Oxidation of Microcystin-LR via Activation of Peroxymonosulfate Using

Mar 7, 2018 - Advanced oxidation processes (AOPs) have been widely used for the destruction of organic contaminants in the aqueous phase. In this stud...
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Oxidation of microcystin-LR via activation of peroxymonosulfate using ascorbic acid: Kinetic modeling and toxicity assessment Shiqing Zhou, Yanghai Yu, Weiqiu Zhang, Xiaoyang Meng, Jinming Luo, LIN DENG, Zhou Shi, and John C. Crittenden Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06560 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Oxidation of microcystin-LR via activation of peroxymonosulfate

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using ascorbic acid: Kinetic modeling and toxicity assessment

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Shiqing Zhou,†,‡ Yanghai Yu,† Weiqiu Zhang,‡ Xiaoyang Meng,‡ Jinming Luo,‡ Lin Deng,†,‡ Zhou

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Shi† and John Crittenden *,‡

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6

Changsha, Hunan, 410082, China

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8

Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

Department of Water Engineering and Science, College of Civil Engineering, Hunan University,

Brook Byer Institute for Sustainable Systems and School of Civil and Environmental

9 10 11

Corresponding Author

12

John Crittenden

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*E-mail: [email protected]. Tel.: +1 404 894 5676; fax: +1 404 894 7896

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PMS

Intensity

SO4·-

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H2O H2A H+ HA-

PMS

HO·

SO4·-

H2O

Concentration of radicals (M)

Magnetic field

A.-

8.0x10

-13

6.0x10

-13

4.0x10

-13

2.0x10

-13

Sulfate radical Hydroxyl radical Ascorbyl radical

0.0

-7

8.0x10

-8

4.0x10

-8

0.0 0

14

1.2x10

5

10

15

20

Time (min)

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ABSTRACT

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Advanced oxidation processes (AOPs) have been widely used for the destruction of

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organic contaminants in the aqueous phase. In this study, we introduce an AOP on

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activated peroxymonosulfate (PMS) by using ascorbic acid (H2A) to generate sulfate

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radicals (SO4•-). Sulfate radicals, hydroxyl radicals (HO•) and ascorbyl radicals (A•-)

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were found using electron spin resonance (ESR). But we found A•- is negligible in the

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degradation of microcystin-LR (MCLR) due to its low reactivity. We developed a

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first-principles kinetic model to simulate the MCLR degradation and predict the

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radical concentrations. The MCLR degradation rate decreased with increasing pH.

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The scavenging effect of natural organic matter (NOM) on SO4•- was relatively small

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compared to that for HO•. Considering both energy consumption and MCLR removal,

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the optimal H2A and PMS doses for H2A/PMS process were determined at 1.0×10-6 M

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and 1.6×10-5 M, respectively. In addition, we determined the toxicity using the protein

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phosphatase 2A (PP2A) test and the results showed that MCLR was readily detoxified

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and its oxidation by-products were not hepatotoxic. Overall, our work provides a new

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type of AOP and a promising, efficient and environmental-friendly method for

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removing microcystins in algae-laden water.

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INTRODUCTION

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In recent years, cyanobacterial blooms frequently occur worldwide and pose a

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serious threat to drinking water.1 Microcystins (MCs), a class of hepatotoxic

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monocyclic heptapeptides, are the most common toxins produced by cyanobacteria.2, 3

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MCs can cause both acute liver damage and chronic diseases, as they potentially

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inhibit protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A).4 More than

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80 variants of MCs have been identified based on their methylation pattern and two

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variable amino acids.3 Microcystin-LR (MCLR) is the most toxic and abundant

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variant and accounts for 46.0-99.8% of the total MC concentrations during a

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cyanobacterial bloom.5 The World Health Organization (WHO) has set a provisional

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concentration limit of 1.0 µg L-1 for MCLR in drinking water.6

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MCLR is a very stable toxin due to its cyclic peptide structure. Conventional water

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treatment processes such as coagulation, flocculation, sedimentation and filtration can

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effectively remove the intact cells and the majority of intracellular MCLR; however,

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these processes do not detoxify the dissolved extracellular MCLR released from cell

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lysis.7-10 Moreover, the conventional processes simply transfer the toxins from one

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phase to another when algal cells are ruptured.11 Advanced treatment technologies are

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required to remove or destroy MCLR from drinking water.

