浊度干扰下硝酸盐浓度紫外导数光谱检测方法研究

doi:10.3964/j.issn.1000-0593(2019)09-2912-05

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Abstract Nitrate is one of the “three nitrogen” (nitrate nitrogen, ammonia nitrogen, total nitrogen) in water, and it is an important indicator to reflect the degree of pollution for water quality. The traditional method for measuring nitrate has many shortcomings, such as operational complexity, time consuming, and it is difficult to meet the real-time online detection requirements of modern water environment. Nitrate has strong UV absorption characteristics in the ultraviolet region. And in recent years, the UV absorption spectroscopy has been widely used for nitrate measurements for its real-time, low cost and easy operation. However, when the ultraviolet absorption spectrum is used to detect the concentration of nitrate, it is easily affected by the turbidity in water, causing the nonlinear lifting of the spectrum and measurement error. At present, the research on turbidity compensation algorithm is mostly used for the detection of the concentration of COD in water, and there are few studies on turbidity interference removal in nitrate detection. In that case, a method for measuring concentration of nitrate based on first derivative ultraviolet absorption spectrum is proposed to reduce turbidity interference and improve the accuracy of rapid detection of the concentration of nitrate. Ultraviolet absorption spectrum of the formalin and sodium nitrate standard solution and their mixed solution are measured in the region from 190 to 300 nm. Its spectrum is processed by first derivative method. In the meantime, Savitzky-Golay filtering is used to smooth the processed spectrum. After comparing the characteristics of turbidity and nitrate ultraviolet absorption gained by first derivative spectrum, studying the effect of turbidity on the first derivative spectrum of nitrate of different region, the results show that the effect of turbidity is small in the region from 220 to 230 nm. So this region is selected as the spectral analysis interval, and 30 kinds of concentrations of mixed solution of formazin and sodium nitrate solution are used as training samples. The partial least squares algorithm is used to establish the nitrate quantitative analysis model. This model is used to predict the concentration of nitrate in the remaining six different concentrations of formalin and sodium nitrate mixed solution. The results show that the predicted coefficient of determination of nitrate measurement is 0.994 3 and RMSEP is 0.346 9 mg·L^-1 under the condition of formazin interference. In order to further verify the accuracy and stability of the method, the model was also used to predict the concentration of nitrate in mixed water samples prepared from kaolin and potassium nitrate. The results showed that the predicted coefficient of determination of nitrate measurement is 0.991 5 and RMSEP is 0.362 8 mg·L^-1 under the condition of kaolin interference. In summary, the method to detect the concentration of nitrate by UV first derivative spectrum is proposed in this paper. The data from the UV derivative spectrum in the region from 220 to 230nm data was adapted, and PLS algorithm was combined. So we can measure the concentration of nitrate in water under turbidity interference quickly and accurately. Moreover, this study laid the foundation for further implementation of online analysis of actual water and further development of equipment.

Key words：Nitrate; Turbidity; First derivative spectroscopy; UV absorption spectrum; PLS; Correction model

Received: 2018-07-10 Accepted: 2018-11-23

ZTFLH:

O433

Corresponding Authors: ZHAO Nan-jing E-mail: njzhao@aiofm.ac.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)09-2912-05 OR https://www.gpxygpfx.com/EN/Y2019/V39/I09/2912