The Role of Fe-Bearing Phyllosilicates in DTPMP Degradation under

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Energy and the Environment

The Role of Fe-bearing Phyllosilicates in DTPMP Degradation under High Temperature and High Pressure Conditions Lijie Zhang, and Young-Shin Jun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02552 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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Environmental Science & Technology

The Role of Fe-bearing Phyllosilicates in DTPMP Degradation under High Temperature and High Pressure Conditions

Lijie Zhang and Young-Shin Jun* Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130

Address: One Brookings Drive, Campus Box 1180 E-mail: [email protected] Phone: (314)935-4539 Fax: (314)935-7211 http://encl.engineering.wustl.edu/ Submitted: July 2018


*Corresponding Author

ACS Paragon Plus Environment


1

ABSTRACT

2

To ensure safer and more efficient unconventional oil/gas recovery and other

3

energy-related subsurface operations, it is important to understand the effects of abundant

4

Fe-bearing phyllosilicates on the degradation of phosphonates, which are applied to inhibit

5

scale formation. In this study, under subsurface relevant conditions (i.e., slightly oxic

6

owing to oxygen-containing injection, 50–95 °C and 102 atm CO2), we reacted 0.5 mM

7

DTPMP (diethylenetriaminepenta(methylene)phosphonate, a model phosphonate) with

8

three phyllosilicates: an Fe-poor muscovite, an Fe(II)-rich biotite, and an Fe(III)-rich

9

nontronite. The three phyllosilicates induced different effects on DTPMP degradation, with

10

no distinguishable effect by muscovite, slight promotion by nontronite, and remarkable

11

promotion by biotite. We found that Fe associated with phyllosilicates is key to the redox

12

degradation of DTPMP: reactive oxygen species (ROS) were generated through the

13

reduction of molecular oxygen by Fe(II) adsorbed on the mineral surface or in the mineral

14

structure, and the hydroxyl radicals further degraded DTPMP to form phosphate, formate,

15

and DTPMP residuals. In addition, DTPMP degradation was favored at higher

16

temperatures, probably due to more exposed reactive Fe(II) sites created by enhanced

17

biotite dissolution and also due to faster electron transfers. Dissolved Fe and Al precipitated

18

with phosphate or degraded DTPMP and formed secondary minerals. This study provides

19

new information about how DTPMP degradation is affected by the presence of Fe-bearing

20

phyllosilicates under high temperature and high pressure conditions and has implications

21

for engineered subsurface operations.

1 ACS Paragon Plus Environment

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22

INTRODUCTION

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Led by new applications of horizontal drilling and hydraulic fracturing, CO2-

24

enhanced unconventional oil and gas recovery from shale reservoirs has been rapidly

25

growing.1-3 Pressurized fracking fluids are pumped into the wellbores to crack the rocks to

26

increase permeability and release natural gas, petroleum, and brine, and allow them to flow

27

more freely.4 To prevent the formation of mineral scales in the wellbores and cracked rocks,

28

which can reduce their porosity and permeability, chemical scale inhibitors are added to

29

the fracking fluids. After scale inhibitors are injected, at a concentration above the

30

minimum inhibitor concentration, they inhibit scale formation by affecting the processes

31

of nucleation or crystal growth of scale minerals.5-7 To maintain an effective concentration

32

of scale inhibitors and ensure their efficacy, it is important to understand their fate and

33

transport under subsurface environments.

34

Phosphonate chelating agents are widely used as scale inhibitors during hydraulic

35

fracturing.6, 8 The average concentrations of scale inhibitors in fracking fluids are reported

36

to be around 0.023% by weight percent (~0.5 mM).9 Phosphonate scale inhibitors contain

37

more than one phosphonate functional group (-PO3H2), and C-P and/or C-N bonds. In

38

general,

39

decomposition.10 However, when phosphonates were complexed with Fe(III),

40

photodegradation occurred under illumination,11-12 but under non-illuminated conditions

41

no degradation was found.13 It is well known that reactive oxygen species (ROS) generated

42

from photolysis are important in affecting the fate of redox-active substances, and that ROS

43

can also be produced in the absence of illumination.14-15 Under non-illuminated conditions,

44

ROS generated from systems containing Cu2+/H2O2 and Fe2+/H2O2 could facilitate the

phosphonates

are

stable

towards

chemical


hydrolysis

and

thermal


45

degradation of phosphonates.16 In this process, phosphate and formate were reported as the

46

two main degradation products of phosphonates. Based on these previous studies, it is clear

47

that the stability of phosphonates greatly depends on the generation of ROS, which is

48

affected by factors, such as light condition, metal ions, and oxygen. Hence, to better

49

understand the stability of phosphonates during engineered subsurface operations, a

50

systematic investigation of phosphonate degradation is needed under conditions relevant

51

to subsurface sites.

