Comparison of humic and fulvic acid on remediation of arsenic contaminated soil by electrokinetic technology

Journal Pre-proof Comparison of humic and fulvic acid on remediation of arsenic contaminated soil by electrokinetic technology Jiangpeng Li, Ying Ding, Kaili Wang, Ningqing Li, Guangren Qian, Yunfeng Xu, Jia Zhang PII:

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https://doi.org/10.1016/j.chemosphere.2019.125038

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Received Date: 26 July 2019 Revised Date:

30 September 2019

Accepted Date: 2 October 2019

Please cite this article as: Li, J., Ding, Y., Wang, K., Li, N., Qian, G., Xu, Y., Zhang, J., Comparison of humic and fulvic acid on remediation of arsenic contaminated soil by electrokinetic technology, Chemosphere (2019), doi: https://doi.org/10.1016/j.chemosphere.2019.125038. 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.

Comparison of humic and fulvic acid on remediation of arsenic contaminated soil by electrokinetic technology

Jiangpeng Li a, b, Ying Ding a, Kaili Wang a, Ningqing Li a, Guangren Qian a, Yunfeng Xu a, *, Jia Zhang a, ** a School of Environmental and Chemical Engineering, Shanghai University, No.99 Shangda Rd., Shanghai 200444, P. R. China. b School of Environmental Science and Engineering, Southern University of Science and Technology, 1088 Xueyuan Blvd, Nanshan District, Shenzhen 518055, P. R. China.

* Corresponding authors: Yunfeng Xu, School of Environmental and Chemical Engineering, Shanghai University, No.99 Shangda Rd., Shanghai 200444, P. R. China (E-mail: [email protected]) Tel.: +86 21 66137745 Fax: +86 21 66137761 ** Corresponding authors: Jia Zhang, School of Environmental and Chemical Engineering, Shanghai University, No.99 Shangda Rd., Shanghai 200444, P. R. China (E-mail: [email protected]) Tel.: +86 21 66137745 Fax: +86 21 66137761

1

Comparison of humic and fulvic acid on remediation of arsenic contaminated

2

soil by electrokinetic technology

3

Abstract

4

The use of humic acid (HA) and fulvic acid (FA) as reinforcing agents to improve

5

the efficiency of electrokinetic remediation (EKR) were investigated for the first time

6

using an artificially contaminated soil. A series of soil leaching tests and bench-scale

7

EKR experiments were performed to elucidate the mechanisms of As removed from

8

artificially contaminated soil. The characterization of total reducing capacity (TRC)

9

and functional group were carried out to reveal the difference of HA and FA. The

10

observations demonstrated that with 0.1 M NaOH and KCl as the anolyte, using both

11

HA and FA enhanced the efficiency of EKR. After 25 days of EKR, the removal

12

efficiency of TAs in HA/FA-enhanced EKR was about 2.0 – 3.0 times greater than

13

when unenhanced. Compared to HA, more As was removed in EKR with FA, which

14

has more TRC and oxygen-containing groups. These EKR experimental results, with

15

the support of data obtained from soil leaching test, indicate that competitive

16

adsorption, reductive dissolution and complexation were the reasons why HA and FA

17

promoted the release of As in the soil and further enhanced the remediation efficiency.

18

Keywords: Arsenic-contaminated soil; Electrokinetic remediation; Humic acid;

19

Fulvic acid; Temperature-programmed decomposition

20

1. Introduction

21

Arsenic-contaminated soil is a worldwide environmental issue(Bundschuh et al.,

22

2011; Das et al., 2013; Sarkar and Paul, 2016; Zhou et al., 2018). Both natural 1

23

(volcanic eruption, rock weathering, et al.) and anthropogenic (mining and metallurgy,

24

applications of insecticide, et al.) activities have resulted in a significant input of

25

arsenic into the environment, especially soil(Bissen and H. Frimmel, 2003; Chary et

26

al., 2008; Khan et al., 2008). The bioaccumulation and non-biodegradability of As

27

cause a variety of adverse health effects, such as hyperpigmentation, keratosis,

28

cardiovascular and neurological diseases, chronic lung diseases, and cancers(S.Y.

29

Choong et al., 2007; Wei and Yang, 2010). Therefore, effective remediation

30

technologies for As-contaminated soil are still needed.

