Revealing the influences of functional groups in azo dyes on the degradation efficiency and power output in solar photocatalytic fuel cell

Abstract

In this study, the degradation efficiency and electricity generation of the azo dyes affected by the functional groups and molecular structure in a solar photocatalytic fuel cell (PFC) system were investigated and discussed in detail. Four different azo dyes such as, Acid Orange 7 (AO7), Acid Red 18 (AR18), Reactive Black 5 (RB5), Reactive Red 120 (RR120) with different molecular structure were evaluated. The degradation efficiency of AO7, AR18, RB5 and RR120 achieved 5.6 ± 0.3%, 11.1 ± 0.6%, 41.9 ± 0.9% and 52.1 ± 1.3%, respectively, after 6 h irradiated under solar light. In addition, the maximum power density, P_max for AO7, AR18, RB5 and RR120 was 0.0269 ± 0.01, 0.111 ± 0.03, 1.665 ± 0.67 and 4.806 ± 1.79 mW cm⁻², respectively. Meanwhile, the concentration of COD for AO7, AR18, RB5 and RR120 reduced to 16 ± 0.1, 10 ± 0.3, 7 ± 0.6 and 3 ± 0.9 mg L⁻¹, respectively. The concentration ratio of benzene / naphthalene, benzene / azo bond and naphthalene / azo bond, respectively, was analyzed to investigate the impact of the functional groups over photodegradation of the azo dyes in PFC. Electron releasing groups (-OH and –NH₂) and electron withdrawing groups (-SO₃Na) which attached to the naphthalene or benzene ring also played a pivotal role in the degradation mechanism.

Introduction

One of the major sources in water pollution could be attributed to the textile industry [1]. The wastewater contains organic contaminants such as phenol, azo dyes and others could be toxic or mutagenic which can cause severe health and environmental issues. Azo dyes are the largest group among the synthetic dyes, which constitute up to 70% of all the known commercial dyes produced [2–4]. The substitutions of aromatic rings joined by one or more azo groups will characterize their chemical structure [5]. Highest concentration of azo dyes in industrial wastewater would affect aquatic life due to the limited light penetration into water for photosynthesis process to be occurred.

The concept of photocatalytic fuel cell (PFC) to eliminate pollution and simultaneously produce renewable energy had been developed in microbial fuel cell (MFC), where MFC can generate electrons during the oxidation of organic matters by microbial community [6]. MFC may convert the energy of organic compounds into electric current and also will produce hydrogen [7]. However, MFC has several limitations, for example, not feasible to treat some persistent organics due to their biorefractory nature and low oxygen reduction kinetics [8]. This system shows advantages over MFC as it is based on metal-oxide semiconductor which acts as photoanode that drives a rapid and direct reaction between highly reactive radicals and organic compounds under irradiation of light [9].

In PFC process, electron and hole pairs are generated through excitation of photoelectrons from valence band to the conduction band. These photoelectrons can be transferred spontaneously to cathode via an external circuit for electricity generation [10]. The production of electrons due to the reaction between photocatalyst and light irradiation which solar light irradiation had been chosen in this study. The photogenerated holes can react with H₂O or OH⁻ and oxidize them into hydroxyl radicals (OH•) [11]. The positive holes could oxidize the organic pollutants, while electrons would reduce the dye or react with electron acceptors to form superoxide radical anion, •O₂⁻ [11, 12]. Some of the photogenerated electrons might be externally transferred from anode to cathode and produce voltage in the cell. The most common photocatalyst that have been studied are titanium dioxide (TiO₂), ZnO, cadmium sulfide (CdS) and tungsten (VI) oxide (WO₃) [13–15]. Among the various semiconductors, TiO₂ is generally considered to be the best photocatalyst and widely used in many wastewater treatments. However, TiO₂ do not show better performance under solar light and ZnO has become a suitable alternative to TiO₂ since the band gap energy of ZnO also similar with TiO₂ which is 3.2 eV [16].

The molecular structure of the azo dyes has considerable effect on the photodegradation of these dyes in the PFC system. Besides, functional groups such as sulfonic group, amine substituent, and alkyl side chains also may influence the degradation efficiency of dyes as well as the voltage output in PFC. The objective of this study was to investigate and compare the influence of functional groups of four different azo dyes in PFC in terms of photodegradation efficiency and power output. The role of electron releasing and withdrawing group along with the ratio of the aromatic compound and azo bonds were also studied to disclose their relationship with the photodegradation efficiency.

Materials and methods

Fabrication of photoanode

A mixture of 0.1 M sodium silicate (Bendosen Laboratory Chemicals, Norway) and 5.0 g of ZnO (HmbG Chemicals, Germany) was stirred until it was dispersed homogeneously. Carbon felt (SG-222) with the size of 12 cm² was immersed into the mixture for 1 h before it was taken out and dried in the oven at 60 °C. It was sustained for three times to ensure ZnO powder fully occupied at the surface of carbon felt.

