Role of Structure and Microporosity in Phenanthrene Sorption by

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Role of Structure and Microporosity in Phenanthrene Sorption by Natural and Engineered Organic Matter Lanfang Han, Ke Sun, Jie Jin, Xin Wei, Xinghui Xia, Fengchang Wu, Bo Gao, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5022087 • Publication Date (Web): 03 Sep 2014 Downloaded from http://pubs.acs.org on September 7, 2014

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Environmental Science & Technology

Role of Structure and Microporosity in Phenanthrene Sorption by Natural and Engineered Organic Matter

Lanfang Han,

†

Ke Sun, †,* Jie Jin, † Xin Wei, † Xinghui Xia, † Fengchang Wu, ‡ Bo

Gao, § and Baoshan Xingǁ

†

State Key Laboratory of Water Environment Simulation, School of Environment,

Beijing Normal University, Beijing 100875, China ‡

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Research Academy of Environmental Sciences, Beijing 100012, China §

State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin,

China Institute of Water Resources and Hydropower Research, Beijing, 100038, China ‖

Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003,

USA

*Corresponding author. Tel: 86-10-58807493; Fax: 86-10-58807493; E-mail: [email protected] (K. Sun).

1

ACS Paragon Plus Environment


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1

ABSTRACT: Natural sorbents including one humic acid (HA), humins (HMs),

2

nonhydrolyzable carbons (NHCs), and engineered sorbents (biochars) were subject to

3

bleaching to selectively remove a fraction of aromatic C. The structural properties and

4

sorption isotherm data of phenanthrene (Phen) by original and bleached sorbents were

5

obtained. Significant correlations between Phen Koc values by all sorbents and their

6

organic carbon (OC)-normalized CO2 cumulative surface area (CO2-SA/OC)

7

suggested that nanopore-filling mechanism could dominate Phen sorption. After

8

bleaching, natural sorbents still contained large amounts of aromatic C, which are

9

resistant to bleaching, suggesting that they are derived from condensed or

10

non-biodegradable organic matter (OM). After eliminating the effect of aromatic C

11

remaining in the bleached samples, a general trend of increasing CO2-SA/OC of

12

natural sorbents with increasing aliphaticity was observed, suggesting that nanopores

13

of natural sorbents are partially derived from their aliphatic moieties. Conversely,

14

positive relationships between CO2-SA/OC or Phen logKoc of engineered sorbents and

15

their aromaticity indicated the aromatic structures of engineered sorbents primarily

16

contribute to their nanopores and dominate their sorption of HOCs. Therefore, this

17

study clearly demonstrated that the role of structure and microporosity in Phen

18

sorption is dependent on the sources of sorbents.

19 20 21 22 23 24 25

2


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26

INTRODUCTION

27

Sorption of hydrophobic organic compounds (HOCs) to soil/sediment organic

28

matter (SOM) is a crucial factor governing their fate in the environment.1 Thus, the

29

sorption behavior of HOCs in soils/sediments is of growing concern in the vast

30

research.2 Numerous findings on sorption mechanisms between HOCs and SOM have

31

been documented. It has been proposed that chemical composition, physical

32

conformation and polarity of SOM affect HOCs sorption.3 The application of

33

cutting-edge nuclear magnetic resonance (NMR) spectroscopy has emphasized the

34

importance of chemical composition at the molecular level in investigating the

35

sorption mechanism of HOCs by geosorbents.2-9 Among them, the relative role of

36

aliphatic and aromatic carbon (C) domains within SOM in HOCs sorption has drawn

37

particularly great research attention from environmental scientists in the past few

38

decades.2 Much work indicated the significant contribution of the aromatic moieties

39

of SOM to the overall sorption of HOCs and the positive correlations between

40

sorption affinities and aromaticity were highlighted in those studies.4, 6, 10-13 However,

41

sorption potential of aliphatic domains has been demonstrated to be largely ignored in

42

sorption interactions of HOCs with SOM.14-16 Chefetz et al.17 observed a positive

43

trend between Koc values and the aliphaticity of a series of sorbents with different

44

levels of aromaticity and aliphaticity. A similar trend was also exhibited with humic

45

substances that humins (HMs) were observed to have higher sorption affinity of

46

Phenanthrene (Phen) than humic acids (HAs), even though HAs had higher

47

aromaticity than HMs.18 More recently, Ran et al.19 and Sun et al.20 proposed that

48

Phen sorption was strongly correlated to the content of aliphatic moieties of

49

nonhydrolyzable carbon (NHC) and coal samples. These divergent findings suggest

50

that a consensus on the relative role of aliphatic and aromatic C within SOM in 3



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51

affecting sorption process of HOCs is urgently needed. Recently, to elucidate the role

52

of aliphatic and aromatic C as sorption domains for HOCs, Chefetz and Xing2

53

collected a large and diverse set of published data on Phen Koc values, aromaticity and

54

aliphaticity of organic sorbents covering natural and engineered sorbents. They found

55

that when a large data set was plotted, no specific correlation was presented between

56

Phen Koc values and aromaticity of natural sorbents, including humic substances from

57

different sources, biopolymers (such as cellulose, chitin, lignin, cutin, and cutan),

58

diagenesized samples like kerogen, and biological samples such as algae, cuticles, and

59

leaves. Interestingly, when the data for engineered sorbents was added to this data set,

60

a general trend of increasing Koc with increasing aromaticity was recorded although a

61

significant linear relationship between them was not obtained. Conversely, only for

62

natural sorbents, a general trend of increasing phanthrene Koc values with increasing

63

aliphaticity was displayed. If data for engineered sorbents was included, no

64

relationship was exhibited between binding coefficients and aliphaticity.

