Method for rapidly measuring nitrate content in water based on COD (Chemical Oxygen Demand) compensation

Abstract

The invention discloses a method for rapidly measuring nitrate content in water based on COD compensation, and belongs to the technical field of water sample determination. Comprising the following steps: 1) collecting ultraviolet absorption spectrums of nitrate nitrogen standard solutions, COD standard solutions and mixed solutions with different concentrations, and calculating to obtain a differential spectrum; 2) determining a COD characteristic wave band, namely a COD compensation model interval, by using the residual sum of squares; 3) determining an optimal nitrate modeling interval; 4) selecting a COD characteristic wavelength interval, performing linear fitting on the spectral integral in the selected wavelength interval and the COD concentration, and establishing a COD concentration prediction model; 5) obtaining a nitrate spectrum after COD compensation; and 6) obtaining the nitrate concentration after COD compensation. Compared with other methods, the method for predicting the nitrate concentration in the water sample by using the ultraviolet spectroscopy does not need chemical agents, does not cause secondary pollution to the environment, is convenient and rapid, and can be used for large-scale detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of water sample determination, and particularly relates to a method for rapidly measuring the nitrate content in water based on COD compensation. Background Art

[0002] Common biological denitrification processes include nitrification-denitrification, shortcut nitrification, shortcut denitrification coupled with anaerobic ammonia oxidation, etc. Nitrate nitrogen, chemical oxygen demand (COD), etc. are conventional indicators for testing the operation of reactors. Therefore, it is necessary to find a method that can accurately and efficiently measure nitrate nitrogen and organic matter in water samples. Common methods for measuring COD include potassium dichromate titration method and potassium permanganate method. In addition to the ultraviolet spectrophotometry method of adding hydrochloric acid and sulfamic acid commonly used in laboratories for the determination of nitrate nitrogen content in water, Yaqoob proposed a method using flow injection method and fluorescence reagent chemiluminescence detection method to determine nitrate and nitrite in fresh water. Shariati-Rad proposed a simple and sensitive spectrophotometry method for simultaneous determination of nitrite and nitrate, which is based on the reaction of nitrite and nitrate with 2,4-xylenol as a reagent in concentrated sulfuric acid. Ion chromatography uses the principle of ion exchange, and qualitative and quantitative analysis is carried out according to the retention time and peak area of nitrate nitrogen in the standard solution through a conductivity detector. However, these methods are accompanied by disadvantages such as the need to use chemical reagents, high cost, and long measurement cycle.

[0003] Combining ultraviolet absorption spectroscopy with advanced optical sensors has the advantages of high detection efficiency, high precision, no need to add excessive chemical reagents, and can perform on-line detection of water quality compared with traditional laboratory chemical methods. Establishing a correlation model between ultraviolet spectra and the concentrations of organic and inorganic substances to evaluate the quality of water has become an important measure for the future application of spectral analysis in the field of water quality monitoring. Establishing a correlation model between absorbance and substance concentration can efficiently and accurately determine the content of pollutants in water. It can be seen that the combination of chemometric methods and ultraviolet absorption spectroscopy for analysis is an important development direction for future water quality detection.

[0004] Ultraviolet spectroscopy does not require pretreatment of water samples and has a short detection time. However, the presence of COD is not conducive to the ultraviolet spectroscopic determination of nitrate because COD also has an absorption peak at the characteristic wavelength of nitrate, resulting in partial overlap of the ultraviolet absorption curves of the two, which is not conducive to accurately determining its concentration. Therefore, it is necessary to eliminate the interference of COD. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for rapidly measuring the nitrate content in water based on COD compensation to solve the problem in the prior art in the background art that the presence of COD is not conducive to the ultraviolet spectroscopic determination of nitrate because COD also has an absorption peak at the characteristic wavelength of nitrate, resulting in partial overlap of the ultraviolet absorption curves of the two, which is not conducive to accurately determining its concentration.

