Kinetic study on degradation of micro-organics by different UV-based advanced oxidation processes in EfOM matrix

Effluent organic matter (EfOM) contains a large number of substances that are harmful to both the environment and human health. To avoid the negative effects of organic matter in EfOM, advanced treatment of organic matter is an urgent task. Four typical oxidants (H2O2, PS, PMS, NaClO) and UV-combined treatments were used to treat micro-contaminants in the presence or absence of EfOM, because the active radical species produced in these UV-AOPs are highly reactive with organic contaminants. However, the removal efficiency of trace contaminants was greatly affected by the presence of EfOM. The degradation kinetics of two representative micro-contaminants (benzoic acid (BA) and para chlorobenzoic acid (pCBA)) was significantly reduced in the presence of EfOM, compared to the degradation kinetics in its absence. Using the method of competitive kinetics, with BA, pCBA, and 1,4-dimethoxybenzene (DMOB) as probes, the radicals (HO·, SO4−·, ClO·) proved to be the key to reaction species in advanced oxidation processes. UV irradiation on EfOM was not primarily responsible for the degradation of micro-contaminants. The second-order rate constants of the EfOM with radicals were determined to be (5.027 ± 0.643) × 102 (SO4−·), (3.192 ± 0.153) × 104 (HO·), and 1.35 × 106 (ClO·) (mg C/L)−1 s−1. In addition, this study evaluated the production of three radicals based on the concept of Rct, which can better analyze its reaction mechanism.


Introduction
Urban sewage contains a large amount of organic matter of various and complex types. Although more than 90% of this organic matter can be effectively removed with conventional biological treatment, some refractory organic residue inevitably remains. These refractory dissolved organic substances remaining in sewage after primary and secondary biochemical treatment are collectively referred to as effluent organic matter (EfOM) (Vigneswaran 2006). While the concentration of emerging contaminants (ECs) in EfOM is extremely low in the environment (Rosal et al. 2010), their high stability in wastewater makes them difficult to degrade with conventional biological treatment (Taoufik et al. 2021). Conventional wastewater treatment processes-including coagulation, sedimentation, filtration, and disinfectioncan remove only a limited amount of EfOM, and sometimes highly toxic intermediate products are produced during treatment. Advanced oxidation process (AOPs) is a new type of high-efficiency pollutant control technology developed in the 1980s (Hisaindee et al. 2013). Because AOP has strong oxidizing ability and low selectivity to pollutants, and can remove trace amounts of harmful chemicals and refractory organics, it has been widely used in the treatment of contaminated groundwater, especially for the removal of some special trace pollutants in water. With the development of these advanced oxidation processes, in addition to the initial application of · OH, other highly reactive free radicals (such as SO 4 −· , O 2 −· , and Cl · ) can undergo electron transfer, or hydrogen addition or substitution, to react with refractory organic matter (Khan and Adewuyi 2010), thereby causing chemical bond breakage of the organic matter. It is even possible to directly mineralize the organic matter into carbon dioxide and water.
UV/H 2 O 2 is a conventional AOP, based on the production of a hydroxyl radical (HO · ) (E 0 = 2.8 V) via UV/H 2 O 2 . The major water constituents known to scavenge HO · are EfOM and inorganic species such as carbonate, bicarbonate, nitrite, and bromide ions (Keen et al. 2014;Wols and Hofman-Caris 2012). HO · water matrix demand is commonly calculated based on measured concentrations of these compounds and the respective second-order rate constants (k ·OH,P , M −1 s −1 ) for their reaction with HO · . The second-order rate constants for the reaction between HO · and EfOM have been reported as 1.0-4.5 × 10 7 MC −1 s −1 (And and Fulkersonbrekken † 1998;Donham et al. 2014;Reisz et al. 2003), and these vary depending on the origin, characteristics, and composition of the EfOM.
An advanced oxidation process based on SO 4 −· (E 0 = 2.6 V) could be applied as an alternative to those based on a hydroxyl radical (HO · ) for the remediation of organic pollutants in surface water, groundwater, or wastewater (Hori et al. 2005;Yang et al. 2014). SO 4 −· is generated via the activation of peroxymonosulfate (HSO 5 − , PMS) or persulfate (S 2 O 8 2− , PS) by UV, heat, or transition metals (Matta et al. 2011;Zhou et al. 2013) (Milh et al. 2021). UV/ persulfate possesses several advantages, including stability of the precursors (PMS or PS), ease of storage and transportation, high water solubility, versatile activation strategies, and a wide operating pH range (And and Dionysiou 2004;Das 2017) (Giannakis et al. 2021).
UV/Cl is an emerging AOP alternative to the UV/H 2 O 2 process, as it produces HO · and reactive chlorine species (RCS). The quantum yields of HOCl and OCl − by UV photolysis and their absorptivity are reported to be higher than those of H 2 O 2 (Feng et al. 2007;Watts and Linden 2007). Compared to HO · , RCS such as Cl · , Cl 2 −· , and ClO · are also powerful oxidants, with oxidation potentials of 2.47 V, 2.0 V, and 1.5-1.8 V, respectively (Alfassi et al. 1988;Beitz et al. 1998).
Previous studies have demonstrated the feasibility of adopting advanced oxidation processes to treat micropollutants (Cong et al. 2015) (He et al. 2020). However, data on this topic are still scarce, especially for UV/oxidant methods for municipal wastewater treatment. In the present work, benzoic acid (BA), para chlorobenzoic acid (pCBA), and 1,4-dimethoxybenzene (DMOB) were chosen as model compounds to investigate the degradation of micropollutants in UV/H 2 O 2 , UV/PS, UV/PMS, and UV/Cl processes. A kinetic model of UV-based AOPs was established for the degradation of micro-pollutants in wastewater, and the second-order rate constants of EfOM with radicals were evaluated.