65

The contribution of pore-filling mechanism to the sorption of HOCs by SOM has identified.21-23 Especially,

66

been previously

it has been showed that the

67

nanopore-filling is the dominant mechanism for sorption of Phen and benzene by

68

NHC and coals.20 It has been mentioned above that a general trend of increasing Phen

69

Koc values with increasing aliphaticity of only natural sorbents and a similar trend

70

between Phen Koc values and aromaticity of engineered samples were reported by

71

Chefetz and Xing.2 If pore-filling mechanism governs the sorption of HOCs by

72

sorbents, it is very reasonable to hypothesized that microporosity of natural sorbents

73

and engineered sorbents should, respectively, be derived from their aliphatic and

74

aromatic moieties. However, how the structure and microporosity of natural and

75

engineered sorbents are related to sorption of HOCs is not well understood. 4


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One chemical degradation technique, referred to as ‘bleaching’, has been

77

previously employed to selectively remove non-condensed aromatic moieties such as

78

lignin-like and polyphenols units in SOM, and simultaneously retain char-derived

79

aromatic C.24 Based on our above hypotheses, bleaching treatment would influence

80

nanopore characteristics, in turn, affect sorption properties of natural and engineered

81

sorbents in a different pattern. Thus, this technique would aid to test our hypothesis.

82

The major works of this study were therefore to: 1) remove the aromatic

83

components of natural and engineered sorbents by bleaching treatment; 2) determine

84

the nanopore properties of original natural and engineered samples (OR) as well as

85

their corresponding bleached samples (BL) using CO2 isotherms at 273 K; 3) obtain

86

the aliphatic and aromatic C characteristics of these OR and BL using cross

87

polarization magic angle spinning C-13 nuclear magnetic resonance (CPMAS

88

13

89

study, natural organic matter fractions (NOM), including HA, HM, and NHC, were

90

selected as natural sorbents; biochars produced from rice straw and pine wood were

91

used as engineered samples.

92

C-NMR); 4) quantify the sorption affinity of HOCs to these OR and BL. In this

MATERIALS AND METHODS

93

Sorbate and Sorbents. Phen was used as a sorbate and purchased from

94

Sigma-Aldrich Chemical Co. One river sediment sample (bulk 1) was collected using

95

a stainless steel grab sampler in July 2008 from one river in the Tongzhou district of

96

Beijing.25 Three soil samples (bulk 5, bulk 7 and bulk 8) were also collected to a

97

depth of 20 cm in July 2007 from the surface soils in the vicinity area of Tianjin near

98

Bohai Bay, China.25 Albic (A) and black (B) soils were sampled from Sanjiang Plain,

99

Heilongjiang province, China.26 The collected samples were subjected to a series of

100

treatment to obtain different organic matter fractions including HA, HM, and NHC, 5



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101

whose extraction along with their purification and homogenization methods were

102

described elsewhere.25, 26 Briefly, HA1 fraction was obtained from mixing extractions

103

with 0.1 M Na4P2O7 for 7 times.27 The soil residue after HAs extraction was

104

demineralized with 1 M HCl and 10% (v/v) HF at 1:5 solid/liquid ratio and shaking at

105

40 oC for 5 d continuously. Finally the supernatant was removed by centrifugation at

106

4500 rpm for 30 min. The same treatment was repeated for six times in order to get

107

HM fraction containing adequate amount of organic carbon (OC) and low mineral

108

content.

109

HCl/HF/trifluoroacetic acid (TFA) method described elsewhere.19,

110

(BC) in this study was obtained by heating an aliquot of the NHC sample at 375 °C

111

for 24 h with sufficient air.29 The six biochars were produced from two kinds of

112

feedstock materials, rice straw and pine wood, respectively. After washing and

113

grinding to obtain a particle size of less than 1.5 mm, these feedstocks were charred at

114

300, 450 and 600 °C, respectively, for 1 h in a closed container under oxygen-limited

115

conditions in a muffle furnace. Then the biochars were washed with 0.1 M HCl

116

followed by deionized (DI) water flushing till neutral pH,30 subsequently oven-dried

117

at 105 °C, and gently milled to pass a 0.25 mm sieve (60 mesh) prior to further

118

analysis. These biochar samples were hereafter abbreviated and referred as to their

119

individual two initial capitals of feedstock source (rice straw and pine wood) (i.e., RI

120

and PI) and heat treatment temperatures (HTT) (300, 450 and 600 °C) (i.e., RI300,

121

RI450, RI600, PI300, PI450 and PI600)

NHC

fraction

was

extracted

from

the

whole

soil 28

using

a

Black carbon

122

The details of bleaching procedures were described elsewhere.24 Briefly, bleaching

123

involved treating 10 g of each sorbent (HA1, NHC1, NHC5, NHC7, NHC8, A-HM,

124

A-NHC, B-HM, B-NHC, RI300, RI450, RI600, PI300, PI450 and PI600) three times

125

with 100 g of sodium chlorite (NaClO2), 100 mL of acetic acid (CH3COOH), and 6


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126

1000 mL of DI water for 7 h for each time. All BL were freeze-dried, ground, and

127

stored for their characterization and sorption work.

