Electrochemical oxidation of Acid Red 3R (AR3R) was investigated with the new catalyst of iron phosphomolybdate (FePMo12) supported on modified molecular sieves type 4 Å (4A) as packing materials in the reactor. The results of the Fourier transform infrared spectroscopy and X-ray diffraction indicated that the heteropolyanion had a Keggin structure. The optimal conditions for decolorization of simulated AR3R wastewater were as follows: current density 35 mA/cm2, initial pH 4.0, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm. With the addition of NaCl to the system, the decolorization efficiency increased. But Na2SO4 had a negative effect on the decolorization efficiency, which was attributed to the negative salt effect. The degradation mechanisms of AR3R were also discussed in detail.

INTRODUCTION

With the rapid growth of dye industry, dye pollutants are considered as a significant source of environmental contamination. It has been estimated that 50,000 metric tons of azo dyes are released into the land and natural water each year (Zhou et al. 2007). The molecular structure of azo dyes contains azo groups (−N = N−) and auxochromes, such as −OH and −SO3Na groups. When azo dyes are discharged into the water body, not only do they bring undesirable colors, but more importantly they are toxic, high oxygen demanding and have rather low biodegradability (Martínez-Huitlea et al. 2012). Therefore, proper treatment of azo dyes wastewater has drawn more attention.

Electrochemical oxidation process (EOP) is an environmentally friendly method for wastewater treatment through direct anodic oxidation or indirect oxidation. On the one hand, there is no consumption of chemicals and no production of sludge during the process. On the other hand, the process is carried out at room temperature and atmospheric pressure generally (Maria et al. 2011). However, the low removal efficiency of azo dyes has limited the development of traditional electrochemical oxidation technology.

It is strongly believed that the use of catalyst provided a promising way to overcome the limitation. Polyoxometalates (POMs) have gained considerable interest due to their versatility for the oxidation of organic substrates (Omwoma et al. 2014). They can be used as dual functional catalysts in both homogeneous and heterogeneous systems. Nevertheless, POMs were limited by small specific surface area and dissolution in polar solvent (Song & Mark 2002). A set of RnPMo12O40 (R = H+, Ba2+, Zn2+, Co2+ and Cu2+) with different counter-cations has caused more interest due to their unique structured properties and application in electrochemical oxidation (Song & Mark 2002). Iron ion was introduced as a counter-cation in order to generate the synergistic effect between redox and Fenton-like reactions in this study.

In this paper, molecular sieves type 4 Å (4A) were selected as carrier to support POMs in order to improve the specific surface area of the catalyst. Iron phosphomolybdate (FePMo12O40, abbreviated as FePMo12) was loaded on the modified 4A with 3-aminopropyltriethoxy silane (APTES) to prepare FePMo12/APTES-4A. Coupling reagent was applied to modify 4A so as to improve the chemical stability and resistance of acid or alkali. The optimal parameters of treating (Acid Red 3R (AR3R)) were studied. The degradation mechanisms of AR3R were also investigated.

EXPERIMENTAL

Materials and reagents

AR3R, a typical azo dye, was provided by Hebei Rising Chemical Co., China. Its purity was above 99%. APTES, H3PMo12O40·24H2O (PMo12) and all the chemical reagents were of analytical grade and purchased from Tianjin Bodi Chemical Holding Company, China. All reagents were used without further purification.

Preparation

FePMo12 preparation

FePMo12 was prepared as follows: 2.02 g Fe(NO3)3·9H2O and 20.0 g CH3COOK were added into PMo12 solution (3%) under stirring at 90 °C. The equivalent volume of methanol was added (under stirring) into the PMo12 solution. The solution was stabilized at 5 °C for 24 hours. After that, it was recrystallized three times in 80 °C water and dried at 55 °C.

FePMo12/APTES-4A preparation

At first, 4A was pretreated to remove impurities on the carrier. It was dipped into dilute sulfuric acid for 12 hours and rinsed with distilled water three times. Subsequently, 4A was boiled in distilled water three times and dried at 120 °C. Then the pretreated 4A was modified by 50.0 mL toluene containing 3.0 mL APTES. The mixture was stirred for 12 hours. The modified 4A was added into 15.0 mL FePMo12 aqueous solution and then kept at room temperature for 24 hours. Finally, the produced FePMo12/APTES-4A was rinsed with distilled water and dried at room temperature.

