Abstract

In this study, coal tar wastewater was treated by electrochemical oxidation technology using lead dioxide anodes. The influence of operating parameters, including applied current density, electrode gap and initial pH value, on the removal ratio of chemical oxygen demand (COD) was investigated. The results demonstrated that the COD removal ratio reached 90.5% after 3.5 h electrolysis with the current density at 3 A dm−2 and electrode gap at 1.0 cm. Correspondingly, the COD decreased from 5,125 mg L−1 to 487 mg L−1, which fitted the wastewater discharge standards of China, and the specific energy consumption (SECCOD) was 35.3 kWh kgCOD−1. Not only was the COD removal ratio only 77.1% after 2 h electrolysis but the BOD5/COD ratio of the wastewater reached 0.44, which could be biochemically treated, and the SECCOD decreased by 34.3%. Moreover, the main composition of pristine wastewater before and after 2 h electrolysis was analyzed by GC-MS, and the disappearance of macromolecules (such as ethyl-2-pyrenemethanol) and the production of small molecules (such as propane-1,3-diol) could improve the biodegradability of the wastewater. Therefore, electrochemical oxidation for 2 h is a promising alternative for pretreatment of coal tar wastewater prior to biological treatment.

INTRODUCTION

Coal tar is an important product of the coking industry, which is commonly used in the production of polymer materials (such as plastics and resins), pharmaceutical reagents, pesticides and paints (Luo et al. 2016; Wang et al. 2016). With the rapid development of coal chemical and coal tar refining industries, the amount of coal tar wastewater has increased significantly, and the composition has become increasingly complex, including phenols, polycyclic aromatic hydrocarbons, and cyanide, all of which are highly toxic to most microorganisms (Ma et al. 2018). Improper disposal of coal tar wastewater results in serious environmental pollution and threats to human health. Therefore, a series of treatment methods, such as ozonation (Yang et al. 2013), biological denitrification (Lu et al. 2011), and Fenton reactions (Zhu et al. 2011), has been employed to degrade coal tar wastewater. However, these approaches have encountered many problems, such as overloading of wastewater treatment equipment, high values of chemical oxygen demand (COD) after treatment, intensive investment and large area requirement by denitrification units, and high operation costs. Therefore, degradation techniques for coal tar with high efficiency are high in emerging demand.

Electrochemical oxidation has been proposed as an efficient and environmentally friendly approach for treating non-biodegradable organic contaminants (Kenova et al. 2018). In addition, this method has exhibited some excellent advantages, such as robustness, versatility, automation amenability, and no necessity to add other chemicals (Kaur et al. 2017). Under appropriate conditions, electrochemical oxidation can significantly remove organic compounds, ammonia nitrogen, and colors (Miyata et al. 2011; Bai et al. 2017). However, the disadvantage of the electrochemical oxidation treatment wastewater process is the high energy consumption. In that respect, anode materials play crucial roles in the electrochemical oxidation process, which strongly affect the current efficiency. In recent decades, a lot of electrode materials have been examined to improve the effectiveness of oxidation and current efficiency, such as graphite, IrO2, RuO2, PbO2 and BDD electrodes (Gao et al. 2017; Kaur et al. 2017; Agustina et al. 2019). Among all the materials, PbO2 and BDD are the most attractive electrodes because of their high oxygen evolution overpotential. However, the high cost of BDD anodes limits their large-scale application. Therefore, PbO2 electrodes have become popular due to their good electrical conductivity, high chemical inertness, and low cost, which have been widely applied to remove organic pollution in wastewater, including medical wastewater (Dai et al. 2016), textile wastewater (Aquino et al. 2014), and coking wastewater (Shen et al. 2012). These processes proved to be efficient methods in the treatment of industrial wastewater.

The primary objective of this study is to evaluate the performance of the electrochemical oxidation treatment of coal tar wastewater with PbO2 anodes. The main influencing factors, such as current density, pH values, and electrode gap, on the COD removal percentage of coal tar wastewater are discussed. After that, the reaction kinetics of coal tar wastewater treatment are also established. Finally, the main components of coal tar wastewater before and after electrochemical oxidation are identified using GC-MS.

