3. Results and discussion
3.1. Effect of electrochemical treatment on COD variation Variations of COD values with respect to time in different electrochemical methods have been investigated and the results are shown in Fig. 2.
As can be seen, the highest COD removal efficiency was attained for EC + Fe + Air + pH 3 method in comparison with that of other methods, which indicated that the combination of Fe particles, air and adjusting the pH have been efficient for the treatment of wastewater.
In the direct electrochemical process (EC), COD value of the wastewater decreased from 4753 mg L−1 to 2040 mg L−1 after 60 min treatment, which decreased radically after the first 20min, and then changed slightly.
The reason is that a small amount of Cl− in the wastewater can be oxidized and form some strong oxidants, such as chlorine and hypochlorite ion (Eqs. (2)–(4)), which can oxidize and decompose some organic compounds resulting in the COD decrease [17].
The test results showed that Cl− in the wastewater has been exhausted from 4633 mg L−1 to 1268 mg L−1 and 1093 mg L−1after 20 min and 60 min, respectively. Moreover, although the COD dropped slowly in the last 40 min, the COD value had still been declining, and 57.08% of COD removal was obtained after 60 min treatment, this means that the main role in this process has to rely on the action of electrodes.