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Preparation, characterization and kinetic evaluation of struvite in various carboxylic acids
Journal of Crystal Growth 531 (2020) 125339
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Preparation, characterization and kinetic evaluation of struvite in various carboxylic acids
T
Sevgi Polat, Perviz Sayan
⁎
Department of Chemical Engineering, Faculty of Engineering, Marmara University, 34722 İstanbul, Turkey
ARTICLE INFO
ABSTRACT
Communicated by T. Nishinaga
In this study, struvite crystallization was investigated using different carboxylic acid media—propionic, tartaric, and trimesic acid. The struvite was synthesized as a result of the reaction between magnesium chloride hexahydrate and ammonium dihydrogen phosphate. In the first part. of the study, the crystals prepared with and without additives were characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and zeta potential measurements. XRD results showed that the struvite prepared in pure media comprised crystals with the normal orthorhombic structure. SEM images confirmed that the struvite underwent morphological changes when prepared in carboxylic acid media. FTIR analysis indicated that the carboxylic acids affected the functional groups of the struvite. Zeta potential measurements showed that struvite crystals have a −13.2 ± 1.8 mV zeta potential value, and that the surface of the crystals in trimesic acid media became more positive than that created in pure media with an increase in value to −2.3 ± 0.9 mV. In the second part of the study, the thermal decomposition of the struvite crystals was investigated in detail. The thermal data were used to calculate the activation energy that corresponded to struvite dehydration prepared with and without additives using a nonisothermal kinetic analysis based on the Flynn-Wall-Ozawa, KissingerAkahira-Sunose, Starink, and Tang models. The mean activation energy calculated for struvite crystallization in pure media was between 56.06 and 52.13 kJ/mol, depending on the kinetic method used.
Keywords: B1. Struvite B1. Crystallization A1. Morphology B1. Carboxylic Acids A1. Kinetics A1. Model free
1. Introduction Struvite crystals (magnesium ammonium phosphate hexahydrate [NH4MgPO4·6H2O]) are hydrated phosphates comprising equimolar amounts of magnesium (Mg), ammonium (NH4+), and phosphate (PO43−) [1,2]. Although struvite is commonly used in industry, struvite crystals are also among the most common of urinary stones in animals, including humans [3]. In industry, such as wastewater treatment plants, struvite formation can clog pipes, heat exchangers, and centrifuges and causes vales to freeze up. Its low solubility in water, creates turbulence that increases the energy needed to move water though the system, and raises pH [4–6]. To resolve these issues, time-consuming and expensive measures must be implemented [6,7]; therefore, prevention of struvite crystallization is a major focus in industrial facilities. Several studies have been conducted to determine the process of struvite crystallization to help develop new mechanisms by which to resolve these issues. To that end, the formation of struvite crystals has been investigated from several aspects with results indicating that several physicochemical parameters, such as pH [8,9], temperature [10], supersaturation [11],
⁎
operating conditions [12,13], and various additives [7,14–23] play a role in crystal formation. Zhang et al. [14] have researched the inhibitory effects of humic acid and citric acid on struvite precipitation and crystal growth. Wei et al. [20] have investigated the adsorption behavior of alginic acid in relation to pH, ionic strength, adsorption time, and its concentration and have assessed its prohibitory effect on struvite, which has been reported to be considerable because of its adsorption mechanism. That is, small amount of these additives changed the nucleation rate, crystal growth, morphology, and physicomechanical properties. In this study, struvite crystallization was investigated using propionic acid, tartaric acid, and trimesic acid as additives. These additives were selected because studies have been a limited on struvite crystallization using carboxylic acids with different carboxyl group number and chain length and because these additives are inexpensive, easy to handle, and nontoxic. In our study, the crystalline structure, morphology, and particle size of the struvite crystals prepared using solutions with and without an additive were determined. In addition, the thermal decomposition behavior of the samples was investigated in
Corresponding author. E-mail address: [email protected] (P. Sayan).
https://doi.org/10.1016/j.jcrysgro.2019.125339 Received 7 August 2019; Received in revised form 20 October 2019; Accepted 5 November 2019 Available online 07 November 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 531 (2020) 125339
S. Polat and P. Sayan
detail and modeled the kinetic precipitation process. 2. Experimental
the solid products were washed with distilled water. Finally, the products were dried, and the prepared crystals were used for further analysis.
2.1. Materials
2.3. Property characterization
Analytical-grade magnesium chloride hexahydrate (MgCl2·6H2O), ammonium dihydrogen phosphate (NH4H2PO4), sodium hydroxide (NaOH), propionic acid (C3H6O2), tartaric acid (C4H6O6), and trimesic acid (C9H6O6) were purchased from Merck. All solutions were prepared using distilled water.
X-ray diffraction (XRD) was conducted using the Bruker D2 Phaser benchtop XRD analyzer with copper (Cu) Kα radiation (λ = 1.5418 Å) at 30 kV and 10 mA in the 2θ range from 10° to 60° for phase identification. The morphologies of the struvite crystals prepared with and without an additive were examined using the Zeiss EVO LS 10 scanning electron microscopy (SEM; Carl Zeiss AGO, Berkochen, Germany). Particle size was analyzed using the Mastersizer 2000 laser particle size analyzer (Malvern Panalytical, Malvern, UK). The functional groups of the crystals were identified using the IRAffinity-1S Fourier-transform infrared spectrometer (FTIR; Shimadzu) with the wavenumber ranging from 4000 to 400 cm−1 at a resolution of 4 cm−1 with eight scans per sample. The surface properties of the struvite were examined using the K-Alpha™ X-ray Photoelectron Spectrometer (XPS) System (ThermoFisher Scientific) with an Al Kα X-ray source. The zeta potential of the struvite was measured using a Malvern Zeta Sizer Nano Series Nano-ZS (Malvern Panalytical). Each measurement was repeated at least 10 times, and the average value was calculated. All experiments were performed at 25 °C and pH 8. The thermal properties of the struvite were examined using thermogravimetric analysis (TGA) and differential thermogravimetric (DTG) analysis conducted on a Setaram LABSYS Evo instrument. In each analysis, approximately 10 mg sample was placed in an aluminum oxide (Al2O3) crucible and heated over temperatures ranging from 30 °C to 400 °C at rates of 5, 10, and 20 °C/ min. A continuous nitrogen flow at a rate of 20 mL/min was supplied to TGA to maintain an inert atmosphere. Kinetic calculations were also conducted using TGA data.
2.2. Experimental method The experiments were conducted in an enclosed glass crystallizer with a working capacity of 1 L. The schematic representation of experimental setup is shown in Fig. 1. The crystallizer was equipped with a thermostatic jacket, a mechanical stirrer, and a pH control system. Two peristaltic pumps and an infusion pump were used for feeding the reactants and the additive into the crystallizer, respectively. The crystallizer environment was maintained at 40 °C and the pH of the system was maintained at 8 by adding dilute NaOH solution using a pH control system. A stirring mechanism comprised a three-blade propeller and mechanical mixer and was maintained at a rate of 500 rpm. The struvite crystals were prepared using the reaction of MgCl2·6H2O and NH4H2PO4. 250 mL each 1.0 M MgCl2·6H2O and 1.0 M NH4H2PO4 were concurrently added into the crystallizer through the peristaltic pumps at 5 mL/min to create the struvite crystals. The additives propionic acid, tartaric acid, and trimesic acid were used at constant concentration for each carboxylic acid (250 ppm) to investigate the influence of the additives and compare their effect on struvite crystallization. In the experiments conducted in the presence of propionic acid, tartaric acid, and trimesic acid, the required quantity of additive was prepared by dissolving it in distilled water, and these solutions were then used in the experiments. 50 mL of additive solution was added to the crystallizer simultaneously during the process of struvite crystallization through an infusion pump at 1 mL/min. At the end of the experiments, the suspension was withdrawn and filtered, and
3. Results and discussion 3.1. X-ray diffraction analysis XRD analysis was used to identify the structure of the struvite
Fig. 1. Experimental setup.
