Natural Organic Matter Exposed to Sulfate Radicals Increases Its

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Natural Organic Matter Exposed to Sulfate Radicals Increases its Potential to Form Halogenated Disinfection By-products Junhe Lu, Wei Dong, Yuefei Ji, Deyang Kong, and Qingguo Huang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00327 • Publication Date (Web): 14 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016

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Environmental Science & Technology

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Natural Organic Matter Exposed to Sulfate Radicals Increases its Potential to

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Form Halogenated Disinfection By-products

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Junhe Lu *, Wei Dong , Yuefei Ji , Deyang Kong , Qingguo Huang§

†

†

†

‡

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†

Department of Environmental Science and Engineering, Nanjing Agricultural University, Nanjing

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210095, China ‡

Nanjing Institute of Environmental Science, Ministry of Environmental Protection of PRC,

8 9

Nanjing 210042, China §

Department of Crop and Soil Sciences, University of Georgia, Griffin, GA 30223, USA

10 11 12 13

*Corresponding author: e-mail: [email protected]

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Telephone: +86-025-84395164; Fax: +86-025-84395210

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1

ACS Paragon Plus Environment


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Abstract

Sulfate radical-based advanced oxidation processes (SR-AOPs) are considered as viable

18

technologies to degrade a variety of recalcitrant organic pollutants. This study demonstrates that

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o-phthalic acid (PA) could lead to the formation of brominated disinfection by-products (DBPs) in

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SR-AOPs in the presence of bromide. However, PA does not generate DBPs in conventional

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halogenation processes. We found that this was attributed to the formation of phenolic

22

intermediates susceptible to halogenation, such as salicylic acid through the oxidation of PA by

23

SO4•−. In addition, reactive bromine species could be generated from Br- oxidation by SO4•−.

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Similar in situ generation of phenolic functionalities likely occurred, by converting carboxylic

25

substituents on aromatics to hydroxyl, when natural organic matter (NOM) was exposed to trace

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level SO4•−. It was found that such structural reconfiguration led to great increase in the reactivity

27

of NOM towards free halogen, and thus its DBP formation potential. After a surface water sample

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was treated with 0.1 µM persulfate for 48 h, its potential to form chloroform, trichloroacetic acid,

29

and dichloroacetic acid increased from 197.8, 54.3, and 27.6 to 236.2, 86.6, and 57.6 µg/L,

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respectively. This is the first report on possible NOM reconfiguration upon exposure to low-level

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SO4•− that has an implication in DBP formation. The findings highlight potential risks associated

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with SO4•−-based oxidation processes, and help to avoid such risks in design and operation.

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TOC Arts

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Introduction

Sulfate radical (SO4•−) generated by activation of persulfate (PS) or peroxymonosulfate (PMS)

40

has a standard reduction potential of 2.5~3.1 V (depending on the pH), making it one of the

41

strongest oxidants 1, 2. SO4•−-based advanced oxidation processes (SR-AOPs) have attracted

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increasing attention in soil/ground water remediation practices as cost-effective technologies.

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SR-AOPs have been assumed to be relatively environmentally friendly until Fang and Shang

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revealed that SO4•− could oxidize bromide (Br-) to generate carcinogenic bromate (BrO3-)3.

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Recently, formation of regulated disinfection by-products (DBPs) including bromoform and

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bromoacetic acids were reported in SR-AOPs in the presence of Br- 4, 5. Formation of DBPs was

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attributed to the transformation of Br- by SO4•− to reactive bromine species (RBS) such as Br•,

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Br2•−, BrOH•−, Br2, and HBrO through a series of reactions (eqs. 1~7) 6-8. These RBS are

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electrophiles and can selectively react with electron-rich compounds and cause bromination7, 9.

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Transformation of Br- and the reactivity of RBS thus formed have been addressed in several

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papers3, 7, 8, 10.

