Development of functionalized proppant for the control of NORM in

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Development of functionalized proppant for the control of NORM in Marcellus Shale produced water Alen Gusa, and Radisav D. Vidic Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05710 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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Environmental Science & Technology

1

Development of functionalized proppant for the control of NORM

2

in Marcellus Shale produced water

3

Alen V. Gusa and Radisav D. Vidic*

4

Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA 15261

5

*Corresponding author

6 7

Abstract

8

One of the major environmental concerns with the recovery of unconventional gas resource

9

from Marcellus Shale is the presence of Naturally Occurring Radioactive Material (NORM) in

10

produced water. Ra-226 is the major component of NORM with a half-life of 1,600 years that is

11

present at concentration as high as several thousand pCi/L. Most of the studies on NORM

12

management are focused on above-ground scenarios. The main focus of this study was on

13

functionalizing the proppant (i.e., quartz sand) that is used in hydraulic fracturing to prevent the

14

closure of induced fractures formed during this process and allow release of natural gas so that it

15

can also sequester NORM from the produced water before it reaches the surface. Five different

16

sulfates and carbonates were tested for their ability to capture Ra-226 from aqueous solution and

17

celestite (SrSO4) was identified as the best choice because of its affinity for Ra-226 sequestration

18

even in the presence of very high total dissolved solids that are characteristic of Marcellus Shale

19

produced water. Among possible ways of coating the proppant with celestite, precipitating celestite

20

directly on the sand surface was found to be the best option as it provided uniform distribution of

21

celestite and high uptake of Ra-226. Although quartz sand can adsorb some radium from the

22

solution due to electrostatic interactions, adding a small amount of celestite on the sand surface

23

(20-30 mg/g) increased radium removal from the solution containing 5,000 pCi/L of Ra-226 to

24

more than 80% in dilute solution and to more than 50% in high salinity solution even in the 1 ACS Paragon Plus Environment


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presence of very high concentrations of competing divalent cations. The results of this study

26

indicate the potential of coated proppant to sequester NORM in the subsurface and prevent adverse

27

environmental impacts when radiogenic produced water is brought to the surface.

28 29

INTRODUCTION

30

Natural gas can be found in both conventional and unconventional reservoirs. The

31

extraction of natural gas from geological formations with low permeability (i.e., unconventional

32

reservoirs) is made economical1 with the development of new technologies and advances in

33

processes, such as horizontal well drilling and reservoir stimulation. During hydraulic fracturing,

34

water mixed with a proppant and chemical additives is injected into the formation at high pressures

35

(i.e., 480-850 bar2) to enlarge existing fractures and create new ones. Stimulation of a single well

36

requires 5-7 million gallons of water and 2,000 tons of proppant.3 Several different proppants are

37

used in this process with sand, resin-coated sand and ceramic proppants being the most common.

38

The proppant makes only about 9% of the total weight of the fracturing fluid4, 5 and is essential to

39

prevent the collapse of microfractures that are formed during the hydraulic fracturing process and

40

enable efficient extraction of natural gas.

41

One of the major environmental concerns with hydraulic fracturing is the very high salinity

42

of flowback water generated during several weeks after the drilling process4 and produced water

43

generated during the life of the well. Total dissolved solids (TDS) in this water are mainly

44

comprised of sodium and chloride as well as alkaline earth metals, such as strontium, barium,

45

calcium and magnesium, and can range from 92-308 g/L6 depending on the location and life of the

46

well. In the case of Marcellus Shale formation in Pennsylvania, the average TDS is 106 g/L.2

47

However, high salinity is not the only concern with produced water as it also contains very high

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concentration of Naturally Occurring Radioactive Material (NORM). Marcellus Shale is the most

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radiogenic formation among all shale plays in the US7 and the produced water from Marcellus

50

wells contains Ra-226 and Ra-228 as decay products of U-238 and Th-232, respectively. Due to

51

low concentration of Th-232 in organic rich shale, Ra-228 usually represents 10-15% of the total

52

NORM in the produced water.7-9 Ra-228 also has a shorter half-life than Ra-226 (5.75 vs 1,600

53

years) and most of the focus in previous studies has been on Ra-226.10-13 Radium content in

54

produced water is a function of a local lithology and its total activity (i.e., combined Ra-226 and

55

Ra-228 activity) can reach as high as 20,000 pCi/L, with a median value of 2,460 pCi/L for the

56

Marcellus Shale in Pennsylvania and 5,490 pCi/L for the Marcellus Shale in New York.7 The

57

existing regulatory limit for radium activity in industrial effluents and drinking water is 60 and 5

58

pCi/L, respectively.14 Radium can cause major environmental and health problems4,

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appropriate management of flowback and produced water is necessary to prevent environmental

60

impacts and enable further development of this industry.

