New insight into the behavior and cost-effectiveness of different radicals in the removal of NO and Hg0

Chemical Engineering Journal 385 (2020) 123885

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New insight into the behavior and cost-effectiveness of different radicals in the removal of NO and Hg0

T

Runlong Haoa,b, , Zhao Maa, Zhen Qiana, Yaping Gonga, Zheng Wanga, Yichen Luoa, Bo Yuana,b, Yi Zhaoa,b ⁎

a

Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, PR China b MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China

HIGHLIGHTS

GRAPHICAL ABSTRACT

were more efficient than • Cl-radicals SO % and HO% in terms of removal of 4

• • •

−

NO and Hg0. NaClO2 possessed the highest absorbance at 254 nm. UV/NaClO2 is an emerging and promising UV-AOP method. The radical evolution under different pH conditions were revealed by ESR analysis.

ARTICLE INFO

ABSTRACT

Keywords: Radicals NO removal Hg0 removal ESR UV-AOP

Radical-induced removal of NO and Hg0 is a hot topic, but few people comparatively study the behavior and costeffectiveness of three categories radicals (Cl-radicals, SO4%− and HO·) in the removal of NO and Hg0. For this, we used ultraviolet (UV) to irradiate NaClO2, NaClO, Na2S2O8, KHSO5 and H2O2, respectively, to produce the desirable radicals, and used electron spin resonance (ESR) to reveal the generation and evolution of the radicals in the five UV-AOP systems under different pH. The results demonstrated that Cl-radicals (ClO2, ClO· and Cl2%−) produced in UV/NaClO2 and UV/NaClO were more efficient than SO4%− and HO· produced in UV/Na2S2O8, UV/KHSO5 and UV/H2O2 in term of removal of NO and Hg0. UV/NaClO2 had the best cost-effectiveness (hundreds to thousand times higher than others), possibly due to NaClO2 possessed the highest absorbance at 254 nm (124.3 abs/mol), implying a possible highest quantum yield. Cl-radicals also exhibited a good selectiveness toward the removal of NO and Hg0 under high concentration of SO2. Hence, Cl-radical-based UV-AOP system, especially UV/NaClO2, is hopefully to be an emerging and promising UV-AOP method which can be used in both air pollution control field and water treatment field.

⁎ Corresponding author at: Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, PR China. E-mail address: [email protected] (R. Hao).

https://doi.org/10.1016/j.cej.2019.123885 Received 24 September 2019; Received in revised form 13 December 2019; Accepted 17 December 2019 Available online 19 December 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 385 (2020) 123885

R. Hao, et al.

1. Introduction

improve the performance of different UV-AOP methods in the removal of NO and Hg0, few people studied the light sensitivity of different radical precursors. As we all know the radical yield in UV-AOP method mainly depends on the absorption of high energy photon by radical precursors. The ability on absorbing UV light surely affects their quantum yield and radical yield. Hence, studying the performance of different radical precursors on absorbing UV light is of significance. Besides, there was few works to comparatively study the generation and evolution of different radicals under different pH conditions by means of electron spin resonance (ESR); as well the cost-effectiveness of different UV-AOP systems in the removal of NO and Hg0 was seldom reported. Hence, the objective of the current paper is to evaluate the behavior and cost-effectiveness of three categories radicals (Cl-radicals, SO4%− and HO·) in the removal of NO and Hg0. We used ultraviolet (UV) to irradiate NaClO2, NaClO, Na2S2O8, KHSO5 and H2O2, respectively, to produce the desirable radicals, and finally gained the best UVAOP method in term of removal of NO and Hg0.

Developing a method that can simultaneously remove SO2, NO and Hg0 in flue gas is a hot topic in the field of air pollution control. Prior to simultaneous removal process, rapidly and efficiently oxidizing NO and Hg0 is a key step since NO and Hg0 are insoluble in aqueous [1–2]. Radical-induced oxidation of NO and Hg0 is considered to be the most efficient method. According to the generation path of radicals, radicalinduced oxidation method can be clarified into two categories: photocatalytic oxidation and advanced oxidation process (AOP). The mechanism for photo-producing radical driven by visible light can be described as a photocatalytic process that O2 and H2O adhered onto the active sites of catalyst surface can capture the photo-produced electrons (e−) and holes (h+) to produce the highly reactive superoxide radical (O2%−) and hydroxyl radicals HO· [3–7]. A series of photo-catalysts have been synthesized and used to photo-catalytically oxidize NO and Hg0, such as blue TiO2 [3], BiOBr/BiOI [4], carbon quantum dots (CQDs)/ZnFe2O4 composite [5], Bi2O3/Bi2O2CO3 heterojunctions [6], g-C3N4 [7], Ag/AgI-Ag2CO3 [8], AgI-BiOI/CoFe2O4 [9], Ag/BiOI/ ZnFe2O4 [10] and Ag@AgCl/Ag2CO3 [11]. Another photocatalytic oxidation method is short wavelength ultraviolet (UV) induced ozonation process: when the light wavelength is shorter than 200 nm it can break the oxygen bond (O-O) to generate two highly reactive oxygen atoms (O·), which can then trigger radical chain reactions to produce a variety of secondary radicals [12]. For instance, (1) O· can react with O2 to produce ozone (O3) [12]; (2) O3 can further react with H2O to produce H2O2; (3) under UV irradiation, H2O2 can be further photo-decomposed into two HO·; (4) UV light lower than 200 nm can also split H2O to H· and HO· [13]. These produced radicals can conduct a homogeneous oxidation of NO and Hg0. Advanced oxidation process (AOP) is a liquid phase radical-induced oxidation method. According to the catalytic means, it can be organized into four categories: (1) transition metal catalysis [14–15], (2) photocatalysis [12,16], (3) thermocatalysis [17] and (4) other physiochemical catalysis (ultrasound [18] and microwave [19]). The oxidants usually used as radical precursors include H2O2 [12], persulfate (PS) [17], oxone (PMS) [16], NaClO2 [20–21] and NaClO [22] etc. The latest developed AOP methods for removing NO and Hg0 are photo-Fenton [14], Fe2+/Co2+/PMS [15], vacuum (VUV)/O2/H2O2 [12], Heat/PS [17], VUV-Heat/PMS [16], UV/NaClO [22] and UV/NaClO2 [20–21], etc. The radicals played leading roles in the removal of NO and Hg0 are mainly HO·, sulfate radical (SO4%−), oxychloride radical (ClO·) and chlorine dioxide (ClO2). UV-AOP method has the advantages of insensitive to the solution pH, less energy consumption and no need to treat the waste water. As aforementioned, previous works mainly focused on how to