Samples and chemicals
Secondary wastewater effluent was obtained from a wastewater treatment plant in Beijing with a capacity of 1,000,000 m 3 /d. The municipal sewage was purified by screens, aerated grit chambers, and primary settling, and an A 2 /O process (anaerobic, anoxic, and oxic conditions) and secondary clarification were carried out. The water parameters of the WWTP effluent are listed in Table 1.

UV-based advanced oxidation experiment
The photo reactor used for the AOPs was equipped with a Xenon lamp peaking at 254 nm (CEL-HXUV300, Zhongjiao Jinyuan). The average UV fluence rate (E 0 ) was 1.217 mW cm −2 . A 200-mL test solution containing EfOM and 1.0 μM BA, pCBA, or 5.0 μM DMOB was dosed with the oxidant stock solution (PS, PMS, H 2 O 2 , or NaClO) and simultaneously exposed to UV irradiation at 25 ± 0.2 °C. The oxidant dosages of the reaction were 0.588, 1.176, 2.352, and 5.880 mM, respectively. Samples were collected at 10-min intervals for an hour, for further analysis. Reactions of UV/ PS, UV/PMS, and UV/H 2 O 2 were quenched with 100 mM sulfite and UV/Cl reactions were quenched with ascorbic acid at a molar ratio of [ascorbic acid]/[chlorine] = 1.5:1. All tests were conducted at least twice. All data plots represent the average of the experimental data of the duplicated test results.  Table S1). The column used in the liquid chromatographic analysis of BA, pCBA, and DMOB was a Waters Acquity UPLC BEH C18 (1.7 μm, 2.1 × 100 mm). The dissolved organic matter in the EfOM was determined using a TOC analyzer (TOC; Shimadzu, Japan), and UV absorbance at 254 nm (UV 254 ) was measured using a UV/Vis (Evolution 300, Thermo Scientific, USA). Ion chromatography (ICS3000, Dionex Corp., USA) was used to determine [Cl − ], [NO 3 − ], and [HPO 3 2− ]. The separation was finished in an IonPac AS11 column with a constant gradient mode. The mobile phase eluent was NaOH solution (30.0 mM) and the flow rate was 1.0 mL/min. Before the separation, 25.0 μL of the sample was injected by an autosampler. The pH was measured using a pH meter (S210 Seven Compact, Mettler Toledo).

