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Technology » Textile Technology » "Textile Wastewater Treatment", book edited by E. Perrin Akçakoca Kumbasar and Ayşegül Ekmekci Körlü, ISBN 978-953-51-2543-3, Print ISBN 978-953-51-2542-6, Published: July 14, 2016 under CC BY 3.0 license. © The Author(s).

Chapter 5

Heterogeneous Photocatalysis Remediation of Wastewater Polluted by Indigoid Dyes

By Enrico Mendes Saggioro, Anabela Sousa Oliveira and Josino Costa Moreira
DOI: 10.5772/63790

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Chemical structure of Indigo carmine.
Figure 1. Chemical structure of Indigo carmine.
Mechanism of semiconductor (TiO2) particle surface.
Figure 2. Mechanism of semiconductor (TiO2) particle surface.
Reactors used on photocatalytic degradation of Indigo carmine dye in water [29].
Figure 3. Reactors used on photocatalytic degradation of Indigo carmine dye in water [29].
Simplified diagram of a compound parabolic collecting (CPC) reactor (CPC) [29].
Figure 4. Simplified diagram of a compound parabolic collecting (CPC) reactor (CPC) [29].
Time dependent UV–visible spectrum of Indigo carmine. Initial concentration of IC: 30 mg L−1; dosage of TiO2: 1×10−2 g L−1. Spectra from top to bottom correspond to irradiation times of 0, 15, 30, 45, 60, 90, 120, 180, and 300 min, respectively. Experiments performed with Reactor 1, in Figure 3A [29].
Figure 5. Time dependent UV–visible spectrum of Indigo carmine. Initial concentration of IC: 30 mg L−1; dosage of TiO2: 1×10−2 g L−1. Spectra from top to bottom correspond to irradiation times of 0, 15, 30, 45, 60, 90, 120, 180, and 300 min, respectively. Experiments performed with Reactor 1, in Figure 3A [29].
Effect of TiO2 suspension concentration on the photocatalytic degradation of 30 mg L−1 of IC [34].
Figure 6. Effect of TiO2 suspension concentration on the photocatalytic degradation of 30 mg L−1 of IC [34].
Photodegradation of Indigo carmine for different type of water mediated by 0.1 g L−1 of TiO2 suspensions [29].
Figure 7. Photodegradation of Indigo carmine for different type of water mediated by 0.1 g L−1 of TiO2 suspensions [29].
Ions and carboxylic acid concentrations formed during photodegradation of Indigo carmine [29].
Figure 8. Ions and carboxylic acid concentrations formed during photodegradation of Indigo carmine [29].
Recycling and reuse of TiO2 on the photocatalytic degradation of Indigo carmine for an accumulated energy, Quv, of 15 kJ L−1 [34].
Figure 9. Recycling and reuse of TiO2 on the photocatalytic degradation of Indigo carmine for an accumulated energy, Quv, of 15 kJ L−1 [34].
Evaluation of the concentrations of carboxylic acids followed by ion chromatography; formed during solar CPC photodegradation of Indigo carmine with supported TiO2 in real MWWTP secondary effluent [34].
Figure 10. Evaluation of the concentrations of carboxylic acids followed by ion chromatography; formed during solar CPC photodegradation of Indigo carmine with supported TiO2 in real MWWTP secondary effluent [34].

Heterogeneous Photocatalysis Remediation of Wastewater Polluted by Indigoid Dyes

Enrico Mendes Saggioro1, Anabela Sousa Oliveira2, 3 and Josino Costa Moreira4

1. Introduction

Textile industry process generates a significant amount of wastewaters containing 5–15% of an untreated dye, which can be released into the environment. Around 100,000 dyes are currently in use by the global textile industry, and 7 × 105 ton of dyestuff is produced annually worldwide. Moreover, the effluent of textile industries has intense color, chemical oxygen demand (COD), suspended solids, and several refractory compounds like heavy metals [1]. Discharges of untreated dye effluent into the water body produce colored effluents that not only cause esthetic deterioration but also affect oxygen and nitrogen cycles through photosynthesis, and they may also be toxic to aquatic biota [2]. Indigo is a commercial dye used mostly for cotton cloth dyeing (blue jeans), and the main constituent is indigotin that is extracted from the leaves of Indigofera tinctoria [35]. Indigo carmine (IC; 3,3-dioxo-2,2-bis-indolyden-5,5-disulfonic acid disodium salt) is also one of the oldest dyes and still employed extensively today for dyeing cotton with annual consumption around 33 million kg [6]. The wastewater-containing indigo is characterized by a dark blue color due to cross-conjugated system or H-chromophore, consisting of a single –C=C– double bond substituted by two NH donor groups and two CO acceptor groups [7]. The structure of IC is shown in Figure 1.


