Ultraviolet Irradiation of Permanganate Enhanced the Oxidation of

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UV Irradiation of Permanganate Enhanced the Oxidation of Micropollutants by Producing HO• and Reactive Manganese Species Kaiheng Guo, Jinsong Zhang, Ailin Li, Ruijie Xie, Zhuojian Liang, Anna Wang, Li Ling, Xuchun Li, Chuanhao Li, and Jingyun Fang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00402 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 26, 2018

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Environmental Science & Technology Letters

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UV Irradiation of Permanganate Enhanced the Oxidation of Micropollutants by Producing

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HO• and Reactive Manganese Species

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Kaiheng Guo†, Jinsong Zhang‡, Ailin Li†, Ruijie Xie†, Zhuojian Liang†, Anna Wang†, Li Ling§,

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Xuchun Liǁ, Chuanhao Li†, Jingyun Fang*,†

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†

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Technology, School of Environmental Science and Engineering, Sun Yat-Sen University,

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Guangzhou 510275, P. R. China

8

‡

Shen Zhen Water Affairs (Group) Co.Ltd., Shenzhen 518031, P. R. China

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§

The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon 999066, Hong

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation

10

Kong

11

ǁ

12

310018, P. R. China

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*Corresponding author: J. Fang, Phone: + 86-18680581522; e-mail: [email protected].

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou

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1

ACS Paragon Plus Environment


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Abstract

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Permanganate was activated by UV photolysis at 254 nm, resulting in the efficient degradation of

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micropollutants. The degradation of four probe molecules (i.e., nitrobenzene, benzoic acid,

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terephthalic acid and parachlorobenzoic-acid) and two micropollutants (i.e., gemfibrozil and

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nalidixic acid) resistant to permanganate oxidation, was enhanced by the UV/permanganate system,

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with pseudo first-order rate constants (k′) of 0.065-0.678 min-1 under the experimental conditions.

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Hydroxyl radicals (HO•) and Mn(V) peroxide were responsible for the enhancement, which were

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produced during the activation of permanganate by UV irradiation. The quantum yield of HO• was

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0.025 (± 0.001) mol Es-1 in the system. HO• oxidation primarily accounted for the degradation of

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nitrobenzene and gemfibrozil, while both HO• and Mn(V) were responsible for the degradation of

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benzoic acid, terephthalic acid, para-chlorobenzoic acid and nalidixic acid. This study is the first

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report on the activation of permanganate by UV irradiation for the abatement of micropollutants in

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water treatment, which may lead to a new advanced oxidation process relying on both HO• and

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reactive manganese species.

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INTRODUCTION

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Permanganate is widely used in water treatment to control organic pollutants.1 Compared with

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other oxidants, the advantages of permanganate are its effectiveness over a wide pH range, easy and

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safe storage and delivery, chemical stability, low cost, and the lack of toxic byproducts.1 However,

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permanganate is a selective oxidant and has been reported to only react with pollutants containing

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electron rich organic moieties (ERM) such as phenols and olefins,2-4 whereas it is less effective to

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some micropollutants such as ciprofloxacin, lincomycin, and trimethoprim.5

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To enhance the degradation of pollutants, some methods have been used to activate

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permanganate to produce reactive manganese intermediates such as trivalent manganese (Mn(III)),

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hypomanganate (Mn(V)) and manganate (Mn(VI)).6, 7 For instance, permanganate can be activated

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by bisulfite to produce Mn(III), which rapidly reacts with aniline and bisphenol A with second rate

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constants of 105−106 M-1 s-1.8, 9 Permanganate can also be activated to produce Mn(III) by some

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ligands, such as pyrophosphate (PP), ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic

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acid (NTA).2, 10 Mn(V) could be generated through the reduction of permanganate by As(III) under

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acidic conditions,11 while Mn(VI) could be generated through the reduction of permanganate by

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sulfite or temperature heating under alkaline conditions.6,

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important intermediates via the transformation of permanganate by some polysaccharides under

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alkaline conditions.14,

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benzylamine and primary/secondary alcohols.11, 16 Mn(VI) rapidly reacts with the phenolate ion via

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electron transfer,17 and primary/secondary alcohols with second rate constants of ~105 M-1 s-1.18 Thus,

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activation of permanganate is a promising way to enhance the utilization of permanganate and

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12, 13

Both Mn(V) and Mn(VI) are

Mn(V) is very reactive toward sulfides, benzaldehydes, formic acid,

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promote the removal of micropollutants. However, the current activation methods, such as bisulfite,

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ligands and As(III), involve the addition of inorganic or organic reagents, which are toxic or lead to

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an increase in organic carbon and thus are not applicable to water treatment.