−•

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SO4 + Br − → Br ⋅ + SO4

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Br ⋅ + Br − → Br2

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Br ⋅ +OH − → BrOH −•

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Br2 + Br⋅ → Br2 + Br −

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2−

k = 3.5 × 109 M-1s-1

(1)

k = 1.2 × 1010 M-1s-1

(2)

k = 1.06 × 1010 M-1s-1

(3)

k = 2.0 × 109 M-1s-1

(4)

Br2 + Br2 → Br2 + 2 Br −

k = 1.9 × 109 M-1s-1

(5)

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Br ⋅ + Br ⋅ → Br2

k = 1.0 × 109 M-1s-1

(6)

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Br2 + H 2O ← → HBrO + H + + Br−

k = 1.6 × 1010 M-1s-1

(7)

−•

−•

−•

−•

4


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Halogenated DBPs have been shown to cause notable health risks and regulated world

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widely11-14. It is well recognized that dissolved natural organic matter (NOM) is the precursor of

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DBPs during water chlorination14-19. More specifically, it is the phenolic moieties in NOM

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molecules that act as the principal reactive sites for halogenation and are responsible for the

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formation of trihalomethanes (THMs) and haloacetic acids (HAAs)14, 18, 20. The reaction was

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hypothesized to proceed through a “haloform-like” mechanism. Rook proposed that moieties such

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as resorcinol, 3,5-dihydroxylbenzoic acid contained “masked” diketons. Carbon atoms in the ortho

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position of these compounds are expected to be halogenated readily and thus function as haloform

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precursors19, 21. This hypothesis was confirmed in an elegant experiment that employed

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isotopically labeled resorcinol by Boyce et al22.

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Presence of precursors having phenolic and/or diketon moieties is considered as a prerequisite

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for DBPs formation in water chlorination. Good correlations have been established between the

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phenolic character of NOM and its DBP formation potential (DBPFP)20, 23. In a previous study, we

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investigated the formation of brominated DBPs (Br-DBPs) in SR-AOPs in the presence of humic

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acid (HA) and Br-. Yields of bromoform and HAAs as high as 0.30 and 0.51 µM/mg TOC,

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respectively, were found24. The values are comparable or even higher than the reported DBPFP for

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NOM25-28. For example, Chen and Westerhoff tested the DBPFP of 50 water samples from 11

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surface rivers. The average formation potential of THMs and HAAs was below 0.29 and 0.32

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µM/mg TOC, respectively28. Because phenolic moieties in NOM molecules are prone to

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degradation by SO4•− 29, the relatively high DBPs formation during SO4•−-based oxidation may

79

have involved different mechanisms. The present study was designed to explore the underlying

80

mechanisms leading to DBP formation in SR-AOPs. We demonstrated for the first time that 5



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phenolic DBP precursors could be generated as intermediates through SO4•−-based oxidation of

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benzoic acid moiety, which was otherwise unavailable for halogen attacking. Such a mechanism

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would dramatically increase the DBPFP of NOM exposed to trace level SO4•−.

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Experimental

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Chemicals and materials. All chemicals were of analytical grade or better. Salicylic acid

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(SA), o-pathalic acid (PA), K2S2O8, KHSO5, KBr, NaClO, and

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N,N-dimethyl-1,4-phenylenediamine monohydrochloride (DPD) were purchased from Aladdin

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(Shanghai, China). THMs and HAAs calibration mixtures were purchased from Sigma-Aldrich (St.

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Louis, MO). Methanol and methyl tert-butyl ether (MTBE) were of HPLC grade and obtained

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from Fisher (Waltham, MA). Surface water sample was taken from Yunhu Lake in Jiangsu

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Province, China, and filtered through 0.45 µM membrane. Water quality parameters of the sample

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can be found in Supporting Information (Table S1). Stock solutions of Co2+, KBr, PA, SA, and

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NaClO were prepared by dissolving the reagents in Milli-Q water (18.2 MΩ/cm) produced in a

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Millipore Super-Q Water Purification system. PS and PMS stock solutions were prepared in

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Milli-Q water and used within a week. Free chlorine concentration of NaClO stock solution was

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assayed using DPD method prior to use 30.

