UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane

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Remediation and Control Technologies

UV Photolysis of Chloramine and Persulfate for 1,4-dioxane Removal in Reverse Osmosis Permeate for Potable Water Reuse Wei Li, Samuel Patton, Jamie M. Gleason, Stephen Peter Mezyk, Kenneth P Ishida, and Haizhou Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06042 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 15, 2018

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Environmental Science & Technology

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UV Photolysis of Chloramine and Persulfate for 1,4-dioxane Removal in

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Reverse Osmosis Permeate for Potable Water Reuse

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Wei Li,†‡ Samuel Patton,†‡ Jamie M. Gleason,ǁ Stephen P. Mezyk,ǁ

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Kenneth P. Ishida§ and Haizhou Liu†‡*

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Department of Chemical & Environmental Engineering, University of California, Riverside, CA 92521 USA

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Environmental Toxicology Program, University of California, Riverside, CA 92521 USA §

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CA 92708 USA

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Research & Development Department, Orange County Water District, Fountain Valley,

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Department of Chemistry and Biochemistry, California State University, Long Beach, CA 90840, USA

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* Corresponding author, phone (951) 827-2076; fax (951) 827-5696

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Email: [email protected]

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Submitted to Environmental Science & Technology

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Abstract

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A sequential combination of membrane treatment and UV-based advanced oxidation processes

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(UV/AOP) has become the industry standard for potable water reuse. Chloramines are used as

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membrane anti-fouling agents and therefore carried over into the UV/AOP. In addition,

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persulfate (S2O82-) is an emerging oxidant which can be added into a UV/AOP, thus creating

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radicals generated from both chloramines and persulfate for water treatment. This study

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investigated the simultaneous photolysis of S2O82- and monochloramine (NH2Cl) on the

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removal of 1,4-dioxane (1,4-D) for potable water reuse. The dual oxidant effects of NH2Cl and

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S2O82- on 1,4-D degradation were examined at various levels of oxidant dosage, chloride, and

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solution pH. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond

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which the scavenging by NH2Cl of HO•, SO4•-, and Cl2•- radicals decreased the 1,4-D

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degradation rate. At the optimal ratio, the degradation rate of 1,4-D increased linearly with the

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total oxidant dose up to 6 mM. The combined photolysis of NH2Cl and S2O82- was sensitive to

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the solution pH, due to a disproportionation of NH2Cl at pH lower than 6 into less photo-

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reactive dichloramine (NHCl2) and radical scavenging by NH4+. The presence of chloride

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transformed HO• and SO4•- to Cl2•- that is less reactive with 1,4-D, while the presence of

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dissolved O2 promoted gaseous nitrogen production. Results from this study suggest that the

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presence of chloramine can be beneficial to persulfate photolysis in the removal of 1,4-D;

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however, the treatment efficiency depends on a careful control of an optimal NH2Cl dosage

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and a minimal chloride residue.

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Introduction

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Potable water reuse provides a viable solution to water scarcity by treating municipal

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wastewater.1,2 UV-based advanced oxidation processes (UV/AOPs) have become an integral

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part of the treatment train to degrade trace organic contaminants.3-9 For example, 1,4-dioxane

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(1,4-D), a solvent widely used in production of adhesives, dyes, textiles and cosmetics, has

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been widely detected in wastewater and classified as a potential human carcinogen. 10 - 13

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Because small and neutral trace organic contaminants like 1,4-D can pass through reverse

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osmosis (RO) membranes, UV/AOP typically takes place after membrane treatment to

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ultimately eliminate recalcitrant trace contaminants from RO permeate.

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Chloramine is often added to the feed water to minimize membrane bio-fouling. Due to their

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small molecular sizes and neutral charge, chloramines also pass through RO membranes and

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are carried over to downstream UV/AOP step where they undergo photolysis.14 In addition,

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hydrogen peroxide (H2O2) is the default oxidant added to the UV/AOP step to produce

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hydroxyl radicals (HO•). Although possessing a high oxidative capacity, HO• is not selective

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and can readily be scavenged by bicarbonate and chloride which adversely affects its efficiency.

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6,15

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In recent years, persulfate (S2O82-) has also been considered as an alternative oxidant in

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UV/AOP.6, 16-20 The primary quantum yield of SO4•- from S2O82- is higher than that of HO•

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from H2O2 (0.7 vs. 0.5), while both SO4•- and HO• have similar oxidizing power (2.5-3.1 V vs.