Chemicals

Four types of azo dyes (Acid Orange 7, Acid Red 18, Reactive Black 5, and Reactive Red 120) were supplied by Sigma Aldrich (Merck KGaA, US) and used without further purification. Table 1 presents the molecular structure and mass of these azo dyes.

Table 1.

Molecular structure and functional groups of Acid Orange 7, Acid Red 18, Reactive Black 5 and Reactive Red 120 used in this study

PFC procedure

The stock solutions of AO7, AR18, RB5 and RR120 were prepared in 1.0 g L⁻¹ concentration. Four azo dyes, AO7, AR18, RB5 and RR120 with concentration 30 mg L⁻¹, respectively were carried out in four 500 mL beakers separately and placed under solar light irradiation. The intensity of the sunlight was 870 ± 100 Lux which was measured by Lux meter. The ZnO/carbon felt as photoanode and Pt/C as cathode were externally connected by copper wire with a resistor of 1000 Ω. The experiment was conducted under solar light for 6 h and sample was collected for every hour. The sample then was analyzed in terms of its decolorization and reduction in absorption spectrum by UV-Vis spectrophotometer (Hitachi, U-2810) as well as its reduction in COD concentration (HACH, DR2800).

Results and discussion

Decolorization of dyes

Figure 1 shows the decolorization of AO7, AR18, RB5 and RR120 over 6 h of reaction time. The final concentration of AO7, AR18, RB5 and RR120 was 28.324 ± 0.573, 26.656 ± 0.289, 17.416 ± 0.925 and 14.370 ± 1.281 mg L⁻¹, respectively. Hydroxyl radicals that generated through reaction between holes and hydroxide ions mostly attacked the azo bond which has higher amounts of electrons, and led to the decolorization of the azo dye solution. Garcia-Segura et al. (2011) in their study about comparative decolorization of monoazo, diazo and triazo dyes by electro-Fenton process found that decolorization rate decreased as number of azo bond increased due to larger and more stable conjugated π system was formed [17]. Besides, Khataee and Kasiri (2010) found that monoazo dyes achieved higher photodegradation rate compared to dyes that contained anthraquinone structure [18]. However, in this study, it was found that the decolorization rate of diazo dyes (RB5 and RR120) is greater than mono azo dyes (AO7 and AR18). Besides, NH group is the fragile group which results from an equilibrium between two tautomeric forms [19, 20] where an H atom is exchanged between O and N. Khataee et al. (2009) [21] reported that the main degradation pathway of these dyes is the abstraction of H atom (carried by an oxygen atom in the azo form or by a nitrogen atom in the hydrazone form) by OH radicals. On top of that, Khalik et al. (2015) [22] also found that higher number of azo bonds and sulfonic groups would increase the decolorization rate of azo dyes. This could be due to that azo bonds are the most active bonds in the molecular structure of azo dyes which could be oxidized by positive hole and hydroxyl radical or reduced by electron in the conduction band. Decolorization of dyes is attributed to the breakup of –N=N– bonds [23]. Besides, the sulfonic group would also increase the adsorbability of the dye molecules on the catalyst contributing to higher photocatalytic degradation rate [23, 24]. As this RR120 possesses four NH groups, two OH groups and six sulfonic groups in its molecular structure, which is the largest number among the four azo dyes. Thus, it revealed the highest photodegradation efficiency.

Fig. 1.

Decolorization of azo dyes under irradiation of solar light for 6 h

Apart from the production of OH•, there are other superoxide radical anions that produced from this process, for example, sulfate radical anions, SO₄²⁻• and chloride radical anions, Cl⁻• [25].

The higher number of sulfonic groups would increase the production of SO₄²⁻• in dye solution and these superoxide radical anions could act as electron acceptor which enhance the decolorization rate of azo dyes. Muruganandham and Swaminathan (2004) stated that electron acceptors enhanced the degradation rate in several ways (i) increasing the OH• concentration, (ii) preventing the electron-hole recombination by accepting the conduction band electron and (iii) generating other oxidizing species (SO₄²⁻) to accelerate intermediate compound oxidation rate [26].

