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Interaction between Dissolved Organic Matter and Long-Chain Ionic Liquids: A Microstructural and Spectroscopic Correlation Study Xiao-Yang Liu, Wei Chen, Chen Qian, and Han-Qing Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05228 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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Environmental Science & Technology

Interaction between Dissolved Organic Matter and Long-Chain Ionic Liquids: A Microstructural and Spectroscopic Correlation Study

Xiao-Yang Liu*, Wei Chen*, Chen Qian, Han-Qing Yu** CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China

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ACS Paragon Plus Environment


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ABSTRACT

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The production and use of ionic liquids (ILs) increase the potential risk after their

3

emission into the environment. After entering the environment, ILs will readily interact

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with dissolved organic matter (DOM), and their environmental behavior will be impacted

5

by DOM, which is abundant in the environment and has various functional groups.

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However, to date, the interaction between DOM and ILs, especially long-chain ILs,

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remains unclear. In this work, the interaction between long-chain ILs and humic acid

8

(HA), a representative DOM, was investigated using synchronous fluorescence, Fourier

9

transform infrared spectroscopy, dynamic light scattering and zeta potential techniques,

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which were integrated with two-dimensional correlation spectroscopy (2DCOS), hetero-

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2DCOS and perturbation-correlation moving-window analyses. The results show that

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cation exchange by the carboxylic groups in humic-like fractions was primarily

13

responsible for interaction at low IL concentrations. As a result, the decrease in

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electrostatic repulsion and the increase in hydrophobicity facilitated the loose aggregation

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of HA. With an increase in IL concentration, the aromatic and carbonyl groups were

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involved in the interaction via the π-π interaction and dipole-dipole interaction

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respectively, which resulted in the disruption of intra-molecular hydrogen bond and

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promoted the compaction of HA under the hydrophobic effect. The intensity transition

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sequence of various groups in HA was elucidated more specifically by 2DCOS. With

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these results, a comprehensive view of the structural changes of DOM in its IL-binding

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process was obtained, and the fate and environmental impact of ILs could be better

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understood. Furthermore, the superior potential of such an integrated approach in

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investigating the complex interactions in the environment was also demonstrated.

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INTRODUCTION

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Ionic liquids (ILs), composed of organic cations and organic/inorganic anions, are

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generally liquid at room temperatures. Recently, ILs have received considerable attention

29

due to their unique physical and chemical properties, such as their inherent

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amphiphilicity, negligible volatility, good thermal stability and good extraction

31

capability.1,

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including catalysis, organic synthesis, oils, electrochemistry and liquid/liquid extraction,

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etc.3, 4 Considering these applications and their potential use in large-scale production

34

processes, ILs will inevitably be released into the environment.5 Although their low vapor

35

pressure minimizes the atmospheric pollution, the impacts of ILs on aquatic and

36

terrestrial ecosystems cannot be ignored or underestimated.6

2

Therefore, the commercial applications of ILs are rapidly increasing,

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Dissolved organic matter (DOM), which is ubiquitous in aquatic and terrestrial

38

environments, is composed of aromatic and aliphatic hydrocarbon structures and contains

39

amide, carboxyl, hydroxyl, ketone and various minor functional groups.7, 8 The presence

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of DOM is able to influence the mobility, retention, transformation and environmental

41

toxicity of ILs.1, 2, 9 Previous studies have proven that ILs can interact with various soils

42

via cation-exchange mechanism and the absorption strength increases with the increasing

43

organic carbon content.2 It is also found that the dispersive interaction occurs with the

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excess cation exchange capacity of soils.10 Several driving forces in the interaction of ILs

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and DOM, i.e. ionic binding, ion-dipole interaction, dipole-dipole interaction and π-π

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interactions, were proposed.10 In extraction experiments, a high affinity of humic acid

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(HA) towards ILs in aqueous solutions was reported.11 Although this interaction

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significantly reduces the freely dissolved concentration and bioavailability of ILs, another

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study indicates that DOM and coexisting organic contaminants could be released from

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soils after addition of ILs, suggesting that DOM could facilitate the transportation of ILs

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in soils and groundwater and thereby increase their sub-surface mobility.9,

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influence of DOM is reported, but there is no molecular-level study to probe such an

53

interaction process in detail. Furthermore, previous studies on DOM-ILs interaction focus

54

on the absorption behavior only. Therefore, elucidating the interaction between ILs and

55

DOM at a molecular level is essential and will lead to a better understanding of the fate

56

and environmental impacts of ILs.