Figure 1.

Chemical structure of Indigo carmine.

IC is a toxic dye once its contact with skin and eyes can cause permanent injury to cornea and conjunctiva. Moreover, the oral exposure can cause a disturbance in the reproductive, developmental, and neuronal systems [8]. Most toxic dyes are recalcitrant to biodegradation, causing a decrease in the efficiency of biological wastewater treatment plants. Furthermore, traditional physical–chemical methods have some operational problems such as sludge generation, membrane fouling, and phase change of the pollutants [2, 9]. To avoid these problems, the use of advanced oxidative processes (AOPs) for wastewater treatment from textile industries has been proposed. AOPs are emergent and promising processes for removal of persistent organic pollutants. The main factor in the degradation of pollutants is by the generation of highly oxidant and nonselective hydroxyl radicals (OH) that promote the reaction of different classes of organic compounds. This technology can lead the complete mineralization or promote the formation of more biodegradable intermediates. The AOPs can be applied to a large set of different matrixes and that decontamination occurs through pollutants of degradation instead of their simple phase transfer. These methodologies become even more attractive when they use the sunlight as the source of energy [10, 11]. Although different advanced oxidation processes use several different reaction systems, all of them have the same chemical characteristic, i.e., the production and use of OH [1214].

Among AOPs, heterogeneous photocatalysis has been very attractive because the use of sunlight activated the process, allowing energy economy [15]. Heterogeneous photocatalysis produces oxidizing species able to promote degradation of organic pollutants through semiconductor as the catalyst. Typically, TiO2, ZnO, CdS, and ZnS semiconductors are employed in photocatalysis due to the electronic structure. It has a fully occupied valence band (VB) and an empty conduction band (CB). In this way, excited electrons can be transferred to chemicals into the semiconductor particle environment, and at the same time, the catalyst accepts electrons of oxidized species [12, 13]. Photocatalysis action mechanism can be visualized in Figure 2.

The semiconductor (TiO2) absorbs photons equal or higher than that of the band gap to promote an electron from the VB to the CB. Consequently, an electron/hole pair is formed as described by the following set of equations:

TiO2+ huh+VB+ e-CB

The hole produced in the VB can oxidize the water presents in the medium producing OH and oxidize hydroxide ions or the substrate itself, according to Figure 2.


Figure 2.

Mechanism of semiconductor (TiO2) particle surface.

TiO2(h+VB) + H2OadsorvTiO2+OHadsorv+ H+
TiO2(h+VB) + OH-adsorvTiO2+OHadsorv
TiO2(h+VB) + RXadsorvTiO2+ RX+adsorv

Moreover, the electrons promoted to the CB are also able to reduce the oxygen available for superoxide radicals. Thus, the presence of oxygen is essential in all oxidative processes:

TiO2(e-CB) + O2TiO2+ O2

Titanium dioxide is a well-known photocatalyst mostly used in photoreactions for wastewater treatment [16]. Fujishima and Honda [17] studied the heterogeneous photocatalysis of water extensively by TiO2. As a consequence, several studies have reported the use of titanium dioxide as process able to degrade all persistent pollutants [1823] and wastewater textile dyes from industries [24, 25]. The TiO2 has low cost and toxicity, high photochemical reactivity, and non-specific oxidative attack. In this way, it can promote the degradation of organic compounds with little change of operational parameters [26]. TiO2 has significant advantages for environmental application at the fact converted persistent organic molecules to safe oxidation products such CO2 and H2O [27]. Additionally, it can be used as an antibacterial agent due to its strong oxidation activity and hydrophilicity [28].