52

Subsurface environments are usually dark. The temperature and pressure are high

53

in subsurface sites for energy-related operations, falling in the ranges of 31–110 °C and

54

73.6–600 atm, respectively.17-21 Shales are the main subsurface reservoirs for

55

unconventional oil and gas recovery, and they contain a large quantity of phyllosilicates.2,

56

22

57

oxidation states, are ubiquitous in subsurface systems. Furthermore, the dissolution and

58

secondary precipitation of phyllosilicate minerals can affect the hydrogeological properties

59

of reservoirs, thus affecting the subsurface operations.23-24 Under ambient conditions, the

60

Fe sites in phyllosilicates have been reported to play a critical role in various redox

61

processes, e.g., reduction of chromium(VI)25-27 and nitrate,28-29 and oxidation of inorganic

62

arsenic (III)30 and organic trichloroethylene31 and 1,4-dioxane32. These redox reactions can

63

occur directly through oxidations by Fe(III) and reductions by Fe(II), or indirectly through

64

Fe(II)-catalyzed ROS generation from molecular oxygen. Although subsurface sites are

65

often under reducing conditions with deficient oxygen, molecular oxygen can be

66

introduced into subsurface environments through natural or artificial processes, such as

67

surface water and groundwater interactions or injection of fracking fluids,33-35 altering the

Iron(II/III)-bearing phyllosilicate minerals, with different iron contents and iron


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subsurface redox conditions. It has been reported that CO2 captured from fossil fuel power

69

plants contains up to 6% O2,36-37 and CO2 injection operations can also introduce oxygen

70

into subsurface sites. However, under these conditions, the effects of phyllosilicates on the

71

generation of ROS, and consequently on the stability of phosphonates, are unclear.

72

The objective of this study was to investigate the impacts of Fe-bearing

73

phyllosilicates, with different Fe contents and Fe oxidation states, on the degradation of

74

phosphonates, employing commonly used DTPMP scale inhibitor as a model, under

75

conditions relevant to subsurface environments. We further elucidated the reaction

76

mechanisms. We report here that DTPMP degradation was promoted in the presence of

77

Fe(II)-bearing phyllosilicates through the structural Fe(II)-catalyzed generation of reactive

78

oxygen species from reduction of molecular oxygen. The DTPMP degradation may affect

79

its scale inhibition performance. In addition, significant secondary precipitates were

80

formed. The DTPMP degradation, mineral dissolution, and secondary precipitation may

81

further impact reservoir porosity and permeability. This study provides new findings about

82

the effects of Fe-bearing phyllosilicates on DTPMP degradation under high temperature

83

and high pressure conditions, and has important implications for understanding the fate and

84

transport of chemical additives during engineered subsurface operations and benefiting

85

their sustainable design.



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EXPERIMENTAL SECTION

87

Chemicals and Minerals

88

Technical-grade

diethylenetriaminepenta(methylene)phosphate

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(DTPMP,

89

C9H28N3O15P5) was obtained from Sigma-Aldrich. The active concentration of DTPMP in

90

the purchased chemical is 50% by weight, and there is 15% HCl (hydrochloric acid) and

91

35% H2O (water). To control the ionic strength and mimic the high salinity in subsurface

92

sites,38 reaction solutions of 0.5 mM DTPMP were prepared in 0.5 M NaCl.

93

Biotite and muscovite purchased from Ward’s Natural Science, and nontronite

94

(NAu-1) purchased from the Source Clays Repository of the Clay Minerals Society, were

95

used to examine the effects of the Fe contents and Fe oxidation states in mineral structure

96

on DTPMP degradation. They are all phyllosilicates, having 2:1 layer structures composed

97

of two opposing tetrahedral sheets and an octahedral sheet with an interlayer filled by

98

cations to balance the charge. However, their Fe contents and Fe oxidation states are

99

different and were characterized as described later. To remove organic matter, the minerals

100

were sonicated in acetone, ethanol, and isopropanol respectively for 5 min, washed with

101

deionized (DI) water (resistivity > 18.2 MΩ·cm, Barnstead Ultrapure Water Systems), and

102

dried with high purity nitrogen gas. Then they were ground with stainless grinders, and

103

particles between 53–106 µm were isolated by sieves for use in further experiments. The

104

specific surface areas measured by the Brunauer-Emmett-Teller (BET) method were 2.99

105

m2/g, 2.49 m2/g, and 75.1 m2/g for biotite, muscovite, and nontronite, respectively. The

106

chemical composition of the biotite was analyzed with X-ray fluorescence (XRF) and

107

found to be K0.91Na0.08Ca0.05Mg1.72Mn0.06Fe1.12Ti0.12Al1.00Si2.98O10(F0.51(OH)0.49)2 (Table

108

S1). The empirical chemical formula for muscovite is KAl2(AlSi3O10)(F,OH)2; for


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109

nontronite it is Na0.3Fe2(AlSi3O10)(OH)2. The contents of Al, Si, and Fe in muscovite and

110

nontronite were measured with scanning electron microscopy with energy-dispersive X-

111

ray microscopy (SEM-EDX, Nova NanoSEM 230) and are shown in Table S2.

