Determination method for dissolved inorganic selenium in seawater

Abstract

The invention relates to the technical field of determination of selenium in seawater, and provides a determination method for dissolved inorganic selenium in seawater. The determination method comprises the following steps: sampling seawater, filtering, adding concentrated hydrochloric acid with the mass concentration greater than or equal to 36%, carrying out a heating reaction at 98-105 DEG C for 12-18 min, cooling, and carrying out a constant volume to prepare a seawater sample to be detected: preparing a plurality of tetravalent selenium standard sample solutions according to gradient concentrations, drawing a standard curve, and testing the seawater sample to be detected. According to the method disclosed by the invention, seawater is treated by concentrated hydrochloric acid, excessive reduction of tetravalent selenium is avoided, a hydride generation-atomic fluorescence method is combined, and a heating reaction process is improved, so that the measurement of the dissolved selenium in the seawater is small in matrix interference, accurate in result, good in precision, high in sensitivity, low in detection limit, high in measurement speed and simple to operate.

Description

Technical Field

[0001] The present invention relates to the technical field of selenium element determination in seawater, and in particular to a method for determining dissolved inorganic selenium in seawater. Background Art

[0002] Selenium is an essential trace element for organisms. On the one hand, it exhibits important biological functions, and on the other hand, it also exhibits toxicity at higher concentrations. Therefore, it is an important index factor in the Seawater Quality Standard (GB3097-1997). Accurately determining selenium in seawater is of great significance for understanding the occurrence characteristics and environmental behaviors of selenium in the marine environment and evaluating its ecological health impacts.

[0003] The valence states of dissolved inorganic selenium in seawater are hexavalent and tetravalent, existing in the forms of selenate and selenite. Currently, the method for determining selenium in seawater in China is based on "Appendix G" in the Technical Specification for Environmental Monitoring of Coastal Waters (HJ 442.3-2020). This standard method uses thiourea to reduce hexavalent selenium in the sample to tetravalent selenium, and then uses hydride generation-atomic fluorescence spectrometry. Tetravalent selenium first reacts with potassium borohydride in an acidic medium to generate hydrogen selenide gas, and then is introduced into the atomizer through a carrier gas (argon gas), and is determined by atomic fluorescence spectrometry. However, in practice, this method requires high experimental skills and relatively strict experimental conditions to accurately convert hexavalent selenium into tetravalent selenium. Otherwise, thiourea will not only convert hexavalent selenium into tetravalent selenium, but also over-reduce tetravalent selenium to a lower valence state. This over-reduction or reducing the amount of thiourea due to fear of over-reduction will both result in the loss of tetravalent selenium, causing the determination result of selenium in seawater to be significantly low, which is not conducive to the accurate determination of selenium. Therefore, it is necessary to develop a simple and reliable determination method to achieve the accurate determination of dissolved inorganic selenium in seawater. Summary of the Invention

[0004] Aiming at the problem of low determination results of the selenium determination method in seawater provided by Standard HJ442.3-2020, the purpose of the present invention is to provide an improved method for determining dissolved inorganic selenium in seawater, with more accurate determination results.

[0005] The technical solution provided by the present invention is as follows.

[0006] A method for determining dissolved inorganic selenium in seawater, comprising the following steps:

[0007] (1) Preparation of seawater test sample: After sampling seawater, filter it, add concentrated hydrochloric acid with a mass percentage ≥ 36%, heat and react at 98-105 °C for 12-18 min, and make up the volume after cooling;

[0008] (2) Preparation of standard samples: Prepare several standard sample solutions of tetravalent selenium according to gradient concentrations;

[0009] (3) Plot the standard curve: Use the hydride generation-atomic fluorescence spectrometry to measure each standard sample solution and plot the standard curve.

[0010] (4) Test the seawater sample to be tested: Use the atomic fluorescence spectrometry to measure the seawater sample to be tested prepared in step (1), and calculate the concentration of inorganic selenium in seawater according to the standard curve in step (3).

