Controlled synthesis of struvite nanowires in synthetic wastewater

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Controlled synthesis of struvite nanowires in synthetic wastewater Han Li, Qi-Zhi Yao, Zemin Dong, Tian-Lei Zhao, Gen-Tao Zhou, and Sheng-Quan Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04393 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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ACS Sustainable Chemistry & Engineering

Controlled synthesis of struvite nanowires in synthetic wastewater

Han Li1, Qi-Zhi Yao2, Ze-Min Dong3, Tian-Lei Zhao1, Gen-Tao Zhou1*, Sheng-Quan Fu4

1

CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and

Space Sciences, University of Science and Technology of China, No.96 Jinzhai Road, Hefei 230026, P. R. China. 2 School

of Chemistry and Materials Science, University of Science and Technology of China,

No.96 Jinzhai Road, Hefei 230026, P. R. China. 3

Jiangxi Institute of Veterinary Drug and Feed Contol, No.698 Jingdong Road, Nanchang

330013, P. R. China. 4

Hefei National Laboratory for Physical Sciences at Microscale, University of Science and

Technology of China, No.96 Jinzhai Road, Hefei 230026, P. R. China.

 Corresponding author：Prof. Dr. Gen-Tao Zhou Email: [email protected] Tel.: 86 551 63600533 Fax: 86 551 63600533

1

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Abstract Controlled struvite crystallization is regarded as a promising route to phosphorus recovery from wastewaters, and has been the topic of extensive research in the last two decades. Most studies made great efforts to elevate the quantity of struvite recovered from various wastewaters, but little attention has been focused on the struvite quality which affects the agronomic response. The improvement of struvite quality can raise its value in fertilizer markets and improve the economical sustainability of struvite crystallization technology. In this study, we report a facile method for the synthesis of struvite nanowires by regulating NaCl concentration and initial pH (pHi) in synthetic wastewater. The identification and characterization of the synthesized products were done using X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Zeta potential Analyzer. Our results reveal that both NaCl concentration and pHi play important roles on the phase composition and morphology of products in the crystallization system. In particular, a mass of struvite nanowires with high yield can be obtained at NaCl concentration ranging from 3.5 to 4.5 wt% at pHi 11.0, and the mechanism for the formation of struvite nanowires was systematically investigated. As struvite has been regarded as a fertilizer, struvite nanowires can potentially act as a new kind of nanofertilizer with higher efficiency.

Keywords: Struvite nanowires, Foreign ions, NaCl, Nanofertilizer, Phosphorus recovery

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Introduction Phosphorus, an indispensable macronutrient for all living organisms, is mostly obtained from phosphate ore which is currently being depleted.1-4 The U.S. Geological Survey estimated in 2001 that the global reserve life of phosphate ore was about 90 years.5 Meanwhile, municipal wastewater contains a large amount of phosphorus, which can cause eutrophication when released into water bodies in excess.6-8 To reconcile the simultaneous shortage and overabundance of phosphorus, struvite crystallization technique was developed.7,9,10 Struvite, known as magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O), crystallizes in the orthorhombic system.10,11 Due to its high phosphorus and nitrogen content and low solubility, struvite has been considered as an ideal slow-release fertilizer.4,9,12,13 In a specific reactor, struvite precipitation could be achieved by additional supplementation of Mg2+ and NaOH into wasterwater, where Mg2+ and pH all tend to be deficient for struvite precipitation.10,14,15 In this way, phosphorus and nitrogen can be removed and recovered from wastewaters as reusable struvite.16,17 In this regard, struvite crystallization technique can not only provide a means of phosphorus recovery, but also mitigate eutrophication of water bodies. Moreover, it is also reported that the deliberate struvite crystallization can help to solve scaling problems in wastewater treatment plants (WWTPs), and reduce sludge volumes.9,18 Therefore, struvite precipitation has been regarded as a promising route to phosphorus recovery from wastewaters, and received increasing attention in recent years.3,10,19-24 However, the successful implementation of struvite precipitation in WWTPs depends on its economical sustainability.10 At present, struvite crystallization can hardly generate profits because of the high costs for the production and the limited value of recovered struvite on the market.10,14,15,21,25-27 For example, the experience with full-scale struvite crystallization plants in Japan showed that the income from struvite only covers one-third of the cost of the chemical inputs (e.g., Mg2+ and alkali sources), thus generating no profits.10 This significantly limits its application at full scale. To reduce the cost of production, numerous effects have 3



