Halogen Radical Oxidants in Natural and Engineered Aquatic Systems

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Critical Review

Halogen radical oxidants in natural and engineered aquatic systems Ke Zhang, and Kimberly M. Parker Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02219 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Environmental Science & Technology

Halogen radical oxidants in natural and engineered aquatic systems.

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By:

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Ke Zhanga and Kimberly M. Parkera*

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Department of Energy, Environmental & Chemical Engineering,

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Washington University in St. Louis, Brauer Hall, 1 Brookings Dr., St Louis, MO 63130

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*[email protected], 1-314-935-3461

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Words: 8,195

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Figures: 3; schemes: 2 (1,800 word-equivalents total)

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Total word count: 9,995

ACS Paragon Plus Environment

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Abstract.

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Photochemical reactions contribute to the transformation of contaminants and biogeochemically

14

important substrates in environmental aquatic systems. Recent research has demonstrated that

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halogen radicals (e.g., Cl•, Br•, Cl2•–, BrCl•–, Br2•–,) impact photochemical processes in sunlit

16

estuarine and coastal waters rich in halides (e.g., chloride, Cl–, and bromide, Br–). In addition,

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halogen radicals participate in contaminant degradation in some engineered processes, including

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chlorine photolysis for drinking water treatment and several radical-based processes for brine

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and wastewater treatment. Halogen radicals react selectively with substrates (with bimolecular

20

rate constants spanning several orders of magnitude) and via several potential chemical

21

mechanisms. Consequently, their role in photochemical processes remains challenging to assess.

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This review presents an integrative analysis of the chemistry of halogen radicals and their

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contribution to aquatic photochemistry in sunlit surface waters and engineered treatment

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systems. We evaluate existing data on the generation, speciation, and reactivity of halogen

25

radicals, as well as experimental and computational approaches used to obtain this data. By

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evaluating existing data and identifying major uncertainties, this review provides a basis to

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assess the impact of halogen radicals on photochemical processes in both saline surface waters

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and engineered treatment systems.

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Introduction.

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Photochemical processes play an important role in sunlit surface waters1 and some water

31

treatment processes.2 Recently, the role of halogen radicals (e.g., X•, X2•–, XY•– where X, Y =

32

chlorine, Cl; bromine, Br; or iodine, I) in photochemistry in surface water and engineered

33

treatment systems has been highlighted in several studies.3-11 In surface waters and treated waters

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rich in halides (e.g., chloride, Cl–; bromide, Br–; iodide, I–), including oceans, estuaries, and


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brines, halogen radicals can occur at concentrations that are orders of magnitude higher than

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other free radicals such as hydroxyl radical (•OH) and drive photochemical reactions.4-6 In waters

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with low halide concentrations, halogen radicals have also been implicated in contaminant

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degradation during treatment by chlorine photolysis.8-11

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Halogen radicals affect substrate transformation rates and mechanisms in environmental

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aquatic systems. In systems dominated by halogen radicals, the rate of substrate transformation

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may be very different from the rate in systems dominated by other reactive species (e.g., •OH).

42

Halogen radicals selectively degrade substrates, focusing oxidation on the most reactive solution

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constituents and thereby altering relative transformation rates.4,5 Consequently, halogen radicals

44

may lead to rapid degradation of some substrates.6,7 However, other substrates react slowly with

45

halogen radicals and will exhibit reduced transformation rates with halogen radicals in

46

comparison to rates of reactions with •OH or other strong oxidants.8,12 In addition to altering

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substrate transformation kinetics, halogen radicals react with some substrates by different

48

mechanisms than other photochemical oxidants and may lead to unique transformation products.

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For example, halogen radicals may drive substrate halogenation.13-16

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The contribution of halogen radicals to photochemical reactions in environmental aquatic

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systems depends on their concentrations and reactivity with substrates (e.g., anthropogenic

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pollutants, biogeochemically important molecules). In this critical review, we analyze the state of

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knowledge on the role of halogen radicals in sunlit surface waters and engineered treatment

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systems. First, we establish the key mechanisms determining the concentrations of halogen

55

radicals and their reactions with substrates. While most reactions are presented as generalized

56

mechanisms, available halide-specific reactions and rates are presented as Supplementary

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Information (SI) (Tables S1-S8). Next, we summarize experimental and computational


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techniques used to investigate halogen radical chemistry and quantitatively examine the

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generation, concentrations and reactions of halogen radicals in natural and engineered systems.