2. Experimental section 2.1. Chemical reagent H2O2 (30%wt), Na2S2O8 (98.0%wt), KHSO5 (47.0%wt), NaClO (8.0% chlorine) and NaClO2 (82.0%wt) were used to prepare the oxidant solutions. 10% (v/v) H2SO4-4% (w/w) KMnO4 was used to totally capture the unreacted Hg0 in tail gas after mercury detector. 2.2. Experimental setup and procedure The experimental setup is shown in Fig. 1. The cylinders of N2, NO and SO2 were used to produce the simulated flue gas. Hg0 was released from a mercury osmotic tube (1000 ng/min, VICI Metronics Co., USA). The gas flow is 2.0 L/min. During the experiment, the prepared the simulated flue gas flows into a UV catalytic reactor which is a cylinder and jacketed quartz-wall reactor, made by quartz and heated by a thermostat water bath (HH-ZK4, Yuhua instrumental Co., China). The diameter and height of the inner and outer cylinder were 60/96 mm and 140/200 mm, respectively. A low-pressure lamp (TUVPS-S, Philips Co., Beijing) with light intensity of 3.82 × 10−4 Einstein·s−1 was inserted into inside cylinder to irradiate the oxidant solution. The temperature and pH of the oxidant solution were detected online by an inside thermocouple and a pH meter. The reacted flue gas then passes through a dryer to dry the tail gas, NO and Hg0 in the inlet and outlet flue gases were detected using a flue gas analyzer (ECOM-J2KN, RBR Company, Germany) and a mercury detector (RA-915 M, Lumex Co. Russia), respectively. The baseline of mercury detector was calibrated

Fig. 1. Flow diagram of the experimental apparatus. 2

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by highly pure N2. The initial concentration of Hg0 was recorded until the Hg0 concentration was stable at least 10 min. The removal efficiencies of Hg0 and NO were calculated via eq. (1).

=

Cin

Cout × 100% Cin

NaClO2 into ClO2 [25], thus ClO2 produced under low pH possibly contributed to the increase of absorbance. By contrast, the absorbance of the NaClO solution decreased with the pH decreasing, indicating that acid was an inhibitor for NaClO absorbing UV254 light. And Cl2/HClO formed in acidic condition could be the main inhibitors for the decreasing of absorbance. The impact of the pH on the absorbance of KHSO5 was complicated: the absorbance obtained at intrinsic pH (1.54) was higher than that obtained at neutral pH, but was comparable to that obtained at alkaline condition. This case is similar to that happens to Na2S2O8. For the case of H2O2, an increasing of the pH from 4 to 10 increased the absorbance, suggesting that HO2–/O2– could absorb more UV254 light. But one thing should be noted that the decomposition of H2O2 to O2 would be accelerated with the pH increasing, which would significantly decrease the utilization rate of H2O2, thus using H2O2 under alkaline conditions was not recommended.

(1)

where η is the removal efficiency; Cin and Cout are the inlet and outlet concentrations, μg/m3 for Hg0 and mg/m3 for NO. 2.3. Analytical method A TU-1900 double beam UV–Vis spectrometer was employed to determine the absorbance. The radical species were identified by using ESR method, and 5, 5-dimethyl-1-pyrroline-Noxide (DMPO) was used as a spin-trapping agent. The operation procedure was available in Supporting Information (SI), as Text S1.

3.2. Radical chemistry of different UV-AOP systems under different pH conditions

3. Results and discussion 3.1. Light sensitivity and UV254 absorbance of different UV-AOP systems

We then used ESR method to identify the kind of radicals produced in different UV-AOP systems. As shown in Fig. 3 (A), using UV254 to irradiate 100 mM NaClO2 solution forms a characteristic spectrum with seven peaks, which can be assigned to the DMPO-Ox’ adduct. The formation of DMPO-x’ adduct was due to the oxidation of DMPO by strong oxidant [26]. The possible radical was ClO2 because the UV/NaClO2 was inclined to produce ClO2 under the intrinsic pH conditions (eqs. (2)–(3)) [25,27]. Reaction (4) could not be the leading reaction path as the signal of DMPO-OCl adduct was not observed [25]. In Fig. 3 (B), when the pH of the NaClO2 solution decreased to 4, the DMPO-Ox’ adduct disappeared, but the DMPO-OH adduct was formed, suggesting the elimination of ClO2 but the formation of HO·. The possible reasons for the formation of HO· under acidic condition could be that (1) the produced ClO2 (eq. (5)) [25] was irradiated by UV254 to produce O· (eq. (6)) which then attacked H2O to yield two HO· (eq. (7)) [28]; (2) another reaction path was due to the reaction between O%− (eqs. (3)–(4)) and the extra H+ (eq. (8)) [29]. The disappearance of the DMPO-Ox’ adduct indicated that the reaction between ClO2 and DMPO was not the leading path under acidic condition, which was because after an irradiation of 5 and 10 min ClO2 was photo-decomposed into ClO· and O· (eq. (6)). Thus reaction (7) became the main reaction path for the formation of HO·, and reaction (8) played a minor role as a result of the reaction (4) was suppressed under acidic condition. ClO2 (eqs. (2)–(3)) mildly produced under UV illumination was the main reason for the formation of DMPO-Ox’ adduct.

Fig. 2 (A) displays the UV–Vis spectra of 0.1 M H2O2, 0.5 M Na2S2O8, 0.1 M KHSO5, 0.025 M NaClO and 0.0125 M NaClO2, the detail UV254 absorbance values are shown in Table S1. The sensitive range of light wavelength for H2O2, Na2S2O8, KHSO5 and NaClO2 is from 210 to 250 nm, while that for NaClO appears at 270–320 nm, which suggested that using 210–250 nm light to irradiate H2O2, Na2S2O8, KHSO5 and NaClO2 solutions might yield more radicals; while 270–320 nm light might be more useful for NaClO. Fig. 2 (B) shows the calculated absorbance of per mole H2O2, Na2S2O8, KHSO5, NaClO and NaClO2 at UV254 under different pH conditions, the value is 18.3, 18.9, 22.7, 78.2 and 124.3 abs/mol, respectively, indicating that NaClO and NaClO2 are better to be used as radical precursors, especially NaClO2. Using UV/NaClO2 and UV/NaClO may realize the deep degradation of contaminants. Then we studied the influence of the pH on the absorbance of different oxidation systems. As the pH of the Na2S2O8 solution increased from the intrinsic 4.87 to 7 and 10, the absorbance decreased at first but then increased, indicating that alkali facilitated Na2S2O8 absorbing UV254 light, which could be the reason why UV/Na2S2O8-OH− was more useful in the degradation of water contaminants [23–24]. When the pH of the NaClO2 solution decreased from the intrinsic 12.05 to 7 and 4, the absorbance increased a little, suggesting that acid promoted NaClO2 absorbing UV254 light. It is known that acid can decompose

