Sorption of Anionic and Zwitterionic Per- and Polyfluoroalkyl Substances Present in Legacy Aqueous Film-Forming Foams

Aqueous film-forming foams (AFFF) are a major source
of polyfluoroalkyl
substances (PFAS) in the environment. Many PFAS in AFFF formulations
lack analytical standards, which has challenged the evaluation of
their fate and transport. In this study, AFFF was used to spike batch
sorption reactors, and experimental samples were analyzed with high-resolution
mass spectrometry to yield the first experimental Kd values (1195 total) for 50 anionic and zwitterionic
AFFF PFAS in 23 US soils. Sorption of most anionic, substituted, and
unsaturated perfluoroalkyl sulfonates (PFSAs) was comparable to PFSAs
of the same chain length. Zwitterionic PFAS with net neutral charge
or terminal positive functional groups showed log Kd values >2 log units higher than anionic PFAS of the
same chain length, whereas those with one or two terminals, negative
functional groups, and net negative charge showed log Kd values closer to anionic AFFF PFAS. The influence of
PFAS charge and charge position was validated using hydrophilic interaction
liquid chromatography (HILIC) retention times collected under conditions
that isolated PFAS electrostatic interactions. These findings provide
a conceptual model of AFFF PFAS retention and mobility that demonstrates
a range of expected behaviorsfrom highly mobile PFAS likely
to undergo downgradient transport to less mobile PFAS likely to be
more highly retained in source zones.

水成膜泡沫（Aqueous Film-Forming Foams, AFFF）是环境中多氟烷基物质（Polyfluoroalkyl Substances, PFAS）的主要来源。诸多存在于AFFF配方中的PFAS缺乏分析标准品，这为其归趋与迁移行为的评估带来了挑战。本研究中，研究人员采用AFFF对批量吸附反应器进行加标处理，并借助高分辨质谱对实验样品开展分析，最终获得了针对23份美国土壤中50种阴离子型与两性离子型AFFF关联PFAS的首批实验吸附分配系数（Kd）值，共计1195个。多数阴离子型、取代型与不饱和型全氟烷基磺酸类（Perfluoroalkyl Sulfonates, PFSAs）的吸附行为与同链长的PFSAs基本相当。带有净中性电荷或末端正官能团的两性离子型PFAS，其log Kd值较同链长阴离子型PFAS高出2个对数单位以上；而带有一个或两个末端负官能团且整体带净负电荷的两性离子型PFAS，其log Kd值则更接近阴离子型AFFF关联PFAS。研究人员通过在隔离PFAS静电相互作用的实验条件下采集的亲水作用液相色谱（Hydrophilic Interaction Liquid Chromatography, HILIC）保留时间，验证了PFAS电荷属性与电荷位置对吸附行为的影响。本研究结果构建了AFFF关联PFAS的保留与迁移性概念模型，该模型涵盖了一系列预期行为模式：从极易发生下游迁移的高流动性PFAS，到更易在源区被高效滞留的低流动性PFAS。