Electrochemical setup

Batch experiments were performed in an open undivided polytetrafluoroethylene plate electrochemical reactor containing AR3R solution. The dimension stable anode (DSA) anode (Ti/RuO2 − IrO2, 6.0 × 12.0 cm) and graphite cathode were fixed vertically and aligned parallel to each other. The effective electrode area was 18 cm2 and their inter-distance could be adjusted. Compressed air was blown into the reactor through a microporous plate at the bottom. A regular DC power supply was equipped to provide power.

Electrochemical oxidation process and analysis procedures

To eliminate the additional adsorption effect, the catalytic particles were dipped into the AR3R solution until a saturation phase. Fifty grams of this catalyst were then placed between the anode and the cathode. Before each run, a fresh solution of AR3R was prepared with deionized water, and initial concentration was kept at 500 mg/L. A control trial without any catalyst present was also done.

The Fourier transform infrared spectroscopy (FT-IR spectrum) and X-ray diffraction (XRD) pattern were performed by Nicolet6700/FT-Raman modules (500–4,000 cm−1) and Rigaku-D/Max-2500 (Cu target, tube voltage 40 kV, tube current 100 mA, 2θ: 0.5–100°), respectively. Elemental composition of catalyst FePMo12/APTES-4A was determined with a Bruker SRS-3400 sequential X-ray fluorescence (XRF) spectrometer. The absorbance value was measured with double beam UV-vis spectroscopy (UV-2600 spectrophotometer, China) at maximum absorption wavelength 507 nm. Total organic carbon (TOC) concentration was measured with a Shimadzu OCT-1 TOC-VCPH analyzer to quantitatively characterize the mineralization degree.

The decolorization efficiency was examined to evaluate the effect of the oxidation process. It was calculated by Equation (1). 
formula
1
where A0 is the initial absorbance value of the solution, and At is the absorbance value at reaction time t (minutes).

RESULTS AND DISCUSSION

Catalyst characterization

The FT-IR spectrum of FePMo12 is shown in Figure S1(a) (available online at http://www.iwaponline.com/wst/071/027.pdf). The major peaks belonging to the Keggin structure were located at: 1,030 cm−1 for stretching vibration v(P–O), 896 cm−1 for v(Mo–Ob–Mo), 842 cm−1 for v(Mo–Oc–Mo) (Song & Mark 2004). Double bond of Mo = O opened and Mo–O–Fe was formed by iron ion. The results showed that Fe3+ worked as counter-cation and PMo12O403− unit had the Keggin structure.

The XRD pattern of FePMo12/APTES-4A is presented in Figure S1(b) (online at http://www.iwaponline.com/wst/071/027.pdf). Three diffraction peaks appeared at 9.0°, 20.0° and 29.0°, which represented that FePMo12 on the carrier had the Keggin structure (Maria et al. 2011). Four peaks appeared at 12.0°, 16.0°, 19.0°and 24.0° in the pattern, which belonged to the characteristic peaks of 4A (Wang et al. 2010). From the evidence of the XRD pattern and FT-IR spectrum, it was shown that the active ingredient FePMo12 on the modified 4A still possessed the Keggin-type structure. Moreover, the XRD result indicated that FePMo12 was dispersed on the 4A surface.

The XRF analysis showed that a certain amount of Mo and Fe was found in the catalyst, which was loaded with 1.723 wt% MoO3 and 1.068 wt% Fe2O3. Approximately 1.87 wt% FePMo12 was loaded on the carrier.

Decolorization of AR3R under different systems

To evaluate the synergetic effect of FeMo12/APTES-4A with the electrochemical process, the degradation of dye wastewater treated in the same electrochemical conditions without the catalyst was investigated in Figure S2 (available online at http://www.iwaponline.com/wst/071/027.pdf). The decolorization efficiency was 48.6% after 90 minutes and 50.3% after 120 minutes in the electrochemical process without the catalyst. Moreover, AR3R decolorization efficiency did not increase obviously after 120 minutes. In contrast, AR3R decolorization efficiency in electrochemical system with FeMo12/APTES-4A was enhanced greatly. Decolorization efficiency with FeMo12/APTES-4A was 77.1% after 90 minutes and 78.4% after 120 minutes. Decolorization efficiency was 28.1% higher than that without the catalyst.

