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Choi, Cha, Cho, Lee, and Lee: Understanding the impacts of oxidant doses and dissolved organic carbon concentration on ozone and hydroxyl radical exposures in the peroxone process
Research

Environmental Engineering Research 2025; 30(3): 240521.

Published online: September 26, 2024

DOI: https://doi.org/10.4491/eer.2024.521

Understanding the impacts of oxidant doses and dissolved organic carbon concentration on ozone and hydroxyl radical exposures in the peroxone process

Jaehan Choi1, *, Dongwon Cha1, *, Junho Cho1, Hye-Jin Lee2, †, Changha Lee1, †

1School of Chemical and Biological Engineering, Institute of Chemical Process (ICP), and Institute of Engineering Research, Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea

2Industrial Water Management Dept., K-water, 200 Sintanjin-ro, Daedeok-gu, Daejeon 34350, Republic of Korea

†Corresponding author: E-mail: leehj2@kwater.or.kr (H.-J.L.), leechangha@snu.ac.kr (C.L.), Tel: +82-42-629-4355 (H.-J.L.), +82-2-880-8630 (C.L.), ORCID: 0000-0002-0142-3406 (H.-J.L.), 0000-0002-0404-9405 (C.L.)

* These authors contributed equally to this work.

Received September 5, 2024       Revised September 19, 2024       Accepted September 23, 2024

© 2025 Korean Society of Environmental Engineers

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This research investigates the effects of ozone (O3) and hydrogen peroxide (H2O2) doses, as well as the dissolved organic carbon (DOC) concentration, on O3 and hydroxyl radical (•OH) exposures (i.e., ∫[O3]dt and ∫[•OH]dt) during the peroxone (O3/H2O2) process. The addition of H2O2 during the ozonation process accelerated the O3 decomposition into •OH, increasing ∫[•OH]dt while decreasing ∫[O3]dt. The optimal [H2O2]/[O3] input ratio for maximizing ∫[•OH]dt was found to be around 0.2; at higher ratios, additional H2O2 consumption did not further increase ∫[•OH]dt. At higher DOC concentrations, both oxidant exposures decreased, and the effect of H2O2 on ∫[•OH]dt was diminished due to competitive reactions between H2O2 and DOC for O3, as well as scavenging of •OH by DOC. An increase in ∫[•OH]dt in the peroxone process exhibited a linear correlation with the [O3]/[DOC] ratio when the [H2O2]/[O3] input ratio was fixed at 0.2.

Keywords: Dissolved organic carbon, Hydrogen peroxide, Hydroxyl radical, Ozone, Peroxone

Graphical Abstract

/upload/thumbnails/eer-2024-521f6.gif

Keywords: Dissolved organic carbon, Hydrogen peroxide, Hydroxyl radical, Ozone, Peroxone

