Photochemical reaction between biphenyl and N(III) in the atmospheric aqueous phase

Chemosphere 167 (2017) 462e468

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Photochemical reaction between biphenyl and N(III) in the atmospheric aqueous phase Jianzhong Ma a, b, Chengzhu Zhu a, b, *, Jun Lu c, Tao Wang a, b, Shuheng Hu a, Tianhu Chen a a

School of Resource and Environmental Engineering, Hefei University of Technology, Hefei 230009, PR China Institute of Atmospheric Environment & Pollution Control, Hefei University of Technology, Hefei 230009, PR China c Center of Analysis & Measurement, Hefei University of Technology, Hefei 230009, PR China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The rate constants of biphenyl with were H2ONOþ, HONO and NO 2 determined.

OH reacts with biphenyl with a rate constant of 9.4  109 L mol1 s1.
Nitro-compounds was generated from the reaction between biphenyl with N(III).

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 January 2016 Received in revised form 2 October 2016 Accepted 3 October 2016 Available online 14 October 2016

The photochemical reaction between biphenyl (Bp) and N(III) under irradiation at 365 nm UV light was investigated. The results showed that Bp conversion efficiency was strongly influenced by N (III) concentration, Bp initial concentration and pH. Species-specific rate constants determined by reaction of Bp 1 with H2ONOþ (k1), HONO (k2) and NO s1), 2 (k3) were k1 ¼ (0.058 ± 0.005 L mol k2 ¼ (0.12 ± 0.06 L mol1 s1) and k3 ¼ (0.0019 ± 0.0003 L mol1 s1), respectively. Laser flash photolysis studies confirmed that OH radical deriving from the photolysis of N(III) attacked aromatic ring to form Bp-OH adduct with a rate constant of 9.4  109 L mol1 s1. The products analysis suggested that Bp-OH adduct could be nitrated by N (III) and NO2 to generate nitro-compounds. © 2016 Elsevier Ltd. All rights reserved.

Handling Editor: R. Ebinghaus Keywords: Biphenyl Hydroxyl radical Photochemistry N(III) Atmospheric aqueous phase

1. Introduction Atmospheric aqueous-phase chemistry has been extensively studied in the last few years to understand secondary organic aerosol (SOA) formation and aging mechanisms (Chameides and

* Corresponding author. School of Resource and Environmental Engineering, Hefei University of Technology, Hefei 230009, PR China. E-mail address: [email protected] (C. Zhu). http://dx.doi.org/10.1016/j.chemosphere.2016.10.010 0045-6535/© 2016 Elsevier Ltd. All rights reserved.

Stelson, 1992; Martin, 2000). In troposphere system, solution reactions may occur in the droplets of fog, clouds and rain at low ionic strength (Herrmann, 2003), which play an important role in determining the conversion, removal and final fate of most of the water-soluble pollutants in the atmosphere (Djouad et al., 2002). Among various reactions, nitration process in the atmospheric aqueous-phase (dark or photoinduced) may be significantly (Vione et al., 2006; Harrison et al., 2005), as they yield for instance mutagenic nitro-PAHs and phytotoxic nitrophenols (Enya et al.,

J. Ma et al. / Chemosphere 167 (2017) 462e468

1997; Atkinson et al., 1992). Gas-phase nitration process has been extensively studied. They are very similar for both PAHs and phenols and include
OH and
NO 3 mediated nitration (Pitts et al., 1985; Sasaki et al., 1997). However, the reports on the nitration routes in the atmospheric liquid-phase are relatively infrequent (Anastasio and Chu, 2009; Vione et al., 2002; Zhu et al., 2007), which may play an important role in the formation of secondary aerosol. Nitrous acid (HONO) has been considered as a major precursor of hydroxyl radical since it has a strong absorption towards the UV light in the 300e400 nm region (Ouyang et al., 2005), contributing up to 80% of OH radical formation in the atmospheric environment (Li et al., 2014; Elshorbany et al., 2010). As one of the most important species in photochemical cycles, OH radical is the key oxidant in the degradation of most air pollutants in the atmospheric environment (Zhou et al., 2001). Therefore, HONO is believed to make a significant contribution to the oxidation capacity of the troposphere (Levy et al., 2014). Huang et al. (2007) performed the study of photochemical reaction between biphenyl (Bp) and nitrous acid in aqueous solution under pH of 1.5 and found that the Bp-OH adduct could either reacted with Hþ or be oxidized by HONO to form nitrosobiphenol. However, their studies was performed in aqueous-phase under a highly acidic condition, and in fact that the chemistry of HONO was complicated since it could both dissociate to nitrite (NO 2 ) and protonated to form nitrous acidium ion (H2ONOþ) in the aqueous phase (Anastasio and Chu, 2009):

