Method for microwave digestion of express packaging material and application

Abstract

The invention discloses a method for microwave digestion of an express packaging material and application, and the method comprises the following steps: S1, crushing and grinding the express packaging material until the particle size is less than 100 [mu] m; s2, adding a composite acid-chitinase mixed solution into the digestion tank, and preliminarily contacting with the sample to weaken the structure of the sample; the composite acid-chitinase mixed solution comprises a composite acid mixed solution and a chitinase aqueous solution; the composite acid mixed solution comprises nitric acid, trifluoroacetic acid, hydrogen peroxide and formic acid; s3, placing the digestion tank treated in the step S2 on a turntable of a microwave digestion instrument, and carrying out pulse type microwave digestion; the microwave time is 25 minutes to 35 minutes; according to the method, pulse type microwaves are matched with the composite acid to digest the express packaging material, the digestion solution with high digestion completeness is obtained, HP-beta-CD and DTPA are used as double ligands to synergistically extract heavy metals such as lead and cadmium, the selectivity is good, and the efficiency is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of the digestion of express packaging materials, and particularly relates to a method and application for microwave digestion of express packaging materials. Background Art

[0002] The heavy metal pollution of express packaging materials (such as cardboard boxes, plastics, tapes, etc.) has become a global issue of food safety and environmental protection that has drawn widespread attention. With the explosive growth of the e-commerce economy, the volume of express deliveries in China has continued to climb, reaching 132 billion pieces in 2023, a year-on-year increase of 19%, consuming approximately 46 million tons of cardboard boxes and 2 million tons of plastic packaging. During the production, circulation, and recycling processes of these packaging materials, they face multiple heavy metal pollution risks: lead (Pb) in printing inks, cadmium (Cd) in recycled fibers, mercury (Hg) residues from electronic waste, etc., which may contaminate food through migration or damage the ecological environment. For example, excessive lead can lead to retarded intellectual development in children, cadmium pollution is associated with osteoporosis, and mercury residues may cause damage to the nervous system.

[0003] The national standard "Solid Waste Leaching Toxicity Leaching Method" (HJ / T 299-2007) uses a sulfuric acid-nitric acid system to leach heavy metals, which requires combining 4 hours of microwave digestion with atomic absorption spectroscopy detection, resulting in a single-sample detection cycle as long as 5 hours. For example, Chinese patent CN112326575A proposes to digest solid hazardous waste through calcination-nitric acid digestion, but the digestion time still needs 2 hours, and the problem of the efficient decomposition of SiO2 in aluminum foil membranes has not been solved, and the XRD detection results in a SiO2 residue rate as high as 15%. Although the hydrofluoric acid system can completely decompose SiO2, its strong corrosiveness is likely to cause equipment damage and environmental pollution. In addition, when using a single acid such as nitric acid or sulfuric acid to digest packaging materials, the lead recovery rate is only 75% (GB / T 38726-2020), which cannot meet the detection requirements for composite packaging materials in GB 43352-2023. Summary of the Invention

[0004] The first object of the present invention is to provide a method for microwave digestion of express packaging materials. The present invention directly enhances the chemical action of the composite acid by using the thermal and non-thermal effects of pulsed microwaves, shortening the digestion time and improving the digestion completeness.

[0005] To solve this technical problem, the technical solution of the present invention is: a method for microwave digestion of express packaging materials, comprising the following steps: S1. Take the express packaging materials, break them, and grind them to a particle size of less than 100 μm; place the express packaging materials in a polytetrafluoroethylene digestion tank; S2. Add a composite acid-chitinase mixed solution to the digestion tank, tighten the tank lid, and let it stand. The composite acid-chitinase mixed solution is initially in contact with the sample to weaken the sample structure; The composite acid-chitinase mixture includes a composite acid mixture and an aqueous chitinase solution; The composite acid mixture includes nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid; S3. Place the digestion tank after S2 on the turntable of the microwave digestion instrument for pulsed microwave digestion; The pulsed microwave drives the vibration of polar molecules in the acid solution to generate heat through the internal Joule effect, raising the system temperature from room temperature to 75°C - 85°C; The microwave electric field causes the dipole moments of acid molecules to be oriented and penetrate into the microscopic voids of the express packaging material particles, triggering local high temperature and high pressure at the solid-liquid interface; The microwave time is from 25 min to 35 min.