The degradation efficiency of azo dyes decreased following the order of: RR120 > RB5 > AR18 > AO7. Both molecular structure of monoazo dyes which was AO7 and NC attached with –OH at ortho position to –N=N-, meanwhile for diazo dye, the molecular structure of RB5 attached with –OH and –NH₂ at ortho position to –N=N-. Both –OH and –NH₂ were relatively strong electron releasing groups had shown positive effect for oxidation [27]. Furthermore, electron withdrawing groups such as –SO₃Na attached at the para position to –N=N- for AO7 at benzene ring, while for AR18, -SO₃Na attached at the meta position to –N=N- at naphthalene ring. The -SO₃Na for both of diazo dyes attached at para and ortho position at naphthalene ring and meta position for benzene ring. When attached to benzene ring, the -SO₃Na will cause the electrophilic aromatic substitution reaction slower and more complex. This phenomenon could be observed in the degradation of AO7 which was the slowest compared to other dyes. Meanwhile, for AR18, the –N=N- attached to two naphthalene rings and the strongly deactivating group would attack at meta position which then leading to the cleavage of azo bond. On the other hand, the electron donating group that attached in molecular structure of RB5 and RR120 would donate some of its electron density and become more nucleophilic. These electrons might undergo photocatalytic process to create more OH• and superoxide radical anions, resulting in enhancing the degradation rate of RB5 and RR120. The formation of electrons through degradation of azo dyes also would affect the voltage generation as further discussed in section 3.2.

Evaluation of solar PFC performance

As ZnO has larger band gap energy compared to energy of solar light, electrons in valence band are excited to the conduction band, and leave holes in the valence band. The electrons generated could be externally transferred and give voltage output to the solar PFC performance as depicted in Fig. 2. The initial voltage output for AO7, AR18, RB5 and RR120 was 4.9, 14.7, 76.9 and 235.7 mV, respectively. The final voltage output generated in PFC system for both AO7 and AR18 was 0 mV, meanwhile, for RB5 and RR120, the voltage output was 25.3 and 168.7 mV, respectively.

Fig. 2.

Voltage outputs vs solar light irradiation time for AO7, AR18, RB5 and RR120

The polarization curve for AO7, AR18, RB5 and RR120 was measured by varying the value of external resistor from 300,000 Ω to 750 Ω as shown in Fig. 3a, b. The open circuit voltage, V_oc for AO7, AR18, RB5 and RR120 was 298, 415, 852 and 1017 mV, respectively. Meanwhile, the short-circuit current density, J_sc for AO7, AR18, RB5 and RR120 was 0.003, 0.005, 0.014 and 0.025 mA cm⁻², respectively. In addition, the maximum power density, P_max for AO7, AR18, RB5 and RR120 was 0.0269 ± 0.01, 0.111 ± 0.03, 1.665 ± 0.67 and 4.806 ± 1.79 mW cm⁻², respectively.

Fig. 3.

Polarization curve a Voltage vs Current density and b Power density vs Current density for different azo dyes

Once the photocatalyst is excited by irradiation of solar light, electron-hole pairs will be created. The positively charged holes will react with water molecule to produce hydrogen molecule and OH•, meanwhile combination between electrons and oxygen will produce O₂•⁻. Some electrons will externally transport from photoanode to cathode, and some of them will react with oxygen. Due to the degradation of AO7 and AR18 is slow, which means that they might generate less electrons, so the amount of electrons transferred from photoanode to cathode would become limited. This resulted in low voltage generation in PFC which contained AO7 and AR18, respectively. Meanwhile, rapid degradation of RB5 and RR120 due to the increase in production of OH• in the dye solution, would lead to large amount of electron transferred from photoanode to cathode. Consequently, the voltage output of each RB5 and RR120 containing PFC was high at the end of the reaction time compared with AO7 and AR18. The voltage output of RR120 higher compared to RB5 due to larger number of sulfonic groups attached at molecular structure of RR120. The degradation of RR120 did not only occur in the reaction between photocatalyst and irradiation of solar light, but it also happened in the dye molecules itself. Both of these reactions would produce electrons and since there are 6 sulfonic groups attached to the molecular structure of RR120, the formation of electrons are higher compared to other dyes. These electrons would then move from photoanode to cathode across the external circuit and resulted in higher output voltage for RR120 than RB5, AR18 and AO7. The difference in J_sc and P_max for AO7, AR18, RB5 and RR120 could be due to the various compounds which might be ascribed to the difference in molecular structure of organic compounds [9]. The oxygen reduction reaction occurs at cathode side depend on the availability of oxygen. The hydrogen revolution reaction occurred in the absence of oxygen, meanwhile with presence of oxygen, oxygen reduction occurred in order to produce water [8]. The number of hydrogen ions (H⁺) might be completely eliminated by interacting with OH⁻ and producing water. Moreover, the reaction between water molecules and photogenerated holes would form H⁺ and OH• which effectively degrade the azo dyes [28].

