Chlorine DioxideâPollutant Transformation and Formation of

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Chlorine dioxide – Pollutant transformation and formation of hypochlorous acid as a secondary oxidant Jens Terhalle, Pascal Kaiser, Mischa Jütte, Johanna Buss, Sermin Yasar, Robert Marks, Helmut Uhlmann, Torsten Claus Schmidt, and Holger Volker Lutze Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01099 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Environmental Science & Technology

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Chlorine dioxide – Pollutant transformation and formation of hypochlorous acid as a

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secondary oxidant

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Jens Terhalle1, Pascal Kaiser1, Mischa Jütte1, Johanna Buss1, Sermin Yasar1, Robert Marks1,

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Helmut Uhlmann3, Torsten C. Schmidt1,2,4, Holger V. Lutze1,2,4*

8 9 10 11 12 13 14 15 16

1

University of Duisburg-Essen, Faculty of Chemistry, Instrumental Analytical Chemistry,

Universitätsstraße 5, D-45141 Essen, Germany 2

IWW Water Centre, Moritzstraße 26, D-45476 Mülheim an der Ruhr, Germany

3

a.p.f Aqua System AG, Friedrich-Ebert-Str. 143 b-c, D-42117 Wuppertal, Germany

4

Centre for Water and Environmental Research (ZWU), Universitätsstraße 5, D-45141 Essen,

Germany *

Corresponding author: Tel.: +49 201 183 6779, Fax: +49 201 183 6773, E-mail address:

[email protected]

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ACS Paragon Plus Environment


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Abstract

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Chlorine dioxide (ClO2) has been used as a disinfectant in water treatment for a long time, and

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its use for micropollutant abatement in wastewater has recently been suggested. Surprisingly,

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a mechanistic understanding of ClO2 reactions in (waste)water matrices is largely lacking.

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The present study contributes to this mechanistic understanding by performing a detailed

23

investigation of ClO2 reactions with organic matter using phenol as a surrogate for reactive

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phenolic moieties. A concept for indirectly determining HOCl using 2- and 4-bromophenol

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was developed. The reaction of phenol with ClO2 formed chlorite (62 ± 4% per ClO2

26

consumed) and hypochlorous acid (HOCl) (42 ± 3% per ClO2 consumed). The addition of

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ClO2 to wastewater (5 × 10-5 M ClO2) resulted in 40% atenolol and 47% metoprolol

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transformation. The presence of the selective HOCl scavenger glycine largely diminished

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their transformation, indicating that atenolol and metoprolol were transformed by a fast

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reaction with HOCl (e.g., k (atenolol + HOCl) = 3.5 × 104 M-1 s-1) that formed in ClO2

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reactions with the wastewater matrix. The formation of HOCl may thus increase the number

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of transformable micropollutants in ClO2 applications. However, chlorine related by-products

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may also be formed.

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Key words: Chlorine dioxide, pollutant transformation, wastewater treatment, hypochlorous

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acid, chlorite, ozone, bromate

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TOC Art



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Introduction

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Wastewater is an important source of micropollutants in the environment.1 To minimize the

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emissions of pollutants in wastewater treatment plant effluents, advanced treatment steps are

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currently implemented at many plants, in particular, ozonation and activated carbon

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treatment.2 In contrast to activated carbon treatment, O3 disinfects wastewater. However, one

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drawback of ozonation is the formation of bromate (BrO3-), a carcinogenic compound.

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An alternative to ozone is chlorine dioxide (ClO2), which has been used for a long time in

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drinking water disinfection.3,

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bromide (k (Br- + ClO2) < 0.01 M-1 s-1)5 and results in negligible formation of biodegradable

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dissolved organic carbon (BDOC).6 The applicability of ClO2 for pollutant control in

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wastewater was demonstrated by Hey et al. 1, who showed that more than 50 pharmaceuticals

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could be transformed by ClO2 in wastewater using ClO2 concentrations from 1.25 – 20 mg L-1

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(1.9 × 10-5 – 3.0 × 10-4 M). However, the overall number of pollutants that can be transformed

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by ClO2 is smaller compared to those of O3 for two main reasons. First, ClO2 is more

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selective compared to O3, and second, O3 forms highly reactive hydroxyl radicals, which can

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transform even very recalcitrant pollutants (such as iopamidol, atrazine or chlorobenzene).7-9

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Pollutants that react quickly with ClO2 include those with activated aromatic systems (e.g.,

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sulfamethoxazole (k (sulfamethoxazole + ClO2) = 6.7 × 103 M-1 s-1 at pH = 7.00)4. Other

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important functional groups that are reactive towards ClO2 are activated double bonds

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(indigotrisulfonate: k (indigotrisulfonate + ClO2) > 2.5 × 105 M-1 s-1)10 and activated neutral

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amines (protonated amines do not react with ClO2).6, 11 The reactivity of amines follows the

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sequence: k (tertiary amines) > k (secondary amines) > k (primary amines).11

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As with many oxidants, ClO2 also forms undesired by-products such as chlorite (ClO2-). The

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maximum contaminant levels (MCL) of chlorite in drinking water range from 0.2 mg L-1

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(3 × 10-6 M) (Germany) to 1 mg L-1 (1.5 × 10-5 M) (United States Environmental Protection

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ClO2 is also a strong oxidant that reacts very slowly with


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Agency (US-EPA)). Although there is no regulation of the maximal chlorite concentrations in

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wastewater, one may expect that such a regulation would be associated with drinking water

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regulations, analogous to bromate formation in ozonation.