Effect of electrolysis conditions on AR3R decolorization

Effect of current density

The current density of the electrochemical system had a major role during the EOP. The decolorization efficiency was enhanced from 38.1 to 72.1% when the current density increased from 5 to 35 mA/cm2 (Figure 1(a)). The strong oxidants, such as H2O2 and •OH, might be electro-generated by water and discharged to oxidize AR3R with increase of the current density (Liu et al. 2012). However, the decolorization efficiency decreased when the current density exceeded 35 mA/cm2. The higher current density that would cause more side reactions such as the anode oxygen evolution can consume more power energy (Xiong et al. 2002). Therefore, to choose an appropriate current density is most important to favor the decolorization efficiency.

Figure 1

(a) Effect of current density on decolorization efficiency of AR3R; initial pH 4.0, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm. (b) Effect of initial pH on decolorization efficiency of AR3R; current density 35 mA/cm2, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm. (c) Effect of airflow on decolorization efficiency of AR3R; current density 35 mA/cm2, initial pH 4.0 and inter-electrode distance 3.0 cm. (d) Effect of inter-electrode distance on decolorization efficiency of AR3R; current density 35 mA/cm2, initial pH 4.0 and airflow 0.08 m3/hour.

Figure 1

(a) Effect of current density on decolorization efficiency of AR3R; initial pH 4.0, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm. (b) Effect of initial pH on decolorization efficiency of AR3R; current density 35 mA/cm2, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm. (c) Effect of airflow on decolorization efficiency of AR3R; current density 35 mA/cm2, initial pH 4.0 and inter-electrode distance 3.0 cm. (d) Effect of inter-electrode distance on decolorization efficiency of AR3R; current density 35 mA/cm2, initial pH 4.0 and airflow 0.08 m3/hour.

Effect of initial pH

The effect of initial pH on decolorization was investigated (Figure 1(b)). Comparing with the decolorization efficiency at pH 4.0, it was lower when pH was 2.0 or 3.0. The decolorization efficiency decreased with increasing pH from 4.0 to 8.0. It was shown that the decolorization efficiency in acidic conditions was slightly higher than that in alkaline and neutral conditions. The optimal initial pH value was 4.0.

On the cathode, oxygen was converted into stronger oxidizing agent H2O2 by the two-electron reduction of oxygen. Fenton-like reaction that H2O2 reacted with Fe3+ to produce •OH was favored to occur in acidic conditions. But low pH caused the evolution of hydrogen on the surface of cathode, which was negative to produce H2O2 (Liu et al. 2012). POMs were strong Brønsted acids, and the suitable pH value for POMs was resistant to hydrolytic decomposition (Omwoma et al. 2014). In addition, POMs can dissociate completely or stepwise in aqueous solution at different pH. UV spectra of FePMo12 aqueous solutions at different pH are shown in Figure S3 (available online at http://www.iwaponline.com/wst/071/027.pdf). Adsorption peaks appeared at pH 2.0, 3.0 and 4.0, which indicated that PMo12O403− was relatively stable at lower pH conditions. Once pH exceeded 4.0, adsorption peaks disappeared. POMs would decompose partly. As a result of PMo12O403− protonation, the decolorization efficiency decreased with increasing pH in neutral and alkaline conditions. And PMo12O403− was subject to alkaline hydrolysis for its low surface negative charge and high dielectric constant of water (Song & Mark 2004). Hydroxyl free radical (•OH) electro-generated in the system was favored to transform into in the alkaline condition, according to Equation (2). But the oxidation capacity of was lower than that of •OH. The electrode surface was passivated more easily in alkaline media than in acidic media (Lv et al. 2009). These reasons led to the result that the decolorization efficiency in acidic conditions was higher than that in alkaline and neutral conditions. 
formula
2

Effect of airflow

Air was blown into the electrochemical system for two purposes: to promote mass transfer and activate the surface of particle electrodes, and to supply the essential oxygen for electrochemical reactions. Oxygen was converted into stronger oxidizing agent H2O2 on the cathode by two-electron reduction, according to Equation (3) (Lv et al. 2009). To investigate the effect of airflow, electrochemical reactions were performed at different airflow rates (Figure 1(c)). The decolorization efficiency increased when the airflow rate changed from 0.02 to 0.08 m3/hour. And at 0.08 m3/hour, the decolorization efficiency reached the maximum value. 
formula
3
Meanwhile, Mo was the coordination atom of phosphomolybdate, which had high oxidation state (Antonaraki et al. 2010). The oxidation state Mo would oxidize the organic molecule, and the reduction state Mo would be oxidized by O2. The typical reversible redox process was exhibited.