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UV is commonly used for disinfection and oxidation in water treatment.19 Additionally, UV has

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been used to activate oxidants such as H2O2,20, 21 chlorine22-24 and persulfate25, 26 to produce highly

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reactive radicals such as HO•, Cl• and SO4•- for the abatement of micropollutants.27,

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hypothesize that UV is a possible method to activate permanganate and produce highly reactive

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species. MnIIIO2- and MnIVO2 have been suggested as the ultimate products of permanganate

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photodecomposition,29 while the formation of MnVO4- and MnVIO42- intermediates in the system has

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also been proposed.29-32 The production of the intermediates from the photolysis of permanganate

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indicates that the UV/permanganate system might be a promising technology for water treatment.

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However, the efficiency and the underlying mechanisms of the UV/permanganate process for

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micropollutant degradation are totally unknown.

28

We

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Thus, the objective of this study was to investigate the feasibility of the UV/permanganate

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process for the abatement of micropollutants in water treatment. Reaction kinetics and participating

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reactive

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para-chlorobenzoic acid were selected as probe molecules, while gemfibrozil and nalidixic acid were

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selected as actual micropollutants.

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MATERIALS AND METHODS

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Materials. All solutions were prepared with reagent-grade chemicals and ultrapure water (18.2 MΩ

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cm). Sources of reagents and the preparation of stock solutions are provided in the Supporting

species

were

investigated.

Nitrobenzene,

benzoic acid,

4


terephthalic

acid and

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Information (SI) Text S1.

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Experimental Procedures. The photochemical experiments were performed in a 700-mL,

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magnetically stirred cylindrical borosilicate glass reactor with a quartz tube in the center, into which

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a low-pressure mercury lamp (Heraeus GPH 212T5L/4, 10 W) was placed, as described previously.24

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The average UV fluence rate was determined to be 2.13 mW cm-2 by iodide/iodate chemical

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actinometry (Text S2).33 The temperature was maintained at 25 (± 0.2) °C.

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A 700-mL testing solution containing 5 µM of the target compound, buffered at pH 7.4 using 2.5

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mM borate buffer, was dosed with the permanganate stock solution to achieve a concentration of 100

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µM and was simultaneously exposed to UV irradiation. Samples were collected at different time

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intervals and the residual oxidants were quenched with ascorbic acid at a molar ratio of ascorbic acid

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to permanganate of 5:1. Control tests, which involved exposure to either UV photolysis or

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permanganate oxidation alone, were carried out in a similar manner. Another test was conducted in a

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similar manner to determine the formation of HO• by spiking t-butanol into the UV/permanganate

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system and monitoring the formation of formaldehyde.34, 35 One data set (shown with error bars in

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each figure) was duplicated for quality control. The error bars in all the data plots represent the

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maximum and minimum of the experimental data of the duplicated test results.

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Analytical Methods. The concentrations of nitrobenzene, benzoic acid, terephthalic acid,

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para-chlorobenzoic acid, gemfibrozil and nalidixic acid were determined using a high-performance

90

liquid chromatography (HPLC, Agilent 1260) equipped with an Agilent C18 column (Poroshell, 4.6

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× 50 mm, 2.7 µm) and a diode array detector. Formaldehyde was determined by HPLC after

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derivatization with 2,4-dinitrophenylhydrazine.36 A UV−visible spectrophotometer (Shimadzu,

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UV-2600) was used to monitor the absorbance at 200−800 nm in the solution in the absence of

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organics. Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMX-E spectrometer,

95

Germany) was applied for in situ investigation of radical generation, using 5,5-dimethyl-1-pyrroline

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N-oxide (DMPO) as the spin trapping reagent. The particle size distribution of MnO2 was

97

determined by a laser particle size analyzer (Malvern, Mastersizer 3000).