[0006] Preferably, the mass ratio of the express packaging material to the volume of the composite acid-chitinase mixture is 0.1 g : 5 ml.

[0007] Preferably, the volume ratio of nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid in the composite acid mixture is (2.5 to 5) : (1.5 to 4) : (1.5 to 3) : 1. Through the optimization of the composite acid system and the double-ligand synergistic extraction technology, the present invention has achieved multiple technological breakthroughs. In terms of environmental protection, traditional hydrofluoric acid digestion produces highly polluted waste liquid (COD > 5000 mg / L) and the treatment cost is as high as 15 yuan / L. In the present invention, trifluoroacetic acid with a 60% reduction in corrosiveness is used to replace part of the hydrofluoric acid. According to the research in Journal of Hazardous Materials (J. Hazard. Mater., 2020), its corrosiveness is 60% lower than that of hydrofluoric acid, and the theoretical decomposition rate of SiO2 is ≥ 99%, significantly reducing environmental pollution and treatment costs.

[0008] Furthermore, the volume ratio of nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid in the composite acid mixture is 5 : 3 : 3 : 1.

[0009] Preferably, the content of the aqueous chitinase solution is 10 U / ml; The volume ratio of the composite acid mixture to the aqueous chitinase solution is (0.9 to 1.3) : 1.

[0010] The present invention combines the specific hydrolysis effect of chitinase (10 U / mL) to decompose silicon-containing organic matter, with a theoretical decomposition rate of ≥ 99%, reducing the acid dosage by 30%, further reducing the waste liquid COD from 200 mg / L to 120 mg / L, and the hydrofluoric acid substitution rate reaching 60%, significantly reducing environmental pollution and treatment costs.

[0011] Preferably, the process parameters of the pulsed microwave digestion are as follows: The microwave power is 300 W and the duty cycle is 50%; The temperature control condition is to raise the temperature to 80 °C in 10 minutes and keep it warm for 20 minutes; After digestion is completed, it is naturally cooled to room temperature to obtain the digestion solution.

[0012] The existing method requires step-by-step digestion for 4 hours and extraction for 1 hour, with a total time of up to 5 hours (GB / T38726-2020). Through the integrated design of pulsed microwave digestion and digestion-extraction, and the thermal and non-thermal effects of microwaves, the chemical action of the composite acid is directly enhanced. Microwaves drive polar molecules such as water and nitric acid in the acid solution to vibrate at a high speed at a frequency of 2.45 GHz, generating heat through the internal Joule effect. The temperature of the system can be raised to 80 °C within 30 minutes, which is more than 5 times faster than traditional heating. Based on the Arrhenius equation, when the temperature rises from 25 °C to 80 °C, the reaction rate constant increases significantly, and the reaction rate is increased by about 10 times, directly enhancing the intensity of the oxidation reaction of nitric acid and the fluorolysis reaction of trifluoroacetic acid.

[0013] The second object of the present invention is to provide an application of a method for microwave digestion of express packaging materials. The present invention uses a pulsed microwave in combination with a composite acid to obtain a digestion solution with high digestion completeness, and is combined with an extractant containing HP-β-CD and DTPA as double ligands for synergistic extraction to detect heavy metals such as lead and cadmium, with good selectivity and high efficiency.

[0014] To solve this technical problem, the technical solution of the present invention is: an application for detecting the heavy metal content in express packaging materials by ICP-MS using the digestion solution obtained by the method for microwave digestion of express packaging materials proposed by the present invention.

[0015] Preferably, the application in the present invention includes the following steps: A1. Place the digestion solution in a centrifuge tube and add an extractant containing HP-β-CD and DTPA as double ligands for synergistic extraction, and extract to form a Pb 2+ -DTPA-HP-β-CD ternary complex; A2. Centrifuge the centrifuge tube extracted in step A1, take the supernatant for ICP-MS detection, and obtain the heavy metal dissolution amount and heavy metal dissolution rate.

