Method for evaluating migration of soil microplastics

Abstract

The present application relates to the technical field of soil new pollutant treatment, and particularly relates to a soil microplastic migration evaluation method, comprising the following steps: saturated quartz sand porous medium microplastic migration simulation based on a migration model, wherein the migration model comprises a blocking model and a ripening model; and non-saturated zone variable mineral component porous medium microplastic migration simulation.The present application comprehensively considers microplastic migration under different soil conditions by simulating saturated and non-saturated zones, accurately evaluates migration, provides a scientific basis for subsequent treatment, and uses the blocking and ripening models to fit microplastic migration behavior for scientific evaluation and guidance for targeted treatment measures; in the present application, the non-saturated zone considers the influence of mineral composition by setting different experimental columns, accurately evaluates the migration risk of microplastics in different soils; finally, the evaluation method system constructed in the present application provides a scientific method for soil microplastic treatment, and targeted measures can be taken to reduce pollution risk.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of soil new pollutant treatment, and particularly relates to a soil microplastic migration evaluation method. BACKGROUND

[0002] With the development of social economy, soil microplastic pollution gradually becomes a problem threatening food safety and drinking water safety. Multiple sources such as agricultural film use, fertilizer application, atmospheric deposition and irrigation cause the enrichment of microplastics in the soil environment. Existing evidence shows that soil is a hot spot for microplastic occurrence, especially in the soil environment, with the action of rainfall infiltration, microplastics occurring in the shallow soil can migrate vertically and eventually enter the deep soil and groundwater aquifer, posing a threat to human health.

[0003] Currently, the evaluation of microplastic migration is still usually based on saturated quartz sand medium simulation experiments, and there is a lack of evaluation and simulation methods for the migration of soil microplastics in the real environment. Existing patents also mostly construct simulation devices for soil microplastic migration, and lack methods for objectively evaluating the migration of microplastics in the real soil environment. In view of the above problems, the present application develops a method for evaluating the migration of microplastics in the soil environment, which uses different types of minerals as soil aeration zone medium to realize the evaluation of the migration of soil microplastics. SUMMARY

[0004] The purpose of the present application is to solve the problem that the existing evaluation of microplastic migration is still usually based on saturated quartz sand medium simulation experiments, and there is a lack of evaluation and simulation methods for the migration of soil microplastics in the real environment, and a soil microplastic migration evaluation method is proposed.

[0005] The technical scheme of the present application: a soil microplastic migration evaluation method, comprising the following steps:

[0006] Saturated quartz sand porous medium microplastic migration simulation based on a migration model, the migration model including a blocking model and a maturation model;

[0007] Non-saturated zone (aeration zone) variable mineral component porous medium microplastic migration simulation;

[0008] According to the simulation results of the saturated quartz sand porous medium microplastic migration simulation and the non-saturated zone variable mineral component porous medium microplastic migration simulation, the migration of microplastics in the soil is rated, and then corresponding treatment measures are taken for different migration of the soil.

[0009] Optionally, the saturated quartz sand porous medium microplastic migration simulation comprises the following steps:

[0010] The experimental column is made of organic glass material, the inner diameter of the experimental column is 3-6 cm, and the length is 2-3 times of the inner diameter, the experimental column is placed horizontally, and a stainless steel filter membrane with a pore size of 5-10 μm is placed on the left side of the experimental column;

[0011] The experimental column is filled with quartz sand with a particle size of 40-80 mesh to control the porosity to 0.4-0.6, and 5-20 mg of microplastic is loaded on the left filter membrane section;

[0012] The sodium chloride salt solution or potassium chloride salt solution with an ionic strength of 0.5-5 mmol / L and a pH of 6-8 is injected at an infiltration rate of 5-20 mm / h as the leaching solution, the injection rate is 1-5 mL / min, and the right side of the filter is collected at different time intervals, and the microplastic concentration in the filter is analyzed by using the microplastic characterization method;

[0013] 2-4 pore volumes of salt solution are injected before the microplastic is placed to make the system reach the hydrochemical equilibrium, and the blocking model and the maturation model are used to fit the migration behavior of the microplastic to judge the migration of the microplastic.