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crystallization medium, most likely from incorporation of the carboxylic acid molecules into the crystal lattice, creating structural imperfections and strains in the lattice symmetry, which resulted in the peak shifts and changes from that in pure media. The lattice parameters for propionic acid, tartaric acid, and trimesic acid were determined to be a = 6.963 Å, b = 6.143 Å, c = 11.218 Å; a = 6.967 Å, b = 6.149 Å, c = 11.236 Å; and a = 6.959 Å, b = 6.143 Å, c = 11.223 Å, respectively. 3.2. Scanning electronic microscopy analysis Fig. 3 shows the SEM images of the struvite crystals prepared with and without a carboxylic acid. As observed in Fig. 3a, the crystals prepared in pure media were rod-like and had a tendency to grow atop each other. The mean particle size of the crystals was 17.6 µm. Fig. 3b shows the SEM image of the struvite crystals prepared with propionic acid, which has a single carboxyl group. As observed from the figure, the morphology of the struvite crystals changed after adding propionic acid to the crystallization medium. The rod-like crystals were replaced with hollowed, bent plate structures. This bending created crystals with holes in their middle that emerged during the advanced stages of development. Although the crystals that were formed were not uniform, they grew atop each other and had a tendency to agglomerate. The hollowed structure with a particle size of 27.4 µm was the common characteristic of the crystals prepared in the presence of propionic acid. Fig. 3c shows the SEM image of the crystals that were produced in the presence of tartaric acid. Compared to those prepared in pure media, the presence of tartaric acid, which has double carboxyl groups similar to those of acids with a single carboxyl group, resulted in the formation of a larger amount of shorter, blunted plate-shaped crystals in nonuniform sizes that had a tendency to develop in layers; there were also holes along these crystals, and their mean particle size was 33.4 µm. Deformations and irregular shapes were also observed on the crystal surfaces. These forms could be broken under hydrodynamic conditions in the media. Although the developed crystals were triangular prisms, holes were observed at the midway to the tip of the triangle. Fig. 3d shows the SEM image of the crystals prepared in the presence of trimesic acid. The developed struvite crystals were rod-like and thicker than those combined with other additives. The sharp edges of the struvite crystals were replaced by more rounded crystals without sharp corners. The surfaces of these crystals were rough with substantial deformation. The hollowed structures observed in the crystals in the trimesic acid medium were smaller than in those combined with other media. The mean particle size of these struvite crystals was 53.7 µm. In addition to particle size analysis, the width-to-length ratios of the struvite crystals were determined in this study. The width-to-length ratios of the crystals obtained in the presence trimesic acid exhibited a considerable transformation as compared to those of the crystals obtained in pure media and with the other additives. The lengths of the crystals were shortened and their widths were increased in the presence of carboxylic acid media. While width-to-length ratios of the struvite obtained in pure media was 0.24, this value was determined as 0.32, 0.45 and 0.53 for propionic acid, tartaric acid, and trimesic acid, respectively. Accordingly, it can be concluded that the additives used in this study are the effective crystal modifiers to change the crystal morphology and size of the struvite.
Fig. 2. XRD spectra for struvite crystals prepared with and without different carboxylic acid media.
crystals. Fig. 2 shows the XRD patterns of the crystals prepared with and without carboxylic acid. As seen from the patterns prepared in pure media, the 2θ values of 14.96° (1 0 1), 20.82° (1 1 1), 27.02° (1 0 3), 33.22° (0 2 2), and 45.96° (3 0 3) suggest that the peaks belong to the orthorhombic Pmn21 space group, and these results were coincident with the literature [24] (i.e., the XRD peaks were attributed to the sole existence of the struvite form in pure media). Materials Analysis Using Diffraction (MAUD) software was used to determine the lattice parameters of the crystals prepared in pure media, which were a = 6.969 Å, b = 6.145 Å, c = 11.241 Å, and α = β = γ = 90°. The XRD patterns of the struvite crystals prepared using different carboxylic acids are shown in Fig. 2. It was observed that the XRD peaks did not include any characteristic carboxylic acid peaks because of the trace amount of the additive, and this result indicated that the presence of the additive within the studied concentration range had no effect on crystal type. Nevertheless, the peak intensities changed and the peaks shifted slightly with the addition of the carboxylic acids to the
3.3. Fourier-transform infrared spectroscopy analysis FTIR spectroscopy analysis was used to determine the chemical structure of the struvite and identify the absorption behavior of the additives. The FTIR spectra obtained for the struvite crystals prepared
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Fig. 3. SEM images of struvite crystals prepared in the absence (a), and presence of different carboxylic acid media: propionic (b), tartaric (c), and trimesic (d) acid.
with and without a carboxylic acid are illustrated in Fig. 4. The wide transmittance band within the ranges of 3500 and 3030 cm−1 for the struvite prepared in pure media represented the hydrogen–oxygen–hydrogen (HeOeH) stretching vibrations of the water of crystallization. The peaks at ~2370 cm−1 and ~1680 cm−1 were assigned to the HeOeH stretching and bending vibrations of the water molecules, respectively. The bands located at ~2900 cm−1 and ~1435 cm−1 were characteristic of the nitrogen NeH stretching vibrations in the NH4+ unit. The band at ~980 cm−1 denoted the presence of a PO43− stretching vibration [25–27]. The FTIR results showed that OeH, NeH, and PeO functional groups were detected mainly on the surface of the struvite crystals prepared in pure media. The FTIR spectra obtained for the struvite crystals prepared with propionic, tartaric, and trimesic acid indicated the characteristic peaks of struvite. In addition, a new slight absorption peak appeared in the crystals developed in additive media compared with the peaks in those developed in pure media. This peak at ~1720 cm−1 showed that the additives reside on the struvite crystals corresponding to the characteristic peak of the carbonyl group of the carboxylic acid [28,29], which indicates that the additives were adsorbed onto the surface of the struvite. To determine the amount of adsorbed carboxylic acids on the crystal’s surface and to characterize the crystals quantitatively, the adsorption properties of the crystals were examined. Upon completion of the process, the samples were collected, filtered and the filtrate analyzed for the residual carboxylic acid concentration using a UV spectrometer. The amounts of propionic acid, tartaric acid, and trimesic acid adsorbed per unit mass of the struvite crystals were determined to be 0.27 mg/g, 0.31 mg/g and 0.44 mg/g, respectively.
3.4. X-ray photoelectron spectrometer analysis XPS analysis was conducted to further determine the adsorption behavior of the carboxylic acids used in the struvite crystallization process, and the Mg(1s), O(1s), N(1s), and P(2p) patterns are provided in Fig. 5. Fig. 5a displays the Mg(1s) peak of the struvite crystals at a binding energy of 1303.3 eV, which is the related to the MgeO bond, while the same signal appeared at 1308.6 eV, 1310.7 eV, and 1302.0 eV after adding propionic acid, tartaric acid, and trimesic acid, respectively. The shifting observed in the binding energy of Mg(1s) displayed the interaction between the additives and the struvite, which was most likely between the eCOOH group of the carboxylic acids and the Mg groups on the struvite crystals. The same change in the Mg(1s) was observed for the O(1s) pattern, as seen in Fig. 5b. In addition, compared to the peaks of the crystals developed in pure media, there was a sharp reduction in peak intensity, which is the indicator of the additives bound to the crystal surface. As seen in Fig. 5c, the peak located at 402.6 eV for pure media can ascribe to the NH4+ group; however, the binding energy of N(1s) shifted to 409.5 eV, 411.1 eV, and 401.9 eV for propionic acid, tartaric acid, and trimesic acid, respectively. As shown in Fig. 5d, P(2p) has a peak at 134.6 eV, which is attributed to the P]O bond for the struvite crystal prepared in pure media. The binding energy of P(2p) slightly decreased from 134.6 to 133.3 eV for trimesic acid; whereas, this energy increased in the presence of propionic acid and tartaric acid by 5.9 and 9.3 eV, respectively; therefore, these results can be suggested as one proof of their interaction with struvite crystals, which led to the adsorption of the additives on the crystal surfaces.
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Fig. 4. FTIR spectra of struvite crystals prepared with and without different carboxylic acids media.
3.5. Zeta potential analysis
3.6. Thermal analysis
As stated in the literature, the zeta potential is an important parameter that could influence the properties of the struvite crystals [30]; therefore, zeta potential measurements were taken to investigate the effect of propionic, tartaric, and trimesic acids on the surface electrical charge of the crystals. In this study, the zeta potential for struvite crystals prepared in pure media in its own saturated solution was negatively charged at −13.2 ± 1.8 mV. The presence of carboxylic acids in the crystallization media drastically affected the surface charge of the struvite, and the zeta potential then became more positive. The zeta potentials of the struvite crystals prepared in propionic, tartaric, and trimesic acid media were −11.6 ± 1.1, −7.2 ± 1.7, and −2.3 ± 0.9 mV, respectively. This result showed that trimesic acid, in particular, was very effective for changing the surface charge of the struvite crystals and that the additives can be adsorbed onto the crystal surface. In addition, the variations in zeta potentials in the additive media were associated with the increasing agglomeration tendency of the crystals, which is consistent with the results of SEM analysis.
