Method for determining T2 cutoff value based on ion change characteristics of soil-water system

Abstract

The invention discloses a method for determining a T2 cutoff value based on soil-water system ion change characteristics. The method comprises the following steps: S1, measuring a T2 distribution curve of a sample; s2, quantitatively describing the potential change in the double electric layers; s3, analyzing ion change characteristics; s4, determining the thickness of an adsorption water film; and S5, determining a T2 cutoff value. According to the method, the T2 cutoff value of the test soil sample can be determined only by acquiring the surface electrification characteristic and the salt-containing condition of the target soil, the operation is simple and convenient, the physical significance is clear, and the accuracy is high.

Description

Technical Field

[0001] The present invention relates to the field of geotechnical engineering technology, and in particular to a method for determining a T2 cutoff value based on ion change characteristics of a soil-water system. Background Art

[0002] The migration and phase change of water in geotechnical media are the key to various engineering problems. Water has a high hydrogen content and has a strong signal and sensitivity in nuclear magnetic resonance. Therefore, nuclear magnetic resonance technology, which can accurately detect water content and distribution characteristics, is gradually being introduced to analyze and solve water problems in geotechnical engineering.

[0003] During the NMR test, the macroscopic magnetization vector will deflect and generate out-of-plane components until the RF field ends, and the system gradually returns to its original state. When the system recovers, the receiving coil continues to monitor and outputs the detected attenuation signal (i.e., FID curve), the peak value of which is proportional to the moisture content. The distribution characteristics of moisture can be described by the T2 spectrum curve, and the peak area in different T2 intervals reflects the moisture content in each distribution area. The T2 cutoff value, as the boundary value between bound fluid and movable fluid, is the prerequisite for the NMR method to distinguish the moisture form of soil and determine the moisture content.

[0004] For the T2 cutoff value, researchers in the past could only observe and obtain the value by step-by-step temperature control or applying staged suction. This is not only cumbersome and time-consuming, but also overly dependent on test conditions and equipment factors, with large errors, and cannot meet the actual needs of the majority of researchers. Therefore, there is an urgent need for a method that can quickly and non-destructively determine the T2 cutoff value of soil adsorbed water and capillary water, so as to break through the many limitations of moisture boundary experimental research, improve the deficiencies of existing studies on the boundary between adsorbed water and capillary water, and thus achieve rapid differentiation of soil moisture forms and accurate calculation of content, providing experimental support for further revealing the intrinsic relationship between salt conditions, surface charge characteristics, and soil water holding capacity. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system. On the basis of quantifying the ion change characteristics of the soil-water system, it is only necessary to obtain the surface charge characteristics and salt conditions of the target soil to quickly and non-destructively determine the T2 cutoff value of the test soil sample, thereby realizing the rapid distinction of soil moisture forms and accurate calculation of the content.

[0006] To achieve the above objectives, the method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system described in the present invention comprises the following steps:

[0007] S1. Determine the T2 distribution curve of the sample.

[0008] S2. Quantitative description of the potential change in the double layer:

[0009] The total negative surface charge (σ) of a clay mineral particle is balanced by the sum of the charges of the Stern and Gouy layers:

[0010]

[0011] Where N i is the number of adsorption sites available per square meter on the surface of clay particles, N i ≈10 19 / m 2 ;M w is the molar weight of the electrolyte (kg / mol); ν is the density of the solvent (kg / m 3 ); n0 is the molar concentration of ions in the solution (mol / m 3 );Φ is the specific adsorption potential (J), Φ=0; is the Stern potential (V); z is the ion chemical valence; F is the Faraday constant, F = 9.6487 × 10 4 C / mol; R is the gas constant, which is 8.314 J / (mol·K); T is the absolute temperature (K); K m is the relative dielectric constant of the solvent medium; ∈0 is the dielectric constant of vacuum, 8.854×10 -12 C 2 / (J·m) or farad per meter (F / m), the dielectric constant of the solution is considered to be uniform everywhere.