[0016] Preferably, the molar concentration of HP-β-CD in the extractant for double ligand synergistic extraction is 0.1 M; the molar concentration of DTPA is 0.03 M; the pH of the extractant is adjusted to 5.5 with a citrate-disodium hydrogen phosphate buffer solution. The masking rate of the traditional EDTA ligand for Al 3+ is only 82%, resulting in a detection error > 8%. To solve the above problems, the present invention uses 0.1 M HP-β-CD and 0.03 M DTPA for synergistic extraction, breaking through the performance limitations of a single ligand through the dual mechanisms of hydrophobic inclusion and coordination, and through structural complementarity and synergistic effects.

[0017] The preferred extraction process parameters are as follows: Place it in a 60°C water bath and oscillate at 200 rpm for 30 minutes; the extraction temperature is set at 60°C; The volume ratio of the extractant to the composite acid-chitinase mixture is 2:1.

[0018] The extraction conditions of oscillating at 60°C for 30 minutes can maximize the mass transfer efficiency, which is 2 times faster than the single EDTA system, and is applicable to 8 types of packaging materials such as aluminum foil membranes and plastics. The test results meet the GB43352-2023 limit standard.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The present invention adopts a composite acid system containing nitric acid, trifluoroacetic acid, hydrogen peroxide and formic acid. Among them, nitric acid is used as the core oxidant. By releasing NO3⁻ and H⁺, it reacts with metal oxides such as PbO and CdO to form soluble salts such as PbO + 2HNO3 → Pb(NO3)2 + H2O, and at the same time decomposes organic substances. Trifluoroacetic acid can not only release F⁻ and react with silicates such as SiO2 to form hexafluorosilicic acid, SiO2 + 6HF → H2SiF6 + 2H2O, but also reduce the surface tension of the acid solution and enhance the penetration of the sample. Hydrogen peroxide decomposes to generate hydroxyl radicals ・OH in an acidic environment, forming a "nitro-oxygen" oxidation system with nitric acid to strengthen the destruction of refractory organic substances such as high molecular resins in packaging materials. Formic acid complexes with cadmium ions through formate (HCOO⁻), and the complexation method is Cd 2+ + 2HCOO − → Cd(HCOO)2, stabilizing the dissolution state of metal ions, and at the same time adjusting the pH of the system to maintain the activity of free radicals. The four acids used in the composite mixture of the present invention construct a multi-dimensional chemical action network through oxidation, fluorolysis, complexation and pH regulation.

[0020] The digestion of express packaging materials is further combined with the microwave process. The non-thermal effect of microwave optimizes the reaction environment from two aspects: First, the microwave electric field makes the acid molecules align in a dipole moment direction, weakening the intermolecular force, promoting the penetration of the acid solution into the microscopic pores of the sample, and making the hydroxyl radicals (・OH) generated by hydrogen peroxide more likely to contact organic substances, improving the oxidation efficiency; Second, it induces local high temperature and high pressure at the solid-liquid interface, destroying the crystal lattice structure of the sample, such as the Al-Si bond in the aluminum foil membrane, reducing the reaction activation energy, accelerating the complexation equilibrium of formic acid and metal ions, making the metal dissolution more complete, and at the same time making it easier for silicates and metal-organic complexes that are difficult to decompose originally to react with acids. This synergy shortens the digestion time from the traditional 240 minutes to 30 minutes, and also improves the digestion completeness, making the decomposition rate of SiO2 reach 99.9% and the decomposition rate of organic substances exceed 99%, achieving a qualitative leap compared with the digestion effect of a single acid system or without microwave assistance; The sufficiently digested express packaging materials are further extracted by the synergistic extraction of HP-β-CD and DTPA. The specific principle is as follows: In the present invention, HP-β-CD, as a cyclic oligosaccharide, its hydrophobic inner cavity can encapsulate hydrophobic metal complexes, while the polycarboxylic acid groups of DTPA can form stable chelates with interfering ions such as Al 3+ (logK = 18.6). The size matching of the two forms a division of labor: DTPA preferentially binds to Al 3+ with a smaller radius (0.535 Å), while HP-β-CD is adapted to Pb 2+ with a larger radius (1.19 Å). At the level of electronic effects, the carboxylic acid groups of DTPA and the hydroxyl groups of HP-β-CD coordinate synergistically to change the electron cloud density of metal ions, making the stability constant of the Pb 2+ -DTPA-HP-β-CD ternary complex logK = 21.3, which is 2.7 orders of magnitude higher than that of the binary complex. Specific Embodiments

[0021] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations to the present invention.

