Textile effluents contain triazine-substituted reactive dyes that cause health problems such as cancer, birth defects, and hormone damage. An electrochemical process was employed effectively to degrade azo reactive dye with the aim of reducing the production of carcinogenic chemicals during biodegradation. Textile dye C.I. Reactive Red 2 (RR2), a model pollutant that contains dichloro triazine ring, was subjected to the electrocoagulation process using aluminium (Al) electrodes. A maximum of 97% of colour and 72% of chemical oxygen demand (COD) removal efficiencies were achieved and 9.5 kWh/kg dye electrical energy and 0.8 kg Al/kg dye electrode consumption were observed. The dye removal mechanism was studied by analysing the results of UV-Vis spectra of RR2 and treated samples at various time intervals during electrocoagulation. Fourier transform infrared (FTIR) spectra and energy dispersive X-ray (EDX) spectral studies were used for analysing the electrocoagulated flocs. The results indicate that in this process the dye gets removed by adsorption and there is no significant carcinogenic by-product formation during the degradation of dye.
Textile industries use different types of fabrics for dyeing and produce large quantities of wastewater. Approximately 1,000–3,000 m3 of wastewater is produced for every 10,886.2–18,143.7 kg of textile fabrics processed (Al-Kdasi et al. 2004). Textile wastewater consists of unfixed dyes, inorganic and organic chemicals and trace metals that are harmful to the environment and can cause haemorrhage, nausea, skin diseases, cancer, and mutagenesis (Ghaly et al. 2014). Textile wastewater is characterised by high colour content due to the presence of unfixed dyes, which affects its biodegradability by reducing the penetration of light and decreases the dissolved oxygen content (Muthukumar et al. 2007). The fixation of dyes on fabrics depends on the nature of the dyes and fabrics. To increase fixation, heterofunctional dyes such as sulfonic and chlorine substituted triazine groups containing azo dyes are used in industries; however, dyes containing chloro-substituted triazines are easily hydrolysed and get converted into their constituent components (Sala et al. 2014). Hence, the removal of these dyes is difficult, and only limited methods can be adopted for the removal of triazine-substituted dyes from textile industrial effluents.
The abatement of RR2 triazine dye from wastewater is carried out using various adsorbents such as Metapenaeus monoceros shell (Thiyagarajan et al. 2013), nanoscale zerovalent iron (Lin & Chen 2017), Fe3O4 nanoparticles (Lin et al. 2016) and textile sludge (Sonai et al. 2016). Catalysts such as anthraquinone-2,6-disulfonate (AQDS) (Costa et al. 2010) and platinised titanium dioxide (Kuo & Lin 2009) are used to remove RR2 from aqueous solution. Catalytic ozonation, iron powder reduction and photooxidation (Feng et al. 2000), biodegradation using Pseudomonas sp. SUK1, advanced oxidation (Wu & Ng 2008) and UV/chlorine advanced oxidation (Wu et al. 2016) have been employed for treating RR2 contained wastewater.
The current study focuses on the use of electrochemical methods for the removal of textile dyes from wastewater. Electrochemical methods are classified with respect to the electrode material, used chemicals and the pollutant removal mechanism. Well known electrochemical methods are electro-oxidation (using stable electrodes such as PbO2, mixed metal oxides (MMO) and RuO2), electro-Fenton, and peroxi-coagulation (using H2O2 as an activator). Electrocoagulation is cost-effective, operationally simple, with no sensitivity to toxicity and has less sludge generation than other processes (Nguyen et al. 2014b, 2016). The theory of the process is described as the generation of metal ions from anodes hydrolysed into metal hydroxides, and the generation of hydrogen (H2) bubbles that occur at the cathode which help the coagulated matters to float (Equations (1)–(4)) (Mollah et al. 2004; Ozyonar & Karagozoglu 2011). The performance of the electrocoagulation process depends on wastewater properties, especially pH, conductivity, and chemical composition. The electrocoagulation process involves three successive stages for pollutant removal: (i) generation of coagulants by the oxidation of electrodes in an electrolyte solution, (ii) destabilisation of pollutants and suspensions by compression of diffuse double layer and charge neutralisation and (iii) agglomeration of destabilised pollutants with coagulants (Singh et al. 2014). The electrocoagulation process is applied for the removal of some triazine substituted azo dyes, such as reactive yellow 84 and reactive blue 49 which are removed using Al, Fe and stainless steel (SS) electrodes. Reactive orange 84 is eliminated using Fe and SS electrodes (Kim et al. 2002; Yuksel et al. 2013). A central composite design (CCD) modelling software is utilised to optimise the removal of reactive red 141 (Salmani et al. 2016) with Fe electrodes and reactive red 198 (Khosravi et al. 2015) with Al electrodes. He et al. (2007) experimented with the decolourisation of reactive yellow 84 by electrocoagulation which is enhanced with ozonation. The degradation of reactive red 120 is studied by using the electrocoagulation process and the dye degradation mechanism is analysed through gas chromatography mass spectrometry (GC-MS) studies (Pirkarami & Olya 2014).
