A method for environmental risk assessment of building material recycling after heavy metal contaminated soil remediation

Abstract

The present application belongs to the technical field of heavy metal contaminated soil environmental risk assessment, and particularly relates to an environmental risk assessment method for heavy metal contaminated soil remediation and building material recycling. The present application adopts simulated rainwater to leach the risk assessment system, calculates the diffusion coefficient of the remediated heavy metal in soil, and thereby constructs a one-dimensional heavy metal diffusion model. Then, according to the dilution and attenuation coefficient of the heavy metal in groundwater, the concentration level of the heavy metal in the building material released into the groundwater during the process of remediated soil recycling into building material is calculated. Finally, the potential drinking groundwater pollution risk possibly caused by the remediated soil recycling into the first type of land (residential land) is quantitatively evaluated. The present application has the advantages of simple method, clear process, short cycle, high efficiency, scientific rationality, and can accurately predict and evaluate the pollution risk of the local groundwater caused by the building material recycling of the remediated contaminated soil, which is beneficial to the protection of the groundwater environment, and the method provided by the present application has wide applicability.

Description

Technical Field

[0001] This invention belongs to the field of environmental risk assessment technology for heavy metal contaminated soil, specifically relating to an environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation. Background Technology

[0002] Urbanization has led to the closure or relocation of numerous industrial enterprises, particularly heavily polluting ones (such as electroplating and dyeing), located in urban areas, leaving behind a large number of sites contaminated with heavy metals. However, given the need for sustainable development and to alleviate soil resource shortages, there is an urgent need to remediate the soil in these contaminated sites. Currently, many soil remediation technologies have been applied to heavy metal-contaminated soils, such as soil washing, stabilization / solidification, and bioremediation. Among these, high-temperature solidification technology has become a feasible and promising technology due to its advantages in economy, operability, and effectiveness. High-temperature solidification technology involves mixing contaminated soil with auxiliary materials such as coal gangue, fly ash, and kaolin, followed by heating and sintering. Through phase transformation during the thermal sintering process, toxic heavy metals interact with the auxiliary materials such as coal gangue, fly ash, and kaolin, and can be incorporated into specific crystal structures, such as spinel crystal structures, thus being fixed. Spinel is typically represented by the general formula "AB₂O₄", where "A" represents a divalent metal, such as Zn, Cu, Cd, or Ni, and "B" represents a trivalent matrix metal, such as Cr and Al. The spinel crystal structure is considered a stable phase in which various metals, such as Zn, Cr, Cu, and Cd, can be fixed over long periods. Currently, high-temperature curing technology is widely used in the remediation of contaminated sites / soil.

[0003] Sintered materials obtained from the high-temperature curing technology for treating and remediating contaminated soil can be used to prepare roadbeds and other building materials, making the reuse of these materials an effective way to address soil resource shortages. However, when remediated contaminated soil is reused as building materials for walkways or roadbeds, the fixed toxic elements may be reactivated and released under long-term rainwater leaching and sunlight exposure, leading to potential secondary environmental risks. What is the stability and time-dependent nature of heavy metals in these roadbed materials during reuse? Current research is limited, and corresponding assessment methods are lacking. Therefore, it is extremely important and urgent to scientifically and accurately evaluate the release potential and environmental risks of originally stable heavy metals in remediated contaminated soil used as bricks and other building materials. Summary of the Invention

[0004] The purpose of this invention is to provide an environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation. The assessment method provided by this invention can effectively and quantitatively predict the potential risk to drinking groundwater from the activation and release of heavy metals in building materials such as bricks after the reuse of heavy metal contaminated soil in building materials after remediation under rainwater leaching conditions, thus providing technical support for the safety assurance of the reuse of contaminated soil in building materials.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention provides an environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation, comprising the following steps:

[0007] (1) A risk assessment model is constructed, which includes a remediated soil layer for reuse as building materials and a pollution-free soil layer stacked from top to bottom; simulated rainwater leachate is sprayed onto the surface of the remediated soil layer for reuse as building materials in the risk assessment system, and the leachate is collected from the bottom liquid outlet of the risk assessment system; the content of heavy metals in the leachate is determined.

[0008] (2) According to Fick's first law, the diffusion coefficient of heavy metals in the remediated and reusable soil is obtained from the content of heavy metals in the leachate.

[0009] (3) According to Fick's second law, a one-dimensional diffusion model of heavy metals in the remediated building material reuse soil is constructed from the diffusion coefficient. The one-dimensional diffusion model of heavy metals is the cumulative release of heavy metals per unit mass of remediated building material reuse soil over time.

[0010] (4) Based on the density of the uncontaminated soil, the water content of the unsaturated soil layer of the uncontaminated soil, the proportion of air in the unsaturated soil layer of the uncontaminated soil, the distribution coefficient of heavy metals in the uncontaminated soil and water, the permeability and porosity of the uncontaminated soil, the thickness of the groundwater mixing zone, the groundwater depth, the width of the remediated building material reuse soil layer in the risk assessment system, and the thickness of the uncontaminated soil layer, obtain the leaching dilution factor of heavy metals in the remediated building material reuse soil in the groundwater; based on the leaching dilution factor and the diffusion coefficient of heavy metals from groundwater to the water intake well, obtain the dilution attenuation coefficient of heavy metals in the groundwater.

[0011] (5) Based on the one-dimensional diffusion model of the heavy metals and the cumulative amount of leachate over time, the cumulative content of heavy metals in the soil reclaimed for building materials reuse per unit mass of leachate over time is obtained; based on the dilution and decay coefficient of heavy metals in groundwater and the cumulative content of heavy metals in the soil reclaimed for building materials reuse per unit mass of leachate over time, the cumulative content of heavy metals in the soil reclaimed for building materials reuse per unit mass of leachate over time is obtained;

[0012] (6) Calculate the groundwater exposure corresponding to the drinking water route according to the "HJ 25.3—2019 Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land". Based on the groundwater exposure corresponding to the drinking water route and the cumulative content of heavy metals in the groundwater in the remediated building material reuse soil over time, obtain the drinking groundwater hazard quotient of the remediated building material reuse soil.

[0013] (7) Based on the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3-2019)" and the hazard quotient of drinking groundwater pathway for remediated soil used in building materials, a risk rating shall be conducted on the remediated soil used in building materials; when the hazard quotient of drinking groundwater pathway for remediated soil used in building materials is <10... -6 After restoration, the soil used for building materials and reuse is at a safe level for drinking groundwater.

[0014] Preferably, the spraying is performed in multiple applications, with a total number of spraying applications ≥ 9 times; the interval between two adjacent spraying applications is 7 to 14 days.

[0015] Preferably, the formula for calculating the diffusion coefficient in step (2) is shown in Equations 1 to 3:

[0016]

[0017]

[0018]

[0019] In Equations 1 to 3, M i The leaching mass of heavy metals per unit area during the time interval between the end time of the i-th spray and the end time of the (i-1)-th spray is expressed in mg / m². 2 C i V represents the heavy metal content in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray, expressed in mg / L; i The volume of leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is expressed in liters (L); A is the cross-sectional area of ​​the bottom liquid outlet of the risk assessment system, expressed in meters (m²). 2 ;D iobs The diffusion coefficient of heavy metals in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is given, in m. 2 / s; Q0 is the initial heavy metal content of the remediated soil for reuse in building materials, in mg / L; ρ is the density of the remediated soil for reuse in building materials, in kg / m³. 3 ;t i t represents the end time of the i-th spray, in seconds. i-1 The end time of the (i-1)th spray is in seconds; n is the total number of sprays; n ≥ i.

