Method for extracting exchangeable base ions from saline soil

Abstract

The application provides a method for extracting exchangeable base ions of saline soil, which comprises the following steps: placing the soil to be measured and a lithium chloride exchange solution on a magnetic stirrer to perform magnetic stirring, filtering and collecting the filtrate, and measuring the exchangeable base ions of the saline soil. Compared with the prior art, the application has the following advantages: high accuracy and strong anti-interference ability; the lithium chloride can effectively replace the ions on the soil colloids, and can reduce the dissolution promoting effect on associated salts; high-concentration ethanol significantly reduces the dissolution ability of water on ionic salts, and also reduces the dissolution power; the two are synergistic, which double inhibits the dissolution of associated salts from the aspects of thermodynamics and kinetics, eliminates the interference of dissolved salts, and realizes the direct and accurate measurement of exchangeable calcium, magnesium and potassium; the operation is simple, the efficiency is extremely high, the reagents and samples are saved, the calculation result of sodium ions is reliable, and the universality is strong.

Description

Technical Field

[0001] This application relates to the field of soil chemical analysis technology, specifically to a method for the extraction and determination of soil exchangeable cations. Background Technology

[0002] Saline soils are widely distributed in coastal areas and arid and semi-arid inland regions of my country. Accurate assessment of their exchangeable basic ion content (Ca²⁺) is crucial. + Mg² + K + Na + The content of exchangeable base ions, especially exchangeable sodium, is crucial for assessing the degree of soil salinization and guiding soil improvement and agricultural utilization. However, the accurate extraction of exchangeable base ions from saline soils has always been a technical challenge in soil analysis.

[0003] The core difficulty lies in the fact that saline soils typically contain a considerable amount of soluble salts (such as sodium chloride and sodium sulfate) and slightly soluble salts (such as calcium sulfate and calcium carbonate). When using traditional neutral salts (such as 1 mol L...),... -1 When performing exchangeable base ion extraction with an ammonium acetate solution (pH 7.0), these associated salts will partially or completely dissolve in the extract, releasing additional Ca²⁺. + Na + Plasma. This part, consisting of ions produced by the dissolution of salts (non-exchangeable state) mixed with ions exchanged from soil colloids (exchangeable state), results in a significantly higher measured content of exchangeable calcium and sodium, failing to reflect the true exchange capacity of the soil.

[0004] Existing technologies attempt to address this interference, but all have significant limitations:

[0005] Physical removal methods: For example, pre-rinsing with a water-ethanol solution to remove soluble salts. This process is not only cumbersome, but during rinsing, the water-ethanol solution may trigger the desorption or hydrolysis of exchangeable ions, causing Mg²⁺ ions to be lost. + K + Plasma loss leads to lower measured values.

[0006] Chemical correction method: for example, by measuring sulfate (SO4) in the filtrate. - The concentration of sulfate ions is used to infer the amount of dissolved calcium sulfate, thereby correcting the calcium content. However, this method cannot distinguish whether the sulfate ions originate from the dissolution of calcium sulfate or sodium sulfate, and it has no corrective ability for the dissolution of sodium chloride and calcium carbonate, thus its accuracy is limited.

[0007] Mineral quantification method: The content of gypsum (calcium sulfate) in soil is determined by methods such as X-ray diffraction (XRD). However, XRD has relatively low quantitative accuracy and cannot confirm whether these minerals can be completely dissolved during the extraction process, so it is difficult to use for accurate calibration.

[0008] Therefore, for saline soils, there is currently a lack of a standard, universally applicable method for extracting exchangeable base ions that can effectively suppress the interference of associated salt dissolution. Developing an extraction technology that can inhibit the dissolution process at its source and effectively distinguish between exchangeable and dissolved states has become a long-standing unresolved technical need in this field. Summary of the Invention

[0009] To address the aforementioned technical limitations, this application proposes a method for extracting exchangeable basic ions from saline soils. This method combines an extractant with specific compositions and optimized extraction conditions to significantly inhibit the dissolution of common sparingly soluble and slightly soluble salts in saline soils, such as calcium carbonate and calcium sulfate, during the extraction process, thereby achieving the extraction of exchangeable Ca²⁺ ions. + Mg² + K + The direct and accurate determination of ions, and the reliable calculation of exchangeable Na+ using the difference method. + Content; This method has the advantages of low reagent consumption, simple operation, and rapid extraction.

