Chlorate induced false reduction in chemical oxygen demand (COD) based on standard dichromate method: Countermeasure and mechanism

Abstract

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.