To perform a heterogeneous photocatalytic reaction is necessary to use semiconductors with the adequate “band gap” to be activated by solar energy. TiO2 has a high band gap, of 3.2 eV, being consequently activated only by radiation below 380 nm. On the other hand, metal oxides, such as TiO2, are resistant to photocorrosion with an adequate application on photocatalysis [28].

The decolorization of a model water effluent containing IC dye mediated by TiO2 indicated that IC photodegradation depends on various parameters, e.g., the initial concentration of the dye, the amount of TiO2, pH of the solution, the presence of inorganic anions, temperature, and the addition of different concentrations of hydrogen peroxide. Furthermore, the efficiency of the photocatalytic process strongly depends on the geometry of the photoreactor, which should enable all photocatalyst particles to be fully illuminated. Different photoreactors under artificial and solar irradiation were used, and their efficiency tested on the photodegradation of IC dye. On the other hand, photocatalytic degradation may generate photoproducts more toxic than their parent compounds. Thus, it is important to assess the toxicity of the resultant solution after treatment to determine potential threats to biodiversity of the treated waste to be released into the environment.

Although most of the experiments with IC reported in the literature were performed with the photocatalysts dispersed in water to enable the post-treatment photocatalysts removal, it is most important to use an immobilized catalyst. This chapter makes a comprehensive approach to the different results on the remediation of model effluents containing IC dye.

2. Heterogeneous photocatalysis degradation of Indigo carmine dye

2.1. Degradation of Indigo carmine dye under different geometry reactors

Heterogeneous photocatalysis processes require the maximum utilization of photons generated by artificial light or solar irradiation. The photochemical solar technology included the geometry design reactors for efficient solar photon collections to promote the photodegradation of organic pollutants presents on wastewater effluents [6]. The degradation of the IC dye was studied under different irradiation source and geometry reactors. Figure 3 shows the reactors that have been used to study the degradation at a laboratory bench top scale. First, Figure 3A shows a batch magnetically stirred reactor, irradiated with a high pressure of 125 W mercury vapor lamp (Reactor 1). Due to lamp geometry, a single point of stirring is applied. Second, Figure 3B shows a batch magnetically stirred with irradiation being ensured by four parallel 20 W daylight lamps; as light distribution is always identical below the lamps, multipoint stirring can be used (Reactor 2). The reactor presented on Figure 3C consists of a glass tubular continuous-flow reactor illuminated by one 20 W daylight lamp, fitted inside the tubular reactor (Reactor 3). The dye solution is pumped through this reactor, between lamp and inside’s reactor wall, circulating to/from a storage beaker.


Figure 3.

Reactors used on photocatalytic degradation of Indigo carmine dye in water [29].

In all cases, the efficiency of photocatalytic degradation of IC was directly related to the amount of photocatalyst. The optimum concentration of TiO2 (i.e., the minimum photocatalyst concentration enabling the highest photodegradation rate) depended on the geometry of the photoreactor that should enable all photocatalyst particles to be fully illuminated. For improvement of the photocatalysis efficiency, especially under solar irradiation, an equipment that makes a more efficient collection of photons could be applied. This equipment, already a pilot plant reactor, is a solar collector coupled to the tubular reactor where degradation itself takes place, and it usually represents the largest source of operating costs of a photocatalysis unit for treatment of effluents. This reactor, whose simplified diagram is presented in Figure 4, is a tubular reactor with compound parabolic solar concentration. These reactors are static parabolic collectors with a parabolic reflective surface that have their axis (where sunlight concentrates the most) a tubular reactor, where wastewater to be remediated flows through as shown in Figure 4. They had demonstrated to provide an excellent efficiency in the treatment of low pollutant’s concentration effluents [29].


Figure 4.

Simplified diagram of a compound parabolic collecting (CPC) reactor (CPC) [29].

Reactor 1 demonstrated the best results in photocatalytic degradation for IC. Photocatalysis efficiency at 96% and 92% was achieved in 30 minutes for concentrations of 1 g L−1 and 1 × 10−1 g L−1 of TiO2, respectively. On the other hand, the photodegradation on Reactors 2 and 3 was very slower with 100% of IC degradation at 1440 minutes of irradiation time under the same TiO2 concentrations [29]. The results obtained for Reactor 1 were very similar to the ones obtained under sunlight irradiation during summer because the photon flux used in both processes was similar. It is possible to replace artificial light source by natural solar light irradiation with the same efficiency. Furthermore, experiments carried out in winter months also demonstrated reasonable efficiencies. The photolysis of the dye was negligible for the compound parabolic reactor (CPC) like for the experiments in batch lab reactors. On the other hand, in distilled water, its photodegradation was observed to be complete for an accumulated UV energy of 15 kJ L−1, correspondingly approximately to only 12 minutes of irradiation time [29].

