Method for evaluating long-term stability of repaired heavy metal polluted soil

Abstract

The invention belongs to the technical field of pollution treatment, and particularly discloses a method for evaluating the long-term stability of repaired heavy metal polluted soil. The method comprises the following steps: crushing and sieving the treated heavy metal polluted soil to obtain a sample, and then mixing the sample with different acid rain accelerating solutions; continuously oscillating the mixture for a preset time length to obtain a mixed solution of a single acceleration test; circularly executing an acceleration test on the mixed solution, and determining the concentration Ci and the chemical form distribution characteristics of the heavy metal element after each time of completion; based on the environmental background values Bi and Ci, calculating a heavy metal polluted soil stability index Ei = Ci / Bi, and constructing a long-term stability index model EN; setting a long-term stability grading standard, and evaluating the long-term stability grade of the repaired heavy metal polluted soil according to the Ei and the EN by comparing the standard. According to the method, the durability of the remediation effect can be accurately quantified, and the problem of risk quantification of reactivation of the heavy metal contaminated soil is effectively solved.

Description

Technical Field

[0001] This invention belongs to the field of pollution control technology, specifically relating to a method for evaluating the long-term stability of remediation of heavy metal contaminated soil that can accurately quantify the durability of remediation effects and effectively solves the problem of difficulty in quantifying the risk of reactivation of heavy metal contaminated soil. Background Technology

[0002] The mining and smelting of heavy metal mineral resources, as well as the overuse of pesticides and fertilizers, will cause heavy metal ions to accumulate in the soil environment. Due to the high mobility and bioaccumulation of heavy metal ions, they seriously threaten the safety of agricultural products and the health of the ecosystem. Therefore, it is urgent to carry out the remediation and treatment of heavy metal contaminated soil.

[0003] Currently, remediation technologies for heavy metal contaminated soil mainly include topsoil replacement, chemical leaching / stabilization, phytoremediation, and electrokinetic remediation. Among these methods, chemical methods are widely used, specifically encompassing chemical leaching and stabilization techniques. Stabilization techniques, in particular, have been widely applied in heavy metal contaminated soil remediation projects due to their advantages such as low cost, small workload, short remediation cycle, and wide applicability.

[0004] Geopolymers, as a novel green cementitious material, have become an important material in the field of solidification / stabilization remediation of heavy metal contaminated soil due to their outstanding advantages such as environmental protection characteristics, solid waste resource utilization capabilities, high efficiency in remediation, and stable solidification performance. This material can directly utilize industrial solid wastes such as fly ash, slag, coal gangue, waste concrete, and shell powder as raw materials to achieve the goal of "treating waste with waste," demonstrating considerable application potential in the solidification / stabilization remediation of heavy metal contaminated soil.

[0005] However, heavy metal contaminated soil that has undergone geopolymer solidification / stabilization treatment faces the challenge of long-term stability risks in practical applications. This is because the fixed heavy metals may be reactivated by external environmental disturbances such as acid rain and pH changes. Therefore, accurately assessing the stability of solidified / stabilized heavy metal contaminated soil is a decisive factor in determining the long-term effectiveness of remediation materials.

[0006] In existing technologies, the stability evaluation of solidified / stabilized heavy metal contaminated soil involves several methods. One method involves preparing coal-based biochar (made from corn straw biomass BM, fly ash FA, and sodium hydroxide), then using the biochar to remediate the heavy metal contaminated soil. This is followed by multiple rounds of wet-dry cycles and freeze-thaw cycles on the remediated soil. Samples are then taken from each remediated soil sample for heavy metal content and speciation analysis, allowing for a qualitative evaluation of the heavy metal stability in the soil after solidification. Another method for evaluating the long-term stability of phosphate-based polymer-contaminated soil with acidic lead contaminated soil uses multiple solidified soil samples of the same specifications as parallel tests. Data on the effects of different solidification agent dosages and freeze-thaw cycles on the toxicity leaching, compressive strength, elastic modulus, resistivity, pH, and conductivity of the solidified soil are obtained. These macroscopic data are then fitted and analyzed to assess the long-term stability of the phosphate-based polymer-contaminated soil under freeze-thaw cycles. Although the aforementioned existing methods can meet the long-term stability assessment of remediation of heavy metal contaminated soil with specific polymers, they are only applicable to the post-remediation assessment of specific polymers or even specific heavy metal contaminated soil, thus having a narrow scope of application. Moreover, the first method can only achieve qualitative assessment, and the assessment results still need to be obtained through manual post-analysis and comparison, which is easily affected by human subjective factors. Although the second method can achieve quantitative assessment, due to the large number of parameters, the data measurement is complicated, costly, and inefficient, making it difficult to provide assessment results in a timely manner.

