# Study on Simulation Test of Peat Soil Environment in Dianchi Lake

Authors: Jing Cao; Siyang Huang; Wenlian Liu; Cheng Kong; Yue Gao; Fangyi Liu
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

The effect and feasibility of peat soil environment (PSE) simulation pose a difficult problem for geotechnical environmental engineering. In this study, the actual content of humic group (HG) in peat soil of Dianchi Lake is determined, and the method of adding humic acid (HA) reagent into cohesive soil and soaking it in fulvic acid (FA) solution is used to simulate PSE of Dianchi Lake. By comparing the HG content of test samples and natural peat soil, the effect and feasibility of simulation test are studied. And the effects of HG on microstructure and material composition of PSE are analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The test results show that the content of HG and its components of PSE in seven sites of Dianchi Lake are quite different. The simulation method used in this study can simulate the PSE with different HA and FA contents, but the simulation method of soaking samples in FA solution cannot reach the actual effect. The SEM test confirm that the pore size and pore connectivity will increase with the HA reagent. However, FA is wrapped on the skeleton of soil through adsorption and fills some pores, which reduces the pore diameter and weakens pore connectivity. The XRD test shows that both HA and FA can reduce the diffraction peak of main substances in the samples, but not including SiO2. The reason is that HG and cohesive soil particles undergo coordination exchange and ion exchange; free HG combines with cohesive soil particles and transforms into bound HG, forming an organic-inorganic complex PSE.


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## Body

## 1. Introduction

Due to the special geographical location and plateau climate, the peat soil layers in the area around Dianchi Lake in Kunming are widely distributed. Peat soil is a special kind of soft soil, which has the characteristics of high organic matter content, large void ratio, low natural weight, high moisture content, high compressibility, and low bearing capacity [1, 2]. Peat soil matter contains a large amount of HG, which is a kind of amorphous, polydisperse organic polymer mixture formed by the decomposition and transformation of the remains of animals, plants, and microorganisms through a series of complex physical, chemical, and biological actions [3, 4]. According to the difference in solubility of HG in acidic and alkaline aqueous solutions, it can be divided into three categories: HA, which is only soluble in alkaline solution but not in acidic solution; FA, which is soluble in both acidic solution and alkaline solution, and humin, which is insoluble in both acid and alkaline solution.With the rapid development of urban construction and the continuous increase of societal demands, more and more engineering projects have to be located in the distribution areas of PSE. Before the design and construction of engineering projects, it is necessary to conduct a series of tests for the reinforcement of peat soil foundations. Therefore, it has certain theoretical and practical value to verify the PSE of Dianchi Lake and to study the effect and feasibility of artificially peat soil through a simulation test.Since the discovery of HG, many experts and scholars have carried out research on the separation, extraction, chemical composition, and microstructure of HG. In 1786, Achard [5] first isolated HG from peat soil. In 1826, Sprengel [6] comprehensively study the origin and chemical properties of humic substances, and he has developed many methods of extracting and preparing HG, which are widely used, such that the soil is pretreated with mineral acid before extraction with lye. In 2009, Song et al. [7] used elemental analysis and Fourier transform infrared spectroscopy to study the chemical composition and structural characteristics of four different sources of HG. The results show that although the chemical composition and structure of HG from different sources have many similarities, they also have obvious differences in structure due to their different biological sources and diagenetic environments. In 2014, Liu Yu [8] first studied the influence of the microstructure and material components of Dianchi peat soil on the mechanical strength through mechanical tests. In 2017, Liu Chaochao [9] performed elemental analysis, Fourier transform infrared spectroscopy, solid 13C nuclear magnetic resonance, and three-dimensional fluorescence spectroscopy on the HG in the excess sludge of sewage treatment plants. The results show that there are great differences in chemical composition and structure between the HG in the residual sludge of the sewage treatment plant and the natural state. In addition, many experts and scholars have used the method of artificially preparing peat soil to study the effect of HG on the strength of cement soil. In 2004, Bertron [10] reinforced the soil containing HA with cement and found that HG will corrode cement soil. In 2019, Liang Shihua [11] used the method of adding HG into cohesive soil to artificially prepare peat soil with different HG content and then studied the effect of HG on the strength of cement soil. The results show that the HG in the cement soil will hinder the hydration reaction of the cement and reduce the strength of the cement soil.In summary, due to the particularity of the PSE, many experts and scholars have studied the chemical composition and microstructure of HA in peat soil. Among them, the method of artificially preparing peat soil by adding HG into the soil has not been combined with the actual PSE, and there is a lack of relevant research to verify the effect and feasibility of simulating the PSE. In this study, the content of HA in the peat soil of Dianchi Lake was verified. Then, the method of adding HA reagent to the cohesive soil with low organic matter content and soaking in FA solution was used to simulate the PSE. By comparing the HG content of test samples and the actual Dianchi peat soil, the effect and feasibility of artificially prepared peat soil are studied. Combined with SEM test and XRD test, the effect of HG on the microstructure and material composition of PSE was explored.


## 2. Determination of Peat Soil Environment in Dianchi Lake

### 2.1. Peat Soil Sampling

Due to its special geographical location and plateau climate, the area around Dianchi Lake in Kunming belongs to a large area of lakes and swamps in ancient time, with deep Quaternary sediment and widespread peat soil [12, 13]. After data collection, on-site investigation, seven representative groups of natural peat soil samples in the area around Dianchi Lake are obtained by drilling and manual soil sampling at the bottom of the foundation pit. The distribution of sampling sites in Dianchi Lake is shown in Figure 1. The test is based on “Determination of Forest Soil Humus Composition” (LY/T 1238–1999) [14] to extract and determine the content and component of HG in peat soil.Figure 1
Distribution of sampling sites in Dianchi Lake.