[0006] The object of the present invention can be achieved by the following technical solutions:

[0007] A method for rapidly measuring the nitrate content in water based on COD compensation, comprising the following steps:

[0008] 1) Collect the ultraviolet absorption spectra of nitrate nitrogen standard solutions, COD standard solutions, and mixed solutions with different concentrations, and calculate the difference spectrum;

[0009] 2) Use the sum of squared residuals to determine the COD characteristic band, that is, the COD compensation model interval;

[0010] 3) Determine the optimal nitrate modeling interval;

[0011] 4) Select the COD characteristic wavelength interval, linearly fit the integral of the spectra within the selected wavelength interval with the COD concentration, and establish a COD concentration prediction model;

[0012] 5) Obtain the nitrate spectrum after COD compensation;

[0013] 6) Obtain the nitrate concentration after COD compensation.

[0014] Further, in step 1), the COD standard solution uses a potassium hydrogen phthalate standard solution; the nitrate nitrogen standard solution uses a potassium nitrate standard solution; the mixed solution is composed of a nitrate nitrogen standard solution and a COD standard solution; the method for preparing the COD standard solution is: diluting the potassium hydrogen phthalate standard solution mother liquor to obtain COD standard solutions with different concentrations; the method for preparing the nitrate nitrogen standard solution is: diluting the potassium nitrate standard solution mother liquor to obtain nitrate nitrogen standard solutions with different concentrations.

[0015] Further, in step 1), the difference spectrum is the reference compensation curve, and its calculation method is: subtracting the spectrum of the nitrate nitrogen standard solution corresponding to the mixed solution from the spectrum of the mixed solution; the calculation formula of the reference compensation curve is:

[0016]

[0017] In the formula, A ref is the reference compensation curve, A mix is the ultraviolet absorption spectrum data of the mixed solution, A N is the ultraviolet absorption spectrum data of the nitrate nitrogen standard solution, Tur ref is the COD concentration of the difference spectrum, n is the number of difference spectra, and λ is the wavelength.

[0018] Further, in step 2), the calculation formula of the sum of squared residuals (RSS) is:

[0019]

[0020] Wherein, y ij is the j-th differential spectrum of the COD concentration at the i-th level, and y i0 is the reference spectrum of the COD concentration at the i-th level. n is the number of differential spectra except the reference spectrum, and m is the number of spectra with different COD concentrations.

[0021] Furthermore, in step 3), the specific steps for determining the nitrate modeling interval are as follows: Divide the COD characteristic band into n sub-intervals, establish PLS models on each sub-interval respectively, compare the RMSECV values of each interval, and the nitrate modeling interval can be determined.

[0022] Furthermore, in step 4), select the characteristic wavelength interval of COD according to the ultraviolet absorption spectra of the nitrate nitrogen standard solution and the COD standard solution.

[0023] Furthermore, the specific process of step 5) is as follows: It can be known from step 2) the COD characteristic band, that is, the COD compensation model interval. Take the average value of the differential spectra with the same COD concentration as the reference compensation curve, and substitute the COD concentration of the differential spectrum as the reference COD value into the following formula to obtain the regression parameters a λ , b λ ;

[0024]

[0025] Wherein, A' ref is the reference compensation curve, Tur' ref is the COD concentration of the differential spectrum, a λ , b λ are the regression parameters;

[0026] After obtaining the regression parameters, substitute the COD concentration of the unknown solution into the following formula to obtain the COD compensation curve;

[0027]

[0028] Wherein, A tur is the absorbance of the unknown solution COD compensation, Tur any is the COD concentration of the unknown solution, which can be obtained from the COD prediction model;

[0029] Subtract the COD compensation curve from the original spectrum to obtain the nitrate spectrum after COD compensation, and the calculation formula is as follows:

[0030] A R = A any - A tur

[0031] Wherein, A R is the absorbance of the nitrate after COD compensation, A anyis the original absorbance of the unknown solution, A tur is the absorbance of the unknown solution with COD compensation.

[0032] Furthermore, the specific process of step 6) is as follows: The optimal nitrate modeling interval can be obtained from step 3). In this interval, the PLS model is used to predict the nitrate concentration in the mixed solution. The nitrate spectrum of the mixed solution after COD compensation is used as the calibration set for modeling, and the nitrate concentration after COD compensation can be obtained.