been made to look for alternative cheaper magnesium source and alkali source. Several magnesium sources such as seawater, bittern solution and MgO have been tested to replace high-grade MgCl2 and MgSO4 salts, and air stripping was used as a way for the pH rise.15,17,21 Meanwhile, the attention was also paid on struvite crystalline quality including the control on the size, morphology and purity in order to elevate its value on the fertilizer market.10,28,29 Recently, nanofertilizers, smaller than 100 nm at least in one dimension, were expected to be far more effective than conventional fertilizers.30-32 Due to the high surface area to volume ratio, nanofertilizers can feed crops gradually in a controlled manner and synchronize the release of nutrients with their uptake by crops, preventing the nutrients from prematurely converting into chemical/gaseous forms that cannot be utilized by crops.30,31 In this way, the utility and efficiency of fertilizers will be significantly enhanced, and thus the crops' growth and yields are markedly improved.33,34 The high efficiency of nanofertilizers can also reduce nutrients losses into surface water and groundwater environments, minimizing the adverse environmental impacts.31-33 As such, Liu and Lal pointed out that research, development and application of nanofertilizers are emergent and have a high research priority.33 Therefore, the successful synthesis of nanoscale struvite will provide a new kind of nanofertilizer and enhance the efficiency and value of recovered struvite. This will help to improve the economical sustainability of struvite crystallization process. It has been reported that several physicochemical parameters such as pH, mixing energy, and foreign ions can influence the particle size of struvite.10,35 Matynia et al. observed that an increase of pH from 8 to 11 can lead to about 5.5-fold decrease of the mean crystal size of struvite.35 Our previous study also found that the struvite crystals become slimmer with the increase of pH.36 Mixing energy is another parameter that can exert an influence on struvite crystal size and shape, and more elongated struvite crystals were harvested in lower mixing speeds.37 Moreover, the presence of foreign ions is also known to affect struvite crystallization.38-40 For example, Le Corre et al. examined the effect of Ca2+ ions on size and shape of the struvite crystals and found that the increase of calcium concentration reduces the crystal size and inhibits struvite growth.38 Suguna et al. reported that fluoride with higher 4


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concentrations can inhibit the nucleation and growth of struvite, resulting in a significant reduction in the number and size of the crystals.39 Muryanto and Bayuseno investigated the influence of Cu2+ and Zn2+ ions on crystallization kinetics of struvite, and found that the crystallization rate is significantly retarded by the foreign Cu2+ or Zn2+ ions.40 They suggested that the inhibition is achieved by the metal ion adsorption onto the developing crystal faces. These studies indicated that foreign ions can exert a profound effect on struvite crystallization and morphogenesis. It appears that the combined interactions of the various physicochemical parameters can potentially lead to the formation of nanoscale struvite during the crystallization. Herein, sodium chloride was selected as an inorganic additive to influence the crystallization and morphogenesis of struvite in synthetic wastewater. In previous studies, sodium chloride has been used to adjust solution ionic strength and influence the crystallization of other minerals such as calcite.41-43 The effects of various physicochemical parameters such as NaCl concentration, initial pH (pHi), and reaction time on struvite formation were systematically investigated. The goal of this study is to explore the potential to recover nanoscale struvite from wastewaters. As a consequence, struvite nanowires can be indeed formed in the synthetic wastewater, and a plausible mechanism for the formation of struvite nanowires was proposed. These findings could provide a facile route to produce struvite nanowires, and will promote the development of struvite recovery technique.