112

A modified digestion-ferrozine method was used to characterize the extents of Fe(II)

113

and Fe(III) in the phyllosilicates used in this study.39-41 A mass of 0.2 g minerals was

114

digested in 30 mL of 9 M HCl for 2 h. The sample was then filtered, and the filtrate was

115

diluted to 40 mL. Taking 0.1 mL of the diluted solution to 4.9 mL of ferrozine solution, we

116

performed ultraviolet-visible spectroscopy (UV-Vis, Thermo Scientific Evolution 60S) to

117

measure the absorbance at 562 nm, which indicated Fe(II). Another 10 mL of the diluted

118

solution was mixed with 1 mL hydroxylamine (NH2OH) (100 g/L), which reduced Fe(III)

119

to Fe(II), and was boiled for 10 min. After the solution cooled, the Fe(II) was again

120

quantified by the UV-Vis method as described above, which indicated the total Fe content.

121

The detailed process and results can be found in S2 in the Supporting Information.

122

DTPMP degradation experiments

123

DTPMP degradation experiments were conducted in a high temperature and high

124

pressure reactor (Parr Instrument Co., IL, Figure S1). The experimental setup allowed in

125

situ sampling after reaction for elapsed times of 1, 3, 5, 10, and 22 h. In a 300 mL Teflon

126

cup, prepared phyllosilicate powders (0.0375 g) were mixed with 150 mL of 0.5 mM

127

DTPMP solution, with a solid to liquid ratio of 0.25 g/L. The mixtures were prepared in an

128

open system, allowing air to dissolve. The measured dissolved oxygen concentration was

129

around 8.9 mg/L. Next, the Teflon cup was sealed in a stainless reactor. In this way, the

130

reaction mixture did not have access to light and photodegradation was halted. The reaction

131

suspensions were stirred during the experiments to obtain a well-mixed system. To



132

examine the effects of temperature on the DTPMP degradation in the presence of biotite,

133

temperature conditions of 50 °C, 70 °C, and 95 °C were used. The pressure was set at 102

134

atm CO2 to simulate the high pressure in subsurface operations. The initial pH of the

135

solutions was adjusted to pH 5.6 under ambient conditions prior to experiments. Right after

136

adjusting the temperature and CO2 pressure to be 95 °C and 102 atm CO2, the in situ pH

137

became around 3.2 (details in S4). The initial pH was within the pH ranges in subsurface

138

sites for engineered operations.24 Control experiments were performed without adding any

139

minerals.

140

To prepare reacted biotite samples for better characterization of surface

141

morphology alteration, DTPMP degradation experiments were also conducted with clean

142

1.0 cm × 0.8 cm biotite flakes of with a thickness of 80 ±10 m at 50 °C, 70 °C, and 95

143

°C, all under 102 atm of CO2.

144

Characterization of aqueous solutions and minerals

145

Aqueous samples were filtered along the reaction course. The concentrations of

146

phosphate and formate, two of the degradation products,13 over time were monitored by

147

ion chromatography (IC, Thermo Scientific Dionex ICS-1600). The concentrations of

148

aqueous cations released from mineral dissolution were measured with inductively coupled

149

plasma-optical emission spectrometry (ICP-OES, PerkinElmer Optima 7300 DV).

150

After 22 h reaction, the reactor was allowed to cool and degas over 30 min. Previous

151

studies have reported that the degassing process did not change the aqueous solutions and

152

secondary minerals.42-43 The solutions were centrifuged at 5000 rpm for 5 min. The

153

supernatant, containing dissolved ionic species, was discarded, then 40 mL of DI water

154

was added, and the suspension was centrifuged again. The process was repeated for 10


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155

times to remove ionic species and wash the mineral powders to prevent secondary

156

precipitation after reactions. The minerals were collected, oven-dried overnight, and

157

analyzed with SEM. A 10.00 kV electron accelerating voltage was used for the

158

measurements. The elemental compositions of the minerals were analyzed with energy-

159

dispersive X-ray spectroscopy (EDX) during the SEM measurements. The reacted biotite

160

flakes were also characterized with SEM to assess the surface morphology changes. The

161

phyllosilicate minerals before reaction were measured for comparison (Figure S2A).

