Molecular Composition and Photochemical Reactivity of Size

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Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter Andrew Chapin Maizel, and Christina K. Remucal Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05140 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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Environmental Science & Technology

1

Molecular Composition and Photochemical

2

Reactivity of Size-Fractionated Dissolved

3

Organic Matter

4 Andrew C. Maizel1 and Christina K. Remucal1, 2*

5 6 7

1

Department of Civil and Environmental Engineering

8

University of Wisconsin - Madison

9

Madison, Wisconsin

10

2

Environmental Chemistry and Technology Program

11

University of Wisconsin - Madison

12

Madison, Wisconsin

13 14 15

* Corresponding author address: 660 N. Park St., Madison, WI 53706; e-mail:

16

[email protected]; telephone: (608) 262-1820; fax: (608) 262-0454; Twitter: @remucal.

ACS Paragon Plus Environment

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Abstract

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The photochemical production of reactive species, such as triplet dissolved

19

organic matter (3DOM) and singlet oxygen (1O2), contributes to the degradation of

20

aquatic contaminants and is related to an array of DOM structural characteristics, notably

21

molecular weight. In order to relate DOM molecular weight, optical properties, and

22

reactive species production, Suwannee River (SRFA) and Pony Lake fulvic acid (PLFA)

23

isolates are fractionated by sequential ultrafiltration and the resultant fractions are

24

evaluated in terms of molecular composition and photochemical reactivity. UV-visible

25

measurements of aromaticity increase with molecular weight in both fulvic acids, while

26

PLFA molecular weight fractions are shown to be structurally similar by Fourier-

27

transform ion cyclotron resonance mass spectrometry. In addition, Bray-Curtis

28

dissimilarity analysis of formulas identified in the isolates and their size fractions reveal

29

that SRFA and PLFA have distinct molecular compositions. Quantum yields of 3DOM,

30

measured by electron and energy transfer probes, and 1O2, decreased with molecular

31

weight. Decreasing [3DOM]ss with molecular weight is shown to derive from elevated

32

quenching in high molecular weight fractions, rather than increased 3DOM formation.

33

This work has implications for the photochemistry of waters undergoing natural or

34

engineered treatment processes that alter DOM molecular weight, such as photooxidation

35

and biological degradation.


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Introduction

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Dissolved organic matter (DOM) is a heterogeneous mixture of biologically

38

derived molecules present in natural and engineered aquatic systems that contributes to

39

environmentally significant processes including carbon cycling,1 disinfection by-product

40

formation,2 and contaminant fate and transport.3 For example, DOM contributes to the

41

indirect

42

pharmaceuticals,5 through the production of excited triplet states (3DOM) and reactive

43

intermediates, such as hydroxyl radical.6,7 3DOM are long-lived species resulting from

44

photon absorption by DOM to form excited, singlet DOM and subsequent partial

45

relaxation via intersystem crossing.8,9 3DOM degrades contaminants by direct reaction

46

through energy10 or electron transfer,11 or through the production of reactive species, such

47

as singlet oxygen (1O2).12

photodegradation

of

aquatic

pollutants,

including

pesticides4

and

48

The production of reactive species varies with DOM optical properties,13-16

49

molecular weight (MW),14,17 and composition.18,19 These structural variations derive from

50

disparities in source20 (i.e., allochthonous or autochthonous) and physical/chemical

51

processing (e.g., irradiation or ozonation).21,22 UV-visible spectra of DOM provide

52

insight into the structure and photochemical reactivity of DOM.13,15,23 For example,

53

measurements of absorbance ratios, such as E2:E3 (i.e., the ratio of absorbance at 250 nm

54

to that at 365 nm), and exponential spectral slope terms decrease with molecular

55

weight,23-26 while molar absorptivity increases.27 The specific-UV absorbance at 254 nm

56

(SUVA254) correlates with DOM aromaticity, as determined by the relative signal

57

intensity in the aromatic region (110 – 160 ppm) of the

58

molecular weight.27-30 Quantum yields of 3DOM14,15 and 1O213-16,31 are consistently shown

13


C NMR spectrum, and with

3


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to increase with E2:E3, while reactive species steady-state concentrations generally

60

increase with solution absorbance (i.e., the concentration of dissolved organic carbon;

61

[DOC]).17,32

62

The environmental activity of DOM is highly influenced by molecular weight, yet

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accurate measurements of MW are confounded by the structural diversity of DOM and its

64

tendency to form supramolecular assemblies.33 Analytical techniques applied to estimate

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DOM MW include diffusivimetry,34 field flow fractionation,35 fluorescence correlation

66

spectroscopy,34 size exclusion chromatography (SEC),27 reactivity with radical species,36

67

small angle x-ray scattering,37 and vapor pressure osmometry.38 Ultrafiltration is also

