Oxidation of Trace Organic Contaminants (TrOCs) in Wastewater

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Oxidation of trace organic contaminants (TrOCs) in wastewater effluent with different ozone-based AOPs: comparison of ozone exposure and #OH formation Ze Liu, Michael Chys, Yongyuan Yang, Kristof Demeestere, and Stijn W.H. Van Hulle Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00293 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Industrial & Engineering Chemistry Research

Oxidation of trace organic contaminants (TrOCs) in wastewater

effluent

with

different

ozone-based

AOPs:

comparison of ozone exposure and OH formation Ze Liu1*, Michael Chys1, Yongyuan Yang1, Kristof Demeestere2, Stijn Van Hulle1

[email protected]

1

LIWET, Department of Green Chemistry and Technology, Ghent University, Campus

Kortrijk, Graaf Karel De Goedelaan 5, B-8500 Kortrijk, Belgium 2

Research Group EnVOC, Department of Green Chemistry and Technology, Ghent

University, Coupure Links 653, B-9000 Ghent, Belgium

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Ozone-based advanced oxidation processes (AOPs), recognized as effective methods for tertiary water treatment in view of trace organic contaminants (TrOCs) removal, were investigated in this study. By assessing and comparing different ozonebased AOPs for the TrOCs degradation in secondary wastewater effluent, criteria for appropriate AOP application and oxidant dose are provided. In this study, the value of the minimum ozone demand for formatted hydroxyl radical (OH) to react with target contaminants was proposed and determined. The ozone exposure and OH exposure were systemically investigated and compared in four types of different ozone-based AOPs (i.e. O3 only, O3/H2O2, O3/UV and O3/H2O2/UV). The significant increase of OH exposure was achieved by means of adding H2O2 and/or UV, which results in the increase of the eliminant efficiency of TrOCs. In particular, ozone-resistance compounds (e.g. atrazine and alachlor) were removed more efficient by combination O3 with H2O2 and UV than O3 only, O3/H2O2 and O3/UV at the equal ozone dose. Such as, the removal of atrazine and alachlor tripled after 2 minutes of treatment in the effluent by combining O3 with H2O2 and UV. Among the four ozone-based AOPs, the highest ozone exposure (3.27 mg min/L) was achieved by applying an O3:DOC of 0.9 O3 only. High ozone exposure is beneficial to enhance the abatement efficiency of high ozone-selective compounds (e.g. pentachlorophenol).

Keywords: ozone; hydroxyl radicals; ozone exposure; TrOCs; advanced oxidation process; secondary wastewater effluent


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1. Introduction The occurrence of trace organic contaminants (TrOCs) in the aquatic environment is a known environmental issue which has been of growing concern in recent years.1 TrOCs, including pharmaceuticals, personal care products, hormones and persistent organic components, have potential impact on the aquatic environment and human health even at very low concentrations (ng/L and μg/L). A well-known example is the potential feminization of male fish.2 TrOCs are typically difficult to remove during conventional municipal (biological) wastewater treatment and are therefore largely present in plant effluents.3 Effluents from these municipal wastewater recovery facilities (WRRFs) have been confirmed as a major route for TrOCs to be discharged into the environment.4,5 Therefore, it is necessary to pay attention to the degradation of TrOCs by for example adding a tertiary treatment step to reduce TrOCs discharge. In recent years, advanced treatment methods such as activated carbon or membrane filtration have received more attention as some types of TrOCs can be removed effectively during these processes,6 but high operation cost and difficult material regeneration limit their implementation. UV-based AOPs have also been reported widely for TrOCs removal, however, UV-based AOPs are not widely applied because of low UV-transmittance and high scavenging rate of wastewater effluents.7–10 Besides, plasma-based AOPs attract attention and are found to remove environmental pollutants effectively11, but require high operational costs, in particular for treatment of industrial wastewater. Amongst these advanced treatment methods, ozone-based advanced oxidation processes (AOPs) are considered as very promising techniques as many studies have demonstrated their high efficiency on TrOCs removal.12 During ozonation, TrOCs are transformed via direct reaction with ozone or with hydroxyl radicals (OH, a second oxidant formed from a chain reaction mechanism during ozone exposure). Ozone, as a selective oxidant, reacts preferentially with ERMs (electron-rich organic moieties), such as activated aromatic compounds and deprotonated amines. In contrast, OH is a non-selective oxidant and reacts with almost any type of TrOCs.13