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Formation of DBPs in SO4•− based oxidation processes. Formation of DBPs in heat

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activated PS oxidation process was investigated in EPA vials (42 mL in volume) as batch reactors.

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Reaction solutions containing 0.1 mM Br- and 0.05 mM PA or SA were pre-heated to the working

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temperature (70°C) in a water bath before appropriated volume of PS stock solution was spiked to

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achieve preset concentrations. Because only a tiny amount of PS stock solution was added, change

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in temperature of the reaction solutions was negligible. The vials were headspace free and

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immersed in a water bath during the reaction period. The reaction solution was maintained at pH 6

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with 10 mM phosphate buffer. After preset reaction time 2 vials (for THMs and HAAs) were taken

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out and chilled in an ice bath for 10 min and kept at 5°C in a refrigerator until further treatment

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and analysis for DBPs. Preliminary experiments demonstrated that rapid decrease of temperature

107

could effectively cease the reactions caused by PS31. PS doses of 1.0, 2.0, 3.0, 4.0, and 5.0 mM

108

were tested.

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The same reaction setup was used to investigate the formation of DBP in Co2+/PMS oxidation

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process at ambient temperature. The reaction solution contained 0.1 mM Br-, 5.0 mM PMS, and

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0.05 mM PA. Co2+ (5.0 µM) was added as the last component to initiate the reaction. The pH of

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the solution was maintained at 6.0 using 10 mM phosphate buffer. After preset reaction time was

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reached, 2 vials were quenched by adding an excess Na2SO3 (1 M) and kept in refrigerator (5°C)

114

until further treatment and analysis for DBPs. Control experiments without Co2+ were run

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concurrently.

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Kinetic studies. Degradation of PA and SA in Co2+/PMS oxidation process was examined in

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the presence and absence of 0.1 mM Br-. The reactions were conducted at ambient temperature in

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EPA vials as batch reactors. Initial concentration of PA or SA was 0.05 mM. PMS and Co2+

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concentrations were 5.0 mM and 5.0 µM, respectively. Aliquots (0.5 mL) were withdrawn at

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predetermined time intervals and mixed with 10 µL Na2SO3 (1 M) immediately to quench the

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reaction. The samples were kept in refrigerator before analysis of residual PA or SA. For

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comparison, controls without Co2+ but other conditions identical were run concurrently.

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Time-dependent formation of SA during Co2+/PMS oxidation of PA was investigated at Co2+

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levels of 0, 1, 2, 3, 4, and 5 µM. The concentration of SA was in situ monitored according to its

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characteristic fluorescence emission. Reaction was initiated by dosing an appropriate amount of

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Co2+ into the solution containing 0.05 mM PA and 5.0 mM PMS. The solution was immediately

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transferred into a quartz cuvette and placed in a Cary Eclipse Fluorescence spectrophotometer

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with the excitation and emission wavelengths at 295 and 400 nm, respectively. A 6-point standard

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curve was used to determine the concentration of SA. This approach was validated by comparing

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the results with those of high-performance liquid chromatography analysis as described in the

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Supporting Information.

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Formation of free bromine. Formation of free bromine was explored at the same conditions

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as described in the kinetic study. An aliquot of 1 mL sample was withdrawn from the reaction

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solution and immediately mixed with 1 mL DPD solution (8 mM). Free bromine was quantified

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colorimetrically by measuring the absorbance at 510 nm on a Varian Cary 50 spectrophotometer 30.

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Controls without Br- were also analyzed and the values were used to correct the absorbance

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created by oxidants other than free bromine. Standard deviation of this approach was within 5%.