[0020] Preferably, the one-dimensional diffusion model of heavy metals in step (3) is shown in Equation 4:

[0021]

[0022] In Equation 4, M mass t represents the cumulative release of heavy metals per unit mass of remediated soil for reuse in building materials, expressed in mg / kg; S represents the area of ​​the remediated soil layer for reuse in building materials within the risk assessment system, expressed in m². 2 V represents the volume of the remediated and reusable soil layer in the risk assessment system, in meters (m³). 3 t represents time, measured in seconds (s).

[0023] Preferably, the calculation formula for the leaching dilution factor is shown in Equation 5;

[0024]

[0025] In Equation 5, LF is the leaching dilution factor; ρ b Density of uncontaminated soil, in kg / dm³ 3 H' is the Henry's constant; θ as The proportion of air in the unsaturated layer of uncontaminated soil; θ ws The water content of unsaturated soil layer in unpolluted soil; K d The distribution coefficient of heavy metals in soil and water, in cm. 3 / g;U gw δ represents the porosity of groundwater medium, expressed in cm / a. gw I represents the thickness of the groundwater mixing zone, in meters. f Soil permeability, measured in cm / a; W gw L1 represents the width of the soil layer that will be reused as building materials after remediation in the risk assessment model, in meters; L2 represents the thickness of the soil layer that will be reused as building materials after remediation in the risk assessment model, in meters; and L3 represents the groundwater depth, in meters.

[0026] The formula for calculating the dilution and attenuation coefficient of the heavy metal in groundwater is shown in Equation 6:

[0027] NAF=LF×DAF Formula 6;

[0028] In Equation 6, NAF is the dilution and attenuation coefficient of heavy metals in groundwater, and DAF is the diffusion coefficient of heavy metals from groundwater to the water well.

[0029] The preferred formula for calculating the cumulative content of heavy metals in groundwater over time in soil reclaimed for reuse as building materials is shown in Equation 7:

[0030] C gw =C sw ×NAF Formula 7;

[0031] In Equation 7, C sw C represents the cumulative heavy metal content per unit mass of soil in the leachate after remediation and reuse as building materials over time, expressed in mg / (kg·L); gw The cumulative content of heavy metals in soil after restoration and reuse as building materials in groundwater over time is expressed in mg / (kg·L).

[0032] Preferably, the groundwater exposure corresponding to the drinking water pathway is the groundwater exposure corresponding to the drinking water pathway for Class I land use; the calculation formula for the groundwater exposure corresponding to the drinking water pathway for Class I land use is shown in Equation 8:

[0033]

[0034] In Equation 8, CGWER nc Groundwater exposure corresponding to the drinking water pathway for Category I land use; GWCR a This refers to the daily water consumption for adults, expressed in L / d; EF a The adult exposure frequency is expressed in days per year (d / a); EDa represents the adult exposure period in years (a); BW a Average adult weight, in kg; AT nc The average time to achieve a non-carcinogenic effect is measured in days (d).

[0035] Preferably, the formula for calculating the drinking groundwater hazard quotient of the reclaimed building material reuse soil is shown in Equation 9:

[0036]

[0037] In Equation 9, HQ cgw To address the risk of contaminated groundwater from soil repurposed for building materials after remediation; C gwThe cumulative content of heavy metals in groundwater over time in soil reclaimed for reuse as building materials is expressed in mg / (kg·L); RfDo is the non-carcinogenic reference dose of heavy metals, expressed in mg / kg / d; WAF is the reference dose allocation factor for exposure to groundwater.

[0038] Preferably, the heavy metals in the remediated and reused soil include at least one of copper, zinc, lead, cadmium, nickel, and chromium.

[0039] Preferably, the pH value of the simulated rainwater leachate is 3 to 6.5.

[0040] This invention provides an environmental risk assessment method for the reuse of remediated heavy metal contaminated soil in building materials, comprising the following steps: (1) constructing a risk assessment model, wherein the risk assessment model includes a remediated soil layer and a non-contaminated soil layer stacked from top to bottom; spraying the surface of the remediated soil layer in the risk assessment system with simulated rainwater leachate, and collecting the leachate from the bottom liquid outlet of the risk assessment system; determining the content of heavy metals in the leachate; (2) according to Fick's first law, determining the heavy metal content in the leachate. (2) Obtain the diffusion coefficient of heavy metals in the remediated soil for reuse as building materials; (3) Construct a one-dimensional diffusion model of heavy metals in the remediated soil for reuse as building materials based on the diffusion coefficient according to Fick's second law. The one-dimensional diffusion model of heavy metals is the cumulative release of heavy metals per unit mass of remediated soil for reuse as building materials over time; (4) Based on the density of the uncontaminated soil, the water content of the unsaturated soil layer of the uncontaminated soil, the proportion of air in the unsaturated soil layer of the uncontaminated soil, the distribution coefficient of heavy metals in the uncontaminated soil and water, the permeability and porosity of the uncontaminated soil, and the groundwater mixing... The thickness of the soil layer, the depth of the groundwater, and the width and thickness of the reclaimed soil layer in the risk assessment system are used to obtain the leaching dilution factor of heavy metals in the reclaimed soil layer in the groundwater. Based on the leaching dilution factor and the diffusion coefficient of heavy metals from groundwater to the water well, the dilution attenuation coefficient of heavy metals in the groundwater is obtained. (5) Based on the one-dimensional diffusion model of heavy metals and the cumulative amount of leachate over time, the cumulative content of heavy metals per unit mass of reclaimed soil layer in the leachate over time is obtained. Based on the dilution attenuation coefficient of heavy metals in groundwater and the cumulative amount of leachate in the leachate, the cumulative content of heavy metals per unit mass of reclaimed soil layer in the leachate over time is obtained. (6) Calculate the groundwater exposure corresponding to the drinking water pathway according to the "HJ25.3-2019 Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land". Based on the groundwater exposure corresponding to the drinking water pathway and the cumulative content of heavy metals in the groundwater in the soil after remediation, obtain the drinking water pathway hazard quotient of the soil after remediation; (7) Based on the "HJ 25.3-2019 Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land" and the drinking water pathway hazard quotient of the soil after remediation, conduct a risk rating of the soil after remediation; when the drinking water pathway hazard quotient of the soil after remediation is <10 -6The drinking groundwater from the remediated soil used for building materials is at a safe level. This invention uses simulated rainwater to leach the risk assessment system and calculates the diffusion coefficient of heavy metals, thereby constructing a one-dimensional diffusion model for heavy metals. This model can effectively predict the diffusion and release of heavy metals in the remediated soil used for building materials. Then, based on the dilution and attenuation coefficient of heavy metals in groundwater, the risk concentration of heavy metals in the remediated soil used for building materials is calculated, thereby further quantitatively calculating the potential drinking groundwater risk of heavy metals from the remediated soil. Finally, referring to the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3—2019)," the potential drinking groundwater risk that may arise from the reuse of the remediated soil in Class I land use (residential land) is quantitatively evaluated. The method of this invention is simple, the process is clear, the cycle is short, the efficiency is high, and it is scientific and reasonable. It can accurately predict and assess the pollution risk of local groundwater caused by the reuse of remediated contaminated soil in building materials. Moreover, the method provided by this invention has wide applicability and can be applied to the quantitative simulation assessment of the potential drinking groundwater risk of remediated heavy metal contaminated soil in building materials under different pH environmental conditions in different regions of the country.

