Electrochemical Precipitation of Struvite from Wastewater: A Sustainable Approach for Nitrogen Recovery

Abstract

This study evaluates the feasibility of nitrogen recovery from wastewater via electrochemical methods as an alternative nutrient source for agricultural applications. Ammonium nitrogen (NH₄⁺-N) and phosphate (PO₄³⁻-P) contamination poses significant environmental risks and challenges water resource management globally. The electrochemical precipitation of struvite (MgNH₄PO₄·6H₂O) offers a promising solution for nutrient recovery, with potential applications as a slow-release fertilizer. Experimental results demonstrate that increased current density (from 2.5 to 7.5 mA/cm²) and reduced electrode distance (1 cm) significantly enhance NH₄⁺ and PO₄³⁻ consumption and struvite precipitation. Increasing the amperage from 2.5 to 7.5 mA·cm⁻² at a 1 cm electrode distance raised the ammoniacal nitrogen incorporation from 1.59 to 5.34 g/100 g, signifying greater struvite production. The Mg and P concentrations were 15.44 and 12.60 g/100 g, respectively, for this higher amperage, although lower than the concentrations seen with 2.5 mA·cm⁻² (22.16 and 14.52 g/100 g). The majority of Mg (60%) and P (93.6%) were, however, incorporated within struvite. Additionally, this study reveals that Mg is primarily incorporated as struvite when using higher current densities, while lower current densities yield greater Mg incorporation in non-struvite forms, such as magnesium carbonate. Findings suggest that optimizing current density and electrode distance can improve nitrogen and phosphorus recovery efficiencies, making electrochemical struvite production a viable, sustainable approach for nutrient recycling. This method not only reduces dependence on synthetic fertilizers but also supports sustainable agricultural practices by transforming wastewater contaminants into valuable resources.

1. Introduction

Water contamination has become an intensifying environmental issue, primarily due to elevated nitrogen levels that pose significant risks to both human health and ecosystems [1]. Water sources are essential for drinking water, landscape irrigation, and various industrial uses. However, contamination with ammonium nitrogen (NH₄⁺-N) and phosphate (PO₄³⁻-P) in these waters has become a widespread global issue, reported extensively across diverse water bodies [2]. NH₄⁺-N serves as an important indicator of surface water pollution, yet it is difficult to transform, which can exacerbate water quality deterioration. Phosphorus (P), commonly present as PO₄³⁻-P, promotes algal blooms and poses significant ecological risks [3,4]. The widespread impact of pollution across numerous water bodies highlights the critical need for nitrogen recovery as a strategy for sustainable nutrient management and energy conservation [5,6].

The swine production chain generates large volumes of wastewater rich in organic matter, nutrients, pathogens, and heavy metals, which have become one of the main sources of surface water pollution [7,8], causing eutrophication both in surface and underground waters [9]. However, the high concentrations of nitrogen and phosphorus have long evoked the interest of the scientific community regarding the possible applications of this effluent in the agricultural area in order to meet the demand for NP fertilizers, which presents some manufacturing problems [10]. The contamination of water bodies by effluents from the swine production chain associated with the large-scale application of effluents from swine farming in agricultural soil has boosted scientific investigations. The studies aimed at the chemical recovery of these nutrients and possible controlled applications [11,12].

NH₃-nitrogen compounds in industrial wastewater pose significant environmental risks. These compounds can lead to eutrophication, which harms aquatic ecosystems, disrupts water disinfection processes, produces unpleasant odors, and may be carcinogenic [13,14]. While various methods exist for ammonium removal—including air stripping, biological denitrification, breakpoint chlorination, and electrolysis—the electrochemical approach has gained particular interest [15,16]. This method offers advantages such as low secondary waste production, ease of operation, and remote-control capabilities. Given that ammonium has higher Gibbs free energy compared to nitrogen gas, it can potentially be decomposed into environmentally benign nitrogen gas through electrochemical processes [17].