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DBPFP test. DBPFP of PA solution and surface water sample pretreated with PS was

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investigated according to the protocol in the Standard Methods for the Examination of Water and

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Wastewater30. The pre-oxidation was conducted in a series of screw-cap vials with Teflon septa.

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PA solutions (0.05 mM) were dosed with appropriate amount of PS stock solutions to achieve

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PS/PA molar ratios from 0 to 10. Phosphate buffer (10 mM) was used to control pH at 7.0. No

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buffer was used for the surface water sample and the PS dose varied from 0 to 1 µM. The

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solutions were incubated at 70°C for 48 h. After the reaction solutions were cooled to room 8


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temperature, aliquots (5 mL) were sampled for the quantification of SA formation using the

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fluorescence approach described above. The rests were transferred to EPA vails for chlorination. A

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chlorine dose of 15 mg/L was used for each of the vials and the solutions were allowed to react for

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72 h at room temperature in the dark. Pre-experiments showed that there was still over 3 mg/L

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residual chlorine after 72 h in the solution. The samples were then quenched with 20 µL 1 M

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Na2SO3. Formation of THMs and HAAs were analyzed. The surface water sample was treated in a

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similar procedure except the pre-oxidation dose of PS varied from 0 to 1.0 µM and no buffer was

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added.

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Analytical methods. DBPs were enriched by liquid-liquid extraction using MTBE as the

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solvent and analyzed by an Agilent 7890 gas chromatography equipped with an electron capture

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detector (GC/ECD) according to EPA method 551.1 and 552.2. Detailed procedure and conditions

156

can be found in Supporting Information. Residual PA and SA were quantified by a Hitachi L-2000

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high performance liquid chromatography system equipped with an L-2455 diode array detector.

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The separation was performed on a C18 reverse phase column (Hitachi LaChrom, 5 µM × 250mm

159

× 4.6 mm) using a gradient of 2 solvents: 0.1% acetic acid (v/v) in methanol (A) and 0.1% acetic

160

acid (v/v) in water (B). The program was 70% A maintained for 10 min and linearly increased to

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100% A in 10~30 min. The flow rate was 1 mL/min. Quantification was determined by using

162

multipoint calibration curves.

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Results and Discussion

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Formation of Br-DBPs in heat activated PS oxidation of PA. PA is not a phenolic

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compound and no appreciable DBPs formation could be found when it was reacted with free

9



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halogen (mixture of HBrO and HClO, Figure S1 in Supporting Information). However, substantial

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bromoform and brominated acetic acids were produced when PA was treated in heated PS solution

168

in the presence of Br-, implying PA could lead to the formation of Br-DBPs in SR-AOPs. Figure 1

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presents the influence of PS concentration and reaction time on the formation and speciation of the

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Br-DBPs in heated PS oxidation process. Bromoform and dibromoacetic acid (DBAA) were the

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dominant species and accounted for more than 95% (in molarity) of the total identified Br-DBPs.

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Yields of monobromoacetic acid (MBAA) and tribromoacetic acid (TBAA) were orders of

173

magnitude less than that of DBAA. Preferential formation of DBAA has been observed in

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previous studies and appeared to be unique in SO4•− based oxidation processes4, 24, because this

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result was conspicuously different from bromination by free bromine where formation of TBAA is

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more favored4, 32, 33. Formation of the four Br-DBPs showed similar trend towards the change of

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PS concentration and reaction time. At PS doses of less than 2.0 mM, formation of Br-DBPs

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increased monotonically with the reaction time. When Br-DBPs were examined at PS doses of 3, 4

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and 5 mM, their yields increased first and then decreased, suggesting further decomposition by

180

SO4•−. Such a formation trend was also observed in our earlier studies where HA and phenol were

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used as the substrate 24, 34, indicating that PA can well mimic NOM for DBPs formation in

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SR-AOPs in the presence of Br-.

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Formation of Br-DBPs and free bromine in cobalt activated PMS oxidation of PA.

