Effects of total organic carbon content and leaching water volume on migration behavior of polycyclic aromatic hydrocarbons in soils by column leaching tests

Journal Pre-proof Effects of total organic carbon content and leaching water volume on migration behavior of polycyclic aromatic hydrocarbons in soils by column leaching tests Ting Cai, Yue Ding, Zhihuan Zhang, Xinwei Wang, Tieguan Wang, Yuanyuan Ren, Yibo Dong PII:

S0269-7491(19)31041-3

DOI:

https://doi.org/10.1016/j.envpol.2019.112981

Reference:

ENPO 112981

To appear in:

Environmental Pollution

Received Date: 24 February 2019 Revised Date:

18 June 2019

Accepted Date: 29 July 2019

Please cite this article as: Cai, T., Ding, Y., Zhang, Z., Wang, X., Wang, T., Ren, Y., Dong, Y., Effects of total organic carbon content and leaching water volume on migration behavior of polycyclic aromatic hydrocarbons in soils by column leaching tests, Environmental Pollution (2019), doi: https:// doi.org/10.1016/j.envpol.2019.112981. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

1

1

Effects of Total Organic Carbon Content and Leaching Water

2

Volume on Migration Behavior of Polycyclic Aromatic

3

Hydrocarbons in Soils by Column Leaching Tests

4 5

Cai Ting1,2, Ding Yue1,2, Zhang Zhihuan1,2 ∗, Wang Xinwei3,4, Wang Tieguan1,2, Ren

6

yuanyuan3,4, Dong Yibo3,4

7

1.College of Geosciences, China University of Petroleum, Beijing 102249, China;

8

2. State Key Laboratory of Petroleum Resources and Prospecting, China University of

9

Petroleum, Beijing 102249, China;

10

3.College of Chemical Engineering and Environment, China University of Petroleum,

11

Beijing 102249, China;

12

4.State Key Laboratory of Petroleum Pollution Control, China University of

13

Petroleum, Beijing 102249, China

14 15

Abstract

16

The risk of soils transferring polycyclic aromatic hydrocarbons

17

(PAHs) into groundwater has caused widespread concern. Research on

18

the leaching behavior of PAHs in soil profiles is very important for

19

assessing this risk. Column leaching tests were carried out to provide

20

insight into the effect of TOC and leaching water volume on leaching

21

behavior of PAHs. Four groups were leached intermittently by deionized

22

water under the same leaching rate for 10 d, 30 d, 90 d and 120 d. These

23

four leaching periods are equivalent to 1 yr, 3 yr, 9 yr and 12 yr of rainfall ∗

Corresponding author. College of Geosciences, China University of Petroleum, Beijing 102249, China E-mail: [email protected]. 1

2

24

time under natural conditions, respectively. To our knowledge, this is the

25

first report to simulate the migration characteristics of PAHs under such

26

long time leaching. The results showed that residual concentrations of

27

PAHs on the surface of soil (0~5 cm) in three columns after 30 d of

28

leaching were 37.9 µg/g, 18.5 µg/g and 3.7 µg/g, respectively, which was

29

consistent with their TOC contents. According to the correlation analysis,

30

both residual concentrations of ∑16PAHs and PAHs with different ring

31

numbers were significantly correlated with the TOC content at depths of

32

5~100 cm after 30 d of leaching. With increased leaching water volume,

33

PAH migration rates significantly decreased (from 3.13 µg/g/d to 0.005

34

µg/g/d) from 10 d to 120 d, which indicates that the initial period of the

35

leaching process has a stronger effect on PAH vertical migration than the

36

later stages of the process. Under long-term leaching, PAHs that were not

37

leached previously were capable of migrating deeper into the soil profile.

38

Therefore, it has the risk of PAH-contaminated soils transferring PAHs

39

into groundwater.

40 41

Key words: PAHs; TOC contents; Migration behavior; Leaching water volume; Column leaching tests

42

Main finding:

43

Under long-term leaching, PAHs that were not leached previously

44

were capable of migrating deeper into the soil profile.

2

3

45

1 Introduction

46

Polycyclic aromatic hydrocarbons (PAHs) are a group of ubiquitous

47

persistent organic pollutants in the environment. Sixteen PAHs have been

48

categorized as priority pollutants by the United States Environmental

49

Protection Agency (US EPA) due to their toxic, mutagenic and

50

carcinogenic properties. Soil is the major sink for PAHs in the

51

environment (Wild and Jones, 1995) and serves as an important medium

52

for the deposition, degradation, migration and volatilization of PAHs. In

53

addition, soils also act as secondary sources of PAHs through re-emission

54

routes, especially for low molecular weight assemblages into the

55

atmosphere (Yang et al. 2015; Daso et al. 2016; Obrist et al. 2015). The

56

pollution levels of PAHs are significantly higher in China than in other

57

Asian countries (Hong et al., 2016). Chen et al. (2014) revealed that 16.1%

58

of the soil in China was contaminated. This phenomenon has attracted the

59

attention of many researchers. Numerous studies have been conducted on

60

many related aspects of PAHs in soils, such as (1) the spatial distributions

61

of PAHs at the different scales (Peng et al. 2016; Xue et al. 2017); (2)

62

characteristics, source identification and risk assessment (Dai et al. 2008;

63

Kamal et al. 2015; Sun et al. 2017); and (3) remediation of

64

PAH-contaminated soils (Falciglia et al. 2016; Kuppusamy et al. 2016;

65

Bezza and Chirwa, 2017).

