chlorine synergistic oxidation process applied to the removal of N, N-diethyl-3-toluamide

Chemical Engineering Journal 380 (2020) 122409

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Ammonia/chlorine synergistic oxidation process applied to the removal of N, N-diethyl-3-toluamide

T

⁎

Bei Yea, Xin-Yang Zhangb, Tao Hec, Wen-Long Wangd, Han-Jing Wub, Yao Lua, Qian-Yuan Wub, , ⁎ Hong-Ying Hua,d, a

Shenzhen Environmental Science and New Energy Technology Engineering Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen 518055, PR China Key Laboratory of Microorganism Application and Risk Control of Shenzhen, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China c South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510000, PR China d Environmental Simulation and Pollution Control State Key Joint Laboratory and State Environmental Protection Key Laboratory of Microorganism Application and Risk Control (SMARC), School of Environment, Tsinghua University, Beijing 100084, PR China b

H I GH L IG H T S

combination of ammonia and chlorine shows synergistic effect for DEET removal. • The DEET removal efficiency is obtained under neutral condition. • Highest reactive species that can be quenched by TBA attribute to DEET removal. • Only from ammonia is detected in the transformation product. • Nitrogen • Hydroxylation is found to be the dominant DEET degradation pathway.

A R T I C LE I N FO

A B S T R A C T

Keywords: Ammonia Chlorine Synergistic oxidation process DEET Degradation mechanism

The ammonia/chlorination process is important for the treatment of ammonia-rich water, such as the ammonia removal, disinfection strategies, and disinfection byproducts controlling. Recently, the simultaneous removal of recalcitrant micropollutants during ammonia/chlorination has attracted attention. In this study, ammonia/ chlorination was employed to remove the chlorine-resistant micropollutant N, N-diethyl-3-toluamide (DEET). Using the optimal chlorine to ammonia molar ratio (Cl/N) of 1.6, nearly 60% of DEET was removed by ammonia/chlorination within 15 min, while chlorination or chloramination alone showed minimal DEET removal. The synergistic effect obtained from the combination of ammonia and chlorine is attributed to reactive species generated from the decomposition of chloramine over Cl/N 1.0 to 1.6, being proved by the significantly inhibited performance (almost 95%) by t-butanol. The aqueous matrix was found to affect the results, and acidic/ basic solutions reduced DEET removal by 70% to 79%, while the presence of natural organic matter (NOM 6 mgC/L) suppressed the removal by 33%. The quantity of DEET removed per mole of decomposed chloramine (α) is proposed as a measure of the efficiency of this synergistic oxidation process, and the α values were determined to be 0.034, 0.022 and 0.010 in the synthetic solution, a solution containing 6 mg-C/L NOM and a wastewater effluent, respectively. Trials using high resolution quadrupole time of flight mass spectrometry and 15N isotope labelling, allowed the transformation products to be identified, and hydroxylation was found to be the dominant DEET degradation pathway.

1. Introduction Increasingly, various pollutants are being found to have deleterious environmental effects, even in the absence of acute toxicity. These emerging environmental contaminants include disinfection byproducts

(DBPs), nanomaterials, pharmaceuticals and personal care products (PPCPs), and antibacterial agents [1,2]. Among these, PPCPs are perhaps used most frequently and universally. Typical treatments for PPCPs include membrane separation, adsorption, oxidation and advanced oxidation processes (AOPs) [2]. Membrane technologies are

⁎ Corresponding authors at: Shenzhen Laboratory of Microorganism Application and Risk Control, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China. (Qian-Yuan Wu). Room 524, School of Environment, Tsinghua University, Beijing, 10084, PR China (Hong-Ying Hu). E-mail addresses: [email protected] (Q.-Y. Wu), [email protected] (H.-Y. Hu).

https://doi.org/10.1016/j.cej.2019.122409 Received 1 May 2019; Received in revised form 4 July 2019; Accepted 2 August 2019 Available online 03 August 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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synergistic oxidation process, using DEET as a model PPCP. The effects of aqueous matrix, such as the pH value and NOM concentration, were also investigated. The major reactive species were determined and the application of this process to a wastewater effluent was assessed. The transformation products (TPs) were identified and a possible reaction pathway was proposed.