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RESULTS AND DISCUSSION

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Degradation of Micropollutants in the UV/Permanganate Process.

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The degradation of four probe molecules (i.e., nitrobenzene, benzoic acid, terephthalic acid

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and para-chlorobenzoic acid) and two micropollutants (i.e., gemfibrozil and nalidixic acid) was

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enhanced by the UV/permanganate system, with pseudo first-order rate constants (k′) of 0.065, 0.182,

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0.293, 0.318, 0.110 and 0.678 min-1 respectively, at a permanganate dosage of 100 µM and pH 7.4

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(Figure 1). Permanganate was insignificant for their degradation. UV photolysis at 254 nm was the

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most efficient for the degradation of para-chlorobenzoic acid with k′ of 0.056 min-1 and the quantum

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yield of 0.106 mol Es-1 (Table S1). The efficient degradation of selected micropollutants by

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UV/permanganate indicated that some reactive species may be produced during the co-exposure of

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UV and permanganate.

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The addition of t-butanol inhibited the degradation of nitrobenzene and gemfibrozil more

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significantly than benzoic acid, terephthalic acid, para-chlorobenzoic acid and nalidixic acid. The k′

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by reactive species of nitrobenzene and gemfibrozil decreased by 100% and 93.5% in the presence of

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2 mM t-butanol, while those of benzoic acid, terephthalic acid, para-chlorobenzoic acid and nalidixic

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acid were suppressed by 71.0%, 48.2%, 52.7% and 0.8% respectively (Figure 2). The nearly

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complete inhibition of nitrobenzene and gemfibrozil degradation by t-butanol might be attributed to

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the scavenging effect on HO•. As for the other compounds, their degradation should be attributable to

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other reactive species in the presence of t-butanol, which might be reactive manganese species

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(RMnS). When the concentration of t-butanol increased from 2 mM to 10 mM, k′s of the four probe

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molecules were not further inhibited, indicating that RMnS may not be scavenged by t-butanol.

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Role of HO• in the UV/Permanganate Process.

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To investigate the existence of HO•, EPR was used. A typical 1:2:2:1 quartet signal of

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DMPO-OH spin adduct with spin Hamiltonian parameters (aN = 14.9 G, aH = 14.9 G, g = 2.006)37

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was generated in UV/permanganate, consistent with that in UV/H2O2 (Figure 3a). As for UV alone or

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permanganate alone, no signal was observed. Note that the other three-line signal of oxidized DMPO

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radical was also observed.38, 39

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To investigate the quantum yield of HO• in the UV/permanganate system, t-butanol was added

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to scavenge HO• and further form formaldehyde, whose formation could indicate the yield of HO•.34,

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35

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generated in the UV/permanganate system increased linearly with increasing reaction time, and

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increasing the t-butanol concentration from 1 mM to 10 mM didn’t enhance the generation of

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formaldehyde (Figure S1a). Thus, the quantum yield of HO• was determined to be 0.0253 (± 0.001)

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mol Es-1 at pH 7.4 (Shown in Text S3 and Figure S1b). The quantum yield of HO• in

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UV/permanganate was much lower than those of UV/H2O2, UV/chlorine and UV/NH2Cl processes

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(Table S2). However, the higher molar absorption coefficient (ε254 nm) of permanganate (637 M-1 cm-1,

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determined in this study) than other oxidants made the generation of HO• and the degradation

The HO• yield is about four-fold of the formaldehyde yield.34 The concentration of formaldehyde

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efficiency of the four probe micropollutants in UV/permanganate comparable with those in other UV

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based advanced oxidation processes (AOPs) (Table S2 and Figure S2).

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Varying conditions could alter the quantum yield of HO•. Increasing pH from 4.0 to 9.5 reduced

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the quantum yield by 50.4% (Figure 3b and Figure S3a). The decreasing quantum yield with

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increasing pH indicated that H+ may be involved in the generation of HO•. Note that the ε254 nm of

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permanganate was stable with increasing pH (Figure S3b). Also, the quantum yield decreased by

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42.8% with increasing permanganate dosage from 20 µM to 100 µM (Figure 3b and Figure S4), due

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to the greater filter effect of UV light at higher permanganate dosage.

