Simultaneous recovery of ammonium, potassium and magnesium from produced water by struvite precipitation

Journal Pre-proofs Simultaneous Recovery of Ammonium, Potassium and Magnesium from Produced Water by Struvite Precipitation Lei Hu, Jiuling Yu, Hongmei Luo, Huiyao Wang, Pei Xu, Yanyan Zhang PII: DOI: Reference:

S1385-8947(19)32411-8 https://doi.org/10.1016/j.cej.2019.123001 CEJ 123001

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Chemical Engineering Journal

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28 July 2019 25 September 2019 28 September 2019

Please cite this article as: L. Hu, J. Yu, H. Luo, H. Wang, P. Xu, Y. Zhang, Simultaneous Recovery of Ammonium, Potassium and Magnesium from Produced Water by Struvite Precipitation, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123001

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Simultaneous Recovery of Ammonium, Potassium and Magnesium from Produced Water by Struvite Precipitation Lei Hu1, Jiuling Yu2, Hongmei Luo2, Huiyao Wang1, Pei Xu1, Yanyan Zhang1, * 1Department

of Civil Engineering, New Mexico State University, Las Cruces, NM 88003, USA

2Department

of Chemical and Materials Engineering, New Mexico State University, Las Cruces,

NM 88003, USA

*Corresponding author: Yanyan Zhang Postal Address: Department of Civil Engineering, New Mexico State University, 3035 S Espina Street, NM 88003, USA. E-mail: [email protected]

1

1

Abstract

2

Unconventional oil and gas industries generate huge amounts of produced water (PW)

3

containing high concentrations of potentially hazardous organic and inorganic contaminants.

4

This study demonstrated the feasibility of simultaneously recovering NH4+, K+ and Mg2+ from

5

PW by struvite precipitation after calcium pretreatment with Na2CO3 addition or CO2 stripping.

6

Without pretreatment, calcium exhibited strong competition for phosphate through the formation

7

of Ca3(PO4)2 precipitate. The pretreatment with a Ca2+:CO32– molar ratio of 1:1.2 achieved a

8

relatively low loss rate of Mg2+ (31.3%) and high Ca2+ removal efficiency (95.9%). The results

9

also revealed that the Mg/N/P molar ratio and solution pH had a remarkable effect on the struvite

10

precipitation, while the seeding dosage and Na+ slightly influenced struvite formation. The

11

combination of pH=9.5 & Mg/N/P molar ratio=1.5:1:1.5 was ideal for struvite recovery from

12

PW, resulting in NH4+, K+ and Mg2+ recovery efficiencies of 85.9%, 24.8% and 96.8%,

13

respectively. The results of X-ray diffraction and scanning electron microscopy further

14

confirmed that the precipitates generated at this optimal condition were orthorhombic struvite.

15

Moreover, along with the struvite recovery, no accumulation of heavy metals and organic

16

contaminants was observed, indicating that the struvite quality was sufficient for field

17

application. Furthermore, struvite recovery process was able to reduce the Microtox toxicity of

18

PW towards Vibrio fischeri by 60%. Considering the low cost and relatively simple technology,

19

struvite precipitation process has the potential to be used for large-scale applications for

20

produced water treatment and resource recovery.

21 22

Keywords: produced water; ammonium recovery; struvite precipitation; pretreatment; toxicity.

2

23

1. Introduction

24

Recent advances in horizontal drilling and hydraulic fracturing have enabled shale oil and

25

gas production from unconventional reservoirs in the United States [1, 2]. The rapid

26

development of unconventional shale oil and gas industry has not only created an energy boom

27

in the U.S. [1, 3, 4], but also offered an opportunity for some other countries around the world to

28

access a relatively new fossil fuel and then reduce their reliance on energy imports [2, 5].

29

However, there are still several environmental problems associated with the extraction of

30

unconventional shale oil and gas, including intense freshwater consumption and significant

31

hazardous wastewater production [5, 6]. For instance, a single horizontal well can consume

32

13,700~23,800 m3 freshwater, and then generate 8,000~22,700 m3 flowback and produced water

33

depending on the geological conditions of the shale formation [7]. The injected fluid returning to

34

the surface prior to shale oil and gas recovery is known as flowback water, while the produced

35

water (PW) [8] is extracted formation water during shale oil and gas production [9]. PW is often

36

more saline than flowback water and contains high concentrations of potentially hazardous

37

organic and inorganic contaminants, which may pose great risks to the environment and human

38

health [9, 10]. PW is the largest waste stream associated with unconventional shale oil and gas

39

production [11], which should be treated properly before reuse or discharge due to significant

40

potential in surface water and groundwater pollution. Two main management options for PW,

41

including deep-well injection and direct discharge into surface water, are usually either

42

constrained or impractical due to the geological and legal restrictions and the negative impacts

43

on natural water resources. Currently, deep-well injection is the main technology for PW

44

disposal in the United States due to the treatment challenges and expenses [9], however, some

45

environmental risks associated with deep-well injection should be noted, such as potential 3

46

seismicity and groundwater contamination [1]. Therefore, the effective and economically

47

feasible management for PW is in urgent need to promote sustainable development of

48

unconventional shale oil and gas industry, while protecting the environment and human health

49

[11].

50

A major challenge for PW treatment and reuse applications is high salt concentrations,

51

measured as total dissolved solids (TDS, up to 390,000 mg/L) [12], including sodium, potassium,

52

calcium, magnesium, ammonium [13-15], chloride, sulfate, bromide, fluoride, nitrate, phosphate,

53

and iodide [13, 16, 17]. For example, high ammonium concentration (up to 432 mg/L) was found

54

in produced waters from the Lower Silurian Oneida Formation in the Appalachian Basin [18].

55

Our preliminary analysis of PW samples from the Permian Basin has also shown that the initial

56

concentration of ammonium is around 600 mg/L. Ammonium is highly toxic and can cause

57

several serious environmental implications, such as eutrophication and dissolved oxygen

58

depletion, thereby degrading water quality, leading to the death of aquatic life, and destroying

59

ecosystem structure [19, 20]. Furthermore, some disinfection processes (such as ozone, chlorine

60

dioxide, UV, and chloramine disinfection) may convert ammonium to other hazardous

61

compounds, such as carcinogenic N-nitrosamines, contributing to cancers and blue body

62

syndrome [21, 22]. For those reasons, effective treatment technologies should be applied for

63

ammonium removal in PW to ensure sustainable development and human health. Recently,

64

various methods, such as nitrification-denitrification, adsorption, ion exchange and air stripping,

65

have been successfully used for ammonium removal from different wastewaters [19, 20, 23].