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Advanced oxidation processes (AOPs) have been shown to possess outstanding

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ability to oxidize and destroy a variety of toxic and refractory organic contaminants in

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the aqueous phase. AOPs seldom generate toxic chlorinated organic compounds

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(known as disinfection by-products, DBPs) that are formed by chlorination.12, 13 Due

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to the high standard redox potentials of hydroxyl radicals (HO•, 2.7 V) and sulfate

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radicals (SO4•-, 3.1 V), AOPs have received much attention for destroying organic

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pollutants; the underlying mechanism involves hydrogen abstraction, double bond

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addition, electron transfer reactions and electrophilic substitution of aromatic

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compounds.14,

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peroxide (H2O2), peroxodisulfate (S2O82-, PDS), and peroxymonosulfate (HSO5-, PMS)

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by transition metals and minerals activation, photolysis, thermolysis and sonolysis

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methods.16-26 In addition to these catalytic methods, it has been reported that some

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reducing organic species such as phenols, hydroxylamine, hydroquinone and

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semiquinone moieties in reduced humic acid and biochar could also activate oxidants

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to generate radicals through an electron transfer mechanism.27-31

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These radicals can be generated through activation of hydrogen

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As an eco-friendly reducing agent, ascorbic acid (C6H8O6, H2A) contains two

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ionizable hydroxyl moieties and can undergo sequential electron transfer reactions.32,

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33

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oxidation by H2O2 to yield HO•, dehydroascorbic acid and an intermediate ascorbyl

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radical (A•-). Curtin et al.34 reported that ascorbate reacted with PDS to initiate SO4•-

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production, which significantly accelerated the reaction rate between PDS and

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formate. PMS and PDS are similar in structure, and both have an O-O bond. PMS can

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be generated by replacing one hydrogen atom in H2O2 with SO3, while PDS is

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generated by replacing two hydrogen atoms in H2O2 with SO3.35 Considering the

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similar PDS and PMS structures, it is hypothesized that PMS could also be activated

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by H2A (HA-), although the mechanism involving PMS and H2A is still unknown.

For instance, Nappi and Vass32 found that ascorbic acid could undergo a two-step

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In this study, we investigate the activation reactions between H2A and PMS and the

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kinetics of MCLR oxidation. Electron spin resonance (ESR) was used to verify the

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generation of HO• and SO4•-. Based on our experimental results and the kinetic

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parameters obtained from the literature, we developed a first-principles kinetic model

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to simulate the MCLR degradation and to predict the radical concentrations. PMS and

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H2A dosages, solution pH values and natural organic matter (NOM) concentrations

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were modeled to improve the understanding of MCLR degradation in the H2A/PMS

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process. The hepatotoxicities of MCLR and its oxidation products were evaluated

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using the PP2A assay.

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MATERIALS AND METHODS

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Chemicals. All chemicals were of analytical grade, and used as received without

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further purification. All solutions were prepared with ultrapure water, unless

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otherwise specified. The specific chemicals and reagents are provided in the

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Supporting Information (SI) Text S1.

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Experimental Procedures. Batch experiments were conducted in the open air and

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in a series of borosilicate glass jars containing 100 mL of solutions at room

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temperature (25± 2°C). The initial concentration of MCLR was fixed at 2.0×10-7 M.

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The pH of the bulk solution containing MCLR and H2A was initially adjusted to pH 4,

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5, 6 and 12 using H2SO4 or NaOH. At each given time interval, sample aliquots were

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harvested and mixed immediately with appropriate amounts of Na2S2O3 to quench the

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reaction. Then the samples were immediately analyzed.

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Analytical Methods. The MCLR concentrations were determined by LC/MS/MS

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(Agilent 1290/6460 Triple Quad, USA) equipped with a Symmetry C18 column (50

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mm × 2.1 mm × 5 mm, Agilent, USA). A mixture of ultrapure water (0.1% formic

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acid) and acetonitrile (v: v = 65:35) was used as the mobile phase. The flow rate was

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0.2 mL min-1 and the injection volume was 10 µL. The intermediates of MCLR were

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also identified using LC/MS/MS (Agilent 1290/6460 Triple Quad, USA) according to

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the method of our previous study.36

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The electron spin resonance (ESR) experiment was conducted using a Bruker A300

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spectrometer

(Bruker

Instrument,

Germany)

with

or

without

5,

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5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin-trapping agent. The detailed ESR

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procedure is available in SI Text S2-S3. The hepatotoxicities of MCLR and its

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oxidation products were evaluated by the PP2A activity assay using a MicroCystest

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kit (ZEU Inmunotec, Spain) according to the method of James et al.,37 and the

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detailed information is presented in SI Text S4.