able to remove high molecular weight organic compounds, large particles and TOC, but have the disadvantages of high cost, fouling and poor rejection of low molecular weight compounds [3]. Adsorption, typically using GAC or BAC, readily captures organic compounds, but these systems are severely impacted by the presence of natural organic matter (NOM) and the regeneration of these activated carbon materials requires significant energy inputs [3]. Oxidation, using ozone, chlorine or chlorine dioxide, efficiently degrades certain compounds, but the selectivity of oxidant should not be neglected [4]. The advanced oxidation processes (AOPs) have been reported to effectively remove PPCPs, such as Fenton, heterogeneous Fenton-like catalysis, persulfate, peroxymonosulfate, photocatalysis, ozonation, UV/chlorine, etc [5–7]. These processes are characterized by the generation of highly oxidizing, non-selective reactive species, including hydroxyl radicals, singlet oxygen and chlorine radicals, which rapidly react with PPCPs [2,5,8–11]. Due to its low cost, chlorine is widely used for the disinfection of drinking water and for wastewater treatment [12–14]. However, the redox potentials of both chlorine and chloramine are insufficient to remove most pollutants, and so ultraviolet is often combined with chlorine (UV/chlorine process) to produce stronger reactive species, such as hydroxyl and chlorine radicals, to remove pollutants [8,15–17]. Even so, the transmittance of UV light through water is poor and a considerable electrical energy input is also required. Ammonia is commonly found in various water sources and is an important pollutant for eutrophication. In addition to nitrification processes, chlorine has also been used for the removal of ammonia [18–20]. In breakpoint chlorination process, a Cl/N mass ratio of 8 is required to transform the nitrogen in ammonia into nitrogen gas, trichloramine and nitrate ions [20]. This process can also be used for disinfection purposes. However, chlorination can also lead to the formation of toxic chlorine-based disinfection byproducts due to the reaction between chlorine and NOM in water sources or to the formation of nitrogen-based disinfection byproducts in the case that ammonia is present [19,21], and both these possibilities are significant concerns. Many studies have demonstrated only minimal formation of trihalomethanes (THMs) and haloacetic acids (HAAs) before the breakpoint is reached, as a result of the lower reactivity of chloramine compared to chlorine [22–24]. In contrast, the highest concentration of N-nitrosodimethylamine (NDMA) was found near breakpoint [25,26]. Previous work by our group determined that the chlorine-resistant pharmaceutical compound cambamazepine (CBZ) could be removed by chlorination if the reaction solution also contained ammonia [13]. Because of the simplicity of this removal process, it could potentially be suitable for the removal of other PPCPs. However, the majority of research regarding breakpoint chlorination has been related to the removal of ammonia, or to disinfection and the subsequent formation of DBPs and N-DBPs. As such, there have been few studies concerning the application of this process to the oxidation of PPCPs. Therefore, the extent to which an ammonia/chlorination treatment is able to degrade different PPCPs, the effects of the aqueous matrix and the associated degradation mechanism remain unclear. Thus, the synergistic effects between ammonia and chlorine at the oxidant dosage similar to disinfection level were studied for removing the recalcitrant pollutants, and the corresponding oxidation mechanism and operational optimization were also investigated. The chemical N, N-diethyl-3-toluamide (DEET) has been used as an insect repellent for more than 60 years and has been shown to be present in a wide range of natural aquatic environments at concentrations in the ng/L to μg/L range [12,27–29]. Some studies have determined that DEET is a potential human nasal mucosal cell carcinogen and this compound is also weakly toxic to fish, birds and invertebrates [28,30,31]. While DEET resists biodegradation [28,30], several studies have reported that DEET cannot be degraded using pure ozone or direct chlorination [12,28,32]. This work targets to evaluate the performance of ammonia/chlorine