66

Ammonium also plays a crucial role in fertilizer production for food security around the world

67

[20]. Ammonium used in fertilizer production is often synthesized by the expensive industrial

68

Haber-Bosch process [20]. Hence, ammonium recovery from wastewaters seems more valuable 4

69

than ammonia removal, snice it can not only supplement fertilizer production but also lead to

70

better resource management. There are three main ammonium recovery approaches, including

71

ammonia stripping coupled with adsorption [24], struvite precipitation [25], and membrane

72

concentration [26]. Compared to ammonia stripping and membrane concentration, struvite

73

precipitation possesses its apparent advantages: firstly, struvite is an effective and safe slow-

74

release fertilizer that can be directly applied to land [27]; and secondly, struvite formation could

75

effectively avoid the scaling problem in the membrane concentration process and benefit the

76

sludge dewatering [28]. Low capital cost and relatively simple operation technique make it a

77

promising process for large-scale application. Thus, struvite precipitation process, as a renewable

78

method to recover ammonium from wastewaters, has been widely employed as a cost-effective

79

approach for fertilizer production [20, 25]. In addition, to improve the sustainability in PW

80

management, treatment technologies need to be changed from removal-centered processes to

81

recovery-centered approaches. According to our preliminary analysis, PW from the Permian

82

Basin is also in rich of Mg2+ and K+. Therefore, precipitation of struvite (MAP as

83

MgNH4PO4∙6H2O, or MPP as MgKPO4∙6H2O) may be considered as an effective and

84

economical approach for simultaneous recovery of ammonium, potassium and magnesium from

85

PW. This will not only minimize the environmental impact of PW, but also produce a valuable

86

fertilizer rich in N, P, and K. The mechanisms of this method are reflected as the following

87

equations [29, 30]:

88

Mg2 + (aq) + NH4+ (aq) + H𝑛PO𝑛4 ― 3(aq) +6H2O→MgNH4PO4 ∙ 6H2O↓ + nH +

(1)

89

Mg2 + (aq) + K + (aq) + H𝑛PO𝑛4 ― 3(aq) +6H2O→MgKPO4 ∙ 6H2O↓ + nH +

(2)

5

90

Both of MAP and MPP have been assessed as a concentrated slow-release fertilizer

91

because of its slow nutrient releasing rate and higher nutrient contents than other commercial

92

fertilizers, thereby attracting special attention from research [27, 30]. Recently, several

93

researchers have investigated the potential of struvite recovery from different waste streams,

94

such as landfill leachate, swine wastewater, anaerobic digester supernatant, and urine [31-36].

95

Due to unique characteristics of PW, the feasibility of struvite recovery from PW needs to be

96

studied. For instance, high concentration of Ca2+ in PW [16] may compete with Mg2+ for PO43-

97

during the struvite crystallization [37]. Wu et al. [32] found that a high Ca2+ concentration in

98

landfill leachate can lead to low struvite purity. In addition, PW is usually rich in Na+, which

99

may compete with NH4+ and K+ for Mg2+ and PO43- to form Na-struvite (MSP, MgNaPO4∙7H2O,

100

Eq. (3)), thereby inhibiting the MAP and MPP precipitation and reducing recovery efficiencies

101

of NH4+ and K+ [38, 39]. Moreover, some heavy metals in produced water, such as Zn, Cu, Cr,

102

Pb and Ni, might be co-precipitated with struvite during the recovery process [40-42]. Heavy

103

metals in waste streams may decrease the purity of the recovered precipitates and theoretically

104

impede struvite reutilization as fertilizer, while posing great threats to planting and environment.

105

Thus, the fates of heavy metals and organic matters in PW during struvite precipitation should be

106

well studied due to its important influence for struvite quality.

107

Mg2 + (aq) + Na + (aq) + H𝑛PO𝑛4 ― 3(aq) +7H2O→MgNaPO4 ∙ 7H2O↓ + nH +

108

Hence, to address this knowledge gap, this study aimed to investigate the feasibility of

109

simultaneous recovery of ammonium, potassium and magnesium from PW by struvite

110

precipitation. Owing to the presence of high calcium concentration in PW, the influence of

111

calcium on the crystallization of struvite was first studied. Next, sodium carbonate addition and

112

CO2 stripping were employed as the calcium pretreatment methods to remove calcium in PW to 6

(3)

113

enhance the purity of the obtained struvite. Subsequently, experiments were carried out to

114

examine the effects of pH, Mg/N/P molar ratio, seeding dosage, and reaction time on struvite

115

recovery from PW. The competition precipitation relationship between Na+ and NH4+ (or K+)

116

was also assessed to clarify the co-precipitation process of struvite-type compounds in PW.

117

Moreover, the fates of heavy metals and organic matters in PW during struvite precipitation were

118

studied in order to evaluate the quality of the obtained struvite. Finally, the toxicity of PW

119

toward Vibrio fischeri was evaluated before and after struvite precipitation. All precipitates were

120

characterized by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) and

121

X-ray diffraction (XRD) for morphology, chemical composition and structure analyses.

122

2. Materials and methods

123

2.1. Materials

124

The PW samples utilized in this study were collected from the Permian Basin at a deep

125

well disposal site in Jal, New Mexico, USA. Prior to use, PW was filtered by filter paper in order

126

to minimize the impact of suspended solids. The main characteristics of the filtered PW are

127

presented in Table 1. CO2 (instrument grade) used in this study was ordered from Airgas. All

128

other chemicals including Na2CO3, NaOH, KCl, Na2SO4, MgCl2·6H2O, NH4Cl, and Na2HPO4

129

were of analytical grade obtained from Sigma-Aldrich and Fisher Scientific.

130

2.2. Experimental procedures

7

131

2.2.1. Struvite recovery without calcium pretreatment

132

A series of batch experiments was performed without calcium pretreatment to evaluate

133

the influence of Mg:P molar ratio and pH on struvite recovery from PW. Na2HPO4 was selected

134

as the phosphorus source to adjust Mg:P molar ratio to the desired value (1:1, 1:2, 1:3, 1:5, and

135

1:7). The solution pH was then adjusted to the desired value (8.0, 8.5, 9.0, 9.5, and 10.0) by

136

using 0.5 M NaOH, which has been suggested as the recommended pH range for struvite

137

recovery from various wastewaters [43]. The mixed solution (PW volume = 150 mL) was stirred

138

at 200 rpm for 30 min, followed by precipitation for another 30 min. When the reaction was

139

complete, 5 mL supernatant was taken and filtered through a 0.22 μm pore size membrane for

140

the composition analysis. Then the obtained precipitates were washed twice with deionized water

141

and dried at room temperature for 36 h before the subsequent characterization. All experiments

142

were carried out at room temperature in triplicate according to procedures described above.

143

2.2.2. Calcium pretreatment

144

Calcium pretreatment by Na2CO3: Na2CO3 stock solution was added to PW to

145

precipitate Ca2+ as CaCO3. In order to determine the optimal dosage of Na2CO3 for Ca2+ removal,

146

the molar ratios of Ca2+:CO32- were designed as 1:1, 1:1.2, 1:1.4, 1:1.5, 1:1.6, and 1:1.8. When

147

the reaction was complete, 5 mL supernatant was taken and filtered through a 0.22 μm pore size

148

membrane to determine the Ca2+ removal efficiency and Mg2+ loss rate in PW.

149

Calcium pretreatment by carbonation process: The initial pH of PW was adjusted to

150

the desired value (pH = 11, 12, and 13, respectively) by using 0.5 M NaOH. Then, CO2 gas

151

(flowrate: 235~260 mL/min) was added into PW with the working volume of 3 L for 150 min in

8

152

each batch. Samples were collected every 30 min and filtered through a 0.22 μm pore size

153

membrane for further analysis.