### 2.2. Measurement and Analysis of Peat Soil Environment

HG is a carbon-containing organic substance in peat soil. Since the different components of HG (humic acid, fulvic acid, and humin) are all mixtures composed of a series of compounds, there is no unified molecular structure and molecular formula. Therefore, this text first determines the carbon content of different components in HG and then calculates its content in combination with its corresponding carbon content. Studies [15–17] show that the carbon content of HA and humin is about 50%–60%, and the FA is about 40%–50%. Comprehensively comparing the molecular structure and molecular formula of each component of HG proposed by previous studies [16], the carbon content of HA and humin is about 60% and the FA is about 50%. By combining the measured carbon content and carbon element content, the HG content of the peat soil sample in Dianchi Lake is calculated, as shown in Table 1. It can be concluded from Table 1 that although those samples are all taken from the area around Dianchi Lake in Kunming, the amount of HG in different sites are quite different, and the lowest amount of HG is only 7.15% (SS-4), and the highest can reach 50.06% (SS-1). Comparing the contents and components of HG in peat soil samples at the same site, it can be clearly concluded that the contents of HA and humin are higher, FA is lower, and the content of FA in each soil sample is no more than 10%.Table 1
Determination results of HG content in natural peat soil samples.
NO.SiteDistance from the shore of Dianchi Lake/kmTotal HG/%HA content/%FA content/%Humin content/%SS-1Intersection of Huachen road and Yikang road3.6850.0628.138.3413.59SS-2Changhong road, Luoya community, Guandu District3.5034.3814.565.1114.71SS-3North of the intersection of Fubao road and Huanhu east road, Guandu district0.8015.186.612.456.12SS-4Intersection of Baiji road and Huanhu east road, Guandu district0.527.152.360.794.00SS-5Intersection of Haidong road and Gudukou road, Guandu district1.3241.7422.793.3915.56SS-6Intersection of Yupu road and Gudian road, Chenggong district1.1040.2622.222.6715.37SS-7North of Haibao mountain, Chenggong district1.369.074.062.922.09


## 2.1. Peat Soil Sampling

Due to its special geographical location and plateau climate, the area around Dianchi Lake in Kunming belongs to a large area of lakes and swamps in ancient time, with deep Quaternary sediment and widespread peat soil [12, 13]. After data collection, on-site investigation, seven representative groups of natural peat soil samples in the area around Dianchi Lake are obtained by drilling and manual soil sampling at the bottom of the foundation pit. The distribution of sampling sites in Dianchi Lake is shown in Figure 1. The test is based on “Determination of Forest Soil Humus Composition” (LY/T 1238–1999) [14] to extract and determine the content and component of HG in peat soil.Figure 1
Distribution of sampling sites in Dianchi Lake.


## 2.2. Measurement and Analysis of Peat Soil Environment

HG is a carbon-containing organic substance in peat soil. Since the different components of HG (humic acid, fulvic acid, and humin) are all mixtures composed of a series of compounds, there is no unified molecular structure and molecular formula. Therefore, this text first determines the carbon content of different components in HG and then calculates its content in combination with its corresponding carbon content. Studies [15–17] show that the carbon content of HA and humin is about 50%–60%, and the FA is about 40%–50%. Comprehensively comparing the molecular structure and molecular formula of each component of HG proposed by previous studies [16], the carbon content of HA and humin is about 60% and the FA is about 50%. By combining the measured carbon content and carbon element content, the HG content of the peat soil sample in Dianchi Lake is calculated, as shown in Table 1. It can be concluded from Table 1 that although those samples are all taken from the area around Dianchi Lake in Kunming, the amount of HG in different sites are quite different, and the lowest amount of HG is only 7.15% (SS-4), and the highest can reach 50.06% (SS-1). Comparing the contents and components of HG in peat soil samples at the same site, it can be clearly concluded that the contents of HA and humin are higher, FA is lower, and the content of FA in each soil sample is no more than 10%.Table 1
Determination results of HG content in natural peat soil samples.
NO.SiteDistance from the shore of Dianchi Lake/kmTotal HG/%HA content/%FA content/%Humin content/%SS-1Intersection of Huachen road and Yikang road3.6850.0628.138.3413.59SS-2Changhong road, Luoya community, Guandu District3.5034.3814.565.1114.71SS-3North of the intersection of Fubao road and Huanhu east road, Guandu district0.8015.186.612.456.12SS-4Intersection of Baiji road and Huanhu east road, Guandu district0.527.152.360.794.00SS-5Intersection of Haidong road and Gudukou road, Guandu district1.3241.7422.793.3915.56SS-6Intersection of Yupu road and Gudian road, Chenggong district1.1040.2622.222.6715.37SS-7North of Haibao mountain, Chenggong district1.369.074.062.922.09


## 3. Simulation Test

### 3.1. Material

Test soil is selected from the alluvial-proluvial cohesive soil near Sanhe Village, Xinjie Town, Jinning County, and Kunming City, which is brownish-yellow and brownish-red. As shown in Figure2 and 3, the FA reagent is purified from young lignite in Yunnan by hydrogen peroxide degradation process. The actual content of FA in the FA reagent is determined to be 30.46%. The HA reagent was obtained from Tianjin Guangfu Chemical Reagent Factory, and the HA content in the product was determined to be 41.68%. In the test, the mixing water and FA solution water are distilled water. The physical and mechanical properties of the test soils are shown in Table 2.Figure 2
Location of borrow site for test soil.Figure 3
Location of soil layer for test soil.Table 2
Physical and mechanical properties index of soil for test.
Test soilNatural moisture contentω (%)Liquid limit wL (%)Plastic limit wP (%)Natural density (g·cm−3)Specific gravity of soil particleGsModulus of compression (MPa−1)Cohesive soil33.539.2620.381.872.734.65


### 3.2. Sample Preparation

The test is carried out according to the “Standards for Geotechnical Test Methods” (GB/T50123-2019) [18]. In order to ensure the accuracy of the test, the moisture content (ω) = 24%, and the void ratio (e) = 0.8. Before sample preparation, the test soil was broken into pieces, and after air-drying, it was packed in a box with an aperture of 2.00 mm. When preparing the sample, a three-lobed mold with an inner diameter of (d) = 39.10 mm and a height of (h) = 80.00 mm is used as the test mold. After mixing the test soil, HA reagent, and water uniformly, the sample is compacted in three valves by layered compaction method to ensure that the sample is uniform and dense.


### 3.3. Test Design

The test simulates the PSE by adding the HA reagent into cohesive soil and soaking it in the FA solution, as shown in Figure4. Since fulvic acid is weakly acidic, the pH value of the solution was controlled by controlling the concentration of FA. The concentration of FA in distilled water (pH 7.0) is 0. The pH value of the FA solution is measured by an electronic pH tester, and the pH value of the solution is kept constant by adding FA reagent during the soaking test [19].Figure 4
Immersion of samples in FA solution.The simulation test is divided into three groups. Test Group 1: the amount of HA reagent is 0%, 15%, and 25%, respectively, the HG content of the sample is determined after sample preparation. Test Group 2: soak the above samples in FA solution with a pH value of 4.5, and when the soaking age reaches 90d, take out those samples, and measure HG content after the samples are completely air-dried. Test Group 3: the mixing amount of HA reagent is 15%, and the samples are soaked in FA solutions with different pH values (4.5, 5.0, 5.5). When the soaking age reaches 90d, the sample is taken out, and the HG content is measured after the sample is completely air-dried. Three test groups are shown in Table3.Table 3
Simulation test conditions of PSE.
Test soilTest groupThe amount of HA reagent addedλ/%FA solution pHImmersion agedWater contentω/%Void ratioeCohesive soil10, 15, 25——2.734.6520, 15, 254.5903155.5, 5.0, 4.590SEM test and XRD test are carried out on the samples with immersion age of 90d. SEM test uses Czech TESCAN-VEGA3 automatic tungsten filament SEM; XRD test uses Holland PANalytical X'Pert3 Powder multifunctional powder X-ray diffractometer. The test conditions are shown in Table 4.Table 4
Microstructure test conditions.
No.The amount of HA reagent addedλ/%FA solution pHWater contentω/%Void ratioeImmersion aged10, 5, 10, 15, 20, 257.0240.8902156.0, 5.5, 5.0, 4.5


