Electro Oxidation with Active Chlorine

A very different behavior is found when the wastewater contains chloride ions since the generated active chlorine species such as Cl2, HClO and/or ClO-, ClO2-, ClO3-and ClO4- can attack the organics in competition with ROS (Bergmann et al. 2009; Martinez-Huitle and Brillas 2009; Panizza and Cerisola 2009). This procedure is usually called electro-oxidation with active chlorine and is based on the direct oxidation of Cl- ion at the anode to yield soluble chlorine by reaction (8.25):

The electrogenerated Cl2 can form bubbles if its local concentration exceeds its solubility, although it partly diffuses away from the anode and can react with chloride ion to form trichloride ion from reaction (8.26); otherwise, it is rapidly hydrolyzed and disproportionated to hypochlorous acid and chloride ion from reaction (8.27):

Hypochlorous acid is then in equilibrium with hypochlorite ion at p^a=7.55:

From the above equilibria, it is found that Cl3- is formed in a very low concentration up to pH ca. 4, while the predominant species are Cl2 until pH near 3, HClO in the pH range 3-8, and ClO- at pH > 8 (Martinez Huitle and Brillas 2009). The mediated oxidation with active chlorine species is expected to be faster in acid than in alkaline media because of the higher standard potential of Cl2 (E° = 1.36 V vs. SHE) and HClO (E° = 1.49 V vs. SHE) compared to ClO- (E°=0.89 V vs. SHE). Nevertheless, only the chemical action of hypochlorite ion and generated ROS is commonly referred for organic mineralization since most electrolyses are performed with alkaline wastewaters. The concentration of hypochlorite ion, however, can be limited by its anodic oxidation to chlorite ion from reaction (8.29), followed by the successive oxidation to chlorate and perchlorate ions from reactions (8.30) and (8.31), respectively:

It is worth mentioning that the formation and accumulation of toxic chloroderiv-atives, trihalomethanes, and chloramines is a possible major drawback of the electro-oxidation in the presence of active chlorine.

As a conclusion from Sects. 8.3.1 and 8.3.2, the complexity arising from both, the generation of ROS in the AO treatment of non-chloride effluents and chlorine active species when using chloride effluents, makes the degradation rate and current efficiency of these processes to be a function of different experimental parameters such as pH, temperature, stirring, substrate concentration, and current density, and their effects need to be studied to reach the best operative conditions of the applied method.

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