JOURNAL OF COAL SCIENCE & ENGINEERING

(CHINA) DOI 10.1007/s12404-011-0413-9

pp 426–430 Vol.17 No.4 Dec. 2011

Electrochemical treatment of COD in biologically pretreated coking wastewater using Ti/RuO2-IrO2 electrodes combined with modified coke

HE Xu-wen, LIU Li-yuan, GONG Jing-wen, WANG Jian-bing, QIN Qiang, WANG Hao School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083,

China

© The Editorial Office of Journal of Coal Science and Engineering (China) and Springer-Verlag Berlin Heidelberg 2011

Abstract The electrochemical treatment of COD contained in biologically pretreated coking wastewater treated by a

three-dimensional electrode system with modified coke as the particle electrode was investigated. And the electrochemical per-

fomance of the coke modified with various active components was studied. The results show that the coke modified with

Fe(NO3)2 has the lowest energy consumption and higher COD removal rate under the same condition, and the modified coke

has better surface characteristics for the purpose of this study. In addition, the kinetic constant was also calculated. The study

shows that the three-dimensional electrode system with Fe (NO3)2-modified coke can give a satisfactory solution in biologically

pretreated coking wastewater.

Keywords three-dimensional electrode, Ti/RuO2-IrO2 anode, Fe (NO3)2-modified coke, coking wastewater

Received: 15 May 2011 Supported by the National High Technology Research and Development Program (2009AA05Z306) E-mail: hexuwen@sina.com

Introduction

There are several conventional coking wastewater treatment methods, including solvent extraction of phenolic compounds, steam stripping of ammonia, and biological treatment (mostly the activated sludge process). Many of these treatment processes have been studied (Zhang et al., 1998; Minhalma and de Pinho, 2002; Li et al., 2003; Vázquez et al., 2007). The bio-logically pretreated coking wastewater contains hard biodegradable aromatic compounds, most of which are refractory, toxic, mutative, and carcinogenic (Zhang et al., 1998; Wang et al., 2002; Lai et al., 2003; Lim et al., 2003; Lai et al., 2005; Zhu et al., 2009). Thus, it is necessary to treat biologically pretreated coking wastewater to reduce any possible impacts on the aquatic environment. The three-dimensional electrodes technology has strong ability to mineralize bio-refractory pollutants and humic substances due to its high capacity of free hydroxyl radicals (Wang et al., 2007). Hence, the three-dimensional electrode system should be able to give a satisfactory solution to the remaining bio-refractory COD from the biological

treatment process of coking wastewater. Some researchers showed that a coating of RuO2

and IrO2 on the titanium electrode exhibited a good effect on electrochemical oxidation and wastewater treatment in a three-dimensional system (Yavuz and Koparal, 2006; Costa et al., 2008). Because of the low cost and the similar properties of active carbon, the coke is always selected as particle electrodes in the three- dimensional electrode technology (Kristóf et al., 1997; Li et al., 2005; Szpyrkowicz et al., 2005). For the aforementioned reasons, the combined process with Ti/RuO2-IrO2 the anode and the coke particle electrodes was selected. In order to improve the elec-trochemical performance, the coke was modified with various transition metal components in this study.

1 Materials and methods

1.1 Reagents and coking wastewater The coking wastewater used for this study was ob-

tained from a secondary sedimentation tank at a cok-ing plant in North China. The wastewater was first treated biologically at the plant using A/O technology.

HE Xuwen, et al. Electrochemical treatment of COD in biologically pretreated 427

A wastewater sample of 1 L was used in each test, and the characteristics of the wastewater were: pH 6~7 and COD 260 mg/L.