## 3.1. Material

Test soil is selected from the alluvial-proluvial cohesive soil near Sanhe Village, Xinjie Town, Jinning County, and Kunming City, which is brownish-yellow and brownish-red. As shown in Figure2 and 3, the FA reagent is purified from young lignite in Yunnan by hydrogen peroxide degradation process. The actual content of FA in the FA reagent is determined to be 30.46%. The HA reagent was obtained from Tianjin Guangfu Chemical Reagent Factory, and the HA content in the product was determined to be 41.68%. In the test, the mixing water and FA solution water are distilled water. The physical and mechanical properties of the test soils are shown in Table 2.Figure 2
Location of borrow site for test soil.Figure 3
Location of soil layer for test soil.Table 2
Physical and mechanical properties index of soil for test.
Test soilNatural moisture contentω (%)Liquid limit wL (%)Plastic limit wP (%)Natural density (g·cm−3)Specific gravity of soil particleGsModulus of compression (MPa−1)Cohesive soil33.539.2620.381.872.734.65


## 3.2. Sample Preparation

The test is carried out according to the “Standards for Geotechnical Test Methods” (GB/T50123-2019) [18]. In order to ensure the accuracy of the test, the moisture content (ω) = 24%, and the void ratio (e) = 0.8. Before sample preparation, the test soil was broken into pieces, and after air-drying, it was packed in a box with an aperture of 2.00 mm. When preparing the sample, a three-lobed mold with an inner diameter of (d) = 39.10 mm and a height of (h) = 80.00 mm is used as the test mold. After mixing the test soil, HA reagent, and water uniformly, the sample is compacted in three valves by layered compaction method to ensure that the sample is uniform and dense.


## 3.3. Test Design

The test simulates the PSE by adding the HA reagent into cohesive soil and soaking it in the FA solution, as shown in Figure4. Since fulvic acid is weakly acidic, the pH value of the solution was controlled by controlling the concentration of FA. The concentration of FA in distilled water (pH 7.0) is 0. The pH value of the FA solution is measured by an electronic pH tester, and the pH value of the solution is kept constant by adding FA reagent during the soaking test [19].Figure 4
Immersion of samples in FA solution.The simulation test is divided into three groups. Test Group 1: the amount of HA reagent is 0%, 15%, and 25%, respectively, the HG content of the sample is determined after sample preparation. Test Group 2: soak the above samples in FA solution with a pH value of 4.5, and when the soaking age reaches 90d, take out those samples, and measure HG content after the samples are completely air-dried. Test Group 3: the mixing amount of HA reagent is 15%, and the samples are soaked in FA solutions with different pH values (4.5, 5.0, 5.5). When the soaking age reaches 90d, the sample is taken out, and the HG content is measured after the sample is completely air-dried. Three test groups are shown in Table3.Table 3
Simulation test conditions of PSE.
Test soilTest groupThe amount of HA reagent addedλ/%FA solution pHImmersion agedWater contentω/%Void ratioeCohesive soil10, 15, 25——2.734.6520, 15, 254.5903155.5, 5.0, 4.590SEM test and XRD test are carried out on the samples with immersion age of 90d. SEM test uses Czech TESCAN-VEGA3 automatic tungsten filament SEM; XRD test uses Holland PANalytical X'Pert3 Powder multifunctional powder X-ray diffractometer. The test conditions are shown in Table 4.Table 4
Microstructure test conditions.
No.The amount of HA reagent addedλ/%FA solution pHWater contentω/%Void ratioeImmersion aged10, 5, 10, 15, 20, 257.0240.8902156.0, 5.5, 5.0, 4.5