[0007] Therefore, in order to address the above problems, there is an urgent need to study a method for assessing the long-term stability of heavy metal contaminated land aggregates after solidification / stabilization that is accurate, widely applicable, and quantifiable. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a method for evaluating the long-term stability of remediation of heavy metal contaminated soil after remediation, which can accurately quantify the durability of remediation effects and effectively solves the problem of difficulty in quantifying the risk of reactivation of heavy metal contaminated soil.

[0009] The method for evaluating the long-term stability of heavy metal contaminated soil after remediation, as described in this invention, includes the following steps: sample preparation, accelerated testing, measurement, model construction, and risk level assessment. The specific processes for each step are as follows: A. Sample preparation: Heavy metal contaminated soil treated with remediation agent was crushed and then sieved to prepare heavy metal contaminated soil remediation samples. Subsequently, the heavy metal contaminated soil remediation samples were mixed with three acid rain acceleration solutions with different pH values ​​according to the preset solid-liquid ratio to obtain three corresponding mixtures. B. Accelerated test: The three mixtures are continuously oscillated for a preset time to obtain the corresponding mixtures that are equivalent to the acid rain leaching effect in a single accelerated test; C. Determination: The mixture is circulated and subjected to the accelerated test in step B. After the accelerated test, the concentration of heavy metal elements in the supernatant of the mixture is determined. C i and its chemical speciation distribution characteristics; D. Model Construction: Based on Environmental Background Values B i and the heavy metal concentrations obtained after each accelerated test in step C. C i Calculate the stability index of heavy metal contaminated soil after each accelerated test. E i = C i / B i Construct a long-term stability index model for heavy metal contaminated soil remediation. Calculate the long-term stability index E N ,in i To accelerate the number of test cycles, i ∈ (0,1,2,3,…, n ); n is the total number of cycles in the accelerated test; N is the number of years in the equivalent simulation; E. Risk Level Assessment: Establish long-term stability level classification standards and calculate the long-term stability index. E N The long-term stability level of remediated heavy metal contaminated soil was assessed in accordance with the long-term stability level classification standard.

[0010] Furthermore, in step A, the remediation agent is a conventional geopolymer or a geopolymer prepared from industrial solid waste, wherein the industrial solid waste includes red mud and blast furnace slag; the remediation agent dosage in the treated heavy metal contaminated soil is less than 20%.

[0011] Further, in step A, the heavy metal contaminated soil treated with the remediation agent is placed in a constant temperature chamber and cured at 20°C and humidity >95% for 28 days. After curing, it is ground, crushed, and passed through a 2 mm square-hole sieve to obtain a heavy metal contaminated soil sample. Simultaneously, natural soil collected from the same area as the heavy metal contaminated soil is treated according to the aforementioned process to obtain a natural soil sample. Then, the heavy metal leaching concentration of the natural soil sample is determined using TCLP as an environmental background value. B i .

[0012] Further, in step A, the pH range of acid rain in the local area where the heavy metal contaminated soil is located is actually measured. Then, two pH values ​​near the endpoints and one intermediate pH value are selected within the pH range of acid rain. Subsequently, three acid rain acceleration solutions are prepared according to the aforementioned pH values. Then, three samples of heavy metal contaminated soil of the same mass are placed in three shaking flasks in sequence. Then, the three acid rain acceleration solutions are added to the three shaking flasks at a solid-liquid ratio of 1:5 to obtain three mixtures.

[0013] Furthermore, in step A, the preparation of the acid rain acceleration solution involves aerating CO2 gas into deionized water at a pressure of 0.3 MPa for 5 minutes, and then slowly adding 1.0 mol / L HNO3 and NaOH using a dropper to adjust the pH value to a predetermined value.

[0014] Furthermore, in step B, the shaking bottle containing the mixture is placed on a horizontal shaker and continuously shaken at 100-120 rpm for 20-28 hours to accelerate the simulated acid rain leaching effect.

[0015] Furthermore, in step B, the shaking bottle containing the mixture is placed on a horizontal shaker and continuously shaken at 110 rpm for 24 hours to obtain a single accelerated test equivalent to 9.48 years of acid rain leaching.

[0016] Furthermore, in step C, after each accelerated test, the mixture is allowed to stand for 12 hours, and then the concentration of heavy metal elements in the supernatant of the shaking flask is determined using TCLP. C i The chemical speciation and its distribution were determined using the BCR sequential extraction method.

[0017] Furthermore, in step E, the long-term stability risk level is divided into five levels: slight, low, medium, high, and very high. The criteria for classifying long-term stability levels are shown in the table below: .

[0018] Furthermore, in step E, the calculated stability index is used... E i The value, compared with the long-term stability grading standard, can be used to assess the stability level of heavy metal contaminated soil after remediation within a certain period of time; the long-term stability index is calculated accordingly. E N By comparing with the long-term stability classification standard, the long-term stability level of heavy metal contaminated soil after remediation can be assessed.