68

capable of estimating DOM molecular weight and, uniquely, can separate fractions for

69

photochemical

70

measurements made with ultrafiltration are often higher and more variable compared

71

with the above techniques.21,34,37,40 Despite an established sensitivity to variation in

72

experimental parameters (e.g., pH and [DOC]),37,40 studies using ultrafiltration to

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fractionate DOM for photochemical experiments typically do not independently evaluate

74

the retention characteristics of their ultrafiltration protocol. Fulvic acid fractions of DOM

75

are generally reported to have average molecular weights near 1 kDa. Authochthonous

76

isolates, such as Pony Lake fulvic acid (PLFA), are typically reported to be lower in

77

molecular weight (186 – 2400 Da)36,41 compared to allochthonous isolates, such as

78

Suwannee River fulvic acid (SRFA; 241 - 4100 Da).27,35,36,38,41

experiments

without

significant

dilution.32,39

However,

MW

In DOM fractionated using ultrafiltration, quantum yields of 3DOM14,42 and

79 80

1

O231,43 are consistently enhanced in low MW fractions. Conversely, 3DOM and 1O2

81

steady-state concentrations have been observed to increase,32 decrease,17,44,45 or remain


4

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82

constant31,46 with molecular weight. The lack of correlation between quantum yields and

83

steady-state concentrations is a product of the conflicting trends of increasing

84

absorbance,27 but decreasing quantum yields, with increasing molecular weight.

85

The application of high resolution mass spectrometry, such as Fourier transform-

86

ion cyclotron resonance mass spectrometry (FT-ICR MS), provides new insight into the

87

molecular composition of DOM.29,47-51 FT-ICR MS can identify thousands of individual

88

molecular formulas in a single DOM sample and is used to compare DOM populations

89

with respect to source20 and processing through engineered52-54 or environmental

90

systems.55-57 FT-ICR MS has been used to demonstrate that lignin-derived formulas are

91

rich in 3DOM-relevant carbonyls18 and to identify the classes of DOM that are

92

susceptible to degradation by solar irradiation,22 but it has not been previously used to

93

link DOM structure and 3DOM photochemistry or to evaluate DOM structural trends with

94

molecular weight.

95

While an array of correlations between photochemistry, molecular structure, and

96

molecular weight have been observed in DOM, previous efforts have largely combined

97

ultrafiltration protocols using single membranes with structural characterization solely by

98

optical spectroscopy. Alternatively, this research presents a calibrated sequential

99

ultrafiltration protocol with fraction characterization by UV-visible spectroscopy and FT-

100

ICR MS. Further, a range of probes that quantify 3DOM and 1O2 production by defined

101

reaction pathways are utilized to resolve changes in DOM photochemical reactivity with

102

molecular weight and to determine the underlying mechanisms of the observed trends

103

between photochemical reactivity and MW.

104


5


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Materials and Methods

106

Materials. SRFA II (2S101F) and PLFA (1R109F) isolates were obtained from

107

the International Humic Substances Society (Denver, CO). Details on other materials and

108

solution preparation are available in Supporting Information (Section S1).

109

Ultrafiltration Evaluation. Ultrafiltration was performed with a HP4750 stirred

110

cell (Sterlitech Corp., Kent, WA). The apparatus and protocol are described in detail in

111

Section S2. Retention of six model compounds (30.0 - 973.7 Da) by 1 and 3 kDa

112

ultrafiltration membranes was evaluated at pH 3, 7, and 10. Model compound

113

concentrations in stock, permeate, and retentate solutions were determined with UV-

114

visible spectroscopy or high-performance liquid chromatography (HPLC) as described in

115

Section S3. Additionally, the role of [DOC] and pH in determining DOM retention by 1,

116

3, and 5 kDa ultrafiltration membranes was evaluated with SRFA solutions (Section S2).

117

Fulvic Acid Isolate Fractionation. PLFA and SRFA solutions (~80 mg-C/L, pH

118

7) were fractionated with a series of ultrafiltration membranes (3, 5, and 10 kDa) into

119

four nominal molecular weight classes: 10 kDa

120

(Section S2). The resultant fractions were evaluated in terms of UV-visible spectroscopy,

121

mass spectrometry, and photochemical analyses, as described below.

122

DOM Characterization. [DOC] was quantified as non-purgeable organic carbon

123

with a Shimadzu TOC-V analyzer, calibrated against potassium hydrogen phthalate.