[0014] Optionally, the blocking phenomenon is reflected in the process of colloid migration as the number of attachable sites on the medium surface decreases with time, resulting in a decrease in the attachment rate of the colloid, and the blocking model includes the following formula:

[0015]

[0016] wherein x is the distance, unit: cm; θ is the volumetric water content, ρ b is the bulk density of the porous medium, unit: g / cm 3 ; s1 is the solid-phase concentration of microplastic in the first type of kinetic site (reversible adsorption without blocking), unit: g / g; s2 is the solid-phase concentration of microplastic in the second type of kinetic site (irreversible adsorption with blocking), unit: g / g; D is the dispersion coefficient determined by tracer experiment, unit: cm 2 / h, c is the concentration of microplastic in the pore water, unit: g / cm 3 ; v is the Darcy flow rate, unit: cm / h; k at,1 (h -1 ), k dt,1 (h -1 ) and k at,2 (h -1 ) are the adsorption constant of the first type of site, the desorption constant of the first type of site, the adsorption constant of the second type of site, respectively, ψ s is a dimensionless blocking function related to the depth of the porous medium, and the depth of the porous medium is as follows:

[0017]

[0018] where d is the median particle size of the porous medium (cm), and β, also known as the shape factor, is an empirical constant. s is the median particle size of the porous medium (cm), and β, also known as the shape factor, is an empirical constant.

[0019] Optionally, the migration of microplastics in the porous medium is also affected by the maturation effect, and maturation and clogging are antagonistic, when the maturation mechanism occurs, the microplastics attached to the surface of the soil mineral particles provide new attachment sites, thereby promoting the deposition of microplastic particles, and the maturation model includes the following formula:

[0020]

[0021] where ψ is a dimensionless maturation function, s is the maximum microplastic concentration of the solid phase, and g / g is the unit. r is a dimensionless maturation function, s is the maximum microplastic concentration of the solid phase, and g / g is the unit. max is a dimensionless maturation function, s is the maximum microplastic concentration of the solid phase, and g / g is the unit.

[0022] Optionally, the non-saturated zone (vadose zone) variable mineral component porous medium microplastic migration simulation includes the following steps:

[0023] Use an organic glass material experimental column, the inner diameter of the experimental column is 5-8 cm, and the filling length is 2-3 times the inner diameter;

[0024] Pure quartz sand experimental column A and experimental column B containing 50-70% clay minerals and 30-50% quartz sand are constructed respectively, and the porosity is consistent with that of the saturated quartz sand porous medium experimental column;

[0025] Place the experimental column from top to bottom, place the microplastics at the upper end of the experimental column, inject a salt solution with the same ion strength and formula as the saturated quartz sand porous medium experiment, and control the injection rate at 9-15 mL / h;

[0026] Take the dry-wet cycle experiment method, pause the salt solution injection every 1-2 pore volumes; place the experimental column in an environment of 40-55 degrees for 48-72 hours, and then take it out and place it at room temperature;

[0027] Input 1-2 pore volumes of leaching solution, a total of 8-12 cycles, and before the first cycle, inject 2-4 pore volumes of salt solution to make the system reach hydrochemical equilibrium;

[0028] After the dry-wet cycle is completed, the solid phase medium located at a depth of 2-4 cm is taken out, and the microplastic abundance in the solid phase medium is tested using a microplastic characterization method, and the concentration ratio of the original concentration of the microplastics is obtained to obtain the migration rate of the microplastics.

[0029] Optionally, the migration of microplastics is rated, and when the simulation result of the saturated quartz sand porous medium microplastic migration simulation conforms to the clogging model:

[0030] If the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is 0-2%, it is determined as migration L1;

[0031] If the migration rate of the experimental column A is >0.5% and the migration rate of the experimental column B is 0-2%, or the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is >2%, it is determined as migration L2;

[0032] If the migration rate of the experimental column A is >0.5% and the migration rate of the experimental column B is >2%, it is determined as migration L3.