[0011] In the test method of the present invention, concentrated hydrochloric acid with a mass percentage of ≥ 36% is used to reduce hexavalent selenium to tetravalent selenium, and the reaction conditions are controlled. The reaction is heated at about 100 °C, so that hexavalent selenium can be converted into tetravalent selenium, and tetravalent selenium will not be over-reduced. Then, combined with the hydride generation-atomic fluorescence spectrometry for determination, the measured results have high accuracy, high precision and good sensitivity, and the detection limit is 0.13 μg / L. The hydride generation-atomic fluorescence spectrometry used in this scheme is the hydride generation-atomic fluorescence spectrometry shown in "Appendix G" of "Technical Specifications for Environmental Monitoring of Coastal Waters" (HJ 442.3-2020), and the specific parameter conditions are adjusted according to the actual situation. The heating reaction time can be selected from the specific times of 12 min, 14 min, 15 min, 16 min, 17 min, 18 min, or within the time range with any two of the above values as endpoints, such as 12 - 18 min, or 14 - 16 min, or 14.5 - 15.5 min.

[0012] As a preference of the method of the present invention, in step (1), the mass percentage of concentrated hydrochloric acid is 39% - 40%, and the heating temperature is 99 - 101 °C. It is found that maintaining as high a hydrochloric acid concentration as possible (i.e., the mass percentage of hydrochloric acid) and controlling the heating temperature close to 100 °C helps to improve the accuracy and precision of the determination method.

[0013] As a preference of the method of the present invention, ultrasonic waves are applied during the heating process, the ultrasonic frequency is 70 - 150 kHz, and the ultrasonic power density ρ is 1.5 - 3 W / cm 2 . It is found that when the mass percentage of concentrated hydrochloric acid is 39% - 40% and the heating temperature is 99 - 101 °C, introducing ultrasonic waves with a low power density during the heating process can significantly improve the precision of the detection results compared with other conditions. This may be because under these conditions, the influence of hydrochloric acid concentration and heating temperature has reached a relatively stable level, and at this time, the cavitation effect of ultrasonic waves appears and plays an important role in promoting the reaction, exceeding the influence of other reaction factors.

[0014] As a preference of the method of the present invention, the ultrasonic power density ρ changes with the heating time t, and the change relationship is as follows:

[0015] When t ≤ 5 min, ρ = 1.5 - 2 W / cm 2 ;

[0016] When 5 < t ≤ 10 min, ρ = 2.5 - 3 W / cm 2 ;

[0017] When t > 10 min, ρ = 1.5 - 2 W / cm 2 . Further research unexpectedly found that by integrating ultrasound with the reaction process, with the key being to adjust the ultrasonic power density to an appropriate level according to the reaction progress, the precision and accuracy of the detection results can be further improved.

[0018] As a preference of the method of the present invention, in step (1), the volume ratio of seawater to concentrated hydrochloric acid is 1.5 - 2.0:1;

[0019] And / or, for seawater filtration, a cellulose acetate filter membrane with a pore size of 0.4 - 0.5 μm is used, preferably a cellulose acetate filter membrane with a pore size of 0.45 μm.

[0020] As a preference of the method of the present invention, in step (2), the concentration gradient of the standard sample solution is at least 6 levels.

[0021] As a preference of the method of the present invention, in step (2), the concentration gradients of the standard sample solution are successively: 0.00 μg / L, 0.45 - 0.55 μg / L, 0.90 - 1.10 μg / L, 1.80 - 2.20 μg / L, 3.50 - 4.50 μg / L, 7.00 - 9.00 μg / L. Preferably, the concentration gradients of the standard sample solution are successively: 0.00 μg / L, 0.50 μg / L, 1.00 μg / L, 2.00 μg / L, 4.00 μg / L, 8.00 μg / L.

[0022] As a preference of the method of the present invention, in step (2), ultrapure water with a resistivity ≥ 18.2 MΩ·cm is used to prepare the standard sample solution.

[0023] As a preference of the method of the present invention, in step (3) and / or step (4), when using the hydride generation - atomic fluorescence method for determination, the carrier solution used is a dilute hydrochloric acid solution with a mass percentage of 9% - 10%.

[0024] As a preference of the method of the present invention, in step (3) and / or step (4), when using the hydride generation - atomic fluorescence method for determination, the reducing agent solution used contains potassium borohydride with a mass percentage of 0.9% - 1.1% and potassium hydroxide with a mass percentage of 0.18% - 2.02%.