The selectivity of ozone limits its application for the abatement of TrOCs which do not contain ERMs, and the non-selective OH is also consumed easily by matrix compounds, resulting in only a part of the OH reaching the targeted TrOCs. This results in a reduction of the TrOCs removal efficiency during AOP application. In addition, ozone is also consumed by matrix compounds containing ERMs and decomposes naturally as well. As such, TrOCs degradation efficiency with ozone-based AOPs is typically subject to the reactivity of these oxidants to target TrOCs and matrix compounds present in the water. Therefore, it is crucial to have the information on the matrix scavenging kinetics and the oxidant kinetics to evaluate the removal of targeted TrOCs and the required oxidant dose. Wert et al. investigated the impact of water quality on ozone exposure during application of O3/H2O2,14 and Rosenfeldt et al. studied the efficiency of OH formation during O3/H2O2 and UV/H2O2 in pure water.15 The results of these studies indicate that OH

and ozone exposure are affected by both the ozone dose and the characteristics of

EfOM (effluent organic matter). However, research on comparing different ozonebased AOPs for ozone exposure characteristics and potential OH formation is still lacking. This kind of information can offer a support for design, modeling and optimization of cost, energy and removal performance during pilot-scale and full-scale application. The aim of this study was to structurally and critically compare several ozonebased AOPs, i.e. O3 only, O3/H2O2, O3/UV and O3/H2O2/UV, with respect to their ozone exposure and OH formation during the treatment of a selection of TrOCs in secondary wastewater effluent. This comparison is meaningful to provide criteria for applying an appropriate ozone-based AOPs and for defining the required ozone dose for TrOCs degradation. To address this goal, bench-scale experiments for these ozonebased AOPs were performed, focusing on six selected (environmental-related and with a wide variety of ozone affinity) TrOCs in wastewater effluents. The kinetics of oxidants scavenging in a wastewater effluent were determined as well.


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2. Experimental methods and materials 2.1 Water sampling and quality Wastewater effluent samples were collected from the WRRF in Harelbeke, Belgium (operated by Aquafin NV).16 All samples were used without any dilution, additional filtration or any other disinfection process prior to the ozone-based AOPs experiments. Samples were refrigerated at 4°C (in plastic barrels) before further usage. Basic information of the wastewater quality is provided in Table 1. The second-order reaction rate constants with oxidants for the OH scavengers in the wastewater effluent are summarized in Table S1. Table 1. Water quality characteristics of the wastewater effluent (‘... ± ...’ indicates the average and standard deviation on triplicate measurements).

Alkalinity Ammonium Nitrate Nitrite COD UVA254 DOCeqa pH a DOC

Units mM mg/L NH4+-N mg/L NO3--N mg/L NO2--N mg O2/L 1/m mg C/L

Effluent 2.3 ± 0.3 0.15 ± 0.03 4.56 ± 0.03 0.209 ± 0.001 25.8 ± 0.6 20.4 ± 0.2 10.3 ± 0.2 7.3 ± 0.1

value based on the relationship with UVA254 and previous experiences with effluent water

from the same WRRF.17

2.2 Chemicals All chemicals were of analytical grade (purity of 98% or above) and used without any further purification. Hydrogen peroxide (H2O2) used in the experiments had a concentration of 30 wt%. H2O2 and six (environmentally-relevant) micropollutants, including atrazine (ATZ), alachlor (ALA), carbamazepine (CBZ), bisphenol-A (BPA), 17α-ethinylestradiol (EE2) and pentachlorophenol (PCP), were purchased at Sigma-



Aldrich (Belgium). Information of application and physical-chemical properties of these six micropollutants are provided in Table S2-S3. All stock solutions were prepared in demineralized water. TrOCs stock solutions were prepared by dissolving 6 selected TrOCs to 10 mg/L. GC-MS reagent grade dichloromethane (CH2Cl2), with a purity of over 99.8%, was purchased at Carl Roth (www.carlroth.com). NaOH and sodium phosphate buffer18 were used for pH adjustment of reaction solutions.

