Introduction

Among various industry types, textile industry is extremely water intensive. Only 85% of coloring matters in textile dyeing process get fixed to cloths, while the remaining 15% of dyes are discarded from dye baths as effluent (Liang et al. 2012; Aravind et al. 2016). This dye wastewater containing synthetic dyes discharged from textile industry exhibits high chrominance and salinity (Zhou et al. 2016; Sun et al. 2009), which would do harm to the growth of aquatic lives and the self-cleaning of waters when releases into water without treatment (Zhang et al. 2016; Bu et al. 2016). Thus, it should be properly treated before discharge, especially for color and salt removal.

A large portion of processes has been devoted to the decolorization of synthetic dyes including physicochemical adsorption (Goel et al. 2011; Mohammadi et al. 2011) and microbiological method (Toro et al. 2013). Adsorption can transfer contaminants from water to another medium which may lead to secondary pollution with difficulties of regenerating used adsorbents (Mrinmoy and Sirshendu 2016), while biological approaches are often limited by the microbial activity and the low biodegradability of dye wastewater (Florêncio et al. 2016; Zhang et al. 2015a, b). Though chemical oxidation processes can realize dye decolorization efficiently (Ma et al. 2014), they are liable to cause secondary pollution or need post-treatment (Alventosa-deLara et al. 2012). Moreover, the generation of carcinogenic compounds and mutagens during chemical oxidation and advanced oxidation processes undoubtedly restricts their application in dye wastewater treatment (Migliorini et al. 2011). In comparison, electrochemical degradation is regarded as a more efficient process to treat dye wastewater with the advantages of simple setup, easy control, ambient operation, and complete degradation (Panizza and Oturan 2011; Subba and Venkatarangaiah 2014). For example, MO has been successfully handled in electrochemical system on Pt-Bi/C nanostructured electrode by a square-wave potential method (Li et al. 2013). Electro-Fenton and electrocoagulation have also been utilized to treat dye wastewater (Martínez-Huitle and Brillas 2009), while ion removal is often ignored in decolorization processes.

The profile of the salt concentration in dye wastewater always maintains higher level. The high salinity enhances the mineralization degree of the receiving water and leads to land salinization (Gabelich et al. 2002). Regarding salt removal, reverse osmosis is frequently employed, while membrane fouling has to be faced (Ghanbari et al. 2015). As an emerging technique, capacitive deionization comes in researchers’ view, but relatively lower efficiency restricts its practical applications (Porada et al. 2012). Electrodialysis has been proved to be an efficient and versatile method for the applications of drinking water production and wastewater purification because it has many advantages over other methods including strong adaptability, low energy consumption, and easier and low cost of operation and maintenance (Smith 2017). Since NaCl is widely used in the textile industry, electrodialysis has been proved to remove NaCl from textile wastewater efficiently (Chandramowleeswaran and Palanivelu 2006). Azo dyes can also migrate with salt as they can be ionized in electrodialysis systems, and higher applied voltages can result in strong oxidation environment, which facilitate dye decolorization. Thus MO, one of the anionic azo dyes, can also be treated in electrodialysis systems simultaneous with salt removal, exhibiting economic and environmental benefits, while few studies concern roles of oxidizing pollutants accompanied with salt removal in these electrochemical treatments.

In this study, a laboratory scale electrochemical cell in constant-voltage mode with three compartments was constructed and its performance of simultaneous color and salt removal was evaluated. A representative azo dye MO was selected as the target pollutant to simulate salt-containing dye wastewater. Operation factors affecting the performance of above cell were studied. Mechanisms were also investigated, especially for decolorization in aspects of generation and functions of active substances and the degradation products of MO. This study emphasizes the feasibility and effectiveness of electrochemical process to treat dye wastewater for simultaneous color and salt removal satisfactorily.

Experimental section

Materials

A three-compartment laboratory scale electrochemical cell was constructed (Fig. 1), which was divided by a pair of anion and cation exchange membranes (AEM and CEM) purchased from Shanghai Shanghua Water Treatment Materials Co., Ltd., China. Properties of membranes were listed in Table 1, with membrane fluxes of 0.1 mL h−1 cm−2 MPa−1 for both AEM and CEM. They were washed three times with deionized water before use. Anodic and cathodic compartments were in cuboid shape with an effective volume of 288 mL (8 cm × 8 cm × 4.5 cm) and the middle compartment of 96 mL (8 cm × 8 cm × 1.5 cm). The boron-doped diamond (BDD) anode (CONDIAS GmbH, German) with geometric dimension of 2 cm × 2 cm was employed noting its unique advantage in the complete mineralization of organic pollutants (Nava et al. 2014), while cathode was made of carbon plate (Beijing Evergrow Resources Co. Ltd., Beijing, China) with geometric dimension of 4 cm × 4 cm. They were placed tightly to membranes in anodic and cathodic compartments, respectively, with electrode spacing of 2.0 cm and connected by titanium wire with a DC power source during experiment.