The four primary sources of nitrogen available for crop production are as follows: (1) nitrogen mineralized from native soil organic matter, (2) nitrogen derived from biological fixation by rhizobial bacteria associated with legumes, (3) synthetic nitrogen fertilizers, and (4) nitrogen-rich sources such as manures, composts, biosolids, and other organic wastes [18]. Currently, synthetic fertilizers are generated through energy-intensive chemical processes, such as the Haber–Bosch method, which captures atmospheric dinitrogen. A more sustainable approach would reduce energy demands and chemical use, redirecting the nitrogen recovered from wastewater into fertilizer production and other applications [1]. In addition, different recovery methods, though varied in technique and output, converge on the goal of capturing ammonium from wastewater for subsequent use. Among these, the recovery of magnesium ammonium phosphate (MgNH₄PO₄·6H₂O), commonly known as struvite, has gained attention [9,19]. Due to its slow-release characteristics, struvite is increasingly valued as a fertilizer, especially for its effectiveness in both slightly alkaline and acidic soils [10].

Struvite (MgNH₄PO₄·6H₂O) is a white mineral that precipitates in supersaturated solutions containing Mg²⁺, NH₄⁺, and PO₄³⁻ ions, as shown in Equation (1). Its role as a controlled-release fertilizer has proven beneficial for nutrient management in agriculture [20].

$${\mathrm{Mg}}^{2+}{+\mathrm{NH}}_4^{+}+{\mathrm{H}}_x{\mathrm{PO}}_4^{\mathrm{x}-3}+{ 6\mathrm{H}}_2\mathrm{O} \to \mathrm{Mg}{\mathrm{NH}}_4\mathrm{P}{\mathrm{O}}_4·{6\mathrm{H}}_2\mathrm{O}\times {\mathrm{H}}^{+}$$

(1)

The crystallization process of struvite involves nucleation and crystal growth, which are complex and influenced by physicochemical parameters such as pH, temperature, and supersaturation with respect to struvite, interfering ions (SO₄²⁻, CO₃²⁻, Ca²⁺, Fe²⁺, Fe³⁺), metals like Cu and Zn species [20,21,22], and total suspended solids [23].

The high concentration of total or dissolved organic carbon present in swine wastewater disturbs struvite precipitation, decreases crystal purity, and slows precipitation kinetics. Thus, one of the ways to facilitate struvite precipitation from swine wastewater is to perform a pre-treatment to remove/minimize these factors that interfere with struvite formation [1,5]. In addition to carbon, the presence of heavy metals can also interfere with struvite morphology and yield, and their removal is preferable before struvite precipitation [9]. Thus, electrocoagulation is a pre-treatment alternative capable of reducing interfering agents without changing the initial concentration of NH₃ [7].

In this context, the electrochemical precipitation of struvite (MgNH₄PO₄) using the sacrificed magnesium anode is a new and effective method to recover phosphorus from wastewater [24]. The electrochemical precipitation of struvite, using Mg electrodes primarily at the anode, eliminates the need for external Mg²⁺ addition and potentially simplifies pH regulation without chemical amendments. In this context, electrochemical methods using sacrificial magnesium anodes were developed to generate Mg²⁺ in situ, facilitating struvite formation, while cathodic electrodeposition enabled the simultaneous separation of the precipitate [25]. The Mg²⁺ ions released from the anode react with NH₄⁺-N and PO₄³⁻-P in the bulk solution to form struvite, which preferentially accumulates on the cathode surface due to localized pH elevation caused by hydrogen evolution, leading to the enhanced recovery and separation of struvite from the liquid phase [25,26].

The optimal pH range for struvite formation is 8–9, so the precursor ions enriched near the cathode are more likely to form struvite and deposit on the cathode under supersaturation and a relatively strong alkaline pH environment (≥8.5) [27].

Given these considerations, this study aims to assess the feasibility of nitrogen recovery from wastewater as an alternative nutrient source for crop production, aiming to reduce the environmental impact of traditional synthetic fertilizers and support sustainable agricultural practices. To achieve this objective, this study evaluates the effectiveness of electrochemical methods for the recovery of ammonium nitrogen (NH₄⁺-N) from wastewater, focusing on the production of struvite (MgNH₄PO₄·6H₂O) as a slow-release fertilizer. For this purpose, the effluent was submitted to an electrochemical pre-treatment with iron (Fe) electrodes to remove possible interferents, such as carbon and heavy metals, followed by electro-precipitation with magnesium electrodes, in which the effects of the distance of the electrodes (1 or 3 cm) and current density (2.5 or 7.5 mA·cm⁻²) were evaluated. In this sense, this study also aims to evaluate the influence of the distance between electrodes and current density on struvite recovery efficiency.

