Experimental study on the effect of ion exchange on solute transport in a sandy tank
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摘要:
为研究河流阶地和洪积扇这类典型水力沉积单元污染物运移规律, 搭建了大尺寸室内渗流槽开展溶质运移实验。通过点投放NaNO3溶液模拟点源污染物在水力沉积物中的运移过程, 测定不同位置主要离子成分随时间的变化, 用于分析离子交换过程对于溶质运移的影响及溶质迁移规律。结果表明: 在运移过程中, NO32-属于保守性离子, 穿透曲线呈尖瘦形, Na+受阳离子交替吸附作用的显著影响, 峰形陡升缓降; 阳离子交替吸附作用降低了Na+的弥散度, 离子交替吸附作用对弥散度的影响随着运移距离的增加愈加明显; 运移初期高浓度Na+在砂层中能交换出相当数量的Ca2+, Mg2+, K+等离子; 运移后期阳离子交替吸附反应方向改变, 沉积砂层吸附水中的Ca2+, Mg2+, K+, 穿透曲线存在3种阳离子低于背景值的情况; 交替吸附作用使得对流-弥散作用下的Na+质量浓度穿透曲线形状更加宽缓, "拖尾"现象更为明显; 渗流砂槽内不同区域的水化学类型在空间上产生了差异性。研究成果对于开展水力沉积单元地下水污染防治具有一定指导意义。
Abstract:Objective To study the transport of pollutants in typical hydraulic sedimentary units such as river terraces or alluvial fans, solute transport experiments were carried out in an indoor seepage tank.
Methods NaNO3 solution were introduced into the tank to simulate the point-pollution in hydraulic sediments. By measuring the concentration of the main ion components at different positions over time, the migration law of pollutants and the ion exchange process are analysed.
Results The results show that NO32- is a conservative ion, and its breakthrough curve (BTC) is sharp and thin. The transport behaviour of Na+ is significantly affected by cation exchange, its concentration rising sharply and decreasing slowly. Cation exchange reduces the dispersion of Na+, and the effect becomes more obvious as the distance increases. In the early stage, the high concentration of Na+ can exchange Ca2+, Mg2+, and K+ in the sand layer. Cation exchange reduces the Na+ dispersion concentration. Due to the adsorption by sediment, the concentrations of Ca2+, Mg2+, and K+ will be lower in the later stage. The change in the reaction direction of cation exchange makes the BTCs of Na+ wider and gentler under the action of advective dispersion, and the phenomenon of "tailing" is more obvious. The water chemistry types in different areas in the seepage sank have different properties in space.
Conclusion The research results have guiding significance for preventing and controlling groundwater pollution in hydraulic sedimentary units.
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Key words:
- solute transport /
- cation exchange adsorption /
- breakthrough curve /
- advective dispersive
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图 1 渗流槽系统物理模型
a.实物图；b.模型示意图；c.定水头装置；d.溶质投放装置；e.取样装置
Figure 1. Physical model of the seepage tank
图 2 监测孔布设位置示意图(1~9为监测孔)
Figure 2. Schematic diagram of the location of the monitoring holes
图 3 阴离子质量浓度穿透曲线(分图中左上角编号②~⑨分别对应图 2中的2~9号监测孔)
Figure 3. Concentration breakthrough curve of anion
图 5 交替吸附作用对Na+质量浓度的影响
Figure 5. Effect of cation exchange adsorption on the concentration of Na+
表 1 渗流槽填充介质设计参数
Table 1. Design parameters of the filling medium of the seepage tank
填充介质 几何参数 物理参数 矿物组成 长/cm 宽/cm 高/cm 容重/(kg·m-3) 孔隙度/% 渗透系数/(m·s-1) 黏土 150 100 40 1 830 58 10-7 高岭石、蒙脱石 中砂 340 100 60~90 2 460 44 10-4 石英、长石 砾石 100 100 20 2 650 28 2×10-4 石英、云母 
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表 2 监测孔布设位置坐标
Table 2. Coordinates of the monitoring holes locations
监测孔编号 1 2 3 4 5 6 7 8 9 x/cm 20 50 80 120 120 150 250 250 300 y/cm 50 50 50 75 25 50 75 25 50 z/cm 50 50 50 50 75 50 75 75 75 注: 以模拟槽前端顶部近视点为坐标原点(见图 2)，长度单位为cm
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表 3 2, 3号监测孔Na+和NO32-的弥散度计算结果
Table 3. Calculation results of the dispersion of Na+ and NO32- in the No.2 and No.3 monitoring holes
离子成分 x/m tm/h 实际流速v/(m·h-1) 弥散系数DL/(m2·h-1) 弥散度α/m Na+ 0.3 1.76 0.1 0.008 384 0.083 841 0.6 3.56 0.016 381 0.163 809 NO32- 0.3 1.75 0.1 0.008 482 0.084 821 0.6 3.36 0.018 384 0.183 847
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表 4 交替吸附作用对到达峰值时间的影响
Table 4. Effect of cation exchange adsorption on the peak time
监测孔编号 2 3 4 6 7 8 9 到达峰值时间tm/h NO32- 1.75 3.36 5.50 5.50 7.50 8.00 8.75 Na+ 1.76 3.56 — — 8.75 9.00 11.00 延迟时间tdelay/h 0.01 0.20 — — 1.25 1.00 2.25 延迟时间比r/% 0.57 5.95 — — 16.67 12.50 25.71 注：延迟时间比$r=\frac{t_{\text {delay }}}{t_{\mathrm{m}}\left(\mathrm{NO}_3^{2-}\right)} \times 100 \% $
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