Fig. 1
figure 1

The laboratory scale electrochemical cell with three compartments employed in present study

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Table 1 Properties of ion exchange membranes used in the present study
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Solutions containing 0.01 M Na2SO4 was introduced into both anodic and cathodic compartments, while the middle compartment was filled with simulated dye wastewater containing MO (C14H14N3O3SNa) and salt in the form of NaCl with the settled initial concentration. All chemicals used in this study were analytical grade without further purification.

Experimental procedure

With initial concentrations of MO and NaCl at 50 and 3000 mg L−1, respectively, feasibilities of simultaneous decolorization and desalination of dye wastewater were evaluated in the middle compartment of the electrochemical cell in a 6-h operation cycle with applied voltage of 6 V first. Its behaviors with only MO or NaCl with the same concentration were also evaluated for comparison. Then operation factors affecting performance of the electrochemical cell were studied, including applied voltage (2, 4, 6, and 8 V), initial MO concentration (20, 50, 80, and 100 mg L−1), and initial salt concentration (1000, 3000, and 5000 mg L−1). After that, concentrations of total oxidizing species (TOS) were measured and different scavengers were added respectively to confirm functions of corresponding active species during the MO decolorization process. Isopropanol, 4-hydroxy-2, 2, 6, 6-tetramethylpiperidin 1-oxyl (TEMPOL), and Fe (II)-EDTA were used to remove diffusing ·OH, ·O2−, and H2O2, respectively (Guo et al. 2015; Chen et al. 2011). Structural transformation and degradation products of MO were also monitored in the end of typical operation cycle, compared with initial samples. Desalination mechanisms were also investigated by monitoring the distribution and mass balance of Na+ and Cl− in different compartments during the operation. The whole experiment was conducted in batch mode at room temperature (22 ± 2 °C). All tests were repeated at least three times at the identical condition and average values were presented.

Analytical methods

MO concentration was monitored by a UV-vis spectrophotometer (DR 6000, HACH, USA) at 464 nm (Guo et al. 2017). A conductometer (Seven Multi S40, Mettler Toledo, Switzerland) was used to measure the conductivity of simulated dye wastewater to reflect the desalination. The concentrations of Na+ and Cl− were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, X Series II, Thermo Fisher Scientific, USA) and ion chromatography (ICS-1100, Thermo Fisher Scientific, USA), respectively. Total organic carbon (TOC) was measured by Multi N/C 3000 TOC analyzer (Analytik Jena AG, Germany) (Zhang et al. 2009). Currents were recorded by a multimeter and energy consumption (EC) was calculated. Current efficiency (CE) was also calculated on the basis of measured TOC (Liang et al. 2017). The concentrations of TOS were determined by N, N-diethyl-p-phenylenediamine (DPD) colorimetric method using a UV-vis spectrophotometer (DR 6000, HACH, USA) (Li et al. 2010), where the added KI was oxidized to I2 and meantime DPD was oxidized to form a red-violet product by generated I2 (Reactions (1) and (2)). This measure gave a global concentration of the oxidants, including H2O2, O3, etc.

$$ 2\mathrm{KI}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{I}}_2+2\mathrm{KOH} $$
(1)
$$ {\mathrm{I}}_2+\mathrm{DPD}\to \mathrm{Red}\hbox{-} \mathrm{violet}\ \mathrm{product} $$
(2)

A wavelength scanning test from 200 to 800 nm was conducted by a UV-visible spectrophotometer (DR 6000, HACH, USA) to reveal the structural transformation of MO during the process. The degradation products were measured by Gas Chromatography-Mass Spectrometric (GC-MS, Trace GC-DSQ, Thermo Fisher, USA) according to (Wang et al. 2015).