2. Materials and methods 2.1. Chemicals and reagents DEET of purity higher than 98% was obtained from J&K Scientific, Ltd. (China), ammonia-15N chloride (15NH4Cl, 98 atom% 15N) were purchased from Sigma Aldrich (USA). Other chemical reagents, such as t-butanol (TBA), 14NH4Cl, NaClO, NaH2PO4, Na2HPO4·12H2O were all of analytical grade. NOM (Suwannee River, 2R101N) was obtained from International Humic Substance Society (IHSS). Ultrapure water (UPW) was prepared by Milli-Q (Millipore, Massachusetts, USA). A wastewater effluent sample was collected from a reclaimed water treatment plant at Shenzhen of China (Water quality parameters were shown in Table S1). 2.2. Experimental procedures Chlorination and chloramination experiments of DEET were conducted by spiking certain dose of chlorine and monochloramine into DEET solution buffered with 20 mM phosphate buffer (PBS) to keep solution pH at 7. Monochloramine was prepared just before chloramination experiment. NaClO solution and NH4Cl solution were mixed at the molar ratio of 0.8:1, then newly prepared solution was detected by UV–vis spectrum (UV-2600, Shimadzu, Japan) under 243 nm and 297 nm to make sure that the absorbance at 243 nm far exceeded the absorbance at 297 nm which meant that the major constituent in the mixture was monochloramine and there was limited dichloramine. The concentration of monochloramine was measured as 1.02 g-Cl2/L. Ammonia/chlorination experiment was conducted in beakers. Reaction solution contains 2 mg-N/L ammonia, 10 μM DEET. Free chlorine was in the form of sodium hypochlorite in the stock solution. The chlorine dosage was between 0 and 40 mg-Cl2/L. No effect of phosphates on DEET removal was found as shown in Fig. S1, thus PBS was added to keep the solution pH stable. Based on the relationships between pH and the formation rate of monochloramine and dichloramine (Figs. S2a and S2b), the experiment investigating the effect of pH on ammonia/chlorination was conducted at pH 5.5 and pH 9.5. To keep the certain solution pH (5.5, 7.0 and 9.5), 20 mM PBS was added into the reaction system, while the concentration of PBS was 5 mM to prepare samples for QTOF-MS. The reaction solution kept stirring at 300 r/min using a magnetic stirrer during the reaction. Before sampling, excessive sodium thiosulfate was added to consume residual chlorine. Addition of sodium thiosulfate didn’t affect the determination of DEET. Scavenging experiments were conducted by adding 10 mM TBA to quench most radical species [13], while other experimental conditions were kept the same. When applied in a wastewater effluent, the ammonia concentration was adjusted to 2 mg-N/L by adding ammonium chloride. Due to the strong buffering capacity of a wastewater effluent, no PBS was added to the reaction system. Other operating conditions were the same as in the synthetic water. 2.3. Analytical methods 2.3.1. DEET concentration analysis The concentration of DEET was determined by high-performance liquid chromatograph (HPLC). The HPLC system was equipped with a C18 reversed-phase column (250 mm × 4.6 mm i.d., 5 µm particle size; BonnaAgela Technologies, China) and a PDA detector (SPD-M20A, 2

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Removal of DEET (%)

Shimadzu, Japan). The mobile phase was a 80:20 (v:v) mixture of methanol and ultrapure water, and the flow rate was 0.7 mL/min with 20 μL of injection volume. The column oven temperature was set as 40 °C. The wavelength of PDA detector was 254 nm. The retention time of DEET was 6.8 min. 2.3.2. Determination of water quality The concentration of ammonia nitrogen was measured by a potable ammonia nitrogen analyzer (HI 96700, Hanna, Italy). The concentrations of free chlorine and total chlorine residual were determined by a spectrophotometer (DR3900, Hach, America) using the DPD method. The dissolved organic carbon (DOC) was measured by a total organic carbon (TOC) analyzer (TOC-L, Shimadzu, Japan). 2.3.3. Identification of transformation products To identify the transformation products after ammonia/chlorination, samples of experimental groups were prepared after 15 min at Cl/ N molar ratio as 1.6, while no chlorine was added in the blank groups and other conditions kept the same. To investigate the function of ammonia nitrogen in the reaction, both 15NH4Cl and 14NH4Cl were used as ammonia source. The transformation products of DEET by ammonia/chlorine synergistic oxidation process were identified by quadrupole time of flight mass spectrometer (QTOF), which was equipped with an Agilent 1200 HPLC (America) coupled to a QTOF (AB QSTAR-Elite, SCIEX, America). Samples were injected directly to the system without column with 20 μL injection volume. The mobile phase was a 80:20 (v:v) mixture of methanol and ultrapure water, and the flow rate was 0.28 mL/min. The ESI ion source in positive ion mode: drying gas temperature 300 °C, ion source gas 1 50 psi, ion source gas 2 30 psi, curtain gas 30 psi, ion spray voltage 5500 V.

a

Ammonia/chlorination Chlorination Chloramination

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Chlorine dosage (mg-Cl2/L) Chlorine dosage (mg-Cl2/L) 10

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Removal of DEET

2.3.4. Detection of DBPs DBPs were detected by gas chromatograph equipped with an electronic capture detector (GC-ECD). The detailed information could be found in Text S1 and Table S2. 3. Results and discussion