Fig.2. UV–Vis spectra of different oxidation solutions (A); effect of the pH conditions on the absorbance of different oxidation systems at 254 nm light. (B). 3

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100mM NaClO2 at 5 min 100mM NaClO2 at 10 min

3000

(A)

2500

2000

2000

1500

1500

1000

1000

500

500

Intensity

Intensity

2500

0 -500

-1000 -1500

-2000

-2000 -2500

-3000 318.4

318.5

318.6

318.7

-3000 318.2

318.8

mT

100mM NaClO at 5 min 100mM NaClO at 10 min

3000

(C)

2500

2000

2000

1500

1500

1000

1000

500

500

Intensity

Intensity

2500

0 -500

-1000 -1500

-2000

-2000

-2500

-2500 318.6

318.7

-3000 317.7

318.8

mT

3000 2500

(E)

100mM Na2S2O8 at 5 min 100mM Na2S2O8 at 10 min

1500 1000

500

500

0 -500

-1000 -1500

-2000

-2000

-2500 318.3

318.4

318.5

318.6

318.7

100mM KHSO5 at 5 min 100mM KHSO5 at 10 min

-3000 318.3

(G)

3000

Intensity

-500

318.1

318.2

318.3

(F)

318.4

318.5

318.6

318.7

mT

318.8

318.9

100mM KHSO5 under pH=10 at 5 min 100mM KHSO5 under pH=10 at 10 min

(H)

500 0 -500

-1000

-1000

-1500

-1500

-2000

-2000

-2500

-2500 318.4

318.5

318.6

318.7

318.8

318.9

mT

1000mM H2O2 at 10 min

-3000 318.3

3500

(I)

1000mM H2O2 at 10 min

3000

1500

1000

1000

500

500

Intensity

2000

1500

0 -500

318.6

mT

318.7

318.8

1000mM H2O2 under pH=10 at 10 min 1000mM H2O2 under pH=10 at 5 min

318.9

(J)

-1000 -1500

-2000

-2000

-2500

-2500

-3000

-3000 318.1

mT

318.2

318.3

-3500 318.3

318.4

Lore

0

-1500

318.0

318.5

-500

-1000

317.9

318.4

2500

2000

Intensity

mT

1000

0

-3500 317.8

318.0

1500

500

2500

317.9

2000

1000

3000

317.8

2500

1500

3500

(D)

100mM NaClO under pH=4 at 5 min 100mM NaClO under pH=4 at 10 min

-2500 318.2

mT

-3000 318.3

318.8

0

-1500

2000

318.7

-500

-1000

2500

318.6

2000

1000

3000

mT

2500

1500

-3000 318.1

318.5

100mM Na2S2O8 under pH=10 at 5 min 100mM Na2S2O8 under pH=10 at 10 min

3000

Intensity

Intensity

2000

318.4

0

-1500

318.5

318.3

-500

-1000

-3000 318.4

Intensity

0

-1500

3000

(B)

-500

-1000

-2500

100mM NaClO2 under pH=4 at 5 min 100mM NaClO2 under pH=4 at 10 min

318.4

318.5

318.6

318.7

318.8

318.9

mT

(caption on next page) 4

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Fig. 3. ESR spectra of UV/NaClO2 (A); acidized UV/NaClO2 (B); UV/NaClO (C); acidized UV/NaClO (D); UV/Na2S2O8 (E); alkalized UV/Na2S2O8 (F); UV/KHSO5 (G); alkalized UV/KHSO5 (H); UV/H2O2 (I); alkalized UV/H2O2 (J).

ClO2− + hv → (ClO2−)* (ClO2−)* + ClO2− → ClO2 + ClO− + O·− ClO2− 5ClO2

+ hv → ClO· + O·

−

+ 4H

+

−

→ Cl

−

+ 4ClO2 + 2H2O

(3) (4) (5)

ClO2* + hv → ClO· + O·

(6)

O· + H2O → HO· + HO·

(7)

−

O%

+H

+

→ HO·

radicals. ClO· and Cl2%− (eqs. (11)–(12)) must be the final fate of Cl· because we had demonstrated that the DMPO-Ox’ adduct could be formed from the reaction between Cl2%− and DMPO in a previous work [35]. And the great contribution of ClO· in the degradation of organic contaminant using UV/ClO− had been also reported by Fang [33]. Thus, under the intrinsic pH, the main radicals produced in UV/ClO− could be Cl·, ClO· and Cl2%−; and Cl· was possibly less selective and efficient than ClO· and Cl2%− in the reaction with DMPO. When we decreased the pH of the NaClO solution to 4 in Fig. 3 (D), the ESR spectrum was significantly changed: the DMPO-Ox’ adduct disappeared, instead the DMPO-OH adduct appeared, which suggested the generation of HO· and the decreased quantity of ClO·/Cl2%−. Under the acidic condition, HOCl was formed and decomposed into HO· and Cl· after UV illumination (eq. (13)) [33–34]; besides, the reaction (8) also occurred and contributed to the increase of the yield of HO·. The yield of ClO· and Cl2%− significantly decreased as the signal intensity of DMPO-Ox’ adduct disappeared, which was because the rate constants of HO· and Cl· reacting with HOCl are 4.4 and 2.7, respectively, times lower than that with ClO− [36]. Hence, HO· and Cl· were the main radicals in UV/HOCl. One thing should be noted is that the intensity of DMPO-OH adduct obtained in UV/HOCl was weaker than that obtained in UV/HClO2 (Fig. 3 (B)), which could be due to the difference in the absorbance between the two reaction systems under acidic condition

(2)

(8)

UV/chlorine process has been widely studied in past years [30–34], herein, we just reported some new findings according to the ESR results. In Fig. 3 (C), under intrinsic alkaline pH, the DMPO-Ox’ adduct appeared in UV/ClO−, while the DMPO-OH adduct was not observed. As we all know, photolysis ClO− can produce O%− and Cl· [32–33], which then can induce the further radical chain reactions (eqs. (10)–(12)) to produce the secondary radicals of HO·, ClO· and Cl2%− [33]. Under alkaline condition, reaction (10) would be suppressed, this could be the main reason why DMPO-OH was not observed in Fig. 3 (C). According to Marcon’s work [26], the reaction between Cl· and DMPO could also produce the adduct of DMPO-OH, but it was not observed in UV/ClO−, implying that most of Cl· might transform to other Cl-

Fig. 4. Behavior (A) and cost-effectiveness (B) of different oxidation systems in NO removal. Simulated flue gas flow is 2.6 L/min; oxidation solution volume is 500 mL; reaction temperature is 50 °C; NO concentration is 300 mg/Nm3.

