Method for analyzing vertical migration rule of soil cadmium based on phosphate passivation condition

Abstract

The invention relates to the technical field of soil pollution treatment, and discloses a phosphate passivation condition-based soil cadmium vertical migration law analysis method, which comprises the following steps: selecting a cadmium-polluted soil column, applying a calcium magnesium phosphate fertilizer as a passivator to the soil column part, carrying out a soil column solute penetration experiment, collecting pore water at different depths through side sampling holes, and determining the vertical migration law of cadmium in the soil column; measuring a measured value; the method comprises the following steps: establishing a one-dimensional migration-reaction model based on a Hydrus-Phreeqc coupling module, simulating migration and transformation of cadmium in soil and chemical reaction with phosphate, and obtaining a simulation value; according to the actual measurement value and the simulation value, the Hybrid-Phreeqc coupling module is corrected, and the vertical migration characteristics of cadmium are analyzed; after the soil column experiment is finished, soil samples are collected from top to bottom, and the distribution of cadmium in the soil is detected and analyzed. According to the method, the soil column experiment is combined with the Hydrus-Phreeqc coupling model, so that the influence of applying the calcium magnesium phosphate fertilizer on the vertical migration of the soil cadmium can be more systematically analyzed; the method is easy to operate, high in practicability and capable of being applied to remediation research of different heavy metals.

Description

Technical Field

[0001] The present invention relates to the technical field of soil pollution control, and specifically to a method for analyzing the vertical migration law of cadmium in soil under phosphate passivation conditions, which provides theoretical support for the remediation of heavy metal contaminated soil. Background Art

[0002] The problem of cadmium pollution in soil is quite prominent in my country. Cadmium has extremely strong mobility in the soil. It can migrate and diffuse between different soil layers through various channels such as the movement of soil moisture and farming activities.

[0003] Phosphate is a commonly used soil passivation material. In practical applications, by adding phosphorus-containing substances such as calcium magnesium phosphate fertilizer to the soil, cadmium reacts with phosphate to form insoluble phosphate precipitation, thereby reducing the bioavailability and mobility of cadmium. However, current research mainly focuses on static adsorption tests, which cannot truly reflect the dynamic analysis of the vertical migration law of cadmium under the long-term action of passivators in the natural environment, making it difficult to comprehensively and deeply study the vertical migration and transformation mechanism of soil cadmium.

[0004] In the field of numerical simulation, although models such as Hydrus can simulate water flow and solute transport, they do not integrate the complex chemical reaction mechanism between cadmium and phosphate, and only treat cadmium as a common solute, resulting in insufficient prediction accuracy and failure to provide a reliable theoretical basis and technical support for the treatment and remediation of soil cadmium pollution. The Hydrus-Phreeqc coupling module can make up for this deficiency by integrating the chemical reaction mechanism to more accurately simulate the migration process of cadmium in the soil. Summary of the invention

[0005] In view of the above shortcomings and deficiencies, the present invention provides a method for analyzing the vertical migration law of cadmium in soil under phosphate passivation conditions. By combining indoor soil column experiments with numerical simulations, the method quantifies the effect of phosphate passivation materials on the migration law of cadmium in soil, clarifies the influence of passivators on the vertical migration of cadmium, and provides a scientific basis for effectively treating heavy metal contaminated soil.

[0006] In order to achieve the above object, the main technical solutions adopted by the present invention include:

[0007] A method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions, characterized in that it includes the following steps:

[0008] S1. Collect cadmium-contaminated soil, dry it, screen it, mix it and homogenize it to remove impurities and plant residues, and measure the physical and chemical properties of the soil;

[0009] S2, prepare a plurality of soil column devices, set sampling holes for collecting pore water on the side walls at different heights on the sides thereof, lay a nylon mesh at the bottom of the soil column device, and be equipped with a collection bottle for collecting the leachate at the bottom; fill the soil treated in step S1 inside the soil column device and evenly apply a calcium magnesium phosphate passivator on the top of the soil and compact it to simulate the actual soil condition;