40

Total chlorine residual

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60 20 40

10 20

3.1. Synergistic effect of ammonia and free chlorine during DEET removal 0

In this work, DEET was removed by direct chlorination, chloramination and ammonia/chlorination, as shown in Fig. 1a. Since dichloramination was stable only at pH 4–5, the direct oxidation of DEET by dichloramine could be neglected at pH 7. Both direct chlorination and chloramination were achieved by spiking a specific dosage of chlorine or chloramine (present as monochloramine in the pH range of 6.5–8.5) to the reaction solution. However, even at free chlorine or chloramine levels equivalent to 40 mg-Cl2/L, only 1.6% to 4% of the DEET was eliminated, indicating that DEET is resistant to these chemicals. Similar result was observed in a previous study, and a rate constant of 0.0016 ± 0.0003 M−1·s−1 between HOCl and DEET was reported [12]. Remarkably, ammonia/chlorination outperformed both chlorination and chloramination in terms of eliminating DEET. DEET removal was found to increase from 4.1% to 55% upon increasing the molar ratio of chlorine/ammonia (Cl/N) from 1.0 to 1.6, then to decrease to 10% as the Cl/N ratio further raised to 4.0. DEET was not removed at Cl/N ratios in the range of 0 to 1.0, because the majority of chlorine was transformed into monochloramine under these conditions. The DEET elimination trends when using ammonia/chlorination conincided with variations in the total residual chlorine, as shown in Fig. 1b. Very little DEET was removed during the chloramination stage (Cl/N < 1.0), while the performance was significantly improved as additional chlorine was spiked (1.0 ≤ Cl/N ≤ 1.6). Over the range of 1.0 ≤ Cl/N ≤ 1.6, the monochloramine was transformed into dichloramine, that subsequently underwent rapid decomposition, such

0

1

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Total chlorine residual (mg-Cl2/L)

100

0

0

Cl/N molar ratio Fig. 1. (a) Comparison of DEET removal by three chlorine oxidation processes, (b) DEET removal and change of total chlorine residual at different molar ratio of Cl/N.

that the total chlorine residual concentration was minimized at a Cl/N ratio of approximately 1.6. At this same point, the maximum extent of DEET elimination was achieved. However, the DOC value of the solution after ammonia/chlorination (Cl/N 1.6) showed no significant decrease. With further increases in the Cl/N ratio from 1.6 to 4.0, all the monochloramine was consumed and free chlorine again became available, while the degree of DEET elimination was lowered relative to that at Cl/N ratio of 1.6. The above results suggest that, at an optimal Cl/N ratio, the ammonia/chlorination process generate powerful reactive species, which are responsible for the degradation of recalcitrant contaminants. It has been proposed that dichloramine is unstable under neutral conditions, and that the decomposition of both chloramine and dichloramine produces more reactive species, such as NOH/NO–, ONOOH/ONOO– and % OH, as summarized in Eqs. (1)–(8) [26,33,34]. Thus, the removal of DEET exhibits a linear correlation with the decomposition of chloramine (the descending section of the total chlorine residual

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(1)

NH2 Cl + H2 O→ HOCl + NH3

(2)

HOCl + NH2 Cl → NHCl2 + H2 O

(3)

NHCl2 + H2 O→ HOCl + NH2 Cl

(4)

NHCl2 + H2 O→ NOH + 2HCl

(5)

HOCl + NHCl2 → NCl3 + H2 O

(6)

NCl3 + H2 O→ HOCl + NHCl2

(7)

NOH + O2 → ONOOH → ·NO2 + ·OH

(8)

α=

Removal of DEET(μmol) Decomposed chloramine(μmol)

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0

(9)

In Eq. (9), the term “Decomposed chloramine” refers to the concentration of chloramine at a given point in time subtracted from the maximum chloramine concentration, while the term “Removal of DEET” refers to the reduction in the amount of DEET. The experimental data were fitted and the slope of the fitting curve (α) was determined to be 0.034 ( ± 0.003), meaning that each mole of decomposed chloramine could remove 0.034 mol of DEET. When it turned to ozonation of DEET, each mole of ozone molecule could degrade 0.027 mol of DEET, and the degradation capacity of ammonia/ chloramination showed a little higher than direct ozonation [28]. In our previous study, fitting the relationship between CBZ removal and consumed chloramine gave α value of 0.043 [13]. It’s not unexpected that different PPCPs would give different α values, and this parameter could potentially be used to evaluate the ease with which a given PPCP can be removed using ammonia/chlorine synergistic oxidation process. In future work, we intend to examine the relationship between the chemical structure of a PPCP and α, the relationship between the rate constants of PPCP and reactive species, so as to obtain a better understanding of this new oxidation process. Based on the present and previous studies [13,35], chlorine efficiently removes various PPCPs in the presence of ammonia, by generating several different reactive species. On this basis, we propose a new chlorine-based oxidation treatment that we term the ammoniachlorine synergistic oxidation process (SOP).