2. Materials and Methods

2.1. Swine Wastewater

The swine wastewater was collected at the outlet from an Upflow Anaerobic Sludge Blanket (UASB) reactor for pig manure treatment at Embrapa Swine and Poultry (Concórdia, Santa Catarina, Brazil, 27°18′ S, 51°59′ W). Swine wastewater was stored in 5 L polyethylene containers and kept frozen at −10 °C until its use in electrocoagulation treatment. The swine wastewater was first submitted to electrocoagulation under continuous flow (hydraulic retention time: 90 min) using Fe electrodes (current density: 38.89 mA·cm⁻²) as described in previous studies [28,29]. Wastewater samples were collected before and after this treatment for analytical determinations.

2.2. Electrochemical Precipitation Reactor for Nitrogen Recovery

A pair of vertically installed Mg electrodes (surface area: 4000 mm²; dimensions: 130 mm height × 70 mm length × 20 mm width; AZ91 alloy; RIMA Industrial, Bocaiúva, Brazil) were placed within a cylindrical batch reactor (1 L). The system featured a magnetic stirrer to ensure periodic homogenization every 20 min, achieved by adding a 0.1 mol·L⁻¹ HCl solution (via a Peristaltic pump—Fisher Scientific (Hampton, NH, USA)) to maintain a pH range of 8.5 (controlled by a JENCO pH controller, San Diego, CA, USA). The electrodes were linked to an FA-3005 DC power supply (Instrutherm, São Paulo, Brazil) for current density regulation. The electrode distance ranged between 1 and 3 cm, with current densities of 2.5 or 7.5 mA·cm⁻². Supernatant samples were collected at intervals of 0, 2, 4, and 6 h, and the resulting white solid was filtered and dried at 30 °C for 24 h.

2.3. Analytical Determinations

Ammonium, nitrite, and nitrate concentrations were analyzed by absorption spectrometry in the ultraviolet–visible region using a FIAlab 2500 flow injection analysis system (Bellevue, WA, USA) equipped with an automatic sampler following the equipment manufacturer’s recommendations and according to the official APHA methods, 4500-NH₃ H nitrogen (ammonium), 4500-Nitrite nitrogen (Nitrite), and APHA 4500-Nitrate nitrogen (Nitrate) [30]. Turbidity was determined by nephelometry using a Hach 2100P portable turbidimeter (Loveland, CO, USA), following the APHA 2130 B [30].

Soluble organic carbon (SOC) was analyzed with the principle of burning at 950 °C, and CO₂ was measured by an NDIR infrared detector by the elemental analysis equipment Multi Elementary Analytic^® Multi N/C 2100, by Analytik Jena (Jena, Germany). The determination of total phosphorus (P_total) was performed by spectrometry in the ultraviolet–visible region, which was based on the official method [28,29].

The pH monitoring was conducted periodically throughout the experiment with a Marconi PA200 pH meter (Piracicaba, SP, Brazil). Specifically, pH measurements were taken every 20 min by extracting a 50 mL aliquot using a plastic beaker. The pH was then measured, and the aliquot was returned to the system, with adjustments made as necessary in the system. Conductivity was determined by direct measurement in a Hanna HI 255 conductivity meter. Alkalinity was determined following the procedures described by the Standard Methods for Examination of Water and Wastewater [30].

The determination of metals, copper (Cu), zinc (Zn), and iron (Fe), which used the analytical procedure for opening liquid samples by nitric–sulfuric digestion, is based on the official method APHA 4500-P B [30]. The analytical determination of the elements Cu, Zn, Mn, Fe, and Al was performed by flame atomic absorption spectrophotometry (AAS) and is based on the official method AOAC 975.03. The percent removal was calculated by Equation (2).

$$CR(\%)={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(2)

where C₀ is the initial concentration of the constituents, and C_t is the final concentration of the constituents (after EC).