5

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(Fig. 3 (B)). Obviously, UV/HClO2 was more efficient in yielding HO·. ClO

−

−

−

+ hv → O%

Cl· + ClO Cl· + Cl

−

(11)

H2O2 + hv → HO· + HO·

(9)

+ H2O → HO· + OH −

(10)

+ Cl· –

O%

NaClO, Na2S2O8 and KHSO5, because the intensity of ESR spectra of UV/H2O2 (1000 mM) was comparable to that obtained in UV/NaClO2 (100 mM), UV/NaClO (100 mM), UV/Na2S2O8 (100 mM) and UV/ KHSO5 (100 mM).

→ ClO· + Cl

−

−

↔ Cl2%

–

HO2 + hv → HO· + O·

(12)

HOCl + hv → HO· + Cl·

S2O82− + hv → SO4%− + SO4%− SO4%

+ H2O → HO· +

HSO5− −

SO4%

+ hv → SO4%

−

SO42−

+ OH → HO· +

Fig. 4 (A) displays the behavior of different oxidation systems in the NO removal without and with UV. Table S2 gives the detail information on the different oxidation systems, NO removal efficiency and the costeffectiveness. In the absence of UV, the NO removal efficiency increases from 8.7% to 25.6%, when the concentration of NaClO2 increases from 0.3 to 3 mM; increases from 2.6% to 7.6% when the concentration of Na2S2O8 increases from 10 to 200 mM; increases from 5.8% to 8.0% when the concentration of KHSO5 increases from 10 to 100 mM; increases from 6.0% to 13.5% when the concentration of NaClO increases from 0.5 to 200 mM; increases from 4.2% to 7.2% when the concentration of H2O2 increases from 1000 to 7000 mM. After UV illumination, the corresponding NO removal efficiency increases to 75.8–98.6%, 6.2–8.9%, 8.9–10.4%, 7.3–86.7% and 12.5–30.3%. The radical contribution to the NO removal was calculated to be 67.1–73%, 2.3–3.6%, 2.4–4.1%, 1.3–73.2% and 8.3–23.1% for the reaction systems of UV/NaClO2, UV/Na2S2O8, UV/KHSO5, UV/NaClO and UV/ H2O2, respectively. Apparently, UV/NaClO2 and UV/NaClO was the best in term of NO removal, thus the Cl-containing radicals were more efficient in NO removal. To accurately depict the economy of different oxidation systems in the NO removal, we calculated the cost-effectiveness of these five UVAOP systems. The cost-effectiveness (CE) is defined as that the cost needed to remove one order of magnitude quantity of pollutant [37], the calculation equation is available in eq (20).

+H

(15) (16)

SO42−

(17)

Fig. 3 (I) and (J) display the ESR spectra of UV/H2O2 under different pH conditions. The intensity of the DMPO-OH adduct obtained in alkalized UV/H2O2 (eq. (18)) is higher than that obtained in the intrinsic UV/H2O2, indicating that the alkalization of H2O2 increased the yield of HO· (eq. (19)). The possible reason was that the alkalized H2O2 could absorb more photons so to increase the radical yield, as illuminated in Fig. 2 (B). Combined the results of ESR and UV–Vis, we could conclude that the quantum yield of H2O2 was far lower than that of NaClO2, 100

(A) H 2O 2

60

NaClO2 40

KHSO5 20

0

100

Na2S2O8

Na2S2O8

KHSO5

NaClO

NO removal efficiency (%)

KHSO5

(C)

80 70 60 50 40 30 20 10

ol

tr Con

3

SO 2

(1 0 0

mg/m

)

3

SO 2

(500

mg/m

)

3

1 00

( SO 2

NaClO H2O2

40

20

0

Con

trol

3

(100 SO 2

mg/m

)

3

SO 2

(500

mg/m

)

3

SO 2

(100

g/m 0m

)

H2O2

90

0

Na2S2O8

60

ri ) ri ) = 7 1 0 ri ) = 7 1 0 ri ) = 7 1 0 = 4 = 7 ri ) 1 0 =4 p H = 7 (O p H = 3 . 4 (O p H p H = . 7 (O p H p H = p H p H 1 2 (O . 6 (O p H p H = = 2 = =2 pH pH pH= pH pH

NaClO2

NaClO2

80

g/m 0m

)

Duration time for highest NO removal (s)

NO removal efficiency (%)

Without UV With UV

(B) NO removal efficiency (%)

NaClO 80

(19)

3.3. Behavior and Cost-effectiveness of different oxidation systems on NO removal

(14) +

+ HO·

–

(18)

(13)

Fig. 3 (E), (F), (G) and (H) display the ESR spectra of UV/Na2S2O8 and UV/KHSO5 under different pH conditions. SO4%− and HO· are known to be the two major radicals in UV/Na2S2O8 and UV/KHSO5 (eqs. (14)–(16)) [16–18,23–24], which can be demonstrated by the adducts of DMPO-OH and DMPO-SO4 in Fig. 3 (E) and (G). Increasing the pH can increase the yields of SO4%− and HO·, as shown in in Fig. 3 (F) and (H), which was mainly due to that the alkaline activation of Na2S2O8 and KHSO5 enhanced the UV254 absorbance, as depicted in Fig. 2 (B); besides, the transformation of SO4%− to HO· (eq. (17)) further enhanced the intensity of DMPO-OH adduct [16–17]. In term of radical yield, UV/KHSO5 was better than UV/Na2S2O8 as the intensity of DMPO-OH adduct obtained in UV/KHSO5 was higher than that obtained in UV/Na2S2O8, which was mostly due to the difference in the absorbance between the two systems. −

−

(D)

1200 1000 800

SO2=0 mg/m3

600

0.7mM NaClO2

400 200 0 3 3 3 3 ) ) M M M M ) M M ) 0.3m 0.5m 0.7m 0.9m 1.0m 3.0m 0 mg/m0 mg/m 0 mg/m0 mg/m ( 0 0 SO 2 SO 2 (1 SO 2 (5 O 2 (100 S

6

Fig. 5. Behavior of different oxidation systems on NO removal under different pH conditions (A); effect of SO2 on the NO removal without UV (B) and with UV (C). Simulated flue gas flow is 2.6 L/min; oxidation solution volume is 500 mL; reaction temperature is 50 °C; NO concentration is 300 mg/Nm3. The concentrations of H2O2, Na2S2O8, KHSO5, NaClO and NaClO2 are 1000, 100, 100, 200 and 0.7 mM, respectively.