128

Sorbent Characterization. The C, H, N, and O contents of all samples were

129

measured using an Elementar Vario ELШ elemental analyzer (Germany). Solid-state

130

cross-polarization magic-angle-spinning

131

performed on a Bruker Avance 300 NMR spectrometer (Karlsruhe, Germany)

132

operated at

133

samples. The NMR running parameters are available in the Supporting Information

134

and the chemical shift assignments were depicted elsewhere.31 Surface area (CO2-SA)

135

were calculated using nonlocal density functional theory (NLDFT) and grand

136

canonical Monte Carlo simulation (GCMC) using CO2 isotherms at 273 K

137

(Quantachrome Instrument Corp, Boynton Beach, FL) (Figure S1) because previous

138

studies show that N2 at 77 K was unable to detect BC microporosity while CO2 at 273

139

K can enter the micropores (0-1.4 nm).26, 32

13

C-NMR spectroscopy analysis was

13

C frequency of 75 MHz to get structural information of all studied

140

Sorption Experiment. All sorption isotherms were obtained using a batch

141

equilibration technique at 23 ± 1 °C. Appropriate amount of investigated samples

142

(0.1-8.0 mg) were added to the background solution containing 0.01 M CaCl2 in DI

143

water with 200 mg/L NaN3 to minimize biodegradation. The amount of sorbents was

144

controlled to result in 20-80% uptake of initially added Phen. The initial

145

aqueous-phase Phen concentrations (C0, 2-1000 µg/L), which was chosen to cover the

146

range between detection limit and aqueous solubility (1.12 mg/L), were added into the

147

vials and shaken for 10 d. Preliminary tests showed that the apparent sorption

148

equilibrium was reached before 10 d. The blanks consisted of Phen solution without

149

sorbents. Headspace was kept minimal to reduce solute vapor loss. After being shaken

150

on the rotary shaker for 10 d, all vials were placed upright for 24 h.30 The supernatant 7



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151

was then withdrawn from each vial and was transferred to a 2 mL vial for analyzing

152

solution-phase sorbate concentration with HPLC (HP model 1100, reversed phase

153

C18, 15 cm × 4.6 mm × 4.6 µm, Supelco, PA, USA) with a diode array detector for

154

concentrations ranging from 2 to 1000 µg/L and a fluorescence detector for

155

concentrations from approximately 0.2 to 50 µg/L.27 Isocratic elution was used at a

156

flow rate of 0.8 mL/min with a mobile phase: 90:10 (v:v) of methanol and DI water.

157

All samples, along with blanks, were measured in duplicate. Data Analysis. The sorption data were fitted to the logarithmic form of Freundlich

158 159

isotherm model: Log qe = log KF + n log Ce

160

(1)

161

where qe [µg/g] is the equilibrium sorbed concentration; Ce [µg/L] is the equilibrium

162

aqueous concentration; KF [(µg/g)/(µg/L)n] is the Freundlich affinity coefficient; and

163

parameter n is the Freundlich exponential coefficient. The investigated correlations

164

among properties of sorbents as well as their sorption coefficients of Phen (Pearson

165

correlation coefficients: r, and significant level: p) were obtained from the Pearson

166

correlation analysis by SPSS 16.0 software (SPSS Inc., USA).

167

RESULTS AND DISCUSSION

168

Characteristics of NOM fractions and Biochars. The elemental composition,

169

atomic ratio, ash content, and surface area of original and bleached samples (NOM

170

fractions and biochars) are shown in Table 1. The appreciable differences in bulk

171

compositions among various original NOM fractions revealed their heterogeneous

172

structures. Moreover, obviously different chemical compositions detected in NHCs

173

from different soil/sediment sources (Table 1) were consistent with the previous

174

literature which postulated that the physicochemical nature of SOM can vary greatly

175

as a function of the origin, age, weathering, maturation, and soil depth.11, 33, 34 As for 8


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176

biochars, with increasing HTT, C content increased, while H and O contents as well

177

as bulk polarity decreased as reported elsewhere (Table 1).35 The removal of aromatic

178

C by bleaching greatly altered bulk composition of all samples, including natural and

179

engineered sorbents (Figure 1 and Table 1). From the OC recovery (%) of the tested

180

samples after bleaching (Table 1 and Figure 1a), most of the C of HA was removed

181

because its recovery of OC was very low (9.3%), suggesting that the HA contained

182

small amounts of BC, which is resistant to bleaching. Additionally, the OC recovery

183

of biochars reduced with the increasing HTT (Figure 1b), indicating that the

184

high-temperature

185

low-temperature biochars. After bleaching, the C content of investigated samples

186

generally declined except for three NOM fractions covering NHC1, NHC5, and

187

A-HM (Table 1), which had high abundance of ash contents (>55%). The ash contents

188

of these three fractions consistently decreased (Table 1), indicating that the increase of

189

bulk C contents in these three samples after bleaching could be partly explained by

190

the fact that NaClO2 used in bleaching treatment can remove a portion of minerals

191

under acidic conditions.24 Furthermore, such a treatment led to the general increase in

192

the polarity (e.g., (N+O)/C) except for A-HM and B-NHC (Figure 1c and d) as a

193

portion of aromatic C and their functional groups had been oxidized during the

194

treatment, suggesting that a fraction of hydrophobic aromatic components was

195

successfully removed.

biochars

contained

more

resistant

C

compared

to

the

196

The 13C-NMR spectra also illustrated that bleaching caused structural modification

197

(Figure S2 and Table S1). According to the distribution of C functional groups, the

198

reduction of the relative content of aromatic C was noted in both NOM fractions and

199

biochars after bleaching (Table S1). Among them, a regular alternation was observed

200

in aromatic C content of biochars after bleaching that the decreased content of 9



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201

aromatic C after bleaching declined with the increasing of HTT (Table S1 and Figure

202

2b), indicating that more condensed aromatic C of the biochars produced at high HTT

203

possibly is more difficult to be bleached compared to the biochars at low HTTs. As a

204

result of the reduction of the aromatic C, the relative intensity of aliphatic C (0-108

205

ppm) was enhanced (Figure 2c and d). Nonetheless, it should be mentioned that most

206

bleached samples still contained a considerable portion of aromatic C (Table S1). For

207

instance, NHC1 still had 25.8% of aromatic C after bleaching, which could be

208

attributed to that large percentages of aromatic moieties of the tested samples were

209

resistant to bleaching. The percentage of the remaining aromatic C after bleaching to

210

the total aromatic C of their untreated counterparts was further calculated (Table S1).