[0012] Electric potential The distance x from the particle surface satisfies:

[0013] Stern layer (x≤d),

[0014]

[0015] Gouy layer (x>d),

[0016]

[0017] Wherein, d is the distance from the surface of the clay particle to the outer surface of the Stern layer (m), d = 0.5 ~ 0.6nm.

[0018] S3. Analysis of ion change characteristics:

[0019] In most ionic solutions, half of the ions are cations and the other half are anions. The closer to the surface of the clay particles, the more this ratio changes due to the influence of the surface electric field. The ion concentration at any interface position (n + ) all satisfy:

[0020] n+ (x)+n - (x) = 2n (5)

[0021] Where n represents the average concentration of anions and cations, mol / m 3 .

[0022] Considering that the Boltzmann density function that describes the change of ion concentration alone has a normalization factor, which is difficult to determine, the ion concentration at different positions is described in the form of a ratio. In order to facilitate the application of the Boltzmann distribution to describe the ion dispersion in the double layer, the potential energy of the Boltzmann distribution exponential term can be corresponded to:

[0023]

[0024] Where k is the Boltzmann constant, 1.38×10 -23 J / K; and are the electric potentials at position B and position A respectively. It is assumed that only the electric field force does work in this process.

[0025] The charges of anions and cations in any salt solution must be equal (Q + =-Q - ), if the anions at a plane x in the diffusion layer are replaced by cations at a plane at infinity, the total work done by the ion exchange process is It can be expressed as:

[0026]

[0027] The ratios of anion and cation concentrations at different positions A and B are:

[0028]

[0029] Further,

[0030]

[0031] x B Considered as infinitely far away, then:

[0032] n + (x B )=n - (x B ) (11)

[0033]

[0034] Then formula (10) can be generalized as:

[0035]

[0036] If only the exchange of 1 mol of ions is considered, equation (13) can be further transformed into:

[0037] Monovalent salt ions:

[0038] Divalent salt ions:

[0039] Where F is the Faraday constant, F = 9.6487 × 10 4 C / mol; R is the gas constant, which is 8.314 J / (mol·K); k is the Boltzmann constant, which is 1.38×10 -23 J / K.

[0040] Combining equations (5), (14), and (15), we can see that:

[0041] Monovalent salt ions:

[0042]

[0043] Divalent salt ions:

[0044]

[0045] S4. Determine the thickness of the adsorbed water film:

[0046] Charge density Q(x)(C / m 3 ) can be expressed as:

[0047] Monovalent salt ions:

[0048]

[0049] Divalent salt ions:

[0050]

[0051] Since the electric potential at infinity is generally 0 (i.e. ), therefore, formula (20) and (21) can be simplified as: 1-valent salt ion:

[0052]

[0053] Divalent salt ions:

[0054]

[0055] Then the adsorbed water film thickness h(m) can be expressed as:

[0056]

[0057] Monovalent salt ions:

[0058]

[0059] Divalent salt ions:

[0060]

[0061] In the formula, l u Represents the zero point (m) of the charge density Q(x) variation curve.

[0062] S5. Determine T2 cutoff value:

[0063] There is the following relationship between the transverse relaxation time T2 (ms) and the aperture r (m):

[0064]

[0065] Where ρ2 is the transverse relaxation rate, which is a factor of the transverse surface relaxation intensity (m / ms) and changes with the change of paramagnetic ions on the surface of soil particles. Usually, the value of ρ2 is between 10 -9 ~10 -8 m / ms; S / V is the ratio of the surface area to the volume of the pores; F s is the shape factor of the pore. For plate-like pores, F s =1.

[0066] The maximum pore radius filled entirely by adsorbed water is the boundary pore diameter between adsorbed water and capillary water (h = r), so the T2 cutoff value satisfies the following relationship:

[0067] Monovalent salt ions:

[0068] Divalent salt ions:

[0069] Starting from the double layer effect on the surface of clay particles, the present invention first defines the Boltzmann form of the ion concentration ratio through the concept of ion exchange work, and then uses the ion balance condition to quantitatively describe the change law of ion concentration. Then, the expression of the adsorbed water film thickness considering the charged characteristics of the soil particle surface and the salt conditions is derived, and on this basis, the adsorbed water film thickness is associated with the critical pore size, and the T2 cutoff value that can distinguish the moisture category and content of the soil is further determined.

