oxidized carbon fiber electrodes electrochemically produced and their influences on Brilliant Green dye degradation

Materials Research Bulletin 122 (2020) 110642

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Titanium dioxide/oxidized carbon fiber electrodes electrochemically produced and their influences on Brilliant Green dye degradation

T

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Lânia Auxiliadora Pereira , Dalva Alves de Lima Almeida, Andréa Boldarini Couto, Neidenei Gomes Ferreira National Institute for Space Research – INPE, Av. dos Astronautas, 1758, Jd. Granja, 12227-010, São José dos Campos, SP, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords: TiO2 Carbon fiber Composite materials Photoelectrocatalysis Dye degradation

Electrochemical advanced oxidation processes to degrade organic pollutants require the study of new anode materials. Porous structure of three-dimensional oxidized carbon fibers (OCF) associated to titanium dioxide (TiO2) becomes an alternative since they can effectively improve the mass transport in their enlarged areas. TiO2/OCF electrodes were produced from an optimized OCF substrate evaluating the influence of oxygen functional groups on its surface in the TiCl3 anodic hydrolysis. From scanning electron microscopy, Raman analyzes, and electro/photoelectrochemical responses, a suitable TiO2/OCF composite was obtained from oxidized CF for 30 min. TiO2 presented several characteristic Raman bands of anatase modes at 150, 398, 511, and 620 cm−1. To improve the TiO2 crystallinity, the samples were also heat-treated at 500 °C in an argon atmosphere for 2, 3, and 4 h. TiO2/OCF composite treated for 2 h presented the highest electrochemically active surface area and photoelectrochemical activity. This is due to its TiO2 rougher morphology with rounded nanoparticles presence on OCF surface. For Brilliant Green dye degradation, an electrolysis process was highly efficient at the current density of 10 mA cm−2 at 300 min of time treatment. Besides, TiO2/OCF composite under UV irradiation shows an electrolysis efficiency improvement presenting the highest kinetic rate associated to the additional %OH radical productions.

1. Introduction

hydroxyl radical (%OH) which attacks organic pollutants in wastewaters converting them into carbon dioxide and pure water (mineralization process) [4,5]. Furthermore, literature reports that the semiconductor electrode utilization combined with UV light illumination, which promotes the photoelectrocatalytic process, significantly improves the efficiency in the wastewater treatment performance [6,7]. To date, titanium dioxide (TiO2) semiconductor is the most commonly used electrode in studies for wastewater cleanup due to its non-toxicity, chemically inert and low cost [8]. However, most photocatalytic processes involve TiO2 suspensions in water [9–11], which for practical applications makes it disadvantageous since it requires an additional treatment to recover and remove it from treated water. Therefore, for photoelectrocatalytic processes, TiO2 immobilization on suitable solid matrices is desirable to create a composite electrode ensuring a higher specific surface area availability, in which %OH is simultaneously generated at the anode surface both electrochemically and photochemically. Nowadays, special attention has been given to developing an electrode material that offers high efficiency in pollutant degradation, high stability under anodic polarization conditions, and

Dramatic changes have been taking place concerning the understanding of pure drinking water due to human civilization expansion into urban areas associated with increasing industrial activity. Population growth, as well as industrial negative impact on natural water sources, have led significant increases in the volumes of wastewater, which has raised widespread apprehension in relation to its toxicity. Hence, natural water contamination has been a huge problem for modern society. Thus, the development and extension of chemical technologies for organic synthesis and wastewater processing have come up. In particular, dye wastewaters present a complex composition where conventional treatments have become inadequate. As a result, several technologies have been developed for more effective dye wastewater treatment [1–3]. Electrochemical advanced oxidation processes (EAOP) seem to be a promising and clean technique, which does not require additional chemical species as they use only the electrons as their main reagent. These methods are based on the electrochemical generation of the

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Corresponding author. E-mail address: lania.fi[email protected] (L.A. Pereira).

https://doi.org/10.1016/j.materresbull.2019.110642 Received 22 April 2019; Received in revised form 30 August 2019; Accepted 22 September 2019 Available online 23 September 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.