MATERIALS AND METHODS

Coal tar wastewater

The coal tar wastewater used in this study was obtained from the coal tar chemical company located in Hebei province, China. The factory produces on average 20 m3 d−1 of wastewater and the characteristics of the wastewater are relatively stable with different seasons. The main characteristics of the wastewater are shown in Table 1.

Table 1

Typical characteristics of coal tar wastewater

ParameterAverage value
COD (mg L−15,125 
BOD5 (mg L−1388 
pH 3.9 
Conductivity (mS cm−125.5 
Chloride ions (mg L−174 
ParameterAverage value
COD (mg L−15,125 
BOD5 (mg L−1388 
pH 3.9 
Conductivity (mS cm−125.5 
Chloride ions (mg L−174 

Experimental procedure

Electrochemical oxidation experiments were carried out under galvanostatic conditions in an undivided Plexiglas reactor (10 × 10 cm, 10 cm of height, 1,000 mL capacity), and the working volume of the electrochemical reactor was 800 mL. The PbO2 electrode (10 × 10 cm, 100 cm2) and a stainless-steel net (10 × 10 cm, 100 cm2) were respectively adopted as the anode and cathode. The pH of the wastewater was adjusted to a desirable value using NaOH and H2SO4 solutions. A digital DC power supply (MP1560D, China) with digital displays was used as the electric current source for all the experiments. Because the initial concentration of coal tar wastewater is certain and has high conductivity, the operating parameters we discussed in this study include the current density, initial pH, and plate spacing. During the experiments, the wastewater was mainly stirred by the H2 gas bubbles generated on the cathode, the current density varied from 2 to 5 A dm−2, initial pH value varied from 2 to 10, and the plate spacing varied from 0.5 to 2.0 cm. After that, the coal tar wastewater was treated under optimized conditions to investigate treatment performance and the variation of COD in the wastewater.

Analytical methods

The electrocatalytic degradation of coal tar wastewater was monitored by a COD test. The COD test was performed according to the Standard Methods for Water and Wastewater Examination of China (HJ/T399-2007). The COD removal efficiency was calculated by Equation (1) and the instantaneous current efficiency (ICE) was calculated by Equation (2):  
formula
(1)
 
formula
(2)
where COD0 is the initial value of the COD of wastewater (g L−1); CODt is COD value (g L−1) at time t; CODt+Δt is COD value (g L−1) at time t + Δt; I is the current (A); F is the Faraday constant (96,487 C mol−1); V is the volume of the treated wastewater (L); and t is the electrolysis time(s).

The pH and the conductivity were determined by a pH-meter (PHS-3C) and conductivity meter (DDS-11A), respectively. Biological oxygen demand (BOD5) was measured by the dilution and seeding method according to HJ 505-2009. The Pb ion concentration in the solution after electrolysis reaction was detected by an atom absorption spectrometer (AAS, Thermo M6).

The specific energy consumption in terms of COD mass removed (SECCOD, kWh kgCOD−1) was calculated by Equation (3) (Cecconet et al. 2019):  
formula
(3)
where P(t) is the recorded power demand by the applied potential, t is time, and mCOD is the mass of removed COD.

The pristine coal tar wastewater and wastewater solution after 2 h electrolysis were extracted by isopyknic dichloromethane (DCM), respectively, and were then injected into the GC-MS (Agilent GC 6890 coupled with Agilent 5973 mass spectrometer) for content analysis. The injected volume was 20 μL. Helium was used as the carrier gas with a flow rate of 3 mL min−1. The injector temperature was set at 300 °C. The column temperature was held at 50 °C for 5 min, and then heated at 20 °C min−1 to 280 °C, and maintained at this temperature for 10 min.