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1.9-2.8 V), respectively.

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contaminants, especially electron-rich contaminants in recycled water.6, 27 , 28 Therefore,

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UV/S2O82- can potentially have a higher efficiency and a lower energy footprint than UV/H2O2

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Furthermore, SO4•- is more selective towards organic

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in water reuse. Given the existence of de facto UV/NH2Cl because of NH2Cl carry-over and

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the future application of UV/S2O82-, it is important to understand the unique radical generation

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under photolysis in mixtures of NH2Cl and S2O82-.

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The mechanism on the simultaneous photolysis of NH2Cl and S2O82- is not well developed.

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NH2Cl has a relatively high UV molar absorbance coefficient (371 M-1cm-1 at 254 nm) and a

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quantum yield comparable to H2O2.4,29-31 NH2Cl can also act as a self-scavenger and will

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decrease its radical yield upon UV irradiation.14 Furthermore, the presence of ammonium in

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wastewater, and as a photo-degradation product of NH2Cl, may also act as a scavenger for

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radicals including Cl•.31,32 Most prior studies have examined chloramine photolysis in aquatic

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conditions unrelated to potable reuse.8,29-31 Our recent study showed that photolysis of NH2Cl

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in RO permeate produces both HO• and Cl2•- that promote 1,4-D degradation.14 Furthermore,

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SO4•- produced from S2O82- photolysis can react with NH2Cl to generate secondary radicals,

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but the extent of radical propagation reactions in RO permeate with the dual oxidants remains

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unknown.

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The objectives of this study were to quantify the formation of reactive radicals during the

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simultaneous photolysis of NH2Cl and S2O82- solutions, to examine the impact of oxidant

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dosage, pH, and chloride on the degradation of 1,4-D, and to develop a kinetic model to predict

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the radical yield and transformation of primary radicals (e.g. SO4•- and Cl•) to secondary

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radicals including Cl2•- and HO•.

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Materials and Methods

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All chemicals used were reagent grade and purchased from Fisher Scientific or Sigma Aldrich.

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All solutions were prepared using deionized water (DI) (resistivity < 18.2 MΩ, Millipore

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System). A fresh 50 mM NH2Cl stock solution was prepared daily by slowly titrating NaOCl

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with (NH4)2SO4 at 1:1.2 molar ratio and buffered with 4 mM borate at pH 8.8.33 The prepared

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NH2Cl solution was equilibrated for 3 hours in the dark and its concentration was verified

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using the standard DPD method with KMnO4.34 An equal-molar chloride residue always co-

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existed with NH2Cl due to chloramine equilibrium chemistry, which was confirmed by a

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Dionex-1000 ion chromatography equipped with a conductivity detector.

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persulfate stock solution was prepared daily using Na2S2O8. Most experiments were conducted

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using N2-purged water to minimize dissolved O2. In some experiments, DI water was purged

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with ambient air for 20 minutes to ensure an air-saturated solution. The solution pH was

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adjusted to a targeted value between 5 to 8 with phosphate buffer, which also maintained the

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ionic strength constant at 50 mM. Chloride concentration in the mixture was adjusted to a final

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value between 0.2 and 4 mM.

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To start a UV experiment, solutions of NH2Cl ranging between 0 and 6 mM and S2O82-

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between 2 and 4 mM were mixed with 1,4-D at a final concentration of 250 µM. In addition,

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10 µM benzoic acid (BA) and 20 µM nitrobenzene (NB) were added as radical probe

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compounds to collectively calculate the radical steady-state concentrations. The concentrations

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of oxidants and 1,4-D used in this study were higher than those observed in RO permeate in

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order to observe the reaction kinetics. In the fixed chloride experiments, chloride concentration

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was adjusted to a final concentration of 4 mM. To examine the effect of NH4+ on Cl• and Cl2•-

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scavenging, the NH4+ concentration was adjusted between 0 and 6 mM with 100 mM

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(NH4)2SO4 stock solution. At the same time, chloride concentration was set at 4 mM. The

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A 100-mM

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prepared reaction solution was quickly mixed, transferred to multiple 8-mL quartz tubes with

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no headspace, and placed in a carousel UV reactor (ACE Glass) equipped with a low-pressure

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monochromatic (λ=254 nm) mercury lamp (Philips TUV6T5, 6W). The UV fluence was

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measured by a multimeter equipped with a thermopile 919P sensor (Newport Power meter) and

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determined to be 1.4 mW/cm2.