Materials Research Bulletin 122 (2020) 110642

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TiO2/OCF composites were investigated by field emission gun scanning electron microscopy (FEG-SEM) from TESCAN MIRA 3 microscope system and by Raman scattering spectroscopy from Horiba Scientific LabRAM HR Evolution microscope system with laser beam line of 514 nm. Electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit potential (OCP) in 0.5 mol L−1 H2SO4 with ± 10 mV of the potential amplitude in the frequency range from 10−3 to 105 Hz. Photoelectrochemical activities were analyzed by linear sweep voltammetry in the potential range from 0.2 V to 1.0 V vs. Ag/AgCl/KCl(sat) using 0.1 mol L−1 KCl at scan rate of 20 mV s−1 frontally irradiating the electrode with an UV source using a mercury lamp 300 W Niewport.

low production costs. Carbonaceous materials are commonly used as a support for heterogeneous metal catalysts because they can provide singular characteristics [12]. CF, in particular, has been a suitable material as catalytic support due to its porosity by promoting high specific surface area not to mention its good conductivity [12,14–16]. In this context, porous structure of three-dimensional oxidized carbon fibers (OCF) associated to TiO2 can effectively improve the mass transport in their enlarged areas as well as explore their photocatalytic properties. This allows producing composite electrodes with high performance in dye degradations. TiO2 deposition on the electrode surface can be prepared by different methods and parameters like chemical vapor deposition (CVD) [13], hydrothermal synthesis [14,15], sol-gel [16–18], sputtering [19,20], and electrodeposition [21–24]. Among them, electrodeposition is the most suitable method to produce composite electrodes because this process allows controlling TiO2 microstructure as a novel electrode for photoelectrocatalytic investigation. Thus, we have studied the feasibility of employing a cost-effective alternative method to form TiO2/OCF binary composite via electrodeposition aiming at their application as an anode for Brilliant Green (BG) dye degradation. Firstly, the emphasis was given to the influence of oxygen functional groups on the OCF surface to obtain TiO2/OCF binary electrodes. Afterward, these electrodes were subjected to heat-treatment to investigate their changes in TiO2 crystalline structure. Hence, the electrocatalytic and photoelectrocatalytic activity of the composite electrodes were studied in the degradation of BG dye in an aqueous medium.

2.4. Electrochemical system For electrochemical and photoelectrochemical degradation experiments, electrolysis was performed using a polypropylene home-made single cell containing 450 mL of 100 mg L−1 of Brilliant Green (BG) dye (∼96% m/m) in 0.1 mol L−1 K2SO4 aqueous solution. The geometric area of all electrodes was 4.4 cm2. The applied current density was 10 mA cm-2 and total treatment time of 300 min. All experiments were carried out under constant stirring at 25 °C. 2.5. UV–vis and HPLC analyzes To obtain information on the dye decolorization degree, aliquots were taken out at time intervals of 0, 15, 30, 45, 60, 75, 90, 120, 150, 180, 240 and 300 min and evaluated by UV–vis spectrophotometer analyzer (Varian Cary 50) at λmax =426 nm. The decolorization percentage was calculated using the following equation [27]:

2. Experimental 2.1. Chemicals and reagents

% decolorization =

Deionized water was used throughout the experiments. Brilliant Green dye, titanium (III) chloride (TiCl3), Methanol (MeOH) and acetonitrile (C2H3N) were purchased from Merck (Brazil). Sulfuric acid (H2SO4) and ammonium acetate were purchased from Dinâmica (Brazil). Potassium chloride (KCl) and potassium sulfate (K2SO4) were purchased from Synth (Brazil).

A0 − At x 100 A0

(1)

where A0 and At are the dye absorbances at the beginning and at the t instant of the electrolysis, respectively. The oxidation degree of the organic matter collected throughout the treatment was analyzed by high performance liquid chromatography (HPLC). The employed system was a HPLC Flexar PerkinElmer, equipped with UV/Vis detector and an Ascentis C18 column (2.7 μm, 150 × 4.6 mm). The injection volume was set at 20 μL and the isocratic eluent, a mixture of methanol/ ammonium acetate 0.01 mol L−1/ acetonitrile (50:35:15, v/v), was pumped at a flow rate of 1.0 mL min−1. Detection was performed with the UV/Vis detector set at λ = 206 nm, and the column was kept at room temperature. The chromatograms displayed a BG dye peak with a retention time of 12 min. The degradation percentage was determined according to the normalized peak chromatographic area.