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Role of Reactive Manganese Species (RMnS) in the UV/Permanganate System.

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Theoretically, permanganate can be reduced to Mn(VI), Mn(V), Mn(IV), Mn(III) and Mn(II).

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Mn(VI) was tested to be not reactive with nitrobenzene and benzoic acid (Figure S5), and it could

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only exist in alkaline condition.6 Also, Mn(II) and Mn(IV) usually act as stable manganese reduction

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products, and they are not reactive with most micropollutants.2, 9 Therefore, the RMnS may contain

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Mn(V) and Mn(III), which are very reactive and unstable. Figure S6a shows the time-resolved

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absorbance spectra of the UV/permanganate system. The absorbance at 525 nm (Mn(VII)) decreased

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from 0.247 cm-1 to 0.209 cm-1 with increasing time from 0 to 30 min, while that at 375 nm increased

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from 0.062 cm-1 to 0.517 cm-1, due to the accumulation of in situ formed MnO2.40 In the presence of

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50 mM PP, the absorbance at 300−500 nm was inhibited significantly (Figure S6b), probably due to

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the complexation of MnO2 with PP (Figure S7). Figure S8 confirms that the MnO2 particles were

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produced in UV/permanganate, but it disappeared when adding 50 mM PP.

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Mn(III) was recently reported to be an important oxidant in permanganate/bisulfite and

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permanganate/Mn(II)/PP systems.9,

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UV/permanganate, additional tests were conducted. (1) The characteristic peak of Mn(III) was

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compared with permanganate/Mn(II)/PP system. By comparing Figures S6 and S9, the featured peak

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of Mn(III) at 260 nm10 was not recorded in UV/permanganate system. Also note that Mn(III) was not

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produced by permanganate alone in the presence of 50 mM PP (Figure S10). (2) The effect of PP on

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the degradation of nitrobenzene and benzoic acid by UV/permanganate was investigated (Figure

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S11). Their degradation was not suppressed by PP, which can react with Mn(III) to form Mn(III)-PP

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complex. This result indicated that Mn(III) was not the dominant species for the degradation of

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nitrobenzene and benzoic acid, as Mn(III)-PP was not reactive toward them (Figure S12). If Mn(III)

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was important for the degradation of nitrobenzene and benzoic acid, the addition of PP would

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compete with the target compounds to form Mn(III)-PP and to inhibit their degradation. Above all,

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Mn(III) was ruled out.

168

10

To

ascertain

whether

Mn(III)

was

important

in

As reported, an excited-state [MnO2(η2-O2)-]* can be formed during the UV photolysis at 254

169

nm of permanganate.30,

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oxidation agent than permanganate, may be formed via O-O formation from two Mn=O bonds of

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permanganate.29,

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micropollutants in UV/permanganate. As for the reactivity of Mn(V), it can oxidize α-C of primary

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and secondary alcohols to aldehydes through hydrogen transfer.11 As such, it is rational to expect that

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t-butanol cannot be oxidized by Mn(V) due to the lack of hydrogen at the α-C. So, the obvious

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degradation of benzoic acid, terephthalic acid, para-chlorobenzoic acid and nalidixic acid by

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UV/permanganate with the presence of t-butanol (Figure 2) was very likely attributable to Mn(V),

30, 42

40, 41

Then Mn(V) peroxide (MnO2(η2-O2)-), which is a more reactive

Thus, Mn(V) was proposed as the main RMnS for the degradation of

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177

while the negligible degradation of nitrobenzene and gemfibrozil indicated their resistance to Mn(V).

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This result also indicates that Mn(V) is a selective oxidant, which reactivity is compound specific.

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In summary, the mechanism for the UV photolysis of permanganate is proposed in Scheme 1.

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The UV photolysis of permanganate forms Mn(V) peroxide. The UV photolysis of O-O in Mn(V)

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peroxide is likely to form HO•, similar like that of the UV photolysis of H2O2. Meanwhile, the

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disproportionation 13 or UV photolysis of Mn(V) peroxide forms MnO2 as the final product.30, 40

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Scheme 1. Proposed mechanism for the UV photolysis of permanganate

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Engineering Implications.