HONO4H þ þ NO 2

(1)

HONO þ H þ 4H2 ONOþ

(2)

Even though aerosol water is more acidic, it is unlikely that all nitrite are in the form of HONO in cloud and rainwater. According to the previous reports (Rubio et al., 2008; Kieber et al., 1999), the N (III) concentration ranges from 0.5 to 50 mM in atmospheric water droplets. Given that the pH of atmospheric waters (cloud, fog, rain, and dew) varies from 1.95 to 7.74 (Arakaki et al., 1999), the atmospheric waters should contain HONO, H2ONOþ and NO 2 . Both H2ONOþ and NO 2 are involved in the production of OH radical via photolysis due to their similar chemical properties with HONO. Biphenyl (Bp) was selected as model compound since its toxicity to the human health (Miyashita et al., 2015; Chakraborty and Das, 2016) and widespread existence in the atmospheric environment (Anderson et al., 1999; Zabel et al., 1995; Bailey et al., 1983). In the present paper, N (III) of total nitrogen concentration, which dissolved NaNO2 as a precursor of OH radical under UV irradiation (l ¼ 365 nm, 8 W). The study was performed at pH of 1.5, 2.8 and 6.3 to evaluate the detail pathway of reaction between different speciation of N (III), nitrite (NO 2 ), nitrous acid (HONO) and nitrous acidium ion (H2ONOþ) with Bp. Laser flash photolysis spectrometer techniques were used to measure the rate constant between Bp and hydroxyl radical. Meanwhile, the steady reaction products of Bp with N (III) after irradiation was analyzed using GC-MS to obtain the detailed removal pathway and reaction kinetics. 2. Experimental section

463

2.2. Photolysis experimental setup The photolysis process was evaluated with a quartz cell (Volume ¼ 600 mL), at room temperature (25 ± 2  C) under an atmospheric pressure. The tests were carried out an ultraviolet lamp (8 W), whose maximum emission was about 365 nm and irradiation intensity was about 1.52 mW cm2. The pH value of reaction solution was adjusted by HClO4. 2.3. Analytical method The concentration of biphenyl was analyzed by a High Performance Liquid Chromatography (HPLC, Dionex UltiMate 3000) system with a Thermo C18 column (4.6  150 mm, 5 mm, Akzo Nobel, Netherlands) under the following conditions: the temperature of column oven was maintained at 30  C, the detection wavelength of UV detector was 254 nm, the mobile phase was consisted of 80% methanol and 20% water, the flow rate was 1.0 mL min1 and the injection volume was 20 mL. The UVeVis absorbance spectrum was recorded by Shimadzu UV1750. Final products from the reaction between the Bp and N (III) were identified by the GC-MS (Agilent 7890A-5975C). The samples were then concentrated to 2 mL before GC-MS analysis for a better signal-to-noise ratio. Gas chromatographic separation was performed on a 30 m HP-5MS capillary column (30 m  0.25 mm, thickness 0.25 mm). The oven temperature was programmed from 60  C to 80  C at a rate of 20  C min1, then 110  C at a rate of 5  C min1, 150  C at a rate of 20  C min1, 230  C at a rate of 5  C min1, and finally to 300  C at a rate of 20  C min1 and held for 4 min. High purity helium gas was served as carrier gas with a flow rate at 1.0 mL min1. The MS ion source (Electron Impact, EI) and quadruple temperature were both 150  C. The transfer line temperature was set at 280  C, and the electron energy was 70 eV. 2.4. Laser flash photolysis spectrometer An Edinburgh Instruments LP920 laser flash photolysis system with a Nd: YAG laser (355 nm, laser pulse 4e6 ns, laser energy was about (30 ± 5) mJ pulse1, diameter of the laser beam cross section was 0.60 cm). The analyzing light was emitted from a pulsed 450 W Xe920 xenon lamp and then entered the 1  1 cm quartz cell at a right angle to the laser beam. The signal was recorded with a Tektronix TDS 3012B oscilloscope and a detector. 3. Results and discussion 3.1. Photo-transformation of Bp in aqueous solution by N(III) under irradiation 3.1.1. Effect of initial Bp concentration The rate expression for the oxidation of Bp by N (III) followed a pseudo-first order reaction with respect to both the organic compound and N(III). Accordingly, the rate expression for the reaction of Bp with N(III) could be expressed as:

2.1. Materials

d½Bp ¼ kapp ½Bp½NðIIIÞ  dt

Biphenyl and NaNO2 (99.0%, Shanghai Chemical Reagents Co. Ltd.), HClO4 (AR, Shanghai Chemical Reagents Co. Ltd.), ethanol (AR, Shanghai Chemical Reagents Co. Ltd.) were used. Solutions were freshly prepared using ultrapure water. When necessary, the solutions were purged with nitrogen (or oxygen) to obtain oxygenfree (or oxygen-saturated) samples.

Where [N(III)] and [Bp] were the concentrations of N(III) and biphenyl and kapp was the overall apparent reaction rate constant. When the influence of initial concentration on Bp conversion was investigated with the ranges from 3.25  105 M to 9.75  105 M at different pH (1.5, 2.8 and 6.3) in 5  103 M N(III) solution, the pseudo-first order rate constants k of Bp degradation with initial concentration of 3.25  105, 6.5  105 and

(3)

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9.75  105 M were obtained by linear fit from Fig. 1 of 4.7  104 s1, 4.08  104 s1 and 3.36  104 s1 at pH of 1.5; 1.75  104 s1, 1.47  104 s1 and 1.17  104 s1 at pH of 2.8; 9.17  106 s1, 6.94  106 s1 and 5.56  106 s1 respectively. The Bp kinetic constant decreased with increasing the initial concentration of Bp at pH of 1.5, 2.3 and 6.3, which was similar with the results for photosensitized degradation of bisphenol A (Zhan et al., 2006).

pH = 1.5 -0.5

ln(Ct/C0)

-1.0




ðkd þ kt $½BpÞ

(4)

-0.4

3.1.2. Effect of N (III) concentration Photolysis experiments of 3.25  105 M Bp was conducted in the presence of 1  104 to 1  102 M nitrite at pH of 1.5, 2.8 and 6.3, respectively (Fig. 2a). The photo-degradation efficiency of Bp was increased with increasing of N(III) concentration from 1  104 to 1  103 M. Ethanol (0.3 M), as an effective
OH acceptor (Ma et al., 2015), was added into photo reaction solution before irradiation, it effectively scavenged about 98% hydroxyl radical and the Bp degradation was completely inhibited (the insert of Fig. 2a), implied that
OH played a major role in the degradation process of Bp. However, when the N (III) concentration was in the range from 1  103 M to 1  102 M, the Bp photodegradation efficiency had just fluctuated slightly (Fig. 2a). The N(III) photolysis induced a complex series of radical reactions. The relevant reactions were follows (Loegager and Sehested, 1993; Ouyang et al., 2005; Zhu et al., 2007):

-0.6

HONO þ $OH/H2 O þ $NO2

-0.8

 NO 2 þ $OH/NO2 þ OH

-1.0

The enhancement of collision frequency between N(III) and OH radical was increased when the amount of N(III) in a high level in the aqueous solution. Beyond the optimum concentration, more OH radicals would consumed by HONO and NO 2 molecules to generate NO2, prohibiting the photo-oxidation process of Bp. These results could be offered as an explanation for increasing N (III) concentration in the experimental range had no effect on Bp transformation efficiency.

-1.5

-2.0

-2.5 0

1000

2000

3000

4000

5000

Time (s) 0.0

pH = 2.8

-0.2

ln(Ct/C0)

Wd ¼ kt $½Bp$Wg

According to equation (4), the degradation rate was changed from 0 to limiting value of Wg with increasing Bp concentration. The process of active species production became the limit step. At large Bp concentration, the improvement of degradation rate (Wd) was slower than concentration growth of Bp, which resulted in the drop of decomposition degree.

