Soil-water chemical tests and action mechanism of acid rain infiltration into expansive soil
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摘要: 弄清酸雨入渗膨胀土所产生的水土化学作用，对获取酸雨区浅表层膨胀土基本性能劣化机理及分析边坡的稳定性非常重要。为此，取广西酸雨重灾区的百色原状膨胀土，研制循环饱水试验装置并模拟不同pH值的雨水入渗环境，开展酸雨入渗膨胀土水土化学试验；运用多种微观测试技术，探究膨胀土的矿物与化学成分演变规律；采用地球化学模拟软件（PHREEQC）及水土化学理论，分析膨胀土与酸雨间水土化学作用机理。结果表明：相比静态水饱和状态，降雨入渗动态水饱和环境能促进膨胀土中矿物质的溶蚀与淋滤，加速土体结构的破坏；酸雨入渗后膨胀土中CaO、Fe2O3、K2O明显大于SiO2、Al2O3等氧化物的溶蚀与淋滤量；雨水的pH值越小，膨胀土片状微结构间溶蚀的方解石及游离氧化物等胶结物越多，离子交换作用增强；酸雨与土中矿物的相互作用加剧，部分伊利石脱钾转变为蒙脱石黏土矿物，导致伊利石含量减小，蒙脱石含量增加，土的亲水性增强而结构不稳定性变大，不利于膨胀土边坡的稳定。Abstract: It is very important to clarify the mechanism of soil-water chemical interaction between acid rain and expansive soil for studying the deterioration mechanism of performance of shallow expansive soil and slope stability in acid rain areas. Therefore, the undisturbed expansive soil in the acid rain-affected areas in Baise City, Guangxi Zhuang Autonomous Region is studied. A cyclic water saturated test device is designed to simulate the rainwater infiltration environment with different pH values to carry out soil-water chemical tests. A variety of micro-test techniques are used to explore the evolution law of minerals and chemical components in the expansive soil. The mechanism of soil-water chemical interaction between expansive soil and acid rain is analyzed by using PHREEQC simulation software and soil-water chemical theory. The results show that compared with the static water saturated environment, the dynamic water saturated environment of rainfall infiltration promotes the dissolution and leaching of minerals in the expansive soil, and accelerates the destruction of soil structure. The dissolution and leaching amount of CaO, Fe2O3, K2O in the expansive soil under acid rain infiltration is significantly greater than that of SiO2, Al2O3. The lower the pH value of rainwater, the more the cements such as calcite and free oxides leached between the sheet microstructures of the expansive soil, and the ion-exchange will be enhanced. The interaction between acid rain and minerals in soil is intensified, and part of illite is converted into montmorillonite clay minerals, resulting in the decrease of illite content, the increase of montmorillonite content, the enhancement of soil hydrophilicity and the increase of structural instability, which is not conducive to the stability of expansive soil slopes.