15

and

61

Current strategies for produced water management include reuse, disposal into Class II

62

Underground Injection Control (UIC) wells, or appropriate treatment prior to disposal.16, 17 Reuse

63

is the most effective option for managing this water in Pennsylvania but it may be of limited value

64

depending on the drilling schedule and field maturity. The lack of disposal wells in Pennsylvania18

65

and risks associated with induced seismicity motivated the development of treatment technologies

66

for this water. One of the most cost-effective strategies for the control of radium is co-precipitation

67

with different sulfates, most commonly with barium sulfate (barite). Zhang et al. showed that

68

radium co-precipitation with both barium sulfate (barite) and strontium sulfate (celestite) can be

69

utilized for NORM removal from produced water.11, 19 Several studies evaluated radium removal



70

by preformed barite where radium uptake occurs during barite dissolution and re-crystallization

71

over a long period.13, 20, 21

72

Contaminant removal from aqueous solutions based on impregnating different surfaces

73

with inorganic materials capable of adsorbing pollutants has been investigated previously. Vaisha

74

and Gupta evaluated arsenic(III) and arsenic(V) removal from solution by barium sulfate modified

75

iron oxide coated sand.22, 23 They showed that this coating complex can significantly enhance

76

arsenic removal and analyzed the impact of process parameters (i.e., alkalinity, pH, temperature

77

and surface area) on the kinetics and equilibrium of arsenic uptake. Arsenic removal by iron-oxide

78

coated polymeric surface was investigated by Katsoyiannis et al.24 while iron-oxide and

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manganese-oxide coated sand were used for arsenic removal from ground water.25, 26 Han et al.

80

investigated the ability of manganese-oxide coated sand to remove heavy metals like Cu2+ and

81

Pb2+ from aqueous solution.27,

82

polyurethane resin coated proppants for radium removal under various conditions.29 Similar idea

83

of NORM removal using resin coated proppant by “in-situ” approach was investigated by

84

McDaniel et al.30

28

Goyal et al. demonstrated the potential of using existing

85

The objective of this study was to develop a functionalized proppant that can be injected

86

in the shale formation to sequester radium from the produced water before it reaches the surface.

87

Although the solid solution theory31 is typically used to analyze co-precipitation equilibrium

88

(Equations S1-S4 in Supporting Information), very high theoretical distribution coefficient for

89

radium-strontium sulfate (Kd=237)11 suggests that strontium sulfate may be used as an effective

90

sequestering agent for radium even if celestite is already present in solid phase. In this study,

91

celestite was compared with other sulfates (i.e., barium sulfate) and carbonates (i.e., strontium

92

carbonate, barium carbonate and calcium carbonate) for its ability to sequester radium from


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aqueous solution when it is dispersed in solution or when it is impregnated on the surface of quartz

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sand that is typically used as proppant in hydraulic fracturing operations in Marcellus Shale.32

95 96

MATERIALS AND METHODS

97

Concentrated RaCl2 solution obtained from the Pennsylvania State University was used to

98

prepare all solutions in this study. Gamma spectroscopy (Canberra BE 202) was used to determine

99

the activity of Ra-226 in the stock solution of 1.44 mCi/L, which was then diluted with deionized

100

(DI) water (Synergy, Millipore, Billerica, MA) to achieve initial Ra-226 activities ranging from

101

5,000-7,750 pCi/L. Strontium sulfate, barium sulfate, strontium chloride hexahydrate, sodium

102

chloride and sodium sulfate decahydrate were purchased from Fischer Scientific (Pittsburgh, PA)

103

and calcium carbonate, strontium carbonate, barium carbonate and quartz sand (50-70 U.S. Mesh

104

size with average particle size of 250 m) were purchased from Sigma Aldrich (St. Louis, MO).

105

All chemicals were analytical grade.

106 107

Adsorption experiments

108

Adsorption experiments were first performed to evaluate the ability of five common

109

minerals of alkaline earth metals (i.e., SrSO4, BaSO4, CaCO3, SrCO3 and BaCO3) to sequester

110

radium from dilute aqueous solutions. The solids at concentrations ranging from 100 to 2,000 mg/L

111

were dispersed in 50 mL of 5,000 pCi/L Ra-226 solution for 1 hour using the sonicator (Aquasonic,

112

West Chester, PA). Dispersed samples were transferred to 50 mL Falcon polypropylene conical

113

centrifuge tubes (Fisher Scientific, Pittsburgh, PA) and mixed for 24 hours in a horizontal shaker

114

(Darts Control Inc., Zionsville, IN) at 25 rpm to achieve adsorption equilibrium. A blank sample

115

without solids was also mixed for 48 hours to confirm that no Ra-226 was adsorbed to centrifuge



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tube walls. Each sample was filtered through 0.45m mixed cellulose ester membrane (Millipore,

117

Billerica, MA) and analyzed using Liquid Scintillation Counter (LSC, LS 6500, Beckman Coulter,

118

Brea, CA). Equilibrium adsorption capacity for Ra-226 was calculated as:

119

q=

(C0 ― Ce)V m

(1)

120

where, q (pCi/mg) represents the adsorption capacity for Ra-226, C0 and Ce (pCi/L) are the initial

121

and final Ra-226 activity in the liquid phase, V (L) is the volume of the sample and m (mg) is the

122

mass of the adsorbent. Freundlich isotherm33 was used to model the experimental results.

123 124

Kinetics of Ra-226 adsorption

125

Kinetics of Ra-226 adsorption by quartz sand, commercial celestite and freshly precipitated

126

celestite, formed by reacting strontium chloride (SrCl2) and sodium sulfate (Na2SO4), was

127

investigated at varying solution conditions (e.g., ionic strength, pH) and at temperature of 21±2°C

128

unless specified otherwise. These solids were added to 50 mL of Ra-226 solution in predetermined

129

amounts and stirred using a magnetic stirrer. Liquid samples taken from the reactor during the

130

experiment were filtered through a 0.45 m mixed cellulose ester membrane and stored in 15 ml

131

Falcon polypropylene centrifuge tubes (Fisher Scientific, Pittsburgh, PA) at room temperature for

132

Ra-226 analysis.