2.4. Struvite Characterization

The samples were analyzed by X-ray powder diffraction (XRD) using a Brucker D2 Phaser (Billerica, MA, USA), with Cu Ka radiation. XRD patterns were recorded in the 2θ scan range of 10° to 80°. A small angular step of 2θ = 0.017° was used, and a fixed counting time of 4 s was used. The morphology of the precipitate was performed using a scanning electron microscope (SEM) (EVO 25 ZEISS (Jena, Germany)). The struvite particles were previously covered with a gold layer with the aid of a metallizer, and the analyses were carried out using a voltage of 5 kV, with different increases, and the surface elemental composition was analyzed using energy dispersive spectroscopy (EDS).

3. Results

3.1. Electrocoagulation Process

The characterization of the wastewater pre-treated by UASB and used as raw pigs’ wastewater in the present study is presented in

Table 1. Chemical characterization of wastewater pre-treated by UASB before and after the electrocoagulation process.

Parameters	Wastewater Pre-Treated by UASB	After Electrocoagulation	Removal (%)
Conductivity (mS·cm−1)	11.5	8.01	30
Turbidity (NTU)	1179.2	22.4	98
SOC (mg·L−1)	4001.0	1572.5	61
P (mg·L−1)	76.5	1.1	98
Cu (mg·L−1)	2.0	0.3	85
Zn (mg·L−1)	7.9	<DL	100
Fe (mg·L−1)	7.8	19.3	-
Ca (mg·L−1)	1214.0	43.7	96
Mg (mg·L−1)	34.4	25.7	25
K (mg·L−1)	609.0	480.0	21
Alkalinity (mgCaCO3 L−1)	3414.7	2099.3	38
Ammonium (mg·L−1)	1221.0	1121.0	8
Nitrate (mg·L−1)	<DL *	<DL *	-
Nitrite (mg·L−1)	<DL *	<DL *	-

* DL = below the detection limit of the method.

. The presence of a high concentration of ammonia (NH₃ = 1221.0 mg·L⁻¹) is observed, in addition to phosphorus (P) and magnesium (Mg) in lesser amounts, among other components. The presence of NH₃, P, and Mg, which are part of the struvite structure of Equation (1), suggests that wastewater pre-treated by UASB may be a potential precursor for the electrochemical production of struvite.

It was not possible to conduct the formation of struvite crystals in the preliminary tests using magnesium electrodes directly in the wastewater pre-treated by UASB. This trend was linked to the presence of high concentrations of soluble and suspended organic matter (indicated by turbidity), calcium (Ca) and, to a lesser extent, the elements Cu, Zn, and Fe (Table 1), which are reported by the literature as possible interferers in the nucleation and growth of struvite crystals [21,23,31,32].

In this sense, seeking to minimize and even eliminate the negative effect of these possible interfering agents, the wastewater pre-treated by UASB was subjected to a treatment step by electrocoagulation (EC) in continuous flow, with iron electrodes, using the conditions optimized by Mores et al. [28]. The results of this treatment are shown in Table 1.

An increase in the concentration of iron (Fe) from 7.8 to 19.3 mg·L⁻¹ was observed, which was linked to the dissolution of Fe⁺² and/or Fe⁺³ of the iron (Fe⁰) electrode used in the electrochemical assay as a precursor of the coagulating agent Fe(OH)₂ and/or Fe(OH)₃ [33].

The EC showed removals of 61%, 98%, 85%, 99%, and 96% for soluble organic carbon (SOC) and in suspension (turbidity), copper (Cu), zinc (Zn), and calcium (Ca), respectively, and with a removal of only 8%, almost all the ammoniacal nitrogen (1121 mg·L⁻¹) in solution was maintained. Through electroflocculation, removals of the same magnitude were observed by other authors for different types of effluents [29,34].