211

It was found that regarding NOM fractions, the contribution of bleaching-resistant

212

aromatic C accounted for 6.8%, 25.8-40.6% and 10.3-49.9% to the total aromatic C of

213

HA, HM and NHC fractions, respectively. Chefetz et al.24

214

bleaching, in the case of aromatic substrates, is effective for decomposing

215

non-condensed aromatic structures such as lignin-like and polyphenols units detected

216

in HAs, while condensed moieties were not susceptible to be bleached. They also

217

proposed that the residual aromatic C after bleaching likely originated from charcoal

218

and/or charred plant materials, collectively referred to as BC. Moreover, as shown in

219

Table S2, the contribution of BC obtained from combustion at 375 °C of each NHC

220

sample represented more than 10% to the NHC fractions except of BC5 (7.6%).

221

Therefore, it could be concluded that the NOM fractions contained a certain amount

222

of BC, which could be also supported by ubiquitous occurrence of BC in

223

soils/sediments: median BC contents as a fraction of total OC are 4% for 90 soils, 9%

224

for 300 sediments and are up to 30-45% in fire-impacted soils.36

225

demonstrated clearly that

The relationship between micropore properties of NOM fractions and 10


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226

biochars and their aromatic and aliphatic C. The microporosity and surface

227

characteristics of organic sorbents in soils/sediments are pivotal for the mechanistic

228

evaluation of sorption. It has been shown that the traditionally recommended N2

229

sorption techniques would underestimate the SA of OM with pores less than 0.5

230

nm.37-40 Since CO2 at 273 K can enter the micropores (0-1.4 nm),41 the application of

231

CO2-SA helps us to gain a better insight into nanoporosity and SA of SOM. The

232

CO2-SA of the natural sorbents ranged from 9.5 to 100.2 m2/g and CO2-SA values of

233

the tested biochars was in the range of 155.0-544.6 m2/g, which was comparable to

234

the CO2-SA of a temperature series of wood biochars reported recently.40 Obviously,

235

biochars exhibit higher CO2-SA than natural sorbents (Table 1). The CO2-SA values

236

of the NOM fractions obtained in this study were lower than those of the eight

237

American Argonne Premium coals (113-225 m2/g) 42-44 and comparable or lower than

238

the CO2-SA values of SOM and coals reported by Ran et al.44 In this study, the

239

CO2-SA of all samples generally decreased after bleaching, except for NHC8, B-NHC

240

(Figure 1e and f). It was reported that CO2-SA of biochars is positively correlated

241

with their OC contents30 and the similar linear correlations were also observed for

242

NOM fractions in other investigation,39, 41, 44, 45 which was consistent with our data

243

(Figure 3a). This suggests that OC is very likely a major contributor to CO2-SA of

244

sorbents. Therefore, to better compare the impact of the removal of aromatic C on the

245

SA of samples, OC-normalized CO2-SA (CO2-SA/OC) was employed instead of

246

CO2-SA. The range of CO2-SA/OC values of the NOM fractions and biochars

247

investigated in this study was 45.9-316.1 m2/g and 239.6-668.7 m2/g (Table 1),

248

respectively, suggesting that besides C content of sorbents, other properties of OM

249

within these investigated sorbents, such as chemical compositions, molecular

250

structure, configuration and maturation as well as geochemical alteration, should exert 11



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251

an influence on the microporosity and SA. It was noted that the CO2-SA/OC values of

252

HA, NHC8, and A-NHC were less than 100 m2/g (Table 1), which is different from

253

the previous results that the range of CO2-SA/OC values (113.3-610.5 m2/g) for a

254

wide range of NOM fractions and their average CO2-SA/OC is 185 m2/g.39, 41, 44, 45 As

255

presented in Figure 1g and h, the bleaching treatment, to a dissimilar extent, exerted

256

an influence on CO2-SA/OC of NOM fractions and biochars. With respect to NOM

257

fractions, CO2-SA/OC of six samples (NHC1, NHC5, NHC7, A-NHC, A-HM and

258

B-HM) decreased after treatment, whereas that of 1HA, NHC8 and B-NHC increased;

259

in contrast, CO2-SA/OC of biochars consistently declined after the removal of

260

aromatic C, implying that the micropores of engineered sorbents were probably

261

derived from aromatic matrix, while those of natural sorbents were not necessarily

262

derived from aromatic moieties. In order to further examine the molecular structure of

263

NOM and its relationship with the micropores of OM within natural and engineered

264

sorbents, the correlations between CO2-SA/OC and the contents of functional groups

265

as indicated by 13C-NMR were conducted (Figure 4 and Figure S3). It was noted that

266

CO2-SA/OC values of both original and bleached biochars were significantly and

267

positively correlated with their aromaticity (Figure 4a) and negative relationships

268

between CO2-SA/OC values of biochars and their aliphaticity were also detected

269

(Figure 4b), providing the robust evidence to support that nanopores of engineered

270

sorbents were majorly contributed by their aromatic moieties. On the other hand, no

271

specific correlations were obtained between aromaticity as well as aliphaticity of only

272

original NOM fractions and their CO2-SA/OC values (Figure S3a and b). However, it

273

was interesting to find that when the data for bleached NOM samples, whose aromatic

274

C was mainly derived from BC, were added to the data set of Figure S3a and b, the

275

general trend of CO2-SA/OC values with aromaticity or aliphaticity was changed 12


Page 13 of 30


276

(Figure S3c and d). Although this change was not very remarkable, it was assumed to

277

be of significance since it seems to indicate that aromatic C of BC which coexists

278

with natural sorbents would, to some degree, affect the relationship between

279

aromaticity or aliphaticity of natural samples and their CO2-SA/OC. Consequently, to

280

eliminate the effect of BC-derived aromatic C as much as possible, the contents of

281

both natural aromatic and aliphatic C of original NOM fractions were obtained by

282

deducting contribution of bleaching-resistant aromatic C to the total OC of each NOM

283

fraction. Additionally, CO2-SA/OC values of original natural samples were also

284

calibrated by deducting the contribution of CO2-SA/OC of bleached counterparts. The

285

calibrated aromaticity, aliphaticity, and CO2-SA/OC values were listed in Table S3.