a

pH = 7.0 pH = 5.5 pH = 9.5

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Time (min) Chlorine dosage (mg-Cl2/L) 100

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pH = 7.0 pH = 5.5 pH = 9.5

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HOCl + NH3 → NH2 Cl + H2 O

Removal of DEET (%)

concentration curve). This relationship can be summarized as in Eq. (9), which is first reported for CBZ removal [13]:

0

Cl/N molar ratio

3.2. Factors affecting DEET removal

Fig. 2. (a) The influence of pH on reaction rate at molar ratio of Cl/N as 1.6, (b) The influence of pH on DEET removal efficiency.

The properties of the aqueous matrix are the most crucial factors affecting the performance of an advanced oxidation process [36]. A great number of organic and inorganic substances can be present in an aqueous environment, and can either promote or suppress the removal of target pollutants. In the work reported herein, the effects of solution pH and of NOM were investigated.

Several studies have demonstrated that solution pH greatly affects the reactions between chlorine and ammonia [34,37]. Fig. S2a shows that the rate of monochloramine formation via the reaction in Eq. (1) at pH 7.0 was approximately 24.6 and 0.63 times as fast as the rates at pH values of 5.5 and 9.5, respectively [38]. In addition, the rate of dichloramine formation via the reaction in Equation (3) at pH 7.0 was approximately 0.78 and 71.9 times as fast as the values obtained at pH 5.5 and 9.5, respectively, as shown in Fig. S2b [38]. Under these conditions, the NOH/NO− pair would be produced from dichloramine according to the reaction in Equation (5), and reactive species such as ONOOH/ONOO− and %OH would be generated via the reaction in Equation (8). The NOH formation rate is proportional to the concentration of hydroxide ions [38], and so this species will be formed more rapidly under basic conditions. In a neutral solution, both monochloramine and dichloramine are rapidly produced, thus accelerating the generation of reactive species and promoting the removal of DEET. The data confirmed that the reaction rate was faster at pH 7.0 compared to the values at pH 5.5 and 9.5.

3.2.1. The effects of solution pH The effect of solution pH on the reaction rate and the DEET removal efficiency when using the ammonia/chlorine SOP are summarized in Fig. 2a and b. It is evident that, at a Cl/N molar ratio of 1.6, the reaction rate was highest at pH 7.0, and that this rate was lower but similar at pH values of 5.5 and 9.5. The equilibration time also changed along with the pH, with times of 10, 15 and 45 min at pH values of 5.5, 7.0 and 9.5, respectively. At equilibrium, the DEET removal efficiencies were quite different, with values of 12.5%, 59.6% and 17.6% at pH 5.5, 7.0 and 9.5, respectively. These results are also significantly different from those obtained when examining the removal of CBZ using this same process [13]. In prior work with CBZ, the same removal efficiency of, approximately 72%, was observed following equilibration regardless of the pH. 4

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Chlorine dosage (mg-Cl2/L) 20

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Fig. 3. The influence of NOM concentration on DEET removal efficiency.

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Cl/N molar ratio

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0 mg/L NOM 3 mg/L NOM 6 mg/L NOM Reclaimed water Linear regression Linear regression Linear regression Linear regression

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Decomposed chloramine (ȝM-Cl2) Fig. 5. (a) Application in reclaimed water, (b) Relationship between removal of DEET and decomposed chloramine.

0

Cl/N molar ratio

removal efficiency. From the data in Fig. 5b, we can obtain α values for 0, 3 and 6 mg-C/L NOM of 0.034 ( ± 0.003), 0.031( ± 0.002) and 0.022 ( ± 0.002), respectively. These results indicate that the amount of DEET removed by a given amount of decomposed chloramine decreased with increasing NOM concentration, which agrees with the preceding discussion.

Fig. 4. Inhibition of ammonia/chlorine synergistic oxidation process by TBA.