Materials and methods Materials Magnesium chloride hexahydrate (MgCl2·6H2O), sodium chloride (NaCl), ammonium chloride (NH4Cl), ammonium dihydrogen phosphate (NH4H2PO4), lithium chloride (LiCl), sodium nitrate (NaNO3), and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd, and are of analytical grade. Deionized water was used in all of the experiments. 5



Synthesis procedure All the experiments were conducted at a constant temperature of 25 ± 1 °C. Synthetic wastewater was prepared according to our previous study.36 That is, 0.0407 g of MgCl2·6H2O was dosed to 40 mL of deionized water in a 50-mL beaker under vigorous stirring to form solution A. Then, 0.0230 g of NH4H2PO4 and 0.0214 g of NH4Cl were dissolved in 10 mL of deionized water to form solution B. Solution B was introduced into solution A under continuous stirring, and homogeneous synthetic wastewater was obtained, with a molar ratio of 1:1:3 for Mg2+:PO43-:NH4+. As for subsequent precipitation of struvite, a typical procedure is as follows: 1.75 g of NaCl was dissolved in 50 mL of the synthetic liquor to reach a NaCl concentration of 3.5 wt%, and then the pH of the solution was adjusted to 11.0 by adding 0.5 M NaOH solution. Afterwards, the beaker was covered with parafilm and stirred for 24 h at 360 rpm on a magnetic stirrer. Finally, the product was isolated by centrifugation (1400 g for 3 minutes), washed with absolute alcohol three times, and dried in vacuum at room temperature for 48 h. The same procedures were employed to study the effect of NaCl concentration, pHi, and reaction time on struvite formation.

Characterization techniques The morphology and size of the obtained precipitates were observed by a JEOL JSM-6700F field emission scanning electron microscope with an energy dispersive X-ray spectroscope. To prepare the samples for FESEM analysis, products were dispersed in anhydrous ethanol, spotted on a 5 mm × 5 mm copper sheet, and dried at room temperature. After platinum sputtering, the crystal size and morphology of the samples were observed by FESEM in a vacuum. Our previous test has confirmed that the FESEM analysis process cannot lead to the change in mineral phase and morphology of struvite. X-ray diffraction pattern was recorded on a Japan MapAHF X-ray diffractometer equipped with Cu Kα irradiation (λ = 0.154056 nm). The Raman spectrum was collected on a confocal microprobe Raman system (LabRam HR Evolution, Horiba Jobin Yvon) with a laser excitation of wavelength of 532 nm and one micrometer spatial resolution. The samples for micro-Raman 6


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analysis were first scattered in absolute ethanol to form suspension. Then the suspension was dropped on a glass slide. After the absolute ethanol volatilized completely, the samples have spread out on the slide and can be observed by optical microscopy and analyzed by micro-Raman. Monocrystalline silicon and polystyrene were analyzed at the beginning of each analysis session to monitor the precision and accuracy of the Raman data. X-ray photoelectron spectrum was recorded on a Thermo-ESCALAB 250 X-ray photoelectron spectrometer with Al Kα radiation. The

31P

MAS/NMR spectrum was acquired at 161.98

MHz with an Avance III 400 WB spectrometer (Bruker). Samples were contained in 4 mm rotors (outer diameter) spinning at 14 kHz. The 31P chemical shifts are reported relative to 85% H3PO4 (aq) standard. The zeta potential of the sample was determined by NanoBrook Omni Zeta Potential Analyzer (Brookhaven Instruments Corporation, USA). The struvite suspensions for the zeta potential measurement were prepared by adding 5 mg of struvite crystals into 100 mL of a solution saturated with respect to struvite at pH 11.0. The suspension was then treated in a supersonic bath for 3 min at constant temperature. Each measurement is the average of five cycles. The reported values are the mean of ten repeated measurements. The zeta potential is calculated by determining the electrophoretic mobility and then applying the Smoluchowski equation.44

Results and discussion The morphology and size of the product synthesized with 3.5 wt% NaCl at pHi 11.0 for 24 h was first examined by FESEM. It can be observed from Figure 1a that a large number of wire-like structures with a length of 10-20 μm and a width of 200-400 nm are obtained, and there are no nanoparticles or nanorods, indicating that the synthetic route can result in a high yield of the nanowires. The magnified image (inset in Figure 1a) shows that the thickness of the wire-like crystals is about 50 nm. EDX analysis (inset in Figure 1a) reveals that the nanowires contain Mg, P, O, and less Na elements. The phase purity of the product was further examined by the XRD technique, and the result (Figure 1b) shows that the synthesized 7



