Deliberate addition of mildly oxidative chlorate (ClO3-), so-called "chemical oxygen demand (COD) remover", into wastewater in China or electrochemical production of ClO3- from Cl- induces the false COD reduction, which would bring about false appearance of effluents meeting the COD discharge standards. In this study, an easy sulfite-based reduction method was developed for the first time to remove ClO3- from the water samples before COD determination to eliminate this interference of ClO3-. In this reaction system, keeping the reaction temperature of sulfite reducing ClO3- at 60 °C was crucial for fast ClO3- removal rate, fixed molar [sulfite]ini/[chlorate]ini ratio value and the synchronous exhaustion of sulfite and ClO3-, which were of great significance for the real application of this improved COD determination method. The ClO3- interference on COD determination could be successfully eliminated after 20 min reduction of ClO3- by sulfite at pHini 4.0∼6.0 with the molar [sulfite]ini/[chlorate]ini ratio value in the range of 5∼6 when concentration of ClO3- was below 5 mM. Despite of the involvement of SO4·- in the sulfite reducing ClO3- system, the degradation of organic matters by SO4·- could be greatly impeded due to the lessened dissolved oxygen for SO4·- production at high reaction temperature and the scavenging of SO4·- by sulfite. In this reaction system, ClO2, ClO2- and ClO- were also generated and could be further reduced by sulfite stoichiometrically via oxygen transfer process with Cl- as the final product. In general, this study pioneered an effective, fast and convenient method for COD determination of the ClO3--laden wastewaters and evaluating the real electrochemical wastewater treatment performance in terms of COD removal.
Keywords: COD; COD remover; Chlorate; False reduction; Sulfite-based reduction method.
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