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Kinetic Model. Table 1 summarizes the reactions in the H2A/PMS system. The

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associated rate constants are generally obtained either from the literature or by fitting

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experimental data. The genetic algorithm (GA) was used to minimize the objective

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function (OF) and to determine the rate constants as described in our former work.38

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The backward differentiation formula (BDF) method [i.e., Gear’s method]39 was used

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to solve the ordinary differential equations (ODEs) by using MATLAB. Our model

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had 30 rate constants, 19 from the literature and 11 determined by fitting the model to

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the data. The detailed modeling approaches and kinetic equations are provided in SI

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Text S5 and S6, respectively.

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OF =  ∑[(C  − C )/C  ]

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(1)

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Here, n is the number of data points, and C  and C are the experimental and

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calculated concentrations of MCLR, respectively. Table 1

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RESULTS AND DISCUSSION

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MCLR Degradation and Reactive Radicals Identification. Figure 1 compares

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the degradation of MCLR by H2A alone, PMS alone and the H2A/PMS process. No

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significant degradation of MCLR was observed in 30 min when the MCLR solution

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was treated by H2A alone, suggesting that H2A does not oxidize MCLR. On the other

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hand, although PMS itself is an oxidant (E0 = +1.82 V/SHE), the loss of MCLR was

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also negligible for a molar ratio of [PMS]/[MCLR] up to 25. However, approximately

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91% of MCLR was destroyed after 30 min using the H2A/PMS process (for the same

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molar ratio of [PMS]/[MCLR] of 25), and the degradation followed pseudo-first-order

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kinetics with a rate constant of 1.2×103 s-1. Based on this phenomenon, H2A reacted

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with PMS and produced other reactive radicals in the H2A/PMS process.

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Figure 1

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To identify the reactive radicals involved in the H2A/PMS process, ESR tests

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with/without DMPO were conducted (Figure 2). DMPO was employed as a

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spin-trapping agent to identify hydroxyl radicals or sulfate radicals, in which HO• and

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SO4•- could be discerned by determining the signals of DMPO-OH adducts and

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DMPO-SO4 adducts, respectively.50 As shown in Figure 2a, both DMPO-OH adducts

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(with α(N) = α(H) = 14.9 G) and DMPO-SO4 adducts (with α(N) = 13.2 G, α(H) = 9.6

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G, α(H) = 1.48 G, α(H) = 0.78 G) were detected during the whole reaction process.

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This result suggests that H2A successfully activated PMS to generate SO4•- and HO• in

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the H2A/PMS process.28, 50

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Figure 2b shows the typical ESR signal of A•- (with α(H) = 1.76 G) during the

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reaction of H2A and PMS.51 The ascorbyl radical is stable and nonreactive, therefore

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directly observable by ESR. Moreover, the ascorbyl radical reacts preferentially with

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itself thus terminating the propagation of free radical reactions. Despite the short

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lifetime of the free radicals, the ESR signals in Figure 2b verified the generation of

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ascorbyl radicals during the H2A/PMS process. Based on the results of HO•, SO4•- and

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A•- verified by ESR tests, the detailed activation mechanism of PMS by ascorbic acid

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is proposed (SI Figure S2) and the elementary reactions are listed in Table 1.

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Figure 2

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Kinetic Modeling and Radical Concentrations. Due to the low reactivity of A•-,

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the contribution of A•- to the degradation of MCLR is negligible in this work. Our

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model was built based on the hypothesis that the degradation of MCLR is primarily

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caused by SO4•- and HO•. The contribution of other reactive species can also be

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neglected due to their low concentration in the H2A/PMS process. For example, the

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concentration of SO5•- is predicted to be 1.4×10-15 M (SI Figure S3), which is lower

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than one-percent of the model-predicted SO4•- concentration. By fitting the

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experimental data to our kinetic model, we determined the rate constants of SO4•- and

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HO• with the targeted compounds and other unknown rate constants using the genetic

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algorithm (an algorithm that can better locate the global minimum with respect to a

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large number of dimensions and computational efficiency). Table 1 provides the fitted

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rate constants for these elementary reactions which have not been previously reported.