[0025] The beneficial effects of the present invention are:

[0026] The present invention uses concentrated hydrochloric acid with a concentration not lower than a certain level to treat seawater, quantitatively converting hexavalent selenium in seawater into tetravalent selenium, while avoiding excessive reduction of tetravalent selenium, solving the problem of low selenium measurement results in Standard HJ442.3-2020. By combining with hydride generation-atomic fluorescence spectrometry and improving the heating reaction process, the measurement matrix interference of dissolved selenium in seawater is small, the results are accurate, the precision is good, the sensitivity is high, the detection limit is low, the measurement speed is fast, and the operation is simple. Specific Embodiments

[0027] The technical solutions of the present invention will be further described below in conjunction with specific embodiments. Unless otherwise specified, the raw materials used in the following embodiments can be purchased from the market or are commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art.

[0028] To maintain the comparability of the test results of each embodiment and comparative example, the main instrument conditions of the atomic fluorescence spectrometer used are as follows (of course, not limited to the following conditions, and can be adjusted according to different brands and models of atomic fluorescence spectrometers):

[0029] Wavelength: 196.0 nm; Negative high voltage: 270 V; Lamp current: 80 mA; Atomizer temperature: 200 °C; Carrier gas flow rate (argon): 400 mL / min; Shielding gas flow rate (argon): 950 mL / min.

[0030] The resistivity of the ultrapure water used is 18.2 MΩ·cm.

[0031] Example 1

[0032] A method for determining dissolved inorganic selenium in seawater, the steps are as follows:

[0033] (1) Preparation of seawater test sample: Accurately weigh 12.00 mL of seawater, filter it through a 0.45 μm cellulose acetate filter membrane, add 8.00 mL of concentrated hydrochloric acid with a mass percentage of 40%, seal it and place it in a water bath at 100 °C for heating for 15 min, take it out and cool it, and then make up the volume to 20.00 mL with ultrapure water;

[0034] (2) Preparation of standard samples: Use a commercially available certified tetravalent selenium standard solution (concentration 100 μg / L) and ultrapure water to prepare standard solutions with concentrations of: 0.00 μg / L, 0.50 μg / L, 1.00 μg / L, 2.00 μg / L, 4.00 μg / L, 8.00 μg / L;

[0035] (3) Preparation of the standard curve: Take 10 mL of the standard sample prepared in step (2) and place it in a stoppered colorimetric tube. Start the atomic fluorescence spectrometer, connect the pipelines of the carrier solution reagent bottle and the reducing agent solution reagent bottle, light the selenium hollow cathode lamp, and after preheating the instrument for 20 min, measure each standard sample and draw a calibration curve. The carrier solution is dilute hydrochloric acid with a mass percentage of 10%, and the reducing agent solution is a solution of 1% potassium borohydride + 0.2% potassium hydroxide;

[0036] (4) Testing of the seawater sample to be tested: Take 10 mL of the seawater sample to be tested prepared in step (1) and place it in a stoppered colorimetric tube. Test it using the same operation method as in step (3), and then calculate the concentration of the seawater sample to be tested according to the concentration curve in step (3).

[0037] Example 2

[0038] The difference from Example 1 is that the mass percentage of the concentrated hydrochloric acid used in step (1) is 36%, and it is heated to 105 °C by oil bath for 15 min. To ensure the uniformity of the oil bath, the heating oil is stirred at a speed of 50 rpm during the oil bath process.

[0039] Example 3

[0040] The difference from Example 1 is that the mass percentage of the concentrated hydrochloric acid used in step (1) is 38%, and the water bath heating temperature is 98 °C for 15 min.

[0041] Comparative Example 1 (too high temperature)

[0042] The difference from Example 1 is that the water bath heating temperature in step (1) is 180 °C.

[0043] Comparative Example 2 (too long time)

[0044] The difference from Example 1 is that the water bath heating time in step (1) is 25 min.

[0045] Comparative Example 3 (concentrated hydrochloric acid concentration 20%)

[0046] The difference from Example 1 is that the hydrochloric acid concentration (mass percentage) in step (1) is 20%.

[0047] Example 4 (ultrasonic on the basis of Example 1)

[0048] The difference from Example 1 is that ultrasound is applied during the water bath heating process, the ultrasonic frequency is 70 kHz, and the power density is 1.5 W / cm 2 .