3.2.2. The effects of NOM concentration Fig. 3 presents the DEET removal profile in the presence of NOM. These data confirm that the addition of NOM significantly suppressed the removal efficiency and also increased the Cl/N molar ratio required to obtained the highest removal efficiency and the lowest total chlorine residual concentration. In the absence of NOM, the DEET removal efficiency was maximized at a Cl/N ratio of 1.6, with a value of approximately 54.6%. In contrast, after the addition of NOM equivalent to 3 and 6 mg-C/L, the removal efficiency was maximized at a Cl/N molar ratio of 2.0, with values of 41.6% and 36.6%, respectively. In solutions to which NOM was added, the DOC concentration was almost 58.6 times the dissolved organic nitrogen (DON) concentration, and so the inhibitory effect of the NOM was attributed to DOC in this study. The NOM evidently reacted with chlorine to shift the optimal Cl/N molar ratio and also competed with DEET to react with the various species generated during the oxidation process, thus lowering the DEET

3.3. Verification of the generation of reactive species The DEET removal efficiency data and variations in the total chlorine residual concentration at different Cl/N molar ratios with and without the addition of TBA are plotted in Fig. 4. The rate constant for DEET with hydroxyl radicals was reported as 7.51 ± 0.08 M−1·s−1, indicating that DEET was easy to react with hydroxyl radicals [39]. TBA is known to scavenge for %OH and reactive chlorine species and can also quench nitroxyl radicals [13]. Following the addition of 10 mM TBA, the DEET removal rate was almost negligible (fluctuating between 1% and 3%), demonstrating that the removal of DEET was greatly inhibited (by almost 95%) by the TBA. Interestingly, prior work has shown that 5

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Table 1 Identified transformation products of DEET by LC-QTOF-MS. Experimental m/z [M + H] +

Experimental m/z [M + Na] +

Molecular Formula

DEET

192.1387

214.1268

C12H17NO

TP 1

208.1341

230.1150

C12H17NO2

TP 2

208.1341

230.1150

C12H17NO2

TP 3

208.1341

230.1150

C12H17NO2

TP 4

206.1231

228.0988

C12H15NO2

TP 5

206.1231

228.0988

C12H15NO2

TP 6 TP 7

148.9247 164.1073

/ 186.0886

/ C10H13NO

TP 8

224.0884

246.1131

C12H17NO3

TP 9

146.0956

/

C6H11NO3

TP TP TP TP

135.0437 80.9471 150.0586 178.1232

/ / / /

/ / C8H7NO2 C11H15NO

10 11 12 13

Proposed Structure

/

/ / /

(continued on next page)

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Table 1 (continued) Experimental m/z [M + H] +

Experimental m/z [M + Na] +

Molecular Formula

TP 14

198.0751

220.0936

C10H15NO3

TP 15

118.0624

/

C5H11NO2

TP 16

226.0991

248.0887

C11H15NO4

TP 17 TP 18

80.9440 /

/ 204.1028

C10H15NO2

TP 19

254.1039

/

C12H15NO5

TP 20

222.1238

244.0834

C12H15NO3

TP 21

180.1358

202.0837

C10H13NO2

Proposed Structure

The DON would be expected to consume chlorine while also generating organic chloramine, which could also be measured as total chlorine residual. This, in turn, would reduce the removal efficiency without affecting the optimal Cl/N ratio. From Fig. 5b, the α value for DEET removal from the wastewater effluent was found to be 0.010 ( ± 0.001), equal to approximately one third of the value obtained in synthesized water. This value confirms that each unit of decomposed chloramine removed only 0.010 units of DEET. This occurred because the DON in the wastewater consumed chlorine and thus competed with the DEET. The presence of DON thus had a greater impact than that of DOC on the ammonia/chlorine SOP.

