One-step synthesis of reduced graphene oxide/activated carbon composite for efficient removal of per- and polyfluoroalkyl substances from drinking water: Adsorption mechanisms and DFT study

Abstract

This study illustrates the synthesis, efficiency, and the mechanism of a novel reduced graphene oxide (RGO)/activated carbon (AC) (RGO/AC) composite to adsorb the lower concentration per- and polyfluoroalkyl substances (PFAS) in drinking water. The study integrates the experimental and Density Functional Theory (DFT) analyses, while emphasizing the role of graphene oxide (GO) on physicochemical properties of AC and enhancement of PFAS adsorption. The synthesized composites were characterized using several techniques prior to static evaluation for the adsorption of long and short-chain PFAS. Results indicated that RGO layers were uniformly anchored on AC surface. The RGO/AC composite exhibited larger specific surface area (S_BET), higher mesoporous ratio, and contact angle than the AC synthesized under similar conditions. The C–H bonds on RGO/AC composite surface disappeared, while H content in RGO/AC composite was reduced compared to that of individual AC. The respective adsorption rates (K₂) of perfluorooctane sulfonates (PFOS), perfluorooctanoic acid (PFOA), perfluorobutane sulfonate (PFBS) and perfluorobutyric acid (PFBA) by RGO/AC composite were 1.67, 1.70, 2.52, and 1.80 times higher than those of AC, individually. Meanwhile, the respective adsorption affinities (K_F) of PFOS, PFOA, PFBS and PFBA using RGO/AC composite were 2.24, 2.14, 1.36 and 1.60 times higher than those of AC. PFAS molecules were adsorbed by RGO/AC composite through multilayer heterogeneous, spontaneous and exothermic process, where adsorption behavior of PFOS and PFOA was significantly altered. The hydrophobic interactions, electrostatic forces and hydrogen bonding were dominant between PFAS molecules and RGO/AC composite. Meanwhile, DFT simulation of PFAS molecular dynamics on RGO/AC composite consistently suggested the spontaneous adsorption. The RGO/AC composite would be a promising material for effectively removing PFAS from drinking water.