133 134

Zeta potential

135

Zeta potential of celestite particles used in this study was determined using Litesizer 500

136

(Anton Paar, Ashland, VA). Celestite particles were suspended in DI water and pH was adjusted

137

by adding HCl and NaOH. Solutions were then completely dispersed in a sonicator and a small

138

volume of each sample (350 L) was placed in a polycarbonate cuvette equipped with a gold 6 ACS Paragon Plus Environment

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electrode on each side. Zeta potential was determined based on electrophoretic mobility of celestite

140

particles. Litesizer 500 was also used to measure particle size distribution by dynamic light

141

scattering technique.

142 143

Radium activity

144

Radium activity was measured using LSC,11, 34, 35 where Ra-226 was first co-precipitated

145

with BaSO4 by adding 364 L of 100 mM BaCl2 to 2 mL of the liquid sample followed by the

146

addition of 20 mL of 1 M H2SO4. This approach was used in order to separate Ra-226 from other

147

ions (i.e., Na, Ca, Sr, Ba) in the solution to avoid any impact of salts on radium measurement.36

148

Samples were heated on a hotplate at 50°C for 1 hour to ensure equilibrium and precipitated solids

149

were separated from the liquid phase using 0.45 m mixed cellulose ester membrane and scraped

150

into

151

ethylenediaminetetraacetic acid (EDTA) to ensure complete transfer of solids. The samples were

152

then heated at 60°C for 1 hour to enhance the dissolution of Ra-Ba-SO4 in EDTA and 14 mL of

153

liquid scintillation Ultima Gold cocktail (Perkin Elmer, Waltham, MA) was added after the

154

samples cooled for 15 min. Each sample was analyzed in LSC for 40 min at 170-230 keV energy

155

range. QA/QC protocol included validation of random samples using Gamma spectroscopy.37

glass

vials

followed

by

washing

the

membrane

with

3

mL

of

0.25

M

156 157

Sand functionalization

158

Sand functionalization was performed by either mixing the commercial celestite (SrSO4)

159

with quartz sand or precipitating celestite on sand surface by adding supersaturated strontium

160

sulfate solution to a reactor already containing sand particles. Both experiments were performed

161

at pH 6 to enhance electrostatic attraction between sand and celestite particles because sand and 7 ACS Paragon Plus Environment


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celestite have opposite surface charges at this pH. The first procedure involved dispersing 2 g of

163

celestite and 5 g of sand in a sonicator followed by mixing in a tumbler for 24 hours. The second

164

procedure involved dissolving a predetermined amount of SrCl2 in DI water at pH 6 followed by

165

the addition of 5 g of sand and sufficient concentration of Na2SO4 to precipitate 2 g of celestite.

166

These samples were also mixed in the tumbler for 24 hours. Sand was separated from the solution

167

by filtering through a 53-m sieve and washed with ethanol (celestite is not soluble in ethanol) to

168

remove loosely attached celestite particles. Sand was then dried in air until all ethanol evaporated

169

and analyzed using scanning electron microscopy (SEM) (JEOL JSM6510, Peabody, MA). Ionic

170

strength of the remaining aqueous sample containing celestite was adjusted to 1 mol/L using NaCl

171

to ensure complete dissolution of celestite (celestite solubility at this ionic strength is close to 4

172

mmol/L)38 and analyzed for total Sr concentration to validate the mass balance. Mass of celestite

173

coated on the sand surface by each of these procedures was calculated using the following

174

equation:

175

mc = mi ― mr

176

where, mc (mg) is the mass of celestite that attached to the surface of the sand, mi (mg) is the total

177

mass of celestite that precipitated in the system, mr (mg) is the mass of celestite that remained in

178

the system, which included celestite in the supernatant and celestite that was washed from the sand

179

surface. Concentration of Sr2+ was measured using Inductively Coupled Plasma – Optical

180

Emission Spectroscopy (ICP-OES) (5100 ICP-OES, Agilent Technologies, Santa Clara, CA).

(2)

181 182

Surface area

183

Surface area of quartz sand, celestite, barite, calcite, strontianite (SrCO3) and witherite

184

(BaCO3) was determined using nitrogen adsorption/desorption measurements (Micromeritics


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ASAP 2020, Micromeritics, Norcross, GA) using a 6-point Brunauer-Emmett-Teller (BET)

186

analysis in the relative pressure range 0.06 < P/P0 < 0.25.

187 188

RESULTS AND DISCUSSION

189

Uptake of radium by sulfates and carbonates.

190

Adsorption isotherms for radium on barite, celestite, strontianite, witherite and calcite are

191

shown in Figure 1. Freundlich adsorption parameters (Equation S5 in Supporting Information)

192

used to describe adsorption capacity and adsorption intensity, KF and 1/n, are shown in Table 1.

193

Higher values of 1/n indicate larger change in adsorption capacity over the range of equilibrium

194

concentrations.39 Celestite and barite had the lowest 1/n parameter which makes them the most

195

reliable adsorbents (i.e., adsorption capacity is less prone to change with changes in liquid phase

196

concentration). It is important to note that the correlation coefficients for all isotherms except for

197

strontianite were at or above 0.9. As can be seen in Figure 1, all five solids showed the potential

198

for removing radium from solution and sulfates exhibited higher adsorption capacity than

199

carbonates. Wang et al.40 also reported that celestite and barite are more efficient radium

200

adsorbents than strontianite.