## 4. Test Results and Analysis

### 4.1. Effect and Feasibility of Simulation Test

Humin is a polymer mixture containing amino groups, phenolic hydroxyl groups, methoxy groups, carboxyl groups, and other groups, which are closely combined with the inorganic parts in the soil and can exist in the soil for a long time and are difficult to decompose [15].According to the method of calculating the content of HG by combining the content of carbon and carbon element, the determination results of HG content of simulation test are obtained, as shown in Table5. It can be obtained from Test Group 1: (1) The contents of HA and FA in cohesive soil selected for test are both low (TS-1), which can significantly reduce the influence of organic matter for test results. (2) Since the content of pure HA in HA reagent is about 41.68%, theoretically, 15% HA reagent can be calculated as about 6.25% (the measured value of HA content in TS-2 is 6.04%). Similarly, the HA content of 25% HA reagent is about 10.42% (the measured value of HA content in TS-3 is 11.50%), so it can be concluded that the measured value of HA content in the sample is close to the calculated value according to the amount of reagent, and the simulation method is effective.Table 5
Determination results of HG content of PSE simulation test.
Test groupSample No.The amount of HA reagent addedλ/%FA solution pHMeasured value of HG content/%HAFA1TS-10—0.320.08TS-215—6.040.08TS-325—11.50<0.022TS-404.50.430.21TS-5154.53.881.34TS-6254.58.671.043TS-7155.54.730.10TS-8155.04.600.74TS-5154.53.881.34The measured results of the HA content in Table5 (Test Group 1 and 2) are plotted to obtain the relationship curve between the measured value of HA content and the amount of HA reagent in the samples, as shown in Figure 5. Plotting the measured results of the FA content in Table 5 (Test Group 3), the relationship curve between measured FA content of samples and pH value of FA solution is obtained. Combining Table 5 and Figure 5, the following is obtained. (1) The measured HA content of the sample increases with the increase of the amount of HA reagent added. (2) Compared with the not soaking sample (Test Group 1), the HA content of the sample soaking in the pH 4.5 FA solution (Test Group (2) is slightly reduced. Combining Table 5 and Figure 6, it can be obtained that (1) when the sample is immersed in the FA solution, the measured value of the FA content in the sample increases slightly. (2) For samples soak in FA solution with pH 5.5, the measured value of FA content increases from 0.08% (TS-1) to 0.10% (TS-7). The measured value of FA in the sample increases with the increases of the concentration of FA solution (or decrease in pH).Figure 5
Relationship curve between the measured value of HA content and the amount of HA reagent in the samples.Figure 6
Relationship curve between measured FA content of samples and pH value of FA solution.The molecular structure of HG contains a variety of active oxygen-containing functional groups, mainly carboxyl, phenolic hydroxyl, and other acidic groups. FA shows more obvious acidity and higher reaction activity because it has more oxygen-containing functional groups (such as carboxyl). Through these functional groups, HG can interact with other substances in the sample in various forms such as coordination exchange and ion exchange, so as to adsorb HG and form organic-inorganic complexes. HG changes from free state to bound state.In Test Groups 1 and 2, with the increase of the amount of HA reagent added, the measured value of HA content in the samples increases. The reason is that insoluble HA can be adsorbed on the surface of the sample clay particles and interact with the clay particles and cations to form the sample soil skeleton. The free HA is converted into the bound HA, so that the HA can remain in the sample. In Test Groups 2 and 3, the measured value of the FA content of the samples soaking in the FA solution increased slightly. And the content of FA in the sample increases with the increase of the concentration of FA solution (or decrease in pH). The reason is that FA can be immersed into the pores of the clay soil with the soaking liquid by virtue of its small molecular structure and low molecular weight. The free FA is wrapped on the soil skeleton through adsorption [20, 21] and undergoes various interaction forms such as coordination exchange and ion exchange with the clay particles in the sample, thereby transforming into the bound FA, and finally remains in the internal pores of the sample. In addition, the decrease of the pH value of the FA solution (or the increase of the concentration) can increase the content of FA in the soaking solution, thereby increasing the adsorption capacity of FA. Comparing the measured values of HA content in Test Groups 1 and 2, the measured value of HA content in Test Group 2 is slightly decreased. The reason is that when FA is immersed into the sample with the soaking solution, it combines with clay minerals through various interaction forms, occupying part of the binding sites that can be combined with HA. The binding sites for HA to contact and adsorb on clay minerals are reduced, resulting in the precipitation of unbound part of HA from the sample.Plot the measured results of the FA content in Table5 (Test Group 2), and obtain the relationship curve between measured FA content and HA reagent content of samples, as shown in Figure 7. Plot the measured results of HA content in Table 5 (Test Group 3), and obtain the relationship curve between measured value of HA content of samples and pH value of FA solution, as shown in Figure 8. Combining Table 5 and Figure 7, it can be seen that when the sample is immersed in a FA solution with a pH value of 4.5, as the dosage of HA reagent increased from 0% to 15%, the measured value of FA content in the sample increased from 0.21% to 1.34%. And as the content of HA reagent increased from 15% to 25%, the measured value of FA content decreased slightly. Combining Table 5 and Figures 6 and 8, it can be seen that when the amount of HA reagent added to 15%, as the pH value of the FA solution decreases, the measured value of the FA content in the sample gradually increases. However, the measured value of HA content has an obvious decreasing trend.Figure 7
Relationship curve between measured FA content and HA reagent content of samples.Figure 8
Relationship curve between measured HA content of samples and pH value of FA solution.In Test Group 2, when the sample is immersed in a FA solution with a pH value of 4.5, the measured value of the FA content increased significantly as the dosage of HA reagent increased from 0% to 15%. The reason is that the increase of HA content causes the dispersion of clay minerals in the sample [22], and the pore size increases, so that more FA molecules can invade into the pores of the clay soil with the soaking solution. As the dosage of HA reagent increased from 15% to 25%, the measured value of FA content decreased slightly. The reason is that the adsorption sites on the surface of clay minerals were occupied by more HA, making it difficult for FA to attach, and the amount of FA adsorption decreased [23]. Compared with Test Group 1 (TS-2) and Test Group 3, with the decrease of pH value of FA solution, the content of FA in test sample gradually increased, and the content of HA decreased significantly. The reason is that as the concentration of the FA solution increases, the adsorption capacity of the sample to FA increases, and more FA occupies the binding site. The number of binding sites for HA to contact and adsorb on the clay particles decreases, resulting in more HA precipitation from the sample.Although the content of HG in the PSE of Dianchi Lake varies greatly, the method of adding HA reagent into the cohesive soil with low organic matter content can simulate different PSE according to actual engineering or experimental requirements. For the PSE with higher HA content, the method of increasing the dosage of HA reagent or selecting a higher purity HA reagent can improve the effect of simulating the PSE. Compared with the content range of FA in Dianchi peat soil measured in Table1 (0.79%∼8.34%), although the method of soaking in FA solution can significantly increase the content of FA in the sample, the simulation effect of this method is limited. It can only satisfy the simulation of the PSE with lower FA content. It should be noted that due to the limited adsorption capacity of HG in cohesive soil and the complex interaction between HG and the material components in cohesive soil, the method of incorporating HA reagent into cohesive soil will lead to the actual HA content in the sample being different from the theoretical incorporation value. The method of soaking in FA solution also needs to be tested to determine the FA content of the sample. Therefore, when simulating the PSE, in order to ensure that the test method has a good simulation effect and feasibility, the HG content of the sample should be measured first.