product is mainly struvite (JCPDS file 77-2303) with a trace of cattiite [Mg3(PO4)2·22H2O] (JCPDS file 83-1486). This result agrees well with the model data calculated by Visual MINTEQ 3.1. The calculated saturation indexes (SI) with respect to struvite and cattiite are 1.116 and 0.399, indicating that the solution is supersaturated in regard to the two minerals, and thus their precipitation can occur. The SI of other minerals such as newberyite and brucite are calculated to be -0.836 and -2.032, showing an unsaturated state. Meanwhile, micro-Raman analysis was also performed for the identification of the nanowires. As shown in Figure 1c and d, the Raman vibrational bands of the nanowire are in good agreement with the reported pure struvite,15 further confirming that the nanowires are struvite. After a series of tedious FESEM analyses, however, a slice of plate-like structure was found (Figure S1), resembling the appearance of cattiite reported in our previous study.36 EDX analysis (inset in Figure S1) reveals that the plate-like crystal contains Mg, P, and O elements, corresponding to the elemental composition of cattiite. Combined with the XRD result (Figure 1b), it can reasonably assign the plate-like structure to cattiite. Hence, plenty of struvite nanowires with minor cattiite can be readily synthesized by current route. To the best of our knowledge, this is the first report on the synthesis of struvite nanowires. As the struvite nanowires can be successfully obtained from the mimetic wastewater, this could provide a novel and facile route to fabricate struvite nanofertilizer from wastewaters. In order to optimize the synthetic conditions of struvite nanowires, the effects of NaCl concentration, pHi, and reaction time on the crystallization of struvite were further investigated. First, a series of experiments with NaCl concentration ranging from 0 to 10.0 wt% were conducted at pHi 11.0 for 24 h. The representative FESEM results are depicted in Figure 2. It can be seen from Figure 2a and its inset that the large tabular structures and aggregates of nanoparticles coexist in the absence of NaCl. Nevertheless, the XRD analysis (Figure S2a) reveals that struvite is the only crystalline phase. Therefore, the tabular structures can be assigned to struvite crystals, while the nanoparticle aggregates may be amorphous matter. Meanwhile, the corresponding

31P

MAS/NMR spectrum (Figure S3) displays a prominent

peak at a chemical shift of +6.1 ppm that can be attributed to struvite, and a broader peak at 8


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+2.0 ppm near the position reported previously for amorphous phosphate phase.2,45 Furthermore, the EDX spectrum of the nanoparticle aggregates (inset in Figure 2a) shows the strong peaks for Mg, P, and O elements, indicating that the aggregates should be magnesian phosphate. It appears that the nanoparticle aggregates formed under current conditions can be assigned as amorphous magnesian phosphate (AMP). However, when 1.0 wt% of NaCl is added to the precipitation system, FESEM analyses found that besides some nanoparticles, massive nanowires appear (Figure 2b and its inset). The corresponding XRD analysis also donates the similar diffraction pattern (Figure S2b) to the precipitate with 3.5 wt% NaCl (Figure 1b). Combined with the FESEM results (e.g., Figure 2b), it can be easily concluded that the nanowire-like structures are struvite, and the nanoparticles are AMP, and that the presence of NaCl can significantly influence the morphology of struvite and promote the formation of struvite nanowires. On further increasing NaCl concentration to 2.0 and 3.0 wt%, the phase composition and morphological structure of the precipitates almost retain unchanged. However, the struvite nanowires without AMP can be harvested at NaCl concentration from 3.5 to 4.0 and 4.5 wt% (e.g., Figure 1a and b), and no nanoparticles can be found, indicating that the addition of NaCl inhibits the formation of AMP. This may be because the increase of NaCl concentration raised solution ionic strength and decreased the activities of Mg2+(aq) and PO43−(aq), thus limiting the precipitation of AMP. According to the model data calculated by Visual MINTEQ 3.1, the SI of Mg3(PO4)2(s) indeed decreases from 2.433 (0 wt% NaCl) to 1.288 (1.0 wt% NaCl) and 0.515 (3.0 wt% NaCl) with the increase in NaCl concentration. Moreover, the much higher NaCl concentrations such as 5.0 and 10.0 wt% lead to the formation of rod-like struvite (Figures 2c and S2c) and the radiating structures assembled by struvite tabulae (Figures 2d and S2d), respectively. These results indicate that NaCl can exert a prominent effect on the phase composition and morphology of the product, and the additive NaCl should be indispensable for the formation of struvite nanowires. The concentration-dependent experiments also reveal that the suitable NaCl concentration range for the synthesis of struvite nanowires is from 3.5 to 4.5 wt%. It has been reported that the pH of the solution can affect morphology, particle size, and 9