31

Until now, As-contaminated soil has usually been treated by many technologies,

32

including soil washing, soil flushing, solidification/stabilization, phytoremediation

33

and bioremediation(Tokunaga and Hakuta, 2002; Verbruggen et al., 2009; Yoon et al.,

34

2010; Banerjee et al., 2011). Among these methods, electrokinetic remediation (EKR),

35

an emerging technique, is considered as a promising in-situ technology, which has

36

proven an effective method even in fine-grained soils(Acar and Alshawabkeh, 1993;

37

Jeon et al., 2015; Fu et al., 2017). In EKR process, the main removal mechanism of

38

contaminant is electrolysis of water, electromigration, electroosmotic flow, and

39

electrophoresis, which govern the mobility and transport of contaminant under an

40

electric field in soil. However, EKR technology only removes mobile contaminant(Xu

41

et al., 2019). It has been challenging to remove contaminants that were adsorbed on

42

the soil surface or precipitated in the soil(Giannis et al., 2009). Therefore, maintaining

43

As in the dissolved phase has been the key problem of treating As-contaminated soil

44

by EKR. 2

45

Many reinforcing agents were developed to enhance the efficiency of EKR

46

technique, and has proven effective in both application, and economically(Baek et al.,

47

2008; Yuan and Chiang, 2008; Isosaari and Sillanpää, 2012). However, these

48

reinforcing agents were usually synthetic chemical reagents that might cause

49

secondary pollution when introduced into the soil, such as synthetic chelator

50

EDTA(Dirilgen, 1998; Giannis et al., 2009). So it has been essential to develop an

51

environmentally friendly and economically viable alternative. Humic acid (HA) and

52

fulvic acid (FA) are ubiquitous in nature without potential toxicity, and the main parts

53

of humic substance (HS). The peculiar feature of HA and FA are polyfunctionality,

54

which enables them to interact with As in soil and has a significant effect on the

55

environmental fate and mobility of As(Buschmann et al., 2006; Weng et al., 2009a;

56

Yang et al., 2016). Hence, the use of HAs and FAs as reinforcing agent has been a

57

promising application in enhanced EKR. To the best of our knowledge, very few

58

reports have documented the application of HA or FA on the removal of heavy metals

59

(Cu(OX)2, Cd, Co, Mn, Ni, Pb and Zn) in soil by EKR technique(Sawada et al., 2003;

60

Giannis et al., 2007; Bahemmat et al., 2016). Moreover, these studies had focused on

61

the application of HA or FA on the removal of cations in the soil. So it is urgent to

62

explore the effect of HA or FA on the fate of oxyanionic As in soil.

63

In this work, HA and FA were applied to improve the efficiency of EKR for the

64

first time using an artificially contaminated soil. During the EKR process, the soil pH,

65

conductivity, accumulated electroosmotic flow and the removal efficiency of TAs

66

were recorded. The characterization of total reducing capacity (TRC) and functional 3

67

group were carried out to reveal the difference of HA and FA. Further more, to

68

evaluate the mechanism of HA and FA on As-contaminated soil, a series of soil

69

leaching tests were performed.

70

2. Materials and methods

71

2.1 soil sample and characterization

72

An artificially As-contaminated soil was used in this experiment. The original soil

73

samples were collected from a disused building site, Shanghai University Baoshan

74

Campus, located in north of Shanghai, China. In original soil, the concentration of As

75

wasn’t detected. Therefore, contaminated soil containing 500 ppm of As was prepared

76

using NaAsO2 (1.73 g), deionized water (1 L), and original soil (2 kg). The

77

As-contaminated soil was air-dried, ground and passed through a 0.85 mm sieve, then

78

homogenized before use. The characteristics of the As-contaminated soil are presented

79

in Table 1. The initial soil conductivity (167.05 µS/cm) was extremely low.

80

2.2 The elemental analysis, FTIR spectroscopy, TRC and TPD of HA and FA

81

Humic acid was purchased from Sinopharm Chemical Reagent Co., Ltd.,

82

(Shanghai, China). Fulvic acid was obtained from Shanghai Aladdin Bio-Chem

83

Technology Co., Ltd., (Shanghai, China). The content of carbon, hydrogen and

84

nitrogen of HA and FA were measured by Leeman China (EuroEA3000) elemental

85

analyzer (Table 2). Fourier transform infrared (FTIR) spectra of HA and FA were

86

obtained on Nicolet 380 spectrometer by the KBr transmission method.

