Determination of Chemical Oxygen Demand in High Chloride Wastewater from Rare Earth Extraction Separation Process by Potassium Dichromate Oxidation-chlorine Correction Method

Abstract

Chemical oxygen demand (COD), is an important indicator for evaluating the degree of water pollution,and is a commonly used analytical project in water quality monitoring. The potassium dichromate standard solution and silver nitrate-bismuth nitrate mixture is quantitatively added to the water sample in a medium of sulfuric acid, with silver sulfate acted as catalyst, heated and boiled, with 1,10-phenanthroline as an indicator, and a standard solution of ammonium ferrous sulfate is used for titration to calculate the apparent COD. The chlorine gas produced by the oxidation of uneliminated chloride ions in the water sample is extracted with a certain flow of nitrogen gas, and then absorbed by sodium hydroxide. After that, potassium iodide, starch, and sodium thiosulfate standard titration solution are added to the mixture in a sulfuric acid medium. The chloride ion correction value is calculated based on the titration result. The difference between the apparent COD and the chloride-corrected value is calculated to determine the chemical oxygen demand of high chloride wastewater. The interference experiment shows that, in high chloride samples, a certain concentration of ammonium ions react in sulfuric acid medium, affecting the accuracy of the results and making it difficult to correct. Therefore, the concentrations of chloride and ammonium ions in the reaction system must be strictly controlled to ensure accurate results. This method is suitable for the determination of chemical oxygen demand (COD) in high chloride wastewater from rare earth extraction processes, with a relative standard deviation (RSD, n = 10) less than 5% and a recovery rate of 95% to 104% for spiked samples.