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Sci Rep
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Scientific Reports
2045-2322
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10.1038/s41598-024-72529-2
Article
Enhanced degradation of nitrate by a combined electrolysis precipitation process
Zhang Yang 1
Li Haokang 1
Li Shuo 1
Li Yifan 2
Ding Yuanhong yhding@nju.edu.cn

1
1 https://ror.org/041j8js14 grid.412610.0 0000 0001 2229 7077 College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao, 266042 China
2 https://ror.org/041j8js14 grid.412610.0 0000 0001 2229 7077 Department of Environmental Design, Qingdao University of Science and Technology, Qingdao, 266061 China
17 9 2024
17 9 2024
2024
14 2164923 5 2024
9 9 2024
© The Author(s) 2024
2024
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Nitrate can be electrolyzed mainly into N2, but the generated ammonia, as its secondary intermediate, is soluble and remained in the wastewater yet, which affects negatively the degradation of nitrate and total nitrogen. In this work, an electrolytic reactor constructed with Ti/RuSn anode and Fe cathode, was applied to electrolyze nitrate, and magnesium chloride was used as electrolyte and precipitant simultaneously, while disodium hydrogen phosphate (DSP) was added only as precipitant of ammonia. The results indicated that, most part of generated ammonia could be precipitated as magnesium ammonium phosphate (MAP) and some residual ammonia may be transformed into N2 by breakpoint chlorination. Thus, the nitrate and total nitrogen (TN) degradation efficiencies could be enhanced obviously by the combined electrolysis precipitation process.

Keywords

Nitrate
Electrolysis
Ammonia
MAP
Subject terms

Environmental sciences
Environmental chemistry
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Nitrate is soluble in waters and difficult to be broken into single component nitrogen compounds, because its N–N bond is high-valence and has high chemical bond energy. The wastewater containing nitrate is generally discharged from industrial factories1 and harmful to the ecological environment. Many national and local environmental standards, such as China “discharge standard of pollutants for municipal wastewater treatment plant (GB18918-2002 A, Class I), stipulate that the total nitrogen (TN) concentration of discharged wastewater effluents is not permitted higher than 15 mg/L. Therefore, for managers, it is crucial to guarantee the treated nitrate wastewater to reach to standards on TN by any feasible technology, regardless of the resource utilization of nitrate.

The nitrate (or nitrogen) wastewater is commonly treated by combined physiochemical and biological processes, in most cases, nitrate is still the biggest-concentration residual TN component in effluent after nitrifying-denitrifying process than any other nitrogen components, in a sense, the target of nitrogen wastewater treatment is actually how to degrade nitrate effectively.

Many practicable methods are used to degrade nitrate wastewater effectively, which include electrolysis, active metals reduction, deep-bed filter, anaerobic ammonia oxidation and aerobic-anoxic denitrification2, but some obvious disadvantages limit its large-scale engineering application yet3, for example, the normal aerobic-anoxic denitrification process demands commonly for enough biodegradable organic pollutants in effluents, while it is already exhausted after the former nitrifying-denitrifying process4, thus, it is necessary to add external carbon sources to maintain the normal denitrification process. In addition, the chemical reduction of nitrate consumes iron or alumina elementary metals, and also produces chemical sludge.

Because of no extra chemical consumption or sludge production, electrolysis is regarded as a green technology to treat nitrate wastewater, most of nitrate is transformed into N2 and released off the wastewater, however, the generated ammonia is soluble and still remained in the wastewater, which affect negatively the degradation of nitrate and TN. Therefore, how to remove the generated ammonia effectively is crucial to treat nitrate wastewater. In effect, the ammonia can be transformed into N2 by breakpoint chlorination or precipitated as MAP5. Thus, the degradation of nitrate could be enhanced by the removal process of generated ammonia.

Ammonia is commonly produced by the Haber–Bosch process and widely used for agriculture and industry, however, the synthetic reaction of ammonia demands for harsh pressure and temperature conditions, and consumes expensive nitrogen and hydrogen materials6, photo-electrocatalytic nitrogen reduction reaction (NRR) are also explored to produce ammonia from nitrate under normal conditions, nevertheless, some NRR factors including high N–N bond energy of ammonia, multi-valences nitrogen products and low solubility of N2 and H2 are negative.