Pseudo-first-order dynamics model
Because UV irradiation showed no effect on the reference compound degradation (shown in Fig. 1), and considering that the degradation reaction of the reference compound (R) may be related to the oxidizing properties of the oxidants (PS, PMS, and H 2 O 2 ), SO 4 −· and HO · , the degradation of R can be assumed to follow second-order kinetics: where k i is the second-order rate constant of the reaction of PS, PMS, H 2 O 2 , SO 4 −· , and HO · with R. It is known that the minimum concentration of PS, PMS, or H 2 O 2 is 0.588 mM, and it can be assumed that the BA and pCBA concentration is 1.0 μM [Oxidant] ≫ [R]. In order to simplify Eqs.

Determination of second-order rate constants of HO · and EfOM
To quantify the reactivity of EfOM with HO · , the secondorder rate constants ( k HO ⋅ ,Ef OM , MC −1 s −1 ) between HO · and EfOM were determined based on the competition kinetics method using BA, pCBA, and MeOH (or t-BuOH) in UV/H 2 O 2 , since the reaction rate constants of BA, pCBA, and MeOH (or t-BuOH) with HO · were known. Then, where k app R is the apparent degradation rate constant of R (s −1 ), and k HO ⋅ ,R is the second-order rate constant for the reaction between HO · and compound R (M −1 s −1 ). [HO · ] ss is the steady-state concentration of HO · (M), and HO ⋅ is the formation rate of HO · (M s −1 ).
In order to eliminate the capture of hydroxyl radicals by inorganic ions in the EfOM and determine the concentrations of Cl − , NO 3 − , and HPO 3 2− , we introduced the parameter β: where k 9 , k 10 , and k 11 are the second-order rate constants of HO · with Cl − , NO 3 − , and HPO 3 2− , respectively (Alfassi et al. 1988;Buxton et al. 1988;Herrmann et al. 1999).

Determination of second-order rate constants of SO 4 −· and EfOM
Similar to calculating the secondary reaction rate of HO · with EfOM, the second-order rate constant ( k SO 4 −⋅ ,Ef OM , MC −1 s −1 ) of the reaction between EfOM and SO 4 −· is the same as above, using BA, pCBA, and MeOH (or t-BuOH) in the UV/PS system and UV/PMS system.
where k 12 , k 13 , and k 14 are the second-order rate constants of SO 4 −· with Cl − , NO 3 − , and HPO 3 2− , respectively; k app R is the apparent degradation rate constant of R (s −1 ); k SO 4 −⋅ ,R is the second-order rate constant for the reaction between SO 4 −· and compound "R" and SO4 −⋅ is the formation rate of SO 4 −· (M s −1 ).

Determination of second-order rate constants of ClO · and EfOM
The second-order rate constants ( k ClO• ) for the reaction of ClO · with EfOM were determined by competition kinetics between EfOM and a reference compound of DMOB, which was selected to be the reference compound because of its available k value with ClO · of 2.1 × 10 9 M −1 s −1 .

