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Advances in Condensed Matter Physics
Volume 2014 (2014), Article ID 961609, 5 pages
Visible-Light-Driven Photocatalytic Degradation of Aniline over NaBiO3
1School of the Environment, Nanjing University, Nanjing 210093, China
2State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China
3Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 610059, China
Received 6 December 2013; Accepted 1 January 2014; Published 13 February 2014
Academic Editor: Haimin Zhang
Copyright © 2014 Guo Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Aniline was almost completely degraded in 30 min in given conditions. It was found that pH and NaBiO3 dosage had played important role in the photocatalytic degradation. To scrutinize the mechanistic details of the aniline photodegradation, several critical analytical methods including spectroscopy and GC/MS were utilized to detect the temporal course of the reaction. Intermediates and several small molecular products were separated and identified, such as C2H5O3N, C10H13O2N, and C12H10N2. Then two possible photodegradation pathways of aniline over NaBiO3 were proposed: ring opening and mineralization.
Aniline is widely used in the chemical industry, mainly as a raw material for obtaining isocyanate. Aniline is also applied to the manufacture of accelerators, antioxidants, pesticides, dyes, and pigments . The negative effects of aniline on human health and the environment are depending on the amount and exposure time. Aniline can enter the aqueous environment via dyes and nitro aromatic compounds and has been identified as a potential carcinogen . Nowadays, more attention is focused to remove it from environment.
Among chemical methods, photocatalysis technology can play an important role in removing harmful organic compounds, which enables human to have comfortable and safe lives . Being the most common photocatalyst among semiconductors, TiO2 possesses various merits such as low cost, high photocatalytic activity, chemical activity, and nontoxicity . As its photoresponse is only limited in the UV region, which accounts for less than 5% of the earth-reaching solar irradiation [5, 6], it requires a high power UV excitation source . Therefore, it is meaningful to explore a new photocatalyst which can utilize visible light. Lots of studies have been devoted to developing visible-light-driven photocatalysis. Apart from TiO2 doped with metallic elements, for instance, Cr3+ and V5+ , new materials such as AgNbO3 and InNbO4 are previously reported to absorb visible light [9, 10]. However, the activity of the photocatalyst was hardly enhanced, and the low reaction rate severely blocked the development of photocatalysis technology.
Seeking for new materials with higher photocatalytic activity has become a key solution to this problem. Particularly, Bi(V)-containing oxides are potential materials as the candidates of visible light sensitive oxides . In fact, some Bi(V)-containing oxides have indicated the ability of absorption of visible light. Among them, NaBiO3 is studied as it displayed the visible light absorption because of its hybridized valence band. The large dispersion of the hybridized orbitals in the conduction band increases the mobility of the photoexcited electrons, thus suppressing the recombination of photoexcited electron-hole pairs and enhancing activities . Kou and coworkers  utilized NaBiO3 in photooxidation of PAHS. Yu and coworkers  utilized NaBiO3 in photooxidation of rhodamine B under visible light irradiation and found that heating temperature significantly influenced the photocatalytic activity of the catalyst. As NaBiO3 has the properties of strong oxidation, electromagnetism, and electrochemical activity, it has been applied to industries such as organic synthesis, superconductor, and electrochemistry . It is meaningful to explore the feasibility of the photodegradation of aniline over NaBiO3.
In this study, aniline was chosen as the target organic pollutant to investigate its degradation behavior over NaBiO3 under visible light irradiation. The goal was aimed at the investigation of aniline degradation and the presentation of mechanistic details of the photochemical process.
2. Experimental Section
2.1. Materials and Reagents
Aniline was bought from Ke Long Chemical Company, and NaBiO3·2H2O (NBH) was purchased from Guang Fu Chemical Products Institute. All other chemicals were of analytical grade and used without further purification.
2.2. Photoreaction Chamber
Photodegradation experiments were performed in a chamber which was displayed in Figure 1. A 500 W xenon lamp (Chengdu Na Pu Photoelectricity Company, China) was positioned to ensure visible light. The reactor was a double-deck beaker equipped with an inlet and an outlet which can draw into water circulation to make sure experiments performed in adequate temperature. A 50 W fan was used to keep the chamber at ambient temperature. A stir was used to ensure that photocatalyst distributes well in the suspension. Oxygen pump was used to provide enough oxygen to ensure that oxidation performed sufficiently.
2.3. Experimental Procedures and Analysis Methods
In the experiments, the dispersions containing aniline and NaBiO3 photocatalyst were prepared by adding NBH to an aqueous solution of aniline in a beaker.
A 500 mL double-deck beaker was used as a reactor. The NBH was added to 200 mL aniline solution, which was magnetically stirred at a constant speed to ensure continuous contact between the solution and NaBiO3 when they were under visible light irradiation emitted from xenon lamp. At given time intervals, samples (10 mL) were centrifuged immediately. The samples were analyzed by a V-1100D spectrophotometer. The intermediates were detected by GC/MS (GCMS-QP2010 Plus).
3. Results and Discussions
3.1. Control Experiment
The decreases of aniline concentration as a function of reaction time were shown in Figure 2. In the control experiment, when utilizing NBH (1 g/L) alone, a slight decrease of aniline concentration was observed; this could be attributed to surface adsorption. Aniline was relatively stable in aqueous solution under visible light irradiation as expected. Therefore, aniline was not significantly decomposed by either under ambient conditions used in the experiment. Aniline can be degraded rapidly only in the circumstance of both NBH and visible light irradiation.
3.2. Nitrogen Transformation
As there is an amidogen in the aniline structure, it will be harmful to environment if it transforms into nitrite nitrogen. It is important to study the transformation of nitrogen in the photodegradation process of aniline.
As it was shown in Figure 3, the removal rose in the first 30 min of the degradation aniline; the concentration of ammonia nitrogen and nitrate were increasing and tended towards stability in 120 min. Nitrites were produced as the intermediates of nitrate, and the concentration was low on the whole.
3.3. Kinetic Model
The reaction mechanism of photocatalytic degradation was the process of absorption-surface-reaction-desorption whose kinetic model was in coincidence with the Langmuir-Hinshelwood model [15, 16]. The kinetic model was as below : where is time and is instantaneous concentration of aniline. The equation below is the integration of it when is from 0 to :
When initial aniline concentration was 30 mg/L, the pseudo first order rate constant for photocatalytic degradation, light reaction, and dark reaction was 0.0402 min−1, 0.00182 min−1, and 0.000987 min−1, respectively. The kinetic study proved that the effect of NBH on aniline degradation is great.
3.4. Influence Factors
3.4.1. pH Value Effect
In the photocatalytic oxidation, the solution pH had an instant influence on the agglomeration of particles, electric charge, and absorption of organic on the surface . In Figure 4, the remainder aniline in the acidity was less than that in the neutral and alkaline condition. When pH was 2, the aniline concentration was reduced to 0.98 mg/L in the 120 min with a removal of 97%. The main reason was that the oxidation was in the dominant position. It was found that the NBH effect was better in the acidity condition. The removal rate can reach 75% in the 5 minutes, whereas the removal rate in the neutral and alkaline condition was 20% and 16%, respectively. In the acidity condition, [BiO3] can turn into [BiO6] more easily, while the latter’s crystal structure was relatively weak. During reduction of Bi(V), oxygen which had higher activity was released . The equation was as below:
In this case, aniline was oxidized mainly by . The reaction progress was not part of photocatalytic oxidation.
The photocatalytic oxidation of aniline was as below:
H+ has played an important role in the photocatalytic oxidation. In the weak acidity condition, H+ can combine with active oxygen to produce more •OH which increases the reaction rate. In the neutral and alkaline condition, the oxidation electric potential decreased as the pH increases. As a result, the removal had a decline .
3.4.2. NBH Dosage Effect
The result of aniline degradation over NBH dosage was given in Figure 5. The result showed that the oxidation efficiency was increasing with NBH dosage increasing correspondingly in the first 30 min. When NBH dosage was 0.5 g, there was a significant rise in the removal, as the aniline concentration was decreased to 12.7 mg/L in the 10 min. However, the removal in the 120 min was 84%, lower when compared with 0.2 g and 0.1 g, which were both around 87%. Referring to the fact that over NBH dosage could hinder the suspension transparency, which would be an obstacle to absorb light and decrease photocatalytic degradation efficiency eventually, as a result, excessive NBH dosage could lead to a downward trend shown in the oxidation effect.
3.4.3. GC/MS Analysis
In the given condition, aniline could be degraded completely in the 30 min. Analysis on the intermediates and final products can help to clarify the details of the reaction. They were identified by GC/MS (GCMS-QP2010 Plus) and shown in Table 1 and Figure 6.
In the chromatograph of initial sample, there was a peak representing aniline, which disappeared in the 30 min. The chromatomap of 60 min resembled that of 30 min, as the peak of C10H13O2N and azobenzene existed, while their top area was decreasing. It proved that the above intermediates of aniline have been degraded to other products. There was a peak at about 7 min in the picture of 60 min, which was C2H5O3N, the product of C10H13O2N and azobenzene. In the chromatomap of 90 min, C2H5O3N was not detected, which meant it had been mineralized into CO2. The peak of C10H13O2N had disappeared in the chromatomap of 120 min. The organics containing nitrogen only was azobenzene, and its peak area had decreased to a small part.
The photohole, generated by NaBiO3 under visible light, could make reaction with aniline and produce azobenzene. Azobenzene produces C10H13O2N by •OH and h+. As the reaction goes on, azobenzene could still react with •OH or h+ until its loop opens. C10H13O2N would go into two parts by the effect of •OH and h+. One could produce C2H5O3N and then be mineralized, while the other one is mineralized after ring opening.
Aniline could be easily degraded by NaBiO3 under visible light irradiation. In the experiment. Aniline was decomposed largely under visible light over NaBiO3 in 120 min, which could reach 95% in some conditions. Moreover, three small molecular products were also identified by GC/MS. On the basis of the data collected, the mechanism of aniline photodegradation on NaBiO3 had been elucidated. Aniline could react with •OH or h+. Its products would go into two parts. One part would be mineralized directly, while the other part opened ring and produced C2H5O3N then was mineralized; meanwhile, N-ionogen turned into mineral nitrogen.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was supported by the National Natural Science Foundation of China (41272266), Natural Science Foundation of Jiangsu Province (BK2012732), and the Natural Science Key Program for Si Chuan Province Education Department of China (10ZA113).
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