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Samples were withdrawn from the UV reactor at pre-determined time intervals. The NH2Cl

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concentration was measured immediately using the DPD colorimetric method.34 The presence

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of S2O82- in the sample had no interference with this measurement. The S2O82- concentration

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was measured using KI titration on a Horiba UV spectrometer after NH2Cl was removed by

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air-purging the solution for 30 minutes.35,36 The concentrations of 1,4-D, BA and NB were

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measured by an Agilent 1200 liquid chromatography equipped with a diode array detector and

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a Zorbax Eclipse SB-C18 column (4.6×150mm, 5-µm particle size). To quantify the generation

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of nitrogen species during NH2Cl photolysis, total nitrogen (TN) was measured by a TOC

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analyzer coupled with a nitrogen detector (OI Analytical, Inc.); ammonium, nitrite, and nitrate

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were measured using colorimetric methods with phenate, sulfanilamide and nicotinamide

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adenine dinucleotide phosphate, respectively,34 and no interference from persulfate was

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observed. Gaseous nitrogen formation was calculated based on the loss of TN; the remaining

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fraction of nitrogen was attributed to organic nitrogen.

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The steady-state concentrations of SO4•-, HO•, and Cl2•- were calculated based on the

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competitive decay kinetics of probe compounds (Text S1 and Table S1). All calculations were

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based on experimentally observed pseudo first-order rates of 1,4-D, BA and NB. The second-

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order rate constants of each radical with 1,4-D, BA and NB were obtained from the prior

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literature. The direct photolysis of 1,4-D, NB and BA was experimentally determined to be

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negligible. A kinetics model was developed using the Kintecus software (calculations for direct

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photolysis rate was presented in Text S2, and all reactions of the model are listed in Table

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S2). 37 The model development and optimization can be found in Text S3 and prior

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literature.18,38 Laser flash photolysis studies were conducted to determine the rate constants of

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SO4•-, Cl• and Cl2•- with NH2Cl and NHCl2 (Text S4 and Figures S1-S3).

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Results and Discussion

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Impact of NH2Cl dosage on UV/S2O82-

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The effect of NH2Cl on 1,4-D degradation by UV photolysis of S2O82- was investigated by

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fixing the S2O82- and 1,4-D concentrations at 2 mM and 0.25 mM, respectively. The UV

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fluence-normalized rate constant39-42 of 1,4-D degradation increased from 7.5×10-4 to 9.7×10-4

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cm2•mJ-1 when the NH2Cl dosage increased from 0 to 0.2 mM; however, the rate decreased to

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5.3×10-4 cm2•mJ-1 when the NH2Cl dosage was increased further to 4 mM (solid bars in Figure

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1A and Figure S4A). Photolysis of S2O82- generated SO4•-, which quickly reacted with Cl- to

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form Cl• (Reactions 1-2 in Scheme 1; all subsequent referred reactions are shown in Scheme 1).

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Cl• was further transformed into Cl2•- (Reaction 3) and finally resulted in HO• production

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(Reactions 4-6). Experimental data from competition kinetics with probe compounds and

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branching ratio calculations showed that HO• contributed the most to 1,4-D degradation in

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UV/S2O82- and NH2Cl, followed by SO4•- and Cl2•- (Figure 1A and Text S5). Cl2•- was assumed

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to be the major radical that contributes to 1,4-D degradation other than HO• and SO4•-. This

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assumption was made based on prior laser flash photolysis study14 and branching ratio

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calculations (Text S5.4). The laser flash photolysis study revealed a second-order rate constant 7


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of 3.3×106 M-1s-1 between Cl2•- and 1,4-D (Text S4), and branching ratio calculation indicated

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that approximately 3-25% of Cl2•- contributed towards 1,4-D (Text S5.4). Because Cl•

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predominately reacted with Cl- and NH2• had negligible reactivity with 1,4-D,14 Cl• and NH2•

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were not considered as important to 1,4-D degradation. Experimental results showed that the

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contribution of HO• reached a maximum at the optimal NH2Cl dosage of 0.2 mM –

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corresponding to a NH2Cl-to-S2O82- molar ratio of 0.1:1, and decreased by 27% when the

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NH2Cl dosage increased to 4 mM. The contribution of SO4•- to 1,4-D degradation dropped

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significantly to negligible levels when the NH2Cl-to-S2O82- ratio increased above 0.1,

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accompanied by a substantial increase in the contribution of Cl2•- to 1,4-D removal (solid bars

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in Figure 1A).

