Determination of Total Dissolved Nitrogen in Geothermal Water by Ultraviolet Second Derivative Spectrometry with Alkaline Persulfate Digestion

Significance: The excessive exploitation and utilization of geothermal water has led to an excessively high total dissolved nitrogen content in the geothermal water，causing water quality deterioration^[8]. At the same time，it is accompanied by the mutual transformation of different forms of nitrogen in the total dissolved nitrogen，which has a serious impact on human health and economic development^[8-9]. To solve the problem of total dissolved nitrogen pollution in geothermal water，it is very necessary to accurately and rapidly determine the content of total dissolved nitrogen. Total dissolved nitrogen is converted into nitrate-nitrogen through digestion reactions，and then quantitatively determined. The alkaline persulfate digestion method^[15-17] is a widely used digestion method，which has the advantages of simple equipment and high oxidation efficiency. Geothermal water has a high mineralization degree and complex composition，with the presence of colloids，organic matter，etc. When using traditional methods^[24-26] to determine total dissolved nitrogen，there are many interfering components and low accuracy. Based on the above problems，the method of alkaline persulfate digestion-ultraviolet second derivative spectrometry was used to determine total dissolved nitrogen in geothermal water. Its second derivative spectrogram can eliminate most of the interfering factors.

Methods:

　　(1) Experimental method: Prepare a standard series solution of nitrate -nitrogen (0.00-5.0mg/L) in 25mL glass colorimetric tube，make up to 10mL，add 5.00mL of alkaline potassium persulfate solution (40.0g/L)，tighten tube plugs，and place it in a high-pressure steam sterilizer (120℃，20min). After cooling，add 1.0mL of 10% hydrochloric acid solution，dilute to 25mL，and shake well. The blank solution is used as a reference，and a 10 mm quartz cuvette is used with a wavelength scanning range of 190-270nm，and the second derivative absorption value of nitrate -nitrogen at a wavelength of 226.6nm is selected and recorded The calibration curve is drawn with the absorbance of the standard solution as the abscissa and the content of nitrate -nitrogen as the ordinate. The sample volume is 10.00mL，and the experimental method is the same as that of the nitrate -nitrogen standard series，and a blank control experiment is also performed.

　　(2) Wavelength and digestion parameters: ①In the ultraviolet second derivative spectrometry，the wavelength plays a role in improving resolution and reducing background interference. Fig.1 showed the ultraviolet spectra，first derivative spectra，and second derivative spectra of the nitrate-nitrogen standard solution and the geothermal water sample solution. Fig.2 showed the second derivative spectra of nitrate-nitrogen standard solutions of different concentrations. ②The selection of digestion parameters is crucial for accurately determining the total dissolved nitrogen in geothermal water. Quality control samples of total dissolved nitrogen，dissolved inorganic nitrogen，and dissolved organic nitrogen were prepared，with concentrations of 15.0，10.0，and 21.0mg/L，respectively. Single-factor level experiments were conducted on digestion time，digestion temperature，and the volume of alkaline potassium persulfate solution using these quality control samples (Fig.3). A two-factor ANOVA experiment was carried out on sample volume and dilution time (Fig.4).

　　(3) Coexisting ions: The geothermal water has a high mineralization degree and a complex matrix composition. Six groups of quality control samples of total dissolved nitrogen (3.0mg/L) were prepared，each containing 2000mg/L sodium chloride，2000mg/L sodium sulfate，800mg/L sodium bicarbonate，10mg/L bromide ions，10mg/L iodide ions，and 10mg/L fluoride ions，respectively. The determination contents of total dissolved nitrogen in these six groups of quality control samples were compared to evaluate the influence of coexisting ions on the determination results of total dissolved nitrogen (Fig.5).

Data and Results: Through the study of the ultraviolet second derivative spectra of nitrate-nitrogen，it was determined that the second derivative value at a wavelength of 266.6nm was the characteristic absorbance，and the characteristic absorbance had a good linear relationship with the nitrate-nitrogen concentration. Through single-factor level experiments and a two-factor variance analysis experiment，the parameters in the digestion reaction were optimized. It was found that within a certain range of conditions，the dilution time was the most important factor affecting the determination of total dissolved nitrogen. It was recommended to select the digestion temperature，digestion time，and volume of alkaline potassium persulfate solution as 120°C，20min，and 5.0mL，respectively; the sample volume should be 10.0mL，and the dilution time should be before sample digestion. In the coexisting ion interference experiment，it was found that high content of bromide ions had a positive interference on the determination of total dissolved nitrogen. Within the linear range of 0.20-5.0mg/L，a calibration curve of nitrate-nitrogen standard solutions was established，with its coefficient of determination R²=0.9996. The actual samples were measured and spiked recovery tests were conducted(Table 1). The recovery rates were between 94% and 103.5%，and the relative deviations were between 0.83% and 3.36%.