[0041] Furthermore, in this invention, the pH value of the simulated rainwater leachate is 3–6.5. This invention, by simulating acid rain and normal rainfall conditions, considers the potential drinking groundwater risk of remediated soil under both acid rain and normal rainfall conditions. It can effectively evaluate the potential drinking groundwater risk that remediated soil may generate under complex pH environments in different actual reuse scenarios under both acid rain and normal rainfall conditions, achieving dynamic assessment of the reuse risk of remediated heavy metal contaminated soil under different pH conditions. Attached Figure Description

[0042] Figure 1 This is a graph showing the changes in Zn and Cr leaching content in soil columns over time under different pH rainfall conditions in Implementation Case 1;

[0043] Figure 2 The risk of groundwater contamination of Zn and Cr in four soil columns under different pH rainfall conditions is investigated in Exercise 1.

[0044] Figure 3 This is a schematic diagram of the risk assessment test device for the reuse of heavy metal contaminated soil after remediation in an embodiment of the present invention.

[0045] Figure 3 In the middle: 1-Rainfall simulator, 2-Leaching column, 3-Leachate collector, 4-Transfer pump, 5-Upper quartz sand layer, 6-Upper layer of brick soil after remediation of contaminated soil, 7-Lower layer of brick soil after remediation of contaminated soil, 8-Upper uncontaminated soil layer, 9-Lower uncontaminated soil layer, 10-Lower quartz sand layer, 11-Sampling port, 12-Outlet, 13-Rapid filter paper, 14-Support frame. Detailed Implementation

[0046] This invention provides an environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation, comprising the following steps:

[0047] (1) A risk assessment model is constructed, which includes a remediated soil layer for reuse as building materials and a pollution-free soil layer stacked from top to bottom; simulated rainwater leachate is sprayed onto the surface of the remediated soil layer for reuse as building materials in the risk assessment system, and the leachate is collected from the bottom liquid outlet of the risk assessment system; the content of heavy metals in the leachate is determined.

[0048] (2) According to Fick's first law, the diffusion coefficient of heavy metals in the remediated and reusable soil is obtained from the content of heavy metals in the leachate.

[0049] (3) According to Fick's second law, a one-dimensional diffusion model of heavy metals in the remediated building material reuse soil is constructed from the diffusion coefficient. The one-dimensional diffusion model of heavy metals is the cumulative release of heavy metals per unit mass of remediated building material reuse soil over time.

[0050] (4) Based on the density of the uncontaminated soil, the water content of the unsaturated soil layer of the uncontaminated soil, the proportion of air in the unsaturated soil layer of the uncontaminated soil, the distribution coefficient of heavy metals in the uncontaminated soil and water, the permeability and porosity of the uncontaminated soil, the thickness of the groundwater mixing zone, the groundwater depth, and the width and thickness of the remediated building material reuse soil layer in the risk assessment system, obtain the leaching dilution factor of heavy metals in the remediated building material reuse soil in the groundwater; based on the leaching dilution factor and the diffusion coefficient of heavy metals from groundwater to the water intake well, obtain the dilution attenuation coefficient of heavy metals in the groundwater.

[0051] (5) Based on the one-dimensional diffusion model of the heavy metals and the cumulative amount of leachate over time, the cumulative content of heavy metals in the soil reclaimed for building materials reuse per unit mass of leachate over time is obtained; based on the dilution and decay coefficient of heavy metals in groundwater and the cumulative content of heavy metals in the soil reclaimed for building materials reuse per unit mass of leachate over time, the cumulative content of heavy metals in the soil reclaimed for building materials reuse per unit mass of leachate over time is obtained;

[0052] (6) Calculate the groundwater exposure corresponding to the drinking water route according to the "HJ 25.3—2019 Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land". Based on the groundwater exposure corresponding to the drinking water route and the cumulative content of heavy metals in the groundwater in the remediated building material reuse soil over time, obtain the drinking groundwater hazard quotient of the remediated building material reuse soil.

[0053] (7) Based on the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3-2019)" and the hazard quotient of drinking groundwater pathway for remediated soil used in building materials, a risk rating shall be conducted on the remediated soil used in building materials; when the hazard quotient of drinking groundwater pathway for remediated soil used in building materials is <10... -6 After restoration, the soil used for building materials and reuse is at a safe level for drinking groundwater.

[0054] In this invention, unless otherwise specified, all raw materials / components used in the preparation are commercially available products well known to those skilled in the art.

[0055] The present invention constructs a risk assessment model, which includes a remediated soil layer for reuse as building materials and a pollution-free soil layer stacked from top to bottom; simulated rainwater leachate is sprayed onto the surface of the remediated soil layer for reuse as building materials in the risk assessment system, and the leachate is collected from the liquid outlet at the bottom of the risk assessment system; the content of heavy metals in the leachate is determined.

[0056] In this invention, the soil used for remediation and reuse as building materials is preferably soil sample obtained by crushing and grinding the corresponding building materials such as bricks after the contaminated soil has been remediated.

[0057] In this invention, the uncontaminated soil is soil that has not been contaminated by heavy metals.

[0058] In this invention, the soil that is remediated and reused as building material is more preferably a soil sample that has been remediated by high-temperature solidification.

[0059] In this invention, the method for preparing the reclaimed soil for reuse in building materials preferably includes the following steps:

[0060] The heavy metal contaminated soil and auxiliary materials are mixed to obtain a mixture;

[0061] The mixture is cured at high temperature to obtain the reclaimed soil that can be reused as building material.

[0062] In this invention, the auxiliary materials preferably include coal gangue and shale. The mass ratio of the heavy metal contaminated soil to the auxiliary materials is preferably 2:1.

[0063] In this invention, the temperature for high-temperature curing is preferably 900℃~1200℃; the holding time for high-temperature curing is preferably 2~5h.

[0064] In this invention, the heavy metals in the remediated and reused soil preferably include at least one of copper, zinc, lead, cadmium, nickel and chromium, and more preferably include zinc and / or cadmium.

[0065] In this invention, the reclaimed soil for reuse as building material is preferably roadbed material prepared after the remediation of contaminated soil and then screened through a 20-mesh sieve.

[0066] like Figure 3 As shown, the risk assessment system constructed by the present invention preferably includes a rainfall simulator, a leaching column, and a leaching collector arranged sequentially from top to bottom. The rainfall simulator is connected to a delivery pump, which is used to deliver the leaching liquid to the rainfall simulator for rainfall simulation. The leaching column is arranged sequentially from top to bottom as follows: an upper quartz sand layer, an upper layer of brick soil after contaminated soil remediation, a lower layer of brick soil after contaminated soil remediation, an upper layer of uncontaminated soil, a lower layer of uncontaminated soil, and a lower quartz sand layer. Four sampling ports are provided on each side of the leaching column. The four sampling ports on the same side correspond to the upper layer of brick soil after contaminated soil remediation, the lower layer of brick soil after contaminated soil remediation, the upper layer of uncontaminated soil, and the lower layer of uncontaminated soil, respectively. The four sampling ports on one side are used to collect water samples, and the four sampling ports on the other side are used to measure pH, temperature, moisture content, and conductivity. A water outlet is provided at the bottom of the leaching column, and the leaching collector is located below the water outlet to collect the leaching liquid flowing out of the water outlet.