154

2.2.3. Effect of pH and Mg/N/P molar ratio

155

To investigate the effect of pH and Mg/N/P molar ratio on struvite precipitation from PW,

156

two runs of batch experiments were performed by using pretreated PW. One run of struvite

157

precipitation was conducted with initial pH of 8.0, 8.5, 9.0, 9.5 and 10.0. The other one was

158

carried out with varying Mg/N/P molar ratio of 1:1:1, 1.2:1:1.2, 1.5:1:1.5, and 1.8:1:1.8. As

159

shown in Table 1, the NH4+-N molar concentration in the PW was greater than Mg2+ molar

160

concentration, making Mg2+ become a limited factor for struvite precipitation from PW.

161

Therefore, MgCl2·6H2O and Na2HPO4 were selected as magnesium and phosphorus sources and

162

added into pretreated PW to meet the desired Mg/N/P molar ratio values. For each experiment,

163

only one parameter was varied while keeping the others constant. The mixed solution was then

164

treated following the same procedures as stated in Section 2.2.1.

165

2.2.4. Effect of seeding dosage and reaction time

166

The seeding experiments of struvite precipitation from PW were conducted at the

167

Mg:N:P = 1.5:1:1.5 and pH = 9.5. Preformed MAP was used as the seeding material, which were

168

prepared by analytical grade MgCl2·6H2O, NH4Cl, and Na2HPO4. First, the chemicals were

169

added into deionized water to prepare stock solutions (0.3M). Next, the stock solutions were

170

mixed by a magnetic stirrer at the Mg:N:P molar ratio of 1:1:1, and the initial pH was adjusted to

171

9.5 by using 0.5 M NaOH. This mixture was stirred at 200 rpm for 30 min, followed by

172

precipitation for another 30 min. Finally, the solution was filtered through a 0.22 μm pore size

9

173

membrane and the obtained precipitates were washed twice with deionized water and then dried

174

at room temperature for 36 h. Five seeding dosages, 0.25, 0.50, 0.75, 1.0, and 1.5 g/L, were

175

selected to investigate the effect of the seeding dosage on the ammonium and potassium recovery

176

efficiency from PW by struvite precipitation process. In order to study the effect of the reaction

177

time on struvite precipitation process from PW, the experiments were further performed for a

178

period of 90 min at pH = 9.5 and Mg:N:P = 1.5:1:1.5, and water samples were collected after 1,

179

2, 5, 10, 15, 20, 25, 30, 50, 70, and 90 min. The following experimental procedures were similar

180

to those described in Section 2.2.1.

181

2.2.5. Effect of competitive cation Na+

182

The PW used in this study has very high sodium concentration (44.2±2.5 g/L).

183

Competitive cation Na+ present in PW may markedly affect the precipitation of MAP and MPP,

184

since Na+ can compete with NH4+ and K+ to form Na-struvite (MgNaPO4·7H2O, magnesium

185

sodium phosphate, MSP). To determine the effect of Na+ on MAP and MPP precipitation, NH4Cl,

186

MgCl2·6H2O, KCl, and Na2SO4 were added to deionized water to prepare the synthetic PW with

187

similar water quality. In the synthetic PW, the concentration of NH4+-N, Mg2+, K+, and SO42-

188

was 570 mg/L, 760 mg/L, 910 mg/L, and 750 mg/L, respectively. The following experiments

189

were performed: first, the Na+ concentration in synthetic PW was adjusted to the desired value

190

(9.2, 18.4, 27.6, 36.8, and 46.0 g/L) by NaCl addition. Next, MgCl2·6H2O and Na2HPO4 were

191

added to the synthetic PW in Mg:N:P molar ratio of 1.5:1:1.5. The mixed solution pH was then

192

adjusted to 9.5 and stirred at 200 rpm for 30 min, followed by precipitation for another 30 min.

193

Finally, 5 mL supernatant was taken and filtered through a 0.22 μm pore size membrane for the

194

composition analysis.

10

195

2.2.6. Microtox® acute toxicity assay

196

Microtox® acute toxicity assay was performed on a Microtox® Model 500 Analyzer

197

(Azur Environmental, Carlsbad, USA) according to the 81.9% Screening Test Protocol [44]. In

198

the Microtox® test, the marine luminescent bacterium Vibrio fischeri was used to assess the

199

toxicity of PW samples. Samples were taken from raw PW, pretreated PW, and PW after struvite

200

precipitation. Before the toxicity assay, the pH of all the samples was adjusted to 6~8 with 0.5 M

201

NaOH and HCl to provide stable conditions for Vibrio fischeri. ZnSO4·7H2O (100 mg/L) was

202

used as the positive control to verify the sensitivity of the luminescent bacterium, and Microtox®

203

diluent was used as the negative control. The percentage inhibition on bioluminescence of the

204

marine luminescent bacterium Vibrio fischeri, with an exposure time of 15 min, was designated

205

as the toxic effect.

206

2.3. Analysis

207

The concentrations of NH4+, Mg2+, K+, Ca2+, Na+, Cl-, and SO42- in PW samples were

208

quantified by using an ion chromatograph (IC, ICS-2100, Dionex, Sunnyvale, CA, USA). A pH

209

and conductivity meter (Model 431-61, Cole-Parmer, Vernon Hills, IL, USA) was used to

210

measure electrical conductivity and pH of all liquid samples. Total organic carbon (TOC) in

211

water samples was determined by a carbon analyzer (Shimadzu TOC-L, Kyoto, Japan). An

212

inductively coupled plasma optical emission spectrophotometer (ICP-OES, PerkinElmer,

213

Waltham, MA, USA) was employed to measure the concentrations of heavy metals (Cr, As, Fe,

214

Pb, Ni, and Mn). The morphology of dried precipitates was characterized using a scanning

215

electron microscope-energy dispersive spectrometer (SEM-EDS, S3400N Type II, Hitachi,

216

Pleasanton, CA, USA). The crystalline structure of the obtained precipitates was identified using 11

217

an X-ray diffraction analyzer (XRD, MiniFlex II, Rigaku, Japan), which was excited with Cu Kα

218

(λ = 1.5406 Å) at 45 kV and 40 mA.

219

In this study, the purity of MAP and MPP in the obtained precipitates was determined

220

according to the following procedures [33, 45, 46]. First, the dried precipitates were dissolved in

221

1% HNO3, and the NH4+ and K+ concentrations were quantified by an ion chromatograph. Next,

222

the quantity of MAP and MPP in the precipitates can be estimated based on the contents of NH4+

223

and K+ present in the dried precipitates. Finally, the purity of MAP (PMAP) and MPP (PMPP) was

224

calculated according to Eqs. (4) and (5), as follows: 𝑀𝑀𝐴𝑃

225

𝑃𝑀𝐴𝑃 = 𝑀𝑇𝑜𝑡𝑎𝑙 × 100%

226

𝑃𝑀𝑃𝑃 = 𝑀𝑇𝑜𝑡𝑎𝑙 × 100%

𝑀𝑀𝑃𝑃

(4)

(5)

227

where, MMAP, MMPP and MTotal represent the masses of MAP and MPP in the dried precipitates

228

and the total mass of the dried precipitates, respectively.