Removal efficiencies of reference compounds in EfOM by UV/ oxidants
BA and pCBA has good stability and will cause pollution to the environment. At present, advanced oxidation technology is mainly used to remove them. By evaluating its degradation effect in various systems, the reaction conditions are determined to provide a reference for subsequent quantitative research on active substances. Figure 1 compares the degradation of BA (or pCBA) by the UV/H 2 O 2 , UV/PS, UV/PMS, and UV/Cl systems at different oxidant concentrations in EfOM and ultrapure water. BA and pCBA are commonly used as radical probe compounds in UV-based AOPs because they have high reactivity with radicals, especially with HO · and SO 4 −· . When the oxidant concentration is 0.588 mM, the probe compound in ultrapure water has the best degradation effect in different systems. The degradation rates of BA in the UV/H 2 O 2 , UV/ PS, UV/PMS, and UV/Cl systems were 86.07%, 79.05%, 56.94%, and 54.91%, respectively, and the degradation rates of pCBA in the UV/H 2 O 2 , UV/PS, UV/PMS, and UV/Cl systems were 69.42%, 50.17%, 44.77%, and 60.02%, respectively. It can be concluded that the AOPs had the potential to degrade the contaminants. The pseudo-first-order reaction rate constants for BA were 4.90 × 10 −4 , 3.54 × 10 −4 , 2.31 × 10 −4 , and 2.14 × 10 −4 cm 2 mJ −1 in the UV/H 2 O 2 , UV/PS, UV/PMS, and UV/Cl systems, respectively, and the pseudo-first-order reaction rate constants for pCBA were 2.73 × 10 −4 , 1.55 × 10 −4 , 1.60 × 10 −4 , and 2.58 × 10 −4 cm 2 mJ −1 in the UV/H 2 O 2 , UV/PS, UV/PMS, and UV/Cl systems, respectively (as shown in Table S2). It can be seen that the reaction rates of BA were higher than those of pCBA in the UV/H 2 O 2 , UV/PS, and UV/PMS systems, indicating that the reaction mechanisms between the two probes and the radicals (HO · and SO 4 −· ) differed. The reaction rate was opposite in the UV/Cl system, where the degradation of  ClO − + HO · → OH − + ClO · 8 Cl · + HClO → H + + Cl − + ClO · 9 Cl · + OCl − → Cl − + ClO 10 Cl − + Cl · → Cl 2 . − pCBA in ultrapure water was significantly faster than the degradation of BA, while the opposite results were obtained in the EfOM background. The degradation of BA and pCBA in the secondary effluent organic matter by UV alone had little effect. The efficiency of the degradation rate of the two probes under UV irradiation showed a significant relationship with the molar absorption coefficient (ε) and quantum yield (Φ) (Kwon et al. 2015). As the concentration of oxidants increased, the degradation efficiency of the probe compounds also increased, indicating that the concentration of radicals increased. But this increase was not continuous. It can be seen that when the concentration reached 5.880 mM, the degradation efficiency was basically the same as for the concentration of 2.352 mM. For example, the pseudo-first-order reaction rate constant for BA was 1.13 × 10 −4 cm 2 mJ −1 with a PS concentration of 2.235 mM, and the reaction rate constant for BA was 1.15 × 10 −4 cm 2 mJ −1 with a concentration of 5.880 mM in the UV/PS system. The reason for this difference is that the concentration of the micro-contaminant   was limited, in the wastewater. When the concentration of free radicals reaches a certain value, the reaction is saturated. Therefore, an oxidant concentration of 2.352 mM was selected for the next experiment. The degradation rate of the UV/H 2 O 2 system was higher than for other processes. For example, the pseudofirst-order reaction rate constants for BA were 1.64 × 10 −4 , 1.13 × 10 −4 , 1.56 × 10 −4 , and 1.57 × 10 −4 cm 2 mJ −1 in UV/ H 2 O 2 , UV/PS, UV/PMS, and UV/Cl systems, respectively, with a 2.352-mM concentration of oxidants in the presence of EfOM. In the UV/H 2 O 2 system, the major oxidant was the hydroxyl radical (HO · ); the reaction is shown in Table 2 (reaction 1). H 2 O 2 is decomposed to generate a powerful oxidant HO · under the irradiation of ultraviolet light and to trigger free radical chain reactions. PS is stable at room temperature, and the UV led to the cleavage of the O-O bond of PS and generated two SO 4 −· molecules (reaction 2), which was efficient at degrading the probes. Compared to PDS, PMS has a shorter bond, and more energy is required to cleave the peroxide bond   and generate HO · and SO 4 −· (reaction 3). The redox potential of SO 4 −· is equal to or even better than HO · (Ghauch and Tuqan 2012) but SO 4 −· is more selective than HO · in degrading the contaminants, which may have led to the lower degradation rate in the UV/PMS and UV/PDS than in the UV/H 2 O 2 system. In the UV/Cl system, there are several kinds of radicals, such as OH · , Cl · , ClO · , and Cl 2 .− , that are responsible for degrading the probes in the effluent organic matter (reactions 5-10).
In particular, a large amount of SO 4 −· is transformed to HO · under basic conditions (reaction 4). In the presence of pCBA, the conditions are more acidic compared to those in the presence of BA, indicating that the degradation rate of BA is higher than that of pCBA. Besides, the main reaction of BA with HO · is the direct attack of the HO · on the aromatic ring to form a hydroxy-substituted compound (Singla et al. 2004). However, the reaction of SO 4 −· with BA first leads to the formation of a radical cation followed by hydrolysis, to form 4-hydroxybenzoic acid (HBA) (Ying-Hong et al. 2011a). This is why the BA degrades faster in UV/H 2 O 2 than in the UV/PS or UV/PMS systems.
In the UV/PS system, the degradation efficiencies of BA and pCBA were 40.66% and 26.85%, respectively, at the concentration of 5.880 mM in the presence of EfOM. Under the same conditions, the degradation efficiencies under UV/ PMS were 44.04% and 28.35%, respectively. Mahdi-Ahmed and Chiron (2014) and Lee et al. (2018) also found that the removal rate of the probe compound in the UV/PS process is higher than that of the UV/PMS process in ultrapure water, whereas the UV/PMS process has a higher removal rate of the probe compound than the UV/PS process in wastewater from a sewage treatment plant. Guan's research results proved that UV/PMS significantly enhances the degradation of BA in the pH range of 9-11, while the concentration of PMS has little effect (Ying-Hong et al. 2011b).
By comparing the degradation results in ultrapure water and EfOM, we observed that the degradation of the probe compound by different oxidant concentrations in EfOM is not as effective as in ultrapure water. It can be reasonably inferred that this is most likely because the organic matter contained in EfOM has a trapping effect on the radicals, which leads to different results in the degradation of the probe compound by different oxidation systems. Therefore, we elected to use the competition kinetics method to calculate the second-order rate constants of EfOM and radicals.