162

Testing for reactive oxygen species

163

Tert-butyl alcohol (TBA) was used to scavenge hydroxyl radicals during DTPMP

164

degradation.44 The boiling point of TBA is around 82 °C under ambient conditions. To

165

retain TBA in our reaction solutions, DTPMP degradation experiments with TBA were

166

conducted at 70 °C and 102 atm of CO2. The concentration of the TBA scavenger added

167

was 50 mM. After desired reaction times, aqueous samples were taken for measurements

168

of phosphate and formate concentrations.

169

To check if superoxide (O2·-) was formed during the reaction, nitro blue tetrazolium

170

(NBT), with a concentration of 0.1 mM, and TBA, with a concentration of 50 mM, were

171

added together into the biotite reaction suspension (0.25 g/L) at 70 °C and 102 atm CO2.45

172

After 22 h reaction, the suspension was sampled and scanned from 400 nm to 800 nm by a

173

UV-Vis spectrometer under ambient conditions. The measurement was based on

174

spectrophotometric measurements quantifying the reduction of yellowish NBT by

175

superoxide into purple formazan derivatives.45 By adding TBA to remove hydroxyl

176

radicals, we could ensure that the reduction of NBT was caused solely by reaction with

177

superoxide in the system.



178

To test for hydrogen peroxide (H2O2), biotite powders were first reacted in 0.5 mM

179

DTPMP solution with a solid to liquid ratio of 0.25 g/L at 95 °C under ambient pressure,

180

to allow more convenient sampling for further measurements. Then, under ambient

181

conditions, a photometric method was used to determine the presence of H2O2.46 After 22 h

182

reaction, 2.7 mL of filtered reaction solution was mixed with 0.3 mL of buffer solution,

183

prepared by mixing 0.5 M Na2HPO4 and 0.5 M NaH2PO4, to achieve pH 6. Next, 50 µL of

184

DPD (N,N-diethyl-1,4-phenylenediammonium sulfate) reagent solution and 50 µL of POD

185

(peroxidase) reagent were added. The DPD reagent was prepared by adding 0.1 g of DPD

186

to 10 mL 0.1 M H2SO4, and the POD reagent was prepared by dissolving 10 mg of POD

187

in 10 mL DI water. The absorption spectrum of the mixed solution was scanned from 400

188

nm to 600 nm by a UV-Vis spectrometer. The basic principle of the test is that H2O2 can

189

oxidize DPD catalyzed by POD to form DPD·+, which has two absorption maximums at

190

around 510 nm and 550 nm.46

191

RESULTS AND DISCUSSION

192

Effects of phyllosilicates on DTPMP degradation

193

The three types of phyllosilicates induced different extents of DTPMP degradation.

194

The concentrations of phosphate and formate after DTPMP reaction with phyllosilicates at

195

95 °C are shown in Figure 1. In the control experiment without adding any minerals, the

196

phosphate and formate concentrations did not change much with time, indicating DTPMP

197

was stable during the experimental period. Compared with the control experiments, the

198

effects of muscovite and nontronite on DTPMP degradation were not obvious, and similar

199

or slightly higher aqueous concentrations of phosphate and formate were observed.

200

However, in the biotite system, the evolutions of the aqueous concentrations of phosphate 9 ACS Paragon Plus Environment

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and formate were much faster than those in the control system, with increases of around

202

0.3 mM phosphate and 0.2 mM formate after 22 h reaction, corresponding to 60%

203

degradation if one DTPMP releases one phosphate. This difference confirms that DTPMP

204

degradation was significantly promoted by biotite. Note that even before the experiments,

205

the concentrations of phosphate and formate were not zero and were around 0.4 mM and

206

0.04 mM in the prepared solutions, respectively. The initial phosphate and formate

207

concentrations were probably impurities introduced during DTPMP production. However,

208

because the same initial DTPMP concentrations, and thus the same initial phosphate and

209

formate concentrations, were prepared for all the degradation experiments with different

210

phyllosilicates, the comparison of the concentration evolutions of phosphate and formate

211

are valid. In sum, the effects of the three phyllosilicates on DTPMP degradation varied:

212

biotite significantly promoted the degradation of DTPMP, nontronite showed slight

213

promotion, and muscovite barely affected DTPMP degradation.

