Molecular Insights into the Transformation of Dissolved Organic Matter

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Molecular insights into the transformation of dissolved organic matter in landfill leachate concentrate during biodegradation and coagulation processes using ESI FT-ICR MS Ziwen Yuan, Chen He, Quan Shi, Chunming Xu, Zhenshan Li, Chengzhai Wang, Huazhang Zhao, and Jinren Ni Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Environmental Science & Technology

1

Molecular insights into the transformation of dissolved organic

2

matter in landfill leachate concentrate during biodegradation and

3

coagulation processes using ESI FT-ICR MS

4

Ziwen Yuan †,‡, Chen He §, Quan Shi §, Chunming Xu §, Zhenshan Li †,‡, Chengzhai Wang †,‡,

5

Huazhang Zhao *,†,‡, Jinren Ni †,‡

6 †

7 ‡

8 9

§

Department of Environmental Engineering, Peking University, Beijing 100871, China

The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

10 11

* Corresponding author. Tel: +86-10-62758748; fax: +86-10-62756526.

12

E-mail address: [email protected] (H.Z. Zhao).

13

1 Environment ACS Paragon Plus


14

ABSTRACT: Landfill leachate concentrate is a type of refractory organic wastewater with high

15

environmental risk. Identification of refractory components and insights into the molecular

16

transformations of the organics are essential for the development of efficient treatment process.

17

In this report, molecular compositions of dissolved organic matter (DOM) in leachate

18

concentrate, as well as changes after anaerobic/aerobic biodegradation and coagulation with

19

salts, were characterized using electrospray ionization (ESI) coupled with Fourier transform ion

20

cyclotron resonance mass spectrometry (FT-ICR MS). DOM in leachate concentrate were more

21

saturated and less oxidized with more nitrogen and sulfur-containing substances (accounting for

22

50.0%), comparing with natural organic matter in Suwannee River. Selectivity for different

23

classes of organics during biodegradation and coagulation processes was observed. Substances

24

with low oxidation degree (O/C < 0.3) were more reactive during biodegradation process,

25

leading to the formation of highly oxidized molecules (O/C > 0.5). Unsaturated (H/C < 1.0) and

26

oxidized (O/C > 0.4) substances containing carboxyl groups were preferentially removed after

27

coagulation with Al or Fe sulfate. The complementary functions of biodegradation and

28

coagulation in the treatment of DOM in leachate concentrate were verified at the molecular level.

29

Lignin-derived compounds and sulfur-containing substances in leachate concentrate were

30

resistant to biodegradation and coagulation treatments. To treat leachate concentrate more

31

effectively, processes aimed at removal of such DOM should be developed.

32

KEYWORDS: Landfill leachate concentrate; DOM; FT-ICR MS; Molecular Characterization;

33

Biodegradation; Coagulation

34


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INTRODUCTION

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Leachate from waste landfill is a type of high concentration and refractory organic wastewater.

37

In recent years, more than 95% of landfill leachate in China is treated with a combined process

38

of biodegradation and membrane filtration (i.e. ultrafiltration, nanofiltration and reverse osmosis)

39

due to its high efficiency of removing contaminants

40

produced during membrane treatment represents typically 15–30% of total incoming leachate 3, 4.

41

The landfill leachate is characterized by a high concentration of dissolved organic matter (DOM)

42

2

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components of the leachate concentrate produced by membrane treatment process 5. Moreover,

44

most of the salt in landfill leachate is also concentrated. Owing to the potential pollution to the

45

surrounding environment, it is prohibited to discharge the untreated leachate concentrate into the

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municipal wastewater treatment plant in China, which means leachate concentrate must be

47

appropriately treated in the sanitary landfill. However, there is no effective treatment for landfill

48

concentrate so far

49

concentrate during treatment process is still unclear.

1, 2

. Nevertheless, leachate concentrate

, and the organic contaminants that have not been removed by biodegradation is the main

1, 4

, partly because the chemical transformation of DOM in leachate

50

It is well known that chemical characteristics and composition of DOM would significantly

51

affect the performance of water treatment techniques 6. Accordingly, detailed characterizations

52

of DOM are of critical importance to develop effective treatment process for leachate

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concentrate 7. However, only few previous studies have analyzed the chemical properties of

54

organic matter in leachate concentrate, and most of them focused on characteristics of different

55

fractions of bulk DOM determined by ultraviolet/visible (UV/vis) spectroscopy, fluorescence

56

excitation/emission matrix (EEM) and GC-MS analysis

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humic substances including humic acid and fulvic acid were the major DOM in the leachate

3-5, 8

. For example, it was found that



3, 5

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concentrate, ranging from 60-75%

. Organic matters including long-chain hydrocarbons and

59

halohydrocarbons are accumulated in the leachate concentrate 3. Previous studies have provided

60

some understanding of the DOM in leachate concentrate, but due to the limitations of the

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methods used, the DOM was only characterized as groups of compounds instead of specific

62

molecules. Therefore, the specific molecular components of DOM in leachate concentrate

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remain unknown 9.