[0010] To achieve the above objectives, this application adopts the following technical solution:

[0011] The inventive point of this application is to provide a method for extracting exchangeable basic ions from saline soil, comprising the following steps:

[0012] S1: Weigh a sample of saline soil and place it in a centrifuge tube;

[0013] S2: Add a magnetic stir bar and lithium chloride-ethanol mixed exchange solution to the centrifuge tube;

[0014] S3: Place the centrifuge tube on a magnetic stirrer and stir magnetically. During the stirring process, maintain a vortex on the liquid surface and stir for a predetermined time to complete a single exchange reaction.

[0015] S4: Perform solid-liquid separation on the mixture after the exchange reaction, and collect the filtrate or supernatant as the test solution;

[0016] S5: Determine the concentrations of exchangeable calcium ions, magnesium ions, and potassium ions in the test solution;

[0017] S6: Calculate the exchangeable content of calcium, magnesium, and potassium ions in the test solution based on their concentrations, and calculate the exchangeable sodium ion content using the difference method: Exchangeable Na + Content = Cation Exchange Capacity (CEC) - (Exchangeable Ca) 2+ Content + Exchangeable Mg 2+ Content + Exchangeability K + content).

[0018] Optionally, in the above extraction method, in step S1, the soil sample is an air-dried soil sample that has passed through a sieve with a pore size of 0.20-0.30 mm, preferably 0.25 mm.

[0019] Optionally, in the above extraction method, in step S1, the sample weight of the saline soil sample is 0.20-0.30g, preferably 0.25g.

[0020] Optionally, in the above extraction method, the concentration of lithium chloride in the lithium chloride-ethanol mixed exchange solution in step S2 is 0.3-0.5 mol / L. -1 The volume concentration of ethanol is 70-80%; preferably, the concentration of lithium chloride is 0.4 mol / L. -1 The volume concentration of ethanol is 75%.

[0021] Optionally, in the extraction method described above, in step S2, the volume of lithium chloride-ethanol mixed exchange medium used for each centrifuge tube soil sample is 24-26 mL, preferably 25 mL.

[0022] Optionally, in the extraction method described above, the predetermined stirring time in step S3 is 4-6 minutes, preferably 5 minutes.

[0023] Optionally, in the extraction method described above, in step S3, the depth of the vortex is 2-5 mm or the stirring speed is 50-2500 r / min, preferably 1500 r / min.

[0024] Optionally, in the above extraction method, the exchange reaction in step S3 is a single exchange reaction.

[0025] Optionally, in the above extraction method, the solid-liquid separation in step S4 is filtration separation; preferably, medium-speed quantitative filter paper is used for filtration.

[0026] The second inventive point of this application is to provide a reagent for extracting exchangeable basic ions from saline soils, wherein the reagent is a lithium chloride-ethanol mixed solution, and the concentration of lithium chloride is 0.3-0.5 mol / L. -1 The volume concentration of ethanol is 70-80%; preferably, the concentration of lithium chloride is 0.4 mol / L. -1 The volume concentration of ethanol is 75%.

[0027] Compared with the prior art, this application has the following advantages:

[0028] 1. High accuracy and strong anti-interference ability: This invention creatively uses 0.4 mol L... - ¹Lithium chloride-75% ethanol solution, as an extractant, exerted a dual synergistic inhibitory effect.