229

3. Results and discussion

230

3.1. Struvite recovery without calcium pretreatment

231

To investigate the feasibility of direct struvite recovery from PW without calcium

232

pretreatment, batch experiments were performed at the Mg:P molar ratio of 1:1~1:7 with a pH

233

range of 8~10. As shown in Fig 1a and b, without calcium pretreatment, the removal efficiencies

234

of NH4+ (0.3~6.1%) and K+ (1.3~5.4%) in PW were very low even though the phosphorus

235

supply was sufficient. In contrast, the removal efficiencies of Ca2+ and Mg2+ increased

236

significantly with an increase in Mg:P molar ratio (Fig 1c). The highest Ca2+ (98.3%) and Mg2+ 12

237

(77.8%) removal efficiencies occurred at pH 10 and Mg:P molar of 1:7. The low removal

238

efficiencies of NH4+ and K+ can be attributed to the high initial concentration of calcium (4779 ±

239

105 mg/L) in raw PW, which was consistent with the findings of previous studies [28, 32]. It was

240

reported that the calcium present in water may affect struvite precipitation [47], since calcium

241

can interact effectively with phosphate to form calcium phosphate (Ca3(PO4)2) and

242

hydroxylapatite (Ca5(PO4)3(OH)) according to the following equations:

243

5Ca2 + (aq) + 3HPO24 ― (aq) + H2O→Ca5(PO4)3(OH)↓ + 4H +

(6)

244

3Ca2 + (aq) + 2HPO24 ― (aq)→Ca3(PO4)2↓ + 2H +

(7)

245

The solubility product constant (Ksp) of calcium phosphate (2.0 × 10-29) is much lower

246

than that of MAP (2.5 × 10-13), that is why the precipitation of calcium phosphate is much easier

247

and faster than MAP precipitation. It is speculated that the effect of calcium on the struvite

248

precipitation in PW was mainly achieved by competition for PO43-, which results in the reduction

249

of the amount of PO43- used to form MAP and MPP in PW. Several studies have reported the

250

effect of calcium on struvite precipitation in wastewaters, which demonstrated that the purity of

251

struvite decreased with an increase in calcium concentration. Gao et al. [45] found that the

252

presence of calcium in the solution markedly influenced struvite precipitation from synthetic

253

urine and reduced the purity of MPP due to the formation of calcium phosphate. Li et al. [48]

254

indicated that calcium had a strong negative impact on struvite recovery for application in dairy

255

wastewaters. Wu et al. [32] investigated the feasibility of struvite recovery from landfill leachate

256

without calcium pretreatment, they found Ca2+ exhibited strong competition for PO43- with Mg2+,

257

leading to an unsatisfied struvite recovery efficiency in the calcium-dominated landfill leachate.

258

Yan et al. [49] also observed that struvite precipitation was completely inhibited by calcium

13

259

when the Ca/Mg molar ratio was more than 2, which was consistent with the finding of this study.

260

In this study, Ca/Mg molar ratio was around 4.3, thereby achieving a low NH4+ removal

261

efficiency. Therefore, a pretreatment to effectively remove calcium is required to recover struvite

262

from PW.

263

3.2. Struvite recovery with calcium pretreatment

264

3.2.1. Calcium pretreatment

265

High calcium concentration in PW can result in an unsatisfied struvite recovery

266

efficiency. To solve this problem, calcium pretreatment experiments were conducted by adding

267

Na2CO3 into raw PW. It should be noted that magnesium could also react with CO32- to form

268

MgCO3 during the calcium pretreatment process. Therefore, experiments should be performed to

269

optimize the CO32- dosage towards a minimal magnesium loss rate. Fig. 2a illustrates the

270

variations of Ca2+ and Mg2+ concentrations and removal rates with different Ca2+:CO32- molar

271

ratios. Average Ca2+ removal of 89.7%, 95.9%, 96.0%, 96.6%, 97.4% and 97.6% were observed

272

when the molar ratio of Ca2+:CO32- was 1:1, 1:1.2, 1:1.4, 1:1.5, 1:1.6 and 1:1.8, respectively.

273

However, magnesium loss also increased with Na2CO3 dose. As shown in Fig 2a, the magnesium

274

loss rate significantly increased from 31.3% to 64.9% when the Ca2+:CO32- molar ratio changed

275

from 1:1.2 to 1:1.4. Thus, considering both the calcium removal efficiency and magnesium loss

276

rate, the molar ratio of Ca2+:CO32- at 1:1.2 was selected as the optimal condition for calcium

277

pretreatment by Na2CO3 addition.

278

Despite the promise of calcium pretreatment by Na2CO3, the use of Na2CO3 to remove

279

calcium from PW would lead to a high treatment cost, which was a major hurdle for the calcium

280

pretreatment of PW. Zhang et al. [50] suggested that CO2 stripping could be considered as a 14

281

feasible and economical method to remove calcium from wastewaters rather than adding Na2CO3,

282

especially when municipal solid wastes incineration flue gas (CO2 volume fraction: 4%) was

283

used as the feed. Here, the effects of imported CO2 gas on the carbonation process were

284

investigated using raw PW. Fig. 2b shows the variations of calcium concentration and removal at

285

different pH over CO2 aeration time. According to Fig. 2b, with pH adjustment alone (at 0 min),

286

higher calcium removal rate was achieved at pH = 13 (46.4%) compared with that of pH = 11

287

(22.2%) and pH = 12 (29.1%), which indicated that less Ca2+ precipitated by CO32- under lower

288

pH conditions. In addition, the maximum calcium removal rate (95.6%) could be achieved at pH

289

= 13 with 120 min of CO2 aeration. The carbonation process by CO2 aeration to form CaCO3

290

precipitates could be described by Eqs. (8)~(11) [51]. During this reaction process, the aqueous

291

CO2 could react with the dissociative OH- in the mixed solution to produce carbonic acid ion. It

292

was reported that the carbonic acid ion will predominantly exist in the form of CO32- in the

293

solution when pH > 10.5 [52], therefore, the calcium concentration in PW decreased significantly

294

when the CO2 gas was imported into the raw PW. However, the magnesium concentration in PW

295

also decreased obviously during the carbonation process (Fig. 2c). The maximum loss rate of

296

72.5% (Fig. 2c) for magnesium occurred at pH = 13 with 120 min of CO2 aeration. Thus, CO2

297

stripping could achieve high calcium removal efficiency under high pH condition, while the high

298

magnesium loss rate was also obtained. Overall, calcium pretreatment by Na2CO3 addition could

299

achieve high calcium removal rate and relatively low magnesium loss rate with high treatment

300

cost, calcium pretreatment by CO2 stripping could obtain high calcium removal rate with less

301

chemicals, but the magnesium loss rate was also very high. Therefore, considering both the

302

calcium removal efficiency and magnesium loss rate, Na2CO3 addition with Ca2+:CO32- = 1:1.2

15

303

was selected as the optimal calcium pretreatment method for the subsequent struvite recovery

304

experiments in PW.

305

CO2(g)↔CO2(aq)

(8)

306

CO2(aq) + OH ― (aq)↔HCO3― (aq)

(9)

307

HCO3― (aq) + OH ― (aq)↔CO23 ― (aq) + H2O(l)

(10)

308

Ca2 + (aq) + CO23 ― (aq)↔CaCO3(s)

(11)

309

To further examine the structure and composition of the obtained precipitates during the

310

calcium pretreatment process by Na2CO3 addition, the dried precipitates were characterized by

311

SEM-EDS and XRD (Fig. 2d, 2e, and 2f). In the EDS spectrum (Fig. 2e), the characteristic peaks

312

of Ca, C, O, Mg, and Na simultaneously appeared, which suggests that the precipitates were the

313

mixture of calcium and magnesium salts. In addition, the XRD analysis (Fig. 2f) revealed that

314

the components of the precipitates were mostly matched as calcium carbonate, indicating that the

315

obtained precipitates were mainly crystalline CaCO3. The precipitates were further investigated

316

using EDS mapping equipped on SEM, as shown in Fig. S1. It is observed that the distributions

317

of Ca, O and Mg elements in the whole precipitates are highly uniform, further verifying the

318

existence of calcium and magnesium carbonate compounds in the obtained precipitates.