2.2. Preparation of TiO2/OCF electrodes CF samples were produced from polyacrylonitrile (PAN) precursor. The fibers were oxidized at 200 °C and submitted to heat treatment temperature of 1000 °C, according to previous paper [25]. CF samples were cut with dimension of 1 cm2. Electrochemical oxidative treatment (EOT) and TiO2 electrodeposition were performed at room temperature using Autolab-Pgstat 302 equipment and a conventional three-electrode glass cell with quartz window, with platinum wire and Ag/AgCl/KCl(sat) as counter and reference electrode, respectively. The EOT was performed on CF surface applying fixed potential of 2.0 V vs Ag/AgCl/ KCl(sat) for 5, 10, 20, and 30 min in 0.5 mol L−1 H2SO4. The TiO2 electrodeposition on CF substrates was performed under potentiostatic mode at a fixed potential of 0.75 V vs Ag/AgCl/KCl(sat) for 30 min deposition in a 5 mmol L−1 TiCl3 (pH = 2) +0.1 mol L−1 KCl aqueous solution. These parameters were studied in a previous paper [26]. Subsequently, to improve the TiO2 crystallinity, the composites with optimized conditions were heat-treated (HT) at 500 °C with a ramp rate of 10 °C min−1 for 2, 3 and 4 h in an argon atmosphere.

3. Results and discussion 3.1. Morphological and structural characterizations of TiO2/OCF CF surface functionalization plays an important role to facilitate Ti nanoparticles binding, embedding or loading on its surface. Thus, the first step was to study EOT at different times investigating its influence on the TiCl3 anodic hydrolysis to produce TiO2/OCF composites. XPS analyzes confirmed the CF functionalization besides assessing the carbon-oxygen groups on its surface. Fig. 1 shows the XPS survey spectra for the binding energy range from 0 to 1000 eV for as-received CF and OCF samples. All samples presented carbon and oxygen as the main peaks. In addition, XPS measurements detected a small peak of N 1s at 400 eV because of the polyacrylonitrile precursor. This peak decreased with the oxidative treatment time increase, indicating that the anodic treatment may remove the CF surface impurities. To further investigate the functional groups formed after different EOT times, high-resolution XPS spectra of the O (1s) region were carried out. The O (1s) spectra were deconvoluted (Fig. S1) assuming five

2.3. Characterization techniques X-ray photoelectron spectroscopy (XPS) measurements were carried out using the spectrometer Kratus Axis Ultra with monochromatic X-ray radiation Al-Kα (λ = 1486.5 eV) to identify the oxygen functional groups present on oxidized carbon fibers (OCF) surface as a function of different EOT times. Spectroscopic data were processed in CasaXPS, version 2.3.16. The morphological and structural characteristics of 2

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amounts of oxygen-containing groups. According to the literature, the general proposed mechanism for TiO2 formation involves the anodic hydrolysis of TiCl3 solution and subsequent heat treatment (Eqs. (2) and (3) [30].

Ti3 + + H2 O → TiOH 2 + + H+ e−

(2) Δ

TiOH 2 + → Ti (IV ) oxo − species → TiO2

The rate-determining step in the sequence of reactions occurs during Ti3+ hydrolytic conversion to TiOH2+ and simultaneously followed by its oxidation to Ti(IV) oxo-species which probably consists to partly dehydrated (cross-linked) polymeric Ti(IV) hydroxide, containing both terminal and bridged OH groups deposited over the electrode surface (as shown in Fig. 3). However, other electrogenerated oxidant like H2O2 can be formed through water discharge on the anode surface (2H2O → H2O2 + 2H+ + 2e) as well as on the cathode surface via oxygen reduction (O2 + H+ + 2e → H2O2). This oxidizing agent can participate in the oxidation hydrolytic of TiCl3 and the following reaction can be considered [31]:

Fig. 1. XPS survey spectra for the binding energy range of 0–1000 eV for: (a) asreceived CF; (b) EOT 5 min; (c) EOT 10 min; (d) EOT 20 min; (e) EOT 30 min.