[0067] The risk assessment system constructed in this invention simulates the leaching effect of rainfall under natural conditions, periodically collects leachate, detects the concentration of pollutants in the leachate, and monitors parameters such as soil moisture content, pH, temperature, and conductivity of soil pore water. Referring to the relevant parameters and calculation models provided in the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3—2019)," it assesses the risk of reusing heavy metal-contaminated soil as building materials after remediation. This achieves risk assessment of the reuse of heavy metal-contaminated soil as building materials after remediation, providing a basis for the safety of such reuse. Furthermore, it is simple to operate and maintain, has low cost, and is applicable to risk assessment of the reuse of most walkway bricks and other building materials, thus having a wide range of applications.

[0068] As a specific embodiment of the present invention, the lower quartz sand layer 10 and the lower uncontaminated soil layer 9 are separated by a rapid filter paper 13 to ensure that the leachate can penetrate quickly and evenly, and to prevent the soil layer from flowing into the lower quartz sand layer 10 with the water flow.

[0069] In one specific embodiment of the present invention, the diameter of the rapid filter paper 13 is 18 cm. During the experiment, rapid filter paper 13 of different diameters was selected according to different sizes of filtration columns 2.

[0070] As a specific embodiment of the present invention, the rainfall simulator 1 is mounted on the support frame 14 and supported by the support frame 14, and the rainfall simulator 1 is placed above the leaching column 2.

[0071] As a specific embodiment of the present invention, the delivery pump 4 is a peristaltic pump, which delivers the leachate to the rainfall simulator 1.

[0072] In one specific embodiment of the present invention, the height of both the upper quartz sand layer 5 and the lower quartz sand layer 10 is 3 cm. In other embodiments, other layer heights may also be provided.

[0073] In one specific embodiment of the present invention, the heights of the upper remediated and reusable soil layer 6, the lower remediated and reusable soil layer 7, the upper uncontaminated soil layer 8, and the lower uncontaminated soil layer 9 are all 15 cm. In other embodiments, other heights may be set.

[0074] In this invention, the pH value of the simulated rainwater leachate is preferably 3 to 6.5.

[0075] In this invention, the simulated rainwater leachate is preferably a mixed aqueous solution of H2SO4 and HNO3, and more preferably a mixed deionized aqueous solution of H2SO4 and HNO3.

[0076] In this invention, the molar ratio of H2SO4 to HNO3 in the mixed aqueous solution of H2SO4 and HNO3 is preferably 3:1.

[0077] In this invention, the spraying is preferably carried out in multiple sprayings, with a total number of sprayings ≥ 9 times; the interval between two adjacent sprayings is preferably 7 to 14 days.

[0078] In this invention, the total volume of the simulated rainwater leachate used for spraying is preferably determined according to the local annual precipitation of the uncontaminated soil.

[0079] In this invention, the spraying and leaching cycle is preferably greater than or equal to the number of days required for the heavy metal concentration to stabilize. In a specific embodiment of this invention, the spraying cycle is preferably 84 days. The total number of sprayings is 9, applied to the surface of the remediated, reusable soil layer on days 1, 7, 14, 21, 28, 42, 56, 70, and 84. The volume of the simulated rainwater leachate is preferably 330 mL per spraying.

[0080] In this invention, prior to spraying, it is preferable to pretreat the remediated soil layer for reuse as building materials and the uncontaminated soil layer in the risk assessment system. The pretreatment preferably includes the following steps: wetting the remediated soil layer for reuse as building materials and the uncontaminated soil layer with deionized water, followed by compaction with deionized water, so that the remediated soil layer for reuse as building materials and the uncontaminated soil layer reach the soil's saturated water-holding capacity. In this invention, the soil's saturated water-holding capacity is preferably 38%. In this invention, the pretreatment time is preferably 4 weeks.

[0081] After obtaining the leachate, the present invention preferably uses inductively coupled plasma optical emission spectrometry (ICP-OES) to determine the content of heavy metals in the leachate.

[0082] After determining the heavy metal content in the leachate, this invention obtains the diffusion coefficient of heavy metals in the remediated and reusable soil based on Fick's first law.

[0083] In this invention, the formula for calculating the diffusion coefficient is preferably as shown in Equations 1 to 3:

[0084]

[0085]

[0086]

[0087] In Equations 1 to 3, M i The leaching mass of heavy metals per unit area during the time interval between the end time of the i-th spray and the end time of the (i-1)-th spray is expressed in mg / m². 2 C i V represents the heavy metal content in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray, expressed in mg / L; i The volume of leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is expressed in liters (L); A is the cross-sectional area of ​​the bottom liquid outlet of the risk assessment system, expressed in meters (m²). 2 ;D i obs The diffusion coefficient of heavy metals in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is given, in m. 2 / s; Q0 is the initial heavy metal content of the remediated soil for reuse in building materials, in mg / L; ρ is the density of the remediated soil for reuse in building materials, in kg / m³. 3 ;t i t represents the end time of the i-th spray, in seconds.i-1 The end time of the (i-1)th spray is in seconds; n is the total number of sprays; n ≥ i.

[0088] After obtaining the diffusion coefficient of heavy metals in the remediated soil for reuse as building materials, this invention constructs a one-dimensional diffusion model of heavy metals in the remediated soil for reuse as building materials based on Fick's second law. The one-dimensional diffusion model of heavy metals represents the cumulative release of heavy metals per unit mass of remediated soil for reuse as building materials over time.

[0089] In this invention, the preferred one-dimensional diffusion model for the heavy metal is shown in Equation 4:

[0090]

[0091] In Equation 4, M mass t represents the cumulative release of heavy metals per unit mass of remediated soil for reuse in building materials, expressed in mg / kg; S represents the surface area of ​​the remediated soil layer for reuse in building materials, as defined in the risk assessment system, expressed in m². 2 V represents the volume of the remediated and reusable soil layer in the risk assessment system, in meters (m³). 3 t represents time, measured in seconds (s).

[0092] This invention obtains the leaching dilution factor of heavy metals in remediated soil for reuse in groundwater based on the density of the uncontaminated soil, the water content of the unsaturated soil layer, the proportion of air in the unsaturated soil layer, the distribution coefficient of heavy metals in the uncontaminated soil and water, the permeability and porosity of the uncontaminated soil, the thickness of the groundwater mixing zone, the groundwater depth, and the width and thickness of the remediated soil layer for reuse in the risk assessment system. Based on the leaching dilution factor and the diffusion coefficient of heavy metals from groundwater to the water intake well, this invention obtains the dilution attenuation coefficient of heavy metals in groundwater.

[0093] In this invention, the calculation formula for the leaching dilution factor is preferably as shown in Formula 5;

[0094]

[0095] In Equation 5, LF is the leaching dilution factor; ρ b Density of uncontaminated soil, in kg / dm³ 3 H' is the Henry's constant; θ as The proportion of air in the unsaturated layer of uncontaminated soil; θ ws The water content of unsaturated soil layer in unpolluted soil; K d The distribution coefficient of heavy metals in soil and water, in cm. 3 / g;U gwδ represents the porosity of groundwater medium, expressed in cm / a. gw I represents the thickness of the groundwater mixing zone, in meters. f Soil permeability, measured in cm / a; W gw L1 represents the width of the soil layer that will be reused as building materials after remediation in the risk assessment model, in meters; L2 represents the thickness of the soil layer that will be reused as building materials after remediation in the risk assessment model, in meters; and L3 represents the groundwater depth, in meters.