The absorbance of the IC dye at 610 nm decreased gradually with prolonged light exposure due to an increase in decolorization and light-induced degradation. Figure 5 shows the fast decolorization of 610 nm and also the changes of the spectra in the UV region. The decolorization of IC solutions is associated with the cleavage of double-bond carbon (–C=C–), characteristic of indigoid dye molecules (Figure 1) [30]. Absorption in the UV region can be assigned to the aromatic rings and exhibited peaks at 286 and 250 nm [31]. The intermediates may have been formed as demonstrated the changes of indigo dye UV–visible spectra. Lower molecular weight (MW) organic compounds or carbon dioxide probably is the most intermediates formed by oxidation of original IC structure [32].


Figure 5.

Time dependent UV–visible spectrum of Indigo carmine. Initial concentration of IC: 30 mg L−1; dosage of TiO2: 1×10−2 g L−1. Spectra from top to bottom correspond to irradiation times of 0, 15, 30, 45, 60, 90, 120, 180, and 300 min, respectively. Experiments performed with Reactor 1, in Figure 3A [29].

The solar irradiation batch lab experiments were very helpful to establish the best conditions for different geometries and light source for the photodegradation of IC dye. It facilitates the correspondence between various reactors with artificial irradiation and solar irradiation on the photodegradation of IC dye [29]. The summarized results indicate the feasibility of solar photocatalysis with TiO2 to the treatment of IC effluents from a textile industry, mainly in regions with available sunlight throughout the year. Moreover, titanium dioxide application to be activated by sunlight is compatible with the green chemistry principles [32].

2.2. Influence of different parameters on the photodegradation of Indigo carmine dye

2.2.1. Effect of catalyst amount

The effect of the TiO2 amount on the photocatalytic degradation of IC was evaluated. Moreover, the significance of adsorption on the catalyst surface should also be assessed from results obtained in the absence of light. The IC adsorption on TiO2 was found to be about 10% after 90 minutes of contact, and adsorption/desorption equilibrium time was 30 minutes in the dark. The color of indigoids dyes is influenced by the presence of associated chromophores and auxochromes groups. IC can be oxidized by positive hole or OH or reduced by electrons in the CB where all processes were leading to the decrease in the color of water [7].

The photocatalysis efficiency is apparently directly proportional to the amount of photocatalyst used, according to Figure 6. These results can be rationalized regarding an availability of active sites on TiO2 surface and on the light penetration for activation of TiO2 suspensions [24]. Moreover, in suspensions containing 1 g L−1 of TiO2 (first-order kinetic = 0.8442 min−1), the depth of light penetration is considerably smaller than in those containing only 0.1 g L−1 of TiO2 (first-order kinetic = 0.9002 min−1). However, the availability of active sites is much higher. Additionally, agglomeration and sedimentation of TiO2 particles also occur in suspensions containing a high concentration of TiO2 [33]. In this way, the optimum amount of TiO2 has to be determined for each solution to be treated to avoid the unnecessary use of a catalyst in excess.


Figure 6.

Effect of TiO2 suspension concentration on the photocatalytic degradation of 30 mg L−1 of IC [34].

2.2.2. Effect of initial dye concentration

The photocatalytic degradation of the dye decreased with increase in its concentration in the sample solution. The reduction in the photodegradation rate constant can be attributed to adsorption of dye molecules on the catalyst surface and consequent decrease on the generation of OH radicals because the active sites were occupied by dye cations [35]. Also, a significant amount of light may be absorbed by the indigo dye rather than TiO2. Probably reducing the efficiency of the catalytic reaction, the concentration of oxidant species decreases [36]. Another possible cause of the decline of decolorization is the competition between intermediate products formed in photocatalytic processes for the limited adsorption and catalytic site on the surface of TiO2 [37].