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As the concentration of NH2Cl increased from 0 to 4 mM, UV photons were preferentially

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absorbed by NH2Cl as compared to S2O82-, which resulted in an increase in the photolysis rate

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of NH2Cl and a decrease in the photolysis rate of S2O82- (Figure S4B). The percentage of light

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absorbed by NH2Cl and S2O82- in the mixture system was calculated (Text S6 and Table S7).

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For instance, in the presence of 0.1 mM NH2Cl, S2O82- absorbed 98% of UV photons if no

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monochloramine were present, but only 49% of light in the presence of 4 mM NH2Cl.

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Therefore, the contribution of S2O82- to 1,4-D degradation decreased by 50% with 4 mM

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NH2Cl addition. Meanwhile, NH2Cl photolysis produced Cl• and NH2• (Reaction 7) that are

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less reactive with 1,4-D compared to SO4•- and HO•.14 As a result, the rate of 1,4-D

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degradation was reduced when the concentration of NH2Cl increased from 0.2 to 4 mM (Figure

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1A). In contrast, an increase in S2O82- dosage in the presence of a constant NH2Cl level always

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increased radical concentrations (Figure S5).

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In addition, when the NH2Cl-to-S2O82- ratio was higher than 0.1, the experimentally observed

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1,4-D degradation rate with both oxidants was 10% to 40% lower than the theoretically

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summed value as if both oxidants existed separately and no radical scavenging by oxidants

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occurred (striped bars in Figure 1A, calculation provided in Text S6 and Table S7). Compared

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to theoretical values, the mixing of NH2Cl and S2O82- led to a strong scavenging effect on SO4•-

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and Cl2•-, especially with higher dosages of NH2Cl. Furthermore, the degradation rate of 1,4-D

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was positively correlated with the photolysis rate of S2O82-, but inversely correlated with that of

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NH2Cl (Figure 1B). These trends strongly suggested that in the mixed oxidant system, S2O82-

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did not strongly scavenge radicals. S2O82- was the driving force for the radical generation and

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1,4-D removal. In contrast, increasing NH2Cl dose was not beneficial to the treatment, due to

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the scavenging effects of NH2Cl on major radicals.

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Radical distribution in UV photolysis of S2O82 and NH2Cl

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Impacts of NH2Cl on the radical generation and scavenging pathways in UV/S2O82- were

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elucidated in Scheme 1. When the NH2Cl dosage was increased up to 0.2 mM, [SO4•-]ss

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decreased by 75%, and [HO•]ss increased by 2-fold (Figure 1C). Since 1,4-D is more reactive

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with HO• than with SO4•-, the overall degradation rate of 1,4-D was enhanced.43 However, as

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the NH2Cl concentration increased from 0.2 to 4 mM, both [HO•]ss and [SO4•-]ss decreased

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substantially, and [Cl2•-]ss increased by 4-fold (Figure 1C). This trend resulted from the

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scavenging effects of NH2Cl on HO• and SO4•- (Reactions 8-9).

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In addition, the co-existence of chloride with NH2Cl also transformed SO4•- and Cl• to Cl2•-

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(Reactions 2-3). HO• reacts with NH2Cl with a rate constant of 5.1×108 M-1s-1, 44 and the

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second-order rate constant between SO4•- and NH2Cl of 2.4 × 107 M-1s-1 was measured by laser

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flash photolysis (Text S4). For instance, up to 68% of HO• and 7% of SO4•- were scavenged by

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4 mM NH2Cl (Texts S5.1 and S5.2, Tables S3-S4). Cl• was less affected by the presence of

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NH2Cl (Text S5.3 and Table S5). Cl2•- was scavenged by NH2Cl (Reaction 10, Text S5.4 and

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Table S6); however, more Cl2•- was produced with increasing NH2Cl dosage, thus [Cl2•-]ss

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reached a plateau after NH2Cl reached 2 mM (Figure 1C). As 1,4-D is more reactive with HO•

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and SO4•- than with Cl2•-, the overall loss rate slowed down significantly at the higher NH2Cl

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concentrations (solid bars in Figure 1A). Although the concentrations of 1,4-D and oxidants

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utilized in this study were higher than the concentrations used in the real treatment processes,

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the calculated steady-state radical concentrations were comparable to those observed in real

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AOPs (~10-10 and 10-13 M).

