Method for predicting exchangeable heavy metal content in soil based on P-wave velocity

Abstract

The invention belongs to the technical field of soil treatment, and particularly discloses a method for predicting exchangeable heavy metal content in soil based on P-wave velocity, which comprises the following steps: collecting a soil sample from a heavy metal polluted plough layer, crushing and sieving; then drying and carrying out vacuum sealing and standing; spraying purified water to the soil sample until the preset water content is reached; pouring the soil sample into a mold, horizontally oscillating to a preset index, and taking down; determining the heavy metal concentration and exchangeable state content in each soil sample after oscillation by adopting a soil basic physicochemical property analysis method; a comprehensive acoustic monitoring system is used for measuring the actual propagation time of P waves passing through the soil, and the wave velocity of the P waves corresponding to the soil sample is calculated; and based on the P-wave velocity, the heavy metal concentration and the exchangeable state content, establishing a fitting linear model of the exchangeable state heavy metal content and the P-wave velocity, and dynamically simulating soil key variables to accurately predict the soil heavy metal pollution level. The method has the characteristics of high pertinence, simplicity and convenience in operation, high detection speed, wide detection range and low cost.

Description

Technical Field

[0001] The present invention relates to the technical field of soil treatment, and particularly relates to a method for predicting the content of exchangeable heavy metals in soil based on P-wave velocity, which has strong pertinence, is easy to operate, has a fast detection speed, a wide detection range, and a low cost. Background Art

[0002] Geophysical exploration techniques such as gravity, magnetism, resistivity, and seismic waves play an important role in understanding soil physical and chemical characteristics and pollution levels. Among them, seismic waves, especially P-waves, stand out due to their high efficiency, rapidity, and non-invasiveness. P-waves are longitudinal waves that penetrate the earth's subsurface and can provide key information about soil physical properties. The velocity of P-waves passing through soil is affected by various factors, and soil porosity is one of the most important factors. A large number of studies have shown that there is a negative correlation between P-wave velocity and soil porosity. Therefore, changes in soil porosity will directly affect the propagation velocity of P-waves in soil, making P-wave velocity an effective indicator for measuring soil porosity.

[0003] With the acceleration of industrialization and urbanization, soil heavy metal pollution has become an important environmental issue of global concern. Heavy metals (such as cadmium, lead, arsenic, zinc, copper, and mercury, etc.) accumulating in soil for a long time will pose a serious threat to human health, the ecosystem, and agricultural productivity. Although the adsorption of heavy metals by soil particles will cause changes in porosity, which in turn affects the propagation velocity of P-waves. However, the impact of heavy metals on soil properties does not solely depend on their total concentration. The exchangeable state of heavy metals is equally important for the degree of soil pollution. Exchangeable heavy metals are prone to diffusion, redox reactions, and chelation in soil, and can thus be directly absorbed by plants. Therefore, understanding the internal relationship between the exchangeable state of soil heavy metals and their physical and chemical properties is crucial for assessing environmental risks and formulating effective remediation strategies.

[0004] Traditional soil pollution detection methods mostly rely on chemical analysis. Although these methods are accurate, they usually take a long time, have a high cost, and are not suitable for large-scale or real-time monitoring. In addition, the operation of these traditional methods is complex and requires professional technicians for sample processing and data analysis, which limits their application in real-time and wide-area environmental monitoring.

[0005] In the prior art, to address the deficiencies of the aforementioned traditional detection methods, a fitting relationship model between mass moisture content and longitudinal wave velocity, and a relationship model between heavy metal concentration and longitudinal wave velocity under different mass moisture content conditions were pre-constructed. Then, a pulsed longitudinal wave signal was introduced into the soil to be measured, and the actual longitudinal wave velocity in the soil to be measured was measured. Since the influence of mass moisture content on longitudinal wave velocity is much greater than that of heavy metal concentration on longitudinal wave velocity, first, the fitting value of mass moisture content was obtained based on the actual longitudinal wave velocity and the fitting relationship model between mass moisture content and longitudinal wave velocity. Then, the relationship model between heavy metal concentration and longitudinal wave velocity under the measured mass moisture content condition with the smallest difference from the fitting value of mass moisture content was selected, and the actual longitudinal wave velocity was substituted into it to obtain the heavy metal concentration range of the soil to be measured. Although the accuracy of the heavy metal concentration measured by this technical solution basically meets the requirements, and it also realizes low cost and high efficiency; as mentioned above, since the influence of heavy metals on soil properties does not only depend on their total concentration, the exchangeable state of heavy metals is equally important for the degree of soil pollution. Therefore, it is particularly important to further study how to quickly and accurately judge the degree of soil heavy metal pollution by means of P-wave velocity. Summary of the Invention

[0006] Aiming at the deficiencies in the prior art, the present invention provides a method for predicting the content of exchangeable heavy metals in soil based on P-wave velocity, which has strong pertinence, is easy to operate, has a fast detection speed, a wide detection surface, and a low cost.

[0007] The present invention is implemented as follows: It includes steps of soil sampling, soil sample drying, soil sample rewetting, soil sample molding, soil sample determination, wave velocity monitoring, and content prediction. The specific content is as follows: A. Soil sampling: Collect soil samples from the heavy metal-polluted plough layer at a depth of 3 - 50 cm from the ground surface, and then crush and sieve them for standby. B. Soil sample drying: Dry the sieved soil samples, then pack them in vacuum bags, seal them, and let them stand for 7 - 14 days. C. Soil sample rewetting: Divide the standing soil samples into several parts evenly, and spray pure water on each soil sample multiple times until the predetermined moisture content is reached. D. Soil sample molding: Pour each soil sample that has reached the predetermined moisture content into the corresponding mold, then horizontally oscillate the soil sample with the mold, and remove it for measurement after reaching the predetermined index. E. Soil sample determination: Use the soil basic physical and chemical property analysis method to measure the heavy metal concentration and exchangeable state content in each soil sample after oscillation. F. Wave velocity monitoring: Use a comprehensive acoustic monitoring system to measure the actual propagation time of P-waves passing through the same soil samples in step E, and calculate the corresponding P-wave velocity of each soil sample. G. Content Prediction: Based on the P-wave velocities and the exchangeable state contents of heavy metals of each soil sample obtained above, a fitting linear model of the exchangeable state heavy metal content and the P-wave velocity is established, and the influence of key soil variables on the exchangeable state content of soil heavy metals is dynamically simulated to accurately predict the key factors of pollution risk.