Results and discussion

Electrochemical cell performance

With initial concentrations of MO and NaCl at 50 and 3000 mg L−1, respectively, feasibilities of simultaneous decolorization and desalination of dye wastewater were evaluated in the middle compartment of the electrochemical cell in a 6-h operation cycle with applied voltage of 6 V first, gradual decolorization, and desalination in the middle compartment of the electrochemical cell were simultaneously observed (Fig. 2a). The removal efficiencies of MO reached 70.2% after 6-h treatment, which was comparable to previous study (Alejandra et al. 2017) and showed advantage to foregoing MO degradation studies conducted by anaerobic-aerobic biofilm reactor (Murali et al. 2013). 43.8% of TOC was removed within 6-h operation (Fig. 2b), suggesting partial mineralization of MO took place, which was higher than that realized by peroxymonosulfate-induced reactive oxidants (Lou et al. 2013). Current gradually decreased during the operation (Fig. 2b) and the calculated maximum instantaneous CE and EC were 9.1% and 11.5 kWh m−3, respectively, both of which were comparable with results from undivided electrochemical reactors (Li et al. 2016; Liang et al. 2017). Regarding desalination, satisfactory results were also obtained in our proposed system as the desalination efficiencies reached as high as 88.7% after 6-h operation, also better than previous desalination experiments conducted by electrodialysis (Turek 2003).

Fig. 2
figure 2

a Decolorization and desalination of dye wastewater in the proposed electrochemical cell and control sets as well as b variations of TOC and current in the electrochemical cell during 6-h operation

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The electrochemical system behaviors with only MO or NaCl with the same concentration were also evaluated for comparison to further evaluate its performance. The decolorization efficiencies significantly increased to 95.0% after 6-h treatment when only MO of 50 mg L−1 was added into the middle compartment (Fig. 2a). The suppression of decolorization was probably due to the competitive consumption of TOS by Cl− with MO when MO and NaCl were presented together in simulated dye wastewater (Martinez-Huitle et al. 2016). Furthermore, desalination efficiencies decreased to 66.4% within 6-h operation with sole NaCl in the middle compartment (Fig. 2a), suggesting that the ion migrations were enhanced with the presentation of ionic dye MO due to the improved solution conductivity. Similar phenomenon was also observed that the presence of negatively charged glutamate promoted the migration of Cl− in the nanofiltration system (Luo and Wan 2006). Therefore, conclusions were drawn that salt-containing dye wastewater could be treated efficiently through this proposed electrochemical process.

Operation factors influences

Effects of applied voltage (2, 4, 6, and 8 V) were studied with initial MO concentration of 50 mg L−1 and initial NaCl concentration of 3000 mg L−1, respectively. Both decolorization and desalination efficiencies were improved with applied voltages increasing from 2 to 8 V. Particularly, the decolorization efficiencies increased significantly when applied voltages increased from 4 to 6 V (Fig. 3a). The increased production of TOS under higher applied voltage was responsible for the increase of color removal as previous studies indicated (Ma et al. 2007). More remarkable improvements in aspect of desalination were noted when applied voltage increased from 4 to 6 V (Fig. 3b). The electrical potential is the driving force during ion transport through the membrane and this driving force was strengthened at higher applied voltages as indicated by previous reports (Barakat 2011).

Fig. 3
figure 3

Effects of different operation factors on decolorization and desalination efficiencies of the electrochemical cell. a, b applied voltage with initial MO concentration of 50 mg L−1 and initial NaCl concentration of 3000 mg L−1; c, d initial dye concentration with initial NaCl concentration of 3000 mg L−1 at voltage of 6 V; e, f initial salt concentration with initial MO concentration of 50 mg L−1 at applied voltage of 6 V

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Four gradients of initial MO concentration (20, 50, 80, and 100 mg L−1) were set with initial NaCl concentration of 3000 mg L−1 at voltage of 6 V. The decolorization efficiencies decreased gradually during 6-h treatment with the increase of initial MO concentration, from 71.3% with 1.4 mg MO removal at 20 mg L−1 to 56.6% with 5.4 mg MO removal at 100 mg L−1 (Fig. 3c). The generated intermediate products with MO decomposition could compete with the original dye molecule for the TOS produced by the electrochemical process, thus reducing the decolorization efficiency under higher initial MO concentration (Panizza and Oturan 2011). Desalination efficiencies were slightly affected by variations of initial MO concentration with their values around 88.0% with dialyzed approximate 253.4 mg NaCl in a 6-h operation cycle (Fig. 3d).