TGA was used to assess the structural evolution and the thermal decomposition behavior of the struvite crystals. The TG and DTG curves of the struvite crystals prepared with and without an additive for three different heating rates are shown in Fig. 6. In addition, specific decomposition temperatures, such as initial temperature, Ti; maximum peak temperature, Tmax; and final temperature, Tf, are provided in Table 1, which clearly shows that the characteristic temperatures shifted to higher values after increasing the heating rate. This relationship has been attributed to different heat-transfer rates, which affect the kinetics of thermal decomposition. In this study, the decomposition zone shifted to within the higher temperature used in the study, and the peak intensity increased with the increasing heating rate. As seen in Fig. 6, the thermal decomposition of the struvite at the different heating rates encompassed simultaneous dehydration and decomposition and liberation of H2O and NH3 molecules without exhibiting distinct steps in the process [31]. The amount of residue from the struvite prepared in pure media was 48.2% of the initial crystal
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Fig. 5. XPS of magnesium (Mg)(1s), oxygen (O)(1s), nitrogen (N)(1s), and phosphorus (P)(2p) of the struvite crystals.
weight, which was comparable to the theoretical value based on a theoretical weight loss of 51.42% for the molecule. This weight loss comprises 44.08% water and 7.34% ammonia [18]. The results for the weight loss were found to be in good agreement literature [32,33]. The residue amounts after adding propionic, tartaric, and trimesic acid were 47.9%, 47.4%, and 46.9%, respectively, which were lower than that of pure struvite. The decrease in the residue amounts indicated that after the acids adsorbed onto the crystal surface and the Mg ions interacted with the struvite surface, the carboxylic acids and struvite coprecipitated.
common isoconversional kinetic methods—FWO [35,36], KAS [37], Starink [38], and Tang [39]—were used to determine the activation energies of the struvite prepared with and without the carboxylic acids, Their linear equations are presented in Eqs. (1)–(4), respectively. FWO:
AE Rg (x )
ln( ) = ln
5.331
1.052
E1 RT
(1)
KAS:
ln
3.7. Kinetic analysis
T2
= ln
AR Eg (x )
E RT
(2)
Starink:
To study the kinetics of struvite’s thermal decomposition and calculate the activation energy, TGA was used to calculate the weight changes of the samples prepared with and without an additive as a function of temperature. Two methods can be used to evaluate these kinetics—isothermal or nonisothermal. The latter method is presumed to be simpler because it does not consider the changes in the chemical and physical properties of the sample and can be conducted using fewer experiments. Model-free or model-based methods are used to calculate kinetic parameters based on TGA data. With the model-free method, the reaction mechanism can be determined without having to calculate the activation energy, which can be an advantage [34]. In this study, four
ln
T1.92
=C
1.008
E RT
(3)
Tang:
ln
T1.8947
=C
1.0015
E RT
(4)
where x is the conversion rate, β is the heating rate, A is the pre-exponential factor, E is the activation energy, R is the gas constant, t is the time, T is the absolute temperature, and g(x) is the integrated reaction
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Journal of Crystal Growth 531 (2020) 125339
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Fig. 6. (a) TGA and (b) DTG curves obtained for the struvite crystals prepared in pure media; (c) TGA and (d) DTG curves obtained in the presence of propionic acid media; (e) TGA and (f) DTG curves obtained in the presence of tartaric acid media; and (g) TGA and (h) DTG curves obtained in the presence of trimesic acid media.
model. For any given value, it is possible to estimate E based on the gradients of the lines obtained from the plot of ln(β) versus 1/T, ln(β/ T2) versus 1/T, ln(β/T1.92) versus 1/T, and ln(β/T1.8947) versus 1/T for
the FWO, KAS, Starink, and Tang models, respectively. Fig. 7 presents the plots of the four models of the struvite crystals prepared in pure media from 0.1 to 0.9 conversion degree at different
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Table 1 Characteristic temperatures of the struvite decomposition process. Media
Heating Rate (°C/min)
Ti (°C)
Tmax (°C)
Tf (°C)
Pure Media
5 10 20 5 10 20 5 10 20 5 10 20
72 76 86 77 86 95 81 89 93 85 89 99
118 133 149 126 139 166 115 142 165 132 141 165
161 186 215 165 192 248 151 193 231 171 201 242
Propionic Acid Media Tartaric Acid Media Trimesic Acid Media
heating rates. As seen from the figure, each line is highly correlated with the four selected models and the experimental data fits well with the model equations, all within a range of 0.9839–0.9999, which suggests that the results were strongly reliable. The parallel lines represent similar kinetic behavior, which suggests that there is a single, identical reaction mechanism from struvite dehydration. Fig. 8 shows the activation energy as a function of the degree of struvite conversion with or without the carboxylic acids. Activation energy is the minimum amount of energy required to initiate a reaction; the lower the value of the activation energy, the more likely the reaction will occur. The results indicate that the minimum mean activation energy values from the FWO, KAS, Starink, and Tang models were 56.06, 52.69, 52.13, and 52.50 kJ/mol, respectively, which were in accordance with those reported in the literature [40]. The calculated values are very similar and the standard deviation among the different models is quite good, which indicates the consistency and reliability of these energies that were calculated using the four kinetic models and multiple heating rates. The minor differences in the activation energy values were the result of approximations, assumptions, and mathematical formulations that varied according to the model. Depending on which model was used, the activation energies were calculated to be 60.9–28.3, 49.7–27.7, and 53.8–32.3 kJ/mol for propionic, tartaric, and trimesic acid, respectively. For the FWO, KAS, Starink, and Tang models, the mean activation energies were calculated to be 45.7, 41.9, 41.3, and 41.4 kJ/mol; 40.8, 36.6, 36.2, and 36.3 kJ/ mol; 45.8, 42.0, 41.4, and 41.5 kJ/mol, respectively, for propionic, tartaric and trimesic acid. According to the calculated activation energy values, the crystals prepared with the additives required less energy to decompose. The energy needed was less for the crystals prepared with additives than for those prepared without additives, and a fluctuation was observed. This lower value exhibited degrees of fluctuations related to conversion, which suggests that the decomposition process comprises complex reactions. To determine the appropriate mechanism by which struvite crystals are degraded and the optimum reaction models for process, 15 reaction models (Table 2) for defining g(x) were used. These reaction models have been proposed by the Coats–Redfern method for kinetic analysis, which considers diffusion, reaction, or nucleation as the rate-limiting steps [41–44]. The plotting method was used to correctly describe the thermal degradation process, with ln(g(x)/T2). This graph should be linear and have a high correlation coefficient. The most probable reaction
Fig. 7. Plots of (a) Flynn-Wall-Ozawa, (b) Kissinger-Akahira-Sunose, (c) Starink, and (d) Tang models of the struvite crystals prepared in pure media.
mechanism is represented by the greatest linearity. The evaluations were conducted based on these models for the struvite crystals developed in all media using three heating rates. The results are provided in Table 3. As is clearly seen in the Table 3, chemical reaction controlled
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Table 2 Different reaction models with various functions of g(x) [41–44]. No
Reaction mechanism
Diffusion controlled models 1 One-dimensional diffusion 2 Two-dimensional diffusion (valensi equation) 3 Three-dimensional diffusion (Jander equation) 4 Anti-Jander equation 5 Three-dimensional diffusion (GinstlingBrounstein equation)
g(x) x2 x + [(1 − x)ln(1 − x)] [1 − (1 − x)1/3]2 [(1 + x)1/3 − 1]2 1 − (2/3)x − (1 − x)2/3
Nucleation reaction models 6 Power law 7 Power law 8 Power law 9–11 n = 2, 3, 4 (Avrami-Erofeev)
X x1/2 x1/3 [−ln(1 − x)]1/n
Chemical reaction models 12 First-order (Mampel) 13 Reaction order (n = 2)
−ln(1 − x) [(1 − x)(1−n) − 1]/(n − 1)
Phase boundary reaction models 14 Contraction of cylinder 15 Contraction of sphere
1 − (1 − x)1/2 1 − (1 − x)1/3
model was the most suitable for characterizing the results. 4. Conclusions In this study, the effects of propionic, tartaric, and trimesic acid on struvite crystallization were investigated and the following conclusions were drawn: (i) Both XRD and FTIR were used to characterize the crystals prepared with and without additives and both analyses showed that the crystals were in the struvite form. (ii) Propionic, tartaric, and trimesic acid distinctively increased the average particle size of the struvite and significantly changed the morphology of the crystals. SEM images showed that different struvite shapes were created from different additives. (iii) When compared to pure media, significant changes in the zeta potentials of the crystals were observed as a result of the adsorption of additives onto the struvite crystals. (iv) The kinetic results of thermal decomposition showed that all the additives caused a decrease in the average activation energy, and that trimesic acid resulted in the lowest activation energy. Using the FWO method, the mean activation energies for propionic, tartaric, and trimesic acid were calculated to be 45.7, 40.8, and 45.8 kJ/mol. Furthermore, the kinetic modeling study fitted the chemical reaction control model (n = 2) with the highest correlation coefficient for all the media. It could be suggested that the results obtained could be important in two aspects. One of them is that propionic, tartaric and trimesic acid may be beneficial to wastewater treatment plants because of their effect on particle size and morphology on struvite crystals. This will help to reduce costs due to the shortening of the processing time as well as the production of slow release struvite fertilizer with the desired particle size. The other is that this study will shed light on additional perspective studies in this area and will provide experimental evidence for the modification of the size and shape of struvite crystals.