[0096] In this invention, the preferred formula for calculating the dilution and attenuation coefficient of the heavy metal in groundwater is as shown in Equation 6:

[0097] NAF=LF×DAF Formula 6;

[0098] In Equation 6, NAF is the dilution and attenuation coefficient of heavy metals in groundwater, and DAF is the diffusion coefficient of heavy metals from groundwater to the water intake well.

[0099] In this invention, the relevant parameters and reference values ​​in Equations 5 and 6 are shown in Table 1:

[0100] Table 1 shows the relevant parameters and reference values ​​for equations 5 and 6.

[0101] parameter name unit Reference value DAF groundwater dilution attenuation coefficient - 1 <![CDATA[ρ b ]]> Soil density <![CDATA[kg / dm 3 ]]> 2.65 H' Henry's constant - 0 <![CDATA[θ as ]]> The proportion of air in the unsaturated layer of soil - 0.12 <![CDATA[θ ws ]]> Unsaturated soil layer water content - 0.26 <![CDATA[K d (Zn)]]> Zinc partition coefficient in soil and water <![CDATA[cm 3 / g]]> 1.2 <![CDATA[K d (Cr)]]> Partition coefficient of chromium in soil and water <![CDATA[cm 3 / g]]> 1.28 <![CDATA[U gw ]]> porosity of groundwater medium cm / a 2500 <![CDATA[δ gw ]]> Thickness of groundwater mixing zone m 0.2 <![CDATA[I f ]]> Soil permeability cm / a 1.87 <![CDATA[W gw ]]> pollution source width m 40 <![CDATA[L1]]> Road thickness m 0.52 <![CDATA[L2]]> groundwater depth m 5

[0102] After constructing a one-dimensional diffusion model of heavy metals in the remediated soil for reuse as building materials, the cumulative content of heavy metals in the remediated soil for reuse as building materials per unit mass in the leachate over time is obtained based on the one-dimensional diffusion model of heavy metals and the cumulative amount of leachate over time. Based on the dilution and attenuation coefficient of heavy metals in groundwater and the cumulative content of heavy metals in the remediated soil for reuse as building materials per unit mass in the leachate over time, the cumulative content of heavy metals in the groundwater over time is obtained based on the dilution and attenuation coefficient of heavy metals in groundwater and the cumulative content of heavy metals in the remediated soil for reuse as building materials per unit mass in the leachate over time.

[0103] In this invention, the preferred formula for calculating the cumulative content of heavy metals in groundwater over time in soil reclaimed for reuse as building materials is shown in Formula 7:

[0104] C gw =C sw ×NAF type 7;

[0105] In Equation 7, C sw C represents the cumulative heavy metal content per unit mass of soil in the leachate after remediation and reuse as building materials over time, expressed in mg / (kg·L); gw The cumulative content of heavy metals in soil after restoration and reuse as building materials in groundwater over time is expressed in mg / (kg·L).

[0106] After obtaining the cumulative content of heavy metals in groundwater over time in soil remediated for reuse as building materials per unit mass, this invention calculates the groundwater exposure corresponding to the drinking water pathway according to the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ25.3—2019)". Based on the groundwater exposure corresponding to the drinking water pathway and the cumulative content of heavy metals in groundwater over time in soil remediated for reuse as building materials per unit mass, the drinking water hazard quotient of the remediated soil for reuse as building materials is obtained.

[0107] In this invention, the groundwater exposure corresponding to the groundwater drinking route is preferably the groundwater exposure corresponding to the drinking groundwater route for Class I land use; the calculation formula for the groundwater exposure corresponding to the drinking groundwater route for Class I land use is preferably as shown in Equation 8:

[0108]

[0109] In Equation 8, CGWER nc Groundwater exposure corresponding to the drinking water pathway for Category I land use; GWCR a This refers to the daily water consumption for adults, expressed in L / d; EF a The adult exposure frequency is expressed in days per year (d / a); EDa represents the adult exposure period in years (a); BW a Average adult weight, in kg; AT nc The average time to achieve a non-carcinogenic effect is measured in days (d).

[0110] In this invention, the preferred formula for calculating the drinking groundwater hazard quotient of the reclaimed building material reuse soil is shown in Equation 9:

[0111]

[0112] In Equation 9, HQ cgw To address the risks to drinking groundwater from reclaimed soil used for building materials; C gw The cumulative content of heavy metals in groundwater over time in soil reclaimed for reuse as building materials is expressed in mg / (kg·L); RfDo is the non-carcinogenic reference dose of heavy metals, expressed in mg / kg / d; WAF is the reference dose allocation factor for exposure to groundwater.

[0113] In this invention, the relevant exposure parameters and reference values ​​for calculating the groundwater exposure / hazard quotient in Equations 8 and 9 are shown in Table 2:

[0114] Table 2 shows the relevant exposure parameters and reference values ​​for calculating groundwater exposure / hazard quotient in Equations 8 and 9.

[0115]

[0116] In this invention, the groundwater exposure corresponding to the groundwater drinking route is for the non-carcinogenic effect of a single pollutant, taking into account the harm to the adult population; the drinking groundwater route hazard quotient refers to the human health hazard quotient of oral exposure.

[0117] After obtaining the drinking water hazard quotient of the remediated soil for reuse in building materials, this invention, based on the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3-2019)" and the drinking water hazard quotient of the remediated soil for reuse in building materials, conducts a risk rating of the remediated soil for reuse in building materials; when the drinking water hazard quotient of the remediated soil for reuse in building materials is <10... -6 After restoration, the soil used for building materials and reuse is at a safe level for drinking groundwater.

[0118] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0119] Explanation of terms in this invention:

[0120] Exposure amount: Exposure amount refers to the total amount of a substance that enters the body through exposure routes such as air, food and water.

[0121] Hazard quotient: The ratio of the daily intake dose of a pollutant to the reference dose, used to characterize the level of harm to a human body from exposure to a non-carcinogenic pollutant via a single route.

[0122] Hazard index: The sum of hazard quotients of a population exposed to a single pollutant through multiple pathways, used to characterize the level of harm caused by human exposure to non-carcinogenic pollutants.

[0123] Acceptable risk level: The risk level that will not produce adverse or harmful health effects on the exposed population, including the acceptable total carcinogenic risk level of carcinogens and the acceptable hazard index of non-carcinogens. According to the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3—2019)," the acceptable total carcinogenic risk level for a single pollutant is 10. -6 The acceptable non-carcinogenic risk for a single pollutant is 1.

