Soil, Water and Environmental Science Department, Hydrology and Atmospheric Sciences Department, School of Earth and Environmental Sciences , University of Arizona , 429 Shantz Building , Tucson , Arizona 85721 , United States.
Environ Sci Technol. 2018 Jul 17;52(14):7745-7753. doi: 10.1021/acs.est.8b02348. Epub 2018 Jun 26.
Miscible-displacement experiments are conducted with perfluorooctanoic acid (PFOA) to determine the contribution of adsorption at the air-water interface to retention during transport in water-unsaturated porous media. Column experiments were conducted with two sands of different diameter at different PFOA input concentrations, water saturations, and pore-water velocities to evaluate the impact of system variables on retardation. The breakthrough curves for unsaturated conditions exhibited greater retardation than those obtained for saturated conditions, demonstrating the significant impact of air-water interfacial adsorption on PFOA retention. Retardation was greater for lower water saturations and smaller grain diameter, consistent with the impact of system conditions on the magnitude of air-water interfacial area in porous media. Retardation was greater for lower input concentrations of PFOA for a given water saturation, consistent with the nonlinear nature of surfactant fluid-fluid interfacial adsorption. Retardation factors predicted using independently determined parameter values compared very well to the measured values. The results showed that adsorption at the air-water interface is a significant source of retention for PFOA, contributing approximately 50-75% of total retention, for the test systems. The significant magnitude of air-water interfacial adsorption measured in this work has ramifications for accurate determination of PFAS migration potential in vadose zones.
进行了全氟辛酸 (PFOA) 的可混相驱替实验，以确定在水非饱和多孔介质中传输过程中，吸附在气-水界面上对保留的贡献。在不同的 PFOA 输入浓度、水饱和度和孔隙水速度下，用两种不同直径的砂进行了柱实验，以评估系统变量对迟滞的影响。不饱和条件下的穿透曲线表现出比饱和条件下更大的迟滞，这表明气-水界面吸附对 PFOA 保留的显著影响。对于较低的水饱和度和较小的粒径，迟滞更大，这与系统条件对多孔介质中气-水界面面积大小的影响一致。对于给定的水饱和度，对于较低的 PFOA 输入浓度，迟滞更大，这与表面活性剂流体-流体界面吸附的非线性性质一致。使用独立确定的参数值预测的迟滞因子与测量值非常吻合。结果表明，气-水界面上的吸附是 PFOA 保留的一个重要来源，对于测试系统，约占总保留量的 50-75%。本工作中测量的气-水界面吸附的显著程度对准确确定 PFAS 在包气带中的迁移潜力有重要影响。
