Water Contaminated by Industrial Textile Dye: Study on Decolorization Process

Pierantonio De Luca; Paola Foglia; Carlo Siciliano; Jànos  B. Nagy; Anastasia Macario

doi:10.3390/environments6090101

De Luca, Pierantonio;Foglia, Paola;Siciliano, Carlo;Nagy, Jànos B.;Macario, Anastasia

2019-09-02 00:00:00

environments Article Water Contaminated by Industrial Textile Dye: Study on Decolorization Process 1 , 2 3 2 Pierantonio De Luca * , Paola Foglia , Carlo Siciliano , Jànos B. Nagy and 2 , Anastasia Macario * Dipartimento di Ingegneria Meccanica, Energetica e Gestionale. Università della Calabria, I-87036 Arcavacata di Rende (CS), Italy Dipartimento di Ingegneria per l’Ambiente ed il Territorio ed Ingegneria Chimica. Università della Calabria, I-87036 Arcavacata di Rende (CS), Italy Dipartimento di Farmacia e Scienze della Salute e della Nutrizione. Università della Calabria, I-87036 Arcavacata di Rende (CS), Italy * Correspondence: pierantonio.deluca@unical.it (P.D.L.); anastasia.macario@unical.it (A.M.); Tel.: +39-09-8449-6757 (P.D.L.); +39-09-8449-6704 (A.M.) Received: 25 July 2019; Accepted: 26 August 2019; Published: 2 September 2019 Abstract: This work aims to investigate possible interferences due to the presence of sodium carbonate on the photodegradation of the reactive Black 5 azoic dye, both in systems containing only titanium oxide and those containing titanium oxide and hydrogen peroxide. The role of hydrogen peroxide is explicitly treated. Sodium carbonate, in fact, is often present in the wastewater of textile industries as it is used in the ﬁber dyeing phases. The use of TiO nanoparticles is emphasized, and the possible danger is underlined. Each system was subjected to ultraviolet irradiation (UV) by varying the exposure time. After the photodegrading tests, the resulting solutions were analyzed by UV-vis spectrophotometry and High-Resolution Nuclear Magnetic Resonance to measure the residual concentrations of dye. The dye degradation curves and reaction rates for dierent UV exposure times were obtained and discussed as a function of the used additives. All the data are repeated three times, and they dier only by a maximum of 5%. The results indicated a reduction of about 50% of the initial concentration of Reactive Black 5 after 30 min under optimal experimental conditions. The NMR analysis indicated the formation of a series of aromatic structures that were generated by the UV-induced photochemical fragmentation of the original molecule. Keywords: reactive black-5; azo-dye; photodegradation; sodium carbonate; titanium dioxide; hydrogen peroxide 1. Introduction There are already many polluting substances that put the environment and human health at risk [1–6]. In order to improve the environmental conditions, many researchers have carried out numerous studies for the development of innovative materials that respect the environment [7–11]. Alongside the development of new materials, particular interests have been turned to the study of eco-friendly processes designed to purify the wastewater using sunlight, a renewable and clean energy. In this “pro environment” context, the photochemistry is having a great development through the application of photocatalysis, a natural phenomenon that is manifested by a chemical reaction that mimics the photosynthesis of plants to absorb and transform pollutants into harmless elements [12–17] in the presence of speciﬁc substances called photocatalysts. In this sense, in-depth studies conducted in recent years have shown that among the substances that can act as photocatalysts, the semiconductor oxides are the most ecient results in the promotion of the transformation reactions of the pollutants Environments 2019, 6, 101; doi:10.3390/environments6090101 www.mdpi.com/journal/environments Environments 2019, 6, 101 2 of 18 in the presence of light. Particularly appreciated for its excellent photocatalytic action is the titanium dioxide, especially in its crystallographic form of anatase [18–20]. Recently, however, one has tried to widen the range of substances that may be used as photocatalysts, and among the new species there are the zeolitic materials [21–29], and adsorption materials such as microporous materials and carbon nanotubes [30–36]. The use of nanoparticle TiO (average size of 15 nm) is inherently a dangerous process due to environmental release [37]. Many studies highlight nanoparticle emission due to nanocoatings [38] and tiles [39,40]. Cases of nanoparticle exposure in the ﬁeld of occupational hygiene in workplaces have been reported [41]. Knowing the large use of these nanoparticles [42] and the risk of the release of these types of products in the environment [43,44], one has to emphasize the exposure risk to the consumer [45]. These aspects are also addressed in relation to metrological challenges related to particle characterization [46]. In particular, textile and paper industries produce wastewater with high concentrations of harmful dyes that are very dicult to degrade. In fact, the eciency of the ﬁxation of dyes is between 60% and 90%, and approximately 1–20% of the dye production is lost during the staining process and released in wastewater. Thus, the development of eective approaches for the treatment of industrial wastewater containing pollutants is a challenge for the protection of the environment and human health. Azo dyes themselves have a minor environmental eect because they make euent streams highly colored. However, their precursors and degradation products, such as aromatic amines, can cause dermal and respiratory diseases, allergies, skin and mucous membrane irritation, and carcinogenesis [47,48]. In general, reactive azo dyes are degraded and removed from the aqueous environment by processes that are photocatalytic-based, and sonication-induced degradation treatments [49]. Zeolites [50–55] have been proposed as eective dye adsorbents. Immobilization techniques of reactive azo dyes on alkyl calixarene materials have also been studied [56– 58]. In this context, synthetic calixarene-based resins were found to be excellent [59]. Absorption by cellulose and synthetic polymeric materials, microgels, magnetic amine/metal oxide functionalized biopolymer resins, and anaerobic/aerobic membrane reactors have recently been investigated [60–63]. Photoelectrochemical treatments by an iron mesh double layer such as an anode, electrocoagulation using sacriﬁcial iron electrodes, or metal plates activated with carbon materials, have been proven to be ecient auxiliary methods for the remediation of azo dye polluted waters [64–66]. Sawdust, coal-based activated carbon, carbon char, and simple or functionalized multi-walled carbon nanotubes (MWCNTs) have been proposed as adsorbents [67,68]. Among the available nanomaterials, cellulose nanocrystals-reinforced keratin bioadsorbents, zero-valent nano-copper, protein nanoﬁbrils, and the loading of Ag/AgCl nanoparticles, have been proposed for the remediation of wastewater. These methods allow for the coagulation and/or homogeneous and heterogeneous oxidation of textile azo dyes through the Fenton reaction, which can also be carried out on nanotube arrays [69]. However, the limitations in using conventional methods for the treatment of dyes are the high resistance of dyes to biodegradation, the high water-solubility of these aromatic compounds, and their poor adsorption capabilities onto the sludge. Hence, most of these methods oer a lower eciency for degradation. As an alternative, studies on the applicability of naturally available adsorbents from agricultural and industry waste materials, fruit and plant waste, and natural inorganic material as bio-adsorbents have recently been reviewed [70]. Among these systems, surfactant