201

As shown in Figure 1, celestite exhibited the highest radium uptake capacity among the

202

solids tested in this study, which is in agreement with its high theoretical distribution coefficient

203

during the co-precipitation reaction (Kd = 237)11 (higher distribution coefficient indicates higher

204

affinity of radium for the solid phase than the aqueous phase31). Carbonates and barite have much

205

lower distribution coefficient ranging from 0.013 to 1.5411, 41 and exhibited uptake capacities that

206

are below that of celestite.

207



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208 209

Figure 1. Adsorption isotherms for Ra-226 uptake by sulfates and carbonates.

210 211 212

Table 1. Freundlich isotherm parameters and correlation coefficients Solids Barite (BaSO4) Celestite (SrSO4) Witherite (BaCO3) Calcite (CaCO3) Strontianite (SrCO3)

KF 1.68×10-3 6.19×10-4 4.08×10-6 1.17×10-6 1.16×10-9

1/n 1.13 1.38 1.70 2.13 2.74

R2 0.89 0.91 0.93 0.92 0.69

213 214

There are many studies on zeta potential of carbonate species that reported different values

215

as a result of complex reactions at carbonate-water interface.42-44 Zeta potential of calcite in the

216

pH range from 2-11 was found to be in the range of 5-10 mV (Figure S1 in Supporting

217

Information). On the other hand, zeta potential of sulfates was close to neutral or slightly negative,

218

which explains higher Ra-226 uptake by sulfates than carbonates.


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Brandt et al.10 showed that radium uptake by dissolution-recrystallization of barite can be

220

higher than the values observed in this study but this process is extremely slow and it can take up

221

to one year to achieve equilibrium. Hence, it is reasonable to disregard dissolution-recrystallization

222

as a possible mechanism for radium uptake in this study that was conducted over a period of 24 h.

223

The concentration of surface sites is a possible reason for higher Ra-226 uptake by celestite than

224

barite45, 46 since BET analysis showed that the specific surface area of celestite (i.e., 0.58 m2/g) is

225

significantly higher than that of barite (i.e., 0.17 m2/g). Higher measured surface area of celestite

226

can be explained by the tendency of barite particles to agglomerate (Figure S2d in Supporting

227

Information).

228 229

Zeta potential of celestite and quartz sand

230

Zeta potential previously reported for celestite47-50 included a wide range of isoelectric

231

points (IEP) from 2.4 to 5.8 because of the difference in experimental conditions. Zeta potential of

232

commercial and freshly precipitated celestite used in this study was measured in DI water at pH

233

range from 1.5 – 10 and is shown in Figure 2.

234

As can be seen from Figure 2, the IEP for celestite under experimental conditions used in

235

this study was 6.3. Zeta potential is positive (up to +10 mV) when the solution pH is below IEP

236

and it becomes negative (up to -20 mV) at higher pH. This behavior of celestite can be explained

237

by the dissolution and hydrolysis processes on the mineral surface.51 Addition of acid will create

238

positively charged surface through the adsorption of H+, while the addition of base will make

239

surface negatively charged through the adsorption of OH-. Several studies that investigated the

240

zeta potential of quartz sand and silica, reported that there is no significant difference between

241

these two species and the IEP to be approximately at pH 2.52-56 Zeta potential of silica SiO2 (i.e.,



242

particle size 1 m) was evaluated in DI water and the results are shown in Figure 2. Increasing the

243

pH of solution above 2 significantly decreases the zeta potential to -80 mV at pH 10 due to

244

deprotonation of silanol (Si-OH) groups.57, 58 Zeta potential of celestite and sand (i.e., SiO2) were

245

measured to elucidate the role of electrostatic interactions in radium uptake and to determine the

246

most suitable pH region for coating proppant sand with celestite. Because quartz sand and celestite

247

have opposite surface charge in the pH range from 2.5-6.3 (Figure 2), impregnation of sand with

248

celestite was performed at pH 6 to maximize the attraction between the positively charged celestite

249

and negatively charged quartz sand.

250

251 252 253 254

Figure 2. Zeta potential of celestite as a function of the pH of the solution; experimental standard deviation calculated based on at least 3 replicates is shown using error bars.

255 256 12 ACS Paragon Plus Environment

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Radium uptake by celestite

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Commercially available celestite and freshly prepared celestite were used to investigate the

259

mechanism and key parameters that influence Ra-226 uptake. As shown in Figure S2 (Supporting

260

Information), both types of celestite had similar particle shape and morphology with an average

261

particle diameter of 3.5 m for freshly precipitated celestite and 4.5 m for commercial celestite.

262

The analysis of particle size distribution at varying ionic strength showed no agglomeration of

263

commercial celestite particles (Figure S2c).

264

Celestite speciation in DI water evaluated using Phreeqc software59 showed that the

265

solubility of celestite remained close to 120 mg/L in the pH range from 2 to 10, which was

266

confirmed experimentally (i.e., measured Sr2+ and SO42- concentrations were 55 mg/L and 62

267

mg/L, respectively). Kinetics of Ra-226 removal by celestite at three different pH values using

268

2,000 mg/L of celestite particles in each experiment was monitored for a period of 96 hours in a

269

batch reactor and the results are shown in Figure 3.

270

Radium removal by celestite occurs either through lattice replacement or surface

271

adsorption.11 Lattice replacement is associated with co-precipitation of a trace contaminant during

272

precipitation and crystal growth of the main solid.60 It is reasonable to conclude that adsorption on

273

active sites (i.e., negatively charged SO42- groups due to incomplete coordination spheres on the

274

mineral surface60) was the main mechanism of radium removal in these experiments because the

275

equilibrium was achieved within two hours (Figure 3), which is typically not sufficient for celestite

276

dissolution and recrystallization to affect radium uptake.10

277



278 279 280 281 282

Figure 3. Radium uptake by celestite in SrSO4 saturated solution (i.e., 120 mg/L) with initial Ra226 concentration of 7,750 pCi/L; experimental standard deviation calculated based on at least 3 replicates is shown using error bars.