0.0

-1.2 -1.4 0

1000

2000

3000

4000

5000

6000

7000

8000

Time (s) 0.0

pH = 6.3

-0.2

-0.6 -0.8

-1.2 -1.4

60

90

120

(6)

(7)

a1 ¼

101:7pH 1 þ 101:7pH þ 10pH2:8

(8)

a2 ¼

1 1 þ 101:7pH þ 10pH2:8

(9)

a3 ¼

10pH2:8 1 þ 101:7pH þ 10pH2:8

(10)

-1.6 30

k ¼ 6  109 L mol1 s1

(5)

kapp ¼ ðk1 a1 þ k2 a2 þ k3 a3 Þ½NðIIIÞ

-1.0

0

k ¼ 1  109 L mol1 s1

3.1.3. Effect of pH The pKa value for equations (1) and (2) were 1.7 and 2.8 (Lide, 1999; Riordan et al., 2005; Crugeiras et al., 2011), respectively. As a result, the value of apparent rate constants kapp was deduced to be (Yang et al., 2011):

-0.4

ln(Ct/C0)

The generation rate of active species (Wg) had to be constant rather than their concentration. The kd was the total frequency of active species loss for all reactions except Bp removal reaction. The kt was the rate constant of Bp degradation and [Bp] was the biphenyl concentration. The degradation rate (Wd) could be written as:

150

3

Time (10 s) Fig. 1. Photodegradation of Bp in three initial concentration ( ) 3.25  105 M ( ), 6.5  105 M ( ), 9.75  105 M ([N(III)] ¼ 5  103 M).

Where, k1, k2 and k3 represented species-specific of N(III) rate

J. Ma et al. / Chemosphere 167 (2017) 462e468

465

the different quantum yield of OH radical from the NO 2 , HONO and H2ONOþ. According to Anastasio and Chu (2009) and Arakaki et al. (1999), the OH quantum from NO 2 was one order of magnitude lower than that of HONO and H2ONOþ. For Bp, the reaction with HONO controlled the overall reaction at pH from 1.7 to 4.5, where the contribution of HONO on the transformation of Bp (blue line), k2a2, was much larger than k1a1 (H2ONOþ) and k3a3 (NO 2 ). The reaction between Bp with H2NOþ dominated at pH < 1.7 (red line). Nitrous acidium ions could only exist in aqueous-phase under a highly acidic condition. It was unlikely that nitrite was in the form of H2ONOþ in cloud and rainwater. Even though aerosol water was more acidic, H2ONOþ had a very minor contribution to conversion of Bp in the atmospheric aqueous solution. The reaction with NO 2 controlled the major reaction at pH > 4.5 (green line). However, the reaction rate of nitrite was relatively low (k3a3 < 0.0019 s1). The direct reaction of nitrite to Bp was unlikely to be a major pathway. Considering that the pH of atmospheric waters varied from 1.95 to 7.74, it suggested that the contribution of HONO to the transformation of Bp in atmospheric solution was more significant than þ NO 2 and H2ONO . 3.1.4. Effect of chloride ions Soluble anions had been identified in field investigations of rain and cloud-water, superficial waters and these compounds could react with OH radical in the troposphere aqueous phase (Herrmann, 2003). The rate constant of OH radical with chloride ions was 4.3  109 L mol1 s1 (George et al., 1988). The overall reaction rate of Bp degradation was determined in the presence and absence of competition kinetics with Cl (Mack and Bolton, 1999; George et al., 1988):

,OH þ Cl /ClOH 

(11)



In Fig. 2c, Cl concentration had a little effect on the kinetic curves of Bp conversion. Chloride ions concentration in the atmospheric solution was very low. Therefore, the effect of OH radical scavenged by Cl was insignificant. 3.2. Laser flash photolysis studies Three main absorption bands with peaks centered at 305 nm, 370 nm and 400 nm were observed as shown in Fig. 3a. Ethanol was an efficient hydroxyl radical acceptor, which scavenged
OH in aqueous solution and consumed
OH immediately.

EtOH þ $OH/CH3 $CHOH þ H2 O

k ¼ 1:9  109 L mol1 s1 (12)

Fig. 2. (a) Effect of N(III) concentration on Bp decomposition. (b) The mole fraction of H2ONOþ (dashed line), HONO (solid line) and NO 2 (dotted line) in solution and apparent rate constants for the reaction of Bp with speciation-species of N(III) as a function of pH (1.5e6.3), (c) Effect of chloride ions concentration on the transformation of Bp under irradiation (The experiments were performed in the conditions of 3.25  105 M Bp and 5  103 M N(III) solution at pH ¼ 1.5).

constants of H2ONOþ, HONO and NO 2 , respectively. a1, a2 and a3 represented the fraction of H2ONOþ, HONO and NO 2 , respectively. The species-specific second rate constants, k1 ¼ (0.058 ± 0.005 L mol1 s1), k2 ¼ (0.12 ± 0.06 L mol1 s1) and k3 ¼ (0.0019 ± 0.0003 L mol1 s1) were calculated from the nonlinear curve of the experimental data kapp by using software Origin 8.5. The model could describe very well with the experimental kapp. As depicted in Fig. 2b, the rate constants kapp of the reaction decreased with increasing pH values. These differences could be explained by