[0124] Example 1

[0125] This embodiment uses laboratory simulations of heavy metal contaminated soil. Uncontaminated soil was collected from the suburbs of Guangzhou, China, and consisted of homogeneous red soil with a Zn concentration of 27.7 mg / kg and a Cr concentration of 66.9 mg / kg. The artificially prepared contaminated soil consisted of 10 g of a ZnO and Cr₂O₃ mixture (Zn:Cr molar ratio of 1:2) and 90 g of uncontaminated soil powder. The sintering material consisted of coal gangue and shale powder in a 1:2 mass ratio. The coal gangue contained 23.0 ± 1.7 mg / kg of Zn and 40.5 ± 8.1 mg / kg of Cr; the shale powder contained 90.6 ± 5.7 mg / kg of Zn and 184.0 ± 14.1 mg / kg of Cr. The Zn concentration in the sample powder mixture was 3413.7 ± 205.6 mg / kg, and the Cr concentration was 12009 ± 435.7 mg / kg. All powder samples were thoroughly air-dried, ground into powder, and pressed into 40mm diameter pellets to ensure consistent compaction during sintering. Zn and Cr were simultaneously solidified within the ZnCr2O4 spinel structure using a high-temperature curing technique. All powder mixtures were heat-treated at pressures above 350MPa in a muffle furnace with controlled heating and cooling rates of 10℃ / min, held at 1000℃ for 4 hours. Finally, the sintered samples were used as remediated soil samples (denoted as RS), ground through a 20mm sieve, and filled into leaching columns as required for further risk assessment of potential drinking water reuse after Zn-Cr contaminated soil remediation.

[0126] (1) First, the soil that has been solidified and remediated at high temperature and the uncontaminated soil are filled into the leaching column. The leaching column is arranged from top to bottom as follows: upper quartz sand layer 5, upper remediated building material reuse soil layer 6 (15cm), lower remediated building material reuse soil layer 7 (15cm), upper uncontaminated soil layer 8 (15cm), lower uncontaminated soil layer 9 (15cm) and lower quartz sand layer 10. There are four sampling ports 11 on each side of the leaching column 2. The four sampling ports 11 on the same side correspond to the upper remediated building material reuse soil layer 6, the lower remediated building material reuse soil layer 7, the upper uncontaminated soil layer 8 and the lower uncontaminated soil layer 9, respectively. The four sampling ports 11 on one side are used to collect water samples, and the four sampling ports 11 on the other side are used to measure temperature, moisture content and conductivity. There is an outlet 12 at the bottom of the leaching column 2. The leachate collector 3 is located below the outlet 12 and is used to collect the leachate flowing out of the outlet 12.

[0127] An acid solution was prepared by mixing H₂SO₄ and HNO₃ in a molar ratio of 3:1, and then diluted with deionized water to pH = 3.0 and pH = 6.5. The lower pH of 3.0 represents the most severe acid rain conditions, while pH = 6.5 serves as a control, representing the pH level of normal rainfall. Before leaching, the leaching column was thoroughly wetted and compacted with deionized water to achieve the soil's saturated water-holding capacity (38%), and maintained stable for 4 weeks. The entire leaching cycle was then set to 84 days. 330 mL of simulated rainwater leachate was slowly sprayed in equal volumes on days 1, 7, 14, 21, 28, 42, 56, 70, and 84 at a rate of approximately 6 mL / min. The total leaching volume was 2970 mL. Finally, the total Zn and Cr content in the bottom leachate was determined using ICP-OES. The results are as follows: Figure 1 As shown, Figure 1 The left side of the graph shows the changes in Zn leaching content in the soil column under different pH and rainfall conditions after remediation; the right side shows the changes in Zn leaching content in the soil column under different pH and rainfall conditions after remediation. Figure 1 It can be concluded that, regardless of acid rain or normal rainfall conditions, the Zn and Cr contents in the leachate obtained from the leaching column increase with increasing leaching time, and their accumulation rate shows a trend from rapid to slowing down. On day 1 of leaching, the Zn and Cr contents in the leachate were 0.37 mg / L and 0.16 mg / L, respectively, at pH 3.0, and 0.12 mg / L for both at pH 6.5. On day 7 of leaching, the Zn content in the leachate reached its maximum value of 1.08 mg / L (pH=3.0) and 0.28 mg / L (pH=6.5), while the Cr content reached its maximum value of 0.31 mg / L (pH=3.0) and 0.19 mg / L (pH=6.5) on day 14 of leaching. The maximum contents of Zn and Cr under acid rain conditions were significantly higher than those under normal rainfall conditions. After 14 days, the Zn and Cr contents gradually decreased under both rainfall conditions and tended to stabilize. On day 84, the Zn and Cr contents were 0.22 mg / L and 0.05 mg / L at pH 3.0, and 0.07 mg / L and 0.05 mg / L at pH 6.5, respectively. Under acid rain conditions (pH 3.0), the cumulative release of Zn and Cr in the soil column reached 4.43 mg / L and 1.31 mg / L, respectively, which were significantly higher than those under normal rainfall conditions (pH 6.5), which were 1.00 mg / L and 0.84 mg / L, respectively.

[0128] (2) After determining the Zn and Cr content of the bottom leachate, it is necessary to calculate the diffusion coefficient D in order to establish a diffusion model. obs The calculation formulas are shown in Equations 1 to 3:

[0129]

[0130]

[0131]

[0132] In Equations 1 to 3, M i The leaching mass of heavy metals per unit area during the time interval between the end time of the i-th spray and the end time of the (i-1)-th spray is expressed in mg / m². 2 C i V represents the heavy metal content in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray, expressed in mg / L; i The volume of leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is expressed in liters (L); A is the cross-sectional area of ​​the bottom liquid outlet of the risk assessment system, expressed in meters (m²). 2 ;D i obs The diffusion coefficient of heavy metals in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is given, in m. 2 / s; Q0 is the initial heavy metal content of the remediated soil for reuse in building materials, in mg / L; ρ is the density of the remediated soil for reuse in building materials, in kg / m³. 3 ;t i t represents the end time of the i-th spray, in seconds. i-1 The end time of the (i-1)th spray is in seconds; n is the total number of sprays; n ≥ i.

[0133] The diffusion coefficients D of Zn and Cr obs It is the arithmetic mean of the diffusion coefficients at each leaching stage, with respect to the diffusion coefficient pD. obs The negative logarithm of pD can characterize the dissolution rate of heavy metals in different soil layers. Referring to the EU's environmental safety assessment standards for building materials, pD... obs ≤11 represents a high diffusion rate level, 11 <pD obs <12.5 represents the average diffusion rate level, pD obsA diffusion rate of ≥12.5 is considered low (EA NEN7375). The calculations are as follows: The diffusion rates of Zn in the upper remediated soil (pH=3.0, pH=6.5) are 11.12 and 12.61, respectively. The diffusion rates of Cr in the upper remediated soil (pH=3.0, pH=6.5) are 12.94 and 14.34, respectively. The diffusion rates of Zn in the lower remediated soil (pH=3.0, pH=6.5) are 11.04 and 12.69, respectively. The diffusion rates of Cr in the lower remediated soil (pH=3.0, pH=6.5) are 12.92 and 14.33, respectively. The diffusion rates of Zn in the upper uncontaminated soil (pH=3.0, pH=6.5) are 12.64 and 13.39, respectively. The diffusion rates of Cr in the upper uncontaminated soil (pH=3.0, pH=6.5) were 13.58 and 14.11, respectively. The diffusion rates of Zn in the lower uncontaminated soil (pH=3.0, pH=6.5) were 12.65 and 13.63, respectively. The diffusion rates of Cr in the lower uncontaminated soil (pH=3.0, pH=6.5) were 13.71 and 14.21, respectively. The results indicate that Zn diffuses faster than Cr in both remediated and uncontaminated soil. Furthermore, pH conditions also affect the diffusion rates of Zn and Cr; under acid rain conditions, the diffusion rates of Zn and Cr are faster than under normal rainfall conditions. To further understand the release of Zn and Cr from contaminated soil bricks over their service life, an effective heavy metal release model for remediated soil was established. The one-dimensional diffusion model is shown in Equation 4.

