*Article* **Optimizing Struvite Crystallization at High Stirring Rates**

**Atef Korchef 1,\*, Salwa Abouda 2,3 and Imen Souid <sup>1</sup>**


**Abstract:** Phosphorus and ammonium can both be recovered in the presence of magnesium through struvite (MgNH4PO4·6H2O) crystallization. The present work aimed to optimize struvite crystallization at turbulent solution flow. Struvite was crystallized by magnetic stirring at different initial phosphorus concentrations between 200 and 800 mg·L−<sup>1</sup> and high stirring rates between 100 and 700 rpm. The crystals obtained were analyzed by powder X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy. For all experiments, the only phase detected was struvite. It was shown that for an initial phosphorus concentration of 200 mg·L<sup>−</sup>1, increasing the stirring rate to 500 rpm accelerated the precipitation of struvite, improved the phosphorus removal efficiency, and obtained larger struvite crystals. A decrease in the phosphorus removal efficiency and smaller struvite crystals were obtained at higher stirring rates. This was attributed to the solution turbulence. The limiting effect of turbulence could be overcome by enhancing the initial phosphorus concentration or by lowering the stirring rate. The highest phosphorus removal efficiency (~99%) through large struvite crystals (~400 μm in size) was obtained for an initial phosphorus concentration of 800 mg·L−<sup>1</sup> and a stirring rate of 100 rpm.

**Keywords:** struvite; fertilizer; phosphorus; ammonium; wastewater; stirring; turbulence

#### **1. Introduction**

The world's surface is covered by water to a great extent, most of it in the oceans. Only 3% of this water is fresh, of which only ~0.5% is available [1]. The rest (2.5%) is locked up in glaciers and soil or is too deep below the earth's surface to be easily extracted at an affordable cost. Unfortunately, freshwater can be polluted by industrial effluents, animal discharges, and chemical fertilizers. Phosphorus and nitrogen are two of the main nutrients found in water. Their presence in high concentrations pollutes the water and caused eutrophication. For this reason, their recycling has gained importance, especially since the European legislation against water pollution has imposed a phosphorus concentration below 2 mg·L−<sup>1</sup> [2]. Phosphorus and nitrogen can be simultaneously recycled, in a basic medium and in the presence of magnesium, through the crystallization of a sparingly soluble salt, struvite (MgNH4PO4·6H2O). This prevents eutrophication and varies phosphate resources. Struvite is recognized as a valuable fertilizer [3]. Recently, it was shown that struvite could be used effectively as a fire-extinguishing agent [4]. For these reasons, struvite crystallization from synthetic solutions and real wastewater has been intensively studied [5–8].

The control of both the nucleation and growth of struvite crystals is difficult because they depend on various physical and chemical parameters such as ion transfer between the liquid and solid phases, reaction kinetics, temperature, supersaturation, pH of the solution, concentrations of struvite constituent ions, foreign ions (Ca2+, Fe2+, Cu2+, and others), and flow turbulence. The optimal temperature reported in the literature for struvite

**Citation:** Korchef, A.; Abouda, S.; Souid, I. Optimizing Struvite Crystallization at High Stirring Rates. *Crystals* **2023**, *13*, 711. https:// doi.org/10.3390/cryst13040711

Academic Editor: Zongyou Yin

Received: 26 March 2023 Revised: 18 April 2023 Accepted: 19 April 2023 Published: 21 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

crystallization was in the range of 15–30 ◦C [9,10]. Indeed, higher temperatures decreased the struvite solubility and affected both its morphology and purity [11]. The precipitation pH used was in the range of 8–11 [9,10,12]. That is, alkaline solutions promoted struvite crystallization.