Fig. 8. Variation in the activation energy versus conversion degree for the struvite crystals prepared in (a) pure media, (b) propionic acid, (c) tartaric acid, and (d) trimesic acid.
provided the highest correlation coefficients for all the crystals and for all heating rates regardless of the media. After considering the correlation coefficient values, it was observed that second order reaction
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Table 3 Values of correlation coefficients for the struvite prepared with and without additives. Model No
Correlation coefficient (R2) Pure media 5 °C/min
Propionic acid media
Tartaric acid media
Trimesic acid media
10 °C/min
20 °C/min
5 °C/min
10 °C/min
20 °C/min
5 °C/min
10 °C/min
20 °C/min
5 °C/min
10 °C/min
20 °C/min
Diffusion controlled models 1 0.8297 2 0.8589 3 0.9003 4 0.8107 5 0.8730
0.8412 0.8694 0.9072 0.8219 0.8825
0.8004 0.8197 0.8619 0.7709 0.8339
0.8670 0.8950 0.9333 0.8485 0.9083
0.8396 0.8148 0.9172 0.8191 0.8875
0.8220 0.8546 0.8957 0.8095 0.8690
0.8713 0.8016 0.9438 0.8521 0.9168
0.8444 0.8775 0.9210 0.8332 0.8929
0.8133 0.8460 0.8893 0.7915 0.8610
0.8386 0.7905 0.9220 0.8179 0.8900
0.8349 0.8685 0.9139 0.8137 0.8844
0.8165 0.8500 0.8962 0.8064 0.8659
Nucleation reaction models 6 0.8129 7 0.7863 8 0.7545 9 0.8967 10 0.9091 11 0.9196
0.8204 0.7868 0.7441 0.8958 0.9122 0.9251
0.7703 0.7324 0.7860 0.8334 0.8574 0.8769
0.8943 0.8243 0.7932 0.9347 0.9437 0.9509
0.8284 0.7934 0.7479 0.9061 0.9218 0.9341
0.8009 0.7737 0.7095 0.8275 0.8688 0.8975
0.8526 0.8287 0.7991 0.8764 0.8513 0.9597
0.8281 0.7875 0.7329 0.9166 0.9322 0.9438
0.8968 0.8766 0.8503 0.9107 0.9261 0.9380
0.8337 0.8011 0.8593 0.9323 0.9426 0.9508
0.8282 0.7878 0.8334 0.9073 0.9248 0.9379
0.7894 0.8335 0.7570 0.7751 0.7877 0.7956
Chemical reaction models 12 0.9387 13 0.9922
0.9822 0.9949
0.8928 0.9922
0.9573 0.9992
0.9510 0.9903
0.9382 0.9865
0.9644 0.9848
0.9477 0.9922
0.9432 0.9879
0.9504 0.9902
0.9515 0.9898
0.9465 0.9841
Phase boundary reaction models 14 0.8959 0.9807 15 0.9170 0.9812
0.8264 0.8479
0.9419 0.9575
0.9419 0.9955
0.9354 0.9436
0.9134 0.9403
0.9359 0.9515
0.9414 0.9546
0.9262 0.9467
0.9415 0.9552
0.9447 0.9559
Declaration of Competing Interest
[14] Q. Zhang, S. Zhao, X. Ye, W. Xiao, Effects of organic substances on struvite crystallization and recovery crystallization and recovery, Desalin. Water Treat. 57 (2016) 10924–10933. [15] J. Prywer, E. Mielniczek-Brzóska, M. Olszynski, Struvite crystal growth inhibition by trisodium citrate and the formation of chemical complexes in growth solution, J. Cryst. Growth 418 (2015) 92–101. [16] L. Wei, T. Hong, H. Liu, T. Chen, The effect of sodium alginate on struvite crystallization in aqueous solution: a kinetics study, J. Cryst. Growth 473 (2017) 60–65. [17] K.S. Le Corre, E. Valsami-Jones, P. Hobbs, S.A. Parsons, Impact of calcium on struvite crystal size, shape and purity, J. Cryst. Growth 418 (2015) 92–101. [18] Z. Zhou, D. Hu, W. Ren, Y. Zhao, L.M. Jiang, L. Wang, Effect of humic substances on phosphorus removal by struvite precipitation, Chemosphere 141 (2015) 94–99. [19] H. Li, Q.Z. Yao, S.H. Yu, Y.R. Huang, X.D. Chen, S.Q. Fu, G.T. Zhou, Bacterially mediated morphogenesis of struvite and its implication for phosphorus recovery, Am. Mineral. 102 (2017) 381–390. [20] L. Wei, T. Hong, X. Li, M. Li, Q. Zhang, T. Chen, New insight into the adsorption behavior and mechanism of alginic acid onto struvite crystals, Chem. Eng. J. 358 (2019) 1074–1082. [21] D.S. Perwitasari, S. Muryanto, J. Jamari, A.P. Bayuseno, Kinetics and morphology analysis of struvite precipitated from aqueous solution under the influence of heavy metals: Cu2+, Pb2+, Zn2+, J. Environ. Chem. Eng. 6 (2018) 37–43. [22] Y.H. Song, Y.R. Dai, Q. Hu, X.H. Yu, F. Qian, Effects of three kinds of organic acids on phosphorus recovery by magnesium ammonium phosphate (MAP) crystallization from synthetic swine wastewater, Chemosphere 101 (2014) 41–48. [23] D.S. Perwitasari, D. Suci, J.S. Jamari, S. Muryanto, A.P. Bayuseno, Influence of citric acid on struvite precipitation, Advance Sci. Let. 23 (2017) 12231–12234. [24] J. Prywer, A. Torzewska, Biomineralization of struvite crystals by Proteus mirabilis from artificial urine and their mesoscopic structure, Cryst. Res. Technol. 45 (2012) 1283–1289. [25] C.K. Chauhan, M.J. Joshi, In vitro crystallization, characterization and growth-inhibition study of urinary type struvite crystals, J. Cryst. Growth 362 (2013) 330–337. [26] E.L. Foletto, W.R.B. dos Santos, M.A. Mazutti, S.L. Jahn, A. Gündel, Production of struvite from beverage waste as phosphorous source, Mat. Res. 16 (2013) 242–245. [27] E. Banks, R. Chianelli, R. Korenstein, Crystal chemistry of struvite analogs of the type MgMPO4.6H2O (M+ = K+, Rb+, Cs+, TI+, NH4 +), Inorg. Chem. 14 (1975) 1634–1639. [28] X. Gao, C. Leng, G. Zeng, D. Fu, Y. Zhang, Y. Liu, Ozone initiated heterogeneous oxidation of unsaturated carboxylic acids by ATR-FTIR spectroscopy, Spectrochim Acta A. 214 (2019) 177–183. [29] A.S. Amarasekara, B. Wiredu, Y.M. Lawrence, Hydrolysis and interactions of Dcellobiose with polycarboxylic acids, Carbohydr. Res. 475 (2019) 3438. [30] N.C. Bouropoulos, P.G. Koutsoukos, Spontaneous precipitation of struvite from aqueous solutions, J. Cryst. Growth 213 (2000) 381–388. [31] E. Heraldy, F. Rahmawati, H. Heriyanto, D.P. Putra, Preparation of struvite from desalination waste, J. Environ. Chem. Eng. 5 (2017) 1666–1675. [32] A.P. Bayuseno, W.W. Schmahl, Thermal decomposition of struvite in water: qualitative and quantitative mineralogy analysis, Environ. Technol. 4 (2019) 1–7. [33] M.I.H. Bhuiyan, D.S. Mavinic, F.A. Koch, Thermal decomposition of struvite and its phase transition, Chemosphere 70 (2008) 1347–1356.