[0019] The present invention has the following beneficial effects: 1. This invention uses acid rain acceleration solutions with different pH values ​​to simulate the acid rain effect (the pH range covers the actual site acid rain conditions). Combined with multiple rounds of accelerated tests (the number of cycles is adjustable), it simulates the stability level under long-term environmental stress (such as acid rain leaching and alternating wet and dry conditions). This allows for dynamic monitoring of the heavy metal release characteristics (including concentration and chemical speciation) at each stage in contaminated soil after geopolymer remediation. This not only accurately quantifies the durability of the remediation effect and effectively solves the problem of quantifying the risk of reactivation of contaminated soil, but also, at its core, assesses the response of soil samples treated with remediation agents under different environmental disturbances. Therefore, it is not limited to a specific geopolymer or heavy metal and can be adapted to various geopolymer materials (conventional geopolymers or geopolymers prepared from industrial solid waste) and different types of heavy metal contaminated soil. This significantly improves the universality of the method and makes it applicable to long-term risk prediction in complex environments, providing a scientific and long-term evaluation framework for heavy metal remediation projects.

[0020] 2. This invention innovatively constructs a long-term stability index model based on heavy metal concentration (… C i ) and environmental background values ​​( B i The ratio of ) is dynamically calculated to determine long-term stability. Combined with the analysis of chemical speciation distribution (BCR sequential extraction method), the reactivation risk of heavy metals is transformed into specific values, which can accurately quantify the durability of remediation effect and reduce the influence of human subjective factors on evaluation results. This realizes the transformation from traditional qualitative or complex macro data fitting analysis to quantifiable and objective evaluation, and effectively solves the problem of quantifying the reactivation risk of heavy metal contaminated soil.

[0021] 3. This invention uses multi-pH acid rain accelerated solution and cyclic accelerated test to scientifically simulate the long-term disturbance of acid rain leaching. It not only effectively simulates the long-term leaching and erosion of solidified bodies by actual environmental factors, but also can complete the assessment of the long-term stability of heavy metal contaminated soil in a short time (such as simulating 9.48 years of acid rain action with a single 24-hour oscillation), thereby improving the evaluation efficiency. It can also predict the persistence of remediation effects many years later, providing early warning for long-term environmental risks.

[0022] 4. This invention mainly constructs a stability index model by measuring the leaching concentration of heavy metals. It requires fewer parameters and the measurement process is simpler. Therefore, compared with the existing technology that requires the measurement of multiple macroscopic performance parameters (such as compressive strength, elastic modulus, etc.) and fitting analysis, this invention significantly improves the evaluation efficiency, reduces the evaluation cost, and can provide evaluation results more promptly.

[0023] 5. This invention establishes a five-level risk level classification standard (slight to extremely high), combined with a single-event stability index ( E i ) and long-term composite index (E N The long-term stability index model constructed can quantitatively output the risk level, making the evaluation results more intuitive and directly used to judge the long-term effectiveness of remediation materials. This provides a reliable basis for subsequent decision-making in heavy metal contaminated soil remediation projects and reduces the risk of heavy metal reactivation in the soil.

[0024] In summary, this invention, through systematic simulation, quantitative model construction, and risk classification, solves the problems of narrow applicability and lagging evaluation of existing technologies, and provides a standardized and scalable technical path for the long-term safe use of heavy metal contaminated soil after remediation, especially suitable for the post-remediation supervision of complex contaminated sites such as mining and metallurgy sites. Attached Figure Description

[0025] Figure 1 This is a diagram showing the concentration of heavy metal ions in the accelerated acid rain solution according to an embodiment of the present invention. Figure 2 This is a graph showing the leaching concentration of heavy metals in contaminated soil after accelerated acid rain testing, as described in this embodiment of the invention. Figure 3 This is a diagram showing the stable efficiency after accelerated acid rain testing in an embodiment of the present invention. Figure 4 This is a diagram showing the chemical morphology evolution after accelerated acid rain testing in an embodiment of the present invention; In the diagram: F1 represents the weakly acid-soluble portion (active state), F2 represents the reducible portion (carbonate-bound state), F3 represents the oxidizable portion, and F4 represents the heavy metal residue portion; in symbol 4-1a9, 4 represents the pH value, 1 represents the proportion of the repair agent as 10%, and a9 represents the acceleration of the solution 9 times. The meanings of the other symbols are deduced accordingly. Figure 5 This is a graph showing the evolution of long-term stability indicators after accelerated acid rain testing in an embodiment of the present invention. In the diagram: N=9.48 i That is, the number of years of equivalent simulation and the number of cycles of accelerated testing. i Related; where N1 = 9.8 years, then N2 = 9.8 × 2 = 19.6 years, and so on. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this does not limit the present invention in any way. Any changes or improvements made based on the teachings of the present invention shall fall within the protection scope of the present invention.