1.2 Preparation of modified coke The coke was obtained from a coke bin in a coking

plant in North China. It was subjected to surface pre-treatment with hot alkaline washing and hydro-chloric acid picking for half an hour, respectively, to avoid the effect of absorption. After cleaning with de-ionized water, the coke was dried at 378 K in an oven for 12 h and then immersed in a 0.5 mol/L solution and oscillated by a shaker at 150 r/min for 4 h. The solutions used in the immersing process included Cu(NO3)2, Zn(NO3)2, Ni(NO3)2, Fe(NO3)3, Ce(NO3)2, Co(NO3)2, SnCl4·5H2O and Mn(NO3)2·4H2O. In addition, SnCl4·5H2O (99.0%) and Mn(NO3)2·4H2O (98.0%) was mixed with concentrated hydrochloric acid before dissolving in ethanol and dried at 358 K for 12 h. The remaining steps were the same for the preparation of the other solutions. After pretreatment, the cokes were obtained by heating at 378~393 K for 12 h and burning at 773 K for 4 h in several cycles. After the burning, the nitrates were degraded to the corresponding metal oxide formed in the burning process at high temperatures. The bulk volume of the coke used in each test was 200 mL.

1.3 The electrolytic system During electrolysis, the current density was adjusted

by a DC power supply (MPS702). The cell used in this study was a synthetic glass container having the di-mensions 140 mm×90 mm×130 mm. The performance of the cell was evaluated in batch mode. The Ti/RuO2-IrO2 electrodes, with an exposed geometric area of 84.5 cm2, were used as the anodes and stainless steel plates of the same size were used as the cathodes. The Ti/RuO2-IrO2 electrodes were prepared by the thermal decomposition technique (Costa et al., 2008).

1.4 Analytical methods and methodology The chemical oxygen demand (COD) was measured

by the titrimetric method using dichromate as the oxi-dant in an acidic solution at 423 K for 2 h (Hach Cor-poration, USA). The NH3—N concentration was ob-tained via a Nessler’s reaction using a 752 N spectro-photometer (Shanghai Jingke, China). The pH was determined using a pH meter (pHS-3C, Hanna Corpo-ration, Italy).

In order to optimize the electrochemical perfo- rmance of the three-dimensional electrode process using Ti/RuO2-IrO2 anodes and the coke, with or without being modified, electrolysis tests of the coking wastewater were carried out (operation condition:

electrode distance 1 cm, current density 4.5 mA/cm2).

2 Results and discussion

2.1 The effect of the coke modified with different

components In order to optimize the electrochemical treatment

of coking wastewater using the three-dimensional electrode process combined with modified coke, the effect of supported components on COD removal was investigated. Fig.1 shows the removal rate of COD with different components supported by the coke. The removal rates of the processes with the coke modified with Ce(NO3)2, Fe(NO3)2 and Ni(NO3)2 were 73.86%, 69.47% and 68.74%, respectively, which were the best results. It also can be that most of the modified coke improved the degradation efficiency.

Fig.1 Effect of supported component on removal rate of

COD and energy consumption

As shown in Fig.1, the coke modified with Fe(NO3)2 had the lowest energy consumption and higher COD removal rate, which was considered as the suitable particle electrode in this study.

2.2 Electrode characterization and coking waste-

water electrochemical treatment The SEM of the coke modified with Fe(NO3)2 was

obtained to study the morphology of prepared amor-phous particles. The porous structures can be seen in Fig.2, which showed the function of particle elec-trodes.

Journal of Coal Science & Engineering (China) 428

Fig.2 SEM of Fe(NO3)2-modified coke (4 500×)

Fig.3(a) shows the degradation efficiency of COD as a function of electrolysis time with three-dimen-sional and two-dimensional electrodes. Previous stud-ies have demonstrated that particle electrodes improve electrolysis efficiency because many small coke parti-cles form charged microelectrodes under the influence of an electric field in the three-dimensional electrode system, which could generate additional free hydroxyl radicals and greatly improve COD removal rate (Kong et al., 2006; Zhou et al., 2009). As shown in Fig.4, the COD degradation using the process with modified coke was 92.6% in 60 minutes, while the other two were only about 52.3% and 72.9%, respectively.

Fig.3 Effect of different processes on the electro-chemical

removal rate of COD, energy consumption and current

efficiency

Fig.3(b) shows the evolution of energy consumption

which is measured in kW•h/kg COD as a function of electrolysis time in the combined processes. The en-ergy consumption of the combined process with modi-fied coke is the lowest, and the removal rate in the combined process is the highest of all.