RESULTS AND DISCUSSION

Effect of current density

Figure 1(a) shows the influence of current density on the electrocatalytic degradation efficiency of coal tar wastewater. When the current density increased from 2 to 3 A dm−2 after reaction for 3.5 h, the COD removal ratio increased from 75.0% to 88.9%, which is ascribed to more hydroxyl radicals generated at higher current density. However, when the current density increased from 3 to 5 A dm−2, the COD removal ratio increased only by 3.5% after 3.5 h, probably due to the fact that the oxidation was controlled by the rate at which organic molecules were transferred from the bulk solution to the electrode surface rather than the generation rate of hydroxyl radicals (Muazu et al. 2015). In addition, the removal of COD was faster at the beginning of the experiment, but gradually slowed down with the extension of degradation time. The possible reason is that the content of aromatic organic matter in the wastewater was higher at the beginning, which decreased after a period of degradation time to produce a large number of byproducts, and the degradation rate of byproducts by the electrode was lower than that of the aromatic organic matter (Hurwitz et al. 2014). Therefore, the removal curve of COD became gradually more gentle with the extension of time.

Figure 1

The effect of current density on (a) COD removal with time, (b) ln(COD0/CODt) with time, (c) ICE variation with time. () 2 A dm−2;() 3 A dm−2;() 4 A dm−2;() 5 A dm−2.

Figure 1

The effect of current density on (a) COD removal with time, (b) ln(COD0/CODt) with time, (c) ICE variation with time. () 2 A dm−2;() 3 A dm−2;() 4 A dm−2;() 5 A dm−2.

Figure 1(b) presents the COD degradation kinetics of coal tar wastewater with different current densities. The values of R2 (correlation coefficient) and k1 (rate constant) are summarized in Table 2. A considerable increase of the rate constant with the increasing current density could be observed. For an applied current density of 2 A dm−2, the rate constant was only 0.3910 h−1, while it was 0.6336 h−1 under the current density of 3 A dm−2. The latter was about 1.6 times higher than the former. The results indicated that the driving force of the electron transfer in the electrode reactions became greater with the increase of current density, and higher current density contributed to a higher rate of hydroxyl radical generation responsible for the coal tar wastewater oxidation processes (Muazu et al. 2015). In addition, the increased anodic current density amplified the oxygen evolution reaction, and then the formation and detachment of oxygen bubbles facilitated the diffusion of organic matter in solution (Duan et al. 2012). For an applied current density of 4 A dm−2, the rate constant was 0.7079 h−1, while it was 0.7663 h−1 under the current density of 5 A dm−2.

Table 2

Kinetics parameters of electrochemical degradation of coal tar wastewater by lead dioxide electrode with different operational parameters

Current density (A dm−2)
Electrode gap (cm)
 2345 0.51.01.52.0
k1 (h−10.3910 0.6336 0.7079 0.7663 k1 (h−10.6938 0.6867 0.6258 0.5480 
R2 0.9961 0.9954 0.9865 0.9826 R2 0.9949 0.9936 0.9931 0.9864 
Removal ratio (%) 75.0 88.9 91.6 92.4 Removal ratio (%) 91.0 90.5 88.4 84.6 
Initial pH value
24710
k1 (h−10.7050 0.6356 0.4976 0.3572      
R2 0.9932 0.9955 0.9967 0.9865      
Removal ratio (%) 91.0 88.9 82.6 72.1      
Current density (A dm−2)
Electrode gap (cm)
 2345 0.51.01.52.0
k1 (h−10.3910 0.6336 0.7079 0.7663 k1 (h−10.6938 0.6867 0.6258 0.5480 
R2 0.9961 0.9954 0.9865 0.9826 R2 0.9949 0.9936 0.9931 0.9864 
Removal ratio (%) 75.0 88.9 91.6 92.4 Removal ratio (%) 91.0 90.5 88.4 84.6 
Initial pH value
24710
k1 (h−10.7050 0.6356 0.4976 0.3572      
R2 0.9932 0.9955 0.9967 0.9865      
Removal ratio (%) 91.0 88.9 82.6 72.1      

Figure 1(c) shows the variation of ICE with time for the degradation assays performed at 2 A dm−2, 3 A dm−2, 4 A dm−2 and 5 A dm−2, respectively. The higher applied current intensity gave higher loss in the current efficiency, revealing that the electrolysis mainly operated under mass transport control. The loss of current efficiency was due to the side reactions, such as oxygen evolution and electrolyte decomposition (Fajardo et al. 2017). These side reactions were enhanced at higher applied current density.