319

Therefore, most of calcium was precipitated as CaCO3 and some magnesium was precipitated as

320

MgCO3 during the calcium pretreatment process.

321

3.2.2. Effect of pH and Mg/N/P molar ratio

322

The pH and Mg/N/P molar ratio have been considered as the crucial parameters affecting

323

the struvite precipitation process [33, 34]. To determine the effects of pH and Mg/N/P molar 16

324

ratio on struvite precipitation from PW, experiments were performed at pH between 8 and 10

325

with the Mg/N/P molar ratio range of 1:1:1~1.8:1:1.8. It is obvious that the NH4+ and K+

326

recovery enhanced with increasing pH and Mg/N/P molar ratio (Fig. 3a). As shown in Fig. 3a,

327

when the Mg/N/P molar ratio increased from 1:1:1 to 1.5:1:1.5, the NH4+ and K+ recovery

328

efficiency increased markedly, however, further increases in the dosages of phosphate and

329

magnesium salts did not induce further increase in the NH4+ recovery efficiency when pH was

330

greater than 9.5. These findings were consistent with those reported earlier [38]. On the other

331

hand, Fig. 3a also shows that when the pH increased from 8 to 9.5 with the Mg/N/P molar ratio

332

of 1.5:1:1.5, the NH4+ recovery efficiency rapidly increased, reaching a maximum value of 85.9%

333

at pH 9.5, and then decreased slightly with further increase in pH. It was reported that pH

334

determined the distribution of different Mg, P, and N species in the solution, such as Mg2+,

335

MgOH+, MgPO4-, H3PO4, H2PO4−, HPO42-, PO43-, NH3, and NH4+ [53]. Thus, this greater

336

recovery efficiency of ammonium at higher pH levels could be assumed to arise from the

337

presence of the species (Mg2+, NH4+, and PO43-) in forms that are essential for the struvite

338

precipitation. However, when pH > 9.5, a proportion of NH4+ is converted into NH3 in the

339

solution [40], which cannot be precipitated during the struvite precipitation process, thereby

340

decreasing the ammonium recovery efficiency slightly. At optimal operating condition (Mg:N:P

341

= 1.5:1:1.5 and pH = 9.5), only 2.9% dissolved NH4+ was converted into NH3 gas during the

342

struvite precipitation process, indicating that most of the ammonium in PW could be recovered

343

as MAP. In addition, it was found that the K+ recovery efficiency was much lower than the NH4+

344

recovery efficiency (Fig. 3b). The pKsp of MAP was 13.6 [35], whereas that of MPP was 11.7

345

[54], indicating that MPP is more soluble than MAP. This could explain why MPP presents

346

lower precipitation potential than MAP under the same conditions. When the Mg/N/P molar ratio

17

347

was 1.5:1:1.5, the K+ recovery efficiency increased rapidly with an increase in pH, and peaked at

348

pH 9.5 (24.8%). These results are consistent with the findings of Xu et al. [39], who investigated

349

the simultaneous removal of potassium and phosphate from synthetic urine through the MPP

350

precipitation and found that the optimal pH for MPP precipitation was 10. As presented in Fig.

351

3c, a significantly higher magnesium recovery efficiency (> 96.8%) was obtained when the

352

Mg/N/P molar ratio escalated from 1:1:1 to 1.5:1:1.5 at pH 9.5~10, indicating that the Mg/N/P

353

molar ratio of 1.5:1:1.5 at pH 9.5 could achieve the highest ammonium recovery efficiency while

354

maximizing magnesium resource recovery in PW. At this optimal operating condition (Mg:N:P =

355

1.5:1:1.5 and pH = 9.5), the mass balance of the main cations and anions (including Ca2+, Mg2+,

356

NH4+, K+, Na+, Cl-, Br- and SO42-) in PW during the whole PW treatment process is shown in Fig.

357

S4. During the calcium pretreatment process, 95.9% of Ca2+ and 31.3% of Mg2+ in PW were

358

removed by forming carbonate precipitates. 85.9% of NH4+, 24.8% of K+ and 96.8% of Mg2+ in

359

PW were recovered by struvite precipitation process. The results in Fig. 3d show the MAP and

360

MPP purity in the obtained precipitates at different pH when the Mg/N/P molar ratio was

361

1.5:1:1.5. It was found that the pH had a significant effect on the struvite (MAP and MPP) purity,

362

which was consistent with the previous study [45]. As the pH increased from 8 to 10, the MAP

363

proportion rapidly decreased from 81.3% to 70.8%, whereas the MPP proportion markedly

364

increased from 10.1% to 23.2%. These results are supported by the findings recorded in the

365

previous study [33], which reported that the increase in the pH could improve the

366

competitiveness of potassium for phosphate during the MAP precipitation, thereby promoting the

367

formation of MPP.

18

368

3.2.3. Effect of seeding dosage and reaction time

369

Crystal seeding material addition has been proposed as a technique to enhance the

370

ammonium recovery efficiency during the struvite precipitation process [55]. Several different

371

types of seeding materials, such as struvite crystals [45], stainless steel [56] and quartz sand [57],

372

have been recommended as potential seeding materials. Among these seeding materials,

373

preformed struvite crystals are the most commonly used seeding material. In this study,

374

preformed MAP was added to PW as a seeding material before the struvite recovery at optimal

375

operating condition (Mg:N:P = 1.5:1:1.5 and pH = 9.5). Five different doses of seeding materials

376

were trialed to determine variation in ammonium and potassium recovery with changes in the

377

amount of seeding material used (Fig. 4a). The findings suggest that the addition of preformed

378

MAP could improve the ammonium and potassium recovery efficiencies. When the dosage of

379

the preformed MAP increased from 0 g/L to 1 g/L, the ammonium and potassium recovery

380

efficiencies increased from 85.9%, 24.8% to 91.9%, 30.5%, respectively. These results were

381

consistent with the findings of Kim et al. [31], who found that the ammonium recovery

382

efficiency in landfill leachate increased from 86.4% to 96.8% when struvite seeding dosage

383

increased from 0 to 40 g/L. However, no further improvement in ammonium and potassium

384

recovery was observed when the seeding dosage was increased from 1 to 1.5 g/L. It was reported

385

that the struvite crystallization process was divided into two phases - nucleation and crystal

386

growth [27]. During the crystallization process, the preformed MAP served as the nucleus on

387

which accumulation of crystallizing material could occur. Seeding materials could promote

388

nucleation by providing greater surface areas, thus reducing induction time for crystal growth

389

and increasing the struvite precipitation rate when the solid-liquid concentration of the preformed

390

MAP was relatively low in the solution. However, when the seeding amount is too high, the 19

391

mechanism of crystal nucleation has a weaker effect than crystal growth, resulting in no

392

improvement in ammonium recovery efficiency [45, 58]. Therefore, no further improvement in

393

ammonium and potassium recovery was obtained when the seeding dosage further increased.