Based on the reported literature, the removal of RR2 dye was studied by varying key process parameters of the electrocoagulation process such as: solution pH (3–11), applied potential (2–6 V), electrolyte (NaCl) concentration (1–3 g/L), dye solution concentration (100–500 ppm), active surface area of electrodes (10–30 cm2) and inter-electrode distance (2–4 cm). The process efficiency was optimised based on parameters of colour, chemical oxygen demand (COD) removal efficiencies, electrical energy and electrode consumption, and operational cost. The dye abatement mechanism during the treatment was analysed with UV-visible, Fourier transform infrared (FTIR) spectra and energy dispersive X-ray (EDX) spectral studies.
MATERIALS AND METHODS
C.I reactive red 2 (RR 2) (C19H10Cl2N6Na2O7S2, MW 615.34 g/mol) was selected as a typical model pollutant, and the molecular structure of dye is depicted in Figure S1 (Supplementary Data). The textile dye was dissolved in distilled water and stored as a stock solution (5,000 ppm). Before the experiment, a certain concentration of dye solution was prepared by diluting the stock solution. The solution pH and conductivity were adjusted using dilute HCl (0.1 N), NaOH (0.1 N) and NaCl (1–3 g/L). Variations in pH (ELICO pH LI 120) and conductivity (CM 180) were measured using respective meters. A working volume of 0.3 L was placed in the reactor and an Al (HE18) electrode (dimensions 15 × 15 × 0.1 cm) was fitted in the reactor with the help of non-conducting holders which maintained the desired effective surface area of the electrode. During the experiment, the residual dye concentration was measured using a UV-Vis Spectrometer (DR3900, Hach Company, USA) and the UV-Vis spectra were recorded with an ELICO (BL198) spectrophotometer. The dye solution was collected at different time intervals. It was filtered using Whatman filter paper (size 41), and the COD was measured as per the standard protocol (APHA/AWWA/WEF 2005). After electrocoagulation, electrocoagulated flocs were collected, filtered, and dried at 105 °C in a hot air oven. Dried flocs were subjected to FTIR analysis which was carried out using a JASCO (FT-IR-V 400 series) spectrophotometer.
RESULTS AND DISCUSSION
Effect of solution pH in the removal of RR2
The highest enhanced colour removal efficiency was achieved at pH 3 due to the combined effect of these mechanisms with the increase of solution pH with time. The lower colour removal efficiency was observed at alkaline (pH 9 and 11), which may be caused by the repulsion between anionic dye molecules and Al (OH)4– ions.
Effect of an applied potential in the removal of RR2
The generation of metal hydroxide flocs and bubble formation is amplified with the increase in applied voltage. The colour removal effectiveness increased from 60.07 to 79.38% whereas the applied potential varied from 2 to 4 V. When further increasing the applied potential from 5 to 6 V, the colour removal efficiency slightly increased (Figure 2). According to Faraday's law, the metal dissolution and floc generations are directly proportional to the current density of the reactor (Daneshvar et al. 2007). The metal hydroxide increases with the increase in applied voltage, which is attributed to colour removal efficiency up to 4 V. The colour removal efficiency attained at 5 and 6 V is due to unsaturated concentration dye and metal hydroxides. The COD removal efficiency was improved from 25.37 to 65.07% by raising the applied potential from 2 to 6 V. The energy consumption was calculated as 0.71 and 12.05 kWh/kg for applied potentials of 2 and 6 V, respectively. The final pH of the treated solution is approximately 8.5, which shows that the applied potential does not affect the reaction scheme.