[0033] Advantages of the present invention:

[0034] 1. Currently, most methods for eliminating COD interference are based on the premise that the nitrate concentration remains unchanged. They change the COD concentration or perform COD compensation by simply subtracting the corresponding spectrum to solve the influence of COD on nitrate, ignoring whether the spectral absorption of different concentrations of nitrate nitrogen is the same when the COD concentration remains unchanged. The present invention proposes the influence of COD on the absorption spectra of different concentrations of nitrate, and proposes a COD compensation method based on the combination of spectral difference method and linear fitting, and establishes a PLS model to predict nitrate nitrogen in water samples.

[0035] 2. The present invention uses ultraviolet spectroscopy to predict the nitrate concentration in water samples. Compared with other methods, it does not require the use of chemical reagents, will not cause secondary pollution to the environment, is convenient and fast, and can be detected in large quantities. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] Figure 1 is the flow chart of predicting nitrate content by the COD compensation method of the present invention;

[0038] Figure 2 is the ultraviolet spectrum diagram of the third group (nitrate concentration is 0.5 mg / L, COD concentration is 50 mg / L) by differential spectrum analysis;

[0039] Figure 3 is the absorption spectra of nitrate nitrogen standard solutions with different concentrations;

[0040] Figure 4 is the absorption spectra of COD standard solutions with different concentrations;

[0041] Figure 5 is the RSS of COD at different wavelengths;

[0042] Figure 6 is the scatter plot of the predicted value and the true value of nitrate in random samples. DETAILED DESCRIPTION OF THE INVENTION

[0043] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0044] Embodiment 1

[0045] As Figure 1 shown, a method for rapidly measuring the nitrate content in water based on COD compensation includes the following steps:

[0046] 1) Prepare standard solutions with a concentration of 500 mg / L using potassium nitrate and potassium hydrogen phthalate respectively, and then dilute them with deionized water to mixed solutions with different concentration gradients. Set 25 groups, and the specific parameters are shown in Table 1. Collect their ultraviolet spectra respectively, and subtract the spectrum of the nitrate standard solution corresponding to the mixed solution to obtain the difference spectrum. The calculation formula is as follows:

[0047]

[0048] In the formula, A ref is the reference compensation curve, A mix is the ultraviolet absorption spectrum data of the mixed solution, A N is the ultraviolet absorption spectrum data of the nitrate nitrogen standard solution, Tur ref is the COD concentration of the difference spectrum, n is the number of difference spectra, and λ is the wavelength.

[0049] Table 1

[0050]

[0051]

[0052] Taking the sample of the 3rd group (nitrate concentration is 0.5 mg / L, COD concentration is 50 mg / L) as an example, the test results are as Figure 2 shown, Figure 2 Among them, the four curves from the inside to the outside are the difference spectrum of subtracting the nitrate solution from the mixed solution, the mixed solution, COD, and the nitrate solution spectrum in sequence. It can be seen from the figure that the presence of COD increases the spectral absorbance, and the absorbance of the mixed solution is not simply the sum of the absorbances of nitrate nitrogen and COD spectra. The spectrum of the mixed solution is less than the sum of the spectra of the nitrate solution and the COD solution. This is because nitrate is not a reducing substance but still has strong ultraviolet absorption, and substances with benzene ring double bonds such as potassium hydrogen phthalate will have a great impact on the absorbance of nitrate.

[0053] According to Figure 3 and Figure 4It can be seen that the characteristic absorption wavelength of nitrate nitrogen is in the range of 190 - 240 nm, and there is basically no absorption after 240 nm. COD mainly has three absorption intervals, namely 190 - 220, 220 - 260, and 260 - 310 nm, corresponding to the E1, E2, and B absorption bands. As the COD concentration continuously increases, the E1 absorption band of the spectrum tends to saturate, the E2 absorption band gradually strengthens and redshifts, and the B absorption band also gradually strengthens. The absorption spectrum of COD covers the absorption interval of nitrate nitrogen, resulting in the spectral cross - overlap of the two, causing the absorption spectrum of the nitrate solution to be interfered by COD.

[0054] 2) The characteristic band of COD, that is, the COD compensation model interval, is determined by the sum of squared residuals. The calculation formula is as follows:

[0055]

[0056] In the formula, yij is the j - th differential spectrum of the i - th level of COD concentration, yi0 is the reference spectrum of the i - th level of COD concentration, n is the number of differential spectra except the reference spectrum, and m is the number of spectra of different COD concentrations.