66

The leaching process can encourage contaminants to migrate 3

4

67

downward, which can lead to surface water and groundwater

68

contamination (Adam et al., 2002). Formerly published results revealed

69

that PAHs accumulated in the soil via long-term irrigation could be

70

revolatilized as secondary emission sources to the atmosphere for LMW

71

PAHs (Cui et al., 2016). In addition, PAHs absorbed in soil colloids (not

72

previously leached) may migrate into the deeper soil undergoing

73

continuous leaching, especially in the horizon having lower TOC.

74

Actually, PAHs in the leachate may be mostly from the soil. The quantity

75

of PAHs that enter into the leachate through the leaching process is

76

determined by the PAH concentrations in the soil as well as the leaching

77

conditions (leaching time and amount). It is very important for an

78

accurate risk assessment to investigate the leaching behavior of PAHs in

79

soil profiles and to reveal whether PAHs could transfer into groundwater.

80

Many studies have focused on the concentration of PAHs (individual and

81

total) in leachate for evaluating the risk of groundwater contamination

82

(Jefimova et al. 2016; Oh et al. 2016). The results from Kadari et al.,

83

(2015) showed oxygenated aromatic compounds are dominated in polar

84

compounds from waters of the leaching. Tian et al. (2015) reported that

85

phenanthrene in the leachate decreased with increasing leaching volume.

86

According to Jefimova et al. (2014), the concentrations of ΣPAHs in field

87

leachate water from aged spent shale clearly decreased during the entire

88

sampling period (from Nov. 2006 to Oct. 2009), which indicated that 4

5

89

long-term leaching could affect the behavior of PAHs, decreasing the

90

concentrations in leachates. In addition, low concentrations of PAHs in

91

leachates could reach unacceptable levels over longer leaching times due

92

to accumulation (Jefimova et al., 2014). Nevertheless, data on the

93

leaching behavior of PAHs in soil profiles are absent from the literature.

94

As demonstrated by Kalbe et al. (2008), once PAH contaminants

95

enter soils, numerous factors including physical parameters (particle size,

96

porosity, and homogeneity), as well as parameters such as TOC content,

97

chemical reaction kinetics, chemical speciation of contaminants, and

98

complexation with other constituents could act together and affect the

99

leachability of PAHs. Among these factors, TOC and clay minerals are

100

considered to be the two most important factors affecting the sorption of

101

organic pollutants in soil (Banach-Szott et al., 2015). Sorption to mobile

102

particles/colloids is the dominant mechanism for PAH mobility (Enell et

103

al., 2016). The effect of sorption is more dominant onto organic matter

104

than onto clay minerals (Chen et al., 2007), and the sorption and

105

desorption of PAHs are primarily regulated by TOC content (Chiou et al.,

106

1998).

107

It has been observed that the high PAH concentration in soils is in

108

accordance with high TOC content (He et al., 2009; Li et al. 2010; Li et al.

109

2014; Daso et al. 2016). Our previous studies (Zhang et al.,2004; He et

110

al.,2009) also found that PAHs in soil profiles are accumulated in topsoils 5

6

111

and are correlated with the TOC (<40 cm), which is most likely caused by

112

the higher TOC content in surface soils. PAHs in the soil horizon which

113

have a lower TOC may easily migrate downward during the leaching

114

process (Adam et al., 2002). The results of Samia et al. (2013) revealed

115

that total PAH concentrations were consistent with the TOC contents

116

under the long-term use of waste water for irrigation. Fei et al. (2017)

117

reported that the transport abilities of PAHs were significantly influenced

118

by TOC. Oleszczuk and Baranet (2005) also indicated that TOC plays a

119

certain role in the transfer of low molecular weight (LMW) PAHs in the

120

initial period (18 months) of their study. Aside from TOC, the ratio of

121

dissolved organic matter (DOM), fulvic acid (FA), humic acid (HA) and

122

humin in organic matter, can also affect the mobility of PAHs during the

123

leaching process (Petruzzelli et al., 2002). Numerous studies have

124

reported the correlation between PAH concentrations and TOC in soils

125

(Simpson et al., 1996; Petruzzelli et al., 2002; Ran et al., 2007; Li et al.,

126

2014). However, little is known about the effect of TOC on the migration

127

behavior (concentrations and characteristics) of PAHs during the leaching

128

process through soil profiles.