[0008] Further, in step B, the sieved soil sample is continuously dried in an oven at 110 ± 2 °C for 10 h, then the soil sample is taken out and placed in a drying dish to cool to room temperature, and then sealed and left standing in a vacuum bag.

[0009] Further, in step C, first, according to the predetermined water content corresponding to each soil sample, the mass of pure water required for each is calculated, then the mass of the required pure water is evenly divided into 2 - 4 portions, and then the evenly divided pure water is sprayed onto the corresponding soil samples at intervals of 10 - 12 h to obtain the corresponding soil samples with the predetermined water content.

[0010] Further, the water content determination is also included in step C. The water content of the corresponding soil sample with the predetermined water content is measured to obtain the actual water content of the soil sample; the predetermined water content is 0 - 20%.

[0011] Further, in step D, the mold is a cube-shaped lidless experimental mold made of polytetrafluoroethylene, polyamide fiber, polycarbonate or polymethyl methacrylate with dimensions of 40×40×40 mm or 50×50×50 mm; the oscillation frequency of horizontal oscillation is 110 ± 10 times / min and the amplitude is 40 mm. After the soil sample oscillates with the mold at room temperature for 10 min, it is taken off for testing.

[0012] Further, in step E, the determination of heavy metal concentration in the soil sample adopts inductively coupled plasma atomic emission spectrometry, inductively coupled plasma optical emission spectrometer, inductively coupled plasma mass spectrometry, graphite furnace atomic absorption spectrometry or flame atomic absorption spectrometry, and the determination of the exchangeable state heavy metal content in the soil sample adopts Tessier five-step sequential extraction method.

[0013] Further, the specific operation process of the determination by Tessier five-step sequential extraction method is as follows: E10. pH Measurement: Take 5 g of the oscillated soil sample, add pure water according to the water-soil ratio of 2.5:1 and mix evenly, and measure the pH value of the soil mixture. E20. Neutralization of the Mixture Ⅰ: When the pH of the soil mixture in E10 is > 7, add Mg(CH3COO)2 solution with pH = 6 and concentration of 1.0 mol / L to the sample at a rate of 1 mL / min until a mixture Ⅰ with pH = 7 is formed. E30, mixed solution neutralization II: When the pH of the soil mixed solution of E10 is less than 7, a MgOH2·MgCl2 solution with a pH of 8 and a concentration of 1.0 mol / L is added to the sample at a rate of 1 mL / min until a mixed solution II with a pH of 7 is formed; E40. Obtain supernatant I: add 5 mL of MgCl2 solution to mixed solution I or mixed solution II, shake for 1-2 hours, and then centrifuge for 5-10 minutes. Then, let the soil mixture obtained by centrifugation stand for 30 minutes, and then filter it with a 0.45 μm filter to obtain supernatant I to be tested for use. E50, obtaining supernatant II: using vacuum filtration to separate the residual liquid in the soil after centrifugation in E40, washing the separated solid with 8 ml of deionized water and shaking for 1-2 h, then centrifuging for 5-10 min and letting stand for 30 min, combining all the liquids obtained from the solid-liquid separation in this step, and then filtering out supernatant II with a 0.45 μm filter, then repeating the above process in this step 3 times and combining supernatant II; E60. Determination of exchangeable heavy metal content: Combine supernatant I and supernatant II, then filter through a 0.45 μm filter, and then use the aforementioned method for determining the concentration of heavy metals in soil samples to determine the heavy metal concentration of the combined supernatant, which is the exchangeable heavy metal content in the soil sample.

[0014] Furthermore, the integrated acoustic monitoring system in step F includes Multi-Channeloscilloscope analysis software of TiePie of the Netherlands, an engineering USB oscilloscope of TiePie of the Netherlands, an acoustic emission preamplifier of Beijing Soft Island Times Technology Co., Ltd., and an acoustic emission sensor of Hunan Endeti Technology Co., Ltd. The engineering USB oscilloscope has a transmission frequency of 5-50 Hz and a pulse width of 1-10 μs for a fixed signal. The acoustic emission preamplifier amplifies the signal by 40 dB and propagates it in the soil sample, with an output noise of 2.1 mV. The acoustic emission sensor has a resonant frequency of 150 kHz, a sensitivity of 72 dB, and a frequency range of 80-350 kHz. The acoustic emission sensor detects the mechanical vibration propagated to the surface by the acoustic emission event inside the soil sample and converts it into an electrical signal. The output voltage V(t,x) of the acoustic emission sensor is the convolution of the surface displacement wave U(x,t) and its response function T(t), that is, V(t,x)=U(t,x)·T(t).

[0015] Further, the comprehensive acoustic monitoring system in step F includes the Multi-Channel oscilloscope analysis software of TiePie Company in the Netherlands, the engineering USB oscilloscope of TiePie Company in the Netherlands, and the acoustic emission sensor of Hunan Enditi Technology Co., Ltd. The fixed signal with a transmission frequency of 5 - 50 Hz and a pulse wave width of 1 - 10 μs is emitted by the engineering USB oscilloscope. The USB oscilloscope amplifies the signal by 40 - 50 dB and then propagates it in the soil sample, with a sampling rate of 0.2 - 1 GSa / s. The resonant frequency of the acoustic emission sensor is 150 kHz, the sensitivity is 72 dB, and the frequency range is 80 - 350 kHz. The acoustic emission sensor converts the mechanical vibration generated by the acoustic source on the surface of the soil sample into an electrical signal. The output voltage V(t,x) of the acoustic emission sensor is the convolution of the surface displacement wave U(x,t) and its response function T(t), that is, V(t,x)=U(t,x)·T(t).