2.2.3. Effect of inorganic ions

Several anions commonly used in dye-containing industrial wastewater such as Cl, HCO3, SO4−2, and HPO4−2 should be tested. Dissolved inorganic ions may compete for the active sites on the TiO2 surface or deactivate the photocatalyst and, subsequently, decrease the degradation efficiency [38]. In the same approach, Chen et al. [39] found that the addition of H2PO4 and HCO3 significantly inhibited the degradation of Acid Orange 7 in the TiO2 system.

For IC photodegradation, the inhibition of decolorization is exhibited:

HCO3-< Cl-< HPO4-2< SO4-2

Inhibition effect of anions can be explained as the reaction of positive holes (hvb+) and resulting from the high reactivity and non-selectivity of OH toward non-target compounds present in the water matrix. The HCO3 and Cl ions were with less inhibition effect on IC decolorization. In TiO2/UV system, HCO3 can trap OH to produce CO3●−, which is less reactive [40]. This reaction appears to be of minor importance on the photodegradation of the IC. Additionally, in the case of Cl under neutral or alkaline conditions, the addition of Cl ion did not influence the reaction [41]. On the other hand, the SO4−2 ions demonstrated more inhibition on the decolorization rate because it is possible for a high competitive adsorption of the dye on the TiO2 surface, and they can trap both positive holes (h+) and OH [41].

2.2.4. Effect of temperature

The IC photodegradation is temperature dependent, and it decreases with the rise of solution temperature.

It is known that an increase in temperature can affect the efficiency of e/h+ recombination and adsorption/desorption processes of dye molecules on the TiO2 photocatalyst surface [42]. Some of the most important surface phenomena are dye molecule aggregation, tautomerization, and geometric (cis–trans) isomerization, and all those processes can be affected by temperature variation. The increase in solution temperature causes disaggregation of the dye molecules [24].

Habib et al. [24] considered MW and anion site (sulfate and a carboxylic group), which can interact with molecules by ion–dipole interactions. According to these authors, the dye has low MW and less anionic showed a significant variation under different temperatures. Consequently, the change of the temperature of the solution has a significant effect on the effective collisions between dyes and the TiO2 photocatalyst. In this case, the IC molecules possess a comparatively low MW (466.36 g mol−1) and only two anion sites (sulfate groups). Therefore, the variation of the temperature should also affect its photodegradation.

2.2.5. Effect of hydrogen peroxide addition

The principal problem found in most photocatalysis processes using TiO2 is the undesired electron/hole recombination, which represents the major energy-wasting step, thus restricting the feasible quantum yield of the photodegradation process. Hydrogen peroxide is an electron donor favoring the photocatalytic process inhibiting electron–hole recombination [43]. In this sense, the effect of H2O2 on the photocatalytic degradation of IC is examined. It was noted that when the hydrogen peroxide concentration increases, the photodegradation suffers an initial increase up to H2O2 0.022M and decrease at higher concentrations. Hydrogen peroxide at low concentration acts mainly as a source of OH and as an electron scavenger inhibiting the electron–hole recombination (Eq. 7) [44].

H2O2+ e-CBOH+ OH-

However, at higher concentrations, H2O2 reacts with OH and acts itself as a scavenger of the photoproduced holes. The H2O2 excess acts itself as a scavenger of the photoproduced holes. In this way, they behave like the self-quenching process of OH to form hydroperoxyl radicals (OOH) (Eq. 8), where oxidation potential is much lower than that of OH [45], leading to a decrease in the photocatalytic efficiency.

H2O2 + OH  H2O + OOH

The optimum dosage of H2O2 is variable and has been reported that it was dependent on the initial dye concentration.

2.2.6. Effect of Initial pH

The effect of pH changes on the photodegradation rate is studied in the range of pH 2–11. The photocatalytic reaction occurs on the surface of the catalyst and is dependent on TiO2 surface charge. In this way, the adsorptive properties of TiO2 particles depended strongly on the solution pH [46]. The degradation of IC is faster in acid solutions (i.e., pH ranges from 2 to 5). At neutral and basic medium, the rate of dye degradation is slow, nevertheless in accumulated energy of Quv 70 kJ L−1, all IC solutions are degraded. This effect can be explained by the surface charge density of TiO2. The point of zero charge (pzc) of TiO2 is at pH = 6.8. Thus, the TiO2 surface is fully protonated in medium acidic solution and negatively charged under alkaline conditions (Eqs. 9 and 10) [47]:

pH < pzc:TiOH + H+TiOH2+
pH < pzc:TiOH + OH-     TiO-+ H2O 

Considering the IC structure has sulfuric groups, which are negatively charged, at pH < 5.0, the positive charge through protonation (Eq. 9) on the photocatalyst surface promotes active interaction of a dye onto the catalyst surface and improves the photocatalytic degradation. On the other hand, for the basic solutions (pH > pzc), the surface of the catalyst is negatively charged through the proton abstraction by hydroxide ion (Eq. 10). Consequently, repulsions between a negative-charged surface of the catalyst and anionic dye fragments retard the surface adsorption, resulting in a low-photodegradation activity [48].

2.2.7. Effect of water matrix

In distilled water IC degradation is observed to be complete for an accumulated UV energy of 15 kJ L−1, correspondingly ~12 minutes of irradiation time. However, for complex matrices of water like freshwater and simulated municipal wastewater treatment plant (MWWTP) secondary effluent, the degradation of the dye is slower. The freshwater and simulated MWWTP secondary effluent required ~25 kJ L−1 for complete photodegradation. When a real MWWTP secondary effluent was carried out, the complete IC photodegradation occurred at accumulated energy around 33 kJ L−1. It is demonstrated that as the matrix more complex, the degradation rate of organic compounds is slower. This fact can be explained the presence of carbonate species on real wastewater that act as scavengers of the OH generated on photocatalysis [49], as showed in Figure 7. Table 1 exhibited a high concentration of inorganic carbon in the real effluent, which is widely found in real wastewater [50].

Type of water Inorg. Carbon (ppm) TOC (ppm) COD (ppm) pH Conduc. (µs cm-1) Ionic species (mM)
Na+ Ca+2 Mg+2 K+ NH+4 PO4 -3 Cl- SO4 -2
Fresh water13.893.4011.67.42060.760.360.440.1 0.090.04
Synthetic MWWTP secondary effluent9.9817.7657.598.02611.270.360.450.170.350.010.040.93
Real MWWTP secondary effluent69.9226.981.28.315048.

Table 1.

Characteristics physical and chemical of different water matrices [29]


Figure 7.

Photodegradation of Indigo carmine for different type of water mediated by 0.1 g L−1 of TiO2 suspensions [29].

The total degradation of organic dyes leads to the conversion of organic carbon into gaseous CO2, whereas nitrogen and sulfur heteroatoms are converted into inorganic ions, such as nitrate or ammonium and sulfate ions, respectively. In distilled water, the mineralization of the IC dye was completed around 90 kJ L−1 of accumulated energy. When complex matrices were tested, the mineralization was not complete. However, considering the theoretic TOC of the IC (12 mg L−1) and initial TOC of the matrices (see Table 1), the dye was totally mineralized even in the presence of scavenger species [29].


Figure 8.

Ions and carboxylic acid concentrations formed during photodegradation of Indigo carmine [29].

The structure of IC dye has two sulfonic groups attached to two aromatic rings, and these results indicate that SO4−2 ions are formed during the process. However, the sulfate ion concentration is lower than expected from theory stoichiometry, because by adsorption of SO4−2 at the surface of titanium dioxide [51]. The evolution of NH4+ suggests that this ion is the primary N-containing mineralization product. It indicates its origin as first products resulted from the initial attack on the carbon-to-carbon double bond of IC. The formic and oxalic acids remain for the UV-accumulated energy of 80 kJ L−1 indicating the evaluation of ecotoxicology tests, according to Figure 8.

2.2.8. Recycling of TiO2

One of today’s main industrial wastewater treatment strategies is focused on the development of green technologies and management practices for environmental benefit. To attend this “new”concept, the recycling of the photocatalyst should be performed. In this study, the TiO2 catalyst was recycled for consecutive reuse on this procedure up to eight times [34].