87

The standard redox potential of Fe(III) is 0.77 V. Owing to higher oxidizing

88

potential, Fe(III) was used to evaluate the total reducing capacity (TRC) of HA and 4

89

FA. TRC of HA and FA were quantified by a series of batched equilibrium tests

90

according to our previous experiment(Xu et al., 2014). In brief, 60 mL of Fe(III)

91

solution (0.00357 mol/L, pH = 2) and 0.02 – 0.03 g of HA and FA were mixed in a

92

250 mL glass flask. After N2 purging (100 mL/min) for 15 min, each bottle was

93

air-tightly sealed and darkly agitated in a constant temperature oscillation incubator

94

(25 °C) at 180 rpm for 96 h. After reaction, the concentration of Fe(II) in solution,

95

separated by filtration, was determined by phenanthroline spectrophotometry (752N

96

ultraviolet−visible spectrophotometer, China). One electron transfer being assumed

97

during the redox reaction, the TRC of HA and FA were determined by the amount of

98

Fe(II).

99

Temperature-programmed decomposition (TPD) was used to evaluate the

100

compositions of the oxygen-containing groups on HA and FA. In detail, TPD was

101

performed under a flow of N2 by heating 0.04 gram of sample from 50 °C to 790 °C at

102

3 °C min−1. The GC 7900 was used to continuously monitor the concentrations of

103

evolved CO and CO2, which would be deconvoluted by PeakFit software to quantify

104

the contents of functional groups. Then, the obtained concentrations (mg/L) were

105

transformed into amounts (mmol/g) of functional groups.

106

2.3 Experimental apparatus

107

Figure 1 shows the schematic diagram of experimental setup, which was adopted

108

to conduct a series of bench-scale experiments of EKR. Generally, the apparatus was

109

composed of one perspex reactor used as a soil cell (Length × Width × Height = 18

110

cm × 7 cm × 10 cm), two electrode compartments (Length × Width × Height = 6.5 cm 5

111

× 7 cm × 10 cm), two graphite electrodes (Length × Width × Height =0.2 cm × 7 cm ×

112

10 cm, Density = 1.8 g/cm3), one direct current (DC) power supply (APS3005S-3D,

113

China). Electrode compartments were filled with the processing fluid (Table 3). Nylon

114

fabric was placed between the soil chamber and electrode compartment to separate the

115

As-contaminated soil and electrolyte. Graphite electrodes were inserted directly into

116

electrode compartment. The soil chamber was homogeneously divided into five

117

sampling areas (S1 – S5) from the cathode to anode.

118

2.4 Electrokinetic remediation test

119

Three tests were carried out under different manipulation patterns of EKR (Table

120

3). In EKR process, the soil conductivity is related with remediation efficiency. When

121

the voltage is constant, the rise of soil conductivity leads to the increase of current

122

density, which improves the amount of electroosmotic flow and further enhances the

123

remediation efficiency. So 0.1 M KCl was used to increase the conductivity of

124

As-contaminated soil(Acar and Alshawabkeh, 1993). What’s more, 0.1M NaOH was

125

used to control the pH of anolyte to avoid focusing phenomenon during EKR(Li et al.,

126

2012). Each soil chamber was filled with about 600 g (dry weight) of

127

As-contaminated soil, mixed with 250 mL of deionized water for EKR1, and with 250

128

mL HA or FA solutions at a concentration of 16.0 g/L for EKR2 and EKR3. To

129

achieve homogeneity, the mixture was stirred manually for several minutes. It was

130

then necessary to wait 24 h as a pretreatment before beginning the EKR test. Three

131

experiments were conducted under a constant voltage (25 V) for 25 days. Soil samples

132

were taken at specified sampling areas (S1 – S5) with a plexiglass tube (Diameter = 6

133

0.9 cm), after 5, 10, 15, 20 and 25 days. The depth of sampling was 4 cm. In all

134

experiments, the anolyte was refreshed daily during the EKR processes. After

135

experiment, soil samples were oven dried at 105 °C for 24 h and then passed through

136

a 0.15 mm sieve to analysis. The determination methods for soil pH, soil conductivity

137

and TAs concentration in soil can be found in our previously published paper(Xu et al.,

138

2019).