[0016] Further, in step G, the fitting linear model of the exchangeable heavy metal content and the P-wave velocity is as follows: In the formula, ΔV is the P-wave velocity; ΔEX is the exchangeable heavy metal content; a is the correlation coefficient. When a > 0, it indicates a positive correlation between ΔEX and ΔV; when a < 0, it indicates a negative correlation between ΔEX and ΔV; b is the P-wave velocity of the soil without heavy metal pollution; a and b are both fitting coefficients, which are obtained by fitting the measured experimental results through Origin software data analysis.

[0017] Further, in step G, the dynamic simulation of the influence of key soil variables on the exchangeable content of soil heavy metals is based on the response surface of the key parameter of the soil - the exchangeable heavy metal content, and a first-order linear model is constructed to accurately predict the key factors of pollution risk. The formula of the first-order linear model is: , In the formula, Y is the exchangeable heavy metal content of the soil; X i are the key parameters of the soil. The key parameters include organic matter content, moisture content, and cation content; β 0 is the intercept term; β i represents the regression coefficients to be estimated for each factor; ε is the random error term, ∣ βThe larger the ∣ value, the stronger the influence of this factor on the content of heavy metals in the soil. Conversely, it indicates that the influence of this factor on the content of heavy metals in the soil is weaker.

[0018] Advantages of the present invention: 1. In view of the important influence of exchangeable heavy metals on soil pollution and the lack of existing research, the present invention extracts the content of exchangeable heavy metals in soil samples by the Tessier five-step method, and combines the measurement of P-wave velocity to innovatively establish a fitting linear model of the content of exchangeable heavy metals and P-wave velocity. It can not only evaluate the change of soil porosity caused by exchangeable heavy metal pollution, so as to accurately evaluate the pollution risk and ecological toxicity by dynamically simulating the key variables of the soil; but also dynamically simulate the key variables of the soil (such as water content, cation exchange capacity and organic matter content) to adapt to different environmental conditions and heavy metal types, thus expanding the applicable scenarios.

[0019] 2. The present invention combines physical wave velocity measurement and chemical speciation analysis (such as determination of exchangeable heavy metal content and spectral analysis of heavy metal concentration, etc.), so as to be able to cross-verify the accuracy of data and ensure that the model prediction results have both high precision and scientific rigor.

[0020] 3. The present invention clarifies the dynamic evolution law of heavy metal speciation transformation and its influence on soil acoustic properties. Through linear fitting analysis, the relationship between heavy metal ion transformation and P-wave velocity is quantitatively evaluated, providing new insights into the interaction between heavy metal pollution and soil acoustic properties, which is of great significance for environmental monitoring and risk assessment.

[0021] 4. The present invention uses an acoustic monitoring system to measure P-wave velocity in real time, which can greatly shorten the detection time; and by measuring the P-wave velocity by physical method and substituting it into the fitting linear model, the content of exchangeable heavy metals can be obtained, which can significantly reduce the dependence on expensive chemical reagents, effectively reduce the detection cost, especially suitable for large-scale soil pollution screening; and the P-wave velocity measurement is realized by using acoustic sensors and portable devices, so as to support in-situ rapid detection, reduce the damage to sampling, and provide a technical basis for real-time monitoring of soil pollution changes, making up for the lag of traditional laboratory analysis.

[0022] 5. From the pretreatment of soil samples (drying, rewetting, molding) to wave velocity monitoring, the parameters of each step (such as oscillation frequency, water content control) are strictly quantified, which can not only reduce human error, but also simplify the operation, thus improving the repeatability of the operation and reducing the technical dependence on professional personnel.

[0023] In summary, the present invention has the characteristics of strong pertinence, simple operation, fast detection speed, wide detection range and low cost. Description of the drawings

[0024] Figure 1 Schematic diagram of the principle of the comprehensive acoustic monitoring system in the embodiment of the present invention; In the figure: 1 - Multi-Channel oscilloscope analysis software, 2 - USB oscilloscope (Handyscope HS5), 3 - received waveform signal, 4 - transmitted pulse wave signal, 5 - soil sample to be measured, 6 - acoustic emission preamplifier (40 dB), 7 - acoustic emission sensor (GTR150a); Figure 2 Interface of the Multi-Channel oscilloscope analysis software in the embodiment of the present invention; Figure 3 Variation diagram of the exchangeable Cd content and P-wave velocity of soil samples with different water contents in Example 1 of the present invention; Figure 4 Linear fitting diagram of the exchangeable Cd content and P-wave velocity in Example 1 of the present invention; Figure 5 Variation diagram of the exchangeable Pb content and P-wave velocity of soil samples with different water contents in Example 2 of the present invention; Figure 6 Linear fitting diagram of the exchangeable Pb content and P-wave velocity in Example 2 of the present invention. Specific implementation manners

[0025] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] The present invention includes steps of soil sampling, soil sample drying, soil sample rewetting, soil sample molding, soil sample determination, wave velocity monitoring, and content prediction. The specific contents are as follows: A. Soil sampling: Collect soil samples from the heavy metal-polluted tillage layer at a depth of 3 - 50 cm from the ground surface, and then crush and sieve them for standby; B. Soil sample drying: Dry the sieved soil samples, then pack them in vacuum bags, seal them, and let them stand for 7 - 14 days; C. Soil sample rewetting: Divide the standing soil samples into several parts evenly, and spray pure water on each soil sample multiple times until the predetermined water content is reached; D. Soil sample molding: Pour each soil sample with the predetermined water content into the corresponding mold respectively, then horizontally oscillate the soil samples with the molds, and take them off for measurement after reaching the predetermined index; E. Soil sample determination: Use the soil basic physical and chemical property analysis method to measure the heavy metal concentration and exchangeable state content in each soil sample after oscillation; F. Wave velocity monitoring: Use an integrated acoustic monitoring system to measure the actual propagation time of P-waves passing through the same soil samples as in step E, and calculate the P-wave velocity corresponding to each soil sample. G. Content prediction: Based on the P-wave velocities of each soil sample and the exchangeable state content of heavy metals obtained above, establish a fitting linear model between the exchangeable state heavy metal content and the P-wave velocity. By dynamically simulating the influence of key soil variables on the exchangeable state content of soil heavy metals, accurately predict the key factors of pollution risk.