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CE =

alkalization of UV/KHSO5 and UV/Na2S2O8 did not greatly promote the NO removal, thus SO4%− and HO· produced in UV/KHSO5 and UV/ Na2S2O8 are less efficient in NO removal. Due to HO· was less important in NO removal, we could conclude that ClO· produced in the acidized UV/NaClO2 was possibly the dominative radical for the NO removal. Decreasing the pH of the NaClO solution was favorable for the NO removal; but after UV irradiation, a decrease of NO removal with the decreasing of the pH was observed. This result indicated that Cl2/HClO produced in the acidized NaClO was more efficient than ClO− in the NO removal; but with UV light, the situation was opposite, ClO% and Cl2%− were more useful in NO removal. It was amazing that the alkalized H2O2 in both without and with UV facilitated the NO removal, suggesting that O2– and UV/O2– were more useful in NO removal. Fig. 5 (B) and (C) display the effect of SO2 on the removal of NO. For the NaClO2 solution without UV, SO2 concentration increasing from 0 to 1000 mg/m3 increased the NO removal efficiency from 9.2% to 51.8%, which was due to the formation of ClO2 (eq. (20)) and ClO·/Cl· via the reaction between SO2 and ClO2 (eqs.21–22) [38–40]. By contrast, the addition of SO2 in Na2S2O8, KHSO5, NaClO and H2O2 did not affect the NO removal. After UV illumination in Fig. 5 (C), the NO removal efficiency for UV/NaClO2 and UV/NaClO greatly increased to over 95% and 80%, respectively, and was slightly affected by the increasing SO2 concentration. The results further demonstrated that Clradicals possessed higher selectivity in NO removal and insensitive to the presence of SO2.

C· P log

( ) C0 Ct

(20)

where, C is the oxidation concentration, g/L; P is the price of oxidant, CNY/g; C0 and Ct is the initial and final concentrations of the pollutant, mg/m3 for NO or μg/m3 for Hg0. The calculated cost-effectiveness values are shown in Fig. 4 (B). UV/ NaClO2 has the best cost-effectiveness, followed by UV/NaClO, UV/ KHSO5, UV/Na2S2O8 and UV/H2O2. The cost-effectiveness for NaClO2 is from 9.87 × 10−4 to 3.04 × 10−3 USD/order-NO, and for UV/ NaClO2 is from 5.36 × 10−5 to 2.10 × 10−4 USD/order-NO; that for NaClO is from 1.23 × 10−3 to 2.09 × 10−1 USD/order-NO, for UV/ NaClO is from 1.00 × 10−3 to 1.08 × 10−2 USD/order-NO. And the cost-effectiveness of UV/KHSO5, UV/Na2S2O8 and UV/H2O2 are lower several orders of magnitude than that of UV/NaClO2 and UV/NaClO. Introducing UV light could increase the cost-effectiveness by 17.4 times for NaClO2 and by 18.3 times for NaClO, thus the contribution rate of UV light on activation of NaClO2 was comparable to that on the activation of NaClO. Another phenomenon is worth noting: though H2O2 has the worst cost-effectiveness, adding UV can still significantly increase its cost-effectiveness. Overall, UV/NaClO2 was demonstrated to be the most promising AOP method in NO removal with respect to the removal efficiency and cost-effectiveness. 3.4. Effects of key factors on the NO removal Fig. 5 (A) displays the influence of the pH on the removal of NO with different UV-AOP systems. It can be found that decreasing the NaClO2 solution pH promoted the NO removal without and with UV, which could be attributed to the formation of ClO2 and the photo-catalysis of ClO2 (ClO·/HO·). Increasing the pH of KHSO5 and Na2S2O8 did not significantly enhance their ability on NO removal, which suggested that the increased quantity of SO4%− and HO· resulting from the

ClO2 + SO2 → ClO· + SO3

(21)

ClO% + SO2 → Cl· + SO3

(22)

UV/NaClO2 was demonstrated to be the best UV-AOP method for NO removal in terms of removal efficiency and cost-effectiveness. But how long can UV/NaClO2 maintain the highest removal of NO under such low concentration? We needed to study this issue. As shown in

100

(A) Without UV With UV

60

40

0

Hg removal efficiency (%)

80

20

NaClO2 0 0 .1

Na2S2O8

NaClO

KHSO5

H2O2

M mM m M .3 m M 5 m M .7 m M .0 m M 0 m M 0 m M 0 m M 0 0 m M 1 0 m M 5 0 m M 0 m M 0 m M .1 m M .5 m M .0 m M .0 m M 0 m M 0 0 m M 0 m M 0 0 m 00 1 1 1 5 7 1 0 0 5 0 1 0 2 0. 10 10 50 10 30

0

0

Cost for Hg removal (USD/order-Hg removal)

0.9 0.8

0.0010

0.7

0.0008

Without UV

(B)

With UV

0.6

NaClO

NaClO2

0.0006

0.5

0.0004

0.4 0.3

0.0002

0.2

0.0000

H2O2

Na2S2O8

0.1

KHSO5

NaClO2

0.0 0 .1

mM

0 .3

mM

0 .5

mM

0 .7

mM

1 .0

mM

10

mM

50

mM

10

0

mM

20

0

mM

10

mM

50

mM

70

mM

NaClO 10

0

mM

0 .1

mM

0 .5

mM

1 .0

M mM m M .0 m M 0 m M 0 0 m M 0 m M 0 0 m 00 1 5 1 50 10 30

Fig. 6. Behavior (A) and cost-effectiveness (B) of different oxidation systems in Hg0 removal. Simulated flue gas flow is 2.6 L/min; oxidation solution volume is 500 mL; reaction temperature is 50 °C; Hg0 concentration is 1000 μg/Nm3. 7