12

The

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Current approaches, e.g., binding isotherm and conductivity titration, which can be

58

used to explore the interaction between ILs and DOM, are limited, and little is known

59

about the binding mechanism between DOM and ILs and the behavior of DOM after

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binding with ILs at a molecular level.12,

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dimensional correlation spectroscopy (2DCOS) analysis could remedy this defect and

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offer a deep insight into the variations of molecular structure. Recently, hetero-2DCOS

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receives growing attention due to the capability of integrating complementary

64

information from the two different types of spectroscopy.7,

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two methods shows detailed information along the perturbation direction. Therefore, it is

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difficult to connect changes in the functional group of analytes with the macroscopic

67

properties, such as the morphology, zeta potential and so on. To solve this problem, an

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emerging technique called perturbation-correlation moving-window (PCMW) is derived

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from the correlation between the spectral intensities and perturbation changes. This

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Optical spectroscopy coupled with two-

14, 15


However, neither of the

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approach can monitor the complicated spectral variations along the perturbation direction

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and determine the transition points and regions.16, 17

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In this work, by using FTIR, synchronous fluorescence, dynamic light scattering

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(DLS) and zeta potential techniques, we aim to provide a comprehensive understanding

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of the interaction between DOM and ILs from both the microstructural and spectroscopic

75

perspectives. 2DCOS was used to analyze the subtle changes in the optical spectroscopy,

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while hetero-2DCOS and PCMW were employed to integrate the results, including the

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functional groups, fluorophore variations and macroscopic characteristics, i.e., the zeta

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potential and hydrodynamic size. To the best of our knowledge, this might be the first

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report about using a combination of macroscopic and microscopic approaches to

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investigate the DOM-related interaction. HA was used as a representative DOM, and a

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typical long-chain imidazolium-based IL, 1-dodecyl-3-methylimidazolium bromide

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[C12mim]Br, was used to represent the ILs due to its good surface activity, hard

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degradation and wide application in various areas.7, 18-22

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MATERIALS AND METHODS

86 87

Sample Preparation. Analytical reagent-grade NaOH and HCl were purchased from

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Sinopharm Chemical Reagent Co., China. [C12mim]Br was obtained from Shanghai

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Cheng Jie Chemical Co., China with a purity of over 99%. Commercial HA (Sigma-

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Aldrich Co., USA) was puriﬁed prior to use (details in Supporting Information, SI). The

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purified HA was dissolved to a final concentration of 100 mg/L at pH=7.0 with 1 M

92

NaOH and 1 M HCl. The [C12mim]Br powder was dissolved to a final concentration of



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25 mM as a stock solution. A series of solutions containing 24 mg C/L HA and

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[C12mim]Br with concentrations ranging from 0 to 0.65 mM were prepared, and the

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background ionic strength was maintained by 0.03 M KBr. The concentrations of IL used

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were far below the critical micelle concentration (10.9 mM).23 The mixed solutions were

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shaken for 30 h at room temperature (25 °C) to ensure the binding equilibrium.11 Then,

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the solutions were agitated gently using a vortex mixer for 5 s and analyzed by DLS, zeta

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potential and synchronous fluorescence spectroscopy. A 35-mL aliquot of each sample

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was freeze-dried for further analysis by FTIR spectroscopy.

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DLS and Zeta Potential Measurements. The DLS and zeta potential

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measurements of HA solutions with different IL concentrations were performed using a

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Malvern Zetasizer ZS instrument (Malvern Instruments Co., UK). During the DLS

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measurement, a He-Ne laser beam operated at λ = 633 nm was incident on the samples,

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and then scattered light was detected and collected by a photodetector at an angle of 173°

106

to utilize the backscattering mode. The DLS instrument could be used to measure

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particles ranging from 0.6 nm to 6 µm. In the DLS and zeta potential measurements, three

108

parallel tests were conducted at 25 °C, and the data were presented as the mean ±

109

standard deviation. In addition, after the reaction the pH values of all the solutions were

110

measured to be 6.14 ± 0.07. Such a slight change of pH should have negligible impact on

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the zeta potential.24

112

Spectroscopy Measurements. Synchronous fluorescence spectra of HA solutions

113

with various IL concentrations were analyzed with a luminescence spectrometer (LS-55,

114

Perkin-Elmer Inc., USA). The synchronous scans were performed over a range from 250

115

to 550 nm with a constant offset △λ of 60 nm.7 The excitation and emission slits were


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both 10 nm, and the scan rate was 1000 nm/min. The background synchronous

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fluorescence spectra of water and IL are shown in Figure S1.

