A method for predicting cation exchange capacity based on changes in soil hygroscopic water content

Abstract

This invention discloses a method for predicting cation exchange capacity (CEC) based on changes in soil hygroscopic water content. The method comprises the following steps: S1: Determining the water activity range based on the soil water vapor adsorption mechanism; S2: Determining two reagents based on the water activity range determined in S1 and the safety and cost considerations of using saturated salt solutions to control water activity, and determining the specific water activities of the two reagents; S3: Selecting multiple samples and determining the soil CEC using conventional methods, while simultaneously determining the soil hygroscopic water content of the samples at the specific water activities of the reagents determined in S2; S4: Establishing a predictive model between the change in hygroscopic water content and CEC within the specific water activity range; S5: Determining the hygroscopic water content of the sample under test at the two specific water activities and calculating the change in hygroscopic water content, then using the predictive model to calculate the CEC of the soil sample. This method can quickly and accurately predict soil CEC when dealing with a large number of samples, saving time, manpower, and testing costs.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of soil surface chemical property prediction, and relates to a method for predicting cation exchange capacity based on the change amount of soil hygroscopic water content. BACKGROUND

[0002] Soil cation exchange capacity refers to the centimole number of all exchangeable cations contained in per kilogram of soil at a certain pH value. It directly reflects the ability of soil to store, supply and buffer cation nutrients, and is closely related to many physical and chemical properties of soil (for example, adsorption and migration of nutrients and pollutants, swelling and shrinking, etc.). Therefore, the cation exchange capacity is often used as an evaluation index of soil quality and soil fertility and an important basis for soil fertilization, improvement, etc.

[0003] At present, the commonly used methods for determining soil cation exchange capacity include neutral ammonium acetate method and barium chloride-sulfuric acid forced exchange method. These methods have the disadvantages of complicated steps and time-consuming, and the determination of cation exchange capacity by the barium chloride-sulfuric acid forced exchange method will also produce some hazardous waste. When the sample amount is large, it is time-consuming, laborious and high-cost to directly determine the soil cation exchange capacity by using the traditional method. Therefore, it is very necessary to design a simple, rapid and economical method for determining the cation exchange capacity.

[0004] Hygroscopic water is a very thin water layer on the surface of soil clay minerals maintained by physical and chemical effects. Cation hydration is an important mechanism for soil to adsorb water molecules, so the soil cation exchange capacity is closely related to the soil hygroscopic water content. Considering that the soil hygroscopic water content is easy to measure and has low cost, some studies have proposed a model for predicting the soil cation exchange capacity using the hygroscopic water content (Arthur E. Rapid estimation of cation exchange capacity from soil water content [J]. European Journal of Soil Science, 2017, 68(3), 365-373). These studies analyze the relationship between the soil hygroscopic water content and the cation exchange capacity under a certain water activity condition, and establish a model for predicting the soil cation exchange capacity using the hygroscopic water content under this water activity condition. Since the soil hygroscopic water content changes with the water activity, the model established under different water activity conditions is different. The relationship between the soil hygroscopic water content and the water activity can be quantified by the soil water vapor adsorption isotherm. Since the factors affecting soil water vapor adsorption are different under different water activity conditions, the accuracy of the model for predicting the cation exchange capacity using the hygroscopic water content will also differ. However, previous studies have not considered this factor in the established model. Therefore, it is necessary to establish a new cation exchange capacity prediction model based on the main factors controlling the soil hygroscopic water content under different water activity conditions. SUMMARY

[0005] The purpose of the present application is to provide a method for predicting the cation exchange capacity based on the change in soil hygroscopic water content, which solves the problems of complicated steps and time-consuming in the prior art.

[0006] The technical solution adopted by the present application is a method for predicting the cation exchange capacity based on the change in soil hygroscopic water content, which is implemented according to the following steps:

[0007] S1: Determine the water activity range according to the soil water vapor adsorption mechanism;

[0008] S2: Determine two reagents and their specific water activities according to the water activity range determined in S1 and the safety and cost of using saturated salt solution to control water activity;

[0009] S3: Select multiple samples and determine the soil cation exchange capacity using the traditional method, and simultaneously determine the soil hygroscopic water content of the samples under the specific water activity of the reagents determined in S2;

[0010] S4: Establish a prediction model between the change in hygroscopic water content and the cation exchange capacity under a specific water activity range;

[0011] S5: measuring the hygroscopic water content of the sample to be tested at two specific water activities, calculating the change in hygroscopic water content, and calculating the cation exchange capacity of the soil sample to be tested using the predictive model between the cation exchange capacity and the change in hygroscopic water content.