Contribution of different radicals to contaminant degradation in EfOM
To demonstrate the HO · and SO 4 −· reactivity with the EfOM, the second-order rate constants between radicals and the EfOM were determined based on the competition kinetics method using probes (BA, pCBA) and inhibitors (MeOH, t-BuOH) in the UV/PS, UV/PMS, and UV/H 2 O 2 systems. The initial concentration of MeOH (or t-BuOH) was varied from 0 mM to 0.10 mM, and the initial concentrations of probes and oxidants were fixed at 1.0 μM and 2.352 mM, respectively. The introduction of MeOH and t-BuOH significantly inhibited the degradation of probes in UV/PS, UV/PMS, and UV/H 2 O 2 compared to results in the absence of quencher, indicating that HO · and SO 4 −· are the main reactive oxidizing species. In the sulfate radical systems, when 0.1 mM of radical scavenger was applied, the removal efficiency of BA and pCBA was reduced by about 30% in the presence of MeOH (Fig. 2), while almost no BA or pCBA decrease was observed with the addition of t-BuOH (Table S3 and Fig. 3). These results indicated that SO 4 −· was the predominant reactive species in the UV/PS and UV/PMS systems, a result consistent with a study by Osburn et al. (2009) . Figures 4 and 5 show the experimental results of the competition kinetics for the calculation of the second-order rate constants. Due to the reactivity of ClO · , the second-order rate constant for the reaction between ClO · and EfOM was determined using DMOB as a reference compound with varying EfOM concentrations, which react with ClO · at the second-order rate constant of 2.1 × 10 9 M −1 s −1 (Alfassi et al. 1988) (Fig. 6). Table S6 lists the second-order rate constants determined for the reactions between the EfOM and radicals. To verify the method and rate constants determined in this study, both probes and inhibitors were measured in each of the different oxidation systems. The second-order rate constant of EfOM and HO · was determined to be (3.192 ± 0.153) × 10 4 (mg C/L) −1 s −1 , which is within the commonly reported range of secondorder rate constants. These rate constants are similar to the 10 3 -10 11 M −1 s −1 range of second-order rate constants for the reaction of organics with HO · presented in the literature: these are compared with other studies in Table S5. The results of Yang and Nagarnaik were 3.3 × 10 4 (mg C/L) −1 s −1 and (7.1 ± 0.81) × 10 4 (mg C/L) −1 s −1 , respectively (Nagarnaik and Boulanger 2011;Yang et al. 2016)-which are very similar to our findings. In the sulfate radical-mediated oxidation system, the error of the determined rate constant ( k SO 4 − ⋅ ,Ef OM =(5.027 ± 0.643) × 10 2 (mg C/L) −1 s −1 ) in this study was small, indicating the reliability of the measurement method. And compared with the other studies shown in Table S4, Yang's samples of EfOM were isolated from RO Brine A by solid-phase extraction and contain a large amount of chloride ions, exceeding the probe compounds concentration by 1300-to 2300-fold. They observed that SO 4 −· can be converted to more selective halogen and carbonate radicals, resulting in a wider range of degradation efficiencies among the contaminants (Yang et al. 2016). Zhou measured the absolute rate constants of the reaction of SO 4 −· with four types of organic matter: two fulvic acids and two types of lake organic matter, and their results were close to those in our research (Zhou et al. 2017). The differences in the organic matter contained in the effluent are the main cause of the difference in research results and are also related to the choice of secondary biochemical reaction process.
Because the UV/Cl process is an emerging advanced oxidation process (AOP) used for the degradation of micropollutants, there has been little research on the secondary reaction rate constant of ClO · reacting with different substances in aqueous systems. The second-order kinetic rate constant for EfOM with a chlorooxyl radical was calculated to be 1.35 × 10 6 (mg C/L) −1 s −1 in this research. This value is two orders of magnitude higher than that from EfOM from the Tai Cang wastewater treatment plant in Shanghai-1.83 × 10 4 (mg C/L) −1 s −1 (Guo et al. 2018). Guo et al. (2017) researched simulated drinking water prepared by spiking NOM in pure water (1 mg L −1 ), and the k value of the organic matter with ClO · was 4.52 × 10 4 (mg C/L) −1 s −1 . The difference in k values between the organic matter of different sources and ClO · may be due to the different components of the wastewater. Calculating the radical production rate in EfOM It is well known that a wastewater matrix such as carbonate species and EfOM affects the removal efficiency of a reference compound (R) in a radical-mediated system (Rosenfeld and Linden 2007;Yuan et al. 2011). von Gunten and Linden introduced the concept of R ct to model these complex matrix effects in different AOPs (Elovitz et al. 2000b;Rosenfeld and Linden 2007). The R ct concept, defined as the experimentally determined radical exposure per UV fluence for a given water matrix and initial oxidants concentration, can characterize the effectiveness of the UV/oxidant AOPs within a specific water matrix (Elovitz et al. 2000a). To quantify the scavenging effect of the EfOM, R HO ⋅ ,UV and R SO 4 − ⋅ ,UV were calculated with Eqs. (12) and (13), respectively: where k D,Ef OM

