Experiment and Simulation on Migration Rule of Arsenic in Soil of Surface Karst Zone in Southwest China
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摘要: 岩溶地下河是我国西南地区的重要水源，工业生产过程中产生的砷污染物，除通过落水洞等直接进入并污染地下水外，还会在表层岩溶带溶缝、溶隙内吸附、滞留及富集，并在特定条件下再次迁移，成为"稳定次生污染源".以广西某砷污染事件为例，采用窄缝槽物理模型装置进行砷的动态吸附、解吸实验，并结合地球化学模拟研究砷污染物在表层岩溶带土壤中的迁移规律.实验结果显示表层岩溶带对砷的吸附以物理吸附（扩散过程）为主，相比吸附过程而言解吸速率则显得缓慢，而酸溶液相比去离子水可促进砷的解吸过程.地球化学模拟结果表明土壤矿物中以针铁矿对砷的吸附贡献最大，而酸溶液通过溶蚀针铁矿等矿物削弱对砷的吸附能力.因此在西南岩溶地区，表层岩溶带系统一旦纳入砷污染物，则解吸过程缓慢，易形成砷污染物的滞留、富集；而酸雨作用下砷的解吸、迁移过程加快，则会提高地下水系统的污染风险.Abstract: Karst underground river is an important water source in Southwest China. In addition to entering and polluting the groundwater directly through the sinkholes, arsenic pollutants produced from the industrial production process will also adsorb, remain, and concentrate in the dissolution cracks and gaps in the surface karst zone, and remigrate under certain conditions to become a "secondary pollution source". Taking an arsenic pollution event in Guangxi as an example, the dynamic adsorption and desorption experiments of arsenic were carried out by using narrow slot physical model device, and the migration rule of arsenic contamination in the soil of surface karst zone was studied, combined with geochemical simulation. The experimental results show that the adsorption of arsenic in the surface karst zone is mainly physical adsorption (diffusion process), and the desorption rate is slow compared with the adsorption process, while the acid solution can promote the desorption process of arsenic compared with deionized water. The results of geochemical simulation show that goethite contributes most to arsenic adsorption in soil minerals, while acid solution weakens the ability of arsenic adsorption by corroding goethite and other minerals. It is concluded that in the karst area of Southwest China, once the surface karst zone system is integrated with arsenic pollutants, the desorption process is slow, and it is easy to form the retention and enrichment of arsenic pollutants; while the accelerated desorption and migration process of arsenic under acid rain will increase the pollution risk of groundwater system.
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图 1 研究区裸露型表层岩溶带发育形态
Fig. 1. Development form of exposed surface karst zone in the study area
图 3 窄缝槽实验装置(a)及其设计图(b)
Fig. 3. (a) Diagram of narrow slot experimental device; (b) design of narrow slot experimental device
图 4 模拟表层岩溶带土壤对As(Ⅲ)的吸附穿透曲线
Fig. 4. Breakthrough curve of As(Ⅲ) in the adsorption experiment
图 5 模拟表层岩溶带对As(Ⅲ)吸附的动力学过程拟合
Fig. 5. Kinetic process fitting of As(Ⅲ) adsorption process
图 6 解吸实验(使用去离子水)中出水浓度变化曲线
Fig. 6. Breakthrough curve of As(Ⅲ) in the desorption experiment with deionized water
图 7 解吸实验(使用去离子水)中As(Ⅲ)解吸过程的动力学拟合
Fig. 7. Kinetic process fitting of As(Ⅲ) desorption process with deionized water
图 8 解吸实验(使用酸溶液)中出水浓度变化曲线
Fig. 8. Breakthrough curve of As(Ⅲ) in the desorption experiment with acidic solution
图 9 解吸实验(使用酸溶液)中As(Ⅲ)解吸过程的动力学拟合
Fig. 9. Kinetic process fitting of As(Ⅲ) desorption process with acidic solution
图 10 基于地球化学模拟的吸附试验出水浓度变化曲线
Fig. 10. Breakthrough curves of As(Ⅲ) in the adsorption experiment based on geochemical modeling
图 11 解吸实验(使用酸溶液)中土壤pH变化模拟
Fig. 11. Simulation of soil pH variation in desorption experiment with acidic solution
表 1 地球化学模拟中所用参数
Table 1. Parameters of soil minerals for geochemical modeling
参数 伊利石 高岭石 针铁矿 比表面积(m2/g) 24.2 21.6 54.0 位点密度(10-4 mol/mol) 4.45 6.30 160.00 lg KS+(int) 3.53 6.28 7.29 lg KS-(int) -7.10 -9.28 -8.93 lg KSAs(Ⅲ)1(int) 4.49 3.97 5.41 lg KSAs(Ⅲ)2(int) -1.85 -3.66 / lg KSAs(Ⅲ)3(int) -11.2 -14.1 / 注：“/”表示针铁矿对最后两个阶段并无吸附. 
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表 2 模拟表层岩溶带土壤吸附/解吸过程参数对比
Table 2. Comparison of parameters in absorption or desorption kinetic process
过程 拟合双常数方程 R2 起始吸附/解吸量(mg/kg) 吸附/解吸速率(mg/kg·min-1) 完全吸附/解吸时间(min) 实验1-吸附过程 lnQt = 0.951 7lnt-5.338 1 1.000 0 0.004 8 0.951 7 999.4 实验2-解吸过程(去离子水) lnQt=0.792 8lnt-6.454 5 0.998 8 0.001 6 -0.792 8 16 313.1 实验3-解吸过程(酸溶液) lnQt=0.844 1lnt-5.587 7 0.999 5 0.003 7 -0.844 1 13 489.6
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表 3 基于地球化学模拟的不同矿物吸附量及解吸试验(酸溶液)前后的矿物含量变化
Table 3. Changes of mineral contents before and after desorption test (acid solution) based on geochemical simulation
单元 伊利石吸附砷量(10-4 mmol) 蒙脱石吸附砷量(10-4 mmol) 针铁矿吸附砷量(10-4 mmol) 伊利石含量(mg/kg) 蒙脱石含量(mg/kg) 针铁矿含量(mg/kg) 实验前 实验后 实验前 实验后 实验前 实验后 0~5 cm 8.059 9.829 40.598 6.936 5.346 5.913 7.015 0.224 0.087 5~10 cm 10.445 8.255 8.216 6.936 6.929 5.913 5.892 0.224 0.018 10~5 cm 10.444 8.255 8.184 6.936 6.929 5.913 5.892 0.224 0.018 15~20 cm 10.444 8.255 8.154 6.936 6.928 5.913 5.892 0.224 0.018 20~25 cm 10.443 8.256 8.125 6.936 6.928 5.913 5.893 0.224 0.017 25~30 cm 10.442 8.256 8.099 6.936 6.928 5.913 5.893 0.224 0.017 30~25 cm 10.442 8.256 8.073 6.936 6.927 5.913 5.893 0.224 0.017 35~40 cm 10.441 8.257 8.048 6.936 6.927 5.913 5.894 0.224 0.017 40~45 cm 10.441 8.257 8.025 6.936 6.927 5.913 5.894 0.224 0.017 45~50 cm 10.441 8.257 8.011 6.936 6.927 5.913 5.894 0.224 0.017
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