[0134]

[0135] In Equation 4, M mass t represents the cumulative release of heavy metals per unit mass of remediated soil for reuse in building materials, expressed in mg / kg; S represents the surface area of ​​the remediated soil layer for reuse in building materials, as defined in the risk assessment system, expressed in m². 2 V represents the volume of the remediated and reusable soil layer in the risk assessment system, in meters (m³). 3 t represents time, measured in seconds (s).

[0136] After the remediated soil was used to make bricks and was used for 15 years (15 years is the service life of a conventional roadbed), the cumulative release of Zn and Cr through leaching from rainfall at different pH levels were 30.375 mg / kg, 5.1 mg / kg (pH=3.0), 4.855 mg / kg, and 1.05 mg / kg (pH=6.5), respectively. Under acidic rainfall conditions (pH=3.0), the cumulative release rates of Zn and Cr were significantly higher than those under normal rainfall conditions (pH=6.5). The maximum release of Zn was 31.96 mg / kg, and the maximum release of Cr was 5.13 mg / kg.

[0137] (3) The migration of heavy metals into groundwater can be divided into two stages. First, Zn and Cr in the roadbed materials are leached into the groundwater by rainwater (leaching dilution factor, LF). Second, the Zn and Cr that have entered the groundwater diffuse further into the well as the groundwater flows (diffusion coefficient of heavy metals from groundwater to the well, DAF). Therefore, the content of Zn and Cr in groundwater can be calculated using equations 5 to 7.

[0138]

[0139] In Equation 5, LF is the leaching dilution factor; ρ b Density of uncontaminated soil, in kg / dm³ 3 H' is the Henry's constant; θ as The proportion of air in the unsaturated layer of uncontaminated soil; θ ws The water content of unsaturated soil layer in unpolluted soil; K d The distribution coefficient of heavy metals in soil and water, in cm. 3 / g;U gw δ represents the porosity of groundwater medium, expressed in cm / a. gw I represents the thickness of the groundwater mixing zone, in meters. f Soil permeability, measured in cm / a; W gw L1 represents the width of the pollution source in meters; L2 represents the road thickness in meters; and L3 represents the groundwater depth in meters.

[0140] NAF=LF×DAF Formula 6;

[0141] In Equation 6, NAF is the dilution and attenuation coefficient of heavy metals in groundwater, and DAF is the diffusion coefficient of heavy metals from groundwater to the water intake well.

[0142] C gw =C sw ×NAF Formula 7;

[0143] In Equation 7, C sw C represents the cumulative heavy metal content per unit mass of soil in the leachate after remediation and reuse as building materials over time, expressed in mg / (kg·L); gw The cumulative content of heavy metals in soil after restoration and reuse as building materials in groundwater over time is expressed in mg / (kg·L).

[0144] Calculate the leaching dilution factor (LF) for Zn and Cr leachate migration into groundwater. zn =9.22E-3, LF cr=8.69E-3. Considering only soil leaching and dilution and the impact of heavy metals on the underlying groundwater, DAF is set to 1. Therefore, the Zn and Cr contents in the groundwater under different pH rainfall conditions are: 0.28 mg / kg, 0.044 mg / kg (pH=3.0) and 0.045 mg / kg, 0.009 mg / kg (pH=6.5), respectively.

[0145] (4) The obtained groundwater pollutant content C gw Substituting the values ​​of heavy metals in the soil into the calculation model provided in the newly issued "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3-2019)" in my country, the impact of heavy metals on drinking groundwater exposure and / or hazard quotient in the soil was calculated. The calculation model is shown in Equations 8 and 9:

[0146] The groundwater exposure corresponding to the drinking groundwater pathway for Class I land use (residential land) is calculated using Formula 8:

[0147]

[0148] In Equation 8, CGWER nc Groundwater exposure corresponding to the drinking water pathway for Category I land use; GWCR a This refers to the daily water consumption for adults, expressed in L / d; EF a The adult exposure frequency is expressed in days per year (d / a); EDa represents the adult exposure period in years (a); BW a Average adult weight, in kg; AT nc The average time to achieve a non-carcinogenic effect is measured in days (d).

[0149] The risk quotient for drinking groundwater is calculated using formula 9:

[0150]

[0151] In Equation 9, HQ cgw To address the risk of contaminated groundwater from soil repurposed for building materials after remediation; C gw The cumulative content of heavy metals in groundwater over time in soil reclaimed for reuse as building materials is expressed in mg / (kg·L); RfDo is the non-carcinogenic reference dose of heavy metals, expressed in mg / kg / d; WAF is the reference dose allocation factor for exposure to groundwater.

[0152] Simultaneously, based on the evaluation standards provided in the "Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land (HJ 25.3-2019)," a risk rating was conducted on the calculated total carcinogenic risk and / or hazard index of heavy metals in the soil: the acceptable total carcinogenic risk level for a single pollutant is 10. -6Alternatively, the acceptable non-carcinogenic risk level for a single pollutant is 1; when the calculated result (total carcinogenic risk of heavy metals in soil samples) is greater than the above acceptable risk level, it indicates that there is a heavy metal health risk. In this example, the heavy metals are zinc and chromium, and the results are as follows... Figure 2 As shown, Figure 2 The left side shows the groundwater risk of Zn and Cr in four soil columns under acid rain conditions (pH=3.0); the right side shows the groundwater risk of Zn and Cr in four soil columns under normal rainfall conditions (pH=6.5). Under acid rain conditions, the maximum risk values ​​for drinking groundwater are 5.97E-06 for Zn and 1.80E-07 for Cr (RS2), significantly higher than those under normal rainfall conditions (9.20E-07 for Zn and 3.73E-08 for Cr (RS2)). Furthermore, the drinking water risk from uncontaminated soil is much lower than that from remediated soil. Under acid rain conditions (CS2), the maximum risk values ​​for Zn and Cr (the hazard quotient for drinking groundwater from remediated, repurposed soil) are 9.59E-07 and 8.23E-08, respectively, while under normal rainfall conditions (CS1), the maximum risk values ​​for Zn and Cr are 3.96E-07 and 4.78E-08, respectively. In this study, even after 15 years, the maximum drinking water pathway hazard quotients (MBRs) of the remediated soil were 7.69E-04 (Zn) and 5.83E-06 (Cr), while those of the uncontaminated soil were 1.22E-04 (Zn) and 2.73E-06 (Cr), both within safe levels (<1). This indicates that the contaminated soil remediated by high-temperature solidification technology has good long-term stability. In other words, high-temperature solidification treatment effectively reduced the hazard indices of Zn and Cr in the contaminated soil samples. This scheme provides a risk assessment method for the reuse of remediated heavy metal contaminated soil in building materials, providing technical support for ensuring the safety of such reuse.