2.3 Current efficiency The current efficiency (CE) for electrochemical

oxidation processes is defined as the current fraction used for organic compound oxidation. It is calculated from the COD values using the following relation (Costa et al., 2008):

E 0[(COD) (COD) ] / 8tC FV I t (1)

The trend of the current efficiency with the retention time during the electrochemical process in this study is shown in Fig.3(c). As shown in this figure, the elec-trochemical process with modified coke has the high-est current efficiency among the three. Compared to the other processes, the process with modified coke has a better performance. Moreover, it can be seen that the current efficiency of the electrochemical degrada-tion with coke particles is much higher than that without it. It may be because the coke particles serve as particle electrodes so that degradation of COD not only takes place at the anode, but also occurs on the surface of the coke particles. Additionally, the current efficiency values in the first 30 min of treatment are all higher than the last 30 min in all the processes, which indicates that the degradation in the first 30 min of treatment is the main contributor to the whole process.

2.4 Kinetic of degradation of COD Some researchers showed that direct oxidation takes

places at the electrode surface in the electro-chemical process (Wang et al., 2007). Moreover, COD could also be adsorbed and oxidized on the coke. Consider-ing that the concentration of active chlorine during electrolysis is assumed to be a constant. The COD degradation may be simplified to Eq.(2):

d[COD]/ d [COD]nt k (2)

With Eq.(2), the value of the kinetic constant in the electrolysis with the modified coke and Ti/RuO2-IrO2 electrode (combined process) and with Ti/RuO2-IrO2 employed singly are calculated in Table 1.

According to the above method, the zero-grade re-action lines (C-t) of the different processes of the cok-ing wastewater are linear, and their reaction rate con-stants were 21.00, 29.49 and 39.08 mg/(L•min) under the assumption that the reaction was a zero-grade re-action with a half-life of 5.85, 4.17 and 3.44 min using Eq.(3), respectively.

1 0/ 2 0.5 /t C k (3)

HE Xuwen, et al. Electrochemical treatment of COD in biologically pretreated 429

Table 1 Calculation of kinetic constant in electrochemical process

Items Ti/RuO2-IrO2 Ti/RuO2-IrO2+Coke Ti/RuO2-IrO2+Modified Coke

Kinetic constant, k (mg/(L·min)) 21.000 29.490 39.080 Zero-grade

Coefficient 0.956 0.958 0.901

Kinetic constant, k (mg/(L·min)) 0.123 0.216 0.438 First-grade

Coefficient 0.988 0.997 0.992

Kinetic constant, k (mg/(L·min)) 0.000 0.001 0.007 Second-grade

Coefficient 0.999 0.967 0.867

The reaction rate constants of the first-grade reac-

tion lines (ln C-t) were 0.123, 0.216 and 0.438 min-1 (Fig.4), under the assumption that the reaction was first-grade with a half-life of 5.63, 3.21 and 1.58 min using Eq.(4), respectively.

t1/2=0.693/k (4)

Fig.4 First-grade reaction lines of different processes on

current efficiency of coking wastewater

The reaction rate constant of the second-grade reac-tion lines (ln C-t) were 0.007, 0.001 and 0 mg/(L•min) under the assumption that the reaction was second- grade with a half-life of ∞, 40 958.3 and 19 222.9 min using Eq.(5), respectively.

t1/2=C0/2k (5)

By comparing the half-life, pseudo-first order was more consistent with the facts, so the degradation of COD in the three-dimensional electrode obeyed the first-order dynamics under the conditions in the ex-periment. Moreover, the In C-t curve was a straight line, while the 1/C-t curve was a curve. So the degra-dation reaction had the characteristic of a first-grade reaction rather than second-grade reaction. Therefore, the kinetics of the treatment on COD at certain condi-tions could be expressed as Eq.(6), here k is the ap-parent pseudo-first order kinetic constant for the elec-trochemical process of COD degradation.

-d(COD)/dt=k(COD) (6)

With Eq.(6) above, the value of kinetic constant k in the electrolysis with modified coke and Ti/RuO2-IrO2 electrode (combined process) and with Ti/RuO2-IrO2 employed singly are calculated in Fig.4. The kinetic

constants of the different processes were 0.123, 0.216 and 0.438 min-1, respectively.

All of these results indicate that the three-dimen-sional electrode system combined with the modified coke particle electrode may be able to provide a more satisfactory solution to degrade the residual COD compounds from the biological treatment process in comparison with the other two processes in this study.