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The model was able to simulate the MCLR degradation (as discussed in the following

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sections) and predict the radical concentrations, especially the concentrations of A•-.

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The model-predicted concentration profiles of HO•, SO4•- and A•- are shown in

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Figure 3. The concentration of SO4•- gradually decreased while the concentration of

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HO• increased and then decreased as the reaction time increased. The predicted results

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are highly consistent with the ESR test results (Figure 2). As the intensity of DMPO

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radical adduct signals are proportional to the concentrations of reactive radicals, it

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was determined that the concentration of SO4•- gradually decreased from 0 min to 30

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min, while the concentration of HO• increased linearly from 0 min to 20 min and then

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decreased from 20 min to 30 min (SI Figure S4). The intensity of the DMPO-OH

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adduct signals was much stronger than that of the DMPO-SO4 adducts, and the

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transformation of DMPO-SO4 adducts to DMPO-OH adducts could occur via

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nucleophilic substitution (by H2O/OH-).52, 53

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The model-predicted concentrations of A•- initially increased to its maximum

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(1.0×10-7 M) and then sharply decreased with the reaction time. Similar to the ESR

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tests, the intensity of A•- signals gradually decreased and disappeared within 30 min

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(SI Figure S4). In the H2A/PMS process, A•- is an important radical formed by a

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reaction between H2A and PMS. Bielski reported that A•- reacts preferentially with

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itself and exhibits a strictly second-order decay that depends on pH.54 Due to the high

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concentration of A•- found in the system, the reaction between A•- and A•- was

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considered in our model (reaction 11 in Table 1). In addition, we also considered the

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A•- reaction with HO• and SO4•- (reactions 12 and 13 in Table 1), which forms

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dehydroascorbic acid. The above proposed reactions have a significant influence on

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the decay of A•-; without considering these reactions, the concentration of A•- would

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be overestimated. To the best of our knowledge, no similar studies have predicted the

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concentration of A•- as a function of time.

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Figure 3

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To further evaluate the kinetic model, we investigate the impact of H2A dosage,

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PMS dosage, solution pH and NOM on MCLR degradation during the H2A/PMS

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process.

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Effect of H2A and PMS Dosage. Figure 4a shows the experimental profiles of

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MCLR degradation and the model fitting under different H2A dosages. The objective

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function (OFH2A) values are listed in SI Table S1. At a given PMS dosage (5.0×10-6

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M), the MCLR degradation increased with an increasing H2A dosage from 1.0 to 5.0

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×10-6 M but then decreased at higher H2A concentrations (i.e., 1.0×10-5 M). This

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phenomenon can be explained by the fact that excess H2A in a high-concentration

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would act as a scavenger of reactive radicals and decrease the destruction of MCLR if

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the dosage is too high, as shown in Table 1, reactions 3 and 4.34

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Figure 4b shows the experimental profiles of MCLR degradation and the model

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fitting under different PMS dosages. The objective function (OFPMS) values are listed

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in SI Table S1. At a given H2A dosage (2.0×10-6 M), the MCLR degradation gradually

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increased as the PMS dosage increased from 1.0×10-6 to 1.0×10-5 M. As shown in

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Table 1, excess PMS could enhance the MCLR degradation due to the generation of

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more SO4•- and HO• in the H2A/PMS system. Moreover, Antoniou et al.24 reported

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that the direct oxidation of MCLR by PMS could occur at a high ratio (50 or 100) of

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[PMS]/[MCLR]. Therefore, the contribution of SO4•- and HO• is significantly affected

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by excess PMS.

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Figure 4

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Effect of pH. Figure 5a shows the experimental profiles of MCLR degradation and

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the model predictions under different initial solution pH values from 4 to 12 which

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cover the pKa range. The objective function (OFpH) values listed in SI Table S1 were

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below or equal to 0.11. The oxidation of MCLR in the H2A/PMS process was strongly

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pH-dependent, and the maximum rate of MCLR degradation was achieved at pH 4.