286

Interestingly, the calibrated CO2-SA/OC values of natural sorbents were closely

287

related to the calibrated aliphaticity, but negatively related to the calibrated

288

aromaticity excluding HA, A-NHC, and NHC8 because their abnormally low

289

CO2-SA/OC values (Figure 4c and d). The above findings not only suggest that

290

aromatic moieties of BC, which coexists with NOM, could affect the structure and

291

microporosity, but also demonstrate that the microporosity of NOM was closely

292

associated with their aliphatic matrix, as we hypothesized.

293

The role of nanopores, aromatic and aliphatic C in sorption of Phen by both

294

NOM fractions and biochars. The Freundlich isotherms were shown in Figure S4

295

and S5, and the fitting parameters were listed in Table S4. The sorption isotherms of

296

Phen by original NOM and biochars were nonlinear with n values being in the range

297

of 0.50-0.89 and 0.38-0.71, respectively, and well fitted with the Freundlich model

298

(Table S4). The isotherms for biochars were all highly nonlinear (n < 0.71), similar

299

results were reported by Lattao et al.,40 reflecting the predominance of

300

adsorption/pore-filling mechanisms. The removal of certain aromatic moieties by 13



Page 14 of 30

301

bleaching resulted in the rise of n values as compared to that of the untreated samples

302

except for RI600. Especially for NOM, the bleached samples nearly exhibited a linear

303

and partition-type sorption behavior (Table S4), which implies that a more expanded

304

sorbent was produced due to the removal of aromatic moieties and also supports that

305

aromatic moieties should be the predominant components responsible for nonlinear

306

sorption process as reviewed by Chefetz and Xing.2

307

Bleaching exercised a great effect upon Koc (OC content-normalized sorption

308

coefficient) (Table S4). Except for NHC8, the Koc of NOM fractions and biochars all

309

decreased compared with their untreated samples (Table S4), which was similar to the

310

results presented by Huang et al.46 Additionally, after removal of aromatic moieties,

311

bulk polarity (e.g., (O+N)/C) of the NOM fractrions and biochars generally increased,

312

which may be responsible for their decreasing Phen Koc values because the polarity of

313

SOMs can significantly affect sorption capacity of HOCs and the SOMs with

314

relatively low polarity show the higher sorption capacity than those with high

315

ploarity.27, 47, 48 The significant and negative correlation of logKoc values of Phen by

316

the original and bleached biochars to their bulk polarity (e.g., (N+O)/C) (Figure S6)

317

supports our hypothesis. However, recently, Lattao et al.40 found that no simple

318

relationship stands out between logKoc values and O/C ratio, surface area (N2 and

319

CO2), and porosity and they demonstrated that sorption is a complex function of

320

biochar properties and solute molecular structure, and not very predictable on the

321

basis of readily determined char properties. It has been widely documented that

322

pore-filling mechanism plays a key role in HOCs sorption by microporous solids of

323

SOM.21, 22 For example, Ran et al.21 reported that sorption behaviors of Phen and

324

dichlorobenzene (DCB) by kerogen were satisfactorily explained by hole-filling

325

mechanism. Like these studies, the significantly positive correlation between Koc 14


Page 15 of 30


326

values of Phen by all original and bleached sorbents and their CO2-SA/OC obtained in

327

our case (Figure 3b) implied that pore-filling could be a major mechanism regulating

328

sorption interactions of HOCs-SOM. Moreover, the slope of the linear regression line

329

for the NOM fractions was higher than that of the biochars (Figure 3b), implying that

330

although the biochars generally have higher CO2-SA per unit mass of their OC than

331

the NOM fractions (Table 1), the sorption capacity of CO2-SA per unit mass of OC

332

within NOM fractions could be remarkably higher than that within the biochars in this

333

study. Therefore, it can be assumed that the sorption capacity of sorbents depends on

334

not only their CO2-SA per unit mass of OC but also other factors such as the chemical

335

composition, structure and configuration of the contributor to CO2-SA. Meanwhile, as

336

we demonstrated before, nanopores of natural sorbents and biochars were perhaps

337

mainly derived from their aliphatic and aromatic moieties, respectively. Thus, CO2-SA

338

associated with the aliphatic moieties within NOM fractions should have higher

339

sorption capacity compared to the CO2-SA derived from the aromatic matrix within

340

the biochars. As a result, we must not think only of how much CO2-SA a sorbent has,

341

but also of its chemical composition (e.g., aliphatic and aromatic moieties) to evaluate

342

its sorption capacity for HOCs. Furthermore, our data showed that the Phen Koc by

343

both original and bleached biochars was strikingly and positively related to their

344

aromaticity but negatively correlated to their aliphaticity (Figure 3c and d). This was

345

exactly the same as the findings by Chefetz and Xing,2 who observed a general trend

346

of increasing Phen Koc values with increasing aromaticity of engineered samples.