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74.6% to 77.1%, respectively. After UV illumination, the Hg0 removal efficiency increases to 95.5%-97.5%, 86.9%-90.2%, 93.0–98.6% and 87.6%-96.9, respectively, which demonstrated the outstanding role of radicals on Hg0 removal. And the radical contributions were 29.3–43.8% for UV/NaClO2, and 32.1–35.7% for UV/Na2S2O8, and 1.3–18.6% for UV/KHSO5, and 13.0–19.8% for UV/NaClO. Obviously, the radicals produced in UV/NaClO2 and UV/Na2S2O8 were more useful in Hg0 removal. It is interesting that Hg0 removal was insensitive to UV light when KHSO5 was used as the oxidant, thus HSO5− alone was sufficient to conduct a deep removal of Hg0 and the radical contribution was weakly. We also calculated the cost-effectiveness of the five UV-AOP systems in term of Hg0 removal. As shown in Fig. 6 (B), UV/NaClO2 and UV/ NaClO have the best cost-effectiveness. In specific, the cost-effectiveness for NaClO2 is from 4.11 × 10−5 to 2.61 × 10−4 USD/order-Hg0, for UV/NaClO2 is from 9.65 × 10−6 to 8.11 × 10−5 USD/order-Hg0. The cost-effectiveness for NaClO is from 1.11 × 10−5 to 1.03 × 10−3 USD/order-Hg0, and for UV/NaClO is from 7.28 × 10−6 to 4.37 × 10−4 USD/order-Hg0. Thus, NaClO2 and NaClO were comparable in term of the economy for Hg0 removal. While given the efficiency of Hg0 removal, UV/NaClO2 is more suitable. The cost-effectiveness for Na2S2O8 is from 6.42 × 10−3 to 1.06 × 10−1 USD/order-Hg0, and for UV/Na2S2O8 is from 2.27 × 10−3 to 3.97 × 10−2 USD/order-Hg0. The cost-effectiveness for KHSO5 is from 5.58 × 10−3 to 2.10 × 10−2 USD/ order-Hg0, and for UV/KHSO5 is from 2.86 × 10−3 to 1.78 × 10−2 USD/order-Hg0. Hence, UV/Na2S2O8 and UV/KHSO5 are also

Fig. 5 (D), increasing the NaClO2 concentration from 0.3 to 3.0 mM increases the duration time from 48 to 1285 s, which suggests that the utilization rate of NaClO2 increases with the increasing of NaClO2 concentration. Then the influence of SO2 concentration on the duration time was also studied. The increasing of the SO2 concentration slightly increased the duration time, which possibly because the secondary radicals such as ClO2/ClO·/Cl· produced between the reactions of ClO2− and SO2 (eqs. (5), 21–22) contributed to the increase of utilization of NaClO2 and the increase of NO removal. It was reported [20,40] that ClO2 had a high selectivity in term of NO oxidation, thus the formation of ClO2 resulting from the presence of SO2 might facilitate the deep and enduring removal of NO. 3.5. Behavior and Cost-effectiveness of different oxidation systems on Hg0 removal Fig. 6 (A) displays the performance of different oxidation systems in Hg0 removal without and with UV. Table S3 gives the detail information about the different oxidation systems, Hg0 removal efficiency and the cost-effectiveness. It can be found that NaClO2, Na2S2O8, KHSO5 and NaClO are capable to conduct the deep removal of Hg0. However, H2O2 alone or even UV/H2O2 is so bad in removal of Hg0. Without UV, when the concentration of NaClO2, Na2S2O8, KHSO5 and NaClO ranges from 0.1 to 1.0 mM, from 10 to 200 mM, from 10 to 100 mM, and from 0.1 to 10 mM, respectively, the Hg0 removal efficiency ranges from 51.7% to 68.2%, from 51.2% to 58.1%, from 74.4% to 97.3%, and from

Fig. 7. Behavior of different oxidation systems in Hg0 removal under different pH conditions (A); effect of SO2 on the Hg0 removal without UV (B) and effect of SO2 on the Hg0 removal with UV (C). Simulated flue gas flow is 2.6 L/min; oxidation solution volume is 500 mL; reaction temperature is 50 °C; Hg0 concentration is 1000 μg/Nm3. The concentrations of H2O2, Na2S2O8, KHSO5, NaClO and NaClO2 are 500, 50, 70, 5 and 0.5 mM, respectively. 8

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comparable in term of the Hg0 removal economy, but was hundreds to thousand times lower than that of NaClO2 and NaClO. Hence, the Clradicals were still the best option for Hg0 removal.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123885.

3.6. Effects of key factors on the Hg0 removal

References

Fig. 7 (A) illuminated the effect of the pH on Hg0 removal. It can be found that acidizing NaClO2 could facilitate the Hg0 removal without and with UV, suggesting the effectiveness of ClO2/ClO·; alkalizing Na2S2O8 did not significantly affect the Hg0 removal without and with UV; alkalizing KHSO5 decreased the Hg0 removal efficiency, but this situation was improved when UV light was added, suggesting that SO4%−/HO· induced oxidation of Hg0 was insensitive to the change in pH; acidizing NaClO slightly decreased the Hg0 removal, indicating that ClO− was better than HClO/Cl2 in term of Hg0 removal, but the situation was opposite when UV was added, implying that Cl· produced in UV/HClO assisted the HO· to conduct a deep removal of Hg0; the alkalized UV/H2O2 also enhanced the Hg0 removal, indicating that UV/ O2– yielded more HO· and then promoted Hg0 removal. Fig. 7 (B) and (C) reveal the effect of SO2 concentration on the removal of Hg0 without and with UV. In the absence of UV, NaClO2 and NaClO were sensitive to the change in SO2 concentration: increasing the SO2 concentration decreased the Hg0 removal efficiency, which was mainly due to the consumption of limited oxidant (0.1–10 mM) by SO2; but the Hg0 removal using KHSO5 and Na2S2O8 was relatively steady with the increasing of SO2 concentration. The presence of SO2 promoted H2O2 removing Hg0, which possible was due to the formation of H2SO4. H2SO4 was proved to be a promoter for Hg0 removal [14,35]. After UV illumination, the inhibition caused by SO2 in the case of NaClO2 was weakened, the Hg0 removal efficiency was steady at over 95%, which further confirmed the outstanding effectiveness of Cl-radicals in Hg0 removal. The Hg0 removal efficiency using UV/Na2S2O8 and UV/KHSO5 were at around 85% and over 96%, respectively. The presence of SO2 still facilitated H2O2 to conduct a deep removal of Hg0 (75%). Compared the results in Fig. 7 (B) and (C), we could conclude that the radicals induced oxidation of Hg0 hold a good resistance to the presence of SO2, and the UV/NaClO2 system is the best UV-AOP method in term of cooperative removal of NO and Hg0, and also is promising to be used for simultaneous removal of SO2, NO and Hg0.

