A Novel Method For Measuring COD Index In Dye-containing Wastewater By Coupling Microwave-assisted Sample Treatment Technique With The Potentiometric Titration Mode

Until now,the COD determination by using national standard methods still has a few of deficiencies,including massive consumption of time,serious secondary pollution to the environment due to the use of hazardous mercury or chromic salts.Therefore,based on our previous research,we develop a new method for measuring COD index in dye-containing wastewater,in which the water sample can be directly degraded through the microwave-assisted two-step wet-chemical oxidation reaction,while the treated sample solution will be determined again by the potentiometric titration mode.All of experimental results have demonstrated that under the each optimum condition,three heterocyclic dye molecules in the simulated dye-containing wastewater samples,such as phenothiazine dyes,phenazine dyes and triphenylmethane dyes can be completely degraded with the total digestion efficiency of more than 95%.Compared with the standard COD method,this method can offers some technical advantages: the high digestion capability for the heterocyclic organic dyes,good precision,a short analysis time(approximately 5 min required per sample),high COD sensitivity for the organic wastewater sample.Especially,there is a new "green" analysis process owing to the no use of any hazardous mercury or chromic salts reagents.The thesis consists of four chapters.Chapter one: ReviewFirst of all,we introduced the classification about the organic synthetic dyes and revealed the harmfulness of dye-containing wastewater to environment and human health.Additionally,some treatment techniques for the dye-containing wastewater are summarized.Because a trace-level amount of dye effluent might lead to a visible pollution of the whole water body,while chemical oxygen demand(COD)is an analytical index to assess the contamination degree of water body.Thus,the research progress for COD analytical method and the some research contents related to the application of oxidants and catalysts in the wet-chemical sample treatment process were also reviewed.Chapter two: A rapid determination method of chemical oxygen demand in dyeing wastewater containing phenothiazine dyes.In this section,we have developed a new method for rapid determination of low-level COD index in dyeing wastewater.In this method,a unique sample treatment that combines the advantages of microwave-assisted digestion and two-step wet-chemical oxidative reaction with paired oxidants such as KMn O4 and Ce(SO4)2 is adopted.By using methylene blue(MB)as the model dye molecule,the determination conditions including the microwave digestion time(t1=110 s,t2=60 s),the addition dosage of both oxidative and catalyst reagents are optimized.In particular,it is found that there exist the specific molar proportion relationships between the added amounts oxidants and catalysts in the acidic and alkali digestion solutions,which can be expressed as n(KMn O4):n(Mn SO4)=6:1 and n(Ce(SO4)2):n(Cu2++Ag+)=4:3.Moreover,we find that potassium tungstate added into the acidic digestion solution can significantly inhibit the form of manganese dioxide precipitate due to the thermal decomposition side reaction of potassium permanganate.Under optimized operating conditions,the total average digestion efficiency of the MB dye molecules is up to99.5%.Then,the COD quantitative ranges of the MB dye are 1.0-75.0 mg O2 L-1with a correlation coefficients(R2)of 0.9996 and its lower limit of detection(LOD)of 0.33 mg O2 L-1(3?).Moreover,the method's application for other types of phenothiazine dyes also has achieved satisfactory results.Chapter three: Rapid determination of chemical oxygen demand in dyeing wastewater containing phenazine dyes.In this section,the proposed method that couples microwave-assisted sample treatment technique with the potentiometric titration has been studied on its applicability for COD determination in the simulated water samples containing phenazine dyes.By using neutral red as the model dye molecule,we have optimized the determination conditions including the digestion time,the addition amounts of oxidant and catalysts,the acidity and alkalinity of digestion solution on the digestion efficiency of phenazine dyes.In comparison with the experimental evidences of the second chapter,it is found that even though the molecular structure of phenazinedye is different to that of phenothiazine dye,the specific molar proportion relationships between the added amounts oxidants and catalysts in the acidic and alkali digestion solutions,which are n(KMn O4):n(Mn SO4)=6:1 and n(Ce(SO4)2):n(Cu2++Ag+)=4:3,still be kept;but except for the slight change of the optimal microwave digestion time(t1=150 s,t2=70 s).Then,under optimized operating conditions,the the total average digestion efficiency of the neutral red dye molecules is up to 96.8%.The COD quantitative ranges of the neutral red dye are0.6-61.0 mg O2 L-1with a correlation coefficients(R2)of 0.9997 and its lower limit of detection(LOD)of 0.21 mg O2 L-1(3?).In addtion,the COD values of the simulated water samples containing neutral red,phenosafranin and Safranine T were determined by the proposed method.The results showed that there were good linear relationships between the gross mass concentration of the dye substance and the found COD values in a certain concentration range,and we can prove that this method is suitable for the determination of COD value in the organic wastewater samples containing phenazine dyes.Chapter four: Rapid determination of chemical oxygen demand in dyeing wastewater containing triphenylmethane dyes.In this section,the proposed method that couples microwave-assisted sample treatment technique with the potentiometric titration has also been studied on its applicability for COD determination in the dyeing wastewater containing triphenylmethane dyes.By using crystal violet as the probe molecule,after a series of experiments,we can obtain the optimized determination conditions including the digestion time,the addition amounts of oxidant and catalysts,the acidity and alkalinity of digestion solution and auxiliary catalyst dosage.Herein,the optimum microwave digestion time will be changed(t1=140 s,t2=80 s)due to difference of the target dye molecules,but the specific molar proportion relationships is still remain in this case.Under optimized operating conditions,the the total average digestion efficiency of the crystal violet molecule is up to 96.2%.The COD quantitative ranges of the crystal violet dye are 1.2-71.0 mg O2 L-1(R2= 0.9994)and its lower limit of detection(LOD)of 0.39 mg O2 L-1(3?).At the same time,the COD values in asimulated water samples containing Crystal violet,bright green and malachite green dye are determined by the proposed method.The results showed that there were good linear relationships between the gross mass concentration of the dye substance and the found COD values in a certain concentration range,these results are very satisfactory.