[0027] In step A, the collected soil samples are first removed of crop roots, branches, leaves and gravel, then crushed and ground and passed through a 2-mm soil sieve for standby.

[0028] In step B, the sieved soil samples are placed in an oven at 110 ± 2 °C and continuously dried for 10 h, then the soil samples are taken out and placed in a drying dish to cool to room temperature, and then sealed and left standing in a vacuum bag.

[0029] In step C, first calculate the mass of pure water required for each soil sample according to the predetermined moisture content corresponding to each soil sample, then divide the mass of the required pure water into 2 - 4 portions, and then spray the evenly divided pure water onto the corresponding soil samples at intervals of 10 - 12 h to obtain the corresponding soil samples with the predetermined moisture content.

[0030] Moisture content determination is also included in step C. The moisture content of the corresponding soil samples with the predetermined moisture content is measured to obtain the actual moisture content of the soil samples; the predetermined moisture content is 0 - 20%.

[0031] In step D, the mold is a cube-shaped lidless experimental mold made of polytetrafluoroethylene, polyamide fiber, polycarbonate or polymethyl methacrylate with dimensions of 40 × 40 × 40 mm or 50 × 50 × 50 mm; the oscillation frequency of horizontal oscillation is 110 ± 10 times / min and the amplitude is 40 mm. After the soil samples oscillate with the mold at room temperature for 10 min, they are taken off for testing.

[0032] In step E, the determination of heavy metal concentration in soil samples uses inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma optical emission spectrometer (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), graphite furnace atomic absorption spectrometry or flame atomic absorption spectrometry, and the determination of the exchangeable state heavy metal content in soil samples uses the Tessier five-step sequential extraction method.

[0033] The specific operation process of the determination by the Tessier five-step sequential extraction method is as follows: E10. pH determination: Take 5 g of the oscillated soil samples, add pure water and mix evenly according to the water-soil ratio of 2.5:1, and measure the pH value of the soil mixture. E20. Neutralization of the mixed solution I: When the pH of the soil mixed solution in E10 is > 7, add a Mg(CH3COO)2 solution with a pH of 6 and a concentration of 1.0 mol / L to the sample at a rate of 1 mL / min until a mixed solution I with a pH of 7 is formed. E30. Neutralization of the mixed solution II: When the pH of the soil mixed solution in E10 is < 7, add a MgOH2·MgCl2 solution with a pH of 8 and a concentration of 1.0 mol / L to the sample at a rate of 1 mL / min until a mixed solution II with a pH of 7 is formed. E40. Obtaining supernatant I: Add 5 mL of MgCl2 solution to the mixed solution I or the mixed solution II, shake for 1 - 2 h, then centrifuge for 5 - 10 min, and then let the centrifuged soil mixed solution stand for 30 min. Subsequently, filter it through a 0.45 μm filter to obtain the supernatant I to be measured for standby. E50. Obtaining supernatant II: Use the method of vacuum filtration to separate the residual liquid in the soil after centrifugation in E40. Wash the separated solid with 8 ml of deionized water and shake for 1 - 2 h, then centrifuge for 5 - 10 min and let it stand for 30 min. Combine all the liquids obtained from the solid - liquid separation in this step, then filter out the supernatant II through a 0.45 μm filter. Subsequently, repeat the above process in this step 3 times and then combine the supernatant II. E60. Determining the content of exchangeable heavy metals: Combine the supernatant I and the supernatant II, then filter through a 0.45 μm filter, and then use the aforementioned method for measuring the heavy metal concentration in the soil sample to measure the heavy metal concentration of the combined supernatant, which is the content of exchangeable heavy metals in the soil sample.

[0034] In the step E40, the added MgCl2 solution is shaken at 110 ± 10 rpm, an amplitude of 40 mm, and at 25 ± 0.5 °C for 1 - 2 h, and then centrifuged at 4500 r / min for 5 - 10 min.

[0035] In the step E50, the solid is washed with deionized water and shaken at 110 ± 10 rpm, an amplitude of 40 mm, and at 25 ± 0.5 °C for 1 - 2 h, and then centrifuged at 4500 r / min for 5 - 10 min and let it stand for 30 min.

[0036] The comprehensive acoustic monitoring system in step F includes the Multi-Channel oscilloscope analysis software of TiePie Company in the Netherlands, the engineering USB oscilloscope of TiePie Company in the Netherlands, the acoustic emission preamplifier of Beijing Ruandao Times Technology Co., Ltd., and the acoustic emission sensor of Hunan Enditi Technology Co., Ltd. The engineering USB oscilloscope emits a fixed signal with a frequency of 5 - 50 Hz and a pulse wave width of 1 - 10 μs. The acoustic emission preamplifier amplifies the signal by 40 dB and then propagates it in the soil sample, with an output noise of 2.1 mV. The resonant frequency of the acoustic emission sensor is 150 kHz, the sensitivity is 72 dB, and the frequency range is 80 - 350 kHz. The acoustic emission sensor detects the mechanical vibration propagated from the internal acoustic emission events of the soil sample to the surface and converts it into an electrical signal. The output voltage V(t, x) of the acoustic emission sensor is the convolution of the surface displacement wave U(x, t) and its response function T(t), that is, V(t, x) = U(t, x) · T(t).

[0037] The model of the engineering USB oscilloscope is Handyscope HS5, and the model of the acoustic emission sensor is GTR150a.