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For the formation of by-products, the reaction of ClO2 with the wastewater matrix is

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important. In this context, dissolved organic matter (DOM) is a key matrix constituent. DOM

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is a complex, partially polymeric material that contains electron rich phenolic moieties, such

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as hesperetin.12 These structures can be considered to be the main reaction partners of ClO2.

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Wajon et al.

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compounds resulting in formation of hypochlorous acid (HOCl) and ClO2-. The phenolic

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anion reacts with ClO2 to produce a phenoxyl radical and ClO2-. The resulting phenoxyl

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radical undergoes a very fast subsequent reaction with ClO2, resulting in p-benzoquinone and

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HOCl (further details of this mechanism can be found in Text S1 in the supporting

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information (SI)). In addition, other authors postulated the formation of HOCl during the

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reaction of organic compounds with ClO2.14-16

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It is important to know whether and how much HOCl is formed to foster our understanding of

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pollutant transformation and by-product formation in ClO2-based processes. However,

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experimental evidence for the formation of HOCl and a suitable approach for quantifying

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HOCl formation in wastewater or drinking water treatment are still lacking. Furthermore,

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HOCl itself can produce halogenated by-products, such as undesired trihalomethanes (THMs)

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(MCL in drinking water is 0.08 mg L-1)17. However, Hua and Reckhow

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the application of ClO2 barely forms THMs in drinking water purification. Nevertheless, other

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halogenated by-products can be formed.18,

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halogenated products during chlorination. When HOCl is formed in the ClO2 system, the

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same products can principally be formed. 20-22

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postulated a reaction mechanism for the reaction of ClO2 with phenolic

19

18

have reported that

Many studies have reported the formation of



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The present paper addresses the formation of HOCl in the reaction of ClO2 with organic

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matter. A new concept for determining HOCl produced by the reaction of ClO2 with phenol

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was developed. Additionally, the formation and influence of HOCl in ClO2-based

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transformation of micropollutants in wastewater treatment were investigated.

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Material and methods

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Chemicals

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All chemicals and solvents were used as received from suppliers. A complete list of all

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chemicals used can be found in Table S1 of the SI.

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Equipment

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The equipment used is summarized in Table S2 of the SI.

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Generation of ozone and chlorine dioxide stock solutions

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For the preparation of O3 stock solutions, oxygen was enriched with O3 by an ozone generator

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(BTM Messtechnik, Berlin, Germany) (Table S2). Gas was bubbled into ice-cooled ultrapure

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water. After an enrichment phase of approximately 45 minutes, the O3 concentration was

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determined by UV absorption of a 1:3-diluted O3 stock solution at 258 nm,

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εO3 = 2950 M-1 cm-1.23 The O3 concentration in the stock solution ranged between 1.3 × 10-3 -

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1.6 × 10-3 M. Continuous purging of the stock solution with gaseous O3 was required to keep

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the O3 concentration constant.

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ClO2 stock solutions were prepared by mixing of 50 mL of a 0.885 M NaClO2 solution with

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50 mL of a 0.164 M Na2S2O8 solution. Further purification steps were performed according to

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a chlorine-free method described by Gates

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were determined by UV absorption measurements of a 1:30-diluted ClO2 stock solution at

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359 nm, εClO2 = 1200 M-1 cm-1.25 The concentrations of the ClO2 stock solutions used during

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this study were in the range of 8.0 × 10-3 – 1.6 × 10-2 M ClO2.

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(Text S2 in SI). The resulting concentrations


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Detection of hypochlorous acid in the reaction of chlorine dioxide with phenol

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Experimental concept:

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In brief, HOCl formed in the reaction of phenol with ClO2 was scavenged by a surplus of

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bromide (Br-), resulting in hypobromous acid (HOBr) and chloride (Cl-). HOBr, in turn, was

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determined by scavenging with phenol and determining the concentration of the products,

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2- and 4-bromophenol. Detailed explanations can be found in the SI (Text S3). According to

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Fischbacher et al. 26 the optimal pH for bromination of phenol by HOBr is pH 4.00 (Text S8).