TiOH 2 + + O2 → Ti (IV ) oxo − species + O2− → TiO2

Table 1 Contribution of oxygen groups resulting from the fitting of components to the O (1 s) photoelectron for different EOT times.

as-received CF OCF_EOT 5 min OCF_EOT 10 min OCF_EOT 20 min OCF_EOT 30 min

C]O

CeOH, CeOeC, C]O esther

CeO esther

COOH

water

12.17 12.48 16.70 13.03 4.20

26.96 13.62 24.55 26.22 47.45

27.65 16.48 31.14 36.92 23.40

29.15 54.86 25.01 22.07 23.76

4.07 2.56 2.60 1.76 1.19

(3)

(4)

By comparing the percentages obtained from these different oxygen groups, OCF surface for EOT 30 min depicted the highest presence of the C–OH group. The surface containing hydroxyl group may promote important shallow trap sites for Ti cations in the anodic hydrolysis of TiCl3 to get polymeric Ti (IV) hydroxide with both terminals and bridges of OH groups leading to the TiO2 deposition. Moreover, the electrodeposition of TiO2 by anodic oxidative hydrolysis of Ti3+ is quantitative at pH values below 3 [30]. Thus, the enhancement in the TiO2 deposition at OCF EOT 30 min may be justified according to Fig. 3. Fig. 4 presents the Raman spectra of TiO2 deposits on OCF for different EOT times. All spectra show characteristic bands of carbonaceous materials, i.e., D and G bands. The D band, at around 1360 cm−1, is related to disordered graphite structure, while the G band, at about 1590 cm−1, is related to ordered graphitic structure [32]. The spectra show a wide D band, since the heat treatment temperature was not enough to reorganize CF structure, producing fiber with high defect levels and misalignment in its lamellar layers [33]. Except for as-received CF surface, which probably present amorphous TiO2, the spectra also exhibit the typical peaks of the TiO2 anatase, at around 150, 398, 511, and 620 cm−1, attributed to the Eg(1), B1g, A1g, and Eg(2) vibrations, respectively [34]. These Eg, B1g and A1g peaks originate from symmetric stretching vibration, symmetric bending vibration, and antisymmetric bending vibration of OeTieO in TiO2, respectively [35]. The main Eg mode arises from the external vibration of the anatase structure, and its intensity increases with EOT time [36]. This welldefined peak points out that the anatase phase was already obtained for TiO2/OCF with 30 min of EOT [37]. Taking into account these results, the 30 min treated TiO2/OCF composite was submitted to heat treatment in an argon atmosphere in order to improve the TiO2 crystallinity and enhance its photoelectrochemical efficiency under UV irradiation. The temperature was kept constant at 500 °C and the heat treatment times were varied for 2, 3, and 4 h. These parameters were chosen since they are suitable to obtain the TiO2 in anatase structure, which present the best response under UV irradiation to form electrons/holes pairs [8]. Their FEG-SEM morphologies are shown in Fig. 5. TiO2/OCF composite heat-treated for 2 h presented significant surface changes with formation of small rounded white grain uniformly distributed throughout the OCF surface. This result is in agreement with the literature for TiO2/CF composites with HT at 500 °C for 2 h [34,38–41]. According to them, a morphology like that can generate an efficient improvement in the charge carrier separation and, consequently, enhance the photoelectrochemical activity of the electrode [42]. On the other hand, for TiO2/OCF samples heat-treated for 3 and 4 h TiO2 particles coalescence probably occurred, which caused a stress increase between CF substrate and TiO2