[0038] Measuring the P wave is recorded by the VALUE window of the Multi-Channel oscilloscope analysis software through the actual propagation time of the soil sample. To calculate the P wave velocity corresponding to each soil sample, use a vernier caliper to measure the distance s between the probe surfaces, and this distance s divided by the actual propagation time t is used to determine the P wave velocity Δ V Specifically, the expression is: Δ V = s / t.

[0039] In step G, the fitting linear model of the exchangeable heavy metal content and the P wave velocity is as follows: In the formula, ΔV is the P wave velocity; ΔEX is the exchangeable heavy metal content; a is the correlation coefficient. When a > 0, it indicates a positive correlation between ΔEX and ΔV; when a < 0, it indicates a negative correlation between ΔEX and ΔV; b is the P wave velocity of the soil without heavy metal pollution; a and b are both fitting coefficients, which are obtained by fitting the measured experimental results through the data analysis of Origin software.

[0040] In step G, the effect of dynamic simulation of key soil variables on the content of exchangeable heavy metals in soil is based on the response surface (RSM) of the key parameter of soil - the content of exchangeable heavy metals, and a first-order linear model (linear regression model) is constructed to accurately predict the key factors of pollution risk. The formula of the first-order linear model is: , where, Y is the content of exchangeable heavy metals in soil; X i is the key parameter of soil, and the key parameters include organic matter content, moisture content and cation content; β 0 is the intercept term; β i represents the regression coefficients to be estimated for each factor; ε is the random error term, and the larger the ∣ β ∣, the stronger the influence of this factor on the content of heavy metals in soil, and vice versa, the weaker the influence of this factor on the content of heavy metals in soil.

[0041] After step G, the proportion of the content of exchangeable heavy metals in the total concentration of heavy metals is also calculated to evaluate the heavy metal activity and potential release risk; when the total concentration of heavy metals is high but the proportion of the content of exchangeable heavy metals is low, the actual harm may be small; while when the total concentration of heavy metals is general but the proportion of the content of exchangeable heavy metals is high, the actual harm may be large.

[0042] Example 1

[0043] S100: Collect 600 g of soil samples from the cadmium (Cd)-polluted plough layer 3 cm below the ground surface. First, remove impurities such as crop roots, branches, leaves and gravel, and then crush, grind and sieve through a 2-mm soil sieve for standby.

[0044] S200: Place the sieved cadmium-polluted soil samples in an oven at 110 ± 2 °C and dry continuously for 10 h. Then take out the soil samples, place them in a drying dish and cool them to room temperature. Subsequently, put them into a vacuum bag, seal and let them stand for 14 d.

[0045] S300: Divide the cadmium-polluted soil samples after standing into 5 parts (numbered A - E). According to the predetermined moisture content corresponding to each soil sample, calculate the mass of pure water required for each, and then divide the mass of pure water required into 4 equal parts. Subsequently, spray different amounts of evenly divided pure water on the surface of the corresponding cadmium-polluted soil samples at intervals of 10 h to ensure uniform water distribution, and repeat four times to obtain 5 soil samples with actual moisture contents of 0%, 1%, 2%, 4% and 8% respectively.

[0046] S400: Pour each soil sample that has reached the predetermined moisture content into the corresponding mold (a cube-shaped lidless polytetrafluoroethylene with dimensions of 40×40×40 mm). Then, subject the soil sample along with the mold to horizontal oscillation at a frequency of 110±10 times / min and an amplitude of 40 mm at room temperature for 10 min, and then remove it. Subsequently, measure the actual moisture content of the 5 soil samples respectively according to the method of GB7172 - 1987. The results are shown in Table 1.

[0047] Table 1 Basic parameters of soil samples S500: Determine the Cd concentration in each soil sample after oscillation by inductively coupled plasma atomic emission spectrometry (ICP - AES); and determine the content of exchangeable Cd in each soil sample after oscillation by Tessier five - step sequential extraction method.

[0048] The specific operation process of the determination by Tessier five - step sequential extraction method is as follows: S510: Take 5 g of the soil sample after oscillation, add pure water according to a soil - water ratio of 2.5:1 and mix evenly, and measure the pH value of the soil mixture.

[0049] S520: When the pH of the soil mixture in S510 > 7, add Mg(CH3COO)2 solution with a pH of 6 and a concentration of 1.0 mol / L to the sample at a rate of 1 mL / min until a mixture solution Ⅰ with a pH of 7 is formed.

[0050] S530: When the pH of the soil mixture in S510 < 7, add MgOH2·MgCl2 solution with a pH of 8 and a concentration of 1.0 mol / L to the sample at a rate of 1 mL / min until a mixture solution Ⅱ with a pH of 7 is formed.

[0051] S540: Add 5 mL of MgCl2 solution to the mixture solution Ⅰ or mixture solution Ⅱ, oscillate it at 110±10 rpm, an amplitude of 40 mm and 25±0.5℃ for 1 h, then centrifuge it at 4500 r / min for 10 min, then let the centrifuged soil mixture stand for 30 min, and then filter it through a 0.45 - μm filter to obtain the supernatant Ⅰ to be measured for standby.

[0052] S550: Use the method of vacuum filtration to separate the residual liquid in the soil after centrifugation in S540. Wash the separated solid with 8 ml of deionized water, then oscillate it for 2 h under the conditions of 110 ± 10 rpm, an amplitude of 40 mm, and 25 ± 0.5 °C. Then centrifuge it at 4500 r / min for 5 min and let it stand for 30 min. Combine all the liquids obtained from the solid-liquid separation in this step, then filter out the supernatant II with a 0.45-μm filter. Subsequently, repeat the above process in this step 3 times and then combine the supernatant II.

[0053] S560: Combine the supernatant I and the supernatant II, then filter them with a 0.45-μm filter. Subsequently, use the aforementioned method for measuring the heavy metal concentration in the soil sample to measure the Cd concentration of the combined supernatant, which is the content of exchangeable Cd in the soil sample.

