Method for detecting total phosphorus adsorbed by aged micro-plastic

Abstract

The invention discloses a method for detecting total phosphorus adsorbed by aged micro-plastics, and belongs to the technical field of total phosphorus detection. According to the method, the citric acid-sodium citrate extracting agent is adopted, so that the release of trace interfering substances of the aged micro-plastic can be reduced; further, efficient removal of interfering substances is realized through sodium thiosulfate pretreatment, and the influence of the interfering substances on a detection result is eliminated; a long-optical-path cuvette is adopted in the detection process, so that the light absorption response performance of low-concentration phosphorus is greatly improved; meanwhile, a standard curve is fitted by low-concentration calibration points, so that the detection result better meets the detection requirement of the trace phosphorus content, and the detection accuracy is effectively guaranteed.

Description

Technical Field

[0001] This application relates to the field of total phosphorus detection technology, and more specifically, to a method for detecting total phosphorus adsorbed by aged microplastics. Background Technology

[0002] Some studies have shown that certain microplastics adsorb phosphorus. With long-term presence and aging in the environment, the physicochemical properties of microplastic surfaces change significantly, especially the increase in surface roughness and the introduction of oxygen-containing functional groups. These changes significantly enhance the adsorption capacity of aged microplastics for nutrients such as phosphorus in the environment. To study the environmental impact of microplastics, it is necessary to investigate their phosphorus adsorption performance. However, during the aging process, microplastics produce a large number of small molecule degradation products, including aldehyde and carbonyl groups. These substances have strong light absorption characteristics, especially in the visible light region. These degradation products may react with molybdate in the ammonium molybdate colorimetric reaction, leading to unstable and fluctuating absorbance in experiments, severely affecting the accurate determination of the true phosphorus content adsorbed on the microplastic surface. While the traditional ammonium molybdate spectrophotometric method is widely used for the detection of total phosphorus in water, it often fails to provide stable and accurate results when measuring low concentrations of phosphorus (especially phosphorus adsorbed by aged microplastics) due to interference from microplastic surface degradation products. The minimum detection concentration of this method is 0.01 mg / L, and the limit of quantitation is 0.04 mg / L. When the phosphorus concentration adsorbed on the surface of aged microplastics is below the detection limit, the absorbance of the colorimetric reaction fluctuates significantly, making it difficult to meet the standard requirements for accuracy and precision (relative deviation ≤10%). Therefore, the existing method cannot accurately reflect the phosphorus adsorption performance of aged microplastics, limiting the further development of microplastic pollution research and remediation. Summary of the Invention

[0003] To address the limitations of existing total phosphorus detection methods in detecting phosphorus adsorbed on microplastics, this invention provides a method for extracting and detecting total phosphorus adsorbed on aged microplastics. This method can achieve efficient extraction and accurate detection of phosphorus adsorbed on the surface of aged microplastics with a particle size of less than 1 mm.

[0004] To achieve the objective of this invention, the technical solution adopted is as follows: a method for detecting total phosphorus adsorbed by aged microplastics, comprising the following steps: 1) Pretreatment: Wash the microplastics with water (preferably deionized water and / or distilled water) to remove impurities (such as attached mud, plankton and other impurities) from the surface of the microplastics; filter through an aqueous filter membrane (preferably 0.45 μm) until there are no free water droplets on the surface, and air dry for later use. The microplastic particle size is less than 1 mm.

[0005] 2) Phosphorus Extraction and Digestion: Add citric acid-sodium citrate extractant (extractant pH 4.0-5.0, citrate concentration 0.1 M) to the pretreated microplastics, place in a constant temperature shaker, and shake at 15-30 ℃ until the phosphorus adsorbed on the surface of the aged microplastics is fully dissociated (at least 60 minutes at 160 r / min for complete dissociation (higher speeds result in faster dissociation)). Then filter to obtain the extract. This extractant efficiently dissociates the phosphorus adsorbed on the surface of aged microplastics through a weakly acidic environment and the complexation effect of citric acid, without damaging the microplastic structure, reducing the interference of small molecules containing aldehyde and carbonyl groups released by the microplastics on the test results. The citric acid-sodium citrate extractant avoids the interference problems caused by strong acids or EDTA: when using strong acids as extractants, the surface structure of aged microplastics may be damaged (such as the breakage of oxygen-containing tube functional groups), resulting in incomplete release of adsorbed phosphorus and the introduction of impurities.