Analysis of UV-Vis spectra

Figure 4 demonstrates the UV-Vis spectra of AO7, AR18, RB5 and RR120 during solar PFC system using ZnO/carbon felt as photoanode. There are three peaks observed which are 228, 309 and 483 nm in the spectra of AO7, meanwhile for AR18, the peaks appeared at 245, 332 and 504 nm. The UV-Vis absorption spectra for RB5 also showed three peaks which were located at 256, 310 and 598 nm, meanwhile for RR120, its major absorbance peaks appear at 235, 290 and 511 nm. The azo bond that gives colorant to the AO7, AR18, RB5 and RR120 was at 483, 504, 598 and 511 nm, respectively. On the other hands, the absorption spectra ranging from 200 until 350 referred to the benzene (255 nm) and naphthalene (289 nm) ring. A decline in the intensity of the maximum absorbance peaks of mono and diazo dyes was observed over 6 h of reaction time. The –SO₃Na at the para and ortho position might attract electrons from naphthalene ring, meanwhile, -SO₃Na at the meta position eager to release the electrons [29]. This indicates that the formation of OH• may firstly attach with the N and C atoms of –N=N- and benzene in azo dyes, respectively, led to dissolution of the N-C bond [30]. The azo bonds are easily to be destroyed compared to the other aromatic compound due to the electrons at azo bond remain as highly reactive site in the dye molecule [24]. Meanwhile, naphthalene and benzene structure would be destructed at the ortho position of OH• and produce different carboxylic acids [31]. However, the absorbance peak at benzene and naphthalene ring for each dye inclined during irradiation time. The breakdown at azo bond would elevate the peak of both benzene and naphthalene structure, but these aromatic compounds required prolonged irradiation time to mineralize completely. This is due to the limited number of OH• present in the reaction and does not have enough energy to break down the benzene and naphthalene ring, which then increased their peak in spectrum. In addition, the inorganic groups of SO₃Na would firstly be desolated from the azo dyes and the increased concentration of SO₄²⁻ in the solution could indicate the breakdown of the C-S bond [23].

Fig. 4.

UV-Vis spectrum for a AO7, b NC, c RB5 and d RR120

The ratio between benzene, naphthalene and azo bond was shown in Fig. 5a–c. The destruction of azo bond might lead to the increase of benzene and naphthalene compounds and its ratio increased due to the breakdown of azo bond. The aromatic compounds and azo bonds of RR120 are relatively easy to be deformed compared with other dyes which lead to the highest ratio of benzene/azo bond and naphthalene/azo bond. Besides, the azo bond and aromatic compounds of AO7 did not break down as fast as other dyes, which resulted in low ratio of benzene/azo bond and naphthalene/azo bond. The different photocatalytic decolorization rate may be due to several factors such as adsorption capacity onto catalyst [32], molecular structure and sulfonic group [21, 33]. The result exhibited that the decolorization depends on both azo groups and molecular structure exist in the dyes. RR120 containing two naphthalene rings are rapidly decolorized compared to AO7 (one benzene and one naphthalene ring), AR18 (two naphthalene rings) and RB5 (two benzene rings and one naphthalene ring). Muthukumar et al. (2005) suggested that azo dyes with higher number of sulfonic groups would have higher solubility, resulting in better interaction between dye molecules with OH• or superoxide radical anions that generated during the reaction [34].

Fig. 5.

Ratio between a benzene and naphthalene, b benzene and azo bond and c naphthalene and azo bond

Mineralization of azo dyes

The mineralization of azo dyes was analyzed through reduction in COD concentration. The COD test is widely used to study the efficiency of dye removal techniques by measuring the amount of organic compounds in water [35]. Figure 6 depicts the COD concentration for AO7, AR18, RB5 and RR120 under solar light irradiation for 6 h. AO7, AR18, RB5 and RR120 achieved COD concentration of 16 ± 0.1, 10 ± 0.3, 7 ± 0.6 and 3 ± 0.9 mg L⁻¹, respectively at the end of the reaction. Compared with monoazo dyes, the diazo dyes showed higher mineralization and significant with their decolorization rate. The result depicts that both monoazo and diazo dyes only partially mineralized during 6 h of solar photocatalytic fuel cell. The final products from degradation of azo dyes are some intermediates products, CO₂ and H₂O. Kansal et al. (2007) also found that COD reduction was less than the percentage of decolorization which may be due to the formation of smaller uncolored products and required longer irradiation time to achieve complete mineralization [36].

Fig. 6.

The reduction in COD concentration of azo dyes over 6 h of irradiation time

Conclusions

Solar photocatalytic fuel cell of monoazo and diazo dyes was investigated. The decolorization of diazo dyes required less irradiation time compared to monoazo dyes due to higher number of sulfonic groups attached to their molecular structure. The concentration of of AO7, AR18, RB5 and RR120 was reduced to 28.324, 26.656, 17.416 and 14.370 mg L⁻¹, respectively, at the end of the reaction. The highest V_oc, J_sc and P_max were achieved in RR120 containing PFC, followed by that of RB5, AR18 and AO7. The mineralization of both monoazo and diazo dyes required longer irradiation time to achieve complete mineralization. Consequently, it could be concluded that the molecular structure, number of sulfonic groups, electron withdrawing group and electron donating group of the dye affect the degradation efficiency and voltage output in the PFC system.