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Similar formation of DBPs was also observed in Co2+/PMS oxidation process in the presence of

185

PA and Br- (Figure S2 in Supporting Information). RBS generated upon the reactions between Br-

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and SO4•− were proposed as the reactive species responsible for Br-DBPs formation in SR-AOPs.

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Thus, in addition to DBPs, we examined the formation of free bromine. As shown in Figure 2, free 10


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bromine concentration increased and reached 53 µM at 25 min before decreasing gradually in

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Co2+/PMS oxidation process. This might be explained by the further oxidation of free bromine to

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bromate by SO4•− 3, 8. When PA was present in the solution, free bromine formation was little

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affected initially. However, significant decrease of free bromine was observed after 10 min

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compared with that in the absence of PA (Figure 2). It was presumed that a fraction of free

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bromine was converted to organic bromine such as Br-DBPs when PA was present. However, it

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seemed that PA per se could not react with free bromine directly because no appreciable

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suppression of free bromine formation was observed during the first 10 min of reaction. Free

196

bromine was most likely consumed by certain compounds derived from the oxidation of PA by

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SO4•−.

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It has been demonstrated in an earlier study that Br- can be oxidized to free bromine by PMS

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without activation but free bromine cannot be further transformed in the absence of SO4•− 34. As

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shown in Figure 2, free bromine concentration increased monotonically in PMS oxidation process

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(Co2+ absent). Its formation was not affected by the presence of PA. This evidently demonstrated

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that no reaction occurred between free bromine and PA. Accordingly, no DBPs were detected in

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PMS oxidation without Co2+ where free bromine was present but SO4•− absent (Figure S2).

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Formation of phenolic intermediates during PA oxidation. Owing to the 2

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electron-withdrawing carboxylic acid groups on the aromatic ring, PA is highly electron-deficient

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thus relatively unreactive towards either free bromine or bromine radical species which are

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electrophiles7. Formation of DBPs using PA as the substrate in SR-AOPs suggests that PA could

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be transformed to certain intermediates susceptible to RBS attacking. SA was detected as a key

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intermediate during the degradation of PA in both heat activated PS and Co2+/PMS oxidation 11



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processes (Figure S3 in Supporting Information). Oxidation of PA was presumed to be initiated by

211

transferring an electron from one of the carboxyl groups to a SO4•− followed by Kolbe

212

decarboxylation to form a benzoic acid radical cation35, 36 which further transform to SA. Detailed

213

reaction pathway is presented in Scheme 1. Note that, SA is only an intermediate and is further

214

degraded in the presence of SO4•−. The reaction scheme of SA formation and degradation can be

215

described by Eq (8). Figure 3 shows the time-dependent formation of SA in Co2+/PMS oxidation

216

process. It is evident that the reaction rate increased with the increasing of Co2+. Assuming the

217

steady-state concentration of SO4•− remained relative stable at a given Co2+ concentration, and that

218

the formation and degradation of SA followed pseudo-first-order reaction kinetics, the data in

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Figure 3 can be described by sequential kinetic model as shown in Eq (9)37, where k1 and k2

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represent the formation and degradation pseudo-first-order rate constants of SA, respectively; α is

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the fraction of this pathway to the total transformation of PA.

222 223

224

225

Scheme 1. Proposed transformation pathway of PA upon oxidation by SO4•−.

k1 k2 PA → SA →  → CO2 + H 2O

[SA ] =

k1α[PA ]0 − k t −k t (e −e ) k 2 − k1 1

2

(8)

(9)

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Fitting the experimental data to Eq (9), values of α, k1 and k2 at each Co2+ concentration can be

227

obtained. It is evident that α is relatively constant (0.072) at varying Co2+ concentration. However,

228

both k1 and k2 are proportional to the concentration of Co2+ (Figure S4 in Supporting Information).

