water-PO43− hydrogen bonded vibrations at 2357 cm−1 combined with NH4+ ν1-ν3 symmetric-antisymmetric stretches and the water ν1-ν3 symmetric-antisymmetric stretches between 3100 and 3246 cm−1. Two distinct peaks at 3498 and 3603 cm−1 were not reported in the literature26 in the analysis of pure struvite crystals though Banks et al.37 observed them as broad shoulders and attributed these peaks to the same water ν1-ν3 vibrations. Literature data, however, suggest that these frequencies can also be associated with

(d) shown in Figure 2 suggest that the local symmetry of surface PO43− ions is much more distorted resulting in a symmetry loss and loss of degeneracy. Interestingly, this phenomenon is also observed in samples obtained via solution growth using the soluble MgCl2 precursor, shown in Figure 2(c), implying that there is hydration-assisted ion transport and increased surface water content. Finally, the peaks at 880 and 748 cm−1 have previously been assigned to NH4+−H2O and H2O−H2O bonding modes, which were confirmed by deuteration labeling experiments although the exact vibrational mode was shown to be difficult to assign.37,39 Time Resolved Study: Analysis of Struvite Using Raman spectroscopy and SEM. Speciation of Struvite Materials. Raman and IR spectroscopies provide complementary information. However, since Raman is more intense in πbonds of symmetric molecules compared to σ-bonds of atoms of different electronegativity, the latter are more intense in IR.40 Therefore, the symmetric PO43− stretch becomes the major vibration detected at ∼950 cm−1, while antisymmetric stretching modes and those involving H2O stretches become almost negligible.26 Ex situ Raman spectroscopy was performed on the insoluble (MgO and MgCO3) and soluble (MgCl2) magnesium sources. The time-resolved conversion into struvite from the insoluble and soluble precursors was accomplished at two different concentration levels shown in Table 2. In particular, 1000 ppm of the magnesium precursor in agreement with previous experiments7 was used while two concentrations of (NH4)2HPO4, 600 and 4000 ppm, were utilized. Experiments with high and low concentrations were performed because the more dilute solutions are more indicative of those that would be found in various wastewater streams5,7 and were also performed to monitor the time-resolved conversion of MgO, MgCO3, and MgCl2 into struvite, as shown in Figures 3−7. 1571


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ACS Sustainable Chemistry & Engineering surface hydroxyl groups originating on the MgO surface.41−47 In particular, stretching frequencies of hydroxyl groups on monatomic steps and corners with four and three surface coordinated oxygen hydroxyls (O4C−H and O3C−H) have been suggested to be responsible for vibrational frequencies observed in the 3500 to 3600 cm−1 region.42 Additionally, peaks have been observed in the vibrational spectra of natural magnesium carbonate minerals, such as dypingite (Mg5(CO3)4(OH)2· 5H2O), between 947 and 1012 cm−1 and have been attributed to the OH deformation modes in the MgOH units.48 The corresponding stretching vibrations of these MgOH units were observed in the 3500 to 3700 cm−1 region, which is in agreement with the data shown in Figure 3 and provides evidence for an alternative assignment of the surface termination. The surface hydroxylated termination is corroborated by the Raman analysis of raw MgCO3 and MgO used in the struvite synthesis. While MgO does not have Raman peaks, MgCO3 exhibits a strong peak due to the symmetric ν1 stretch of CO32−.49 Additionally, three peaks at 3456, 3527, and 3661 cm−1 were observed as shown in Figure 3. These also can be attributed to the stretching vibrations of residual hydroxyl groups on the sample surface. In particular, sharp peaks at 3593 and 3648 cm−1 have been observed for artinite (Mg2(CO3)(OH)2·3H2O) and dypingite, respectively. Peaks due to the −OH stretching vibrations generally were broader and observed below 3520 cm−1.49 The broad nature of peaks at 3498 and 3603 cm−1 in struvite samples synthesized from MgCl2 in Figure 3 also suggest a complex coordination environment. Overall, these data suggest that surface termination of magnesium containing materials, such as struvite, can potentially be different from the bulk due to the presence of the hydroxyl groups. Morphological Analysis via Scanning Electron Microscopy. SEM images of (a) MgO, (b) 1000/600 ppm MgO/ (NH4)2HPO4, (c) 1000/4000 ppm MgO/(NH4)2HPO4, (d) MgCO3, (e) 1000/600 ppm MgCO3/(NH4)2HPO4, (f) 1000/ 4000 ppm MgCO3/(NH4)2HPO4, (g) 1000/600 ppm MgCl2/ (NH4)2HPO4, and (h) 1000/4000 ppm MgCl2/(NH4)2HPO4 are shown in Figure 4. The morphology of the unreacted MgO and MgCO3 particles is shown in Figure 4a and d, respectively. As can be observed from the images, the particles are not uniform and are comprised of micron size aggregates. Upon conversion of these solid magnesium precursors in the presence of 600 and 4000 ppm (NH4)2HPO4 for 10 min new morphologically distinct particles were observed. In particular, elongated needlelike particles formed from the reaction of 600 ppm of (NH4)2HPO4 with both 1000 ppm of MgO and MgCO3, as shown in Figure 4b and e, respectively. This is in agreement with some reports of the crystallite shape of struvite reported in the literature.24 The nature of the residual material in contact with struvite crystals is difficult to rationalize. The XRD data in Figure 1 suggests only the presence of struvite with no MgO or MgCO3 crystalline peaks, which would be expected at 2θ of 42° for (200) reflection of cubic rocksalt MgO50 and 33° for magnesite.33 It can be proposed that some transient material is being formed that possesses an amorphous structure and serves as a precursor to struvite under aqueous solutions. Similar observations of different physical properties with the same chemical composition amorphous materials have been made for calcite single crystal surfaces using a combination of atomic force microscopy and X-ray photoelectron spectroscopy in the presence of water as relative humidity.51,52 Reaction of the magnesium precursors for 10 min