But it is still significant to reduce nitrate of wastewater into ammonia, because it could resolve the problems on production of ammonia and resource utilization of nitrate, thus, to catalyze the reduction of nitrate to ammonia has achieved great progress. However, some defects limit its prevail application in nitrate wastewater treatment yet, these include with, the stability and selectivity of catalysts and the purity of nitrogen production7.

The main object of present industrial nitrate wastewater treatment is to reach the TN standard, therefore, all the practicable denitrification methods are focused on how to transform nitrate into N2 effectively, for many intermediates including ammonia are soluble in wastewater, which is meaningless to the degradation of TN in practice. If the generated ammonia could be removed off wastewater simultaneously, all the degradation of nitrate and TN can be enhanced.

In the electrolysis process, sodium chloride is usually used as electrolyte, and the relative reaction processes of Cl− and NO3− are as follows, shown in Eqs. (1)–(5)1:

by anodes:1 2Cl-→Cl2+2e-

by cathodes:2 NO3-+6H2O+6e→NH4++8OH-

Some ammonia can be changed to N21,8 by breakpoint chlorination as follows, expressed by Eqs. (3), (4) and (5).3 Cl2+H2O→HOCl+Cl-+H+

4 HOCl+NH4+→NHCl2+H2O+H+

5 NHCl2+NOH→N2+HOCl+H++Cl-

The oxidation–reduction cycles of Cl− → Cl2 → HOCl → Cl− is likely processed on the surface of selective anode, combined with ammonia, some ammonia may be degraded by breakpoint chlorination.

To improve the anode’s selectivity for ammonia and nitrate, some catalytic elements including Ru (Ruthenium) and Sn (Stannum) are coated on its surface. To construct a third-dimensional electrical reactor (TDER) to increase the electrolytic current density and accelerate the degradation of nitrate, some surface-modified conductive particles electrodes are also tried to add into the electrolytic reactor. However, it is difficult to maintain the electrodes’ selectivity for nitrate and ammonia simultaneously.

In addition, the generated ammonia can be changed into gaseous ammonia molecules by air stripping process, or precipitated as magnesium ammonium phosphate (MAP) when pH value of wastewater is adjusted to higher than 11. In conclusion, besides breakpoint chlorination into N2, the ammonia also can be blown off as gaseous NH3 or precipitated as MAP9,10, however, the air striping method has some obvious defects comparing to MAP precipitation process, such as high air striping towers and large air water ratio, while the MAP process can be simply carried out at normal temperatures and air pressures in one compact reactor added merely with two kinds of precipitants, which are commonly designated as magnesium chloride (MgCl2) and disodium hydrogen phosphate (DSP). The MAP precipitation process is shown in Eq. (6).6 Mg2++NH4++HPO42-=MgNH4PO4(s)+H+

Thus, it is feasible to degrade ammonia by MAP precipitation process, in which Magnesium chloride is used as precipitant and worked as electrolyte in the electrochemical process of nitrate either, furthermore, some chloride ions of it participate in the breakpoint chlorination to transform ammonia into N2. It seems reasonable to degrade nitrate more completely by a combined electrolysis-precipitation process than by electrolysis only. Besides breakpoint chlorination, the MAP precipitation process of ammonia is conducive to the degradation of nitrate and TN.

In this work, an electro-catalytic reactor constructed with Fe cathode-Ti/RuSn anode is used to treat nitrate, by which nitrate is reduced into N2 continuously11. Here, magnesium chloride is used as the electrolyte instead of sodium chloride in the electrolysis process and worked as one precipitant simultaneously in the ammonia MAP precipitation process, while disodium hydrogen phosphate (DSP), as another precipitant, is also added into the wastewater. A series of experiments are carried out to explore the differences and advantages between conventional electrolysis process and combined electrolysis-precipitation process12.

Materials and methods

Four types of electrolytic reactors, composed of Al, Fe, Ti and Ti/RuSn plates, are carried out for the degradation of nitrate wastewater, Fe plate is designated as cathode, and Al, Fe, Ti and Ti/RuSn plates are used as anodes respectively. Fe cathode and Ti/RuSn anode are selected finally. Every piece of plate electrodes is 100 mm long and 48 mm wide which is placed parallelly about 20 mm spacing distance, and the volume of electrolytic reactor is about 120 ml. The DC power is Agilent E3649A Programmable DC Power Supplies with 30–100 W, and its output voltages is about 5–15 V.