[0070] Compared with the prior art, the present invention has the following advantages:

[0071] 1. The present invention can obtain the T2 cutoff value of the sample by measuring the T2 distribution curve through nuclear magnetic resonance test, abandoning the previous research mode that can only observe and obtain values ​​through step-by-step temperature control or applying staged suction, breaking through the limitations of experimental conditions and equipment, and greatly simplifying the operation process.

[0072] 2. The present invention takes into account the surface charge characteristics of soil particles and the salt conditions, and can determine the T2 cutoff value of adsorbed water and capillary water in any saline soil. It has clear physical meaning and high accuracy, and has great application value for further studying the theoretical model of soil adsorbed water content. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 The present invention is a flow chart of the method.

[0074] Figure 2 is the T2 cutoff value of adsorbed water and capillary water in different saline soils. DETAILED DESCRIPTION

[0075] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited to the following.

[0076] like Figure 1 As shown, a method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system includes the following steps:

[0077] S1. Determine the T2 distribution curve of the sample.

[0078] S2. Quantitative description of the potential change in the double layer:

[0079] The total negative surface charge (σ) of a clay mineral particle is balanced by the sum of the charges of the Stern and Gouy layers:

[0080]

[0081] Where N i is the number of adsorption sites available per square meter on the surface of clay particles, N i ≈10 19 / m 2 ;M w is the molar weight of the electrolyte (kg / mol); v is the density of the solvent (kg / m 3 ); n0 is the molar concentration of ions in the solution (mol / m 3 );Φ is the specific adsorption potential (J), Φ=0; is the Stern potential (V); z is the ion chemical valence; F is the Faraday constant, F = 9.6487 × 10 4 C / mol; R is the gas constant, which is 8.314 J / (mol·K); T is the absolute temperature (K); K m is the relative dielectric constant of the solvent medium; ∈0 is the dielectric constant of vacuum, 8.854×10 -12 C 2 / (J·m) or farad per meter (F / m), the dielectric constant of the solution is considered to be uniform everywhere.

[0082] Electric potential The distance x from the particle surface satisfies:

[0083] Stern layer (x≤d),

[0084]

[0085] Gouy layer (x>d),

[0086]

[0087] Wherein, d is the distance from the surface of the clay particle to the outer surface of the Stern layer (m), d = 0.5 ~ 0.6nm.

[0088] S3. Analysis of ion change characteristics:

[0089] In most ionic solutions, half of the ions are cations and the other half are anions. The closer to the surface of the clay particles, the more this ratio will change due to the influence of the surface electric field. The ion concentration (n + ) all satisfy:

[0090] n + (x)+n - (x) = 2n (5)

[0091] Where n represents the average concentration of anions and cations, mol / m 3 .

[0092] Considering that the Boltzmann density function that describes the change of ion concentration alone has a normalization factor, which is difficult to determine, the ion concentration at different positions is described in the form of a ratio. In order to facilitate the application of the Boltzmann distribution to describe the ion dispersion in the double layer, the potential energy of the Boltzmann distribution exponential term can be corresponded to:

[0093]

[0094] Where k is the Boltzmann constant, 1.38×10 -23 J / K; and are the electric potentials at position B and position A respectively. It is assumed that only the electric field force does work in this process.

[0095] The charges of anions and cations in any salt solution must be equal (Q + =-Q - ), if the anions at a plane x in the diffusion layer are replaced by cations at a plane at infinity, the total work done by the ion exchange process is It can be expressed as:

[0096]

[0097] The ratios of anion and cation concentrations at different positions A and B are:

[0098]

[0099] Further,

[0100]

[0101] x B Considered as infinitely far away, then:

[0102] n + (x B )=n - (x B ) (11)

[0103]

[0104] Then formula (10) can be generalized as:

[0105]

[0106] If only the exchange of 1 mol of ions is considered, equation (13) can be further transformed into:

[0107] Monovalent salt ions:

[0108] Divalent salt ions:

[0109] Where F is the Faraday constant, F = 9.6487 × 10 4 C / mol; R is the gas constant, which is 8.314 J / (mol·K); k is the Boltzmann constant, which is 1.38×10 -23 J / K.