in the more concentrated (NH4)2HPO4 solution with MgO and MgCO3, shown in Figure 4c and f, respectively, allowed for the formation of crystals to occur with no visible fine irregular phase. Rather irregular shape crystals were obtained from MgO, while more rhomboidlike crystals were obtained from MgCO3. These are in agreement with a recent study by Li et al.53 where polyaspartic acid was used to modify the crystal habit of struvite grown from MgCl2 at a molar ratio of [Mg2+][PO43−][NH4+] of 1:1:3. The growth mechanisms in this study, however, are completely different since our solutions do not contain any organic growth inhibitors that preferentially bind to specific high-energy crystal surfaces. More likely this crystal shape dichotomy can be attributed to the local pH resulting from the ion flux to the crystal, provided the mixing rate is sufficient to achieve reaction, not diffusion limited mass transfer conditions. A high precipitation rate in the 4000 ppm (NH4)2HPO4 solution can decrease the concentration of the participating ions (Mg2+, PO43−, NH4+) thus affecting the local pH of the system at the double layer. Additionally, various struvite crystal faces possess varying concentration of the ions exposed for reaction ((010) and (101) faces of struvite have high density of Mg2+ ions)35 and thus provide a positively charged environment for enhanced growth. While pinpointing the exact reason for varying crystal morphologies is not trivial from SEM data, the SEM images obtained of struvite formed from MgCl2, e.g. via homogeneous nucleation, provide some additional information. These experiments, shown in Figure 4g and h for the reaction of MgCl2 with 600 and 4000 ppm of (NH4)2HPO4 solution, respectively, show very well-defined crystals with needlelike and rhomboidlike geometries which are very similar to those observed for MgO and MgCO3 derived struvite. One striking suggestion from these images is that heterogeneously nucleated insoluble magnesium source derived struvite exhibits the same crystal morphology as that of the homogeneously nucleated struvite under identical reaction conditions. The marked difference in shape morphology when using 600 and 4000 ppm of (NH4)2HPO4 as a reactant suggests different growth conditions. In particular, low concentrations of the precursor would lead to a crystal growth without induction of the new crystalline facets. At higher aqueous reactant concentration a higher driving force occurs with the resulting shape transformation because of a need to minimize the total interfacial free energy at a fixed volume. This is observed at 4000 ppm where needle morphology changes into rhomboidlike. Thus, we propose that the struvite growth at 4000 ppm is kinetically controlled where the shapes of the crystals deviate from those favored thermodynamically via the formation of higher order (higher energy) planes,54 as shown in Figure 4c, f, and h. Observed Kinetics for Struvite Formation on MgO and MgCO3. The growth kinetics of crystals are proposed to depend on nucleation rate and crystal growth. The growth of struvite crystals has been previously proposed to depend on the solution supersaturation and follows a linear growth model.7 This model, however, relies on the volumetric nuclei molar volume (in m3), rather than a surface (surface sites per m2) model, which is an approximation when dealing with water insoluble particles such as MgO and MgCO3. We performed time-resolved ex situ Raman spectroscopy to monitor struvite formation on 1000 ppm MgO and MgCO3 using both 600 and 4000 ppm of (NH4)2HPO4 solutions to monitor the early stages of the solid magnesium source conversion into struvite. Samples were taken at the appropriate times and dried, which 1572