modiﬁed barley straw, sun ﬂower seed shells, rice agricultural waste microbes, bacterial strains, crude enzymes, marine microorganisms and yeasts, and -cyclodextrins synergically combined with metal oxides, have been used for the aerobic decolorizing and biological degradation of azo dyes [71]. However, the eciency of these eco-friendly adsorption methods showed a high dependence on the pH of the medium in the treatment process. In general, all reported studies have proven that the treatment of textile dyeing wastewater by various combinations of biological methods might be inadequate. Traditional methods mainly provide a phase transfer of the contaminants from wastewater to the sludge. Alternatively, advanced oxidation processes are promising technologies that aim at the decolorizing and mineralization of a wide range of azo dyes into stable inorganic compounds, or, Environments 2019, 6, 101 3 of 18 at least, their transformation into biodegradable and harmless products. Many studies have been carried out for the design of innovative and eco-friendly processes and technologies based on the use of sunlight as a renewable and clean UV source to repair waters [72]. In these approaches, semiconductor oxides have been found to be the most ecient photocatalysts in the transformation of azo dyes in the presence of light. The potential of photocatalysis by TiO for the removal of dyes from aqueous solutions has been widely studied [73], and heterogeneous systems based on the use of TiO nanoparticles supported on dierent materials in the presence of UV irradiation have attracted interest due to their eciency [74,75]. However, low quantum yields limit the practical application of titanium dioxide photocatalytic processes. The photonic eciency in the decolorization of azo dyes can be improved by using eco-friendly redox additives, such as peroxymonosulfate or H O 2 2 under UV irradiation. These systems have been investigated with the aim of supplementing the low photonic eciency of conventional UV photocatalysis in the presence of TiO , and the eciency of these photooxidation techniques have clearly been highlighted [76–82], although possible interferences on the photodegradation of the azo dye, by salts such as sodium carbonate, have been little investigated. It is known that euents of textile dyeing industries contain signiﬁcant amounts of inorganic salts, such as chlorides and carbonates [83,84]. Inorganic anions occur naturally in wastewater or may be added to facilitate the dyeing process [85]. Thus, the presence of inorganic anions might play an important role in the photooxidation kinetics of dierent azo dyes. The research in the ﬁeld of treatment of waste water produced by textile industries is challenging. Today, the exploitation of bio-sustainable and chemical bio-compatible methods for the remediation of waters containing azo dyes needs new impulses. This study is focused on the photooxidation of Reactive Black 5, an azo dye that is commonly used in the textile industry. The process was carried out under UV irradiation, in the presence of TiO and the redox additive H O , in an aqueous solution 2 2 2 containing Na CO . The main objective of the present work was to investigate, by ultraviolet irradiation 2 3 (UV) laboratory-scale experiments, the inﬂuence of the additive H O and the inorganic salt Na CO 2 2 2 3 on the catalytic activity of TiO during the decolorization of Reactive Black 5 azo dye. The peroxide additive H O has been studied with the aim to improve the photodegradation process. The basic 2 2 salt sodium carbonate was used to investigate whether its presence might produce eects in the photodegradation of Reactive Black 5. Experiments were performed by simulating the composition of wastewater produced from the textile industry, which employs Na CO in the tissue color ﬁxing process. 2 3 The novelty of the present work consists in the investigation of the Reactive Black 5 degradation, when this dye is treated with well-known economic and environmentally friendly methods that can be modiﬁed by the presence of basic salts in waste water generated by the industrial color ﬁxing process. Being aware that the photocatalytic system TiO /H O /UV-light has been widely experimented on, 2 2 2 the use of inorganic salts as cheap, smart, and chemically bio-compatible partners of the above cited system has only seldom been reported and investigated. 2. Materials and Methods The azo-dye Reactive Black-5 (Sigma-Aldrech, Darmstadt, Germany) (Figure 1, empirical formula C H N Na O S , molecular weight 991.82) was used in all experiments. 26 21 5 4 19 6 Titanium dioxide (TiO ) (Anatase- 99.6% Alfa Aeser, Karlsruhe, Germany) was used as the photocatalytic agent (15 nm average particle size, surface area 50 m /g), together with hydrogen peroxide (30%(w/w), Sigma-Aldrich, Germany) and the basic inorganic salt sodium carbonate (Sigma-Aldrich, Darmstadt, Germany). All systems were irradiated with a UV lamp (100 W, = 365 nm). The intensity of irradiation was low (1 mW/cm ), because a recombination of electron-hole pairs is occurring at higher intensity [86]. The concentration of 30% w/w is chosen for H O solution, because an excess can have a 2 2 detrimental eect acting as a scavenger of Valence Band holes and OH, producing hydroperoxide radicals HO that have a less oxidizing power than OH [86]. 2 Environments 2019, 6, x FOR PEER REVIEW 3 of 18 products. Many studies have been carried out for the design of innovative and eco-friendly processes and technologies based on the use of sunlight as a renewable and clean UV source to repair waters [72]. In these approaches, semiconductor oxides have been found to be the most efficient photocatalysts in the transformation of azo dyes in the presence of light. The potential of photocatalysis by TiO2 for the removal of dyes from aqueous solutions has been widely studied [73], and heterogeneous systems based on the use of TiO2 nanoparticles supported on different materials in the presence of UV irradiation have attracted interest due to their efficiency [74,75]. However, low quantum yields limit the practical application of titanium dioxide photocatalytic processes. The photonic efficiency in the decolorization of azo dyes can be improved by using eco-friendly redox additives, such as peroxymonosulfate or H2O2 under UV irradiation. These systems have been investigated with the aim of supplementing the low photonic efficiency of conventional UV photocatalysis in the presence of TiO2, and the efficiency of these photooxidation techniques have clearly been highlighted [76–82], although possible interferences on the photodegradation of the azo dye, by salts such as sodium carbonate, have been little investigated. It is known that effluents of textile dyeing industries contain significant amounts of inorganic salts, such as chlorides and carbonates [83,84]. Inorganic anions occur naturally in wastewater or may be added to facilitate the dyeing process [85]. Thus, the presence of inorganic anions might play an important role in the photooxidation kinetics of different azo dyes. The research in the field of treatment of waste water produced by textile industries is challenging. Today, the exploitation of bio-sustainable and chemical bio-compatible methods for the remediation of waters containing azo dyes needs new impulses. This study is focused on the photooxidation of Reactive Black 5, an azo dye that is commonly used in the textile industry. The process was carried out under UV irradiation, in the presence of TiO2 and the redox additive H2O2, in an aqueous solution containing Na2CO3. The main objective of the present work was to investigate, by ultraviolet irradiation (UV) laboratory-scale experiments, the influence of the additive H2O2 and the inorganic salt Na2CO3 on the catalytic activity of TiO2 during the decolorization of Reactive Black 5 azo dye. The peroxide additive H2O2 has been studied with the aim to improve the photodegradation process. The basic salt sodium carbonate was used to investigate whether its presence might produce effects in the photodegradation of Reactive Black 5. Experiments were performed by simulating the composition of wastewater produced from the textile industry, which employs Na2CO3 in the tissue color fixing process. The novelty of the present work consists in the investigation of the Reactive Black 5 degradation, when this dye is treated with well-known economic and environmentally friendly methods that can be modified by the presence of basic salts in waste water generated by the industrial color fixing process. Being aware that the photocatalytic system TiO2/H2O2/UV-light has been widely experimented on, the use of inorganic salts as cheap, smart, and chemically bio-compatible partners of the above cited system has only seldom been reported and investigated. 