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The freeze-dried HA samples with various IL concentrations were ground and

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homogenized. Afterwards, 50 mg of each sample was pressed under the irradiation of an

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infrared lamp to eliminate the influence of moisture. Their transmission IR spectra were

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recorded on a Vertex 70 spectrometer (Bruker Co., Germany) with a deuterated triglycine

122

sulfate detector, and each spectrum was obtained after 64 scans with a 4 cm-1 resolution

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and finally transformed to absorbance spectra using OPUS 5.5 software for the

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subsequent analysis. The IL samples at different concentrations in the absence of HA

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were treated by the same procedure referring to literature.7, 25 No extra KBr was added as

126

ground KBr powder was added into the solution (30 mM) before freeze drying. The FTIR

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spectra of the purified HA and IL are shown in Figure S2.

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Data Analysis. To probe the structural variation in the interaction between HA and

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the IL, 2DCOS, hetero-2DCOS and PCMW analyses were performed using the

130

synchronous fluorescence and FTIR spectra (details in SI). The concentration of the IL

131

was used as an external variable. Prior to the analysis, the spectra of each sample were

132

baseline-corrected and smoothed, and the background signal (without the addition of HA)

133

was subtracted with the peak at 860 cm-1 as a reference band (Figure S2). 2DCOS,

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hetero-2DCOS and PCMW maps were calculated and plotted by 2D Shige (Kwansei-

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Gakuin University, Japan) and Origin 8.5 software.

136 137

RESULTS

138



139

Change of HA Stability with the IL. HA is a complex mixture of negatively charged

140

organic compounds present in the environment. The impact of the tested IL on the

141

stability of HA was evaluated by examining the hydrodynamic radii Rh and zeta

142

potentials of HA at variable IL concentrations (Figure 1). Three steps of aggregation

143

could be distinguished from Figure 1a: (i) Rh of the HA-IL complexes rapidly increased

144

from 236.4 ± 2.1 nm at [IL] = 0.05 mM to 3275.8 ± 234.3 nm at [IL] = 0.20 mM; (ii) Rh

145

slightly decreased from 0.20 to 0.30 mM IL; and (iii) the mean size of the HA-IL

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complexes remained almost unchanged at approximately 2500 nm. The three distinct

147

steps were designated Periods 1, 2 and 3. There was significant difference between the

148

sizes of HA in the IL concentration ranges (0.15 mM, 0.20 mM), (0.20 mM, 0.25 mM)

149

and (0.25 mM, 0.30 mM) (p ＜ 0.05, Table S1). The difference was not significant at IL

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concentrations higher than 0.30 mM. These results further confirm that there were three

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periods when the IL with increasing concentrations bound to HA, and the turning points

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were around 0.20 and 0.30 mM. The increase in the HA size in Period 1 likely resulted

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from the decrease in electrostatic repulsion and the increase in hydrophobicity.26 Then,

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the aggregates became dense and compact in Periods 2 and 3, which was attributed to the

155

hydrophobic effect of IL as well as the disruption of intramolecular hydrogen bond in HA,

156

which will be discussed later with the spectroscopy results.8, 11, 27

157

The zeta potential profiles of HA in the presence of the IL are shown in Figure 1b.

158

With the addition of the IL, the zeta potential of HA became less negative, indicating that

159

the IL imparted positive charges to HA and decreased the absolute surface potential.28

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Moreover, according to the DLVO theory, the decrease in surface potential would reduce

161

the repulsive forces between HA molecules, and subsequently, large aggregates were


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formed, which is exactly in agreement with the DLS results. In Period 2, the zeta

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potentials of the HA-IL complexes remained almost unchanged, which was likely related

164

to the constriction and rearrangement of HA. There was significant difference for the zeta

165

potentials at IL concentrations of 0-0.10 mM and 0.40-0.55 mM (p < 0.05, Table S1) due

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to the absorption of IL, while the difference was not significant at 0.10-0.40 mM.

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Although Na+, K+ and IL+ are all mono-valent cations, the absorption energy of ILs

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with HA is much higher than that of Na+ and K+ due to the hydrophobicity of the alkyl

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chain.11 Thus, IL+ could be absorbed closer to the HA surface, leading to the less negative

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zeta potential. Previous studies have shown that Na+ forms very weak outer-sphere

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complexes with DOM.29 By calculating the occupancy of the Stern layer with ionic

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surfactant and Na+ in the sorption on glass, Dimov et al. also found that, although cation

173

exchange took place, the increase in the occupancy of Stern layer with ionic surfactant

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was larger than the decreased amount of occupancy with Na+.30 Considering the similarity

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between ILs and ionic surfactants, the ion replacement ratio of IL+ and Na+ or K+ in the