124

Solution pH was measured with a Mettler Toledo EL20 pH meter before and after

125

irradiations, and typically varied by 10 kDa fraction (E2:E3 = 4.04), which reports lower formation rates than the other

406

SRFA fractions (Figure 4b). When this data point is removed, the p-value of the linear

407

regression between F3DOM and E2:E3 in SRFA fractions increases from 0.08 to 0.43,

408

demonstrating the weakness of the correlation. Conversely, Ra,UV-A strongly decreases

409

with E2:E3 (Figure 4c), suggesting that a structural trait of larger DOM results in

410

increased absorbance. Therefore, the lower 3DOM quantum yields of the high MW

411

fractions are due to increased light absorbance, likely from chromophores that do not take

412

part in 3DOM production, rather than decreased production of 3DOM.

413 414 415

Similarly, 3DOM steady-state concentrations with HDA are determined as the ratio of 3DOM formation rates and quenching rate constants:92

[ 3 DOM]ss =

F3DOM kd

(2)

416

As discussed above, F3DOM are essentially constant between the molecular weight

417

fractions of each isolate. However, kd decreases linearly with E2:E3 in both isolates

418

(Figure 4d). Therefore, the decreased [3DOM]ss observed in high molecular weight

419

fractions is likely attributable to increased quenching of 3DOM, rather than decreased

420

formation of 3DOM. However, it should be noted that kd measurements rely on estimated

421

rate constants for reaction between 3DOM and t,t-HDA, and that variations in this rate

422

constant could influence apparent trends. The quenching rate constants reported here (2.2

423

- 7.3 x 106 s-1) are similar to those determined in SRNOM by a similar method (2.5 - 3 x


19


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424

106 s-1),66 but higher than quenching rates determined by measurements of 1O2 formation

425

rates and TMP loss rates (5 – 50 x 104 s-1).8,93 While inaccuracies in the estimated value

426

of kp introduce some uncertainty, these results suggest shorter 3DOM lifetimes for higher

427

energy 3DOM that is capable of reaction with t,t-HDA. The above observations agree with a model of 3DOM photochemistry in which

428 429

3

430

borohydride-reducible carbonyls,93 while light absorption is controlled by intramolecular

431

charge-transfer interactions.94,95 In molecular weight fractionated isolates,

432

formation rates correlate with the percentage of lignin-like formulas (Figure S16a), which

433

have previously been identified as high in concentration of borohydride-reducible

434

carbonyl groups.18 However, the lack of correlation between Φ3DOM and the percentage of

435

lignin-like formulas (Figure S16b), coupled with the strong correlations between

436

molecular weight and optical properties, such as E2:E3 and the rate of light absorbance

437

(Table 1), agree with the assertion that DOM light absorbance and photochemical

438

efficiency are largely determined by the mutual accessibility of charge-transfer

439

partners.94,95

DOM formation is related to the concentration of specific functional groups, such as

3

DOM

440

Environmental Implications. The molecular weight of DOM decreases as

441

natural waters flow from uplands to the open ocean due to the combined action of bio-

442

and photodegradation.23,83,96 Similarly, water treatment processes, such as UV-irradiation

443

and ozonation, lower the average MW of DOM.21 We observed increased light

444

absorbance with MW in both isolates, which suggests processes that lower the MW of

445

DOM will deepen the photic zone and enhance direct contaminant photodegradation,

446

regardless of DOM source or other co-occurring structural trends.


20

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447

Additionally, we demonstrate that structural trends with molecular weight are not

448

necessarily shared between DOM samples from divergent sources and that resolving

449

these trends may require specific analytical methods or approaches. For example, SRFA

450

aromaticity increases with molecular weight by both UV-visible spectroscopy and FT-

451

ICR MS. The same trend is observed in PLFA using UV-visible spectroscopy, while FT-

452

ICR MS analyses suggest that the PLFA molecular weight fractions are structurally

453

similar. Accordingly, the use of only UV-visible spectroscopy or FT-ICR MS to evaluate

454

DOM structural modifications may result in the determination of trends that would be

455

contradicted by other analytical methods.

456

While 3DOM and 1O2 quantum yields increase with E2:E3, [3DOM]ss, which

457

determines the rate of indirect contaminant photodegradation, increases with E2:E3 when

458

measured by HDA, but not TMP, at a constant [DOC]. This demonstrates that the

459

mechanism by which a contaminant reacts with 3DOM may influence how indirect

460

photolysis rates change with DOM molecular weight. Further, photochemical studies that

461

rely on a single probe compound to quantify 3DOM production amongst diverse DOM

462

samples may overlook trends in photoreactivity. This has been observed in natural

463

systems as a previous study of DOM photochemistry identified divergent trends in 1O2

464

and 3DOM production as DOM moved through an estuary.90 Our observation that 3DOM

465

formation rates are constant with molecular weight suggests that environmental or

466

engineered processes that modify molecular weight may not alter the carbon-normalized

467

3

468

decreasing DOM molecular weight may allow indirect photodegradation to occur deeper

469

in the water column and increase the overall contaminant loss rate.

