Study On A New Wet-chemical Oxidative Digestion Technique For The Determination Of Chemical Oxygen Demand In The Environmental Water

In this thesis, based on the experimental procedures of national standard methods to determine the chemical oxygen demand?COD?, we had developed a new multistep wet-chemical oxidative digestion technique. The new digestion technique consisted of two digestion steps: firstly, the water sample was digested under the open-reflux condition in the alkaline digestion solution which containing potassium permanganate, manganese sulfate and potassium persulfate; secondly, the treated water sample was then acidified by adding a little of sulfuric acid, and continuously digested again in the acidic digestion media containing potassium dichromate, copper sulfate and silver sulfate, in order to achieve the complete degeneration for all of organic compounds in the water sample; finally, the redox potentiometric titration method was performed upon the treated aqueous sample to calculate the COD value quantitatively. Thus, a new method for the accurate determination of COD index in environmental water samples was successfully established. In addition, the proposed method could remedy some deficiencies of the current national standard titration methods about the determination of CODCr and CODMn index in environmental waters, and provided some experimental foundation to develop a new method for the rapid determination of COD in environmental waters with merits of low cost and high accuracy.This thesis consists of four chapters.Chapter one: ReviewFirst of all, the significance of the determination of COD was introduced briefly, and the two international standard methods of COD determination?Dichromate method and Permanganate index? were summarized. Then, some progresses in the use of oxidants, the selection of digestion systems and catalysts were described. At last, the current approaches on the instrumental determination method of COD, including chemiluminescence method, flow injection analysis, electrochemical method, photocatalytic method, photoelectrocatalytic oxidative degradation method and indirectly spectrometric method were reviewed.Chapter two: Accurate determination of low-level chemical oxygen demand using a multistep wet-chemical oxidation digestion process for treating drinking water samplesIn this section, a special wet-chemical oxidative digestion technique that combines the advantages of both alkaline potassium permanganate and acidic potassium dichromate digestion mode was adopted. And then the digestion mode coupled with the automatic potentiometric titration method and developed a new method for the determination of chemical oxygen demand?COD? in drinking water samples. In the continuing work to optimize the operation conditions, it can be found that there were some certain molar proportion relationships between the optimal added amounts of the oxidants and catalysts in the two kinds of digestion solutions, that were n?KMnO₄?:n?MnSO₄?=5:2, n?KMnO₄?:n?K₂Cr₂O₇? = 2:1, n?K₂Cr₂O₇?: n(Cu²⁺+Ag⁺)=1:2. Under the optimal experimental conditions, the digestion ratios of potassium hydrogen phthalate standard substance samples can reach 100%. The new method offered the outstanding advantages of a low detection limit?0.11 mg O2 L-1?, less sampling volume?only 10.00 m L? and a broad linear range of COD values?0.2-25.0 mg O2 L-1? and the use of mercury-free catalysts. Moreover, the tolerable interference concentration of chloride ions for a drinking water sample also was confirmed to be up to the level of 90 mg L-1. Thus, the proposed method was successfully applied to determine the ultra-low COD values of tap water and bottled water samples with satisfactory results.Chapter three: A fessibility study about the determination of chemical oxygen demand in the simulated water samples containing methylene blue by using a multistep wet-chemical oxidation digestion techniqueIn this section, the applicability of the multistep wet-chemical oxidation digestion method for treating the simulated water samples containing phenothiazine compounds was studied. With methylene blue as the probe molecule, we optimized the determination conditions including the digestion time, the addition amounts of oxidant and catalysts, the acidity of digestion solution and the interreact ratios of the added digestion catalysts through the experimental exploration. It can be found that there were some certain molar proportion relationships between the added amounts of oxidants and catalysts in the two kinds of digestion solutions, which were n?KMnO₄?:n?MnSO₄? =3:1, n?KMnO₄?:n?K₂Cr₂O₇?= 3:2, n?K₂Cr₂O₇?:n(Cu²⁺+Ag⁺)= 6:5. Under the optimized conditions, the degradation ratios of the methylene blue in the water samples could be validated to reach 93.6%. In addtion, the COD values of the simulated water samples containing methylene blue, toluidine blue and promethazine hydrochloride were determined by the proposed method. The results showed that there were good linear relationships between the mass concentration of the three phenothiazine compounds and the found COD values in a certain concentration range.Chapter four: A feasibility study about the determination of chemical oxygen demand in the simulated water samples containing some organic compounds by using a multistep wet-chemical oxidation digestion techniqueIn this chapter, the applicability of the multistep wet-chemical oxidation digestion technique for treating the simulated water samples containing small molecule alcohols and carbohydrates was studied. In this experiment, with ethanol as the probe substance, we examined the effect of some COD determination conditions such as the digestion time, the addition amounts of oxidant and catalysts, the acidity of digestion solution and the interreact ratios of the added catalysts. It can be found that there were some certain molar proportion relationships between the added amounts of oxidants and catalysts in the two kinds of digestion solutions, which were n?KMnO₄?:n?MnSO₄?=3:1, n?KMnO₄?:n?K₂Cr₂O₇?=3:2 and n?K₂Cr₂O₇?:n(Cu²⁺+Ag⁺) =12:5. Under the optimized conditions, the proposed method was applied to determine the COD values of several simulated water samples containing small molecule alcohols, and validated the degradation ratios of 96.3% for ethanol in water samples. Simultaneously, it was showed that there were good linear relationships between the mass concentration of the alcohols in the simulated water samples and the measured COD values in a certain concentration range. Furthermore, the proposed method could measure the COD values of the simulated water samples containing some kind of carbohydrates such as sucrose, lactose, trehalose, sorbitol and xylitol with the degradation ratios more than 96%, respectively.