[0010] S3, inject water into the soil where the calcium magnesium phosphate fertilizer passivator is applied for maintenance, set a test period, irrigate the soil at a fixed frequency after the maintenance period, collect pore water samples and leachate samples after the soil becomes stable after irrigation, process and test them, and record the measured values;

[0011] S4. A one-dimensional migration-reaction model is established based on the Hydrus-Phreeqc coupling module, the model boundary conditions are set, and the initial conditions are input. The reaction model is used for prediction to obtain the initial simulation value. The soil water flow and soil solute transport parameters of the Hydrus-Phreeqc coupling module are adjusted to make the initial simulation value consistent with the measured value, and the final simulation value after fitting is obtained.

[0012] S5. Cut and sample the soil columns after the test period, dry, sieve and weigh the collected soil, and detect the vertical changes of the effective state and Tessier morphology of soil cadmium;

[0013] S6. Combine the final simulation value in S4 with the effective content of soil cadmium obtained in S5 after the end of the experimental period compared with the initial soil and the vertical changes in the Tessier morphology distribution of heavy metal cadmium in each layer of soil to analyze the vertical migration characteristics of heavy metal cadmium in soil under the action of phosphate.

[0014] The physical properties of the soil in S1 include water content, pH value, soil bulk density, particle composition and porosity; the chemical properties of the soil include water-soluble cadmium, water-soluble phosphorus, water-soluble calcium, water-soluble magnesium, cation exchange capacity and iron and manganese oxides;

[0015] In S2, the soil column device is an acrylic plexiglass soil column device with a height of 35 cm. Sampling holes are set at 15 cm and 25 cm from the bottom to the top of the side of the soil column device. 10 g / kg of calcium magnesium phosphate passivator is evenly applied to the soil layer 10 cm deep in the upper part of the soil by stirring and compacting to make the soil compaction in the soil column uniform.

[0016] The specific step of irrigating the soil in S3 is as follows: recording the transpiration of the soil column every day, and adjusting the soil moisture content according to the actual soil moisture content each time irrigation is carried out to maintain the soil moisture content at 60%-70% of the field water holding capacity, simulating the real moisture conditions.

[0017] The pore water sample is collected in S3 using an in-situ collection technique, in which a pore water collector is placed in a side sampling hole to automatically collect soil solution pore water by vacuuming.

[0018] The pore water collector device is made of hydrophilic porous polyester material, the pores are between 0.12-0.18 microns, and is directly connected to a medical syringe.

[0019] The processing and detection of the collected pore water samples and leachate samples in S3 specifically includes filtering the pore water samples and leachate samples through a 0.45 μm filter membrane, and then measuring the filtered solutions using inductively coupled plasma mass spectrometry.

[0020] The initial conditions input to the Hydrus-Phreeqc coupling module in S4 include the physical and chemical properties of the soil measured in S1, the cadmium ion concentration of the soil solution, the transpiration rate, the amount of phosphate added, and the chemical reactions involving cadmium and phosphate.

[0021] The S5 specifically involves cutting and collecting soil columns at intervals of 10 cm from top to bottom in the soil column device.

[0022] In the S5, inductively coupled plasma mass spectrometry and inductively coupled plasma emission spectrometry are used to measure the effective content of different layers of soil and the Tessier form distribution of heavy metal cadmium in each layer of soil.

[0023] The present invention has the following beneficial effects and advantages:

[0024] (1) The present invention sets up a soil column experiment and applies phosphate passivation materials at a specific depth to simulate the vertical migration of soil cadmium under natural irrigation conditions. By combining the soil column experiment with the Hydrus-Phreeqc coupling model, the effect of applying calcium magnesium phosphate fertilizer on the vertical migration of soil cadmium can be more systematically analyzed. The method is simple to operate and highly practical, and can be applied to the remediation research of different heavy metals.