The removal of calcium (Ca²⁺) prior to struvite precipitation is of critical importance, as the presence of Ca²⁺ ions has been shown to negatively affect the properties and purity of the struvite formed. Elevated concentrations of calcium during the precipitation process promote the formation of undesired amorphous calcium phosphate phases, such as CaHPO₄·2H₂O (brushite, DCPD), Ca₄H(PO₄)₃·2.5H₂O (octacalcium phosphate, OCP), Ca₃(PO₄)₂·nH₂O (amorphous calcium phosphate, ACP), and Ca₅(PO₄)₃OH (hydroxyapatite, HAP). These competing phases not only reduce the crystallinity and quality of the struvite but also decrease the phosphorus recovery efficiency, thereby lowering the phosphorus content in the final product [35,36]. For this purpose, electrocoagulation is a pre-treatment alternative capable of reducing interfering agents without changing the initial concentration of NH₃ [7].

3.2. Electrochemical Struvite Precipitation

The evolution of phosphorus and ammonia removal from wastewater after electrochemical precipitation and the phosphorus content correction values for the different conditions of current density and distance of electrodes are shown in

Table 2. Concentration of phosphorus (P) and ammonia (NH₃) in the electrochemical struvite precipitation process in mg·L⁻¹ and removal percentage.

	2.5 mA·cm−2 and 1 cm		2.5 mA·cm−2 and 3 cm		7.5 mA·cm−2 and 1 cm		7.5 mA·cm−2 and 3 cm
Time (h)	NH3(mg·L−1)	P (mg·L−1)	NH3(mg·L−1)	P (mg·L−1)	NH3(mg·L−1)	P (mg·L−1)	NH3(mg·L−1)	P (mg·L−1)
0	1168 (0%)	2063 (0%)	1116 (0%)	1935 (0%)	2000 (0%)	1184 (0%)	1740 (0%)	1550 (0%)
2	984 (16%)	1853 (10%)	1217 (0%)	1830 (1%)	1760 (12%)	948 (20%)	1800 (0%)	1326 (14%)
4	983 (16%)	1763 (15%)	1087 (3%)	1920 (5%)	1140 (43%)	420 (60%)	1235 (29%)	1065 (31%)
6	891 (24%)	1725 (16%)	894 (20%)	1660 (14%)	860 (57%)	469 (65%)	1079 (38%)	933 (40%)
Yield (g)	5.12		4.33		15.19		11.32

.

All evaluated conditions demonstrated a similar trend for both responses: increased phosphorus (P) and ammonia (NH₃) consumption during the electrochemical struvite precipitation process. Increasing the current density from 2.5 to 7.5 mA·cm⁻² for both electrode distances (ED) assessed (1 cm and 3 cm) enhanced phosphorus consumption from 16% to 57% at a 1 cm ED and from 14% to 38% at a 3 cm ED. For ammonia, consumption rose from 24% to 65% at a 1 cm ED and from 20% to 40% at a 3 cm ED. This increase in consumption directly impacted the mass of precipitate generated, showing a positive correlation with N and P levels.

An increase in ED, regardless of current density, reduced P and NH₃ consumption, with decreases from 16% to 14% for P and from 24% to 20% for NH₃ at 2.5 mA·cm⁻² and from 57% to 38% for P and from 65% to 40% for NH₃ at 7.5 mA·cm⁻². Examining these variables independently, current density showed the most significant effect, with variations in the P and NH₃ consumption of approximately 3.5 and 2.7 times, respectively, for tests conducted at a 1 cm ED.

In specific values, the test conducted at 7.5 mA·cm⁻² with a 1 cm ED achieved the highest removal rates for P (57%) and NH₃ (65%), yielding the largest precipitate mass (15.19 g). Conversely, tests at 2.5 mA·cm⁻² with a 3 cm ED yielded the lowest P (14%) and NH₃ (20%) removals and the smallest precipitate mass (4.33 g).

These results align with expectations, as increasing the current density and decreasing the ED between electrodes enhance electron flow within the reaction medium. This, in turn, promotes the dissolution of the Mg⁰ electrode as Mg²⁺ ions in solution, which is essential for struvite precipitation (Equation (1)) [37].