purity of struvite.2,10 In this context, the effect of different pHi values on the formation of struvite nanowires was investigated at the pHi ranging from 9.0 to 11.5, and a NaCl concentration 3.5 wt% and stirring time 24 h were used. The experimental results show that the tabular crystals with a mean dimension of 50 × 5 μm , as well as rod-like crystals with an average size of 20 × 2 μm are obtained at pHi 9.0 (Figure 3a). On further increasing the pHi to 10.0, the tabular crystals, with about 30 μm in length and 3-4 μm in width, become much more thinner (Figure 3b), and more rod-like crystals with an average length and width of about 20-30 and 1 μm can be found relative to the pHi 9.0. The XRD analyses confirm that the two kinds of products are all struvite (e.g., Figure S4a and b). At pHi 10.5, the product exhibits the rod-shaped structures with a length of ca. 30-50 µm and a width from 500 nm to 2 µm (Figure 3c), and the XRD analysis (Figure S4c) reveals that the product consists of struvite with minor cattiite, similar to the product obtained at pHi 11.0 with 3.5 wt% NaCl (Figure 1a and b). However, further raising pHi to 11.5, a mass of aggregates of nanoparticles and a few nanowires coexist (Figure 3d). The XRD pattern (Figure S4d) reveals that the crystalline phase is still dominated by struvite, but the strong reflection background and noises relative to the XRD patterns in Figure S4a-c indicate the presence of amorphous matter in the product. In short, it is obvious that struvite crystals get slenderer with pHi increase in the presence of 3.5 wt% NaCl. This variation trend of struvite agrees well with our previous study dealing with the effect of polyaspartic acid on struvite growth 36 and the results reported by Ma et al.2 in the absence of NaCl. This is because high precipitation rate and changes in aqueous speciation decrease the activities of Mg2+(aq), NH4+(aq), and PO43−(aq), leading to the limitation of struvite growth at higher pHi.2 However, further increasing pHi to 11.5 results in a significant reduction of struvite yield. This may because of the reduction of NH4+(aq) activity due to increased formation of NH3 species.46 Therefore, the pH-dependent experiments demonstrate that the slightly higher pH is required for the formation of struvite nanowires relative to the optimum pH range from 8.5 to 9.5 for struvite precipitation in the absence of NaCl,10 and the ideal pHi is 11.0. In order to clarify the formation details of struvite nanowires, a series of time-resolved 10


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experiments were carried out at pHi 11.0 with 3.5 wt% NaCl. The first batch of precipitate was collected when the pHi was just adjusted to 11.0. As shown in Figure 4a, only the mud-like aggregates can be observed. The magnified image (inset in Figure 4a) further unveils that the aggregates consist of nanoparticles. EDX analysis (inset in Figure 4a) shows that the aggregates contain elements O, Mg, P, and Na, and the XRD analysis identifies its amorphous nature (Figure S5a). Therefore, the initial precipitate is an amorphous magnesian phosphate with minor sodium incorporated. At 10 min, massive mud-like aggregates and wire-like crystals are obtained (Figure 4b). EDX analysis (inset in Figure 4b) reveals that the irregular aggregates also contain Mg, P, O, and Na elements. However, the XRD analysis of the precipitate (Figure S5b) donates the similar pattern to that depicted in Figure S4d, indicating that except for crystallized struvite and cattiite, the mud-like aggregates is still AMP. Further extending the reaction time to 30 min results in a marked reduction of the amount of AMP and the formation of a large amount of struvite nanowires, even though a few aggregates can still be observed, as indicated by the red arrows (Figure 4c). When the reaction time is prolonged to 3 h, only plenty of nanowires are harvested (Figure 4d), indicating that the AMP has completely transformed into struvite nanowires at that time. Therefore, AMP should be the precursor in the crystallization of struvite nanowires. In fact, a number of studies have found that many minerals are formed via an amorphous precursor pathway.47,48 For example, Rodriguez-Blanco et al. reported the crystallization of nanoparticulate amorphous calcium carbonate (ACC) to calcite, via vaterite.48 Jiang et al. observed that the hydroxyapatite hollow microspheres are synthesized following initial formation and subsequent transformation of amorphous calcium phosphate (ACP) spheres.49 According to the NaCl concentration-dependent experiments, the products are tabular struvite and AMP without the addition of NaCl, while struvite nanowires and AMP are obtained with low NaCl concentration. With the increase in NaCl concentration, more struvite nanowires appear and the amount of AMP is decreased. This indicates that NaCl is critical for struvite crystallization and nanowires formation. In fact, numerous crystallization studies have shown that foreign inorganic ions have similar effects on crystal growth of minerals as more 11