[0025] (2) The migration and distribution process of the passivation material solute in the soil over time and space in the cadmium ion concentration measured in the present invention; by continuously adjusting the model parameters, the simulated value and the measured value are more closely matched, which can effectively solve the problem that the solute migration parameters are difficult to measure and the accuracy cannot be guaranteed, and at the same time improve the accuracy of parameter estimation.

[0026] (3) The Hydrus-Phreeqc coupling module was used for simulation, which took into account the geochemical reactions involved in the interaction between calcium magnesium phosphate passivation materials and cadmium in the soil, providing a reliable theoretical basis for the remediation of cadmium-contaminated soils. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of the method for analyzing the vertical migration law of cadmium in soil under phosphate passivation conditions based on the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below in conjunction with the accompanying drawings. Figure 1 As shown, the present invention is a method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions, comprising the following steps:

[0029] S1. Collect cadmium-contaminated soil, dry it, screen it, mix it and homogenize it to remove impurities and plant residues, and measure the physical and chemical properties of the soil;

[0030] Specifically, the physical properties of the soil include water content, pH value, soil bulk density, particle composition and porosity; the chemical properties of the soil include water-soluble cadmium, water-soluble phosphorus, water-soluble calcium, water-soluble magnesium, cation exchange capacity and iron and manganese oxides.

[0031] S2. Prepare multiple acrylic plexiglass soil column devices with a height of 35 cm, set sampling holes for collecting pore water on the side walls at 15 cm and 25 cm from bottom to top, lay a 100-mesh nylon mesh at the bottom of the soil column device, and be equipped with a collection bottle for collecting the leachate at the bottom; fill the soil treated in step S1 with a height of 30 cm inside the soil column device, divide the soil column into 3 layers, and evenly apply 10 g / kg calcium magnesium phosphate passivator at a height of 10 cm on the first layer of the upper soil and compact it, use the soil bulk density of the sampling area as the compaction standard to ensure that the thickness of each layer of soil is uniform, simulating the actual soil conditions.

[0032] S3. After the soil column is filled, in order to promote the calcium magnesium phosphate fertilizer passivator to fully react with the cadmium in the soil, water is injected into the soil where the passivation material is applied for 7 days of maintenance.

[0033] The test cycle was set at 28 days. After a 7-day curing period, the test entered the stage of simulating natural irrigation. The soil was irrigated at a frequency of once every four days, and the transpiration of the soil column was recorded every day. During each irrigation, the soil moisture content was adjusted according to the actual soil moisture content to maintain the soil moisture content at 60%-70% of the field water holding capacity to simulate real moisture conditions.

[0034] After irrigation is completed, a series of complex physical, chemical and biological reactions will be triggered inside the soil. In order to obtain stable data, it is necessary to wait for the soil column to stabilize for 12 hours. After the soil column stabilizes, pore water samples and leachate samples are collected separately. Specifically, the pore water samples are collected using in-situ collection technology. The pore water collector is placed in the side sampling hole, and the soil solution pore water is automatically collected by vacuuming. The pore water collector device is made of hydrophilic porous polyester material with pores between 0.12-0.18 microns and is directly connected to a medical syringe.

[0035] After collection, the pore water samples and leachate samples were filtered through a 0.45 μm filter membrane, and the filtered solutions were measured using inductively coupled plasma mass spectrometry, and the measured values ​​were recorded;

[0036] S4. Use the Hydrus-Phreeqc coupling module to establish a one-dimensional migration reaction model, set the upper boundary conditions of atmospheric water accumulation and the lower boundary conditions of the leakage surface, and input the physical and chemical properties of the soil, the cadmium ion concentration of the soil solution, the transpiration amount, the amount of phosphate added, and the chemical reactions involving cadmium and phosphate measured in S1 as initial conditions, and use the Hydrus-Phreeqc coupling module reaction model to predict and obtain the initial simulation value; repeatedly adjust the soil water flow and soil solute transport parameters of the Hydrus-Phreeqc coupling module to make the initial simulation value consistent with the measured value to achieve the best fitting effect, and obtain the final simulation value after fitting;

