Study On Key Technologies Of On-line Detection Of Total Chromium In Water Quality Based On Advanced Oxidation Processes

With the continuous development of society and the deepening of modernization,the pollution of water environment is also increasing.In order to protect the water resources that human beings depend on for survival,it is very important to reflect the water pollution through the automatic on-line monitor of water quality.Among them,the automatic on-line monitor of total chromium in water quality is an indispensable instrument in water quality monitoring.At present,the method used for automatic on-line monitor of total chromium in water quality in domestic and foreign markets is potassium persulfate oxidation-diphenylcarbazide spectrophotometry.The potassium persulfate,a strong oxidant used in this method has certain toxicity and deliquescence,is not safe and easy to preserve.In addition,the method also has problems such as high reaction temperature,long reaction time of pre digestion and oxidation,excessive use of chemical reagents,high cost of operation and maintenance and complicated operation,which makes it difficult to meet the new requirements such as environmental friendly,safety and efficiency of on-line detection of total chromium in water quality at this stage.In order to find a new method of total chromium determination with safety,environment friendly,energy saving,high efficiency,simple operation and high detection accuracy,this study focused on the design of total chromium oxidation digestion method and its on-line digestion and detection device.At the same time,the validity of this method was experimentally verified,and the design of the on-line detection device of total chromium in water quality and computer control system were carried out,which laid a solid foundation for the development of the new automatic on-line monitor of total chromium in water quality.The main research content of the paper is as follows:（1）Based on the analysis of the importance of automatic on-line monitor of water quality to the protection of water resources and the harm of heavy metal chromium pollution in water environment,the national standard method and other methods of determination method for the total chromium in water quality,the development status of automatic on-line monitor of total chromium in water quality in domestic and foreign markets was analyzed and summarized,which established a new method for oxidative digestion of total chromium in water quality.（2）The oxidation digestion reaction and color reaction in the detection process of total chromium were studied,and a new method for the determination of total chromium in water quality based on UV/O₃ advanced oxidation-DPC spectrophotometry was proposed.In view of the problems existing in potassium permanganate or potassium persulfate oxidation-DPC spectrophotometry,such as high reaction temperature,long reaction time of pre-digestion and oxidation,excessive use of chemical reagents,high cost of operation and maintenance,tedious operation,etc.,and the comparative analysis of the applicability of various common oxidants such as KMn O₄,K₂S₂O₈,（NH₄）₂S₂O₈,KCl O₃,H₂O₂ and applicability of various advanced oxidation processes,the·OH with stronger oxidation ability was selected as the oxidant for total chromium oxidation digestion and the UV/O₃ method was determined as the basic method of total chromium oxidation digestion.The problem of negative interference of ozone oxidants on the measurement results of total chromium is specifically manifested in that hydrogen peroxide produced by ozone degradation will promote the reduction of hexavalent chromium ions under acidic conditions and lead to low detection.This problem can be effectively avoided by adjusting the sample p H.At the same time,the design of an experimental device for the detection of total chromium in water quality was explored,and the effects of the volume of the reaction solution,p H,reaction time,ozone gas volume,and ultraviolet light intensity on the total chromium oxidation rate,as well as the effects of solvent of chromogenic agent,acidity,dosage and chromogenic time on the measurement results of total chromium,in this way,the better process operating conditions was obtained.（3）The performance index of the experimental device for detecting total chromium in water quality was tested and evaluated.The results show that the calibration curve of total chromium has good linearity when the concentration of total chromium is in the range of 0～1.25 mg/L,R²=0.9998;At the same time,the precision-RSD,indication error-δ,method detection limit-MDL and conversion-P were1.14%,1.17%,0.0007 mg/L and 98.99%,respectively,which were better than the basic requirements stipulated in the national environmental protection standard:≤5%,±5%,≤0.01 mg/L and≥90%.The feasibility and effectiveness of UV/O₃ advanced oxidation-DPC spectrophotometry for the determination of total chromium in water quality were verified.In addition,the influence of common metal ions in industrial wastewater on the determination results of total chromium based on advanced oxidation processes was further studied and verified,which provides a reference for the analysis of automatic on-line detection results of total chromium in water quality.（4）According to the specific operation flow of the automatic on-line detection device of total chromium in water quality,the pretreatment unit,oxidation digestion/color development unit,sample injection/metering unit,analysis unit and control unit were selected and designed in detail.At the same time,according to the specific process control requirements,the corresponding automatic control system scheme was put forward.Finally,based on the statistics of system measurement and control points,a microcomputer control system based on S7-200 SAMRT PLC was designed,and the hardware of the system was selected and configured,which lays a solid foundation for the development of automatic on-line detection device of total chromium in water quality.