complex organic modifiers. That is, some inorganic ions can entirely inhibit crystal growth or enhance it, while others may exert a highly selective effect on some specific crystal faces and thus modifying the crystal habit.50,51 For instance, Wang et al. studied the growth of calcite in the presence of Li+ ions by in situ AFM observations, and found that the site-selective interactions between Li+ ions and growing calcite surface stabilize the energetically unfavourable (001) face, leading to the development of calcite with {0001} faces.50 Benages-Vilau et al. proved that both K+ and Li+ ions can modify the morphology of nitratine from simple form {10.4} to a crystallographic combination consisting of {00.1} and {10.4} forms.51 Suguna et al. reported that high NaF concentration inhibits growth of struvite and leads to a significant reduction in the number and size of struvite crystals.39 They speculated that the inhibition effect results from the adsorption of fluoride on crystal faces of struvite, but the exact mechanisms remain to be investigated. Here, to further identify the exact role of Na+ and Cl- ions in the formation of struvite nanowires, two control experiments were carried out. That is, an equimolar concentration of LiCl or NaNO3 is added to replace the 3.5 wt% NaCl while other conditions remain unchanged. After 24 h of reaction, only huge tabular aggregates are obtained in the presence of LiCl (Figure 5a). The magnified image (inset in Figure 5a) displays that the aggregates consist of curly lamellar structures. EDX analysis (inset in Figure 5a) shows that the tabular aggregates contain Mg, P, and O elements, and the XRD pattern exhibits an amorphous feature (Figure S6a). Therefore, only AMP is formed with LiCl. In contrast, plenty of nanowires are harvested with the addition of NaNO3 (Figure 5b). XRD analysis confirms that the nanowires are struvite (Figure S6b). That is to say, struvite nanowires can still be synthesized with NaNO3. Combining with the experimental result with LiCl, it can be concluded that it is Na+ ions rather than Cl- ions that play a central role in the formation of struvite nanowires. Meanwhile, the surface element composition of the struvite nanowires obtained with 3.5 wt% NaCl was determined by XPS over the energy range of 0-1350 eV. As depicted in Figure S7, the typical XPS wide-scan spectrum shows that all four elements comprising struvite (MgNH4PO4·6H2O), i.e., Mg, O, P, and N are observed. It is worth 12


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noticing that Na also appears in the spectrum while no Cl signal is found. Moreover, the mass fraction of Na detected by XPS (9.07%) is much higher than that of the EDX analysis (4.68%), indicating that much more Na ions were concentrated on the surfaces of struvite. Hence, NaCl mediates the formation of struvite nanowires by the selective adsorption of Na+ ions onto struvite surface, resulting in the decrease of the growth rate of certain crystal faces and the alteration of crystal morphology. In fact, several researchers have disclosed that struvite crystals exhibit high negative zeta potential in the pH range 8.5-10.5.52-54 Usually, the higher the absolute value of zeta potential, the greater is the charge density on the particles surface.54,55 Therefore, struvite crystals with high negative surface charges could potentially attract positive charged Na+ ions in solution. Herein, the zeta potentials of a series of struvite crystals synthesized with different NaCl concentrations were also determined in deionized water at pH 11.0. To avoid the interference of amorphous nanoparticles, the sample without NaCl was synthesized at pHi 9.0, and others were synthesized at pHi 11.0. The zeta potential-NaCl concentration curve is shown in Figure 6. The results show that zeta potential of pure struvite synthesized without NaCl is about -26 mV, in agreement with the results reported by other researchers.52,53 With increasing NaCl concentration, the zeta potential gradually becomes less negative, suggesting the adsorption of more and more positive charged Na+ ions onto the crystal surface. The analogous phenomena have been reported by other investigators.55,56 For instance, Wan et al. used zeta potential to characterize the interaction between lecithin liposome and calcium ions.56 They found that zeta potential of lecithin liposome varies toward the positive direction as the concentration of Ca2+ increased, thus proving the adsorption of calcium ions onto the negatively charged liposome surface. Huang et al. observed the zeta potential of calcium oxalate crystals becomes more negative with increasing concentration of negative charged polysaccharides, resulting from the adsorption of more polysaccharides on the crystal surface.55 Therefore, our results demonstrate that the selective electrostatic accumulation of Na+ ions on some crystal faces of struvite occurs, leading to the orientation growth of struvite and the final formation of struvite nanowires. It is also noticed that no nanoscale struvite forms when NaCl concentration 13