64

Recently, high-resolution mass spectrometry, such as Fourier transform ion cyclotron

65

resonance mass spectrometry (FT-ICR MS) at high magnetic field (>9 T), has been utilized in

66

the studies of environmental samples and shown to be a remarkable technique for the analysis of

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DOM at the molecular level 10, 11. Since this technique can determine the molecular formulas for

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components of DOM, it provides better understanding of the molecular transformations of DOM

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in aquatic environments including fresh water 12, 13, marine water

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and refinery wastewater

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Studies have suggested that structural properties, oxygen and heteroatom contents of individual

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DOM molecules play an important role in treatment processes. For instance, a large portion of

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aliphatic and aromatic compounds containing nitrogen and oxygen atoms can be preferentially

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degraded by microorganisms10, 16, while carbon-rich compounds, which have been suggested as

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the major component of humic acids and characterized by heavily carboxylated, condensed

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aromatic structures, are resistant to microbial degradation

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that coagulation by hydrolyzing metals (e.g., Al or Fe hydroxides) selectively removed DOM

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compounds high in unsaturation or rich in oxygen-containing functional groups 9, 19, 20. There are

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limited reports that comparatively analyzed the molecular transformation of DOM in the same

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water system during biodegradation and coagulation processes.

16

14, 15

, municipal wastewater 10,

during the biological, chemical and physical treatment processes.

17, 18


. Moreover, other studies show

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81

In the current study, we investigated the molecular composition of DOM in landfill leachate

82

concentrate and its difference from NOM using ESI FT-ICR MS. In addition, the molecular

83

transformations of DOM in leachate concentrate during biodegradation and coagulation

84

processes were investigated to better understand the removal and transformation preference of

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DOM during treatment processes. To our knowledge, this is the first study to comprehensively

86

explore the DOM in leachate concentrate at molecular level, as well as comparatively analyze

87

the change tendency of molecular composition of DOM during biodegradation and coagulation.

88

The results of this study can contribute to the assessment of the environment risk of the leachate

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concentrate and guide the development of its effective treatment process.

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EXPERIMENTAL SECTION

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Source of Leachate Concentrate

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Leachate concentrate samples from waste landfill were obtained from a municipal solid waste

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(MSW) landfill leachate treatment plant located at An Ding District in Beijing, China. The

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landfill was on stream and most of the solid wastes were general MSWs in China. The leachate

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plant applied physical/chemical and biological process as the first step and membrane filtration

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process including nanofiltration (NF) and reverse osmosis (RO) as the second step treatment,

97

which were typical processes for landfill leachate treatment in China. The flow diagram of the

98

leachate treatment plant is shown in Figure S1 (see Supporting Information). The leachate

99

concentrate was from the collecting basin, which stored all RO concentrate and part of NF

100

concentrate. The samples were kept in dark at 4 °C before analysis. The chemical characteristics

101

(including pH, COD, DOC, NH4+-N, TN, TP) of leachate concentrated investigated in this study

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were similar with leachate concentrate from other landfill leachate treatment plants in China

103

(see Table S1 in Supporting Information).


[10]


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Leachate Concentrate Treated by Biodegradation and Coagulation

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Batch experiments for the biodegradation of leachate concentrate were conducted in a

106

laboratory-scale anaerobic/oxic (A/O) system which consisted of an anaerobic reactor (2.0 L)

107

and an aerobic reactor (2.0 L) (Figure S2 in Supporting Information). The anaerobic and aerobic

108

activated sludge used in the system were collected from a domestic waste water treatment plant

109

in Beijing, China. Before experiments, the activated sludge was domesticated for about 70 d to

110

adapt to the high salinity of the concentrate. Coagulation experiments of leachate concentrate

111

were performed using a program-controlled TA6-1 jar test apparatus with six paddles, and metal-

112

salt coagulants [Al2(SO4)3 and Fe2(SO4)3,] were used. The detailed procedures of A/O

113

biodegradation and coagulation experiments are included in Supporting Information.

114

All samples including raw leachate concentrate, effluents from anaerobic bioreactor, aerobic

115

bioreactor, and the coagulation by Al or Fe salts were collected immediately after experiments,

116

filtered with 0.45-µm nylon filters, and kept in the dark at 4 °C before analysis. The dissolved

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organic carbon (DOC) of the samples was analyzed using a 5000A total organic carbon (TOC)

118

analyzer (Shimadzu, Japan), and Chemical Oxygen Demand (COD) value of the samples was

119

measured by standard methods (PRC EPA, 2002). Experiments were carried out in three

120

replicates.