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It should be noted that at this pH, HOCl and HOBr are the most abundant chlorine and

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bromine species.27, 28 The other chlorine species (such as Cl2 and OCl-) are present at very

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small fractions at this pH. According to Deborde and von Gunten

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complete at pH 4 in presence of 5 mM chloride (reaction 1) and when the pH is below the pKa

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value of HOCl (see below). Cl2 + H2O

HOCl + Cl- + H+

27

Cl2 hydrolysis is almost

(reaction 1)

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Considering the reaction kinetics, even small fractions of a highly reactive species can still be

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important for a chemical reaction. To that end, one cannot exclude the possibility that free

125

chlorine species other than HOCl (i.e., Cl2 and Cl2O) may also have oxidized bromide.

126

However, the excellent recovery rates of HOCl in the validation experiments (SI Fig. S10 –

127

S12) indicated that this did not affect the determination of HOCl. The same applies to HOBr.

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Validation of the method

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To verify that the concentrations of 2- and 4-bromophenol resembled the HOCl concentration,

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experiments were performed in which HOCl was dosed to the reaction solution. For these

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experiments, the concentration of the stock solution of sodium hypochlorite (NaOCl) was

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determined by measuring its UV-absorption. Therefore, an extinction coefficient of

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362 M-1 cm-1 at 292 nm of the hypochlorite anion (OCl-) was used.29 To ensure that OCl- is



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the predominant species, the pH was adjusted with NaOH to 10.0, which is considerably

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above the pKa value of HOCl (pKa (HOCl) = 7.47).29

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For the validation experiments, three different approaches were used:

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A: The reaction solution of phenol (1.0 × 10-3 M), bromide (1.0 × 10-2 M) and phosphate

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buffer (5.0 × 10-3 M) was prepared and adjusted with H3PO4 and NaOH to pH 4.00 ± 0.05.

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Aliquots of 10 mL of the solution were placed in 20 mL headspace vials. Different volumes

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of the HOCl stock solution were added to this solution (30 – 190 µL) for final concentrations

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of 2.0 × 10-5 – 1.2 × 10-4 M.

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B: A solution of 1.0 × 10-3 M phosphate buffer was prepared and adjusted to pH 4.00 ± 0.05.

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After this solution was placed in 20 mL headspace vials, different HOCl concentrations

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(2.0 × 10-5 – 1.2 × 10-4 M) were added. In the last step, a solution containing phenol and

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bromide was added for a final concentration of 1.0 × 10-3 M phenol and 1.0 × 10-2 M bromide.

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C: Here, the same approach was used as in B, except higher concentrations of phenol

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(6.0 × 10-3 M) and bromide (6.0 × 10-2 M) were used.

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Dosage of chlorine dioxide

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The reaction solution consisted of phenol (1.0 × 10-3 M), bromide (1.0 × 10-2 M) and

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phosphate buffer (5.0 × 10-3 M). Aliquots of 10 mL of the solution were placed in 20 mL

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headspace vials. The solutions in the headspace vials were treated with 6 different dosages of

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ClO2 (29 – 175 µL), resulting in final concentrations from 2.0 × 10-5 – 1.2 × 10-4 M ClO2.

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Samples were analyzed 24 h after the experiment was performed. After that reaction time,

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ClO2 was completely consumed. Experiments were performed in triplicate at each dose.

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Sample measurement:

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The samples were measured by ion chromatography (Text S4 in SI) for the determination of

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chloride and chlorite and with HPLC-UV for the determination of the bromophenols (Table ACS Paragon Plus Environment

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S3 in SI). Calibration of each investigated substance was integrated in each measurement

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sequence.

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Experiments in wastewater

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Based on the results of Wajon et al.

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wastewater organic matter. To investigate the effect of HOCl on the transformation of

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micropollutants during oxidation with ClO2 and for comparison to the ClO2-based process

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with ozonation, four pollutants were added to samples from a municipal wastewater effluent

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(DOC = 8.11 ± 0.12 mg L-1, [NH3] < 0.1 mg L-1, Text S5 in SI) at a concentration of

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1 × 10-6 M (Figure S3 in SI) prior to oxidant addition. Four compounds were chosen: one

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pollutant that reacts quickly with HOCl and slowly with ClO2 (atenolol, metoprolol)7, one

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pollutant that reacts quickly with HOCl and ClO2 (sulfamethoxazole)7 and one pollutant that

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reacts slowly with HOCl and ClO2 (5,6-dimethyl-1-H-benzotriazol) (for second order reaction

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rate constants see Table 1). All of the compounds react quickly with ozone7. The pH of the

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wastewater was adjusted to a value of 7.90 ± 0.05, which is typical for this wastewater.

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Different concentrations of oxidants were added to the wastewater (2.0 × 10-5 – 1.2 × 10-4 M).

