胶体作用下饱和土壤中Cd运移的数值模拟

NUMERICAL SIMULATION OF Cd TRANSPORT IN SATURATED SOILS AS AFFECTED BY COLLOID

通过室内土柱出流实验探讨了饱和壤砂土中Cd在SiO2胶体作用下的运移行为,并用对流-弥散方程(Convection-Dispersion Equation,CDE)对实验结果进行了数值模拟。结果表明,土壤中SiO2胶体的存在抑制了Cd的运移,使Cd穿透时间较晚,出流浓度较低,土壤对Cd的吸附以动力学反应为主。耦合Freun-dlich等温吸附的单点非平衡模型(One Site Nonequilibrium Model,OSM)能较好地模拟Cd在壤砂土中的运移,模拟得到的吸附特性参数β较小,表明壤砂土吸附表面的不均一性较强。SiO2胶体的穿透时间较早,耦合沉淀-释放反应的CDE模型可以很好地描述SiO2胶体在壤砂土中的运移。胶体作用下的CDE模型成功地模拟了SiO2胶体存在时Cd的运移;同时,参数敏感性分析结果表明,胶体对Cd的吸附速率系数kamc、kaic越小,解吸速率系数kdmc、kdic越大,越有利于Cd的运移。

Mobile colloid may play an important role in the transport of contaminants in soils and ground water, especially for strongly adsorbed heavy metals. Effect of colloid SiO2 on transport of Cd in saturated soil was studied in an indoor soil column and effluent experiment. Results of the experiment were simulated numerically with the CDE （ convectiondispersion equation）. It was found that the colloid inhibited mobility of Cd, making it spend longer time getting through the column and lower in concentration in the effluent and that Cd adsorption was mainly controlled by dynamic reaction. The OSM （one site nonequilibrium model） coupled with the Freundlich isothermal adsorption could better simulate Cd transport in the loamy sand soil. The Cd adsorption feature parameter β obtained through simulation was quite small which indicates that the adsorption surface of the soil was rather nonuniform. SiO2 colloid penetrated the column pretty quickly. Its transport in the soil could well be described with the CDE coupled with kinetic attachment-detachment reaction. As affected by colloid, the CDE could successfully simulate transport of Cd in the soil with SiO2 in presence. Meanwhile the parameter sensitivity analysis indicated that the smaller the k kamc and the kaic, the larger the kdmc and the kdic and the more convenient the transport of Cd.