[1] R.L. Hao, Y.M. Mao, X.Z. Mao, Z. Wang, Y.P. Gong, Z.L. Zhang, Y. Zhao, Cooperative removal of SO2 and NO by using a method of UV-heat/H2O2 oxidation combined with NH4OH-(NH4)2SO3 dual-area absorption, Chem. Eng. J. 365 (2019) 282–290. [2] H.M. Xu, N.Q. Yan, Z. Qu, W. Liu, J. Mei, W.J. Huang, S.J. Zhao, Gaseous heterogeneous catalytic reactions over Mn-Based oxides for environmental applications: a critical review, Environ. Sci. Technol. 51 (2017) 8879–8892. [3] H. Shang, M.Q. Li, H. Li, S. Huang, C.L. Mao, Z.H. Ai, L.Z. Zhang, Oxygen vacancies promoted the selective photocatalytic removal of NO with blue TiO2 via simultaneous molecular oxygen activation and photogenerated hole annihilation, Environ. Sci. Technol. 53 (2019) 6444–6453. [4] X. Shi, P.Q. Wang, L. Wang, Y. Bai, H.Q. Xie, Y. Zhou, L.Q. Ye, Change in photocatalytic NO removal mechanisms of ultrathin BiOBr/BiOI via NO3- adsorption, Appl. Catal. B-Environ. 243 (2018) 322–329. [5] Y. Huang, Y.L. Liang, Y.F. Rao, D.D. Zhu, J.J. Cao, Z.X. Shen, W.K. Ho, S.C. Lee, Environment-Friendly carbon quantum dots/ZnFe2O4 photocatalysts: characterization, biocompatibility, and mechanisms for NO removal, Environ. Sci. Technol. 51 (2017) 2924–2933. [6] Y.F. Lu, Y. Huang, Y.F. Zhang, J.J. Cao, H.W. Li, C. Bian, S.C. Lee, Oxygen vacancy engineering of Bi2O3/Bi2O2CO3 heterojunctions: implications of the interfacial charge transfer, NO adsorption and removal, Appl. Catal. B-Environ. 231 (2018) 357–367. [7] J.Y. Li, W. Cui, Y.J. Sun, Y.H. Chu, W.L. Cen, F. Dong, Directional electron delivery via a vertical channel between g-C3N4 layers promotes photocatalytic efficiency, J. Mater. Chem. A. 5 (2017) 9358–9364. [8] A.C. Zhang, L.X. Zhang, X.Z. Chen, Q.F. Zhu, Z.C. Liu, J. Xiang, Photocatalytic oxidation removal of Hg0 using ternary Ag/AgI-Ag2CO3 hybrids in wet scrubbing process under fluorescent light, Appl. Surf. Sci. 392 (2017) 1107–1116. [9] L.X. Zhang, A.C. Zhang, Q.F. Zhu, H. Wang, C.J. Zhang, Effects of experimental parameters on Hg0 removal over magnetic AgI-BiOI/CoFe2O4 photocatalysts using wet process, J. Fuel. Chem. Tech. 46 (2018) 365–374. [10] C.W. Li, A.C. Zhang, L.X. Zhang, J. Song, S. Su, Z.J. Sun, J. Xiang, Enhanced photocatalytic activity and characterization of magnetic Ag/BiOI/ZnFe2O4 composites for Hg0 removal under fluorescent light irradiation, Appl. Surf. Sci. 433 (2018) 914–926. [11] A.C. Zhang, L.X. Zhang, Q.F. Zhu, B.J. Dai, W. Sheng, S. Su, J. Xiang, Photocatalytic oxidation removal of Hg0 by ternary Ag@AgCl/Ag2CO3 hybrid under fluorescent light, Fuel. Process. Tech. 159 (2017) 222–231. [12] Y.X. Liu, Q. Wang, J.F. Pan, Novel process of simultaneous removal of nitric oxide and sulfur dioxide using a vacuum ultraviolet (VUV)-activated O2/H2O/H2O2 system in a wet VUV-spraying reactor, Environ. Sci. Technol. 50 (2016) 12966–12975. [13] J.H. Ye, J. Shang, Q. Li, W.W. Xu, J. Liu, X. Feng, T. Zhu, The use of vacuum ultraviolet irradiation to oxidize SO2 and NOx for simultaneous desulfurization and denitrification, J. Hazard. Mater. 271 (2014) 89–97. [14] Y.X. Liu, J. Zhang, Y.S. Yin, Removal of Hg0 from flue gas using two homogeneous photo-Fenton-like reactions, AIChE. J. 61 (2015) 1322–1333. [15] Y.X. Liu, Y. Wang, Elemental mercury removal from flue gas using heat and Co2+/ Fe2+ coactivated oxone oxidation system, Chem. Eng. J. 348 (2018) 464–475. [16] Y.X. Liu, Y. Wang, Q. Wang, J.F. Pan, J. Zhang, Simultaneous removal of NO and SO2 using vacuum ultraviolet light (VUV)/heat/peroxymonosulfate (PMS), Chemosphere 190 (2018) 431–441. [17] Y.X. Liu, Q. Wang, Removal of elemental mercury from flue gas by thermally activated ammonium persulfate in a bubble column reactor, Environ. Sci. Technol. 48 (2014) 12181–12189. [18] Y.X. Liu, Z.Y. Liu, Y. Wang, Y.S. Yin, J.S. Pan, J. Zhang, Q. Wang, Simultaneous absorption of SO2 and NO from flue gas using ultrasound/Fe2+/heat coactivated persulfate system, J. Hazard. Mater. 342 (2018) 326–334. [19] A. Mudhoo, S.K. Sharma, Microwave irradiation technology in waste sludge and wastewater treatment research, Environ. Sci. Tech. 41 (2011) 999–1066. [20] R.L. Hao, X.Z. Mao, Z. Wang, Y. Zhao, T.H. Wang, Z.H. Sun, B. Yuan, Y.K. Li, A novel method of ultraviolet/NaClO2-NH4OH for NO removal: Mechanism and kinetics, J. Hazard. Mater. 368 (2019) 234–242. [21] R.L. Hao, Z. Wang, X.Z. Mao, Y.P. Gong, B. Yuan, Y. Zhao, B.J. Tian, M. Qi, Elemental mercury removal by a novel advanced oxidation process of ultraviolet/ chlorite-ammonia: Mechanism and kinetics, J. Hazard. Mater. 374 (2019) 120–128. [22] Y.X. Liu, Y. Wang, Z.Y. Liu, Q. Wang, Oxidation removal of nitric oxide from flue gas using UV photolysis of aqueous hypochlorite, Environ. Sci. Technol. 51 (2017) 11950–11959. [23] M.A. Lominchar, A. Santos, E. de Miguel, A. Romero, Remediation of aged diesel contaminated soil by alkaline activated persulfate, Sci. Total Environ. 622–623 (2018) 41–48. [24] C.M. Dominguez, V. Rodriguez, E. Montero, A. Romero, A. Santos, Methanol-enhanced degradation of carbon tetrachloride by alkaline activation of persulfate: Kinetic model, Sci. Total Environ. 666 (2019) 631–640. [25] R.L. Hao, X.Z. Mao, Z. Qian, Y. Zhao, L.D. Wang, B. Yuan, K.M. Wang, Z.H. Liu, M. Qi, J. Crittenden, Simultaneous removal of SO2 and NO using a novel method of ultraviolet irradiating chlorite-ammonia complex, Enviro. Sci. Technol. 53 (2019)

