Study On The Masking Mechanism Of Chlorate On Electroplating Wastewater COD(cr) Detection

The COD（cr）concentration of electroplating wastewater is very high due to the utilization of massive surfactants and other substances,which are extremely difficult to be handled with traditional physical,chemical and biological methods.Based on the extensive field research,the effluent COD（cr）concentration could be decreased greatly and thus meet the discharge standard by the addition of chlorate into electroplating wastewater.However,there is more and more doubt on whether or not the organic pollutants were truly removed through this process.Therefore,it has an important and practical significance to develop the research on the organics removal by chlorate.Firstly,the real electroplating wastewater was applied as the test raw water and the organic matter indexes,such as chemical oxygen demand（CODcr）,total organic carbon（TOC）,biochemical oxygen demand（BOD）and UV₂₅₄ were analyzed and compared after adding the sodium chlorate into the wastewater.It turned out that the COD concentration was reduced by 61.1%from 822 mg/L to 320 mg/L at the sodium chlorate dosage of 4.0g/L,while the reaction time almost had no effect on the results and little or no changes in TOC,BOD and UV₂₅₄were found.So it could be preliminarily confirmed that the chlorate had the masking effect on COD detection.To verify the reaction mechanism of this process,the influencing factors,such as chlorate dosage,pH value and composition of organics in wastewater on COD detection were discussed,and the masking mechanism of chlorate on COD detection was further revealed according to the results of COD,TOC and chlorine elements（Cl^-,ClO₂ and ClO₃^-）,that is,the ClO₃^-and organic substances were subjected to oxidation-reduction reaction when the samples were digested by acid under high temperature,resulting in the decreased consumption of Cr₂O₇^2-;meanwhile most of the ClO₃^-were reduced to Cl^-which was then stabilized by mercuric sulfate（chloride ion masking agent）through complexing reaction,which could finally lead to the reduction of detected COD value.If the wastewater treated by this method were discharged continuously,massive organic pollutants and chlorate would eventually reach the aquatic environment,which gave rise to a severe damage to the local ecological environment.In order to assess the potential ecotoxicological effect caused by this process,the relevant studies were carried out using two typical kinds of algae,Scenedesmus obliquus and Chlorella.The results showed that when the sodium chlorate dosage was 4.0 g/L,the 50%effective concentration(EC₅₀)values of Scenedesmus obliquus and Chlorella by electroplating wastewater were decreased from 5.69%and 4.91%without addition of sodium chlorate to 4.98%and 4.43%,respectively,which indicated that the toxicity of electroplating wastewater was enhanced by chlorate.Although the addition of chlorate into the wastewater could reduce the COD value to the level of discharge standard,large amounts of organic matters produced from electroplating industry still exist without any effective treatment,which definitely has serious impacts on the ecological environment.In this study,the preliminary mechanism of masking effect of chlorate on COD（cr）detection was unveiled and provided the theoretical basis for the relevant environmental protection administrations on environmental analysis.