[0012] The application is also characterized in that:

[0013] The specific process of S1 is: selecting a plurality of soil samples of different mineral types and clay contents, measuring the water vapor adsorption isotherm and cation exchange capacity of the soil sample, calculating the correlation between the water vapor adsorption isotherm slope and the cation exchange capacity at different water activities, analyzing the change of the determination coefficient with the water activity, and determining the water activity range according to the change of the determination coefficient.

[0014] The water vapor adsorption analyzer and the traditional neutral ammonium acetate method or barium chloride-sulfuric acid forced exchange method are used to measure the water vapor adsorption isotherm and cation exchange capacity of the soil sample, respectively.

[0015] The reagents in S2 are potassium acetate and sodium bromide, and the specific water activities of potassium acetate and sodium bromide are 0.23 and 0.57, respectively.

[0016] The specific process of measuring the soil hygroscopic water content in S3 is: first, the cation exchange capacity of the sample is measured by the traditional method; at the same time, the sample is placed in the saturated salt solution containing the reagent in S2, and the hygroscopic water content of the soil in the specific water activity in S2 is measured after equilibrium.

[0017] The number of samples in S3 is not less than 20.

[0018] The specific process of S4 is: calculating the change in hygroscopic water content in the specific water activity range, and establishing a cation exchange capacity prediction model with the change in hygroscopic water content as the input variable by regression method.

[0019] The prediction model can use the mean absolute error MAE and the root mean square error RMSE formula to evaluate the simulation accuracy.

[0020] The formula of the mean absolute error MAE is:

[0021]

[0022] In the formula, P represents the predicted value, O represents the measured value, and n represents the number of samples.

[0023] The formula of the root mean square error RMSE is:

[0024]

[0025] In the formula, P represents the predicted value, O represents the measured value, and n represents the number of samples.

[0026] The method for predicting cation exchange capacity based on the change amount of soil hygroscopic water content of the present application, by analyzing the relationship between soil hygroscopic water content and cation exchange capacity under a certain water activity condition, establishes a prediction model for predicting soil cation exchange capacity based on hygroscopic water content, and further realizes the determination of cation exchange capacity, solves the problems of existing determination methods such as complicated steps, time-consuming and labor-consuming, can quickly and accurately predict the soil cation exchange capacity, saves time, manpower and test cost. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is the graph of the determination coefficient between the slope of the water vapor adsorption isotherm and the cation exchange capacity of the present application with the change of water activity;

[0028] Figure 2 is the cation exchange capacity prediction model established by the hygroscopic water content of the water activity in the range of 0.23-0.57 in the embodiment of the present application;

[0029] Figure 3 is the cation exchange capacity prediction model established by the hygroscopic water content of the water activity in the range of 0.23-0.57 in the embodiment of the present application;

[0030] Figure 4 is the cation exchange capacity prediction model established by the hygroscopic water content of the water activity in the range of 0.23-0.57 in the embodiment of the present application;

[0031] Figure 5 is the cation exchange capacity prediction model established by the hygroscopic water content of the water activity in the range of 0.23-0.57 in the embodiment of the present application;

[0032] Figure 6 is the cation exchange capacity prediction model established by the hygroscopic water content of the water activity in the range of 0.23-0.57 in the embodiment of the present application;

[0033] Figure 7 is the cation exchange capacity prediction model established by the hygroscopic water content of the water activity in the range of 0.23-0.57 in the embodiment of the present application. DETAILED DESCRIPTION

[0034] The present application will be described in detail below in combination with the drawings and specific embodiments.

[0035] The method for predicting cation exchange capacity based on the change amount of soil hygroscopic water content of the present application is specifically implemented according to the following steps:

[0036] S1: According to the soil water vapor adsorption mechanism, determine the water activity range closely related to soil water vapor adsorption and cation exchange capacity;

[0037] According to the mechanism of soil water vapor adsorption, soil water vapor adsorption can be divided into monolayer adsorption, multilayer adsorption and capillary condensation, in which the monolayer adsorption coefficient is less than 0.2, the multilayer adsorption coefficient is in the range of 0.2-0.6, and the capillary condensation coefficient is greater than 0.6. Cation hydration is the main mechanism of monolayer adsorption. There are differences in the exchangeable cations adsorbed on the surface of different soil clay minerals, and because the hydration forces of different exchangeable cations are different, the relationship between the hygroscopic water content and the cation exchange capacity in the monolayer adsorption range will be affected by the different types of cations. In addition, capillary condensation mainly occurs in soil pores, so the cation exchange capacity is not the main control factor of soil water vapor adsorption in the capillary condensation process. In the process of multilayer adsorption, soil adsorbs water molecules through hydrogen bonds between water molecules, which reduces the influence of the difference between exchangeable cations on the relationship between hygroscopic water content and cation exchange capacity. Therefore, the correlation between the change of hygroscopic water content and the cation exchange capacity in the process of multilayer adsorption is higher than that in the processes of monolayer adsorption and capillary condensation.