4. Conclusion This paper systematically studied the behavior and cost-effectiveness of three categories radicals in the removal of NO and Hg0. NaClO2 possessed the highest absorbance at UV254 light, implying a possible highest quantum yield. ESR analyses confirmed the generation and evolution of the Cl-radicals (ClO2, ClO· and HO·) in different oxidation systems. The experimental results suggested that Cl-radicals hold great reactivity and selectivity towards the oxidation of NO and Hg0 as compared with SO4%− and HO·. Not only to that, Cl-radicals also exhibited a good adaptability to the high concentration of SO2 during the cooperative removal of NO and Hg0. The results obtained in this paper indicated that developing Cl-radical-based AOP methods is more promising, it is not only useful for air pollution control but also is of great significance for the development of the water treatment technology. Acknowledgments The authors appreciate the financial support provided by the Natural Science Foundation of China (No. 51978262 and 51708213), the Natural Science Foundation of Hebei (No. E2018502058), the Young Elite Scientists Sponsorship Program by CSEE (CSEE-YESS2018018), the National Key Research and Development Plan (No. 2016YFC0203705) (No. 2017YFC0210600) and the Fundamental Research Funds for the Central Universities (No. 2019MS103 and 2019MS128). 9

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R. Hao, et al. 9014–9023. [26] J. Marcon, G. Mortha, N. Marlin, F. Molton, C. Duboc, A. Burnet, New insights into the decomposition mechanism of chlorine dioxide at alkaline pH, Holzforschung 71 (2017) 599–610. [27] H. Cosson, W.R. Ernst, Photodecomposition of chlorine dioxide and sodium chlorite in aqueous solution by irradiation with ultraviolet light, Ind. Eng. Chem. Res. 33 (1994) 1468–1475. [28] E.A. Albers, K. Hoyermann, H.G.G. Wagner, J. Wolfrum, Absolute measurements of rate coefficients for the reactions of H and O atoms with H2O2 and H2O, Symp. Int. Combust. Proc. 13 (1971) 158–163. [29] S. Javoy, V. Naudet, S. Abid, C.E. Paillard, Elementary reaction kinetics studies of interest in H2 supersonic combustion chemistry, Expt. Thermal Fluid Sci. 27 (2003) 371–377. [30] J.Y. Fang, Y. Fu, C. Shang, The roles of reactive species in micropollutant degradation in the UV/free chlorine system, Environ. Sci. Technol. 48 (2014) 1859–1868. [31] K.H. Guo, Z.H. Wu, C. Shang, B. Yao, S.D. Hou, X. Yang, W.H. Song, J.Y. Fang, Radical chemistry and structural relationships of PPCP degradation by UV/chlorine treatment in simulated drinking water, Environ. Sci. Technol. 51 (2017) 10431–10439. [32] P.Z. Sun, W.N. Lee, R.C. Zhang, C.H. Huang, Degradation of DEET and caffeine under UV/chlorine and simulated sunlight/chlorine conditions, Environ. Sci. Technol. 50 (2016) 13265–13273. [33] X.J. Kong, Z.H. Wu, Z.R. Ren, K.H. Guo, S.D. Hou, Z.C. Hua, X.C. Li, J.Y. Fang,

[34] [35] [36] [37]

[38] [39] [40]

10

Degradation of lipid regulators by the UV/chlorine process: Radical mechanisms, chlorine oxide radical ClO-mediated transformation pathways and toxicity changes, Water. Res. 137 (2018) 242–250. J.Y. Fang, Q. Zhao, C.H. Fan, C. Shang, Y. Fu, X.R. Zhang, Bromate formation from the oxidation of bromide in the UV/chlorine process with low pressure and medium pressure UV lamps, Chemosphere. 183 (2017) 582–588. R.L. Hao, X.H. Dong, Z. Wang, L. Fu, Y. Han, B. Yuan, Y.P. Gong, Y. Zhao, Elemental mercury removal by a method of ultraviolet-heat synergistically catalysis of H2O2–halide complex, Environ. Sci. Technol. 53 (2019) 8324–8332. Z.H. Wu, J.Y. Fang, Y.Y. Xiang, C. Shang, X.C. Li, F.G. Meng, X. Yang, Roles of reactive chlorine species in trimethoprim degradation in the UV/chlorine process: Kinetics and transformation pathways, Water. Res. 104 (2016) 272–282. S.Q. Zhou, Y.H. Yu, W.Q. Zhang, X.Y. Meng, J.M. Luo, L. Deng, Z. Shi, J. Crittenden, Oxidation of microcystin-LR via activation of peroxymonosulfate using ascorbic acid: kinetic modeling and toxicity assessment, Environ. Sci. Technol. 52 (2018) 4305–4312. B.R. Deshwal, D.S. Jin, S.H. Lee, S.H. Moon, J.H. Jung, H.K. Lee, Removal of NO from flue gas by aqueous chlorine-dioxide scrubbing solution in a lab-scale bubbling reactor, J. Hazard. Mater. 150 (2008) 649–655. J.C. Wei, Y.B. Luo, P. Yu, B. Cai, H.Z. Tan, Removal of NO from flue gas by wet scrubbing with NaClO2/(NH2)2CO solutions, J. Ind. Eng. Chem. 15 (2009) 16–22. J. Johansson, A.H. Hultén, S. Ajdari, P. Nilsson, M. Samuelsson, F. Normann, K. Andersson, Gas-phase chemistry of the NO–SO2–ClO2 system applied to flue gas cleaning, Ind. Eng. Chem. Res. 57 (2018) 14347–14354.