2. Materials and Methods The azo-dye Reactive Black-5 (Sigma-Aldrech, Darmstadt, Germany) (Figure 1, empirical Environments 2019, 6, 101 4 of 18 formula C26H21N5Na4O19S6, molecular weight 991.82) was used in all experiments. NaO SOCH CH S N N SO Na 3 2 2 3 HO H N NaO SOCH CH S N N SO Na 3 2 2 3 Figure 1. Molecular structure of Reactive Black-5. At the end of the photocatalytic tests, the resulting solutions were ﬁltered by microﬁlters (0.45 m) and analyzed by UV-VIS spectrophotometry (UV-3100 PC-VWR), in order to measure the residual concentrations of dye as observable after photodegradation. All experiments were performed in triplicate, and the results were reported as the mean values. All graphics report the mean values obtained for all measurements. A signiﬁcant standard deviation less/equal to 5% was determined for all runs (not reported in the ﬁgures in order to avoid embedding). High-Resolution Nuclear Magnetic Resonance analysis (HR-NMR) methods used for the structural determination of organic compounds in complex mixtures [87,88] were also applied on samples of the starting water solutions and the resulting solutions obtained from the photodegrading process, in order to assess the eciency of the UV treatment. NMR experiments were performed with a Bruker Advance AC300 instrument, by ﬁxing the probe temperature at 25 C. Samples (480 L) of the aqueous solutions were spiked with a small amount of DMSO-d (20 L) as the spectral calibrating agent. All chemical shifts were reported in ppm and referenced to the signal of residual DMSO protons (2.50 ppm). The spectra were recorded as previously reported [89,90]. The isotherm curves and reaction rates were analyzed for dierent UV exposure times and discussed as a function of the additives used. Initially, an aqueous solution of Reactive Black-5 dye was prepared with a 0.003% weight-to-weight concentration, and the same solution was used to prepare dierent systems that were subsequently subjected to photodegradation under UV irradiation, during dierent times (Table 1). Table 1. Composition of the initial systems subjected to a photocatalytic test; Concentrations of the dye solution = 30 mg/L; H O solution= 30% (w/w). 2 2 System Composition 0 10 mL dye solution 1 10 mL dye solution + 0.01 g TiO 2 10 mL dye solution + (0.01 g TiO + 0.01 g Na CO ) 2 2 3 3 10 mL dye solution + 0.5 mL H O 2 2 4 10 mL dye solution + (0.01 g TiO + 0.5 mL H O ) 2 2 2 5 10 mL dye solution + (0.01 g TiO + 0.5 mL H O + 0.01 g Na CO ) 2 2 2 2 3 Initially, the azo-dye photostability was investigated either in the absence or in the presence of titanium dioxide, without adding other reagents. Further tests were carried out on the same systems containing TiO , after adding other additives such as sodium carbonate (determining in all cases a salt concentration of sodium carbonate equal to 1 g/L) and hydrogen peroxide. This was made in order to study and improve the photocatalytic activity of titanium dioxide. Each system was prepared by adding the required amounts of components in a batch reactor, operating at atmospheric pressure. The following order was applied for reactants: dye Environments 2019, 6, 101 5 of 18 solution, titanium oxide, hydrogen peroxide, and sodium carbonate. The systems were stirred at room temperature for two minutes. Subsequently, all aqueous systems were irradiated with a UV lamp for dierent times under Environm magnetic ents stirring. 2019, 6, x FOR Table PEER 1 reports REVIEW the compositions of all of the studied systems. 5 of 18 Environments 2019, 6, x FOR PEER REVIEW 5 of 18 3. Results and Discussion 3. Results and Discussion 3. Results and Discussion 3.1. UV Analysis 3.1. UV Analysis 3.1. UV Analysis The results were plotted to have a clear vision of the residual concentration of dye after the UV The results were plotted to have a clear vision of the residual concentration of dye after the UV irradiation, and the variation of pH. The plots obtained for all systems are shown below, together with The results were plotted to have a clear vision of the residual concentration of dye after the UV irradiation, and the variation of pH. The plots obtained for all systems are shown below, together the system coloring. irradiation, and the variation of pH. The plots obtained for all systems are shown below, together with the system coloring. with the system coloring. 3.1.1. System 0 (10 mL of Dye Solution) 3.1.1. System 0 (10 mL of Dye Solution) 3.1.1. System 0 (10 mL of Dye Solution) This system, containing only the dye and no photocatalytic agent, was initially tested to study This system, containing only the dye and no photocatalytic agent, was initially tested to study the natural photocatalytic stability of the dye (Figure 2). UV irradiation alone was not sucient to This system, containing only the dye and no photocatalytic agent, was initially tested to study the natural photocatalytic stability of the dye (Figure 2). UV irradiation alone was not sufficient to degrade the dye in suitable times. In fact, after 7 h of irradiation, the dye was degraded only to a 10% the natural photocatalytic stability of the dye (Figure 2). UV irradiation alone was not sufficient to degrade the dye in suitable times. In fact, after 7 h of irradiation, the dye was degraded only to a 10% extent. This proves the very limited tendency of the dye to photodegradation, at least in the absence of degrade the dye in suitable times. In fact, after 7 h of irradiation, the dye was degraded only to a 10% extent. This proves the very limited tendency of the dye to photodegradation, at least in the absence added photocatalytic agents. The pH value remained close to neutrality, and no color variations of the extent. This proves the very limited tendency of the dye to photodegradation, at least in the absence of added photocatalytic agents. The pH value remained close to neutrality, and no color variations of aqueous system were observed (Figure 3), conﬁrming the evidence for the photocatalytic resistance of of added photocatalytic agents. The pH value remained close to neutrality, and no color variations of the aqueous system were observed (Figure 3), confirming the evidence for the photocatalytic the dye. the aqueous system were observed (Figure 3), confirming the evidence for the photocatalytic resistance of the dye. resistance of the dye. System (0) System (0) 100 14 100 14 0 0 0 0 0 0.5 127 Time (h) 0 0.5 127 Time (h) Figure 2. Residual concentration (%) and pH for System 0, as a function of the UV irradiation Figure 2. Residual concentration (%) and pH for System 0, as a function of the UV irradiation time exposure. Figure 2. Residual concentration (%) and pH for System 0, as a function of the UV irradiation time exposure. time exposure. Figure 3. Variation of the color solution for System 0, as a function of the UV irradiation time exposure. Figure 3. Variation of the color solution for System 0, as a function of the UV irradiation time exposure. Figure 3. Variation of the color solution for System 0, as a function of the UV irradiation time exposure. 