The authors declared that there is no conflict of interest. Acknowledgements This work was supported by Marmara University Scientific Research Projects Commission under the funding FEN-A-110718-0396. References [1] A. Moulessehoul, D. Gallart-Mateu, D. Harrache, S. Djaroud, M. de la Guardia, M. Kameche, Conductimetric study of struvite crystallization in water as a function of pH, J. Cryst. Growth 471 (2017) 42–52. [2] B. Tansel, G. Lunn, O. Monie, Struvite formation and decomposition characteristics for ammonia and phosphorous recovery: a review of magnesium-ammonia-phosphate interactions, Chemosphere 194 (2018) 504–514. [3] R.R. Sadowski, J. Prywer, A. Torzewska, Morphology of struvite crystals as an evidence of bacteria mediated growth, Cryst. Res. Technol. 49 (2014) 478–489. [4] D.S. Perwitasari, L. Edahwati, S. Sutiyono, S. Muryanto, J. Jamari, A.P. Bayuseno, Phosphate recovery through struvite-family crystals precipitated in the presence of citric acid: mineralogical phase and morphology evaluation, Environ. Technol. 38 (2017) 2844–2855. [5] J.B. Neethling, M. Benisch, Struvite control through process and facility design as well as operation strategy, Water Sci. Technol. 49 (2004) 191–199. [6] K.P. Fattah, Assessing struvite formation potential at wastewater treatment plants, Int. J. Environ. Sci. Technol. 3 (2012) 548–552. [7] A.N. Kofina, K.D. Demadis, P.G. Koutsoukos, The effect of citrate and phosphocitrate on struvite spontaneous precipitation, Cryst. Growth Des. 7 (2007) 2705–2712. [8] Y. Liu, H. Qu, Effects of solution composition and polymer additive on the crystallization of struvite, Chem. Eng. Technol. 7 (2017) 1293–1299. [9] J. Prywer, L. Sieroń, A. Czylkowska, Struvite grown in gel, its crystal structure at 90 K and thermoanalytical, Study Crystals. 89 (2019) 1–12. [10] A.I. Adnan, M. Dastur, D.S. Mavinic, F.A. Koch, Preliminary investigation into factors affecting controlled struvite crystallization at the bench scale, Environ. Eng. Sci. 3 (2004) 195–202. [11] X. Ye, Z.L. Ye, Y. Lou, S. Pan, X. Wang, M.K. Wang, S. Chen, A comprehensive understanding of saturation index and upflow velocity in a pilot-scale fluidized bed reactor for struvite recovery from swine wastewater, Powder Technol. 295 (2016) 16–26. [12] L. Pastor, D. Mangin, R. Barat, A. Seco, A pilot-scale study of struvite precipitation in a stirred tank reactor: conditions influencing the process, Bioresource Technol. 99 (2008) 6285–6291. [13] M.S. Rahaman, N. Ellis, D.S. Mavinic, Effects of various process parameters on struvite precipitation kinetics and subsequent determination of rate constants, Water Sci. Technol. 57 (2008) 647–654.
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S. Polat and P. Sayan [34] S. Vyazovkin, A.K. Burnham, J.M. Criado, L.A. Pérez-Maqueda, C. Popescu, N. Sbirrazzuoli, ICTAC kinetics committee recommendations for performing kinetic computations on thermal analysis data, Thermochim Acta 520 (2011) 1–19. [35] T. Ozawa, A new method of analyzing thermogravimetric data, Bulletin Chem. Soc. Japan. 38 (1965) 1881–1886. [36] J.H. Flynn, L.A. Wall, General treatment of the thermogravimetry of polymers, J. Res. Nat. Bur. Stand. 70 (1966) 487–523. [37] T. Akahira, T. Sunose, Method of determining activation deterioration constant of electrical insulating materials, Res. Rep. Chiba Inst. Technol (Sci Technol) 16 (1971) 22–31. [38] M. Starink, A new method for the derivation of activation energies from experiments performed at constant heating rate, Thermochim. Acta 288 (1996) 97–104. [39] L.C. Nicolescu, B. Tita, T. Jurca, E. Marian, D. Tita, Thermal stability of piroxicam – active substance and tablets. I. Kinetic study of the active substance under nonisothermal conditions, Rev. Chim. – Bucharest. 66 (2015) 1802–1806.
[40] E. Ariyanto, T.K. Sen, H.M. Ang, The influence of various physico-chemical process parameters on kinetics and growth mechanism of struvite crystallisation, Adv. Powder Technol. 25 (2014) 682–694. [41] A. Khawam, D.R. Flanagan, Role of isoconversional methods in varying activation energies of solid-state kinetics I. isothermal kinetic studies, Thermochim Acta 429 (2005) 93–102. [42] R. Ebrahimi-Kahrizsangi, M.H. Abbasi, Evaluation of reliability of Coats-Redfern method for kinetic analysis of non-isothermal TGA, Trans. Nonferrous Met. Soc. China 18 (2008) 217–221. [43] C.C. Su, R.R.M. Abarca, M.D.G. de Luna, M.C. Lu, Phosphate recovery from fluidized-bed wastewater by struvite crystallization technology, J. Taiwan Inst. Chem. E. 45 (2014) 2395–2402. [44] Z. Lei, S. Wang, H. Fu, W. Gao, B. Wang, J. Zeng, J. Xu, Thermal pyrolysis characteristics and kinetics of hemicellulose isolated from Camellia Oleifera Shell, Bioresource Technol. 282 (2019) 228–235.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Preparation, characterization and kinetic evaluation of struvite in various carboxylic acids
T
Sevgi Polat, Perviz Sayan
⁎
Department of Chemical Engineering, Faculty of Engineering, Marmara University, 34722 İstanbul, Turkey
ARTICLE INFO
ABSTRACT
Communicated by T. Nishinaga
In this study, struvite crystallization was investigated using different carboxylic acid media—propionic, tartaric, and trimesic acid. The struvite was synthesized as a result of the reaction between magnesium chloride hexahydrate and ammonium dihydrogen phosphate. In the first part. of the study, the crystals prepared with and without additives were characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and zeta potential measurements. XRD results showed that the struvite prepared in pure media comprised crystals with the normal orthorhombic structure. SEM images confirmed that the struvite underwent morphological changes when prepared in carboxylic acid media. FTIR analysis indicated that the carboxylic acids affected the functional groups of the struvite. Zeta potential measurements showed that struvite crystals have a −13.2 ± 1.8 mV zeta potential value, and that the surface of the crystals in trimesic acid media became more positive than that created in pure media with an increase in value to −2.3 ± 0.9 mV. In the second part of the study, the thermal decomposition of the struvite crystals was investigated in detail. The thermal data were used to calculate the activation energy that corresponded to struvite dehydration prepared with and without additives using a nonisothermal kinetic analysis based on the Flynn-Wall-Ozawa, KissingerAkahira-Sunose, Starink, and Tang models. The mean activation energy calculated for struvite crystallization in pure media was between 56.06 and 52.13 kJ/mol, depending on the kinetic method used.
Keywords: B1. Struvite B1. Crystallization A1. Morphology B1. Carboxylic Acids A1. Kinetics A1. Model free
1. Introduction Struvite crystals (magnesium ammonium phosphate hexahydrate [NH4MgPO4·6H2O]) are hydrated phosphates comprising equimolar amounts of magnesium (Mg), ammonium (NH4+), and phosphate (PO43−) [1,2]. Although struvite is commonly used in industry, struvite crystals are also among the most common of urinary stones in animals, including humans [3]. In industry, such as wastewater treatment plants, struvite formation can clog pipes, heat exchangers, and centrifuges and causes vales to freeze up. Its low solubility in water, creates turbulence that increases the energy needed to move water though the system, and raises pH [4–6]. To resolve these issues, time-consuming and expensive measures must be implemented [6,7]; therefore, prevention of struvite crystallization is a major focus in industrial facilities. Several studies have been conducted to determine the process of struvite crystallization to help develop new mechanisms by which to resolve these issues. To that end, the formation of struvite crystals has been investigated from several aspects with results indicating that several physicochemical parameters, such as pH [8,9], temperature [10], supersaturation [11],
⁎
operating conditions [12,13], and various additives [7,14–23] play a role in crystal formation. Zhang et al. [14] have researched the inhibitory effects of humic acid and citric acid on struvite precipitation and crystal growth. Wei et al. [20] have investigated the adsorption behavior of alginic acid in relation to pH, ionic strength, adsorption time, and its concentration and have assessed its prohibitory effect on struvite, which has been reported to be considerable because of its adsorption mechanism. That is, small amount of these additives changed the nucleation rate, crystal growth, morphology, and physicomechanical properties. In this study, struvite crystallization was investigated using propionic acid, tartaric acid, and trimesic acid as additives. These additives were selected because studies have been a limited on struvite crystallization using carboxylic acids with different carboxyl group number and chain length and because these additives are inexpensive, easy to handle, and nontoxic. In our study, the crystalline structure, morphology, and particle size of the struvite crystals prepared using solutions with and without an additive were determined. In addition, the thermal decomposition behavior of the samples was investigated in
Corresponding author. E-mail address: [email protected] (P. Sayan).
https://doi.org/10.1016/j.jcrysgro.2019.125339 Received 7 August 2019; Received in revised form 20 October 2019; Accepted 5 November 2019 Available online 07 November 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 531 (2020) 125339
S. Polat and P. Sayan
detail and modeled the kinetic precipitation process. 2. Experimental
the solid products were washed with distilled water. Finally, the products were dried, and the prepared crystals were used for further analysis.
