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Low Permeability Zone Remediation via Oxidant Delivered by Electrokinetics and Activated by Electrical Resistance Heating: Proof of Concept Ahmed I.A. Chowdhury, Jason Ian Gerhard, David A Reynolds, and Denis Michael O'Carroll Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02231 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Environmental Science & Technology

1

Low Permeability Zone Remediation via Oxidant Delivered

2

by Electrokinetics and Activated by Electrical Resistance Heating:

3

Proof of Concept

4 5 6

Ahmed I. A. Chowdhurya, Jason I. Gerharda, David Reynoldsb, Denis M. O’Carrolla,c,*

7 8 9

a

10 11 12 13

b

Department of Civil and Environmental Engineering, Western University, 1151 Richmond St., London, ON, Canada. N6A 5B9

Geosyntec Consultants, 130 Stone Road W., Guelph, ON, Canada. N1G 3Z2 School of Civil and Environmental Engineering, Connected Water Initiative, University of New South Wales, Manly Vale, NSW, 2093, Australia. email: [email protected]; Tel: +61 2 8071 9800; Fax: +61 2 9949 4188 c

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

*Corresponding author

Keywords: electrokinetics, persulfate, ISCO, thermal activation, electrical resistance heating, ERH, low permeable zone, PCE, remediation.

1 ACS Paragon Plus Environment


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Abstract

34

This study proposes and proves (in concept) a novel approach of combining electrokinetic

35

(EK)-assisted delivery of an oxidant – persulfate (PS) - and low temperature electrical

36

resistivity heating (ERH) – to activate PS – to achieve remediation of contaminated, low

37

permeability soil. This unique combination is able to overcome existing challenges in

38

remediating low permeability materials, particularly associated with delivering

39

remediants. A further benefit of the approach is the use of the same electrodes for both

40

EK and ERH phases. Experiments were conducted in a laboratory-scale sand tank packed

41

with silt and aqueous tetrachloroethene (PCE) and bracketed on each side by an electrode.

42

EK first delivered unactivated PS throughout the silt. ERH then generated and sustained

43

the target temperature to activate the PS. As a result, PCE concentrations decreased to

44

below detection limit in the silt in a few weeks. Moreover, it was found that activating PS

45

at ~36 °C eliminated more PCE than activating it at >41 °C. It is expected this results

46

from the reactive SO4●- radical being generated more slowly, which ensures more

47

complete reaction with the contaminant.. The novel application of EK-assisted PS

48

delivery followed by low temperature ERH appears to be a viable strategy for low

49

permeability contaminated soil remediation.


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50

1

51

Transport of constituents in low permeability zones is dominated by diffusion-limited

52

mass transport. As such, low permeability zones can store significant contaminant mass

53

over extended periods (e.g., since a historical release) and slowly release that mass into

54

adjacent, more permeable (i.e., transmissive) zones (i.e., back diffusion). Low

55

permeability zones can therefore act as long term sources of contamination even after the

56

transmissive zone has been remediated

57

target the transmissive zones via advection of fluids (e.g., in-situ chemical oxidation

58

(ISCO)) are unable to adequately penetrate, and therefore remediate, low permeability

59

zones sites

60

permeability zones to comply with regulatory standards at contaminated sites 7.

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ISCO is a remediation technology where an oxidant is injected into the subsurface to

62

intercept and react with the contaminant

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much interest due to its high reduction potential ( = 2.01 ), its stability in aqueous

64

solution at neutral pH, and the production of non-toxic by-products after reaction with

65

chlorinated solvents 11-13. PS can react with the contaminant by direct electron transfer via

66

self-decomposition or, upon activation, can produce free radicals (e.g., SO4●-); the latter

67

reactions are faster 12 since the sulfate radical (SO4●-) is a more aggressive oxidant (Eo =

68

2.6 V)

69

slowly, use of activated PS is preferred as remediation times are reduced.

Introduction

13

7, 8

1-6

. Traditional remediation technologies that

. Therefore, it is necessary to develop methods to remediate low

9, 10

. Use of persulfate (PS) ( ) has gained

. Although PS self-decomposition should be able to degrade contaminants



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70

PS can be activated using heat, ultra-violet light, high pH (>11), hydrogen peroxide, or

71

dissolved or chelated metals (e.g., Fe2+)

72

effectiveness by quantifying contaminant degradation. For example, Fan, et al.

73

observed more degradation of polychlorinated biphenyls (PCBs) from high pH activation

74

of PS compared to other methods (thermal activation not studied). Other studies quantify

75

PS activation by measuring PS disappearance in the absence of contaminants. It is noted

76

that most studies examine PS activation in ideal systems, using continuously stirred

77

reactors with no soil 14, 17-21. At elevated temperatures, PS decomposes to reactive sulfate

78

radicals (SO4●-) and finally to sulfate anion (SO42- ) 11, 22:

79

2 ⦁ → 2 SO4

80

SO4●- initiates a chain of reactions producing reactive intermediates (e.g., hydroxyl

81

radicals (OH●) and peroxymonosulfate ( )) 17, 20, 23, 24. The activation energy (Ea) for

82

PS decomposition is reported to be between 118 and 140.2 kJ/mol in systems with just

83

water and 120±7 kJ/mol in systems that included soil

84

and without soil suggest that Ea for thermal activation of PS is independent of the

85

presence of soil. Dissolved phase activators such as Fe2+ or alkaline solutions require that

86

both the PS and activator ions come into contact to activate PS. The presence of soil

87

would affect mixing and collisions between PS and activators, resulting in different rate

88

constants and Ea. By comparison, application of heat is expected to be more uniform than

89

other activation methods.