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Stern layer should also be larger than 1:1.10

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Spectral Response of HA to the IL. There are abundant functional groups in HA,

178

including carboxyl, phenol, hydroxyl, quinone, ester, ketone and amino groups. Thus,

179

synchronous fluorescence and FTIR spectroscopies were used to probe the spectral

180

changes of HA during interaction with the IL (Figure 2). In order to show the changes of

181

synchronous fluorescence spectra with the dose of IL more clearly, the contour map is

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shown in Figure 2a and the corresponding spectra are shown in Figure S3. There were

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three fluorescence regions at 250-300, 300-380 and 380-550 nm, which could be assigned

184

to the protein-, fulvic- and humic-like fluorescence fractions of HA, respectively.



185

Compared to the fulvic- and humic-like regions, the fluorescence intensity of the protein-

186

like fraction was relatively low. After dosing the IL, the fluorescence intensity in the

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humic-like fraction decreased remarkably, which suggests that the interaction sites were

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mainly from the humic-like fraction, and the decrease is probably due to the charge-

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transfer effect. Numerous studies show that the fluorescence properties of DOM result

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from the intra-molecular charge-transfer interactions between hydroxy-aromatic donors

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and quinoid or other acceptors formed by the partial oxidation of lignin precursors.31

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Especially, in the wavelength region where the local donor states should no longer absorb

193

(beyond λex 375-400 nm), the fluorescence emission is primarily attributed to an

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ensemble of the charge-transfer states of differing energies.32

195

The dominant infrared spectral characteristics of HA with the addition of the IL are

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presented in Figure 2b. At low IL concentrations, the characteristic peak at 1385 cm-1

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assigned to the symmetric stretching vibrations of COO- or phenolic C-O-H was shifted

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to approximately 1408 cm-1. The drastic blue-shift might be attributed to the breaking of

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hydrogen bonding. The structural changes of HA upon IL addition are illustrated more

200

clearly in the differential IR absorbance spectra, in which the spectrum of HA without IL

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addition was subtracted from each other spectrum (Figure 2c). For example, the peak

202

intensity at 1200 cm-1 attributed to the stretching vibrations of C-O of aromatic acid and

203

aliphatic acid ester gradually increased with IL addition. Even so, some of the IR bands

204

remained overlapped and could hardly be distinguished owing to the diverse groups of

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HA. Thus, to determine the conformational changes of HA in the binding process with

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the IL, 2DCOS, hetero-2DCOS and PCMW analyses were performed using the

207

synchronous fluorescence and FTIR spectra.


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Correlation Analysis of the Structural Change in the HA-IL Interaction.

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2DCOS enables the decomposition of overlapping peaks and enhances the spectral

210

resolution by spreading the spectra in a second dimension.14 When two or more

211

overlapping peaks behave differently under perturbation, the cross-peaks, which are

212

located in the off-diagonal region, could provide clues that aid the assignment of their

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frequencies and components. In addition, 2DCOS may reveal the trend of intensity

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changes that are not evident in the original spectra.14 The synchronous 2DCOS map for

215

the synchronous fluorescence spectra of HA in the 280-550 nm region (Figure 3a) shows

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that two predominant autopeaks centered at 450 and 320 nm along the diagonal line, and

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a small peak at 266 nm can be identified from the cross-peaks, which indicates that the

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fluorescence of fulvic- and humic-like fractions were readily affected by IL addition.

219

Moreover, the intensity at 450 nm changed in the opposite direction compared to those at

220

266 and 320 nm, considering the negative cross-peaks of (450, 266) and (450, 320). The

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asynchronous map provided the sequential relationship among these fractions (Figure 3b).

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The cross-peaks (450, 266) and (450, 320) exhibited positive and negative signs,

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respectively, which implies that the intensity variation followed the order: 266 nm > 450

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nm > 320 nm, i.e., protein- → humic- → fulvic-like fractions.

225

Overlapping IR peaks are major obstacles to assigning bands and analyzing the

226

conformational changes of HA, but 2DCOS could effectively resolve this problem.