121

For the FT-ICR MS measurement, all samples were extracted by Sep-pak C18 solid phase

122

extraction cartridges (1 g, 6 mL, Waters, USA) to remove salt. Details about the extraction

123

procedure are provided in supporting information.

124

Negative Ion ESI FT-ICR MS Analysis

125

All extracted DOM samples and Suwannee River natural organic matter (SRNOM) standard

126

were analyzed using a Bruker Apex ultra FT-ICR MS equipped with a 9.4 T superconducting


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magnet. The SRNOM standard was purchased from the International Humic Substances Society

128

(IHSS). The DOM samples and SRNOM standard were diluted with methanol solution and

129

injected into the electrospray source at 180 µL h-1 using a syringe pump. The operating

130

conditions for negative-ion formation consisted of a 3.5 kV spray shield voltage, 4.0 kV capillary

131

voltage, and - 320 V capillary column end voltage. Ions accumulated in the ion source for 0.001

132

s in a hexapole. All of the ions passed through a quadrupole, accumulated in an argon filled

133

hexapole collision pool, in which ions accumulated for 0.2 s. The delay was set to 1.0 ms to

134

transfer the ions from the collision poll to an ICR cell by electrostatic focusing of transfer optics.

135

The mass range was set at m/z 150-600.The data size was set to 4 M words, and a data set of 32

136

FT-ICR scans were accumulated to enhance the signal-to-noise ratio (S/N) and dynamic range.

137

Solvent blanks and C18 SPE extraction blanks were measured. Methanol blank analyses were

138

performed to check whether the instrument was clean prior to analyzing the samples. Very few

139

peaks overlapped between the C18 extraction blank and the sample mass spectra and all peaks

140

found in the blank were removed from the sample peak lists.

141

The procedures for FT-ICR MS mass calibration, data acquisition, and processing are provided

142

in Supporting Information. The majority of peaks in this work contained C, H, O, N, and S

143

elements. The compounds containing P and Cl were excluded because of the low levels found in

144

our samples (< 0.5%).

145

RESULTS AND DISCUSSION

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Molecular Characterization of DOM in Leachate Concentrate

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Figure S3 (a) and (b) in the Supporting Information show the negative ion mass spectra of

148

SRNOM and the C18 extracted raw leachate, respectively. Although all samples were analyzed

149

in broadband mode (m/z 100-2000), the mass spectra displayed a similar pattern as in previous



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21-23

150

DOM studies, where peaks are generally distributed over the mass range of 150-600 m/z

151

The eventual loss of peaks below 150 m/z and above 600 m/z can be attributed to the limitation

152

of FT-ICR MS

153

Supporting Information). The complexity of the spectra is apparent from the detection of 10-20

154

peaks per nominal mass at nearly every mass throughout that range. Typical mass spacing

155

patterns such as 14.0156 Da for CH2- groups and an increase of 36.4 mDa for the replacement of

156

O by CH4 were observed for both raw leachate and SRNOM. This implied that DOM in leachate

157

concentrate and SRNOM may be largely composed of molecules belonging to chemically related

158

families, or homologous series, which have also been described by other researchers for various

159

humic-rich DOM samples 17, 20, 22. Formulas were assigned to the peaks defined by the detection

160

limit (S/N ≥ 6) and we found that both raw leachate and SRNOM mainly contained CHO,

161

CHON, CHOS and CHONS compounds, indicating that they were the major subcategories in the

162

DOM of leachate concentrate.

10

and the molecular sizes of humic substances

9, 24

.

(details are provided in the

163

According to Table S2, the intensity weighted averaged values of H/C, O/C and double bound

164

equivalent (DBE) of SRNOM standard sample were 1.066, 0.497 and 9.450 respectively, which

165

were close to the values obtained in other study

166

leachate were quite different. Relatively higher average H/C, lower average O/C and DBE of raw

167

leachate (H/Cwa = 1.225, O/Cwa = 0.358, DBEwa = 8.248) implied that DOM in leachate

168

concentrate was mainly associated with more saturated and less oxidized compounds. Moreover,

169

according to Figure 1b, the percentage of C, H, O-only components in SRNOM was around

170

80%, whereas it was decreased to 50% in raw leachate and the other 50% was the components

171

belonging to CHON, CHOS, CHONS subcategories, indicating more N and S-containing

172

compounds in DOM of leachate concentrate..

15

. However, these general parameters of raw


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In addition, the molecular composition of DOM can be visualized in a van Krevelen diagram.