129

Most leaching tests have focused on surface soils (<30cm)

130

(Petruzzelli et al., 2002; Zhang et al., 2011; Song et al., 2016), but

131

0~100cm soil profiles have been examined in our study. We propose that

132

samples from the 100 cm depth profile can reveal more useful 6

7

133

information about the influence of PAH retention, partitioning, transport,

134

and fate processes in the vertical soil profile. Leaching tests are

135

fundamental tools for the assessment of contaminant pathways in soils

136

(Krüger et al., 2012). Furthermore, the reproducibility of column tests is

137

reported to be better for the investigation of the leachability of organic

138

contaminants (Grathwohl and Sloot, 2007; Kalbe et al., 2008).

139

This study provided insight into the behavior of 16 priority PAHs in

140

soil profiles (0~100 cm) with different TOC content and different

141

leaching water volumes through leaching tests. The aims of the present

142

study were (1) to study the effect of TOC content and leaching water

143

volume on the migration behavior of PAHs in soils and (2) to describe the

144

characteristics of PAHs with different ring numbers under the effect of

145

TOC and leaching volume at different depths.

146

2 Materials and methods

147

2.1 Sampling

148

Soil samples were collected from three profiles at different sites with

149

different soil types and TOC contents in Beijing. The locations and

150

descriptions of sampling sites are shown in Fig. S1 of the Supplementary

151

Information and in Table S1. To eliminate randomicity, the “quincunx

152

sampling method” was used to collect samples from each site. For each

153

location, ten samples were taken from the soil surface downward at

154

depths of 0-5, 5-10, 10-15, 15-20, 20-25, 25-30, 30-40, 40-50, 50-75 and 7

8

155

75-100 cm. Soil samples were wrapped in precleaned aluminum foil,

156

air-dried at room temperature, ground in a mortar and sieved by passing

157

through a 10-mesh sieve after removing stones and residual roots.

158 159

2.2 Soil column leaching experiments

160

Column leaching tests were performed with glass columns of 105 cm

161

length and 8 cm internal diameter (a scale was marked on the outside of

162

the column). On the bottom and top of the columns, a piece of filter

163

paper and two layers of glass beads (5~6 mm grain size) were placed.

164

Sprinklers and flowmeters were installed vertically above the columns to

165

control the flow rate. The experimental device is shown in Fig. S2, and

166

the experimental design is given in Table 1. First, PAHs were added to

167

the surface soil (0-5 cm) of the column. Then, the soil was stirred evenly

168

and aged in the dark for 15 d (keeping the humidity consistent with that

169

of the original soils). The samples were filled into the columns from the

170

bottom layer to the top layer successively as in the original soil profiles

171

and were compacted slightly so that the bulk density was very similar for

172

all columns. In order for the soil in the column to be closer to the

173

humidity and pore structure characteristics of the underground soil,

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columns were saturated with deionized water prior to the test. The aged

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soils were placed at the surface of the column (0~5 cm) and subjected to

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leaching at a 1 mL/min rate for 4.5 h per day. According to a Beijing 8

9

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Water Resources Bulletin from the Beijing Water Authority, the average

178

annual rainfall from 2009~2015 was 536 mm, and these data are

179

available from . Therefore, the average annual

180

rainfall volume was approximately 2700 mL using the superficial area of

181

the soil column in the tests. Leaching ended when the desired leaching

182

volume had been applied, and then the remaining water in the column

183

was drained, the column was separated and stratified sampling was

184

conducted. Soil samples were wrapped in precleaned aluminum foil and

185

air-dried at room temperature. The content of PAHs in different soil

186

layers was determined by Soxhlet extraction and GC-MS after passing

187

soils through a sieve (100 mesh). Three groups were set as control

188

columns to illustrate the effect of PAHs on leaching tests results in the

189

original soil. Our previous study demonstrated that the reproducibility of

190

leaching test results is good (Cai et al., 2018).

191 192

2.3 Sample pretreatment and GC-MS analysis

193

The extraction was performed by placing soil samples into a Soxhlet

194

apparatus with redistilled dichloromethane/methanol (9:1 v/v) in a 60°C

195

water bath for 36 h. Activated copper powder was added to remove sulfur

196

during the extraction process. The extracts were concentrated by a rotary

197

evaporator to 2~3 mL, transferred into weighing bottles with

198

dichloromethane and dried at room temperature. The extracts were 9

10

199

fractionated

200

chromatography (silica gel: alumina of 3:2 v/v) using n-hexane,

201

dichloromethane/n-hexane (2:1 v/v), and dichloromethane/methanol (98:2

202

v/v) as respective eluents. Aromatics were concentrated to 1 mL under

203

steam of pure nitrogen and prepared for GC-MS analysis.

