Electrocoagulation Applied for Textile Dye Oxidation Using Iron Slag as Electrodes


 The Indigo Blue dye is widely used in the textile industry, specifically in jeans dyeing, the effluents of which, rich in organic pollutants with recalcitrant characteristics, end up causing several environmental impacts, requiring efficient treatments. Several pieces of research have been conducted in search of effective treatment methods, among which is electrocoagulation. This treatment consists of an electrochemical process that generates its own coagulant by applying electric current on metallic electrodes, bypassing the use of other chemical products. The objective of this work was to evaluate the potential use of iron slag in the electrocoagulation of a synthetic effluent containing commercial dye Indigo Blue and the effluent from a textile factory. The quantified parameters were color, turbidity, pH, electrical conductivity, sludge generation, phenol removal, chemical oxygen demand (COD), and total organic carbon (TOC). The electrocoagulation treatment presented a good efficiency in removing the analyzed parameters, obtaining average removal in the synthetic effluent of 85 % of color and 100 % of phenol after 25 min of electrolysis. For the effluent from the textile factory, average reductions of 80 % of color, 91 % of turbidity, 100 % of phenol, 55 % of COD, and 73 % of TOC were measured after 60 min of electrolysis. The results obtained demonstrate the potential of using iron slag as an electrode in the electrocoagulation process in order to reuse industrial waste and reduce costs in the treatment and disposal of solid waste.


Introduction
The textile industry stands out as one of the most polluting sectors in terms of the volume and complexity of the e uents produced. The dyeing and nishing processes contribute signi cantly to the generation of wastewater through the use of dyes during the production process, promoting an e uent with speci c characteristics, such as intense color, high chemical oxygen demand (COD), a large amount of dissolved solids, and pH variation (Zaroual et al., 2006;Cerqueira et al., 2009;Mook et al., 2017).
Due to the presence of dyes, several environmental impacts may be caused when this e uent is discharged without meeting the regulatory requirements. One of these impacts occurs by partially reducing the penetration of sunlight into the aquatic environment, hindering the photosynthesis process and the production of oxygen by aquatic plants, thus directly affecting the amount of dissolved oxygen present in the environment, causing serious effects on the life cycle of sh (Chakraborty, 2014). Also, high concentrations of nitrogen, phenols, iron, and chromium, which end up harming aquatic life, and high Indigo Blue dye is widely used in the coloring of jeans (Abdelileh et al. 2020). The structural and chemical properties of indigo blue make it poorly soluble in water (Volkov et al., 2020;Yin et al., 2020). Thus, the ber dyeing process occurs through a series of oxidation-reduction processes: immersion in a sodium dithionite bath entails the reduction of the dye, which takes the leuco-indigo form, presenting high solubility in water and a nity with the ber for dyeing; after immersion, the ber is exposed to air to return to the original insoluble form (Albuquerque et al., 2013;Abdelileh et al., 2020;Hendaoui et al., 2021).
There are numerous physicochemical, chemical, and biological techniques for the treatment of these textile e uents. Due to the environmental implications, in addition to conventional treatments, research has emerged using new treatment technologies for the degradation of these compounds, among which electrochemical processes have stood out (Holkar et al., 2016;Bener et al., 2019).
Electrocoagulation is a widely applied technique for pollutant removal widely used in the treatment of textile e uents due to its environmental friendliness, versatility, energy e ciency, safety, and costeffectiveness. It uses simple equipment of easy operation, which facilitates its maintenance (Khandegar and Soroha, 2013). The electrochemical reactor is composed of an electrolyte and two electrodes (cathode and anode). Oxidation-reduction reactions occur at the electrodes through the application of a continuous electric current, and a coagulant is released into the e uent by the anode through the oxidation of the material. At the cathode, the electrode reduction process occurs, releasing hydroxyl ions to the medium (Gendel and Lahav, 2010). After this combination of reactions, water electrolysis occurs, producing oxygen at the anode and hydrogen at the cathode, forming microbubbles and causing the occulation of the particles present in the e uent (Mollah et al., 2004;Verma, 2017).
The most commonly used materials in electrochemical treatments are aluminum and iron; however, other materials have been used as electrodes in various studies, such as zinc, stainless steel, galvanized steel, steel, graphite, platinum, and diamond (Lee and Gagnon, 2015;Silva et al., 2016;Zazou et al., 2019), and industrial waste such as scrap metal may also be used. The foundry industry in Brazil in the rst half of 2018 was responsible for producing approximately 800,000 tons of castings, of which 600,000 tons represent the production of cast iron, a volume 6 % higher than in 2017 (Carmelio et al., 2018). In the process of casting metal parts, a solid residue called slag is produced. This residue is rich in iron and, because it has no other application, ends up in industrial land lls.
Aiming at the sustainability of the environment and cost reduction for the industry through using the waste generated and reducing land ll disposal, the present work shows the evaluation of the application of iron slag from a foundry as electrodes in an electrochemical process employed as a secondary treatment for color removal in synthetic and real e uents containing textile dye.