227

Figure 3c and d shows the 2D IR COS of the interaction between HA and the IL at

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various concentrations. The sign and assignment of each cross-peak are listed in Tables

229

S2 and S3, respectively.33 There were eight autopeaks in the synchronous map, centered

230

at 1730, 1600, 1570, 1490, 1408, 1375, 1201 and 1060 cm-1 (Figure 3c). The band



231

centered at 1600 cm-1 changed most significantly, which was attributed to the asymmetric

232

stretching of the carboxylate group. The smallest changes were at 1730 and 1060 cm-1,

233

which was assigned to the stretching of C=O of the carboxylic group and C-O of

234

polysaccharides, respectively. The autopeak at 1644 cm-1 corresponding to the C=O

235

stretching of amide, quinone or ketone could be overlapped by the peak at 1600 cm-1,

236

according to the cross-peaks. Most of the cross-peaks in the synchronous map were

237

positive, except for those associated with 1570 and 1375 cm-1. This indicates that most IR

238

bands changed in the same direction, whereas the bands at 1570 and 1375 cm-1 changed

239

in the reverse direction. The 1570 and 1375 cm-1 bands could be assigned to the

240

stretching of imidazole C=C and C-N-C as well as the wagging vibration of CH2 of the

241

IL, suggesting the formation of HA and IL complexes. Combined with the asynchronous

242

map in Figure 3d, the final sequence order of HA conformational changes could be

243

concluded on the basis of Noda’s rule: 1570, 1375 > 1730 > 1490 > 1201 > 1644 >

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1060 > 1600 > 1408 cm-1 or ν(C=C of imidazole), νas(C=C of imidazole) or ω(CH2 of IL)

245

→ ν(C=O of COOH) → ν(C=C of aromatic ring) → ν(C-O of aromatic acid, aliphatic

246

acid ester) → ν(C=O of amide, quinone or ketone) → ν(C-O of polysaccharide) →

247

νas(COO-) → νs(COO-) or phenolic ν(C-O), δ(O-H).

248

To determine the IR band assignments and the correlation between the fluorescence

249

and FTIR spectra, fluorescence/IR hetero-2DCOS analysis was performed (Figure S4a

250

and b). In the synchronous map, six cross-peaks were located remarkably in the IR

251

regions 1644, 1600, 1570, 1490, 1375, and 1201 cm-1 and the corresponding fluorescence

252

region 450 nm (Figure S4a). The cross-peaks of 1570 and 1375 cm-1 with 450 nm were

253

positive, while the others were negative. These results indicate that the carbonyl and


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aromatic groups are the basic fluorescent units for the humic-like fraction of HA, and

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their IR intensities increased after the IL addition. In the asynchronous map, two new

256

cross-peaks were observed at 1730 and 1010 cm-1 with the fluorescence peak at 450 nm,

257

which indicates that carboxylic groups and some polysaccharide groups were also related

258

to the humic-like fraction (Figure S4b). In addition, the fulvic-like fraction had a higher

259

content of carboxylate and phenolic groups due to the two cross-peaks at 1600 and 1408

260

cm-1 with the fluorescence region at 380-300 nm.34

261

Because the IR and fluorescence spectra reflect the different aspects of molecular

262

responses towards an identical stimulation, asynchrony was observed between the

263

spectroscopic data of the same species. Five positive signs at coordinates (1644, 450),

264

(1600, 450), (1201, 450), (1600, 380-300) and (1408, 380-300) appeared in the

265

asynchronous map, which implies that the fluorescence response of the fulvic-like

266

fraction occurred before the carboxylate groups, and the fluorescence response of the

267

humic-like fraction occurred before the carboxylate groups, C-O of aromatic acid and

268

aliphatic acid ester, and C=O of amide, quinone or ketone. In addition, the cross-peaks

269

(1490, 450), (1570, 450) and (1375, 450) indicate that the fluorescence response of the

270

humic-like fraction changed after the addition of IL and C=C groups. The order of IR

271

intensity changes was in good agreement with the 2DCOS results.

272

Correlation between the Spectral Variable and the IL Concentration. Although

273

the spectral resolution was enhanced greatly by 2DCOS, the correlation between the

274

characteristic spectral variable and the IL concentration remained unclear. Thus, PCMW

275

was used to determine the transition point and monitor the complicated spectral variations

276

along the perturbation. Furthermore, the changes of the HA functional groups observed



277

by the PCMW method could be readily related to other data, such as the DLS and zeta

278

potential.

279

Figure 4a and b shows the PCMW synchronous and asynchronous maps based on

280

the FTIR spectra of HA with the IL at various concentrations. A window size of 2m + 1 =

281

7 was applied for the calculation. For convenience, the transition concentrations and

282

transition regions of HA that can be read from these maps are plotted in Figure 4c. The

283

transition concentrations of various functional groups, which are the local maximum

284

gradient of the spectral intensity change along the concentration, were determined by the

285

synchronous peaks: carboxylic (1730 cm-1) and IL functional groups (1375, 1570 cm-1)

286

below 0.15 mM IL→ aromatic groups (1490, 1201 cm-1) at 0.35 mM IL→ carboxylate

287

groups (1600, 1408 cm-1) and C-OH of polysaccharide (1060 cm-1) at 0.4 mM IL. These

288

transition concentrations were mostly gathered around 0.35-0.4 mM, indicating that the

289

changes of the functional groups slowed down after the size of HA-IL reached a limit

290

(Period 3).