283

As expected, commercially available and freshly precipitated celestite exhibited similar

284

trends in Ra-226 removal due to their similar size and morphology (Figure S2). At pH 2 and 5.9,

285

radium removal from dilute solutions was very fast and equilibrium was achieved within a few

286

hours with no significant removal observed during the next 4 days. However, the results at pH 9.6

287

indicate that the kinetics of radium uptake is slower compared to that observed at lower pH. Such

288

behavior can be explained by the reduction in double layer thickness from 6 nm at pH 9.6 to 2.6

289

nm at pH 2.53, 61 At higher pH, radium ions need more time to diffuse through the electrical double

290

layer resulting in slower overall adsorption kinetics. Results in Figure 3 indicate that the zeta

291

potential of celestite, which can vary from +10 mV at pH 2 to -15 mV at pH 9.6 (Figure 2) with a

292

corresponding surface charge varying from +0.37 mC at pH 2 to -0.39 mC at pH 9.6 (calculated 14 ACS Paragon Plus Environment

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using Equations S6-S9 of Supporting Information), has limited impact on radium removal at

294

equilibrium.

295

Because the produced water from unconventional gas wells has very high salinity, which

296

is predominantly due to NaCl2, radium uptake by celestite was also evaluated in 1.6 M NaCl

297

(93,500 mg/L) solution using the experimental conditions shown in Table 2. Figure S3 (Supporting

298

Information) compares Ra-226 uptake by celestite in DI water and 1.6 M NaCl solution at the

299

initial Ra-226 concentrations of 5,000 pCi/L.

300

Table 2. Experimental conditions and Ra-226 uptake at low and high ionic strength

301 302

Sample name A B C

Solution DI water 1.6 M NaCl 1.6 M NaCl

Ra-226 concentration (pCi/L) 5,000

Total SrSO4 added (mg/L) 1,000

5,000

1,000

800

200

1.51

8.78

5,000

2,000

800

1,200

0.59

3.41

Dissolved SrSO4 (mg/L)

SrSO4 Solids (mg/L)

Ra-226 uptake (pCi/cm2)

Ra-226 uptake (pCi/mg)

120

880

0.83

4.82

303 304

As can be seen in Table 2 (and Figure S3), addition of 1,000 mg/L of celestite to solution

305

A was sufficient to remove 85% of radium at equilibrium. Moreover, equilibrium was achieved

306

within 1 hour of contact. However, when the ionic strength was adjusted to 1.6M with NaCl,

307

radium removal decreased to just 35%. The increase in ionic strength increases the solubility of

308

celestite38 from 120 mg/L to 800 mg/L as calculated by Phreeqc using Pitzer activity corrections,

309

which was validated experimentally by measuring the Sr2+ concentration in NaCl solution.

310

Increased solubility of celestite at high ionic strength lead to a decrease in the surface area of

311

celestite available in solution by 77% and a corresponding decrease in radium removal. Higher

312

radium uptake per unit surface area in sample B compared to sample A (Table 2) can be explained 15 ACS Paragon Plus Environment


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by the higher equilibrium concentration of radium in sample B and greater adsorption driving

314

force. Competition for active sites on celestite surface between radium and high concentration of

315

Sr2+ present in solution at high ionic (i.e., eight orders of magnitude higher Sr2+ concentration

316

compared to radium) also contributed to reduced radium removal under these conditions. The rate

317

of radium removal at high ionic strength decreased compared to that observed in DI water and it

318

took 7 hours to achieve adsorption equilibrium (Figure S3). Such behavior can be attributed to a

319

decrease in radium diffusivity at high ionic strength62 and reduction in the surface area of

320

celestite.63

321

A second adsorption test was conducted under identical experimental conditions as sample

322

B except that 2,000 mg/L of celestite was added to the batch reactor (Sample C, Table 2). This

323

modification increased the surface area available for radium adsorption 6 times and the removal

324

of radium was almost identical to the removal observed in DI water (i.e., above 80%). Although

325

higher radium removal was achieved with the addition of more celestite solids, the rate of

326

adsorption was still lower than in DI water and was almost identical as in the case of lower

327

concentration of celestite solids (i.e., equilibrium was achieved after 7 hours). While the addition

328

of 2,000 mg/L celestite increased radium removal, radium uptake per unit surface area decreased

329

from 0.83 pCi/cm2 in sample A to 0.59 pCi/cm2 in sample C (Table 2) despite having identical

330

equilibrium Ra-226 concentration. This decrease in capacity can be explained by the competition

331

from Na+ and Sr2+ for active sites on celestite surface.

332 333

Radium uptake by quartz sand

334

Because quartz sand is widely used as proppant in hydraulic fracturing,32 its ability to

335

remove radium from DI water was evaluated at three different pH levels using 5 g of sand in the


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336

batch reactor and the initial Ra-226 activity of 5,000 pCi/L. Figure 4 shows the impact of pH and

337

ionic strength on radium uptake by quartz sand.