[0027] like Figures 1 to 5As shown, the long-term stability evaluation method for heavy metal contaminated soil remediation of this invention is implemented as follows: it includes sample preparation, accelerated testing, measurement, model construction, and risk level assessment steps, and the specific processes of each step are as follows: A. Sample preparation: Heavy metal contaminated soil treated with remediation agent was crushed and then sieved to prepare heavy metal contaminated soil remediation samples. Subsequently, the heavy metal contaminated soil remediation samples were mixed with three acid rain acceleration solutions with different pH values ​​according to the preset solid-liquid ratio to obtain three corresponding mixtures. B. Accelerated test: The three mixtures are continuously oscillated for a preset time to obtain the corresponding mixtures that are equivalent to the acid rain leaching effect in a single accelerated test; C. Determination: The mixture is circulated and subjected to the accelerated test in step B. After the accelerated test, the concentration of heavy metal elements in the supernatant of the mixture is determined. C i and its chemical speciation distribution characteristics; D. Model Construction: Based on Environmental Background Values B i and the heavy metal concentrations obtained after each accelerated test in step C. C i Calculate the stability index of heavy metal contaminated soil after each accelerated test. E i = C i / B i Construct a long-term stability index model for heavy metal contaminated soil remediation. Calculate the long-term stability index E N ,in i To accelerate the number of test cycles, i ∈ (0,1,2,3,…, n ); n is the total number of cycles in the accelerated test; N is the number of years in the equivalent simulation; E. Risk Level Assessment: Establish long-term stability level classification standards and calculate the long-term stability index. E N The long-term stability level of remediated heavy metal contaminated soil was assessed in accordance with the long-term stability level classification standard.

[0028] In step A, the remediation agent is a conventional geopolymer or a geopolymer prepared from industrial solid waste, wherein the industrial solid waste includes red mud and blast furnace slag; the remediation agent dosage of the treated heavy metal contaminated soil is less than 20%.

[0029] In step A, the heavy metal contaminated soil treated with the remediation agent is placed in a constant temperature chamber and cured at 20°C and humidity >95% for 28 days. After curing, it is ground, crushed, and passed through a 2 mm square-hole sieve to obtain a heavy metal contaminated soil sample. Simultaneously, natural soil collected from the same area as the heavy metal contaminated soil is treated according to the aforementioned process to obtain a natural soil sample. Then, the heavy metal leaching concentration of the natural soil sample is determined using TCLP as an environmental background value. B i .

[0030] In step A, the pH range of acid rain in the local area where the heavy metal contaminated soil is located is actually measured. Then, two pH values ​​near the endpoints and one intermediate pH value are selected within the pH range of acid rain. Subsequently, three acid rain acceleration solutions are prepared according to the aforementioned pH values. Then, three samples of heavy metal contaminated soil of the same mass are placed in three shaking bottles in sequence. Then, the three acid rain acceleration solutions are added to the three shaking bottles at a solid-liquid ratio of 1:5 to obtain three mixtures.

[0031] In step A, the acid rain acceleration solution is prepared by aerating CO2 gas into deionized water at a pressure of 0.3 MPa for 5 min, and then slowly adding 1.0 mol / L HNO3 and NaOH with a dropper to adjust the pH value to the predetermined value.

[0032] In step B, the shaking bottle containing the mixture is placed on a horizontal shaker and continuously shaken at 100-120 rpm for 20-28 hours to accelerate the simulated acid rain leaching effect.

[0033] In step B, the shaking bottle containing the mixture is placed on a horizontal shaker and shaken continuously at 110 rpm for 24 hours to obtain a single accelerated test equivalent to 9.48 years of acid rain leaching.

[0034] In step C, after each accelerated test, the mixture was allowed to stand for 12 hours, and then the concentration of heavy metal elements in the supernatant of the shaking bottle was determined using TCLP. C i The chemical speciation and its distribution were determined using the BCR sequential extraction method.

[0035] It should be noted that the chemical speciation distribution is mainly used for qualitative evaluation in the risk assessment process, with the aim of understanding the process by which heavy metals are transformed into low-valence or stable forms during acid rain leaching, which are non-toxic or low-toxic.

[0036] In step E, the long-term stability risk level is divided into five levels: slight, low, medium, high, and very high. The criteria for classifying the long-term stability level are shown in the table below: .

[0037] In step E, the stability index is calculated. E i The value, compared with the long-term stability grading standard, can be used to assess the stability level of heavy metal contaminated soil after remediation within a certain period of time; the long-term stability index is calculated accordingly. E N By comparing with the long-term stability classification standard, the long-term stability level of heavy metal contaminated soil after remediation can be assessed.

