Material Evidence Technology Research For The Regulation Of Wastewater Illegal Discharge And Electrodes Development For Chemical Oxygen Demand Monitoring By Electrochemical Method

Water pollution and water security problem is becoming the environment problem of our Chinese nation. Water resources in our country are insufficient and the pollution of water resources is also serious. The development of multi-function, economical online monitoring equipment is to provide a powerful technical support for water environment regulation, to strengthen the monitoring of surface water pollution of the environment supervision, to eradicate illegal discharge and to promote the fundamental improvement of surface water quality in China. As the phenomenon of wastewater illegal discharge is serious, which has led to serious water pollution in our country, it is an urgent issue to strengthen the regulation of water environment. To investigate and eradicate the behavior of illegal discharge and meet the environmental protection requirements of our country, is the focus and the basic content of our research work in this thesis. In this paper, the important scientific and technical problems which are proposed to be solved by the developed equipment are including: （1） to solve the problem that the monitoring parameters are too little or even a single of the traditional water quality monitoring equipment; （2） to solve the problem that the sensor interface in the traditional water quality monitoring equipment cannot be spliced or displaced by the other sensors; （3） to solve the problem of function simplification in the traditional instruments and to diversify instrument functions, which can realize on-line monitoring, wireless alarming and automatic sampling; （4） to solve the problem of high error rate for alarming and sampling in the traditional instrument by using "multiple factors-double level" decision-making alarming system in our research and design; （5） to solve the problem of time-consuming, no real-time online monitoring, generating secondary pollution, bulky instrument and expensive in traditional water quality monitoring equipment for detecting chemical oxygen demand （COD）. These work, has important practical significance for the online monitoring and seamless supervision of surface water environmental pollution, for effectively curb illegal discharge phenomenon, for finding and treating water pollution accident in time and for protecting our precious limited water resource from pollution.The main results were as follows:（1） The design and development of the automatically monitoring-sampling-alarming equipment with "in situ float type" and "pump flow shore leave type". Design of the intelligent devices that with low error rate for sampling-alarming, and can be fit for various typical form of illegal discharge by equipping with the functions of online monitoring and supervision, automatic sampling of evidence and the remote wireless alarm of monitoring. The intelligent devices are installed in a necessary point for online monitoring and sampling. In order to adapt and meet the need of different monitoring sites we design two kinds of intelligent monitoring devices with low error rate and low-cost, the "in situ float type" and the "pump flow shore leave type". By designing and debugging the five parts of "online monitoring sensor system", "illegal discharge decision system", "remote wireless alarm system", "sample collection system" and "assistant peripheral module" for the automatic sampling device respectively, we developed two kinds of intelligent monitoring devices with low error rate and low-cost, the "in situ float type" and the "pump flow shore leave type". The device can automatically sample illegal discharge wastewater by the way of time-instant and time-delay as using the program control and synchronous wireless alarm. The devices can provide important valuable material evidence for the environmental workers to find out who is to blame for the pollution discharge. Also realized man-machine two-way dialogue to rewrite equipment decision command through remote control, by this way the use of the floating type automatic monitor is very convenient.（2） The performance test of the "in situ float type" and "pump flow shore leave type" automatic monitoring-sampling-alarming equipment. We selected the north canal at Beijing Tongzhou district, the central sewage treatment plant at Tongzhou district Huo County and four pollution enterprises near it to be a long time experiment sites to carry out the performance test of the devices. Also, we selected the outfall of YuLong River at Century road Zibo city and Huashan River at Fourbaoshan town Zibo city to be a short-term experiment sites. Experimental results show that the device is successfully realized the functions of on-line monitoring（monitoring temperature, conductivity, turbidity, dissolved oxygen, pH, ammonia nitrogen, lead ions, fluorine ion and COD indexes at the same time）, remote alarm （the synchronous wireless alarm let the environmental regulators at monitoring station receive the alarm timely） and automatic sampling （The device can automatically sample illegal discharge wastewater by the way of time-instant and time-delay as using the program control）. The average error rate< 10%.