At a given phosphate concentration, enhancing the magnesium concentration in the solution lowered the struvite crystallization pH and considerably increased the effectiveness of phosphate and ammonium removal [13]. Depending on the added concentration, the addition of phosphate to the solution at a fixed magnesium concentration can either delay or entirely inhibit struvite crystallization [13]. An increase in magnesium or phosphate concentration affected the struvite crystals' purity, shape, and size. However, adding ammonium in excess to the solution favored struvite crystallization without affecting the purity of the obtained struvite crystals [13]. Jaffer et al. [14] showed that struvite crystallization occurred at a sewage treatment plant when the molar ratio of Mg:P was at least 1.05:1 and for lower ratios, phosphorus removal decreased but not exclusively as struvite. Kruk et al. [15] studied the crystallization of struvite from the supernatant of fermented waste-activated sludge using magnesium sacrificial anode at N:P molar ratios between 1.98 and 2.05. They found that up to 98% of soluble phosphorus was recovered through struvite crystallization. Korchef et al. [13] demonstrated that there was no optimal value for the Mg:P:N ratio for struvite crystallization that could be considered independently of the initial constituent ions concentrations, but it should be adjusted with respect to the initial phosphate, ammonium, and magnesium concentrations and operating parameters.

The presence of foreign ions in the solution such as the ions of calcium [16], copper [16,17], zinc [17], iron [18], aluminum [18,19], cadmium [19], and nickel [20], among other ions, affected the ammonium and phosphate recovery and limited struvite crystallization. Indeed, these ions can be incorporated with struvite, adsorbed on struvite surfaces, or precipitated as separated phases. Unlike heavy metal ions, the presence of phenolic organics enhanced the struvite crystallization rate [21].

Different techniques, such as stirring [22,23], CO2 repelling [24,25], electrochemical deposition [26], and ion exchange [27], have been used to precipitate struvite from synthetic solutions and real wastewater. In this work, struvite was precipitated by magnetic stirring. It was shown that stirring strongly affected struvite crystallization. Indeed, increasing the stirring rate affected the mass transfer between the solution and the struvite crystals and enhanced the struvite crystal growth rate [28]. Capdevielle et al. [22] found that more than 90% of phosphorus was recovered through large struvite crystals at a high N:P ratio of 3:1 and moderate stirring rates between 45 and 90 rpm. Perera et al. [29] studied the precipitation of struvite in a stainless steel reactor at pH 9, and a N:P:Mg molar ratio equal to 1:1.2:1.2. They found that stirring at 50 rpm did not allow sufficient mixing, thus affecting the struvite growth. They found that the removal efficiencies of nitrogen and phosphate were 97% and 71%, respectively, for a stirring rate of 500 rpm. Xu et al. [30] investigated the effect of stirring on laboratory-scale recovery of phosphorus and potassium from urine. They showed that for stirring rates between 100 and 200 rpm, 68% of the phosphorus was recovered in the form of struvite and 76% of the potassium in the form of K-struvite (MgKPO4·6H2O). Zhang et al. [31] investigated the effect of stirring and experiment time on struvite crystallization from swine wastewater as pretreatment to anaerobic digestion. They found that 38% and 44% of ammonium were recovered after 10 min at stirring rates of 160 and 400 rpm, respectively, and with increasing the reaction time no remarkable changes in the recovered amounts were observed. The optimal operating conditions for struvite crystallization were a P:Mg:N molar ratio of 1:1:1.2, pH = 10, and initially stirring at 400 rpm for 10 min and then stirring for 30 min at 160 rpm. From an experimental point of view, the precipitation of struvite under magnetic stirring is one of the easiest methods to implement on a laboratory scale. It requires space-saving equipment whose price remains reasonable. It saves consumables and time on the preparation and progress of manipulations.

The present work aimed to investigate struvite crystallization at turbulent solution flow caused by magnetic stirring. It is not unfounded to expect that the effect of turbulence due to high stirring rates on struvite crystallization can be overcome by controlling the solution volume or the concentrations of the struvite constituent ions. For this reason, the effect of stirring at a fixed volume and initial phosphorus concentration was first investigated. Then, the effect of the solution volume on the crystallization of struvite at a fixed initial phosphorus concentration and stirring rate was investigated. Finally, the effect of the initial phosphorus concentration on struvite crystallization was studied at a fixed stirring rate and solution volume. Phosphorus concentration and solution pH were monitored over time. Struvite crystals were examined by powder X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy. Their morphology was observed by scanning electron microscopy (SEM).