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Finally, we provide guidelines for ongoing research on halogen radicals in aquatic

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photochemistry and identify critical areas for future research.

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Reactions of Halogen Radicals in Environmental Aquatic Systems.

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Generation & speciation of halogen radicals.

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Halogen radicals in environmental samples were first observed in the early 1970s when

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Rao17 and Zafiriou18 performed flash photolysis on natural seawater. Both researchers reported

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the formation of a transient species with an adsorption maximum near 350 nm. Based on

67

comparisons to earlier observations of halogen radicals of the form X2•– – namely Cl2•–, Br2•–, I2•–

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– in single-halide solutions,19 the transient species in seawater was identified as either Cl2•– or

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Br2•–.17,18 In these experiments, X2•– formed by photo-induced electron transfer from the halide,

70

X–, to the solvent (water) to produce the monoatomic halogen radical, X•, (i.e., Cl•, Br•, I•) (Eq.

71

1) followed by the rapid reaction of X• with X– (Eq. 2).20,21

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X– + hν X• + e–

Eq. 1

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X• + X– X2•–

Eq. 2

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Since these first observations of halogen radicals in seawater, halogen radical generation

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by several different mechanisms relevant to environmental aquatic systems has been explored: 1. Photolysis of halogen-containing oxidants (e.g., HOX22) under UV light generates X• and

77

•

OH (Eq. 3) with high quantum yields (Φ254nm = 0.2-0.6 mol/Einstein).23 Due to low

78

concentrations of halogen-containing oxidants, oxidant photolysis has not been widely

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considered in sunlit surface waters. However, in some engineered systems (e.g.,

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UV/chlorine treatment),24 oxidant photolysis is an important source of halogen radicals.


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HOX + hν X• + •OH

Eq. 3

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2. Oxidation of halides by non-halogen radicals (e.g., •OH; sulfate radical, SO4•–) generates

83

halogen radicals through an intermediate (HOX•–, Eq. 4-6)21,25-30 or directly (Eq. 7).31-36

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Oxidation of halides by •OH occurs in both surface water37 and engineered treatment.4,5

85

•

OH + X– HOX•– HO– + X•

Eq. 4

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HOX•– + H+ H2O + X•

Eq. 5

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HOX•– + X– (or Y–) HO– + X2•– (or XY•–)

Eq. 6

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SO4•– + X– SO42– + X•

Eq. 7

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3. Oxidation of halides by excited triplet state organic sensitizers (3SENS*) generates

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halogen radicals (Eq. 8, 9).38,39 Triplet state dissolved organic matter (3DOM*)40 in sunlit

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surface water may directly oxidize halides to generate halogen radicals,6,41,42 a potential

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seawater-specific radical generation pathway.

93

3

SENS* + X– SENS•– + X•

Eq. 8

94

3

SENS* + 2X– (or X– + Y–) SENS•– + X2•– (or XY•–)

Eq. 9

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4. Photolysis of metal-halide complexes (e.g., FeCl2+, FeCl2+) generates X• (Eq. 10, 11)

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under UV light;43,44 however, these pathways have only been investigated at acidic pH

97

values ( 5×10-5.47 As indicated by Eq. 4-6, halogen radicals are present at higher

106

concentrations relative to •OH in more acidic solutions (Figure S1).

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X• + Y– XY•–

Eq. 12

108

X2•– + Y– XY•– + X–

Eq. 13

109

Halogen and non-halogen radicals also react via termination reactions (Eq. 14-18),

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resulting in the formation of non-radical halogen oxidants (e.g., diatomic halogen, X2 (or XY);

111

hypohalous acid, HOX). While these reactions are fast (k = 108-109 M-1 s-1),20,30,33,50 radical

112

concentrations, even in treatment systems,4 are too low for these reactions to be a dominant loss

113

process for halogen radicals. However, even if unimportant for overall halogen radical loss,

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halogen oxidant products from these reactions may contribute to subsequent reactions (e.g.,