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In addition, the co-existence of chloride with NH2Cl acts as a scavenger to transform SO4•- and

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Cl• to Cl2•- (Reactions 3-4). To eliminate the confounding effects of chloride, the initial NH2Cl

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dosage was varied but the chloride level was fixed at 4 mM. These data showed that the 1,4-D

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degradation rate decreased by 43% with increasing NH2Cl concentrations (Figure S6). This

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trend further confirmed that scavenging reactions with NH2Cl decreased the radical

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concentrations (i.e., Reactions 8-10).

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Products from photolysis of S2O82- and NH2Cl

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The major products of NH2Cl photolysis were ammonium, nitrate, gaseous nitrogen and

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organic nitrogen (Figure 2). The formation of ammonium likely resulted from the oxidation of

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NH2Cl by NH2• (Reaction 11). The NHCl• radical is also formed via HO•, SO4•- and Cl2•-

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reactions with NH2Cl (Reactions 8-10). Recombination of this radical and its eventual 10



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oxidation can produce N2 gas (Reactions 12-13). A small amount of nitrate was produced

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throughout the experiment, possibly due to the reaction between NH2Cl and nitrite or H2O.31,45

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Organic nitrogen was likely the degradation products from the interaction of NH2• and NHCl•

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with 1,4-D. 46 - 48 Another possible pathway is that dissolved O2 reacts with NH2• forming

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intermediate products NH2O2•, which reacted with other radicals such as SO4•-, HO•, and Cl• to

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form nitrate.40 The formation of NO2• from nitrate photolysis subsequently resulted in the

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formation of organic nitrogen.49-51

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In the absence of S2O82-, photolysis converted 60% of NH2Cl to NH4+, 32% to organic nitrogen,

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3% to gaseous nitrogen and 5% to nitrate, respectively (Figure 2). In contrast, in the presence

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of S2O82-, NH2Cl photolysis generated more gaseous nitrogen and less organic nitrogen species.

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SO42-, with a 2:1 stoichiometry, was the only sulfur species observed from S2O82- photolysis

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(Figure S7). SO4•- directly oxidizes NH2Cl (Reaction 9) and yields N2 gas (Reaction 13). SO4•-

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could also withdraw elections from NH3 or NH4+ and oxidize them to N2.52 Consequently, the

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gaseous nitrogen formation almost doubled with the inclusion of S2O82- (Figure 2). The

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presence of O2 reacted with NH2• and promoted the formation of nitrate and subsequent

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organic nitrogen. The results suggest that O2 and S2O82- aid in the production of more highly

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oxidized nitrogen species.

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Impact of total oxidant dosage on 1,4-dioxane degradation

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The effect of total oxidant dosage on 1,4-D degradation was examined at two NH2Cl-to-S2O82

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molar ratios, specifically 0.1:1 and 1:1. The 0.1:1 ratio generally achieved 33% to 240% more

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1,4-D removal as compared to the 1:1 ratio (Figure 3A). The [SO4•-]ss was approximately 5

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times higher at the 0.1:1 ratio (Figure 3B), and [HO•]ss was 70% to 300% higher when the total 11


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oxidant dosage exceeded 4 mM (Figure 3C). At the optimal ratio of 0.1:1, when the total

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oxidant (i.e., S2O82- and NH2Cl) dosage exceeded 6 mM, major scavenging by S2O82- reduced

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the [HO•]ss by 15% (Reaction 14, Figure 3C), and consequently 1,4-D degradation rate

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dropped by 15% (Figure 3A). In contrast, at the NH2Cl-to-S2O82- ratio of 1:1, the 1,4-D

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removal rate exhibited a bell-shape curve with the maximal rate observed at 4 mM (Figure 3A).

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At this oxidant ratio, more SO4•- was transformed to Cl2•- that was less reactive with 1,4-D.

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Despite the conversion of Cl2•- to HO• (Reactions 3-7), the 1:1-ratio generally resulted in

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higher [Cl2•-]ss concentration and lower [SO4•-]ss compared to 0.1:1-ratio (Figures 3B and 3C).