139

2.5 Soil leaching test

140

To explore the potential mechanism of HA and FA on As-contaminated soil, five

141

groups of soil leaching tests were conducted in duplicate. The experimental condition

142

was shown in Table 4. The As-contaminated soil was submitted to leaching test

143

performed in a 50 ml centrifuge tube at room temperature. 20 ml of the leaching agent

144

was mixed with a certain amount of As-contaminated soil, followed by shaking for 24

145

h in constant temperature oscillation incubator. The mixture would be centrifuged

146

when the shaking was over. The concentration of Non-Purgeable Organic Carbon

147

(NPOC) in the supernatant after filtration was determined by total organic carbon

148

analyzer (Multi N/C2100). The content of metal in the precipitate after drying and

149

grinding was measured by X-ray fluorescence spectrometer (XRF-1800).

150

3. Results and discussion

151

3.1 Total reduction capacity and function group of HA and FA

152

Figure S1 presents standard curve for Fe(II). The total reducing capacity (TRC)

153

of HA and FA measured by Fe( ) reduction method is shown in Figure S2. TRC of

154

HA (0.50 mmol/g) is smaller than that of FA (0.56 mmol/g). The TRC of FA was 1.1 7

155

times that of HA. This result could be explained by the difference of function group

156

between HA and FA.

157

FTIR of HA and FA (Fig. 2) were performed to qualitatively compare the

158

difference of function group. Both spectra were approximately similar. Compared

159

with FA, HA has a weaker band at 2920 cm-1, which was due to the vibration of C-H

160

bond in -CH2 or -CH3. At approximately 3226 cm−1, FTIR spectra of HA (Fig. 2(a))

161

and FAs (Fig. 2(b)) presented a strong band, which can be ascribed to

162

hydroxyl-containing groups, including phenols and carboxyls(Martinez et al., 2016).

163

The band at 1566 cm−1 in HA was attributed to carbonyl groups(Senesi et al., 1989). A

164

similar band in FA was centred at 1557 cm−1, indicating a redshift of 9 cm−1. The band

165

close to 1370 cm−1 was ascribed to the vibration of the C-H bond of -CH3. The band

166

near 1007 cm−1 indicates the presence of C-O of alcohol or carbohydrate(Christensen

167

et al., 1996).

168

When HA and FA were subjected to TPD in inert atmosphere, oxygen-containing

169

groups desorbed as CO and CO2. During thermal decomposition of these groups, CO2

170

was formed from the decomposition of carboxylic acid (around 200 °C for weak

171

carboxylic acid and around 300 °C for strong carboxylic acid), carboxylic anhydride

172

(400 – 500 °C) and lactones (600 – 900 °C). The CO originated from the evolution of

173

groups such as carboxylic acid (around 200 °C for weak carboxylic acid and around

174

300 °C for strong carboxylic acid), lactone (600 – 700 °C) and phenolic hydroxyl

175

(650 °C) and keto (around 800 °C), in which one carbon atom was bonded to one

176

oxygen atom(Zazo et al., 2009; Shen et al., 2010). 8

177

Deconvoluted CO/CO2-TPD spectra of HA and FA are compiled in Fig. 3. As

178

shown in Fig. 3(a), the deconvolution of the CO-TPD profile failed to obtain the

179

phenolic hydroxyl group content, probably because the phenolic hydroxyl group

180

content was too low. The released amount of CO usually varied with the reaction

181

temperature. After about 700 °C, the intensity of CO increased sharply, and the CO

182

content of HA and FA increased to 4538 and 8599 mg/L, respectively. This was

183

probably because of the extremely high levels of ketone or benzoquinone in HA and

184

FA. However, the CO content of FA increased faster than HA, indicating that the

185

content of ketone or benzoquinone in FA is higher than that of HA. Deconvolution of

186

the CO2-TPD curve (Fig. 3(b) and Fig. 3(c)) reveals that HA has four peaks (301

187

404

188

peaks are produced by carboxylic acids, carboxylic anhydride and lactones.