[0054] S600: Use the comprehensive acoustic monitoring system shown in Figure 1 and 2 (including the Multi-Channel oscilloscope analysis software of the Dutch company TiePie, the Handyscope HS5 of the Dutch company TiePie, the acoustic emission preamplifier of Beijing Soft Island Times Technology Co., Ltd., and the GTR150a of Hunan Enditi Technology Co., Ltd.) to measure the actual propagation time of the P-wave passing through the same soil sample in step S500, and calculate the P-wave velocity corresponding to each Cd-contaminated soil sample.

[0055] S700: Based on the P-wave velocities and the contents of exchangeable Cd of the foregoing soil samples, establish a fitting linear model of the content of exchangeable Cd and the P-wave velocity: where ΔV is the P-wave velocity; ΔEX is the content of exchangeable Cd; Then, by dynamically simulating the influence of the key variables of the soil on the content of exchangeable Cd in the soil heavy metals, accurately predict the key factors of the pollution risk. Among them, dynamically simulating the influence of the key variables of the soil on the content of exchangeable heavy metals in the soil is based on the response surface (RSM) of the key parameter of the soil - the content of exchangeable heavy metals in the soil, and constructing a first-order linear model (linear regression model) to accurately predict the key factors of the pollution risk. The formula of the first-order linear model is: , where Y is the content of exchangeable heavy metals in the soil; X i is the i th key parameter of the soil. The key parameters include the organic matter content, the moisture content, and the cation content; β 0 is the intercept term, which is obtained through the mean formula, that is ; β i is X i the corresponding regression coefficient; k is the total number of key soil parameters; ε is the random error term; ∣ β ∣ The larger it is, the stronger the influence of this factor on the content of heavy metals in soil, and vice versa, the weaker the influence of this factor on the content of heavy metals in soil.

[0056] Such as Figure 3 As shown in the change diagram of the exchangeable Cd content ΔEX and the P-wave velocity ΔV of soil samples with different water contents, it is not difficult to see that the P-wave velocity ΔV decreases with the increase of soil water content. The change of the P-wave velocity ΔV is affected by the combined action of water conditions and the content of exchangeable Cd in the soil. The change of water content will cause the species transformation of Cd, resulting in an increase in soil porosity, thus leading to a decrease in the P-wave velocity.

[0057] Such as Figure 4 As can be seen from the linear fitting diagram of the exchangeable Cd content ΔEX and the P-wave velocity ΔV, the determination coefficient (R 2 ) is 0.98, and the correlation coefficient is less than 0, indicating that there is an obvious negative correlation between the Cd content ΔEX and the P-wave velocity ΔV. This finding provides a theoretical basis for using the P-wave velocity as an index to monitor the content of exchangeable heavy metals in soil.

[0058] It should be noted that the proportion of the exchangeable Cd content ΔEX in the total Cd concentration is also calculated to evaluate the Cd activity and potential release risk; when the total Cd concentration is high but the proportion of the exchangeable Cd content is low, the actual harm may be small; while when the total Cd concentration is general but the proportion of the exchangeable Cd content is high, the actual harm may be large.

[0059] Example 2

[0060] S100: Collect 600 g of soil samples from a depth of 50 cm in the plow layer polluted by lead (Pb). First, remove impurities such as crop roots, branches, leaves, and gravel, and then crush, grind, and pass through a 2-mm soil sieve for standby.

[0061] S200: Place the sieved lead-polluted soil samples in an oven at 110 ± 2 °C and dry them continuously for 10 h. Then take out the soil samples, place them in a drying dish and cool them to room temperature, and then put them into a vacuum bag, seal them, and let them stand for 7 d.

[0062] S300: Divide the static lead-contaminated soil samples into 5 portions (numbered F - J). Calculate the mass of pure water required for each soil sample according to the corresponding predetermined water content, then divide the mass of the required pure water into 2 equal parts. Subsequently, spray different amounts of the evenly divided pure water onto the surface of the corresponding cadmium-contaminated soil samples at intervals of 12 h to ensure uniform water distribution, and repeat twice to obtain 5 soil samples with actual water contents of 0%, 1%, 3%, 5%, and 7% respectively.

[0063] S400: Pour each soil sample that has reached the predetermined water content into the corresponding mold (a 50×50×50 mm cube-shaped lidless polycarbonate mold). Then, horizontally oscillate the soil sample with the mold at an oscillation frequency of 110 ± 10 times / min and an amplitude of 40 mm at room temperature for 10 min and then remove it. Then, measure the actual water contents of the 5 soil samples respectively according to the method of GB7172 - 1987, and the results are shown in Table 2.

[0064] Table 2 Basic parameters of soil samples S500: Determine the Pb concentration in each soil sample after oscillation by inductively coupled plasma mass spectrometry (ICP-MS); and determine the content of exchangeable Pb in each soil sample after oscillation by the Tessier five-step sequential extraction method.

[0065] The specific operation process of the determination by the Tessier five-step sequential extraction method is as follows: S510: Take 5 g of the soil sample after oscillation, add pure water according to a soil-water ratio of 2.5:1 and mix evenly, and measure the pH value of the soil mixture.

[0066] S520: When the pH of the soil mixture in S510 > 7, drop the Mg(CH3COO)2 solution with a pH of 6 and a concentration of 1.0 mol / L into the sample at a rate of 1 mL / min until a mixture I with a pH of 7 is formed.

[0067] S530: When the pH of the soil mixture in S510 < 7, drop the MgOH2·MgCl2 solution with a pH of 8 and a concentration of 1.0 mol / L into the sample at a rate of 1 mL / min until a mixture II with a pH of 7 is formed.

[0068] S540: Add 5 mL of MgCl2 solution to mixture I or mixture II, oscillate at 110 ± 10 rpm, an amplitude of 40 mm, and 25 ± 0.5 °C for 2 h, then centrifuge at 4500 r / min for 5 min, then let the centrifuged soil mixture stand for 30 min, and then filter it through a 0.45 μm filter to obtain the supernatant I to be measured for standby.