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Figure 5. Time resolved Raman spectra of powder products from the reaction of 1000/600 ppm MgO/(NH4)2HPO4, 1000/4000 ppm MgO/ (NH4)2HPO4, 1000/600 ppm MgCO3/(NH4)2HPO4, and 1000/4000 ppm MgCO3/(NH4)2HPO4. Sharp peak at 0 cm−1 is due to the elastic (Rayleigh) scattering.

are shown in Figure 5. In the 1000/4000 ppm experiments (where 1000 ppm MgO or MgCO3 was reacted with 4000 ppm (NH4)2HPO4), an excess of NH4+ and PO43− ions was in solution above the stoichiometric 1:1:1 ratio necessary for formation with some NH4+ in excess to account for any possible NH3 volatilization in a basic environment. Lower (NH4)2HPO4 conditions of 600 ppm represented PO43− in an undersaturation scenario or below the stoichiometric ratio amount. Spectra were collected at 1, 2, 3, 4, 5, 7, and 10 min for the high and low concentrations of (NH4)2HPO4 with the two insoluble magnesium sources. The low (NH4)2HPO4 concentration regime of 600 ppm showed important information. In particular, 1000/600 ppm MgCO3/(NH4)2HPO4 spectra exhibits a strong peak due to ν1 of CO32− at 1125 cm−1 after the first 4 min of reaction while also maintaining the hydroxyl group region spectral shape with a sharp peak at 3661 cm−1. As gradual struvite growth begins the peak at 950 cm−1 starts increasing in intensity, but after 10 min the conversion is still incomplete, as inferred from the presence of the 1125 cm−1

peak. This suggests that the hydroxylation of the MgCO3 surface is necessary via Mgx(CO3)y(OH)z as a prerequisite for struvite formation. Also shown in Figure 5, the MgO data for the same low (NH4)2HPO4 regime − 1000/600 ppm MgO/(NH4)2HPO4 − on the other hand, shows relatively faster conversion and struvite formation, as inferred from the peak at 950 cm−1. The two sharp peaks − the stronger at 3659 and the weaker at 3720 cm−1 − are very pronounced at 1 min into the reaction even though they were absent on the dry initial MgO spectrum shown in Figure 3. These two peaks are due to the reactive intermediates involved in struvite formation on MgO with the former due to the same species as that in pure MgCO3. In contrast to the MgCO3 conversion, both the struvite and the hydroxyl peaks in the 3000−3700 cm−1 region coexist up to 10 min of conversion. In the MgCO3 conversion case, the struvite peaks gradually displace those due to the proposed Mgx(CO3)y(OH)z· as the conversion progresses. The peak at 3720 cm−1 can be attributed to strongly undercoordinated hydroxyl groups, e.g. O1C−H in convex areas of 1573


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ACS Sustainable Chemistry & Engineering MgO.42 This suggests a gradual conversion of O4C−H and O3C−H into O1C−H via dissolution or surface reaction processes. The O1C−H peak is absent in the 1000/600 ppm MgCO3/(NH4)2HPO4 reaction suggesting a different reaction mechanism. Here the diffusion/removal of the structural carbonate group to form isolated O4C−H and O3C−H might be suggested as the rate-limiting step. For the high (NH4)2HPO4 concentration regimes − 1000/ 4000 ppm MgO/(NH4)2HPO4 and 1000/4000 ppm MgCO3/ (NH4)2HPO4 struvite formation experiments − faster conversion of MgO to struvite than MgCO3 is again observed as inferred from the presence of both the 1125 and 3661 cm−1 peaks in MgCO3 experiments. No undercoordinated hydroxyl group intermediates can be observed in the 1000/4000 ppm MgO/(NH4)2HPO4 experiment except for those due to the stable surface termination with peaks at 3498 and 3603 cm−1. Partial conversion in 1000/4000 ppm MgCO3/(NH4)2HPO4 can still be seen even after 10 min providing additional evidence for the slower kinetics of the MgCO3 conversion to struvite than for MgO. A more detailed analysis of the spectral data shown in Figure 5 suggests two different struvite growth regimes for the two (NH4)2HPO4 concentration investigated. In particular, if a reaction can be approximated as a first order it would follow an exponential decay, which can potentially be monitored via the decrease in the CO32− peak at 1125 cm−1 and the increase in the PO43− peak at 950 cm−1. To minimize the extraneous effects of fluctuating laser intensity between the samples, the ratio of both peak intensities − I(CO32−)/ I(struvite) − was calculated and plotted as a function of time in Figure 6. It can be seen that in the low (NH4)2HPO4