[0029] The role of lithium chloride: Li + It has a small ionic radius, low hydration energy, and extremely weak hydrolysis, resulting in a near-neutral solution. As an exchange ion, it can effectively displace Ca²⁺ from soil colloids. + Mg² + K + Na + Meanwhile, due to its weak destructive effect on the salt lattice and the resulting common ion effect, it can minimize the dissolution-promoting effect on associated salts such as calcium carbonate and calcium sulfate.

[0030] The effect of high-concentration ethanol: Ethanol's dielectric constant is much lower than that of water. Adding 75% high-concentration ethanol to the extract greatly reduces the polarity of the system, thereby significantly reducing the solubility of water for ionic salts. Simultaneously, ethanol lowers the H₂O content. + The activity of H further inhibited H + It has a dissolving effect on salts such as calcium carbonate and calcium sulfate. The presence of ethanol also alters the interfacial properties between the solution and soil particles, reducing the dissolution kinetics.

[0031] The two work together to inhibit the dissolution of associated salts from both thermodynamic and kinetic perspectives, resulting in the extract mainly containing ions exchanged from the colloid, which essentially eliminates the interference of dissolved salts, thus enabling the direct and accurate determination of exchangeable Ca, Mg, and K.

[0032] 2. Simple operation and extremely high efficiency: This invention optimizes the extraction process into a single 5-minute magnetic stirring. Compared to the lengthy process of traditional methods that require repeated washing and centrifugation 3-5 times, this method reduces the number of steps by more than 80%, shortening the processing time for a single sample from tens of minutes to less than ten minutes. It is particularly suitable for scenarios involving large batches of samples, such as saline soil improvement and saline-alkali land survey.

[0033] 3. Reagent and sample savings: Only 25 mL of extraction solution and 0.25 g of soil sample are required per sample. Compared with traditional methods (which often require hundreds of milliliters of extraction solution and several grams of soil sample), this greatly saves expensive lithium chloride, ethanol reagents and sample consumption, which is in line with the green and economical detection concept.

[0034] 4. Reliability of sodium ion calculation: By directly and accurately measuring exchangeable Ca, Mg, and K, and utilizing the relatively stable and easily accurately measured parameter of soil-inherent cation exchange capacity (CEC), the difference method (CEC - Σ(Ca)) is used. + Mg + K + This method cleverly avoids the direct measurement of Na, calculating exchangeable Na. + The problem of being unable to distinguish between exchange and dissolved states due to interference from large amounts of soluble sodium chloride and sodium sulfate is solved, and the results are reliable.

[0035] 5. High applicability: As shown in the examples, this method shows good adaptability to saline soils containing different proportions and types of associated salts (calcium carbonate, calcium sulfate, sodium sulfate, sodium chloride), and the test results are stable and accurate, indicating that this method has wide applicability to a variety of saline soil types. Attached Figure Description

[0036] Figure 1 The figure shows the effect of different ethanol concentrations (0%, 60%, 75%) on the exchangeable Ca extraction value and pH value of the extract in standard saline soil SRS-5 in Experiment Example 1.

[0037] Figure 2 In Experiment 2, the method of the present invention (0.4 mol L) was used. -1 Comparison of the results of the determination of exchangeable basic ions extracted from standard saline soil SRS-5 using LiCl (75% ethanol) with the standard value range.

[0038] Figure 3 The figure shows the results of extracting exchangeable base ions using the method of this invention after adding different proportions (20%, 50%, 80%) of calcium carbonate to standard saline soil SRS-5 in Experiment Example 3.

[0039] Figure 4 The figure shows the results of extracting exchangeable base ions using the method of this invention after adding different proportions (20%, 50%, 80%) of calcium sulfate to standard saline soil SRS-5 in Experiment Example 4.

[0040] Figure 5 The figure shows the results of extracting exchangeable base ions using the method of this invention after adding different proportions (20%, 50%, 80%) of sodium sulfate to standard saline soil SRS-5 in Experiment Example 5.