[0038] In this example, 8 different mineral types and clay content soil samples are selected, and the water vapor adsorption isotherm and cation exchange capacity of these samples are determined by using a water vapor adsorption analyzer and a traditional method, respectively. The correlation between the slope of the soil water vapor adsorption isotherm and the cation exchange capacity at different water activities is calculated, and the change of the determination coefficient with water activity is analyzed.

[0039] The determination coefficient between the slope of the water vapor adsorption isotherm and the cation exchange capacity changes with the water activity as shown in Figure 1 , it can be seen that the determination coefficient between the slope of the soil water vapor adsorption isotherm and the cation exchange capacity is low in the low water activity range less than 0.15 and in the high water activity range greater than 0.75, and is high in the moderate water activity range of 0.15-0.75. This also proves that the relationship between soil water vapor adsorption and cation exchange capacity is close in the moderate water activity range of 0.2-0.6, so the water activity range is determined to be 0.2-0.6.

[0040] S2: Determine two specific water activities according to the water activity range determined in S1 and the safety and cost of using saturated salt solution to control water activity;

[0041] Because potassium acetate and sodium bromide reagents are small in danger, low in cost and easy to obtain, the water activities corresponding to their saturated salt solutions are 0.23 and 0.57 respectively. According to the water activity range of 0.2-0.6 closely related to the cation exchange capacity determined in S1 and the safety and cost of using saturated salt solution to control water activity, potassium acetate and sodium bromide reagents meet the requirements, and the two specific water activities selected are 0.23 and 0.57 respectively.

[0042] S3: Select part of the samples in a large sample, and determine the soil cation exchange capacity and the hygroscopic water content of the soil at two specific water activities by using the traditional method;

[0043] In this example, 30 samples were selected, but in practice, no fewer than 20 samples can be selected. The cation exchange capacity of the 30 samples was determined using traditional methods. Simultaneously, these samples were placed sequentially in saturated salt solutions containing potassium acetate and sodium bromide, and after equilibration, the hygroscopic water content of the soil was measured at water activities of 0.23 and 0.57, respectively.

[0044] S4: Establish a predictive model for the relationship between the change in hygroscopic water content and cation exchange capacity within this water activity range;

[0045] Calculate the change in hygroscopic water content (WCC) within the water activity range of 0.23–0.57. 0.23-0.57 A regression analysis was used to establish the change in hygroscopic water content (WCC). 0.23-0.57 For example, a cation exchange capacity prediction model with input variables, such as Figure 2 As shown. For ease of use later, it is named Model 1.

[0046] To compare the prediction model proposed in this patent with the prediction model established using hygroscopic water content, hygroscopic water content WC was established under conditions of water activity of 0.23 and 0.57, respectively. 0.23 and WC 0.57 Predictive models that predict cation exchange capacity for input variables, such as Figure 3 and Figure 4 As shown, they are named Model 2 and Model 3 respectively. Based on the determination coefficients of the three models, Model 1 is superior to Model 2 and Model 3.

[0047] S5: Determine the hygroscopic water content of the sample under two specific water activities and calculate the change in hygroscopic water content. Calculate the cation exchange capacity of the soil sample using a predictive model between the hygroscopic water content and the cation exchange capacity.

[0048] To evaluate the predictive performance of the cation exchange capacity model established by S4, ten independent samples were selected, and the hygroscopic water content was measured under two specific water activity conditions of 0.23 and 0.57. Model 1 established by S4 (input variable WCC) was then used. 0.23-0.57 Model 2 (input variable WC) 0.23 Model 3 (input variable WC) and Model 3 0.57 The cation exchange capacity of the sample to be tested was predicted. The simulation accuracy of the three models was evaluated using mean absolute error (MAE) and root mean square error (RMSE).

[0049]

[0050]

[0051] In the formula, P represents a predicted value, O represents a measured value, and n represents the number of samples.

[0052] The prediction performance of Model 1 (input variable WCC 0.23-0.57 ), Model 2 (input variable WC 0.23 ), and Model 3 (input variable WC 0.57 ) is shown in Table 1. Figures 5-7 As shown in the table, the MAE and RMSE of Model 1 are lower than those of Model 2 and Model 3. Therefore, when a large number of samples need to be measured for cation exchange capacity in soil investigation or soil fertility evaluation, the cation exchange capacity prediction model with WCC 0.23-0.57 as the input variable can be used to predict the cation exchange capacity of the samples.