Effect of an electrolyte dosage in the removal of RR2
The addition of electrolytes leads to an increase in the conductivity of the dye solution which reduces the ohmic resistance among the electrodes (Nguyen et al. 2014a; Nandi & Patel 2017). The electrolyte concentration increases the current density of electrocoagulation reactor at the same applied voltage (Şengil & Özacar 2009). In major studies, NaCl is selected as an electrolyte to perform electrocoagulation as it is used for dye fixation in textile industries. It also acts as a pitting agent to remove oxide films from the electrode's surface (Pajootan et al. 2012; Pirkarami & Olya 2014). In this study, the conductivity of the dye solution ranges from 3.36 to 8.24 mS/cm for NaCl dosages of 1–3 g/L, respectively. It leads to a change in voltage drop from 2.5 to 3.07 V which simultaneously influences the increase in energy and electrode consumption from 3.86 to 10.14 kWh/kg dye and 0.32 to 0.85 kg Al/kg dye. The high concentration of electrolyte leads to more metal dissolution (Mbacké et al. 2016) which enhances the colour removal efficiency from 79.38 to 86.93% by raising the electrolyte concentration from 1 to 3 g/L (Table 1). From the above results, it can be concluded that increasing electrolyte concentration greatly affects the energy and electrode consumption.
Effect of an initial dye concentration in the removal of RR2
Characterisation of real-time textile effluents shows fluctuating values of COD and different colour intensities (Manikandan et al. 2015). Concentration of RR2 varies between 100 and 500 ppm to study the colour parameters, COD abatement efficiencies, and kinetics. With an increase in the dye concentration from 100 to 500 ppm, the colour and COD removal efficiencies decrease from 97.65 to 63.89% and 71.43 to 35.92%, respectively (Figure 3). The pseudo first-order kinetics was studied to determine the rate of colour removal efficiency. This rate constant value decreases from 0.082 to 0.026 min–1 while the dye concentration increases from 100 to 500 ppm. The decrease in colour, COD abatement efficiencies and rate constant at different dye concentrations are due to the generation of coagulants. The coagulant generation rate is found to be constant at the same current density. The generation of an inadequate amount of coagulants at a constant current density prevents the removal of dye molecules at high concentration of dyes from aqueous solution (Nandi & Patel 2014).
Effect of an active surface area of electrode in the removal of RR2
The electrode active surface area in the electrocoagulation reactor varies from 10 to 30 cm2. The colour removal efficiency, electrical energy, and electrode consumption are shown in Table 2. The colour removal efficiency increases from 53.24 to 84.10% with an increase in the electrode active surface area of 10–30 cm2. A similar trend has been observed by Daneshvar et al. (2003). The colour removal efficiency due to the floc density and distribution increases with the active surface area of electrode. At certain voltages, the passage of current increases with the increase in the active surface area of the electrode which affects the electrical energy use. The highest value of electrical energy consumption was 4.18 kWh/kg when the electrode active surface area was 30 cm2. It was observed that the dye removal capacity per unit area of electrodes decreased from 16.77 to 8.50 ppm/cm2 when the surface area of the electrode increased from 10 to 30 cm2 (Table 2). The increasing concentration of flocs is unsaturated with the decreasing concentration of dye during the course of the experiment (Sakthisharmila et al. 2017). Therefore, the dye removal capacity decreases while increasing the active surface area of the electrode.
Effect of inter-electrode distance (IED) in the removal of RR2
The charge loading decreases from 12.45 to 4.95 F/m3 by increasing IED from 2 to 4 cm (Figure 4). The increase in electrode distance causes resistance to the passage of current into the solution. This leads to low current intensity during electrocoagulation by simultaneously reducing charge loading. The decrease in charge loading directly affects the quantity of floc generation and colour removal efficiency.
The effective removal of RR2 using Al electrodes ranges from 98 to 100%. While varying the interelectrode distance from 2 to 4 cm, this observation can be attributed to the possibility that an increase in the inter-electrode distance increases the discharge of Al ions into the solution. Similar results have been observed by Singh et al. (2014).