Considering the solubility equilibrium of struvite (Equation (1)), an increase in the concentration of Mg in solution—supplied by dissolution from the magnesium electrode—promotes struvite precipitation by shifting the chemical equilibrium toward the insoluble form. In terms of cost, using magnesium electrodes as a source of Mg²⁺ ions is comparable to the addition of magnesium salts such as MgCl₂ and MgSO₄ [38].

The magnesium concentration in solution was monitored under all evaluated conditions, but none showed detectable levels of magnesium. This absence suggests that all of the magnesium that leached from the electrode during the electrochemical process was incorporated into struvite crystals and/or other compounds. Figure 1 shows the SEM images of struvite crystals formed under the 7.5 mA cm⁻² current density and 1 cm spacing conditions, revealing coarse, irregularly shaped crystals.

The morphology of struvite crystals is influenced by the arrangement of its constituent ions—ammonium (NH₄⁺), phosphate (PO₄³⁻), and magnesium (Mg²⁺) (Equation (1)). Specifically, the crystal shape results from various sequences of ionic and non-covalent interactions, rearrangements, and clustering during crystallization, leading to diverse morphologies and occasionally the formation of other struvite structures. Additional factors influencing crystal structure and shape include temperature, pH, residence time, and the type and concentration of ions in solution, which can facilitate the incorporation of impurities into struvite crystals [39]. These impurities may alter crystal growth patterns, resulting in irregularities or variations in morphology [40].

Swine wastewater contains a broader range of ions beyond those directly involved in struvite formation (Equation (1)), introducing ions that may affect the dynamics of ionic interactions and clustering [9,40], thereby explaining the heterogeneous crystal shapes observed in SEM analysis.

The shape of struvite crystals is related to the arrangement of ionic groups (ammonium—NH₄⁺, phosphate—PO₄³⁻, and magnesium—Mg²⁺) that constitute struvite (Equation (1)); more specifically, due to the different sequences of ionic and non-covalent interactions, the rearrangement and clustering of ions during crystal formation results in different morphologies and/or the formation of other struvite crystals. Other parameters that control the formation of crystals and that can affect their structure and shape are temperature, pH, residence time, and the variety and concentration of ions present in the solution, which favor the incorporation of impurities in the composition of struvite crystals [39]. These impurities are responsible for possible changes in crystal growth patterns, resulting in irregularities and/or changes in their morphology [40].

As shown in Table 1, the composition of swine wastewater is not limited to the ions that are present in the struvite composition (Equation (1)); it includes other ions, which may be affecting the dynamics of ion interactions and clusters [9,40], justifying the heterogeneity of the shapes of struvite crystals observed in the SEM analysis. Figure 2 presents the XRD crystalline material of the struvite sample obtained using the CD condition of 7.5 mA·cm⁻² and an ED of 1 cm.

The chemical composition of struvite crystals was assessed qualitatively via EDS and quantitatively for the primary components (Mg, NH₃, and P) using atomic spectroscopy and UV–Vis spectrophotometry. The EDS spectra, obtained from the surface of the struvite crystals, showed compositional consistency across different conditions, revealing additional elements such as carbon (C) and sodium (Na) (Figure 3), which appear as impurities alongside the primary elements (N, Mg, P, and O) that form the struvite structure (NH₄MgPO₄·6H₂O). The results regarding the quantitative composition, more specifically in relation to the contents of Mg, N, and P, are presented in

Table 3. Quantitative composition of the elements Mg, N, and P present in the struvite crystals obtained under the different conditions evaluated.

Elements	2.5 mA·cm−2 and 1 cm	2.5 mA·cm−2 and 3 cm	7.5 mA·cm−2 and 1 cm	7.5 mA·cm−2 and 3 cm
Mg (g/100 g)	22.16 ± 0.15	15.44 ± 0.85	17.78 ± 0.15	19.25 ± 0.08
N_NH3 (g/100 g)	1.59 ± 0.25	4.49 ± 0.17	5.34 ± 0.03	4.73 ± 0.14
P (g/100 g)	14.52 ± 0.19	11.48 ± 0.48	12.60 ± 0.19	11.83 ± 0.36

.