204

into

Aromatic

saturates,

hydrocarbons

aromatics

were

and

resins

determined

by

column

with

gas

205

chromatography-mass spectrometry (GC-MS) (Agilent 6890/5975, the

206

USA). Helium was used as the carrier gas. The gas chromatograph (GC)

207

operating conditions for aromatic hydrocarbons were 50ºC to 120ºC at

208

20ºC/min and then 120ºC to 310ºC at 3ºC/min. The oven temperature was

209

held for 1 min at 80ºC then increased to 300ºC at 3ºC/min and held for 18

210

min. The injector temperature was programmed at 300ºC. The injection

211

volume was 1.0 microliters. The syringe size was 10.0 microliters. The

212

mass spectrometer (MS) was operated in electron impact ionization (EI)

213

mode with 70 eV electron energy and a scanning range of 50~600 Da.

214

2.4 TOC analysis

215

TOC content was analyzed using a WR112 LECO CS-230

216

carbon/sulfur instrument (LECO Corp., Michigan, USA). Micronized

217

samples (ca. 0.2 g) were weighed and placed into a crucible. Carbonate

218

mineral removal was performed using 7 mol/L HCl in a 60°C water bath

219

for 1 h. The residue was washed with distilled water until the solution pH

220

reached neutral, and then the sample was oven dried at 70°C for 8 h. The 10

11

221

residual powder was analyzed in the LECO instrument.

222 223

2.5 Identification and quantification

224

All of the organic solvents were analytically pure, distilled by a

225

Soxhlet extraction apparatus and tested with gas chromatography. The

226

silica (100~200 mesh) was extracted until it no longer fluoresced before

227

being activated at 150°C for 8 h. The alumina was extracted and activated

228

at 450°C for 8 h. Both silica and alumina were kept in a dryer.

229

Identification of PAHs was performed by matching characteristic ions and

230

their retention times with those of authentic standards, and quantification

231

was performed with the combination of an internal standard

232

(terphenyl-d14) and external standards (16 PAH mixed standard). The

233

standard solution was a combination of external standards and internal

234

standards with dichloromethane in a gradient of six concentrations.

235

A deuteration PAH multi-compound standard was added to air-dried

236

soil samples randomly prior to extraction as a recovery indicator.

237

Recovery standards, internal standards, and external standards were

238

purchased from J & K Chemical Company Limited. The average

239

recoveries of phenanthrene-d10, chrysene-d12, and perylene-d12 were 90%,

240

94% and 96%, respectively. A calibration curve was established based on

241

the standard solutions, and the correlation coefficient R2 ranged from

242

0.9871 to 0.9997 with an average of 0.9920.

243

2.6 Statistical analysis

244

Pearson correlation analysis was conducted using SPSS to examine 11

12

245

the relationship between concentrations PAHs and TOC contents for soil

246

profile layers ranging from depths of 5~100 cm.

247

3 Results and discussion

248

3.1 Distribution of Σ16PAH concentrations in original and control soil

249

columns

250

The Σ16PAH concentrations of the original soil profiles from MTG,

251

FS and HD ranged from 0.044 to 0.552 µg/g, 0.021 to 0.502 µg/g and

252

0.024 to 0.365 µg/g, respectively (Fig.1). These values were significantly

253

lower than the mean concentration in urban soils in Beijing (1802.6 ng/g)

254

(Liu et al. 2010) but still higher than the soil background value for China

255

(1~10 ng/g) (Edwards, 1983). According to the classification criteria of

256

Maliszewska-Kordybach (1996), which were based on an investigation of

257

PAHs in European agricultural soils, ΣPAH concentrations of < 200,

258

200~600, 600~1000, and >1000 ng·g−1 could be classified as

259

noncontaminated, weakly contaminated, contaminated, and heavily

260

contaminated, respectively. Based on this classification, the soils in the

261

sampling sites were weakly contaminated.

262

The concentrations of Σ16PAHs in original (before leaching) and

263

control (after leaching) soil columns are shown in Fig. 1. The maximum

264

concentrations of Σ16PAHs in all soil profiles were in the surface soil

265

(0~40 cm) and concentrations declined with depth, which is consistent

266

with the pattern seen in TOC contents. After leaching, the Σ16PAH

267

concentrations in every soil horizon were clearly lower than their 12

13

268

counterparts in the original soil profiles, suggesting that PAHs could

269

migrate downward during the leaching process. Similar results were

270

reported by Jefimova et al. (2016).

271

The components were divided into three groups according to the ring

272

numbers of the PAHs as follows: 3-ring, 4-ring, and 5~6-ring PAHs.