The decolorization efficiency declined when the airflow rate exceeded 0.10 m3/hour. It indicated that aeration at unsuited flow rate had a negative effect on the decolorization efficiency. The aeration may cause severe fluid short-circuiting due to the volume taken up by the air in the cell. It forced AR3R to react with active site of FePMo12/APTES-4A at short retention time and led to a decrease in the decolorization efficiency (Saleh 2009).

Effect of inter-electrode distance

Inter-electrode distance was an indispensable factor to optimize the electrochemical system. The effect of inter-electrode distance on the AR3R decolorization was determined in Figure 1(d). The decolorization efficiency decreased with reducing inter-electrode distance from 3.0 to 2.0 cm. Short inter-electrode distance could cause more side reactions and energy consumption and then a decrease in decolorization efficiency (Yue et al. 2013). When the inter-electrode distance exceeded 3.5 cm, the decolorization efficiency decreased. With the distance increasing, the movement of produced ions got slower and ions would have fewer opportunities to aggregate. The decrease of decolorization efficiency was due to less hydroxyl free radical (•OH) (Wang et al. 2009).

Effect of electrolyte

Effect of NaCl

Electrolytes could improve the solution conductivity and accelerate the electron transfer, which benefitted the electrochemical oxidation. The decolorization efficiency was investigated by adding different concentrations of supporting electrolytes (Figure 2). The decolorization efficiency increased and then decreased with an increase in the NaCl concentration. By adding a NaCl concentration of 0.05 mol/L, the decolorization efficiency was enhanced from 74.6 to 95.3% because hypochlorous acid or hypochlorite, as strong oxidizing species, which was generated in the system, could oxidize organic compounds (Equations (4) and (5)). 
formula
4
 
formula
5
Figure 2

Effect of electrolyte concentration on the decolorization efficiency; current density 35 mA/cm2, initial pH 4.0, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm.

Figure 2

Effect of electrolyte concentration on the decolorization efficiency; current density 35 mA/cm2, initial pH 4.0, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm.

On the one hand, the high decolorization efficiency was attributed to the indirect electrochemical oxidation by the active chlorine (Mijin et al. 2012). On the other hand, NaCl increased the ionic strength of PMo12O403− (Song & Mark 2004). It was reported that NaCl in the POMs solution screened out the electrostatic repulsions, which allowed the anions to move more closely (Song & Mark 2004).

Effect of Na2SO4

The decolorization efficiency was exhibited to be lower than that of no electrolyte, when Na2SO4 was added. When the concentration of Na2SO4 was 0.04 mol/L, decolorization efficiency reached the peak value of 42.5%, which was still lower than that without supporting electrolyte (Figure 2). The presence of Na2SO4 showed an obvious inhibition effect on the AR3R degradation. It should be mentioned that negative salt effect occurred. The difference between two supporting electrolytes was mostly due to the aggregation or association with ionic dye in wastewater. High concentration of salts and −SO3 group of AR3R molecule were favorable for aggregation or association. Therefore, aggregation was enhanced by the addition of Na2SO4 and subsequently gave rise to lose the chance of AR3R reacted with catalysts (Niu et al. 2014). But the amount of persulfate electro-generated in a higher concentration of Na2SO4 solution was larger than that in a lower concentration of Na2SO4 solution. The result that the decolorization efficiency increased gradually with increasing concentration of Na2SO4 was attributed to the generation of persulfate ions that can oxidize azo dyes, according to Equation (6). Although persulfate ions were beneficial to the decolorization, the amount was little. Na2SO4 cannot play a decisive role in the AR3R decolorization (Martínez-Huitlea et al. 2012). 
formula
6

Electrochemical mechanism

UV-vis spectra of AR3R at different reaction time were investigated in order to clarify the changes of molecular and structural characteristics (Figure 3). Three absorption peaks appeared in UV-vis spectra. In the visible region, there was a rapid decrease in absorbance at 507 nm due to azo bond (−N = N−) cleavage in the dye molecules. The azo bond was the most active site for oxidation. It was confirmed that the electrochemical process was able to remove color in a short time. The adsorption peaks at 331 and 215 nm were due to the π–π* transition of the benzene and naphthalene rings. The absorbance in the UV region increased within 30 minutes and then decreased. This absorbance increase was related to intermediate formation. Aromatic amine compounds were generated at initial reaction process because of the breakage of azo bond. The absorption of amino group existed within this region. The adsorption peaks were weakened after 30 minutes, indicating that the relevant structures were destroyed gradually. On the basis of the results, the removal of TOC was analyzed. After the 30 minutes it was found that the removal of TOC was slow. In this stage, the azo bond was broken. Subsequently, a series of consecutive degradation steps occurred, including desulfonation, denitrogenation, opening loop reaction and mineralization. After 90 minutes, the removal efficiency of TOC reached 46.0%.

