Adsorption of perfluorocarboxylic acids by carbon nanotube and its derivative: Insight into mechanisms based on...

We systematically investigate the adsorptive interaction between perfluoroalkyl carboxylates (PFCAs) and pristine as well as oxidized carbon nanotubes (refer to as CNT and O-CNT, respectively), through experimental and modeling methods, to get an in-depth understanding of the potential risk of perfluoroalkyl and polyfluoroalkyl substances (PFASs) in the environment. The adsorption affinity was correlated well with the perfluorocarbon chain length of PFCAs, and CNT outcompeted O-CNT to present a stronger adsorption capability to the same PFCA. The observations suggest that hydrophobic effect and electrostatic interaction were the predominant adsorption mechanisms, and oxidation treatment increased the negative surface charge and decreased the hydrophobicity of CNT, which led to inhibitory adsorption of PFCAs to the carbonaceous adsorbents. The adsorption of PFOA (a representative PFCA) on the (O-)CNT adsorbents at varied pH was strongly EPM-dependent, indicating that the electrostatic repulsion between PFOA and (O-)CNT was reasonable for the weak adsorption under alkaline condition. However, the influence of ionic strength on adsorption was more complicated. Increasing Na⁺ concentration caused more favorable aggregation of CNT than O-CNT, therefore generated more available adsorption sites for PFOA. While the adsorption of PFOA to O-CNT was more likely to be affected by Ca²⁺, which benefited from the cation-bridging between the O-functional groups of O-CNT and the deprotonated PFOA molecules. The findings of this study imply that longer-chained PFCAs prefer to be adsorbed on carbonaceous adsorbents compared with shorter-chained ones, accelerating the combined pollution of colloids and PFASs, which was highly related to the water chemical factors of the aquatic environment. Meanwhile, the transformation of carbonaceous colloids under environmentally relevant conditions can alter their properties and behaviors, which further influence their adsorptive interaction with PFASs, and consequently impact the fate and risk of the pollutants.