, 571

and 735

) and FA has three peaks (331

, 510

and 690

,

). These

189

In order to get a detailed comparison on the amounts of oxygen-containing

190

groups between HA and FA (Fig. 3), the CO/CO2-TPD profiles were further

191

deconvoluted considering the following contributions: carboxylic acid (301

and

192

331

), carboxylic anhydride (404

and

193

735

). The fittings are summarized in Table 5.

and 510

) and lactone (571

, 690

194

According to the deconvolution results of CO2-TPD curves, the contents of

195

carboxylic acid (8.33 mmol/g) and carboxylic anhydride (16.27 mmol/g) in FA were

196

significantly higher than that of carboxylic acid (5.58 mmol/g) and carboxylic

197

anhydride (11.26 mmol/g) in HA. What’s more, the total content of carboxylic acid,

198

carboxylic anhydride and lactone in the HA and FA were 26.53 mmol/g and 32.22 9

199

mmol/g, respectively. Therefore, according to the results of CO/CO2-TPD, the content

200

of the oxygen-containing groups in the FA was higher than that in the HA.

201

3.2 Accumulated electroosmotic flow during EKR

202

Fig. 4 presents the accumulated electroosmotic flow (EOF) in electrokinetic

203

remediation process. The accumulated EOF gradually increased with time. This result

204

is consistent with what Ryu observed(Ryu et al., 2017). As shown in Fig. 4, the slope

205

of three curves were almost invariable, indicating that the EOF rate was nearly

206

constant during the remediation. In these experiments, the direction of EOF was from

207

the anode to the cathode, this phenomenon was related to soil zeta potential(Tadros,

208

1982). After the treatment, the accumulated EOF of EKR1, EKR2 and EKR3 were

209

1035 mL, 2229 mL and 3009 mL, respectively. It should be point out that the

210

accumulated EOF of EKR2 and EKR3 are 2.2 times and 2.9 times of the EOF

211

accumulated in EKR1, respectively. This indicates that the EOF was significantly

212

enhanced when electrokinetic technology combined with HA or FA. This result is

213

similar to what Baek observed(Baek et al., 2008). Moreover, the enhancement effect

214

of FA on electroosmotic flow was stronger than that of HA. Generally, the EOF rate is

215

influenced by the pore water property, the zeta potential of the soil, and so on. In

216

alkaline condition, the zeta potential of soil surface is negative, and more negative

217

zeta potential enhances the EOF rate(Kim et al., 2005).

218

3.3 Soil pH variation during EKR

219

Fig. 5 shows the soil pH as a function of time and space during electrokinetic

220

experiments. The initial As-contaminated soil pH was 8.59. It can be seen in Fig. 5 10

221

that there is no significant change in soil pH before and after treatment in EKR1. This

222

result suggests that the soil has a buffering capacity. In these three experiments, there

223

was no significant change in the distribution of soil pH from S1 to S5 over time. In

224

EKR1, the soil pH near the cathode was about 9, due to the production of OH− by

225

electrolysis of water at the cathode. The soil pH near the anode varied in 6.75 – 8.18,

226

due to H+ production by electrolysis of water at the anode. From S1 to S5, the soil pH

227

gradually decreased. Because the H + generated at the anode migrated toward the

228

cathode, leading to the decrease of soil pH, and the OH− derived from cathode

229

resulted in the increase of soil pH(Acar and Alshawabkeh, 1993; Xu et al., 2019). In

230

EKR2 and EKR3, the experiments were carried out under different EKR manipulation

231

patterns, with a strong alkali solution (0.1 M NaOH) and electrolyte (0.1 M KCl) used

232

as anolyte solution to maintain an alkaline condition for soil. It can be seen in Fig. 5

233

that the change of soil pH at samples (S1 – S5) in these later experiments were

234

significantly different from those in EKR1. There was no significant change of soil

235

pH in S1 – S4 regions of EKR2 and EKR3, which around 10.4 and 10.2, respectively.

236

But in S5 region, the soil pH decreased sharply, varying in 7.66 – 9.23 and 7.06 – 8.44,

237

respectively. This suggests that the soil pH was significantly increased when 0.1 M

238

NaOH and KCl were used as anolyte in electrokinetic remediation. Moreover, there

239

was no significant difference in the effect of pretreatment of soil with HA or FA on

240

soil pH.