[0057] In the present invention, the differential spectrum with a nitrate concentration of 0.5 mg / L is used as the reference spectrum, and the sum of squared residuals of each wavelength between it and other differential spectra is as Figure 5 shown. At wavelengths greater than 230 nm, RSS is less than 0.5. Therefore, it is optimal to establish a COD compensation model in the 230 - 400 nm band, and the concentration of the nitrate solution in this band will not affect the spectrum of the mixed solution.

[0058] 3) In order to improve the accuracy of predicting the nitrate content in the water sample, the modeling interval of the nitrate solution should be further narrowed. The 230 - 400 nm ultraviolet spectra of standard nitrate solutions with different concentration gradients are divided into 17 intervals, and PLS models are established in each sub - interval respectively. The interval with the smallest RMSECV value is selected as the best modeling interval for nitrate, and at this time, the best modeling interval is 230 - 240 nm.

[0059] 4) According to Figure 2 、 Figure 3 and Figure 4 the spectra of the nitrate standard solution and the COD standard solution, it can be seen that after 250 nm, the absorbance of nitrate is almost 0, and the absorbance of the mixed solution in this interval is only related to the COD concentration. Therefore, the modeling interval for predicting the COD model is selected as 250 - 400 nm. A COD prediction model is established by combining linear regression with the spectral area. The 25 spectral curves of the mixed solution are divided into five groups, and those with the same COD concentration are grouped together. Five spectral curves are obtained by averaging each group, and then the five spectral curves in the range of 250 - 400 nm are integrated to obtain the spectral area. A COD concentration prediction model is established using the integral value and the COD concentration.

[0060] 5) As can be seen from Step 2, the modeling range of the COD compensation model should be 230 - 400 nm. The average value of the differential spectra with the same COD concentration is used as the reference compensation curve, and the COD concentration of the differential spectra is used as the reference COD value and substituted into the following formula to obtain the regression parameters a λ , b λ .

[0061]

[0062] In the formula, A' ref is the reference compensation curve, Tur' ref is the COD concentration of the differential spectra, a λ , b λ are the regression parameters.

[0063] After obtaining the regression parameters, substituting the COD concentration of the unknown solution into the following formula can obtain the COD compensation curve;

[0064]

[0065] In the formula, A tur is the absorbance of the COD compensation of the unknown solution, Tur any is the COD concentration of the unknown solution, which can be obtained from the COD prediction model.

[0066] Subtracting the COD compensation curve from the original spectrum can obtain the nitrate spectrum after COD compensation, and the calculation formula is as follows:

[0067] A R = A any - A tur

[0068] In the formula, A R is the absorbance of the nitrate after COD compensation, A any is the original absorbance of the unknown solution, A tur is the absorbance of the COD compensation of the unknown solution.

[0069] 6) As can be obtained from Step 3), the best modeling range for nitrate is 230 - 240 nm. In this range, the PLS model is used to predict the nitrate concentration in the mixed solution. Using the nitrate spectrum of the mixed solution after COD compensation as the calibration set for modeling, the nitrate concentration after COD compensation can be obtained.

[0070] Example 2

[0071] Verify the nitrate prediction model:

[0072] Collect the ultraviolet absorption spectra of 10 groups of water samples in Table 2 to obtain the COD compensation curve. Subtract the original spectrum of the mixed solution from the COD compensation curve to obtain the corrected nitrate curve. Perform PLS modeling at 230 - 240 nm to obtain the predicted nitrate concentration. Table 3 shows the relative error between the predicted result and the actual value, and the average error is 8.4465%. Figure 6 It is the regression line graph of the predicted value and the actual value of nitrate, and the coefficient of determination reaches 0.9831, and the RESEP value is 0.1330. The above content indicates that the COD compensation method can effectively eliminate the influence of COD on the nitrate solution and can more accurately predict the nitrate content.