3.1.2. System 1 (10 mL of Dye Solution/0.01 g TiO2) 3.1.2. System 1 (10 mL of Dye Solution/0.01 g TiO ) 3.1.2. System 1 (10 mL of Dye Solution/0.01 g TiO2) The results obtained using TiO2 as the photocatalyst are reported in Figure 4. The results obtained using TiO as the photocatalyst are reported in Figure 4. The results obtained using TiO2 as the photocatalyst are reported in Figure 4. In the presence of TiO , a signiﬁcant dye photodegradation was observed. After 2 h of UV irradiation, the extent of dye photodegradation was at almost 80%. The UV irradiated aqueous solution showed an evident discoloration (Figure 5). As already observed for System 0, the pH value did not change signiﬁcantly. Residual cconcentration Residual cconcentration (%) (%) pH pH Environments 2019, 6, x FOR PEER REVIEW 6 of 18 System (1) Environments 2019, 6, 101 6 of 18 100 14 Environments 2019, 6, x FOR PEER REVIEW 6 of 18 System (1) 100 14 0 0 00.5 1 2 Time (h) 4 Figure 4. Residual concentration (%) and pH for System 1, as a function of the UV irradiation time exposure. 0 0 00.5 1 2 In the presence of TiO2, a significant dye photodegradation was observed. After 2 h of UV Time (h) irradiation, the extent of dye photodegradation was at almost 80%. The UV irradiated aqueous solution showed an evident discoloration (Figure 5). As already observed for System 0, the pH value Figure 4. Residual concentration (%) and pH for System 1, as a function of the UV irradiation Figure 4. Residual concentration (%) and pH for System 1, as a function of the UV irradiation time exposure. did not change significantly. time exposure. In the presence of TiO2, a significant dye photodegradation was observed. After 2 h of UV irradiation, the extent of dye photodegradation was at almost 80%. The UV irradiated aqueous solution showed an evident discoloration (Figure 5). As already observed for System 0, the pH value did not change significantly. Figure 5. Variation of the color solution for System 1, as a function of the UV irradiation time exposure. Figure 5. Variation of the color solution for System 1, as a function of the UV irradiation time exposure. 3.1.3. System 2 (10 mL Dye Solution + (0.01 g TiO + 0.01 g Na CO ) 2 2 3 3.1.3. System 2 (10 mL Dye Solution + (0.01 g TiO2 + 0.01 g Na2CO3) The study of this system, containing sodium carbonate, was undertaken taking into account that The study of this system, containing sodium carbonate, was undertaken taking into account Figure 5. Variation of the color solution for System 1, as a function of the UV irradiation time exposure. the basic inorganic salt is generally used to ﬁx the color on the textiles, since it induces a pH increase. that the basic inorganic salt is generally used to fix the color on the textiles, since it induces a pH In fact, it is important for the dye molecules to ﬁx up with the ﬁber during the industrial manufacturing increase. In fact, it is important for the dye molecules to fix up with the fiber during the industrial 3.1.3. System 2 (10 mL Dye Solution + (0.01 g TiO2 + 0.01 g Na2CO3) process. Therefore, it was reputed interesting to test any variation of the photoactivity of titanium manufacturing process. Therefore, it was reputed interesting to test any variation of the The study of this system, containing sodium carbonate, was undertaken taking into account dioxide by simulating aqueous environments containing Reactive Black-5 and sodium carbonate. photoactivity of titanium dioxide by simulating aqueous environments containing Reactive Black-5 that the basic inorganic salt is generally used to fix the color on the textiles, since it induces a pH The collected data (Figure 6), showed that the activity of titanium dioxide was not aected by and sodium carbonate. increase. In fact, it is important for the dye molecules to fix up with the fiber during the industrial the presence of sodium carbonate. In fact, the photodegradation activities of Systems 1 (TiO only) The collected data (Figure 6), showed that the activity of titanium dioxide was not affected by manufacturing process. Therefore, it was reputed interesting to test any variation of the and the presence 2 (TiO and of so Nadium carbonate. In CO ) were found fact, th to be similar e photodegradat . In this case, ion activi the pH tiesvalue of Systems 1 (TiO increased due 2 only) to the 2 2 3 photoactivity of titanium dioxide by simulating aqueous environments containing Reactive Black-5 and 2 (TiO2 and Na2CO3) were found to be similar. In this case, the pH value increased due to the presence of sodium carbonate. Although the color of the basic solution was aected by the presence of and sodium carbonate. presence of sodium carbonate. Although the color of the basic solution was affected by the presence Na CO , the discoloration trend after the UV irradiation was comparable to that shown by the system 2 3 The collected data (Figure 6), showed that the activity of titanium dioxide was not affected by of Na2CO3, the discoloration trend after the UV irradiation was comparable to that shown by the containing only TiO (Figure 7). Environments 2019, 6, x FOR PEER REVIEW 7 of 18 the presence of sodium carbonate. In fact, the photodegradation activities of Systems 1 (TiO2 only) system containing only TiO2 (Figure 7). and 2 (TiO2 and Na2CO3) were found to be similar. In this case, the pH value increased due to the presence of sodium carbonate. Although the color of the basic solution was affected by the presence System (2) of Na2CO3, the discoloration trend after the UV irradiation was comparable to that shown by the 100 14 system containing only TiO2 (Figure 7). 0 0 00.5 1 2 Tim (h) Figure 6. Residual concentration (%) and pH for System 2, as a function of the UV irradiation Figure 6. Residual concentration (%) and pH for System 2, as a function of the UV irradiation time exposure. time exposure. Figure 7. Variation of the color solution for System 2, as a function of the UV irradiation time exposure. 3.1.4. System 3 (10 mL Dye Solution + 0.5mL H2O2) System 3 contained H2O2, and no TiO2 and Na2CO3 were added. This system was tested in order to investigate the effect of the peroxide on the dye photodegradation. The system was maintained under UV irradiation for 2 h, and after this time the dye photodegradation was limited to 20% (Figure 8). Although no evident changes in color were observed (Figure 9), the use of hydrogen peroxide was not satisfactory for obtaining an appreciable diminution of the dye. The presence of hydrogen peroxide maintained the solution pH at 4. System (3) 100 14 0 0 00.5 1 2 Time (h) Figure 8. Residual concentration (%) and pH for System 3, as a function of the UV irradiation time exposure. Residual cocnentration (%) Residual concent Residual concent ration ( ration ( Residual concentration (%) %) %) pH pH pH pH Environments 2019, 6, x FOR PEER REVIEW 7 of 18 Environments 2019, 6, x FOR PEER REVIEW 7 of 18 System (2) System (2) 100 14 100 14 80 12 20 2 0 0 0 0 00.5 1 2 Tim (h) 00.5 1 2 Tim (h) Environments 2019, 6, 101 7 of 18 Figure 6. Residual concentration (%) and pH for System 2, as a function of the UV irradiation time exposure. Figure 6. Residual concentration (%) and pH for System 2, as a function of the UV irradiation time exposure. Figure 7. Variation of the color solution for System 2, as a function of the UV irradiation time exposure. Figure 7. Variation of the color solution for System 2, as a function of the UV irradiation time exposure. Figure 7. Variation of the color solution for System 2, as a function of the UV irradiation time exposure. 