Per- and poly-fluoroalkyl substances (PFAS) adsorb at air-water interfaces during transport in unsaturated porous media. This can cause surfactant-induced flow and enhanced retention that is a function of concentration, which complicates characterization and modeling of PFAS transport under unsaturated conditions. The influence of surfactant-induced flow and nonlinear air-water interfacial adsorption (AWIA) on PFAS transport was investigated with a series of miscible-displacement transport experiments conducted with a several-log range in input concentrations. Perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and ammonium perfluoro 2-methyl-3-oxahexanoate (GenX) were used as model PFAS. The results were interpreted in terms of critical reference concentrations associated with PFAS surface activities and their relationship to the relevancy of transport processes such as surfactant-induced flow and nonlinear AWIA for concentration ranges of interest. Analysis of the measured transport behavior of PFAS under unsaturated-flow conditions demonstrated that AWIA was linear when the input concentration was sufficiently below the critical reference concentration. This includes the absence of significant arrival-front self-sharpening and extended elution tailing of the breakthrough curves, as well as the similarity of retardation factors measured for a wide range of input concentrations. Independently-predicted simulations produced with a comprehensive flow and transport model that accounts for transient variably-saturated flow, surfactant-induced flow, nonlinear rate-limited solid-phase sorption, and nonlinear rate-limited AWIA provided excellent predictions of the measured transport. A series of simulations was conducted with the model to test the specific impact of various processes potentially influencing PFOS transport. The simulation results showed that surfactant-induced flow was negligible and that AWIA was effectively linear when the input concentration was sufficiently below the critical reference concentration. PFAS retention associated with AWIA can be considered to be ideal in such cases, thereby supporting the use of simplified mathematical models. Conversely, apparent nonideal transport behavior was observed for experiments conducted with input concentrations similar to or greater than the critical reference concentration.

全氟烷基物质和多氟烷基物质 (PFAS) 在不饱和多孔介质中传输过程中会吸附在空气-水界面上。这可能会导致表面活性剂引起的流动和增强的保留，这是浓度的函数，这使得不饱和条件下 PFAS 传输的表征和建模变得复杂。通过在输入浓度的几个对数范围内进行一系列混相置换传输实验，研究了表面活性剂诱导流动和非线性空气-水界面吸附 (AWIA) 对 PFAS 传输的影响。全氟辛烷磺酸 (PFOS)、全氟辛酸 (PFOA) 和全氟 2-甲基-3-氧己酸铵 (GenX) 被用作模型 PFAS。结果根据与 PFAS 表面活性相关的临界参考浓度及其与传输过程（例如表面活性剂引起的流动和感兴趣浓度范围的非线性 AWIA）的相关性的关系来解释。对不饱和流条件下测量的 PFAS 传输行为的分析表明，当输入浓度充分低于临界参考浓度时，AWIA 呈线性。这包括不存在显着的到达前沿自锐化和突破曲线的延长的洗脱拖尾，以及针对各种输入浓度测量的延迟因子的相似性。使用综合流动和传输模型生成的独立预测模拟考虑了瞬态可变饱和流动、表面活性剂诱导流动、非线性限速固相吸附和非线性限速 AWIA，为测量的传输提供了出色的预测。使用该模型进行了一系列模拟，以测试可能影响 PFOS 传输的各种过程的具体影响。模拟结果表明，当输入浓度充分低于临界参考浓度时，表面活性剂引起的流动可以忽略不计，并且 AWIA 有效地呈线性。在这种情况下，与 AWIA 相关的 PFAS 保留被认为是理想的，从而支持使用简化的数学模型。相反，在输入浓度类似于或大于临界参考浓度的实验中观察到明显的非理想传输行为。