The applied current depends on the enterprise's requirements for wastewater discharge and the treatment cost of wastewater. In the process of electrolysis, a further increase of current density up to 5 A dm−2 did not produce significant improvements on COD removal, but gave higher loss current efficiency. In order to obtain the best results, both in COD removal and in electrolytic efficiency, the optimum value for the current density in this study was 3 A dm−2.

Effect of pH

Figure 2 shows the COD removal ratio of coal tar wastewater versus time under different pH values. It can be seen that the COD removal ratio decreased with increasing pH, and the highest value was observed at pH = 2 after 3.5 h degradation. Figure 2(a) shows that along with the pH of coal tar wastewater increasing from 2 to 10, the COD removal percentage decreased from 91.0% to 72.1% after 3.5 h. Figure 2(b) shows the first-order reaction kinetics fitting plots for different pH. The values of R2 and k1 are shown in Table 2. These results indicated that the coal tar degradation for different pH was in agreement with pseudo-first-order reaction kinetics. There was a decrease in k1 with the increase of initial pH, which might be because the electrocatalytic oxidation reaction always competes with the anodic oxygen evolution reaction in the electrochemical degradation process for coal tar wastewater. In an alkaline environment, the oxygen evolution potential is relative low, and an oxygen evolution side reaction easily occurs to compete with the electrocatalytic oxidation of wastewater. However, under acidic conditions, the oxygen evolution potential of the electrode is high, and the oxygen evolution reaction will be suppressed (Wang et al. 2011). Moreover, some researchers have found that active hydroxyl radicals are of stronger oxidative capability toward organic molecules under acidic conditions, further improving the degradation effect of coal tar wastewater (Li et al. 2018). However, the low initial pH exhibits a corrosive effect on the electrode, which will decrease the catalytic activity of the electrode and increase the cost. Although the maximum COD removal ratio was 91.0% obtained at pH = 2.0, we chose pH = 4.0 as the optimal pH value because of the similar COD removal (88.9%) at the more implementable subacidic condition, which means the pH adjustment is not required and thus treatment costs will be reduced in practical applications.

Figure 2

The effect of pH, i.e. () pH = 2; () pH = 4; () pH = 7; () pH = 10, on (a) the removal of COD with time, (b) ln(COD0/CODt) with time.

Figure 2

The effect of pH, i.e. () pH = 2; () pH = 4; () pH = 7; () pH = 10, on (a) the removal of COD with time, (b) ln(COD0/CODt) with time.

Effect of electrode gap

Figure 3 shows the influence of electrode gap on the electrocatalytic degradation efficiency of coal tar wastewater. As shown in Figure 3(a), the electrode gap has a slight effect on the COD degradation of coal tar wastewater. When the electrode distance was less than 1.0 cm, the COD removal ratio of coal tar wastewater was no different under different electrode gaps. However, when the distance was greater than 1.0 cm, the COD removal ratio gradually decreased as the electrode distance increased. For example, when the electrode gap increased from 1.0 cm to 2.0 cm, the removal ratio decreased from 90.5% to 84.6%, and the k1 value also decreased from 0.6867 h−1 to 0.5480 h−1. This is because larger electrode distance will bring on the vanishing and quenching of highly active hydroxyl radicals generated on the surface of the electrode.

Figure 3

The effect of electrode gap, i.e. () 0.5 cm; () 1.0 cm; () 1.5 cm; () 2.0 cm, on (a) the removal of COD with time and (b) the SECCOD with time.

Figure 3

The effect of electrode gap, i.e. () 0.5 cm; () 1.0 cm; () 1.5 cm; () 2.0 cm, on (a) the removal of COD with time and (b) the SECCOD with time.

As shown in Figure 3(b), as the distance between anode and cathode increased, the energy consumption also increased. Similarly, the increase on energy consumption was not obvious when the electrode distance was less than 1.0 cm. But when the electrode gap was greater than 1.0 cm, the energy consumption was significantly enhanced. For example, when the COD removal ratio of coal tar wastewater was 80%, the energy consumption was 19.6, 20.1, 28.7 and 35.3 kW h kgCOD−1 if the electrode gaps were 0.5 cm, 1.0 cm, 1.5 cm and 2.0 cm, respectively. Short circuits can be made in the smaller electrode gap, and wastewater treatment costs increased with the larger electrode gap (He et al. 2011). After considering the COD removal ratio and energy consumption, the 1.0 cm distance was selected as the most suitable electrode gap.