（3） The preparation and performance evaluation of high performance graphite felt substrate lead dioxide electrode （CF/β-PbO₂（CP）） of "constant voltage-current monitoring" method of detecting COD. The traditional method of detecting COD is the potassium dichromate titration reflux method, but this method is time-consuming, using poisonous and harmful reagent, complicated operation, and cannot be real-time online monitoring. So it is difficult to be used popularly and to be applied in the real-time online monitoring of water. But using the electrochemical method of detecting COD have the advantages of fast, efficient, simple operation, no secondary pollution, and can realize online real-time monitoring, etc. Traditional lead dioxide electrode which are used to detect COD, due to the specific surface area is small, lead to low COD detection sensitivity, narrow linear range and big different current response of different types of organic matter which with the same COD concentration, therefore, it is not conducive to the detection of COD. In this paper, through the design of a three-dimensional network structure of graphite felt （CF） substrate PbO₂ electrode. The micron size of lead dioxide crystals are successfully electrodeposited on a 3D woven mesh structure of graphite felt base by using the plating method of constant potential （CP）, and get CF/β-PbO₂（CP） electrode. Using CF/β-PbO₂（CP） electrode to detect COD, the results show that under constant potential at 1.45 V （vs. SCE）, the sensitivity is 2.05 x 10’3 mA cm-2/mg L^-1, the linear range is 50-5000 mg L^-1, R= 0.993, and the COD detection limit is 3.9 mg L^-1 （S/N=3）. The performance of CF/β*PbO₂（CP） electrode is superior to the currently published lead dioxide electrode in COD detection. At the same time, we use this 3D electrode to make a comparison with 3 kinds of 2D plane electrodes for the research of COD detection mechanism and COD detection performance. These 3 kinds of 2D plane electrodes are:titanium substrate lead dioxide electrode （Ti/Sb2O3-SnO₂/β-PbO₂）, graphite sheet substrate of lead dioxide electrode （GS/β-PbO₂） and boron doped diamond （BDD） electrode. The results showed that the COD detection performance of CF/β-PbO₂（CP） electrode is far superior to Ti/Sb203-SnO₂/β-PbO₂,GS/β-PbO₂ and BDD electrode.（4） The preparation and performance evaluation of high performance graphite felt substrate lead dioxide electrode （CF/CeO₂-β-PbO₂（CV）） of "constant voltage-current monitoring" method of detecting COD. Through the study of CF/β-PbO₂（CP） electrode, we find that due to the large size of micro grain （3 to 8 μm） which are plated with constant potential,in the use process of COD test in the long-term,crystal defects appear on the structure, and it is also easy to crack between crystal particles.In order to change the structure defects between single crystal, this paper puts forward of mixing nano-sized ceria particles in the plating solution, then adopt the method of co-deposition, in the electroplating process, CeO₂ and PbO₂ deposit together to form an active layer. At the same time, when using cyclic voltammograms （CV） plating method, under high potential is advantageous to the crystals’ nucleation, but under low potential is conducive to the growth of the crystals.Through changing the plating potential continuously, we can realize the transformationof PbO₂ from micron size to nano scale.Through electrochemical testing, the results show that the doping of CeO₂, can improve the combination performance between the crystals and prolong the life of the electrode, and improve the electric-generationrate of hydroxyl free radicals on the electrode surface. In addition, through the electrodeposition by cyclic voltammetry method, the grain size of CF/CeO₂-β-PbO₂（CV） electrodechange into 20-50 nm, thus greatly improves the detection sensitivity, linear range and detection limit of electrode.Use of CF/CeO₂-β-PbO₂（CV） electrode to detect COD, results show that under constant potential at 1.45 V （vs. SCE）, CF/CeO₂-β-PbO₂（CV） electrode, the COD detection sensitivity is 3.01 x 10-3 mA cm-2/mgL^-1, the linear range is 30-8500 mg L"1, R= 0.9985, the detection limit is 3.6 mg I/1 （S/N=3）, significantly better than that of CF/β-PbO₂（CV） and CF/β-PbO₂（CP） electrode. Using CF/CeO₂-β-PbO₂（CV） electrode as the work electrode, to design the on-line COD monitoring equipment. The COD online monitoring device is composed of the three electrodes detection system, constant potential system, signal conversion system, fluid conveying system, alarm system and sampling system. The device is with high degree of automation, and also use the principle of "double potentiostat-double working electrode" and work alternately, which make full use of the principle of "detection-regeneration-detection" features of CF/CeO₂-β-PbO₂（CV） electrode. We use this principle for the instrument design and debugging, thus shortening the COD testing time between sample and sample, then greatly improve the efficiency of the device. Using the online monitoring device for the COD testing of simulated water sample and actual water, the results show that it has good consistency with the COD test values of this method and national standard method of COD test, and which can be used for the real water COD detection. The design and development of COD online monitoring device, expecting to be a powerful weapon for real-time online monitoring of surface water.（5） The preparation and performance evaluation of high performance graphite felt substrate lead dioxide electrode （Ni/α-PbO₂-CeO₂-β-PbO₂（CV）） of "constant voltage-electric quantity monitoring" method of detecting COD. There are two kinds of constant potential method to detect COD, namely "constant potential-current monitoring" and "constant potential-electric quantity monitoring". This paper puts forward using the flexibility characteristics of nickel foam, as which can be easily bended and molded. We bend the nickel foam around and form the concentric ringsstructure, and then use cyclic voltammetry method for electroplating, firstly, in alkaline solution,a layer of a-PbO₂ is electroplating on the 3D structure of nickel foam substrate, and get Ni/a-PbO₂（CV） electrode.And then, under the condition of acid, an active layer of CeO₂-β-PbO₂is electroplating on the Ni/a-PbO₂（CV） electrode, and getNi/a-PbO₂-CeO₂-P-PbO₂（CV） electrode with a high specific surface and high electric catalytic activity. Using the "constant voltage-electric quantity monitoring" method of detecting COD Ni/a-PbO₂-CeO₂-β-PbO₂（CV） electrode.Results show that the electrode under constant potential at 1.45 V （vs. SCE） in 5 ml of sodium sulfate solution, injecting of 1 mL of COD samples with the concentration of 15-1000 mg L^-1, the single sample detection time is 10 to 20 min, and compared to a single sample test time （more than 2 h） of the national standard method （potassium dichromate titration reflux method）,the time shorten at least 6 to 12 times. It has good consistency with the COD test values of this method and national standard method of COD test, and it can be used for the COD detectionof real water.