the presence of the same concentration of TBA, still allows the removal of 10% of the CBZ in solution [13]. These results confirm that only those reactive radicals that can be quenched by TBA, such as hydroxyl radicals, reactive chlorine species and nitroxyl radicals (generated by the decomposition of chloramine), play important roles in the removal process of DEET. 3.4. Application to a wastewater effluent This ammonia/chlorine SOP was subsequently applied to a wastewater effluent in which the initial DOC, DON and ammonia concentrations were equivalent to 3.93 mg-C/L, 1.67 mg-N/L and 0.1 mgN/L, respectively. The final ammonia concentration in this sample was adjusted to approximately 2 mg-N/L so as to have a value equivalent to those in the previous experiments. As shown in Fig. 5a, the DEET removal efficiency from this wastewater effluent was greatly decreased (to 14.1%) compared to that obtained from the synthesized water (from which 54.6% DEET was removed). This value was also much lower than the removal efficiency in the presence of 6 mg-C/L NOM (36.6%). Interestingly, these trials with wastewater did not show a change in the optimal Cl/N ratio as was observed during the NOM trials. That is, the highest removal efficiency and the lowest total chlorine residual concentration in the wastewater effluent experiments appeared at a Cl/N molar ratio of 1.6. The significant decrease in the DEET removal efficiency from the wastewater effluent is ascribed to the presence of DON.

3.5. Transformation products and degradation pathway The DEET TPs after the ammonia/chlorine SOP were identified by quadrupole time of flight mass spectrometry (QTOF-MS). Samples were injected directly into the system without a column, so that TPs were not separated prior to analysis. By comparing the results from experimental and blank samples, we identified the 21 possible TPs summarized in Table 1. These TPs are provided in the order of decreasing intensity, along with their proposed molecular structures according to the literature [6,12,29,30,40–43]. The TPs identified in this study are similar to those found in previous research using different treatments in which %OH was the predominant reactive species for DEET removal. However, it should be noted that the identifies of four of the TPs (TP 6, TP 10, TP 7

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Fig. 6. Proposed possible degradation mechanism.

to decarboxylation and further ring opening. The decarboxylation reaction of TP 20 could also lead to the formation of TP 13, from which tri-hydroxylation gives TP16. Aromatic ring opening also occurs via a second attack by %OH on the same carbon atom [40,43]. The conjugated π bonds promote the transfer of a single electron and, after additional oxygenation and decarboxylation reactions, TP 18 is produced. The hydroxylation of this compound could give TP 14, while reaction with ONOO− and a subsequent attack by %OH produces TP 9 from TP 18. Chlorination usually leads to the formation of toxic chlorine-based disinfection byproducts (Cl-DBPs) due to the reaction between chlorine and organic matters in water [19,21]. To investigate the formation of Cl-DBPs in water samples after ammonia/chlorine synergistic oxidation process, 11Cl-DBPs were selected as detection targets (detailed in Table S2, comparing with those after chlorination. Results were shown in Fig. S4. Three Cl-DBPs, including trichloromethane (tCM), chloral hydrate (CH), and dichloroacetonitrile (dCAN), were both detected in water samples after chlorination and ammonia/chlorine SOP. CH was the predominant Cl-DPB in water samples after ammonia/chlorination, with the concentration of 12.34 μg/L, almost 12 times of that after chlorination. The concentrations of tCM and dCAN in water samples after ammonia/chlorine SOP were slightly higher than those after chlorination, being 3.83 and 1.37 μg/L, respectively. Trichloronitromethane was only detected after ammonia/chlorine SOP with a quite small concentration of 0.12 μg/L. Taken DEET removal efficiency into consideration, though ammonia/chlorine SOP had higher Cl-DBPs absolute yields than chlorination, the ratio of Cl-DBPs yield after ammonia/chlorination to removed DEET was only 2.8%,

11, TP 17) could not be determined with certainty. Based on these data, a possible degradation mechanism is proposed in Fig. 6. Eight of the TPs detected are hydroxylated derivatives of DEET: TP 1 (m/z [M + H]+ = 208), TP 2 (m/z [M + H]+ = 208), TP 3 (m/z [M + H]+ = 208), TP 4 (m/z [M + H]+ = 206), TP 5 (m/z [M + H] + = 206), TP 8 (m/z [M + H]+ = 224), TP 19 (m/z [M + H]+ = 254) and TP 20 (m/z [M + H]+ = 222), generated either through hydroxylation reaction or further oxidation. Due to the limitations of our analytical methodology, the substitution site of hydroxyl groups (-OH) on the aromatic rings could not be determined, and so both of TP 2 and TP 8 could potentially consist of a combination of up to three different isomers. TP 7 (m/z [M + H]+ = 164) and TP 21 (m/z [M + H]+ = 180) are derived from the elimination of the vinyl group (eCH]CH2) attached to the nitrogen atom, followed by additional hydroxylation. TP 21 also has the potential for up to four isomers based on variations in the substitution site of the hydroxyl groups. TP 12 (m/z [M + H]+ = 150) and TP 15 (m/z [M + H]+ = 118) are generated from the attack of %OH on the bond near the carbonyl group and simultaneous bond cleavage. It should be noted that TP 12 confirms the participation of nitrogen from ammonia nitrogen in the removal of DEET. In trials using 15NH4Cl as the ammonia source, the m/ z [M + H] + value of TP 12 changed from 150 to 151, indicating the successful incorporation of 15N isotope and confirming the participation of ammonia in the reaction, as shown in Fig. S3. The remaining five TPs, TP 9 (m/z [M + H]+ = 146), TP 13 (m/z [M + H]+ = 178), TP 14 (m/z [M + Na]+ = 198), TP 16 (m/z [M + Na]+ = 226) and TP 18 (m/z [M + Na]+ = 204), are attributed 8