Performance of the electrochemical cell was also evaluated under different initial NaCl concentration (1000, 3000, and 5000 mg L−1) with initial MO concentration of 50 mg L−1 at applied voltage of 6 V. Similar tendencies of color removal were also observed (Fig. 3e), with around color removal of 70.0% (MO removal of 3.4 mg) under tested conditions in the end of the 6-h operation, indicating that the decolorization was rarely affected by salt profile, similar to the existing report (Buscio et al. 2016). As to salt removal, a significant higher desalination efficiency was exhibited in the first 2 h with the initial NaCl of 5000 mg L−1 than that of 1000 and 3000 mg L−1 (Fig. 3f). This phenomenon could be attributed to a larger transmembrane concentration gradient that could enhance the transport rate of ions across the membrane (Moya 2017), while their final desalination efficiencies reached similar level of about 88.0% after 6-h treatment, with dialyzed NaCl of 84.8, 253.4, and 422.4 mg, respectively.

Mechanisms investigations

Regarding decolorization in electrochemical systems, indirect electrochemical oxidation induced by the generated active species played important roles (Kariyajjanavar et al. 2011). Levels of TOS concentration in the three compartments of the electrochemical cell without MO addition were monitored. TOS in anode compartment were gradually accumulated as expected, reaching maximum concentration of 28.6 mg L−1 at 6 h (Fig. 4), which was measured reflectometrically based on the relationship of absorbance of generated red-violet product through DPD oxidation by active species via I2/KI mediator. Previous study reported that active species were mainly generated on the anode surface (Li et al. 2010). Nevertheless, TOS in anode compartment showed nonlinear growth over time, indicating the concomitant consumption of active species as well as the generation of new ones (Li et al. 2010). Few TOS were detected in the middle compartment and cathode compartment implied that the generated active species on anode surface hardly transferred across membranes used in this study. Therefore, it was elucidated that MO dissociated into anions transferred through the anion exchange membrane into the anode compartment and reacted with the active species generated in anode compartment, leading to color removal. These reactions took place so fast that there were few MO that were accumulated as the anolyte remained transparent during the operation.

Fig. 4
figure 4

Decolorization of MO with the presence of different scavengers and accumulations of TOS in three compartments of the electrochemical cell during 6-h operation

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Electrochemical processes can generate reactive species such as ·OH, H2O2, ·O2−, and so on; the following Reactions (3)–(6) show their formation (Li et al. 2010). Some of them may be responsible for the MO decolorization during the electrochemical process.

$$ {\mathrm{H}}_2\mathrm{O}\to \cdotp \mathrm{OH}+{\mathrm{H}}^{+} $$
(3)
$$ 2\cdotp \mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O}}_2 $$
(4)
$$ \mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}\mathrm{O}}_2\cdotp +{\mathrm{H}}_2\mathrm{O} $$
(5)
$$ {\mathrm{H}\mathrm{O}}_2\cdotp \to \cdotp \mathrm{O}+{\mathrm{H}}^{+} $$
(6)

To determine the significance of each active specie on the decolorization of MO in the proposed electrochemical cell, different scavengers were added to remove the corresponding active species with initial MO concentration of 50 mg L−1 at applied voltage of 6 V. MO decolorization was nearly the same as that without any scavengers when TEMPOL was introduced, indicating that ·O2− was not involved in the decolorization (Fig. 4). Adding isopropanol caused the decrease of decolorization efficiencies of MO to 59.7% in 6 h, which demonstrated that ·OH was definitely involved in this decolorization process. Similar phenomenon was also observed in electrochemical oxidation of Tl(I) with BDD anode (Li et al. 2016). Only 52.1% of MO was decolorized with Fe (II)-EDTA, suggesting that the generated H2O2 through Reaction (4) also contributed to MO decolorization. Though H2O2 was commonly produced in the cathode of electrochemical systems through oxygen reduction (Zhang et al. 2015a, b), it could also be generated in anode compartments by the dimerization of ·OH and take part in processes for pollutants removal (Yu et al. 2014).

Whereas, simultaneous addition of all scavengers did not completely inhibit the decolorization, which indicated that there were other active substances produced in the anode compartment functioning during decolorization, such as Cl2 and ClO− produced from electrochemical oxidation of Cl− presented in anolyte according to Reactions (7) and (8) (Sala and Gutiérrez-Bouzán 2014). With aids of this active chlorine, MO was further oxidized (Reaction (9)). The positive effect of the generation of active chlorine on the performance of electrochemical oxidation process had been proved (Pereira et al. 2015), especially in the electrochemical decolorization of Acid Red14 (Thiam et al. 2015). Moreover, MO would be intercepted when it went through the anion exchange membrane, causing slight color removal, while the membrane fluxes rarely changed after the whole experiment, further implying that MO decolorization was realized by electrochemical oxidation with the aids of generated active species.