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substrate halogenation).49

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X• + X• X2

Eq. 14

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X• + X2•– X2 + X–

Eq. 15

118

X2•– + X2•– X3– + X– (or X2 + 2X–)

Eq. 16

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X• + •OH HOX

Eq. 17

120

X2•– + •OH HOX + X–

Eq. 18

121

In most environmental aquatic systems, the dominant sinks for halogen radicals are

122

reactions with carbonates (k = 105-108 M-1 s-1,14,35,51 Eq. 19-21) or with DOM (k = 101-103 L

123

mgC-1 s-1 (estimated from phenol),41,42

124

carbonate (CO32–) result in the formation of a carbonate radical (CO3•–),35 a weak oxidant.52-54

125

Halogen radicals may extract an electron from DOM to form a carbon-centered radical,

126

DOM•+,18 or add to the DOM structure (X-DOM).16,55,56

Eq. 21-24). Reactions with bicarbonate (HCO3–) or


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X• + CO32– (or HCO3–) X– + CO3•– (+ H+)

Eq. 19

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X2•– + CO32– (or HCO3–) 2X– + CO3•– (+ H+)

Eq. 20

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XY•– + CO32– (or HCO3–) X– + Y– + CO3•– (+ H+)

Eq. 21

130

X• + DOM X– + DOM•+ (or X-DOM)

Eq. 22

131

X2•– + DOM 2X– + DOM•+ (or X– + X-DOM)

Eq. 23

132

XY•– + DOM X– + Y– + DOM•+ (or Y– + X-DOM)

Eq. 24

133

Ultimately, specific chemical and photochemical attributes of sunlit surface waters and

134

engineered treatment systems will determine halogen radical generation rates, loss rate constants,

135

and concentrations, as detailed in later sections of this review.

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Reactivity of halogen radicals towards substrates.

137

The role of halogen radicals in aquatic photochemistry is determined by both their

138

concentrations (Table S9) and their reactivity towards environmentally relevant substrates.

139

Reactivity towards substrates varies greatly among halogen radical species. For example, Cl•,

140

with a reduction potential (E = 2.5 VNHE)57 similar to •OH (E = 2.7 VNHE)57 (Figure 1a), reacts

141

rapidly with many substrates (k = 108-1010 M-1 s-1)58-63 (Figure 1b). Other halogen radicals (E =

142

1.1-2.2 VNHE)57 are selective oxidants and react with substrates across broad range of bimolecular

143

rate constants (0.1 M)108 (with maximum yields ~6-50%).38 In

369

seawater, radical generation may be promoted by mixed-halogen reactions due to (i) more

370

favorable primary interactions of Br– (relative to Cl–) with triplets38 and

371

concentrations that increase radical yields.6,39

(ii) high Cl–

372

At this time, there is no direct evidence for halogen radical generation by 3DOM*.

373

However, estimated 3DOM* reduction potentials (E(3DOM*/DOM•–) = 1.3-1.9 VNHE)40,83,84 fall

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within the range of reduction potentials of 3SENS* shown to oxidize halides (E(3SENS*/SENS•–)

375

= 1.1-2.4 VNHE),38,108 and degradation of substrates in synthetic seawater were consistent with

376

3

377

production of H2O2, resulting from superoxide (O2•–) produced from the reaction of DOM•– and

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oxygen, was enhanced by addition of Cl– and Br– to DOM-containing solutions.109

DOM* as the key intermediate in halogen radical formation.6 In addition, the photochemical


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Because halogen radical generation by 3DOM* oxidation of halides has not been directly

380

observed, our estimate of this rate relies on data for halide interactions with model 3SENS*

381

(Figure 2). The rate constant (kq) for quenching of 3SENS* by halides increases (i) with

382

increasing 3SENS* reduction potential and (ii) with decreasing halogen reduction potential

383

(kq(Cl–)< kq(Br–)0.7 mgC L-1 for DOM to

421

serve as the dominant sink for Br2•–. Therefore, both carbonate and DOM likely contribute to

422

halogen radical scavenging in estuarine and marine waters.

423

Role of halogen radicals in pollutant degradation and biogeochemical cycling.