Energy consumption and biochemical study

As shown in Figure 4(a), after electrochemical oxidation with current density at 3 A dm−2, pH = 4 and electrode gap at 1.0 cm for 3.5 h, the COD decreased from 5,125 mg L−1 to 487 mg L−1, and the COD removal reached 90.5%. The remaining COD was 487 mg L−1, which was lower than the industrial wastewater third-class discharge standard of China at 1,000 mg L−1. The effective removal of coal tar wastewater by other oxidation methods, such as the ozonation and Fenton processes, have reported by previous literature.

Figure 4

Under the optimal conditions, (a) the variation of COD and BOD5 with electrolysis time; (b) SECCOD with removal of COD.

Figure 4

Under the optimal conditions, (a) the variation of COD and BOD5 with electrolysis time; (b) SECCOD with removal of COD.

Table 3 presents the removal percentages of COD in the same coal tar wastewater with different methods. The COD removal efficiency of electrochemical oxidation was compared with the ozonation and Fenton processes. As can be seen, under the best conditions, the highest COD removal was obtained by electrochemical oxidation, followed by Fenton and ozonation. These results indicate that three methods were feasible for the removal of COD from wastewater. However, as proposed previously, due to insufficient hydroxyl radical generation by ozone in the oxidation process and high investment, the chemical oxidation approach requires costly strong oxidizers. Therefore, anodic oxidation with PbO2 electrodes is an effective method to degrade coal tar wastewater and has promising application prospects in wastewater treatment.

Table 3

Comparison of COD removal efficiency with different methods under optimal conditions

MethodExperimental conditionsCOD removal efficiency
Electrochemical oxidation Current density: 3 A dm−2; COD0: 5,215 mg L−1; Volume: 500 mL; pH: 4; Time: 3.5 h 90.8% 
Ozonation Ozone concentration: 50 g m−3; gas flow rate 0.7 L min−1; COD0: 5,215 mg L−1; pH: 7; Time: 3.5 h 58.6% 
Fenton C0 (FeSO4·7H2O): 0.67 mol L−1; C0 (30% H2O2): mol L−1; COD0: 5,215 mg L−1; Volume: 500 mL; pH: 3; Time: 3.5 h 84.3% 
MethodExperimental conditionsCOD removal efficiency
Electrochemical oxidation Current density: 3 A dm−2; COD0: 5,215 mg L−1; Volume: 500 mL; pH: 4; Time: 3.5 h 90.8% 
Ozonation Ozone concentration: 50 g m−3; gas flow rate 0.7 L min−1; COD0: 5,215 mg L−1; pH: 7; Time: 3.5 h 58.6% 
Fenton C0 (FeSO4·7H2O): 0.67 mol L−1; C0 (30% H2O2): mol L−1; COD0: 5,215 mg L−1; Volume: 500 mL; pH: 3; Time: 3.5 h 84.3% 

For industry application, it is very important to estimate the cost of the electrochemical oxidation treatment of wastewater. As shown in Figure 4(a), the initial BOD5 value of coal tar wastewater was below 390 mg L−1, and the initial COD value was 5,125 mg L−1. Hence the BOD5/COD ratio was only 0.08, which revealed that the coal tar wastewater was non-biodegradable and microbiologically toxic. According to the reported literature, when the BOD5/COD ratio exceeds 0.35, the solution is regarded as biodegradable (Tan et al. 2016). When the electrocatalytic time reached 2 h, the BOD5/COD ratio of the coal tar wastewater reached 0.44, which could be biochemically treated. At this time, the SECCOD was 23.7 kWh kgCOD−1. The results revealed that combined treatment of electrochemical oxidation for 2 h and then a biological process was less expensive in comparison with a single stage of electrochemical oxidation for 3.5 h. It has been reported in previous literature that biological methods (such as aerobic and anaerobic) can effectively degrade coal tar wastewater. Lu et al. (2011) treated coal tar wastewater by the aerobic biological method, and the efficiencies of COD and NH4+-N removal reached 92.8%–96.0% and 71.3%–100%. Park et al. (2012) treated coal tar wastewater using a membrane-less tubular microbial fuel cell (MFC); the MFC achieved a high level of COD removal (88%) and the electrical voltage reached as high as 543 mV (10 kΩ) on day 7, and the MFC system was effective for removing coal tar compounds. However, the large amounts of activated sludge that are produced during aerobic processes require further treatment and disposal. Thus, considering the cost of treatment and the environmental compatibility of biological treatment, a two-stage electrochemical oxidation/anaerobic biological treatment could be an effective treatment approach for coal tar wastewater degradation.