12, 14, 15

. Some studies examined activation

2−

16

(Equation 1)

11, 22, 25

. Similar Ea in systems with


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90

Thermally activated PS has been employed to degrade a number of contaminants in batch

91

experiments

92

(PCE) as follows 14:

93

+ 2" "# + 8 → 2 + 8"# + 4" + 10 % + 3

94

It is noted here that the reaction given by Equation (2) involves the generation of reactive

95

sulfate radicals (SO4●-) that react with PCE. Therefore, the reaction is dependent on the

96

availability of SO4●- upon activation. Previous studies demonstrated that the observed rate

97

constants (determined from PCE concentration data after PS- PCE reaction for 80 min)

98

were greater in stirred reaction vials with water at 50 °C (80 d-1) in comparison to when

99

F-70 sand or kaolinite clay were present (12 and 3 d-1, respectively)

14, 17-21, 26

. For example, PS has been proposed to degrade tetrachloroethene

(Equation 2)

14

suggesting

100

contaminant degradation will be slower in situ in comparison to constantly stirred-reactor

101

conditions. More recently, Quig

102

degradability of heat activated PS in a column study. In this particular study, thermally

103

activated PS (at 60 °C) was injected into sand with TCE at residual, reporting 33% TCE

104

mass destruction. Although PS has the potential to degrade contaminants in transmissive

105

zones, difficulties in transporting any remediant through low permeability soil has to date

106

limited applications in low permeability zones.

107

The use of electrokinetics (EK) for enhanced remediant (e.g., nano-scale zero valent iron,

108

permanganate, persulfate, lactate) delivery to low permeability zones has been

109

investigated

110

or more electrodes (positive/anode and negative/cathode) inducing two important

111

transport mechanisms

8, 27-31

26

studied the transport behavior as well as TCE

. EK is the application of a low voltage direct current (DC) across two

32

. Electromigration (EM) is the transport of ionic species in bulk



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solution. For example, PS will be repelled by the cathode and migrate to the anode

113

through EM due to its negative charge. Electroosmosis (EO) results in bulk pore fluid

114

migration in the opposite direction (i.e., anode to cathode), including dissolved species 32.

115

EK-induced transport mechanisms are independent of the intrinsic permeability of a

116

porous medium 29. Therefore, EK has significant potential for delivering remediants into

117

low permeability zones.

118

EK-induced PS transport has been examined in a number of laboratory-scale studies 15, 16,

119

31, 33-36

120

through clayey soil based on the appearance of 50% of the maximum observed

121

concentration at the anode. A similar transport rate (approximately 1.0 cm/day) was

122

observed by Fan, et al. 34 suggesting that EK can deliver PS into low permeability zones.

123

However, delivery of aqueous-based PS activators (e.g., alkaline solution, dissolved or

124

chelated metals) into low permeability zones poses an additional challenge. Hence,

125

thermal activation, with temperatures between 30-60 °C, is a promising alternative for

126

activating EK-delivered PS to low permeability zones

127

influenced by intrinsic permeability. This has been examined to a limited extent by

128

Yukselen-Aksoy and Reddy

129

experimental system using a silicone heating tape. This heating approach, however,

130

cannot be used in the field. Waldemer, et al. 20 proposed, but did not investigate, thermal

131

activation of PS using in-situ thermal remediation (ISTR). Electrical resistance heating

132

(ERH), a form of ISTR, applies an alternating current (AC) across electrodes with soil

133

resistance causing in situ heating 37, 38. ERH has been applied for subsurface remediation,

134

including low permeability zones, where the subsurface is heated to temperatures between

. Robertson

31

reported an EM-induced transport rate of approximately 0.5 cm/day

36

12

since heat transport is not

who used EK-induced PS transport and then heated their


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39

135

60 and 110 °C

. In those studies, ERH was used to volatilize contaminants. To the

136

authors’ knowledge, ERH has never been used for thermal activation of PS. Here, the

137

novel concept is proposed that EK-delivered PS is thermally activated by low temperature

138

ERH.

139

This work presents a proof-of-concept of PS oxidation of chlorinated solvent-

140

contaminated low permeability soil where EK was used to deliver the PS followed by

141

ERH for heat activation. Moreover, the EK and ERH phases are both accomplished using

142

the same pair of electrodes. The specific objectives were to: (i) investigate the extent of

143

EK-assisted PS delivery through a fine-grained porous medium, (ii) evaluate the ability of

144

ERH to activate PS, and (iii) quantify the resulting PCE degradation. To this end, a two-

145

dimensional, multi-stage, 91 days long bench-top experiment was conducted. This study

146

provides the basis for up-scaling this novel technology for remediation of contaminated

147

low permeability zones.

148

2

149

Two experiments, a control experiment and an electrokinetically delivered and thermally

150

activated persulfate (EKTAP) experiment, were conducted to accomplish the objectives

151

of the study. All experiments were conducted in a custom built 2D laboratory-scale sand

152

tank (workable area of 36 cm × 15 cm × 10 cm; hereafter referred to as “sand tank”) made

153

of acrylic (1.5 cm thick) panels with a water-tight top cap (Figure 1). The porous medium

154

was contained in the central cell of the sand tank with the “anode” (left) and “cathode”

155

(right) cells on each side, each having 5L liquid capacity (Figure 1). Additional sand tank

156

and PCE stock solution information can be found in the SI.