components corresponding to the bonds: (1) C]O (carbonyl); (2) C–OH (hydroxyl), C–O (ether), and C]O in ester; (3) C–O ester; (4) COOH (carboxyl), and (5) water, at 531.1, 532.3, 533.3, 534.2, and 536.1 eV, respectively [28]. Table 1 shows the calculated percentages of these functional oxygen peaks. Relative percentages of the oxygen functional groups such as carbonyl group (C]O), hydroxyl group (C–OH), and carboxyl group (COOH) were found on the as-received CF surface. After 5 min of EOT, the OCF sample presented an increase in carboxyl group content. For samples with 10 min and 20 min of EOT, the carbonyl and hydroxyl groups increased compared to those of OCF 5 min while the highest content of hydroxyl group was observed for OCF 30 min. According to the literature, synthesis of TiO2/CF requires mainly hydroxyl functional group on the surface, which can facilitate Ti nanoparticles binding with the substrate leading to the subsequent TiO2 formation [29]. It is noteworthy that we did Raman analyzes in the samples after the electrochemical oxidative treatment (Fig. S2). However, ID/IG ratios did not present significant changes. This behavior showed that the oxidative treatment times used in this study did not generate relevant structural changes on the carbon fibers, but only superficial modifications related to oxygen functional groups. FEG-SEM images compared the TiO2 deposits morphologies regarding the EOT times as shown in Fig. 2. The as-received CF surface showed the lowest particle density and a smooth thin TiO2 layer covering the fiber (Fig. 2(a)). With EOT 5 min and EOT 10 min (Fig. 2(b and c)), a thicker TiO2 layer appeared, but no other significant change was observed. Nonetheless, for EOT 20 and EOT 30 min samples the TiO2 deposits promoted drastic changes on OCF surfaces (Fig. 2(d and e)). A rougher film texture is dominant with small rounded TiO2 nanoparticles totally covering the OCF surface. TiO2 on the OCF_EOT 30 min showed the highest density and the best homogeneity besides its roughest texture. Since the experimental conditions were the same for all samples, TiO2 morphology and its particle density strongly depend on the EOT times on CF surface as shown in XPS analyzes. They revealed different 3

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Fig. 2. FEG-SEM images of TiO2/OCF binary composites as a function of EOT on CF surface: (a) as-received CF; (b) EOT 5 min; (c) EOT 10 min; (d) EOT 20 min; (e) EOT 30 min.

Fig. 3. Representative illustration of possible interaction between Ti cations and OCF in the synthesis of TiO2/OCF composites.

promoting possible film delamination, in the case of Fig. 5(c).

structure by the presence of rounded TiO2 nanoparticles on OCF surface evidenced by FEG-SEM image. For better visualization, the composite performances in relation to their generated photocurrents are shown in Fig. 6(b) where ΔI means the difference between the photocurrent (Iph) and dark current (Id) at 1.8 V vs. Ag/AgCl/KCl(sat) plotted as a function of heat treatment time. Clearly, the 2 h heat-treated TiO2/OCF composite possesses the highest photocurrent density, which is at about 0.46 mA cm−2 and significantly higher than that of composites heat-treated for 3 and 4 h. This best efficiency in the photoactivity performance can be attributed to the morphology of TiO2/OCF_HT2 composite because of the TiO2 nanoparticles homogeneously distributed over the entire OCF surface, which can contribute to minimizing the photogenerated electron recombination, enhancing its photoelectrochemical response. According to literature, the smaller TiO2 particle sizes and their uniform distribution can generate an efficient improvement in the charge carrier separation,

3.2. Photoelectro/electrochemical characterizations of TiO2/OCF Afterwards, the photoelectrochemical activities of these composites were carried out by linear sweep voltammetry (I vs. V curves), under intermittent UV irradiation. The photoelectrochemical responses as function of heat treatment time considering the dark current (Id) and the photocurrent (Iph) densities are shown in the Fig. 6(a). Each color represents a curve pair for each sample where the solid line represents the voltammograms under UV illumination while the dotted line refers to the voltammograms in dark conditions. All voltammograms showed current-potential responses for a typical oxide semiconductor associated to the TiO2 presence on the electrode surfaces. The highest dark current was observed for TiO2/OCF_HT2 composite, presumably due to a higher electrochemically active surface area, in line with its rougher 4

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Fig. 4. Raman spectra of TiO2 deposits on CF/OCF samples for (a) as-received CF; (b) EOT 5 min; (c) EOT 10 min; (d) EOT 20 min; (e) EOT 30 min.