[0038] Example 1

[0039] Preparation of samples after remediation of S100 and Cu contaminated soil: Taking a soil remediation project near a heavy metal mining area in Yunnan Province as an example, the main pollutant in the contaminated soil was the heavy metal Cu. First, Cu-contaminated soil samples were collected from a certain depth, naturally air-dried, pulverized, and sieved through a 2 mm square-hole sieve for later use. Then, using red mud and blast furnace slag as raw materials, a red mud-blast furnace slag geopolymer was prepared through an alkaline activation reaction (water glass was used as the alkaline activator). The mass ratio of red mud to blast furnace slag was 2:3, and the mass ratio of water glass with a modulus of 1.5 to the red mud-blast furnace slag mixture was 1:5. Simultaneously, distilled water of equal mass to water glass with a modulus of 1.5 was added to prepare the contaminated soil remediation agent, namely the red mud-blast furnace slag geopolymer. Then, the red mud-blast furnace slag geopolymer was uniformly mixed with the previously sieved contaminated soil, and the proportion of the remediation agent (i.e., the mass ratio of the geopolymer to the mass of the geopolymer-contaminated soil mixture) was set to 0%, 10%, and 20%, respectively. After mixing, the sample was placed in a constant temperature curing room at 20℃ and humidity greater than 95% for 28 days. It was then ground, crushed, and passed through a 2 mm square-hole sieve to obtain Cu-contaminated soil samples. Simultaneously, natural soil samples were collected from the same area as the heavy metal-contaminated soil samples and processed using the same procedure. The heavy metal leaching concentration of the natural soil samples was then determined using TCLP as an environmental background value. B i .

[0040] Based on the actual measured pH range of acid rain in the aforementioned mining area (the pH value of natural rainwater is 4.13 to 8.19), the acid rain acceleration solution was set into three pH values: 4.00, 6.00, and 8.00.

[0041] Preparation of acid rain acceleration solutions: CO2 gas was aerated into deionized water at a pressure of 0.3 MPa for 5 min. Then, 1.0 mol / L HNO3 and NaOH were slowly added with a dropper to adjust the pH value to 4, 6 and 8, respectively, to obtain three acid rain acceleration solutions.

[0042] Finally, nine samples of the aforementioned Cu-contaminated soil of equal mass were placed in nine shaking flasks. Then, the three acid rain acceleration solutions were added to the nine shaking flasks at a solid-liquid ratio of 1:5 (the acid rain acceleration solutions with the same pH value were added to three different shaking flasks). Nine accelerated mixtures were obtained after the remediation of Cu-contaminated soil. The accelerated mixtures obtained from the Cu-contaminated soil samples without the addition of geopolymers were designated as 4-0, 6-0, and 8-0, respectively. The accelerated mixtures obtained from the Cu-contaminated soil samples with the addition of geopolymers were designated as 4-1, 6-1, 8-1 and 4-2, 6-2, 8-2, respectively, according to the pH value of the added acid rain acceleration solutions.

[0043] S200. The above 9 accelerated mixtures were placed on a horizontal shaker along with the shaking bottle and shaken continuously at 110 rpm for 24 h to simulate the acid rain leaching effect of 9.48 years, thus obtaining the corresponding mixtures of 9 single accelerated tests that simulated the acid rain leaching effect of 9.48 years.

[0044] S300. The above 9 mixtures are subjected to the accelerated test of step S200 in a cycle, and after each accelerated test, they are allowed to stand for 12 hours. Then, the Cu ion concentration in the mixture is determined by TCLP. Figure 1 and the concentration of Cu ions in the supernatant C i ( Figure 2 ) and stabilization rate ( Figure 3 The chemical speciation of Cu ions in the supernatant was determined using the BCR sequential extraction method. Figure 4 ).

[0045] S400, Environmental background value based on Cu contaminated soil B i And the Cu ion concentration obtained after each of the aforementioned accelerated tests. C i Calculate the stability index of Cu-contaminated soil after each accelerated test. E i = C i / B i Constructing a long-term stability index model for Cu-contaminated soil remediation And calculate the long-term stability index. E N ;in i To accelerate the number of test cycles, i ∈ (0,1,2,3,…, n (n is the total number of cycles in the accelerated test; N=9.48) i That is, the number of years of equivalent simulation and the number of cycles of accelerated testing. i Related.

[0046] S500, establish long-term stability level classification criteria (Table 1), and obtain the long-term stability index based on the calculation. E N The long-term stability level of Cu-contaminated soil after remediation was assessed according to Table 1. Figure 5 ).