For magnesium (Mg) and phosphorus (P), the highest concentrations were observed during the test conducted at 2.5 mA·cm⁻² with an electrode distance of 1 cm, whereas for nitrogen (N), the peak concentration was recorded during the test at 7.5 mA·cm⁻² and 1 cm. Across all elements assessed, the concentration patterns varied among the four tested conditions, with no consistent trend observed for Mg, P, and N across all tests.

This compositional variation may be attributed to two factors: (i) differences in the relative proportions of the three primary ions (NH₄⁺, Mg²⁺, PO₄³⁻) within the struvite crystals, and (ii) the possible co-formation of other compounds incorporating additional ions present in the solution [39].

A mass balance was also conducted based on molecular weights and the specific contributions of Mg, N, and P within the molecular composition of struvite (NH₄MgPO₄·6H₂O, MW = 245.3 g/mol). The contributions of each element by mass and percentage in the struvite structure are presented in

Table 4. Contribution of the elements Mg, N, and P in the formation of struvite (NH₄MgPO₄·6H₂O) and Mg₃(PO₄)₂.

Compounds	NH4MgPO4·6H2O			Mg3(PO4)2
Molecular Weight	245.3 g/mol			264.0 g/mol
Elements	Mg	N	P	Mg	P
Contribution of mass to the molecular weight of struvite	24.3	14.0	31.0	72.9	62.0
Percentage contribution (%)	9.9	5.7	12.6	27.6	23.5
Stoichiometry	1.74	1.0	2.21	1.2	1.0
P/Mg ratio	1.28			0.85

.

Among the three primary elements, nitrogen (N) and phosphorus (P) contribute the least (10.2%) and most (22.6%) to the composition of struvite, respectively. Regardless of whether the response monitored is mass or percentage contribution (calculated based on mass), the stoichiometry of Mg:N:P is 1.74:1:2.21 (

Table 5. Stoichiometric contribution observed for N, P, and Mg, calculated considering their percentage contributions in the composition of the material analyzed from the masses determined in the characterization step.

Conditions	Mg (g/100 g)	N_NH3 (g/100 g)	P (g/100 g)	Stoichiometry			Linked to Struvite		Not Linked to Struvite
				Mg	N	P	Mg * (%)	P * (%)	Mg * (%)	P * (%)	Relação Mg/P
2.5 mA·cm−2 and 1 cm	22.16 ± 0.15	1.59 ± 0.25	14.52 ± 0.19	13.92	1	9.12	12.50	24.23	12.28	6.91	0.56
7.5 mA·cm−2 and 1 cm	15.44 ± 0.85	5.34 ± 0.03	12.60 ± 0.19	2.89	1	2.36	60.17	93.65	1.25	0.15	0.12
2.5 mA·cm−2 and 3 cm	19.25 ± 0.08	4.49 ± 0.07	11.48 ± 0.48	4.28	1	2.56	40.61	86.48	2.64	0.35	0.13
7.5 mA·cm−2 and 3 cm	17.78 ± 0.15	4.73 ± 0.14	11.83 ± 0.36	3.76	1	2.50	46.24	88.32	2.12	0.29	0.14

* calculated considering stoichiometry 1.74:1:2.21 for Mg:N:P.

).

Comparing this stoichiometric ratio with that of pure struvite (Table 4), it was evident that in all tested conditions, Mg and P consistently showed higher stoichiometries at 1.74 and 2.21, respectively. This suggests that nitrogen exists in a lower proportion, meaning all nitrogen in the samples is incorporated within the struvite structure. Additionally, the elevated stoichiometric values for Mg and P indicate that these elements may coexist as additional compounds, such as magnesium phosphate (as reflected in Table 4) or magnesium carbonate.

By analyzing the Mg ratio specifically, the presence of Mg in 40–60% and P in 84–94% proportions aligns with struvite composition, indicating that a significant portion of Mg is not part of struvite but likely exists as magnesium carbonate, supported by the carbon identified in the EDS analysis (Figure 3).

Among the samples, the one generated at the lowest current density (2.5 mA·cm⁻²) and electrode distance (1 cm) had the highest Mg and P values but the lowest nitrogen incorporation, indicating that Mg and P are likely present in forms other than struvite. The observed Mg/P ratio of 0.56 for Mg and P outside of struvite supports this, suggesting their formation as magnesium phosphate (Mg₃(PO₄)₂), where the typical ratio is 0.85 (Table 5).