[0033] Optionally, when the simulation result of the saturated quartz sand porous medium microplastic migration simulation meets the maturation model, the following measures are taken:

[0034] If the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is 0-2%, it is determined as migration L2;

[0035] If the migration rate of the experimental column A is >0.5% and the migration rate of the experimental column B is 0-2%, or the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is >2%, it is determined as migration L3;

[0036] If the migration rate of the experimental column A is >0.5% and the migration rate of the experimental column B is >2%, it is determined as migration L4.

[0037] Optionally, for the soil with migration L1 and L2, source control measures are taken to reduce the enrichment of microplastics in the surface soil, and for the soil with migration L3 and L4, corresponding risk control and end treatment measures are taken on the premise of source control to prevent the vertical migration of microplastics.

[0038] In summary, the present application includes at least one of the following beneficial technical effects:

[0039] 1、The present application considers the migration of microplastics in different soil conditions through two steps of saturated quartz sand porous medium microplastic migration simulation and non-saturated zone variable mineral component porous medium microplastic migration simulation, comprehensively considers the migration of microplastics in different soil conditions, the saturated zone experiment mainly evaluates the influence of pure polymer-medium interaction on the migration, and the migration of microplastics in the groundwater environment is simulated; the non-saturated zone experiment mainly simulates the influence of the common dry-wet cycle process in nature on the migration, constructs an experimental column with different mineral compositions, and is more close to the actual soil environment. This comprehensive simulation method can accurately evaluate the migration of microplastics in the soil, and provides a scientific basis for subsequent treatment measures;

[0040] 2、The application adopts the blocking model and the ripening model to fit the migration behavior of microplastics, can deeply understand the migration mechanism of microplastics in soil, and can judge the migration of microplastics to belong to different levels, such as L1, L2, L3 and L4, through the fitting of experimental data, the scientific rating method can accurately evaluate the migration risk of microplastics, and provide guidance for targetedly taking control measures.

[0041] 3、In the present application, the unsaturated zone experiment sets a pure quartz sand experimental column A and an experimental column B rich in clay minerals, the influence of different mineral compositions on the migration of microplastics is considered, since the clay minerals have good swelling property, the soil fissures are more easily induced in the process of dry-wet cycle, and the soil fissures are important factors causing the vertical migration of microplastics, therefore, the migration of the B column rich in clay minerals is stronger than that of the A column, and this design can more accurately evaluate the migration risk of microplastics under different soil conditions.

[0042] 4、The soil microplastic migration evaluation method system constructed in the present application provides a scientific method and basis for the control of soil microplastics, through accurate evaluation of the migration of microplastics, source control and risk control end treatment measures can be taken, and the pollution risk of microplastics to the soil environment can be effectively reduced.

[0043] The present application comprehensively considers the migration of microplastics under different soil conditions through the saturated zone and the unsaturated zone, accurately evaluates the migration to provide a scientific basis for subsequent control, and adopts the blocking and ripening model to fit the migration behavior of microplastics for scientific rating to provide guidance for targeted control measures, in the present application, the unsaturated zone considers the influence of mineral composition by setting different experimental columns, accurately evaluates the migration risk of microplastics in different soils, and finally, the evaluation method system constructed in the present application provides a scientific method for the control of soil microplastics, and targeted measures can be taken to reduce the pollution risk. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 The penetration curve graphs of different types of polymers in the saturated porous medium in the embodiment of the present application are given: (a) PE; (b) PP; (c) PA; (d) PET; (e) PVC; (f) PU. DETAILED DESCRIPTION

[0045] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application, obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor belong to the protection scope of the present application.

[0046] EMBODIMENT

[0047] The application provides a soil microplastic migration evaluation method.

[0048] In the embodiment, PE, PP, PA, PET, PVC and PU microplastics with particle sizes in the range of 20-26 μm are used to evaluate the migration thereof in soil.

[0049] The migration of the microplastics in the saturated quartz sand porous medium is simulated based on a migration model, and the migration model includes a blocking model and a maturation model.

[0050] Firstly, the migration behaviors of different types of microplastics in the saturated quartz sand medium are studied, the quartz sand filling porosity is controlled to be 0.5, the leaching solution is 1 mmol / L NaCl, and the migration column experiment is carried out under the condition of pH=7.

