Electro-oxidative decolorization and treatment of model wastewater containing Acid Blue 80 on boron doped diamond and platinum anodes
Introduction
Acid Blue 80, registered as CAS No. (4474-24-2) is a disodium salt of 3-[[9,10-dioxo-4-(2,4,6-trimethyl-3-sulfonatoanilino)anthracen-1-yl]amino]-2,4,6-trimethylbenzenesulfonic acid. This amino-anthraquinone dye, see structure in Scheme 1, is produced in bulk by 76 manufacturers. The majority of producers (67) come from China (e.g. Hubei Jusheng Technology Co., Ltd., Hubei xin bonus chemical Co., Ltd.) and a further five come from the USA (e.g. Alfa Chemistry, Honeywell International Inc.). Acid blue 80 is known under several commercial names, such as Alizarine blue BL; Alizarine fast blue R; Coomassie blue B; Nylosan blue F-L; Polar brilliant blue RAW. This dye is used in weak acid baths to dye wool, silk and polyamide. Using Acid Blue 80 results in a good to very good color stability. The dye can also be used in all stages of wool processing, where it is particularly useful for detailed wool finishing in various fiber blends [1,2].
Both the production and use of amino-anthraquinone dyes have a negative impact on the environment, especially due to residues in the wastewater. The problem lies in the colorization of wastewater traveling to the effluent pond, as well as in the toxicity of the dye and its degradation products. The decolorization of the produced wastewater can be approached in various ways. Biodegradation and decolorization of wastewater containing Acid Blue 80 is discussed in [[3], [4], [5], [6]]. For example, Briganti et al. used laccase and cellobiose dehydrogenase enzymes [6]. The problem can also be alleviated by adsorption of the dye on a suitable sorbent [[7], [8], [9], [10]].
Although the decomposition of Acid Blue 80 chromophore leads to substrate decolorization, this does not solve the toxicity of the intermediates produced. Advanced Oxidation Processes can achieve a thorough degradation or even mineralization of complex organic compounds including dyes [11]. The degradation of Acid Blue 80 has been demonstrated using TiO2-assisted photoelectrocatalysis on BDD electrodes [12], photoelectrolytic oxidation under visible light illumination on semiconducting WO3 film photoanode [13], or microwave-assisted degradation of Acid Blue 80 trapped on the surface of powdered activated carbon [14]. Other studies dealing with Acid Blue 80 focus on its decomposition using hydroxyl radicals in the reaction systems Fe2+/H2O2, Mn2+/HCO3−/H2O2, and Cu2+/phenanthroline/H2O2 [15], UV/H2O2 [16], and O3, Fe2+/H2O2 [17,18].
This study focuses on the decolorization and degradation of the anthraquinone dye Acid Blue 80 in model wastewater from its production and application, using indirect electro-oxidation in the presence of sodium chloride on the Pt and BDD anodes, following up on our previous work with Acid Blue 62 [19]. Sodium chloride was selected as a common electrolyte that helps to increase an insufficient conductivity of wastewater. It is commonly applied in dye baths to improve dye fixation. In addition, this salt is produced in the synthesis itself and is frequently added to the reaction mixture even later, during dye isolation using the salting-out effect. The fact remains that not all manufacturers use suitable nanofiltration membrane processes to decrease wastewater salinity [20,21]. That is why it seemed compelling to verify Acid Blue 80 degradation using indirect oxidation with chloride ions [22,23]. Using suitable electrodes such as BDD, it is possible to oxidize both Acid Blue 80 and the aromatic compounds produced after the breakdown of dye chromophore. These display a high degree of stability under UV–VIS radiation and high resistance to microbial attack, which makes them hard to break down in conventional wastewater treatment plants.
When waste is treated using indirect electrochemical oxidation, the polluting substance is blasted by an oxidizing agent generated in situ on anode. With chloride anions present in the aqueous environment, the anode firstly generates chlorine. This agent, probably the most commonly used oxidant in the water treatment industry, produces other chlorine-based oxidizing agents, dependent on environment temperature and pH [19,24,25], as shown in Fig. 19 ref. [22].
The selection of platinum and BDD anodes [26] was made with the intention of comparing the two different electrode materials with regard to their effectiveness in both the decolorization and oxidation of Acid Blue 80, as well as its mineralization in the model wastewater. This effectiveness was compared in the presence of chloride ions depending on the pH environment and applied current density. The use of an active Pt anode leads to a higher decolorization efficiency, yet it is usually accompanied by a lower substrate mineralization level, caused by the low number of hydroxyl radicals accumulated on its surface. This type of anode shows a high efficiency in the oxidation of chloride ions hence it can be used to process solutions of organic dyes through active forms of chlorine [27,28]. In contrast, the non-active BDD anode generates a high amount of hydroxyl radicals and when applied to the model solution of Acid Blue 80, it potentially leads to dye mineralization. Using this type of anode, several organic dyes have successfully been degraded to elementary molecules. In several cases, degradation has been observed by indirect oxidation in the presence of chlorine compounds. BDD is reported to be very efficient in the formation of secondary oxidants [29,30].
Section snippets
Chemicals
The anthraquinone dye Acid Blue 80 (commercial name is Rybacid Blue 150, purity >99%,) was supplied by Synthesia a.s. and was used without further treatment. All of the used chemicals were of the analytical grade purity and originated from Penta, Prague, Czech Republic if not stated otherwise. All solutions were prepared in deionized water with the electrolytic conductivity about 1.0 μS cm−1 and correspond to resistivity ~1 MΩ cm (Milli-Q Plus system, Millipore, USA) and were stored in the dark
Results and discussion
The aim of this work was to determine the decolorization rate of model wastewater containing Acid Blue 80 on an active Pt anode and an inactive BDD anode in the commonly occurring Cl− ion environment. Emphasis was placed on the effects of the environment's pH, current density, and the concentration of the Cl− supporting electrolyte. The effect of these parameters on AOX was also observed as a function of time.
The course of decolorization was followed by UV–VIS spectrometry. The wavelengths of
Conclusion
In the course of our experiments examining the decolorization and degradation of Acid Blue 80, we monitored the decolorization rate, current density, initial concentration of NaCl and the effect of pH. In order to evaluate the effect of different materials and anode efficiency, we selected an active platinum and non-active BDD. During oxidation on the platinum and BDD anodes, prepared using the HF CVD method over an 8-hour growth of strongly doped polycrystalline BDD thin film, we observed the
CRediT authorship contribution statement
Gabriela Kuchtová:Visualization, Investigation, Validation.Jaromíra Chýlková:Investigation.Jiří Váňa:Investigation.Marian Vojs:Resources.Libor Dušek:Conceptualization, Methodology, Investigation, Validation, Supervision, Writing - original draft.
Declaration of competing interest
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Electrochemical behavior of the textile dye Acid Blue 80 was firstly studied using boron doped diamond and platinum anodes.
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The influence of salts typical for the textile effluents, specifically chloride and sulfate ions, was discussed in relation to the use of indirect anodic oxidation.
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Oxidation and mineralization parameters COD and TOC as well as the formation of adsorbable organohalogenic compounds (AOX) in the presence of chloride ions were monitored during the process of electrolysis.
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Acknowledgements
This work was supported by The Ministry of Education, Youth and Sports of the Czech Republic (Project No. SGS_2019_001).
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