Chlorinated phenols compounds used widely in industry were intentionally or inadvertently released into environment in large quantities. These chlorinated aromatic compounds have strong resistance to physical, chemical and biological treatments and have been listed as a priority pollutant by the U.S. Environmental Protection Agency (Lin and Tseng, 1999). Wastewater containing chlorophenols has malodorous odor, high toxicity, bioaccumulation and carcinogenic potential. As the accumulation of chlorinated aromatic compounds in the environment has become a serious problem nowadays, it is very urgent to develop effective method to treat these contaminants.

Traditional treatment processes such as biological treatments were not very effective for degradation of chlorinated phenols. Recently, advanced oxidation processes (AOPs) have attracted much attention as they can be used to effectively treat wastewater containing chlorophenols. Electrochemical methods for degrading chlorophenol compounds have been widely studied (Brillas et al., 1995; Tahar and Savall, 1998; Houk et al., 1998; Polcaro et al., 1999; Cong et al., 2004).

The electrochemical reduction approach was suggested in the 1970s as a promising method for detoxification of chlorine-containing aromatic hydrocarbons. This method ensured the selective removal of chlorine atoms from various chloroaromatics under mild experimental conditions without using the highly reactive reducing agents (Huang and Rusling, 1995; Tsyganok et al., 1999). Reductive dechlorination promoted by zero-valent metals has been a very active research area (Zhang et al., 2001; Cheng and Wu, 2001). However, metal catalyst fouling by some oxide precipitates was a problem, which decreased the effectiveness of the catalyst over time (Li and Farrell, 2000). Most reductive approaches focus on the removal of chlorine atoms from the aromatic structure but no further measures were applied simultaneously to treat the organics after the dechlorination.

Electrochemical oxidation has also been used for the treatment of chlorinated compounds. Some high oxygen over-potential anodes have shown potential for rapid oxidation of organics (Comninellis and Pulgarin, 1993; Wu et al., 2002a). Hydroxyl radicals formed in the process of oxidation are very powerful oxidizing agents that can open the aromatic nucleus of chlorinated compounds and bring out the chlorinated aliphatic intermediates (Oturan et al., 2001). It is well known that most chlorinated compounds are toxic due to the chlorine contained in their structure. Therefore, the toxicity of intermediates containing chlorine might not decrease or even increase.

The objective of this study was to probe the feasibility of using a combination process of reduction and oxidation and to develop an effective and friendly method for treating contaminants in chlorine compounds. Simulated wastewater containing chlorophenols was introduced into cathodic region where the chlorine atoms of chlorophenols were removed from the aromatic structure and chlorophenols were reduced to non-chlorinated intermediates. Then the wastewater from the cathodic region was introduced into anodic region where the non-chlorinated intermediates were oxidized and degraded.

Fig. 1 Chromatogram of degradation by single electrochemical reduction and oxidation (a) The initial 2-chlorophenol; (b) Treatment by the single electrochemical reduction; (c) Treatment by the single electrochemical oxidation

Table 1 Identification of 2-chlorophenol degradation intermediates

Fig.1b shows that there was only phenol intermediate during the degradation of chlorophenol by the single electrochemical reduction. That is to say, 2-chlorophenol was degraded to phenol and no other intermediates were generated. The chlorophenol intermediate obtained by the single electrochemical reduction was quite simple, but there were many intermediates during the degradation of chlorophenol by single electrochemical oxidation. Fig.1c and Table 1 show that benzoquinone, catechol, organic acids and a chlorinated intermediate were produced during the oxidation of chlorophenol. Apparently the intermediates were much more complicated by the electrochemical oxidation than by the electrochemical reduction. This observation result was similar to that of other investigations by the electrochemical oxidation methods. Hirvonen et al.(2000) noticed that chlorophenols could be treated efficiently, but the dechlorination of the compounds was insufficient. Hydroxylation of chlorophenols and the formation of dimeric products were involved in the oxidation of chlorophenols. Other investigators also detected the complex intermediates including those containing the chlorine during the degradation of chlorophenols by oxidation method (Mylonas and Papaconstantinou, 1996; Antonaraki et al., 2002). The presence of a complex mixture of intermediates suggested that the toxicity of wastewater could not be but might even be increased.

Fig.1 shows that phenol was generated by the electrochemical reduction, but no phenol peak was observed under electrochemical oxidation. On the other hand, benzoquinone, organic acids and intermediate containing chlorine were detected by electrochemical oxidation and they did not occur during electrochemical reduction. In view of the two cases, it may be better to remove the chlorine atom from the benzene ring and degrade chlorophenol to phenol through electrochemical reduction as the phenol produced during electrochemical reduction can be degraded easily by electrochemical oxidation. Through the combination of reduction and oxidation, chlorophenol can be removed efficiently. Therefore, it is feasible to degrade contaminants containing chlorine by electrochemical reduction before electrochemical oxidation.

Fig. 2 Chlorophenol degradation by electrochemical reduction

Fig. 3 Chlorophenol degradation by electrochemical oxidation

Fig. 4 Chlorophenol degradation by combination of reduction and oxidation

Fig. 1s Description: Structural formulas in the article, expressed as a supplementary figure

The current efficiency for the degradation of CP could be calculated using the following Eq.(1):

(1)

where N is the number of electrons used for degrading chlorophenol (mol/(s·cm²)), N ₀ is the total electron numbers per electrode area per time supplied by power source (mol/(s·cm²)), n is the number of electrons required for degrading one mole chlorophenol (here it was 1), V is the volume of wastewater of chlorophenol (L), I is the current (A), t is the reaction time (s), F is the Faraday constant (96480 C), C_t and C_{t +Δt} are the concentration of chlorophenol at the reaction time t and Δt, respectively.

Table 2 lists the current efficiency of anodic oxidation, cathodic reduction and their combination process calculated according to the Eq.(1). Table 2 shows that cathodic reduction had a higher current efficiency than anodic oxidation at the identical time span of degradation. The efficiency of the combination process was higher than that of anodic oxidation and was close to that of cathodic reduction alone. These results show that cathodic reduction made a large contribution to the degradation of chlorophenol. However, the current efficiency decreased very rapidly with time. Comparison of Figs.2, 3 and 4 shows that the concentration of chlorophenol decreased after 3 h degradation, while the concentration of organic acids was still high after 5 h. In fact, low toxicity organic acids can be degraded by the economical biological method. Therefore, it is not advisable to completely degrade chlorophenol to CO₂ and H₂O. The detailed discussion was given in our previous work (Wu and Zhou, 2001) which suggested partial degradation.

Table 2 Current Efficiency of 2-Chlorophenol Degradation

Anodic oxidation, cathodic reduction and the combined process of reduction and oxidation were used to degrade the wastewater of chlorophenol. Results indicated that the combination of electrochemical reduction and oxidation was better than single reduction and single oxidation for the degradation of chlorophenol and intermediates. More than 90% chlorophenol could be degraded in 5 h. Cathodic reduction had better effect on the degradation of chlorophenol than anodic oxidation. Hydrogen atoms played an important role in reducing chlorophenol. It was found that cathodic reduction had higher current efficiency than anodic oxidation. Results showed that the energy cost of anodic oxidation was greater than that of cathodic reduction. Cathodic reduction was the main contributor to the degradation of chlorophenol. The combination of electrochemical reduction and oxidation was effective and feasible method for degrading contaminants containing chlorine.