[0022] Example 1

[0023] This example discloses a method for microwave digestion of express packaging materials and its application, including the following steps: S1. Take the aluminum foil film of the express packaging material, break it with scissors, grind it to a particle size of less than 100 μm, referring to GB / T38726-2020; place 0.1 g of the crushed aluminum foil film particles in a polytetrafluoroethylene digestion tank; S2. Add a composite acid-chitinase mixed solution to the digestion tank, tighten the tank lid, and let it stand for 10 minutes for pre-reaction to make the acid solution contact the sample initially, weaken the sample structure, and improve the subsequent digestion efficiency.

[0024] The mass ratio of the express packaging material to the volume of the composite acid-chitinase mixed solution is 0.1 g:5 ml; The composite acid-chitinase mixed solution includes a composite acid mixed solution and a chitinase aqueous solution; The composite acid mixed solution includes nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid. The volumes of nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid are shown in Table 1. Add 10 ml of the chitinase aqueous solution to the composite acid mixed solution, and the content of the chitinase aqueous solution is 10 U / ml.

[0025] The preparation method of the chitinase aqueous solution is as follows: Weigh 0.1 g of chitinase, dissolve it in 10 mL of deionized water to prepare a 10 U / mL solution, slowly add the enzyme solution to the composite acid mixed solution, and stir magnetically for 10 minutes until uniform.

[0026] S3. Place the digestion tank that has undergone S2 on the turntable of a microwave digestion instrument for microwave digestion; The process parameters of microwave digestion are as follows: microwave power 300 W, duty cycle 50%; Heat up to 80 °C in 10 minutes and keep warm for 20 minutes; monitor the pressure in real time, and the expected peak value ≤ 300 psi; After the digestion is completed, naturally cool to room temperature in about 20 min.

[0027] Apply the digestion solution obtained by the method of microwave digestion of express packaging materials in this example to detect the heavy metal content in express packaging materials by ICP-MS, including the following steps: A1. Place the digestion solution obtained in S3 in a centrifuge tube and add an extractant containing HP-β-CD and DTPA as double ligands for synergistic extraction to form a Pb 2+ -DTPA-HP-β-CD ternary complex; where HP-β-CD is hydroxypropyl-β-cyclodextrin and DTPA is diethylenetriaminepentaacetic acid.

[0028] The preparation of the extractant is as follows: Weigh 14.76 g of HP-β-CD (hydroxypropyl-β-cyclodextrin, hydroxypropyl substitution degree n = 6, purity ≥ 98%, molar mass calculated to be 1476 g / mol), dissolve it in 100 mL of deionized water to prepare a 0.1 M solution; weigh 1.18 g of DTPA (diethylenetriaminepentaacetic acid, analytical pure, molecular formula C 14 H 23 N3O 10 , molar mass 393.27 g / mol), dissolve it in 100 mL of deionized water to prepare a 0.03 M solution; take 50 mL each of the HP-β-CD solution and the DTPA solution, place them in a 100 mL beaker, and mix well. At this time, the concentration of HP-β-CD in the mixed solution is 0.05 mol / L, and DTPA is 0.015 mol / L. Adjust the pH to 5.5 with a citric acid-disodium hydrogen phosphate buffer solution.

[0029] The process parameters of extraction are as follows: Place it in a 60 °C water bath and oscillate at 200 rpm for 30 minutes; the extraction temperature is set at 60 °C; The volume ratio of the extractant to the composite acid-chitinase mixture is 2:1.

[0030] A2. Centrifuge the centrifuge tube extracted in step A1 at 5000 rpm for 5 minutes, take the supernatant for ICP-MS detection, and obtain the heavy metal dissolution amount and the heavy metal dissolution rate.

[0031] The process parameters for ICP-MS detection in A2 are as follows: RF power: 1550 W; carrier gas flow rate: 1.0 L / min; sampling depth: 8 mm; internal standard element: Ge (10 ppb); standard curve: 0.01 - 100 ppm, R² ≥ 0.999.