2.3 Experimental setup Ozonation tests were performed with freshly prepared ozone stock solution (± 70 mg O3 L-1). The ozone stock solution was added to the spiked effluent to obtain a varying ozone dose between 0 and 9 mg/L (O3:DOC mass ratios of 0, 0.1, 0.3, 0.5, 0.7, and 0.9). H2O2 (0.25, 0.5, 1.0, 1.5 and 2.0 mM) was added to the reactors prior to the addition of ozone in the O3/H2O2 and O3/H2O2/UV experiments. For the UV based (O3/UV and O3/H2O2/UV) experiments, a circulating reactor system, consist of an ultraviolet (UV) light at 254 nm, a peristaltic pump (Master Flex model 7518-00, ColeParmer Instrument) and an ozone reactor, was used. More detail on ozone stock solution preparation and the setup of ozone-based and UV-based experiments are shown in Text S1-S2. Samples were taken for the determination of residual dissolved ozone, OH exposure, and TrOCs residual concentrations. During experiments, samples for OH exposure and TrOCs residual concentrations measurement were collected and stored at 4°C in the fridge until they were measured the following day. OH exposure was measured by means of the degradation of pCBA during the oxidation processes.17 Samples for TrOCs residual concentrations measurement were analysed after addition of Na2SO3 (20 mM) to quench residual ozone19and tert-butanol (15 mM) to quench the residual OH radicals.20


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2.4 Analytical methods Water quality was determined according to standard methods.21 Nitrite (NO2−-N), nitrate (NO3−-N), ammonium (NH4+-N) and COD were determined using Hach-Lange cuvettes and a DR2800 spectrophotometer (Hach, Belgium). A Metrohm 600 pH-meter (Metrohm, Belgium) was used to determine the pH of the original solution before treatment and the reaction solution during experiments. A Shimadzu UV-1601 spectrophotometer with 1 cm quartz cuvettes was used to obtain UV−visible (UV−vis) absorption spectra analysis between 200 and 800 nm with 0.5 nm increments. Ozone concentrations in the stock solution were determined by the indigo method according to Bader and Hoigné, et al22. Concentrations of the hydroxyl radicals probe compound p-chlorobenzoic acid (pCBA) were determined by HPLC-DAD (Agilent 1100 series, USA)15,23. All concentrations of TrOCs in the solutions were determined by GC-MS analysis after extraction. Details are given in Text S3-S4. t-BuOH was also determined by GC-MS according to Garoma and Gurol24.

3. Results and discussion 3.1 Comparison of the ozone exposure in wastewater effluent during different ozonebased AOPs During all ozone-based AOPs experiments, dissolved ozone was completely decomposed within 5 min (Fig.1 and Fig.S1). The research from Wert et al 14 showed that ozone decomposition follows the apparent first-order kinetics after the initial instantaneous ozone demand (IOD) phase, which is in agreement with this study. In this study, ozone exposure was calculated by integrating the residual concentration of ozone dissolved in solutions over time (O3 CT (∫[𝑂3]𝑑𝑡)). The first-order rate constants (𝑘′) and O3 CT are shown in Table 2. Based on the results, O3 CT was not able to be measured in the wastewater effluent when a O3:DOC ratio of 0.1 was applied for all ozone-based AOPs or when a O3:DOC ratio of 0.3 was applied for O3/H2O2, O3/UV



and O3/H2O2/UV. This indicated that ozone decomposition finished during the IOD phase when the ozone dose was not sufficient. O3 CT was measured in all ozone-based AOPs when a O3:DOC radio of 0.9 was applied. 3 O₃/DOC=0.3

2.5 DO₃ (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O₃/DOC=0.5 O₃/DOC=0.7

2

O₃/DOC=0.9

1.5 1

0.5 0 0

1

2

Time3(min)

4

5

6

Fig.1 Residual DO3 (dissolved ozone) concentrations over time during experiments with only O3 added as oxidant.