Electrodes
The iron slag was obtained at a foundry located in Erechim, RS, Brazil. The collected raw material needed to be broken manually for obtaining smaller parts to be used as electrodes (Fig. 1). After breaking, the raw scoria pieces were weighed, and cathode and anode pairs with similar masses were formed. The characterization of the raw iron slag has already been reported in previous work by Bragagnolo et al. (2018), and the main constituents of the material are presented in Table 1.

Electrochemical reactor
The reactor used for the treatment was made of glass with the dimensions 15 cm x 30 cm, totalizing a volume of 3 L of e uent per treatment (Fig. 2). The wires used were of a 6 mm gauge, stripped 10 cm at one end to wrap the slag and 5 cm at the other end to connect to the power source, being changed at g L -1 of NaCl was added to each treatment. All trials were performed at room temperature of 20 °C to 25°C (± 1.0 °C), and unadjusted pH from 7.5 to 8.7 (± 1.2) was used.

Synthetic e uent
The synthetic e uent consisted of commercial Indigo Blue dye, requiring grinding and sieving before use in order to achieve its homogenization. The concentration limits used were based on the literature, The experiments were performed following a 2² factorial experiment design (Table 2), analyzing the variables of electric current (0.3 A to 0.9 A) and dye concentration (50 mg L -1 to 80 mg L -1 ). The combination of these variables using experimental planning resulted in a total of eleven trials, as shown in Table 3. Table 2 Levels and variables used in the experimental design of the electrocoagulation process for the oxidation of textile dye from synthetic e uent.
Variables Level Dye Concentration (mg L -1 ) 50 58 65 73 80 Table 3 Matrix of the 2² factorial experiment design with coded values for the electrocoagulation process for the oxidation of textile dye from synthetic e uent.

Real textile e uent
The industrial textile e uent was obtained from a Jeans company located in Erechim, RS, Brazil. It was collected in an equalization tank after a sieve system. The experiments using the real e uent totalized fteen runs, and only the electric current was analyzed as a variable since the initial color concentration was determined in the e uent characterization stage. Thus, each current used in the real e uent (0.3 A to 0.9 A) was analyzed in triplicate to understand the in uence of the electric current on the electrochemical treatment. The electrolysis time was set at 1 h; after analyzing the color removal kinetics using the actual e uent, samples were collected every 10 min until the color removal stabilized (Fig. S2).

Analytical Methodology
The electric current applied to the treatments was based on the literature, ranging from 0.3 A to 0.9 A . The pieces of equipment used in this study are available in the electronic supplementary material 1 (Text S1). In addition to these parameters, sludge production and electrode wear were also analyzed using the Gravimetric Method. The statistical analysis was performed using software Minitab 15, and graphs were made using GraphPad Prism 8.