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Experiments were performed in the presence and absence of 5 × 10-4 M glycine to assess the

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effect of HOCl, which can be formed in-situ with the application of ClO2. Glycine can

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selectively scavenge HOCl (k = 1 × 105 M-1 s-1)25 since it slowly reacts with ClO2

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(k = 1 × 10-3 M-1 s-1)30. Therefore, the effect of the glycine dose on pollutant transformation

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indicates the presence of HOCl. Samples were measured >24 hours after the experiment was

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performed to provide sufficient time for complete ClO2 consumption.

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To investigate whether oxidants other than ClO2 are involved in pollutant transformation, the

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following approach was used. Using equation 1,31 pollutant abatement was calculated on the

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basis of oxidant exposure (∫[Ox]dt). Exposure was determined by measuring oxidant depletion

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over time and calculating the time based integral. For determination of the oxidant

13

HOCl might also form in the reaction of ClO2 with



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concentration, the indigo method was used for both oxidants following the procedure

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described by Hoigné and Bader 23. The exact approach is described in Text S6 in the SI. In the

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case of the ClO2 experiment, only ClO2 exposure was used, and the calculated pollutant

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transformation was compared with experimental results.

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The case in which the calculated data did not match the measured data indicated that other

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(secondarily formed) oxidants, such as HOCl, were involved. The same approach was applied

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for O3.

= × [ ]

(equation 1)

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c = concentration of the pollutant at time t [M]

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c0 = concentration of the pollutant at time 0 [M]

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k = second order reaction rate constant of the pollutant with the oxidant [M-1 s-1]

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[] = oxidant exposure in this wastewater [M × s]

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The second order reaction rate constants of atenolol and sulfamethoxazole were taken from

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Lee and von Gunten 7. The reaction rate constants of metoprolol and 5,6-dimethyl-1-H-

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benzotriazol (DMBT) and the oxidant exposure of ClO2 and O3 were determined in this study.

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Determination of the second order reaction rate constant of DMBT with chlorine dioxide:

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To determine the reaction rate constant of DMBT with ClO2, the concept of the pseudo-first

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order kinetics was used, a model that is described in “Method I” section in the SI of Dodd et

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al.

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was present at 50 times excess of the ClO2 concentration (2 × 10-5 M). The pH was adjusted

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with H3PO4 and NaOH to 8.00 ± 0.05 and buffered by 5 × 10-3 M phosphate buffer. The

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DMBT solution was added to a quartz cuvette (1 cm path length), and ClO2 was directly

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added to that cuvette. After turning the cuvette upside down twice, the kinetics of ClO2

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degradation was measured by UV absorption measurements at 359 nm.

32

. Thus, ClO2 depletion was determined in the presence of DMBT (1 × 10-3 M), which


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Determination of the second order reaction rate constant of metoprolol with chlorine dioxide:

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It was not possible to use pseudo-first order kinetics because a 50-fold metoprolol excess over

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ClO2 could not be achieved due to the low solubility of metoprolol in water. Therefore,

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competition kinetics was used to determine the reaction rate constant of metoprolol with ClO2

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according to “Method IV” of the SI of Dodd et al. 32. Here, atenolol was used as a competitor.

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The reaction solution consisted of 1 × 10-5 M atenolol, 1 × 10-5 M metoprolol, 1 × 10-3 M

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glycine and 1 × 10-3 M phosphate buffer. The pH was adjusted with H3PO4 and NaOH to

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8.00 ± 0.05, and different concentrations of ClO2 were added to the reaction system

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(5 × 10-6 – 3 × 10-5 M ClO2). The concentrations of atenolol and metoprolol were measured

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with HPLC-UV (Table S3 in SI).

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Sample measurement:

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The samples were measured by ion chromatography (Text S4 in SI) for the determination of

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chloride, chlorite, bromide and bromate. Additionally, to determine the pollutants in

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wastewater, the samples were measured with HPLC-UV (Table S3 in SI). The calibration of

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each investigated substance was integrated into each measurement sequence.

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Results and discussion

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Validation of HOCl detection

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Validation experiments were performed to assure that the concentrations of 2- and 4-

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bromophenol represented the HOCl concentration. Therefore, three different approaches were

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used (see chapter 2.4). A comparison of these three approaches is shown in Figure 1. In

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experiment A, HOCl was dosed into the reaction solution of phenol and Br- at pH 4.00. The

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results of this experiment show that the dosage of HOCl does not match the yields of

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2-bromophenol and 4-bromophenol (Figure 1 and Figure S9 in SI). Only 9 ± 1% of 4-

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bromophenol was formed with respect to the dosage of HOCl (2-bromophenol was below the



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limit of quantification (LOQ)). Indeed, most HOCl formed 2,4,6-tribromophenol (75 ± 2%

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average of all samples and HOCl dosages). The formation of 2,4-dibromo- and

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2,4,6-tribromophenol requires 2 and 3 equivalents of HOCl. Considering this, the sum of all

233

bromophenols formed moderately agreed with the dosage of HOCl (87 ± 8% recovery, on

234

average, of all samples and HOCl dosages).