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Figure 5b presents the experimental and predicted pseudo-first-order rate constants of

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MCLR degradation under different pH values. The overall degradation rate constant

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decreased from 1.2×10-3 s-1 to 6.3×10-4 s-1 as the pH increased from 3 to 6. As the

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dissociation constants of H2A are 4.2 and 11.6 (SI Figure S5), H2A can dissociate to

223

different species under different pH values.55 As the pH increased from 4 to 6, the

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percentage of HA- increased, while the percentage of H2A decreased dramatically. In

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addition, HA- could react with PMS to generate SO4•- but more slowly than H2A

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(reactions 1 and 2 in Table 1). Therefore, the degradation of MCLR decreased with

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increasing pH. Furthermore, at pH 12, HA- almost transformed into dianionic ascorbic

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acid (C6H6O62-, A2-), and A2- became the dominant form of H2A. As A2- could not

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activate PMS to produce SO4•- or HO•, the degradation of MCLR was negligible at pH

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12 (Figure 5a).

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Figure 5

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Effect of NOM. Figure 6a shows the experimental profiles of MCLR degradation

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and the model predictions under different NOM concentrations. The objective

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function (OFNOM) values listed in SI Table S1 were below or equal to 0.11. MCLR

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degradation decreased as the NOM concentration increased. Figure 6b presents the

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experimental and predicted pseudo-first-order rate constants of MCLR degradation in

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the presence of different NOM concentrations. The overall degradation rate constant

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decreased from 1.2×10-3 s-1 to 3.2×10-4 s-1 as the NOM concentration increased from 0

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mg L-1 to 5.0 mg L-1, because NOM could compete with MCLR to scavenge SO4•- and

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reduce the SO4•- concentration in the H2A/PMS system. The second-order rate

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constants of NOM reacting with HO• and SO4•- were 3.0×108 Mc-1 s-1 and 2.35×107

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Mc-1 s-1, respectively (reaction 28 and 29 in Table 1).49

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To further clarify the scavenging effect of NOM on MCLR degradation, we also

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modeled and presented the HO• and SO4•- concentrations (SI Figure S6). As the NOM

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concentration increased, the concentrations of HO• and SO4•- were gradually reduced.

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Compared with the SO4•- concentration reduction, the reduction of HO• was much

247

higher. Indeed, according to our model predictions, the concentration of HO• was

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reduced by more than 90%, while the concentration of SO4•- decreased by

249

approximately 50% in the presence of 5.0 mg L-1 of NOM. The smaller decrease in

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the concentration of SO4•- could be attributed to the difference in the second-order rate

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constants of NOM reacting with HO• and SO4•-.

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Figure 6

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Optimization of MCLR removal in the H2A/PMS process. As discussed above,

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our kinetic model has a good agreement with experimental data. In order to optimize

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MCLR removal, electrical energy per order (EE/O) was applied to evaluate the energy

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and cost in the H2A/PMS process.38, 56 The consumption of H2A and PMS which

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would be required for an order of MCLR removal (i.e., 90% destruction of MCLR)

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was calculated using Eqs. 2 and 3, respectively.

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H A/O =

[ ] [ ]

260

PMS/O =

[&'(] [&'(]

!  ( " ) !

(mg H2A L-1)

(2)

(mg PMS L-1)

(3)

!

 (!" ) 

261

Here, Ci and Cf are the MCLR concentration (mol L-1) at reaction time of 0 and t,

262

respectively; [H2A]0 and [H2A]f are the concentration of H2A at reaction time of 0 and

263

t, respectively; [PMS]0 and [PMS]f are the concentration of PMS (mg L-1) at reaction

264

time of 0 and t, respectively. According to the method of Rosenfeldt et al.,57 the

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electrical energy use to produce H2A and PMS were calculated to be 17.44 and 20.93

266

kWh lb-1, respectively (SI Text S7). Thus, H2A/O and PMS/O were converted to

267

EE/OH2A and EE/OPMS in the energy unit of kWh L-1, and the total energy

268

consumption can be calculated by Eqs. 4.