achieved. This is in contrast to the 1000/4000 ppm MgCO3/ (NH4)2HPO4 − high (NH4)2HPO4 concentration regime − where exponential decay is discernably observed for struvite conversion. We attribute these differences in the struvite formation kinetics to the complex interplay between the surface reactions and the diffusion of the reacting molecules. Time Resolved Morphological Changes during Struvite Formation. A detailed SEM morphological analysis was performed of the time-resolved struvite formation experiments with Raman spectra shown in Figure 5. SEM images are presented for all of the 600 and 4000 ppm of (NH4)2HPO4 reaction experiments with 1000 ppm of MgO and MgCO3 at the time points from 1 to 10 min, as shown in Figure 7. The low concentration regime exhibited a significant amount of rather fine irregular material that was observed to be present at all time points, which coincided with 3659 and 3720 cm−1 peaks in Raman spectra shown in Figure 5. In contrast, virtually no irregular materials were observed in the 4000 ppm experiments when reacted with either the 1000 ppm MgO or MgCO3, providing additional evidence of the abrupt conversion to struvite. The combination of XRD, Raman, and SEM data suggests that at low concentrations of (NH4)2HPO4 the reactive intermediate is an amorphous hydroxylated material resembling that of Mg(OH)2. The spectroscopic data reported here provides additional support to a recently proposed reactive struvite intermediate by Stolzenburg et al.22 The suggestion that an amorphous intermediate is not some form of struvite is supported by the absence or late presence of the 947 cm−1 Raman peak in Figure 5 for 600 ppm MgCO3. Additionally, the SEM images in Figure 7 that were collected for the 600 ppm MgO experiment also correlate very well with the relative abundance of the amorphous material indicated by the Raman data. In the high (NH4)2HPO4 concentration regime, an irregular needle crystal shape for the 1000/4000 MgO/ (NH4)2HPO4 reaction is observed to only be slightly impacted by the growth time, while the corresponding crystal shape for the 1000/4000 MgCO3/(NH4)2HPO4 in Figure 7 is more rhombohedral. This suggests that positively charged high growth rate struvite crystal faces are potentially inhibited by some other ions, such as CO32−, and growth takes place on other facets departing from the needle geometry. This would be complicated by the fact that most likely the growth already proceeds in a kinetically controlled regime at these concentrations, as inferred from the MgCl2 SEM images in Figure 4.

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CONCLUSIONS A combination of XRD, ATR-FTIR, Raman, and SEM was used to explore the speciation, reactive intermediates and morphology of struvite crystals grown from MgO and MgCO3 particles. It was shown that well-defined struvite crystals can be grown from these virtually insoluble magnesium sources which indicates these reactions have potential to be used for nutrient recovery from various sources of wastewater. Time resolved Raman spectroscopy and SEM experiments suggested growth regimes dependent on the concentration of the model nitrogen and phosphorus precursor, (NH4)2HPO4. Additionally, a reactive intermediate was identified using Raman peaks at 3659 and 3720 cm−1 in support of a recent literature report.22 It is noted to be comprised of an amorphous structure that contains magnesium hydroxide structural units implying a common reactive intermediate between homo- and heterogeneously nucleated struvite. This suggests that for a sustainable nutrient recovery using insoluble MgO and MgCO3 additional