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Figure 1: Yield of bromophenols per HOCl consumed. A: 60 µM HOCl was added to the solution of phenol (1.0 × 10-3 M) and bromide (1.0 × 10-2 M). B: The solution of phenol (1.0 × 10-3 M) and bromide (1.0 × 10-2 M) was added to the solution of HOCl with a concentration of 60 µM. C: The solution of phenol (6.0 × 10-3 M) and bromide (6.0 × 10-2 M) was added to the solution of HOCl with a concentration of 60 µM (pH = 4.00 ± 0.05).

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The formation of multiple bromophenols can be explained as follows. After addition of HOCl

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to the solution containing phenol and bromide, time is needed for complete mixing of HOCl.

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In this mixing time, the fast reaction between the HOCl of the dosed solution and bromide in

246

the bulk solution can occur at the interface of these two solutions. However, at this interface

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the HOCl concentration is much higher relative to that of the completely mixed solution. The

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spots of high HOCl concentration may result in locally high HOBr concentrations, which may

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in turn have favored the formation of 2,4-di- and 2,4,6-tribromophenol. Tee et al.


33

reported

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that at pH 4.00, mono- (e.g., 2-bromophenol: k (HOBr) = 6.8 × 105 M-1 s-1)33 and di-

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bromophenols (2,4-dibromophenol: k (HOBr) = 7.24 × 105 M-1 s-1)33 were more reactive

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towards HOBr than phenol (k (HOBr) = 5.0 × 102 M-1 s-1)34 by several orders of magnitude

253

because of the decrease in pKa of the phenols with the increasing degree of halogenation (pKa

254

(phenol) = 10;

255

dibromophenol) = 7.79).25, 35

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To prevent excess HOCl in the validation experiment at the interface of the dosed and bulk

257

solutions, the following procedure was used in subsequent experiments (approaches B and C

258

in chapter 2.4). A small volume of a solution containing high concentrations of phenol and

259

bromide was added to 15 mL of HOCl solutions at different concentrations. Using this

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approach, the above described interface effect resulted in a local surplus of bromide and

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phenol, suppressing multiple bromination. With a phenol and bromide concentration of

262

1.0 × 10-3 M and 1.0 × 10-2 M, the surplus was not high enough to completely suppress

263

multiple bromination (Figure 1 and Figure S10 in SI). At a HOCl concentration of 6.0 × 10-

264

5

pKa (2-bromophenol) = 8.43,

pKa (4-bromophenol) = 9.17;

pKa (2,4-

M, 30% of multiple brominated phenols were formed. With increasing phenol and bromide

265

concentrations (approach C), the yield of 2,4,6-tribromophenol was below the LOQ (Figure 1

266

and Figure S11 in SI). Only at the highest HOCl concentration (120 µM) were 2,4,6-

267

tribromophenol (7 ± 1%) and 2,4-dibromophenol (3 ± 0.3%) formed. Below 120 µM HOCl,

268

the ratio of 2-bromophenol and 4-bromophenol was the same as the ratio of 2-bromophenol

269

and 4-bromophenol after ClO2 addition (see below). Additionally, the recovery of HOCl was

270

nearly complete (90 ± 2%). The results show that our method for determination of HOCl via

271

bromophenol formation can result in different products. However, the concentration of

272

bromine attached to the bromophenols caused complete HOCl recovery under all

273

experimental conditions and with all combinations of products formed, which strongly

274

corroborates our hypothesis that HOCl formation can be determined in the reaction of ClO2

275

with phenol. The results also indicate that in mechanistic studies, the experimental approach ACS Paragon Plus Environment


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can have a very strong effect on product formation. Especially in the case of fast reactions, the

277

envisaged surplus of compounds under study over e.g., an oxidant may not be prevalent in the

278

case in which oxidant stock solutions are dosed into reaction mixtures of the parent

279

compounds. These results are similar to the results of Schreiber and Mitch 36. They found the

280

same phenomena in the formation of NDMA during the reaction of chloramine and

281

hypochlorous acid. The concentration and order of reagent addition had an influence on

282

product formation. Shah et al. 37 used 2,6-dichlorophenol for the determination of HOBr. This

283

setup has the advantage that dibromophenol and tribromophenol cannot be formed; only one

284

product can be formed (4-bromo-2,6-dichlorophenol).

285

Detection of hypochlorous acid from reactions of chlorine dioxide with phenol

286

As explained above, the formation of bromophenols indicates the presence of HOCl. Indeed,

287

bromophenols were observed. The yield of the sum of 4- and 2-bromophenol was 42 ± 3%

288

regardless of the ClO2 dose. 4-bromophenol was the main product, at 35 ± 2% (Figure S8 in

289

the SI). Multiple bromination was not observed, which can be explained as follows. Initially,

290

ClO2 reacts with phenol with a slow second order reaction rate constant (k = 49 M-1 s-1, at pH

291

4) to ClO2- and HOCl. This reaction is slow enough to provide sufficient time for mixing.