394

Furthermore, the variations in ammonium and potassium recovery and solution pH with

395

reaction time were also investigated (Fig. 4b). As illustrated in Fig 4b, a sharp decline in solution

396

pH within 10 min (from 9.5 to 7.6) was observed due to the high degree of struvite formation.

397

During the struvite crystallization process, H+ was released into the solution to decrease pH. It

398

was also observed that the ammonium and potassium recovery efficiencies rapidly increased

399

during the initial 10 min, and then plateaued until the reaction equilibrium state was reached

400

within the subsequent 20 min. Correspondingly, approximately 75.1% and 21.9% of NH4+ and

401

K+ were recovered from PW within the initial 10 min, whereas little change was observed in

402

NH4+and K+ recovery from PW after the first 10 min. These results are supported by the previous

403

study [53], which suggested that the optimal duration for struvite crystallization was 20~30 min,

404

and further increase in reaction time could not enhance the struvite crystallization in PW.

405

3.2.4. Effect of competitive cation Na+

406

High concentration of Na+ present in PW may affect the precipitation of MAP and MPP

407

by competing with NH4+ and K+ to form Na-struvite. Previous studies showed that magnesium

408

sodium phosphate heptahydrate (MSP, MgNaPO4·7H2O) could co-precipitate with MAP and

409

MPP when Na+ concentration was high, thereby reducing the recovery efficiencies of NH4+and

410

K+ from urine [38, 45]. To understand the effect of Na+ on the precipitation of MAP and MPP in

411

PW, batch experiments were performed at pH = 9.5 and Mg:N:P = 1.5:1:1.5 with varying Na+

412

dosage (0~46 g/L). The changes in the recovery efficiencies of NH4+ and K+, and the purity of 20

413

MAP and MPP, were presented in Fig. 5a. It can be found that the NH4+ recovery efficiency

414

decreased from 92.0% to 78.4% when the Na+ dosage increased from 0 to 46 g/L, which

415

suggested co-precipitation of MSP inhibited the MAP crystallization in PW. And the purity of

416

MAP in the precipitates correspondingly decreased with an increase in the Na+ dosage (Fig. 5a).

417

These results can be ascribed to the increased Na+ dosage that enhanced the driving force of

418

MSP crystallization reaction and therefore inhibited MAP crystallization. However, K+ recovery

419

efficiency was between 28.2% and 29.5% when Na+ dosage varied from 0 to 46 g/L, which

420

suggested that the formation of MPP was almost not affected by high sodium concentration.

421

Overall, in this study, it is speculated that the formation of MSP could compete for Mg2+ and

422

PO43- with the formation of MAP due to the high sodium concentration in PW.

423

To further understand the results mentioned above, the precipitates formed at different

424

Na+ dosage (9.2, 27.6, and 46.0 g/L) were characterized by XRD, and the precipitates obtained at

425

the Na+ dosage of 46.0 g/L were characterized by SEM-EDS. The results shown in Fig. 5b and

426

5c confirmed that these precipitates are all orthorhombic struvite. MAP and MSP were

427

simultaneously found in the obtained precipitates (Fig. 5b). In addition, it was observed that

428

more MSP peaks occurred when Na+ dosage increased from 9.2 to 46.0 g/L (Fig. 5b), suggesting

429

that more MSP formed with the increase of sodium concentration. These results are supported by

430

the findings of Huang et al. [29], who investigated the effect of Na+ on phosphate and potassium

431

recovery from source-separated urine and found that Na+ amount in the precipitates increased

432

rapidly with increasing sodium concentration. Moreover, as presented in SEM image (Fig. 5c,

433

Na+ = 46.0 g/L), the obtained precipitates were orthorhombic structure crystals structure with

434

different thickness and length, which was consistent with the previous study [29]. Furthermore, it

435

can be observed that the characteristic peaks of Mg, N, K, Na, P, and O could simultaneously 21

436

occur in the EDS spectrum (Fig. 5d), which further confirms the formation of MAP, MPP and

437

MSP mixtures.

438

3.3. Precipitate characterization

439

To compare the obtained precipitates with and without calcium pretreatment, the

440

morphologies and compositions of the precipitates were analyzed by SEM-EDS and XRD (Fig.

441

6). The SEM image in Fig. 6a indicated that some crystals with chaotic structure were present in

442

the precipitates without calcium pretreatment. As shown in Fig. 6c, an amorphous diffraction

443

peak between 10° and 50° (2 theta degree) was observed in the XRD patterns. It was reported

444

that some calcium phosphate compounds, such as calcium phosphate (Ca3(PO4)2), dicalcium

445

phosphate (CaHPO4·2H2O) and hydroxylapatite (Ca5(PO4)3(OH)), were possibly generated in the

446

solution due to the high calcium concentration in the wastewater [59]. Moreover, the EDS

447

analysis (Fig. 6b) and EDS mapping (Fig. S3) indicated that the major elements of the crystals

448

were Ca, O and P, suggesting the obtained precipitates may be mainly calcium-phosphorus based

449

compounds (37.54% Ca, 32.33% O, and 17.75% P). Based on the struvite recovery and

450

characterization results mentioned above, it can be confirmed that high calcium concentration in

451

PW substantially inhibited the occurrence of struvite crystals and led to the formation of

452

amorphous matters (mainly calcium phosphate compounds). In the presence of calcium

453

pretreatment, the SEM image (Fig. 6d and Fig. S5) showed that the morphology of the

454

precipitates was orthorhombic-shaped, which was similar to that of the struvite crystal reported

455

earlier [33]. As shown in Fig. 6e, the characteristic peaks of Mg, N, K, P, and O simultaneously

456

appeared in the EDS spectrum of the obtained precipitates, suggesting the formation of MAP and

457

MPP in PW. In addition, the peak of Na was also detected in the precipitates, which implies that

22

458

MSP may also co-precipitate with MAP and MPP during the struvite recovery process. The XRD

459

pattern (Fig. 6f) generated from the precipitates (with calcium pretreatment, Mg:N:P = 1.5:1:1.5

460

and pH = 9.5) matched well with the reference struvite diffractogram from the International

461

Center for Diffraction Data (positions and intensities of the peaks), suggesting that most of the

462

obtained precipitates were pure struvite. As presented in Fig. 6d, Mg, N, K and P exhibited a

463

uniform spatial distribution within the orthorhombic frameworks, revealing the formation of

464

MAP and MPP. In addition, the distribution of sodium was quite sparse compared to other

465

elements, suggesting that few MSP may be generated due to the co-precipitation with MAP and

466

MPP in PW. Therefore, all these results have demonstrated the feasibility of recovering high-

467

purity struvite from PW when calcium pretreatment was applied prior to struvite precipitation.