The composition analysis of coal tar wastewater and its degradation byproducts

Next, we employed GC-MS to analyze the composition of the DCM extracting solution from coal tar wastewater before and after electrolysis for 2 h. As shown in Figure S1 (Supplement), after extraction of DCM from pristine coal tar wastewater, the GC spectrum exhibited multiple strong signals corresponding to the series of organic contents in the wastewater. By analyzing the mass spectra relating to the GC peak signals with different retention times (Figure S2-S8, Supplement), the component and molecular structure could be confirmed and are summarized in Table 4. The strong peak at the retention time of 1.032 min belongs to the solvent DCM. As shown in Table 4, from a retention time of 1.593 min to 10.46 min, the GC signal is relatively low and composed of small molecules with low concentration, such as ethylene, ethanol, acetaldehyde, propionic acid, N,N-dimethylformamide, 1-butylene, valeric acid, diethyl sulfide, and aniline. These molecules are easily biodegraded. After that, the multiple peaks with strong intensity from 13.279 min to 17.841 min should be the main content of coal tar wastewater, which are mainly polycyclic aromatic compounds, including derivatives of naphthalene, quinoline, anthracene, pyrene, and coronene. These aromatic molecules are chemically structurally stable and non-biodegradable.

Table 4

The composition of the extracting solution from pristine coal tar wastewater

Retention time (min)Molecular ion peakCompoundMolecular structure
1.593 29 Ethylene  
 57 1-Butylene  
2.005 91 Diethyl sulfide  
6.269 47 Ethanol CH3CH2OH 
6.734 45 Acetaldehyde  
 74 N,N-Dimethylformamide  
 94 Aniline  
8.217 103 Valeric acid  
10.398–10.460 75 Propionic acid  
13.279–17.841 131 1,2-Dihydronaphthalene  
 159 2-Methylnaphthalen-1-ol  
 188 6-Quinolineacetic acid  
 205 9-Vinylanthracene  
 233 2-Pyrenemethanol  
 261 Ethyl-2-pyrenemethanol  
 335 1-Chlorocoronene  
 363 1-Chlorodimethylcoronene  
Retention time (min)Molecular ion peakCompoundMolecular structure
1.593 29 Ethylene  
 57 1-Butylene  
2.005 91 Diethyl sulfide  
6.269 47 Ethanol CH3CH2OH 
6.734 45 Acetaldehyde  
 74 N,N-Dimethylformamide  
 94 Aniline  
8.217 103 Valeric acid  
10.398–10.460 75 Propionic acid  
13.279–17.841 131 1,2-Dihydronaphthalene  
 159 2-Methylnaphthalen-1-ol  
 188 6-Quinolineacetic acid  
 205 9-Vinylanthracene  
 233 2-Pyrenemethanol  
 261 Ethyl-2-pyrenemethanol  
 335 1-Chlorocoronene  
 363 1-Chlorodimethylcoronene  

After electrolysis for 2 h by PbO2 electrodes, as shown in Figure S9 (Supplement), the intensity of GC signals decreased sharply, especially peaks in the small molecular zone from 6.229 min to 8.222 min, and the polycyclic aromatic peaks from 13 min to 18 min could hardly be detected. As shown in Table 5, after 2 h electrolysis, ethylene, ethanol, propionic acid, 1-butylene, diethyl sulfide, and aniline could still exist in solution, but the concentration decreased dramatically. Different from pristine coal tar wastewater, the polycyclic aromatic compounds could not be found after treatment by electrochemical degradation, and might have been decomposed to small linear or aromatic molecules, such as acetic acid, propane-1,3-diol, N,N-dimethyl ethanol amine, benzene, monomethylaniline, xylenes, and phenylacetylene. All these byproducts are more biodegradable than polycyclic aromatic compounds. Therefore, electrochemical treatment of coal tar wastewater using PbO2 electrodes for 2 h could promote the biodegradability and the BOD5/COD ratio of the solution, which is in accordance with the experimental results we discussed above.