273

PAHs with 2-rings were not discussed due to their instability. The

274

variation in the proportions of PAHs with different ring numbers in the

275

soil profiles is shown in Fig. S3 of the Supplementary Information. The

276

proportion of 3-ring PAHs at deeper depths was higher than that at 0~30

277

cm depths, but 5~6 ring PAHs were higher at 0~30 cm. The majority of

278

PAHs were dominated by 3 rings in three original soil profiles (except the

279

FS 0~30 cm depth). This fact may be related to the fact that 3-ring PAHs

280

have a much higher enrichment factor in organic soil layers (Xue et al.,

281

2017). The results from Kadari et al., (2015) also showed that the

282

aromatic fraction of the contaminated soil is marked by a preponderance

283

of LMW PAHs. After leaching, the percent of PAHs with different ring

284

numbers changed to different extents, and 4-ring PAHs became dominant,

285

making up more than 60% of the total PAHs (except for MTG at a depth

286

of 0~40 cm). In addition, the percent of 3-ring PAHs clearly decreased

287

and the percent of 4-ring and 5~6-ring PAHs increased in all columns at

288

all depths after leaching, possibly because the physicochemical properties

289

of the PAHs resulted in discrepancies in their relative abundance. 13

14

290

Middle-high ring PAHs have a higher octanol-water partition coefficient,

291

causing them to readily combine with organic matter in soil and adsorb

292

onto the topsoil due to the high content of organic matter (OM). The

293

adsorption and partitioning of PAHs onto OM are considered their

294

primary mechanisms for movement (Zhang et al. 2011). Soil OM has also

295

been speculated to be one of the most important factors affecting PAH

296

leaching in simulation experiments (Zheng et al., 2012).

297

3.2 Effects of total organic carbon content on leaching results

298

3.2.1 PAH residual concentrations at depths of 0~5 cm

299

3.2.1.1 Σ16PAH residual concentrations

300

As shown in Fig. 2a, Σ16PAH residual concentrations in the 0~5 cm

301

horizon markedly decreased after leaching. PAH residual concentrations

302

in MTG, FS and HD after 30 d of leaching were 37.9 µg/g, 18.5 µg/g and

303

3.7 µg/g, respectively, which was consistent with the order of TOC

304

contents. Furthermore, the migration percent of PAHs varied among the

305

columns and was highest in HD, followed by FS and MTG, which was in

306

accordance with the order of TOC content from low to high. These results

307

indicated that TOC content could influence the migration of PAHs. The

308

higher the TOC content was, the lower migration percent of PAHs. A

309

linear correlation of total PAH concentrations versus TOC contents was

310

observed in Li et al. (2010). It has been recognized that TOC plays a 14

15

311

critical role in the fate of PAHs in soils (Yang et al., 2010).

312

The variation of PAHs with different ring numbers in different

313

columns is presented in Fig. 2b. 4-ring PAHs became the dominant

314

species after leaching, constituting more than 60% of the total PAHs.

315

Furthermore, in all the columns, the proportion of 3-ring PAHs decreased

316

while 4-ring and 5-6 ring PAHs increased after leaching. The reasons for

317

this may be as follows: (1) with increasing molecular weight, PAHs have

318

a higher octanol-water partitioning coefficient (Kow), and a positive

319

correlation has been observed between log Kow and log KOC (Villholth,

320

1999). The higher these values are, the more PAHs would be absorbed,

321

the less migrated downward. (2) LMW PAHs are probably mainly

322

transported by leachate water, while HMW PAHs tend to engage in soil

323

particle- or colloid-associated transport (Zhang et al., 2008). These

324

discrepancies make 3-ring PAHs readily migrate downward than 4-ring

325

and 5-6 ring PAHs, resulting in a decreased proportion of 3-ring PAHs.

326

3.2.1.2 Individual PAH concentrations

327

It should be noted that understanding the individual PAH leaching

328

behavior is very important to study the environmental risk of PAHs. The

329

concentrations and residual percent of individual PAHs varied in the

330

different columns after leaching (Fig. 3). Clearly, the residual

331

concentrations of PAHs decreased along the profile in the following order: 15

16

332

MTG> FS> HD, the residual percent followed a similar pattern, which is

333

in accordance with the values of TOC. This result is consistent with the

334

relationship between Σ16PAHs and TOC content. Therefore, TOC plays an

335

important role in regulating the migration behavior of PAHs in surface

336

soil. Additionally, the residual percent increased with increasing ring

337

number, suggesting that 3-ring PAHs exhibit a higher migration ability

338

than 4-6 ring PAHs.

339

A significant discrepancy in residual percent was observed among

340

the PAH compounds (Fig. 3). For 3-ring PAHs, the residual percent in

341

MTG, FS and HD ranged from 20% to 60%, 5% to 30% and 3% to 10%,

342

respectively. Phe and Ant had a similar residual percent that were

343

significantly higher than that of the other 3-ring PAHs. This result

344

indicated that Phe and Ant possess similar migration characteristics. For

345

4-ring PAHs, compounds with higher molecular weights (MW: 228) had

346

higher residual concentrations and residual percentages than lower

347

molecular weights (MW: 202) PAHs. In addition, compounds with the

348

same molecular weight also showed different residual characteristics. For

349

example, the residual concentrations and residual rates of Fla were higher

350

than Pyr, and Chr was higher than BaA. Therefore, Pyr and BaA had a

351

higher migration ability than Fla and Chr, respectively. The two lowest

352

residual percentages among the 5-6 ring PAHs were observed for DahA

353

and BghiP. As shown by Shi et al. (2017), the leaching behaviors of PAHs 16

17

354

during rainfall are mainly affected by the compounds themselves.