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At the NH2Cl-to-S2O82- ratio of 1:1, when the total oxidant dosage exceeded 6 mM, more than

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90% of HO• was scavenged by NH2Cl and Cl- (Text S5.1), which resulted in a 30% decrease of

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[HO•]ss (Figure 3D).

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Impact of chloride on the photolysis of S2O82- and NH2Cl, and 1,4-D removal

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An increase of chloride concentration from 0.2 to 2 mM slowed 1,4-D degradation by 28%,

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and preferentially reduced the contribution from HO• (Figure 4A). There was a 40% and 80%

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reduction in [HO•]ss and [SO4•-]ss, respectively, whereas [Cl2•-]ss increased by 240% (Figure

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4B). The change of radical distribution was attributable to the chloride-assisted conversion of

249

HO• and SO4•- to Cl2•- (Reactions 3-4). Up to 87% of HO• and 97% of SO4•- were scavenged

250

by 2 mM of chloride (Texts S5.1 and 5.2); however, the ClOH•- quickly dissociated to generate

251

HO• (Reactions 5-7). The presence of 2 mM chloride accelerated the reaction between HO• and

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Cl- (Reaction 15) and also accelerated the reaction of HClOH• with Cl- (Reaction 16).

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Approximately 9% of HClOH• was scavenged by 2 mM chloride forming Cl2•- (Text S7).

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These two reverse reactions led to elevated [Cl2•-]ss and reduced [HO•]ss. 12



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Impact of pH on the photolysis of S2O82- and NH2Cl, and 1,4-D removal

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RO permeate is typically acidic due to the application of acids as the scale inhibitors to the feed

257

water. Therefore, it is important to understand the effect of a wider range of pH on the

258

performance of UV/AOP (Figure 5A). The experimental data at the higher pH 8 showed

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enhanced treatment efficiency. The increase of 1,4-D degradation kinetics from pH 5 to 8 was

260

accompanied by an increase of [HO•]ss and [SO4•-]ss (Figure 5B). The enhancement observed at

261

higher pHs (e.g. pH 7 and 8) was likely due to the increased stability of NH2Cl. NH2Cl

262

disproportionates into NHCl2 at pH 5 via an acid catalyzed reaction, which is further converted

263

to trichloramine (NCl3).53,54 Dark control experiments showed that approximately 36% and 11%

264

of NH2Cl decayed into NHCl2 after 20 minutes at pH 5 and 6, respectively (Figure S8).

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Furthermore, the disproportionation of NH2Cl also generates NH4+ which scavenges Cl•

266

(Reaction 17).31,32

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To further test the hypothesis that NH4+ could scavenge reactive chlorine species, additional

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UV/S2O82- experiments were conducted by varying the NH4+ concentration between 0 and 6

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mM with a constant chloride concentration of 4 mM. The presence of NH4+ up to 6 mM

270

decreased the 1,4-D degradation by up to 30% (Figure S9). Collectively, the disproportionation

271

of 0.2 mM NH2Cl to 0.14 mM NH4+ scavenged Cl• and Cl2•- (Reactions 17-18), and resulted in

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approximately 10% decrease of 1,4-D degradation rate at pH 5 compared to pH 8.

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Environmental Implications

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The study demonstrated the application of an alternative oxidant, S2O82- in UV/AOP for 1,4-D

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removal for potable reuse. The findings suggest that UV/S2O82- is an efficient UV/AOP

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technology that may help the water utility to comply with ever strengthening regulations. 13


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Specifically, UV/S2O82- can be operated efficiently in the presence of monochloramine, a

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disinfectant that is widely used in recycled water treatment trains. With careful control of the

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NH2Cl-to-S2O82- molar ratio at 0.1:1, the overall UV/S2O82- performance is enhanced. However,

280

beyond this optimal ratio, the performance is hindered, because high NH2Cl dosage has a

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photon filtering effect and also significantly scavenges generated radicals, which decreases the

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yields of HO• and Cl2•-. In addition, UV/S2O82- decreases the formation of undesirable NH2Cl

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decay products, ammonium, and organic nitrogen, and promoted gaseous nitrogen production.