[0006] Add sodium thiosulfate reducing agent to the extract and shake at room temperature until the trace interfering substances in the extract are completely reduced (the reaction is usually complete in 10-15 minutes). Then filter through a 0.45 μm filter membrane to remove excess reducing agent, leaving no residual interference (this can be verified by iodine-starch reaction, which shows no blue color).

[0007] Transfer the filtrate to a 50 mL colorimetric tube, add excess potassium persulfate (generally 4 mL of 50 g / L potassium persulfate) to oxidize organic phosphorus to inorganic phosphorus, and then perform high-pressure steam digestion (preferably 121±2 ℃, 1.1 kg / cm³). 2 Digest for at least 15 minutes to ensure that total phosphorus is completely converted into orthophosphate form and to obtain digested water.

[0008] 3) Color reaction: After the digestion treatment water is cooled, a reducing agent ferrous ammonium sulfate solution is added to it, followed by a mixture of ammonium molybdate and potassium antimony tartrate. The mixture is left to stand at room temperature for at least 15 minutes to form a blue phosphomolybdic heteropolyacid complex (characteristic absorption peak at 700 nm).

[0009] 4) Concentration detection and calculation The absorbance was measured at 700 nm using a spectrophotometer with a 6 cm cuvette (to increase the optical path and improve sensitivity at low concentrations), and the total phosphorus content was calculated by substituting the absorbance into the standard curve equation. The standard curve was obtained as follows: 50 ml of phosphorus standard solutions with concentrations of 0.0, 0.005, 0.01, 0.02, 0.04, and 0.05 mg / L were digested and subjected to color development. The absorbance was measured, and a linear equation fitting curve (R²≥0.999) was established with concentration as the x-axis and absorbance as the y-axis to plot the standard curve. Calculation: Obtain the total phosphorus concentration from the standard curve based on the absorbance of the test solution, and calculate the content using the formula ω=(C×V) / m (V=50 mL).

[0010] By adopting the above technical solution, the detection error of total phosphorus was reduced from 15%-20% to 3%-8%, indicating that the method has stronger anti-interference ability. Furthermore, optimization of the colorimetric reaction system further improved the accuracy of the measurement results. The specific form of the linear equation fitting is y=ax+b, where a and b are fitting coefficients, x represents the concentration, and y represents the absorbance value.

[0011] Calibration points were set within the concentration range of 0.0-0.1 mg / L (calibration point concentrations of 0.0, 0.005, 0.01, 0.02, 0.04, and 0.05 mg / L), followed by colorimetric determination. A linear equation curve was fitted with concentration as the x-axis and absorbance as the y-axis, with the blank solution as the zero point, to plot a standard curve. The absorbance of the sample was then measured, and the total phosphorus content of the sample was calculated based on the standard curve. The results show that the above technical solution improves the accuracy of low-concentration total phosphorus detection and also enhances detection efficiency.

[0012] Sodium thiosulfate can reduce small molecules containing aldehyde and carbonyl groups generated during the degradation of microplastics into small alcohol molecules. These small molecules are highly polar, readily soluble in water, and will not undergo side reactions with subsequent molybdates or ferrous ammonium sulfate, nor will they adsorb onto the surface of microplastics or clog the filter membrane. Subsequent filtration and digestion steps can remove them normally without affecting detection. Preferably, the concentration of the sodium thiosulfate solution is 0.1M.