Reaction of hydroxyl radical with ethanol did not generate any significant intermediate absorption spectra. The transient absorption in the range of 280e380 nm was strongly suppressed by addition of ethanol (insert figure of Fig. 3a) implied that the transient species in these region should stem from the
OH either directly or indirectly. The peak absorptions in the range of 300e350 nm were the characteristic of the reaction between
OH and the aromatic ring to form hydroxycyclohexadienyl radicals (Merga et al., 1994; Song et al., 2008). Therefore, the absorption bands with peaks at 305 nm and 370 nm were assigned to the BpOH adducts generated from the following reactions.

Bp þ $OH/Bp  OH adduct

(13)

Kinetics study on the transient species at 305 nm indicated that its growth and decay obeyed the pseudo-first order reaction mechanism with kgrowth ¼ 2.7  106 s1 and kdecay ¼ 4.5  104 s1 (Fig. 3b). The second-order rate constant of Bp with
OH was

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0.005

3.5

0.005

-1 0.002 0.001

0.003

0.000

-5

Absorbance

a

pH = 1.5

3.0

0.003

0.004

Concentration (10 mol L )

a

without ethanol 0.03 mol L ethanol

0.004

280

300

320

340

360

380

400

420

0.3 μs 8.3 μs 16 μs 24 μs 32 μs

0.002

0.001

0.000 280

2.5 2.0

biphenyl hydroxybiphenyl nitrobiphenol

1.5 1.0 0.5 0.0

300

320

340

360

380

400

0

420

20

40

Wavelength (nm) 0.012

Time (min)

0.012

0.004

-5

0.008

0.000

0.006

pH = 2.8

3.0

0.008

b

-1

Absorbance

b

80

3.5

Concentration (10 mol L )

Absorbance

0.010

60

0.0

0.5

1.0

1.5

2.0

Time (μs)

0.004

0.002

2.5 2.0

biphenyl hydroxybiphenyl nitrobiphenyl nitrobiphenol

1.5 1.0 0.5 0.0

0.000 0

20

40

60

80

0

100

20

40

60

80

100

120

Time (min)

Time (μs) 3.5

c Concentration (10 mol L )

c

-1

8

-5

5

10

6 4 2 0 0.00

pH = 6.3

3.0

-1

First-order growth rate (10 s )

12

0.02

0.04

0.06

0.08

0.10

0.12

0.14

-1

Bp concentration (mmol L ) Fig. 3. Transient absorption spectra following 355 nm laser flash photolysis of mixed solution containing 5  103 M of N(III) and 3.25  105 M of Bp (a) N2, (b) with ethanol at pH of 1.5, (c) plot of the pseudo-first order growth rate constant of the BpOH adduct at 305 nm and against concentration of Bp.

derived to be 9.4  109 L mol1 s1 by the fitting curves of transient species at 305 nm (Fig. 3c), which was in good agreement with (0.9 ± 0.1)  1010 L mol1 s1 reported by Sehested and Hart (1975) and (1.04 ± 0.05)  1010 L mol1 s1 given by Chen and Schuler (1993) using pulse radiolysis technique.

2.5 2.0

biphenyl hydroxylbiphenyl nitrobiphenol nitrobiphenyl

1.5 1.0 0.5 0.0 0

10

20

30

40

Time (h) Fig. 4. Time profile of Bp and byproducts concentration at different pH, (a) pH ¼ 1.5, (b) pH ¼ 2.8, (c) pH ¼ 6.3 (3.25  105 M Bp and 5  103 M N(III)).

Further studies with peak centered at 400 nm showed that the lifetime of the transient species was much longer than Bp-OH adducts. We verified it by checking the transient absorption spectrum following the 355 nm photolysis of sole HONO solution (without Bp). The transient absorbance of 400 nm could not be quenched completely when adding ethanol and the shape of band resembled

J. Ma et al. / Chemosphere 167 (2017) 462e468

467

OH OH

O 2N

O 2N

NO2-

nitrobiphenol m/z=215

OH -H

OH

OH biphenyl m/z=155

HONO

hydroxybiphenyl m/z=171

NO OH

-H2O

NO2

O2

NO

nitrobiphenyl m/z=199 Fig. 5. Proposed schemes for the reaction pathway of Bp and N(III).