[0041] Figure 6 The figure shows the results of extracting exchangeable base ions using the method of this invention after adding different proportions (20%, 50%, 80%) of sodium chloride to standard saline soil SRS-5 in Experiment Example 6. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this application clearer, a more detailed description is provided below. However, it should be understood that the description herein is merely for explaining this application and is not intended to limit its scope.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. All reagents and instruments used herein are commercially available, and the characterization methods involved can be found in relevant descriptions in the prior art, and will not be repeated here.

[0044] To further understand this application, the following detailed description is provided in conjunction with the preferred embodiments.

[0045] Example 1

[0046] (I) General steps of the method of the present invention

[0047] A method for extracting exchangeable base ions from saline soils, comprising the following general steps:

[0048] 1. Sample pretreatment: Take saline soil samples, air dry, remove plant roots and other impurities, grind, pass through a 0.25mm nylon sieve, and mix well for later use.

[0049] 2. Weighing the sample: Using a 0.01 g balance, accurately weigh 0.25 g of sieved and air-dried soil sample and place it in a 50 mL polyethylene centrifuge tube.

[0050] 3. Preparation of the extraction solvent: Accurately prepare a solution with a concentration of 0.4 mol / L. -1 A lithium chloride-75% ethanol (v / v) mixed solution was used as the exchange extraction solution. For example, a certain amount of LiCl was weighed and dissolved in an appropriate amount of deionized water, and then anhydrous ethanol was added to bring the volume to the required level, ensuring that the final ethanol concentration was 75%.

[0051] 4. Adding Liquid and Stirring: Place a clean PTFE magnetic stir bar into the centrifuge tube containing the soil sample. Accurately add 25 mL of the prepared lithium chloride-ethanol exchange solution using a graduated cylinder or pipette. Immediately place the centrifuge tube vertically on the magnetic stirrer. Start the stirrer, gradually increasing the speed until a clear and stable vortex is observed on the liquid surface inside the centrifuge tube, controlling the vortex depth within the range of 2-5 mm. Continue stirring for 5 minutes.

[0052] 5. Solid-liquid separation: After stirring, immediately remove the centrifuge tube. Filter the suspension using pre-prepared medium-speed quantitative filter paper (or by quickly pouring the supernatant after centrifugation). Collect the filtrate in a clean plastic bottle or test tube. This filtrate is the test solution A, used to determine Ca, Mg, and K.

[0053] 6. Ion Concentration Determination: The concentrations of Ca, Mg, and K in the test solution A were determined by atomic absorption spectrometry (AAS) or inductively coupled plasma optical emission spectrometry (ICP-OES) (unit: mg / L). -1 or μg mL -1 It is recommended to perform two parallel determinations for each sample.

[0054] 7. Subtraction of content calculation and sodium ion difference:

[0055] Based on the measured concentrations, extract volume (0.025 L), and soil sample mass (0.00025 kg), calculate the contents of exchangeable Ca, Mg, and K in the soil, typically expressed in cmol. c kg -1 .

[0056] Calculation of exchangeable Na: The difference method was used. First, the cation exchange capacity (CEC, unit: cmol) of the soil sample was accurately determined by other independent methods (e.g., determination of cation exchange capacity of forest soils LY / T 1243-1999, or a specific CEC method applicable to saline soils). c kg -1 Then, calculate using the following formula:

[0057] Exchangeable Na content (cmol) c kg -1 = CEC - (Exchangeable Ca content + Exchangeable Mg content + Exchangeable K content).

[0058] Experimental Example 1

[0059] Determination of suitable ethanol concentration

[0060] The certified standard saline soil sample SRS-5 was selected, with a pH of 8.35 ± 0.09 and a cation exchange capacity of 8.11 ± 0.75 cmol. c kg -1 The exchangeable values ​​of Ca, Mg, K, and Na were 3.45 ± 0.6, 3.75 ± 0.62, 1.18 ± 0.18, and 1.30 ± 0.40 cmol, respectively. c kg -1 Prepare 0.4 mol / L solutions with 0%, 60%, and 75% ethanol concentrations. -1 The exchangeable Ca, Mg, and K of SRS-5 were extracted with lithium chloride extract, and the exchangeable Na content was calculated by the difference method.

