Preferential Adsorption of Perfluoroalkyl Carboxylic Acids with Different Chain Lengths via Hydrophilic-Hydrophobic Interface Regulation

Recently, per- and polyfluoroalkyl substances (PFAS) pollution has gradually shifted from a pattern dominated by traditional long-chain PFAS to a more complex situation where both short-chain and long-chain PFAS coexist. This poses significant challenges to conventional adsorbents such as activated carbon (AC) and ion exchange resins, whose adsorption performance is strongly influenced by PFAS chain length, adsorbent surface chemistry, and multiple interactions. Herein, we designed and prepared aminated polyurethane (PU) sponge materials by introducing alkylamines with different carbon-chain lengths, denoted as PPU-C₂, PPU-C₆, and PPU-C₁₄ according to the carbon-chain lengths of the grafted alkylamines, thereby constructing interfaces with progressively tuned hydrophilic-hydrophobic characteristics. The resulting material exhibited chain-length-dependent adsorption preferences toward perfluoroalkyl carboxylic acid (PFCA) of different chain lengths. Specifically, PPU-C₂ achieved a removal efficiency of 72% for ultra-short-chain trifluoroacetic acid (TFA) with a maximum adsorption capacity of 596 mg g^-1, whereas PPU-C₆ exhibited the highest adsorption capacity toward perfluorohexanoic acid (PFHxA) (829.2 mg g^-1) with a removal efficiency of 89%. In contrast, PPU-C₁₄ showed superior affinity toward long-chain perfluorononanoic acid (PFNA), achieving a removal efficiency of 96% and a maximum adsorption capacity of 848.2 mg g^-1. Experimental and theoretical results supported a chain-length-dependent matching relationship between PFCA molecular structure and local interfacial polarity. Appreciable PFCA adsorption performance was also retained in natural water samples. The material also exhibited good reusability. This study highlights the role of interfacial hydrophilic-hydrophobic regulation in governing chain-length-dependent PFCA adsorption and provides a basis for designing adsorbents with tunable affinity toward PFCA of different chain lengths.