Experimental Methodology



157 158 159 160

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Figure 1: Photograph of 2D experimental setup where P1 to P6 refer to sampling ports and T1 to T4 represent the installed thermocouples. PS solution was loaded in the cathode cell and migrated towards the anode during EK-Delivery phases.

161 162

The sand tank was filled using a wet packing method as described by 40. Dry fine silt (Sil-

163

Co-Sil 106, US Silica, d50 = 0.045 mm, K = 0.2 m/day) was added to the soil cell in

164

approximately 2 cm height increments. A gear pump (Model no: 75211-30, Barnant Co.,

165

IL, USA) connected to PTFE tubing (3/16th inch diameter) was used to inject the PCE

166

solution into the silt layer. The PCE solution was injected by placing the PTFE tubing at

167

three locations (9, 18 and 27 cm; not shown in Figure 1 for clarity of the photograph)

168

along the length of the soil compartment. The PCE solution level was then raised 2 cm

169

above the top of the silt layer in the soil compartment. The silt was then allowed to settle

170

in the aqueous PCE solution for two hours and gently tapped with a custom built Teflon

171

hammer to further compact the silt layer. These steps were repeated until the soil cell was 8 ACS Paragon Plus Environment

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full. Additional dry silt was added before the top cap was compressed into place to ensure

173

no void space in the system. Estimated soil porosities were 0.60 and 0.63 for EKTAP and

174

Control experiments, respectively. Given the experimental setup it was not possible to

175

hydraulically flush the aqueous PCE stock solution through the sandbox after it was

176

packed; initial PCE emplacement was achieved through wet packing.

177

For both of the experiments a stock buffer solution with no PCE was constantly injected

178

into the anode and cathode cells while packing the soil compartment. However, the

179

solution levels in the anode/cathode cells were maintained below the silt height in the soil

180

compartment. This minimized infiltration of the buffer solution into the soil cell which

181

would have diluted PCE in the silt. The head in the anode and cathode cells was raised to

182

2 cm above the silt once the soil compartment was sealed, ensuring no head gradient.

183

These heads were maintained throughout the experiments. The sand tank was then

184

allowed to sit undisturbed for a period of time to allow the silt-PCE solution system to

185

equilibrate. This equilibration period, as discussed later on, is referred to as Phase 1 for

186

the Control as well as EKTAP experiment.

187

For the EKTAP experiment, a 22L buffer solution (same recipe as above) was prepared in

188

a 26L container (referred to as the “anode reservoir”) and 22L persulfate (98%, Alfa

189

Aesar, MA, USA) (10 or 40 g/L) with phosphate buffer solution was prepared in a

190

separate container (referred to as the “cathode reservoir”). At the start of this experiment,

191

previously used buffer solution (i.e., during PCE loading) from the anode and cathode

192

cells was syphoned out and approximately 4L of buffer and PS solution was added to

193

anode and cathode cells, respectively. Two mixed metal oxide (MMO) electrodes (3 mm

194

diameter, 0.45 m length; Titanium Electrode Products Inc, Stafford, Texas) were inserted 9 ACS Paragon Plus Environment


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into the anode and cathode cells. The electrodes were connected to either (i) a DC power

196

supply (BK Precision–1623A, USA) for EK-induced transport of PS (hereafter referred to

197

as “EK delivery”), or (ii) to an AC variable voltage controller (Startco Energy Products

198

Co., Model: 3PN1010B, OH, USA) for applying ERH to thermally activate the PS

199

(hereafter referred to as “ERH activation”). For the ERH applications, constant power

200

was supplied and AC voltage and current varied with time due to changes in the cell’s

201

electrical conductivity.

202

The experiments were divided into 8 phases as summarized in Table 1. Phase 1, as

203

mentioned above, was the PCE equilibration stage of 18 days for the control experiment

204

and 5 days for the EKTAP experiment, the end of which was established as time = 0 days

205

for the experiment. It is noted here again that the equilibration period refers to silt-PCE

206

solution system equilibrium within the sand tank. The experiments began with 21 days of

207

EK application (Phase 2). In the EKTAP experiment, this was the first PS delivery phase,

208

achieved with a constant head of 10 g/L buffered PS solution maintained in the cathode

209

cell and reservoir. In the control experiment, conditions were identical but the cathode

210

cell and reservoir contained buffer solution with no PS. In both experiments, a constant

211

head was maintained in the anode cell of only buffer solution. A constant current (25mA)

212

was applied throughout this “EK-PS” phase to enable PS migration due to EM through

213

the silt during the EKTAP experiment. It is noted that application of EK would change

214

the electrical conductivity of the pore fluid; therefore, the DC voltage between the anode-

215

cathode would change with time to maintain the constant current condition. The DC

216

voltage varied between 12 and 18V for the EKTAP experiment and between 11 and 26V

217

for the Control experiment. pH was monitored in the anode and cathode cells. These cells, 10 ACS Paragon Plus Environment

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and the reservoirs feeding them, were completely replaced with new solutions every 4 to

219

6 days of EK application to maintain buffering capacity.