291

Combining the signs of the PCMW synchronous and asynchronous spectra, the

292

spectral variations along the concentration perturbation could be probed by the signs of

293

correlation peaks.16 The transition concentration region could be determined by the peak

294

positions in the asynchronous spectra, which are all the turning points of the curves.17

295

Based on these rules, it was concluded that, with the IL addition, the aromatic, COO- and

296

C-O of the polysaccharide groups exhibited S-shaped spectral changes. These groups are

297

related to the hydrogen bonding capability of HA, and the onset concentrations were

298

focused in the region of 0.2-0.35 mM, in which the HA-IL complexes became compact.

299

This result suggests that the constriction of HA might be related to the hydrogen bonds


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300

and hydrophobic environment.8 In addition, at an [IL] of less than 0.15 mM, the peaks at

301

1570 and 1375 cm-1 decreased, while the intensity of the peak at 1730 cm-1 increased.

302

This confirms that the carboxylic group of HA was the main group that interacted with

303

the IL at low concentrations.

304 305

DISCUSSION

306 307

Evolution of the Binding between ILs and DOM. DOM has a great impact on the

308

absorption, transport and transfer of ILs, as demonstrated by various binding

309

experiments.2,

310

interaction between DOM and ILs. HA, a representative DOM, is generally considered as

311

a supramolecular association of self-assembling heterogeneous and relatively small

312

molecules stabilized by weak dispersive forces, such as hydrophobic (i.e., van der Waals,

313

π-π and CH-π) forces and hydrogen bonds. Thus, the structure of HA could be readily

314

destroyed and reconstructed by ILs. The interaction between HA and the IL can be

315

divided into three periods based on the DLS results. The hydrodynamic size of the HA-IL

316

complexes underwent a rapid increase, experienced condensation and was almost

317

constant in Periods 1, 2, and 3, respectively.

11, 12

However, robust evidence is still required to further elucidate the

318

In Period 1 (0-0.2 mM IL), the increased size and zeta potential indicate the loose

319

aggregation of HA. These structural changes of HA have also been observed when HA

320

interacts with other cations.26 According to the non-ideal competitive adsorption (NICA)-

321

Donnan model, carboxylic and phenolic groups of HA are the low and high proton-

322

affinity sites, respectively. The low proton-affinity carboxylic sites tend to interact with



323

cations, as reported in a recent study related to clarithromycin.35 In addition, upon

324

interaction with HA, the critical aggregation concentration of the IL-HA complexes was

325

approximately 0.10 mM, which is equivalent to the average amount of carboxylic groups

326

(3.17 mol/kg) in HA.36 Therefore, the carboxylic groups are supposed to dominate the

327

initial binding process of ILs with HA. In Period 2 (0.2-0.3 mM IL), ILs continued to be

328

bound to HA in the compression of the HA-IL complexes; otherwise, the absolute zeta

329

potential value would increase. In Period 3 (0.3-0.65 mM IL), although the size of the

330

HA-IL complexes remained constant, the ILs still tended to be close to HA, as evidenced

331

by the zeta potential results.

332

The combination of further experimental data and previous literature findings allows

333

us to examine the detailed HA-IL interaction. By combining the DLS and zeta potential

334

results with PCMW analysis, the interaction process can be elucidated from the point of

335

view of structural change of HA. In Period 1, the carboxylic groups of the humic-like

336

fraction are bound to the cationic head of the IL, which is consistent with the proposed

337

conjecture obtained from the DLS results. In Period 2, C=C of aromatic ring, C-O of

338

aromatic acid and aliphatic acid ester, C=O of amide, quinone or ketone, and C-O of

339

polysaccharides related to the humic-like fraction changed in succession, implying that π-

340

π and dipole-dipole interactions occurred and made IL close to HA. A previous study

341

demonstrates that intra-molecular hydrogen bonds helped to stabilize the structure of HA

342

and hinder its compaction, with carboxylic acids and phenols as proton donors, while

343

carbonyl groups (carboxylic acids and ketones) and π-electrons in aromatic rings as

344

proton acceptors.8, 37 The π-hydrogen bonds in HA, which resulted from stereochemical

345

arrangements in primary structures, were rather weaker (< 4 kcal/mol) compared to the π-