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which meant that each mole of removed DEET only produced 0.028 mol of Cl-DBPs, which was significantly lower than that after chlorination, being 167%. It would be the focus of our future work to optimize operation parameters for ammonia/chlorine synergistic oxidation process.

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4. Conclusions A combination of ammonia and free chlorine shows a significant synergistic effect during the removal of DEET, such that nearly 60% of the DEET is removed after 15 min at a Cl/N molar of 1.6. This effect is attributed to the reactive species generated from the decomposition of chloramine, based on trials incorporating TBA as a radical scavenger. We propose that α can be used to quantitatively describe the removal efficiency resulting from ammonia/chlorine synergistic oxidation and to investigate the effects of variations in the sample matrix. Under neutral conditions, both the removal efficiency and reaction rate were highest, due to the increased rate of formation of chloramines. NOM was found to compete with DEET with regard to reaction with various species, and so decreases in removal efficiency were observed with increasing NOM concentration. The transformation products were identified by QTOF-MS and a possible degradation mechanism, in which hydroxylation was the dominant degradation pathway was proposed. Nitrogen from ammonia was confirmed to participate in the DEET removal process based on the detection of a 15N-labeled transformation product. Acknowledgements This work was supported by the National Natural Science Foundation of China [No. 51738005/51678332], the special support program for high-level personnel recruitment in Guangdong Province [No. 2016TQ03Z384], Fundamental Research Program for the state level Public Welfare Research Institutes [No. PM-zx097-201602-05], Science and Technology Program of GuangDong, China [No. 2017A020216003], the Development and Reform Commission of Shenzhen Municipality (urban water recycling and environment safety program). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122409. References [1] J.L. Acero, et al., Degradation of selected emerging contaminants by UV-activated persulfate: Kinetics and influence of matrix constituents, Sep. Purif. Technol. 201 (2018) 41–50. [2] S.D. Richardson, S.Y. Kimura, Emerging environmental contaminants: challenges facing our next generation and potential engineering solutions, Environ. Technol. Innovation 8 (2017) 40–56. [3] S.A. Snyder, et al., Role of membranes and activated carbon in the removal of endocrine disruptors and pharmaceuticals, Desalination 202 (1–3) (2007) 156–181. [4] Y. Lee, U. von Gunten, Oxidative transformation of micropollutants during municipal wastewater treatment: comparison of kinetic aspects of selective (chlorine, chlorine dioxide, ferrate VI, and ozone) and non-selective oxidants (hydroxyl radical), Water Res. 44 (2) (2010) 555–566. [5] J. Li, et al., Improving the degradation of atrazine in the three-dimensional (3D) electrochemical process using CuFe2O4 as both particle electrode and catalyst for persulfate activation, Chem. Eng. J. 361 (2019) 1317–1332. [6] W. Dong, et al., Enhanced emerging pharmaceuticals removal in wastewater after biotreatment by a low-pressure UVA/FeIII-EDDS/H2O2 process under neutral pH conditions, Chem. Eng. J. 366 (2019) 539–549. [7] Z. Xiong, et al., Removal of nitrophenols and their derivatives by chemical redox: a review, Chem. Eng. J. (2019) 13–31. [8] K. Guo, et al., Radical chemistry and structural relationships of PPCP degradation by UV/chlorine treatment in simulated drinking water, Environ. Sci. Technol. 51 (18) (2017) 10431–10439. [9] Z. Xiong, B. Lai, P. Yang, Insight into a highly efficient electrolysis-ozone process for N, N-dimethylacetamide degradation: quantitative analysis of the role of catalytic ozonation, fenton-like and peroxone reactions, Water Res. 140 (2018) 12–23.

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