269

EE/O** = EE/O + EE/O&'(

(4)

270

The operational parameters, H2A and PMS doses, were varied to determine the

271

most energy efficient condition. Figure 7 presents the EE/Ototal of H2A/PMS process

272

vary with H2A and PMS doses. For the same H2A dose, EE/Ototal first decreased and

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then increased as the PMS dose increased from 1.0×10-6 to 5.0×10-5 M (SI Figure

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S7a). Similarly, for the same PMS dose, EE/Ototal first decreased and then increased as

275

the H2A dose increased from 1.0×10-7 to 1.0×10-5 M (SI Figure S7b). On the basis of

276

calculated EE/Ototal values, the optimal H2A and PMS doses for the H2A/PMS process

277

were determined at 1.0×10-6 M and 1.6×10-5 M, respectively. Of note, the economic

278

analysis based on EE/O only provides some theoretical instructions rather than full

279

economic analysis of the process. Further economic analysis would be conducted in

280

field water and at some scale-up processes in our future studies.

281

Figure 7

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Oxidation Products and Toxicity Assessment. To gain insight into the mechanism

283

of MCLR oxidation by the H2A/PMS process, the oxidation products generated

284

during MCLR degradation were analyzed using LC/MS/MS analysis. Table S2 (SI)

285

summarizes the 11 identified major oxidation byproducts, and the primary oxidation

286

pathways are presented in SI Figures S8 and S9. While the H2A/PMS process is

287

effective for the oxidation of MCLR, the hepatotoxicities of MCLR and its oxidation

288

products should not be neglected. As MCLR is a powerful inhibitor of PP2A, the

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PP2A activity is usually used as an indicator of hepatotoxicity.58 As shown in Figure 8,

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the decrease in the hepatotoxicities of the oxidation by-products showed a similar

291

trend with the decrease in the MCLR concentration measured by LC/MS/MS. The

292

results suggest that MCLR was readily detoxified by the H2A/PMS process and the

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oxidation by-products of MCLR were not hepatotoxic.

294 295

Figure 8 Environmental Implications. Ascorbic acid (vitamin C) is a non-toxic and

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eco-friendly organic acid that is commercially available worldwide. Using ascorbic

297

acid to activate PMS for water treatment offers a promising capability of eliminating

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organic contaminants in the aqueous phase. In this work, MCLR was used as a model

299

compound, and we develop a first-principles kinetic model to simulate the MCLR

300

degradation and to predict the radical concentrations. The predicted concentrations of

301

HO• and SO4•- ranged from 10-14 M to 10-12 M, which were similar to the

302

concentrations of radicals in other AOPs in practical water treatments.59 Therefore, the

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H2A/PMS process showed potential application for other selective contaminant

304

destruction. Our model can be used to predict and simulate the degradation of other

305

organic compounds by using this technique. Moreover, our model can determine the

306

optimization of the operational parameters, such as H2A and PMS doses. Our work

307

provides a promising alternative method to other AOPs for water treatment.

308

ASSOCIATED CONTENT

309

Supporting Information

310

The supporting information is available free of charge via the Internet at

311

http://pubs.acs.org.

312

Supplemental text describing chemicals, ESR procedures for detection of sulfate,

313

hydroxyl and ascorbyl radicals, toxicity assessment of MCLR, modeling

314

approach and rate constants determination, kinetic equations, and energy costs.

315

Tables showing objective function values for kinetic model, and oxidation

316

products of MCLR. Figures showing calibration curve for toxicity assessments,

317

and proposed activated mechanism of PMS by ascorbic acid; model-predicted

318

peroxymonosulfate radical concentrations; intensity profiles of hydroxyl radical,

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sulfate radical and ascorbyl radical; fractions of H2A, HA- and A2- species under

320

different pH values; model-predicted sulfate radical and hydroxyl radical

321

distributions under different NOM concentrations; EE/Ototal (in kWh L-1) of

322

H2A/PMS process vary with H2A and PMS doses; proposed pathways of MC-LR

323

degradation in the H2A/PMS process.

324

AUTHOR INFORMATION

325

Corresponding Authors

326

* Phone: 404-894-5676; fax: 404-894-7896; e-mail: [email protected]

327

Notes

328

The authors declare no competing financial interest.

329

ACKNOWLEDGEMENT

330

This work was financially supported by the National Natural Science Foundation

331

(51508174). Financial support from the China Scholarship Council for Zhou’s visiting

332

research in Georgia Institute of Technology is especially acknowledged. This research

333

was also supported by the Brook Byers Institute for Sustainable Systems, Hightower

334

Chair, and the Georgia Research Alliance at the Georgia Institute of Technology. W.