Page 16 of 33

Page 17 of 33


346

π interaction (4-17 kcal/mol).10, 38 Thus, the π-π interaction could readily destroy the π-

347

hydrogen bond. As a result, the decreased stability of HA and the increased

348

hydrophobicity led to the compression of HA, which is supported by the DLS and zeta

349

potential results. The π-π and dipole-dipole interaction are also reported to be involved in

350

the interaction between ILs and soil.10, 39

351

In addition, intra-molecular π-hydrogen bonds formed by the aromatic ring and

352

electron acceptors in HA were more readily destroyed by imidazole ring of IL than those

353

formed by the C=O groups.8, 40 The fluorescence of the humic-like fraction, which has a

354

higher aromatic degree, was quenched due to the increased charge-transfer effect

355

resulting from the π-π interaction and the decreased distance between donors and

356

acceptors because of contraction, according to the sequence derived from hetero-2DCOS

357

and PCMW analyses.31, 41 Furthermore, due to the increased synchronous fluorescence

358

intensity of the fulvic-like fraction after the change of the humic-like fraction in Period 3,

359

the constriction may lead to the release of the low molecular weight and relatively

360

hydrophilic HA fraction, i.e., the fulvic-like fraction, from the supermolecular structure.42,

361

43

362

eliminate self-quenching because of the charge-transfer interactions.31 On the other hand,

363

the aggregation of HA mainly takes place in the hydrophobic region under the influence

364

of dilution or the addition of metal cations.44, 45 It is reported that the hydrated humic

365

components could be hardly incorporated in aggregates.45 Thus, the release of fulvic-like

366

fraction could occur in the HA-IL interaction process. In addition, the release of the small

367

molecular weight and hydrophilic fraction has also been observed in the interaction

368

between HA and the cationic surfactant dodecylpyridinium chloride.46 As a result, the

On the one hand, the release of fulvic-like fraction from the HA aggregate could



369

carboxylate groups related to the fulvic-like fraction were changed. Owing to the

370

similarity in basic chemical structure, it is reasonable to assume that the mechanism for

371

the fulvic acid (FA)-IL interaction is similar to that for the HA-ILs interaction. The

372

binding affinity between FA and IL is weaker, as the structure of FA is less hydrophobic.

373

Inter- and Intra- Molecular Interactions of DOM with the IL at Various

374

Concentrations. The interaction between ILs and mineral surfaces or organic matter has

375

been studied, and various mechanisms have been proposed, e.g., cation exchange,

376

hydrophobic force and van der Waals interaction.1, 2 The interaction mechanisms depend

377

on the physicochemical properties of ILs and soils/sediments/mineral surfaces. The

378

cation exchange was primarily responsible for the initial binding process, as confirmed

379

by our DLS, zeta potential and FTIR results.1 HA became destabilized and formed loose

380

aggregates due to the reduced intermolecular electrostatic repulsion and the increased

381

hydrophobicity in Period 1. Furthermore, our results suggest that the breaking of intra-

382

molecular hydrogen bonds and hydrophobic forces might drive the structural changes of

383

the HA-IL complexes in Periods 2 and 3 (Figure 1). The condensation of fulvic acid by

384

the hydrophobic effect of long alkyl chains was also reported for the interaction between

385

fulvic acid and cation surfactants.47 The transition and turning concentrations of the

386

groups were distinct, which indicates that the energy of hydrogen bonds varied with the

387

functional groups in HA.8 Afterwards, the rearrangement of HA might lead to the release

388

of the small molecular weight fraction in Period 3, because the HA structure is soft and

389

permeable and the aggregation mainly takes place in the hydrophobic region.37, 45

390

In order to understand the effect of alkyl chain length, the interaction between

391

[C6mim]Br and HA in the same concentration region (0-0.65 mM) was investigated, and


Page 18 of 33

Page 19 of 33


392

the results are shown in Figures S5 and S6. With an increase in IL concentration, the

393

hydrodynamic size of HA increased slightly, but far less significantly compared with

394

[C12mim]Br. Meanwhile, the zeta potential and synchronous fluorescence spectra of HA

395

remained almost unchanged. According to the p value in Table S4, there was almost no

396

significant difference for the hydrodynamic size and zeta potential with [C6mim]Br

397

addition (p > 0.05). Due to the hydrophobicity of the alkyl chain, long-chain IL+ could be

398

absorbed closer to the HA surface, which makes the zeta potential less negative.11, 30 As a

399

result, the decrease in electrostatic repulsion and the increase in hydrophobicity promotes

400

the aggregation of HA. However, as for the short-chain ILs, their absorption energies are

401

lowered, thus, cation exchange may not effectively occur. Therefore, compared to long-