[0032] The specifications of nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid in the mixed complex acid solution are as follows: Nitric acid, Sinopharm Chemical Reagent Co., Ltd., specification: mass fraction 65% - 68%, analytical pure (AR), concentration approximately 15 mol / L; Trifluoroacetic acid, Shanghai Aladdin Biochemical Technology Co., Ltd., specification: mass fraction ≥ 99%, analytical pure (AR), concentration 12 mol / L; Hydrogen peroxide, Macklin Biochemical Technology Co., Ltd., specification: mass fraction 30%, analytical pure (AR), concentration approximately 9.7 mol / L; Formic acid, Tianjin Kemiou Chemical Reagent Co., Ltd., specification: mass fraction ≥ 98%, analytical pure (AR), concentration approximately 23 mol / L.

[0033] Example 2

[0034] The main differences between this example and Example 1 are shown in Table 1 and Table 2.

[0035] Example 3

[0036] The main differences between this example and Example 1 are shown in Table 1 and Table 2.

[0037] Example 4

[0038] The main differences between this example and Example 1 are shown in Table 1 and Table 2.

[0039] Example 5

[0040] The main differences between this example and Example 1 are shown in Table 1 and Table 2.

[0041] Comparative Example 1 The main difference between this comparative example and Example 1 is that: according to the GB / T 38727 - 2020 standard, nitric acid and hydrogen peroxide are mixed to obtain the mixed complex acid solution, and the extractant used is 0.15 M EDTA. The specific dosage is shown in Table 1, and the other process conditions are the same as those in Example 1.

[0042] Comparative Example 2 The main difference between this comparative example and Example 1 is that the mixed complex acid solution is used alone, and the chitinase aqueous solution is not used, and the other process conditions are the same as those in Example 1.

[0043] Comparative Example 3 The main difference between this comparative example and Comparative Example 1 is that microwave heating was not used, and the digestion tank that had passed through S2 was directly heated to 80 °C by electric heating and kept warm for 30 min for digestion. The remaining process conditions were the same as those in Example 1.

[0044] Table 1 Composition and dosage of the composite acid mixture used in Examples 1 to 5 and the comparative example System number Nitric acid (mL) Trifluoroacetic acid (mL) Hydrogen peroxide (mL) Formic acid (mL) Total acid amount (mL) Example 1 5 3 3 1 12 Example 2 4 2 2 1 9 Example 3 5 4 3 1 13 Example 4 5 3 2 1 11 Example 5 5 3 3 2 13 Control example 3 0 2 0 5 Table 2 Changes in the composition and pH value of the extractant used in Examples 1 to 5 Experimental group HP-β-CD (mol / L) DTPA (mol / L) pH value Example 1 0.05 0.015 5.0 Example 2 0.05 0.02 6.0 Example 3 0.06 0.01 5.0 Example 4 0.06 0.015 5.5 Example 5 0.06 0.02 6.0 The heavy metal dissolution amount, heavy metal dissolution rate, and theoretical total amount in the aluminum foil film were respectively tested in Examples 1 to 5 and the comparative example through microwave digestion combined with extraction. The specific calculation methods are as follows: ① Calculation of heavy metal dissolution amount ; Among them, Q —Heavy metal dissolution amount (mg / kg); C —ICP-MS detection concentration (mg / L); C 0—Blank experiment detection concentration (mg / L); V —Volume of the digested solution after constant volume (L); m —Sample mass (kg);

[0045] ② Heavy metal dissolution rate calculation formula ; Among them, η —Dissolution rate (%), M —Theoretical total amount of heavy metals in the sample (mg / kg); ③ Theoretical total amount calculation ; Among them, ρ —Sample density (kg / m³); ω —Heavy metal mass fraction (%).

[0046] The specific test and calculation data are shown in Tables 3 and 4.