220

Phase 3 involved heating of both experimental systems with the objective of activating PS

221

in the EKTAP experiment. For this, the DC power source was disconnected and the AC

222

voltage controller was attached to the electrodes. In this phase, heating via ERH was

223

performed such that the soil temperature averaged approximately 50 °C over its duration

224

of 8 days. This was achieved by setting the Variac output voltage to 78% (i.e., 110 V,

225

AC). At the end of Phase 3, the system was allowed to cool down to room temperature

226

over the course of 4 days (Phase 4). This was followed by Phase 5, which involved a

227

second EK application for 3 days (DC 25 mA constant current); in this case, there was no

228

addition of PS because only buffer solution was placed in cathode cell and reservoir. This

229

“EK-only” phase was conducted to evaluate the changes in PS and PCE concentrations

230

existing in the silt due to EK application.

231

Control

-18

0-21

-0

(No PS)

21-29

29-33

33-36

36-61 (No PS)

61-91

(Phase 8)

Cooling Period (No EK, No PS)

Period 2) (Phase 7)

ERH Activation (Heating

(Phase 6)

EK Delivery (EK-PS Period 2)

(Phase 5)

EK-only (No PS)

(Phase 4)

Cooling Period (No EK, No PS)

Period 1) (Phase 3)

ERH Activation (Heating

(Phase 2)

EK Delivery (EK-PS Period 1)

PCE Equilibration (Phase 1)

Table 1: Different Phases during the Experiments

Experiments/Days

232

91-101



EKTA P

-5-0

0-21 (10 g/L)

21-29

29-33

33-36

Page 12 of 32

36-61 (40 g/L)

61-91

-

233 234

As discussed in the results and discussion section, PCE remained in the system at the end

235

of Phase 5 of the EKTAP experiment. Consequently, Phase 6 of this experiment involved

236

a second application of EK-assisted PS delivery; this “EK-PS Period 2” lasted for 25 days

237

using a higher dose of PS in the cathode cell and reservoir (40 g/L) but the same applied

238

DC current (25 mA) (Table 1). Phase 7 involved activating the PS using ERH, but with a

239

lower target temperature of 35 °C. The reduced activation temperature was designed to

240

evaluate the impact of temperature on PS decomposition and PCE degradation. To

241

achieve this, the Variac output voltage was gradually increased from 10% (13 V, AC) to

242

30% (45 V, AC) between day 61 and 64.5, then held at 30% until day 79.5 and then

243

reduced to 20% (28 V, AC) until the end of the phase. This ERH activation phase lasted

244

for 30 days.

245

A similar approach was followed in the Control experiment except for the use of PS in the

246

cathode cell and reservoir during Phases 2 and 6. Therefore, the anode and cathode cells

247

as well as reservoirs contained only buffer solution. Phases 2, 5 and 6 in the Control

248

experiment (i.e., EK only, no PS) were used to quantify PCE loss due to EO sweeping in

249

the absence of PS; “EO sweeping” refers to the process by which an aqueous species

250

(e.g., PCE) is removed from the porous medium by electroosmotically-induced flow of

251

water into the cathode cell and then to the reservoir. Phases 3 and 7 (i.e., heating phases)

252

enabled quantifying any PCE loss due to ERH application in the absence of PS. In the 12 ACS Paragon Plus Environment

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Control experiment only, the second ERH activation phase was followed by a 10-day

254

cooling period (Phase 8).

255

Six sampling ports with luer lock valves were installed on the front panel of the central

256

soil cell to collect aqueous samples from the soil (P1 to P6, Figure 1). Aqueous PCE

257

concentrations were quantified using a GC-ECD system and PS concentrations were

258

determined using the UV-spectrometer method at 352 nm

259

analysis information can be found in the SI.

260

Four thermocouples (Dial Thermometers, Taylor, NM, USA) were installed (T1 to T4,

261

Figure 1). The exposed metal portions outside the cell were insulated with silicon. The

262

temperatures from all four were manually recorded at 12 hr intervals, decreasing to 4 hr

263

intervals between days 21 and 29 during ERH Heating Period 1 and between days 61 and

264

64.5 during ERH Heating Period 2. In addition, thermal images were taken during Phase

265

7 using an infrared camera (A320, FLIR Systems Ltd., Burlington, ON, Canada) to map

266

the distribution of temperature. No gas evolution was observed in either experiment.

267

3

268

3.1 Persulfate migration

269

During the first period of EK-Delivery in the EKTAP experiment (Phase 2), PS migrated

270

from the cathode to P1 (6.5 cm away) in 1 day, to P4 (29 cm away) after 4 days, and to

271

the anode cell (36 cm away) in 5.5 days of applied DC electric field (Figure 2a). An

272

average transport rate of 2.0±0.7 cm/day was estimated based on the arrival times of 50%

273

of the maximum PS concentration observed at a given sampling location. During that

274

experiment’s second EK-Delivery period (Phase 6) the average PS transport rate was

41

. Additional sampling and

Results and Discussion



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1.9±1.3 cm/day. The transport rates observed in the current study were similar to that

276

observed by Fan, et al. 34 (1.0 cm/day) and Robertson 31(0.5 cm/day).