1. Introduction

Ozonation is a widely applied method in drinking water treatment processes for oxidizing refractory micropollutants (MPs) and disinfecting pathogenic microorganisms [1–4]. One of the two oxidants utilized during ozonation is molecular ozone (O3), which is capable of oxidizing various organic compounds. Additionally, O3 serves as a precursor for the generation of hydroxyl radicals (•OH), which is also capable of oxidizing various organic compounds but considered more reactive (i.e., less selective) than O3 [5–7].
Determining the optimal O3 dose is a crucial issue to address during the ozonation process. Insufficient O3 would result in inadequate MP removal, while an excessive dose may produce harmful by-products (e.g., bromate, NDMA) and increase operating costs [8,9]. In order to identify the optimal dose of O3, understanding the extent of MP abatement during ozonation is essential. MP abatement during ozonation can be expressed as Eq. (1):
(1)
ln[MP]0[MP]t=kO3∫0t[O3] dt+k•OH∫0t[•OH] dt
which is comprised of the rate constants of MPs with the two oxidants O3 and •OH (i.e., kO3 and k•OH) and their oxidant exposures (i.e., ∫[O3]dt and ∫[•OH]dt) [10–12]. kO3 and k•OH values for various compounds, including MPs of interest, have been extensively covered by previous works [6,13,14]. On the other hand, ∫[O3]dt and ∫[•OH]dt values can be determined experimentally or predicted through empirical models [10,15–17].
During ozonation, MPs are oxidized by both O3 and •OH produced through the decomposition of O3, as depicted in Eq. (1). Although ozonation is deemed adequate for MP abatement, O3-resistant MPs with slow reaction rates with O3 (i.e., kO3) may not be effectively oxidized. Lee and von Gunten demonstrated through chemical kinetics modeling that micropollutants with lower kO3 values required higher specific ozone doses (gO3/gDOC) to achieve similar abatement when compared to other MPs [10]. Furthermore, it was determined that for micropollutants with a kO3 value smaller than 5 M−1 s−1, O3 was responsible for less than 10% of the total abatement (i.e., •OH was the significantly more effective oxidant) [18]. To efficiently degrade such O3-resistant MPs, O3-based advanced oxidation processes (AOPs) can be employed instead of traditional ozonation to generate more •OH or to accelerate its production; these processes include, but are not limited to, O3/UV, O3/H2O2, catalytic ozonation, and electrochemical oxidation [19–22].
The O3/H2O2 process, commonly known as the “peroxone process”, employs ozonation in conjunction with H2O2. The peroxone process is particularly effective in oxidizing O3-resistant compounds by potentially enhancing •OH generation, consequently increasing ∫[•OH]dt, especially in waters with higher O3 stability [14,23]. In the traditional ozonation process, the initial reaction where the O3 molecule interacts with a hydroxide ion (OH−) to produce a hydroperoxide ion (HO2−) proceeds relatively slowly (Eq. (2); k = 70 M−1 s−1). The O3 molecule then reacts fairly quickly with HO2− to generate •OH (Eq. (3); k = 2.8 × 106 M−1 s−1), perpetuating the chain reaction for O3 decomposition and further generation of •OH [5]. When H2O2 is introduced in the peroxone process, it deprotonates to form HO2−, thus bypassing the slower reaction of Eq. (2) and advancing directly to Eq. (3), thereby accelerating O3 decomposition [24].
(2)
O3+OH-→HO2-+O2
(3)
O3+HO2-→O•H+O2•-+O2
However, previous studies have yielded inconsistent results regarding the enhancements of the peroxone process in comparison to the ozonation process [25–28]. This may result from the influence of water quality parameters such as pH, temperature, alkalinity, and dissolved organic carbon (DOC) concentration on •OH generation and oxidant exposure during the peroxone process. Specifically, the DOC concentration and initial H2O2 dose in the target water are believed to significantly affect •OH generation during the peroxone process.
Numerous studies have sought to optimize the peroxone process by altering O3 and H2O2 doses, modifying [H2O2]/[O3] input ratio, or using different H2O2 injection methods such as single, multiple, or continuous injection. Cruz-Alcalde et al. found that continuous H2O2 injection yielded higher MP removal compared to a single initial dose [29]. Ferguson et al. explored the decomposition of various MPs by changing the [H2O2]/[O3] input ratio from 0.1 to 0.3 [30]. The optimal ratio varied based on the water source used in the peroxone experiments.
In this study, we explored the effects of O3 and H2O2 doses/ratios, as well as DOC concentration, on ∫[O3]dt and ∫[•OH]dt during the peroxone process. The optimal [H2O2]/[O3] input ratio is proposed to maximize ∫[•OH]dt. This research provides insights into determining the appropriate O3 and H2O2 doses during the peroxone process at varying DOC concentrations by suggesting a correlation between O3 dose and DOC concentration on ∫[•OH]dt. Additionally, to the best of our knowledge, the residual amount of H2O2 after the completion of the peroxone process has not yet been measured to evaluate the efficiency of H2O2 utilization (i.e., the actual amount of H2O2 consumed during the peroxone process compared to the initial H2O2 dose). This could offer valuable information regarding the economic feasibility of the peroxone process for target water, as O3 and H2O2 doses can significantly impact operating costs.