3.1.4. System 3 (10 mL Dye Solution + 0.5mL H O ) 2 2 3.1.4. System 3 (10 mL Dye Solution + 0.5mL H2O2) 3.1.4. System 3 (10 mL Dye Solution + 0.5mL H2O2) System 3 contained H O , and no TiO and Na CO were added. This system was tested in order 2 2 2 2 3 System 3 contained H2O2, and no TiO2 and Na2CO3 were added. This system was tested in order System 3 contained H2O2, and no TiO2 and Na2CO3 were added. This system was tested in order to investigate the eect of the peroxide on the dye photodegradation. The system was maintained to investigate the effect of the peroxide on the dye photodegradation. The system was maintained to investigate the effect of the peroxide on the dye photodegradation. The system was maintained under UV irradiation for 2 h, and after this time the dye photodegradation was limited to 20% (Figure 8). under UV irradiation for 2 h, and after this time the dye photodegradation was limited to 20% under UV irradiation for 2 h, and after this time the dye photodegradation was limited to 20% Although no evident changes in color were observed (Figure 9), the use of hydrogen peroxide was not (Figure 8). Although no evident changes in color were observed (Figure 9), the use of hydrogen (Figure 8). Although no evident changes in color were observed (Figure 9), the use of hydrogen satisfactory peroxide w for as not satisfact obtaining anoappr ry for obtaining an eciable diminution appreciab of the le d dye. imin The ution of the presencedye. The pre of hydrogen sence of peroxide peroxide was not satisfactory for obtaining an appreciable diminution of the dye. The presence of maintained hydrogen per the o solution xide maint pH ained the so at 4. lution pH at 4. hydrogen peroxide maintained the solution pH at 4. System (3) System (3) 100 14 100 14 0 0 0 0 00.5 1 2 00.5 1 2 Time (h) Time (h) Figure 8. Residual concentration (%) and pH for System 3, as a function of the UV irradiation time exposure. Figure 8. Residual concentration (%) and pH for System 3, as a function of the UV irradiation Figure 8. Residual concentration (%) and pH for System 3, as a function of the UV irradiation time exposure. Environm time exposur ents 2019, e. 6, x FOR PEER REVIEW 8 of 18 Figure 9. Variation of color for System 3, as a function of the UV irradiation time exposure. Figure 9. Variation of color for System 3, as a function of the UV irradiation time exposure. 3.1.5. System 4 (10 mL Dye Solution + (0.01 g TiO2 + 0.5mL H2O2) 3.1.5. System 4 (10 mL Dye Solution + (0.01 g TiO + 0.5mL H O ) 2 2 2 The addition of titanium oxide to the solution of System 3 determined a gradual decreasing of The addition of titanium oxide to the solution of System 3 determined a gradual decreasing of the the dye concentration (Figure 10). dye concentration (Figure 10). These results were compared to those reported in Figure 4 for System 1, containing only TiO as a System (4) photocatalyst. In this case, an increased activity of titanium dioxide was registered, since the solution 100 14 showed an evident decolorizing after only 30 min (0.5 h) of irradiation (Figure 11), while the pH value did not show appreciable variations (Figure 10). 0 0 00.5 1 2 Time (h) Figure 10. Residual concentration (%) and pH for System 4, as a function of the UV irradiation time exposure. These results were compared to those reported in Figure 4 for System 1, containing only TiO2 as a photocatalyst. In this case, an increased activity of titanium dioxide was registered, since the solution showed an evident decolorizing after only 30 minutes (0.5 h) of irradiation (Figure 11), while the pH value did not show appreciable variations (Figure 10). H2O2 is an oxidant species that positively influences the photocatalytic reaction, due to the formation of hydroxyl radicals °OH. The production of °OH radicals could be written as: TiO2–H2O2 → TiO2–°OH + °OH [86]. Figure 11. Variation of the color solution for System 4, as a function of the UV irradiation time exposure. The decolorizing, caused by the low concentration of the solution that still retains 30% of dye, was obtained after 1 h, and prolonged times of treatment were useless. As in the study case, the fully decolored water solution obtained after 1 h of UV exposure was subjected to a high resolution H NMR analysis, in order to evaluate the dye photoconversion extent. An aliquot of the water residue was placed in the NMR, and the calibrating agent was added. The recorded spectrum (Figure 12) showed a high complexity for the regions of aromatic and methylene protons. Residual cocnentration (%) Residual cocnentration (%) Residual concent Residual concent rat rat ion ion (%) (%) Residual concentration (%) pH pH pH pH pH Environments 2019, 6, x FOR PEER REVIEW 8 of 18 Figure 9. Variation of color for System 3, as a function of the UV irradiation time exposure. Environments 2019, 6, x FOR PEER REVIEW 8 of 18 3.1.5. System 4 (10 mL Dye Solution + (0.01 g TiO2 + 0.5mL H2O2) The addition of titanium oxide to the solution of System 3 determined a gradual decreasing of the dye concentration (Figure 10). System (4) 100 14 Figure 9. Variation of color for System 3, as a function of the UV irradiation time exposure. 3.1.5. System 4 (10 mL Dye Solution + (0.01 g TiO2 + 0.5mL H2O2) Environments 2019, 6, 101 8 of 18 The addition of titanium oxide to the solution of System 3 determined a gradual decreasing of the dye concentration (Figure 10). System (4) 100 14 0 0 00.5 1 2 Time (h) Figure 10. Residual concentration (%) and pH for System 4, as a function of the UV irradiation time exposure. These results were compared to those reported in Figure 4 for System 1, containing only TiO2 as a photocatalyst. In this case, an increased activity of titanium dioxide was registered, since the 0 0 solution showed an evident decolorizing after only 30 minutes (0.5 h) of irradiation (Figure 11), 00.5 1 2 while the pH value did not show appreciable variations (Figure 10). Time (h) H2O2 is an oxidant species that positively influences the photocatalytic reaction, due to the formation Figure 10. of Residual concentration (%) and p hydroxyl radicals °OH. The produc H for Syst tiem 4, as a on of °O function H radica ol fs the UV couldirradiation time be written as:exposure. TiO2–H2O2 Figure 10. Residual concentration (%) and pH for System 4, as a function of the UV irradiation → TiO2–°OH + °OH [86]. time exposure. These results were compared to those reported in Figure 4 for System 1, containing only TiO2 as a photocatalyst. In this case, an increased activity of titanium dioxide was registered, since the solution showed an evident decolorizing after only 30 minutes (0.5 h) of irradiation (Figure 11), while the pH value did not show appreciable variations (Figure 10). H2O2 is an oxidant species that positively influences the photocatalytic reaction, due to the formation of hydroxyl radicals °OH. The production of °OH radicals could be written as: TiO2–H2O2 → TiO2–°OH + °OH [86]. Figure 11. Variation of the color solution for System 4, as a function of the UV irradiation time exposure. Figure 11. Variation of the color solution for System 4, as a function of the UV irradiation time exposure. The decolorizing, caused by the low concentration of the solution that still retains 30% of dye, H O is an oxidant species that positively inﬂuences the photocatalytic reaction, due to the 2 2 was obtained after 1 h, and prolonged times of treatment were useless. As in the study case, the fully formation of hydroxyl radicals OH. The production of OH radicals could be written as: TiO –H O 2 2 2 decolored water solution obtained after 1 h of UV exposure was subjected to a high resolution H ! TiO – OH + OH [86]. NMR analysis, in order to evaluate the dye photoconversion extent. An aliquot of the water residue The decolorizing, caused by the low concentration of the solution that still retains 30% of dye, was placed in the NMR, and the calibrating agent was added. The recorded spectrum (Figure 12) Figure 11. Variation of the color solution for System 4, as a function of the UV irradiation time exposure. was obtained after 1 h, and prolonged times of treatment were useless. As in the study case, the fully showed a high complexity for the regions of aromatic and methylene protons. The decolorizing, caused by the low concentration of the solution that still retains 30% of dye, decolored water solution obtained after 1 h of UV exposure was subjected to a high resolution H NMR was obtained after 1 h, and prolonged times of treatment were useless. As in the study case, the fully analysis, in order to evaluate the dye photoconversion extent. An aliquot of the water residue was decolored water solution obtained after 1 h of UV exposure was subjected to a high resolution H placed in NMR the aNMR, nalysis, i and n order to the calibrating evaluate the agent dye photoconversi was added. The on extent. An al recorded spectr iquot of um the wa (Figur ter e resi 12due ) showed a was placed in the NMR, and the calibrating agent was added. The recorded spectrum (Figure 12) high complexity for the regions of aromatic and methylene protons. showed a high complexity for the regions of aromatic and methylene protons. The signal set appearing between 7.60 and 8.20 ppm, associated with the resonances of aromatic protons, indicated the formation of a series of aromatic structures that were generated by the UV-induced photochemical fragmentation of the original molecule. The photochemical diazotization of the dye produced a further set of not well resolved signals in the range between 3.00 and 5.50 ppm, due to the formation of a series of compounds characterized by dierently substituted short polar alkyl chains. All these signals were strongly overlapped by the water proton signals at 4.50 ppm. In general, a complete recognition of all polar compounds arising from the photochemical disruption of bonds sensible to UV irradiation could be performed by applying an in situ chemical functionalization together with a suitable instrumental analysis and theoretical data treatments [91–94]. In this case, all polar compounds originating from the dye photo-induced dissociation are water soluble; thus, the pre-analysis chemical functionalization of the sample could be unsatisfactory, due to the possible non-complete recovery of products that are formed in low concentrations, and to the unwanted hydrolysis of functionalizing reagents. As a consequence, NMR spectroscopy only gave information about the qualitative composition of the samples, and no other speciﬁc data related to the structure of any single compound were obtained. NMR was used as a qualitative method in order to establish any possible change of the resonances attributable to the residual dyes in the solutions, and to detect newly formed degradation products coming from the decolorizing process. Residual concentration (%) Residual concentration (%) pH pH Environments 2019, 6, 101 9 of 18 Environments 2019, 6, x FOR PEER REVIEW 9 of 18 1 1 Figure 12. H NMR spectrum recorded for a sample obtained from the experiment performed with Figure 12. H NMR spectrum recorded for a sample obtained from the experiment performed with System 4. System 4. The signal set appearing between 7.60 and 8.20 ppm, associated with the resonances of aromatic 3.1.6. System 5 (10 mL Dye Solution + (0.01 g TiO + 0.5mL H O + 0.01 g Na CO ) 2 2 2 2 3 protons, indicated the formation of a series of aromatic structures that were generated by the System 5 was prepared by adding sodium carbonate to System 4. The basic inorganic salt was UV-induced photochemical fragmentation of the original molecule. The photochemical used in order to evaluate its inﬂuence in water environments where titanium dioxide and hydrogen diazotization of the dye produced a further set of not well resolved signals in the range between 3.00 peroxide could simultaneously be present. and 5.50 ppm, due to the formation of a series of compounds characterized by differently substituted The short polar resultsalkyl ch reported ain in s. A Figur ll these e 13sign show als were that sodium strongly over carbonate lapped by the water proton sig did not aect the photodegradation nals at 4.50 ppm. In general, a complete recognition of all polar compounds arising from the photochemical process in the presence of TiO and H O . An increase of the turbidity and pH value of the solution 2 2 2 disruption of bonds sensible to UV irradiation could be performed by applying an in situ chemical was observed, as expected due to the presence of sodium carbonate (Figure 14). Environments 2019, 6, x FOR PEER REVIEW 10 of 18 functionalization together with a suitable instrumental analysis and theoretical data treatments [91–94]. In this case, all polar compounds originating from the dye photo-induced dissociation are water soluble; thus, the pre-analysis chemical functionalization of the sample could be System (5) unsatisfactory, due to the possible non-complete recovery of products that are formed in low 100 14 concentrations, and to the unwanted hydrolysis of functionalizing reagents. As a consequence, NMR spectroscopy only gave information about the qualitative composition of the samples, and no other specific data related to the structure of any single compound were obtained. NMR was used as a qualitative method in order to establish any possible change of the resonances attributable to the residual dyes in the solutions, and to detect newly formed degradation products coming from the decolorizing process. 3.1.6. System 5 (10 mL Dye Solution + (0.01 g TiO2 + 0.5mL H2O2 + 0.01 g Na2CO3) System 5 was prepared by adding sodium carbonate to System 4. The basic inorganic salt was used in order to evaluate its influence in water environments where titanium dioxide and hydrogen peroxide could simultaneously be present. 0 0 The results reported in Figure 13 show that sodium carbonate did not affect the 00.5 1 2 Time (h) photodegradation process in the presence of TiO2 and H2O2. An increase of the turbidity and pH value of the solution was observed, as expected due to the presence of sodium carbonate (Figure 14). Figure 13. Residual concentration (%) and pH for System 5, as a function of the UV irradiation time exposure. Figure 13. Residual concentration (%) and pH for System 5, as a function of the UV irradiation time exposure. Figure 14. Variation of the color solution for System 5, as a function of the UV irradiation time exposure. 3.2. Comparison Between Systems The data plotted in Figures 15–18 report the amount of degraded dye/amount of titanium dioxide ratios, as a function of the UV time exposure. In particular, each figure compares this relation between the analyzed systems. The scope is to study the role played by additives, hydrogen peroxide and sodium carbonate, on the titanium dioxide activity in the reactive black-5 photodegradability. 