Figure 3

UV-vis spectra and TOC changes of AR3R sample at different degradation time; current density 35 mA/cm2, initial pH 4.0, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm.

Figure 3

UV-vis spectra and TOC changes of AR3R sample at different degradation time; current density 35 mA/cm2, initial pH 4.0, airflow 0.08 m3/hour and inter-electrode distance 3.0 cm.

Intermediate products were indentified by liquid chromatography–mass spectrometry (LC–MS) according to previous research in our team (Yue et al. 2015). Briefly, AR3R was degraded to aromatic and organic acids gradually, followed by final mineralization to CO2 and H2O. Organic pollutants were oxidized directly on the anode surface by electron transfer. The activity of electrode and the current density determined the rate of electron transfer in the electrochemical process. In addition, H2O2 was continuously produced by two-electron reduction of oxygen on the cathode surface. Radicals •OH and HO2• with strong ability of oxidation were produced, accompanying electron exchange between electrode and pollutants. It is worth mentioning that PMo12O403− had coordination atom Mo, which had the highest oxidation state. On the catalyst, the transient species would interact rapidly with AR3R to form the Mo-oxygen-organism intermediate. In the subsequent step, the bonds between the oxygen atom and Mo were broken, and quinones as oxydates of intermediate products were produced. Quinones produced in the system were confirmed by LC–MS (Yue et al. 2015). The reduction states of Mo could be oxidized reversibly to the oxidation state by molecular state O2. The reaction processes were deduced in Figure 4. However, the valence state change of Mo atom has not been measured. Thus, the degradation mechanism for the FePMo12/APTES-4A combined system needed further investigation.

Figure 4

Illustration of the reaction mechanisms for electrochemical oxidation of AR3R wastewater with FePMo12/APTES-4A.

Figure 4

Illustration of the reaction mechanisms for electrochemical oxidation of AR3R wastewater with FePMo12/APTES-4A.

Reusability and leaching properties

To prove the reuse potential of the catalyst, cycle experiments were performed, as shown in Figure S4 (available online at http://www.iwaponline.com/wst/071/027.pdf). The decolorization efficiency decreased from 95.3 to 82.1% after five runs. The catalyst was characterized by XRF after five runs, indicating that 16.7% of FePMo12 was lost in total. Reduction of the catalytic effect was mainly due to the loss of catalyst. The decolorization remained at more than 80% even after recycle five times. Therefore, the reusability of the catalyst was possible.

Performance based on TOC and decolorization efficiency was compared with previous studies, as shown in Table S1 (available online at http://www.iwaponline.com/wst/071/027.pdf). Conventional particle electrode had good electrical conductivity and large surface area, but the removal of TOC was not high. The catalyst loaded on the supporter was introduced to extend the promotion of mass transfer, which was regarded to improve the electrochemical oxidation. Similar decolorization efficiency was obtained under the conditions of higher wastewater concentration and lower current density in this paper (He et al. 2014). FePMo12/APTES-4A showed high efficiency of decolorization and removal of TOC, which can be ascribed to the enhancement of stability and catalytic activity compared to other studies.

CONCLUSIONS

This study demonstrated that employing the Keggin-type FePMo12 as catalyst, AR3R wastewater can be effectively decolorized by electrochemical oxidation. Employing the constructed experimental facility, a series of experiments were conducted to examine the effect of the catalyst on the decolorization reaction, and through which the operating parameters were greatly optimized. The decolorization efficiency was increased by 28.1% compared with treatment without any catalyst. Introducing NaCl into the dye solution could improve the decolorization during the electrochemical process and this study found that adding Na2SO4 would cause a negative effect. The adsorption peaks were weakened, indicating that azo bond (−N = N−) and naphthalene rings were destroyed gradually in UV-vis spectra. AR3R degradation reaction contained direct oxidation, catalytic reaction and indirect oxidation in this work. The electrochemical method coupled with POMs catalyst could be considered as an effective and promising technical option for azo dye wastewater treatment.

ACKNOWLEDGEMENTS

This study was supported by the Program for Hundred Outstanding Innovative Talents in universities of Hebei Province (BR2-211), Hebei Natural Science Foundation of Youth Fund (Grant No. B2014208096) and the Program for New Century Excellent Talents in University (Grant No. NCET-10-0127).

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Supplementary data