229

This implies that the pseudo-steady state concentration of SO4•− was linearly correlated to Co2+ 12


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concentration in Co2+/PMS oxidation process, which is in accordance with the finding of previous

231

studies38. We hypothesize that the phenolic intermediates such as SA served as the direct

232

substrates for bromination and formation of DBPs in Co2+/PMS oxidation process in the presence

233

of PA and Br-.

234

Reactivity of PA and SA in SR-AOPs. Reactivity of PA and SA in Co2+/PMS oxidation

235

process (without Br-) was explored and compared (Figure 4). It appeared that the degradation of

236

both PA and SA followed pseudo-first-order reaction kinetics with the rate constants of 4.8 × 10-4

237

and 1.0 × 10-3 s-1, respectively (PMS 5.0 mM, Co2+ 5 µM). Such degradation was ascribed to the

238

generation of SO4•- because no removal of PA or SA occurred in systems without Co2+. Adding 0.1

239

mM Br- to the solution greatly inhibited the degradation of PA in the Co2+/PMS process. The

240

pseudo-first-order kinetic constant reduced to 8.3×10-5 s-1 in the presence of 0.1 mM Br-. Such a

241

reduction was indicative of the scavenging of SO4•- by Br-. Br- reacts with SO4•- at a second-order

242

rate constant of 3.5 × 109 M-1s-1 which is near diffusion controlled limit7, 10. The high reaction rate

243

in combination with its relatively high concentration over PA made Br- being the major sink of

244

SO4•-. Thus, the degradation of PA was inhibited. In contrast, degradation of SA was significantly

245

enhanced in the presence of Br-. We presume that this was due to the reactions with RBS

246

generated through the oxidation of Br-. Although SA reacts with SO4•- rapidly with the

247

second-order rate constant estimated to be close to that of p-hydroxybenzoic acid (3.5 × 10-9

248

M-1s-1)10, there was still a significant fraction of SO4•- scavenged by Br-, which led to the

249

formation of RBS including radical bromine species and free bromine. The concentration of RBS

250

could be several orders of magnitude higher than that of SO4•- according to kinetic analysis7. In

251

addition, free bromine was generated by the oxidation of Br- by PMS per se. Giving the relatively 13



252

high concentration of PMS, free bromine formation via this pathway could be significant. Thus,

253

degradation of SA due to reactions with RBS outweighed the reduced availability of SO4•- caused

254

by Br- scavenging.

255

SA was also rapidly removed in PMS oxidation process in the presence of Br- but absence of

256

Co2+ as illustrated in Figure 4b. This was presumably ascribed to the reactions with free bromine

257

formed upon the oxidation of Br- by PMS because PMS could not react with SA directly. It is

258

noted in Figure 4b that the reaction rate was even slightly higher than that with SO4•- (Co2+/PMS

259

system without Br-). This suggests that the reaction between SA and free bromine was indeed

260

rapid and the rate limiting step was the transformation of Br- to free bromine during the reaction

261

process. On the contrary, no degradation of PA was observed in PMS oxidation system in the

262

presence of Br- (Figure 4a), corroborating the fact that PA cannot react with free bromine. Figure 2

263

also illustrates the influence of SA and PA on the formation of free bromine in PMS oxidation

264

system (Co2+ absent). In accordance with their degradation behavior (Figure 4), presence of SA

265

greatly reduced the formation of free bromine, which was distinct from PA. The reduced free

266

bromine was probably transformed to organic bromine because of its reaction with SA. In

267

Co2+/PMS system, the suppression of free bromine formation in the presence of SA was

268

significant in the initial stage of reaction (Figure 2), suggesting RBS was consumed immediately

269

by SA. This is again conspicuously distinct from PA which could not react with RBS and showed

270

little influence on free bromine formation during the first few minutes of reaction.

271

Formation of DBPs was investigated and compared using PA and SA as the substrates in

272

heated PS oxidation system. Similar to PA, bromoform, and DBAA were the dominant species.