2.1. Materials
2.3. Property characterization
Analytical-grade magnesium chloride hexahydrate (MgCl2·6H2O), ammonium dihydrogen phosphate (NH4H2PO4), sodium hydroxide (NaOH), propionic acid (C3H6O2), tartaric acid (C4H6O6), and trimesic acid (C9H6O6) were purchased from Merck. All solutions were prepared using distilled water.
X-ray diffraction (XRD) was conducted using the Bruker D2 Phaser benchtop XRD analyzer with copper (Cu) Kα radiation (λ = 1.5418 Å) at 30 kV and 10 mA in the 2θ range from 10° to 60° for phase identification. The morphologies of the struvite crystals prepared with and without an additive were examined using the Zeiss EVO LS 10 scanning electron microscopy (SEM; Carl Zeiss AGO, Berkochen, Germany). Particle size was analyzed using the Mastersizer 2000 laser particle size analyzer (Malvern Panalytical, Malvern, UK). The functional groups of the crystals were identified using the IRAffinity-1S Fourier-transform infrared spectrometer (FTIR; Shimadzu) with the wavenumber ranging from 4000 to 400 cm−1 at a resolution of 4 cm−1 with eight scans per sample. The surface properties of the struvite were examined using the K-Alpha™ X-ray Photoelectron Spectrometer (XPS) System (ThermoFisher Scientific) with an Al Kα X-ray source. The zeta potential of the struvite was measured using a Malvern Zeta Sizer Nano Series Nano-ZS (Malvern Panalytical). Each measurement was repeated at least 10 times, and the average value was calculated. All experiments were performed at 25 °C and pH 8. The thermal properties of the struvite were examined using thermogravimetric analysis (TGA) and differential thermogravimetric (DTG) analysis conducted on a Setaram LABSYS Evo instrument. In each analysis, approximately 10 mg sample was placed in an aluminum oxide (Al2O3) crucible and heated over temperatures ranging from 30 °C to 400 °C at rates of 5, 10, and 20 °C/ min. A continuous nitrogen flow at a rate of 20 mL/min was supplied to TGA to maintain an inert atmosphere. Kinetic calculations were also conducted using TGA data.
2.2. Experimental method The experiments were conducted in an enclosed glass crystallizer with a working capacity of 1 L. The schematic representation of experimental setup is shown in Fig. 1. The crystallizer was equipped with a thermostatic jacket, a mechanical stirrer, and a pH control system. Two peristaltic pumps and an infusion pump were used for feeding the reactants and the additive into the crystallizer, respectively. The crystallizer environment was maintained at 40 °C and the pH of the system was maintained at 8 by adding dilute NaOH solution using a pH control system. A stirring mechanism comprised a three-blade propeller and mechanical mixer and was maintained at a rate of 500 rpm. The struvite crystals were prepared using the reaction of MgCl2·6H2O and NH4H2PO4. 250 mL each 1.0 M MgCl2·6H2O and 1.0 M NH4H2PO4 were concurrently added into the crystallizer through the peristaltic pumps at 5 mL/min to create the struvite crystals. The additives propionic acid, tartaric acid, and trimesic acid were used at constant concentration for each carboxylic acid (250 ppm) to investigate the influence of the additives and compare their effect on struvite crystallization. In the experiments conducted in the presence of propionic acid, tartaric acid, and trimesic acid, the required quantity of additive was prepared by dissolving it in distilled water, and these solutions were then used in the experiments. 50 mL of additive solution was added to the crystallizer simultaneously during the process of struvite crystallization through an infusion pump at 1 mL/min. At the end of the experiments, the suspension was withdrawn and filtered, and
3. Results and discussion 3.1. X-ray diffraction analysis XRD analysis was used to identify the structure of the struvite
Fig. 1. Experimental setup.
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crystallization medium, most likely from incorporation of the carboxylic acid molecules into the crystal lattice, creating structural imperfections and strains in the lattice symmetry, which resulted in the peak shifts and changes from that in pure media. The lattice parameters for propionic acid, tartaric acid, and trimesic acid were determined to be a = 6.963 Å, b = 6.143 Å, c = 11.218 Å; a = 6.967 Å, b = 6.149 Å, c = 11.236 Å; and a = 6.959 Å, b = 6.143 Å, c = 11.223 Å, respectively. 3.2. Scanning electronic microscopy analysis Fig. 3 shows the SEM images of the struvite crystals prepared with and without a carboxylic acid. As observed in Fig. 3a, the crystals prepared in pure media were rod-like and had a tendency to grow atop each other. The mean particle size of the crystals was 17.6 µm. Fig. 3b shows the SEM image of the struvite crystals prepared with propionic acid, which has a single carboxyl group. As observed from the figure, the morphology of the struvite crystals changed after adding propionic acid to the crystallization medium. The rod-like crystals were replaced with hollowed, bent plate structures. This bending created crystals with holes in their middle that emerged during the advanced stages of development. Although the crystals that were formed were not uniform, they grew atop each other and had a tendency to agglomerate. The hollowed structure with a particle size of 27.4 µm was the common characteristic of the crystals prepared in the presence of propionic acid. Fig. 3c shows the SEM image of the crystals that were produced in the presence of tartaric acid. Compared to those prepared in pure media, the presence of tartaric acid, which has double carboxyl groups similar to those of acids with a single carboxyl group, resulted in the formation of a larger amount of shorter, blunted plate-shaped crystals in nonuniform sizes that had a tendency to develop in layers; there were also holes along these crystals, and their mean particle size was 33.4 µm. Deformations and irregular shapes were also observed on the crystal surfaces. These forms could be broken under hydrodynamic conditions in the media. Although the developed crystals were triangular prisms, holes were observed at the midway to the tip of the triangle. Fig. 3d shows the SEM image of the crystals prepared in the presence of trimesic acid. The developed struvite crystals were rod-like and thicker than those combined with other additives. The sharp edges of the struvite crystals were replaced by more rounded crystals without sharp corners. The surfaces of these crystals were rough with substantial deformation. The hollowed structures observed in the crystals in the trimesic acid medium were smaller than in those combined with other media. The mean particle size of these struvite crystals was 53.7 µm. In addition to particle size analysis, the width-to-length ratios of the struvite crystals were determined in this study. The width-to-length ratios of the crystals obtained in the presence trimesic acid exhibited a considerable transformation as compared to those of the crystals obtained in pure media and with the other additives. The lengths of the crystals were shortened and their widths were increased in the presence of carboxylic acid media. While width-to-length ratios of the struvite obtained in pure media was 0.24, this value was determined as 0.32, 0.45 and 0.53 for propionic acid, tartaric acid, and trimesic acid, respectively. Accordingly, it can be concluded that the additives used in this study are the effective crystal modifiers to change the crystal morphology and size of the struvite.
Fig. 2. XRD spectra for struvite crystals prepared with and without different carboxylic acid media.
crystals. Fig. 2 shows the XRD patterns of the crystals prepared with and without carboxylic acid. As seen from the patterns prepared in pure media, the 2θ values of 14.96° (1 0 1), 20.82° (1 1 1), 27.02° (1 0 3), 33.22° (0 2 2), and 45.96° (3 0 3) suggest that the peaks belong to the orthorhombic Pmn21 space group, and these results were coincident with the literature [24] (i.e., the XRD peaks were attributed to the sole existence of the struvite form in pure media). Materials Analysis Using Diffraction (MAUD) software was used to determine the lattice parameters of the crystals prepared in pure media, which were a = 6.969 Å, b = 6.145 Å, c = 11.241 Å, and α = β = γ = 90°. The XRD patterns of the struvite crystals prepared using different carboxylic acids are shown in Fig. 2. It was observed that the XRD peaks did not include any characteristic carboxylic acid peaks because of the trace amount of the additive, and this result indicated that the presence of the additive within the studied concentration range had no effect on crystal type. Nevertheless, the peak intensities changed and the peaks shifted slightly with the addition of the carboxylic acids to the
3.3. Fourier-transform infrared spectroscopy analysis FTIR spectroscopy analysis was used to determine the chemical structure of the struvite and identify the absorption behavior of the additives. The FTIR spectra obtained for the struvite crystals prepared
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Fig. 3. SEM images of struvite crystals prepared in the absence (a), and presence of different carboxylic acid media: propionic (b), tartaric (c), and trimesic (d) acid.