Increasing the amperage from 2.5 to 7.5 mA·cm⁻² at a 1 cm electrode distance raised the ammoniacal nitrogen incorporation from 1.59 to 5.34 g/100 g, signifying greater struvite production. The Mg and P concentrations were 15.44 and 12.60 g/100 g, respectively, for this higher amperage, although lower than the concentrations seen with 2.5 mA·cm⁻² (22.16 and 14.52 g/100 g). The majority of Mg (60%) and P (93.6%) were, however, incorporated within struvite (Table 5).

Approximately 40% of Mg remained outside of struvite, with the low Mg/P ratio (0.12) suggesting that this Mg exists predominantly as magnesium carbonate rather than magnesium phosphate, further supported by the presence of carbon in the EDS analysis. This contrasts with the 2.5 mA·cm⁻² condition, where Mg/P was 0.56.

Regardless of current density (2.5 or 7.5 mA·cm⁻²), the samples with a 3 cm electrode distance showed similar incorporation patterns, reflecting the 7.5 mA·cm⁻² and 1 cm setup, with unincorporated Mg primarily in the form of magnesium carbonate. However, nitrogen and phosphorus incorporation levels were lower than those observed in the 7.5 mA·cm⁻² and 1 cm condition, which yielded the highest nitrogen incorporation and, therefore, enhanced struvite production (Figure 4).

An important observation during the electrochemical process was the material loss from the magnesium electrode as struvite crystals accumulated. Detached struvite crystals retained a portion of magnesium, leading to partial electrode wear. The struvite precipitates cover the Mg anode surface, forming a passivating layer, firstly acting as a second cathode phase to accelerate Mg²⁺ release, but at some point, it would reduce the ability of the alloy to corrode and release Mg²⁺ as well [24].

In another study, the potential formation of a Mg(OH)₂ passivation layer on the magnesium anode surface can hinder the release of Mg²⁺ ions, thereby slowing the struvite crystallization process. Nevertheless, applying a current slightly above the pitting potential can disrupt this passive film, increasing the active surface area and accelerating both anodic and cathodic reactions. This enhancement promotes higher magnesium dissolution rates and improved phosphate removal efficiency. As a result, electrochemical systems employing magnesium anodes have demonstrated phosphate removal efficiencies exceeding 90%, along with the production of high-purity struvite [41].

4. Conclusions

Our study shows the potential of electrochemical methods for the efficient recovery of nitrogen and phosphorus from wastewater through struvite (MgNH₄PO₄·6H₂O) precipitation, offering a sustainable alternative for nutrient recovery and fertilizer production. Our findings demonstrate that both current density and electrode distance significantly influence the formation and composition of struvite crystals, directly affecting nitrogen and phosphorus incorporation rates. Higher current densities and shorter electrode distances enhanced the crystallization process, increasing ammoniacal nitrogen incorporation and supporting struvite formation, while unincorporated magnesium tended to precipitate as magnesium carbonate rather than magnesium phosphate, as confirmed by EDS analyses.

The results suggest that optimal electrochemical parameters can effectively control the stoichiometry of struvite production, achieving a Mg:N:P ratio close to that of pure struvite and maximizing nutrient recovery for agricultural applications. Variations in the Mg:P ratio and the presence of carbon impurities highlight the impact of solution composition on struvite crystal morphology, suggesting that additional ions in wastewater influence crystal formation through impurity incorporation and morphological alterations.

While our study emphasizes the benefits of struvite as a slow-release fertilizer and its environmental advantages over synthetic fertilizers, it also underscores the operational challenges associated with electrode degradation and material retention within detached struvite crystals. Future research should focus on refining electrochemical parameters and exploring cost-effective electrode materials to optimize recovery rates and improve the sustainability of this approach. Overall, the integration of electrochemical struvite precipitation in wastewater treatment facilities offers a promising avenue for advancing nutrient recycling, reducing reliance on synthetic fertilizers, and supporting sustainable agricultural practices.