### 4.2. SEM Test Results and Analysis

Figure9 is microstructure images of samples with different amounts of HA reagent (×500). By comparing Figure 9(a)∼9(f), it can be concluded that (1) the internal microstructure of the cohesive soil sample (with 0% of HA reagent) has good integrity and uniformity, with small pore size and poor connectivity. (2) When the dosage of HA reagent increased in the range from 5% to 25%, the pores inside the sample gradually increased and gradually connected. The soil particles of the sample are gradually overhead, forming an obvious loose skeleton, and the integrity of the sample deteriorates. The reason is that there are various functional groups such as carboxyl group and alcoholic hydroxyl group. On the aromatic nucleus structure of HA monomer. In the process of mixing HA, cohesive soil, and water, after HA is wetted by water, the molecular structure and functional groups of HA show higher reactivity, so that HA can interact with pores. The high valence cations in the water react, reducing the cation valence in the pore water, weakening the electrostatic attraction between the cations in the pore water and the negative charges on the surface of the soil particles, increasing the thickness of the diffusion layer and weakening the connection between the soil particles. [23] On the other hand, HA particles have strong adsorption, if there is a certain amount of HA in the cohesive soil, the HA particles will be adsorbed on the surface of the clay particles, increasing the dispersibility of the cohesive soil [22], hindering the coagulation between clay particles, and resulting in the deterioration of the physical properties and structure of the soil.Figure 9
Microstructure images of samples with different amounts of HA reagent (× 500). (a) 0% HA reagent, (b) 5% HA reagent, (c) 10% HA reagent, (d) 15% HA reagent, (e) 20% HA reagent, and (f) 25% HA reagent.
(a)(b)(c)(d)(e)(f)Figure10 is microstructure images of samples soaking in FA solution with different pH values (× 500). By comparing Figure 10(a)∼10(e), it can be concluded that with the decrease of the pH value of the FA solution (or the increase of the solution concentration), the pore size inside the sample gradually decreases, the pore connectivity gradually weakens, the density of the sample increases, and the integrity increases. The reason is due to its small molecular structure and low molecular weight, FA can enter the pores of the sample and the FA in the pore water is gradually wrapped on the soil skeleton through adsorption. Due to its own colloidal properties [21, 23], colloidal connections are generated between clay particles. On the other hand, the soluble FA particles will remain in the pores as fillers, which will reduce the pore size of the sample and weaken the connectivity. At the same time, by reducing the pH value (or increasing the concentration) of the FA solution, the content of FA in the sample can be increased and the adsorption capacity of FA can be increased. In addition, the decrease of pH will weaken the protonation of acidic functional groups, increase the hydrophobicity, and weaken the water solubility [23], and the electrostatic repulsion in the organic matter-mineral system will also weaken, which is favorable for the adsorption of FA.Figure 10
Microstructure images of samples soaked in FA solution with different pH values (× 500). (a) 7.0 pH, (b) 6.0 pH, (c) 5.5 pH, (d) 5.0 pH, and (e) 4.5 pH.
(a)(b)(c)(d)(e)


### 4.3. X-Ray Diffraction Test Results and Analysis

Figure11 is the XRD pattern of alluvial-proluvial cohesive soil. It can be seen from Figure 11 the main material components in the alluvial-proluvial cohesive soil. Among them, the primary minerals are silica (SiO2) and orthoclase (K2O·Al2O3·6SiO2). Secondary minerals are kaolinite (Al2O3 2SiO2 2H2O), serpentine (6MgO 4SiO2 4H2O), and oxides (TiO2, Ti2O3, Fe2O3, FeOOH).Figure 11
XRD pattern of cohesive soil.HG is a high molecular polymer, and its molecular structure is very complex. The different components of HG (humic acid, fulvic acid) can be regarded as composed of a series of molecules of different sizes, rarely with specific and precise structural shape and active functional group sequence [24]. On the other hand, HG in peat soil is divided into free and bound, and most of them are bound HG. Usually 52%–98% of the HG in peat soil is bound in the clay part [25], which constitutes the PSE in which the organic-inorganic complexes are combined in the peat soil. Figure 12 is relationship curve between main diffraction peaks of SiO2 and the amount of HA reagent. It can be seen from Figure 12 that with the increase of the dosage of HA reagent, the main diffraction peak curve of SiO2 in the sample has a large fluctuation and no obvious regularity. Figure 13 is relationship curve between main diffraction peaks and SiO2 and pH value of FA solution. It can be seen from Figure 13 that with the increase of the pH value of the FA solution (or the decrease of the solution concentration), the main diffraction peaks of SiO2 have an obvious upward trend. SiO2, as a primary mineral in cohesive soil, has relatively stable chemical properties. In addition, SiO2 is an acidic oxide and does not react with various acidic substances other than hydrofluoric acid. According to Emerson's [16] theory of clay particle structure, since HG has a variety of active functional groups, HG can cement the clay unit (or structural surface) and SiO2 together to form agglomerates. The active functional groups on the HG structure are linked with the SiO2 surface in the form of bonds [26], forming SiO2-organic matter complexes. However, due to the relatively stable chemical properties of SiO2, this type of action is not easy to occur and the chemical stability is poor. Therefore, with the increase of the dosage of HA reagent, the main diffraction peak curve of SiO2 in the sample fluctuates greatly. Compared with HA, FA contains more active functional groups dominated by carboxyl groups in its molecular structure, showing more significant weak acidity and surface reaction activity than HA [24], which makes it have stronger binding ability and higher binding bond stability with SiO2 surface. Therefore, when the amount of HA reagent reaches a certain level, with the decrease of pH value (or the increase of concentration) of FA solution, the main diffraction peak of SiO2 in the sample has an obvious decreasing trend.Figure 12
Relationship curve between main diffraction peaks of SiO2 and the amount of HA reagent.Figure 13
Relationship curve between main diffraction peaks and of SiO2 and pH value of FA solution.Figure14 is the relationship curve between diffraction peak value of main substances (except SiO2) and the amount of HA reagent. It can be concluded from Figure 14 that when the sample is immersed in distilled water, with the increase of HA reagent added, the diffraction peaks of the main substances in the sample (except SiO2) have a clear trend of decreasing. Figure 15 is the relationship curve between diffraction peak value of main substances (except SiO2) and pH value of FA solution. It can be concluded from Figure 15 that as the pH value of the FA solution decreases (or the solution concentration increases), the diffraction peaks of the main substances in the sample (except SiO2) gradually decrease.Figure 14
Relationship curve between diffraction peak value of main substances (except SiO2) and the amount of HA reagent.Figure 15
Relationship curve between diffraction peak value of main substances (except SiO2) and pH value of FA solution.From the chemical mechanism analysis: the interaction between HA and metal oxides and silicate minerals mainly includes hydrophobic interaction, Coulomb attraction, coordination exchange, ion exchange, cation bridge, and hydrogen bond interaction [25–32]. Among them, (1) due to the hydrophobicity of some nonpolar components in HG, such components are alienated from water molecules, and HG can be contacted and adsorbed on the surface of minerals. On the other hand, HG is a polydisperse electrolyte that can attract each other with the mineral surface through Coulomb attraction. (2) Coordination exchange is the most important mechanism [27, 29], which means that acidic functional groups such as carboxyl (- COO - or - COOH) in HG and hydroxyl (- OH) on the surface of clay minerals form complexes through coordination reaction. The reaction process mainly includes three stages: (1) surface protonation; (2) forming an outer ring compound; and (3) forming inner ring complexes. The main reaction formula is as follows:(1)S—OH+H+⇌S—OH2+,(2)S—OH2++Hu—COO−⇌Hu—COOS+H2O,where S–OH is the hydroxyl group on the mineral surface and Hu COO− is the carboxyl group of HG. The hydroxyl group on the mineral surface first adsorbs H+ in the solution to complete the basic process of surface protonation (1). The protonated mineral surface forms an outer ring compound with the carboxyl group in HG, and then OH2+ is coordinated and exchanged with Hu COO− to form an inner ring compound formula (2). (3) Ion exchange and cation bridges mainly occur between negatively charged mineral surfaces and negatively charged HG molecules. Metal oxides are usually positively charged. They cover the negatively charged clay mineral surface as a glue film [29, 30]. Negatively charged HG molecules can be combined with metal oxides through ion exchange reaction. Metal cations and their oxides form a bond bridge between the negatively charged mineral surface and the anions or polar functional groups of HG, playing the role of “cation bridge”. There is usually a layer of water molecules between the functional groups of metal cations and HG, which is mostly caused by hydrogen bonding [27]. Although clay minerals are generally negatively charged, they may also be positively charged at the edge of clay minerals, and negatively charged HG molecules can also be directly adsorbed with positively charged clay minerals [25]. In this test, kaolinite is positively charged [25] in acidic environment and can be directly adsorbed with HG molecules.In the indoor simulation test of PSE, when the sample is immersed in distilled water with pH value of 7.0, the diffraction peak of main substances (except SiO2) in cohesive soil decreases with the increase of the amount of HA reagent. When the sample is immersed in FA solution, the diffraction peaks of main substances (including SiO2) in cohesive soil decrease with the decrease of FA pH solution value (or the increase of solution concentration). The main reason is that HG and clay particles have a variety of functions such as coordination exchange and ion exchange. At the same time, it shows that simulation test can simulate PSE. That is, the free HG combines with clay particles and changes into bound HG, forming a HG environment combined with organic-inorganic complex similar to PSE.