[0110] Combining equations (5), (14), and (15), we can see that:

[0111] Monovalent salt ions:

[0112]

[0113] Divalent salt ions:

[0114]

[0115] S4. Determine the thickness of the adsorbed water film:

[0116] Charge density Q(x)(C / m 3 ) can be expressed as:

[0117] Monovalent salt ions:

[0118]

[0119] Divalent salt ions:

[0120]

[0121] Since the electric potential at infinity is generally 0 (i.e. ), therefore, formula (20) and (21) can be simplified as: 1-valent salt ion:

[0122]

[0123] Divalent salt ions:

[0124]

[0125] Then the adsorbed water film thickness h(m) can be expressed as:

[0126]

[0127] Monovalent salt ions:

[0128]

[0129] Divalent salt ions:

[0130]

[0131] In the formula, l u Represents the zero point (m) of the charge density Q(x) variation curve.

[0132] S5. Determine T2 cutoff value:

[0133] There is the following relationship between the transverse relaxation time T2 (ms) and the aperture r (m):

[0134]

[0135] Where ρ2 is the transverse relaxation rate, which is a factor of the transverse surface relaxation intensity (m / ms) and changes with the change of paramagnetic ions on the surface of soil particles. Usually, the value of ρ2 is between 10 -9 ~10 -8 m / ms; S / V is the ratio of the surface area to the volume of the pores; F s is the shape factor of the pore. For plate-like pores, F s =1.

[0136] The maximum pore radius filled entirely by adsorbed water is the boundary pore diameter between adsorbed water and capillary water (h = r), so the T2 cutoff value satisfies the following relationship:

[0137] Monovalent salt ions:

[0138] Divalent salt ions:

[0139] Embodiment 1:

[0140] The original loess in Yan'an, Shaanxi Province is used as an example for experimental research. Its physical and mechanical properties are shown in Table 1. Five pore solution samples with different NaCl contents (1000, 2000, 3000, 4000, 5000 mol / m 3 ), and placed in a polytetrafluoroethylene test tube to measure its T2 response spectrum curve. In order to distinguish and define the adsorbed water content of the five different salt-containing samples, the T2 curves of the test soil samples at each stage of the heating process were first measured by the NMR freeze-thaw cycle system. By analyzing the change law of the horizontal axis drop point of the signal peak, the T2 cutoff value was determined to be 1.65ms.

[0141] Table 1 Physical properties of experimental loess

[0142]

[0143] Based on the above physical and mechanical characteristics of the soil samples, the T2 cutoff values ​​of the above five salt distribution ratio samples can be determined using the method of the present invention. The results are as follows: Figure 2 shown. Figure 2 In, T 2l Indicates the left boundary of the determined interval, T 2r It indicates the right boundary of the determination interval, and thus it can be seen that the determination interval of the T2 cutoff value basically envelops the true value. Through error analysis, the maximum difference between the determination result and the true value is only 0.61ms, as shown in Table 2. It can be seen that for different saline soils, the T2 cutoff values ​​of adsorbed water and capillary water can be efficiently and accurately predicted by the method of the present invention.

[0144] Table 2 Error summary

[0145]

[0146] Table 3 T2 cutoff value for judging soil moisture content

[0147]

[0148] In summary, the calculation method of the present invention is easy to operate, has clear physical meaning, high accuracy, and wide applicability. It can meet the prediction requirements of the T2 cutoff value of adsorbed water and capillary water in saline soils, and can be further promoted and applied to the study of adsorbed water content in soils.

[0149] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system, characterized in that: The following steps are involved: S1. Determine the T2 distribution curve of the sample; S2. Quantitatively describe the potential changes within the double layer; S3. Analyze the ion change characteristics; S4. Determine the thickness of the adsorbed water film; S5. Determine the T2 cutoff value.