277

Maximum PS concentrations differed between sampling locations with a gradual

278

decreasing trend from P1 (5.5 g/L; near the source) towards P4 (2.5 g/L; farthest from the

279

source) at the end of Phase 2 (Figure 2a). Recall that the sampling locations P1 to P4

280

were located in the same horizontal plane (Figure 1). Note that significant PS

281

decomposition was not expected during this phase since the setup was at room

282

temperature (20 °C). This expectation was corroborated by the observed absence of

283

sulfate that would have evolved from PS decomposition. A similar trend, decreasing PS

284

concentration from P1 to P4, was observed in Phase 6 (Figure 2a). Concentrations were

285

similar along the same vertical transect (e.g., sampling ports P2 and P5 as well as P3 and

286

P6).

287

PS concentration in the source cell as well as reservoir was increased to 40 g/L during

288

Phase 6 to evaluate the impact of higher PS dosage. Maximum PS concentrations at the

289

observation ports were approximately 60% and 29%, when normalized to source

290

concentrations, during Phases 2 and 6, respectively. The lower normalized maximum PS

291

concentrations during Phase 6 in comparison to Phase 2 suggests that injection at higher

292

PS dosages does not necessarily translate to a proportional increase in mass transport

293

when normalized to injection concentration. Fan, et al.

294

PS transported into the soil, due to EM of PS, which was 22.5% of the injected PS

295

concentration (200 g/L). Higher PS source concentrations, ranging from 100 to 400 g/L,

296

were used in previous EK-PS studies

297

concentration (i.e., 10 g/L) would increase reagent cost and may not result in increased

33, 34, 36, 42

34

observed an average of 45 g/L

. The use of a higher PS source


Page 15 of 32


43

298

mass delivery. Wu, et al.

299

permanganate) concentration by 50% resulted in only a 10% decrease in PCE mass

300

destruction. The same study further suggested that use of lower permanganate

301

concentrations resulted in reduced remediation time as well as lower energy consumption.

302

Further work is therefore required to optimize PS injection concentrations to balance the

303

mass delivered, within a given time, and the cost associated with PS loss in the porous

304

medium during migration for field applications.

suggested that decreasing the oxidant (in that case,

305 15 ACS Paragon Plus Environment


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306 307 308 309 310 311 312

Figure 2: Observed data for a) aqueous persulfate concentrations; b) measured temperatures (at locations T1 to T4, Figure 1); c) sulfate concentrations, and d) PCE concentrations (at locations P1 to P6, Figure 1). The “blue boxes” represent EK-Delivery periods for PS migration while the “orange boxes” represent PS thermal activation periods. The region bounded between vertical green lines represent the TG1 to TG5 temperature regimes from left to right (discussed in Section 3.2).

313

3.2 Thermal activation and persulfate decomposition

314

PS that was delivered to the silt during Phases 2 and 6 was heat activated during Phases 3

315

and 7, respectively. Application of a constant, high current (i.e., 190 mA of AC source)

316

resulted in rapid increase in temperature at all four thermocouple locations (Figure 2b).

317

For example, the temperature increased 13 °C/day on average (T1 through T4) during the

318

first 1.5 days of Phase 3 and then 5 °C/day until the end of this phase. The final

319

temperature at the end of Phase 3 (i.e., experiment day 29) was 62±7 °C. For Phase 7,

320

when power was increased gradually, temperature increased at 2.3 °C/day during the first

321

11.5 days. The rate of temperature increase then reduced to 0.6 °C/day as the input AC

322

power was held constant to maintain the targeted temperature (35 °C) between

323

experiment days 72.5 and 79 (Figure 2b). The average temperature was then decreased

324

from 36±1 °C, at day 79, to 27±2 °C at day 91 to further evaluate PS decomposition

325

sensitivity to temperature. Temperatures in the upper part of the soil cell were higher than

326

the lower part of the cell at the initial stages of ERH application (e.g., 72 °C at T3 and 55

327

°C at T1 on day 28.5) (Figure 2b). However, the temperature distribution became more

328

uniform as ERH continued. A similar trend was observed during Phase 7. Thermal

329

images taken during Phase 7 confirm this increase in average temperature and more

330

uniform temperature distribution with heating duration (Supplementary Information,

331

Figure S1). 16 ACS Paragon Plus Environment

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332

Based on the rates of temperature increase, the heating periods (Phases 3 and 7) were

333

divided into five temperature groups (referred to as TG1 to TG5), each representative of a

334

distinct average temperature; the divisions are marked by vertical green lines in Figure 2.

335

The average temperatures of all temperature measurement locations during TG1, TG2,

336

TG3, TG4 and TG5 were 42±5 (day 21-22.5), 53±6 (day 22.5-24.5), 32±4 (day 63-72.5),

337

36±1 (day 72.5-79), and 27±2 °C (day 79-91), respectively (Table S1). Increased

338

temperature resulted in PS concentration decrease due to decomposition (Figure 2 a,b)

339

with faster PS decomposition during Phase 3 in comparison to Phase 7 due to its higher

340

temperatures. For example, complete PS decomposition was observed in 4 days following

341

heat activation in Phase 3 whereas in Phase 7 considerable PS was observed at the end of

342

experiment (i.e., an average of 10% of the source concentration at day 91). Consistent

343

with literature studies, these data suggest that PS decomposition rate was a function of

344

temperature 20, 25.

345

Pseudo-first order rate constants (kobs,PS), fitted to observed PS concentration at each

346

sampling location, were between 0.023±0.006 at 27 °C and 1.19±0.47 day-1 at 52 °C

347

(Table S1, Figure S2 a,b). Activation energy (Ea, equations shown as Equation S1 and S2)

348

for thermal PS decomposition was calculated using fitted kobs,PS values, observed

349

temperatures measured at the nearest thermocouple and equation S2 (Table S1, Figure

350

S3a). Ea for PS decomposition ranged between 126 to 154 kJ/mol at the different

351

measurement locations with an average of 142±10 kJ/mol (Table S1). This value is

352

similar to that obtained by Johnson, et al.