347

However, in our work, there was no significant correlation between Phen Koc of these

348

tested NOM fractions and their aromaticity or aliphaticity (Figure S3e and f). Similar

349

conclusions were previously reported by Yang et al.49 They performed experiments

350

with sorption of Phen by HA and HM fractions isolated from a single soil sample and 15



Page 16 of 30

351

showed that neither aromatic nor aliphatic components of HAs and HMs could serve

352

as predictors of the soil’s ability to sorb Phen. It has been above-mentioned that the

353

aromatic C in NOM fractions might partly originate from BC materials, which would

354

interfere in exploring where (aromatic or aliphatic C) the nanopores of NOM originate

355

from. Additionally, it was noted that BC appeared particularly higher sorption affinity

356

to Phen with logKoc (Ce = 0.01Sw) ranging from 5.67 to 6.51 than NHC because of

357

high CO2-SA/OC (150.0-887.7 m2/g) resulted by ubiquitous micropores (Table S2).

358

As long as BC materials enter into soils and sediments, they would therefore influence

359

the sorption properties of HOCs by NOM and strengthen the importance of aromatic

360

C of NOM in HOCs sorption by soils and sediments contaminated by BC, thus, the

361

role of aliphatic C within NOM in HOCs sorption could be correspondingly masked.

362

Therefore, we propose that the ‘pollution’ of NOM by BC materials could, to a large

363

degree, account for no clear relationship between Phen Koc values by NOM fractions

364

and their aliphaticity, consequently, influence on investigating the role of aliphatic

365

moieties within NOM fractions.

366

Environmental Implications. This study demonstrated that the nanopores of

367

natural (NOM) and engineered sorbents (biochars) are closely related to their aliphatic

368

and aromatic matrix, respectively. Significant and positive correlations between Phen

369

Koc values by the NOM fractions or biochars and their CO2-SA/OC in this study

370

suggest that nanopore-filling mechanism plays a dominant role in the sorption of

371

HOCs by these sorbents, which are found to be microporous solids. In addition,

372

aliphatic C of the NOM fractions and aromatic C of the investigated biochars,

373

respectively, are demonstrated to be key factors affecting their microporosity and

374

sorption behaviors of HOCs. Moreover, BC is almost composed of aromatic moieties

375

and is characterized by structural stability and high sorption capacity. It inevitably 16


Page 17 of 30


376

changes the structures of NOM. Hence, the importance of aliphatic C within NOM in

377

the sorption of HOCs has often been masked. We used a novel approach by

378

combining fractionation, bleaching, and

379

findings of this work can explain the ongoing debate on the relative role of aromatic

380

and aliphatic C in the sorption of HOCs by SOM and uncover that how the aliphatic

381

and aromatic C within both natural and engineered sorbents play the role in the

382

sorption of HOCs, which is important for correctly predicting the fate of HOCs in

383

soils and sediments. The results described in this study provide important implications

384

for the interpretation of sorption mechanisms of organic contaminants in SOM.

385

386

Supporting Information. Figure of Carbon dioxide (CO2) adsorption isotherm on the

387

various NOM factions and biochars, figure of

388

bleached NOM fractions and biochars, figure of correlations between CO2-SA/OC of

389

original NOM fractions and their aromaticity and aliphaticity, between CO2-SA/OC of

390

original and bleached NOM fractions and their aromaticity and aliphaticity as well as

391

between logKoc values of Phen by original NOM fractions and their aromaticity and

392

aliphaticity, figure of sorption isotherms of Phen by NOM fractions; figure of sorption

393

isotherms of Phen by biochars, figure of correlation of logKoc values of Phen by

394

sorbent to their bulk polarity; table of Functional Groups from the 13C NMR Spectra,

395

table of properties of BC obtained from combustion of NHC at 375 °C, table of the

396

calibrated aromaticity, aliphaticity and CO2-SA/OC values of NOM fractions, table of

397

Freundlich isotherm parameters. This material is available free of charge via the

398

Internet at http://pubs.acs.org.

399

400

Corresponding Author

13

C-NMR to estimate the effect of BC. The

ASSOCIATED CONTENT

13

C NMR spectra of original and

AUTHOR INFORMATION

17



401

*E-mail: [email protected] (K.S.)

402

403

This research was supported by National Natural Science Foundation of China

404

(41273106), Beijing Higher Education Young Elite Teacher Project (YETP0273),

405

and the Scientific Research Foundation for the Returned Overseas Chinese

406

Scholars, State Education Ministry.

407

408 409 410 411 412 413 414

ACKNOWLEDGEMENTS

REFERENCES

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acids as determined by chemical modifications and carbon-13 NMR, pyrolysis-, and

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thermochemolysis-gas chromatography/mass spectrometry. Soil. Sci. Soc. Am. J. 2002, 66, (4),

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disrupting chemicals by condensed organic matter in soils and sediments. Chemosphere 2010, 80, (7),

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characterization of different organic matter fractions from a same soil source and their phenanthrene

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sorption. Environ. Sci. Technol. 2013, 47, (10), 5138-5145.

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(28) Gelinas, Y.; Prentice, K. M.; Baldock, J. A.; Hedges, J. I., An improved thermal oxidation method

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for the quantification of soot/graphitic black carbon in sediments and soils. Environ. Sci. Technol. 2001,

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deashing treatment on biochar structural properties and potential sorption mechanisms of phenanthrene.

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17α-ethinyl estradiol and phenanthrene on thermally and hydrothermally produced biochars. Bioresour.

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properties of environmental black carbon (char): Pseudo pore blockage by model lipid components and

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its implications for N2-probed surface properties of natural sorbents. Environ. Sci. Technol. 2005, 39,

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Sci. Technol. 2005, 39, (18), 6881-6895.

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Nature of porosity, surface area, and diffusion mechanisms. Environ. Sci. Technol. 1996, 30, (2),

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(45) Li, J.; Werth, C. J., Evaluating competitive sorption mechanisms of volatile organic compounds

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(46) Huang, L.; Boving, T. B.; Xing, B., Sorption of PAHs by Aspen wood fibers as affected by

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535

organic matter structural properties and sorption mechanisms of phenanthrene. Environ. Sci. Technol.