In these electrolytic experiments of nitrate, magnesium chloride is used as electrolyte instead of sodium chloride13, and also used as one precipitant of ammonia, the other one is DSP (disodium hydrogen phosphate).

The precipitation process of ammonia is started on the time when the generated ammonia is accumulated enough to be precipitated as MAP, because all the measured pH values of electrolyzed wastewater are bigger than 10, thus, so the DSP is added directly without pH regulation.

To investigate the effects of precipitation on the degradation of nitrate and TN, as showed in Fig. 1, three types of electrolysis and precipitation processes are experimented in detail, which are named as “electrolysis only”, “staged process” and “combined process” respectively, among them, the electrolysis only process is run without MAP precipitation process. The staged process is started with electrolysis for a period and then stop it to begin only precipitation for another period, in the staged process, DSP is added on the designated time and since then the electrolysis process is stopped completely. To the combined process, the DSP is also added on the proper time, but the electrolysis is carried out simultaneously all the time till end.Fig. 1 The schematic chart of two kinds of electrolysis processes of nitrate wastewater.

The initial nitrate concentrations of synthetic wastewater are about 300–700 mg/L in theory, the initial concentrations of TN are equal to that of nitrate, all the sample water parameters including nitrate, nitrite, ammonia, TN, pH, sludge and ions, are measured according to national standard methods, the main apparatus include with UV-2600 (Japan Shimadzu), Thermo SCIENTIFIC ESCALAB Xi + (US Thermo) and Ion Chromatograph ICS-600 (US Dionex).

Results and discussions

Effects of anodes on the electrochemical degradation of nitrate

To select the propriate electrodes, electrolytical reactors constructed by Al–Fe, Ti-Fe, Fe–Fe and Ti/RuSn-Fe electrodes, were used to electrolyze nitrate, Fig. 2a,b showed that, more N2 and smaller ammonia were generated by Ti/RuSn-Fe electrodes than by any other electrodes couples. Thus, Ti/RuSn anode–Fe cathode were determined ultimately as the desired electrodes couples14.Fig. 2 Variation of nitrate, N2 and ammonia by different electrodes couples. (a) N2; (b) ammonia; (c) intermediates’ components of nitrate by Ti/RuSn anode-Fe cathode.

In the electrolysis only processes of nitrate by Ti/RuSn anode -Fe cathode, the degradation of nitrate actually depends on the co-effects by Ti/RuSn anode and Fe cathode, on the surface of Fe cathode, the oxygen atoms of nitrogen–oxygen bonds of nitrate ions are absorbed by Iron atoms and then reduced to nitrite and ammonia mainly by capturing electron or by colliding with hydrogen free radicals15,16. These radicals are generated from the decomposition of H2O on the catalytic surface layer of Ti/RuSn anode, which could frequently attack the nitrogen–oxygen bonds and break it ultimately. Thus, nitrate can be continuously transformed into gaseous nitrogen, ammonia and nitrite, as shown in Eqs. (7), (8), (9) and (10)17, for the concentrations of generated nitrite is almost negligible, ammonia becomes the secondary intermediate to be removed necessarily.7 catalyst+H2O+e-→catalyst+H.+OH-

8 NO3-+H.→NO2-+H2O

9 NO2-+H.→N2+H2O+OH-

10 NO2-+H.→NH4++H2+OH-

The surface layer of Ti/RuSn anode has good selectivity for oxygen, if the oxygen atoms of H2O are absorbed on it, the hydrogen–oxygen bonds are likely broken to generate active hydrogen radicals, which can attack and break nitrate into N218,19, while some corresponding hydroxyl ion from the decomposition of H2O is possibly transformed into hydroxyl free radicals by losing electron on Ti/RuSn anodes, it likely reacts with ammonia to produce N2, thus, most of ammonia may be transformed into N2 by Ti/RuSn anode, as showed in Eqs. (11), (12) and (13).11 H2O-2e→2OH-+2 H+

12 OH--e→OH.

13 NH4++3OH.→0.5N2+3H2O+3e

As Fig. 2c showed, although more than 50% of nitrate was transformed into N2 which escaped off the waters phase instantly, the generated ammonia was accumulated in the waters and listed as the secondary main component of nitrate intermediates by electrolysis process. Provide that the generated ammonia was removed from water simultaneously, the degradation efficiencies of nitrate was possibly improved.