0.03 0.025 0.02 0.015 0.01 0.005 0 0.5 1 1.5 2 2.5 Time (h) System 2: TiO2/Na2CO3 Figure 15. Comparison of the dye degradation curves for Systems 1 and 2. g / g degraded dye TiO2 Resdual concentration (%) pH Environments 2019, 6, x FOR PEER REVIEW 10 of 18 Environments 2019, 6, x FOR PEER REVIEW 10 of 18 System (5) 100 14 System (5) 100 14 0 0 00.5 1 2 0 0 Time (h) 00.5 1 2 Time (h) Figure 13. Residual concentration (%) and pH for System 5, as a function of the UV irradiation time exposure. Environments 2019, 6, 101 10 of 18 Figure 13. Residual concentration (%) and pH for System 5, as a function of the UV irradiation time exposure. Figure 14. Variation of the color solution for System 5, as a function of the UV irradiation time exposure. Figure 14. Variation of the color solution for System 5, as a function of the UV irradiation time exposure. Figure 14. Variation of the color solution for System 5, as a function of the UV irradiation time exposure. 3.2. Comparison Between Systems 3.2. Comparison between Systems 3.2. Comparison Between Systems The data plotted in Figures 15–18 report the amount of degraded dye/amount of titanium The data plotted in Figures 15–18 report the amount of degraded dye/amount of titanium dioxide The data plotted in Figures 15–18 report the amount of degraded dye/amount of titanium dioxide ratios, as a function of the UV time exposure. In particular, each figure compares this ratios, as a function of the UV time exposure. In particular, each ﬁgure compares this relation between dioxide ratios, as a function of the UV time exposure. In particular, each figure compares this relation between the analyzed systems. The scope is to study the role played by additives, hydrogen the analyzed systems. The scope is to study the role played by additives, hydrogen peroxide and relation between the analyzed systems. The scope is to study the role played by additives, hydrogen peroxide and sodium carbonate, on the titanium dioxide activity in the reactive black-5 sodium carbonate, on the titanium dioxide activity in the reactive black-5 photodegradability. peroxide and sodium carbonate, on the titanium dioxide activity in the reactive black-5 photodegradability. photodegradability. 0.03 0.03 0.025 0.025 0.02 0.02 0.015 0.015 0.01 0.01 0.005 0.005 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 Time (h) Time (h) System 2: TiO2/Na2CO3 System 2: TiO2/Na2CO3 Figure 15. Figure 15. Comparison of th Comparison of the dye e dye degradat degradation cu ion curves rves for for Sy System stem s 1 s 1 and and 2. 2. Figure 15. Comparison of the dye degradation curves for Systems 1 and 2. Environments 2019, 6, x FOR PEER REVIEW 11 of 18 0.03 0.025 0.02 0.015 0.01 0.005 00.5 11.522.5 Time (h) System 4: TiO2/H2O2 Figure 16. Comparison of the dye degradation curves for Systems 4 and 5. Figure 16. Comparison of the dye degradation curves for Systems 4 and 5. Figures 15 and 16 compare systems that differ only in presence or absence of sodium carbonate. It can be observed that sodium carbonate does not make a clear improvement in photodegradation, but can be considered as an inert agent, and in fact the curves are almost overlapping. Instead, Figures 17 and 18 below compare systems that differ only in the presence or absence of hydrogen peroxide. 0.03 0.025 0.02 0.015 0.01 0.005 00.511.522.5 Time (h) System 1: TiO2 System 4: TiO2 / H2O2 Figure 17. Comparison of the dye degradation curves for Systems 1 and 4. g / g degraded dye TiO2 g / g g / g degraded dye TiO2 degraded dye TiO2 g / g degraded dye TiO2 Resdual concentration (%) Resdual concentration (%) pH pH Environments 2019, 6, x FOR PEER REVIEW 11 of 18 0.03 0.025 0.02 0.015 0.01 0.005 00.5 11.522.5 Time (h) System 4: TiO2/H2O2 Figure 16. Comparison of the dye degradation curves for Systems 4 and 5. Figures 15 and 16 compare systems that differ only in presence or absence of sodium carbonate. It can be observed that sodium carbonate does not make a clear improvement in photodegradation, but can be considered as an inert agent, and in fact the curves are almost overlapping. Instead, Environments 2019, 6, 101 11 of 18 Figures 17 and 18 below compare systems that differ only in the presence or absence of hydrogen peroxide. 0.03 0.025 0.02 0.015 0.01 0.005 00.511.522.5 Time (h) System 1: TiO2 System 4: TiO2 / H2O2 Figure 17. Comparison of the dye degradation curves for Systems 1 and 4. Figure 17. Comparison of the dye degradation curves for Systems 1 and 4. Environments 2019, 6, x FOR PEER REVIEW 12 of 18 0.03 0.025 0.02 0.015 0.01 0.005 00.511.5 22.5 Time (h) System 2: TiO2 / Na2CO2 System 5: TiO2 / Na2CO3/ H2O2 Figure 18. Comparison between the dye degradation curves for Systems 2 and 5. Figure 18. Comparison between the dye degradation curves for Systems 2 and 5. This comparison confirmed that the presence of hydrogen peroxide, in all systems where Figures 15 and 16 compare systems that dier only in presence or absence of sodium carbonate. titanium dioxide was added, accelerated the dye photodegradation process. The observed trend was It can be observed that sodium carbonate does not make a clear improvement in photodegradation, but most noticeable in the interval between 0.5 and 1 h of UV exposure. Finally, the reactivity of the two can be considered as an inert agent, and in fact the curves are almost overlapping. Instead, Figures 17 systems did not show modifications after two hours of irradiation. The graphs reported in Figure 19 and 18 below compare systems that dier only in the presence or absence of hydrogen peroxide. show the change of residual molar concentrations of the dye on a logarithmic scale as a function of This comparison conﬁrmed that the presence of hydrogen peroxide, in all systems where titanium the UV exposure time. The kinetic results obtained for all Systems 1–5 are reported in Table 2. dioxide was added, accelerated the dye photodegradation process. The observed trend was most noticeable in the interval between 0.5 and 1 h of UV exposure. Finally, the reactivity of the two systems System 1: TiO 2 System2: TiO -Na CO did not show modiﬁcations after two hours of irradiation. The graphs reported in Figure 19 show 2 2 3 -10 -10 the change of residual molar concentrations of the dye on a logarithmic scale as a function of the UV exposure time. The kinetic results obtained for all Systems 1–5 are reported in Table 2. -10.5 -10.5 -11 -11 -11.5 -11.5 -12 -12 -12.5 -12.5 01 23 UV Time (h) UV Time (h) System 4: TiO -H O 2 2 2 System 3: H O 2 2 -10.4 -10 -10.45 -10.5 -10.5 -11 -10.55 -11.5 -10.6 -12 -10.65 -12.5 012 3 0 123 UV Time (h) UV Time (h) ln C ln c g / g degraded dye TiO2 g / g degraded dye TiO2 g / g degraded dye TiO2 ln C ln C Environments 2019, 6, x FOR PEER REVIEW 12 of 18 0.03 0.025 0.02 0.015 0.01 0.005 00.511.5 22.5 Time (h) System 2: TiO2 / Na2CO2 System 5: TiO2 / Na2CO3/ H2O2 Figure 18. Comparison between the dye degradation curves for Systems 2 and 5. This comparison confirmed that the presence of hydrogen peroxide, in all systems where titanium dioxide was added, accelerated the dye photodegradation process. The observed trend was most noticeable in the interval between 0.5 and 1 h of UV exposure. Finally, the reactivity of the two systems did not show modifications after two hours of irradiation. The graphs reported in Figure 19 show the change of residual molar concentrations of the dye on a logarithmic scale as a function of Environments 2019, 6, 101 12 of 18 the UV exposure time. The kinetic results obtained for all Systems 1–5 are reported in Table 2. System 1: TiO System2: TiO -Na CO 2 2 3 -10 -10 -10.5 -10.5 -11 -11 -11.5 -11.5 -12 -12 -12.5 -12.5 01 23 UV Time (h) UV Time (h) System 4: TiO -H O 2 2 2 System 3: H O 2 2 -10.4 -10 -10.45 -10.5 -10.5 -11 -10.55 -11.5 -10.6 -12 -10.65 -12.5 012 3 0 123 Environments 2019, 6, x FOR PEER REVIEW 13 of 18 UV Time (h) UV Time (h) System5: TiO -H O -Na CO 2 2 2 2 3 -10 -10.5 -11 -11.5 -12 -12.5 00.511.522.5 UV Time (h) Figure 19. Residual molar concentrations of the dye versus the UV irradiation exposure time Figure 19. Residual molar concentrations of the dye versus the UV irradiation exposure time (logarithmic scale). (logarithmic scale). Table 