[0049] Example 5 (ultrasonic on the basis of Example 1)

[0050] The difference from Example 1 is that ultrasound is applied during the water bath heating process, the ultrasound frequency is 70 kHz, and the power density is 2.5 W / cm 2 .

[0051] Example 6 (ultrasound based on Example 1)

[0052] The difference from Example 1 is that ultrasound is applied during the water bath heating process, the ultrasound frequency is 70 kHz, and the power density is 3 W / cm 2 .

[0053] Comparative Example 4 (non-optimal heating reaction conditions)

[0054] The difference from Example 4 is that the hydrochloric acid concentration (mass percentage) is 36%, the heating temperature is 105 °C, the heating time is 15 min, and it is heated in an oil bath and the heating oil is stirred at a speed of 50 rpm.

[0055] Comparative Example 5 (non-optimal heating reaction conditions)

[0056] The difference from Example 4 is that the hydrochloric acid concentration (mass percentage) is 38%, the water bath temperature is 98 °C, and the heating time is 15 min.

[0057] Comparative Example 6 (excessive ultrasound power density)

[0058] The difference from Example 6 is that the ultrasound power density is 5 W / cm 2 .

[0059] Example 7

[0060] The difference from Example 1 is that ultrasound is applied during the water bath heating process, the ultrasound frequency is 70 kHz, and the application method is as follows:

[0061] When the water bath heating time t ≤ 5 min, the ultrasound power density is 1.5 W / cm 2 ;

[0062] When the water bath heating time 5 < t ≤ 10 min, the ultrasound power density is 2.5 W / cm 2 ;

[0063] When the water bath heating time is 10 < t ≤ 15 min, the ultrasound power density is 1.5 / cm 2 .

[0064] (1) Verification of the test method of the present invention

[0065] Taking Example 1 as an example, the detection limit, precision and accuracy of the method of the present invention are described as follows.

[0066] 1. Detection limit

[0067] The blank test or low-concentration spike (adding a tetravalent selenium standard sample) determination was carried out using the steps (2)-(4) of Example 1, as shown in Table 1 below.

[0068] Table 1 Detection limit

[0069]

[0070] It can be seen from the table that the detection limit of the method of the present invention is 0.13 μg / L, and the linear coefficient r of the standard curve fitting is r≥0.999, meeting the requirements of "HJ442.3-2020".

[0071] 2. Precision

[0072] Take duplicate seawater samples, and after processing them into seawater test samples to be tested using the method of step (1) in Example 1, based on the standard curve plotted in Table 1 above, the determination was carried out according to step (4), and the results are shown in Table 2.

[0073] Table 2 Determination results of seawater parallel duplicates

[0074]

[0075] It can be seen from the table that the relative deviation RD of the seawater parallel samples is 8.0%, meeting the requirements of HJ442.3-2020.

[0076] 3. Accuracy

[0077] 1) Appropriately dilute the certified reference material of selenium and then determine it. The measured values are all within the given range, as shown in Table 3 below.

[0078] Table 3 Determination of seawater reference material

[0079]

[0080] 2) Add a certain amount of hexavalent selenium to three seawater samples, and after processing according to step (1) in Example 1, the determination was carried out, as shown in Table 4. The obtained spike recovery rate is 90.0% - 96.0%. Under the same conditions, the spike recovery rate of the thiourea treatment method in "Appendix G" of the standard HJ442.3-2020 is only 10 - 80%. Therefore, the accuracy of this method is higher than that of the thiourea treatment method in the standard.

[0081] Table 4 Determination of spike recovery rate

[0082]

[0083] From the above results, it can be seen that the results obtained by the test method based on the present invention meet the requirements of HJ442.3-2020, and have high accuracy and precision.

[0084] (2) Precision and spike recovery rate under different process conditions.

[0085] For the remaining examples and comparative examples, the relative deviation RD and spike recovery rate were measured using the same seawater sample as in Example 1, and the results are shown in Table 5 below.