Effects of magnesium chloride on the electrochemical degradation of nitrate

As known, ammonia could be precipitated as MAP at the suitable alkalinity, as Fig. 3a showed, all the beginning pH values of electrolytic processes were about at the level of 7.75, while the final pH values were about 11.57, and its alkalinity was enough for the MAP precipitation. The results indicated that, the pH values of nitrate wastewater may be increased rapidly after 30 min of electrolysis, which were high enough to precipitate ammonia into MAP effectively. The variation of pH values also demonstrated that, some hydrogen ions were produced by Fe cathode and participated in the degradation of nitrate, because the newly-generated hydrogen in atomic state has strong reducibility to high-valence nitrate. The ionization equilibrium of H2O was moved in the direction of generating more hydroxyl ion, which resulted in the strong alkaline of wastewater and the removal of some ammonia.Fig. 3 Variations of pH, nitrate and ammonia in the course of electrolytic process of nitrate. (a) pH of series of electrolysis processes; (b) nitrate by electrolyte of MgCl2 and NaCl; (c) ammonia by electrolyte of MgCl2 and NaCl.

The degradation efficiencies of nitrate and ammonia were affected by magnesium chloride, as Fig. 3b,c showed, although nitrate was degraded more slowly by magnesium chloride than by sodium chloride, the nitrate removal efficiencies of 97.18% and 98.24% were still achieved respectively, and the similar ammonia generating-removing trends were also obtained, about 20.86% and 16.30% of nitrate was ultimately transformed into ammonia respectively. Thus, magnesium chloride was ultimately selected as electrolyte and precipitant instead of sodium chloride.

Effects of precipitants on the electrochemical degradation of nitrate

Differences between the combined process and staged process

Ammonia was generated and accumulated during the electrolysis of nitrate, provided that it was precipitated simultaneously as MAP, and then more nitrate was likely electrolyzed. As a result, the removal efficiencies of nitrate and TN would be increased obviously20,21. Therefore, it was reasonable to speculate that, the process combined electrolysis with MAP precipitation was possibly conducive to thorough degradation of nitrate. Thus, two type of processes, named combined process and stage process respectively, were carried out to investigate its effects on the degradation of nitrate. To the combined process, its electrolysis process was run all the time along with the precipitation process, but to the staged process, its electrolysis process was stopped completely when precipitation process was started to run.

As Fig. 4a,b showed, nitrate and TN could be degraded more completely by the combined process than by staged process, and it took shorter time for the combined process to achieve lower-concentrations of nitrate and TN than by the staged process, both the nitrate and TN concentrations of combined process were already smaller than 50 mg/L on Time 90 min, while that of staged process was still bigger than 50 mg/L on Time 255 min.Fig. 4 Nitrate and ammonia removal efficiencies by combined process and staged process. (a) Nitrate electrolysis without and combined process; (b) TN, electrolysis without and combined process; (c) nitrate; (d) ammonia; (e); nitrite; (f) TN; (g) nitrate and ammonia; (h) ammonia generated, removed and remained in treated wastewater.

It could be explained as follows, the ammonia generated by the combined process was transformed into MAP and separated from the wastewater continuously, which resulted in more nitrate to be electrolyzed into N2 and ammonia than by the staged process.

Besides N2, ammonia was the secondary intermediate transformed from nitrate, Fig. 4c–g showed the variation curves of nitrate, nitrite, ammonia, TN and N2 with reacting time by the combined process and the staged process respectively, among these curves, the concentrations of generated nitrite were less than 1 mg/L and negligible. In addition, it took less time to reach the minimum concentrations for residual nitrate, ammonia and TN by combined process than by staged process either, as Fig. 4h showed, firstly, the ammonia concentrations increased rapidly to maximum concentration by combined process, and then decreased to no more than 10 mg/L on Time 120 min, it took least time than by any other processes, as control, the ammonia concentration level was still higher than 30 mg/L by electrolysis only. The results indicated that, the generated ammonia may be precipitated by combined process simultaneously, and resulted in the enhanced degradation efficiencies of nitrate and TN. Furthermore, the generated ammonia was likely degraded more effectively by combined process than by electrolysis only. The biggest ammonia removal mass and efficiencies were also achieved by the combined process, which positively resulted in excellent degradation efficiencies of nitrate and TN.