[0013] The detection limit (DL) can be reduced to 0.004 mg / L. The specific data derivation process is as follows: First, parallel determinations of blank samples were carried out. Twelve phosphorus-free blank solutions (containing extractant, reducing agent, oxidant, and colorimetric agent, completely simulating the entire detection process) were prepared. The solutions were measured at a characteristic wavelength of 700 nm and a pH of 60. Absorbance was measured under conditions of a long optical path length cuvette (mm), with data of 0.002, 0.003, 0.002, 0.003, 0.002, 0.003, 0.002, 0.003, 0.002, 0.003, 0.002, 0.003 (unit: A). Statistical calculations were then performed on the blank data, yielding an average blank absorbance of 0.0025 and a standard deviation S = 0.00058. Subsequently, using the general formula for environmental testing, DL = 3.143 × S (where 3.143 is the normal distribution characteristic coefficient of the blank signal), the absorbance detection limit was calculated to be 0.0018. Finally, combining the slope of the standard curve fitted by a linear equation in the low concentration range (0.0-0.1 mg / L) of 4.5019, and through the linear correspondence between absorbance and concentration, the absorbance detection limit was converted to concentration units, ultimately determining the method detection limit to be 0.004. mg / L, method detection limit (mg / L) = standard curve slope / absorbance detection limit.

[0014] The aforementioned technical solution, combined with absorbance measurement at a wavelength of 700 nm, significantly improves the accuracy of detecting low-concentration total phosphorus. This approach results in more precise absorbance readings, thereby enhancing the reliability and repeatability of total phosphorus content determination results and further reducing measurement errors. Specifically, this improvement helps ensure better linear response of the detection system in the low concentration range, ultimately improving the data precision and reliability of the entire determination method.

[0015] Compared with the prior art, this application has the following beneficial effects: First, using a 0.1M citrate-sodium citrate buffer solution (pH 4-5), along with long-path cuvettes, adding calibration points in the low concentration range, and constructing a linear equation fitting standard curve, as well as sodium thiosulfate, we achieved two key benefits. First, we gently dissociated adsorbed phosphorus from the surface of aged microplastics with minimal damage to the microplastic structure. Second, we reduced small molecules containing aldehydes and carbonyl groups generated during microplastic degradation into highly polar and water-soluble alcohol molecules. These alcohol molecules do not react with subsequent molybdates or ferrous ammonium sulfate, nor do they adsorb onto the microplastic surface or clog the filter membrane. Subsequent filtration and digestion steps can remove these molecules normally or without affecting detection. The addition of ferrous ammonium sulfate further stabilized the reduction reaction, preventing over-reduction or incomplete color development due to trace amounts of reducing impurities. Ultimately, while achieving effective phosphorus extraction, we reduced the relative deviation from 15%-20% to 3%-8%, improving data reliability and precision. Attached Figure Description

[0016] Figure 1 This is a schematic flowchart of the method for detecting total phosphorus adsorbed by aged microplastics in an embodiment of the present invention. Figure 2 This is a standard curve for phosphorus content in the low concentration range; Figure 3 The images show SEM images of PE before and after aging. (a) is the original sample, and (b) is the aged sample. Figure 4 XPS images of PE before and after aging: (a) O1s spectrum of the original sample, (b) C1s spectrum of the original sample, (c) O1s spectrum of the 354nm aged sample, (d) C1s spectrum of the 354nm aged sample, and (e) full spectrum of PE before and after aging. Figure 5 This is a colorimetric plot of the standard curve for the low-concentration region. Detailed Implementation

[0017] To further understand the purpose, content, and advantages of this invention, specific embodiments of the invention are described in detail below. However, these embodiments are not limited to the examples described below and should be freely combined according to actual circumstances. The endpoints and values ​​of the ranges disclosed herein are not limited to the precise ranges and values. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0018] The present invention will be further described in detail below with reference to the embodiments: Example 1 (Environmentally Aging Polyethylene Microplastics)

[0019] Standard curve: Preparation of 50 ug / mL phosphorus standard stock solution: Weigh 0.2197±0.001 g of potassium dihydrogen phosphate that has been dried at 110 ℃ for 2 h and cooled, dissolve it in deionized water and transfer it to a 1000 mL volumetric flask, add 2.5 mL of sulfuric acid with a density of 1.84 g / mL, and make up to volume and mix well.

[0020] Preparation of standard curve: Take 6 colorimetric tubes of 50 mL each, and transfer 0 (blank), 5, 10, 20, 40 and 50 μL of phosphorus standard stock solution into each of the 6 colorimetric tubes. Add distilled water to each tube to a final volume of 50 mL and mix well.