3 Conclusions

The three-dimensional electrode system with Ti/ RuO2-IrO2 anode and modified coke can be success-fully carried out for treating biologically pretreated coking wastewater. The results showed that the coke modified with Fe(NO3)2 had the lowest energy con-sumption and higher COD removal rate among other modifiers under the same conditions, and the modified coke had good surface characteristics for the purpose of this study. In addition, the kinetic constant was also calculated. Results showed that the three-dimensional electrode system with Fe (NO3)2 -modified coke gave a satisfactory solution in biologically pretreated cok-ing wastewater.

References

Costa C R, Botta C M R, Espindola E L G, Olivi P, 2008.

Electrochemical treatment of tannery wastewater using

DSA electrodes. Journal of Hazardous Materials,

153(1-2): 616-627.

Kong W P, Wang B, Ma H Z, Gu L, 2006. Electrochemical

treatment of anionic surfactants in synthetic wastewater

with three-dimensional electrodes. Journal of Hazardous

Materials, 137(3): 1 532-1 537.

Kristóf J, Mihály J, Daolio S, De Battisti A, Nanni L, Piccirillo

C, 1997. Hydrolytic reactions in hydrated iridium chloride

coatings. Journal of Electroanalytical Chemistry, 434(1-

2): 99-104.

Li X Y, Cui Y H, Feng Y J, Xie Z M, Gu J D, 2005. Reaction

pathways and mechanisms of the electrochemical

degradation of phenol on different electrodes. Water Re-

search, 39(10): 1 972-1 981.

Li Y M, Gu G W, Zhao J F, Yu H Q, Qiu Y L, Peng Y Z, 2003.

Journal of Coal Science & Engineering (China) 430

Treatment of coke-plant wastewater by biofilm systems

for removal of organic compounds and nitrogen.

Chemosphere, 52(6): 997-1 005.

Lim B R, Hu H Y, Fujie K, 2003. Biological degradation and

chemical oxidation characteristics of coke-oven

wastewater. Water Air Soil Pollut, 146(1-4): 23-33.

Minhalma M, De Pinho M N, 2002. Development of nanofiltra-

tion/steam stripping sequence for coke plant wastewater

treatment. Desalination, 149(1-3): 95-100.

Szpyrkowicz L, Kaul S N, Neti R N, Satyanarayan S, 2005.

Influence of anode material on electrochemical oxidation

for the treatment of tannery wastewater. Water Research,

39(8): 1 601-1 613.

Vázquez I, Rodríguez-Iglesias J, Marañón E, Castrillón L,

Álvarez M, 2007. Removal of residual phenols from coke

wastewater by adsorption. Journal of Hazardous Material,

147(1-2): 395-400.

Wang B, Kong W P, Ma H Z, 2007. Electrochemical treatment

of paper mill wastewater using three-dimensional

electrodes with Ti/Co/SnO2-Sb2O5 anode. Journal of

Hazardous Materials, 146(1-2): 295-301.

Wang J L, Quan X C, Wu L B, Qian Y, Hegemann W, 2002.

Bioaugmentation as a tool to enhance the removal of

refractory compound in coke plant wastewater. Process

Biochem, 38 (5): 777-781.

Yavuz Y, Koparal A S, 2006. Electrochemical oxidation of

phenol in a parallel plate reactor using ruthenium mixed

metal oxide electrode. Journal of Hazardous Materials.

136(2): 296-302.

Zhang M, Tay J H, Qian Y, Gu X S, 1998. Coke plant

wastewater treatment by fixed biofilm system for COD

and NH3—N removal. Water Research, 32(2): 519-527.

Zhou M H, Wang W, Chi M L, 2009. Enhancement on the

simultaneous removal of nitrate and organic pollutants

from groundwater by a three-dimensional bio-electro-

chemical reactor. Bioresource Technology, 100 (20):

4 662-4 668.

Zhu X P, Ni J R, Lai P, 2009. Advanced treatment of

biologically pretreated coking wastewater by electro-

chemical oxidation using boron-doped diamond electrodes.

Water Research, 43(17): 4 347-4 355.