[0073] Table 2

[0074] Number Nitrate concentration (mg / L) COD concentration (mg / L) 1 0.6 27 2 1.7 36 3 3 44 4 2.4 63 5 1.4 85 6 0.8 15 7 1.8 30 8 1.2 45 9 2.2 55 10 1.6 65

[0075] Table 3

[0076]

[0077]

[0078] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0079] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for rapidly measuring nitrate content in water based on COD compensation, characterized in that: The steps include: 1) Collecting ultraviolet absorption spectra of nitrate nitrogen standard solutions, COD standard solutions and mixed solutions of different concentrations, and calculating difference spectra; 2) Determine the COD characteristic band, i.e., the COD compensation model interval, using the residual sum of squares; 3) determine the optimal nitrate modeling interval; 4) Selecting a COD characteristic wavelength range, performing a linear fit between the spectrum integral within the selected wavelength range and the COD concentration, and establishing a COD concentration prediction model; 5) Obtaining the nitrate spectrum after COD compensation; 6) Obtain the nitrate concentration after COD compensation.

2. A method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: In step 1), the COD standard solution adopts potassium hydrogen phthalate standard solution; the nitrate nitrogen standard solution adopts potassium nitrate standard solution; and the mixed solution consists of the nitrate nitrogen standard solution and the COD standard solution.

3. A method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: In step 1), the method for preparing the COD standard solution is: using the mother liquor of potassium hydrogen phthalate standard solution to dilute to obtain COD standard solutions of different concentrations.

4. The method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: In step 1), the method for preparing the nitrate nitrogen standard solution is: using the potassium nitrate standard solution mother solution for dilution to obtain nitrate nitrogen standard solutions of different concentrations.

5. The method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: In step 1), the difference spectrum is the reference compensation curve, which is calculated by subtracting the spectrum of the corresponding nitrate nitrogen standard solution from the spectrum of the mixed solution; the calculation formula of the reference compensation curve is: In the formula, A ref is the reference compensation curve, A mix is the ultraviolet absorption spectrum data of the mixed solution, A N is the UV absorption spectrum data of nitrate nitrogen standard solution, Tur ref is the COD concentration of the difference spectrum, n is the number of difference spectra, and λ is the wavelength.

6. The method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: In step 2), the residual sum of squares (RSS) is calculated as: In the formula, y ij is the jth level difference spectrum of the i-th level COD concentration, y i0 is the reference spectrum of the i-th level COD concentration, n is the number of difference spectra except the reference spectrum, and m is the number of spectra of different COD concentrations.

7. The method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: In step 3), the specific steps for determining the nitrate modeling interval are: dividing the COD characteristic band into n sub-intervals, establishing a PLS model on each sub-interval, and comparing the RMSECV values ​​of each interval to determine the nitrate modeling interval.

8. The method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: In step 4), the characteristic wavelength range of COD is selected according to the ultraviolet absorption spectra of the nitrate nitrogen standard solution and the COD standard solution.

9. The method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: The specific process of step 5) is as follows: From step 2), the COD characteristic band, i.e., the COD compensation model interval, is known. The difference spectrum average value of the same COD concentration is used as the reference compensation curve, and the difference spectrum COD concentration is used as the reference COD value and substituted into the following formula to obtain the regression parameter a λ 、b λ ; In the formula, A' ref is the reference compensation curve; Tur' ref is the COD concentration of the difference spectrum; a λ 、b λ is the regression parameter; After obtaining the regression parameters, substitute the COD concentration of the unknown solution into the following formula to obtain the COD compensation curve; In the formula, A tur The absorbance compensated for COD of the unknown solution; Tur any is the COD concentration of the unknown solution; it can be obtained from the COD prediction model; Subtracting the COD compensation curve from the original spectrum can obtain the nitrate spectrum after COD compensation. The calculation formula is as follows: A R =A any -A tur In the formula, A R is the absorbance of nitrate after COD compensation, A any is the original absorbance of the unknown solution, A tur The absorbance compensated for COD of the unknown solution.

10. The method for rapidly measuring nitrate content in water based on COD compensation according to claim 1, characterized in that: The specific process of step 6) is: The optimal nitrate modeling interval can be obtained from step 3). In this interval, the PLS model is used to predict the nitrate concentration in the mixed solution. The nitrate spectrum of the mixed solution after COD compensation is used as a calibration set for modeling to obtain the nitrate concentration after COD compensation.

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