## 4.1. Effect and Feasibility of Simulation Test

Humin is a polymer mixture containing amino groups, phenolic hydroxyl groups, methoxy groups, carboxyl groups, and other groups, which are closely combined with the inorganic parts in the soil and can exist in the soil for a long time and are difficult to decompose [15].According to the method of calculating the content of HG by combining the content of carbon and carbon element, the determination results of HG content of simulation test are obtained, as shown in Table5. It can be obtained from Test Group 1: (1) The contents of HA and FA in cohesive soil selected for test are both low (TS-1), which can significantly reduce the influence of organic matter for test results. (2) Since the content of pure HA in HA reagent is about 41.68%, theoretically, 15% HA reagent can be calculated as about 6.25% (the measured value of HA content in TS-2 is 6.04%). Similarly, the HA content of 25% HA reagent is about 10.42% (the measured value of HA content in TS-3 is 11.50%), so it can be concluded that the measured value of HA content in the sample is close to the calculated value according to the amount of reagent, and the simulation method is effective.Table 5
Determination results of HG content of PSE simulation test.
Test groupSample No.The amount of HA reagent addedλ/%FA solution pHMeasured value of HG content/%HAFA1TS-10—0.320.08TS-215—6.040.08TS-325—11.50<0.022TS-404.50.430.21TS-5154.53.881.34TS-6254.58.671.043TS-7155.54.730.10TS-8155.04.600.74TS-5154.53.881.34The measured results of the HA content in Table5 (Test Group 1 and 2) are plotted to obtain the relationship curve between the measured value of HA content and the amount of HA reagent in the samples, as shown in Figure 5. Plotting the measured results of the FA content in Table 5 (Test Group 3), the relationship curve between measured FA content of samples and pH value of FA solution is obtained. Combining Table 5 and Figure 5, the following is obtained. (1) The measured HA content of the sample increases with the increase of the amount of HA reagent added. (2) Compared with the not soaking sample (Test Group 1), the HA content of the sample soaking in the pH 4.5 FA solution (Test Group (2) is slightly reduced. Combining Table 5 and Figure 6, it can be obtained that (1) when the sample is immersed in the FA solution, the measured value of the FA content in the sample increases slightly. (2) For samples soak in FA solution with pH 5.5, the measured value of FA content increases from 0.08% (TS-1) to 0.10% (TS-7). The measured value of FA in the sample increases with the increases of the concentration of FA solution (or decrease in pH).Figure 5
Relationship curve between the measured value of HA content and the amount of HA reagent in the samples.Figure 6
Relationship curve between measured FA content of samples and pH value of FA solution.The molecular structure of HG contains a variety of active oxygen-containing functional groups, mainly carboxyl, phenolic hydroxyl, and other acidic groups. FA shows more obvious acidity and higher reaction activity because it has more oxygen-containing functional groups (such as carboxyl). Through these functional groups, HG can interact with other substances in the sample in various forms such as coordination exchange and ion exchange, so as to adsorb HG and form organic-inorganic complexes. HG changes from free state to bound state.In Test Groups 1 and 2, with the increase of the amount of HA reagent added, the measured value of HA content in the samples increases. The reason is that insoluble HA can be adsorbed on the surface of the sample clay particles and interact with the clay particles and cations to form the sample soil skeleton. The free HA is converted into the bound HA, so that the HA can remain in the sample. In Test Groups 2 and 3, the measured value of the FA content of the samples soaking in the FA solution increased slightly. And the content of FA in the sample increases with the increase of the concentration of FA solution (or decrease in pH). The reason is that FA can be immersed into the pores of the clay soil with the soaking liquid by virtue of its small molecular structure and low molecular weight. The free FA is wrapped on the soil skeleton through adsorption [20, 21] and undergoes various interaction forms such as coordination exchange and ion exchange with the clay particles in the sample, thereby transforming into the bound FA, and finally remains in the internal pores of the sample. In addition, the decrease of the pH value of the FA solution (or the increase of the concentration) can increase the content of FA in the soaking solution, thereby increasing the adsorption capacity of FA. Comparing the measured values of HA content in Test Groups 1 and 2, the measured value of HA content in Test Group 2 is slightly decreased. The reason is that when FA is immersed into the sample with the soaking solution, it combines with clay minerals through various interaction forms, occupying part of the binding sites that can be combined with HA. The binding sites for HA to contact and adsorb on clay minerals are reduced, resulting in the precipitation of unbound part of HA from the sample.Plot the measured results of the FA content in Table5 (Test Group 2), and obtain the relationship curve between measured FA content and HA reagent content of samples, as shown in Figure 7. Plot the measured results of HA content in Table 5 (Test Group 3), and obtain the relationship curve between measured value of HA content of samples and pH value of FA solution, as shown in Figure 8. Combining Table 5 and Figure 7, it can be seen that when the sample is immersed in a FA solution with a pH value of 4.5, as the dosage of HA reagent increased from 0% to 15%, the measured value of FA content in the sample increased from 0.21% to 1.34%. And as the content of HA reagent increased from 15% to 25%, the measured value of FA content decreased slightly. Combining Table 5 and Figures 6 and 8, it can be seen that when the amount of HA reagent added to 15%, as the pH value of the FA solution decreases, the measured value of the FA content in the sample gradually increases. However, the measured value of HA content has an obvious decreasing trend.Figure 7
Relationship curve between measured FA content and HA reagent content of samples.Figure 8
Relationship curve between measured HA content of samples and pH value of FA solution.In Test Group 2, when the sample is immersed in a FA solution with a pH value of 4.5, the measured value of the FA content increased significantly as the dosage of HA reagent increased from 0% to 15%. The reason is that the increase of HA content causes the dispersion of clay minerals in the sample [22], and the pore size increases, so that more FA molecules can invade into the pores of the clay soil with the soaking solution. As the dosage of HA reagent increased from 15% to 25%, the measured value of FA content decreased slightly. The reason is that the adsorption sites on the surface of clay minerals were occupied by more HA, making it difficult for FA to attach, and the amount of FA adsorption decreased [23]. Compared with Test Group 1 (TS-2) and Test Group 3, with the decrease of pH value of FA solution, the content of FA in test sample gradually increased, and the content of HA decreased significantly. The reason is that as the concentration of the FA solution increases, the adsorption capacity of the sample to FA increases, and more FA occupies the binding site. The number of binding sites for HA to contact and adsorb on the clay particles decreases, resulting in more HA precipitation from the sample.Although the content of HG in the PSE of Dianchi Lake varies greatly, the method of adding HA reagent into the cohesive soil with low organic matter content can simulate different PSE according to actual engineering or experimental requirements. For the PSE with higher HA content, the method of increasing the dosage of HA reagent or selecting a higher purity HA reagent can improve the effect of simulating the PSE. Compared with the content range of FA in Dianchi peat soil measured in Table1 (0.79%∼8.34%), although the method of soaking in FA solution can significantly increase the content of FA in the sample, the simulation effect of this method is limited. It can only satisfy the simulation of the PSE with lower FA content. It should be noted that due to the limited adsorption capacity of HG in cohesive soil and the complex interaction between HG and the material components in cohesive soil, the method of incorporating HA reagent into cohesive soil will lead to the actual HA content in the sample being different from the theoretical incorporation value. The method of soaking in FA solution also needs to be tested to determine the FA content of the sample. Therefore, when simulating the PSE, in order to ensure that the test method has a good simulation effect and feasibility, the HG content of the sample should be measured first.


