Degradation of 4-Nitrophenol Using a Coupled Photocatalytic

Abstract

4-Nitrophenol (4-NP) was degraded using a coupled photocatalytic–biological aerated filter (BAF) process. Effects of different initial 4-NP concentrations on its degradation by using solo photocatalytic oxidation or BAF process are discussed. Degradation efficiency of 4-NP using BAF process after a short-duration photocatalytic pretreatment (PC) were studied in detail. Intermediates of 4-NP degradation were characterized by UV–vis spectrophotometry, high-performance liquid chromatography, high-performance liquid chromatography/mass spectrometry, and ion chromatography. Possible mechanisms of 4-NP degradation using photocatalytic–BAF process were discussed. Short-duration PC was used to decrease chemical oxygen demand and improve biodegradability of 4-NP. It was found that PC of 4-NP for 2.5 h, during which period about 20%–50% chemical oxygen demand removal occurred, could be coupled to second-stage biological treatment for achieving enhanced biodegradation of 4-NP. The as-prepared N-TiO₂/γ-Al₂O₃ granules were used as photocatalyst, photocatalytic–BAF technique was used to degrade 4-NP, and then the catalyst separated from wastewater automatically after photoreaction, which addressed the problem of in catalyst separation. Results indicated that this comprehensive process was simple and effective for the treatment of this typical nonbiodegradable nitrophenols wastewater.

Introduction

Nitrophenols are some of the most refractory pollutants that can be present in industrial wastewater. Among the mononitrophenols, 4-nitrophenol (also known as PNP or 4-NP), is the most common and important in terms of quantities manufactured and extent of environmental contamination. 4-NP is used for synthesis of medicines, dyes, explosives, synthetic dyes and plasticizers, wood preservatives, and rubber chemicals. It is also generated during formulation, distribution, and field application of pesticides or photodegradation of pesticides that contain the nitrophenol moiety (Kulkarni and Chaudhari, 2006).

These pollutants have high toxicity and carcinogenic character. They have caused considerable damage to the ecosystem and human health. Hence, the U.S. Environmental Protection Agency lists 4-NP and 2-NP as priority pollutants (as HR-3 grade) and restricts their concentrations in natural waters to <10 ng/mL (USEPA, 1976).

The presence of substituted groups, such as nitro-, on phenols increases the toxic effects on ecosystem and human health due to their persistence in the environment. Most of these compounds are resistant to microbial degradation, especially at high concentrations.

Traditional treatment technologies such as biological treatment processes (Bhatti et al., 2002; Tome et al., 2005), chemical oxidation (Gu et al., 2008), adsorption (Nevskaia et al., 2004; Koubaissy et al., 2008), and so on have been proposed to treat wastewater containing 4-NP. Various innovative technologies, such as Photo-Fenton (Oturan et al., 2000; Chang et al., 2008), Electrochemical oxidation (Zhu et al., 2007), Sonochemical (Hua et al., 1995), and Heterogeneous photocatalytic oxidation (Shen et al., 2008; Wang et al., 2009), have also been proposed for treating 4-NP in aqueous solutions at low concentration. All these processes can guarantee high removal efficiencies, whereas adsorption has the main disadvantage that it does not provide a real degradation of the compounds but only their transfer from a diluted to a concentrated stream to be disposed; on the other hand, chemical oxidation and electrochemical oxidation can produce intermediates characterized by a toxicity level similar to the original substance (Slokar et al., 1998). The biological treatment is economical, but it is not very effective in a high concentration of 4-NP wastewater due to its toxicity and poor degradability.

Many researchers have been seeking suitable methods to treat poor degradability wastewater. Coupling of photocatalytic oxidation and biological methods is the main treatment process with its own traits (Martin et al., 2003; Mohanty et al., 2005). Mohanty et al. (2005) use photocatalytic process in tandem with a biological process to treat H-acid. The results show that the method could not only achieve degradation of H-acid molecules but also economize the energy and cost required.

In this article, a coupled process of photocatalytic oxidation with biological aerated filter (BAF) was used to treat 4-NP wastewater. 4-NP was treated by short duration photocatalytic pretreatment (PC) to generate intermediates, which were further taken care of by a second-stage biological process (BAF). After photoreaction, catalyst separates from wastewater automatically, which deals with the problem of difficulty in catalyst separation. The effects of photocatalytic and BAF reaction conditions on the degradation of 4-NP were studied, and the possible degradation mechanism was discussed.

Experimental

Chemicals

4-NP was procured from Sigma Chemical Co. All the other chemicals used for the determination of chemical oxygen demand (COD), biochemical oxygen demand (BOD), and nitrate were high-purity analytical reagents from Shanghai Chemicals Co., China. High-performance liquid chromatography (HPLC) grade acetonitrile and methanol solvent was used for collecting HPLC and HPLC/mass spectrometry (MS) data.

Photocatalytic oxidation of 4-NP

Preparation of N-TiO₂/γ-Al₂O₃ granules photocatalyst

Preparation of nitrogen doped TiO₂ sol

Preparation of nitrogen doped TiO₂ sol was followed as Huang (Huang et al., 2007). Tetrabutyl titanate was used as a starting material, and triethylamine as a nitrogen source. First, tetrabutyl titanate, ethanol, and acetic acid were put into a flask with stirring for 30 min to form solution A; triethylamine, ultrapure deionized water, nitric acid, and ethanol were mixed, stirring for 10 min to form solution B. Second, solution B was added dropwise into solution A under vigorous stirring. After the completion of addition, slow stirring continued until the solution formed a transparent immobile gel. The gel was dispersed in 0.5% nitric acid solution to form a homogenous N-TiO₂ sol.

Preparation of N-TiO₂/γ-Al₂O₃ granules

First, the concentration of the as-prepared N-TiO₂ sol was diluted with deionized water to 0.01%. Second, the γ-Al₂O₃ granules whose diameter is 1–3 mm were put into the diluted N-TiO₂ sol and then taken out after 2 min. Third, the N-TiO₂ sol dippingγ-Al₂O₃ granules was dried in an oven at 110°C and calcined in Muffle Furance at 450°C for 2 h. To form a thick film on the granules' surface, the process was repeated thrice.

Photocatalytic oxidation of 4-NP

Photocatalytic experiments were carried out in an internal circulation three-phase fluidized photoreactor, and the device is shown in Fig. 1. The wastewater stored in reservoir (1) was pumped into the bottom of the photoreactor by a wastewater pump (2), with a flow rate 15 L/h. The air influx produced by the air compressor (11) was measured with an air rotameter (4) varying from 5.0 to 13.3 L/min to keep internal circulation of photocatalytic granules moving in the photoreactor during the experiment.

FIG. 1.

Schematic diagram of coupled photoreactor–biological aerated filter (BAF) setup: (1) influent tank, (2) pump, (3) liquid rotameter, (4) air rotameter, (5) photoreactor, (6) thimble, (7) quartz thimble, (8) UV-lamp, (9) N-TiO₂/γ-Al₂O₃ granules, (10) wire, (11) air compressor, (12) storage tank for pretreated wastewater, (13) filter medium (ceramisite), (14) BAF reactor, (15) storage tank for treated wastewater, and (16) dissolved oxygen probe.

Photocatalysis of 4-NP was performed in two phases. Initially the photocatalytic degradation on 4-NP was assessed. Later, it was used as a pretreatment before biodegradation. For the initial phase, 4-NP solutions (60, 80 and 120 mg/L) were stirred by air from the bottom of the photoreactor before commencing UV irradiation, respectively. A UV lamp (36 W; xinyate Illuminate Factory) was used to irradiate the test solutions. The reacted samples were taken out after a time interval of 1, 2, 4, 6, and 12 h. After treatment, the solutions were filtered through a 0.45 μm membrane filter, and their absorbance was scan measured at the range from 200 to 600 nm using a UV–vis spectrophotometer. The test samples were analyzed for COD and BOD. Simultaneously, the HPLC, HPLC-MS, and ion chromatography (IC) were also recorded.

For biodegradation study under the second phase, PC experiments were carried out with 4-NP solutions of different concentrations (60, 80, and 120 mg/L) for 2.5 h. The pH before and after PC was 7.0 and 6.7, respectively.

Biodegradation of 4-NP

BAF reactor

The BAF reactor was made of polymethylmethacrylate and cylindrical with 8.5 L total volume, whose diameter and height were 100 mm and 1,100 mm, respectively. The cobblestones whose diameter are 5–6 mm were primed on the bottom of the reactor for about 80 mm high. The ceramisites whose diameter are 3–4 mm were on the cobblestones for about 900 mm. The working volume is 6.0 L. The inflow velocity of water and air could be measured by flow meters. The scheme of treatment process is shown in Fig. 1.

BAF reactor start-up

4-NP degrading seed sludge

Cultivation of 4-NP degrading seed sludge was carried out by fill-and-draw technique under total oxidation conditions for 3 months. The used seed sludge was activated sludge from a laboratory reactor fed on synthetic organic wastewater mainly composed of peptone (8.0 g/L) and meat extract (3.0 g/L). The 4-NP concentration in synthetic organic wastewater was stepwise increased during the seed sludge cultivation process.

Ceramisites and biomass attachment

At the start of the experimental period, about 8.0 L of the cultivated 4-NP degrading sludge was attached to the Ceramisites with gentle aeration on the bottom of the reactor. Sludge attached mostly to the Ceramisites in 3–4 h. Sludge attachment was monitored by the decrease in reactor's MLSS concentration.

Operational and environmental conditions

The reactor was operated as follows: Only 4-NP was used as the sole carbon and energy source for a continuous operational period of 20 days. The reactor was subjected to stepwise increases in volumetric loading rate by changing the influent 4-NP concentration.

Substrate and nutrients

4-NP, with and without glucose, was used as carbon and energy as per requirements of BAF reactor start-up phase. (NH₄)₂SO₄ (0.1 g/L), CaCl₂ · 2H₂O (0.062 g/L), FeCl₃ · 6H₂O (0.016 g/L), MgSO₄ · 7H₂O (0.205 g/L), K₂HPO₄ (0.8 g/L), and KH₂PO₄ (0.2 g/L) were added as nutrients and buffer in this phase.

Biodegradation of 4-NP

Wastewaters were put into BAF reactor by sewage pump. The 4-NP concentrations of inflow were 60, 80, and 120 mg/L, respectively; pH value was 7.0; and the velocity of air was 10 L/h. At the same time, potassium dibasic phosphate was added to the reaction liquid according to the mass ratio of 4-NP: P = 100: 1. Samples were collected after filtration through a 0.45 μm membrane filter. The samples were analyzed for their absorbance on the UV–vis spectrophotometer; COD, BOD₅, and HPLC were also recorded for these samples.

Coupling experiments

The 4-NP concentrations of inflow were 60, 80, and 120 mg/L, respectively, which were subjected to PC as previously described. The coupling photocatalytic–BAF (“PC+BAF”) reactor setup is shown in Fig. 1. Influent from tank (1) was pumped into photoreactor (5), and pretreatment was given. The pretreated effluent from photoreactor (5) was collected in tank (12). Then, the pretreated effluent from tank (12) was carried into BAF reactor (14) (8.5 L capacity; feed rate: 4.0 mL/min; dissolved oxygen (DO): 2.5–3.0 mg/L; pH: 7.0) using a panel mounted peristaltic pump. The overflow from BAF reactor (14) was collected into (15) storage tank for treated wastewater. Since there was no significant pH change due to the short-duration (2.5 h) pretreatment step, the pH of the pretreated effluent was not adjusted. Unlike flask studies, there was no nutrient supplementation during the confederative experiments. The samples were collected at different detention times and analyzed for their COD, BOD_5, NO₃⁻ ion, HPLC, and HPLC/MS.

Analysis

The COD was measured by a titrimetric method using dichromate as the oxidant in acidic solution. BOD₅ was detected with dilution inoclulation methods.

UV–vis spectra of 4-NP degradation after both photocatalytic and BAF were scanned on a UV–vis spectrophotometer (UV–vis 2501PC; Shimadzu).

4-NP concentration was measured by reverse phase HPLC (Waters 2695; Waters) equipped with a UV spectrophotometer (210–400 nm). Aliquots of 10 μL were injected automatically into the HPLC to determine the concentration of 4-NP, methanol : 0.1% phosphoric acid = 50: 50 was used mobile phase, the mobile phase was filtered and sonicated to remove dissolved gas. The separation was performed using a Vp-ODS reversed phase column at the flow rate of 1 mL/min under 10.7 MPa pressure and column temperature of 303 K. In this case, 318.6 nm was chosen as UV wavelength.

The concentrations of NO₃⁻ ion were measured with an ion chromatograph (Dionex ICS-2500) equipped with an electrical conductivity detector, on an anionic exchange column (Ion Pack AS18, 2 × 250 mm). Aliquots of 20 μL were injected into the IC and carried with a mobile phase containing 0.015 mol/L NaOH solution with a flow rate of 0.25 mL/min.

The intermediates of 4-NP degradation after both “PC” and “PC+BAF” were identified by HPLC-MS (ZQ4000/2695; Waters) equipped with an ESI source. As for the LC condition, acetonitrile : 0.1% methanoic acid = 40:60 was used as mobile phase and flow rate was set to 0.2 mL/min without a separation column. Full-scale MS spectra in both positive and negative modes in the mass range between 50 and 1,000 m/z were recorded.

Results and Discussion

Photocatalytic degradation of 4-NP

Figure 2 shows a stack plot of UV–Vis absorption spectra corresponding to different irradiation time intervals. The spectra correspond to photocatalytic degradation of 80 mg/L 4-NP solutions using N-TiO₂/γ-Al₂O₃ granules as catalyst. This catalyst dose was 6% (V/V), and the air flow velocity that kept the granules circulate moving in the reactor was 6.67 min⁻¹. 4-NP displayed three UV absorption bands at wavelengths 224 nm, 317 nm, and 401 nm. The band at 224 nm and 401 nm completely disappeared within 270 min of treatment, whereas the intensity of the band at 317 nm decreased significantly during 300 min.

FIG. 2.

UV–vis spectra of photocatalytic treated 4-nitrophenol (4-NP) solutions. [4-NP]₀ = 80 mg/L, pH (initial) = 7, air-saturated. Color images available online at www.liebertonline.com/ees

Figure 3 shows the HPLC of photocatalytically treated 4-NP solutions at different time intervals. The chromatogram of the initial 4-NP sample showed a peak at a retention time of 5.6 min. Subsequently, this peak disappeared when photoreaction time was up to 240 min; and three new peaks appeared at 4.0, 2.2, and 1.9 min retention time (as shown in Fig. 3a, d). The new peaks may be attributed to the formation of intermediates (Bhatti et al., 2002). These intermediates were found to persist in all the subsequent samples collected from 240 to 300 min photocatalytic treatment.

FIG. 3.

High-performance liquid chromatography chromatogram of 4-NP solutions degradation. (a) [4-NP]₀ = 5.0 mg/L, without photoreaction; (b) [4-NP]₀ = 80 mg/L, BAF reaction time = 84 h, dilution factor = 5.0; (c) [4-NP]₀ = 120 mg/L, BAF reaction time = 136 h, dilution factor = 4.0; (d) [4-NP]₀ = 80.0 mg/L, photoreaction time = 4.0 h, dilution factor = 5.0. Experimental conditions: methanol : 0.1% phosphoric acid = 50 : 50, flow rate 1.0 mL/min; detection wavelength, 318.6 nm.

The photocatalytic degradation of 4-NP also resulted in the reduction of COD. Figure 4 shows the COD removal percentage with regard to irradiation time. More than 65–85% reduction in COD occurred during 20 h when the initial 4-NP concentration varies from 60 to 80 mg/L. It was found that the COD removal percentage reduction when initial 4-NP concentration is greater than 120 mg/L. The higher the 4-NP concentration is, the deeper the color is. Deep color affects the transmission of light.

FIG. 4.

Trend of chemical oxygen demand (COD) removal percentage of photocatalytically pretreated as a function of irradiation time at different 4-NP concentrations (pH [initial] = 7.0, air-saturated).

BAF degradation of 4-NP

Figure 5 shows the UV–vis spectra of 4-NP samples treated under different BAF reaction conditions. The acclimatized biomass was capable of utilizing 4-NP, as evidenced from the UV–vis spectra (Fig. 5); the intensity of the band at 401 nm decreased gradually and completely disappeared within 96 h BAF reaction. The result of Fig. 5 shows that 4-NP can be degraded by individual BAF process completely, but the reaction time is too long in the test condition.

FIG. 5.

UV–vis spectra of BAF treated 4-NP solutions (pH [initial] = 7, air-saturated).

Figure 3 shows the HPLC of individual BAF process treated 4-NP solution (initial concentration: 80 mg/L) at different time intervals. The peak at 5.6 min disappeared when the BAF reaction time was up to 84 h; and three new peaks appeared at 4.0, 2.2, and 1.8 min retention time (as shown in Fig. 3a, b). When the initial concentration was 120 mg/L, the peak at 5.6 min disappeared within 136 h (as shown in Fig. 3a, c). This phenomenon shows that high concentration 4-NP has inhibited action to microbial growth (Tomei and Annesini, 2005). The new peaks may be attributed to the formation of intermediates (Bhatti et al., 2002). This intermediate was found to persist in all the subsequent samples collected range from 84 to 136 h BAF treatment in the test condition.

Coupling experiments

The results of pilot experiments performed in the coupled photocatalytic–BAF bioreactor setup (Fig. 1) using 8.5 L 4-NP solution of 60, 80, or 120 mg/L are given in Figs. 6 and 7 and Table 1. Comparison of the spectra labeled as “BAF” suggests that 4-NP was treated by sole BAF. In contrast, photocatalysis pretreatment brought about a transformation of the 4-NP molecule (spectrum labeled as “PC”). Moreover, PC coupled with biodegradation (spectrum labeled as “PC+BAF”).

FIG. 6.

UV–vis spectra of photocatalytically pretreated 4-NP followed by BAF. (a) [4-NP]₀ = 60 mg/L. (b) [4-NP]₀ = 120 mg/L.

FIG. 7.

(a) Trend of COD removal percentage of photocatalytically pretreated 4-NP. (b) BAF as a function of irradiation time at different 4-NP concentrations. A, [4-NP]₀ = 60 mg/L; B, [4-NP]₀ = 80 mg/L; C, [4-NP]₀ = 120 mg/L. Photocatalytic pretreatment (PC) time = 2.5 h.

Table 1.

Results of Pilot-Scale Experiments

Sample/treatment	4-NP(mg/L)	COD (mg/L)	COD reduction (%)	NO₃⁻ (mg/L)	BOD₅ (mg/L)
Initial	60	106.5	—	—	13.5
PC (2.5 h)	18.1	52.0	51.2	2.6	23.5
PC (2.5 h) + BAF (3 h)	1.2	20.8	80.5	3.5	1.75
PC (2 h) + BAF (4 h)	0	11.8	88.9	3.6	0.6
Initial	80	145.5	—	—	17.5
PC (2.5 h)	30.8	74.2	49.0	5.8	31.5
PC (2.5 h) + BAF (24 h)	6.9	25.3	82.6	6.3	2.3
PC (2 h) + BAF (36 h)	0	17.8	87.8	6.7	1.0
Initial	120	215.9	—	—	23.5
PC (2.5 h)	45.6	175.4	18.7	8.6	68.5
PC (2.5 h) + BAF (48 h)	12.5	38.5	80.8	9.4	3.5
PC (2 h) + BAF (60 h)	0	25.2	88.3	10.2	1.7

BAF reaction conditions: capacity = 8.5 L; feed rate = 4.0 mL/min; dissolved oxygen = 2.5–3.0 mg/L; pH = 7.0.

BAF, biological aerated filter; BOD, biochemical oxygen demand; PC, photocatalytic pretreatment.

Figure 6 shows the UV–vis spectra of 4-NP samples treated under different pretreatment and biodegradation conditions; when 4-NP initial concentration was 60 mg/L and photocatalytic treatment time was 2.5 h, the peak at 401 nm completely disappeared after BAF reaction for 12 h (as shown in Fig. 6a); in the same conditions, when the initial concentration of 4-NP was 120 mg/L and BAF reaction time was 36 h, the peak at 401 nm also completely disappeared (as shown in Fig. 6b). Compared with solo “BAF” process, the degradation speed of 4-NP by “PC+BAF” coupling process was evidently increased.

When the PC pretreatment time was 2.5 h and the initial concentration was 60, 80 and 120 mg/L, respectively, the results of COD reduction were shown in Fig. 7. The COD removal percentage after 50 h BAF reaction could range from 80% to 95%. Correspondingly, the ratio of BOD/COD of these samples was a greater increase after PC pretreatment and a less decrease after “PC+BAF”(as shown in Table 1). It may be noted that no nutrients were added to the biological reactor during these pilot experiments. The reasons may be that the intermediates produced by photocatalytic reaction were propitious to maintain the high-activity of microbe under pilot experiments. It was clearly indicated that photocatalytic treatment was a useful technique for generating intermediates that were more biodegradable than the original substrate (4-NP).

The obtained HPLC data also confirmed the enhancement of biodegradation after photocatalytic treatment. In chromatograms, 4-NP was eluted at a retention time of 5.6 min. When the initial concentrations of 4-NP were 60, 80, and 120 mg/L, respectively, and under different “PC,” “BAF,” and “PC+BAF” conditions, the concentrations of 4-NP and the COD decrease percentage were shown in Fig. 8 and Table 1. The data of Table 1 indicated that the effectiveness of the coupled system “PC+BAF” was evident from the substantial decrease found in the concentration of 4-NP when compared with only “PC” or “BAF” process.

FIG. 8.

Comparison of 4-NP degradation by PC, BAF, and PC+BAF.

To explore the fate of nitrogen in 4-NP and provide further information on the degradation mechanism, nitrate was measured by ion chromatograph (as shown in Fig. 9). Nitrate was eluted at a retention time of 5.6 min. There were no nitrites to be detected in all ion chromatographs of 4-NP degraded by “PC” or “PC+BAF” process. The concentrations of −NO₃⁻ were in Table 1. Whether produced by “PC” or “PC+BAF” process, the concentrations of –NO₃⁻ were less than the corresponding theoretic values that 4-NP was completely degraded. During the “PC” process, 4-NP would produce other nitrophenol compounds (as shown in Table 2); in the process of “PC+BAF,” some nitrogen released from 4-NP was used as a nitrogen source for the growth of microbes.

FIG. 9.

Ion chromatogram of photocatalytically pretreated 4-NP and followed by BAF (Ion Pac AS18 [2 × 250 mm], 0.015 mol/L NaOH, flow rate = 0.25 mL/min).

Table 2.

Observed Ions and Possible Compounds for Analysis of High-Performance Liquid Chromatography/Mass Spectrometry of 4-Nitrophenol Degradation During “PC + BAF” Process

m/z	Ion assignment	Compound
Positive mode
105	[M+H]⁺	HOOC-CH₂-COOH
119	[M+H]⁺	HOOC-CH₂-CH₂-COOH
131	[M+H]⁺	HOOC-CH₂-CH = CH-COOH
140	[M+H]⁺
Negative mode
59	[M-H]⁻	CH₃COOH
93	[M-H]⁻	C₆H₆O
107	[M-H]⁻
109	[M-H]⁻
115	[M-H]⁻	HOOC-CH₂-COOH
141	[M-H]⁻
154	[M-H]⁻

Proposed mechanism

To propose a tentative degradation mechanism of 4-NP, it was essential to detect the intermediates and its evolution as the “PC” and “PC+BAF” process. HPLC/MS, HPLC, and IC were used to monitor the intermediates during the oxidation processes qualitatively and quantitatively, respectively. The result of HPLC/MS for 4-NP degradation was presented in Table 2.

Table 2 indicated that two kinds of intermediates were detected, that is, polyhydroxylated intermediates and carboxylic acids. With regard to the formation of polyhydroxylated intermediates, the most possible reason was as follows: the attack of hydroxyl radicals belongs to electrophilic attack, so the reaction took place preferentially at ortho-position and para-position of hydroxyl in 4-NP molecule, or owing to those positions with higher electron density. Since the ortho-positions were vacant, the addition of hydroxyl group to the benzene ring would result in the formation of polyhydroxylated intermediates. The para-position was occupied by a nitro group, and the denitration from aromatic rings occurred. On the other hand, the substitution of nitro group and hydrogen atom on the aromatic rings by hydroxyl radical also took place. The polyhydroxylation would subsequently lead to the opening of aromatic rings to form a variety of carboxylic acids. Finally, these carboxylic acids were oxidized into CO₂ and H₂O. According to the results just mentioned, possible degradation mechanism of 4-NP was proposed as shown in Fig. 10.

FIG. 10.

Possible degradation mechanism of 4-NP during “PC+BAF” process.

Conclusions

The conclusions drawn from this study can be summarized as follows:

4-NP can be degraded in a TiO₂/UV internal circulation three-phase fluidized photoreactor; after photoreaction, the catalyst separates from wastewater automatically, which deals with the problem of difficulty in catalyst separation; and 70–90% COD reduction is attainable in 5 h under the experimental conditions maintained in this article. During photodegradation of 4-NP, an intermediate builds up, indicating transformation of the 4-NP molecule. Depending on the duration of pretreatment, the reactor can be used to transform 4-NP into intermediates that are propitious to maintaining the high activity of microbes in pilot experiments.

A two-stage treatment comprising initial PC followed by a BAF process can ensure enhanced biodegradation of 4-NP wastewater. For this purpose, a PC to bring down COD by ≤20% is adequate to achieve enhanced biodegradation of 4-NP wastewater in second-stage biological treatment.

The results of HPLC/MS and HPLC suggest that two kinds of intermediates are generated, that is, polyhydroxylated intermediates and carboxylic acids. The degradation pathway of 4-NP can be depicted as follows: The denitration and substitution by hydroxyl radicals on aromatic rings seem to be the first stage. As a consequence, some polyhydroxylated intermediates are formed. These compounds are successively oxidized into catechol and hydroquinone, followed by the opening of aromatic rings and the formation of a series of carboxylic acids. Finally, these carboxylic acids are oxidized into CO₂ and H₂O.

Footnotes

Acknowledgments

The authors gratefully acknowledge the financial support of the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (no. 508057), the Scientific Research Foundation of Jiangxi Provincial Department of Education (GJJ08029), and the Department of Science and Technology (2010BSB03210).

Author Disclosure Statement

No competing financial interests exist.

References

Bhatti

Z.I.

, Toda

, Furukawa

2002. p-Nitrophenol degradation by activated sludge attached on nonwovens. Water Res., 36:1135.

Chang

M.W.

, Chen

T.S.

, Chern

J.M.

2008. Initial degradation rate of p-nitrophenol in aqueous solution by Fenton Reaction. Ind. Eng. Chem. Res., 47:8533.

, Zhang

X.W.

, Lei

L.C.

2008. Degradation of aqueous p-nitrophenol by ozonation integrated with activated carbon. Ind. Eng. Chem. Res., 47:6809.

Hua

, Hochemer

R.H.

, Hoffmann

M.R.

1995. Sonochemical degradation of p-nitrophenol in a parallel-plate near-field acoustical processor. Environ. Sci. Technol., 29:2790.

Huang

D.G.

, Liao

S.J.

, Quan

S.Q.

, Liu

, He

Z.J.

, Wan

J.B.

, Zhou

W.B.

2007. Preparation and characterization of anatase N–F-codoped TiO₂ sol and its photocatalytic degradation for formaldehyde. J. Mater. Res., 22:2389.

Koubaissy

, Joly

, Magnoux

2008. Adsorption and competitive adsorption on zeolites of nitrophenol compounds present in wastewater. Ind. Eng. Chem. Res., 47:9558.

Kulkarni

, Chaudhari

2006. Biodegradation of p-nitrophenol by P. putida. Bioresour. Technol., 97:982.

Martin

, Headley

D.W.J.

, Thomas

V.N.

, Friesen

R.A.

2003. Photolysis and biodegradation of selected resin acids in river Saale water, Germany. J. Environ. Sci. Health A, 12:2727.

Mohanty

, Rao

N.N.

, Khare

, Kaul

S.N.

2005. A coupled photocatalytic–biological process for degradation of 1-amino-8-naphthol-3, 6-disulfonic acid (H-acid) Water Res., 39:5064.

10.

Nevskaia

D.M.

, Lopez

E.C.

, Muñoz

, Ruiz

A.G.

2004. Adsorption of aromatic compounds from water by treated carbon materials. Environ. Sci. Technol., 38:5786.

11.

Oturan

M.A.

, Peiroten

, Chartrin

, Acher

A.J.

2000. Complete destruction of p-nitrophenol in aqueous medium by electro-Fenton method. Environ. Sci. Technol., 34:3474.

12.

Shen

X.T.

, Zhu

L.H.

, Liu

G.X.

, Yu

H.W.

, Tang

H.Q.

2008. Enhanced photocatalytic degradation and selective removal of nitrophenols by using surface molecular imprinted titania. Environ. Sci. Technol., 42:1687.

13.

Slokar

Y.M.

, Le

, Marechal

1998. Methods of decoloration of textile wastewaters. Dyes Pigments, 37:335.

14.

Tomei

M.C.

, Annesini

M.C.

2005. 4-Nitrophenol biodegradation in a sequencing batch reactor operating with aerobic–anoxic cycles. Environ. Sci. Technol., 39:5059.

15.

U.S. Environmental Protection Agency (USEPA). 1976. Water Quality Criteria. Washington, DC: USEPA.

16.

Wang

, Wang

H.L.

, Jiang

W.F.

, Li

Z.Q.

2009. Photocatalytic degradation of 2,4-dinitrophenol (DNP) by multi-walled carbon nanotubes (MWCNTs)/TiO₂ composite in aqueous solution under solar irradiation. Water Res., 43:204.

17.

Zhu

X.P.

, Shi

S.Y.

, Wei

J.J.

, Lv

F.X.

, Zhao

H.Z.

, Kong

J.T.

, He

, Ni

J.R.

2007. Electrochemical oxidation characteristics of p-substituted phenols using a boron-doped diamond electrode. Environ. Sci. Technol., 41:6541.

Degradation of 4-Nitrophenol Using a Coupled Photocatalytic–Biological Aerated Filter Process

Abstract

Abstract

Introduction

Experimental

Chemicals

Photocatalytic oxidation of 4-NP

Preparation of N-TiO2/γ-Al2O3 granules photocatalyst

Preparation of nitrogen doped TiO2 sol

Preparation of N-TiO2/γ-Al2O3 granules

Photocatalytic oxidation of 4-NP

Biodegradation of 4-NP

BAF reactor

BAF reactor start-up

4-NP degrading seed sludge

Ceramisites and biomass attachment

Operational and environmental conditions

Substrate and nutrients

Biodegradation of 4-NP

Coupling experiments

Analysis

Results and Discussion

Photocatalytic degradation of 4-NP

BAF degradation of 4-NP

Coupling experiments

Proposed mechanism

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Preparation of N-TiO₂/γ-Al₂O₃ granules photocatalyst

Preparation of nitrogen doped TiO₂ sol

Preparation of N-TiO₂/γ-Al₂O₃ granules