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UV and permanganate have been widely used for water treatment. This study is the first to show

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the synergistic effect of co-exposure to UV photolysis and permanganate for micropollutant

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degradation. The UV/permanganate system can produce reactive species of HO• and Mn(V) peroxide.

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The former is a broad-spectrum oxidant, while the latter is selective. The multiple reactive species in

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the system can complement each other in degrading a variety of contaminants. HO• is firstly

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identified and proved in the UV/permanganate system. Mn(V) is a new reactive species in water

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treatment, whose mechanism on micropollutant degradation is on-going in our group. The higher

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removal efficiencies of benzoic acid, terephthalic acid and para-chlorobenzoic acid by

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UV/permanganate compared with UV/H2O2 were responsible for both Mn(V) and HO• in 10


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UV/permanganate (Figure S2).

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Further research is needed to incorporate the effects of operational conditions (such as

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permanganate dosage) and water matrix components (such as pH, dissolved organic matter and

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alkalinity), to evaluate the application potential of the UV/permanganate process in real water

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treatment. Initial experiments with treatment of nitrobenzene and benzoic acid indicated that: (1) the

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degradation of nitrobenzene and benzoic acid was favored at acidic condition (Figure 4a) and higher

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dosages of permanganate (Figure 4b); (2) nitrobenzene and benzoic acid could be efficiently

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degraded with the presence of 1 mg L-1 natural organic matter (NOM), which are ubiquitous in

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natural water and drinking water (Figure 4c). Thus, the UV/permanganate system can be a promising

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AOP for the abatement of micropollutants in water treatment.

206 207

ASSOCIATED CONTENT

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Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at

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http://pubs.acs.org.

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Sources of reagents and the preparation of stock solutions (Text S1), determination of UV

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photon flux and effective path length of UV light (Text S2), determination of the HO• quantum yields

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of in the UV/permanganate system (Text S3), properties of selected micropollutants (Table S1), the

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quantum yields of HO•, the molar absorption coefficients of oxidants at 254 nm and the formation

215

rates of HO• in different AOPs (Table S2), the formation of formaldehyde by the UV/permanganate

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system in the presence of t-butanol (Figure S1), comparison of the degradation kinetics of

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nitrobenzene, benzoic acid, terephthalic acid and para-chlorobenzoic acid by the UV/H2O2 and

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UV/permanganate systems (Figure S2), effect of pH on the formation of formaldehyde (Figure S3),

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effect of permanganate dosage on the formation of formaldehyde (Figure S4), degradation of

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nitrobenzene and benzoic acid by pre-synthesized MnVIO42- in pure water (Figure S5), UV−vis

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spectra at the wavelengths of 200−800 nm in the UV/permanganate system in the absence/presence

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of PP (Figure S6), effect of PP on UV−vis spectra of pre-synthesized MnO2 (Figure S7), particle size

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distribution of the solution during UV/permanganate treatment in the absence and presence of PP

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(Figure S8), UV−vis spectra at the wavelengths of 200−800 nm (Figure S9), UV−vis spectra at the

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wavelengths of 200−800 nm in the permanganate/PP system (Figure S10), effects of PP on

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degradation of nitrobenzene and benzoic acid by UV/permanganate (Figure S11), degradation of

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nitrobenzene and benzoic acid by Mn(III)-PP (Figure S12).

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AUTHOR INFORMATION

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*Corresponding author: J. Fang, Phone: + 86-18680581522; e-mail: [email protected].

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ACKNOWLEDGMENTS

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This work was financially supported by the Major Science and Technology Program for Water

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Pollution Control and Treatment in China (No. 2015ZX07406004).