353

(i.e., 120 kJ/mol). To the authors’ knowledge, the cited study is the only other study to

354

quantify Ea for PS decomposition with soil present and the current study is the only one to

25

for continuously stirred reactors with soil



Page 18 of 32

355

provide an in-situ value. Literature values for Ea from batch experiments with water and

356

no soil varied between 118 to 140 kJ/mol

357

both with and without the soil, as well as in the current study suggest that presence of the

358

soil may have less impact on thermal activation of PS than other activators. Activators

359

such as Fe2+ or high pH require that the PS and the activator ion come into contact for PS

360

activation. Mixing processes would be affected by the presence of soil, thus Ea might

361

differ for systems with and without soil. In contrast, heat can be applied more uniformly

362

in a system resulting in more homogeneous PS activation.

11, 22, 25

. Similar Ea in the batch experiments,

363 364

To further highlight the importance of temperature on PS decomposition, predicted PS

365

concentrations are shown in Figure S6 at different temperatures using the Arrhenius

366

equation and the parameters obtained from the PS decomposition data of the current work

367

(Table S1). This demonstrates that PS decomposition would be very slow at the average

368

ambient groundwater temperature (approximately 10 °C for Canada), with a half-life of

369

1326 days. Application of ERH to increase the temperature to 20 and 32°C results in

370

decomposition half-lives of 170 and 17 days, respectively. Increasing the temperature to

371

50°C results in a PS half-life of 1 day. This is consistent with the work of Johnson, et al.

372

25

373

no other activation sources (e.g., iron, high pH) were present.

showing that unactivated PS has a half-life of approximately 790 days at 20 °C when

374 375

Sulfate ( ) concentrations increased during Phases 3 and 7 due to PS decomposition

376

(Figure 2c). Theoretically, each mole of PS decomposes to 2 moles of SO4●-, ultimately 18 ACS Paragon Plus Environment

Page 19 of 32


377

resulting in 2 moles of (Equation 1). Observed and theoretical concentrations

378

were in good agreement at all locations (Figure S4). As discussed earlier, PS rapidly

379

decomposed during Phase 3 due to the high temperatures, resulting in rapid increases in

380

which reached a plateau when the PS concentration was depleted (Figure 2a).

381

concentrations increased more gradually during Phase 7 due to the lower

382

temperatures and did not reach a plateau due to excess PS. Once produced, is

383

subject to transport by both EM and EO. EM-induced mass transport is at least 10 times

384

higher than that induced by EO32. Therefore, EM is expected to be responsible for the

385

decrease in concentrations during Phase 5 (EK-only phase; days 33 to 36), whereby

386

migrated into the anode cell and reservoir removing them from the soil

387

compartment. concentrations were not quantified in the anode and cathode cells or

388

reservoirs due to significant dilution and since their fluids were replenished every four

389

days.

390

3.3 PCE degradation

391

In the Control experiment, the setup was left idle to equilibrate for 18 days following

392

packing with PCE-contaminated water (Phase 1, Table 1). Average PCE concentrations

393

were 27±14 mg/L and 21±12 mg/L at day -18 and 0, respectively (Figures 2d and S5). In

394

the EKTAP experiment, the setup was left idle for 5 days (Figure 2d, S5 and Table 1,

395

Phase 1) to allow the PCE to equilibrate within the silt. Average PCE concentrations were

396

7.4±3 mg/L and 8.7±2 mg/L at the beginning and end of this phase, respectively (Figures

397

2d and S5). This suggests that the system was effectively closed after packing of silt was

398

completed (i.e., minimal volatile losses). 19 ACS Paragon Plus Environment


Page 20 of 32

399

PCE concentrations decreased during EK delivery periods (Phases 2 and 6) in both

400

experiments, however the decrease was at a much lower rate than due to reaction with PS

401

during the heating phases (discussed below). The average PCE concentration decreased

402

from 8.7±2 mg/L to 4.7±0.8 mg/L during the 21 days of Phase 2 and from 2±0.5 mg/L to

403

1.1±0.4 mg/L during the 25 days of Phase 6 (Figures 2d and S5) in the EKTAP

404

experiment. Note that no significant PCE degradation was expected at ambient

405

temperatures due to lack of PS decomposition (Figure 3, discussion in Section 3.2).

406

The observed decreases during the EK-Delivery phase were likely due to electroosmotic

407

sweeping of PCE out of the silt pack and into the cathode cell and reservoir. Contaminant

408

(e.g., phenol, TCE) removal due to EO sweeping has been observed in previous studies 44,

409

45

410

rate was determined to be 0.62 and 0.09 mg/day, during Phases 2 and 6, respectively. For

411

comparison, the theoretical EO sweeping rate was also calculated. The in-situ

412

electroosmotic permeability (kEO) could not be measured in the current study due to

413

limitations associated with the experimental setup. Reported EO permeability (kEO)

414

values vary from 4x10-10 to 1x10-8 (m/s)/(V/m) for all soils 46-48. This range of kEO values

415

would result in EO-induced PCE migration rates, using Equation S1, varying between

416

0.05 and 1.2 mg/day. The observed EO sweeping rates are within the range of these

417

calculated EO sweeping rates. PCE was not detected in the cathode cell likely due to

418

dilution effects in the large reservoir.