[0021] Add 4 mL of 50 g / L potassium persulfate solution (consistent with sample digestion) to each of the above 6 colorimetric tubes, place them in a high-pressure steam cooker at 121±2 ℃ for 15 min to digest, cool to room temperature, add 1 mL of 100 g / L ferrous ammonium sulfate solution and 2 mL of 14.1 g / L molybdate solution, and let stand at 20~25 ℃ for 15 min to develop color.

[0022] Using the phosphorus concentration (mg / L) of each tube as the x-axis and the absorbance measured at 700 nm wavelength using a 6 cm long optical path cuvette as the y-axis, a linear equation was fitted (fitting formula y=ax+b, where x is the concentration and y is the absorbance), requiring a correlation coefficient R²≥0.999, which served as the calibration basis for subsequent sample quantification.

[0023] Standard curve plotting: Plot the standard curve by performing a curve fitting operation with concentration (mg / L) on the x-axis and absorbance on the y-axis.

[0024] The standard curve obtained by following the above steps is y = 4.5019x - 0.0033, R0 2 >0.999.

[0025] Sample collection: Samples were collected from surface water in urban rivers and filtered through a 200-mesh planktonic net to collect microplastics with a particle size of less than 1 mm.

[0026] Pretreatment: Place the microplastic in a 50 mL beaker, add 30 mL of deionized water, and ultrasonically clean it at 300 W for 1 minute, repeating twice; filter it with a 0.45 μm aqueous filter membrane until there are no free water droplets on the surface, and then let it air dry naturally and weigh it as 0.5 g.

[0027] Extraction and digestion: Add 20 mL of 0.1 M citrate-sodium citrate buffer (pH 4-5), and shake at 160 r / min for 60 minutes at room temperature. Filter to obtain the extract (using any commonly used filtration method or membrane filtration; in this example, a 0.45 μm membrane is used). Add 1 mL of 0.1 M sodium thiosulfate solution to the extract, and shake at room temperature for 10-15 minutes to completely reduce any trace interfering substances that may be present in the extract. Then, filter through a 0.45 μm membrane to remove excess reducing agent, ensuring no residual interference (verified by the absence of a blue color development with iodine-starch).

[0028] The filtrate was transferred to a 50 mL colorimetric tube and 4 mL of 50 g / L potassium persulfate solution was added. The tube was then placed in a high-pressure steam boiler (1.1 kg / cm²). 2 Digested at 121 °C for 15 minutes, then cooled to room temperature to obtain digested water.

[0029] Color development: Add 1 mL of 100 g / L ferrous ammonium sulfate solution to the digestion water, then add 2 mL of 14.1 g / L molybdate solution (ammonium molybdate-potassium antimony tartrate mixture, prepared according to GB11893-89), and dilute to 50 mL with deionized water. Let stand at room temperature for 15 minutes to develop color.

[0030] Detection: The absorbance was measured at 700 nm using a UV spectrophotometer and a 6 cm cuvette, and was 0.051. Substituting into the standard curve equation y=4.5019x-0.0033 (R²>0.999), the total phosphorus content was calculated to be 0.012 ug / mL, and the total phosphorus mass adsorbed by the microplastics was further calculated to be 0.6 ug.

[0031] Result: Total phosphorus content ω=(C×V) / m=0.012×50 / 0.5=1.2 ug / g. Example 2 (Laboratory Simulation of Aging of Polypropylene Microplastics)

[0032] Standard curve: Same as in Example 1 A method for determining the total phosphorus adsorbed by aged microplastics is disclosed in this embodiment, which differs from Example 1 in that the sample is a laboratory-grade virgin polypropylene (PE) microplastic with a particle size of 50 μm. This microplastic was irradiated in a UV aging chamber (365 nm wavelength, 100 mW / cm²) for 108 hours. XPS analysis showed that the surface oxygen-to-carbon ratio increased by 0.036 compared to the original PE, confirming aging. The sample was then immersed in a 5 ug / mL phosphorus solution for 2 days before extraction and phosphorus content analysis. The final measured total phosphorus content was 1.25 ug / g, with a relative error of 4.5%. Comparative Example 1 (using a strong acid extractant)

[0033] Standard curve: Same as in Example 1 Sample: The same aged microplastic as in Example 2, weighed 0.5 g dry weight.