with and without a carboxylic acid are illustrated in Fig. 4. The wide transmittance band within the ranges of 3500 and 3030 cm−1 for the struvite prepared in pure media represented the hydrogen–oxygen–hydrogen (HeOeH) stretching vibrations of the water of crystallization. The peaks at ~2370 cm−1 and ~1680 cm−1 were assigned to the HeOeH stretching and bending vibrations of the water molecules, respectively. The bands located at ~2900 cm−1 and ~1435 cm−1 were characteristic of the nitrogen NeH stretching vibrations in the NH4+ unit. The band at ~980 cm−1 denoted the presence of a PO43− stretching vibration [25–27]. The FTIR results showed that OeH, NeH, and PeO functional groups were detected mainly on the surface of the struvite crystals prepared in pure media. The FTIR spectra obtained for the struvite crystals prepared with propionic, tartaric, and trimesic acid indicated the characteristic peaks of struvite. In addition, a new slight absorption peak appeared in the crystals developed in additive media compared with the peaks in those developed in pure media. This peak at ~1720 cm−1 showed that the additives reside on the struvite crystals corresponding to the characteristic peak of the carbonyl group of the carboxylic acid [28,29], which indicates that the additives were adsorbed onto the surface of the struvite. To determine the amount of adsorbed carboxylic acids on the crystal’s surface and to characterize the crystals quantitatively, the adsorption properties of the crystals were examined. Upon completion of the process, the samples were collected, filtered and the filtrate analyzed for the residual carboxylic acid concentration using a UV spectrometer. The amounts of propionic acid, tartaric acid, and trimesic acid adsorbed per unit mass of the struvite crystals were determined to be 0.27 mg/g, 0.31 mg/g and 0.44 mg/g, respectively.
3.4. X-ray photoelectron spectrometer analysis XPS analysis was conducted to further determine the adsorption behavior of the carboxylic acids used in the struvite crystallization process, and the Mg(1s), O(1s), N(1s), and P(2p) patterns are provided in Fig. 5. Fig. 5a displays the Mg(1s) peak of the struvite crystals at a binding energy of 1303.3 eV, which is the related to the MgeO bond, while the same signal appeared at 1308.6 eV, 1310.7 eV, and 1302.0 eV after adding propionic acid, tartaric acid, and trimesic acid, respectively. The shifting observed in the binding energy of Mg(1s) displayed the interaction between the additives and the struvite, which was most likely between the eCOOH group of the carboxylic acids and the Mg groups on the struvite crystals. The same change in the Mg(1s) was observed for the O(1s) pattern, as seen in Fig. 5b. In addition, compared to the peaks of the crystals developed in pure media, there was a sharp reduction in peak intensity, which is the indicator of the additives bound to the crystal surface. As seen in Fig. 5c, the peak located at 402.6 eV for pure media can ascribe to the NH4+ group; however, the binding energy of N(1s) shifted to 409.5 eV, 411.1 eV, and 401.9 eV for propionic acid, tartaric acid, and trimesic acid, respectively. As shown in Fig. 5d, P(2p) has a peak at 134.6 eV, which is attributed to the P]O bond for the struvite crystal prepared in pure media. The binding energy of P(2p) slightly decreased from 134.6 to 133.3 eV for trimesic acid; whereas, this energy increased in the presence of propionic acid and tartaric acid by 5.9 and 9.3 eV, respectively; therefore, these results can be suggested as one proof of their interaction with struvite crystals, which led to the adsorption of the additives on the crystal surfaces.
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Fig. 4. FTIR spectra of struvite crystals prepared with and without different carboxylic acids media.
3.5. Zeta potential analysis
3.6. Thermal analysis
As stated in the literature, the zeta potential is an important parameter that could influence the properties of the struvite crystals [30]; therefore, zeta potential measurements were taken to investigate the effect of propionic, tartaric, and trimesic acids on the surface electrical charge of the crystals. In this study, the zeta potential for struvite crystals prepared in pure media in its own saturated solution was negatively charged at −13.2 ± 1.8 mV. The presence of carboxylic acids in the crystallization media drastically affected the surface charge of the struvite, and the zeta potential then became more positive. The zeta potentials of the struvite crystals prepared in propionic, tartaric, and trimesic acid media were −11.6 ± 1.1, −7.2 ± 1.7, and −2.3 ± 0.9 mV, respectively. This result showed that trimesic acid, in particular, was very effective for changing the surface charge of the struvite crystals and that the additives can be adsorbed onto the crystal surface. In addition, the variations in zeta potentials in the additive media were associated with the increasing agglomeration tendency of the crystals, which is consistent with the results of SEM analysis.
TGA was used to assess the structural evolution and the thermal decomposition behavior of the struvite crystals. The TG and DTG curves of the struvite crystals prepared with and without an additive for three different heating rates are shown in Fig. 6. In addition, specific decomposition temperatures, such as initial temperature, Ti; maximum peak temperature, Tmax; and final temperature, Tf, are provided in Table 1, which clearly shows that the characteristic temperatures shifted to higher values after increasing the heating rate. This relationship has been attributed to different heat-transfer rates, which affect the kinetics of thermal decomposition. In this study, the decomposition zone shifted to within the higher temperature used in the study, and the peak intensity increased with the increasing heating rate. As seen in Fig. 6, the thermal decomposition of the struvite at the different heating rates encompassed simultaneous dehydration and decomposition and liberation of H2O and NH3 molecules without exhibiting distinct steps in the process [31]. The amount of residue from the struvite prepared in pure media was 48.2% of the initial crystal
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Fig. 5. XPS of magnesium (Mg)(1s), oxygen (O)(1s), nitrogen (N)(1s), and phosphorus (P)(2p) of the struvite crystals.
weight, which was comparable to the theoretical value based on a theoretical weight loss of 51.42% for the molecule. This weight loss comprises 44.08% water and 7.34% ammonia [18]. The results for the weight loss were found to be in good agreement literature [32,33]. The residue amounts after adding propionic, tartaric, and trimesic acid were 47.9%, 47.4%, and 46.9%, respectively, which were lower than that of pure struvite. The decrease in the residue amounts indicated that after the acids adsorbed onto the crystal surface and the Mg ions interacted with the struvite surface, the carboxylic acids and struvite coprecipitated.
common isoconversional kinetic methods—FWO [35,36], KAS [37], Starink [38], and Tang [39]—were used to determine the activation energies of the struvite prepared with and without the carboxylic acids, Their linear equations are presented in Eqs. (1)–(4), respectively. FWO:
AE Rg (x )
ln( ) = ln
5.331
1.052
E1 RT
(1)
KAS:
ln
3.7. Kinetic analysis
T2
= ln
AR Eg (x )
E RT
(2)
Starink:
To study the kinetics of struvite’s thermal decomposition and calculate the activation energy, TGA was used to calculate the weight changes of the samples prepared with and without an additive as a function of temperature. Two methods can be used to evaluate these kinetics—isothermal or nonisothermal. The latter method is presumed to be simpler because it does not consider the changes in the chemical and physical properties of the sample and can be conducted using fewer experiments. Model-free or model-based methods are used to calculate kinetic parameters based on TGA data. With the model-free method, the reaction mechanism can be determined without having to calculate the activation energy, which can be an advantage [34]. In this study, four
ln
T1.92
=C
1.008
E RT
(3)
Tang:
ln
T1.8947
=C
1.0015
E RT
(4)
where x is the conversion rate, β is the heating rate, A is the pre-exponential factor, E is the activation energy, R is the gas constant, t is the time, T is the absolute temperature, and g(x) is the integrated reaction
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Journal of Crystal Growth 531 (2020) 125339
S. Polat and P. Sayan
Fig. 6. (a) TGA and (b) DTG curves obtained for the struvite crystals prepared in pure media; (c) TGA and (d) DTG curves obtained in the presence of propionic acid media; (e) TGA and (f) DTG curves obtained in the presence of tartaric acid media; and (g) TGA and (h) DTG curves obtained in the presence of trimesic acid media.