419

EO sweeping of PCE was also observed in the Control experiment, measured at rates of

420

1.8 and 0.2 mg/day during Phases 2 and 6, respectively. The lower EO sweeping rate

421

observed in the EKTAP experiment compared to the control experiment is not surprising.

. Based on observed PCE concentrations in the EKTAP experiment, the EO sweeping


Page 21 of 32


422

The ionic strength of the electrolyte solution used in the control and EKTAP experiments

423

were 37 mM and 163 mM (Phase 2), respectively. Increased ionic strength decreases the

424

diffused double layer resulting in lower zeta potential on the soil surfaces, which in turn

425

results in decreased EO flow

426

where a higher ionic strength was used in Phase 6 (163 mM and 667 mM during Phases 2

427

and 6, respectively) resulting in lower EO sweeping rates (0.62 and 0.09 mg/day in

428

Phases 2 and 6, respectively). Similar decreased EO transport of phenanthrene due to

429

increased ionic strength was reported in a previous study 49.

430

Figures 2 and S4 show the comparison of PCE concentrations during different phases of

431

EKTAP and Control experiments. In the EKTAP experiment, the average PCE

432

concentration decreased by two orders of magnitude (from 1.1±0.4 mg/L at day 61 to less

433

than 0.01 mg/L on day 91) during the second ERH application (Phase 7). Thermal

434

activation of PS produces reactive sulfate radicals SO4●- that oxidize PCE (Equations 1

435

and 2). PCE concentrations decreased rapidly at the onset of PS activation (Phases 3 and

436

7). During Phase 3 of the EKTAP experiment, PCE concentrations decreased from an

437

average of 4.7±0.8 mg/L at day 21 to an average of 2.7±0.8 mg/L at day 29, when all PS

438

had decomposed (Figure 2). During Phase 7, PCE concentrations decreased from an

439

average of 1.1±0.4 mg/L at day 61 to below the detection limit (0.02 mg/L) after day

440

85.5. During these intervals (i.e., day 21 to 29, and day 61 to 85.5) sulfate concentrations

441

increased due to PS decomposition (Figures 2 a,c; discussion in Section 3.2).

442

comparison average PCE concentrations decreased by only 10% (from 9.41±2.4 to 8.8±2

443

mg/L) and 8% (from 7.1±1.2 to 6.6±0.9 mg/L) during Phases 3 and 7, respectively, of the

444

Control experiment. The minimal decreases in PCE concentrations during the two ERH

46

. This is further corroborated in the EKTAP experiment,

By



Page 22 of 32

445

phases in the Control experiment and the significant increases in sulfate concentration in

446

the EK-TAP experiment suggest that PCE oxidation by sulfate radical was the dominant

447

PCE removal mechanism and PCE volatilization due to ERH application was not a

448

significant contributing factor to PCE removal.

449

The rate of temperature increase was slower during Phase 7 resulting in slower PS

450

decomposition and more sustained and complete PCE degradation. During the first two

451

days of Phase 7 (day 61-63) the rate of temperature increase was very low with negligible

452

generation due to very limited PS decomposition. As temperature further increased

453

after day 63, PCE degradation was observed (Figure 2 a,d). PCE concentrations at P3 and

454

P4 decreased to below the MDL on (i) day 68.5 at P1 and P2, (ii) day 73 at P6, and (iii)

455

day 85.5 at P5 (Figure 2d). Consistently higher PCE concentrations, and slower

456

degradation, were observed at P5, likely due to its lower temperatures (at T1) compared

457

to the other locations (Figure 2b,d). PCE concentrations decreased sharply at all locations

458

1 to 2 days before reaching the MDL. Similar behavior has been observed in previous

459

studies

460

contaminant concentrations could be due to the combined reactivity of SO4●- and other

461

reactive species (e.g., Cl●) resulting from oxidation of chlorinated ethenes.

462

Average pseudo-first order rate constant (kobs,PCE) values obtained in the current study

463

(0.14±0.05 d-1 at 52 °C) were 1 to 2 orders of magnitude lower than those reported in

464

batch experiments in the presence of sand (12 d-1) or kaolinite (3 d-1) at comparable

465

temperature (50 °C)

466

0.9% and 6.4%, suggesting that the reaction in those batch experiments was not limited

467

by availability of reactive SO4●-. The differences in the observed rate constant were not

19, 20

. Waldemer, et al.

14

20

proposed that this distinct downward curvature at low

. The cited study reported that PS decomposition varied between


Page 23 of 32


468

surprising considering the current study was performed in conditions more representative

469

of field applications whereas the literature study was conducted under ideal conditions

470

with continuous mixing.

471

In the current study, complete PS decomposition was observed at the highest temperature

472

(TG2, 52°C average) and PCE degradation at this time, while significant, was less robust

473

and sustained than observed for lower temperatures during thermal activation (Figure 2,

474

Table S1). SO4●- has a half-life of only 4 sec 50, while the half-life of PCE reacting with

475

thermally activated PS is 3 orders of magnitude larger (5940 sec at 50 °C)

476

extremely reactive and the PCE degradation reaction is competing with the tendency for

477

SO4●- to oxidize water

478

50ºC appears to rapidly consume PS with only a fraction of the sulfate radicals achieving

479

PCE oxidation. It suggests that if the sulfate radical is produced in excess (such as at high

480

temperatures), then only a fraction can be consumed by PCE-radical reaction, due to

481

limitations imposed by kinetics and mixing, before competing oxidation reactions

482

consume the remaining radicals. These results suggest that the PS needs to be activated at

483

an intermediate temperature of 25 to 30 ºC, such that SO4●- is generated at a rate more

484

amenable to effective mixing and reaction with the target contaminant.