Electrocoagulation of the synthetic textile e uent
The results of applying the electrocoagulation process with iron slag electrodes to the synthetic e uent are presented in Table 4, showing the values of the initial and nal concentrations for color removal, according to the experimental design. One may observe an e cient color removal in the e uent, varying from 68 % to 95 %, presenting a removal average of 85 %. This variation in color removal may occur due to the composition of the iron slag electrodes because the concentrations of iron and other constituents of the electrodes are not known with accuracy since they are made of solid foundry residues and replaced at each new treatment; therefore, there may be variability in their composition. In the present study, the color removal e ciency using the proposed treatment was veri ed. Even with the complex composition of the iron slag electrodes, a high color removal e ciency reaching 95 % was obtained. In work developed by Verma (2017), electrocoagulation was applied to a synthetic e uent containing 200 mg L − 1 of dye using one iron and one aluminum electrode, resulting in 86 % color removal after 1 h of treatment.
One of the main parameters for the e ciency of the electrocoagulation treatment is the amount of coagulant generated, which is related to the applied electric current density and the electrolysis time removal from the synthetic e uent. This statement may be con rmed through the Pareto chart with 95 % con dence (Fig. 3).
The pH and electrical conductivity values obtained in the eleven tests performed are shown in Fig. 4a.
One may observe that the pH values after the electrocoagulation treatment are above the value allowed for discharging liquid e uents into water bodies (pH between 5 and 9) according to Resolution No. 430 of the Brazilian National Environment Council (CONAMA) (Conama, 2011).
In electrochemical processes, the pH increases during the treatment due to the generation of OH − ions during the water reduction step (Cerqueira et al., 2009). In electrocoagulation, the nal pH is high compared to the initial pH, corroborating the values obtained in this study. Cerqueira  The wear of the iron slag electrodes (Fig. 5) was determined by weighing them before and after the treatment. For there not to occur mass loss with the washing of the electrodes, the use of abrasive materials was avoided and, after each test, the electrodes only received distilled water to remove the generated sludge that could have adhered to the iron slag for subsequent weighing and disposal.
In the study by Zazouli and Taghavi (2012), the wear of the electrodes was remarkable at higher current densities. In this study using iron slag, the current density showed no effect on the electrode wear; however, it should be taken into account that the maximum limit of the current applied in this study was 9 A, unlike the maximum current of 25 A applied by Zazouli and Taghavi (2012).
The sludge production in the electrocoagulation process using iron slag electrodes in a synthetic e uent is represented in Fig. 6. One may observe the increase in sludge generated as the intensity of the electric current is raised; this behavior was expected since there is a direct relationship between the amount of sludge formed and the current applied in the electrochemical process. Studies by Zodi et al. (2009) and Chen (2004) con rm the increase observed in this study. The authors reported that, when using iron and aluminum electrodes in electrocoagulation, they obtained results where the sludge formation is directly related to the density of the current applied. The amount of sludge formed is also related to the volume of coagulant material produced with the removal of total suspended solids and other compounds present in the e uent.
A small phenol presence was observed with an initial concentration ranging from 0.3 mg L − 1 to 9.1 mg L − 1 ; these concentrations were 100 % removed. Despite the complexity of the composition of the iron slag used, the removal was very e cient for this type of pollutant. The study conducted by Silva et al. (2016) corroborates the results obtained in this study: the authors evaluated the removal of contaminants in a textile e uent through electrochemical degradation, obtaining approximately 100 % of phenol removal.
The TOC concentration was also evaluated initially and at the end of the trials. In the treatment, there occurred a TOC removal of approximately 18 %. The other assays did not present TOC removal. The initial TOC concentrations found ranged from 6 mg L − 1 to 14 mg L − 1 . For evaluating this parameter, it is necessary to consider the chromophore group breakdown; such chemical groups are responsible for the coloration of the dye, being the structure responsible for its xation to the textile ber (Paschoal and Temiliosi-Filho, 2005). The results obtained in the TOC analyses suggest that the electrocoagulation process may only be breaking this chromophore group bond and, therefore, only removing the color and not the organic matter present in the e uent considering the low TOC removal obtained.