R,UV
is the apparent first-order rate constant (s −1 ) of R destruction in EfOM under UV conditions, and k D,EfOM R,UV∕Oxidants is the apparent first-order rate constant (s −1 ) of R degradation in the UV/oxidants system in the presence of EfOM. The superscript "D" indicates that the value is fluence-based. UV fluence (H in the unit of mJ cm −2 ) is simply the product of E 0 and t. The detailed calculation method refers to the study of Gao et al. (2019). Figure 7 displays the first-order degradation kinetics of probe compounds as a function of applied UV fluence at EfOM in the different UV/oxidant systems. The decay is first order with UV fluence, with only one kinetic regime throughout. Through Fig. 7, we can use formulas (13) and (14) to calculate the data in Table S7, which illustrates the R ct values of SO 4 −· and HO · to BA or pCBA degradation in a UV/PS, UV/PMS, or UV/H 2 O 2 system. When BA is used as a probe, UV/PS produces an SO 4
Funding This work is supported by the National Water Pollution Control and Management Technology Major Project (2018ZX07110005), the National Natural Science Foundation of China (51578037), the Guangxi Province Technology Major Project (AA17202032), the Scientific Research Program of Beijing Municipal Education Commission (KM201610016001), and the Fundamental Research Funds for Beijing University of Civil Engineering and Architecture (X18288, X18289, and X20137).
Data availability All data generated or analyzed during this study are included in this published article and its supplementary information files.

Declarations
Ethics approval and consent to participate Not applicable.

Competing interests
The authors declare no competing interests.