338

As can be seen in Figure 4, increase in solution pH resulted in greater radium uptake by

339

quartz sand because of the more negative surface charge (Figure 2) due to deprotonation of silanol

340

groups.57 Radium uptake by celestite was not influenced by the solution pH as much (Figure 3)

341

because the zeta potential of celestite is much lower than that of sand (Figure 2). These results may

342

lead to a conclusion that sand alone would serve as an excellent adsorbent for radium, especially

343

at higher pH. However, radium uptake by sand decreased dramatically at high ionic strength that

344

is representative of produced water (i.e., I = 1.6 mol/L) because of the screening of electrostatic

345

forces at high ionic strength.

346

347 348 349 350

Figure 4. Radium removal by 5 g of quartz sand at initial Ra-226 concentration of 5,000 pCi/L in DI water; analytical procedure standard deviation is presented using error bars.



351

Page 18 of 30

Radium uptake by celestite coated sand

352

As the proppant itself cannot remove significant amount of Ra-226 from produced water

353

under relevant process conditions (i.e., high salinity), in-situ sequestration of radium would require

354

functionalization of the proppant so that radium uptake is not relying only on electrostatic

355

interactions with quartz sand. Two different approaches for depositing celestite on the proppant

356

surface were evaluated using identical masses of celestite (i.e., 2 g) and sand (i.e., 5 g) that were

357

contacted using identical mixing conditions (i.e., end over end tumbling) and reaction time (i.e.,

358

24 h). Each test was performed 5 times and the average amount of celestite deposited on sand

359

surface is reported in Table 3.

360

Table 3. Mass balance for two celestite impregnation techniques

361 362 Method

Mixing with Preformed SrSO4 Direct Precipitation of SrSO4

SrSO4 input (mg)

Filtered SrSO4 (mg)

Washed SrSO4 (mg)

Coated SrSO4 (mg)

2,000

1,806 ± 31

94.5 ± 19

99.5 ± 16

2,000

1,762 ± 30

89 ± 14

149 ± 21

363 364

As seen from Table 3, direct (heterogeneous) precipitation of celestite on the surface of

365

quartz sand as “seed” resulted in about 50% increase in celestite retention on the sand surface when

366

compared to simple mixing of sand and preformed celestite. SEM images of sand particles with

367

celestite coating shown in Figure 5 illustrate differences in morphology under different

368

experimental protocols.

369

As can be seen in Figure 5a, the roughness of the sand surface plays a critical role in

370

celestite deposition when mixing preformed celestite and sand with significantly more celestite

371

deposited in crevices than on the smooth parts of the sand surface. In addition, celestite particles 18 ACS Paragon Plus Environment

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372

were deposited in multiple layers in these crevices, which reduces the total exposed surface area

373

available for the adsorption of Ra-226. On the other hand, coating sand surface by heterogenous

374

celestite precipitation shows significantly better dispersion of celestite particles (Figure 5b) that

375

appear to be larger than in the case of simple mixing of preformed celestite with sand. These results

376

indicate that the sand surface served as an effective "seed" for celestite precipitation and

377

subsequent crystal growth. It is important to note that optimization of celestite impregnation on

378

sand surface through direct precipitation in terms of supersaturation level and kinetics of celestite

379

precipitation could possibly yield much higher celestite loading on sand surface.64 However, the

380

initial objective of this study was to demonstrate the proof of concept with follow-on work

381

designed to optimize the impregnation protocol.

382 383 384 385

Figure 5. SEM images of quartz sand coated with celestite by a) mixing with preformed celestite and b) direct precipitation of celestite.

386

Because both celestite and quartz sand (in DI water) demonstrated significant adsorption

387

of radium in the absence of any additional competing ions (Figures 3 and 4), it is reasonable to

388

expect that celestite impregnated sand would show even better removal of radium due to

389

synergistic effects. The ability of sand and two types of celestite coated sand to remove radium

390

from celestite saturated solution was the investigated under identical conditions and it was found



391

that sand coated with celestite by direct precipitation showed significantly better radium uptake

392

than sand coated with celestite by mixing (Figure S4, Supporting Information). Such behavior can

393

be explained by greater mass of celestite that was impregnated on the sand surface through direct

394

precipitation method (Table 3). Due to its higher coating efficiency and uniform distribution of

395

celestite particles on the sand surface (Table 3 and Figure 5), sand coated through precipitation is

396

better suited for Ra-226 removal.

397

It was surprising to see that Ra-226 removal by celestite coated sand was not additive for

398

both celestite coating methods evaluated in this study. The Ra-226 uptake by sand was 0.025

399

pCi/mg and by celestite impregnated sand was 0.033-0.041 pCi Ra226/mg (Figure S4) while Ra-

400

226 uptake by celestite alone was 3.3-3.7 pCi/mg (Figure 3). Possible reason for such behavior

401

could be very high affinity of celestite for Ra-226 and inability of quartz sand to contribute to Ra-

402

226 uptake because of the celestite layer that is covering sand surface.

403

To investigate these hypotheses, the kinetics of Ra-226 uptake by celestite and sand alone

404

was measured during the first 60 min using 5 g of quartz sand in DI water (K1), 5 g of quartz sand

405

in 0.65 mmol/L of Sr2+ (K2) and 75 mg of celestite in 0.65 mmol/L of Sr2+ (K3). Mass of celestite

406

in experiment K3 was 50% of the mass that was present in the experiment with sand coated by

407

direct precipitation of celestite to test the hypothesis that only half of the celestite impregnated on

408

the sand surface will participate in radium uptake because half of the surface area of celestite is

409

not accessible for Ra-226 uptake (Figure 5). Strontium concentration in experiments K2 and K3

410

corresponds to the solubility equilibrium of celestite in DI water.