Operational cost analysis for the removal of RR2
The operational cost was analysed with respect to the applied voltage. The derived analysis shows that the increase in applied voltage from 2 to 6 V leads to an operational cost increase from 0.92 to 3.54 $/kg dye. The increase in applied voltage leads to more energy and electricity consumption by increasing the current density. Hence, the current density needs to be optimised before the application of the electrocoagulation at industrial scale.
UV-visible spectral studies
The UV-Vis spectra of RR2 recorded at an initial dye concentration of 300 ppm and an initial pH of 7 at frequent time intervals during the electrocoagulation process is depicted in Figure 5. Before the treatment of RR2 dye, the peaks observed at 214, 232, 282, 314 and 535 nm represent the presence of aromatic substituted benzenes, substituted naphthalene ring and n → π* transition of the N = N group. Further into the electrocoagulation process, the peaks at time intervals 10–60 min show a gradual decrease in intensity, no-peak shifts are also observed. These observations support the adsorption mechanism followed for the removal of RR2 using Al electrodes.
FTIR spectral studies of the electrocoagulated flocs
The FTIR spectra of dye before treatment (RR2 alone) and after treatment with electrocoagulated Al flocs are shown in Figure 6(a) and 6(b). The peaks at 3,452.56 and 3,428.81 cm–1 of the dye alone and electrocoagulated flocs with dye correspond to the existence of O-H and N-H stretching vibrations. The peak shift from 3,452 and 3,428 cm–1 is due to the adsorption of dye on the electrocoagulated flocs. Similarly, the peaks at 1,630.16 and 1,638.23 cm–1 indicate the presence of the C=O group in the dye and electrocoagulated flocs with dye molecules. The stretching peak at 1,555.14 cm–1 shows the presence of C = C groups, and it shifted to 1,540.99 cm–1 for dye containing electrocoagulated flocs. The peak of the chromophoric group (N = N) is observed at 1,496.93 cm–1 which is shifted to 1,419.81 cm–1 in electrocoagulated flocs with dye which showed that there is no degradation of the chromophoric group during electrocoagulation. It indicates dye adsorption on flocs. The stretching vibrations of S=O peaks are observed at 1,186.54 cm–1 for dye alone and electrocoagulated flocs with dye. The peaks at 1,129.39, 1,048.24, and 668.21 cm–1 of dye are shifted to 1,156.15, 1,072.22, and 630.60 cm–1 in electrocoagulated flocs with dye. The above derived points supported the adsorption mechanism for the removal of RR2 using A1 electrodes (Coates 2000).
EDX analysis of electrocoagulated flocs
The electrocoagulated flocs were collected with and without dye. The flocs were dried and used for EDX analysis. The electrocoagulated flocs without dye contain 31.06% aluminium, 62.35% oxygen, 2.77% sodium, and 3.82% chlorine while the electrocoagulated flocs with dye contain 14.04% carbon, 50.38% oxygen, and 26.28% aluminium. The presence of carbon in electrocoagulated flocs supports that the dye molecules are adsorbed on the electrocoagulated flocs. The adsorption mechanism is followed for the elimination of RR2 from aqueous solution using Al electrodes (Singh et al. 2014; Sakthisharmila et al. 2017).
The buffering capacity of electrocoagulated aluminium flocs maintained a pH of 9 irrespective of the initial pH. The colour removal efficiency decreased with the increase of the solution pH from acidic to alkaline medium. The increase in applied voltage led to high energy consumption and COD removal efficiency. Moreover, an increase in electrolyte concentration resulted in slight colour removal efficiency, but electrical energy, electrode consumption, and voltage drop were found to be increased. The high concentration of dye present in the solution affects the electrocoagulation process which is supported by the decrease in the pseudo first-order kinetics rate constant. The effective surface area of the electrode has a better impact on colour removal effectiveness and the amount of dye removed per unit area of the electrode decreases. The operational cost of the electrocoagulation process is favoured at low voltages of the electrocoagulation reactor. The adsorption of dyes over the electrocoagulated flocs is supported by UV-visible studies of treated samples, FTIR, and EDX analysis of dried electrocoagulated flocs.
The authors gratefully acknowledge the collaborative research conducted among the groups, institutes and universities of authors.
The Supplementary Material for this paper is available at https://dx.doi.org/10.2166/aqua.2020.109.