[0034] Extraction: Use 20 mL of 40% nitric acid as the extraction solvent and shake at room temperature for 60 minutes (other steps are the same as in Example 1).

[0035] Results: The microplastic surface showed damage (observed by scanning electron microscopy), and the total phosphorus content was 0.43 ug / g, which was lower than that in Example 1 (due to incomplete phosphorus release caused by strong acid damaging the structure). The relative error was as high as 18%, which is far lower than the detection performance of the method of this invention. Comparative Example 2 (without pretreatment of interfering substances)

[0036] Standard curve: Same as in Example 1 Sample: 0.5 g of aged microplastics, the same as in Example 2. The relative error was as high as 21%, far lower than the detection performance of the method of this invention. Comparative Example 3 (without using a long optical path cuvette)

[0037] Standard curve: Same as in Example 1, but a 3 cm short cuvette was used during the test.

[0038] Sample: PE microplastics in the sediment of the same example 2, weighed 0.5 g dry weight.

[0039] Detection: Using a 3 cm cuvette, the relative error is as high as 19%, which is far lower than the detection performance of the method of this invention.

[0040] The experimental results of R² values ​​and errors in Examples 1-2 and Comparative Examples 1-3 are shown in Table 1: The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention. It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

Claims

1. A method for detecting total phosphorus adsorbed by aged microplastics, characterized in that, Includes the following steps: 1) Pretreatment: Wash the microplastics with water to remove impurities from the surface of the microplastics; filter through an aqueous filter membrane until there are no free water droplets on the surface, and air dry for later use. The microplastic particle size is less than 1 mm. 2) Phosphorus extraction and digestion: Add citric acid-sodium citrate extractant with pH 4.0-5.0 to the pretreated microplastics, place them in a constant temperature shaker, and shake at 15-30 ℃ until the phosphorus adsorbed on the surface of the aged microplastics is fully dissociated, and then filter to obtain the extract; Add sodium thiosulfate solution to the extract and shake at room temperature until the trace interfering substances in the extract are completely reduced. Then filter through a 0.45 μm filter membrane to remove excess reducing agent. Excess potassium persulfate was added to the filtrate to oxidize organic phosphorus into inorganic phosphorus, and then high-pressure steam digestion was performed to ensure that total phosphorus was completely converted into orthophosphate form, thus obtaining digested water. 3) Color reaction: After the digestion water is cooled, a reducing agent ferrous ammonium sulfate solution is added to it, followed by a mixture of ammonium molybdate and potassium antimony tartrate. The mixture is left to stand at room temperature for at least 15 minutes to form a blue phosphomolybdic heteropolyacid complex. 4) Concentration detection and calculation The absorbance was measured at 700 nm using a spectrophotometer and a 6 cm cuvette. The total phosphorus content was then calculated by substituting the absorbance into the standard curve equation.

2. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, In step 1), a 0.45 μm aqueous filter membrane is used for vacuum filtration.

3. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, In step 2), the concentration of citrate ions in the citric acid-sodium citrate extractant is 0.1 M.

4. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, In step 2), the concentration of sodium thiosulfate solution is 0.1 M.

5. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, In step 2), the shaking speed is 160 r / min, and the shaking time is at least 60 minutes.

6. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, The reduction time in step 2) is 10-15 min.

7. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, In step 2), the digestion temperature is 119-123 ℃ and the pressure is 1.1 kg / cm². 2 Digest for at least 15 minutes.

8. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, In step 3), the concentration of the ferrous ammonium sulfate solution is 100 g / L.

9. The method for detecting total phosphorus adsorbed by aged microplastics according to claim 1, characterized in that, The standard curve equation in step 4) is obtained as follows: digestion and color development are performed on 50 ml of phosphorus standard solutions with concentrations of 0.0, 0.005, 0.01, 0.02, 0.04, and 0.05 mg / L, respectively. The absorbance is measured, and a linear equation fitting curve (R²≥0.999) is established with concentration as the abscissa and absorbance as the ordinate. The standard curve is then plotted.

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