Ozone lifetime was increased from 1 min to 5 min with the ozone dose increasing from 3 mg/L to 9 mg/L during experiments with O3 only (Fig.1). This confirmed that ozone lifetime in the wastewater effluent changed when the ozone dose was varied. As ozone decomposition kinetics and O3 CT are mainly depended on ozone dose, the rate of ozone decomposition decreased and the O3 CT increased (Fig.1 and Table 2) when an increasing ozone dose was applied to the wastewater effluent. Table 2. Summary of dosage of ozone and H2O2, ozone exposure and first-order rate constants of ozone decomposition for ozone-based AOPs in a wastewater effluent. O3:DOC

Ozone only O3 dose mg/L

0.1

1.1

O3/H2O2 k’

O3 CT

1/min

mg min/L

-

-

H2O2 dose mM 0.25


O3 dose

k’

O3 CT

mg/L

1/min

mg min/L

1.2

-

-

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0.3

3.0

2.92

0.02

3.3

-

-

0.5

5.0

2.35

0.31

5.0

3.9

0.11

0.7

7.0

1.12

1.42

7.2

1.47

0.6

0.9

9.0

0.84

3.27

9.0

0.82

1.25

O3/H2O2 H2O2 dose

O3/H2O2

O3 dose

k’

O3 CT

H2O2 dose

O3 dose

k’

O3 CT

mg/L

1/min

mg min/L

mM

mg/L

1/min

mg min/L

1.3

0

-

1.0

1.2

0

-

3.2

0

-

3.2

0

-

5.0

5.54

0.03

5.1

0

-

7.0

2.27

0.19

7.0

5.29

0.06

9.0

1.73

0.35

9.0

4.74

0.26

mM 0.5

O3/UV

O3/H2O2/UV

O3 dose

k’

O3 CT

H2O2 dose

O3 dose

k’

O3 CT

mg/L

1/min

mg min/L

mM

mg/L

1/min

mg min/L

0.1

1.0

0

-

0.25

9.0

2.9

0.02

0.3

3.0

0

-

0.5

9.0

4.18

0.07

0.5

5.1

4.0

0.04

1.0

9.0

5.87

0.14

0.7

7.0

1.72

0.51

1.5

9.0

0

-

0.9

9.0

0.95

1.73

2.0

9.0

0

-

As shown in Fig.S1, ozone was decomposed at a lower rate when applying O3 only in the wastewater effluent, compared to the other three ozone-based AOPs at an equal



ozone dose. No residual ozone was measured during application of O3/H2O2/UV in the wastewater effluent after 1 min, and ozone was decomposed the fastest during application of O3/H2O2/UV among all applied AOPs. Based on these results, it is demonstrated that applying methods that combine O3 with UV and/or H2O2 improves the ozone decomposition kinetics and leads to a shorter ozone lifetime. As shown in Fig.2, the ozone decomposition efficiency sharply increased after adding H2O2 and/or UV irradiation at the first minute compared to applying ozone alone. As a result, the value of ozone exposure decreased when UV and H2O2 were introduced in ozone-based AOP treatment.

3.5

ozone only ozonation followed by adding H₂O₂

3

DO₃ (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ozonation followed by adding UV irradiation ozonation followed by adding H₂O₂ and UV irradiation

2.5 2 1.5 1 0.5 0 0

1

2

Time3(min)

4

5

6

Fig.2 Ozone decomposition (initial ozone dose=9mg/L, O3:DOC=0.9) for ozone alone and ozone followed by adding 0.5mM H2O2, UV irradiation, and 0.5mM H2O2 and UV irradiation, respectively, 1 min after O3 addition.