According to the results (Figure 9), the effectiveness of the TiO2 decreased from 98% (first cycle) to 80% (fifth cycle) and subsequently to 50% (sixth cycle onward). However, the rate of degradation was kept significant even after eight cycles of TiO2 reuse. The effectiveness of complete separation of photocatalysts from treated water is a critical step required to maintain a satisfactory degradation [52]. Moreover, agglomeration and sedimentation of IC around TiO2 particles after each cycle of photocatalytic degradation are a possible cause of the observed decrease in its efficiency [53]. This study further shows that the reuse of the TiO2 presents a promising photocatalytic performance with little variation of decay rate after eight consecutive usages and also high photochemical stability.


Figure 9.

Recycling and reuse of TiO2 on the photocatalytic degradation of Indigo carmine for an accumulated energy, Quv, of 15 kJ L−1 [34].

2.3. Supported TiO2 on photodegradation of Indigo carmine dye

In the large-scale applications, the use of TiO2 suspensions requires the separation and recycling of TiO2 particles, mainly of nanometric dimension, from treated wastewater before discharge into the water bodies. This fact can be a great drawback for the application of this treatment once it is a time-consuming and an expensive process. Alternatively, the catalyst may be immobilized on a suitable solid inert material, which eliminates the need for catalyst removal step [1] and permits its reuse for several times.

On another study [29], the solar CPC photocatalytic degradation of IC using TiO2 slurry for a treatment of real MWWTP secondary effluent demonstrated the greater efficiency of this procedure than with supported TiO2. Probably, this indicates that the catalyst coated on glass spheres is not fully illuminated. Indeed, there are large amounts of TiO2 particles inside the CPC tube, and a considerable amount of TiO2 supported on glass spheres having some sites not activated for photocatalysis and lower surface area of the catalyst. However, supported TiO2 has the enormous advantage of eliminating the catalyst removal step and thus reducing the costs of treatment considerably.

It should be emphasized that the decrease in the color of the IC solutions not provide complete data on the IC dye degradation. The TOC decrease suggests that during the irradiation processes of supported TiO2, a large number of low MW compounds are formed. Furthermore, hydroxylation of aromatic reaction products leads to cleavage of the aromatic ring, resulting in the formation of oxygen-containing aliphatic compounds [31]. On the other hand, after the fading stage, a breakdown of carbon-to-carbon double bond of IC may form inorganic ions. Formate, acetate, and oxalate are detected during degradation in real MWWTP secondary effluent containing IC as shown in Figure 10.


Figure 10.

Evaluation of the concentrations of carboxylic acids followed by ion chromatography; formed during solar CPC photodegradation of Indigo carmine with supported TiO2 in real MWWTP secondary effluent [34].

2.4. Decolorization of Indigo carmine dye using activated carbon/TiO2/UV

First, the use of activated carbon (AC) is an efficient water and wastewater treatment; the treatment is based on the use of adsorbent substrates and has many applications on textile dye’s wastewater treatments [54]. AC is probably the most versatile adsorbent because of its large surface area, polymodal porous structure, high adsorption capacity, and variable surface chemical composition. Furthermore, what makes ACs attractive to (textile) wastewater treatment is the possibility of tailoring their physical and/or chemical properties to optimize their performance on top of its, already powerful, strong hydrophobic and amorphous character [55]. The synergic role of AC with TiO2 is also a parameter to investigate once it opens the opportunity to combine adsorption and photocatalytic remediation. In this way, the decolorization of the model effluent containing indigo dye is revised using AC in the dark and under irradiation and AC/TiO2/UV.

The AC is used in two different situations. First, the AC in various concentrations is add in the solution of IC dye and kept in the dark. Furthermore, AC is submitted to the same conditions but kept under artificial irradiation during the all-time experiment. Similar decolorization rates are obtained when comparing experiments with different amounts of AC in the dark. Only in high concentrations, AC is able to remove the dye, but in this case, the determinant removal mechanism is adsorption of the dye in AC microporous structure. On the other hand, AC is not able to produce strong photocatalytic activity and had only adsorptive properties with an adsorption capacity of 28.5 mg IC per gram of carbon. The decolorization of AC/TiO2/UV following the same profile of TiO2/UV, but the constant K in TiO2/UV was by itself more efficiency when compared with AC/TiO2/UV. TiO2 activity decreased in the presence of AC not only because AC can absorb light—i.e., by reducing the light flux in the sample—but also because AC can adsorb TiO2, reducing the amount of photocatalyst available.