468

3.4. Fate of heavy metals and organic contaminants

469

Previous researches usually focused on the optimization of struvite recovery conditions,

470

while there is less attention being paid to the possible contaminants existing in the wastewaters

471

that can affect the quality of struvite. Since heavy metals and organic contaminants could impact

472

the quality of struvite for possible agricultural use [60], changes in heavy metals (Table 2) and

473

TOC (Fig. S2) concentrations in PW before and after struvite precipitation were examined. It

474

was found that calcium pretreatment could reduce heavy metal concentrations considerably. The

475

concentrations of As, Fe, Mn, and Ni in PW before calcium pretreatment were 0.8758, 1.66,

476

0.6583, and 0.0244 mg/L, respectively. However, these concentrations were reduced to less than

477

the method detection limit (MDL) after calcium pretreatment process, indicating that no heavy

478

metals were accumulated in the formed struvite. To further verify these results, the obtained

479

CaCO3 precipitates and recovered struvite precipitates were dissolved in 1% HNO3 for the

23

480

detection of heavy metals. As shown in Table 2, the noteworthy accumulation of As, Mn, and Ni

481

(16.97, 42.94, and 0.87 mg/kg, respectively) was found in the CaCO3 precipitates obtained after

482

calcium pretreatment. In contrast, heavy metals were not detected in the recovered struvite

483

precipitates, demonstrating that the heavy metals in PW could have been removed during the

484

calcium pretreatment process due to the co-precipitation or sorption to the surface of CaCO3 [32,

485

40].

486

The quality of struvite formed from PW should be appropriate for agricultural use,

487

depending on the presence of both heavy metal and hazardous organic contaminants in PW [61].

488

Thus, in addition to the accumulation of heavy metals, the fate of organic contaminants during

489

the struvite recovery process was evaluated (Fig. S2). As illustrated in Fig. S2, there was almost

490

no difference in the TOC concentration of PW before and after struvite precipitation, so it infers

491

that organic matters were also not accumulated in the obtain struvite during the struvite

492

precipitation process. Therefore, it can be concluded that the quality of struvite obtained from

493

PW was sufficient for use as a slow-release fertilizer in the field. For the large-scale application,

494

sludge lagoon or drying bed could be used to dewater the struvite slurry before obtaining the

495

final products.

496

3.5. Microtox® acute toxicity assay

497

Finally, the reduction of the toxicity of PW after the struvite precipitation process was

498

evaluated, which could provide information for PW risk assessment after struvite recovery. As

499

an effective pre-screening measurement for acute toxicity, Microtox 81.9% screening protocol

500

was used to evaluate the toxicity towards Vibrio fischeri [44]. The marine bacterium Vibrio

501

fischeri has been studied extensively for evaluating the toxicity of different types of wastewaters, 24

502

such as oil sands process-affected water [62, 63], pharmaceutical wastewater [64] and textile

503

wastewater [65], demonstrating that it should be quick and sensitive for toxicity monitoring of all

504

types of matrices. Vibrio fischeri is a natural inhabitant of seawater, thus, to minimize the impact

505

of salinity on its activity, PW samples were diluted five times to maintain a reasonable seawater

506

salinity (3.5%) before the toxicity assay [65]. The levels of Vibrio fischeri bioluminescence

507

inhibition after 15 min of exposure to the PW samples (raw PW, pretreated PW, and PW after

508

struvite recovery) are shown in Fig. 7. The percentage inhibition of raw PW and pretreated PW

509

on Vibrio fischeri bioluminescence (75.5% and 71.4%, respectively) was very high, which may

510

be attributed to the toxicity of high ammonium concentration in PW. It was reported that the

511

inhibition level of landfill leachates on Vibrio fischeri bioluminescence was up to 100% due to

512

the ammonium concentration (0.64~4.99 g/L) [66]. Yu et al. [67] evaluated the acute toxicity of

513

antibiotic wastewater using Vibrio fischeri, and found that ammonium was the greatest

514

contributor to toxicity statistically, with a high Pearson's correlation of 0.981. In this study, high

515

toxicity reduction towards Vibrio fischeri (60%) was achieved after struvite recovery at the

516

optimal operating condition (Mg:N:P = 1.5:1:1.5 and pH = 9.5), suggesting that struvite

517

precipitation process may be a very effective option for the detoxification of PW due to the high

518

ammonium removal efficiency. These results were supported by the findings of Yu et al. [64],

519

who found the toxicity of pharmaceutical wastewaters towards Vibrio fischeri was significantly

520

and positively correlated with ammonium concentration, and high average acute toxicity removal

521

efficiency (72.47%) was obtained after ammonium removal. Therefore, struvite precipitation

522

process could not only recover the beneficial resource from PW effectively, but also lower the

523

risk of PW discharge or reuse by reducing toxicity.

524

3.6. Economic analysis 25

525

In this study, an economic estimation for recovering NH4+, K+ and Mg2+ from PW by

526

struvite precipitation was performed at optimal operating condition (Mg:N:P = 1.5:1:1.5 and pH

527

= 9.5). To simplify the calculation, the labor and maintenance costs were not taken into

528

consideration in this assessment; only the costs of the chemicals and energy consumption were

529

included in the calculation. The market prices of the chemicals and energy consumption are

530

given in Table S1, which reflect the average price in the United States market in 2019. As shown

531

in Table S1, the total cost of the proposed process was $7.41/m3 PW when using pure chemicals

532

to generate struvite precipitates from PW. However, the chemical cost of proposed processes

533

varies significantly based on PW composition and the chemicals being used. Cheaper phosphate

534

source and carbonate sources would help reduce the overall cost. A previous study [30] found

535

that the use of low-cost MgO and waste phosphoric acid as magnesium and phosphorus sources

536

could save chemical costs by 68.0% compared with the use of pure chemicals. Therefore, further

537

study should be conducted to reduce the chemical cost in future experiments. In terms of the

538

market value of the recovered struvite, it is assumed to be 0.57 USD per kg dry weight based on

539

its application as a fertilizer [68]. The potential revenue of the recovered struvite precipitates was

540

estimated $4.92/m3 PW. Considering both environmental benefits and recovery of a renewable

541

resource, struvite precipitation may be an economically feasible process for PW treatment.

542

Future research on what kind of PW is economically suitable and exploration of the low-cost

543

chemicals are needed for the large-scale application of this proposed process.Conclusions

544

This study has demonstrated the feasibility of NH4+, K+ and Mg2+ recovery from PW as

545

struvite (MAP and MPP), with simultaneous toxicity reduction towards Vibrio fischeri. The

546

quantification results indicated that the struvite precipitation was significantly inhibited without

547

calcium pretreatment. Mg/N/P molar ratio and solution pH posed a greater effect on struvite 26

548

recovery than seeding dosage and Na+ concentration. Mg/N/P molar ratio=1.5:1:1.5 & pH=9.5

549

was determined to be the optimal condition for struvite recovery from PW, reaching the NH4+,

550

K+ and Mg2+ recovery efficiencies of 85.9%, 24.8% and 96.8%, respectively. The precipitated

551

struvite had high purity and showed the absence of heavy metals, indicating the sufficient quality

552

for applications as fertilizers. Overall, the struvite precipitation process could remove toxic

553

ammonia in PW and recover the beneficial minerals as resource. The outcomes of this study will

554

encourage further efforts to develop new methods for resource recovery from PW. In addition,

555

considering excellent removal of calcium and magnesium, this approach is also a promising

556

pretreatment process to minimize the scale formation in membrane-based desalination for the

557

treatment of produced water and fracking flowback water.

558

Acknowledgement

559

The author would like to acknowledge the New Mexico State University and New

560

Mexico Water Resources Research Institute 2018-2019 Student Water Research Grant Program

561

for funding this research (NMWRRI-SG-2018).