241

3.4 Soil conductivity variation during EKR

242

The initial conductivity of As-contaminated soil was 167.05 µS/cm. Fig. 6 11

243

exhibits the soil conductivity as a function of time and space during electrokinetic

244

experiments. In EKR1, the soil conductivity was lower than the initial soil

245

conductivity during the whole experiment. This indicates that the soil conductivity

246

reduced when the As-contaminated soil was treated by electrokinetic technology using

247

deionized water as the electrolyte. On the 25th day, the soil conductivity was the

248

highest, and the average soil conductivity in the five sampling areas (S1 – S5) was

249

122.62 µS/cm. In EKR 2 and EKR3, the soil conductivity was significantly higher

250

than the initial soil conductivity during the electrokinetic process. What’s more, the

251

soil conductivity increased with time. This suggests that with 0.1 M NaOH and KCl

252

as the anolyte, the soil conductivity was significantly increased when the

253

As-contaminated soil was treated by electrokinetic technology combined with HA or

254

FA. However, compared with the results of EKR2, soil conductivity in EKR3 was

255

lower, indicating that the effect of FA combined EKR on the increase of soil

256

conductivity was worse than that of HA combined EKR. It was primarily because FA

257

has more oxygen-containing groups than HA, promoting the release of more soluble

258

salts, which was then removed by electrokinetic technology.

259

3.5 Effect of EKR on TAs distribution in soil as a function of time and space

260

Fig. 7 describes the TAs concentration in the soil as a function of time and space

261

after during EKR. Due to the movement of negative charged As toward the anode by

262

electromigration, the concentration of TAs was lower at near cathode (S1, S2) than at

263

the region close to anode (S4, S5). Moreover, the removal efficiency of TAs in EKR1,

264

EKR2 and EKR3 gradually increased over time. By the end of the experiment, 13.8 %, 12

265

33.8 % and 38.5 % of TAs in EKR1, EKR2 and EKR3, respectively, had been

266

removed from the As-contaminated soil. The results are comparable with the findings

267

in other studies in which As-contaminated soil was treated by EKR(Suzuki et al.,

268

2013; Ryu et al., 2017; Xu et al., 2019). The removal efficiency of TAs in EKR2 and

269

EKR3 were 2.5 and 2.8 times greater than in EKR1, respectively. It should be point

270

out that when HA and FA were used as soil pre-treatment, a significant improvement

271

compared to the EKR1 result was observed, which indicates that more As was

272

released from contaminated soil in EKR2 and EKR3. This may be due to the

273

following two points: (1) The extent of As desorption from the contaminated soil was

274

highly dependent on pH(Suzuki et al., 2013). Under alkaline conditions, the soil

275

surface was negatively charged, which resulted in the release of more As from the soil

276

surface; (2) There was a large number of oxygen-containing groups in HA and FA,

277

which promoted the release of As from contaminated soil(Chen et al., 2006; Weng et

278

al., 2009a). However, the removal efficiency of TAs in EKR3 was 13.9% higher than

279

that in EKR2. This may be ascribed to the fact that the oxygen-containing groups

280

content and TRC of FA were larger than HA (Figure S2 and Table 5).

281

3.6 Effect of HA and FA on As-contaminated soil

282

Five groups soil leaching tests (Table 4) were carried out in duplicate to explore

283

the effect of HA and FA on As-contaminated soil. The ability of HA and FA to

284

enhance the release and mobility of As has been well demonstrated(Chen et al., 2006;

285

Weng et al., 2009a; Mikutta and Kretzschmar, 2011). Therefore, this soil leaching test

286

focused on the mechanism by which HA and FA promote the release of As from 13

287

contaminated soil. Table 6 lists the content of NPOC in the leachate. The NPOC

288

concentration in the leachate of the blank group (Test 1) was 15.2 mg/L, indicating

289

that 15.2 mg/L NPOC of soil was released into the leachate after leaching with

290

deionized water for 24 h. According to Test 4 and Test 5, the NPOC concentration in

291

the leachate were 1582.5 mg/L and 4645.0 mg/L, respectively. This suggests that part

292

of HA and FA were dissolved into leachate. Therefore, if there was no interaction

293

between humic substance (HA or FA) and As-contaminated soil, the NPOC

294

concentration in the leachate should be 1597.7 mg/L and 4663.2 mg/L in Test 2 and

295

Test 3, respectively. However, in fact, the NPOC concentration of the leachate were