3.2 Comparison of OH formation in secondary wastewater effluent during different ozone-based AOPs pCBA, as a probe compound, was used to determine indirectly OH exposure. The results of pCBA decomposition were shown in Fig.3. There was no obvious pCBA decrease during treatment with 1.0 mg/L (O3:DOC=0.3). It can be explained that OH


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is completely consumed in the instantaneous O3 demand (IOD) phase (similar to the results of Wert et al.14), resulting in an unchanged concentration of pCBA. In contrast, more than 30% pCBA was decomposed after 5 min when approximately 3.0 mg/L (O3:DOC=0.3) was applied as initial ozone dose. For experiments with adding more than 3.0 mg/L ozone, 90% pCBA was degraded (Fig.3) and its elimination followed the first-order kinetics (shown in Fig.S2). In this study, an O3:DOC of 0.3 seems to be the minimum ozone demand for enhancing target contaminants elimination. Only when the ozone dose is more than the minimum ozone remand, OH formatted from ozone decomposition could contribute to TrOC degradation. Thus, the value of the minimum ozone demand could be developed as a factor to evaluate and predict the ozone requirement in pilot- or full-scale TrOCs treatment. 1

O₃:DOC=0.1 O₃:DOC=0.3

0.8

O₃:DOC=0.5

[pCBA]/[pCBA]₀

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O₃:DOC=0.7

0.6

O₃:DOC=0.9

0.4 0.2 0 0

1

2

Time3(min)

4

5

6

Fig.3 The degradation of pCBA during experiments applying O3 only at different ozone doses (initial pCBA concentration 0.5 μg/L).

pCBA residual concentrations obviously decreased at higher ozone doses for all four ozone-based AOPs, indicating that the ozone dose is a critical factor influencing OH

exposure (Fig. 3). As such, the pCBA degradation rate was increased from 59% to

more than 99% after 5 min when the ozone dose was increased from 1 mg/L to 7 mg/L during O3/UV treatment, which is in in agreement with Katsoyiannis et al.25 Even



though the final concentrations of pCBA after treatment at the equal ozone dose were almost same for all ozone-based experiments (results shown in Fig.4 and Fig.S3), the reaction time to research 90% pCBA removal was different greatly. For experiments applying O3/H2O2, O3/UV and O3/H2O2/UV, it was only less than 3 min that the pCBA removal researched more than 90%, which was shorter than that for experiments applying only O3. As shown in Fig.4, pCBA degradation completed in 1 min for O3/H2O2 and O3/UV, and in less than 1 min for O3/H2O2/UV when approximately 0.3 mg/L ozone (O3:DOC=0.3) was added to the effluent. It demonstrates that the efficiency of OH formation is enhanced by combining O3 with UV or/and H2O2, and this combination can be used to achieve a high OH exposure even at a lower ozone dose (e.g. a O3:DOC radio of 0.3). Based on the results shown in Fig.4 and Fig.S3, O3/H2O2/UV is the method with the most effective OH exposure among the investigated ozone-based AOPs. In addition, the results of pCBA decomposition were no obvious change during O3/H2O2 experiments with 0.1 mg/L ozone dose. It shows that adding H2O2 does not contribute to increase OH formation when the ozone dose is lower than a minimum ozone demand, and additionally applying H2O2 is not able to get an increase in OH formation during ozone-based AOPs. 1

O₃ only O₃/H₂O₂

0.8

pCBA]/[pCBA]₀

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O₃/UV O₃/H₂O₂/UV

0.6 0.4 0.2 0 0

0.5

1

1.5

2

Time (min)


2.5

3

3.5

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Fig.4 The degradation of pCBA (0.5μg/L) during O3 only, O3/H2O2, O3/UV and O3/H2O2/UV experiments with ozone dose of approximately 0.3 mg/L (O3:DOC) in the wastewater effluents. (initial Conc. H2O2 0.5mM)