174

Formulas that would align at a certain point on the diagram are related to a specific class of

175

compounds

176

regions corresponding to the seven classes of compounds found in NOM, including lipids,

177

aliphatic/proteins,

178

carbohydrates, unsaturated hydrocarbons, aromatic structures and tannins, according to previous

179

literature reports which concerned organic compounds in other water samples

180

stoichiometric ranges used to establish the boundaries of the classification are described in

181

Supporting Information. The points representing the DOM in raw leachate in Figure 1a

182

distributed centrally in the lignins/CRAM-like structures, lipids and aliphatic/proteins region,

183

while SRNOM molecules distributed more dispersedly in the high O/C and low H/C regions of

184

the diagram (e.g. the region relating to the aromatic structures and tannins). These differences

185

between the two samples were illustrated more clearly when the contribution of the relative

186

abundance of major compound classes was compared (Figure 1c). Although ESI FT-ICR MS is

187

not a quantitative technique for the analysis of NOM in the absence of standards, many studies

188

have demonstrated that comparison of relative peak intensities of molecules in ESI FT-ICR MS

189

spectra of different samples analyzed under the same instrumental conditions can be used for

190

semiquantitative analysis of similar types of compounds 28, 29. The lignins/CRAM-like structures

191

showed higher proportion in raw leachate (83.0%) than in SRNOM (71.8%), and more

192

compounds in SRNOM (totally 25.3%) belong to aromatic structures and other classes

193

(including tannins and carbohydrates).

194

Transformation and Removal of DOM in Leachate Concentrate during Biodegradation

195

and Coagulation

20, 25

. As shown in Figure 1a, the van Krevelen diagrams are divided into several

lignins/carboxylic rich

alicyclic molecules


(CRAM)-like structures,

10, 26, 27

. The


196

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Although leachate concentrate is recognized as a type of wastewater with low biodegradability

197

30

198

illustrated in Figure 2a. Removal efficiency of DOC concentrations after A/O biodegradation

199

was approximately 15% (the removal efficiencies of DOC concentrations of anaerobic and

200

aerobic reactor were about 5% and 10% respectively). The addition of salts, however, resulted in

201

higher DOC removal than biological treatment, as shown in Figure 2b. When coagulant

202

concentration was more than 20 mM, iron salt achieved more than 50% removal of DOC and

203

aluminum salt achieved 35% removal. Iron salt precipitated more DOM in the leachate

204

concentrate than aluminum salt. The effluent samples from anaerobic reactor, aerobic reactor and

205

the coagulation by 20 mM Al or Fe salts were chosen for further investigation.

, its DOC gradually decreased during the anaerobic and aerobic biodegradation process, as

206

The mass spectra of the leachate concentrate samples treated by A/O biodegradation and

207

coagulation show almost the same peak distribution as raw leachate (Figure S3 in Supporting

208

Information). However, the number and the intensities of the peaks were quite different. To

209

further analyze the mass differences, all identified peaks (S/N ≥ 6, excluding the isotopic peaks)

210

at the nominal mass of 293 were expanded in Figure S4 and the compounds corresponding to the

211

peaks were sorted by apparent molecular series, which is related with the replacement of CH4 by

212

oxygen in Table S3 (see Supporting Information). It seems that the A/O biodegradation

213

preferentially affected those parts with higher mass defect (m/z = 293.1 - 293.2). On the

214

contrary, the addition of Fe and Al salts preferentially affected those parts with lower mass

215

defect (m/z = 293.0 – 293.1). In general, the lower mass defect always means more oxygen and

216

less hydrogen because of the exchange of CH4 with O. Therefore, this interesting phenomenon

217

indicates that for DOM with the same molecular weight, molecules with comparatively high O/C

218

and low H/C ratios were less biodegradable, but they were more easily precipitated by Al or Fe


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19, 20

219

addition. This result is consistent with previous studies on DOM in other water samples

.

220

Similar conclusion can be extended to the full mass range. According to Table S2, the average

221

O/C value increased and average H/C value decreased after anaerobic and aerobic treatment,

222

while the reverse trend is shown after coagulation treatment. This observation could be caused

223

by the different DOM removal preference between biodegradation and coagulation.

224

As samples after coagulation by Fe2(SO4)3 and Al2(SO4)3 showed similar pattern of ESI FT-

225

ICR MS spectra, further analysis of DOM removal during coagulation was based on the results

226

of concentration by Fe2(SO4)3. We used the van Krevelen diagram to provide a visual display of

227

compound distribution. In order to elucidate clearly the changes occurring within each

228

subcategory, the removed (peak lost), resistant (peak retained) and produced (new peak)

229

formulas after biodegradation (anaerobic and aerobic) and coagulation were plotted against that

230

in raw leachate in Figure 3 (CHO and CHON) and Figure S5 (CHOS and CHONS). The green

231

dots corresponding to the removed compounds during biodegradation were mainly distributed in

232

the relatively low O/C region (O/C = 0.1-0.3); the red dots corresponding to resistant compounds

233

in bioprocess were concentrated in the region where the value of O/C was from 0.3 to 0.5; the

234

blue dots corresponding to the new compounds gained after biodegradation were distributed in

235

the high O/C region (O/C = 0.5-0.7). As for coagulation, the resistant compounds were mainly

236

distributed in the region with O/C = 0.2 – 0.4, while the other regions (O/C < 0.2 or O/C > 0.4)

237

in the van Krevelen diagram were mainly occupied with the removed DOM after coagulation.