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Although the concentration of chloride is relatively low in RO permeate, its presence may slow

285

down 1,4-D degradation through the transformation of reactive HO• and SO4•- to less reactive

286

Cl2•-. Elevated pH could enhance treatment efficiency. Results from this study elucidate the

287

fundamental mechanisms of radical generation during the unique photolysis of dual oxidants,

288

quantify the effects of incidental presence of NH2Cl on the application of S2O82- in water reuse,

289

and assist in the development of more efficient UV/AOP technologies.

290

Acknowledgment

291

This work was supported by grants to H.L. from the U.S. National Science Foundation (CHE-

292

1611306), and to W.L from the U.S. National Science Foundation Graduate Research

293

Fellowship and UC Riverside IGERT Water Sense Fellowship. We thank Alex Numa from UC

294

Riverside and Guadalupe Lara from La Verne University for participation in this project. Laser

295

flash photolysis studies were conducted at the Notre Dame Radiation Research Laboratory,

296

which is supported by Office of Basic Energy Sciences within the U.S. Department of Energy.

297

Supporting Information Section

14



Page 16 of 35

298

Additional description of monochloramine preparation, calculations of steady-state

299

concentration and branching ratio of reactive radical species, tables of reactions for model

300

development, and figures of 1,4-D first-order decay and oxidant decay, the effect of pH on 1,4-

301

D degradation in UV/S2O82 and UV/NH2Cl are provided in the Supporting Information Section.

15


Page 17 of 35


302 303

Scheme 1 Radical generation and reaction pathways in UV photolysis of persulfate and

304

monochloramine. All reactions are listed in Table 1.

16



305

Page 18 of 35

Table 1 Rate constants and elemental reactions for Scheme 1.

No.

Rate Constant (M-1 s-1)

Reaction ௛ఔ

1 2

Sଶ Oଶି ሮ 2SO∙ି ସ ଼ ሱ ∙ି ି ∙ SOସ + Cl → SOଶି ସ + Cl

3

Cl∙ + Clି → Cl∙ି ଶ ି . Cl∙ି ଶ + Hଶ O → Cl + HClOH

4 5

HClOH . → H ା + ClOH .ି

6

ClOH .ି → Clି + OH ∙

7

NHଶ Cl ሱሮ NHଶ. + Cl∙ NHଶ Cl + OH . → NHCl. + Hଶ O

௛ఔ

8

Reference*

See Text S3

Calculated

3.1×108

55

8.5×109

56

1.3×10

3a

57

1.0×10

8a

57

6.1×10

9a

58

See Text S3

Calculated

5.1×108

59

9

ି . NHଶ Cl + SO∙ି ସ → NHCl + HSOସ

(2.4±0.2)×107

Measured

10

NHଶ Cl + Clଶ∙ି → NHCl. + H ା + 2Clି

(6.5±3.5)×106

Measured

11

NHଶ Cl + NHଶ∙ → NHCl. + NHଷ

(1.0±0.8)×105

Measured

∙ି . ∙ 12 NHCl. + SO∙ି ସ /OH /Clଶ /Cl → Nଶ + Prodcut 13 NHCl. + NHCl. → Nଶ + 2HCl ∙ି . ି 14 Sଶ Oଶି ଼ + OH → Sଶ O଼ + OH

1.0×109 1.0×109

Assumedb Assumedb

1.4×107

60

15

OH ∙ + Clି → ClOH .ି

4.3×109

61

16

HClOH . + Clି → Cl∙ି ଶ + Hଶ O

5.0×109

57

17

NHସା + Cl. → NHଶ. + HCl + H ା

(1.3±0.5)×105

Measured

18

. NHସା + Cl.ି ଶ → NHଶ + HCl + HCl

(1.3±0.5)×105

Measured

19

1,4 − D + Cl∙ି ଶ → product 1,4 − D + SO∙ି ସ → product 1,4 − D + OH . → product

3.3×106

14

4.1×107 3.1×109

62 63

20 21 306

a

rate constants are in unit of s-1

307

b

Rate constants are assumed based on known reactivity of the species with other compounds.