2. Materials and Methods

2.1. Reagents

All chemicals used in this study were of reagent grade (Sigma-Aldrich) and were used as received, without further purification. Aqueous stock solutions were prepared using deionized (DI) water supplied by a water purification system (Millipore Milli-Q Integral 5). For the preparation of the O3 stock solution, oxygen gas containing O3 was generated using an O3 generator (Ozonetech Lab-II) and then bubbled into DI water in a glass reactor immersed in an ice bath.

2.2. Natural Water Samples

Two natural water samples were collected from a drinking water treatment plant (DWTP) and the Jinwi River in Gyeonggi Province, Republic of Korea. The DWTP sample exhibits a relatively low DOC concentration, while the river sample features a relatively high DOC concentration. Prior to conducting experiments, all water samples were filtered through 0.45 μm nylon membrane filters (GE Healthcare Whatman) and stored at 4°C until used. For the experiments, the river water sample was diluted twofold with DI water to achieve the desired DOC concentration, subsequently measured with a DOC analyzer (Sievers Instruments M5310 C). The pH was measured using a pH meter (Thermo Fisher Scientific Orion Star A220). Water quality parameters for the natural water samples are presented in Table 1.

2.3. Ozonation Experiments

All experiments were conducted in a flask containing 40 mL of target water. The target water was spiked with 1 μM deethylatrazine (DEA), an •OH probe compound [31]. •OH probe compounds like DEA are utilized to measure ∫[•OH]dt, which can be determined by assessing the extent of •OH probe decomposition as expressed in Eq. (4).
(4)
ln [DEA]0[DEA]t=k•OH∫0t[•OH] dt
where k•OH is the second-order rate constant for DEA with •OH (1.2 × 109 M−1 s−1) [32].
Experiments were conducted at room temperature with slow stirring at 200 rpm, and the ozonation reaction was initiated by adding a measure of O3 stock solution to the target water. In peroxone experiments, H2O2 was added beforehand. Samples (2.5 mL) were collected at predetermined time intervals and immediately treated with indigo trisulfonate reagent (0.28 mL) to quench any residual O3 and assess the concentration of O3 at the timed sampling points. Following complete decomposition of O3 in the solution, the residual H2O2 concentration was quantified. All experiments were conducted in duplicate.

2.4. Analytical Methods

O3 concentration was determined by measuring the decolorization of indigo trisulfonate (indicated by changes in absorbance at 600 nm) using UV/Vis spectrophotometry (PerkinElmer Lambda 465) [33]. ∫[O3]dt was measured by integrating O3 concentration over time. DEA concentration was analyzed using rapid separation liquid chromatography (RSLC) (Thermo Fisher Scientific UltiMate 3000) with UV absorbance detection. A C18 column (Thermo Fisher Scientific Acclaim 120) was used for separation, utilizing a mixture of 0.1 wt% phosphoric acid solution and acetonitrile as the eluent. H2O2 concentration was determined using the N,N-diethyl-p-phenylenediamine (DPD) method [34].

3. Results and Discussion

3.1. Effects of Adding H2O2 During Ozonation

The effects of the peroxone process (i.e., addition of H2O2) on ∫[O3]dt and ∫[•OH]dt in DWTP water were evaluated by monitoring the time-dependent decomposition of O3 and DEA (Fig. 1). The peroxone process resulted in significantly faster O3 decay compared to ozonation (Fig. 1a). Consequently, ∫[O3]dt decreased markedly (Fig. 1c). In a similar trend to O3 decay, DEA decomposition was substantially quicker in the peroxone process, concluding when no further •OH was generated after complete decomposition of O3 (Fig. 1b). Nonetheless, a greater proportion of DEA was decomposed in a shorter time, leading to an almost three-fold increase in ∫[•OH]dt for the peroxone process (Fig. 1d). Adding H2O2 during ozonation expedited the decomposition of O3, with results indicating that more •OH was generated for DWTP water.