[0069] S550: Separate the residual liquid in the soil after centrifugation in S540 by vacuum filtration. Wash the separated solid with 8 ml of deionized water, then oscillate it for 1 h under the conditions of 110 ± 10 rpm, an amplitude of 40 mm, and 25 ± 0.5 °C. Then centrifuge it at 4500 r / min for 10 min and let it stand for 30 min. Combine all the liquids obtained from the solid-liquid separation in this step, then filter out the supernatant II with a 0.45 μm filter. Subsequently, repeat the above process in this step 3 times and then combine the supernatant II.

[0070] S560: Combine the supernatant I and the supernatant II, then filter them with a 0.45 μm filter. Subsequently, measure the Pb concentration of the combined supernatant using the aforementioned method for measuring the heavy metal concentration in the soil sample, which is the content of exchangeable Pb in the soil sample.

[0071] S600: Use the comprehensive acoustic monitoring system shown in Figure 1 and 2 (including the Multi-Channel oscilloscope analysis software of TiePie Company in the Netherlands, the Handyscope HS5 of TiePie Company in the Netherlands, the acoustic emission preamplifier of Beijing Ruandao Times Technology Co., Ltd., and the GTR150a of Hunan Enditi Technology Co., Ltd.) to measure the actual propagation time of the P-wave passing through the same soil sample in step S500, and calculate the P-wave velocity corresponding to each Pb-contaminated soil sample.

[0072] S700: Based on the P-wave velocities, Pb concentrations, and exchangeable Pb contents of the soil samples obtained above, establish a fitting linear model between the exchangeable Pb content and the P-wave velocity: In the formula, ΔV is the P-wave velocity; ΔEX is the content of exchangeable Pb; Then, by dynamically simulating the influence of the key variables of the soil on the content of exchangeable Cd in soil heavy metals, accurately predict the key factors of pollution risk. Among them, dynamically simulating the influence of the key variables of the soil on the content of exchangeable heavy metals in soil is based on the response surface (RSM) of the key parameter of the soil - the content of exchangeable heavy metals, and construct a first-order linear model (linear regression model) to accurately predict the key factors of pollution risk. The formula of the first-order linear model is: , In the formula, Y is the content of exchangeable heavy metals in soil; X i is the i th key parameter of the soil. The key parameters include organic matter content, moisture content, and cation content; β0 is the intercept term, obtained through the mean formula, i.e., ; β i is X i the corresponding regression coefficient; k is the total number of key soil parameters; ε is the random error term; | β | The larger it is, the stronger the influence of this factor on the content of heavy metal forms in soil, and vice versa, indicating that the influence of this factor on the content of heavy metal forms in soil is weaker.

[0073] Such as Figure 5 As shown in the figure of the change of exchangeable Pb content ∆EX and P-wave velocity ∆V of soil samples with different water contents, it is not difficult to see that the P-wave velocity ΔV decreases with the increase of soil water content. The change of P-wave velocity ΔV is affected by the combined action of water conditions and the content of exchangeable Pb in soil. The change of water content will cause the species transformation of Pb, resulting in an increase in soil porosity, thus leading to a decrease in P-wave velocity.

[0074] Such as Figure 6 As can be seen from the linear fitting graph of exchangeable Pb content ΔEX and P-wave velocity ΔV, the determination coefficient (R 2 ) of the fitting equation is 0.92, and the correlation coefficient is less than 0, indicating that there is an obvious negative correlation between Pb content ΔEX and P-wave velocity ΔV. This finding further provides a theoretical basis for using P-wave velocity as an index to monitor the content of exchangeable heavy metals in soil.

[0075] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, and all should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for predicting the content of exchangeable heavy metals in soil based on P-wave velocity, characterized in that: It includes steps of soil sampling, soil sample drying, soil sample rewetting, soil sample molding, soil sample determination, wave velocity monitoring, and content prediction. The specific contents are as follows: A. Soil sampling: Collect soil samples from the heavy metal - polluted plough layer at a depth of 3 - 50 cm from the ground surface, and then crush and sieve them for standby; B. Soil sample drying: Dry the sieved soil samples, then seal them in vacuum bags and let them stand for 7 - 14 d; C. Soil sample rewetting: Divide the standing soil samples into several equal parts, and spray pure water on each soil sample in multiple times until the predetermined moisture content is reached; D. Soil sample molding: Pour each soil sample that has reached the predetermined moisture content into the corresponding mold, then horizontally oscillate the soil samples together with the molds, and take them off for testing after reaching the predetermined index; E. Soil sample determination: Use the methods for analyzing the basic physical and chemical properties of soil to determine the heavy metal concentration and exchangeable state content in each soil sample after oscillation; F. Wave velocity monitoring: Use a comprehensive acoustic monitoring system to measure the actual propagation time of P - waves passing through the same soil samples in step E, and calculate the P - wave velocity corresponding to each soil sample; G. Content prediction: Based on the P - wave velocities and exchangeable state contents of heavy metals of each soil sample obtained above, establish a fitting linear model between the exchangeable state heavy metal content and the P - wave velocity, and accurately predict the key factors of pollution risk by dynamically simulating the influence of key soil variables on the exchangeable state content of soil heavy metals.

2. The method for predicting the content of exchangeable heavy metals in soil based on the P-wave velocity according to claim 1, wherein: In step B, the sieved soil samples are continuously dried in an oven at 110 ± 2 °C for 10 h, then taken out and placed in a drying dish to cool to room temperature, and then sealed in vacuum bags and left standing.

3. The method for predicting the content of exchangeable heavy metals in soil based on the P-wave velocity according to claim 1, wherein: In step C, first calculate the mass of pure water required for each soil sample according to the corresponding predetermined moisture content, then divide the mass of the required pure water into 2 - 4 equal parts, and then spray the evenly divided pure water on the corresponding soil samples at intervals of 10 - 12 h to obtain the corresponding soil samples with the predetermined moisture content.