$$ 2{\mathrm{Cl}}^{\hbox{-}}\to {\mathrm{Cl}}_2\left(\mathrm{aq}\right)+2{\mathrm{e}}^{\hbox{-} } $$
(7)
$$ {\mathrm{Cl}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Cl}\mathrm{O}}^{\hbox{-} }+{\mathrm{Cl}}^{\hbox{-} }+2{\mathrm{H}}^{+} $$
(8)
$$ \mathrm{MO}+{\mathrm{Cl}\mathrm{O}}^{\hbox{-}}\to \mathrm{Intermediates}\to {\mathrm{CO}}_2+{\mathrm{Cl}}^{\hbox{-} }+{\mathrm{H}}_2\mathrm{O} $$
(9)

UV-vis spectra scan exhibited the peak at the wavelength of 464 nm decreased continuously with time for the simulated dye wastewater (Fig. S1, Supporting Information), confirming that azo bond was gradually destroyed (Riaz et al. 2014). Meanwhile, the peak at 272 nm corresponding to aromatic ring also declined considerably (Zhang et al. 2006), indicating the simultaneous destructions of the MO molecular structure (Devi et al. 2009). GC/MS was also performed after 6-h operation and more organics with smaller molecule were detected in the exhausted anolyte such as phenol and indole, compared with original simulated dye wastewater (Table S1, Supporting information). Similar intermediate products were also found previously (Li et al. 2013; Cai et al. 2017). Additionally, some organics were also detected in the middle compartment and even in the exhausted catholyte due to their dispersions.

As to desalination, ion balances were monitored. Concentrations of both Cl− and Na+ in the middle compartment decreased with time (Fig. 5), thus realizing the satisfactory desalination for the simulated dye wastewater. Driven by external voltage, Cl− and Na+ moved to the anode and the cathode compartments through the anion exchange membrane and the cation exchange membrane, respectively, with the observed increase of Cl− in the anolyte and Na+ in the catholyte, respectively (Fig. 5). It was found that migration kinetics of Cl− and Na+ followed linear correlation expressed as follows:

$$ {\mathrm{C}\mathrm{l}}^{\hbox{-} }:\ln\ {\mathrm{C}}_{\mathrm{t}}/{\mathrm{C}}_0=\hbox{-} 0.4696\ \mathrm{t}\ \left({\mathrm{R}}^2=0.9669\right) $$
(10)
$$ {\mathrm{Na}}^{+}:\ln\ {\mathrm{C}}_{\mathrm{t}}/{\mathrm{C}}_0=\hbox{-} 0.4289\ \mathrm{t}\ \left({\mathrm{R}}^2=0.9708\right) $$
(11)

where C0 and Ct are the initial and remaining concentrations of Na+ and Cl−, respectively; k is the removal rate constant.

Fig. 5
figure 5

Concentrations of Na+ and Cl− with time in three compartments of the electrochemical cell during 6-h operation

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The desalination rates were undoubtedly reduced due to the gradual decrease of conductivities in the middle compartment (Shabbir et al. 2017). Unlike Na+, loss of Cl− was observed (Fig. 5), probably due to the direct electrochemical oxidation of Cl− on the anode surface (Reaction (7)), which were also reported in the electrophoretic ion exchange desalination system (Shkolnikov et al. 2012). The produced active substances during Cl− oxidation in this desalination process could be utilized efficiently for MO decolorization, thus achieving the simultaneous decolorization and desalination in one step. Further efforts could be made to combine the proposed electrochemical cell with traditional/emerging techniques, for example, microbial fuel cells as renewable power source and/or for handling dye wastewater simultaneously (Zhang et al. 2009; Guo et al. 2014) to form hybrid systems for practical applications in dye wastewater treatment by further improving the efficiency and reducing the costs.

Conclusions

In this study, we constructed an effective electrochemical cell for simultaneous decolorization and desalination of salt-containing dye wastewater. The decolorization efficiencies of 70.2% and the desalination efficiencies of 88.7% were obtained with applied voltage of 6 V after 6-h electrochemical treatment of solution containing 50 MO and 3000 mg L−1 NaCl, respectively. Higher applied voltages improved the performance of both color and salt removal, while higher MO concentrations suppressed decolorization and higher NaCl concentration accelerated desalination rate. MO dissociated into anions transferred into the anode compartment, where the generated active species as ·OH, H2O2, and ClO− contributed to color removal, while salt removal were driven by external voltage based on ions balance analysis. This work further elucidates the possibility for simultaneous decolorization and desalination of salt-containing dye wastewater treatment by electrochemical process.