[0051] The experimental column is made of organic glass, the inner diameter is preferably 5 cm, the length is preferably 10 cm, a separate experimental column is used for each type of microplastic, and a stainless steel filter membrane with a pore size of 10 μm is placed at the bottom of the experimental column.

[0052] The filled experimental column is placed horizontally to reduce the influence of density difference on the experimental results of the microplastics, and the 10 μm filter membrane end is placed on the left side.

[0053] First, 3 PV of the leaching solution is used to reach the water chemical equilibrium, then 10 mg of the microplastics is loaded on the left side of the 10 μm filter membrane, 1 mmol / L NaCl solution is injected from left to right at a rate of 1.364 ml / min, and a total of 10 pore volumes (PV) are injected.

[0054] The above injection rate is equivalent to the actual flow rate of 1 m / d of groundwater, which is equivalent to the actual flow rate of groundwater in the real coarse sand aquifer, and the filtrate is collected at the right side of the experimental column at different time periods, and the concentration of the microplastics in the filtrate is analyzed.

[0055] The migration curves of different microplastics are as shown in Figure 1 The model fitting results are as shown in Table 1, with the increase of PV, the concentration of the microplastics in the filtrate of the experimental column presents a trend of first increasing and then decreasing, and it can be seen that the migration processes of PE, PP, PA, PVC and PU can be described by the blocking model, R 2 = 0.836-0.961, but the migration process of PET is more consistent with the assumption of the maturation model, R 2 = 0.968, as shown in Table 1.

[0056] The possible reason for this phenomenon is that the quartz-PET system has a higher adsorption capacity than quartz and other polymer systems, which is conducive to the adsorption of PET. After the adsorption sites are rapidly occupied by PET, the remaining suspended polymer particles in the liquid phase can be deposited on the surface of the adsorbed PET through hydrogen bonds and other interactions, resulting in the maturation phenomenon.

[0057] Table 1 migration model fitting parameters

[0058]

[0059] Secondly, the non-saturated zone (vadose zone) mineral composition porous medium microplastic migration simulation experiment was carried out, including the following steps:

[0060] An experimental column with an inner diameter of 5 cm and a length of 12 cm was used, the experimental column was placed from top to bottom, and the same ionic strength and pH leaching solution was injected from the top;

[0061] For each type of PE, PP, PA, PET, PVC, PU, two different vadose zone media were set as follows: the porosity was constant at 0.5, and the effect of different vadose zone types on microplastic migration was studied: experimental column A was pure quartz sand (100% sand particles), and experimental column B was 40% quartz sand and 60% kaolinite (40% sand particles and 60% silt particles);

[0062] In the vadose zone migration column experiment, kaolinite particles with a particle size range of 0.2-2.4 μm and an average particle size of 0.8 μm were selected; quartz sand particles with a particle size range of 90-100 μm (150-180 mesh) were selected;

[0063] For the vadose zone composed of mixed minerals, the mixture was thoroughly mixed before filling and filled by dry method, the filling height was 10 cm, and after filling, 3 PV of leaching solution was added from top to bottom at a rate of 5 mm / h (9.82 mL / h), followed by 10 times of dry-wet cycles: first inject 1 PV of leaching solution, then place the experimental column in a 50 degree environment for 48 h and take it out;

[0064] After 10 cycles, for experimental columns A and B, respectively, the concentration of microplastics was analyzed at a depth of 2 cm, the concentration ratio of the original concentration of microplastics was obtained, and the migration rate of microplastics was obtained.

[0065] The vadose zone migration simulation results are shown in Table 2:

[0066] Table 2 vadose zone migration simulation experiment results

[0067] Microplastic type Experiment column A mobility Experiment column B mobility PE 0.28％ 1.27％ PP 0.27％ 1.31％ PA 0.43％ 2.05％ PET 0.65％ 3.09％ PVC 0.37％ 2.60％ PU 0.55％ 2.66％

[0068] Finally, according to the simulation results of microplastic migration in saturated quartz sand porous medium and non-saturated zone variable mineral composition porous medium, the migration of microplastics in soil is rated, and corresponding treatment measures are taken for different migration of soil.