decreasing bulk recombination of electron/hole pairs [42]. From the results above, we established the best conditions to produce TiO2/OCF binary composite as 30 min of EOT and TiO2/OCF heat-treated at 500 °C for 2 h. We named it as TiO2/OCF_HT. EIS measurements of TiO2/OCF and TiO2/OCF_HT samples were performed for a more detailed electrochemical investigation considering the HT influence. Fig. 7 shows the Nyquist diagrams for both composites while the figure inset depicts a higher magnification of this figure in high and medium frequency regions. The first extrapolation in the real part of impedance (Z’) is related to electrolyte resistance Re while the second extrapolation, in the end of the semi-circle, also in Z’ axis, refers to charge transfer resistance (Rct), which is associated with electrode/electrolyte interface. Rct values were very low, at around 0.5 Ω, confirming the good conductivity for both electrodes. Nonetheless, TiO2/OCF presented the highest impedance module pointing out its highest resistive behavior. On the other hand, the TiO2/OCF_HT presented a more capacitive response that is associated to a larger charge accumulation with high electronic density in the electrode surface favoring its electrochemical performance. In this way, the heat treatment may promote two contributions related to TiO2 nanoparticles formation, its crystallinity improvement as well as the electrode surface area increase. According to the literature, a morphology composed of TiO2 nanoparticles would lead to a higher surface roughness enhancing the electrode active area [42].

Fig. 6. Linear sweep voltammetry curves (A) and photocurrent values ΔI (B) for samples (a) TiO2/OCF; (b) TiO2/OCF_HT2; (c) TiO2/OCF_HT3; and (d) TiO2/ OCF_HT4.

3.3. Brilliant Green dye decolorization by anodic electrooxidation and photoelectrooxidation using TiO2/OCF anode Comparative experiments were performed to find out the effect of electrode modification in addition to the UV irradiation influence on the improvement of BG dye degradation. For this purpose, the dye color removal using 450 mL of 100 mg L−1 in 0.1 mol L−1 K2SO4 applying a j = 10 mA cm−2 for 300 min was assessed by UV–Vis measurements. Fig. 8(A) shows the normalized BG dye concentration decay as a

Fig. 5. FEG-SEM images of TiO2/OCF composite submitted to heat treatment times of: (a) 2 h; (b) 3 h; (c) 4 h. 5

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decolorization process could be explained just by %OH radicals originated from water electrooxidation by Reaction (1) described to Comminellis et al [43]:

H2 O → •OH + H+ + e−

(5)

Upon UV irradiation an enhancement of the degradation process in the order of TiO2/OCF < TiO2/OCF_HT occurred, achieving ∼85% of decolorization for TiO2/OCF_HT anode. This behavior suggests that the generation of strong oxidizing %OH radicals was promoted at the anode surface both electrochemically and photochemically, which can be expressed by Reactions (6) and (7) [44]: − + TiO2 + h ϑ → eCB + hVB

(6)

+ hBV + H2 O → •OH + H+

(7)

The superior behavior of TiO2/OCF_HT over TiO2/OCF under UV irradiation can be associated to the additional formation of %OH since HT improves the TiO2 crystallinity and can generate more hydroxyl radicals when excited under UV light irradiation. Generally, heterogeneous electrocatalytic and/or photoelectrocatalytic reactions are described by the Langmuir-Hinshelwood kinetic model. However, in the case where the analyte concentration is low, the kinetic model can be simplified and studied as a pseudo-first order reaction [45]. Therefore, this study considered that the BG dye color removal also followed the pseudo first-order reaction and then, the apparent kinetic constant (kapp) was calculated. Table 2 shows the kinetics result values obtained from the linear regression analyzes for the ln (C0/C) vs. time (min) plots (Fig. 8(B)) for all electrodes studied with correlation coefficient of 0.99. Upon UV irradiation the electrode submitted to heat treatment presented higher degradation efficiency than that of TiO2/OCF with higher kinetic constant value. In addition, electrochemical oxidation rate increased with UV and HT contributions confirming their synergistic effect for organic compound degradation. Table 3 presents a comparison of some results published in the literature with respect of electrochemical and photoelectrochemical oxidation of organic compounds, using different electrode material and experimental conditions. In general, an enhancement of the overall rate of decolorization and a significant organic oxidation are observed when the electrolysis is carried out with higher current density, which depends on higher energy consumption. Among the presented anode materials tested for electrochemical and/or photoelectrochemical oxidation, the TiO2/OCF_HT_UV composite obtained in this work presented the greatest performance in color removal with the highest kinetic rate taking into account the current density of 10 mA.cm−2. In view of the fact that dyes and pigments are very resistant to biodegradation, color removal with low current density proved to be an important feature in electrochemical treatment of textile wastewater. Thus, the separation technique by HPLC is very important, because it allows efficient and reliable results, besides its stability for quantitative determinations. Taking into account that the study of by-products of BG degradation represents a significant contribution, the electrolysis behavior was analyzed by HPLC measurements, since the band associated to π − π∗ at 206 nm is related to the aromatic ring breaking of this dye [52]. Fig. 9 depicts the HPLC chromatograms of BG dye degraded solutions for 0, 45, 90, 120 and 300 min of electrolysis using TiO2/ OCF_HT electrode under UV irradiation. A decrease in the dye peak