3.2. Effects of H2O2/O3 Ratio and Initial O3 Dose During the Peroxone Process

To evaluate the effects of [H2O2]/[O3] input ratio during the peroxone process, ∫[O3]dt and ∫[•OH]dt were analyzed for three O3 doses with [H2O2]/[O3] input ratios ranging from 0 (i.e., ozonation) to 0.6 (Fig. 2). Time-dependent profiles for O3 and DEA decomposition across various [H2O2]/[O3] input ratios are documented (Figs. S1 and S2 of the supplementary materials). Up to a [H2O2]/[O3] input ratio of 0.2, ∫[O3]dt decreased significantly due to the accelerated decomposition of O3 upon addition of H2O2 (Fig. 2a), whereas ∫[•OH]dt increased compared to ozonation (Fig. 2b). Both oxidant exposures exhibited greater changes with increasing initial O3 doses.
However, when the [H2O2]/[O3] input ratio increased from 0.2 to 0.6, ∫[O3]dt and ∫[•OH]dt remained roughly constant across different O3 doses. These findings suggest that optimizing the peroxone process for enhanced •OH generation occurs at an [H2O2]/[O3] input ratio of approximately 0.2. No significant increase in •OH production was noted with higher [H2O2]/[O3] input ratios, likely due to the limited utilization of excess H2O2. Consequently, further experiments were conducted to explore any correlation between H2O2 consumption and •OH production during the peroxone process.
Residual H2O2 was monitored following the completion of the peroxone process, and the remaining amount was compared to the initial H2O2 dose (Fig. 2c). The proportion of residual H2O2 relative to the initial H2O2 dose was found to decrease as the initial O3 dose increased, and it increased when the [H2O2]/[O3] input ratio also increased. Moreover, the actual amount of H2O2 consumed versus the enhancement in •OH generation was quantified (Fig. 2d). It was noted that the enhancement of •OH production during the peroxone process was somewhat linearly correlated with the actual amount of H2O2 utilized. Irrespective of the input ratio, the results indicated that higher consumption of H2O2 led to a significant increase in •OH generation and consequently, ∫[•OH]dt. This led to the conclusion that the actual used amount of H2O2 is a more crucial factor for generating additional •OH compared to simply the initial dose of H2O2 in the peroxone process.