[0069] The migration of microplastics is classified and judged by Table 3, wherein L1 level is the weakest migration, and L4 level is the strongest migration. It should be pointed out that the method is suitable for microplastics of various particle sizes and various shapes, and the type of microplastic polymer is not limited. It is an evaluation method developed from the internal cause of microplastic-medium interaction affecting the migration of microplastics in soil, and considers the influence of dry-wet cycle in nature on the migration of microplastics, and has universality.

[0070] Table 3: Microplastic migration rating table

[0071]

[0072] According to the above results, the migration of six kinds of microplastics in soil is rated as follows:

[0073] PE-L1, PP-L1, PA-L2, PET-L4, PVC-L2, PU-L3;

[0074] The reason for the above phenomenon is that PET, PU and PA have oxygen-containing functional groups, strong hydrophilicity and strong negative surface properties, so the migration is greater than PE, PP and PVC.

[0075] For soil with migration L1 and L2, source control measures are taken to reduce the enrichment of microplastics in the surface soil, and for soil with migration L3 and L4, corresponding risk control and end treatment measures are taken under the premise of source control to prevent vertical migration of microplastics.

[0076] The present application considers microplastic migration under different soil conditions by saturated zone and non-saturated zone simulation, accurately evaluates the migration, provides scientific basis for subsequent treatment, and uses blocking and curing model to fit the migration behavior of microplastics for scientific rating and guidance for targeted treatment measures. In the present application, the non-saturated zone considers the influence of mineral composition by setting different experimental columns to accurately evaluate the microplastic migration risk of different soils. Finally, the evaluation method system constructed in the present application provides a scientific method for soil microplastic treatment, and can take targeted measures to reduce pollution risk.

[0077] The above specific embodiments are only several optional embodiments of the present application, and based on the technical solutions of the present application and the related inspiration of the above embodiments, those skilled in the art can make various alternative improvements and combinations on the above specific embodiments.

Claims

1. A method for evaluating the mobility of soil microplastics, characterized by, The method comprises the following steps: The saturated quartz sand porous medium microplastic migration simulation experiment based on a migration model is used to study the migration behavior of different types of microplastics in the saturated quartz sand porous medium, and the migration behavior of different types of microplastics is fitted by using a blocking model and a maturation model to determine the migration of the microplastics; The non-saturated zone variable mineral component porous medium microplastic migration simulation experiment is used to obtain the migration rate of the microplastics; According to the simulation results of the saturated quartz sand porous medium microplastic migration simulation and the non-saturated zone variable mineral component porous medium microplastic migration simulation, the migration of the microplastics in the soil is rated, and the soil is treated according to different migration properties; The migration of the microplastics is rated, and when the simulation result of the saturated quartz sand porous medium microplastic migration simulation conforms to the blocking model: When the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is 0-2%, the migration property is determined as L1; When the migration rate of the experimental column A is greater than 0.5% and the migration rate of the experimental column B is 0-2% or the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is greater than 2%, the migration property is determined as L2; When the migration rate of the experimental column A is greater than 0.5% and the migration rate of the experimental column B is greater than 2%, the migration property is determined as L3; When the simulation result of the saturated quartz sand porous medium microplastic migration simulation conforms to the maturation model: When the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is 0-2%, the migration property is determined as L2; When the migration rate of the experimental column A is greater than 0.5% and the migration rate of the experimental column B is 0-2% or the migration rate of the experimental column A is 0-0.5% and the migration rate of the experimental column B is greater than 2%, the migration property is determined as L3; When the migration rate of the experimental column A is greater than 0.5% and the migration rate of the experimental column B is greater than 2%, the migration property is determined as L4.