[0047] Table 3 Detection indexes of aluminum foil film in Examples 1 to 5 and the comparative example System number Lead dissolution amount (mg / kg) Lead dissolution rate (%) Cadmium dissolution amount (mg / kg) Cadmium dissolution rate (%) Example 1 248.8 ± 2.1 99.5 ± 0.8 148.2 ± 1.5 98.8 ± 1.0 Example 2 230.0 ± 3.5 92.0 ± 1.4 138.0 ± 2.3 92.0 ± 1.5 Example 3 245.5 ± 2.8 98.2 ± 1.1 145.3 ± 1.8 96.9 ± 1.2 Example 4 242.3 ± 3.2 96.9 ± 1.3 143.5 ± 2.0 95.7 ± 1.3 Example 5 238.6 ± 3.0 95.4 ± 1.2 141.8 ± 2.2 94.5 ± 1.5 Control example 1 200.5 ± 4.0 80.2 ± 2.5 120.3 ± 3.0 81.0 ± 2.0 Control example 2 235.0±3.0 94.0 ± 1.5 140.0 ± 2.5 93.0 ± 1.8 Control example 3 210.0 ± 4.5 84.0 ± 3.0 125.0 ± 3.5 83.0 ± 2.5 Table 4 Digestion efficiency and residual interfering ions of aluminum foil film in Examples 1 to 5 and the comparative example System number <![CDATA[Decomposition rate of SiO2 (%)]]> Organic matter decomposition rate (%) <![CDATA[Al 3+ Residual amount (mg / kg)]]> Example 1 99.9 ± 0.1 99.9 ± 0.1 0.5 ± 0.1 Example 2 98.0 ± 0.3 95.0 ± 0.5 3.2 ± 0.4 Example 3 99.9 ± 0.1 99.5 ± 0.2 0.8 ± 0.2 Example 4 99.8 ± 0.2 97.0 ± 0.3 1.2 ± 0.3 Example 5 99.8 ± 0.2 98.5 ± 0.3 1.5±0.3 Control example 1 85.3± 0.2 95.0± 0.5 5.0± 0.4 Control example 2 95.1±0.2 97.0 ± 0.5 1.8 ± 0.3 Control example 3 90.0 ± 0.5 96.0 ± 0.8 3.5 ± 0.5 The present invention also sets up a blank experiment. 0.1 g of blank quartz sand is processed according to the same steps as in Example 1, and the reagent background is deducted to ensure that the reagent has no interference (the expected blank value < 0.01 ppm). The present invention also sets up a standard addition recovery experiment: 0.5 mg / kg of lead and 0.3 mg / kg of cadmium are added to 0.1 g of the sample as a standard solution to verify the accuracy of the method, and the expected recovery rate ≥ 98%.

[0048] Table 5 Standard Addition Recovery Experiment Heavy metal Added concentration (mg / kg) Recovery rate (%) RSD (%) Lead 0.5 99.2 1.2 Cadmium 0.3 98.7 1.5 Comparing the detection results of heavy metals in Comparative Examples 1 to 5 and Comparative Examples 1 to 3, it shows that the S1 group with a volume ratio of nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid of 5:3:3:1 performs optimally in terms of heavy metal dissolution efficiency and digestion effect: the lead dissolution rate reaches 99.5% ± 0.8%, the cadmium dissolution rate is 98.8% ± 1.0%, the SiO2 decomposition rate is 99.9% ± 0.1%, the organic matter decomposition rate is 99.9% ± 0.1%, and the Al 3+ residual amount is only 0.5 mg / kg ± 0.1. This ratio provides strong oxidation ability through nitric acid (ΔH = -55.2 kJ / mol), releases fluoride ions through trifluoroacetic acid to promote the fluorolysis reaction (ΔG = -87.3 kJ / mol), generates hydroxyl radicals through hydrogen peroxide to strengthen the oxidation of organic matter, and the formic acid stabilizes metal complexes with a logK of 21.3, and an average of 2.3 hydrogen bonds per molecule form a hydrogen bond network to synergistically interact with free radicals in a three-dimensional effect. Statistical analysis shows that the lead dissolution rate of the S1 group is significantly higher than that of other groups (p < 0.001), and response surface fitting confirms that its ratio combination maximizes the dissolution rate. The standard addition recovery rates for lead are 99.2% and for cadmium are 98.7%, with RSD < 2%, verifying the accuracy and precision of the method. The present invention uses dual-ligand synergistic extraction. Combining the experimental data in Tables 3 to 5 shows that the synergistic system increases the Al 3+ masking rate from 82% to 95%, increases the lead recovery rate from 90% to 99.5%, reduces the detection limit to 0.01 ppm, and remains stable in the pH range of 5.0 - 6.0. This synergistic mechanism not only improves selectivity and kinetic efficiency but also enhances the ability to resist matrix interference, providing an efficient solution for the detection of trace heavy metals in complex environments.