355

3.2.2 PAH residual concentrations at depths of 5~100 cm

356

Residual concentrations of Σ16PAHs after leaching in the 5~100 cm

357

depth of the columns with different TOC contents are presented in Fig. 4a.

358

The Σ16PAH residual concentrations decreased with depth increased. PAH

359

concentrations at depths of 5~30 cm were highest in MTG, followed by

360

FS and HD, which is in accordance with the pattern in TOC contents (Fig.

361

1), suggesting that the migration behavior of PAHs in surface soils could

362

be strongly affected by TOC. The higher the TOC value was, the less

363

PAHs migrating downward, the higher PAHs residual concentrations. In

364

this study, Σ16PAH residual concentrations at depths of 40~50 cm were

365

slightly higher than those at 30~40 cm. One possible explanation for this

366

result is that PAHs in the upper layers just migrated into the 40~50 cm

367

layer and accumulated at this depth under current leaching conditions.

368

However, the concentrations of Σ16PAHs at deeper depths were very

369

similar. According to Nam et al.(1998), sequestration of the contaminant

370

was evident in soils or sand with >2.0% organic carbon. The residuals of

371

PAHs is higher when sequestration is stronger. This finding indicates that

372

the residual content of PAHs may be affected by TOC when it reached a

373

certain value. In addition,

374

some factors that could affect the fate of PAHs may act together as PAHs 17

18

375

migrate downward. Many studies have confirmed that soil type, grain size,

376

composition, etc. can also affect the distribution of PAHs (Zhang et al.

377

2008; Liao et al. 2013). The leaching results from López-Piñeiro et al.

378

(2013) indicated that not only did the sorption capacity and macropore

379

structure affect the leaching behavior of MCPA but the amount of

380

water-soluble organic carbon also played an important role in acting as a

381

carrier.

382

After leaching, residual PAHs accumulated at a depth of 0~40 cm,

383

and declined with depth, which is consistent with previous studies (He et

384

al., 2009; Zhang et al., 2004). However, the sum of Σ16PAHs

385

concentrations at depth 0~100cm after leaching was significant lower

386

than the addition PAHs concentrations. This result indicated that a part of

387

PAHs have migrated downward with leachate. The TOC content

388

decreased with depth increased, resulting in soil in deeper horizon unable

389

to capture much PAHs (He et al., 2009). Therefore, PAHs which can’t be

390

absorbed would migrate into deeper soil profile and it would have the risk

391

of groundwater contamination.

392

As shown in Fig. 4b-4d, the proportion of PAHs from different sites

393

shows a similar trend in the soil profile. With increasing depth, the

394

proportion of 3-ring PAHs increased, while 4-ring PAHs decreased, 5-

395

and 6-ring PAHs remained relatively stable. It is deduced to be related to

396

the physicochemical properties of the PAHs. In order to further explain 18

19

397

the effect of TOC on PAHs with different rings, Pearson correlation

398

analysis was conducted. The results indicate that both of the residual

399

concentrations of ∑16PAHs and PAHs with different ring numbers have

400

significantly positive correlation with TOC at depth 5~100cm after

401

leaching 30d (Table 2). Similar results were also observed by other

402

studies (Li et al., 2014; Daso et al., 2016).

403

3.3 Effects of leaching water volume on leaching results

404

3.3.1 Residual characteristics of PAHs at depths of 0~5 cm under different

405

leaching water volumes

406

The effect of leaching time (corresponding to leaching water volume)

407

on PAH content in the 0~5 cm soil layer (per 100g soil) is depicted in

408

Fig.5. The residual content of PAHs at depths of 0~5 cm after leaching

409

was significantly lower than the addition content of PAHs. Furthermore,

410

the migration content of PAHs was obviously higher than residual content.

411

And the higher the leaching volume was, the more migration of PAHs

412

within a certain range. Therefore, leaching water volume is an important

413

factor affecting PAH vertical migration (Tian et al., 2015). However,

414

migration rates significantly decreased with increased leaching volume.

415

During the first leaching process (leaching 10d), the migration rate was

416

3.13 µg/g/d, while with the leaching time reached up to 30d, migration

417

rates decreased to 0.30 µg/g/d in this period. The leaching time 19

20

418

continually increased, when it up to 90d, the PAHs migration contents

419

was 11.71 µg/g, and migration rates was 0.195 µg/g/d. When the leaching

420

time increased to 120 d, the rates decreased to only 0.005 µg/g/d. This

421

result indicated that the initial period of leaching has a stronger effect on

422

PAH vertical migration than the later period.

