基于紫外光谱的水体硝酸盐氮浊度补偿研究

doi:10.3964/j.issn.1000-0593(2020)02-0472-06

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Abstract Nitrate nitrogen is one of the most important pollution indicators in water environment monitoring and can be detected quicklywithout pollution by absorption spectroscopy. In view of the fact that the ultraviolet absorption spectrum is susceptible to turbidity interference, the influence of formaldehyde turbidity standard solution on the ultraviolet absorption spectrum of nitrate nitrogen standard solution is analyzed by experimental method. Based on this, a turbidity compensation method based on compensation curve method is proposed to compensate and correct the ultraviolet absorption spectrum. Then this method is validated by experiments, and the results are good. Firstly, the ultraviolet absorption spectra of 12 groups of nitrate nitrogen standard solution with concentration of 0.2～10 mg·L^-1, 10 groups of formaldehyde turbidity solution with concentration of 5～50 NTU and formaldehyde turbidity solution mixed with nitrate nitrogen solution were collected in the laboratory. Theoretically, according to Lambert’s law, the absorbance of the solution should be the superposition of the absorbance of different solutes, but through the experimental analysis, the absorbance of the mixed solution in the main absorption spectrum region of nitrate nitrogen was not equal to the sum of the absorbance of nitrate nitrogen and turbidity. This was because turbidity particles break the coplanarity of nitrate nitrogen molecules, resulting in steric hindrance, which destroys the conjugate system and leads to the decrease of nitrate nitrogen absorbance. Therefore, the compensation coefficient between 0 and 1 was introduced to characterize the effect of turbidity on the absorption spectrum of nitrate nitrogen. The closer to 0, the greater the influence of turbidity on the absorption spectrum of nitrate nitrogen at this wavelength. Based on the measured spectral data, the compensation coefficients of different turbidities in the main absorption spectral region of nitrate nitrogen can be obtained. According to the experimental analysis, the absorbance of nitrate nitrogen in 350～400 nm band is basically 0. The absorbance of mixed solution is only related to turbidity, and the absorbance of both is basically the same. Therefore, the spectral integral of this band can be selected to establish the turbidity regression model, and the turbidity value of mixed solution can be calculated. Compared with single wavelength regression, the spectral integral regression model has good stability and is not easily disturbed by other factors. The square of correlation coefficient R of turbidity calculation model is 0.998 5, and the calculation effect is good. Turbidity compensation can be carried out after the turbidity value is obtained. The compensation method is validated by experiments and compared with single wavelength turbidity compensation and uncompensated of turbidity. The validation results show that after turbidity compensation by compensation curve method, the nitrate nitrogen prediction model based on partial least squares (PLS) algorithm is established. The RMSEP is 0.124 and the average absolute error (MAE) between the predicted value and the real value is 5.3%. The compensation effect is pretty good, and the other two methods will deviate greatly. In contrast, the turbidity compensation method proposed in this paper is obviously better than the other two. This method can provide an effective technical reference for the turbidity compensation of nitrate nitrogen ultraviolet absorption spectrum.

Key words：Nitrate nitrogen; Ultraviolet spectrum; Compensation coefficient; Turbidity solution model; PLS

Received: 2018-12-17 Accepted: 2019-04-21

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O433.4

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)02-0472-06 OR https://www.gpxygpfx.com/EN/Y2020/V40/I02/472