485

4

486

Oxidant delivery into low permeability porous media is challenging given the poor

487

hydraulic accessibility of these soils. The current study overcame these limitations by

488

employing a novel combination of EK-assisted PS delivery and low temperature ERH-

489

activation of the delivered PS. Electromigration was shown to successfully deliver the PS

22

20

. SO4●- is

. This work shows that activating PS at temperatures around

Environmental Implications



Page 24 of 32

490

at a rate of approximately 2.0 cm/day through the silt used in the experiments, during the

491

EK-assisted PS delivery phases. Low temperature ERH was successful in activating PS

492

homogenously throughout the silt, resulting in the complete destruction of PCE contained

493

in the silt within 18 days after activation. This combination of technologies has never

494

been previously demonstrated and can be efficiently scaled-up for field application. Both

495

techniques use the same equipment, commonly available electrodes, and requires only

496

switching from a DC electric field (for PS delivery) to an AC electric field (for PS

497

activation).

498

This study demonstrated that increasing the source PS concentration resulted in an

499

increased PS mass transport rate, delivering more mass at a given distance within a given

500

time. However, increased source PS concentration does not necessarily result in a

501

proportional increase in pore fluid PS concentration. In a field setting it is desirable to

502

deliver the maximum oxidant mass at the fastest rate that can be achieved within the

503

budget. The results of the current study suggest that PS would slowly decompose at

504

ambient groundwater temperatures. This suggests PS can be delivered to the targeted

505

contaminated zone without any appreciable loss due to PS decomposition, where it can be

506

thermally activated to degrade in-situ contaminants. This study also suggests that PS

507

activation at temperatures around 30 °C are likely more effective than the high

508

temperatures (≥40 °C) applied in most lab-scale studies with heat assisted PS activation.

509

Application of higher temperatures (~50 °C) rapidly decomposed the available PS and

510

achieved significantly less degradation of PCE than around 30 °C. Field trials are

511

underway to evaluate this technology for chlorinated solvent treatment at contaminated

512

industrial sites. 24 ACS Paragon Plus Environment

Page 25 of 32


513

Although this technology holds significant promise, as with any remediation technology

514

there are a number of challenges that need to be considered when moving to the field

515

scale. For example, natural oxidation demand (NOD) can be a source of unwanted PS

516

consumption, reducing the efficacy of ISCO. Another consideration is increasing

517

electrode spacing would increase power consumption. Finally, in highly transmissive

518

zones adjacent to low permeability zones it may be difficult to maintain high persulfate

519

concentrations for delivery to the low permeability zones due to the large advective flux

520

in the highly transmissive zones. It is anticipated that these challenges can be overcome

521

through good engineering design and testing at the pilot scale.

522



Page 26 of 32

523

ASSOCIATED CONTENT

524

Supporting Information: Calculation of activation energy, electroosmotic flux,

525

temperature maps and additional constituent concentration data.

526

ACKNOWLEDGEMENTS

527

This work was funded by Ontario Research Fund—Research Excellence Program

528

(Ontario, Canada) for the ORF-RE-WR01 Project Innovative, and Natural Sciences and

529

Engineering Research Council of Canada (NSERC) Engage Program (Grant Number:

530

449311-14) in collaboration with Geosyntec Consultants, Canada. We appreciate the

531

assistance of Dr. Cjestmir deBoer and Dr. Hardiljeet Boparai during the experiments.

532

Also, special thanks to David Gent, U.S. Army Corps of Engineers for supplying the test

533

apparatus and advice as well as Nicole Soucy. EK-TAP is patented by Geosyntec

534

Consultants

535

2014249598 and Canadian Filing 2,901,360, Granted patents EUR 2969275, and US

536

9,004,816 B2)

(International

PCT

Filing

PCT/US2014/021033,

Australian

Filing


Page 27 of 32


537

5

538 539 540

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541 542 543

2. Parker, B. L.; Chapman, S. W.; Guilbeault, M. A., Plume persistence caused by back diffusion from thin clay layers in a sand aquifer following TCE source-zone hydraulic isolation. Journal of Contaminant Hydrology 2008, 102, (1–2), 86-104.

544 545 546 547

3. Seyedabbasi, M. A.; Newell, C. J.; Adamson, D. T.; Sale, T. C., Relative contribution of DNAPL dissolution and matrix diffusion to the long-term persistence of chlorinated solvent source zones. Journal of Contaminant Hydrology 2012, 134–135, 6981.

548 549 550

4. Chapman, S. W.; Parker, B. L., Plume persistence due to aquitard back diffusion following dense nonaqueous phase liquid source removal or isolation. Water Resources Research 2005, 41, (12), 1-16.

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5. Ball, W. P.; Liu, C.; Xia, G.; Young, D. F., A diffusion-based interpretation of tetrachloroethene and trichloroethene concentration profiles in a groundwater aquitard. Water Resources Research 1997, 33, (12), 2741-2757.

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