273

Figure 5 presents the formation of the Br-DBPs with initial 5.0 mM PS and 0.1 mM Br- at 70°C. 14


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As it can be seen, the time-dependent concentration profiles showed an increasing and decreasing

275

process in both systems. However, significantly less DBPs were generated from SA than PA. The

276

maximum yields of bromoform and DBAA of 2.74 and 6.58 µM, respectively, were found for PA.

277

In comparison, the maximum yields of bromoform and DBAA were only 0.74 and 4.46 µM,

278

respectively, for SA. Higher DBP formation was presumably attributed to more availability of

279

both DBP precursors and RBS in the solution. Kinetic analysis shows that SO4•− is more readily

280

reacted with Br- to generate RBS in the presence of PA than equal quantity of SA, because SA

281

reacts more rapidly with SO4•− than PA with other conditions identical. Moreover, oxidation of SA

282

results in destruction of DBP precursors while oxidation of PA generates phenolic compounds

283

prone to bromination. Taking both factors into consideration, benzoic acids tend to have higher

284

DBP formation potential than the phenolic analogs in SR-AOPs in the presence of Br-.

285

Changes in DBPFP of PA and NOM subjected to SO4•− pretreatment. Figure 6 shows the

286

DBPFP of PA solution pretreated with SO4•−. As it can be seen, PA per se did not contribute any

287

DBP formation. However, significant DBPFP was achieved after PA was exposed to trace level

288

SO4•−. DBPFP underwent an increasing phase before decreasing with the increase of PS dose. The

289

maximum DBPFP was found at PS/PA molar ratio of 0.8. Formation potential of chloroform,

290

monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), and trichloroacetic acid (TCAA)

291

were 344.2, 119.1, 320.9, and 563.1 µg/L, respectively. It should be noted that the formation of

292

DBPs was solely attributed to the reactions with free chlorine since PS was completely consumed

293

during the pretreatment. The concentration of SA in PA solution subjected to trace level PS

294

pretreatment was also analyzed and the values are given in Figure 6. The highest formation of SA

295

(7.1 µM) was found at PS/PA molar ratio of 1.0. The value corresponded to a yield of 14.2%. It is 15



296

evident that the change of DBPFP was consistent with the SA formation in the solution. Thus, the

297

data presented in Figure 6 demonstrate that it was the phenolic intermediates such as SA generated

298

upon PA oxidation responsible for the DBPFP of the solution.

299

PA is a commonly used model compound to mimic the reactivity of NOM. In fact, NOM

300

molecules are rich of benzoic acid moieties39-41. We hypothesize that similar transformation from

301

benzoic acid to phenolic functionalities occurs when NOM is exposed to trace level SO4•−. In other

302

words, trace levels of SO4•− are expected to lead to in situ generation of DBP precursors, in

303

addition to degrading them. To verify this, DBPFP of a surface water sample pre-treated with trace

304

PS was examined and the data are reported in Figure 7. The water was collected from a reservoir

305

with a TOC of 2.7 mg/L. Formation potential of chloroform, DCAA, and TCAA were 197.8, 54.3,

306

and 27.6 µg/L, respectively, for the raw water. The values increased to 236.2, 86.6, and 57.6 µg/L,

307

respectively, after the water was subjected to 0.1 µM PS pre-oxidation. Generally the change of

308

DBPFP followed a similar trend as observed for PA (Figure 6), i.e. it increased first then

309

decreased with elevated PS dose. Increase of DBPFP indicates the in situ generation of DBP

310

precursors upon reaction with SO4•−. We believe the underlying mechanism is similar to that of the

311

transformation of PA to SA. However, the phenolic moieties thus generated are to be degraded

312

when excessive SO4•− is present.