[0153] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. An environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation, characterized in that, Includes the following steps: (1) A risk assessment model is constructed, which includes a remediated soil layer for reuse as building materials and a pollution-free soil layer stacked from top to bottom; simulated rainwater leachate is sprayed onto the surface of the remediated soil layer for reuse as building materials in the risk assessment model, and the leachate is collected from the bottom liquid outlet of the risk assessment model; the content of heavy metals in the leachate is determined. (2) According to Fick's first law, the diffusion coefficient of heavy metals in the remediated and reusable soil is obtained from the content of heavy metals in the leachate; the formulas for calculating the diffusion coefficient of heavy metals are shown in Equations 1 to 3: M i = Formula 1; =π formula 2; = Formula 3; In Equations 1 to 3, M i The leaching mass of heavy metals per unit area during the time interval between the end time of the i-th spray and the end time of the (i-1)-th spray is expressed in mg / m². 2 ; C i V represents the heavy metal content in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray, expressed in mg / L; i The volume of leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is expressed in liters (L); A is the cross-sectional area of ​​the bottom liquid outlet of the risk assessment model, expressed in meters (m²). 2 ; D i obs The diffusion coefficient of heavy metals in the leachate during the time interval between the end of the i-th spray and the end of the (i-1)-th spray is given, in m. 2 / s; Q0 is the initial heavy metal content of the remediated soil for reuse in building materials, in mg / L; ρ is the density of the remediated soil for reuse in building materials, in kg / m³. 3 ; t i t represents the end time of the i-th spray, in seconds. i-1 The end time of the (i-1)th spray is in seconds; n is the total number of sprays; n ≥ i; (3) According to Fick's second law, a one-dimensional diffusion model of heavy metals in the reclaimed building materials reused soil is constructed from the diffusion coefficient of heavy metals obtained in step (2). The one-dimensional diffusion model of heavy metals is shown in Equation 4: Equation 4; In Equation 4, t represents the cumulative release of heavy metals per unit mass of remediated soil for reuse in building materials at time t, expressed in mg / kg; S represents the surface area of ​​the remediated soil layer for reuse in building materials in the risk assessment model, expressed in m². 2 ; V represents the volume of the remediated and reusable soil layer in the risk assessment model, in meters (m³). 3 ; t represents time, measured in seconds (s). (4) Based on the density of the uncontaminated soil, the water content of the unsaturated soil layer of the uncontaminated soil, the proportion of air in the unsaturated soil layer of the uncontaminated soil, the distribution coefficient of heavy metals in the uncontaminated soil and water, the permeability and porosity of the uncontaminated soil, the thickness of the groundwater mixing zone, the groundwater depth, and the width and thickness of the remediated building material reuse soil layer in the risk assessment model, the leaching dilution factor of heavy metals in the remediated building material reuse soil in the groundwater is obtained; based on the leaching dilution factor and the diffusion coefficient of heavy metals from groundwater to the water intake well, the dilution attenuation coefficient of heavy metals in the groundwater is obtained; the calculation formula of the leaching dilution factor is shown in Equation 5. LF= Formula 5; In Equation 5, LF is the leaching dilution factor; ρ b Density of uncontaminated soil, in kg / dm³ 3 H ' θ is the Henry's constant; as The proportion of air in the unsaturated layer of uncontaminated soil; θ ws The water content of unsaturated soil layer in unpolluted soil; K d The distribution coefficient of heavy metals in soil and water, in cm. 3 / g;U gw δ represents the porosity of groundwater medium, expressed in cm / a. gw I represents the thickness of the groundwater mixing zone, in meters. f Soil permeability, measured in cm / a; W gw L1 represents the width of the reclaimed soil layer for reuse as building materials in the risk assessment model, in meters; L2 represents the thickness of the reclaimed soil layer for reuse as building materials in the risk assessment model, in meters. L2 is the depth of groundwater, in meters (m). The formula for calculating the dilution and attenuation coefficient of the heavy metal in groundwater is shown in Equation 6: NAF=LF×DAF Formula 6; In Equation 6, NAF is the dilution and attenuation coefficient of heavy metals in groundwater, and DAF is the diffusion coefficient of heavy metals from groundwater to the water well. Steps (1) to (3) and step (4) are not in any particular chronological order; (5) Based on the one-dimensional diffusion model of the heavy metals and the leaching time, the cumulative content of heavy metals per unit mass of reclaimed soil for reuse as building materials over time is obtained; based on the dilution and decay coefficient of heavy metals in groundwater and the cumulative content of heavy metals per unit mass of reclaimed soil for reuse as building materials over time in the leaching solution, the cumulative content of heavy metals per unit mass of reclaimed soil for reuse as building materials in groundwater over time is obtained; the calculation formula for the cumulative content of heavy metals per unit mass of reclaimed soil for reuse as building materials in groundwater over time is shown in Equation 7: C gw =C sw ×NAF Formula 7; In Equation 7, C sw C represents the cumulative heavy metal content per unit mass of soil in the leachate after remediation and reuse as building materials over time, expressed in mg / (kg·L); gw The cumulative content of heavy metals in soil after remediation and reuse as building materials in groundwater over time is expressed in mg / (kg·L). (6) Calculate the groundwater exposure corresponding to the drinking water route according to the "HJ 25.3—2019 Technical Guidelines for Soil Pollution Risk Assessment of Construction Land". Based on the groundwater exposure corresponding to the drinking water route and the cumulative content of heavy metals in the groundwater in the remediated building material reuse soil over time, obtain the drinking water hazard quotient of the remediated building material reuse soil; the groundwater exposure corresponding to the drinking water route is the groundwater exposure corresponding to the drinking water route for Class I land use; the calculation formula for the groundwater exposure corresponding to the drinking water route for Class I land use is shown in Equation 8: CGWER nc = Formula 8; In Equation 8, CGWER nc Groundwater exposure corresponding to the drinking water pathway for Category I land use; GWCR a This refers to the daily water consumption for adults, expressed in L / d; EF a The adult exposure frequency is expressed in days per year (d / a); EDa represents the adult exposure period in years (a); BW a Average adult weight, in kg; AT nc The average time to achieve non-carcinogenic effects is expressed in days (d). The formula for calculating the hazard quotient of the reclaimed soil for use in building materials and its impact on drinking groundwater is shown in Equation 9: HQ cgw = Equation 9; In Equation 9, HQ cgw To address the risk of contaminated groundwater from soil repurposed for building materials after remediation; C gw The cumulative content of heavy metals in soil remediated and reused as building materials over time in groundwater is expressed in mg / (kg·L); RfDo is the non-carcinogenic reference dose of heavy metals, expressed in mg / kg / d; WAF is the reference dose allocation factor for exposure to groundwater. (7) Based on the "HJ 25.3-2019 Technical Guidelines for Risk Assessment of Soil Pollution in Construction Land" and the risk hazard to drinking groundwater from remediated soil used for building materials, risk rating is conducted on the remediated soil used for building materials. When the soil is reclaimed and reused as building materials, the risk of damage to drinking groundwater is less than 10. -6 After restoration, the soil used for building materials and reuse is at a safe level for drinking groundwater.

2. The environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation, as described in claim 1, is characterized in that... The spraying is performed in multiple applications, with a total of ≥9 applications; the interval between two consecutive applications is 7 to 14 days.

3. The environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation, as described in claim 1, is characterized in that... The heavy metals in the remediated and reused soil include at least one of copper, zinc, lead, cadmium, nickel, and chromium.

4. The environmental risk assessment method for the reuse of heavy metal contaminated soil in building materials after remediation, as described in claim 1, is characterized in that... The pH value of the simulated rainwater leachate is 3~6.5.

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