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A few studies have specifically investigated the role of halogen radicals in the

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transformation of biogeochemically important substrates including diene-containing algal

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toxins6,7 and the organosulfur species dimethylsulfide6,113 in estuarine and marine waters. In

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these studies, the presence of halides (particularly Br–) increased the rate of substrate

428

phototransformation sensitized by either DOM6,7 or nitrate113 (an •OH source). Using additional

429

quenching experiments and computational modeling, the halide-specific increase in substrate

430

phototransformation rates was shown to be consistent with halogen radical-mediated

431

degradation.6,7

432

Additional studies have investigated the photodegradation of anthropogenic pollutants

433

(e.g., pharmaceuticals, pesticides) and biogeochemically important substrates in authentic

434

estuarine and seawater samples76,114-125 or in halide addition experiments.124,126-130 Among

435

substrates that photodegraded faster in seawater or in the presence of halides, many contained

436

functional groups that would serve as targets for halogen radicals, included olefinic, aromatic,

437

and organosulfur moieties (Figure 1b).6,7,125,126,130 However, in many studies, several variables

438

(e.g., ionic strength, pH, DOM) differed between seawater and control samples (or, in other

439

cases, only Cl– was considered), such that the role of halogen radicals cannot be specifically

440

determined.

441

In addition to the phototransformation of specific substrates, halogen radicals may

442

accelerate rates of DOM photobleaching3 due to the reaction of halogen radicals with electron-

443

rich chromophoric DOM moieties (e.g., aromatic groups). Accelerated DOM photobleaching

444

impacts the global carbon cycle,131 as well as the DOM-sensitized photodegradation of

445

contaminants.132 When only Cl– was added to a DOM-containing solution, no consistent increase


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446

in photobleaching rates was observed,133 suggesting the important role of Br– and mixed-halogen

447

reactions in halogen radical generation and reactivity towards DOM.

448

Beyond affecting rates of photochemical reactions, halogen radicals may also promote

449

halogenation reactions. Calza and colleagues134-138 investigated the halogenation of phenol in

450

artificial and natural seawater under simulated sunlight, while Sankoda and colleagues reported

451

pyrene halogenation in seawater under UV light.121 In addition to halogenating specific

452

substrates, halogen radicals have been proposed to directly or indirectly halogenate DOM.16,55,56

453

Halogenation may result from halogen radical addition to the DOM structure (e.g., to an

454

aromatic moiety13). To meet the reported rate of bromination,16 we estimate that only ~0.1% of

455

reactions between halogen radicals and DOM would be required to yield halogenated products

456

(SI Section 2.2). However, because mechanistic information on halogen radical reactions with

457

substrates is limited, we cannot determine if this yield is reasonable. Alternatively, halogenation

458

may result from non-radical halogen oxidants.139 Because these oxidants are produced through

459

radical termination reactions (Eq. 14-18), high concentrations of radicals (e.g., due to enhanced

460

photo-Fenton processes55,56) may increase the formation rates and concentrations of non-radical

461

halogen oxidants relative to halogen radicals and thereby promote DOM halogenation by these

462

non-radical oxidants. In addition to bulk DOM halogenation, radical and non-radical halogen

463

oxidants may also contribute to the flux of gas phase halogen oxidants140-143 and volatile

464

halomethanes144-146 to the atmosphere.

465

Halogen Radicals in Engineered Aquatic Systems.

466

Halogen radical generation.

467

Halogen radicals have been implicated in radical-based water treatment initiated by the

468

photolysis of chlorine-containing oxidants (e.g., chlorine (HOCl/OCl–), chloramine (NH2Cl)) or,


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in brines and other high-halide solutions, by the photolysis of non-chlorine oxidants (e.g., H2O2,

470

persulfate (S2O82–)). Relative to sunlit surface water, the generation of radicals in engineered

471

treatment systems is well-defined based on oxidant photolysis rates and solution chemistry.

472

The photolysis of HOCl (Eq. 25), OCl– (Eq. 26, 27) or NH2Cl (Eq. 28) generates Cl•,22,45,147-152

473

along with non-halogen radical species: •OH, O•–, excited singlet oxygen atom (O(1D)), or amino

474

radical (•NH2).24 Both O•– and O(1D) react with water to form •OH or H2O2 at environmentally

475

relevant pH values.153-155 The reactivity of •NH2 is poorly understood; •NH2 may react with

476

ground-state dissolved oxygen (O2) to form the aminoperoxy radical (NH2O2•) (k = 3 × 108 M-1 s-

477

1 156

).