Figure 6. Plotted kinetics data for 1000/600 ppm MgCO3/ (NH4)2HPO4 and 1000/4000 ppm MgCO3/(NH4)2HPO4 obtained from Raman measurements. The x-axis represents reaction time and minutes, whereas the y-axis is the ratio of I(CO32−)/I(struvite) where I(CO32−) and I(struvite) are relative populations of the magnesium carbonate or struvite, as obtained from the area under the 1125 and 950 cm−1 peaks, respectively. Data for 1000/4000 ppm MgCO3/ (NH4)2HPO4 follows first order exponential decay, whereas that for 1000/600 ppm MgCO3/(NH4)2HPO4 has two distinct regions I and II representing nucleation and growth phases. See the text for more details.

concentration regime of 1000/600 ppm MgCO 3 / (NH4)2HPO4 two distinct reaction regions can be observed. In particular, reaction region I shows essentially no conversion up to the 5 min reaction time followed by an almost linear increase (seen as a negative slope) in region II in the struvite content in the particles analyzed. This can be viewed as a constant concentration case where supersaturation is not yet 1574


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Figure 7. Time resolved scanning electron microscope images acquired from t = 1 to t = 10 min of 1000/600 ppm MgO/(NH4)2HPO4, 1000/4000 ppm MgO/(NH4)2HPO4, 1000/600 ppm MgCO3/(NH4)2HPO4, and 1000/4000 ppm MgCO3/(NH4)2HPO4. The corresponding Raman spectra are shown in Figure 5.

into struvite. In summary, these data provide newly identified mechanistic aspects of struvite growth using MgO and MgCO3, which will be necessary if these materials are going to be used for efficient nutrient recovery from vast diversity wastewater streams. Experiments are under way to simulate more complex model wastewater solutions including those containing organic compounds, such as humic substances, and their effect on struvite formation kinetics and product speciation.

preparative steps can be taken that result in an enhanced number of the surface magnesium hydroxide groups. This is especially important for MgCO3 where diffusion/removal of the structural carbonate group to form isolated O4C−H and O3C− H might be suggested as the rate-limiting step. The presence of low coordination O4C−H, O3C−H, and O1C−H groups shown in this work to be reactive toward struvite formation on MgO has been associated with the surface steps, edges, and kinks,42,55 and synthesis or processing methods yielding their high distribution would result in a more efficient struvite formation. High surface area, disordered MgO with a high number of very basic defect sites has been synthesized before56 using widely available natural mineral magnesium precursors suggesting potential routes for sustainable struvite growth. Additionally, stabilization of a magnesium hydroxide-like layer, such as hydromagnesite (3MgCO3·Mg(OH)2·3H2O)57 on MgO, can also potentially provide the reactive groups necessary for improved reactivity toward struvite formation. Two different crystal morphologies (needle and rhomboidal) were observed for reactions with 600 and 4000 ppm of (NH4)2HPO4, which were proposed to form due to the kinetic control of the reaction at higher concentrations. Finally, Raman spectroscopy was utilized to measure the relative kinetics of struvite formation utilizing the relative populations of the magnesium carbonate and struvite, as obtained from the area under the 950 and 1125 cm−1, respectively, peaks providing a spectroscopic method to monitor reactive solid magnesium source conversion

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +1-610-758-6836. E-mail: [email protected] (J.B.). *Phone: +1-920-424-7101. E-mail: [email protected] (J.S.C.). ORCID

Jonas Baltrusaitis: 0000-0001-5634-955X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.K., A.L., and J.S.C. would like to thank the University of Wisconsin Oshkosh (UWO) and the Office of Student Scholarly and Creative Activities for their financial support of this project through a 2016 Student/Faculty Collaborative Grant and the Student Titan Employment Program (STEP). Additionally, the authors would like to thank Professors 1575