17


Page 19 of 35


308

kNH Cl (x10-6 M•cm2•mJ-1) 2

0

2

4

6

B k1,4-D (x10-4 cm2•mJ-1)

10

8

6 S2O82Persulfate NH2Cl Monochloramine 4 1.6

309

2.0

kS O ( 2

8

2-

2.4 x10-7

M•cm2•mJ-1)

18


2.8


Page 20 of 35

310 311

Figure 1 Impact of NH2Cl dosage on the treatment of 1,4-dioxane by UV/ S2O82-/NH2Cl. (A)

312

Contribution of radicals to 1,-4-dioxane degradation; (B) Correlation between 1,4-D

313

degradation rates and oxidants photolysis rates; (C) Steady-state radical concentrations. [S2O82-

314

]=2 mM, [1,4-dioxane]=250 µM, [Cl-]=0-4 mM, [benzoic acid]=10 µM, [nitrobenzene]=20 µM,

315

pH=5.8. UV dose=2760 mJ•cm-2. Assumption on Cl2•- contribution in Figure 1A was based on

316

the laser flash photolysis study and branching ratio calculations (Text S5.4). Dashed lines in

317

figure 1C represent the modeled results. In Figure A, the theoretical calculations did not

318

considered radical scavenging reaction by NH2Cl (Reactions 8-10 in Table 1).

19


Page 21 of 35


N 2-purged

Air-saturated

Distribution of Nitrogen Products

100%

Organic c Organi nitrogen n nitroge

80%

Gaseous N2 60%

nitrogen

40%

NO3NO3-

20%

NH4+ NH4

0%

319

UV/NH 0 2Cl

UV/S2 O822-+NH2 Cl

UV/NH 0 2Cl

UV/S2 O28 2 +NH2 Cl

320

Figure 2 Impact of persulfate and dissolved oxygen on the distribution of nitrogen products

321

during UV photolysis of NH2Cl. [S2O82-]=0 or 2 mM, [NH2Cl]=2 mM, [Cl-]=2 mM, [1,4-

322

dioxane]=250 µM, pH=5.8, UV dose=2760 mJ•cm-2. Gaseous nitrogen formation was obtained

323

by subtracting the final total nitrogen from the initial total nitrogen, and organic nitrogen was

324

determined as the difference between total nitrogen and other nitrogen species including NH4+,

325

NO3-, gaseous nitrogen and NH2Cl.

20



326

327

21


Page 22 of 35

Page 23 of 35


328

329 330

Figure 3 Impact of zetal oxidant dosage on the treatment of 1,4-dioxane by UV/S2O82/NH2Cl.

331

(A) Effect of total oxidant dosage on 1,4-dioxzne degradation rates; (B-D) Steady-state

332

concentration of SO4•-, HO• and Cl2•-, respectively. [1,4-dioxane]=250 µM, [Cl-]=0-4 mM,

333

[benzoic acid]=10 µM, [nitrobenzene]=20 µM, pH=5.8, UV dose=2760 mJ•cm-2. Assumption

334

on Cl2•- contribution in Figure 3A was based on the laser flash photolysis study and branching

335

ratio calculations (Text S5.4) 22



Page 24 of 35

336

337 338

Figure 4 Impact of chloride on 1,4-dioxane treatment by UV/S2O82-/NH2Cl. (A) Impact of

339

chloride on 1,4-dioxane degradation rate and radical contribution; (B) steady-state

340

concentrations of radicals. [S2O82-z=2 mM, [NH2Cl]=0.2 mM, z1,4-dioxane]= 250 µM,

341

[benzoic acid]=10 µM, [nitrobenzene]=20 µM, pH=5.8. UV dose=2760 mJ•cm-2. Assumption

23


Page 25 of 35


342

on Cl2•- contribution in Figure 4A was based on the laser flash photolysis study and branching

343

ratio calculations (Text S5.4). Dashed lines in Figure B represent the modeled results.

344

345 346

Figure 5 Impact of pH on 1,4-dioxane treatability by UV/S2O82-/NH2Cl. (A) Impact of pH on

347

1,4-dioxane degradation rate and radical contribution; (B) steady-state concentration of radicals.

348

[S2O82-]=2 mM, [NH2Cl]=0.2 mM, [Cl-]=0.2 mM, [1,4-dioxane]=250 µM, [benzoic acid]=10

349

µM, [nitrobenzene]=20 µM. UV dose=2760 mJ•cm-2. Assumption on Cl2•- contribution in 24



Page 26 of 35

350

Figure 5A was based on the laser flash photolysis study and branching ratio calculations (Text

351

S5.4).

Dashed

lines

in

Figure

B

25

represent


the

modeled

results.

Page 27 of 35

352


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UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane

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