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图 4 循环饱水试验溶液中沉淀质量
Figure 4. Mass of sediments in solution of cyclic water saturated tests
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图 5 循环饱水试验中参与反应的主要矿物质量
Figure 5. Mass of main minerals involved in reaction in cyclic water saturated tests
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图 6 试样主要化合物成分及质量分数
Figure 6. Composition and mass fraction of main compounds in samples
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图 7 沉淀物主要化合物成分及质量分数
Figure 7. Composition and mass fraction of main compounds in sediments
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图 8 溶液中沉淀物主要化学成分质量
Figure 8. Mass of main chemical components of sediments in solution
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图 10 循环饱水试验中主要阳离子成分及其质量浓度
Figure 10. Main cation components and its mass concentration in cyclic water saturated tests
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图 12 不同pH值（pH=3, 5, 7）降雨环境下膨胀土的微观结构图
Figure 12. Microstructure of soil samples under rainwater environment with pH = 3, 5, 7
表 1 膨胀土的基本性质参数
Table 1 Characteristic indices of expansive soil
相对质量密度Gs 天然含水率/% 天然密度/(g∙cm-³) 液限wL/% 塑限wp/% 塑性指数IP 颗粒质量分数/% 蒙脱石
质量分数/%比表
面积/(m2∙g-1)自由膨胀率δef/% 伊/蒙混层比/% > 0.07 mm 0.005~0.075 mm < 0.005 mm 2.70 20.6 2.09 56.3 21.4 34.9 0.10 52.02 47.88 16.58 130.77 82.00 45.00 
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表 2 循环饱水试验中试样及沉淀物的质量
Table 2 Mass of samples and sediments in cyclic water saturation tests
试验条件 试验前试样质量/g 试验后试样质量/g 溶液中沉淀质量/g 参与反应质量/g pH=7（静态水） 103.214 97.170 3.291 2.753 pH=7 101.368 85.272 9.124 6.972 pH=5 104.782 84.782 11.331 8.135 pH=3 102.312 74.014 16.215 12.083
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表 3 循环饱水试验中试样主要矿物成分及质量分数
Table 3 Composition and mass fraction of main minerals of samples in cyclic water saturation tests
(%) 主要矿物成分 原状样 pH=7（静态水） pH=7 pH=5 pH=3 石英 19.02 19.21 19.17 19.21 19.75 方解石 22.53 20.78 18.02 15.54 10.01 伊利石 20.42 20.63 20.49 19.99 17.95 蒙脱石 18.81 19.01 19.48 20.52 22.25 高岭石 17.22 17.39 17.54 17.31 17.44
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表 4 循环饱水试验中沉淀物主要矿物成分及质量分数
Table 4 Composition and mass fraction of main minerals of sediment in cyclic saturation tests
(%) 主要矿物成分 原状样 pH=7（静态水） pH=7 pH=5 pH=3 石英 19.02 — 20.48 21.91 22.13 方解石 22.53 — 15.08 11.87 8.01 伊利石 20.42 — 22.31 21.73 21.24 蒙脱石 18.81 — 20.56 20.77 21.52 高岭石 17.22 — 18.93 19.17 19.01
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表 5 循环饱水试验中主要矿物的质量
Table 5 Mass of main minerals in cyclic saturation tests
试验条件 实测参数 矿物质量/g 石英 方解石 伊利石 蒙脱石 高岭石 pH=7（静态水） 试验前试样质量 20.072 23.776 21.550 19.851 18.173 试验后试样质量 19.739 21.352 21.198 19.533 17.869 溶液中沉淀质量 — — — — — 参与反应的矿物质量 — — — — — pH=7 试验前试样质量 19.740 23.383 21.193 19.522 17.872 试验后试样质量 18.163 17.073 19.413 18.456 16.614 溶液中沉淀质量 1.216 0.659 1.206 1.152 1.064 参与反应的矿物质量 0.208 5.651 0.574 -0.086 0.194 pH=5 试验前试样质量 20.228 23.961 21.717 20.005 18.314 试验后试样质量 18.279 14.787 19.021 19.526 16.471 溶液中沉淀质量 1.741 0.630 1.671 1.693 1.495 参与反应的矿物质量 0.310 8.544 1.025 -1.214 0.347 pH=3 试验前试样质量 20.095 23.804 21.575 19.874 18.194 试验后试样质量 17.669 8.955 16.059 20.128 15.777 溶液中沉淀质量 2.117 0.358 1.776 2.067 1.764 参与反应的矿物质量 0.362 14.490 3.740 -2.322 0.653 注：正值为反应物，负值为生成物。
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表 6 参与化学作用各反应相的反应量模拟结果
Table 6 Simulated results of reaction amount of each reaction phase involved in chemical reaction
(mol) 反应相 pH=7 pH=5 pH=3 蒙脱石 -8.873×10-6 -1.050×10-5 -2.207×10-5 方解石 1.287×10-4 2.453×10-4 8.799×10-4 伊利石 -4.987×10-5 -5.132×10-5 -6.845×10-5 高岭石 -7.886×10-5 -3.229×10-5 -3.158×10-5 石英 1.550×10-5 2.137×10-6 5.983×10-6
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