## 4.2. SEM Test Results and Analysis

Figure9 is microstructure images of samples with different amounts of HA reagent (×500). By comparing Figure 9(a)∼9(f), it can be concluded that (1) the internal microstructure of the cohesive soil sample (with 0% of HA reagent) has good integrity and uniformity, with small pore size and poor connectivity. (2) When the dosage of HA reagent increased in the range from 5% to 25%, the pores inside the sample gradually increased and gradually connected. The soil particles of the sample are gradually overhead, forming an obvious loose skeleton, and the integrity of the sample deteriorates. The reason is that there are various functional groups such as carboxyl group and alcoholic hydroxyl group. On the aromatic nucleus structure of HA monomer. In the process of mixing HA, cohesive soil, and water, after HA is wetted by water, the molecular structure and functional groups of HA show higher reactivity, so that HA can interact with pores. The high valence cations in the water react, reducing the cation valence in the pore water, weakening the electrostatic attraction between the cations in the pore water and the negative charges on the surface of the soil particles, increasing the thickness of the diffusion layer and weakening the connection between the soil particles. [23] On the other hand, HA particles have strong adsorption, if there is a certain amount of HA in the cohesive soil, the HA particles will be adsorbed on the surface of the clay particles, increasing the dispersibility of the cohesive soil [22], hindering the coagulation between clay particles, and resulting in the deterioration of the physical properties and structure of the soil.Figure 9
Microstructure images of samples with different amounts of HA reagent (× 500). (a) 0% HA reagent, (b) 5% HA reagent, (c) 10% HA reagent, (d) 15% HA reagent, (e) 20% HA reagent, and (f) 25% HA reagent.
(a)(b)(c)(d)(e)(f)Figure10 is microstructure images of samples soaking in FA solution with different pH values (× 500). By comparing Figure 10(a)∼10(e), it can be concluded that with the decrease of the pH value of the FA solution (or the increase of the solution concentration), the pore size inside the sample gradually decreases, the pore connectivity gradually weakens, the density of the sample increases, and the integrity increases. The reason is due to its small molecular structure and low molecular weight, FA can enter the pores of the sample and the FA in the pore water is gradually wrapped on the soil skeleton through adsorption. Due to its own colloidal properties [21, 23], colloidal connections are generated between clay particles. On the other hand, the soluble FA particles will remain in the pores as fillers, which will reduce the pore size of the sample and weaken the connectivity. At the same time, by reducing the pH value (or increasing the concentration) of the FA solution, the content of FA in the sample can be increased and the adsorption capacity of FA can be increased. In addition, the decrease of pH will weaken the protonation of acidic functional groups, increase the hydrophobicity, and weaken the water solubility [23], and the electrostatic repulsion in the organic matter-mineral system will also weaken, which is favorable for the adsorption of FA.Figure 10
Microstructure images of samples soaked in FA solution with different pH values (× 500). (a) 7.0 pH, (b) 6.0 pH, (c) 5.5 pH, (d) 5.0 pH, and (e) 4.5 pH.
(a)(b)(c)(d)(e)