[0053] The main theoretical basis of the method for predicting cation exchange capacity based on the change in soil hygroscopic water content is that the soil water vapor adsorption process is closely related to the hydration of cations adsorbed on the negative charges on the soil surface. In the multilayer water vapor adsorption process, the soil adsorbs water molecules through hydrogen bonds between water molecules, which reduces the influence of the difference in exchangeable cations on the relationship between hygroscopic water content and cation exchange capacity. Therefore, the change in hygroscopic water content is closely related to the cation exchange capacity in the multilayer adsorption process, and the prediction accuracy of the cation exchange capacity is higher when WCC 0.23-0.57 is used than when WCC 0.23 and WCC 0.57 are used alone. When the sample size is large, the method proposed in this study can save time, labor, and cost for determining the soil cation exchange capacity.

Claims

1. A method for predicting cation exchange capacity based on the amount of change in soil water content, characterized by, The method is implemented according to the following steps: S1: determining a water activity range according to a soil water vapor adsorption mechanism, wherein the water activity range is 0.2-0.6; S2: determining two reagents and specific water activities of the two reagents according to the water activity range determined in S1 and the safety and cost of using saturated salt solutions to control water activity, wherein the two reagents are potassium acetate and sodium bromide, and the specific water activities of the potassium acetate and the sodium bromide are 0.23 and 0.57 respectively; S3: selecting a plurality of samples and determining soil cation exchange capacity by using a traditional method, and simultaneously determining soil hygroscopic water content of the samples under specific water activities of the reagents determined in S2; S4: establishing a prediction model between a change amount of the hygroscopic water content under the specific water activity range and the cation exchange capacity; S5: determining hygroscopic water content of a to-be-tested sample under two specific water activities, calculating a change amount of the hygroscopic water content, and calculating the cation exchange capacity of the to-be-tested soil sample by using the prediction model between the change amount of the hygroscopic water content and the cation exchange capacity.

2. The method of predicting cation exchange capacity based on the amount of change in soil hygroscopic water content according to claim 1, characterized by, The specific process of S1 is as follows: selecting a plurality of soil samples of different mineral types and clay contents, determining water vapor adsorption isotherms and cation exchange capacities of the soil samples, calculating correlations between soil water vapor adsorption isotherm slopes and cation exchange capacities under different water activities, analyzing changes of determination coefficients with water activities, and determining a water activity range according to changes of the determination coefficients.

3. The method of predicting cation exchange capacity based on the amount of change in soil hygroscopic water content according to claim 2, characterized by, The water vapor adsorption analyzer and the traditional neutral ammonium acetate method or the barium chloride-sulfuric acid forced exchange method are used to determine the water vapor adsorption isotherms and the cation exchange capacities of the soil samples respectively.

4. The method of predicting cation exchange capacity based on the amount of change in soil hygroscopic water content according to claim 1, characterized by, The specific process of determining the soil hygroscopic water content in S3 is as follows: first, the cation exchange capacity of the sample is determined by using a traditional method; meanwhile, the sample is sequentially placed in saturated salt solutions containing the reagents specified in S2, and the soil hygroscopic water content in the two saturated salt solutions under the specific water activities in S2 is determined after equilibrium.

5. The method of predicting cation exchange capacity based on the amount of change in soil hygroscopic water content according to claim 1, characterized by, The number of the samples in S3 is not less than 20.

6. The method of predicting cation exchange capacity based on the amount of change in soil hygroscopic water content according to claim 1, characterized by, The specific process of S4 is as follows: calculating a change amount of the hygroscopic water content under the specific water activity range, and establishing a cation exchange capacity prediction model with the change amount of the hygroscopic water content as an input variable by using a regression method.

7. The method of predicting cation exchange capacity based on the amount of change in soil hygroscopic water content according to claim 1, characterized by, The prediction model uses mean absolute error (MAE) and root mean square error (RMSE) formulas to evaluate simulation accuracy.

8. The method of predicting cation exchange capacity based on the amount of change in soil hygroscopic water content according to claim 7, characterized by, The formula of the mean absolute error (MAE) is as follows: In the formula, P represents a predicted value, O represents an observed value, and n represents the number of samples.

9. The method of predicting the cation exchange capacity based on the change amount of soil hygroscopic water content according to claim 8, characterized by, The formula of the root mean square error (RMSE) is as follows: In the formula, P represents a predicted value, O represents an observed value, and n represents the number of samples.

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