2. The method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system according to claim 1, characterized in that: The step S2 comprises: The total negative surface charge (σ) of a clay mineral particle is balanced by the sum of the charges of the Stern and Gouy layers: Where N i is the number of adsorption sites available per square meter on the surface of clay particles, N i ≈10 19 m 2 ;M w is the molar weight of the electrolyte (kg / mol); v is the density of the solvent (kg / m 3 ); n0 is the molar concentration of ions in the solution (mol / m 3 );Φ is the specific adsorption potential (J), Φ=0; is the Stern potential (V); z is the ion chemical valence; F is the Faraday constant, F = 9.6487 × 10 4 C / mol; R is the gas constant, which is 8.314 J / (mol·K); T is the absolute temperature (K); K m is the relative dielectric constant of the solvent medium; ∈0 is the dielectric constant of vacuum, 8.854×10 -12 C 2 / (J·m) or farad per meter (F / m), the dielectric constant of the solution is considered uniform; Electric potential The distance x from the particle surface satisfies: Stern layer (x≤d), Gouy layer (x>d), Wherein, d is the distance from the surface of the clay particle to the outer surface of the Stern layer (m), d = 0.5 ~ 0.6nm.

3. The method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system according to claim 1, characterized in that: The step S3 comprises: In most ionic solutions, half of the ions are cations and the other half are anions. The closer to the surface of the clay particles, the more this ratio changes due to the influence of the surface electric field. The ion concentration at any interface position (n + ) all satisfy: n + (x)+n - (x)=2n (5) Where n represents the average concentration of anions and cations, mol / m 3 ; Considering that the Boltzmann density function that describes the change of ion concentration alone has a normalization factor, which is difficult to determine, the ion concentration at different positions is described in the form of a ratio. In order to facilitate the application of the Boltzmann distribution to describe the ion dispersion in the double layer, the potential energy of the Boltzmann distribution exponential term can be corresponded to: Where k is the Boltzmann constant, 1.38×10 -23 J / K; and are the electric potentials at position B and position A respectively, assuming that only the electric field force does work in this process; The charges of anions and cations in any salt solution must be equal (Q + =-Q - ), if the anions at a plane x in the diffusion layer are replaced by cations at a plane at infinity, the total work done by the ion exchange process is It can be expressed as: The ratios of anion and cation concentrations at different positions A and B are: Further, x B Considered as infinitely far away, then: n + (x B )=n - (x B ) (11) Then formula (10) can be generalized as: If only the exchange of 1 mol of ions is considered, equation (13) can be further transformed into: Monovalent salt ions: Divalent salt ions: Where F is the Faraday constant, F = 9.6487 × 10 4 C / mol; R is the gas constant, which is 8.314 J / (mol·K); k is the Boltzmann constant, which is 1.38×10 -23 J / K; Combining equations (5), (14), and (15), we can see that: Monovalent salt ions: Divalent salt ions:

4. The method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system according to claim 1, characterized in that: The step S4 comprises: Charge density Q(x)(C / m 3 ) can be expressed as: Monovalent salt ions: Divalent salt ions: Since the electric potential at infinity is generally 0 (i.e. ), therefore, equations (20) and (21) can be simplified as: Monovalent salt ions: Divalent salt ions: Then the adsorbed water film thickness h(m) can be expressed as: Monovalent salt ions: Divalent salt ions: In the formula, l u Represents the zero point (m) of the charge density Q(x) variation curve.

5. The method for determining the T2 cutoff value based on the ion change characteristics of the soil-water system according to claim 1, characterized in that: The step S5 comprises: There is the following relationship between the transverse relaxation time T2 (ms) and the aperture r (m): Where ρ2 is the transverse relaxation rate, which is a factor of the transverse surface relaxation intensity (m / ms) and changes with the change of paramagnetic ions on the surface of soil particles. Usually, the value of ρ2 is between 10 -9 ~10 -8 m / ms; S / V is the ratio of the surface area to the volume of the pores; F s is the shape factor of the pore. For plate-like pores, F s =1; The maximum pore radius filled entirely by adsorbed water is the boundary pore diameter between adsorbed water and capillary water (h = r), so the T2 cutoff value satisfies the following relationship: Monovalent salt ions: Divalent salt ions: The T2 cutoff value of the target soil is thus determined.

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