[0047] Table 1. Long-term stability level classification criteria Example 2

[0048] Preparation of samples after remediation of S100 and Pb contaminated soil: Taking a soil remediation project around a heavy metal mining area in Yunnan Province as an example, the main pollutant in the contaminated soil at this site is the heavy metal Pb. The preparation of the accelerated mixture after remediation of 9 types of Pb-contaminated soil is similar to that in Example 1 (the Cu-contaminated soil in Example 1 is replaced with Pb-contaminated soil, and everything else is the same).

[0049] The preparation of S200 and the nine mixtures is the same as in Example 1.

[0050] S300. The above 9 mixtures are subjected to the accelerated test of step S200 in a cycle, and after each accelerated test, they are allowed to stand for 12 hours. Then, the concentration of Pb ions in the mixture is determined by TCLP. Figure 1 and the concentration of Pb ions in the supernatant C i ( Figure 2 ) and stabilization rate ( Figure 3 The chemical speciation of Pb ions in the supernatant was determined using the BCR sequential extraction method. Figure 4 ).

[0051] S400, Environmental background values ​​based on Pb-contaminated soil B i And the Pb ion concentration obtained after each of the aforementioned accelerated tests. C i Calculate the stability index of Pb-contaminated soil after each accelerated test. E i = C i / B i Construct a long-term stability index model for Pb-contaminated soil remediation. And calculate the long-term stability index. E N ;in i To accelerate the number of test cycles, i ∈ (0,1,2,3,…, n (n is the total number of cycles in the accelerated test; N=9.48)i That is, the number of years of equivalent simulation and the number of cycles of accelerated testing. i Related.

[0052] S500, Set the long-term stability level classification criteria (Table 1), and base the calculated long-term stability index on... E N The long-term stability level of Pb-contaminated soil after remediation was assessed according to Table 1. Figure 5 ).

[0053] Example 3

[0054] Preparation of samples after remediation of S100 and Cd contaminated soil: Taking a soil remediation project around a heavy metal mining area in Yunnan Province as an example, the main pollutant in the contaminated soil at this site is the heavy metal Cd. The preparation of the accelerated mixture after remediation of 9 types of Cd-contaminated soil is similar to that in Example 1 (the Cu-contaminated soil in Example 1 is replaced with Cd-contaminated soil, and everything else is the same).

[0055] The preparation of S200 and the nine mixtures is the same as in Example 1.

[0056] S300. The above 9 mixtures are subjected to the accelerated test of step S200 in a cycle, and after each accelerated test, they are allowed to stand for 12 hours. Then, the Cd ion concentration in the mixture is determined by TCLP. Figure 1 and the concentration of Cd ions in the supernatant C i ( Figure 2 ) and stabilization rate ( Figure 3 The chemical speciation of Cd ions in the supernatant was determined using the BCR sequential extraction method. Figure 4 ).

[0057] S400, Environmental Background Values ​​Based on Cd-Contaminated Soil B i and the Cd ion concentration obtained after each of the aforementioned accelerated tests. C i Calculate the stability index of Cd contaminated soil after each accelerated test. E i = C i / B i Constructing a long-term stability index model for Cd-contaminated soil remediation And calculate the long-term stability index. E N ;in i To accelerate the number of test cycles, i ∈ (0,1,2,3,…, n(n is the total number of cycles in the accelerated test; N=9.48) i That is, the number of years of equivalent simulation and the number of cycles of accelerated testing. i Related.

[0058] S500, Set the long-term stability level classification criteria (Table 1), and base the calculated long-term stability index on... E N The long-term stability level of Cd-contaminated soil after remediation was assessed according to Table 1. Figure 5 ).

[0059] Comparative analysis of results: Depend on Figure 1 It was found that in the accelerated aging test, heavy metal ions readily leach from the soil in the untreated heavy metal contaminated soil (groups 4-0, 6-0, and 8-0). Furthermore, under acidic conditions (pH=4), the leach concentrations of Cu, Pb, and Cd were higher than at pH=6 and pH=8. This indicates that at pH=4, H⁺ ions in acid rain can strongly react with and dissolve heavy metals such as Cu, Pb, and Cd in the soil. After two accelerated aging tests, the heavy metal leachation approached zero.

[0060] according to Figure 2 The toxicity leaching results showed that in the heavy metal contaminated soils (groups 4-0, 6-0, and 8-0) without added remediation agents, the leaching concentrations of Cu, Pb, and Cd far exceeded the leaching standard values ​​for solid waste, and the toxicity leaching concentrations decreased with increasing number of accelerated aging tests. However, after adding remediation agents, the toxicity leaching of Cu, Pb, and Cd began to stabilize after the second accelerated aging test. Specifically, in the heavy metal contaminated soils (groups 4-1, 6-1, and 8-1) with 10% added remediation agents, the leaching concentrations of Cu and Pb began to increase after the ninth accelerated aging test, indicating a weakening of the geopolymer's solidification effect; the heavy metal contaminated soils with 20% added remediation agents remained stable after the ninth accelerated aging test. Although some Cd ions were fixed, their concentration still exceeded the solid waste leaching identification standard value.