[0049] The digestion time of the present invention is shortened to 30 minutes, and the total detection time is compressed to 1.5 hours, which is 3 times faster than the traditional method. Compared with the 2-hour roasting-nitric acid digestion in Chinese Patent CN112326575A, the expected digestion time of the present invention is shortened by 87.5%. In addition, the synchronous extraction process without transferring the digestion solution further reduces the operation steps and the risk of contamination. The theoretical organic matter decomposition rate > 99.9, and the lead recovery rate is 99.5% (RSD < 2.5%), meeting the high-throughput requirements of more than 300 samples per day in the logistics industry.

Claims

1. A method for microwave digestion of express packaging materials, characterized in that: It includes the following steps: S1. Take the express packaging materials, break them, and grind them to a particle size less than 100 μm; place the express packaging materials in a polytetrafluoroethylene digestion tank. S2. Add a composite acid-chitinase mixture to the digestion tank, tighten the tank lid, and let it stand. The composite acid-chitinase mixture makes initial contact with the sample to weaken the sample structure. The composite acid-chitinase mixture includes a composite acid mixture and an aqueous chitinase solution. The composite acid mixture includes nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid. S3. Place the digestion tank after S2 on the turntable of a microwave digester for pulsed microwave digestion. The pulsed microwave drives the vibration of polar molecules in the acid solution to generate heat through the internal Joule effect, raising the system temperature from room temperature to 75°C - 85°C. The microwave electric field makes the acid molecules' dipole moments align directionally, penetrate into the microscopic voids of the express packaging material particles, and trigger local high temperature and high pressure at the solid-liquid interface. The microwave time is from 25 min to 35 min.

2. A method for microwave digestion of express packaging materials according to claim 1, characterized in that: The mass ratio of the express packaging materials to the volume of the composite acid-chitinase mixture is 0.1 g:5 ml.

3. A method for microwave digestion of express packaging materials according to claim 1, characterized in that: The volume ratio of nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid in the composite acid mixture is (2.5 to 5):(1.5 to 4):(1.5 to 3):

1.

4. A method for microwave digestion of express packaging materials according to claim 3, characterized in that: The volume ratio of nitric acid, trifluoroacetic acid, hydrogen peroxide, and formic acid in the composite acid mixture is 5:3:3:

1.

5. A method for microwave digestion of express packaging materials according to claim 1, characterized in that: The content of the aqueous chitinase solution is 10 U / ml; The volume ratio of the composite acid mixture to the aqueous chitinase solution is (0.9 to 1.3):

1.

6. A method for microwave digestion of express packaging materials according to claim 1, characterized in that: The process parameters of the pulsed microwave digestion are as follows: The microwave power is 300 W, and the duty cycle is 50%; The temperature control condition is to rise to 80°C in 10 minutes and keep warm for 20 minutes; After digestion is completed, naturally cool to room temperature to obtain the digestion solution.

7. An application of using the digestion solution obtained by the method for microwave digestion of express packaging materials according to any one of claims 1 to 6 to detect the heavy metal content in the express packaging materials by ICP-MS.

8. The application according to claim 7, characterized in that: It includes the following steps: A1. Place the digestion solution in a centrifuge tube and add an extractant containing HP-β-CD and DTPA as double ligands for synergistic extraction, and then perform extraction to form a Pb 2+ -DTPA-HP-β-CD ternary complex; A2. Centrifuge the centrifuge tube extracted in step A1, take the supernatant for ICP-MS detection, and obtain the heavy metal dissolution amount and the heavy metal dissolution rate.

9. The application according to claim 7, characterized in that: In the extractant for double-ligand synergistic extraction, the molar concentration of HP-β-CD is 0.05 M to 0.06 M; the molar concentration of DTPA is 0.01 M to 0.02 M; use a citric acid-disodium hydrogen phosphate buffer solution to adjust the pH of the extractant to 5.0 to 6.

0.

10. The application according to claim 7, wherein: The process parameters for extraction are as follows: Place it in a water bath at 60 °C, shake, and extract; The volume ratio of the extractant to the composite acid-chitinase mixture is 2:1.