3.3 Scavenging kinetics of the oxidants in wastewater effluent Typically, OH are scavenged by the water matrix during the treatment, resulting in only a fraction of OH that reacts with the targeted TrOCs. DOC, nitrite and carbonate/bicarbonate are the major OH scavengers in the water matrix in this study. The scavenging kinetics is of crucial importance for the evolution of the minimum ozone demand to generate OH for degrading TrOCs. To understand the minimum ozone demand during AOPs, the total OH scavenging rate constant (k1) and the OH scavenging rate constant without alkalinity (k2) were determined as 6.4 × 104 s-1 and 1.9 × 104 s-1. More details on the derivation of the scavenging kinetics of the oxidants are given in Text S5. It is important to calculate k1 and k2, as the water matrix OH scavenging kinetics will contribute to the prediction of TrOCs abatement and oxidants consumption. For example, a wastewater with high scavenging rate will consume more OH,

and more ozone dose will be required for OH generation and reaction with target

TrOCs. 3.4 Removal of selected TrOCs in wastewater effluent In this case, a comparison of removal efficiency of selected TrOCs in the effluent with different ozone-based AOPs and a variety of ozone doses was done (Fig. 5). Due to the different ozone and OH reactivity of each selected compound, a significantly different removal efficiency was observed.



CBZ

100

80

60

60

40

40

20

20

0

0

C/C₀ (%)

80

0

30

60 Time (s)

90

120

BPA

100

0

60

60

40

40

20

20

0

0

C/C₀ (%)

80

30

Time60 (s)

90

120

PCP

100

30

0

30 Time 60 (s)

60

40

40

20

20

0

0 0

30

Time60 (s)

90

120

120

90

120

O₃ only O₃/H₂O₂

80

60

90

EE2

100

80

60 Time (s)

ATZ

100

80

0

ALA

100

C/C₀ (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O₃/UV O₃/H₂O₂/UV

0

30 Time 60 (s)

90

120

Fig.5 Concentration profiles of selected compounds obtained with different ozone-based AOPs in wastewater effluent (O3:DOC=0.5, 0.5mM H2O2, pH=8).


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As shown in Fig.5, during the first 30 s, only a small decrease of the concentration of the selected TrOCs during all four ozone-based AOPs was achieved. The degradation rate began to increase after 30 s. This is consistent with the results on OH exposure. It indicates that OH is generated significantly in the IOD phase. Matrix compounds competed with target TrOCs for oxidants occurring in the first 30 s, which resulted in a lag-phase with ozone-based AOPs. When dissolved O3 was completely decomposed after 2 min, degradation for all six selected TrOCs during O3 and O3/H2O2 stopped. It is noteworthy that relatively slow decreases of TrOCs residual concentration in the wastewater occurred during the lag-phase and even after complete O3 decomposition during experiments with O3/UV and O3/H2O2/UV. This can be attributed to (direct) UV photolysis. Table 3. Summary of first-order rate constants k ( × 10 ―4𝑚𝑖𝑛 ―1) for ozone-based AOPs in the wastewater effluent (O3:DOC=0.5, 0.5mM H2O2, pH=8). Compound

O3 only × 10

―4

𝑚𝑖𝑛

O3/H2O2 ―1

× 10

―4

𝑚𝑖𝑛

O3/UV ―1

× 10

―4

𝑚𝑖𝑛

O3/H2O2/UV ―1

× 10 ―4𝑚𝑖𝑛 ―1

ATZ

13

76

80

98

ALA

35

257

160

473

BPA

214

362

340

921

CBZ

342

412

375

938

EE2

349

401

389

1515

PCP

93

46

80

12

Based on the results in Fig.5, Table S3 and Table 3, BPA, CBZ and EE2 were removed effectively during all ozone-based AOPs, because of their high reaction rates with OH and their high first-order rate constants with ozone during ozone-based AOPs. The removal of BPA, CBZ and EE2 followed the following order: O3/H2O2/UV > O3/H2O2 > O3/ UV > O3 only. As shown in Fig.5, only 8% and 25% removal for ATZ and ALA, respectively, is obtained after 120 s by only ozone. ATZ could not be removed by more than 40% during all ozone-based AOPs in the wastewater effluents. Due to kO₃, ATZ and kO₃, ALA are less than 10 M-1s-1, ATZ and ALA are poor ozone-selective compounds. As such, it is difficult to remove these components during treatment with O3 alone. However, comparing with the results for O3 only, the removal of ATZ and ALA was 2-3 times



