Among emerging contaminants, per- and polyfluoroalkyl substances (PFAS) have captured public attention based upon their environmental ubiquity and potential risks to human health. Due to their typical surface release conditions and amphiphilic properties, PFAS tend to sorb to soil and accumulate at the air-water interface within the vadose zone. These processes can result in substantial plume attenuation. Although there is a growing body of literature on vadose zone transport, few studies have explored PFAS mixture transport, particularly under conditions where nonlinear sorption processes are important. The present study aims to advance our understanding of PFAS transport in variably saturated porous media through integration of experiments and mathematical modeling. Experiments include batch studies to quantify sorption to the solid phase, interfacial tension (IFT) measurements to estimate adsorption at the air-water interface (AWI), and column studies with F-70 Ottawa sand at 100% and ca. 50% water saturation to explore transport mechanisms. Employed PFAS solutions encompass individual solutes and binary mixtures of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) at concentration levels spanning four orders of magnitude to assess competitive and nonlinear sorption at the AWI. Observations demonstrate that concentration levels and competitive effects substantially influence PFAS transport in unsaturated systems. In the presence of PFOS, PFOA experienced less retention than would be anticipated based on single-solute behavior, and effluent breakthrough curves exhibited chromatographic peaking. The presented mathematical model for simultaneous flow and transport of PFAS was able to capture experimental observations with a consistent set of parameters and minimal curve fitting. These results demonstrate the robustness of the model formulation that included rate-limited interfacial mass transfer, an extended Langmuir-Szyszkowski model for adsorption at the AWI, and a scaled Leverett thermodynamic model to predict the AWI specific area. Overall, the results of this work underscore the importance of the AWI in PFAS transport and highlight the relevance of competition effects in adsorption formulations.

中文翻译：

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PFOA 和 PFOS 在不饱和多孔介质中的输运和竞争性界面吸附：实验和建模


在新兴污染物中，全氟烷基和多氟烷基物质 （PFAS） 因其在环境中无处不在和对人类健康的潜在风险而引起了公众的关注。由于其典型的表面释放条件和两亲性特性，PFAS 往往会吸附到土壤中，并在包气带内的气-水界面处积累。这些过程会导致大量的羽流衰减。尽管关于包气带迁移的文献越来越多，但很少有研究探讨 PFAS 混合物迁移，尤其是在非线性吸附过程很重要的条件下。本研究旨在通过整合实验和数学建模来促进我们对 PFAS 在可变饱和多孔介质中传输的理解。实验包括用于量化固相吸附的批量研究、用于估计气-水界面 （AWI） 吸附的界面张力 （IFT） 测量，以及使用 100% 和约 50% 水饱和度的 F-70 Ottawa 沙子进行塔研究以探索传输机制。所采用的 PFAS 解决方案包括全氟辛酸 （PFOA） 和全氟辛烷磺酸盐 （PFOS） 的单个溶质和二元混合物，浓度水平跨越四个数量级，以评估 AWI 的竞争性和非线性吸附。观察结果表明，浓度水平和竞争效应会显著影响 PFAS 在不饱和系统中的传输。在存在 PFOS 的情况下，PFOA 的保留率低于基于单溶质行为的预期，并且流出物突破曲线表现出色谱峰。 所提出的 PFAS 同时流动和传输的数学模型能够捕获具有一致参数集和最小曲线拟合的实验观察结果。这些结果表明了模型公式的稳健性，该模型公式包括速率限制界面传质、用于 AWI 吸附的扩展 Langmuir-Szyszkowski 模型以及用于预测 AWI 比面积的缩放 Leverett 热力学模型。总体而言，这项工作的结果强调了 AWI 在 PFAS 传输中的重要性，并强调了吸附制剂中竞争效应的相关性。