296

592.5 mg/L and 4132.5 mg/L in Test 2 and Test 3, respectively. It is obvious that

297

1005.2 mg/L and 527.7 mg/L NPOC of leachate disappeared, respectively, after

298

leaching 24 h of As-contaminated soil with HA or FA. This indicates that HA and FA

299

interacted with As-contaminated soil, resulting in a significant decrease of NPOC

300

content in the leachate. This phenomenon may be attributed to the fact that part of the

301

HA and FA in the leachate has entered into solid phase. In other words, the adsorption

302

site on the solid phase was occupied by part of HA or FA, which promoted the release

303

of As through competitive adsorption and led to a decrease in NPOC content in the

304

leachate(Weng et al., 2009b). When HA was used as the leaching agent, the

305

concentration of NPOC in leachate decreased as compared with FA. This was

306

primarily because HA is usually composed of macromolecules with a long carbon

307

chain, while FA is often composed of small molecules with a short carbon chain.

308

When HA and FA were combined with the same amount of adsorption sites of the soil, 14

309

the NPOC content in the HA leachate decreased more significantly due to the long

310

carbon chain of HA.

311

The precipitate after drying and grinding was analyzed by X-ray fluorescence

312

spectrometer to determine the content of metal. Fig. 8 shows the content of metal in

313

the precipitate. The total content of Mn, Al, Ca and Fe in Test 1 was the highest,

314

reaching 31.8%. The total content of Mn, Al, Ca and Fe in Test 2 and 3 were 30.4%

315

and 29.5%, respectively. Compared with Test 1, the total metal content of precipitates

316

in Test 2 and Test 3 was decreased by 1.4% and 2.2%, respectively. This indicates that

317

the metal content in the soil reduced significantly when leaching As-contaminated soil

318

with HA or FA. It should be point out that As in soil often existed in combination with

319

metal minerals. Therefore, it can be inferred that when HA or FA was used as the

320

leaching agent, HA or FA interacted with the metal minerals in As-contaminated soil,

321

promoting the release of metal into the soil liquid phase. Finally, the total metal

322

content of precipitate decreased. One potential mechanism is that the reductive

323

dissolution of metal minerals by HA or FA promoted the release of As(Yang et al.,

324

2016). Another possible mechanism is that HA or FA formed a ternary complex with

325

As and metal cation to promote the release of As in solid phase. By far, the most

326

accepted mechanism of ternary complexation is the polyvalent metal cation forming a

327

bridge between negatively charged As oxyanions and HA or FA(Redman et al., 2002;

328

Liu and Cai, 2010; Mikutta and Kretzschmar, 2011). Compared with HA as the

329

leaching agent, the amount of metal (especially Ca and Fe) in soil decreased

330

significantly when leaching 24 h of As-contaminated soil with FA. This suggests that 15

331

the interaction between FA and metal in solid phase was stronger than that of HA.

332

4. Conclusions

333

This study provides a new electrokinetic technology to treat As-contaminated soil

334

by using HA and FA as reinforcing agents and controlling anolyte. The following

335

conclusions were drawn:

336

(1) The total reducing capacity (TRC) of FA (0.56 mmol/g) was 1.1 times that of

337

HA (0.50 mmol/g). The total content of carboxylic acid, carboxylic anhydride and

338

lactone in the HA and FA were 26.53 mmol/g and 32.22 mmol/g, respectively.

339

(2) In EKR experiments, the removal efficiency of TAs was enhanced using HA

340

and FA as reinforcing agents and controlling anolyte. The removal efficiency of TAs

341

in EKR2 and EKR3 were 2.5 and 2.8 times as great as in EKR1, respectively.

342

Compared to HA, the removal efficiency of TAs was increased significantly in EKR

343

with FA, which has more oxygen-containing groups and TRC.

344

(3) Through soil leaching test, three ways of HA or FA interacting with

345

As-contaminated soil were concluded: (a) The adsorption site on the solid phase was

346

occupied by HA or FA through competitive adsorption to promote the release of As;

347

(b) The reductive dissolution of metal minerals by HA or FA promoted the release of

348

As, which combined with the metal minerals; (c) HA or FA formed a ternary complex

349

with As and metal cation to promote the release of As in solid phase.