214

Regarding the mechanisms of the different effects of phyllosilicates, we considered

215

the roles of the contents and oxidation states of Fe sites in phyllosilicate structures in

216

DTPMP degradation. The total Fe contents by weight percent in biotite, muscovite, and

217

nontronite were characterized to be approximately 15%, 0.13%, and 21%, respectively

218

(Table S3). The Fe in biotite structure was determined by the ferrozine method to be mainly

219

Fe(II) (14%), while nontronite contained almost all Fe(III) in its structure. Although

220

nontronite contains the highest amount of Fe, and the Fe is in its oxidized form, nontronite

221

was much less effective in promoting DTPMP degradation than biotite. During the high

222

temperature and high pressure reactions, biotite dissolved and released Fe(II) into the

223

aqueous solutions (Figure 2). It was possible that aqueous Fe(II) or Fe(II) that adsorbed on



224

mineral surfaces affected DTPMP degradation. Therefore, the effects of aqueous Fe(II) and

225

adsorbed Fe(II) on muscovite on DTPMP degradation were also tested (Figure 1). To make

226

the aqueous conditions comparable with the DTPMP-biotite reaction system and to have

227

enough aqueous Fe(II) or adsorbed Fe(II), the concentration of Fe(II) added was about

228

twice the highest aqueous Fe concentration in the DTPMP-biotite reaction system (i.e., 0.3

229

mM Fe(II)). The results showed that aqueous Fe(II) did not affect the degradation of

230

DTPMP. The released phosphate might precipitate with Fe(II) added,43 and so it was

231

difficult to characterize the amount of Fe(II) adsorbed onto muscovite surfaces. However,

232

it is clear that Fe(II) associated with muscovite promoted DTPMP degradation. We

233

hypothesized that Fe(II) associated with mineral structure was key to promoting DTPMP

234

degradation. It has been reported that, under ambient conditions, abundant ROS were

235

generated from oxygenation of phyllosilicates containing Fe(II), and hydroxyl radicals

236

could degrade organic compounds.31-33, 47 Hence, ROS could have also been generated in

237

our reaction systems and involved in the degradation of DTPMP.

238

DTPMP oxidation by reactive oxygen species

239

No increases in phosphate and formate concentrations over time were observed

240

with 50 mM TBA, as shown in Figure 3. The presence of TBA scavenger removed ·OH in

241

the reaction systems and thus inhibited the degradation of DTPMP. This finding suggests

242

that ·OH was generated in the reaction system and played the dominant role in DTPMP

243

degradation. Note that the high concentration of Cl−, used in this study to simulate the high

244

salinity in subsurface brine, might react with ·OH to form reactive halogen species (e.g.,

245

Cl·and Cl2·−).48-49 Cl− might remove some of the ·OH generated in the biotite reaction

246

system. However, the reactive chloride species have second-order rate constants (k) with 11 ACS Paragon Plus Environment

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most organic compounds within the range of 103–108 M-1s-1, and ·OH is more reactive in

248

terms of redox potential and typical electron transfer rates, which are most often in the

249

diffusion-controlled regime (k=109–1010 M-1s-1).50-51 Therefore, reaction of ·OH with Cl−

250

would reduce ·OH concentration in the systems and decrease the efficacy of DTPMP

251

degradation. Even so, we did observe DTPMP degradation by ·OH (Figure 1 and Figure

252

3), indicating that there were sufficient ·OH which were not fully removed by Cl −

253

competing with DTPMP.

254

Molecular oxygen can be reduced stepwise through one-electron transfer to form

255

ROS, including superoxide (O2·-), hydrogen peroxide (H2O2), and hydroxyl radical (·OH),

256

with the first step of the single electron transfer to molecular oxygen being

257

thermodynamically unfavorable.52-53 It has also been reported that catalysis by Fe(II) in

258

pyrite (FeS2) can allow two electrons to be added to molecular oxygen in one step, leading

259

to the formation of H2O2 and avoiding the endergonic first step of forming O2·-.54-55 The

260

involvement of O2·- radicals was tested in the presence of both 0.1 mM NBT and 50 mM

261

TBA. We observed that the color of the suspension changed from yellowish before reaction

262

to purple after 22 h of reaction (Figure S3), indicating the reduction of NBT to formazan.

263

The absorption band in the range of 450–800 nm corresponded to formazan from NBT

264

reduction, but NBT was not reduced by 0.5 mM DTPMP without biotite (Figure 4A). The

265

results suggested the generation of O2·- in the biotite suspension, indicating that different

266

from pyrite, molecular oxygen was reduced by the one-electron transfer mechanism to form

267

O2·- in the biotite system. Additionally, we observed two absorption peaks at around 510

268

nm and 550 nm from the UV-Vis spectra for the reaction suspension mixed with DPD and

269

POD (Figure 4B). These two peaks were consistent with two absorption maxima of DPD·+,



270

formed from DPD oxidation by H2O2 in the presence of POD, a catalyst.46 There was no

271

absorption peak at 550 nm in the spectrum for the reaction solution mixed with DPD only,

272

although there was absorbance at around 510 nm. The peak at 510 nm might be due to DPD

273

oxidation by ROS (e.g., O2·-, H2O2, and ·OH),56-57 but the oxidation did not proceed all the

274

way to forming DPD·+. Further DPD oxidation by H2O2 in the presence of POD could

275

finally result in a new absorbance at 550 nm and the decreasing absorbance at 510 nm.

276

Therefore, in the biotite system, both O2·- and H2O2 were intermediates in the production

277

of ·OH from reduction of O2 by biotite. From the DTPMP degradation results with TBA

278

scavenging ·OH, both O2·- and H2O2 did not affect DTPMP degradation. Moreover, recent

279

studies reported that Fe(II), either as a structural component of silicate minerals or as

280

adsorbed Fe(II), shifts the redox potential of the ferrous/ferric couple downward, making

281

it a better electron donor.58-62 Consistent with the references, we found that aqueous Fe(II)

282

did not affect DTPMP degradation (Figure 1), further suggesting that Fe(II) associated with

283

phyllosilicate structures is more effective in transforming O2 to ·OH.

284

Therefore, by a one-electron transfer mechanism, molecular O2 was reduced to

285

form O2·- and H2O2, and then ·OH through reduction by Fe(II) associated with minerals

286

(biotite structural Fe(II), adsorbed Fe(II), or neoformed Fe(II) minerals). Hydroxyl radicals

287

were responsible for the degradation of DTPMP. Previous studies reported that cleavages

288

of C-N and/or C-P bonds occurred during phosphonate degradation by hydroxyl radicals,16,

289

63-65

290

abstraction mechanisms for the hydroxyl radical-mediated degradation. Although the goal

291

of this study was not to examine the detailed intermediate products of DTPMP degradation,

292

the release of phosphate and formate suggested that the degradation of DTPMP occurred

and they proposed electron abstraction, hydroxyl radical addition, and hydrogen


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293

with the cleavages of both C-N and C-P bonds. The DTPMP structure has five phosphonate

294

groups, and thus, to identify which specific location of phosphonate group degraded first

295

or to discuss the detailed degradation pathways, a more dedicated future study is needed.

296

Coupling biotite dissolution and DTPMP degradation at different temperatures

297

To further couple DTPMP degradation and biotite dissolution, DTPMP degradation

298

experiments with biotite were conducted at 50 °C, 70 °C, and 95 °C. With biotite, at 50 °C,

299

the evolutions of the phosphate and formate concentrations were very slow, while faster

300

increases of phosphate and formate concentrations were observed at higher temperatures

301

(Figure 3). The results suggested that higher temperature favored DTPMP degradation with

302

biotite. In addition to DTPMP degradation, biotite dissolution was monitored during the

303

experiments.

304

With DTPMP, increasing aqueous concentrations of K, Si, and Mg (Figure 2) were

305

observed by ICP-OES at higher temperatures, indicating promoted biotite dissolution at

306

higher temperatures. However, at 70 °C and 95 °C, the aqueous concentrations of Al and

307

Fe first increased to around 90 µM and 150 µM, respectively, and then decreased. The

308

decreases of Al and Fe concentrations were probably due to secondary precipitation of Al-

309

and Fe-bearing secondary minerals. As confirmed by the SEM images in Figure 5A, after

310

22 h reaction at 70 °C and 95 °C, aggregated precipitates (indicated by the red box) were

311

formed with sizes smaller than the original biotite powders (Figure S2) were formed. The

312

SEM images show both homogeneous precipitates, formed in the reaction solution, and

313

precipitates on biotite surfaces, formed either heterogeneously or by attachment of

314

homogeneous precipitates. Characterized by EDX, the reacted biotite powders (yellow box

315

in Figure 5A) had an elemental composition similar to unreacted biotite, but P was 14 ACS Paragon Plus Environment


316

observed, indicating the adsorption of DTPMP or its degradation products. The secondary

317

precipitates were abundant in P, Fe, and Al (Figure S4). Thermodynamic calculations of

318

secondary precipitation and solute speciation were not made because detailed

319

thermodynamic data for DTPMP is not available. However, as reported in recent studies

320

of DTPMP/phosphate−biotite interactions, the homogeneous precipitates could be

321

amorphous P-, Fe-, and Al-bearing minerals, and the heterogeneous precipitates were

322

probably strengite (FePO4·2H2O), berlinite (AlPO4), and gibbsite (Al(OH)3).43, 66 It was

323

also possible that DTPMP or degraded DTPMP was incorporated into the precipitates as

324

organic templates for the secondary precipitation. However, in the muscovite and

325

nontronite reaction systems, after 22 h reaction, there was no significant secondary

326

precipitation compared to the biotite system (Figure S2).

327

To better capture the effects of dissolution on biotite surface morphology changes,

328

reacted biotite flakes were measured with SEM (Figure 5B). Consistent with our previous

329

studies,42-43, 66-67 cracks were formed on reacted biotite basal surfaces, and cracked layers

330

bent outwards, probably due to the adsorption of large DTPMP molecules. At elevated

331

temperatures, enhanced biotite dissolution kinetics induced deeper and more abundant

332

cracks after biotite reacted with DTPMP (Figure 5B), exposing inner surface sites. The

333

differences in the extent of secondary precipitation in the biotite powder and biotite flake

334

reaction systems could result from biotite dissolution. Biotite powders possess a much

335

higher surface area than biotite flakes, exposing more surface sites (especially more

336

reactive edges42) to contact with solutions. In our experiments, the result was more biotite

337

dissolution66 in the powder system and higher saturations of secondary minerals. However,


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Page 17 of 34


338

the trends of promoted biotite dissolution and the formation of deeper and more abundant

339

cracks at higher temperatures should be the same for biotite powders and flakes.

340

Figure 6 provides a schematic diagram of proposed reaction pathways for coupling

341

biotite dissolution and secondary precipitation with DTPMP degradation. To relate the

342

DTPMP degradation to biotite dissolution and secondary precipitation, the following

343

aspects were considered: First, faster biotite dissolution at higher temperatures could create

344

deeper and more abundant cracks and expose Fe(II) sites. Although we did not measure

345

the time-course change of ROS, which was not our focus, the exposure of Fe(II) sites could

346

facilitate the generation of ROS through interactions between Fe(II) associated with biotite

347

and molecular oxygen, and thus promoted DTPMP degradation. It has been reported that

348

electron transfers between phyllosilicate structural Fe(II) sites and redox-active substances

349

can occur at both basal planes and edges.47, 68 The enhanced DTPMP degradation at higher

350

temperature indicated that the electron transfers probably occurred through edge surfaces

351

reacting with oxygen to generate ROS. Second, a higher temperature could favor electron

352

transfer kinetics69 within the biotite’s structure and between Fe(II) sites and oxygen to

353

produce ROS, which could then further react with DTPMP. The slow DTPMP degradation

354

with biotite at 50 °C could result from limited electron transfers or limited reactive Fe(II)

355

sites owing to slow biotite dissolution and less formation of cracks. Originally, biotite

356

contained mainly Fe(II), and there were little to no Fe(II)-O-Fe(III) linkages for

357

transferring electrons from inner to edge surface sites along the octahedral sheet. During

358

the reaction, once surface Fe(II) sites were oxidized, Fe(II)-O-Fe(III) linkages could be

359

formed and the inner Fe(II) could serve as an electron pool to regenerate the Fe(II) on the

360

edges.47 Third, secondary precipitation of cations with phosphate released from DTPMP



361

degradation could cause forward biotite dissolution and facilitate the exposure of more

362

Fe(II) sites, thus enhancing DTPMP degradation by hydroxyl radicals.

363

IMPLICATIONS FOR SUBSURFACE ENVIRONMENTAL REACTIONS

364

Although degradation of various chemical compounds by ROS produced from

365

phyllosilicates under ambient conditions has been recently reported, this study focused on

366

engineered subsurface operations with characteristic high temperature and high pressure

367

conditions. Here, we provide new insights into the fate and transport of phosphonates used

368

in engineered subsurface operations by studying the effects of different phyllosilicates on

369

DTPMP phosphonate degradation and the mechanisms involved. The degradation of

370

phosphonates promoted by biotite will reduce their available concentrations in subsurface

371

sites, thus affecting their performances in mineral scale inhibition. Moreover, significant

372

secondary precipitation may clog pores, reducing the porosity and impacting the

373

permeability of wellbores and reservoir rocks and influencing the transport of reactive

374

fluids. Although we could not eliminate the possibility of DTPMP degradation by

375

nontronite and muscovite, even if not all the way to phosphate and formate, secondary

376

precipitation was minor in these two systems. The differences in secondary precipitation

377

caused by phyllosilicate-DTPMP interactions suggest that the porosity and permeability

378

would be altered to different extents.

379

Phyllosilicates are abundant in shales, which are the main reservoirs for

380

unconventional oil/gas recovery. The reported generation of reactive oxygen species by the

381

interaction between oxygen and Fe(II) sites associated with phyllosilicates under high

382

temperature and high pressure conditions suggests that we need to pay more attention to

383

the role of Fe-bearing phyllosilicates, especially the contents and oxidation states of Fe in 17 ACS Paragon Plus Environment

Page 18 of 34

Page 19 of 34


384

phyllosilicates, during the engineered processes. Although subsurface environments have

385

low access to O2, a small amount of O2 can be imported to subsurface sites by surface water

386

and groundwater interactions or by engineered subsurface processes. The generation of

387

reactive oxygen species through oxygenation of Fe(II)-containing minerals may affect the

388

fate and transport of redox-active substances in subsurface sites for energy-related

389

engineered operations. Additionally, Fe-reducing bacteria can reduce Fe(III) in minerals to

390

Fe(II),70-71 making Fe(III)-bearing phyllosilicates also effective in the transformation of

391

redox-active substances.

392

SUPPORTING INFORMATION

393

Details on the elemental compositions of phyllosilicates (S1), characterization of

394

Fe(II) and Fe(III) in phyllosilicates (S2), the high temperature and high pressure stirring

395

system (S3), in situ pH measurements and thermodynamic calculations (S4), SEM

396

measurements (S5), Images of samples for UV-Vis measurements to test the presence of

397

superoxide (S6), and EDX measurements (S7), mass balance calculation of oxygen

398

consumption by DTPMP degradation (S8), effects of pH drifts (S9).

399

ACKNOWLEDGMENTS

400

This work was supported by the Center for Nanoscale Controls on Geologic CO2,

401

an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of

402

Science, Office of Basic Energy Sciences, via Grant DE-AC02-05CH11231. We also thank

403

the National Science Foundation’s CAREER Award (EAR-1057117). The authors

404

acknowledge Washington University’s Nano Research Facility for use of SEM



405

and ICP-OES. We also thank Professor James Ballard for carefully reviewing our

406

manuscript.


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Page 21 of 34


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List of Figures Figure 1

Aqueous concentrations of phosphate and formate resulting from DTPMP degradation, at a salinity of 0.5 M NaCl and initial DTPMP concentration of 0.5 mM at T = 95 °C and PCO2 = 102 atm. Error bars are the standard deviation of triplicate experiments.

Figure 2

Dissolved concentrations of K, Si, Mg, Al, and Fe from biotite powders after reaction in 0.5 M DTPMP solution with a salinity of 0.5 M NaCl at different temperatures (50, 70, and 95 °C) and 102 atm of CO2. Error bars are the standard deviation of triplicate experiments.

Figure 3

Aqueous concentrations of phosphate and formate from DTPMP degradation at different temperatures (50, 70, and 95 °C) with a salinity of 0.5 M NaCl and initial DTPMP concentration of 0.5 mM at 102 atm of CO2 in the presence of biotite. TBA was added as scavenger for hydroxyl radicals during DTPMP degradation at 70 °C. Error bars are the standard deviation of triplicate experiments.

Figure 4

(A) UV-Vis absorption spectra obtained for 0.5 mM DTPMP after reaction with 0.1 mM NBT with/without biotite at 70 °C and 102 atm of CO2. 50 mM TBA was added to scavenge hydroxyl radicals in the with biotite system. Samples were not filtered because reduction product of NBT strongly adsorbed onto the biotite surface. The yellow shadow in (A) indicates the absorption band of NBT reduced by O2·-. (B) UV-Vis absorption spectra of the filtered solution of 0.5 mM DTPMP, after 22 h reaction with biotite powders under 95 °C and ambient


Page 26 of 34

Page 27 of 34


pressure, mixed with DPD and/or POD reagents, and the spectrum of the unreacted biotite suspension. The two arrows in (B) indicate the absorption maximums of DPD oxidized by H2O2 and catalyzed by POD. Figure 5

SEM measurements of samples collected after 22 h reaction in 0.5 M DTPMP solution with biotite powders (A) and biotite flakes (B) at 102 atm of CO2 and different temperatures (50, 70, and 95 °C). The EDX results of the spots indicated by the yellow and red boxes are shown in Figure S4.

Figure 6

Schematic diagram of coupling DTPMP degradation with biotite dissolution and secondary precipitation.



Table of Content (TOC)


Page 28 of 34


Phosphate (mM)

0.8 0.7

0.27

No mineral Muscovite Biotite Nontronite 0.3 mM Fe(II) 0.3 mM Fe(II)+Muscovite

Formate (mM)

Page 29 of 34

0.6 0.5 0.4 0.3 0

5 10 15 20 Reaction Time (h)

0.18 0.09 0.00 0

25

No mineral Muscovite Biotite Nontronite 0.3 mM Fe(II) 0.3 mM Fe(II)+Muscovite


Figure 1


25


Figure 2


Page 30 of 34

Page 31 of 34


0.2

0.8

50 oC 70 oC 90 oC 70 oC_TBA

Phosphate (mM)

Formate (mM)

0.3

0.1 0.0 0


0.7 0.6 0.5 0.4 0.3 0

25

50 oC 70 oC 95 oC 70 oC_TBA

Figure 3



25


Figure 4


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Figure 5



Figure 6


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The Role of Fe-Bearing Phyllosilicates in DTPMP Degradation under

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