DOM formation rates. However, the decrease in light absorbance that accompanies


21


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470

Supporting Information.

471

Additional experimental details, Figures S1-S16, and Tables S1-S6 are included in

472

Supporting Information. The material is available free of charge via the Internet at

473

http://pubs.acs.org.

474

Acknowledgements.

475

We would like to acknowledge Jing (Juno) Li for assistance with photochemical

476

assays and optical spectroscopy and Matt Lawrence for assistance with FT-ICR MS

477

analysis and formula assignment. We also acknowledge funding from the UW-Madison

478

Graduate School.

479

TOC/Abstract Art

480 481


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482 483 484 485 486 487 488


Table 1. Organic carbon, UV-visible spectroscopy, and FT-ICR MS characterization of fulvic acid isolates and their molecular weight fractions. The overall carbon mass balances of fractionation procedures are included in parentheses. Rates of light absorbance are calculated based on the irradiance of the UV-A light source (Ra,UV-A) used in photochemistry experiments, as well under modeled sunlight (Ra,solar).

Carbon Mass balance

UV-visible spectroscopy E2:E3 SUVA254 (L mg-C-1 m-1)

S275-295 (nm-1)

FT-ICR MS

Ra,UV-A Ra,solar (10-8 E (10-10 E cm-3 s-1) cm-3 s-1)

Identified Average formulas DBE (% CHON)

AI >0.5

Ligninlike

Pony Lake fulvic acid Bulk 10

(100%) 39% 35% 22% 4%

4.74 5.86 5.31 4.58 3.79

3.32 2.66 2.37 3.38 5.12

0.0123 0.0132 0.0129

10.6 6.48 6.02 10.5 19.9

651 (33%) 584 (30%) 642 (32%) 560 (38%) 266 (44%)

7.9 7.5 7.9 8.0 8.0

4.2% 5.0% 5.5% 7.9% 8.3%

33% 31% 29%

0.0125 0.0114

5.79 3.67 3.63 6.07 10.8

4.83 3.80 3.68 4.96 5.93

0.0113 0.0118 0.0126 0.0114 0.0106

8.26 5.61 5.68 8.28 11.8

13.9 9.38 9.27 13.7 20.6

963 (4%) 1066 (4%) 922 (3%) 848 (6%) 515 (7%)

11.5 10.3 10.7 12.2 13.8

39% 26% 32% 48% 69%

42% 46% 46% 34% 19%

29% 35%

Suwannee River fulvic acid Bulk 10

(96%) 25% 12% 39% 19%

4.58 5.30 5.06 4.66 3.91

489


23


490 491 492 493 494 495

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Figure 1. van Krevelen diagrams for all identified DOM molecular formulas in the bulk (a) PLFA and (b) SRFA fulvic acid isolates, as well as (c) the 10 kDa fractions of SRFA.


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496 (a)

13

DBE

average

2

12 R = 0.94 -3 p = 5.8 x 10 11 2

10

R = 0.37 p = 0.28

9 8

(b) 80 2

R = 0.91 -2 p = 1.2 x 10

60 40

2

R = 0.47 p = 0.20

20 0

7 2

3

SUVA

497 498 499 500 501 502 503 504 505

Fraction of Formulas, AI > 0.5 (%)

100

14

4 254

5

6

7

2

(L/cm mg-C)

3

SUVA

4 254

5

6

7

(L/cm mg-C)

Figure 2. (a) The average number of double bond equivalents in FT-ICR MS identified formulas against SUVA254 and (b) the fraction of FT-ICR MS identified formulas with AI >0.5 against SUVA254. Solid lines denote linear regressions of bulk fulvic acid isolates (solid markers) and molecular weight fractions (hollow markers) for SRFA (squares) and PLFA (circles). Dotted lines denote 95% confidence intervals of the linear regressions.


25


506 507 508 509 510 511 512

Page 26 of 35

Figure 3. (a) Quantum yield of 3DOM quantified using HDA, (b) the quantum yield coefficient fTMP, and (c) the quantum yield of 1O2 against E2:E3 in bulk fulvic acid isolates (solid markers) and molecular weight fractions (hollow makers). Dashed lines indicate linear regressions of all samples from each isolate.


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513

514 515 516 517 518 519 520 521 522 523

Figure 4. The (a) steady-state concentration of 3DOM, (b) formation rate of 3DOM, and (d) solution quenching rate of 3DOM against E2:E3, all measured with HDA. (c) Rates of light absorbance (Ra,UV-A) of bulk fulvic acid isolates and molecular weight fractions at the start of photolysis experiments as a function of E2:E3. Dashed lines indicate linear regressions within each isolate, with bulk fulvic acid isolates denoted with solid markers and molecular weight fractions denoted with hollow markers.


27


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