[0061] The measured values ​​of the four basic ions are as follows: Figure 1 As shown. By Figure 1As can be seen, for this standard saline soil, the measured value of exchangeable Ca showed a highly significant decreasing trend with increasing ethanol concentration. The measured values ​​at 0% and 60% concentrations were above the standard range, while the measured value at 75% concentration was within the standard range. When the ethanol concentration increased from 0% to 60%, the measured value decreased by 10.6 cmol. c kg -1 When the concentration increased from 60% to 75%, the measured value decreased by 8.88 cmol. c kg -1 The decreases were all extremely significant.

[0062] Depend on Figure 1 As shown in b, with increasing ethanol concentration, the pH value of the extract showed a highly significant increasing trend, rising significantly from 6.31 at 0% ethanol concentration to 6.70 at 60% ethanol concentration and 7.27 at 75% ethanol concentration, while the exchangeable Ca content showed a highly significant decreasing trend. The fitting equation between the pH of the extract and the exchangeable Ca is: y = -19.815x + 146.79 (coefficient of determination R). 2 =0.97, correlation coefficient r=0.985), indicating a highly significant negative correlation between the two. This suggests that H under ethanol concentration regulation... + Concentration is the main factor controlling the dissolution of calcium salts in saline soils. This indicates that, for lithium chloride as an extractant, the H+ concentration can be affected by adjusting the ethanol concentration. + The activity of ethanol and lithium chloride as extractants can inhibit the dissolution of calcium salts in saline soils. By synergistically controlling the concentration of ethanol and using lithium chloride as an extractant, accurate extraction of exchangeable Ca from saline soils can be achieved.

[0063] Experiment Example 2

[0064] Extraction of exchangeable base ions from standard saline soil

[0065] Certified saline soil sample SRS-5 was selected, and exchangeable basic ions were extracted and measured using the method proposed in this invention. The measured values ​​of the four basic ions are as follows: Figure 2 As shown, the exchangeable Ca, Mg, K, and Na values ​​are all within the standard range, indicating that the proposed method is suitable for the extraction of exchangeable basic ions from saline soils.

[0066] Experimental Example 3

[0067] Effect of calcium carbonate content on the extraction process

[0068] Certified standard saline soil sample SRS-5 was selected, and calcium carbonate was added at concentrations of 20%, 50%, and 80%. Exchangeable basic ions were extracted and measured using the method proposed in this invention, and the effect of calcium carbonate content on the extraction process was investigated. The measured values ​​of the four basic ions are shown below. Figure 3As shown, the exchangeable Ca, Mg, K, and Na values ​​are all within the standard range, indicating that the method proposed in this invention can effectively inhibit the dissolution of calcium carbonate during the extraction process and eliminate the impact caused by calcium carbonate dissolution.

[0069] Experiment Example 4

[0070] Effect of calcium sulfate content on the extraction process

[0071] Certified standard saline soil sample SRS-5 was selected, and calcium sulfate was added at concentrations of 20%, 50%, and 80%. Exchangeable basic ions were extracted and measured using the method proposed in this invention, and the effect of calcium sulfate content on the extraction process was investigated. The measured values ​​of the four basic ions are shown below. Figure 4 As shown, the exchangeable Ca, Mg, K, and Na values ​​are all within the standard range, indicating that the method proposed in this invention can effectively inhibit the dissolution of calcium sulfate during the extraction process and eliminate the impact caused by calcium sulfate dissolution.

[0072] Experimental Example 5

[0073] Effect of sodium sulfate content on the extraction process

[0074] Certified standard saline soil sample SRS-5 was selected, and sodium sulfate was added at concentrations of 20%, 50%, and 80%. Exchangeable basic ions were extracted and measured using the method proposed in this invention, and the effect of sodium sulfate content on the extraction process was investigated. The measured values ​​of the four basic ions are shown below. Figure 5 As shown, the exchangeability of Ca, Mg, K, and Na are all within the standard range, indicating that the method proposed in this invention can eliminate the influence caused by the dissolution of sodium sulfate during the extraction process.