335

Zhang also acknowledges financial support from the China Scholarship Council. The

336

views and ideas expressed herein are solely of the authors and do not represent the

337

ideas of the funding agencies in any form.

338

REFERENCES

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Page 26 of 35

Table 1 Elementary reactions in the H2A/PMS system. No.

Elementary reactions

k (M-1 s-1)

Reference

1

. H A + HSO- → H / + H O + SO. 0 + A

3.1×103

fitted

2

HA + HSO- → H O + SO0. + A.

2.9×103

fitted

3

 SO. 0 + H O → HO ∙ +HSO0

k8[H2O]=1.8×103 s-1

4

 SO. - + H O → HO ∙ +HSO-

k8[H2O]=1.0×103 s-1

38

5

/ .  SO. 0 + H A → 2H + SO0 + A

1.9×109

fitted

6

 /  . SO. 0 + HA → H + SO0 + A

8.0×109

fitted

7

HO ∙ +H A → H / + H O + A.

4.3×109

fitted

8

HO ∙ +HA → H O + A.

9.9×109

fitted

9

 .  SO. 0 + HSO- → SO- + HSO0

1.0×106

41

10

HO ∙ +HSO- → SO. 0 + H O + 0.5O

1.7×107

42

11

A. + A. → A  

2.2×105

fitted

12

A. + SO0. → SO 0 +A

1.0×109

fitted

13

A. + HO ∙→ OH  + A

6.9×109

fitted

14

 SO0. + SO. 0 → S O6

1.6×108

15

.  SO. 0 + SO- → S O6 + 0.5O

8.96×109

16

HO ∙ +HO ∙→ H O

5.0×109

17

HO ∙ +H O → H O + HO ∙

2.7×107

18

HO ∙ +HO ∙→ H O + O

6.6×109

19

HO ∙ +HO ∙→ H O + O

8.3×105

20

HO ∙ +H O → HO ∙ +O + OH 

3.0

21

SO0. + H O → HSO0 + HO ∙

1.2×107

22

 SO. 0 + HO ∙→ HSO0 + O

3.5×109

23

 SO. 0 + HO ∙→ HSO-

1.0×1010

24

SO-. + SO-. → S O 6 + O

2.2×108

25

SO-. + SO-. → SO0. + SO 0 + O

2.1×108

26

.  S O 6 + HO ∙→ SO0 + HSO0 + 0.5O

1.2×107

In the presence of MCLR

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40

43

38

44

44

45

46

44

47

47

41

41

41

48

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27

SO0. + MCLR → Products + SO 0

7.5×109

fitted

28

HO ∙ +MCLR → Products

2.6×109

fitted

In the presence of NOM 29

SO0. + NOM →

2.35×107 Mc-1 s-1

49

30

HO ∙ +NOM →

3.0×108 Mc-1 s-1

49

516

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MCLR concentration (M)

Environmental Science & Technology

2.0x10

-7

1.5x10

-7

1.0x10

-7

5.0x10

-8

Page 28 of 35

H2A in dark PMS in dark H2A/PMS process in dark

0.0 0

5

10

15

20

25

30

Time (min)

517 518 519 520

Figure 1 Concentration profiles of MCLR with H2A alone, PMS alone, and the H2A/PMS process. Experimental conditions: [MCLR]0 = 2.0×10-7 M, H2A = 2.0×10-6 M, PMS = 5.0×10-6 M, and pH0 = 4.0.

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(a)

t=2.0 min t=4.0 min

Intensity

t=10.0 min

t=20.0 min

t=30.0 min

3320

3340

3360 3380 Magnetic field (G)

3400

521

(b) t=2.0 min

Intensity

t=4.0 min

t=10.0 min

t=20.0 min

t=30.0 min

3480

522 523 524

3500 3520 Magnetic field (G)

3540

Figure 2 ESR spectra of DMPO-OH and DMPO-SO4 (a) and the ascorbyl radical (b) during the H2A/PMS process. DMPO-OH adducts; DMPO-SO4 adducts; ascorbate radicals.