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TOC Art

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(b) Benzoic acid

(a) Nitrobenzene

1.0

1.0 y = e -0.005x R2 = 0.9962

0.8 UV photolysis Permanganate UV/permanganate

0.6

C/C0

C/C0

0.8

y = e -0.006x R2 = 0.9968

0.6 0.4

0.4 y = e -0.065x R2 = 0.9813

0.2

y = e -0.182x R2 = 0.9943

0.2 0.0

0.0 0

5

10

15

20

25

0

30

5

10

15

25

30

(d) Para-chlorobenzoic acid

(c) Terephthalic acid

1.0

1.0 0.8 0.6

UV photolysis Permanganate UV/permanganate

0.4 y = e -0.293x R2 = 0.9996

0.2


0.8

y = e -0.011x R2 = 0.9996

C/C0

C/C0

20

Time (min)

Time (min)

0.6 0.4

y = e -0.056x R2 = 0.9955

0.2

y = e -0.318x R2 = 0.9985

0.0

0.0 0

5

10

15

20

25

0

30

5

10

15

20

25

30

Time (min)

Time (min)

(f) Nalidixic acid

(e) Gemfibrozil

1.0

1.0 y = e -0.048x R2 = 0.9977

0.8


0.6 y = e -0.11x R2 = 0.9811

0.4

y = e -0.047x R2 = 0.9976

0.8 C/C0

C/C0


0.6 UV photolysis Permanganate UV/permanganate

0.4 0.2

0.2

y = e -0.678x R2 = 0.9998

0.0

0.0 0

5

10

15

0

20

2

4

6

8

10

Time (min)

Time (min)

Figure 1. Comparison of the degradation kinetics of (a) nitrobenzene, (b) benzoic acid, (c) terephthalic acid, (d) para-chlorobenzoic acid, (e) gemfibrozil, and (f) nalidixic acid by UV, permanganate and UV/permanganate at pH 7.4. Conditions: [MnO4-]0 = 100 µM, [target compound]0 = 5 µM.

19



Page 20 of 22

0.8

0.6 k' (min-1)

Nalidixic acid

UV photolysis Permanganate Reactive species

Terephthalic Para-chlorobenzoic acid acid

0.4

Benzoic acid

0.2 Gemfibrozil Nitrobenzene

0.0 0 2 10

0 2 10

0 2 10

0 2 10

0 2

0 2

t-butanol concentration (mM)

Figure 2. Effects of t-butanol on the first order degradation rate constants (k′) of nitrobenzene, benzoic acid, terephthalic acid, para-chlorobenzoic acid, gemfibrozil, and nalidixic acid by the UV/permanganate system at pH 7.4. Conditions: [MnO4-]0 = 100 µM, [target compound]0 = 5 µM.

20


Page 21 of 22


(b) 20

40

HO· quantum yield (mol Es-1)

0.05

60

80

100

Permanganate dosage (µM)

0.04

0.03

0.02

0.01

0.00 4

5

6

7

8

9

10

pH

Figure 3. (a) Electron paramagnetic resonance (EPR) spectra for the UV/permanganate system after 2 min reaction at room temperature. Conditions: [MnO4-]0 = 0.5 mM, [DMPO]0 = 0.45 M. The EPR signals are marked as follows: red circles - hydroxyl radicals; green squares - oxidized DMPO radicals. (b) Effects of pH (blue symbol, [MnO4-]0 = 100 µM) and permanganate dosages (red symbol, pH 7.4) on HO• quantum yield by the UV/permanganate system at the initial concentration of t-butanol of 10 mM.

21



(a) Nitrobenzene

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(c)

(b) Benzoic acid

Nitrobenzene

Benzoic acid

Nitrobenzene Benzoic acid

0.3

k' (min-1)

UV photolysis Permanganate Reactive species

0.2

0.1

0.0 4.0 7.4 9.5

4.0 7.4 9.5

pH

50 100 200

50 100 200

Permanganate dosage (µM)

0

1

0

1 -1

NOM (mg L )

Figure 4. Effects of (a) pH, (b) permanganate dosages and (c) natural organic matter (NOM) on the degradation of nitrobenzene and benzoic acid by the UV/permanganate system. Conditions: [target compound]0 = 5 µM, (a) [MnO4-]0 = 100 µM; (b) pH = 7.4; (c) [MnO4-]0 = 100 µM, pH = 7.4.

22


Ultraviolet Irradiation of Permanganate Enhanced the Oxidation of

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