423

As shown in Fig. 6a, the residual concentration of PAHs with

424

different numbers of rings decreased into different degrees with leaching

425

time. At the beginning of leaching (10 d), 3-ring PAHs residual

426

concentrations showed a rapid decline, then it decreased slowly with

427

leaching time increased. Most individual PAHs with 3-ring showed the

428

similar trend (except Acy). However, for both of individual and whole

429

4-ring PAHs, their migration concentrations increased with leaching

430

volume within a certain leaching time (less than 90d) (Fig. 6a and Fig.

431

S4). It lead to the proportion of 4-ring PAHs gradually decreased (Fig.

432

6b). As shown in Fig.6a and Fig. S4, although the residual concentrations

433

of PAHs after leaching 120d is slightly less than that after leaching 90d.

434

But there is PAHs still migrating downward. In this study, leaching time

435

of 90d and120d is equivalent to 9yr and 12yr of rainfall time under

436

natural conditions. Therefore, under a long leaching time, PAHs that were

437

not leached earlier were capable of migrating downward.

438

The proportion of PAHs with different ring numbers also showed

439

different characteristics after leaching for 10~120 d (Fig. 6b). For 3-ring 20

21

440

PAHs, their proportion slightly increased with increased leaching volume,

441

but still less than that of the additives. For 4-6 ring PAHs, their proportion

442

displayed an increase comparing with additives. A plausible explanation

443

is PAHs with more rings and larger logKow are generally more

444

hydrophobic and prone to be adsorbed (Zheng et al., 2012).

445

3.3.2 Residual concentrations of Σ16PAHs and PAHs with different ring numbers

446

at depths of 5~100 cm

447

The residual concentrations of Σ16PAHs and PAHs with different

448

ring numbers showed similar distribution characteristics (Fig. 7). Most of

449

the PAHs accumulated at depths of 0~40 cm, which is in accordance with

450

Zhang et al. (2016). The concentrations at a specific depth was higher

451

than adjacent depths and this depth was called accumulation depth. For

452

example, after leaching 10d, the accumulation depth of 3-ring, 4-ring,

453

5-and 6-ring PAHs was 25~30cm, 15~20cm and 15~20cm, respectively.

454

And it could be clearly seen that 3-ring PAHs showed a deeper

455

accumulation depth than HWM PAHs. This result suggested that the

456

physicochemical properties of PAHs could play a role during leaching,

457

especially when the leaching volume was lower. However, when the

458

leaching volume continually increased, PAHs with different ring numbers

459

accumulated at the same depth. For example, PAHs with different rings

460

were all accumulated at 20~25cm after leaching 90d. This result indicates 21

22

461

that under a longer-term leaching process, the effect of the PAH

462

properties on the vertical migration of PAHs is no longer evident. In

463

addition, when the leaching volume further increased (leaching for 120 d),

464

accumulation depth appeared at 15~20 cm. One possible reason for this

465

phenomenon may be that when the leaching volume reached a certain

466

value, PAHs that were not leached earlier became capable of migrating,

467

resulting in a decrease in accumulation depth. It is worth noting that the

468

concentration of Σ16PAHs and PAHs with higher ring numbers increased

469

at depths of 50~75 cm after 120 d of leaching, suggesting that PAHs in

470

the upper layers could migrate to this depth and accumulate under current

471

leaching conditions. These results indicated that PAHs that were not

472

leached earlier could become capable of migrating deeper into the soil

473

profile, which is comparable to the findings achieved by Oleszczuk and

474

Baran (2005). Therefore, we assume that if the leaching water volume

475

increased, it is possible for PAHs which accumulated at this depth to

476

migrate downward even further. Furthermore, the addition content of

477

PAHs are the same in four columns, but the sum of residual content of

478

PAHs decreased with leaching volume. That is, the larger the leaching

479

volume was, the more migrations of PAHs. Therefore, under long time

480

leaching, it would have the risk of PAH-contaminated soils transferring

481

PAHs into groundwater.

22

23

482

4. Conclusions

483

The effects of TOC and leaching water volume on migration

484

behavior of PAHs in soils were tested using a series of laboratory column

485

leaching tests. Major findings include the following. First, TOC could

486

play an important role in regulating PAHs migration behavior in soil

487

profile during leaching process. At depths of 0~5 cm, PAH residual

488

concentrations were in accordance with the value of TOC. The higher the

489

TOC content was, the lower migration percent of PAHs. The residual

490

concentrations of both ∑16PAHs and PAHs with different ring numbers

491

were significantly correlated with TOC at depths of 5~100 cm. Second,

492

leaching water volume is an important factor affecting PAH vertical

493

migration. With leaching volume increased, migration rates of PAHs

494

significantly decreased, which indicates that initial periods of leaching

495

(before leaching 10d) have a stronger effect on PAH vertical migration.

