Rapid Measurement System Of Nitrate In Seawater Based On Ultraviolet Absorption Spectrometry

Nitrate is an important nutrient and the dominant form of inorganic nitrogen in the ocean.It is listed as one of the important seawater quality monitoring indicators because its excessive concentration in seawater can easily lead to the environmental disaster such as eutrophication and red tide.With the rapid development of coastal economy,nutrients are discharged into the sea,leading to the aggravation of eutrophication and frequent disasters such as red tide,hindering the development of coastal economy.Inorganic nitrogen exceeding standard is the most important reason for marine environmental monitoring of seawater quality exceeding standard in recent years.It is important to strengthen the monitoring of nitrate in seawater for controlling marine pollution and preventing and mitigating disasters.The traditional detection method of the marine nitrate concentration in China is chemical method,which is complicated and has the problem of secondary pollution.The nitrate monitor based on optical method has the advantages of fast measurement speed,no pollution and long-time continuous measurement without water sample pretreatment.In this background,this paper established the measurement algorithm for nitrate concentration in seawater that can calibrate the interference of bromide ion,organic matter and turbidity,and completed the construction of the measurement system.The system developed by ourselves has a good performance compared with similar foreign instruments and laboratory chemical methodsThe main research contents and results of this paper are as follows:（1）An algorithm of nitrate concentration measurement in seawater based on Back Propagation Neural Network（BPNN）was established.It realizes the calibration of the influence of bromide ion and organics disturbing the measurement of nitrate concentration in seawater and the deduction of the influence of turbidity.The Mean Absolute Error（MAE）of the calibrated BPNN model is 11.18μg/L for NO₃^--N solutions with concentrations 0-900μg/l,and the linear correlation coefficient（R²）between the measured values and the actual values is 0.998 under the interference of bromide and organic matter.Under the common interference of bromide ion,organic matter and turbidity,the R² between the measured value and the actual value of BPNN model was 0.965 for the solution with NO₃^--N concentration 0-900μg/l after the turbidity interference elimination and bromide ion and organic matter calibration.Compared with the traditional Partial Least Squares（PLS）model,the accuracy of BPNN model is higher.（2）A rapid measurement system was completed for nitrate measurement in seawater,which including the nitrate concentration measurement algorithm,full-spectrum monitor instrument and the upper computer software.The developed system was compared with the TRIOS-OPUS（made in German）production in the laboratory,and the R² between the measured values of the developed system and the OPUS monitor was 0.969 in cross interference environment.（3）The developed system was used in the 2020 autumn voyage of Yantai Ecological Environment Monitoring Center in Shandong Province to measure the nitrate of the actual seawater in the coastal area of Yantai.The results are compared with laboratory chemical methods and similar instruments abroad.The R² between the measured value of the self-developed system and that of the laboratory chemical method is 0.707,which further proves the performance of the self-developed seawater nitrate concentration measurement system,and the system is competent for the task of rapid measurement of seawater nitrate concentration.