2.5. Ecotoxicological assessment

Photocatalytic degradation may generate toxic photoproducts. Thus, it is important to assess the toxicity of the solution after treatment. The ecotoxicological tests of IC and its photoproducts obtained by photocatalytic remediation treatment were evaluated. Two essays, two aquatic organisms, and one terrestrial organism were used. Aquatic organism represented by algae Pseudokirchneriella subcapitata (Chlorophyceae), a primary consumer Daphnia similis (Cladocera), and earthworm Eisenia andrei as terrestrial organism [29, 56, 57].

For all organisms, ecotoxicity tests are performed comparing the effects of solutions containing IC before and after photocatalytic treatment with TiO2. Chronic toxicity tests with P. subcapitata indicated no significant toxic effect for any tested samples (Table 2), but sample containing IC, the sample with photoproducts in pH 7 and sample with TiO2 filtered strongly stimulated algal growth indicating nutritional effects. Thus, the release of IC dye or its photoproducts into aquatic ecosystems may be expected to cause algal growth.

Samples P. subcapitata chronic test D. similis acute test
Mean ± SD
(#cells ml−1)
Observed Mortality (%) pH
Blank119± 5None07.8
Indigo carmine only315± 99164% growth207.1
Photoproducts generated by TiO2 photocatalysis (pH 4)113± 205% inhibition1004.2
Photoproducts generated by TiO2 photocatalysis (pH 7)395± 48231% growth1007.0
Indigo carmine + TiO2 without photodegradation (time 0)135± 1513% growth07.1
TiO2 + ultrapure water389± 37227% growth207.2

Table 2.

Number of Pseudokirchneriella subcapitata cells per ml after 72h exposition, and percent immobilization of Daphnia similis after 48h exposition [29]

The species D. similis had a different response. Acute toxicity tests showed that IC dye caused a low toxicity, but photoproducts are highly toxic (Table 2). Photoproducts in pH 4 and 7, respectively, caused a mortality of 100%. According to Vautier et al. [7], photocatalysis of IC produces mainly aromatic metabolites such as 2-nitrobenzaldehyde and anthranilic. Moreover, carboxylic acids fragments are also present, such as malic and tartaric acids [29]. On the other hand, studies demonstrated that in the presence of TiO2, absorption of some toxicants can be increased [29, 58]. In this approach, it a significant challenge to complete the removal of nano-TiO2 after releasing treated effluents to prevent its contamination of aquatic ecosystems [29].

Furthermore, terrestrial acute ecotoxicological tests with E. andrei earthworms are performed. The earthworms are affected by various organic and inorganic compounds, which may cause bioaccumulation, and their preliminary results serve as a rapid indicator of the toxicity of the compounds. Moreover, it can be used as a complementary test for risk assessment of polluted areas [59]. Effects of different IC concentration and its photoproducts on earthworms are studied. In all cases, no mortality is observed. No significant difference (p > 0.05) of a reduction in mean weight earthworms is found from the paper treated with different concentrations of IC. Toxicity tests with earthworms are also carried out for the photodegradation products of IC, and no mortality is observed after 24 h exposure to different treatments. These results suggest that the presence of IC and its photoproducts do not demonstrate an effect on the earthworms for acute contact test (24 h). However, more tests must be performed for a better understanding of the IC toxicity for E. andrei.

3. Conclusions

Photodegradation of IC by TiO2 was successful to remove colour from water; nevertheless, the degradation of IC in water with powdered TiO2 depends on various parameters. Among the systems evaluated, the Reactor 1 (125 W mercury vapour lamp) was the most efficient. Solar photocatalysis demonstrated better efficient in the summer but degraded the dye in the winter completely; then both seasons allow the solar photocatalysis efficient. CPC pilot plant photocatalysis simulated real situations of environmental remediation, reducing the duration and costs of the treatment. Moreover, a TiO2 catalyst supported on glass spheres proved to have high efficiency to remove IC in different water matrices even in the presence of various ions that acted as scavengers. Ecotoxicological tests revealed that photoproducts generated on photocatalysis promoted different biological responses to both tested organisms as growth effect on the algae and toxicity higher for D. similis. These results show the importance of photoproducts toxicity evaluation and the need for a complete removal process for TiO2 before its release in the environment.


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