536

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22


Page 23 of 30


537 538

Figure Captions:

539

Figure 1. Organic carbon (OC) recovery % (a and b) of natural organic matter (NOM)

540

fractions (left) and biochars (right) after bleaching; comparison of bulk polarity (c and

541

d), CO2-derivded calculative surface area (CO2-SA) (e and f) and OC-normalized

542

CO2-SA (CO2-SA/OC) (g and h) between NOM fractions (left) or biochars (right) and

543

their corresponding bleached fractions.

544 545

Figure 2. Comparison of aromatic C (a and b) and aliphatic C (c and d) between

546

natural organic matter (NOM) fractions (left) as well as biochars (right) and their

547

corresponding bleached fractions.

548 549

Figure 3. Correlations between CO2-derivded calculative surface area (CO2-SA) of

550

original and bleached natural organic matter (NOM) fractions and biochars and their

551

bulk C content (a); correlations between logKoc values (mL/g) of Phen by original and

552

bleached NOM fractions and biochars and their organic carbon (OC)-normalized

553

CO2-SA (CO2-SA/OC) (b); correlations between logKoc values (mL/g) of Phen by

554

original and bleached biochars and their aromaticity (c) and aliphaticity (d).

555 556

Figure 4. Correlations between CO2-SA/OC of original and bleached biochars and

557

their aromaticity (a) or aliphaticity (b); Correlations between calibrated organic

558

carbon (OC)-normalized calculative surface area (SA) (CO2-SA/OC) of original

559

natural organic matter (NOM) fractions excluding HA1, A-NHC, and NHC8 and their

560

calibrated aliphaticity (c) or calibrated aromaticity (d).

561 562 563 23



Page 24 of 30

564 565 566 567 568 569 570 571

Table Captions:

572 573

Table 1. Yields by bleaching treatment, elemental compositions and surface area

574

analysis of NOM fractions and biochars

575 576 577 578 579 580 581 582 583 584 585 586 587 588

24


Page 25 of 30


589

609 610 611 612 613 614 615 616 617 618

g

300 200 100 0

PI600

PI450

PI300

RI600

RI450

RI300

PI600

PI450

PI300

RI600

RI450

900

PI600

0

PI450

200

Original Bleached

h

600 300 0

PI600

2

608

400

Original Bleached

400

PI450

2

500 CO2-SA/OC, m /g

607

f

Original Bleached

PI300

50

606

0.0

PI300

100

0

605

0.2

RI600

604

150

0.4

RI600

603

CO2-SA, m /g

602

d

0.6

600

e

Original Bleached

Original Bleached

0.8

RI300

200

1.0

RI450

0.4 0.0

10

RI450

0.8

600 601

Bulk polarity, (O+N)/C

599

30 20

2

598

c

CO2-SA, m /g

597

1.2

Original Bleached

CO2-SA/OC, m2/g

Bulk polarity, (O+N)/C

596

1.6

b

40

0

HA1 A-HM B-HM NHC1 NHC5 NHC7 NHC8 A-NHC B-NHC

594

60 50

RI300

10 0

595

OC recovery, %

20


593

30


592

40


OC recovery, %

591

a

50

RI300

60

590

Figure 1. Organic carbon (OC) recovery % (a and b) of natural organic matter (NOM) fractions (left) and biochars (right) after bleaching; comparison of bulk polarity (c and d), CO2-derivded calculative surface area (CO2-SA) (e and f) and OC-normalized CO2-SA (CO2-SA/OC) (g and h) between NOM fractions (left) or biochars (right) and their corresponding bleached fractions.

25



Page 26 of 30

619 620

60 40 20

PI600

PI450

RI600

PI300

d

60 40 20 0

0

Original Bleached

80

PI600

632

100

PI450

631

0

PI300

630

c

20

RI600

629

80

Original Bleached


628

100 Aliphatic C, %

627

40

RI450

626

60

RI450

0

625

b

80

RI300

20

Original Bleached

100

RI300

40

Aliphatic C, %

624

60


623

Aromatic C, %

622

120

a

Original Bleached

Aromatic C, %

80

621

633 634

Figure 2. Comparison of aromatic C (a and b) and aliphatic C (c and d) between

635

natural organic matter (NOM) fractions (left) or biochars (right) and their

636

corresponding bleached fractions.

637 638 639 640 641 642 643 644 645 646 26



All sorbents (original and bleached)

2

CO2-SA, m /g

600 400

r = 0.82, p < 0.01 200 a

8.0

0

Original and bleached NOMs

r = 0.65, p < 0.01 7.0

Original and bleached biochars

r = 0.82, p < 0.01 6.0 5.0 b

4.0 0

6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4

20

40 60 80 Bulk C content, %

0

100

Biochars (original and bleached)

LogKoc, Ce=0.01Sw

LogKoc, Ce=0.01Sw

647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676

LogKoc, Ce=0.01Sw

Page 27 of 30

r = 0.82, p < 0.01

c 0

20

40 60 80 Aromaticity, %

100

6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4

200

400

600 2 CO2-SA/OC, m /g

800

Biochars (original and bleached)

r = 0.52, p = 0.085

d 0

5

10 15 20 Aliphaticity, %

25

677 678

Figure 3. Correlations between CO2-derivded calculative surface area (CO2-SA) of

679

original and bleached natural organic matter (NOM) fractions and biochars and their

680

bulk C content (a); correlations between logKoc values (mL/g) of Phen by original and

681

bleached NOM fractions and biochars and their organic carbon (OC)-normalized

682

CO2-SA (CO2-SA/OC) (b); correlations between logKoc values (mL/g) of Phen by

683

original and bleached biochars and their aromaticity (c) and aliphaticity (d).