402

chain ILs, the interaction between short-chain IL and HA was much weaker. This could

403

be used to explain the observation that long-chain ILs would be more readily extracted

404

from aqueous solutions by HA than the short-chain ones.12 Koopal et al. also found that

405

the phase separation of HA-surfactant induced by compaction became stronger with the

406

increase in hydrophobicity.46

407

Significance of This Work. To date, staple ILs are manufactured at the metric ton

408

scale, and big chemical companies (e.g., BASF, Degussa, and IoLiTec/Wandres) are

409

using ILs in various processes, implying that the production volumes and demand of ILs

410

continue increasing. As a result, the emission of ILs into aqueous environments is

411

inevitable. Therefore, insight into the interaction between ILs and DOM is essential,

412

because the widespread DOM in aqueous environments will remarkably affect the

413

mobility, retention, transformation and environmental toxicity of ILs. Combining various

414

correlation analyses with DLS and zeta potential results, this study reveals that DOM



415

could interact with IL through cation-exchange, π-π and dipole-dipole interactions. The

416

disruption of intra-molecular hydrogen bond and the hydrophobic effect of ILs promoted

417

the compression of the HA-IL complex. These findings suggest that, the mobility of long-

418

chain ILs will likely be reduced and sedimentation may occur in the presence of high-

419

hydrophobicity DOM and at high ionic strengths, e.g., in estuaries. DOM can obviously

420

diminish the environmental impact and even toxicity of ILs. In addition, relatively

421

hydrophilic fulvic-like fraction in DOM could be released after compression. Likewise,

422

the other coexisting contaminants in DOM may also occur, as the release of polycyclic

423

aromatic hydrocarbons from soil under the IL addition has already been reported.11 The

424

in-depth elucidation of such an interaction process will enable better understanding of the

425

fate and transformation of ILs in environments and even promote the design of more

426

environmentally friendly IL products. Furthermore, such an integrated approach could

427

also be applied to probe other complicated interaction processes in natural and engineered

428

environments.

429 430

AUTHOR INFORMATION

431

*These authors contributed equally to this work.

432

**Corresponding author: Prof. Han-Qing Yu, Fax: +86 551 63601592; E-mail:

433

[email protected]

434 435

ACKNOWLEDGEMENTS


Page 20 of 33

Page 21 of 33


436

We thank the National Natural Science Foundation of China (21261160489, 21590812

437

and 51538011) and the Collaborative Innovation Center of Suzhou Nano Science and

438

Technology of the Ministry of Education of China for the support of this study.

439 440

ASSOCIATED CONTENT

441

Supporting Information Available. Experiment details, p values of hydrodynamic size

442

and zeta potential with [C12mim]Br addition (Table S1), sign and assignment of

443

characteristic bands in the 2DCOS maps (Tables S2 and S3), p values of hydrodynamic

444

size and zeta potential with [C6mim]Br addition (Table S4), synchronous fluorescence

445

spectra of water and ILs (Figure S1), FTIR spectra of purified HA and IL (Figure S2),

446

synchronous fluorescence spectra of HA with the dose of ILs at various concentrations

447

(Figure S3), synchronous (a) and asynchronous (b) hetero-2DCOS maps calculated from

448

the FTIR spectra of HA interacting with the IL at various concentrations (Figure S4),

449

average particle size, zeta potential (Figure S5) and synchronous fluorescence spectra of

450

HA with the addition of [C6mim]Br (Figure S6). This information is available free of

451

charge via the Internet at http://pubs.acs.org/.

452 453

REFERENCES

454 455

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593



Figure captions

594 595 596 597

Figure 1. Effect of IL concentrations on (a) average particle size; and (b) zeta potential of HA

598 599

Figure 2. Spectral responses of HA to ILs: (a) synchronous fluorescence contour maps of

600

HA in the 250-550 nm region; (b) FTIR absorbance spectra; and (c) differential

601

IR absorbance spectra of the freeze-dried HA

602 603 604

Figure 3. Synchronous and asynchronous 2DCOS maps generated from the synchronous fluorescence (a, b) and FTIR (c, d) analysis of HA with IL binding

605 606

Figure 4. PCMW analysis generated from the IL concentration-dependent FTIR spectra

607

of HA (a) synchronous map; (b) asynchronous map; and (c) transition

608

concentration (□) as well as the onset and end concentrations (▲, ▼) of

609

various groups of HA


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Figure 1



Figure 2


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Figure 3



Figure 4


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Table of Contents (TOC) art


Interaction between Dissolved Organic Matter and ... - ACS Publications

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