292

After complete mixing, HOBr is formed in a follow-up reaction from the reaction of HOCl

293

with bromide, and thus, experience a surplus of phenol likewise the validation experiment C.

294

Since the yield of bromophenols represents the yield of HOCl, it can be included in the

295

chlorine mass balance of ClO2 (Figure 2a). Therefore, with 62 ± 4% ClO2- and 42 ± 3%

296

HOCl, the chlorine balance is complete. In the reaction of HOCl with Br-, stoichiometric

297

concentrations of Cl- are formed. Hence, the sum of Cl- and ClO2- must also resemble the

298

concentration of dosed ClO2. This is shown in Figure 2b, corroborating the experimental

299

concept, albeit slightly exceeding the chlorine mass balance. These results are consistent with

300

the postulated reaction mechanism of Wajon et al. 13 (Figure S1 in SI).


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14

301 302 303 304 305 306

Figure 2: a) Yield of chlorite and hypochlorous acid, and b) yield of chlorite and chloride in the reaction of phenol with ClO2 in presence of bromide at pH 4.00 ([phenol] = 1 × 10-3 M, [bromide] = 1 × 10-2 M, [phosphate buffer] = 5 × 10-3 M, different ClO2 dosages, T = 25°C, in ultrapure water). The horizontal lines mark 100% yield, and the error bars are the standard deviation of triplicates.

307 308

Pollutant transformation in ClO2 based wastewater treatment

309

To interpret the transformation of micropollutants upon addition of ClO2 to wastewater, it is

310

necessary to know the associated reaction rate constants. Table 1 compiles the reaction rate

311

constants of the micropollutants, which were investigated in the present study.

312 313

Table 1: Second order rate constants of the compounds under study at pH 8, with the exception of DMBT pH 7*. Compound

k (ClO2)

Ref.

[M-1 s-1]

k (HOCl)

Ref.

[M-1 s-1]

Atenolol (ATL)

1.0

7, 38

3.5 × 104

7, 38

5,6-dimethyl-1-H-benzotriazol

4.5 × 10-1

This study

1.2 × 102 *

Calculated from39

(DMBT) Metoprolol (MET)

1.3

This study

4.5 × 104



Page 16 of 24

15 Sulfamethoxazole (SMX)

7.9 × 103

7

5.7 × 102

7

314 315

The reaction rate constants indicate that atenolol, metoprolol and DMBT react slowly with

316

ClO2 and cannot be readily transformed by ClO2. However, the high reaction rate constant of

317

SMX with ClO2 suggests that this compound is readily transformed by ClO2 which was also

318

shown by Lee and von Gunten 7.

319

Figure 3 shows the experimentally determined and calculated transformation of atenolol in

320

wastewater. With the known oxidant exposure for a ClO2 dose of 5 × 10-5 M at hand (for

321

further details, see Text S7 in the SI), the transformation of atenolol is calculated to be only

322

1%. This contradicts the experimental data, which indicated a transformation of 40%. The

323

higher degree of transformation suggests the formation of a secondary oxidant. As previously

324

stated, it is possible that during the reaction of ClO2 with organic matter, HOCl is formed

325

which can react with atenolol (k = 3.5 × 104 M-1 s-1 (at pH 8))7. Moreover, the product of

326

atenolol transformation by HOCl may reform atenolol upon further reactions, which was

327

observed in the presence of a reductive agent (e.g., thiosulfate)7. The presence of HOCl can be

328

investigated by addition of a selective scavenger that predominantly reacts with HOCl.

329

Glycine is such a scavenger, and further experiments were performed in the presence of

330

5 × 10-4 M glycine. In the presence of glycine, 7% atenolol was transformed after dosage of

331

5 × 10-5 M ClO2, which moderately matched the calculated transformation based on ClO2

332

exposure.


Page 17 of 24


16

333 334 335 336 337 338 339

Figure 3: Transformation of atenolol by ClO2 in the absence (circles) and presence (triangles) of glycine. The predicted value was calculated with equation 1 and the exposure of ClO2 (star) (dashed line: linear regression forced through x/y = 0/1, [atenolol]0 = 1 × 10-6 M, [glycine] = 5 × 10-4 M, DOC = 8.11 ± 0.12 mg L-1; [NH3] < 0.1 mg L-1; pH = 7.90 ± 0.05, T = 22°C, error bars are the standard deviation of the triplicates, recovery = 99 ± 5%, reaction time > 24 h).

340

From the results shown in Figure 3, the fraction of atenolol transformed by HOCl can be

341

calculated (ClO2 based treatment) (equation 2). Thus, 83% of transformed atenolol can be

342

attributed to reactions with HOCl and 17% of transformed atenolol to reactions with ClO2

343

(Figure S15 in SI). Together, these results strongly suggest that intrinsically formed HOCl is

344

most important for the transformation of atenolol.

