Tailoring graphene oxide nanosheets by alkyl amine grafting for enhanced adsorption of PFASs in drinking water: a combined theoretical and experimental study

Per- and polyfluoroalkyl substances (PFASs) are a class of persistent organic pollutants present in natural water showing environmental and health risks. Most of the currently available purification technologies fail in the removal of short and medium perfluoroalkyl chain length PFASs. Here, we report the design and synthesis of graphene oxide (GO) modified with different alkyl- and alkylamine pendants including <i>N</i>,<i>N</i>-dimethylethylenediamine (GO-DMEN) and investigate the relationships between the chemical structure of the alkyl pendant and the adsorption of PFAS molecules of different sizes and end group type. GO-DMEN shows higher removal (up to 98%) after 15 minutes toward medium and long chain PFASs, such as perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA), poorly removed by unmodified GO. Molecular modeling shows that van der Waals interactions are the driving forces for the adsorption. Indeed, the quaternarization of the amine moieties, with the consequent creation of positive charges on graphene nanosheet surfaces, does not enhance the adsorption capacity. The key role of the modification with an amine ended chain such as DMEN was demonstrated by comparing GO-DMEN adsorption properties to those of an octyl chain modified GO (GO-OcA) characterized by poorer binding energy contribution as a result of the flattening of the hydrophobic octyl chain on GO nanosheets (self-poisoning). This work contributes to a deeper understanding of the chemical interactions driving the adsorption of amphiphilic and charged PFAS molecules.