684 685 686 687 688 27



Page 28 of 30

689 690 691

694 695

900

r = 0.71, p = 0.11

2

696

1500

a 2

1200

r = 0.95, p < 0.01

CO2-SA/OC, m /g

693

Original biochars Bleached biochars

600 300

r = - 0.86, p < 0.05

900

r = - 0.39, p = 0.44

40 60 80 Aromaticity, %

Bleached biochars

600 300

Original NOMs

c

r = 0.80, p = 0.057 Excluding NOM samples

10

20 30 40 50 Calibrated Aliphaticity, %

0

100 2

2

400 350 300 250 200 150 100 50 0

20

Calibrated CO2-SA/OC, m /g

0 Calibrated CO2-SA/OC, m /g

b

0

0

697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713

Original biochars

1200

CO2-SA/OC, m /g

1500

692

60

400 350 300 250 200 150 100 50 0

5

10 15 20 Alipahticity, % Original NOMs

25 d

r = - 0.75, p = 0.084 Excluding NOM samples

20 40 60 80 Calibrated Aromaticity, %

714

Figure 4. Correlations between CO2-SA/OC of original and bleached biochars and

715

their aromaticity (a) or aliphaticity (b); correlations between calibrated organic carbon

716

(OC)-normalized calculative surface area (SA) (CO2-SA/OC) of original natural

717

organic matter (NOM) fractions excluding HA1, A-NHC, and NHC8 and their

718

calibrated aliphaticity (c) or calibrated aromaticity (d).

719 720 721 722

28


Page 29 of 30

723 724


Table 1. Yields by bleaching treatment, elemental compositions and surface area analysis of NOM fractions and biochars Samples

Mass

OC

Recovery

Recovery

(%)

a

(%)

b

C (%)

H (%)

N (%)

O (%)

(O+N) /C

CO2-SA 2

CO2-SA/OC 2

Ash

(m /g)

(m /g)

(%)

NOM fractions (Natural sorbents) HA1

54.2

4.0

2.9

26.5

0.41

24.9

45.9

12.4

NHC1

22.4

2.3

1.0

3.6

0.15

57.0

254.7

70.7

NHC5

15.7

1.2

1.2

6.7

0.39

44.4

282.8

75.1

NHC7

21.4

1.5

1.0

4.8

0.21

40.7

190.4

71.3

NHC8

12.1

1.6

0.9

4.4

0.13

9.5

78.1

81.1

A-NHC

42.2

4.3

0.7

19.2

0.36

31.4

74.4

33.6

A-HM

20.5

2.6

1.0

19.8

0.77

42.5

207.3

56.1

B-NHC

50.8

4.0

1.2

25.7

0.40

100.2

197.2

18.3

B-HM

19.3

2.4

1.3

19.9

0.83

61.0

316.1

57.1

HA1-BL

25.3

9.3

19.9

2.1

1.1

17.7

0.70

17.1

85.8

59.2

NHC1-BL

42.5

52.2

27.5

3.3

0.4

16.4

0.46

45.0

163.4

52.4

NHC5-BL

18.3

27.3

23.4

1.9

0.8

16.7

0.56

36.2

154.5

57.2

NHC7-BL

51.6

49.2

20.4

2.3

0.2

15.5

0.58

25.6

125.6

61.6

NHC8-BL

43.2

26.4

7.4

0.9

0.2

13.4

1.38

15.0

203.7

78.2

A-NHC-BL

29.9

19.1

27.0

2.1

0.6

15.0

0.43

8.0

29.7

55.3

A-HM-BL

30.2

41.7

28.3

2.9

1.7

20.0

0.57

27.1

95.9

47.1

B-NHC-BL

20.5

17.2

42.6

3.1

0.9

18.5

0.34

175.9

413.1

34.9

B-HM-BL

71.8

43.9

11.8

1.8

1.0

16.8

1.13

33.4

282.2

68.5

188.5

354.3

17.6

Biochars (Engineered sorbents) RI300

53.2

3.9

1.1

24.2

0.36

RI450

57.0

2.6

1.2

15.6

0.22

293.4

514.5

23.6

RI600

60.4

1.7

1.1

8.9

0.13

390.6

647.1

27.9

PI300

64.7

4.8

0.0

28.6

0.33

155.0

239.6

1.9

PI450

73.1

2.8

0.1

20.1

0.21

408.1

558.3

3.9

PI600

81.4

2.3

0.1

11.7

0.11

544.6

668.7

4.4

RI300-BL

24.4

12.3

26.9

3.0

0.4

24.9

0.70

85.2

316.3

44.9

RI450-BL

52.6

36.2

39.2

2.2

0.7

27.8

0.55

130.7

333.5

30.1

RI600-BL

66.1

54.9

50.2

1.6

0.8

19.6

0.31

257.2

512.8

27.8

PI300-BL

12.4

8.3

43.1

5.0

0.1

45.4

0.79

16.6

38.6

6.4

PI450-BL

57.1

40.3

51.6

2.4

0.0

36.6

0.53

110.9

215.0

9.4

PI600-BL

69.2

55.4

65.1

2.2

0.0

25.8

0.30

402.1

617.5

6.8

a

b

Mass Recovery (%) = M(BL)/M(OR) × 100, OC Recovery (%) = OC(BL) × M(BL)/[ OC(OR) × M(OR)] × 100, where M

is the weight of original or bleached sample (HA, HM, NHC, biochars); Mass Recovery denotes the bleaching treatment yields; humic acids (HA), humins (HM), nonhydrolyzable carbons (NHC), pine wood (PI), rice straw (RI), Original samples :OR; Bleached samples: BL.

29



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