[0037] Furthermore, the soil water flow parameters of the Hydrus-Phreeqc coupling module are iteratively adjusted using the one-dimensional Richards equation as follows:

[0038]

[0039] where h is the matrix potential, θ is the volumetric water content, t is the time, x is the spatial coordinate (vertical direction is positive), S is the sink term, α is the angle between the water flow direction and x, and K is the unsaturated hydraulic conductivity.

[0040] The soil solute transport parameters of the Hydrus-Phreeqc coupling module are repeatedly adjusted using the one-dimensional advection-dispersion solute transport equation as follows:

[0041]

[0042] i (=1, ..., Nm) is the number of water-containing substances (Nm is the total number of water-containing substances), c i is the concentration of the aqueous solution of the i-th substance, q is the volume flux density, S is the sink term in the water flow equation (1), cr is the concentration of the sink term, and D w is the diffusion coefficient in the liquid phase, Ri is a general source / sink term due to geochemical reactions. This sink / source term includes heterogeneous equilibrium reactions, as well as homogeneous and heterogeneous kinetic reactions.

[0043] The chemical reactions involving cadmium and phosphate are shown in the following table:

[0044] Formula LogK H++X-=HX 1 Ca+2+2X-=CaX2 0.8 Mg+2+2X-=MgX2 0.6 Cd+2+2X-=CdX2 0.8 Cd+2+2H2O=Cd(OH)2+2H+ -20.35 3Cd+2+2PO4-3=Cd3(PO4)2 -32.6 Hfo_sOH+Cd+2=Hfo_sOCd++H+ 0.47 Hfo_wOH+Cd+2=Hfo_wOCd++H+ -2.91

[0045] S5. After the test period, the soil columns were cut and collected from top to bottom at intervals of 10 cm. The collected soil was dried, sieved and weighed. The effective content of different layers of soil and the Tessier form distribution of heavy metal cadmium in each layer of soil were determined by inductively coupled plasma mass spectrometry and inductively coupled plasma emission spectrometry respectively.

[0046] S6. Combine the final simulation value in S4 with the effective content of soil cadmium obtained in S5 after the end of the experimental period compared with the initial soil and the vertical changes in the Tessier morphology distribution of heavy metal cadmium in each layer of soil to analyze the vertical migration characteristics of heavy metal cadmium in soil under the action of phosphate.

[0047] The invention sets a soil column experiment, applies a phosphate passivation material at a specific depth, simulates the vertical migration of soil cadmium under natural irrigation conditions, and combines the soil column experiment with a Hydrus-Phreeqc coupling model to more systematically analyze the influence of applying calcium magnesium phosphate fertilizer on the vertical migration of soil cadmium. The method is simple to operate and highly practical, and can be applied to the restoration research of different heavy metals. The migration and distribution process of the solute of the passivation material added in the soil over time and space in the cadmium ion concentration in the measured soil solution is determined. The model parameters are continuously adjusted to make the simulated value more consistent with the measured value, which can effectively solve the problem that the solute migration parameter is difficult to measure and the accuracy cannot be guaranteed, and the accuracy of parameter estimation is improved. The Hydrus-Phreeqc coupling module is used for simulation, and the geochemical reaction involved in the action of the calcium magnesium phosphate passivation material on the cadmium in the soil after the calcium magnesium phosphate passivation material is applied is considered, so as to provide a reliable theoretical basis for the restoration of cadmium-contaminated soil.