2. The method for evaluating the migration property of soil microplastics according to claim 1, characterized in that, The saturated quartz sand porous medium microplastic migration simulation comprises the following steps: An experimental column made of organic glass is used, the inner diameter of the experimental column is 3-6 cm, the length is 2-3 times of the inner diameter, the experimental column is placed horizontally, a stainless steel filter membrane with a pore size of 5-10 m is placed on the left side; The experimental column is filled with quartz sand with a particle size of 40-80 mesh to control the porosity at 0.4-0.6, and 5-20 mg of microplastics is loaded on the left filter membrane section; A sodium chloride salt solution or a potassium chloride salt solution with an ionic strength of 0.5-5 mmol / L and a pH controlled at 6-8 is injected as a leaching solution at an infiltration rate of 5-20 mm / h, 5-15 pore volumes are injected at an injection rate of 1-5 mL / min, and the right side of the filter effluent is collected at different time periods, and the microplastic concentration in the filter effluent is analyzed by using a microplastic characterization method; 2-4 pore volumes of salt solution are injected before the microplastics are placed to make the system reach a hydrochemical equilibrium.

3. The method for evaluating the migration of soil microplastics according to claim 1, characterized in that, The blocking phenomenon is embodied in the process of colloid migration as the number of attachable points on the surface of the medium decreases with time, which causes the colloid attachment rate to decrease, and the blocking model comprises the following formula: （1） （2） （3） wherein, is the distance, unit cm; is the volumetric water content, is the bulk density of the porous medium, unit g / cm 3 ; is the solid-phase concentration of microplastics in the first type of kinetic point, unit g / g; is the solid-phase concentration of microplastics in the second type of kinetic point, unit g / g; is the dispersion coefficient determined by tracer experiment, unit cm 2 / h; is the concentration of microplastics in pore water, unit g / cm 3 ; is the Darcy flow rate, unit cm / h; (h -1 ), (h -1 ) and (h -1 ) are the adsorption constant of the first type of point, the desorption constant of the first type of point, the adsorption constant of the second type of point, is the dimensionless blocking function, and the depth of the porous medium, which is as follows: （4） wherein is the median particle diameter of the porous medium in cm, is the shape factor.

4. The method for evaluating the migration of soil microplastics according to claim 1, characterized in that, The migration of the microplastics in the porous medium is also affected by maturation, and maturation and blocking are antagonistic, when the maturation mechanism occurs, the microplastics attached to the surface of the soil mineral particles provide new attachment points, thereby promoting the deposition of the microplastic particles, and the maturation model comprises the following formula: （5） （6） （7） wherein, is a dimensionless ripening function, is the maximum microplastic concentration in the solid phase in g / g.

5. The method for evaluating the migration of soil microplastics according to claim 1, characterized in that, The non-saturated zone variable mineral component porous medium microplastic migration simulation comprises the following steps: The experimental column is filled with organic glass material, the inner diameter of the experimental column is 5-8 cm, and the filling length is 2-3 times the inner diameter; Pure quartz sand column A and column B containing 50-70% clay minerals and 30-50% quartz sand were constructed respectively, and the porosity of the two columns was consistent with that of the saturated quartz sand porous medium column; The experimental column was placed from top to bottom, and the microplastics were placed at the top of the column. The salt solution with the same ionic strength and formula as the saturated quartz sand porous medium experiment was injected at a rate of 9-15 mL / h; The dry-wet cycle experiment method was adopted, and the salt solution injection was paused every 1-2 pore volumes. The experimental column was placed in an environment of 40-55 degrees for 48-72 hours and then taken out and placed at room temperature; 1-2 pore volumes of leaching solution were input, and a total of 8-12 cycles were carried out. 2-4 pore volumes of salt solution were injected before the first cycle to achieve water chemical equilibrium in the system; After the dry-wet cycle, the solid phase medium located at a depth of 2-4 cm was taken out, and the microplastics characterization method was used to test the abundance of microplastics in the solid phase medium. The concentration of the solid phase medium and the original concentration of the microplastics were compared to obtain the migration rate of the microplastics.

6. The method for evaluating the migration of soil microplastics according to claim 1, characterized in that, For soils with migration L1 and L2, source control measures are taken to reduce the enrichment of microplastics in the surface soil. For soils with migration L3 and L4, corresponding risk management and control measures are taken on the premise of source control to prevent the vertical migration of microplastics.

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