313

Environmental significance. SR-AOPs are considered as promising strategies to degrade a

314

number of recalcitrant contaminants in soil and subsurface1, 42-46. Generally, SO4•− is a highly

315

oxidizing species that can destruct or even mineralize organic contaminants. Ambient NOM is an

316

important factor affecting the removal efficiency because it competes SO4•− with targeted

317

contaminants. Nonetheless, little attention has been paid to the structural change of NOM per se 16


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after the reactions with SO4•− as well as its implications. In principle, NOM can be mineralized by

319

SO4•− but this comprises multiple steps. It is revealed in this study that when insufficient SO4•− is

320

available, the reactions would result in the reconfiguration of NOM molecules. Such scenarios are

321

highly possible given the fact that the availability of NOM in the environment is relatively

322

unlimited compared with the supply of SO4•−. In addition, diffusion of PS or PMS to the

323

surrounding soil or downstream of ground water would expose NOM to trace level SO4•−. The

324

reconfiguration of NOM structure thus caused would in turn lead to subtle changes of its

325

physicochemical properties and reactivity in both natural and engineering systems. We

326

demonstrated the changed reactivity towards free chlorine in this study, which is expected in water

327

disinfection process. The behavior of NOM would also change in other water treatment processes

328

such as coagulation/sedimentation, membrane filtration, etc. In particular, we believe the

329

reconfiguration of NOM will lead to the change of its mobility and reactivity in natural systems.

330

For example, reconfiguration of functional groups can potentially affect its water solubility. In

331

addition, such change of phenolic to carboxylic acid groups is expected to affect its interactions

332

with heavy metals, which is likely to influence their speciation, mobility, and bioavailability.

333

Hence, the environmental implications should be considered comprehensively in the application of

334

SO4•− based processes in pollution control and remediation practices.

335

Acknowledgements

336

This research was supported by the Natural Science Foundation of China (51578294) and the

337

Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institute. The

338

content of the paper does not necessarily represent the views of the funding agencies.

17



339

Supporting Information

340

Additional experimental details, figures, and table. This material is available free of charge

341

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

342

References

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461

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465 466

Figure 1. Formation of Br-DBPs at varying PS concentration and reaction time in heat activated

467

PS oxidation process in the presence of PA and Br-. Initial PA was 0.05 mM, Br- 0.1 mM.

468

Temperature 70°C

469

25



470 471

Figure 2 Influence of SA and PA on the formation of free bromine in PMS and Co2+/PMS

472

oxidation processes. Initial PMS was 5.0 mM; Br- 0.1 mM; Co2+ if present was 5 µM; PA or SA, if

473

present, was 0.05 mM; pH 6.0.

474

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476 477

Figure 3 Time-dependent formation of SA in Co2+/PMS oxidation process. Initial PMS was 1.0

478

mM; PA 0.05 mM; varying Co2+ from 0 to 5 µM; pH 6.0.

479

27



480 481

482

483 484

Figure 4 Degradation of (a) PA and (b) SA in PMS oxidation processes in the presence and

485

absence of Br-. Initial PA or SA was 0.05 mM; PMS 5.0 mM; Br- 0.1 mM; Co2+ 5 µM; pH 6.0.

486

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488 489

Figure 5. Time-dependent formation of Br-DBPs in heat activated PS oxidation of PA/SA and Br-.

490

Initial PA or SA concentration 0.05 mM and Br- = 0.1 mM, PS dose 5.0 mM, T = 70°C, PA

491 492

29



493 494

Figure 6 Change of DBPFP and formation of SA after PA subjected heat activated PS pretreatment.

495

Initial PA concentration 0.05 mM; reaction temperature 70 °C; reaction time was 48 h; pH 7.0.

496

After treatment, the sample was dosed with 15 mg/L free chlorine and incubated for 72 h before

497

DBP analysis.

498

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500 501

Figure 7 Change of DBPFP of surface water sample pretreated with varying PS. The sample was

502

allowed for 48 h reaction at 70 °C before chlorination.

503

31


Natural Organic Matter Exposed to Sulfate Radicals Increases Its

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