496

Third, the sum of residual contents of PAHs in each soil layer after

497

leaching was significantly lower than addition contents. Fourth, under

498

long-term leaching, PAHs that were not leached earlier could become

499

capable of migrating deeper into the soil profile. Therefore, the risk of

500

PAH-contaminated soils transferring PAHs into groundwater remains to

501

be determined.

502

23

24

503

Acknowledgments

504

This work was supported by the National Natural Science

505

Foundation of China (Grant No. 41373126). We are grateful to the

506

anonymous reviewers for their constructive comments and suggestions.

507

We thank Lei Zhu and Shengbao Shi for laboratory assistance.

508

References

509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540

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697 698 699 700

Sampling sites

Figures and Tables

Table 1 Experimental design of soil column leaching tests

Soil types

Name of column

Additions

MTG control Men Tougou

Cinnamon soil

MTG

PAHs

Leaching volume/mL

Leaching time/d

Approximate precipitation time under natural conditions/yr

8100

30

3

8100

30

3

8100

30

3

MTG original Fang Shan

Brown soil

FS control 28

29

FS 10 d FS 30 d FS 90 d FS 120 d

PAHs PAHs PAHs PAHs

2700 8100 24300 32400

10 30 90 120

1 3 9 12

8100

30

3

8100

30

3

FS original HD control Hai Dian

Paddy soil

HD

PAHs

HD original 701 702 703 704

Table 2 Correlation of residual concentrations of Σ16PAHs and PAHs with different ring numbers and TOC at depths of 5~100 cm.

Σ16PAHs 3-ring 4-ring 5-ring and 6-ring 705

MTG 0.808b 0.879b 0.738a 0.788a

FS 0.964b 0.962b 0.963b 0.963b

a<0.05; b<0.01; correlation is significant (two-tailed)

706 707

708

29

HD 0.802b 0.833b 0.720a 0.924b

30

709 710 711 712 713 714

Fig. 1. TOC and Σ16PAH concentrations in original and control soil columns. Original and control soil columns correspond to columns before leaching and columns after leaching, respectively.

715 716 717 718 719

720

a

721

b

722 723

Fig. 2 Residual concentrations and migration percent of Σ16PAHs (a), and proportion of PAHs with different rings (b) at the 0~5 cm depth in column with different TOC contents after leaching. 30

31

724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740

Migration percent= (1-PAH residual concentrations/ PAH additive concentrations) ×100%

741

742

743

31

32

744 745 746 747 748 749 750

Fig. 3 Residual concentrations and residual percent of individual PAHs in different columns at depths of 0-5 cm after leaching. (Acy: acenaphthylene, Ace: acenaphthene, Flu: fluorene, Phe: phenanthrene, Ant: anthracene, Fla: fluoranthene, Pyr: pyrene, BaA: benzanthracene, Chr: chrysene, BbF: benzo[b]fluoranthrene, BkF: benzo[k]fluoranthrene, BaP: benzo[a]pyrene, DahA: dibenz[a,h]anthracene, IcdP: indenopyrene, BghiP: benzo[g,h,i]perylene) Residual percent= (PAH residual concentrations/ PAH additive concentrations) ×100%

751

752 753 754

Fig. 4 Σ16PAH residual concentrations (a) and proportion of PAHs with different rings (b-d) in soil columns with different TOCs after leaching 30d. 32

33

755 756 757 758 759 760 761

762 763 764 765 766 767 768 769

Fig. 5 Additive, residual and migrated contents of PAHs at depths of 0-5 cm per 100 g soil with different leaching times (volume) Migration rates= (Mi-Mi-1)/ {100*(Ti-Ti-1)} Mi, Mi-1: PAHs migration contents per 100g soil under corresponding leaching time, Ti, Ti-1: Leaching time, i=4,3,2,1, corresponding to leaching time of 120d, 90d, 30d and 10d.

770

771

33

34

772 773 774

Fig. 6 Concentrations and proportional characteristics of PAHs with different ring numbers at depths of 0-5 cm under leaching for 10~120 d.

775

776 777 778

Fig. 7 Concentrations of Σ16PAHs and PAHs with different ring numbers at depths of 5-100 cm under leaching for 10~120 d. 34

1

PAH residual concentrations were in accordance with the TOC contents. The higher the TOC content was, the lower migration percent of PAHs.

2

PAHs with different ring numbers had significantly positive correlation with the TOC content at depths of 5~100 cm after leaching 30 d.

3

With increasing leaching volume, PAHs migration rates significantly decreased.

4

Under long-term leaching, PAHs that were not leached earlier were capable of migrating deeper into the soil profile.

Dear Editor: On behalf of all co-authors, we declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Thank you and best regards. Yours sincerely Corresponding author: Zhang zhihuan E-mail: [email protected].