the original one without ethanol. The similar transient optical absorption spectrum was also observed in the 355 nm photolysis of N2-saturated aqueous solution of HONO reported by Ouyang et al. (2005). Therefore, the absorption bands centered at 400 nm could be assigned to NO2 which was formed by hydrogenabstraction reaction of
OH with nitrous acid and nitrite as equations (3) and (4) (Treinin and Hayon, 1970; Mack and Bolton, 1999). 3.3. Analysis of reaction products The reaction products were identified by GC-MS measurements. 100 mL aqueous solution containing 3.25  105 M Bp and 5  103 M N(III) at different pH (pH ¼ 1.5, 2.8 and 6.3) were exposed to a UV lamp (8 W) with 365 nm wavelength. HPLC chromatograms recorded the concentration of main products during all reaction times. Based on the results of laser flash photolysis, hydroxyl radicals added onto the aromatic ring of Bp to form Bp-OH adducts, and subsequent elimination of H atom from benzene ring of Bp resulted in the generation of hydroxybiphenyl. Under the conditions of pH ¼ 1.5, the hydroxybiphenyl was the most abundant product (Fig. 4a), which was accounted for 24e35% according to following equation:

hydroxylation ð%Þ ¼

½hydroxybiphenyl  100% ½biphenyl

(14)

Where, [hydroxybiphenyl] was the concentration of hydroxybiphenyl. [biphenyl] was the initial concentration of Bp. Reaction products nitrobiphenol was poorly accumulated as shown in Fig. 4a. The lower concentration of nitro-compounds indicated that H2ONOþ played a minor role in the nitration process of Bp and its conversion products. The concentration of products of nitrobiphenol and nitrobiphenyl were increased with increasing pH value of reaction solution (Fig. 4b). Since the Bp did not react with N (III) in the dark, nitro-compounds could not be formed directly from Bp. Fischer and

Warneck (1996) detected the generation of nitrosophenol when studying the photochemical reaction between benzene and HONO, and assumed that nitrosophenol was produced by the further nitration of hydroxycyclohexadienyl radical. Vione et al. (2005) also reported that the nitration of naphthalene upon irradiation of nitrous acid which was a photoinduced process due to HONO did not nitrate naphthalene in the dark. So the nitro-products of nitrobiphenol and nitrobiphenyl should be came from the reaction between HONO with Bp-OH adducts (Zhu et al., 2007). The contribution of nitration and hydroxylation on the Bp transformation at pH of 2.8 were about 22% and 26%, respectively according to the similar calculation process of equation. Given that N (III) was mainly existed in the form of HONO at pH of 2.8, HONO was supposed to play an important role in the formation of nitrobipheny. In a near neutral solution (pH ¼ 6.3), almost all N(III) were existed in the form of NO 2 , the number of OH radical consumed by NO 2 was larger than in the acid solution. Therefore, $NO2 was produced according to Eq (4). NO2 radical was involved in the nitration of aromatic compound (Vione et al., 2002; Noriko et al., 2008). In addition, some of hydroxybiphenyl were existed in the form of phenoxyl radicals and then be oxidized to nitrobiphenol by $NO2. The contribution of nitration by $NO2 on the transformation of Bp was more than 50%. Therefore, nitrobiphenol was the main products at near neutral solution, while nitrobiphenyl and hydroxybiphenyl were poorly accumulated (Fig. 4c). In summary, a plausible scheme for the reaction pathway between Bp and N (III) under irradiation of 365 nm UV light was proposed in Fig. 5. 4. Conclusions The photochemical reaction between Bp and N (III) under irradiation within the wavelength of 365 nm was dependent on N (III) concentration, Bp initial concentration and pH. The species-specific rate constants, k, were determined for reaction of Bp with H2ONOþ

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1 1 (k1), HONO (k2) and NO s ), k2 2 (k3) were k1 (0.058 ± 0.005 L mol (0.12 ± 0.06 L mol1 s1) and k3 (0.0019 ± 0.0003 L mol1 s1) calculating from the nonlinear curve of the experimental data kapp. Laser flash photolysis studies confirmed that OH radical deriving from the photolysis of N (III) would attack Bp to form Bp-OH adduct with a rate constant of 9.4  109 L mol1 s1. Nitrobiphenyl, nitrophenol, hydroxyldiphenyl and o-nitrobiphenyl were identified as the steady products of the photo-reaction.

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