[0061] Depend on Figure 3It can be seen that under pH=4 conditions, when the proportion of the remediation agent increased from 10% to 20%, the stabilization rate of Cu significantly improved in the later accelerated aging test stage. For example, in the ninth accelerated aging test, the Cu stabilization rate of group 4-2 was 97.1%, an increase of 9.1% compared to group 4-1 (89%); the Pb stabilization rate also increased from 90.3% (group 4-1) to 96.7% (group 4-2), an increase of 7.1%. Similar trends were observed under pH=6 and pH=8 conditions, indicating that a higher proportion of the remediation agent has a better long-term stabilization effect on Cu / Pb. The stabilization efficiency of Cd was generally lower than that of Cu / Pb. For example, under pH=4 conditions, the Cd stabilization rate of group 4-2 was only 57.9% in the ninth accelerated aging test, even lower than the 69.3% of group 4-1; while under pH=8 conditions, the Cd stabilization rate of group 8-2 (73.8%) was only slightly higher than that of group 8-1 (0.731), with a slight increase (0.95%). Under low pH conditions, the stabilization rate of Cd decreased with increasing number of accelerated aging tests. However, under pH=6 and pH=8 conditions, the stabilization rate of Cd tended to stabilize with increasing number of accelerated aging tests.

[0062] Depend on Figure 4 It can be seen that the residual states of Cu, Pb, and Cd show a trend of first increasing and then decreasing, indicating that heavy metals exhibit significant speciation characteristics during accelerated aging tests. For example, the active state (F1) of Cu decreases from 53.26% (4-0a1) to 5.51% (4-2a9) with prolonged aging time, while the residual state (F4) increases to 41.25%. Alkaline conditions (pH=8) further enhance the passivation effect by promoting Cu(OH)2 precipitation and organic matter complexation. The carbonate-bound state (F2) of Pb sharply decreases to 24.68% (4-2a9) when the remediation agent concentration is increased to 20%, while the residual state (F4) increases to 30.15%. A neutral environment (pH=6) is more conducive to its stabilization. The speciation of Cd shows a clear bottleneck. Although the active state (F1) decreases from 81.40% (4-0a1) to 31.94% (4-2a9), the F1+F2 ratio is still as high as 55.12%. Cu / Pb completes 80% of its form transformation within the first 60 years, while Cd takes 90 years to achieve a similar effect.

[0063] Depend on Figure 5 It can be seen that Cu is at a slight risk in the 4-0 and 6-0 samples for a long period of time. Eᵢ <50), its stability index reached 35.50 in year 66.36 (seventh test), reflecting the continuous settlement effect of Cu in the soil; although the ecological risk of Pb is generally low (most Eᵢ<10), but the Pb value of sample 4-0 reached 8.23 ​​in year 66.36 (seventh test), suggesting that under long-term exposure (>60 years), attention should be paid to its synergistic toxicity effect with Cd and Cu. From year 9.48 to year 85.32 (ninth test), the spatial heterogeneity of Cd risk remained significant (the difference between sample 4-0 and sample 8-2 was 1.7 times); the stability indices of Cu and Pb did not change much, suggesting that their migration pathways are relatively stable on a decades-long timescale.

[0064] All samples had Cd stability indices exceeding the extremely high risk threshold (750), for example, sample 4-0 at year 9.48 (first test). Eᵢ The value reached 1555.56, with the 8-0 sample peaking at 1571.85 in year 66.36 (the seventh test). In terms of time: the Cd stability index of sample 4-2 decreased from 914.07 in year 9.48 to 610.37 in year 85.32 (the ninth test), a decrease of 33.2%, possibly reflecting the phased effectiveness of long-term pollution control; the Cd stability index of sample 8-0 surged by 47.3% in year 66.36 compared to year 56.88 (the sixth test), indicating cyclical fluctuations in pollution release, possibly related to smelting production intensity or intermittent failure of remediation measures.

[0065] Cd, as a core risk driver, has a long-term stability index ( E Cd The concentration of Cd in sample 4-0 without remediation agent intervention reached 11474.1, far exceeding the threshold, confirming the significant activating effect of an acidic environment (pH=4) on Cd activity (Table 2). Adding 10% remediation agent (group 4-1) resulted in... E Cd The concentration decreased to 9942.5 (a decrease of 13.4%), but its passivation efficiency for Cu and Pb was more significant. E Cu The decrease was 67.8%. E Pb (Reduction of 74.4%). Long-term stability analysis revealed a decline in repair efficacy; sample 4-1 underwent 85 years of simulated aging. E Cd 86.6% of the samples still do not meet the repair criteria, indicating that periodic replenishment of repair materials is necessary to maintain passivation stability.