345

% transformation by HOCl =

346

Metoprolol showed a similar behavior (Figure S16 in SI); 47% and 9% of metoprolol were

347

transformed in the absence and presence of glycine, respectively, at a dose of 5 × 10-5 M

348

ClO2. In Figure S15 (SI), the relative contribution of HOCl and ClO2 to metoprolol

349

transformation is shown. 81% of the transformed metoprolol can be attributed to reactions

350

with HOCl and 19% of the transformed metoprolol to reactions with ClO2. Thus, HOCl also

351

has a strong influence on the transformation of metoprolol. The similar results of atenolol and

352

metoprolol can be explained by the similar structures of the two compounds, which have the

$%&'()*%+&$,*' ,' &-(.'/. *) 012/,'.$%&'()*%+&$,*' ,' 3%.(.'/. *) 012/,'. $%&'()*%+&$,*' ,' &-(.'/. *) 012/,'.


(equation 2)


Page 18 of 24

17 353

same functional groups. Both compounds probably react with ClO2 and HOCl at their

354

secondary amine. Since activation of nitrogen by methyl groups is rather weak, the reactivity

355

towards ClO2 is small. However, HOCl reacts quickly with nearly all amines. Indeed, even

356

ammonia,

357

(k (NH3 + O3) = 2.2 × 101 M-1 s-1)5, is rapidly converted into chloramines by HOCl. Thus, the

358

formation of HOCl in ClO2-based water treatment may increase the overall number of

359

pollutants that can be transformed in this process. The system becomes more complex in the

360

presence of Br-, which can scavenge HOCl, resulting in HOBr.40 Many organic compounds

361

react with HOBr even faster than with HOCl. Thus, HOBr may become important for

362

pollutant transformation in applications of ClO2 as well.35, 41

363

The transformation of SMX (Figure S17 in SI) was not affected by the presence of glycine,

364

which corroborates that under the present experimental conditions, glycine did not affect the

365

ClO2 concentration and HOCl did not contribute to SMX transformation. These results can be

366

explained by the lower reactivity of SMX with HOCl (k (SMX + HOCl) = 5.7 × 102 M-1 s-1)7

367

compared with ClO2 (k (SMX + ClO2) = 7.9 × 103 M-1 s-1)7.

368

The comparison of wastewater ozonation with the application of ClO2 revealed that SMX was

369

transformed better in the latter process, even though O3 reacted 2 orders of magnitudes faster

370

with

371

7.9 × 103 M-1 s-1)7. At a dose of 3 × 10-5 M, ClO2 transformed 96% and O3 only transformed

372

58% of SMX. This result can be explained by the higher exposure of ClO2 in wastewater

373

(Text S7 in SI), which compensated for the lower reactivity of SMX with ClO2 compared to

374

O3. These results readily agree with the study of Lee and von Gunten

375

transformation.

376

Furthermore, the reaction of DMBT was studied (Figure S18 in SI). Nika et al. 39 reported that

377

DMBT reacted slowly with ClO2 and HOCl. The transformation of DMBT by ClO2 was ~1%

SMX

which

is

slowly

degraded

by

(k (SMX + O3) = 5.7 × 105 M-1 s-1)7

even

than

strong

with


oxidants,

ClO2

such

as

O3,

(k (SMX + ClO2) =

7

on SMX

Page 19 of 24


18 378

in experiments in the presence and absence of glycine. These results confirmed the slow

379

reaction rate constants for the reaction of DMBT with ClO2 and HOCl.

380

The above results strongly suggest that HOCl may be formed during ClO2-based wastewater

381

treatment and, subsequently, may contribute to the transformation of micropollutants that

382

slowly react with ClO2. A further example from the literature is ciprofloxacin (CIP). Hey et

383

al.

384

ClO2 dosage of 1.8 × 10-5 M (1.25 mg L-1) even though CIP reacts slowly with ClO2

385

(k (CIP + ClO2) = 7.9 M-1 s-1)42. The experimentally determined transformation of atenolol in

386

the absence of glycine allows for the calculation of the exposure of HOCl after solving

387

equation 1 for exposure. Then, exposure can be calculated from atenolol transformation (the

388

reaction of ClO2 was neglected). With the exposure of ClO2 and HOCl, the transformation of

389

CIP in the wastewater of the present study can be estimated (using equation 1 again). At a

390

dosage of 1.5 × 10-5 M ClO2, the HOCl exposure in the wastewater of the present study was

391

2.3 × 10-6 M × s (> 24 h after ClO2 dosage). The estimated transformation of CIP in the

392

present wastewater was 82% by HOCl (k (CIP + HOCl) = 7.6 × 105 M-1 s-1)27 and 9% by ClO2

393

(Figure S15 in SI). Hence, CIP would also be readily transformed upon ClO2 dosage in the

394

wastewater in the present study, mainly through its reaction with HOCl. The high

395

transformation degree of CIP reported by Hey et al.