350

Acknowledgements

351 352

This work was supported by National Nature Science Foundation of China (21878183, 41472312) and Innovative Research Team in University (IRT13078). 16

353

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22

Table 1 Properties of As-contaminated soil Table 2 Elemental composition of HA and FA Table 3 Experimental condition of electrokinetic remediation test Table 4 Experimental condition of soil leaching test Table 5 Results of deconvolution of CO2-TPD curves Table 6 The content of NPOC in the leachate

Table 1 Properties of As-contaminated soil Property

Result

pH

8.59

Conductivity µS/cm

167.05

O

%

28.09

C

%

2.85

Na mg/kg

1603.6±0.18

K mg/kg

2045.6±1.03

Ca mg/kg

2253.8±1.61

Mg

256.4±3.38

mg/kg

As (mg/kg)

496.7±6.31

Table 2 Elemental composition of HA and FA Sample

C

%

H

%

N

%

H/C

N/C

HA

43.66

5.372

0.164

0.1230

0.0038

FA

39.21

0.199

5.564

0.0051

0.1419

Table 3 Experimental condition of electrokinetic remediation test Experiment

Anolyte

Catholyte

Duration

Voltage

(days)

(V)

Soil saturation 5, 10, 15, 20,

EKR1

Deionized water

Deionized water

Deionized water

25 25

0.1 M NaOH, EKR2

5, 10, 15, 20, Deionized water

16.0 g/L HA

0.1 M KCl 0.1 M NaOH, EKR3

5, 10, 15, 20, Deionized water

0.1 M KCl

25 25

16.0 g/L FA

25 25

Table 4 Experimental condition of soil leaching test Number

As-contaminated soil

Leaching agent

Duration

Test 1

2g

Deionized water

24 h

Test 2

2g

16 g/L HA

24 h

Test 3

2g

16 g/L FA

24 h

Test 4

0g

16 g/L HA

24 h

Test 5

0g

16 g/L FA

24 h

Table 5 Results of deconvolution of CO2-TPD curves Oxygen-containing groups

HA

FA

Carboxylic acids (mmol/g)

5.58

8.33

Carboxylic anhydride (mmol/g)

11.26

16.27

Lactone (mmol/g)

9.69

7.62

Sum total (mmol/g)

26.53

32.22

Table 6 The content of NPOC in the leachate Number

Leaching agent

NPOC concentration

Test 1

DW

15.2 mg/L ± 2.63%

Test 2

HA

592.5 mg/L ± 1.71%

Test 3

FA

4132.5 mg/L ± 0.97%

Test 4

HA

1582.5 mg/L ± 0.30%

Test 5

FA

4645.0 mg/L ± 0.42%

Figure 1 The schematic diagram of EKR test set-up Figure 2 Fourier transform infrared (FTIR) spectra of (a) Humic acids and (b) Fulvic acids Figure 3 Deconvolution of CO and CO2 profiles from TPD curves of HA and FA: (a) HA and FA, CO; (b) HA, CO2; (c) FA, CO2. Figure 4 Accumulated EOF toward the cathode in EKR Figure 5 Soil pH as a function of time and space during EKR Figure 6 Soil conductivity as a function of time and space during EKR Figure 7 TAs residual in the soil as a function of time and space during EKR Figure 8 The content of metal in the precipitate

Fig. 1. The schematic diagram of EKR test set-up

Fig. 2. Fourier transform infrared (FTIR) spectra of (a) Humic acids and (b) Fulvic acids

Fig. 3. Deconvolution of CO and CO2 profiles from TPD curves of HA and FA: (a) HA and FA, CO; (b) HA, CO2; (c) FA, CO2.

Fig. 4. Accumulated EOF toward the cathode in EKR

Fig. 5 Soil pH as a function of time and space during EKR

Fig. 6 Soil conductivity as a function of time and space during EKR

Fig. 7 TAs residual in the soil as a function of time and space during EKR

Fig. 8. The content of metal in the precipitate

Highlights


The characterization of total reducing capacity (TRC) and functional group were carried out to reveal the difference of HA and FA




Soil pH and conductivity as a function of time and space were recorded in EKR process.




The effect of HA and FA as reinforcing agents on the efficiency of electrokinetic remediation (EKR) were compared for the first time using an artificially contaminated soil.




The mechanism of HA and FA on As-contaminated soil was concluded.

Declaration of interests √ ☐The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: