Treatment of Polyaniline Wastewater by Coupling of Photoelectro

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Article Cite This: ACS Omega 2019, 4, 9664−9672

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Treatment of Polyaniline Wastewater by Coupling of PhotoelectroFenton and Heterogeneous Photocatalysis with Black TiO2 Nanotubes Bin Ou,†,‡,§,∥ Jixiao Wang,*,†,‡,§,∥ Ying Wu,†,‡,§,∥ Song Zhao,†,‡ and Zhi Wang†,‡,§,∥ Chemical Engineering Research Center, School of Chemical Engineering and Technology, ‡Tianjin Key Laboratory of Membrane Science and Desalination Technology, §State Key Laboratory of Chemical Engineering, and ∥Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, PR China

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†

ABSTRACT: In this study, mineralization of organic pollutants in polyaniline (PANI) wastewater was achieved by a combination of photoelectro-Fenton (PEF) and heterogeneous photocatalysis with black TiO2 nanotubes. Organics were decomposed by the •OH radicals and SO4−• radicals formed on the black TiO2 nanotube photocatalyst and at the IrO2/Ti anode during water oxidation and from the Fenton’s reaction between Fe2+ and generated H2O2. Poor chemical oxygen demand (COD) removal was obtained utilizing visible light/TiO2, anodic oxidation with generated H2O2 (AO-H2O2), visible light/ black TiO2/AO-H2O2, and electro-Fenton. The mineralization was strongly increased by the PEF/black TiO2 process because of the photolysis of products under visible light irradiation; it was improved by coupled heterogeneous photocatalysis due to the additional •OH radicals at the black TiO2 nanotube surface. The results demonstrated that about 96.4% COD and 85% total organic carbon of PANI wastewater were removed after 360 min by the PEF/black TiO2 process under the optimal conditions. The black TiO2 nanotubes exhibited good stability as the mineralization ratio in 360 min which was still above 80% after 10 cycles’ degradation, showing that the black TiO2 nanotubes are powerful and promising photocatalysts for improving the removal efficiency of PANI wastewater.

1. INTRODUCTION Polyaniline (PANI) is a conductive polymer because of its high conductivity and redox property and could be applied in various aspects including the metal anticorrosion coatings, capacitors, batteries, electrochromic windows, antistatic materials, photovoltaic cells, and light-emitting electrochemical cells.1−4 PANI could be synthesized by electrochemical and chemical oxidation methods.3,5 However, in the synthesis process, inorganic salts and organic byproducts were generated (e.g., aminodiphenylamine, benzidine, hydrazobenzene, azobenzene, hydro-quinone, and benzoquinone), which have high toxicity for human health and the environment. Therefore, it is necessary to establish a practical method to treat the PANI wastewater. In recent years, the advanced oxidation processes (AOPs) were widely used for the wastewater treatment.6 Among the AOPs, anodic oxidation with electrogenerated H2O2 (AOH2O2), the electro-Fenton (EF), and photoelectro-Fenton (PEF) are promising methods because they are environ© 2019 American Chemical Society

mentally friendly and could generate hydroxyl radicals to oxidize pollutants.7−9 In the EF process, hydroxyl radicals generated from the Fenton’s reagent (Fe2+ and H2O2) at the cathode10 are given as follows Fe 2 + + H 2O2 + H+ → Fe3 + + •OH + H 2O

(1)

2+

The Fe and H2O2 are electrogenerated by the reduction of the oxygen and ferric ions at the cathode. O2 + 2H+ + 2e− → H 2O2

(2)

Fe3 + + e− → Fe 2 +

(3)

The oxidation capacity of the EF process could be promoted by using light illumination, which is known as the PEF process. The improvement of the PEF process is due to the Received: February 7, 2019 Accepted: May 20, 2019 Published: June 3, 2019 9664

DOI: 10.1021/acsomega.9b00352 ACS Omega 2019, 4, 9664−9672

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significantly improved by adding Fe2+ to the solution to produce •OH.24 In this study, this is the first time to use the black TiO2 nanotubes as the photocatalyst to treat the PANI wastewater by coupling of PEF and heterogeneous photocatalysis. The effects on applied current density, Fe2+ concentration, and pH were evaluated during the treatment of PANI wastewater. The mineralization and cycling stability of the black TiO 2 nanotubes were investigated.

photocatalytic performance related to the photolysis of Fe(OH)2+, which could produce a greater amount of Fe2+ and generate more quantity of •OH from the photoreduction of Fe(OH)2+ (eq 4) and the photolysis of complexes from Fe3+ reaction (eq 5).11−15 The main disadvantage of the PEF process is the excessive energy cost of the UV lamps used. However, this could be overcome by using the sunlight as an environmentally friendly and renewable energy source. Fe(OH)2 + + hν → Fe 2 + + •OH

(4)

2. EXPERIMENTAL SECTION 2.1. Chemicals. Aniline (C6H7N, 99.5%, Aladdin), barium sulfate (BaSO4, 99.5%), phosphoric acid (H3PO4, 99.5%, Aladdin), ethanol (C2H5OH, 99.5%), ammonium fluoride (NH4F, 99.5%), and ammonium persulfate ((NH4)2S2O4, 99.5%, Aladdin) were used. Ferrous sulfate heptahydrate (FeSO4·7H2O, 99.5%, Aladdin), used as the catalyst in EF and PEF, was obtained from Aladdin. Sulfuric acid (H2SO4, 99.5%, Aladdin) and sodium hydroxide (NaOH, 99.5%, Aladdin) were used to adjust the value of pH and were supplied by Aladdin. 2.2. Synthesis of Black TiO2 Nanotubes. The Ti sheets (10 cm × 10 cm) were first ultrasonically cleaned in ethanol and deionized water, followed by drying in a vacuum drying oven. The anodization was operated utilizing a two-electrode cell with the Ti sheet as an anode and Pt as a cathode, ethylene glycol containing 0.5 wt % NH4F, and 10 vol % H2O as the electrolyte applying a constant potential of 30 V for 3 h. The distance between the two electrodes was fixed at 3 cm. The asformed samples were washed with deionized water, dried at 80 °C, and were crystallized in a muffle furnace at 150 °C for 2 h and then up to 450 °C for 4 h. The samples underwent a brief “activation” treatment (at 30 V for 60 s) in the electrolyte mentioned above. A cathodic voltage (at −40 V for 200 s) was used to electrochemically dope the sample in an ethylene glycol solution of 0.5 wt % NH4F. In the electrochemical reduction process, the TiO2 nanotubes capture electrons, which could reduce Ti4+ to Ti3+. The thus-prepared the black TiO2 nanotubes were obtained by washing with deionized water and drying in a pure nitrogen stream. 2.3. Preparation of PANI Wastewater. Aniline (0.3 M) and (NH4)2S2O8 (APS, 0.375 M) were added to 1 M H3PO4 solution to start the reaction. After reacting for 24 h at −30 °C, the mixture was filtered with a microporous membrane (pore size 0.22 μm), and the PANI wastewater [chemical oxygen demand (COD) = 1582 mg/L] was obtained for further study. The PANI wastewater has a proton concentration at about 2 M with a very high conductivity of about 15 430 μS/cm because of the high concentration of H+, SO42−, PO43−, and NH4+. 2.4. Photocatalytic and Electrolytic Systems. The treatment of PANI wastewater was operated at room temperature in an undivided cell with a volume of about 600 mL and operated at a constant current density. The cathode, anode, and lamp were centered in the cell, surrounded by the black TiO2 nanotubes, which covered the four sides of the inner wall of the cell. Irradiation was carried out with a 500 W Xe lamp (Hayashi Tokei, Luminar Ace 210). A cutoff filter of 420 nm was used for the visible-light irradiation, and the light intensity was adjusted to 1 Sun (100 mW/cm2). A schematic diagram of the used electrolytic−photocatalytic system is shown in Figure 1. A piece of graphite felt (8 cm × 14 cm × 5 mm) and IrO2/Ti sheet (8 cm × 8 cm × 2 mm) were utilized as the cathode and the anode, respectively. A DYK-300V15A

R(CO2 )−Fe3 + + hν → R(•CO2 ) + Fe 2 + → • R + CO2 (5)

On the other hand, heterogeneous photocatalysis under illumination of UV or sunlight on the semiconductor surface is a promising AOP for the decomposition of organic pollutants. TiO2 has a wide band gap energy of 3.2 eV and good stability.16 Although many semiconductors have been used for photocatalysis, the TiO2 had the best performance.17 When TiO2 is irradiated with light photons (λ < 390 nm), an electron from the filled valence band is excited to the empty conduction band (eCB−) with an energy gap of 3.2 eV, generating a positively charged vacancy or hole (hVB+) by reaction 6. The holes generated at the TiO2 surface could oxidize water to produce •OH from reactions 7 and 8, which can destroy the organics.8,18,19 + TiO2 + hν → TiO2 (e−CB + hVB )

(6)

h+VB + H 2O → •OH + H+

(7)

h+VB + OH− → •OH

(8)

The TiO2 has three crystal phases: anatase, rutile, and brookite, and all of them have large electronic band gaps of 3.0−3.2 eV.16 This restricts their absorption in the UV region of the solar spectrum. However, this part only accounts for about 5% of the whole sunlight energy.20 Even if TiO2 is very efficient in using the UV region, the use of sunlight energy is limited. The photocatalytic activity relates to the working electrons and holes on the surface of the photocatalyst for the photocatalyst reaction. In the photocatalytic process, the more the light was absorbed, the more the excited charges were generated on the TiO2 surface. Therefore, it is significant to improve the optical absorption properties of TiO2. Recently, the black TiO2 with doped Ti3+ was used as an effective material for enhancing the electronic conductivity and photoresponse property.21 Self-doping with homo-species is regarded as an effective method to improve the TiO 2 photoactivity. According to self-doping, the method does not cause structural disturbance and defect formation as compared with heteroelement-doped methods. It has been identified that with the self-doping of Ti3+, (1) a series of Ti3+ interstitial bands are generated with energies 0.27−0.87 eV less than the band energy minimum of TiO2, which could bring about a strong visible light absorption;22 (2) the donor density in TiO2 is remarkably promoted, which could improve the electrical conductivity and the photoresponse performance on TiO2.23 Therefore, the Ti3+-self-doped TiO2 improved the photoresponse performances under the whole solar spectrum compared with intrinsic TiO2. Graphite felt cathodes are used for the efficient generation of H2O2 from reaction 2. In EF, the oxidation capacity of electrogenerated H2O2 is 9665


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oxalate method (λ = 402 nm). TOC measurements were determined by the TOC analyzer of Shimadzu TOC-VCSH (North America). The diffuse reflectance UV−vis adsorption spectra were conducted by a spectrophotometer (TU-1900, China, Beijing), using BaSO4 as reference. The average current efficiency (CE) for the PANI wastewater treatment was determined using the following eq 925 CE (%) =

(COD0 − CODt ) × F × V × 100 8I Δt

(9)

where COD0 (mg/L) is the initial COD in the PANI wastewater, CODt is the COD at the time t, F is the Faraday constant (96 486 C/mol), V is the solution volume (L), I is the applied current (A), and 8 is the equivalent mass of oxygen (g/ mol). The energy consumption (ECCOD, kW h/g COD) was determined from the expression

Figure 1. Sketch of the experimental setup of the electrolytic cell.

button-type switching power supply (Shenzhen, China) was used as the potentiostat. The graphite felt cathode was fed with O2 gas at 200 mL/min for the generation of H2O2 by the reaction 2. The sample was centrifuged for 10 min at 8000 rpm to get the supernatant liquid for the analysis of COD and total organic carbon (TOC). 2.5. Apparatus and Analytical Procedures. The pH was measured with a METTLER TOLEDO pH meter. The conductivity was measured by a DDSJ-308A conductivity meter. The COD was measured by a HACH DR900 COD analyzer. Samples were taken out from solutions at regular time intervals and were filtrated with microporous membrane (pore size 0.22 μm). The morphologies of PANI and TiO 2 nanotubes were analyzed by scanning electron microscopy (SEM, JEOL, JSM6700F). The crystalline structure of the samples was determined by X-ray diffraction (XRD, Rigaku D/ max 2500v/PC diffractometer, using Cu Kα radiation, λ = 1.54056 Å). The H 2 O 2 concentration was measured spectrophotometrically using the potassium titanium(IV)

ECCOD =

UI Δt (COD0 − CODt ) × V

(10)

where U is the applied voltage (V), and other parameters are the same as stated above. The overall COD removal percentage was calculated according to the following eq 11 COD removal (%) =

(COD0 − CODt ) × 100 COD0

(11)

3. RESULTS AND DISCUSSION 3.1. Characterization of Black TiO2 Nanotubes. The morphology for the black TiO2 nanotubes is shown in the SEM image of Figure 2a. The coating on the Ti sheet demonstrated a homogeneous morphology and was composed of TiO2 nanotubes of about 56 nm average diameter, which is

Figure 2. Scanning electron microscopy image of black TiO2 nanotubes (a) and TiO2 nanoparticles (Degussa P25) (b); XRD pattern of black TiO2 nanotubes and TiO2 nanoparticles (Degussa P25) (c); UV−vis absorption spectra of black TiO2 nanotubes and TiO2 nanoparticles (Degussa P25) (d); inset in (d) shows the band gap energy of black TiO2 nanotubes and TiO2 nanoparticles (Degussa P25). 9666


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Figure 3. Ti 2p XPS spectra (a) and O 1s XPS spectra (b) of the black TiO2 nanotubes.

Table 1. Characterization of PANI Wastewater Obtained in This Study parameters

values

conductivity (μS/cm) TOC (mg/L) BOD (mg/L) COD (mg/L) NH4+ (mol/L) SO42− (mol/L) PO43− (mol/L) H+ (mol/L)

15 430 496 630 1582 0.75 0.75 1 2.5

Figure 6. Evolution of H2O2 in the function of the O2 flow rate. Experimental conditions: pH = 3.0, [Na2SO4] = 0.75 mol/L, I = 16 mA/cm2, and T = 25 °C.

Figure 4. UV−vis spectra of the raw and treated PANI wastewater (the PANI wastewater was treated using the PEF/black TiO2 process for 360 min. Experimental conditions: COD0 = 1584 mg/L, I = 16 mA/cm2, [Fe2+] = 0.2 mM, pH = 3.0, O2 flow rate = 200 mL/min, T = 25 °C).

Figure 7. COD removal percentage at various applied current densities. Experimental conditions: pH = 3.0, [Fe2+] = 0.2 mM, O2 flow rate = 200 mL/min, T = 25 °C.

average thickness of the uniform coating is about 600 nm. As shown in Figure 2b, the TiO2 nanoparticles (Degussa P25) exhibited a morphology of nanoparticles with an average diameter of 20 nm. Figure 2c exhibits the XRD profiles of the TiO2 nanoparticles (Degussa P25) and black TiO2 nanotubes. Peaks typically located at 25.4, 37.2, 48.2, 54.5, and 55.2 can be indexed to the (101), (004), (200), (105), and (211) crystallographic phases of anatase TiO2, respectively.22,26 In addition, the peaks located at 27.51 and 36.3 correspond to the (110) and (101) planes of rutile, respectively. All these peaks were very sharp in accordance with the formation of black nanotubes, and the strong XRD diffraction peaks also showed that the TiO2 nanocrystals were highly crystallized. Furthermore, XRD indicated that the black TiO2 nanotubes were in the pure anatase phase. As shown in Figure 2d, in comparison to the TiO2 nanoparticles (Degussa P25), the spectra of the Ti3+ selfdoped TiO2−x samples (black TiO2 nanotubes) obtained presents a broad absorption band from 200 to 800 nm,

Figure 5. COD removal for PANI wastewater. Experimental conditions: pH = 3.0, I = 16 mA/cm2, [Fe2+] = 0.2 mM, O2 flow rate = 200 mL/min, T = 25 °C. (■) visible light/black TiO2; (●) AO-H2O2; (▲) visible light/black TiO2/AO-H2O2; (▼) EF; (⧫) PEF; PEF/black TiO2(◀).

the typical nanotube size (nm) for this material when prepared by the electrochemical reduction method. In addition, Figure 2a illustrates a SEM image of the sectional view showing a lateral view of a deposited black TiO2 nanotube film and the 9667


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Figure 8. COD removal percentage at various initial Fe2+ concentrations. Experimental conditions: pH = 3.0, I = 16 mA/cm2, O2 flow rate = 200 mL/min, T = 25 °C.

X-ray photoelectron spectroscopy (XPS) was carried out to determine the presence and chemical states of Ti in the black TiO2 nanotubes. The XPS spectra of Ti 2p (Figure 3a) demonstrates two peaks at binding energies of 458.81 eV (Ti 2p3/2) and 464.67 eV (Ti 2p1/2). The Ti 2p peaks are well split into four peaks as Ti3+ 2p3/2 at 458.61 eV, Ti4+ 2p3/2 at 459.02 eV, Ti3+ 2p1/2 at 464.23 eV, and Ti4+ 2p1/2 at 465.08 eV. In Figure 3b, the O 1s peak observed for the black TiO2 nanotubes could be split into two peaks at 530.18 and 532.06 eV, which correspond to characteristic peaks of Ti−O− Ti and Ti−OH, respectively. The oxygen vacancies and Ti3+ species could restrain the recombination of electron−hole pairs and be benefit to form Oads− by oxygen photo-adsorption and improve the catalytic activity. 3.2. Characterization of PANI Wastewater. Different analyses were operated to determine the characteristics and composition of PANI wastewater. The results are shown in Table 1. As shown in Figure 4, the PANI wastewater presented claybank coloration, corresponding to the high absorbance at 230−300 nm, which the aliphatic intermediates and different aromaticity compounds could absorb in this region of wavelengths.27 The aliphatic intermediates and different aromaticity compounds could include aniline, benzidine, aminodiphenylamine, hydrazobenzene, azobenzeno, benzoqui-

Figure 9. COD removal percentage at various pH values. Experimental conditions: I = 16 mA/cm2, [Fe2+] = 0.2 mM, O2 flow rate = 200 mL/min, T = 25 °C.

covering the whole visible range. The inset in Figure 2d shows that the E g (band gap energy) values for the TiO 2 nanoparticles (Degussa P25) and black TiO2 nanotubes are 3.2 and 2.89 eV, respectively. The absorption in the visiblelight region and Eg indicated that the prepared samples could be effectively activated by visible light and then more photogenerated electrons and holes could be generated to destroy pollutants.

Figure 10. Changes with time of COD removal and regular CE (a) and changes with time of energy consumption per unit COD mass removed for the PEF/black TiO2 process (b). Experimental conditions: COD0 = 1584 mg/L, I = 16 mA/cm2, [Fe2+] = 0.2 mM, pH = 3.0, O2 flow rate = 200 mL/min, T = 25 °C. 9668


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Figure 11. TOC removal after 360 min treatment by coupling of PEF and heterogeneous photocatalysis (a) and TOC removal after 360 min treatment using the black TiO2 nanotubes for 10 cycles by coupling of PEF and heterogeneous photocatalysis (b). Experimental conditions: TOC0 = 496 mg/L, I = 16 mA/cm2, [Fe2+] = 0.2 mM, pH = 3.0, O2 flow rate = 200 mL/min, T = 25 °C.

none, and hydroquinone.28−30 The PANI wastewater was characterized in Table 1. 3.3. Comparative Degradation by Visible Light/Black TiO2, AO-H2O2, Visible Light/Black TiO2/AO-H2O2, EF, PEF, and PEF/Black TiO2 Processes. The decolorization of PANI wastewater was investigated at constant current density (16 mA/cm2), 0.7 M (NH4)2SO4, and initial Fe2+ concentration of 0.2 mM for visible light/black TiO2, AO-H2O2, visible light/black TiO2/AO-H2O2, EF, PEF, and PEF/black TiO2 processes in acid solution (pH = 3.0).11,31−33 Figure 5 shows the removal efficiency for PANI wastewater under electrolysis at 16 mA/cm2 performed for 360 min at room temperature (25 °C). It can be observed that the decay rate of PANI wastewater was higher in the PEF/black TiO2 process than others. The results also demonstrated that the COD removal after 360 min treatment was in the following order: PEF/black TiO2 > PEF > EF > visible light/black TiO2/AOH2O2 > AO-H2O2 > visible light/TiO2. The COD was removed during the 360 min of the PEF/black TiO2 process, yielding 96.4% of COD removal efficiency, whereas at the same time, PEF, EF, visible light/black TiO2/AO-H2O2, AO-H2O2, and visible light/black TiO2 processes led to 85, 76, 65, 53, and 18% COD removal efficiencies, respectively. These results demonstrated that combination of PEF with the photocatalytic process (visible light/black TiO2) could significantly improve the removal efficiency. In the AO-H2O2 process, organics could be oxidized by reactive oxygen species like H2O2 and its oxidation product, HO2•. Moreover, the persulfate anion (S2O82−) could be generated at the anode (eq 12) and then could promote the decomposition of the persulfate anion to produce sulfate radicals (eq 13). Sulfate radical (SO4−•) is a strong oneelectron oxidant (E° ≈ 2.6 V) that is used in oxidation of many organic compounds, especially, aromatic compounds. In addition, the sulfate radicals could react with H2O to produce • OH radicals (eq 14)34−36 2SO4 2 − → S2O82 − + 2e−

(12)

S2 O82 − + Fe 2 + → Fe3 + + SO4 −• + SO4 2 −

(13)

SO4 −• + H 2O → •OH + SO4 2 − + H+

(14)

tion using EF could be explained by the additional generation of •OH via homogeneous catalysis with added Fe2+ by Fenton’s reaction 1. Then, the organic pollutants are oxidized by •OH. The faster mineralization in PEF is ascribed to the photolysis of some byproducts under visible light irradiation, which causes the photoreduction of Fe(OH)2+ to Fe2+ via reaction 4 and the photolysis of Fe3+ complexes with final carboxylic acids by reaction 5.37,38 The COD removal demonstrated that the PEF/black TiO2 process could effectively degrade the PANI wastewater. The results can be explained by the fact that the decolorization enhancement in the PEF/black TiO2 process can be ascribed to the generation of reactive matters in the aqueous solution. Hydroxyl radicals could be generated by photocatalysis and EF processes, which are produced by reactions 6−8 in the photocatalytic process. The synthesized black TiO2 nanotubes present a strong broad absorption in the visible light, which can increase black TiO2 nanotube adsorption capacity and offer more active sites for photocatalysis. In addition, as more hydroxyl radicals are produced from the photocatalysis (eqs 4, 7, and 8), the Fe3+/ Fe2+ cycle was accelerated under light irradiation through reactions 1 and 2.39,40 The above results demonstrated the superiority of PEF/black TiO2 for removing the COD from PANI wastewater. In all cases, the increase of COD removal values gradually declined at prolonged time ascribe to the loss of organic content and the production of more recalcitrant byproducts. 3.4. Influence of Operation Parameters on the PEF/ Black TiO2 Process. 3.4.1. Effect of O2 Flow Rate on the Electrochemical Generation of H2O2. The EF process is closely related to the electroreduction of H2O2 at the cathode. Figure 6 presents the influence of the O2 flow rates on the H2O2 generation. After 360 min electrolysis, the concentration of the H2O2 is about 180, 220, 250, 289, and 295 mg/L under the oxygen sparge flow rates of 80, 120, 160, 200, and 240 mL/ min, respectively. The curves shown in Figure 6 clearly indicates that the H2O2 concentration gets higher when the O2 flow rate becomes higher and the electrolytic time gets longer, and the enhancement degree of H2O2 concentration becomes less when the oxygen flow rate becomes higher and the electrolytic time gets longer. This result could be explained that the oxygen concentration is tends to be saturated in the solution at high flow rate. This phenomenon was also closely related to the positive effect of the increase of dissolved oxygen concentration for the H2O2 generation, and the negative selfdecomposing effect of H2O2 (eq 15) at higher concentrations and followed by oxidation of H2O2 at the anode (eq 16)

The COD removal efficiency of the visible light/black TiO2/ AO-H2O2 process is higher than that of the visible light/black TiO2 and AO-H2O2 processes. This could be explained by the fact that H2O2 promotes photocatalytic degradation due to the scavenging of photoexcited electrons and further yield of reactive oxygen species. The improvement of the mineraliza9669


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2H 2O2 → 2H 2O + O2

(15)

H 2O2 → O2 + 2H+ + 2e−

(16)

needs further treatment and then increases the cost of the PEF/black TiO2 process. 3.4.4. Effect of pH. Figure 9 demonstrates the effect of the PANI wastewater pH on the PEF/black TiO2 process by treating PANI wastewater pH 1.0, 2.0, 3.0, 4.0, and 5.0 at 16 mA/cm2. The efficiency of the PEF/black TiO2 process is low with initial pH of 4.0 and 5.0. This could be ascribed to the low production of hydroxyl radicals at these pH values because the optimum pH for the generation of these radicals is 3.0. In contrast, the PEF/black TiO2 process could effectively decolorize PANI wastewater at the pH = 3.0. Furthermore, the pH value is a significant factor affecting the rate of degradation of organic pollutants in the photocatalytic process. The mechanism of the pH effects on the efficiency of the PANI wastewater photodegradation process is very difficult to explain because of its multiple roles. First, it is related to reactant organics and products such as acids and amines. The pH changes could affect the adsorption of aniline derivative molecules onto the TiO2 surface.19,45,46 Second, hydroxyl radicals could be generated through the reaction between hydroxide ions and positive holes, and the positive holes are the major oxidation matters at low pH values. 3.5. Energy Cost and Regular Current Efficiency. To better analyze the viability of the PEF/black TiO2 process, the energy cost and CE for the trials of PANI wastewater was calculated from the eqs 9 and 10. The relationships between CE, ECCOD, COD removal, and reaction time are displayed in Figure 10. As shown in Figure 10a, the highest CE (43.2%) was obtained for the first 60 min (46% COD removal), and it decreased gradually with an increase in the COD removal. After 360 min treatment, the CE was dropped to 10.5%. In Figure 10b, it can be seen that about 0.6 kW h/kg COD power consumption was required to reach 40% COD removal during 30 min treatment. However, the energy consumption increased significantly thereafter; 1.7 (kW h/kg COD) power consumption was required to achieve 96.4% COD removal at the end of the 360 min treatment. This could be explained by the fact that hardly oxidizable products were generated, such as short chain organic acids. These results demonstrated that the PEF/black TiO2 degradation of PANI wastewater is viable at low currents (16 mA/cm2) because this can yield higher efficiencies with smaller energy costs than operating at greater applied current density, although longer time is needed to decontaminate the PANI wastewater solution. 3.6. Mineralization and Cycling Stability of the Black TiO2 Nanotubes. To evaluate the efficiency of the black TiO2 nanotubes in mineralizing the PANI wastewater, the TOC of the PANI wastewater was removed by the coupling of the PEF and photocatalysis process. As shown in Figure 11a, within the first 2 h, faster mineralization was observed using the black TiO2 nanotubes, compared to TiO2 nanoparticles (P25), which could be ascribed to a wider adsorption in the visible light region than the TiO2 nanoparticles and a higher production of •OH in the system and then faster degradation of PANI wastewater intermediate products. TOC removal after 6 h treatment reached 85% on the black TiO2 nanotubes, and this was 14.3% higher than that of the TiO2 nanoparticles. For their environmental applications in the real wastewater treatment, the long-term stability of the black TiO2 nanotubes is also a crucial property. In Figure 11b, after 10 cycles of use (60 h), the mineralization ratio of the black TiO2 nanotubes was only decreased by 5%.

On considering the H2O2 concentration in the electrochemical cell and cost, the O2 flow rate at 200 mL/min should be the optimal value to be used to treat wastewater. 3.4.2. Effect of Applied Current Density. The applied current density is an important operation parameter on the electrochemical treatments. To investigate the effect of applied current density on the COD removal efficiency, the PEF/black TiO2 process was operated applying various current densities from 8 to 24 mA/cm2. As shown in Figure 7, the applied current demonstrates a significant influence on the decolorization rate of PANI wastewater. The COD removal percentages were 65, 82, 96.4, 96.9, and 97.6% for the current density at 8, 12, 16, 20, and 24 mA/cm2, respectively. With the increase of the current density from 8 to 16 mA/cm2, the COD removal efficiency was increased, which could be corresponded to the generation of a large amount of H2O2 by the reaction 2. The quicker mineralization rate in the PEF/black TiO2 process is due to the generation of more •OH radicals in the medium from reactions 2 and 9 because more H2O2 was electrogenerated at the cathode. However, for the current density at 20 and 24 mA/cm2, the enhancement on the COD removal was not obvious. The further increase in the current density would promote the side reactions of hydrogen and water formation and increase the nonoxidizing reaction rates of generated radicals, causing their destruction without any attack over organics (eqs 17−20)41−43 2H+ + 2e− → H 2

(17)

O2 + 4H+ + 4e− → 2H 2O

(18)

2•OH → H 2O2

(19)

H 2O2 + •OH → HO•2 + H 2O

(20)

These competitive reactions probably cause the decrease of the COD removal. Consequently, more electric energy was wasted at applied potential greater than 16 mA/cm2. Considering of these results, it would be optimum to choose the suitable current density 16 mA/cm2 for the PANI wastewater treatment. 3.4.3. Effect of Initial Fe2+ Concentration. A crucial factor in the EF process is the concentration of Fe2+ which was regarded as the catalyst. In Figure 8, the COD removal was explored at different Fe2+ concentrations when initial pH was 3.0 and applied current density was 16 mA/cm2. The optimum Fe2+ content was about 0.2 mM and the maximum production of hydroxyl radicals was obtained at this content. At higher Fe2+ concentration, the oxidant generation is progressively inhibited because the side reaction 21 was promoted significantly.44 Fe 2 + + •OH → Fe3 + + OH−

(21) 2+

The addition of large amounts of Fe could act as direct scavengers of hydroxyl radicals via promoting the formation of the H2O• radicals. Also, when high amounts of Fe2+ were added to the wastewater, Fe(OH)3 could be precipitated, which restrains the production of the Fe2+ catalyst. Therefore, the use of catalytic amounts of Fe2+ is crucial to solve the problem of sludge formation and iron contamination, which 9670


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4. CONCLUSIONS The synthesized back TiO2 nanotubes have a strong broad absorption band between 200 and 800 nm, covering nearly the whole visible light region. Under the conditions of current density 16 mA/cm2, initial pH 3.0, the concentration of Fe2+ 0.2 mM and the O2 flow rate of 200 mL/min, the PANI wastewater was quickly destroyed after 360 min of PEF/black TiO2 process treatment, yielding 96.4% of COD removal efficiency. In addition, 85% TOC could be removed, the CE was 43.2% and the EC was 1.7 kW h/kg COD. These results demonstrated that this method could decolorize and mineralize the PANI solution effectively. The black TiO2 nanotubes exhibited good stability and photocatalytic performance for a potential application in the treatment of industry wastewater containing recalcitrant organic pollutants by the PEF/black TiO2 process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bin Ou: 0000-0001-9917-0947 Zhi Wang: 0000-0002-8465-687X Notes

The authors declare no competing financial interest.

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REFERENCES

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Treatment of Polyaniline Wastewater by Coupling of Photoelectro

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