At beginning, there was no ammonia in the combined process, but it could be generated gradually when the electrolysis process of nitrate was started to run, then some generated ammonia would be transformed into MAP sludge by precipitation process, but the anions PO43− and OH− were also able to form as Mg3(PO4)2 and Mg(OH)2, which likely inhibited the reaction of ammonia into MAP precipitation22. Furthermore, some ammonia was possibly transformed into N2 by breakpoint chlorination process. As Table 1 showed, about 30.95% of ammonia was removed off by breakpoint chlorination process, and the rest ammonia was likely precipitated as MAP sludge. Table 1 The possible ways of ammonia generated and removed.

Ammonia	Concentration mg/L	Percent %	
CMax	61.85		
CMin	3.18		
CMax–CMin	58.67		
CMAP	40.51	69.05	
CMax–CMin–CMAP	18.16	30.95	
CMax electrolysis process only, CMin combined process, CMax–CMin total removed ammonia, CMAP ammonia removed by precipitation, CMax–CMin–CMAP ammonia removed by breakpoint chlorination.

Effects of dosing time of DSP on denitrification

The generated ammonia was accumulated and re-degraded during the electrolysis-precipitation process, because MgCl2 was already existed as electrolyte, then DSP dosing time to precipitate ammonia need to determine.

As Fig. 5a,b showed, the DSP dosing time was designated at 0, 30, 60, 90 and 120 min respectively, the removal efficiencies of nitrate and TN dosing on 30 min was better than that on 0 and 60 min respectively. Although the DSP dosing time could be shortened, or even dosed on the starting time of electrolysis, the generated rate of ammonia was more higher than its removed rate, thus, the DSP dosing time was finally designated at 30 min.Fig. 5 Effects of dosing time of DSP. (a) Nitrate; (b) TN.

Effects of ratio of Mg to N on denitrification

In the electrolysis process of nitrate, when DSP was added into the electrolytic process of nitrate, then MgCl2 acted as one precipitants of MAP simultaneously. To investigate the appropriate molar ratio of Mg to N, three initial concentrations of MgCl2 were designated at 250 mg/L, 400 mg/L and 500 mg/L respectively, which expressed three molar ratios of 1.1, 0.9 and 0.6 respectively. Here, the molar ratio of Mg to N was calculated instead of MgCl2-Mg to NH4-N. As Fig. 6a–d showed, while the initial concentration MgCl2 was designated at 400 mg/L, nitrate and ammonia could be degraded more effectively than that as 250 mg/L and 500 mg/L respectively, for the generated nitrite concentration was so negligible that the optimal ratio of Mg to N was designated as 0.9:1.Fig. 6 Effects of ratio of Mg to ammonium on denitrification. (a) Nitrate; (b) ammonia; (c) nitrite; (d) TN.

Effects of ratio of P to N on denitrification

In this work, the molar ratio of P to N was calculated instead of DSP-P to NH4-N, under the condition of 500 mg/L nitrate and 400 mg/L MgCl2 respectively, the effects of ratios of DSP-P to NH4-N on denitrification was investigated. As Fig. 7a–c showed, the nitrate, ammonia and TN were degraded more completely at ratio of 1:1 than at any other ratios, combined with ratio of Mg to N, the appropriate ratio of P:Mg:N was designated ultimately as 1:0.9:1, while compared with the conventional ratio of 1:1:1, the consumption of DSP and MgCl2 in the combine process was smaller. The possible reason was as follows, the concentration of generated ammonia was variable, while the dosing concentration of DSP was calculated, besides degradation of ammonia by MAP precipitation, some ammonia was possibly removed by breakpoint chlorination and successive electrolysis.Fig. 7 Effects of ratio of DSP to NH4-N on denitrification. (a) Nitrate; (b) ammonia; (c) TN.

Characteristics of sludge and residual precipitants

In this work, the molar ratio of P to N was calculated instead of DSP-P to NH4-N, under the condition of 500 mg/L nitrate and 400 mg/L MgCl2 respectively, the effects of ratios of DSP-P to NH4-N on denitrification was investigated. As Fig. 7a–c showed, the nitrate, ammonia and TN were degraded more completely at ratio of 1:1 than at any other ratios, combined with ratio of Mg to N, the appropriate ratio of P:Mg:N was designated ultimately as 1:0.9:1, while compared with the conventional ratio of 1:1:1, the consumption of DSP and MgCl2 in the combine process was smaller. The possible reason was as follows, the concentration of generated ammonia was variable, while the dosing concentration of DSP was calculated, besides degradation of ammonia by MAP precipitation, some ammonia was possibly removed by breakpoint chlorination and successive electrolysis.