27

562

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563

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739

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742

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743

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745 746

35

747

Figure captions

748

Fig. 1. The removal efficiency of (a) NH4+, (b) K+, (c) Ca2+, and (d) Mg2+ during struvite

749

recovery without calcium pretreatment.

750

Fig. 2. The concentrations and removal rates of Ca2+ and Mg2+ after calcium pretreatment by (a)

751

Na2CO3, (b) and (c) CO2 stripping; the (d) SEM photo, (e) EDS peaks, and (f) XRD spectrum of

752

the precipitates obtained at the Ca2+ : CO32- molar ratio of 1:1.2.

753

Fig. 3. The variations of (a) NH4+, (b) K+, (c) Mg2+ recovery efficiency, and (d) MAP or MPP

754

purity with different solution pH and Mg:N:P molar ratio.

755

Fig. 4. (a) The variations in the NH4+ and K+ recovery efficiencies with the dosage of the

756

preformed MAP, and (b) the changes in pH, NH4+ and K+ recovery efficiency with the reaction

757

time.

758

Fig. 5. (a) The changes in the NH4+ and K+ recovery efficiencies with different Na+ dosage in

759

the synthetic PW, (b) representative XRD patterns of the precipitates obtained at different Na+

760

dosage, (c) the SEM photo and (d) EDS peaks of the precipitates obtained at the Na+ dosage of

761

46.0 g/L.

762

Fig. 6. (a) The SEM photo, (b) EDS peaks and (c) XRD spectrum of the precipitates obtained

763

without Ca pretreatment; (d) SEM photo and EDS mapping result of Mg, P, N, K and Na, (e)

764

EDS peaks, and (f) XRD spectrum of the precipitates obtained at Mg:N:P = 1.5:1:1.5 & pH=9.5

765

with Ca pretreatment.

766

Fig. 7. The toxic effect of raw PW, pretreated PW, and PW after struvite recovery on Vibrio

767

fischeri bioluminescence after 15 min exposure (PWAT1: pH=10.0 & Mg/N/P molar

768

ratio=1.8:1:1.8, PWAT2: pH=10.0 & Mg/N/P molar ratio=1.5:1:1.5, PWAT3: pH=9.5 &

769

Mg/N/P molar ratio=1.5:1:1.5).

770 771

36

772 773

Fig. 1. The removal efficiency of (a) NH4+, (b) K+, (c) Ca2+, and (d) Mg2+ during struvite

774

recovery without calcium pretreatment.

775

37

776 777

Fig. 2. The concentrations and removal rates of Ca2+ and Mg2+ after calcium pretreatment by (a)

778

Na2CO3, (b) and (c) CO2 stripping; the (d) SEM photo, (e) EDS peaks, and (f) XRD spectrum of

779

the precipitates obtained at the Ca2+ : CO32- molar ratio of 1:1.2.

780

38

781 782

Fig. 3. The variations of (a) NH4+, (b) K+, (c) Mg2+ recovery efficiency, and (d) MAP or MPP

783

purity with different solution pH and Mg:N:P molar ratio.

39

784 785

Fig. 4. (a) The variations in the NH4+ and K+ recovery efficiencies with the dosage of the

786

preformed MAP, and (b) the changes in pH, NH4+ and K+ recovery efficiency with the reaction

787

time.

788

40

789 790

Fig. 5. (a) The changes in the NH4+ and K+ recovery efficiencies with different Na+ dosage in

791

the synthetic PW, (b) representative XRD patterns of the precipitates obtained at different Na+

792

dosage, (c) the SEM photo and (d) EDS peaks of the precipitates obtained at the Na+ dosage of

793

46.0 g/L.

41

794 795

Fig. 6. (a) The SEM photo, (b) EDS peaks and (c) XRD spectrum of the precipitates obtained

796

without Ca pretreatment; (d) SEM photo and EDS mapping result of Mg, P, N, K and Na, (e)

797

EDS peaks, and (f) XRD spectrum of the precipitates obtained at Mg:N:P = 1.5:1:1.5 & pH=9.5

798

with Ca pretreatment.

42

799 800

Fig. 7. The toxic effect of raw PW, pretreated PW, and PW after struvite recovery on Vibrio

801

fischeri bioluminescence after 15 min exposure (PWAT1: pH=10.0 & Mg/N/P molar

802

ratio=1.8:1:1.8, PWAT2: pH=10.0 & Mg/N/P molar ratio=1.5:1:1.5, PWAT3: pH=9.5 &

803

Mg/N/P molar ratio=1.5:1:1.5).

804

43

805

Table captions

806

Table 1. The main characteristics of raw produced water.

807

Table 2. Heavy metal contents in PW, pretreated PW, PW after struvite precipitation, obtained

808

precipitates after calcium pretreatment, and recovered struvite precipitates.

809

44

810

Table 1. The main characteristics of raw produced water. Parameter Physiochemical characteristics pH Total organics

Unit

Value

-

-

7.35 ± 0.10

TOC

mg

TDS

g L-1

129.3 ± 8.5

Ammonium

NH4+

mg L-1

598.6 ± 10.2

Potassium

K

mg L-1

968.5 ± 30.5

Total dissolved solids Macronutrients

L-1

Ca

mg

Magnesium

Mg

mg L-1

TP

mg

L-1

Sodium

Na

mg L-1

44200 ± 2500

Lithium

Li

mg L-1

18.8 ± 0.3

Chloride

Cl

mg L-1

65800 ± 1600

Bromide

Br

mg L-1

591.1 ± 15.8

Silicon

Si

mg L-1

16.3 ± 1.4

Strontium

Sr

mg L-1

256.8 ± 19.7

Iron

Fe

mg L-1

1.66 ± 0.03

Manganese

Mn

mg L-1

0.66 ± 0.02

Nickel

Ni

mg L-1

0.02 ± 0.004

As

mg L-1

0.88 ± 0.09

Arsenic ND: Non-Detect.

812

45

L-1

21.9 ± 1.2

Calcium Total phosphorus Others

811

Symbol

4779.4 ± 105.4 763.9 ± 25.4 ND

813

Table 2. Heavy metal contents in PW, pretreated PW, PW after struvite precipitation, obtained

814

precipitates after calcium pretreatment, and recovered struvite precipitates.

815

Elem

Raw PW (mg/L)

Pretreated PW (mg/L)

PW after struvite precipitation (mg/L)

CaCO3 precipitates (mg/kg)

Struvite precipitates (mg/kg)

MDL (mg/L)

As

0.8758

ND

ND

16.97

ND

0.1

Cr

ND

ND

ND

ND

ND

0.003

Fe

1.66

ND

ND

ND

ND

0.0287

Mn

0.6583

ND

ND

42.94

ND

0.0017

Ni

0.0244

ND

ND

0.8744

ND

0.002

Pb

ND

ND

ND

ND

ND

0.007

ND: Non-Detect; MDL: Method Detection Limit.

816

46

Highlights

817 818



Struvite recovery from produced water was investigated for the first time

819



Calcium pretreatment by Na2CO3 achieved low Mg2+ loss and high Ca2+ removal

820



High-purity struvite was obtained under Mg/N/P molar ratio=1.5:1:1.5 & pH=9.5

821



Struvite precipitate was free of heavy metals and organic contaminants

822



High toxicity reduction towards Vibrio fischeri (60%) was observed

823

47

824

Graphical abstract

825 826 827

48