model. For any given value, it is possible to estimate E based on the gradients of the lines obtained from the plot of ln(β) versus 1/T, ln(β/ T2) versus 1/T, ln(β/T1.92) versus 1/T, and ln(β/T1.8947) versus 1/T for
the FWO, KAS, Starink, and Tang models, respectively. Fig. 7 presents the plots of the four models of the struvite crystals prepared in pure media from 0.1 to 0.9 conversion degree at different
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Journal of Crystal Growth 531 (2020) 125339
S. Polat and P. Sayan
Table 1 Characteristic temperatures of the struvite decomposition process. Media
Heating Rate (°C/min)
Ti (°C)
Tmax (°C)
Tf (°C)
Pure Media
5 10 20 5 10 20 5 10 20 5 10 20
72 76 86 77 86 95 81 89 93 85 89 99
118 133 149 126 139 166 115 142 165 132 141 165
161 186 215 165 192 248 151 193 231 171 201 242
Propionic Acid Media Tartaric Acid Media Trimesic Acid Media
heating rates. As seen from the figure, each line is highly correlated with the four selected models and the experimental data fits well with the model equations, all within a range of 0.9839–0.9999, which suggests that the results were strongly reliable. The parallel lines represent similar kinetic behavior, which suggests that there is a single, identical reaction mechanism from struvite dehydration. Fig. 8 shows the activation energy as a function of the degree of struvite conversion with or without the carboxylic acids. Activation energy is the minimum amount of energy required to initiate a reaction; the lower the value of the activation energy, the more likely the reaction will occur. The results indicate that the minimum mean activation energy values from the FWO, KAS, Starink, and Tang models were 56.06, 52.69, 52.13, and 52.50 kJ/mol, respectively, which were in accordance with those reported in the literature [40]. The calculated values are very similar and the standard deviation among the different models is quite good, which indicates the consistency and reliability of these energies that were calculated using the four kinetic models and multiple heating rates. The minor differences in the activation energy values were the result of approximations, assumptions, and mathematical formulations that varied according to the model. Depending on which model was used, the activation energies were calculated to be 60.9–28.3, 49.7–27.7, and 53.8–32.3 kJ/mol for propionic, tartaric, and trimesic acid, respectively. For the FWO, KAS, Starink, and Tang models, the mean activation energies were calculated to be 45.7, 41.9, 41.3, and 41.4 kJ/mol; 40.8, 36.6, 36.2, and 36.3 kJ/ mol; 45.8, 42.0, 41.4, and 41.5 kJ/mol, respectively, for propionic, tartaric and trimesic acid. According to the calculated activation energy values, the crystals prepared with the additives required less energy to decompose. The energy needed was less for the crystals prepared with additives than for those prepared without additives, and a fluctuation was observed. This lower value exhibited degrees of fluctuations related to conversion, which suggests that the decomposition process comprises complex reactions. To determine the appropriate mechanism by which struvite crystals are degraded and the optimum reaction models for process, 15 reaction models (Table 2) for defining g(x) were used. These reaction models have been proposed by the Coats–Redfern method for kinetic analysis, which considers diffusion, reaction, or nucleation as the rate-limiting steps [41–44]. The plotting method was used to correctly describe the thermal degradation process, with ln(g(x)/T2). This graph should be linear and have a high correlation coefficient. The most probable reaction
Fig. 7. Plots of (a) Flynn-Wall-Ozawa, (b) Kissinger-Akahira-Sunose, (c) Starink, and (d) Tang models of the struvite crystals prepared in pure media.
mechanism is represented by the greatest linearity. The evaluations were conducted based on these models for the struvite crystals developed in all media using three heating rates. The results are provided in Table 3. As is clearly seen in the Table 3, chemical reaction controlled
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Journal of Crystal Growth 531 (2020) 125339
S. Polat and P. Sayan
Table 2 Different reaction models with various functions of g(x) [41–44]. No
Reaction mechanism
Diffusion controlled models 1 One-dimensional diffusion 2 Two-dimensional diffusion (valensi equation) 3 Three-dimensional diffusion (Jander equation) 4 Anti-Jander equation 5 Three-dimensional diffusion (GinstlingBrounstein equation)
g(x) x2 x + [(1 − x)ln(1 − x)] [1 − (1 − x)1/3]2 [(1 + x)1/3 − 1]2 1 − (2/3)x − (1 − x)2/3
Nucleation reaction models 6 Power law 7 Power law 8 Power law 9–11 n = 2, 3, 4 (Avrami-Erofeev)
X x1/2 x1/3 [−ln(1 − x)]1/n
Chemical reaction models 12 First-order (Mampel) 13 Reaction order (n = 2)
−ln(1 − x) [(1 − x)(1−n) − 1]/(n − 1)
Phase boundary reaction models 14 Contraction of cylinder 15 Contraction of sphere
1 − (1 − x)1/2 1 − (1 − x)1/3
model was the most suitable for characterizing the results. 4. Conclusions In this study, the effects of propionic, tartaric, and trimesic acid on struvite crystallization were investigated and the following conclusions were drawn: (i) Both XRD and FTIR were used to characterize the crystals prepared with and without additives and both analyses showed that the crystals were in the struvite form. (ii) Propionic, tartaric, and trimesic acid distinctively increased the average particle size of the struvite and significantly changed the morphology of the crystals. SEM images showed that different struvite shapes were created from different additives. (iii) When compared to pure media, significant changes in the zeta potentials of the crystals were observed as a result of the adsorption of additives onto the struvite crystals. (iv) The kinetic results of thermal decomposition showed that all the additives caused a decrease in the average activation energy, and that trimesic acid resulted in the lowest activation energy. Using the FWO method, the mean activation energies for propionic, tartaric, and trimesic acid were calculated to be 45.7, 40.8, and 45.8 kJ/mol. Furthermore, the kinetic modeling study fitted the chemical reaction control model (n = 2) with the highest correlation coefficient for all the media. It could be suggested that the results obtained could be important in two aspects. One of them is that propionic, tartaric and trimesic acid may be beneficial to wastewater treatment plants because of their effect on particle size and morphology on struvite crystals. This will help to reduce costs due to the shortening of the processing time as well as the production of slow release struvite fertilizer with the desired particle size. The other is that this study will shed light on additional perspective studies in this area and will provide experimental evidence for the modification of the size and shape of struvite crystals.
Fig. 8. Variation in the activation energy versus conversion degree for the struvite crystals prepared in (a) pure media, (b) propionic acid, (c) tartaric acid, and (d) trimesic acid.
provided the highest correlation coefficients for all the crystals and for all heating rates regardless of the media. After considering the correlation coefficient values, it was observed that second order reaction
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Journal of Crystal Growth 531 (2020) 125339
S. Polat and P. Sayan
Table 3 Values of correlation coefficients for the struvite prepared with and without additives. Model No
Correlation coefficient (R2) Pure media 5 °C/min
Propionic acid media
Tartaric acid media
Trimesic acid media
10 °C/min
20 °C/min
5 °C/min
10 °C/min
20 °C/min
5 °C/min
10 °C/min
20 °C/min
5 °C/min
10 °C/min
20 °C/min
Diffusion controlled models 1 0.8297 2 0.8589 3 0.9003 4 0.8107 5 0.8730
0.8412 0.8694 0.9072 0.8219 0.8825
0.8004 0.8197 0.8619 0.7709 0.8339
0.8670 0.8950 0.9333 0.8485 0.9083
0.8396 0.8148 0.9172 0.8191 0.8875
0.8220 0.8546 0.8957 0.8095 0.8690
0.8713 0.8016 0.9438 0.8521 0.9168
0.8444 0.8775 0.9210 0.8332 0.8929
0.8133 0.8460 0.8893 0.7915 0.8610
0.8386 0.7905 0.9220 0.8179 0.8900
0.8349 0.8685 0.9139 0.8137 0.8844
0.8165 0.8500 0.8962 0.8064 0.8659
Nucleation reaction models 6 0.8129 7 0.7863 8 0.7545 9 0.8967 10 0.9091 11 0.9196
0.8204 0.7868 0.7441 0.8958 0.9122 0.9251
0.7703 0.7324 0.7860 0.8334 0.8574 0.8769
0.8943 0.8243 0.7932 0.9347 0.9437 0.9509
0.8284 0.7934 0.7479 0.9061 0.9218 0.9341
0.8009 0.7737 0.7095 0.8275 0.8688 0.8975
0.8526 0.8287 0.7991 0.8764 0.8513 0.9597
0.8281 0.7875 0.7329 0.9166 0.9322 0.9438
0.8968 0.8766 0.8503 0.9107 0.9261 0.9380
0.8337 0.8011 0.8593 0.9323 0.9426 0.9508
0.8282 0.7878 0.8334 0.9073 0.9248 0.9379
0.7894 0.8335 0.7570 0.7751 0.7877 0.7956
Chemical reaction models 12 0.9387 13 0.9922
0.9822 0.9949
0.8928 0.9922
0.9573 0.9992
0.9510 0.9903
0.9382 0.9865
0.9644 0.9848
0.9477 0.9922
0.9432 0.9879
0.9504 0.9902
0.9515 0.9898
0.9465 0.9841
Phase boundary reaction models 14 0.8959 0.9807 15 0.9170 0.9812
0.8264 0.8479
0.9419 0.9575
0.9419 0.9955
0.9354 0.9436
0.9134 0.9403
0.9359 0.9515
0.9414 0.9546
0.9262 0.9467
0.9415 0.9552
0.9447 0.9559
Declaration of Competing Interest
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