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(20) Senbayram, M.; Gransee, A.; Wahle, V.; Thiel, H. Role of magnesium fertilisers in agriculture: plant−soil continuum. Crop Pasture Sci. 2015, 66, 1219−1229. (21) Baltrusaitis, J.; Sviklas, A. M. From insoluble minerals to liquid fertilizers: magnesite as a source of magnesium (Mg) nutrient. ACS Sustainable Chem. Eng. 2016, 4 (10), 5404−5408. (22) Stolzenburg, P.; Capdevielle, A.; Teychené, S.; Biscans, B. Struvite precipitation with MgO as a precursor: Application to wastewater treatment. Chem. Eng. Sci. 2015, 133, 9−15. (23) Capdevielle, A.; Sýkorová, E.; Béline, F.; Daumer, M.-L. Kinetics of struvite precipitation in synthetic biologically treated swine wastewaters. Environ. Technol. 2014, 35 (10), 1250−1262. (24) Le Corre, K. S.; Valsami-Jones, E.; Hobbs, P.; Jefferson, B.; Parsons, S. A. Struvite crystallisation and recovery using a stainless steel structure as a seed material. Water Res. 2007, 41 (11), 2449− 2456. (25) Ichihashi, O.; Hirooka, K. Removal and recovery of phosphorus as struvite from swine wastewater using microbial fuel cell. Bioresour. Technol. 2012, 114, 303−307. (26) Frost, R. L.; Weier, M. L.; Martens, W. N.; Henry, D. A.; Mills, S. J. Raman spectroscopy of newberyite, hannayite and struvite. Spectrochim. Acta, Part A 2005, 62 (1−3), 181−188. (27) Bhuiyan, M. I. H.; Mavinic, D. S.; Koch, F. A. Thermal decomposition of struvite and its phase transition. Chemosphere 2008, 70 (8), 1347−1356. (28) Zeng, L.; Li, X. Nutrient removal from anaerobically digested cattle manure by struvite precipitation. J. Environ. Eng. Sci. 2006, 5 (4), 285−294. (29) Gunay, A.; Karadag, D.; Tosun, I.; Ozturk, M. Use of magnesit as a magnesium source for ammonium removal from leachate. J. Hazard. Mater. 2008, 156 (1−3), 619−623. (30) Huang, H. M.; Xiao, X. M.; Yang, L. P.; Yan, B. Removal of ammonium as struvite using magnesite as a source of magnesium ions. Water Pract. Technol. 2010, 5 (1), wpt2010007. (31) Haynes, W. M. CRC handbook of chemistry and physics: A readyreference book of chemical and physical data; 2009. (32) Stratful, I.; Scrimshaw, M.; Lester, J. Conditions influencing the precipitation of magnesium ammonium phosphate. Water Res. 2001, 35 (17), 4191−4199. (33) Swanson, H. E.; Morris, M. C.; Evans, E. H.; Ulmer, L. Standard X-ray diffraction powder patterns. NBS Monogr. (United States) 1964, Monograph (Sect. 3), 64. (34) Lafuente, B.; Downs, R. T.; Yang, H.; Stone, N. The power of databases: the RRUFF project. In Highlights in Mineralogical Crystallography; Armbruster, T., Danisi, R. M., Eds.; W. De Gruyter: Berlin, Germany, 2015; pp 1−30. (35) Li, H.; Yao, Q.-Z.; Wang, Y.-Y.; Li, Y.-L.; Zhou, G.-T. Biomimetic synthesis of struvite with biogenic morphology and implication for pathological biomineralization. Sci. Rep. 2015, 5, 7718. (36) Henrist, C.; Mathieu, J.-P.; Vogels, C.; Rulmont, A.; Cloots, R. Morphological study of magnesium hydroxide nanoparticles precipitated in dilute aqueous solution. J. Cryst. Growth 2003, 249 (1−2), 321−330. (37) Banks, E.; Chianelli, R.; Korenstein, R. Crystal chemistry of struvite analogs of the type MgMPO4.6H2O (M+ = potassium(1+), rubidium(1+), cesium (1+), thallium(1+), ammonium(1+). Inorg. Chem. 1975, 14 (7), 1634−1639. (38) Max, J.-J.; Chapados, C. Aqueous ammonia and ammonium chloride hydrates: Principal infrared spectra. J. Mol. Struct. 2013, 1046, 124−135. (39) Stefov, V.; Šoptrajanov, B.; Kuzmanovski, I.; Lutz, H. D.; Engelen, B. Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate (struvite) and its isomorphous analogues. III. Spectra of protiated and partially deuterated magnesium ammonium phosphate hexahydrate. J. Mol. Struct. 2005, 752 (1−3), 60−67. (40) Sharma, B. K. Instrumental Methods of Chemical Analysis; Krishna Prakashan Media: Meerut, India, 2000.

Jennifer Wenner and Nenad Stojilovic at UWO for their help with using the XRD facility. J.B. acknowledges Lehigh University for startup support and a CREF grant by Lehigh University that supported the Raman confocal microscope acquisition.

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Spectroscopic and Microscopic Identification of the Reaction Products

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