Transient and steady-state photolysis of p-nitroaniline in aqueous solution

Journal of Hazardous Materials 165 (2009) 867–873

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Transient and steady-state photolysis of p-nitroaniline in aqueous solution Hongjuan Ma a,b , Min Wang a , Changyong Pu a,b , Jing Zhang a,b , Sufang Zhao a,b , Side Yao a,∗ , Jie Xiong c a

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P. O. Box 800-204, Shanghai 201800, PR China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China c Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, PR China b

a r t i c l e

i n f o

Article history: Received 3 September 2008 Received in revised form 15 October 2008 Accepted 15 October 2008 Available online 28 October 2008 Keywords: p-Nitroaniline Photo-degradation Laser Flash Photolysis UV/H2 O2

a b s t r a c t Both transient photolysis and steady-state photo-degradation experiments were performed to gain insight into the kinetics and mechanisms of degradation of p-nitroaniline (p-NA) in aqueous solutions. Monophotonic photo-ionization of p-NA was characterized by 266 nm Laser Flash Photolysis (LFP). The quantum yield of eaq − was calculated. Radical cations and neutral radicals of p-NA were identified, respectively. The LFP of p-NA with H2 O2 in aqueous solutions was investigated for the first time. For steady-state photodegradation, degradation of p-NA was observed at diverse irradiation conditions under 254 nm UV light. Direct photo-degradation of p-NA by 254 nm UV light in aqueous solution was very difficult. Once H2 O2 was added into the experimental system, degradation of p-NA was enhanced remarkably. In the absence of O2 , degradation rate increased rapidly along with the irradiation time and reached 80% at 10 min. p-NA could be totally removed after 15 min in UV/H2 O2 process. In the presence of O2 and H2 O2 , degradation rate increased linearly along with the irradiation time and reached 80% at 7.5 min. p-NA could be totally removed after 10 min. The mechanisms behind the photo-degradation of p-NA were discussed. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Biological treatment is a technique broadly applied to the treatment of wastewater from urban areas. It uses microorganisms to metabolize the pollutants. However a large number of compounds are not biodegradable or cannot be destroyed by biological treatment due to their toxicity. p-NA is an important compound either manufactured or used as an intermediate such as in the synthesis of dyes, antioxidants, pharmaceuticals, gum inhibitors, poultry medicines, pesticides etc. Unfortunately, p-NA has been found to be harmful to aquatic organisms and may cause long-term damage to the environment. It is highly toxic with a TLV (threshold limit value) of 0.001 kg m−3 , which is lower than that of aniline (0.002 kg m−3 ). The presence of a –NO2 in the aromatic ring enhances its stability to resist chemical and biological oxidation degradation, while the anaerobic degradation produces nitroso- and hydroxylamines compounds which are known as carcinogen [1–3]. Therefore, chemical remediation of p-NA contained wastewater is one of the main aspects in modern environmental chemistry [2,4,5]. In recent years, advanced oxidation processes (AOPs) have been identified as an attractive option for wastewater purification, particularly in cases where the contaminant species are difficult to remove using biological or physicochemical processes. Gen-

∗ Corresponding author. Tel.: +86 2159554681; fax: +86 2159554681. E-mail address: [email protected] (S. Yao). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.10.077

erally, the processes involve the generation of highly reactive hydroxyl radical (• OH), which can oxidize and mineralize almost all organic molecules owing to its high oxidation potential (E⍜ = +2.8 V) [6–8]. Since the late 1960s, many studies have indicated that the UV/H2 O2 process is able to oxidize a wide variety of organic pollutants in aqueous solutions [9,10]. During the past decade, photo-induced oxidation methods using UV light and H2 O2 have been improved markedly for the remediation of groundwater and industrial waste-streams [11]. The effectiveness of homogeneous light-driven oxidation processes is associated with • OH radicals, which are generated in the reaction mixture by the direct photolysis of an added component (e.g., H2 O2 ) under UV irradiation. Because of • OH, the major advantages of these technologies are as follows: (a) high rates of pollutant oxidation (most organic contaminants are eventually completely mineralized to harmless chemicals, such as CO2 , H2 O, and halide ions); (b) large range of applicability regardless of water quality; (c) convenient dimensions of the equipment [12]. The success of photolysis is highly dependent on the photoreactivity of the organic compounds. Hence, properties of the pollutants and the photo-degradation mechanism should be investigated beforehand. The degradation mechanism of organic substances is often complex, especially in diverse reaction conditions. Consequently, individual degradation system must be designed to study the effects of primary species. Besides, some reactive intermediates might undergo further reactions and be present at undetectable concentrations. Hence, their transient existence

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can only be inferred from the kinetic data and/or other means. Laser Flash Photolysis and pulse radiolysis techniques have been used to study the mechanism involved. Early in 1977, Van Der Linde had determined the reaction rate constant of p-NA with • OH using pulse radiolysis [13]. The data has never been used to associate with any practical degradation research nevertheless. Although some researches such as photo-catalytic degradation etc. have been done to understand the degradation mechanism [4,5], to the authors’ knowledge, there is little information available in the literatures about the detailed degradation mechanism of p-NA in aqueous solution, especially the cooperation of • OH with O2 . In this paper, both steady-state and transient photolysis experiments were performed to gain insight into the kinetics and mechanisms of photo-induced degradation of p-NA in aqueous solution. In the first part of this paper, transient photolysis of pNA in aqueous solution was investigated using 266 nm LFP. Then, degradation of p-NA under various conditions was investigated. The main objectives of the present work were to elucidate the mechanisms of the photo-induced degradation of p-NA and consequently to provide kinetic insight into utilization for remediation of p-NAladen wastewaters. 2. Experimental 2.1. Chemical reagents p-Nitroaniline (p-NA), tert-butanol (t-BuOH), 30% hydrogen peroxide (H2 O2 ), anhydrous sodium sulfate (Na2 SO4 ), potassium iodide (KI) and potassium peroxydisulfate (K2 S2 O8 ) were of analytical grade purchased commercially. Methanol (CH3 OH) was chromatographic purity grade. N2 O, O2 and N2 gases were of high purity (99.99%). t-BuOH was redistilled prior to use, other reagents were used directly without further purification. All solutions were prepared using water from a Milli-Q purification system. 2.2. Laser Flash Photolysis experiments Laser Flash Photolysis (LFP) experiments were performed with a ND:YAG laser which provided 266 nm pulse with a duration of 5 ns. The maximum laser power was 50 mJ/pulse. Samples for photolysis experiments were all bubbled with high-purity N2 or N2 O in quartz cuvettes for 20 min before use. The time-resolve experiments were all carried out at 15 ± 2 ◦ C. Detailed description of the facility and experimental conditions can be found elsewhere [14,15]. To determine the quantum yield of photo-ionization, a series of KI solution (the quantum yield of eaq − , ˚e− = 1 at 15 ◦ C) was used as reference. Their absorbances at 266 nm were measured by a Hitachi UV-3010 spectrophotometer. p-NA solutions were prepared whose concentrations were adjusted to make the absorption of the sample at 266 nm same to that of KI solutions (absorbance = 1.042). The yield of eaq − formed by ionization was measured from the absorbance at 650 nm immediately after the pulse. Dependence of absorbance measured at 650 nm on absorbance of p-NA and KI solution is obtained. By comparing the slope of the two straight lines, the quantum yield of photo-ionization of p-NA is determined. 2.3. Photo-degradation procedures and analysis The irradiation source is hexagonal in cross-section of 9 cm side length. Six low-pressure Hg lamps (8 W), which emit 254 nm UV light, are arranged on each side of the device made of stainless steel and coated by aluminum foil for light reflection. Samples in quartz cuvettes were located in the center of the lamps. Details of the device can be referred to a Chinese Patent [16]. All steady-state

Fig. 1. Transient absorption spectra of N2 saturated 1 mM p-NA aqueous solution at () 0.05 ␮s; () 0.5 ␮s; () 5 ␮s after the 266 nm laser pulse. Inset: formation and decay traces of the transient absorption at () 640 nm; () 380 nm; () 400 nm.

experiments were carried out at ambient temperatures (25 ± 2 ◦ C) and the pH values of the solution are of natural (pH 6.0). Samples in quartz cuvettes were taken at appropriate time to analyze the concentration of p-NA. The concentrations of p-NA were determined by gas chromatography (Varian CP 3800) with a column (VF-5MS, 30 m × 0.25 mm × 0.25 ␮m). The column temperature was initially held at 50 ◦ C for 2 min and then programmed up to 300 ◦ C at 20 ◦ C min−1 . Prior to the GC analysis, the samples were extracted with a solid-phase microextraction (SPME) method from Supelco (Bellefonte, PA, USA). The fibre used in the study was polydimethylsiloxane–divinylbenzene 65 ␮m (PDMS–DVB). The concentrations of nitrate ions (NO3 − ) and ammonia ions (NH4 − ) formed during irradiation were determined by ion chromatography. The analyses of the inorganic explosive residues were performed on the Metrohm (Switzerland) MIC. The analysis of NO3 − was carried out using METROSEP A SUPP 5-250 column (i.d. 250 mm × 4.0 mm, 5 ␮m particle size), column temperature: 35 ◦ C, eluent: 3.2 mM Na2 CO3 /1.0 mM NaHCO3 , eluent flow rate: 0.7 mL/min, injection volume: 20 ␮L. The analysis of NH4 + was carried out using METROSEP C 2-250 column (i.d. 250 mm × 4.0 mm, 7 ␮m particle size), column temperature: 35 ◦ C, eluent: 4 mM tartaric acid; eluent flow rate: 0.7 mL/min, injection volume: 100 ␮L. The pH values of the solutions were measured with a PHSJ-4A pH meter. All experiments were carried out at ambient temperatures (25 ± 2 ◦ C). 3. Results and discussion 3.1. Characterization of photo-ionization of p-NA in aqueous solution 3.1.1. Monophotonic photo-ionization and the quantum yield of hydrated electrons (eaq − ) Fig. 1 shows the transient absorption spectra observed from laser photolysis of a N2 saturated 1 mM p-NA aqueous solution. After the pulse, in a very short time (0.05 ␮s) a strong absorption appeared in a broad wavelength region of 580–800 nm. The strong, broad range of 580–800 nm disappeared when the system was saturated with N2 O with addition of t-BuOH (Eqs. (3) and (4)). Hence, the transient species with maximum absorption around 680 nm could be assigned as eaq − . It meant that p-NA could be photoionized directly by 266 nm UV light in aqueous solution (Eq. (1)).

H. Ma et al. / Journal of Hazardous Materials 165 (2009) 867–873

Fig. 2. The reaction rate constant for p-NA with eaq − after the 266 nm Laser Flash Photolysis was determined as 3.3 × 1010 M−1 s−1 .

A transient absorption bleaching at 380 nm was recorded (Fig. 1), which represented the depletion of ground states of p-NA. Depletion recorded at 0.05 ␮s corresponded to ground states of p-NA in the reaction 1 (Eq. (1)). At 0.5 ␮s the depletion increased as another part of p-NA reacted with eaq − to form p-NA•− (Eq. (2)). As observed at 680 nm, kinetic analyses showed that eaq − decayed in deaerated solutions with the first-order kinetics. This also indicated that eaq − reacted with unphoto-ionizated molecules of p-NA to form radical anions (p-NA•− ) (Eq. (2)). The slope of the line (ln(OD) − time) was kobs values of the reaction of p-NA with eaq − at a certain concentration. A series of kobs values were obtained with different concentrations of p-NA. Plotting of kobs against the concentration yielded a straight line. The slope of the line was the reaction rate constant of p-NA with eaq − which was determined to be 3.3 × 1010 M−1 s−1 (Fig. 2) 266 nm h

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Fig. 3. Dependence on laser intensity of the absorbance at 650 nm 0.01 ␮s after 266 nm laser pulse: () KI and () p-NA.

sient absorption of p-NA•+ observed at 320 nm was investigated by changing pH value of the solution as shown in Fig. 4. And pKa value of p-NA•+ is determined as 1.5. This meant that in natural pH condition (pH 6.0), p-NA•+ would changed into p-NA• via deprotonation (Eq. (5)). The long-lived species observed at 320 nm could be assigned as p-NA• . (p-NA–H)•+ → p-NA• + H+

(5)

Comparing the transient absorbance OD0 after laser pulse at 380 and 320 nm, the molar extinction coefficients (ε) of p-NA•+ can be deduced via the following equalities as the concentration of bleached p-NA equals to that of formed p-NA•+ . OD0=380 nm = ε=380 nm C=380 nm L

(6)

OD0=320 nm = ε=320 nm C=320 nm L

(7) (8)

p − NA −→ p-NA•+ + e− aq

(1)

(6)/(7) ε=380 nm OD0=320 nm = ε=320 nm OD0=380 nm

p-NA + eaq − → p-NA•−

(2)

eaq − + N2 O + H2 O → N2 + OH− + • OH

(3)

• OH

(4)

where ε␭=380 nm and OD0␭=380 nm are the molar extinction coefficient and the OD0 value of p-NA recorded at the wavelength of 380 nm, respectively. ε␭=320 nm and OD0␭=320 nm are the molar extinction coefficient and the OD0 value of p-NA•+ recorded at the wavelength of 320 nm. ε␭=380 nm of p-NA has been determined to

+ (CH3 )3 COH → CH2 (CH3 )2 COH + H2 O

In order to check whether photo-ionization of p-NA with 266 nm photons was caused by a one-photon or a two-photon process, the yield of eaq − was calculated from OD0 (optical density value recorded at 0 time after laser pulse) at 650 nm by changing intensities of laser pulse. OD0 value had good linear dependence with the incident laser intensity (IL ) as shown in Fig. 3. The ionization was therefore concluded to be a monophotonic process under our experimental condition. By comparing with photolysis of KI solution [14,17], the quantum yield of hydrated electron (˚e− ) is determined to be 0.05. 3.1.2. Identification of p-NA radical cation (p-NA•+ ) The transient absorption spectra observed from laser photolysis of a N2 saturated 1 mM p-NA aqueous solution exhibited an absorption peak at 320 nm (Fig. 1). This might be three kinds of transient species: radical cation (p-NA•+ ) (Eq. (1)), radical anion (p-NA•− ) (Eq. (2)) or neutral radical (p-NA• ) via deprotonation (Eq. (5)). With addition of N2 O and t-BuOH, eaq − was scavenged, p-NA•− would not exist in the system. However, the intensity of the absorbance at 320 nm did not decrease. Hence, the transient species at 320 nm could be assigned as a mixture of p-NA•+ and p-NA• . The tran-

Fig. 4. Absorbance at 320 nm as a function of pH dependence, recorded at 0.01 ␮s after the laser pulse.

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Fig. 5. Transient absorption spectra recorded at () 0.05 ␮s; () 0.5 ␮s; () 5 ␮s in 266 nm LFP of N2 saturated aqueous solution containing 0.475 M K2 S2 O8 and 0.05 mM p-NA. Inset: transient time profile observed at 300 nm.

be 11.970 M−1 cm−1 [18], hence molar extinction coefficient of transient species of p-NA•+ can be calculated as 898.3 M−1 cm−1 from Eq. (8). To further identify p-NA•+ , reaction of p-NA with sulfate radical (SO4 •− ) was investigated. As a one-electron oxidant, SO4 •− can be generated easily with 266 nm LFP from S2 O8 2− (Eq. (9)). Photolysis of 0.1 M K2 S2 O8 aqueous solution showed the formation of SO4 •− with maximum absorption spectra at 320 and 460 nm (data not shown) [19–22]. Concentrations of S2 O8 2− and p-NA must be carefully designed to make sure that S2 O8 2− is principal absorber of 266 nm light and the photolysis of p-NA can be ignored. Then the transient absorption spectra of reaction 10 (Eq. (10)) can be recorded. The transient absorption spectra recorded at 0.05, 0.5 and 5 ␮s after 266 nm LFP of N2 saturated aqueous solution containing 0.475 M K2 S2 O8 and 0.05 mM p-NA are shown in Fig. 5. An increment of absorbance at 300 nm was observed, which indicated that new transient species was formed. As shown in inset of Fig. 5, the growth trace of p-NA• at 300 nm was obtained by subtracting the absorbance of SO4 •− at 320 nm. The transient species with absorption peak at 300 nm could be assigned as p-NA• formed according to the following mechanism: S2 O2− 8

266 nm h

• −→ 2SO− 4

SO4 •− + p-NA → SO4

Fig. 6. Transient absorption spectra recorded at () 0.05 ␮s; () 1 ␮s; () 20 ␮s in 266 nm LFP of N2 saturated 0.1 mM p-NA and 0.022 M H2 O2 aqueous solution. Inset: transient time profile observed at 310; 380 and 500 nm.

tion of p-NA with • OH from photo-decomposition of H2 O2 . Kinetic analyses showed that the growth trace at 500 nm followed the first order kinetics, which was the apparent rate constant of the reaction (Fig. 7, Eq. (15)). Here the transient trace at 320 nm was not suited to determine rate constant for Eq. (15), because there was an overlap of p-NA•+ and p-NA• from photo-ionization of p-NA. By changing the concentration of p-NA (0.025–0.5 mM), a series of apparent rate constants were obtained. And the rate constant for Eq. (15) was determined to be 1.3 × 109 M−1 s−1 (Inset Fig. 7). H2 O2 is weak absorbing compounds in the UV range, and the absorption increases as the wavelength decreases. For example, at  = 254 and 266 nm, the molar absorption coefficients (ε) is 21.2 M−1 cm−1 and 10.7 M−1 cm−1 , respectively. For p-NA at  = 254 and 266 nm, ε is 2.110 and 1.180 M−1 cm−1 , respectively. Hence, in aqueous solutions of 0.1 mM p-NA in the presence of 0.022 M H2 O2 , both H2 O2 and p-NA are principal absorber of UV light. H2 O2 can absorb the [(10.7 × 22)/(10.7 × 22 + 1180 × 0.1)] × 100% = 68% of 266 nm photons and p-NA can absorb 32% 266 nm photons. The photolysis of H2 O2 in pure water has been studied extensively by various groups of researchers in the 1950s [23–27] in order to

(9) 2−

+ p-NA•+ → p-NA• + H

+

(10)

3.2. Characterization of photo-ionization of p-NA with addition of H2 O2 The transient absorption spectra recorded at 0.05, 1.5 and 20 ␮s were characterized by absorption bands with maximum at 320 nm and a weaker one at 500 nm after the 266 nm LFP of N2 saturated 0.1 mM p-NA with 0.022 M H2 O2 aqueous solutions (Fig. 6). Bleaching in the range of 350–420 nm was observed, while the strong broad range of 580–800 nm which should be assigned as eaq − disappeared. The growth–decay trace at 320 nm differed from that with the absence of H2 O2 . A transient species with a lifetime of 30 ␮s showed a clear formation in 2.5 ␮s. Meanwhile, it was synchronous with the decay of ground state of p-NA at 380 nm (inset in Figs. 1 and 6). The strong initial absorption bleaching at 380 nm reflected that pump-induced ground-state depletion dominating the observed dynamics. This indicated that the transient species recorded at 320 and 500 nm could probably assigned as the reac-

Fig. 7. Formation and decay traces of the transient absorption at 500 nm after 266 nm LFP of N2 saturated p-NA and 0.022 M H2 O2 aqueous solution, concentrations of p-NA are at the range of 0.025–0.5 mM. Inset: the reaction rate constant for p-NA with • OH after the 266 nm LFP was determined as 1.3 × 109 M−1 s−1 .

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elucidate the reaction mechanism and to determine the primary and overall quantum yields. The general consensus is that the reaction scheme below describes H2 O2 photo-decomposition in pure water: H2 O2 • OH

266 nm h

−→ 2OH

(11)

+ H2 O2 → HO2 • + H2 O

(12)

kH2 O2 = 2.7 × 107 M−1 s−1 HO2 • + • OH → H2 O + O2

(13)

k = 6 × 109 M−1 s−1 HO2 • + HO2 • → H2 O2 + O2 5

k = 8.3 × 10 M

(14) Fig. 8. Degradation rates of 1 mM p-NA in different aqueous solutions: () N2 saturated; () O2 saturated; () N2 saturated with addition of H2 O2 and () O2 saturated with addition of H2 O2 .

−1 −1

s

p-NA + • OH → p-NA-OH

(15)

H2 O2 + eaq − → • OH + OH−

(16)

k = 1.2 × 1010 M−1 s−1 Thus, H2 O2 photolysis generates the very highly oxidizing and reactive • OH radicals. eaq − produced in photo-ionization of p-NA as mentioned above can be scavenged by H2 O2 (Eq. (16)) and the transient absorption in broad range of 580–800 nm disappeared. The new transient species observed at 320 and 500 nm may be due to the • OH adducts undergoing unimolecular reaction (Eq. (15)). Transient reaction of p-NA with • OH was investigated in previous study using pulse radiolysis. The reaction rate constant indicates that • OH can react with p-NA at a rate near the diffusion-controlled limit. There is general consensus that UV/H2 O2 -promoted oxidations involve mainly • OH radicals produced by photolysis of H2 O2 . However, the actual situation may be much more complex. Consequently, individual steady-state photo-degradation experiments were designed to study the degradation of p-NA. 3.3. Degradation of p-NA under 254 nm UV light in aqueous solution 3.3.1. Direct photo-degradation of p-NA in aqueous solution Series of p-NA aqueous solutions were irradiated at 254 nm UV light and the degradation rates were shown in Fig. 8. It has been found that in N2 saturated experimental system, with the progress of photolysis, the color of the reaction solution became brownish gradually, and precipitates were obtained. The degradation rate increased along very slowly with the irradiation time and could only reach 10% after 120 min. In the N2 saturated aqueous solution, p-NA can be photo-ionizated to form p-NA•+ , p-NA•+ will be changed into p-NA• via deprotonation (Eqs. (1) and (5)). Then p-NA• interact spontaneously to form dimers and eventually resulting in higher oligomers and polymers which could be easily separated from the wastewater (Eq. (17)). Degradation rate of p-NA was almost the same as that of in the presence of O2 as shown in Fig. 8, which meant it was very difficult to remove p-NA directly by 254 nm UV light in water. 2p-NA• → p-NA–p-NA

(17)

3.3.2. Photo-degradation of p-NA in aqueous solution with addition of H2 O2 and O2 Once H2 O2 was added into the experimental system, degradation of p-NA was enhanced remarkably (Fig. 8). A significant role for atmospheric oxygen was ruled out by control experiments carried out under a nitrogen atmosphere. In N2 saturated experimental system, degradation rate increased rapidly along with the irradiation time and reached almost 80% at 10 min. p-NA could be totally removed after 15 min. In the presence of O2 , degradation rate increased linearly along with the irradiation time and reached almost 80% at 7.5 min. p-NA could be totally removed after 10 min. It implied that the effect of O2 could not be neglected. H2 O2 photolysis generates very highly oxidizing and reactive • OH radicals, which are involved in the fast degradation of organic pollutants. • OH, because of its proclivity for electron-rich aromatic compounds, could be mainly engaged in the hydroxylation of phenolic species. These species would suffer further hydroxylation and/or one-electron oxidation by • OH to give semiquinones and, hence, quinones by disproportionation. These latter would undergo muconic-type cleavage, and in part also extradiolic cleavage, by the large excess of H2 O2 through a UV-assisted mechanism [28,29]. When O2 cooperates with • OH, peroxide is formed when O2 adds to the nitro substituted hydroxylcyclohexadienyl radicals. Most of the ring cleavages come from these peroxide compounds producing alcohols, aldehydes and acids [30–33]. As a result, a more prominent degradation pathway is engendered leading to the final evolution of CO2 and NO3 − [34–37]. As an alternative, denitration and deamination occurred with the formation of intermediates during the degradation process. Fig. 9 depicts the change of NO3 − and NH4 + concentrations in the degradation process. Obviously, • OH and O2 played an extraordinary role during the process of denitration and deamination. Almost 80% –NO2 dissociation from p-NA was observed in either O2 or N2 saturated solution after 15 min. Deamination in O2 saturated system increased linearly along with the irradiation time and reached 100% at 8 min. And deamination in N2 saturated system even reached 100% after 15 min irradiation. The mechanism of • OH in the degradation process of p-NA has been discussed in previous study and found that –NO2 dissociation was very efficient once • OH attack p-NA while –NH2 dissociation is not facile. However, it was very exceptional to find that deamination was more efficient than denitration in this study.

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p-NA and cooperation of O2 with • OH cannot be neglected. The removal of p-NA by UV light with addition of H2 O2 resulted exclusively from the direct oxidation by UV light. Based on the results of the above, it can be concluded that the sequential use of H2 O2 and O2 have remarkable advantage in the treatment of p-NA-containing wastewaters.

Acknowledgement The work is supported by National Natural Science Foundation of China (Contract No. 10676036).

References

Fig. 9. The change of NO3 − and NH4 − concentrations in different 1 mM p-NA aqueous solutions with addition of H2 O2 : () NH4 − , N2 saturated; () NH4 − , O2 saturated; () NO3 − , N2 saturated; (䊉) NO3 − , O2 saturated.

The structure of p-NA, with a donor group (–NH2 ) linked via a phenyl ring to a strong acceptor group (–NO2 ), makes electron density of C1 on benzene higher in the conjugated bond structures. Photo-ionization by 254 nm UV light initiates losing of a charge from C1. Then the dipole moment was changed corresponding to a higher electron density on C4 (Eq. (18)). In light of the electrophilic of • OH, hydroxylation might be engaged with C4. Hence, –NH2 dissociation occurs easily before the formation of • NH2 in this UV/H2 O2 process (Eq. (19)).

(18)

(19) 4. Conclusions It is totally a new insight to study the properties of the pollutants and the photo-degradation mechanism by transient photolysis. Degradation process and reactive intermediates were detected by LFP. p-NA can be photo-ionized by 266 nm LFP in aqueous solution. The transient absorption spectra were recorded and bimolecular reaction rate constant for p-NA with eaq − was determined to be 3.3 × 1010 M−1 s−1 . p-NA•+ from photo-ionization of p-NA was further identify by the reaction with SO4 •− . The pKa value of the p-NA•+ is determined as 1.5. For steady-state photo-degradation, degradation rates of p-NA in aqueous solutions by 254 nm UV light is in the order of (O2 + H2 O2 ) > H2 O2 > direct photo-degradation. It means that • OH played the main role in the photo-degradation of

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