Chlorate induced false reduction in chemical oxygen demand (COD) based on standard dichromate method: Countermeasure and mechanism. (1st August 2022)

• Record Type: Journal Article
• Authors: Xiao, Huiji; Yan, Wei; Zhao, Zekun; Tang, Yizhen; Li, Yifan; Yang, Qipeng; Luo, Siyi; Jiang, Bo
• Is Part Of: Water research. Volume 221(2022)
• Publication Date: 2022-08-01
• Subjects: COD -- Chlorate -- COD remover -- False reduction -- Sulfite-based reduction method; Water -- Pollution -- Research -- Periodicals; 363.7394
• DOI: 10.1016/j.watres.2022.118732
• Languages: English
• ISSNs: 0043-1354

Abstract

Highlights: A rapid test paper and spectrophotometric methods were developed to detect ClO3 − concentration. Sulfite-based reduction method was developed to eliminate ClO3 − interference on COD test. The reaction condition was optimized for the removal of ClO3 − by sulfite. Temperature was crucial for the successful application of sulfite-based reduction method. 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. Graphical abstract: Image, graphical abstract