Claims

1. A method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions, characterized in that: The following steps are involved: S1. Collect cadmium-contaminated soil, dry it, screen it, mix it and homogenize it to remove impurities and plant residues, and measure the physical and chemical properties of the soil; S2, prepare a plurality of soil column devices, set sampling holes for collecting pore water on the side walls at different heights on the sides thereof, lay a nylon mesh at the bottom of the soil column device, and be equipped with a collection bottle for collecting the leachate at the bottom; fill the soil treated in step S1 inside the soil column device and evenly apply a calcium magnesium phosphate passivator on the top of the soil and compact it to simulate the actual soil condition; S3, inject water into the soil where the calcium magnesium phosphate fertilizer passivator is applied for maintenance, set a test period, irrigate the soil at a fixed frequency after the maintenance period, collect pore water samples and leachate samples after the soil becomes stable after irrigation, process and test them, and record the measured values; S4. A one-dimensional migration-reaction model is established based on the Hydrus-Phreeqc coupling module, the model boundary conditions are set, and the initial conditions are input. The reaction model is used for prediction to obtain the initial simulation value. The soil water flow and soil solute transport parameters of the Hydrus-Phreeqc coupling module are adjusted to make the initial simulation value consistent with the measured value, and the final simulation value after fitting is obtained. S5. Cut and sample the soil columns after the test period, dry, sieve and weigh the collected soil, and detect the vertical changes of the effective state and Tessier morphology of soil cadmium; S6. Combine the final simulation value in S4 with the effective content of soil cadmium obtained in S5 after the end of the experimental period compared with the initial soil and the vertical changes in the Tessier morphology distribution of heavy metal cadmium in each layer of soil to analyze the vertical migration characteristics of heavy metal cadmium in soil under the action of phosphate.

2. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: The physical properties of the soil in S1 include water content, pH value, soil bulk density, particle composition and porosity; the chemical properties of the soil include water-soluble cadmium, water-soluble phosphorus, water-soluble calcium, water-soluble magnesium, cation exchange capacity and iron and manganese oxides.

3. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: In S2, the soil column device is an acrylic plexiglass soil column device with a height of 35 cm. Sampling holes are set at 15 cm and 25 cm from the bottom to the top of the side of the soil column device. 10 g / kg of calcium magnesium phosphate passivator is evenly applied to the soil layer 10 cm deep in the upper part of the soil by stirring and compacting to make the soil compaction in the soil column uniform.

4. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: The specific step of irrigating the soil in S3 is as follows: recording the transpiration of the soil column every day, and adjusting the soil moisture content according to the actual soil moisture content each time irrigation is carried out to maintain the soil moisture content at 60%-70% of the field water holding capacity, simulating the real moisture conditions.

5. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: The pore water sample is collected in S3 using an in-situ collection technique, in which a pore water collector is placed in a side sampling hole to automatically collect soil solution pore water by vacuuming.

6. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 5, characterized in that: The pore water collector device is made of hydrophilic porous polyester material, the pores are between 0.12-0.18 microns, and is directly connected to a medical syringe.

7. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: The processing and detection of the collected pore water samples and leachate samples in S3 specifically includes filtering the pore water samples and leachate samples through a 0.45 μm filter membrane, and then measuring the filtered solutions using inductively coupled plasma mass spectrometry.

8. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: The initial conditions input to the Hydrus-Phreeqc coupling module in S4 include the physical and chemical properties of the soil measured in S1, the cadmium ion concentration of the soil solution, the transpiration rate, the amount of phosphate added, and the chemical reactions involving cadmium and phosphate.

9. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: Specifically, S5 includes cutting and collecting soil columns at intervals of 10 cm from top to bottom in the soil column device.

10. The method for analyzing the vertical migration law of soil cadmium under phosphate passivation conditions according to claim 1, characterized in that: In the S5, inductively coupled plasma mass spectrometry and inductively coupled plasma emission spectrometry are used to measure the effective content of different layers of soil and the Tessier form distribution of heavy metal cadmium in each layer of soil.

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