[0066] Table 2 Long-term stability index of various contaminated soils after 85 years of acceleration

[0067] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for evaluating the long-term stability of heavy metal contaminated soil after remediation, characterized in that: The process includes sample preparation, accelerated testing, measurement, model construction, and risk level assessment. The specific steps for each step are as follows: A. Sample preparation: Heavy metal contaminated soil treated with remediation agent was crushed and then sieved to prepare heavy metal contaminated soil remediation samples. Subsequently, the heavy metal contaminated soil remediation samples were mixed with three acid rain acceleration solutions with different pH values ​​according to the preset solid-liquid ratio to obtain three corresponding mixtures. B. Accelerated test: The three mixtures are continuously oscillated for a preset time to obtain the corresponding mixtures that are equivalent to the acid rain leaching effect in a single accelerated test; C. Determination: The mixture is circulated and subjected to the accelerated test in step B. After the accelerated test, the concentration of heavy metal elements in the supernatant of the mixture is determined. C i and its chemical speciation distribution characteristics; D. Model Construction: Based on Environmental Background Values B i and the heavy metal concentrations obtained after each accelerated test in step C. C i Calculate the stability index of heavy metal contaminated soil after each accelerated test. E i = C i / B i Construct a long-term stability index model for heavy metal contaminated soil remediation. Calculate the long-term stability index E N ,in i To accelerate the number of test cycles, i ∈ (0,1,2,3,…, n ); n is the total number of cycles in the accelerated test; N is the number of years in the equivalent simulation; E. Risk Level Assessment: Establish long-term stability level classification standards and calculate the long-term stability index. E N The long-term stability level of remediated heavy metal contaminated soil was assessed in accordance with the long-term stability level classification standard.

2. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 1, characterized in that: In step A, the remediation agent is a conventional geopolymer or a geopolymer prepared from industrial solid waste, wherein the industrial solid waste includes red mud and blast furnace slag; the remediation agent dosage of the treated heavy metal contaminated soil is less than 20%.

3. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 1, characterized in that: In step A, the heavy metal contaminated soil treated with the remediation agent is placed in a constant temperature chamber and cured at 20°C and humidity >95% for 28 days. After curing, it is ground, crushed, and passed through a 2 mm square-hole sieve to obtain a heavy metal contaminated soil sample. Simultaneously, natural soil collected from the same area as the heavy metal contaminated soil is treated according to the aforementioned process to obtain a natural soil sample. Then, the heavy metal leaching concentration of the natural soil sample is determined using TCLP as an environmental background value. B i .

4. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 1, 2, or 3, characterized in that: In step A, the pH range of acid rain in the local area where the heavy metal contaminated soil is located is actually measured. Then, two pH values ​​near the endpoints and one intermediate pH value are selected within the pH range of acid rain. Subsequently, three acid rain acceleration solutions are prepared according to the aforementioned pH values. Then, three samples of heavy metal contaminated soil of the same mass are placed in three shaking bottles in sequence. Then, the three acid rain acceleration solutions are added to the three shaking bottles at a solid-liquid ratio of 1:5 to obtain three mixtures.

5. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 4, characterized in that: In step A, the acid rain acceleration solution is prepared by aerating CO2 gas into deionized water at a pressure of 0.3 MPa for 5 min, and then slowly adding 1.0 mol / L HNO3 and NaOH with a dropper to adjust the pH value to the predetermined value.

6. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 4, characterized in that: In step B, the shaking bottle containing the mixture is placed on a horizontal shaker and continuously shaken at 100-120 rpm for 20-28 hours to accelerate the simulated acid rain leaching effect.

7. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 6, characterized in that: In step B, the shaking bottle containing the mixture is placed on a horizontal shaker and shaken continuously at 110 rpm for 24 hours to obtain a single accelerated test equivalent to 9.48 years of acid rain leaching.

8. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 4, characterized in that: In step C, after each accelerated test, the mixture was allowed to stand for 12 hours, and then the concentration of heavy metal elements in the supernatant of the shaking bottle was determined using TCLP. C i The chemical speciation and its distribution were determined using the BCR sequential extraction method.

9. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 4, characterized in that: In step E, the long-term stability risk level is divided into five levels: slight, low, medium, high, and very high. The criteria for classifying the long-term stability level are shown in the table below: 。 10. The method for evaluating the long-term stability of heavy metal contaminated soil after remediation according to claim 9, characterized in that: In step E, the stability index is calculated. E i The value, compared with the long-term stability grading standard, can be used to assess the stability level of heavy metal contaminated soil after remediation within a certain period of time; the long-term stability index is calculated accordingly. E N By comparing with the long-term stability classification standard, the long-term stability level of heavy metal contaminated soil after remediation can be assessed.

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