Characteristics of sludge and residual precipitants

Based on the XPS reports of sludge, MAP was not the exclusive main component in the dry sludge, showed in Fig. 8a, c, d, e, other components such as Magnesium hydroxide, Magnesium phosphate and Ferric phosphate, were also detected, among these components containing Mg, the content of Magnesium hydroxide was the second main solid containing Magnesium. It means that, part of precipitants such as MgCl2 and DSP was transformed into sludge not as MAP, about 78% of Mg2+ and 35.69% of PO43−-P was removed from waters and transformed into sludge containing Mg and P. In total, about 69.05% of removed ammonia by the combined process, was precipitated as MAP.Fig. 8 Sludge production and the main Mg2+ and PO43−-P remained in the wastewater. (a) XPS, main components of sludge; (b) the concentrations of Mg2+ and PO43− remained in the wastewater yet; (c) XPS of Mg; (d) XPS of P; (e) XPS of N.

As Fig. 8b showed, when the combined process started, the initial concentrations of Mg2+ and PO43−-P were 74.47 mg/L and 49.95 mg/L respectively, after this treatment, about 16.34 mg/L Mg2+ and 32.11 mg/L PO43−-P were detected in the treated wastewater yet, it means that, the present dosage of added precipitants was still excess and its actual consumption was possibly smaller than that.

Based on literature, the Mg2+ or PO43− containing in dry sludge can be released out by dissolution and reused to the combined process again23,24.

Inhibitive effects on denitrification

The electrolysis mechanism of nitrate by Ti/RuSn anode-Fe cathode25,26, was investigated in detail, hydrogen radicals was thought as the main active radicals, it was possibly generated on the Fe cathodes and participated in the electrolysis of nitrate. Because hydrogen radicals could be quenched by Tert-butanol, so it was commonly used to inhibit the activities of target radicals. Figure 9a showed that, the degradation efficiencies of nitrate and TN by electrolysis only process were inhibited obviously by Tert-butanol, while that by combined process, as shown in Fig. 9b, was more severely inhibited than by electrolysis only15,27. The results indicated that, the degradation of nitrate and TN was catalyzed by strong active free radicals in the combined process.Fig. 9 Inhibitive effects of Tert-butanol on denitrification (nitrate, TN). (a) Electrolysis only; (b) combined electrolysis-precipitation process.

Mechanism of denitrification by combined process

Nitrate can mainly be transformed into N2 and ammonia by electrolysis, its degradation mechanism is showed in Fig. 10, the newly-generated ammonia concentration increased into a peak value and then decreased to a relatively stable and low value. This curve may be explained that most part of ammonia is likely precipitated into MAP and some residual ammonia may be simultaneously removed off by breakpoint chlorination. Thus, the degradation effects of nitrate and TN are enhanced actually by the united processes of ammonia MAP precipitation and ammonia breakpoint chlorination.Fig. 10 The mechanism of nitrate degradation by combined process.

Conclusions

Nitrate could be electrolyzed mainly into N2 and ammonia by Fe-Ti/RuSn electrodes, and ammonia was remained in the wastewater yet.

The degradation efficiencies of nitrate could be enhanced by the combined process, about 69.05% of generated ammonia was continuously transformed into MAP, while about 30.95% of it was likely oxidized into N2 by breakpoint chlorination.

MgCl2 could be used as electrolyte and precipitant simultaneously. The appropriate Ratio of P to Mg to N was about 1: 0.9: 1. The precipitation was mainly composed of MAP, Mg(OH)2, Mg3(PO4)2 and Fe2(PO4)3, besides MAP, Mg(OH)2 was the second main component.

Author contributions

Yang Zhang, Hao-Kang Li: writing original draft preparation, data curation, formal analysis. Shuo Li, Yi-Fan Li: administration, investigation, visualization, project administration. Yuan-Hong Ding: conceptualization, review and editing, funding acquisition.

Funding

This research was supported by Shandong Science and Technology Project (2022TSGC2539).

Data availability

The datasets used during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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