396

played an important, albeit unnoted, role in their experiments.

397

Comparing the oxidative wastewater treatment based on ClO2 with that based on O3 shows

398

that ClO2 is a much more selective oxidant. On one hand, the high selectivity of ClO2 may

399

result in the better removal of certain pollutants (e.g., SMX, see above). On the other hand,

400

this will reduce the overall number of micropollutants that can be transformed by ClO2, as

401

demonstrated in Figure 4, which shows the transformation of atenolol, metoprolol, SMX and

402

DMBT in wastewater treated by ClO2 and ozone. Ozone is capable of transforming all

403

compounds with a similar efficiency, with SMX being somewhat better transformed than the

1

reported a > 90% transformation of CIP in wastewater in a ClO2 application at a small

1

also suggests that HOCl formation



Page 20 of 24

19 404

other compounds. In contrast to ClO2, O3 can react by very different pathways (i.e., electron

405

transfer, 1,2-cycloaddition and oxygen transfer), enabling a fast reaction with several

406

functional groups (cf. reaction kinetics of O3 with the compounds under study Lee and von

407

Gunten7). Furthermore, hydroxyl radicals are formed in the reaction of O3 with organic

408

matter, which can transform a large variety of pollutants.31,

409

transformation, the formation of by-products, such as ClO2- or BrO3-, should be considered.

410

The dose of 3.0 × 10-5 M ClO2 resulted in an exceedance of the US-EPA drinking water

411

standard of ClO2- (1 mg L-1). At this dosage, micropollutants such as SMX can be largely

412

transformed (>90%). Ozonation formed BrO3- to a level above the drinking water standard at

413

a dose of 1.2 × 10-4 M, and a lower ozone dose should be recommended to avoid elevated

414

BrO3- concentrations. A dose of 9.0 × 10-5 M may enable safe use of ozone in this particular

415

wastewater sample, which also results in nearly complete transformation of the pollutants

416

under study (> 90%). However, these pollutants react quickly with ozone 7, and other

417

pollutants may be transformed to a smaller extent.

418

The applicability of ClO2 depends on the relevance of ClO2– and how it would be regulated in

419

wastewater treatment, as well as on the formation of undesired chlorinated by-products. In

420

both aspects, there is currently a lack of information to make a feasibility statement.

421

Furthermore, the presence of HOCl has not been taken into account in process studies using

422

ClO2 as an oxidant to date, which may have caused a bias on reaction- and disinfection

423

kinetics determined in the past and should be considered when using such data. In addition,

424

knowledge of the presence of other oxidative species, such as HOCl or HOBr, next to ClO2

425

might be useful for the detection of new by-products during treatment with ClO2.

426

The present study showed that the reaction of ClO2 with matrix constituents resulted in the

427

formation of other oxidants. In that regard, HOCl and HOBr can be formed which may react

428

with ammonia to another oxidative species, i.e., halamines. The complex interplay of all

429

reactive species in real water matrices requires further research in future studies. ACS Paragon Plus Environment

43, 44

Beside pollutant

Page 21 of 24


20 430

431 432 433 434 435 436 437 438

Figure 4: a) Transformation of micropollutants in wastewater after different ClO2 dosages and formation of chlorite (absence of glycine). b) Transformation of micropollutants in wastewater after different O3 dosages and formation of bromate ([micropollutant]0 = 1 × 10-6 M, DOC = 8.3 ± 1 mg L-1, pH = 7.80 ± 0.05, T = 20 °C, [Br-]0 = 225.5 ± 2.7 µg L-1 [NH3] < 0.1 mg L-1, error bars are the standard deviation of the triplicates; recovery rate = 82 – 102%). Filled squares show the concentration of the formed chlorite (4a) and bromate (4b).

439 440

Acknowledgements

441

We thank the “Entsorgungsgesellschaft Krefeld” for the kind cooperation. The work was

442

performed in frame of a project funded by the Federal Ministry for Economic Affairs and

443

Energy (ZIM aif). We are thankful for their financial support.

444

Supporting Information

445

The supporting information contains additional information regarding the reaction mechanism

446

of phenol with chlorine dioxide. The SI also contains information on the chemicals,

447

equipment, chlorine dioxide production and handling, structure of the organic compounds

448

under study, details of the detection of hypochlorous acid in the reaction of chlorine dioxide

449

with phenol, HPLC- and IC-methods, DOC-Analysis, indigo-methods. Figures for the yield of

450

bromophenols that are mentioned in the control experiment. Additionally, the description of

451

the determination of the oxidant exposure and an additional figure for the transformation of

452

trace pollutants (Metoprolol, SMX, DMBT).

453 ACS Paragon Plus Environment


21 454

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455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501

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