Fig. 7. Nyquist plots for TiO2/OCF and TiO2/OCF_HT binary composites obtained in 0.5 M H2SO4 solution.

Fig. 8. (A) Normalized BG dye decay and (B) Logarithm of the normalized BG dye concentration during the electrolysis for samples: (a) TiO2/OCF; (b) TiO2/ OCF_UV; and (c) TiO2/OCF_HT_UV.

Table 2 Apparent kinetic constants (kapp) with R2 and electrical energy consumption in order (EEO) obtained for BG dye electro/photoelectrochemical oxidation using TiO2/OCF composites.

function of electrolysis time for the TiO2/OCF and TiO2/OCF_HT composites in the dark and under UV irradiation. As it can be seen, TiO2/OCF anode was attained at the end of the essays of anodic electrooxidation, without illumination, a color removal of 66%. Considering that the studied TiO2/OCF anode is an active electrode, the 6

Electrode

kapp (x10−3 min-1)

R2

EEO (kWh L−1)

TiO2/OCF TiO2/OCF_UV TiO2/OCF_HT_UV

5.09 6.28 7.43

0.998 0.996 0.999

1.66 × 10−2 1.35 × 10−2 5.24 × 10−3

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Table 3 Overview of some results published in the literature for electrochemical and photoelectrochemical oxidation of organic compounds. Anode

Process

TiO2/OCF_HT_UV SS/PbO2 Ti/Pt TiRuSnO2 Au/TiO2/Au/TiO2 TiO2 TiO2

PhoE E E E PhoE PhoE PhoE

Dye Brilliant Green Amaranth Malachite Green Methyl Orange Methyl Orange Methyl Red Malachite Green

Operating parameters −2

2

100 ppm. 10 mA.cm . 4.4 cm 0.015 mM. 25 mA.cm−2 4 cm2 150 ppm. 56 mA.cm−2. 90 cm2 100 ppm. 1.5 A. 50 cm2 5 ppm. 30 mA.cm−2. 7.1 cm2 25 ppm. 30 mA.cm−2. 1 cm2 30 ppm. 4 V. 40 cm2

% decolorization

kapp (x10−3 min-1)

Ref.

85 70 90 69 68 40 30

7.4 5.3 – 5.6 5.5 × 10−6 – 4.8

This work [46] [47] [48] [49] [50] [51]

E = Electrochemical Process; PhoE = Photoelectrochemical process.

4. Conclusions TiO2/OCF composite was produced for the first time by the anodic hydrolysis of TiCl3. OCF oxidized for 30 min significantly improved the TiO2 deposits on its surface because of the highest presence of C–OH group. Linear sweep voltammetry evaluated the photoelectrochemical activities of the deposits under UV irradiation. These results showed that the use of 2 h heat treatment in the TiO2/OCF composite achieved the highest photocurrent because of its morphology. Here, TiO2 nanoparticles presence, uniformly distributed on OCF surface, minimized the recombination of the photo-generated electrons, improving the charge carrier separation efficiency. TiO2/OCF_HT_UV composite yielded higher Brilliant Green dye degradation compared to that of TiO2/ OCF_UV electrode since it has the highest electrochemical active surface area. Besides, it took advantage of the synergy between electrocatalysis and photocatalysis to produce •OH radicals which attack organic matter enhancing the dye removal.

Fig. 9. HPLC chromatograms of BG dye degraded solutions as a function of electrolysis time.