3.3. Effects of H2O2/O3 Ratio and DOC Concentration During the Peroxone Process

To assess the impact of DOC concentration during the peroxone process, ∫[O3]dt and ∫[•OH]dt were studied in waters with relatively low and high DOC concentrations (0.99 and 2.12 mg/L for DWTP and river waters, respectively). Similar to prior experiments, [H2O2]/[O3] input ratios ranged from 0 to 0.6 with a constant O3 dose of 3 mg/L for both water types (Fig. 3). An increase in the [H2O2]/[O3] input ratio up to 0.2 led to a reduction of ∫[O3]dt in both waters (Fig. 3a), while simultaneous generating a rise in ∫[•OH]dt (Fig. 3b). Furthermore, the addition of H2O2 with an input ratio of 0.2 significantly enhanced •OH production in low DOC concentration DWTP water compared to river water (enhancement in ∫[•OH]dt by factor of 2.8 and 1.3, respectively). When [H2O2]/[O3] input ratio was increased from 0.2 to 0.6, both ∫[O3]dt and ∫[•OH]dt showed no significant changes, following a pattern consistent with previous findings that used initial O3 dose as a variable. These findings, along with previous results, demonstrated that •OH production during the peroxone process is optimized at a [H2O2]/[O3] input ratio of 0.2, independent of O3 dose or DOC concentration.
The effect of DOC concentration on two oxidant exposures was significant for both ozonation and peroxone processes. River water with a relatively high DOC concentration typically showed lower oxidant exposures under the same experimental conditions. This can be attributed to a higher presence of dissolved organic matter (DOM) in waters with higher DOC concentration, leading to accelerated O3 decay due to direct reactions between O3 and DOM as expressed in Eq. (5) and Eq. (6). Consequently, less O3 is available to react with H2O2 to generate additional •OH, leading to a decrease in ∫[•OH]dt. Additionally, scavenging of •OH through its reactions with DOM, acting as either a promoter as shown in Eq. (7) and Eq. (8) or an inhibitor as shown in Eq. (9) and Eq. (10), reduces the availability of •OH for MP abatement [5,15].
(5)
O3+DOM1→DOM1,ox
(6)
O3+DOM2→DOM2+•+O3•-
(7)
O•H+DOM3→DOM3•+H2O or OH-
(8)
DOM3•+O2→DOM3-O2•→DOM3++O2•-
(9)
O•H+DOM4→DOM4•+H2O
(10)
DOM4•+O2→DOM4-O2•→no O2•- formation
Similar to previous experiments, residual H2O2 levels post-peroxone process were measured (Fig. 3c). It was observed that residual H2O2 concentrations were higher in river water with relatively high DOC concentrations compared to DWTP water. Notably, the consumption of H2O2 was greater in waters with relatively low DOC concentrations, generally leading to a better enhancement of ∫[•OH]dt compared to waters with high DOC concentrations (Fig. 3d). This indicates that H2O2 is utilized more efficiently in conditions of lower DOC concentration to enhance •OH production.

3.4. Impact of O3 Dose and DOC Concentration on •OH Production

∫[•OH]dt during the ozonation and peroxone process was compared by varying O3 doses for both natural waters (Fig. 4). The [H2O2]/[O3] input ratio in the peroxone process was set at 0.2 to maximize •OH production, as established from previous experiments. Time-dependent profiles for O3 and DEA at various initial O3 doses with a fixed [H2O2]/[O3] input ratio in water with relatively high DOC concentrations are provided (Figs. S3 and S4 of the supplementary materials). For water with relatively low DOC concentrations, ∫[•OH]dt increased significantly in the peroxone process compared to ozonation for all initial O3 doses, suggesting that the peroxone process could be advantageous for enhancing •OH generation in low DOC concentration waters (Fig. 4a). Conversely, for water with relatively high DOC concentrations, the increase in ∫[•OH]dt during the peroxone process was less significant, particularly at lower initial O3 doses (Fig. 4b). These findings imply that the added H2O2 was not effectively utilized due to the rapid decay of O3 in water with high DOC concentration, resulting in less residual O3 for enhanced •OH generation. Nonetheless, when the initial O3 dose was sufficiently high (i.e., 3 mg/L), a significant enhancement in ∫[•OH]dt was observed for both waters.
From experiment results, a linear correlation between specific O3 dose (i.e., gO3/gDOC) and the increase in ∫[•OH]dt could be established (Fig. 5). At low gO3/gDOC (i.e., gO3/gDOC < 0.5), introducing H2O2 to convert ozonation into a peroxone process did not noticeably enhance •OH generation. As gO3/gDOC increased, ∫[•OH]dt increased linearly, with a coefficient of determination (R2) of 0.93. This linear correlation can potentially predict ∫[•OH]dt enhancement levels when utilizing the peroxone process, given the initial O3 dose and DOC concentration in target water.

4. Conclusions

This study explored how [H2O2]/[O3] input ratio, initial O3 dose, and DOC concentration influence oxidant exposures during the peroxone process. Key findings reveal:

  • Adding H2O2 during the ozonation phase, or the peroxone process, increases the generation of •OH particularly in waters with low DOC concentrations to effectively remove O3-resistant MPs.