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increases to 5.0 wt% or higher. This may result from the inhibition of struvite nucleation at high ionic strength. We indeed observed the delay in initial struvite precipitation and the reduction of struvite yield at 5.0 wt% and 10.0 wt% NaCl. The inhibition of nucleation in turn causes the increase of struvite dimension.

Conclusions In summary, uniform struvite nanowires can be manipulatively synthesized by regulating NaCl concentration and pHi in synthetic wastewater. Our results show that both NaCl and pHi play a crucial role during the formation of struvite nanowires. In particular, NaCl acts as an inorganic modifier by the selective adsorption of Na+ ions onto struvite faces, inducing its orientation growth. As the concentrations of N, P, and Mg in our synthetic liquors approach to the values in real wastewater, current route can potentially be applied to struvite nanowires recovery from wastewater. This will help to raise the quality of recovered struvite, thus increasing its value in fertilizer markets and improving the economic feasibility of struvite crystallization technology for phosphorus recovery. Moreover, as seawater is rich in Mg2+ and NaCl,57 the results could be also applied to the recovery of struvite nanowires when cheap seawater is selected as Mg2+ source.

Acknowledgements This work was partially supported by the Natural Science Foundation of China (Nos. 41672034 and 41702034), the Fundamental Research Funds for the Central Universities, the China Postdoctoral Science Foundation (No. 2016M602023), and Anhui Provincial Natural Science Foundation (No. 1808085MD103).

Supporting information The characterizations of the products, including FESEM, EDX, XRD, XPS. 14


31P

MAS/NMR, and

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References (1) Cordell, D.; Drangert, J.O.; White, S. The story of phosphorus: Global food security and food for thought. Global Environ. Change 2009, 19, 292-305. (2) Ma, N.; Rouff, A.A.; Phillips, B.L. A

31P

NMR and TG/DSC-FTIR investigation of the

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Figure captions Figure 1 Typical FESEM image (a) of nanowires formed with 3.5 wt% NaCl at pHi 11.0 for 24 h, corresponding XRD pattern (b), optical microscopy image (c) and Raman spectrum (d) of the nanowires. The analyzed nanowire by Raman spectroscopy was marked by the red arrow. Insets in image (a) display the further magnification and EDX analysis of the nanowires. The (hkl) indices of struvite and cattiite (marked as C) are shown in image (b). Figure 2 FESEM images of the products synthesized at pHi 11.0 for 24 h in the presence of 0 (a), 1.0 (b), 5.0 (c), 10.0 (d) wt% NaCl. Insets display the further magnified images and/or EDX analyses. Figure 3 FESEM images of the products synthesized with 3.5 wt% NaCl for 24 h at pHi 9.0 (a), 10.0 (b), 10.5 (c), 11.5 (d). Figure 4 FESEM images of the precipitates collected when pHi was just adjusted to 11.0 (a) and after 10 (b), 30 (c), and 180 (d) min in the presence of 3.5 wt% NaCl. Insets in images (a) and (b) display the further magnification and/or EDX analyses. Figure 5 FESEM images of the products synthesized at pHi 11.0 for 24 h in the presence of LiCl (a) and NaNO3 (b). Insets in (a) display the further magnified image and EDX analysis. Figure 6 Zeta potential of struvite crystals synthesized with different NaCl concentrations. The vertical lines are mean standard deviation.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5


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Figure 6

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Synopsis and TOC graphics

Struvite nanowires were successfully synthesized from synthetic wastewater, and can be used as a nanofertilizer with high efficiency.

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Controlled synthesis of struvite nanowires in synthetic wastewater

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