478

HOCl + hν •OH + Cl•

Eq. 25

479

OCl- + hν O•- + Cl•

Eq. 26

480

OCl- + hν O(1D) + Cl-

Eq. 27

481

NH2Cl + hν •NH2 + Cl•

Eq. 28

482

Once generated, Cl• reacts with Cl– to form Cl2•– (k = 2.11010 M-1 s-1)28 even at low Cl-

483

concentrations (~0.1 mM).8,104 Cl• (as well as Cl2•–) reacts with water or hydroxide to generate

484

•

485

•

486

VNHE).35 ClO• has been proposed to account for some observed substrate degradation in

487

UV/HOCl treatment.23,157,158

OH via the intermediate HOCl•–, particularly in circumneutral to alkaline waters.28,32,45 Cl• and OH may also react with HOCl/OCl– to generate the oxychlorine radical (ClO•)147,148 (E=1.4

488

During radical-based treatment of waters with high halide concentrations (>30 mM Cl–,

489

>0.02 mM Br–; e.g., brackish industrial wastewaters, wastewater concentrates from reuse

490

facilities, brines, saltwater swimming pools),4,5,8,12 halogen radicals are produced by halide

491

scavenging of non-halogen radical species (e.g., •OH (Eq. 4-6), SO4•– (Eq. 7)). As in sunlit


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492

surface waters (Scheme 2a,b), Br– scavenges •OH to produce halogen radicals to a much greater

493

extent than Cl–, despite much higher concentrations of Cl– relative to Br–.4,5,8 In contrast, both Cl–

494

and Br– directly scavenge SO4•– to form halogen radicals (Eq. 7).31-36 Due Cl– scavenging of

495

SO4•–, halogen radical generation is much more significant in UV/S2O82– treatment relative to

496

UV/H2O2 treatment, particularly in the absence of Br–.5

497

In addition to affecting radical speciation, halides at elevated concentrations may also

498

alter the speciation of the oxidant undergoing photolysis. For example, the addition of Br– during

499

UV/HOCl treatment will shift the oxidant speciation from HOCl to HOBr, with several potential

500

implications for radical generation and reactivity.8,10 In comparison to HOCl, HOBr undergoes

501

photolysis at different rates and produces alternative products (i.e., Br• instead of Cl•).

502

Furthermore, HOBr and OBr– will exhibit different reactivity than HOCl and OCl– with radical

503

species (e.g., forming BrO• instead of ClO•)159 and substrates96-98 (altering the rates and

504

mechanisms of non-radical oxidation reactions co-occurring in treatment systems).

505

In engineered treatment using oxidant photolysis to generate radicals, the radical

506

generation rate (R) in treatment processes is determined by the oxidant molar extinction

507

coefficient (ελ) and quantum yield of radical generation Φλ (each dependent on wavelength,

508

λ),22,151,160,161 as well as irradiation intensity. Importantly, radical generation is reflected by Φλ of

509

radical generation (rather than the Φλ of oxidant photolysis) determined at the corresponding

510

wavelength.24 In addition, a recent study23 determined that several reported Φλ may be

511

overestimated by up to a factor of 2 due to the inclusion of radical chain propagation

512

mechanisms in earlier measurements. We recommend that estimates of R in treatment systems

513

employ corrected Φλ of radical generation (Φλ = 0.62, 0.55, 0.20, and 0.5 mol/Einstein for HOCl,

514

OCl–, NH2Cl, and H2O2, respectively)23 and incorporate chain propagation mechanisms using


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515

validated kinetic modeling. Once determined, R values can be compared among different oxidant

516

systems104 and employed in computational kinetic modeling of radical concentrations.5,23

517

Halogen radical scavenging and concentrations.