4. The method for predicting the content of exchangeable heavy metals in soil based on the P-wave velocity according to claim 3, wherein: Step C also includes moisture content determination, which is to measure the moisture content of the corresponding soil samples that have reached the predetermined moisture content to obtain the actual moisture content of the soil samples; the predetermined moisture content is 0 - 20%.

5. The method for predicting the content of exchangeable heavy metals in soil based on the P-wave velocity according to claim 1, wherein: In step D, the mold is a cube - shaped lidless experimental mold made of polytetrafluoroethylene, polyamide fiber, polycarbonate or polymethyl methacrylate with dimensions of 40×40×40 mm or 50×50×50 mm; the oscillation frequency of horizontal oscillation is 110 ± 10 times / min and the amplitude is 40 mm. The soil samples are oscillated with the molds at room temperature for 10 min and then taken off for testing.

6. The method for predicting the content of exchangeable heavy metals in soil based on P-wave velocity according to claim 1, wherein: In step E, the determination of heavy metal concentration in soil samples uses inductively coupled plasma atomic emission spectrometry, inductively coupled plasma optical emission spectrometer, inductively coupled plasma mass spectrometry, graphite furnace atomic absorption spectrometry or flame atomic absorption spectrometry, and the determination of exchangeable state heavy metal content in soil samples uses Tessier five - step sequential extraction method.

7. The method for predicting the content of exchangeable heavy metals in soil based on the P-wave velocity according to claim 6, wherein: The specific operation process of the determination by the Tessier five - step sequential extraction method is as follows: E10. pH determination: Take 5 g of the oscillated soil sample, add pure water according to the water - soil ratio of 2.5:1 and mix evenly, and measure the pH value of the soil mixture. E20. Neutralization of the mixed solution I: When the pH of the soil mixed solution in E10 is > 7, add a Mg(CH3COO)2 solution with pH = 6 and a concentration of 1.0 mol / L to the sample at a rate of 1 mL / min until a mixed solution I with pH = 7 is formed; E30. Neutralization of the mixed solution II: When the pH of the soil mixed solution in E10 is < 7, add a MgOH2·MgCl2 solution with pH = 8 and a concentration of 1.0 mol / L to the sample at a rate of 1 mL / min until a mixed solution II with pH = 7 is formed; E40. Obtaining supernatant I: Add 5 mL of MgCl2 solution to the mixed solution I or mixed solution II, shake for 1 - 2 h, then centrifuge for 5 - 10 min, and then let the centrifuged soil mixed solution stand for 30 min. Subsequently, filter it through a 0.45 μm filter to obtain the standby supernatant I to be measured; E50. Obtaining supernatant II: Use the method of vacuum filtration to separate the residual liquid in the soil after centrifugation in E40. Wash and shake the separated solid with 8 ml of deionized water for 1 - 2 h, then centrifuge for 5 - 10 min and let it stand for 30 min. Combine all the liquids obtained from the solid - liquid separation in this step, then filter out the supernatant II through a 0.45 μm filter. Subsequently, repeat the above process in this step 3 times and then combine the supernatant II; E60. Determining the content of exchangeable heavy metals: Combine the supernatant I and supernatant II, then filter through a 0.45 μm filter, and then use the aforementioned method for measuring the heavy metal concentration in the soil sample to measure the heavy metal concentration of the combined supernatant, which is the content of exchangeable heavy metals in the soil sample.

8. The method for predicting the content of exchangeable heavy metals in soil based on P-wave velocity according to claim 1, wherein: The comprehensive acoustic monitoring system in the F step includes the Multi - Channel oscilloscope analysis software of the Dutch TiePie company, the engineering USB oscilloscope of the Dutch TiePie company, the acoustic emission pre - amplifier of Beijing Soft Island Times Technology Co., Ltd., and the acoustic emission sensor of Hunan Enditi Technology Co., Ltd. The emission frequency of the engineering USB oscilloscope is a fixed signal with a frequency of 5 - 50 Hz and a pulse wave width of 1 - 10 μs. The acoustic emission pre - amplifier amplifies the signal by 40 dB and then propagates in the soil sample, with an output noise of 2.1 mV. The resonant frequency of the acoustic emission sensor is 150 kHz, the sensitivity is 72 dB, and the frequency range is 80 - 350 kHz. The acoustic emission sensor detects the mechanical vibration of the acoustic emission event inside the soil sample propagating to the surface and converts it into an electrical signal. The output voltage V(t,x) of the acoustic emission sensor is the convolution of the surface displacement wave U(x,t) and its response function T(t), that is, V(t,x)=U(t,x)·T(t).

9. The method for predicting the content of exchangeable heavy metals in soil based on the P-wave velocity according to any one of claims 1 to 8, characterized in that: In the G step, the fitting linear model of the content of exchangeable heavy metals and the P - wave velocity is: Where, ΔV is the P-wave velocity; ΔEX is the content of exchangeable heavy metals; a is the correlation coefficient. When a > 0, it indicates a positive correlation between ΔEX and ΔV; when a < 0, it indicates a negative correlation between ΔEX and ΔV; b is the P-wave velocity of soil without heavy metal pollution; a and b are both fitting coefficients, which are obtained by fitting the measured experimental results through data analysis of Origin software.

10. The method for predicting the content of exchangeable heavy metals in soil based on the P-wave velocity according to claim 9, wherein: In the step G, the impact of dynamic simulation of key soil variables on the content of exchangeable heavy metals in soil is based on the response surface of the key parameter of soil - the content of exchangeable heavy metals, and a first-order linear model is constructed to accurately predict the key factors of pollution risk. The formula of the first-order linear model is: ， Wherein, Y is the content of exchangeable heavy metals in soil; X i is the i th soil key parameter; ε is the random error term; β 0 is the intercept term; β i is the X i corresponding regression coefficient; k is the total number of soil key parameters.

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