离子交换行为对溶质运移影响作用的实验研究

Experimental study on the effect of ion exchange on solute transport in a sandy tank

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NaNO_(3)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,NO_(3)^(2-)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Ca^(2+),Mg^(2+),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Ca^(2+),Mg^(2+),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。

[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]NaNO_(3) 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 NO_(3)^(2-) 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 Ca ^(2+),Mg^(2+),and K^(+)in the sand layer.Cation exchange reduces the Na^(+)dispersion concentration.Due to the adsorption by sediment,the concentrations of Ca^(2+),Mg^(2+),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.