411

As can be seen in Figure 6, both kinetics and equilibrium of radium adsorption by sand are

412

significantly inhibited by the presence of eight orders of magnitude higher concentration of Sr2+

413

cations in solution compared to radium. It was previously shown that radium uptake by quartz sand


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414

is highly dependent on the pH (Figure 4). Furthermore, positively charged ions have the ability to

415

screen negatively charged surface and significantly reduce adsorption capacity of silica.65 Sr2+

416

cations with a total charge of 6.27 C would screen the surface charge of quartz sand (i.e., total

417

charge of approximately 0.05 C) and slower diffusion of Ra2+ through this Sr2+ layer would hinder

418

radium uptake kinetics.66 On the other hand, radium uptake by celestite is almost instantaneous

419

and equilibrium is achieved after 1-2 minutes. Celestite capacity for radium uptake in experiment

420

K3 is 2.86 pCi/mg, which is very close to the values calculated from Figure S4 (i.e., 2.92-3.60

421

pCi/mg) under the assumption that celestite on the sand surface adsorbed all the radium removed

422

from solution and that only half of the surface area of celestite was available for adsorption. Slower

423

kinetics of radium uptake and significantly lower Ra-226 removal by sand in the presence of Sr2+

424

observed in these tests confirm the hypothesis that most of the radium is adsorbed by the celestite

425

impregnated on the sand surface before it has a chance to reach the sand surface. This hypothesis

426

is further confirmed by the fact that the Debye length in celestite saturated solution (i.e., 120 mg/L)

427

is approximately 6 nm while celestite particles that are evenly distributed on the sand surface are

428

approximately 3.50 m (i.e., radium cations will likely be adsorbed by celestite particles before

429

they even have a chance to reach the sand surface). Therefore, contribution of the sand surface to

430

radium removal is negligible and there is no synergy between sand and celestite towards radium

431

removal when celestite is impregnated on sand surface.



432

433 434 435 436 437 438 439

Figure 6. Kinetics of Ra-226 removal during initial 60 min at pH 5.5 and at initial Ra-226 concentration of 5,000 pCi/L (K1: 5 g of quartz sand in DI water; K2: 5 g of quartz sand in 0.65 mmol/L of Sr2+; K3: 75 mg of celestite in 0.65 mmol/L of Sr2+; error bars indicate standard deviation of the analytical procedure).

440

Produced water has a high concentration of sodium and divalent cations and temperature

441

that can range from 35-51 °C.16, 67 Radium removal by the proppant coated by direct celestite

442

precipitation was investigated in the presence of high concentration of competing ions (i.e., Na+ =

443

33,000 mg/L, Mg2+ = 1,600 mg/L and Ca2+ = 16,000 mg/L) at room temperature (i.e., 21 °C) and

444

at elevated temperature (i.e., 40 °C) and the results are shown in Figure 7.


Page 22 of 30

Page 23 of 30


445 446 447 448 449 450

Figure 7. Ra-226 removal by celestite coated proppant in synthetic high salinity solution at pH 5.5 and at different temperatures with initial Ra-226 concentration of 5,000 pCi/L (error bars indicate standard deviation of the analytical procedure).

451

Uptake of Ra-226 by quartz sand decreased from 0.025 pCi/mg in celestite saturated

452

solution (Figure S4) to 0.016 pCi/mg in high salinity solution that contained high concentration of

453

divalent cations (Figure 7). The increase in temperature to 40 oC induced further reduction in Ra-

454

226 removal capacity of quartz sand to 0.008 pCi/mg. It is evident from Figure 7 that celestite

455

coating on sand surface significantly improved Ra-226 removal even in the presence of competing

456

divalent cations. Radium removal by celestite impregnated sand at room temperature was 56%

457

(adsorption capacity of 0.027 pCi/mg) and 43% (adsorption capacity of 0.021 pCi/mg) at 40 °C.

458

Decrease in Ra-226 removal at elevated temperature points to exothermic nature of adsorption

459

reaction. These results are similar to the results obtained by Vinograd et al.68 where radium uptake



460

during co-precipitation with barite was inhibited at higher temperatures. Radium adsorption

461

kinetics was also affected by the high ionic strength of the solution and it took 6 hours to achieve

462

equilibrium compared to about 1 hour observed in dilute solution (Figure S4). Such behavior can

463

be explained by decreased diffusivity of Ra-226 ions at elevated ionic strength. Although some

464

celestite was dissolved due to the higher solubility at elevated ionic strength (i.e., 5.8 mmol/L at I

465

= 2.8 mol/L), it appears that most of the celestite coating remained on the sand surface throughout

466

the experiment. This is an indication of mechanical stability of the coating (i.e., proppant was

467

exposed to shearing in the kinetic experiments during vigorous mixing using a stir bar that was

468

required to suspend 5 g of sand in a beaker). SEM images of the proppant after the experiment are

469

shown in Figures S5 and S6. It is important to note that sulfate resulting from the solubilization of

470

celestite would cause precipitation of barite if barium ions were present in solution, which would

471

lead to additional Ra removal in the form of Ba-Ra-SO469-71 and it would make it difficult to assess

472

the ability of functionalized proppant to remove radium by itself. Therefore, we excluded barium

473

as a competing ion.