## 4.3. X-Ray Diffraction Test Results and Analysis

Figure11 is the XRD pattern of alluvial-proluvial cohesive soil. It can be seen from Figure 11 the main material components in the alluvial-proluvial cohesive soil. Among them, the primary minerals are silica (SiO2) and orthoclase (K2O·Al2O3·6SiO2). Secondary minerals are kaolinite (Al2O3 2SiO2 2H2O), serpentine (6MgO 4SiO2 4H2O), and oxides (TiO2, Ti2O3, Fe2O3, FeOOH).Figure 11
XRD pattern of cohesive soil.HG is a high molecular polymer, and its molecular structure is very complex. The different components of HG (humic acid, fulvic acid) can be regarded as composed of a series of molecules of different sizes, rarely with specific and precise structural shape and active functional group sequence [24]. On the other hand, HG in peat soil is divided into free and bound, and most of them are bound HG. Usually 52%–98% of the HG in peat soil is bound in the clay part [25], which constitutes the PSE in which the organic-inorganic complexes are combined in the peat soil. Figure 12 is relationship curve between main diffraction peaks of SiO2 and the amount of HA reagent. It can be seen from Figure 12 that with the increase of the dosage of HA reagent, the main diffraction peak curve of SiO2 in the sample has a large fluctuation and no obvious regularity. Figure 13 is relationship curve between main diffraction peaks and SiO2 and pH value of FA solution. It can be seen from Figure 13 that with the increase of the pH value of the FA solution (or the decrease of the solution concentration), the main diffraction peaks of SiO2 have an obvious upward trend. SiO2, as a primary mineral in cohesive soil, has relatively stable chemical properties. In addition, SiO2 is an acidic oxide and does not react with various acidic substances other than hydrofluoric acid. According to Emerson's [16] theory of clay particle structure, since HG has a variety of active functional groups, HG can cement the clay unit (or structural surface) and SiO2 together to form agglomerates. The active functional groups on the HG structure are linked with the SiO2 surface in the form of bonds [26], forming SiO2-organic matter complexes. However, due to the relatively stable chemical properties of SiO2, this type of action is not easy to occur and the chemical stability is poor. Therefore, with the increase of the dosage of HA reagent, the main diffraction peak curve of SiO2 in the sample fluctuates greatly. Compared with HA, FA contains more active functional groups dominated by carboxyl groups in its molecular structure, showing more significant weak acidity and surface reaction activity than HA [24], which makes it have stronger binding ability and higher binding bond stability with SiO2 surface. Therefore, when the amount of HA reagent reaches a certain level, with the decrease of pH value (or the increase of concentration) of FA solution, the main diffraction peak of SiO2 in the sample has an obvious decreasing trend.Figure 12
Relationship curve between main diffraction peaks of SiO2 and the amount of HA reagent.Figure 13
Relationship curve between main diffraction peaks and of SiO2 and pH value of FA solution.Figure14 is the relationship curve between diffraction peak value of main substances (except SiO2) and the amount of HA reagent. It can be concluded from Figure 14 that when the sample is immersed in distilled water, with the increase of HA reagent added, the diffraction peaks of the main substances in the sample (except SiO2) have a clear trend of decreasing. Figure 15 is the relationship curve between diffraction peak value of main substances (except SiO2) and pH value of FA solution. It can be concluded from Figure 15 that as the pH value of the FA solution decreases (or the solution concentration increases), the diffraction peaks of the main substances in the sample (except SiO2) gradually decrease.Figure 14
Relationship curve between diffraction peak value of main substances (except SiO2) and the amount of HA reagent.Figure 15
Relationship curve between diffraction peak value of main substances (except SiO2) and pH value of FA solution.From the chemical mechanism analysis: the interaction between HA and metal oxides and silicate minerals mainly includes hydrophobic interaction, Coulomb attraction, coordination exchange, ion exchange, cation bridge, and hydrogen bond interaction [25–32]. Among them, (1) due to the hydrophobicity of some nonpolar components in HG, such components are alienated from water molecules, and HG can be contacted and adsorbed on the surface of minerals. On the other hand, HG is a polydisperse electrolyte that can attract each other with the mineral surface through Coulomb attraction. (2) Coordination exchange is the most important mechanism [27, 29], which means that acidic functional groups such as carboxyl (- COO - or - COOH) in HG and hydroxyl (- OH) on the surface of clay minerals form complexes through coordination reaction. The reaction process mainly includes three stages: (1) surface protonation; (2) forming an outer ring compound; and (3) forming inner ring complexes. The main reaction formula is as follows:(1)S—OH+H+⇌S—OH2+,(2)S—OH2++Hu—COO−⇌Hu—COOS+H2O,where S–OH is the hydroxyl group on the mineral surface and Hu COO− is the carboxyl group of HG. The hydroxyl group on the mineral surface first adsorbs H+ in the solution to complete the basic process of surface protonation (1). The protonated mineral surface forms an outer ring compound with the carboxyl group in HG, and then OH2+ is coordinated and exchanged with Hu COO− to form an inner ring compound formula (2). (3) Ion exchange and cation bridges mainly occur between negatively charged mineral surfaces and negatively charged HG molecules. Metal oxides are usually positively charged. They cover the negatively charged clay mineral surface as a glue film [29, 30]. Negatively charged HG molecules can be combined with metal oxides through ion exchange reaction. Metal cations and their oxides form a bond bridge between the negatively charged mineral surface and the anions or polar functional groups of HG, playing the role of “cation bridge”. There is usually a layer of water molecules between the functional groups of metal cations and HG, which is mostly caused by hydrogen bonding [27]. Although clay minerals are generally negatively charged, they may also be positively charged at the edge of clay minerals, and negatively charged HG molecules can also be directly adsorbed with positively charged clay minerals [25]. In this test, kaolinite is positively charged [25] in acidic environment and can be directly adsorbed with HG molecules.In the indoor simulation test of PSE, when the sample is immersed in distilled water with pH value of 7.0, the diffraction peak of main substances (except SiO2) in cohesive soil decreases with the increase of the amount of HA reagent. When the sample is immersed in FA solution, the diffraction peaks of main substances (including SiO2) in cohesive soil decrease with the decrease of FA pH solution value (or the increase of solution concentration). The main reason is that HG and clay particles have a variety of functions such as coordination exchange and ion exchange. At the same time, it shows that simulation test can simulate PSE. That is, the free HG combines with clay particles and changes into bound HG, forming a HG environment combined with organic-inorganic complex similar to PSE.


## 5. Conclusion

Firstly, the actual content of HG in peat soil of Dianchi Lake is determined. Then, the laboratory test is used to simulate the PSE by mixing HA reagent into cohesive soil and soaking in FA solution. By comparing the HG content of test samples and natural peat soil, the effect and feasibility of simulation test are studied. Combined with SEM test and XRD test, the effect of HG on the microstructure and material composition of peat soil was explored. The conclusions are as follows.(1)
The total amount of HG in peat soil of Dianchi Lake is quite different, and the content of different components of HA is also quite different. But the common trend is that the contents of HA and humin in peat soil are higher, while the content of FA is lower.(2)
The PSE with different HA and FA contents can be simulated by mixing HA reagent into cohesive soil with less organic matter and soaking in FA solution, but the simulation method of soaking samples in FA solution cannot reach the actual effect.(3)
The results of SEM test show that the increase of HA reagent will gradually increase the internal pores of the sample. The pores are gradually connected, and the soil particles are gradually overhead, forming an obvious loose skeleton, and the integrity of the soil becomes worse. Soluble FA can be wrapped on the soil skeleton through adsorption and fill some pores, which reduces the pore size and weakens the connectivity between pores.(4)
The results of XRD test show that the existence of HG will reduce the diffraction peak of main substances (except SiO2) in soil, and FA can reduce the diffraction peak of main substances including SiO2 in soil, which is mainly due to the coordination exchange and ion exchange between HG and clay particles. The free HG combined with clay particles is transformed into bound HG, which is formed in the HG environment combined with the organic-inorganic complex similar to PSE.



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*Source: 1437733-2022-06-02.xml*