238

Besides the difference of O/C, the removed molecules after coagulation were more likely

239

scattered in the low H/C region (H/C < 1.0) and the resistant molecules were mainly distributed

240

in the high H/C region (H/C > 1.0). This trend was more obvious for CHO and CHON

241

compounds (Figure 3) and illustrated more clearly in Figure S6 and S7 (see Supporting



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Information), which shows the quantitative distributions of O/C and H/C of the removed,

243

resistant and produced molecules in leachate concentrate after bioprocess and coagulation. This

244

result further indicates that biological degradation affected the O/C ratios of DOM in leachate

245

concentrate by consuming the oxygen-deficient substances and producing the oxygen-rich

246

substances, while coagulation process altered the average O/C and H/C value of DOM through

247

preferentially removing the high O/C (> 0.4) and low H/C (< 1.0) compounds. In addition,

248

another group of compounds with low O/C value (< 0.2) in Figure 3b, was also found to be

249

readily removed by coagulation. Compared with SRNOM (Figure 1a) and DOM investigated in

250

other studies

251

These compounds may contain few or no hydrophilic oxygen functional groups. Thus they

252

would be hydrophobic and would be easily precipitated by hydrolyzing metals 31.

19, 20

, DOM in leachate concentrate contained more compounds with O/C < 0.2.

253

Figure S8 shows the relative abundance of four major subgroups (CHO, CHON, CHOS,

254

CHONS) in DOMs of raw leachate and treatment (biodegradation and coagulation) samples. The

255

results show that the CHO and CHON compounds can be more easily removed, while the sulfur-

256

containing compounds (CHOS, CHONS) seem resistant to both biodegradation and coagulation

257

process, which can be evidenced by the rise of average S/C value of biodegradation and

258

coagulation effluence samples shown in Table S2. The removal preference for the major class of

259

compounds in the two processes seems different. For example, compared with other classes,

260

lipids were easily removed in biodegradation process, while aromatic structures were readily

261

removed by coagulation.

262

Furthermore, because the lignins/CRAM-like structures of CHO and CHON formulas made up

263

the largest proportion of DOM in leachate concentrate (Figure 3), we performed Kendrick mass

264

defect (KMD) analysis to further analyze the transformation and removal of these compounds


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during the water treatment process. Figure 4 and Figure S9 show the plots of KMD (COO)-

266

number of O for lignins/CRAM-like organic matter of CHO and CHON formulas respectively

267

during biodegradation and coagulation process. Numerous series of lignins/CRAM-like formulas

268

that differ only by a carboxyl group are present on the same horizontal line. The molecules with

269

more carboxyl groups were mainly distributed in the right side of the KMD plot. In addition, as

270

the number of carbon and hydrogen in the formula mainly decreased with the increased value of

271

KMD (COO), the upper part of a vertical line in the KMD (COO) diagram are the relatively

272

unsaturated molecules with more oxidized carbon. The removed compounds during bioprocess

273

were concentrated in the left part of the diagram (O < 4), and the formed compounds were

274

scattered in the right part (O > 8), implying increased carboxyl groups in lignins/CRAM-like

275

compounds after biodegradation. As for coagulation, a large proportion of the removed

276

compounds were distributed in the upper right part of the diagram (O > 8 and KMD > -0.15),

277

demonstrating that the oxidized and unsaturated lignins/ CRAM-like with more carboxyl groups

278

were preferentially removed by coagulation. Similar observations have been made by other

279

researchers 9, 19.

280

Recalcitrant DOM in Leachate Concentrate

281

DOM in leachate concentrate could not be removed completely by the water treatment

282

processes used in the study. We regarded the substances that were resistant in both

283

biodegradation and coagulation process as recalcitrant DOM. It is therefore expected that further

284

analysis of the recalcitrant DOM could provide more useful information for improving the

285

treatment process.

286

As shown in Figure 5a, although CHON and CHO compounds occupied a large proportion of

287

recalcitrant DOM, their proportions dropped obviously compared with those in raw leachate



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288

shown in Figure 1b. By contrast, the contribution of relative abundance of sulfur-containing

289

compounds (CHOS and CHONS) increased dramatically from 14.0% to 39.8%, further

290

indicating that the sulfur-containing substances in the leachate concentrate were resistant to the

291

treatment process, and thus remained in the final effluent. As shown in Figure 5b, relative

292

abundances of S1O3 – S1O7 and N1S1O5 – N1S1O7 classes of species were higher than that of

293

other sulfur-containing classes. We deduced that these S-containing compounds could be partly

294

assigned to sulfonates and sulfonamides, as well as their metabolites. Because these compounds

295

are widely used in consumer products and industrial processes, they are probably introduced into

296

landfill leachate by municipal solid waste

297

compounds are resistant to biodegradation

298

are mostly concentrated in neutrals isolates

299

coagulation processes 38, 39.