474

The utility of impregnated proppant to remove Ra-226 from the produced water generated

475

during the lifetime of a single gas well can be estimated with a simplified analysis. Assuming a

476

lifetime of 20 years, produced water flow of 1.5 m3/day, and Ra-226 concentration of 2,500 pCi/L,

477

the total radioactivity of produced water brought to the surface during this period would be 2.75 *

478

1010 pCi. On the other hand, assuming the uptake capacity of coated proppant of 0.021 pCi/mg at

479

40 °C and 2,000 tons of proppant that is typically used for each well, it can be estimated that the

480

total Ra removal capacity of the celestite impregnated proppant under relevant process conditions

481

would be 4.2 * 1010 pCi (see Supporting Information for additional details). These preliminary

482

calculations suggest that the proposed solution for controlling NORM in the produced water is of


Page 24 of 30

Page 25 of 30


483

significant practical relevance and it is possible that further optimization of the impregnation

484

process could yield even more efficient radium sorbent.

485

The findings of this study suggest that it is possible to significantly increase the ability of

486

proppant sand to remove NORM from the produced water by impregnating celestite on its surface.

487

Furthermore, uniform distribution of celestite achieved when the impregnation is performed by

488

heterogenous precipitation in supersaturated solution suggests that this method is better suited

489

compared to simple mixing of celestite with sand. Faster adsorption kinetics, higher surface area

490

available for adsorption and significantly higher capacity for radium uptake suggest that most of

491

the radium will be adsorbed by celestite even when it is impregnated on sand and that quartz sand

492

would not play a significant role in radium uptake. Radium uptake by celestite impregnated

493

proppant was investigated in synthetic high salinity water and it was shown that high TDS limit

494

Ra-226 removal to 43-56% because of significantly lower (i.e., 6-9 order of magnitude) Ra-226

495

concentration compared to the concentration of competing ions in the solution. Impact of organics

496

and surfactants that are typically present in produced water, as well as the impact of high

497

temperatures and high pressures on radium uptake and coating stability will have to be further

498

evaluated. The idea of coating sand with celestite is presented as a possible alternative to other

499

methods for Ra-226 removal from wastewater produced by the extraction of unconventional gas

500

resource.

501 502

Acknowledgements:

503

Financial support from National Science Foundation (Grant No. NSF CBET #1510764) is

504

gratefully acknowledged.

505



506

Supporting Information Available

507 508 509

Additional details on important experimental results, calculations and SEM images of interest in this study. This information is available free of charge via the Internet at http://pubs.acs.org.

510

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59. Parkhurst, D. L., and Appelo, C.A.J., Description of input and examples for PHREEQC version 3-A Computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. In book 6, chap. A43 ed.; 2013; p 497. 60. Harvey, D., Modern Analytical Chemistry In McGraw-Hill: New York: 2000. 61. Chorom, M.; Rengasamy, P., Dispersion and zeta potential of pure clays as related to net particle charge under varying pH, electrolyte concentration and cation type. European Journal of Soil Science 1995, 46, (4), 657-665. 62. Vinograd, J. R.; McBain, J. W., Diffusion of Electrolytes and of the Ions in their Mixtures. Journal of the American Chemical Society 1941, 63, (7), 2008-2015. 63. Li, Z.; Shi, B.; Su, Y.; Wang, D., Effect of particle size on adsorption kinetics of phenanthrene in water by powered activated carbon. Huanjing Kexue Xuebao / Acta Scientiae Circumstantiae: 2013; Vol. 33, p 67-72. 64. Ali, I., Schneider, P., Crystallization of struvite from metastable region with different types of seed crystal. Journal of Non-Equilibrium Thermodynamics 2005, 30(2), 95-111. 65. Rezwan, K.; Meier, L. P.; Gauckler, L. J., Lysozyme and bovine serum albumin adsorption on uncoated silica and AlOOH-coated silica particles: the influence of positively and negatively charged oxide surface coatings. Biomaterials 2005, 26, (21), 4351-4357. 66. Dąbrowski, A., Adsorption—from theory to practice. Advances in colloid and interface science 2001, 93, (1-3), 135-224. 67. Halliburton The Marcellus Shale Gas/Electric Partnership; 2009, https://www.eia.gov/maps/pdf/MarcellusPlayUpdate_Jan2017.pdf. 68. Vinograd, V.; Kulik, D.; Brandt, F.; Klinkenberg, M.; Weber, J.; Winkler, B.; Bosbach, D., Thermodynamics of the solid solution-Aqueous solution system (Ba, Sr, Ra) SO4+ H2O: II. Radium retention in barite-type minerals at elevated temperatures. Applied Geochemistry 2018, 93, 190-208. 69. Zhang, T.; Gregory, K.; Hammack, R. W.; Vidic, R. D. J. E. s.; technology, Co-precipitation of radium with barium and strontium sulfate and its impact on the fate of radium during treatment of produced water from unconventional gas extraction. 2014, 48, (8), 4596-4603. 70. Brandt, F.; Klinkenberg, M.; Poonoosamy, J.; Weber, J.; Bosbach, D. J. M., The Effect of Ionic Strength and Sraq upon the Uptake of Ra during the Recrystallization of Barite. 2018, 8, (11), 502. 71. Klinkenberg, M.; Weber, J.; Barthel, J.; Vinograd, V.; Poonoosamy, J.; Kruth, M.; Bosbach, D.; Brandt, F. J. C. G., The solid solution–aqueous solution system (Sr, Ba, Ra) SO4+ H2O: A combined experimental and theoretical study of phase equilibria at Sr-rich compositions. 2018, 497, 1-17.



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Development of functionalized proppant for the control of NORM in

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