Table 5

The composition of the extracting solution from coal tar wastewater after electrolysis for 2 h

Retention time (min)Molecular ion peakCompoundMolecular structure
1.566 29 Ethylene  
 57 1-Butylene  
1.988 91 Diethyl sulfide  
4.219–4.584 61 Acetic acid CH3COOH 
 207 9-Anthracenecarboxaldehyde  
6.229 47 Ethanol CH3CH2OH 
 75 Propionic acid  
6.560 94 Aniline  
7.726 79 Benzene  
 90 N,N-Dimethyl ethanol amine  
 108 Monomethylaniline  
8.004 77 Propane-1,3-diol  
 107 Xylenes  
8.222 103 Phenylacetylene  
Retention time (min)Molecular ion peakCompoundMolecular structure
1.566 29 Ethylene  
 57 1-Butylene  
1.988 91 Diethyl sulfide  
4.219–4.584 61 Acetic acid CH3COOH 
 207 9-Anthracenecarboxaldehyde  
6.229 47 Ethanol CH3CH2OH 
 75 Propionic acid  
6.560 94 Aniline  
7.726 79 Benzene  
 90 N,N-Dimethyl ethanol amine  
 108 Monomethylaniline  
8.004 77 Propane-1,3-diol  
 107 Xylenes  
8.222 103 Phenylacetylene  

The reusability and safety of lead dioxide electrodes

Figure 5 shows the removal efficiency of COD by PbO2 electrodes after 3.5 h for 20 successive reactions. Although a slight decrease in COD removals can be observed, the degradation efficiency was still up to 86.15% after 20 cycles. The results demonstrate that PbO2 electrodes possess excellent reusability in organic pollutant degradation. The main drawback of PbO2 electrodes is the possible release of Pb ions in the electrolysis process, which would present a potential risk for industrial applications of electrochemical technology. To evaluate the safety of PbO2 electrodes, the Pb ion concentration in the electrolyte after 3.5 h of electrolysis was analyzed by AAS as 0.020 mg L−1, which was below the integrated wastewater discharge standard of China (GB 8978-1996, 1 mg L−1). Even after ten cycles of usage, the concentration of leaching Pb2+ was measured as 0.022 mg L−1, which was basically equal to the first running. The results indicate that PbO2 electrodes have excellent safety and reusability in degrading coal tar wastewater.

Figure 5

Electrochemical degradation of coal tar for 20 successive reactions using PbO2 electrodes.

Figure 5

Electrochemical degradation of coal tar for 20 successive reactions using PbO2 electrodes.

CONCLUSION

Electrochemical oxidation technology was adopted for the treatment of coal tar wastewater. The COD removal ratio reached 90.5%, and the COD decreased from 5,125 mg L−1 to 487 mg L−1 after electrolysis for 3.5 h by PbO2 anodes with current density at 3 A dm−2 and electrode gap at 1.0 cm, which met the discharge standard of industrial wastewater of China (GB 8978-1996), and the SECCOD was 35.3 kWh kgCOD−1. The electrochemical oxidation process followed the pseudo-first-order kinetics model. The results revealed that the electrochemical oxidation method could effectively treat coal tar wastewater, but the energy consumption was relatively high. It was noteworthy that the COD removal ratio of coal tar wastewater reached 77.1% and the BOD5/COD ratio reached 0.44 after 2 h electrolysis, which could be biochemically treated, and the SECCOD was only 23.7 kWh kgCOD−1. The composition analysis of coal tar wastewater by GC-MS proved that 2 h electrolysis could promote the BOD5/COD ratio and biodegradability of the solution.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (Nos. 21576065, 21402038).

NOTES

The authors declare no competing financial interest.

SUPPLEMENTARY DATA

The Supplementary Data for this paper is available online at http://dx.doi.org/10.2166/wst.2019.323.

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Supplementary data