Per- and poly-fluoroalkyl substances (PFAS), such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), are significant environmental pollutants due to their toxicity, persistence, and bioaccumulation. The US Environmental Protection Agency (EPA) advises that the combined concentration of PFOA and PFOS in drinking water should not exceed 70 ppt. Existing technologies for PFAS removal, including oxidation, UV irradiation, sonochemical, and electrochemical methods, are often less efficient than adsorption. While various adsorbents (activated carbon, ion exchange resins, minerals, molecularly imprinted polymers, biosorbents, carbon nanotubes, metal-organic frameworks, and covalent organic frameworks) have been explored, they often suffer from limitations such as low capacity, slow kinetics, weak binding affinity, poor stability, and low selectivity. To address these issues, the development of high-capacity, rapid, and selective PFAS adsorbents is crucial. This research proposes using porous organic polymers (POPs), specifically porous aromatic frameworks (PAFs), as a platform for creating improved adsorbents. POPs offer advantages including high surface areas, adjustable structures, tunable pore sizes, readily functionalized pore walls, and exceptional stability. The study hypothesizes that incorporating both electrostatic and hydrophobic binding sites onto POPs will synergistically enhance PFOA adsorption. This synergistic approach aims to improve PFOA uptake capacity, adsorption rate, selectivity, and stability.

The literature review extensively covers existing methods for PFAS removal, highlighting the advantages and shortcomings of various adsorption-based techniques. It discusses the limitations of conventional adsorbents, such as activated carbon, which often show low capacity and slow kinetics for PFOA adsorption. The authors reviewed different types of adsorbents and their reported PFOA adsorption capacity and rate constants (k2 values) to showcase the need for more efficient adsorbents. The review also underscores the importance of considering factors like hydrophilicity, strong interactions with PFAS, appropriate pore size, and stability in the design of ideal PFAS adsorbents. The existing literature establishes the need for a novel approach that overcomes the limitations of current technologies, leading to the proposed synergistic binding sites strategy.

The researchers synthesized a series of quaternary ammonium-functionalized porous aromatic frameworks (PAFs) by appending different alkyl chains (N,N-dimethylpropylamine (NDMP), N,N-dimethyl-butylamine (NDMB), N,N-dimethylhexylamine (NDMH)) onto the pore walls of PAF-1. This functionalization introduced both positively charged (electrostatic) and hydrophobic binding sites. The synthesis involved chloromethylation of PAF-1 followed by reaction with the respective amine. The materials were characterized using various techniques, including FTIR, XPS, solid-state 13C NMR, SEM, DLS, EDS, and nitrogen adsorption isotherms. These techniques confirmed the successful functionalization and assessed the morphology, particle size distribution, surface area, and pore size of the synthesized materials. PFOA adsorption experiments were conducted by adding the synthesized materials to aqueous PFOA solutions of varying concentrations. The amount of PFOA adsorbed was determined by measuring the residual concentration in the solution after a specific time interval or at equilibrium. The adsorption kinetics were evaluated using pseudo-second-order kinetic models. Adsorption isotherms were fitted to the Langmuir model to determine the maximum adsorption capacity. The influence of pH on the adsorption efficiency was also assessed. The interaction between PFOA and the adsorbents was investigated using EDS, zeta potential measurements, FTIR, 19F MAS NMR, and 13C NMR spectroscopy. Density functional theory (DFT) calculations were employed to study the binding interactions at the molecular level. Breakthrough experiments were conducted to evaluate the performance of the adsorbents in the presence of humic acid (HA), a model natural organic matter (NOM). Finally, experiments were performed using contaminated water samples collected from the Xiaoqing River, a heavily PFOA-polluted area in China, to demonstrate the practical applicability of the adsorbent. The regeneration ability of the adsorbent was tested through multiple cycles of adsorption and desorption.

The key findings demonstrate that the material PAF-1-NDMB, modified with N,N-dimethyl-butylamine, exhibited superior PFOA removal capabilities. It achieved an unprecedented PFOA uptake capacity exceeding 2000 mg g⁻¹, significantly surpassing the performance of benchmark materials such as DFB-CDP (62.5 mg g⁻¹) and activated carbon (83 mg g⁻¹). This represents a 32 and 24 times improvement respectively. PAF-1-NDMB also demonstrated exceptionally fast kinetics, with a k2 value of 24,000 g mg⁻¹ h⁻¹, the highest reported value for PFOA adsorption. Remarkably, it reduced PFOA concentration from 1000 ppb to 54 ppt (below the EPA advisory level of 70 ppt) in just 2 minutes. The high PFOA removal efficiency was maintained across a wide range of pH values (2–11). Furthermore, PAF-1-NDMB showed a high PFOS uptake capacity (2381 mg g⁻¹) and fast kinetics (k2 = 19,200 g mg⁻¹ h⁻¹). Characterization studies indicated that the exceptional adsorption performance stemmed from the synergistic combination of electrostatic and hydrophobic interactions between the positively charged quaternary ammonium groups and the hydrophobic alkyl chains of PAF-1-NDMB and the PFOA molecule. Breakthrough experiments confirmed the adsorbent's effectiveness in the presence of humic acid (HA), demonstrating high selectivity for PFOA. Importantly, practical application tests using contaminated water from the Xiaoqing River showed PFOA removal efficiency reaching 99.99% within 10 seconds, reducing concentrations below the EPA advisory level. The adsorbent was also shown to be readily regenerable and reusable for at least six cycles without significant loss of performance. The cost analysis showed PAF-1-NDMB to be more cost effective than DFB-CDP for the same adsorption amount.

The results clearly demonstrate the success of the proposed 'synergistic binding sites' strategy for designing highly effective PFOA adsorbents. The exceptional performance of PAF-1-NDMB highlights the importance of combining electrostatic and hydrophobic interactions to enhance PFOA adsorption. The remarkably high uptake capacity and fast kinetics are attributed to the efficient functionalization and the synergistic effect of the multi-functionalized sites. The ability to maintain high PFOA removal efficiency across a wide pH range and in the presence of HA signifies the practical relevance of this approach. The successful application in real-world contaminated water further validates the adsorbent's potential for effective water purification. This work advances the development of efficient and cost-effective PFAS removal technologies, addressing a significant environmental challenge.

This study successfully demonstrates a novel strategy for creating highly efficient and cost-effective PFOA adsorbents using a synergistic combination of electrostatic and hydrophobic interactions within porous organic polymers. The exceptional performance of PAF-1-NDMB, showcasing remarkably high PFOA uptake capacity, rapid kinetics, and excellent regeneration ability, highlights the potential of this approach for practical applications in water purification. Future research could explore the application of this strategy to other POPs and explore the removal of other PFAS contaminants.

While this study demonstrates the exceptional performance of PAF-1-NDMB, some limitations should be noted. The study focused primarily on PFOA and PFOS; further investigation is needed to evaluate the effectiveness against a wider range of PFAS. Long-term stability and potential leaching of functional groups under continuous use conditions warrant further examination. Scale-up and cost optimization for large-scale applications require further investigation. The influence of other co-contaminants in complex water matrices on the adsorption performance could also be further examined.