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-13

-7

Concentration of radicals (M)

8.0x10

1.2x10 Sulfate radical Hydroxyl radical Ascorbyl radical

-13

6.0x10

-8

8.0x10 -13

4.0x10

-8

4.0x10 -13

2.0x10

0.0

0.0 0

5

10

15

20

25

30

Time (min)

525 526 527

Figure 3 Model-predicted sulfate radical, hydroxyl radical and ascorbyl radical concentrations during the H2A/PMS process. Conditions: H2A = 2.0×10-6 M, PMS = 5.0×10-6 M, and pH0 = 4.0.

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Concentration of MCLR (M)

(a) -7

1.0E-6 M H2A

2.0x10

2.0E-6 M H2A 5.0E-6 M H2A

-7

1.5x10

1.0E-5 M H2A -7

1.0x10

-8

5.0x10

0.0 0

5

10

15

20

25

30

Time (min)

528

Concentration of MCLR (M)

(b) 1.0E-6 M PMS 2.0E-6 M PMS 5.0E-6 M PMS 1.0E-5 M PMS

-7

2.0x10

-7

1.5x10

-7

1.0x10

-8

5.0x10

0.0 0

5

10

529 530 531 532 533

15

20

25

30

Time (min)

Figure 4 (a) Concentration profiles of MCLR under different H2A concentrations. (b) Concentration profiles of MCLR under different PMS concentrations. The dots show the experimental results, and the solid lines are model fits. Conditions: [MCLR]0 = 2.0×10-7 M, H2A = 1.0×10-6-1.0×10-5 M, PMS = 1.0×10-6-1.0 ×10-5 M, and pH0 = 4.0.

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Concentration of MCLR (M)

(a) -7

2.0x10

pH 4 pH 5 pH 6 pH 12

-7

1.5x10

-7

1.0x10

-8

5.0x10

0.0 0

5

10

15

20

25

30

Time (min)

534 (b)

Experimental result Model prediction

2.4

-3

-1

Rate constants (10 s )

2.8

2.0 1.6 1.2 0.8 0.4 0.0 3

4

5

6

7

pH

535 536 537 538

Figure 5 (a) Concentration profiles of MCLR under different initial solution pH values. (b) Effect of pH on the pseudo-first-order rate constant for the oxidation of MCLR. The dots show the experimental results, and the solid lines represent the model predictions. Note that the

539 540

concentration profile of MCLR at pH 12 was not modeled because of the non-reactivity of A . Conditions: [MCLR]0 = 2.0×10-7 M, H2A = 2.0×10-6 M, and PMS = 5.0×10-6 M.

2-

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(a) Concentration of MCLR (M)

-1

0 mg L NOM -1 1.0 mg L NOM -1 3.0 mg L NOM -1 5.0 mg L NOM

-7

2.0x10

-7

1.5x10

-7

1.0x10

-8

5.0x10

0.0 0

5

10

15

20

25

30

Time (min)

541

1.6

Experimental result Model prediction

-3

-1

Rate constants (10 s )

(b) 2.0

1.2

0.8

0.4

0.0 0

1

2

3

4

5

-1

542 543 544 545 546

NOM (mg L )

Figure 6 (a) Concentration profiles of MCLR under different NOM concentrations. (b) Effect of NOM concentrations on the pseudo-first-order rate constant for the oxidation of MCLR. The dots show the experimental results, and the solid lines represent the model predictions. Conditions: [MCLR]0 = 2.0 ×10-7 M, H2A = 2.0×10-6 M, PMS = 5.0×10-6 M, and pH0 = 4.0.

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-5

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-4

x 10

x 10

5 10 4

PMS Dose (M)

8 3 6 2 4 1 2 2

4

6

H2A Dosage (M)

8

10 -6

x 10

547 548 549

Figure 7 EE/Ototal (in kWh L-1) of H2A/PMS process vary with H2A and PMS doses. Conditions: [MCLR]0 = 2.0 ×10-7 M, H2A = 1.0×10-7-1.0×10-5 M, PMS = 1.0×10-6-5 ×10-5 M, and pH0 = 4.0.

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-7

MCLR concentration (M)

2.0x10

LC/MS/MS analysis PP2A assessment

-7

1.5x10

-7

1.0x10

-8

5.0x10

0.0 0

2

5

10

20

30

Time (min)

550 551 552

Figure 8 Variations of MCLR concentrations and hepatotoxicity by the H2A/PMS process. Conditions: [MCLR]0 = 2.0 ×10-7 M, H2A = 2.0×10-6 M, and PMS = 5.0×10-6 M.

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