518

In addition to generation rates, radical concentrations in water treatment are influenced

519

by the concentration of radical scavengers, which vary greatly among waters to be treated. In

520

water treatment, DOM162 and bicarbonate are likely dominant sinks for radicals (e.g., Eq. 19-24);

521

however, in some systems (e.g., experiments conducted in laboratory-grade water), the oxidant

522

itself may serve as an important radical scavenger. Beyond affecting absolute radical

523

concentrations, the dominant scavengers will also affect relative radical concentrations by

524

scavenging certain radicals to a greater extent than others.

525

Several

studies

have

employed

computational

kinetic

modeling

to

estimate

526

concentrations of halogen and non-halogen radicals in radical-based water treatment (Figure 3).

527

We note that additional radical species not reported in these prior studies may also occur and

528

contribute to substrate transformation. Among studies that have estimated radicals generated by

529

UV/HOCl,8,23,104 absolute radical concentrations are variable (e.g., modeled •OH concentrations

530

varied by a factor of ~30); however, the relative concentrations are overall in good agreement

531

(Figure 3a). Similar radical concentrations are estimated to be generated by UV/NH2Cl;23

532

notably, concentrations of radicals produced by UV/NH2Cl are relatively pH-independent

533

compared to radicals produced by UV/HOCl. Typically, at circumneutral pH, the modeled Cl•

534

concentration is ~10% of the •OH concentration. Modeled Cl2•– concentrations are highly

535

dependent on the inclusion of Cl– even at low concentrations ([Cl–] ~ 0.1 mM) and can occur at

536

concentrations similar to or exceeding •OH.8,104 Similarly, one study104 also reported low Br–

537

concentrations (0.2 µM) resulted in modeled concentrations of Br-containing radicals at or


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538

exceeding concentrations similar to •OH and Cl-only radical species, indicating that the inclusion

539

of halides even at very low concentrations may influence radical speciation in UV/HOCl

540

treatment due to the rapid reaction of Cl• with these constituents.

541

Currently, two studies8,23 have reported modeled ClO• concentrations to be several orders

542

of magnitude higher than other halogen and non-halogen radicals (Figure 1a). This result may be

543

due to slow (or potentially incomplete) scavenging reactions considered in the model. Unlike

544

other halogen radicals, ClO• has been suggested to be scavenged only slowly by bicarbonate163

545

but relatively quickly by DOM.9 Additional validation is needed to verify ClO• concentration and

546

its proposed contribution to substrate degradation8,9,23,157 in UV/HOCl treatment.

547

Multiple studies have also modeled halogen radical concentrations in UV/H2O2 treatment

548

under high halide concentrations relevant to brines and some wastewaters (Figure 3b).4,5,164 As

549

halide concentrations increase to seawater-relevant levels, •OH concentration drops by ~2 orders

550

of magnitude. Cl• is present at very low concentrations relative to other radicals. In these

551

systems, radicals such as Br2•– and BrCl•– (as well as CO3•–) dominate, while Cl2•– and Br• are

552

present at concentrations similar to •OH. Modeling and experimental results indicates that Br–

553

(rather than Cl–) is largely responsible for •OH scavenging and speciation of halogen radicals

554

during UV/H2O2 treatment.4,5,165

555

A more limited number of studies have investigated the impact of halides on radical

556

concentrations during UV/S2O82– treatment.5,12,165,166 In general, relative to the UV/H2O2 system,

557

halogen radical concentrations are higher due, in part, to the direct scavenging of SO4•– by Cl–

558

and are largely pH-independent.5,165

559

Role of halogen radicals in radical-based treatment.


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560

Assessing contribution of halogen radicals to substrate transformation (as well as

561

formation of disinfection by-products167) during UV/HOCl or UV/NH2Cl treatment at low halide

562

concentrations (e.g., in simulated drinking water) is challenging due to the co-occurrence of

563

other radicals (e.g., •OH), as well as non-radical-mediated transformation processes (e.g., direct

564

UV photolysis, HOCl oxidation). Some studies have estimated potential halogen radical

565

contributions by calculating the difference between total observed rates constants and measured

566

rate constants due to reaction with UV, non-radical oxidants, and •OH.9,157 Using this method,

567

chlorine radicals (i.e., Cl•, Cl2•–, ClO•) were proposed to contribute

Halogen Radical Oxidants in Natural and Engineered Aquatic Systems

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