[0086] Table 5 Precision and spike recovery rate under different parameters

[0087] Experimental Scheme Relative Deviation of Parallel Samples RD / % Spiked Recovery Rate / % Example 1 8.0 90.0～96.0 Example 2 11.2 87.5～92.8 Example 3 9.5 89.0～95.4 Comparative Example 1 52.6 72.5～82.5 Comparative Example 2 30.5 60.0～70.6 Comparative Example 3 14.4 50.5～61.4 Example 4 7.4 91.5～97.0 Example 5 6.9 92.0～97.3 Example 6 7.5 91.4～96.8 Comparative Example 4 11.3 87.5～93.0 Comparative Example 5 9.4 89.2～95.2 Comparative Example 6 22.7 83.1～88.5 Example 7 6.2 92.6～98.2

[0088] As can be seen from the above table, the heat treatment process is the key step in converting hexavalent selenium to tetravalent selenium in the present invention. The precision and accuracy of the detection method are closely related to the heat treatment process. As shown in Examples 1-3, the detection method provided by the present invention can meet the requirements of HJ442.3-2020. At the same time, as can be seen from the comparison between Examples 1-3 and Examples 4-6, and Comparative Examples 4-6, under the condition of maintaining the mass percentage of concentrated hydrochloric acid at 39% - 40% and the heating temperature at 99 - 101 °C, introducing ultrasound and controlling the ultrasonic power density at 1.5 - 3 W / cm 2 helps to further improve the precision of the detection method and slightly improve the accuracy. When the application process of ultrasound is integrated with the heating process, as shown in Example 7, the reliability of the detection method can be further improved, and both the precision and accuracy are more significantly improved.

Claims

1. A method for determining dissolved inorganic selenium in seawater, characterized in that: The following steps are involved: (1) Preparation of seawater samples: Filter the seawater sample, add concentrated hydrochloric acid with a mass concentration of ≥36%, heat at 98-105°C for 12-18 minutes, and cool to a constant volume; (2) Preparation of standard samples: Prepare several tetravalent selenium standard sample solutions according to gradient concentration; (3) Drawing a standard curve: using hydride generation-atomic fluorescence spectrometry to measure each standard sample solution and draw a standard curve; (4) Testing of seawater samples: The seawater samples prepared in step (1) are measured by hydride generation-atomic fluorescence spectrometry in step (3), and the concentration of inorganic selenium in the seawater is calculated based on the standard curve in step (3).

2. The measuring method according to claim 1, characterized in that The mass concentration of concentrated hydrochloric acid in step (1) is 39% to 40%, and the heating temperature is 99 to 101°C.

3. The measuring method according to claim 2, characterized in that The heating process is accompanied by the application of ultrasound, the ultrasound frequency is 70-150KHz, and the ultrasound power density ρ is 1.5-3W / cm 2 .

4. The measuring method according to claim 3, characterized in that The ultrasonic power density ρ changes with the heating time t, and the relationship is as follows: t≤5min,ρ=1.5~2W / cm 2 ; 5<t≤10min,ρ=2.5~3W / cm 2 ; t>10min,ρ=1.5~2W / cm 2 。 5. The assay method according to claim 1, 2, 3 or 4, characterized in that: In step (1), the volume ratio of seawater to concentrated hydrochloric acid is 1.5-2.0:1; And / or, in step (1), the seawater is filtered using a 0.4-0.5 μm cellulose acetate filter membrane.

6. The measuring method according to claim 1, characterized in that The gradient concentration of the standard sample solution in step (2) is at least 6 levels.

7. The measuring method according to claim 1 or 6, characterized in that The concentration gradient of the standard sample solution in step (2) is: 0.00 μg / L, 0.45-0.55 μg / L, 0.90-1.10 μg / L, 1.80-2.20 μg / L, 3.50-4.50 μg / L, 7.00-9.00 μg / L; And / or, ultrapure water with a resistivity of ≥18.2 MΩ·cm is used when preparing the standard sample solution in step (2).

8. The measuring method according to claim 7, characterized in that The concentration gradient of the standard sample solution in step (2) is: 0.00 μg / L, 0.50 μg / L, 1.00 μg / L, 2.00 μg / L, 4.00 μg / L, and 8.00 μg / L.

9. The measuring method according to claim 1, characterized in that In step (3) and / or step (4), the carrier liquid used for hydride generation-atomic fluorescence spectrometry is a dilute hydrochloric acid solution with a mass concentration of 9% to 10%.

10. The measuring method according to claim 1, characterized in that In step (3) and / or step (4), the reducing agent solution used for determination by hydride generation-atomic fluorescence spectrometry contains 0.9% to 1.1% potassium borohydride and 0.18% to 2.02% potassium hydroxide.

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