  • The optimal [H2O2]/[O3] input ratio is approximately 0.2 for maximizing •OH production during the peroxone process, irrespective of initial O3 dose or DOC concentration. An increase in [H2O2]/[O3] input ratio beyond 0.2 did not significantly enhance •OH production and only led to higher residual levels of H2O2 post-peroxone process. This indicates the need for further refining the actual amount of H2O2 utilized, rather than merely the initial amount added.

  • The degree of ∫[•OH]dt enhancement in the peroxone process is greater with higher initial O3 doses and lower DOC concentrations of target water. This study offers crucial insights into how [H2O2]/[O3] input ratio, initial O3 dose, and DOC concentration affect enhancement of •OH generation in the peroxone process.

We believe that the findings from this research on optimizing the [H2O2]/[O3] input ratio and the influence of various operating parameters during the peroxone process may serve as a valuable reference for DWTP operators currently using or considering the peroxone process. From an economic perspective, optimizing oxidant doses can help reduce operating costs while still ensuring sufficient oxidation to achieve micropollutant abatement. Furthermore, the discovery of a linear correlation between gO3/gDOC and ∫[•OH]dt enhancement could be further developed to empirically predict ∫[•OH]dt during the peroxone process in future studies.

Supplementary Information

eer-2024-521-supplementary.pdf

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (2023R1A2C200408), and the Korea Environmental Industry and Technology Institute (KEITI) through the Developing Innovative Drinking Water and Wastewater Technologies Project (2022002710001).

Notes

Conflict-of-Interests

The authors declare that they have no conflict of interest.

Author Contributions

J.C. (M.S.) conducted all experiments and wrote the manuscript. D.C. (Ph.D. student) wrote the manuscript and prepared figures. J.C. (Ph.D. student) provided academic support for experiments. H.-J.L. (Ph.D.) and C.L. (Professor) reviewed and edited the manuscript

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Fig. 1
Time-dependent profiles of (a) O3 and (b) DEA concentrations, along with resulting (c) O3 and (d) •OH exposures for ozonation and peroxone processes (DOC = 0.99 mg/L, [O3]0 = 2 mg/L, [DEA]0 = 1 μM, [H2O2]0/[O3]0 = 0.2 where applicable).
/upload/thumbnails/eer-2024-521f1.gif
Fig. 2
Profiles dependent on the [H2O2]/[O3] input ratio of (a) O3 exposure, (b) •OH exposure, and (c) residual H2O2 across various initial O3 doses. (d) Increase rate of •OH exposure as a function of H2O2 consumption (DOC = 0.99 mg/L, [O3]0 = 1, 2, or 3 mg/L, [DEA]0 = 1 μM).
/upload/thumbnails/eer-2024-521f2.gif
Fig. 3
Profiles dependent on the [H2O2]/[O3] input ratio of (a) O3 exposure, (b) •OH exposure, and (c) residual H2O2 across different DOC concentrations. (d) •OH exposure increase rate as a function of H2O2 consumption (DOC = 0.99 mg/L for low DOC, 2.12 mg/L for high DOC, [O3]0 = 3 mg/L, [DEA]0 = 1 μM).
/upload/thumbnails/eer-2024-521f3.gif
Fig. 4
•OH exposures with varying initial O3 doses in (a) water with a relatively low DOC concentration (0.99 mg/L) and (b) water with a relatively high DOC concentration (2.12 mg/L) ([H2O2]0/[O3]0 = 0.2, [DEA]0 = 1 μM).
/upload/thumbnails/eer-2024-521f4.gif
Fig. 5
Increase of •OH exposure as a function of gO3/gDOC in waters with different DOC concentrations (Low DOC = 0.99 mg/L, High DOC = 2.12 mg/L, [H2O2]0/[O3]0 = 0.2, [DEA]0 = 1 μM).
/upload/thumbnails/eer-2024-521f5.gif
Table 1
Water quality parameters of natural water samples
Water sample pH DOC concentration (mg/L)
DWTP water 7.29 0.99
River water (diluted) 7.03 2.12
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