32, 33

34-36

. It has been shown that these S-containing

. Moreover, the S-containing molecules in DOM 37

, which had low removal efficiency during

300

The recalcitrant DOM formulas, especially the CHO and CHON recalcitrant compounds, were

301

mainly plotted in the region of O/C = 0.3 – 0.5, H/C = 1.0 – 1.75, especially the lignins/CRAM-

302

like structural region in van Krevelen diagram (Figure 5a). These results are consistent with the

303

proposed refractory nature of CRAM and lignin substances

304

derived species have similar H/C and O/C molar ratios, DBE, and molecular weights, different

305

structural characteristics were also found between them. CRAM consist of the majority of

306

carbonyl-containing species with isolated, aliphatic ketones and several carboxyl groups; Lignin-

307

derived species, in contrast, are complex biopolymers that contain reducible groups in

308

conjugation with alkoxy- or hydroxy-substituted aromatics primarily linked by ether-linkages 26,

309

41

310

removed by coagulation treatment, we speculated that lignin-like species might contribute a

10, 40

. Although CRAM and lignin-

. Considering that the CRAM in leachate concentrate with carboxyl groups were preferentially


Page 15 of 28


311

larger proportion of the recalcitrant DOM in leachate concentrate. Figure 5b shows that O4 – O6,

312

N1O5 – N1O7, N2O5 – N2O7, S1O4 – S1O6 and N1S1O5 - N1S1O7 classes of species were

313

predominant in recalcitrant DOM. Thus, we selected some recalcitrant DOM molecules in

314

leachate concentrate, which were plotted in the lignins/CRAM-like structural region in van

315

Krevelen diagram with formulas of CHO4-6, CHN1-2O5-7, CHS1O4-6 and CHN1S1O5-7 (see Table

316

S4 in Supporting Information). Some possible stuctures for each formula were also listed in the

317

table. Functional groups including aromatic ketones/aldehydes, alkoxy aromatics, benzyl ester 41,

318

amines 42 and sulphonates 43 are present as derivatives of lignin.

319

Technique and Environment Implications

320

High-resolution and accurate mass measurements were used to investigate DOM in leachate

321

concentrate in this study, which demonstrated that this analytical technique was effective to

322

characterize the molecular transformation of DOM during treatment processes. Biological and

323

coagulation combined processes are effective for landfill leachate

324

wastewater

325

two major categories of processes. Our results further support this complementary relationship at

326

molecular level. Employment of anaerobic and aerobic bio-oxidation as a pretreatment step

327

could increase O/C ratio of the organic substances by adding oxygen functional groups such as

328

carboxyl groups, which ultimately increase the potential binding to cationic species thereby

329

enhancing coagulation of DOM with hydrolyzing metals. Besides, coagulation process could

330

remove the unsaturated and oxidized organic substances which would resist biodegradation, thus

331

reducing toxicity and enhancing biodegradability of the wastewater. Therefore, the combination

332

of biodegradation and coagulation is preferred when selecting treatment process for landfill

333

leachate concentrate.

45, 46

44

and other types of

. Such effectiveness is attributed to the complementary relationship between the



Page 16 of 28

334

The significant differences in molecular compositions between the DOM in leachate

335

concentrate and SRNOM suggested that leakage of leachate concentrate would have a large

336

effect on the NOM in surrounding aquatic environment by changing its concentration and

337

molecular diversity. The recalcitrant DOM in leachate concentrate, such as sulfur-containing

338

surfactant and lignin-derived species, would be persistent in aquatic environment. The release of

339

these contaminants may have negative impacts on water quality and aquatic ecology

340

exert potential toxicity to aquatic organisms

341

Therefore, efforts should be made to develop highly efficient coagulants and identify novel

342

microorganisms for the treatment of recalcitrant DOM in leachate concentrate.

343

Supporting Information

34

32, 35, 47

, and increase diffusion of other pollutants

,

48

.

344

This material is available free of charge via the Internet at http://pubs.acs.org.

345

Additional details of the experiments, the causes for the peak loss of analysis, boundaries of

346

regions in van Krevelen diagrams and the equations used in present study can be found in

347

Supporting Information. Additional figures and tables as noted in the text include the treatment

348

process flow of the landfill (Figure S1), scheme of bioreactor (Figure S2), negative ion mass

349

spectra and of samples (Figure S3 and S4), van Krevelen diagrams (Figure S5), KMD plots

350

(Figure S9), other additional plots (Figure S6, S7 and S8) and tables (Table S1, S2, S3 and S4).