Acknowledgments

intensity is evident for 45 min of electrolysis where the appearance of other peaks with lower retention times is associated with organic compounds with lower aliphatic chains. This decrease is very important for environmental point of view since these molecules tend to be biodegraded [53]. This behavior corroborated the above electrochemical results concerning the efficiency of this electrode for EAOP with low current density. Moreover, the HPLC peak associated with the dye chromophore group can be considered to analyze the process efficiency from electrical energy consumption in order (EEO), associated with the operating costs. This is because the peak area is directly proportional to the dye concentration whose chromophore group was not degraded yet, as may be observed in the peak evolutions of Fig. 9. According to the literature and considering the low concentration of pollutants, EEO is defined as the electrical energy, in kilowatt-hour (kWh), necessary to degrade a contaminant by volume following the equation:

EEO =

The authors acknowledge CAPES, CNPQ (grant 302017/2015-1), and FAPESP (grants 2016/13393-9 and 2017/10118-0) by the financial support. We are also so grateful to Dr. M.R. Baldan by XPS analyzes. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2019. 110642. References [1] S.V. Mohite, V.V. Ganbavle, K.Y. Rajpure, Photoelectrocatalytic activity of immobilized Yb doped WO3 photocatalyst for degradation of methyl orange dye, J. Energy Chem. 26 (2017) 440–447, https://doi.org/10.1016/j.jechem.2017.01.001. [2] C.B.D. Marien, C. Marchal, A. Koch, D. Robert, P. Drogui, Sol-gel synthesis of TiO2 nanoparticles: effect of Pluronic P123 on particle’s morphology and photocatalytic degradation of paraquat, Environ. Sci. Pollut. Res. 24 (2017) 12582–12588, https://doi.org/10.1007/s11356-016-7681-2. [3] Y. Sun, G. Wang, Q. Dong, B. Qian, Y. Meng, J. Qiu, Electrolysis removal of methyl orange dye from water by electrospun activated carbon fibers modified with carbon nanotubes, Chem. Eng. J. 253 (2014) 73–77, https://doi.org/10.1016/j.cej.2014. 05.017. [4] S. Alcocer, A. Picos, A.R. Uribe, T. Pérez, J.M. Peralta-Hernández, Comparative study for degradation of industrial dyes by electrochemical advanced oxidation processes with BDD anode in a laboratory stirred tank reactor, Chemosphere. 205 (2018) 682–689, https://doi.org/10.1016/j.chemosphere.2018.04.155. [5] Z. Mohammed Redha, H. Abdulla Yusuf, H.A. Ahmed, P.R. Fielden, N.J. Goddard, S.J. Baldock, A miniaturized injection-moulded flow-cell with integrated conducting polymer electrodes for on-line electrochemical degradation of azo dye solutions, Microelectron. Eng. 169 (2017) 16–23, https://doi.org/10.1016/j.mee. 2016.11.016. [6] S. Garcia-Segura, E. Brillas, Applied photoelectrocatalysis on the degradation of organic pollutants in wastewaters, J. Photochem. Photobiol. C Photochem. Rev. 31 (2017) 1–35, https://doi.org/10.1016/j.jphotochemrev.2017.01.005. [7] Z. Wei, F. Liang, Y. Liu, W. Luo, J. Wang, W. Yao, Y. Zhu, Photoelectrocatalytic degradation of phenol-containing wastewater by TiO2/g-C3N4hybrid heterostructure thin film, Appl. Catal. B Environ. 201 (2017) 600–606, https://doi.org/ 10.1016/j.apcatb.2016.09.003. [8] A. Chatzitakis, A. Papaderakis, N. Karanasios, J. Georgieva, E. Pavlidou, G. Litsardakis, I. Poulios, S. Sotiropoulos, Comparison of the photoelectrochemical

P. t V . log

() Ai Af

(8)

where: P is the electric power (kW), t is the time of electrolysis (h), V is The volume treated (L), Ai and Af are the initial and final area of the chromatographic peak associated to the studied pollutant [54,55]. EEO values are presented in third column of Table 2 for experiments using TiO2/OCF, TiO2/OCF_UV, and TiO2/OCF_HT_UV. As expected, the results followed the same trend of kapp values where the EEO abruptly decreased when the thermally treated electrode under UV irradiation was used. This tendency confirmed the synergism between photocatalysis and heat treatment for the TiO2/OCF binary composite, which improved the BG dye degradation rate.

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