higher for O3/H2O2, O3/UV or O3/H2O2/UV. Furthermore, the first-order rate constants of ATZ and ALA were respectively at least 6 times and 4 times higher for O3/H2O2, O3/UV or O3/H2O2/UV treatment than that for O3 only. These results illustrate that combining ozone with UV and H2O2 has a higher efficiency than only applying ozonation when degrading compounds which are less-selective to ozone but moreselective to OH . In contrast, the residual concentration of PCP was 61% after 120 s experiments where only O3 was applied in the wastewater effluent, but less than 40% degradation efficiency was observed at 120 s during treatments with O3/H2O2, O3/UV and O3/H2O2/UV. As shown in Table 3, the first-order rate constants were reduced when applying H2O2 or UV in PCP abatement experiments. The results in Fig.5 show that the removal of PCP decreased with increasing of the OH exposure: O3 only > O3/ UV > O3/H2O2 > O3/H2O2/UV. It indicates that combining ozone with UV and H2O2 is not always more efficient than O3 only for TrOCs removal. Ozone is a selective oxidant and has a fast reaction rate with ERMs. PCP, with its activated phenolic-moiety, is an high ozoneselective compound (kO₃,

PCP

=3.78 × 107M-1s-1)26,27, so that the treatment by applying

O3 only achieved a higher PCP removal (around 40%). In this case, adding H2O2 into experiments resulted in a decrease of PCP elimination, 20% O3/H2O2 removal for and 10% for O3/H2O2/UV, which is lower than that for O3 only. This determines that the presence of H2O2 limits the PCP degradation, and this result is consistent with the research of Zimbron and Reardon28.

4. Conclusions In the present study, four ozone-based AOPs are compared systematically regarding ozone and OH exposure, OH formation and TrOCs degradation efficiency. It is demonstrated that combining O3 with UV and/or H2O2 could decrease ozone exposure and shorten the ozone lifetime in wastewater effluent. A more significantly


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decrease of ozone exposure and a more effective ozone decomposition could be achieved for O3/H2O2/UV compared to other ozone-based AOPs. In view of future application for water treatment, the value of the minimum ozone demand for TrOCs decomposition is presented and the OH scavenging rate of the effluent is determined (k1 = 6.4 × 104 s-1 and k2 = 1.0 × 104 s-1). For example, an O3:DOC of 0.3 is the minimum ozone demand for TrOCs elimination experiments. When the dosage of ozone is lower than the minimum ozone demand, ozone will be decomposed completely during the IOD phase and cannot contribute to the target contaminants decomposition. The value of the minimum ozone demand for degrading target contaminants and the OH

scavenging rate of the effluent can be important factors for evaluation ozone

requirement during further application of ozone-based AOPs in wastewater treatment. Furthermore, even though combining O3 with H2O2 and/or UV doesn’t change the OH yield, it can enhance the OH exposure and achieve a higher OH exposure than O3/only, O3/UV and O3/H2O2 at the equal ozone dose, which leads to a higher removal efficiency of ozone-resistance compounds. In the further study, the conclusions of this study can be applied to real wastewater treated by ozone-based AOP and to predict the TrOCs elimination efficiency in various wastewater effluents. Acknowledgement Ze Liu is financially supported by the China Scholarship Council (CSC) by a CSC PhD grant. This research fits within the LED H2O project. The LED H2O is financially supported by The Flanders Knowledge Centre Water.



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Oxidation of Trace Organic Contaminants (TrOCs) in Wastewater

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