351

ACKNOWLEDGMENTS

352

The authors are grateful to the financial support from the Major Program of the National

353

Natural Science Foundation of China (Grant No. 91434132), National Natural Science

354

Foundation of China (Grant No. 51578006 and 51378020), and the Collaborative Innovation

355

Center for Regional Environmental Quality.


Page 17 of 28

356


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499


Page 23 of 28

(a)


(b)100%

2.5

4.5 4.5 % 3.2 % 12.5%

6.1 % 7.9 %

80%

2.0

H/C

1.0

40%

0.5

20%

79.8% 50.0 %

Raw leachate

0.0

0.2

0.4

2.5 2.0

0.6

0.8

1.0

1.2

(c) 100%

Lignins/CRAM-like Aliphatic/ Proteins

Lipids

Raw leachate CHO

Carbohydrates

CHON 1.7 % 5.5%

SRNOM CHOS

CHONS

17.2%

80% 8.1 %

1.5

60%

1.0

83.0 % 71.8%

40%

Tannin

0.5 0.0

0%

O/C

Unsaturated hydrocarbon

H/C

36.0%

60%

1.5

0.2

0.4

0.6

0.8

SRNOM 1.0 1.2

CHO

CHON

7.3 % 2.6 %

0% 2.5 2.0 1.5Raw leachate 1.0 0.5

Aromatic structures O/C

Subcategories

20%

CHOS

CHONS

1.7 % 1.2%

SRNOM

0.0 2.5 2.0 0.2 0.4 0.6 Lignins/CRAM-like 0.8 1.0 1.2 Lipids0.0 Aliphatic/ Proteins 1.5 1.0 0.5 Others 0.0 Aromatic structures 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Figure 1. Comparison of DOM compositions in raw leachate concentrate and Suwannee River natural organic matter (SRNOM). Van Krevelen diagram of CHO, CHON, CHOS, CHONS of raw leachate concentrate and SRNOM (a). The black lines in the Van Krevelen diagram correspond to major classes of compounds that can be expected in DOM, which were illustrated in Van Krevelen diagram of SRNOM. Bar diagrams show the contribution of the major subcategories (b) and major classes (c) in the two samples.

ACS Paragon Plus Environment


DOC COD

(a)

2000

Coagulant: Al2(SO4)3

(b) 400

1200 200

800

100

400

0

DOC value (mg/L)

300

COD value (mg/L)

DOC value (mg/L)

1600

Fe2(SO4)3

300

2000 DOC COD DOC COD

1600

1200 200

800

100

COD value (mg/L)

400

Page 24 of 28

400

0 Raw leachate

Anaerobic

Aerobic

0

0 0

20

40

60

80

Coagulant dosage (mM Al or Fe)

Figure 2. DOC and COD values of the leachate concentrate after treatment in anaerobic and aerobic reactor (a), and after coagulation by Al2(SO4)3 or Fe2(SO4)3 (b). The raw leachate concentrate, the effluents from anaerobic reactor, aerobic reactor, and the coagulation by 20 mM Fe or Al salts were chosen for further investigation.


Page 25 of 28


2.5

2.5

2.0

2.0

1.5

1.5

H/C

H/C

(a)

1.0

1.0

0.5

0.5

CHON

CHO 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.2

0.4

0.6

O/C

2.5 2.0 1.5 1.0 0.5 0.0

2.0

H/C

0.8

1.0

1.2

2.5

2.5

1.5

2.0

0.2

1.0 0.5 0.0

1.0

O/C

CHO 0.2

0.4

0.6

0.8

1.0

1.5

0.4 H/C

(b)

0.8

0.6

0.8

1.0

1.2

1.0 0.5 0.0

1.2

CHON 0.2

0.4

O/C

0.6

1.2

O/C Removed

Resistant

Produced

Figure 3. Van Krevelen diagrams of CHO and CHON of DOM in leachate concentrate after A/O biodegradation (a) and coagulation by Fe2(SO4)3 (b). Points in green represent raw leachate peaks 0

200

400

600

that disappeared after biodegradation or coagulation (removed), points in red represent raw leachate peaks that were unchanged (resistant), and points in blue denote new peaks that appeared during biodegradation (produced).



(a)

scale-expanded segments

-0.1

-0.2

-0.3

CHO Lignins/CRAM-like

-0.4 0

2

4

6

8

10

12

14

-0.135

Kendrick mass defect (COO)


0.0

-0.140 -0.145 -0.25 -0.26 -0.27 2

4

Oxygen number

8

Formula [M – H]-: C14H13O3 C14H13-nO3(COOH)n O/C = 0.21 (n = 0,1,2,3) H/C = 1.00

O/C = 0.33 H/C = 0.93

C15H13-nO2(COOH)n C15H13O2 O/C = 0.13 (n = 0,1,3,4) H/C = 0.93

O/C = 0.25 H/C = 0.87


O/C = 0.25 H/C = 1.25


O/C = 0.20 H/C = 1.40


-0.2

-0.3

CHO Lignins/CRAM-like 2

4

6

8

10

C20H25O5

C20H27O4

C18H13O8 O/C = 0.44 H/C = 0.82

C19H13O10 O/C = 0.52 H/C = 0.78

C21H25O7 O/C = 0.33 H/C = 1.24

C22H25O9 O/C = 0.41 H/C = 1.18

C21H27O6 O/C = 0.28 H/C = 1.33

C17H13O9 O/C = 0.53 H/C = 0.83

C22H27O8 O/C = 0.36 H/C = 1.27

Formula [M – H]-:

-0.135

2.5 2.0 -0.140 1.5 1.0 0.5

-0.1

0

C16H13O4

C16H13O7 O/C = 0.43 H/C = 0.87

10

scale-expanded segments 0.0

-0.4

C15H13O5

Oxygen number

(b) Kendrick mass defect (COO)

6

Page 26 of 28

12

14

0.0

0.2

-0.145

0.4

0.6

0.8

-0.25

O/C = 0.21 H/C = 1.00

C15H13-nO2(COOH)n 1.0 (n = 0,1,3) 1.2

O/C = 0.13 H/C = 0.93

C19H25-nO3(COOH)n (n = 0,1,2)

-0.26 -0.27 2

4

Oxygen number

6

8

10

C14H13O3

C14H13-nO3(COOH)n (n = 0,1,2)

C19H27-nO2(COOH)n (n = 0,1,2,3)

C15H13O2

C19H25O3 O/C = 0.15 H/C = 1.37

C19H27O2 O/C = 0.11 H/C = 1.47

C15H13O5 O/C = 0.33 H/C = 0.93

C16H13O4 O/C = 0.25 H/C = 0.87

C20H25O5 O/C = 0.25 H/C = 1.25

C20H27O4 O/C = 0.20 H/C = 1.40

C16H13O7 O/C = 0.43 H/C = 0.87

C18H13O8 O/C = 0.44 H/C = 0.82

C21H25O7 O/C = 0.33 H/C = 1.24

C21H27O6 O/C = 0.28 H/C = 1.33

C22H27O8 O/C = 0.36 H/C = 1.27

Oxygen number

Removed

Resistant

Produced

Figure 4. KMD (COO)-number of O in the formulas plots and its scale-expanded segments for lignins/CRAM-like structures of CHO formulas in the DOM after A/O biodegradation (a) and 0

200

400

600

coagulation by Fe2(SO4)3 (b). The assigned formulas (as [M – H]- ions) of some randomly selected points in the segments are also displayed. The calculation equations of the KMD (COO) are shown in Supporting Information.


Page 27 of 28


(a) 2.5

100% CHONS 13.4%

H/C

2.0

80%

1.5 60%

CHOS 26.4%

1.0 40%

0.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

O/C CHO

CHON

CHOS

CHONS

20% CHO

2.5 2.0 1.5 1.0 30.3% 0.5 0.0 2.5 2.0 0.0 0% 1.5 0.2 1.0 0.5 0.0 0.0 0.2

0.4 0.4

S1 O S1 1 O S1 3 O S1 5 O S1 7 S1 O9 O S1 11 O S2 1 5 O S2 2 O S2 5 S2 O7 O S2 10 O S3 1 4 O S3 3 S3 O8 O 13

O 0 O 1 O 2 O 3 O 4 O 5 O 6 O 7 O 8 O O 9 1 O0 1 O1 1 O2 1 O3 1 O4 15 N 1O N 1 1O N 3 1O N 5 1O N 7 1O N 9 1O N 11 1O 1 N 4 2O N 1 2O N 4 2O N 6 2O N 8 2O N 10 2O 1 N 2 3O N 0 3O N 2 3O N 4 3O N 6 3O 8

(b)

Subcategories

CHON 29.9%

S N1

4 9 7 0 4 4 6 2 7 0 3 0 3 3 1O S1O S2O S3O 3O1 S1O 1O1 S2O 2O1 S3O S1O S1O 1O1 2O1 1 1 1 2 2 2 3 3 S S S S S N N N N N N N N N1 N2 N2 N3 N3

Figure 5. Van Krevelen diagram and the contribution of the major subcategories (CHO, CHON, CHOS, CHONS) (a) and relative abundance of identified classes (b) of recalcitrant DOM in leachate concentrate.


0.6 0.6

0.8


TOC/Abstract Art ESI FT-ICR MS

Different H/C

H/C

Landfill leachate Concentrate

SRNOM

DOM

O/C

O/C

Removed H/C

Residual Produced Resistant O/C

Removed O/C H/C

Coagulation

Complementary

H/C

Biodegradation

Raw leachate

Raw Water

Lignin-derived compounds Sulfur-containing substances

Resistant O/C


Page 28 of 28

Molecular Insights into the Transformation of Dissolved Organic Matter

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