2. Kinetic results for Systems 1–5. Table 2. Kinetic results for Systems 1–5. Additives K (h ) ln C R −1 2 Additives K (h ) ln C R System 1 TiO 0.80 10.5 0.992 System 1 TiO2 0.80 10.5 0.992 System 2 TiO –Na CO 0.81 10.5 0.973 2 2 3 System 2 TiO2–Na2CO3 0.81 10.5 0.973 System 3 H O 0.08 10.4 0.823 2 2 System 3 H2O2 0.08 10.4 0.823 System 4 TiO –H O 0.87 10.6 0.952 2 2 2 System 5 TiO –H O –Na CO 0.85 10.6 0.948 System 4 TiO2–H2O2 0.87 10.6 0.952 2 2 2 2 3 System 5 TiO2–H2O2–Na2CO3 0.85 10.6 0.948 Except for System 3, which was subjected to a photodegradation process assisted only by H O , 2 2 Except for System 3, which was subjected to a photodegradation process assisted only by H2O2, all other Systems’ study cases can be carefully described by a ﬁrst order kinetics. For Systems 1 and 2, all other Systems’ study cases can be carefully described by a first order kinetics. For Systems 1 and the kinetic parameters were found to be similar, which means that the presence of sodium carbonate 2, the kinetic parameters were found to be similar, which means that the presence of sodium carbonate did not modify the reactivity. Systems 4 and 5 showed an increased reactivity with respect to Systems 1 and 2 due to the presence of H2O2. The rate constant increase ranged from 6 to 9%. The rate constant of System 3 was ten times smaller than that observed for all other Systems, although the reaction cannot appropriately be described by applying a pseudo-first order rate model. 4. Conclusions From the set of results obtained, it is clear that the Reactive Black 5 azo dye in the absence of photocatalytic agents is fairly photostable; in fact, after thirty minutes of UV radiation, a reduction of only 8.9% is recorded. Even in the presence of hydrogen peroxide only, the dye does not change its photostability much, which is around 11.2% after thirty minutes of UV radiation. Titanium oxide instead plays a fundamental role in the UV-induced photodegradation of Reactive Black 5 azo dye, and the photocatalytic activity of titanium dioxide can be improved through the simultaneous use of hydrogen peroxide. The use of only hydrogen peroxide is not sufficient for the complete dye photodegradation, although the presence of peroxides improved the photoactivity of titanium dioxide. The photocatalytic process with titanium dioxide could be limited by the fast recombination of electron-hole pairs, and the addition of hydrogen peroxide to water systems promoted the dye photodegradation. The hydrogen peroxide, in fact, is used to increase the amount of hydroxyl radicals, which are responsible for the UV-induced degradation. Comparing the system with only titanium oxide and the system with titanium oxide and hydrogen peroxide after thirty minutes of UV irradiation, the percentage reduction of dye is respectively 40.3% and 50.1%, therefore with an improvement of about 10% with respect to the system with only titanium oxide. Adding sodium carbonate to water systems does not exert a significant impact on the photodegradation of the dye. In this case, only an increase of the solution’s turbidity is observed. The observed decolorizing of water solutions containing the Reactive Black 5 dye is mainly due to ln C ln c g / g degraded dye TiO2 ln C ln C ln C Environments 2019, 6, 101 13 of 18 did not modify the reactivity. Systems 4 and 5 showed an increased reactivity with respect to Systems 1 and 2 due to the presence of H O . The rate constant increase ranged from 6 to 9%. The rate constant of 2 2 System 3 was ten times smaller than that observed for all other Systems, although the reaction cannot appropriately be described by applying a pseudo-ﬁrst order rate model. 4. Conclusions From the set of results obtained, it is clear that the Reactive Black 5 azo dye in the absence of photocatalytic agents is fairly photostable; in fact, after thirty minutes of UV radiation, a reduction of only 8.9% is recorded. Even in the presence of hydrogen peroxide only, the dye does not change its photostability much, which is around 11.2% after thirty minutes of UV radiation. Titanium oxide instead plays a fundamental role in the UV-induced photodegradation of Reactive Black 5 azo dye, and the photocatalytic activity of titanium dioxide can be improved through the simultaneous use of hydrogen peroxide. The use of only hydrogen peroxide is not sucient for the complete dye photodegradation, although the presence of peroxides improved the photoactivity of titanium dioxide. The photocatalytic process with titanium dioxide could be limited by the fast recombination of electron-hole pairs, and the addition of hydrogen peroxide to water systems promoted the dye photodegradation. The hydrogen peroxide, in fact, is used to increase the amount of hydroxyl radicals, which are responsible for the UV-induced degradation. Comparing the system with only titanium oxide and the system with titanium oxide and hydrogen peroxide after thirty minutes of UV irradiation, the percentage reduction of dye is respectively 40.3% and 50.1%, therefore with an improvement of about 10% with respect to the system with only titanium oxide. Adding sodium carbonate to water systems does not exert a signiﬁcant impact on the photodegradation of the dye. In this case, only an increase of the solution’s turbidity is observed. The observed decolorizing of water solutions containing the Reactive Black 5 dye is mainly due to the photo-induced cleavage of the azo bond in its structure, aording several dierently hydroxylated by-products. In all experiments, it can be supposed that dye molecules are oxidized by hydroxyl radicals generated by the TiO -assisted photocatalytic process in the presence of H O as an additive 2 2 2 under UV irradiation [95,96]. This process is also pH-sensitive, and the mechanism has already been reported [97]. As a consequence, the photo-degradation mechanism was not further investigated. Although the photo-degradation was not performed on acidic pH values, the basic conditions adopted for the set of experiments, in order to simulate the textile industry dye ﬁxing process, highlighted that the action of the system TiO /H O /CO was not accelerated when compared with the same 2 2 2 3 process performed in the absence of the basic salt. By considering the anionic form of the Reactive Black 5 dye at a basic pH, the formation of an excess of anionic charges on the photocatalyst surface was supposed to be responsible for the kinetics observed in the case of water samples that did not contain the inorganic salt. This evidence was ﬁnally supported by the literature data [97]. Moreover, the time of the UV exposure can play a fundamental role. In fact, a more prolonged UV irradiation increases the percentage of dye photodegradation. Therefore, this method can be proposed for the elimination of other dyes used in the industry. Moreover, it could be coupled with the use of adsorbents (zeolites, carbon nanotubes) in order to recover the dyes one could use for further applications. Author Contributions: Conceptualization, P.D.L.; Data curation, P.F.; Formal analysis, C.S., P.F.; Methodology, P.D.L., C.S.; Supervision, P.D.L., A.M. and J.B.N.; Writing—original draft, P.D.L. Funding: This research received no external funding. 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Water Contaminated by Industrial Textile Dye: Study on Decolorization Process

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Publisher: Multidisciplinary Digital Publishing Institute
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DOI: 10.3390/environments6090101
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Water Contaminated by Industrial Textile Dye: Study on Decolorization Process

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