[0075] Experimental Example 6

[0076] Effect of sodium chloride content on the extraction process

[0077] Certified standard saline soil sample SRS-5 was selected, and sodium chloride was added at concentrations of 20%, 50%, and 80%. Exchangeable basic ions were extracted and measured using the method proposed in this invention, and the effect of sodium chloride content on the extraction process was investigated. The measured values ​​of the four basic ions are shown below. Figure 6 As shown, the exchangeability of Ca, Mg, K, and Na are all within the standard range, indicating that the method proposed in this invention can eliminate the influence caused by sodium chloride dissolution during the extraction process.

[0078] In summary, the method proposed in this invention can accurately extract exchangeable basic ions from saline soils through the interaction between lithium chloride and ethanol. It features low reagent usage, simple operation, short extraction time, and accurate results. It is suitable for the rapid extraction of exchangeable basic ions from large quantities of saline soils and significantly improves work efficiency.

[0079] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for extracting exchangeable base ions from saline soil, characterized in that, Includes the following steps: S1: Weigh a sample of saline soil and place it in a centrifuge tube; S2: Add a magnetic stir bar and lithium chloride-ethanol mixed exchange solution to the centrifuge tube; S3: Place the centrifuge tube on a magnetic stirrer and stir magnetically. During the stirring process, maintain a vortex on the liquid surface and stir for a predetermined time to complete a single exchange reaction. S4: Perform solid-liquid separation on the mixture after the exchange reaction, and collect the filtrate or supernatant as the test solution; S5: Determine the concentrations of exchangeable calcium ions, magnesium ions, and potassium ions in the test solution; S6: Calculate the exchangeable content of calcium, magnesium, and potassium ions in the test solution based on their concentrations, and calculate the exchangeable sodium ion content using the difference method: Exchangeable Na + Content = Cation Exchange Capacity (CEC) - (Exchangeable Ca) 2+ Content + Exchangeable Mg 2+ Content + Exchangeability K + content).

2. The extraction method according to claim 1, characterized in that, In step S1, the soil sample is an air-dried soil sample that has passed through a sieve with a pore size of 0.20-0.30 mm, preferably 0.25 mm.

3. The extraction method according to claim 1, characterized in that, In step S1, the sample weight of the saline soil is 0.20-0.30g, preferably 0.25g.

4. The extraction method according to claim 1, characterized in that, In the lithium chloride-ethanol mixed exchange solution of step S2, the concentration of lithium chloride is 0.3-0.5 mol / L. -1 The volume concentration of ethanol is 70-80%; preferably, the concentration of lithium chloride is 0.4 mol / L. -1 The volume concentration of ethanol is 75%.

5. The extraction method according to claim 1, characterized in that, In step S2, the volume of lithium chloride-ethanol mixed exchange medium used for each centrifuge tube soil sample is 24-26 mL, preferably 25 mL.

6. The extraction method according to claim 1, characterized in that, In step S3, the predetermined stirring time is 4-6 minutes, preferably 5 minutes.

7. The extraction method according to claim 1, characterized in that, In step S3, the depth of the vortex is 2-5 mm or the stirring speed is 50-2500 r / min, preferably 1500 r / min.

8. The extraction method according to claim 1, characterized in that, In step S3, the exchange reaction is a single exchange reaction.

9. The extraction method according to claim 1, characterized in that, In step S4, the solid-liquid separation is filtration separation; preferably, medium-speed quantitative filter paper is used for filtration.

10. A reagent for extracting exchangeable base ions from saline soils, characterized in that, The reagent is a lithium chloride-ethanol mixed solution, wherein the concentration of lithium chloride is 0.3-0.5 mol / L. -1 The volume concentration of ethanol is 70-80%; preferably, the concentration of lithium chloride is 0.4 mol / L. -1 The volume concentration of ethanol is 75%.

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