Electrocoagulation in a real industrial textile e uent
The e uent collected from the textile industry was characterized in order to know its properties (Table 5). Through the application of the electrocoagulation process using iron slag electrodes in the industrial e uent, it was possible to observe the reduction of color, turbidity, TOC, and COD under each current applied. Figure 7 presents the mean values of the removals in triplicate.
The best results obtained for the removal of color and turbidity were observed with the smallest electric current applied (0.3 A): 80 % of color and 91 % of turbidity were removed. These values show that the applied electric current did not present signi cance in the electrocoagulation treatment under the evaluated conditions, with it being possible to use low electric currents and obtain high treatment e ciencies, saving costs with applied electric energy.
According to the CONAMA resolution No. 430, which provides for the liquid e uent discharge standards (Conama, 2011), the color parameter has no concentration limit, with the only condition being that it should not change the color of the water body. Thus, CONAMA resolution No. 357, which provides for the classi cation of water bodies and e uent discharge standards (Conama 2005), was used as a reference for discharge. For Brazilian Class I rivers, the natural true color of the water body is required, while for Class II the true color has a limit of 75 mg Pt L − 1 . The nal concentration values after the electrocoagulation treatment are all below the limit allowed for Class II rivers, which would allow the release of the treated e uent into the water body without the need for secondary treatment. The sludge production in the electrocoagulation process using iron slag electrodes in industrial e uents showed values directly related to the increase in electric current (Fig. 8).
A large standard deviation of the sludge production is noticeable. This is attributed to the intrinsic characteristics of iron slag since it is a solid waste of varied composition, making it di cult to predict sludge production. However, even with the high deviation observed, the amount of sludge generated is reduced, which would justify its application in the process.
When applying a high current density in the treatment, an increase in the anodic metal dissolution is observed and, consequently, an increase in the amount of sludge produced (Zodi et al., 2009). This reaction of increased sludge production is evident in the tests performed using iron slag: the higher the electric current density, the greater the amount of sludge produced. When studying the electrocoagulation process for the decolorization of textile wastewater, Bener et al. (2019) showed that the wear of the electrodes is directly related to the applied electric current density. However, when using iron slag, the observed behavior was not similar: the most considerable wear presented was at the intermediate current of 0.6 A (Fig. 9), possibly due to the characteristics of the slag itself.

Conclusions
The electrocoagulation process using iron slag as electrodes showed good e ciency in treating a synthetic e uent and an industrial e uent containing textile dye. This process presents a great advantage related to cost reduction due to the non-addition of inputs since it is a residue produced in the foundry industry, besides being environmentally and nancially interesting given that its application in e uent treatment would also reduce its disposal in industrial land lls, thus contributing to the foundry industry by reducing the cost of the disposal of this material.
Some points still have to be investigated in order to reach the release standards, such as the pH that was above the allowed by the Brazilian regulations in both analyzed e uents and the TOC removal, which did not show signi cant results in the evaluated tests with the synthetic e uent, requiring greater attention to the chromophore group. From the statistical analysis of the data obtained, it is observed that none of the variables evaluated (i.e., electric current and dye concentration) presented statistical signi cance in the treatment (p < 0.05).
The use of iron slag as an electrode was limited because it presents uncertainties in the concentration of iron in each fraction used in the treatment, which leads to varying results for each test performed. However, electrocoagulation is a method that has many strengths, thus requiring a better evaluation of the use of iron slag in the process for understanding its behavior in the electrochemical treatment.

Declarations
Ethics approval and consent to participate: Not applicable Consent for publication: Not applicable Availability of data and materials: All data generated or analysed during this study are included in this published article [and its supplementary information les].

Figure 1
Iron slag used as electrodes for the electrocoagulation system of textile dye and industrial e uent Figure 2 Schematic representation of the electrocoagulation system using iron slag electrodes applied for the oxidation of textile dye in synthetic and industrial e uents Figure 3 Pareto chart for the analysis of the experimental data obtained for the removal of color from the synthetic textile e uent by the electrocoagulation process using iron slag electrodes with 95 % con dence.

Figure 4
Initial and nal pH (a) and electrical conductivity (b) during the electrocoagulation process for the oxidation of textile dye in a synthetic e uent using iron slag electrodes.

Figure 5
Wear of iron slag electrodes in the electrocoagulation process for the oxidation of textile dye in a synthetic e uent.
Page 19/21 Sludge production by the electrocoagulation process for the oxidation of textile dye in a synthetic e uent using iron slag electrodes.

Figure 7
Removal color, turbidity, TOC, and COD by the electrocoagulation process of an industrial textile dye using iron slag electrodes.