Sorptive remediation of perfluorooctanoic acid (PFOA) using mixed mineral and graphene/carbon-based materials

Abstract

Environmental contextPer- and poly-fluoroalkyl substances (PFASs) are contaminants of emerging concern, creating a need to develop efficient multi-functional adsorbents for improved remediation performance. By exploiting the versatility of graphene technology, we demonstrate that combining mineral and carbonaceous phases greatly increases and strengthens PFAS-binding to the adsorbent. The study highlights the benefits and potential applications of mixed adsorbents in PFAS-remediation. AbstractAs the degradation of perfluorooctanoic acid (PFOA) and related per- and poly-fluoroalkyl substances (PFASs) is energy-intensive, there is a need to develop in situ remediation strategies to manage PFAS-contamination. The sorption of PFOA by graphene oxide (GO), an iron-oxide-modified reduced-GO composite (FeG) and an activated-carbon(C)/clay/alumina-based adsorbent, RemBindTM (RemB), are evaluated. Sorption by FeG and RemB (>90%) is much greater than GO (60%). While an increase in pH hinders PFOA-sorption by GO, owing to the increased repulsion of anionic PFOA, variations in pH and ionic strength do not significantly influence PFOA-sorption by FeG and RemB, which indicates that binding is predominantly controlled by non-electrostatic forces. Hydrophobic interactions are assumed at the graphene or C-surface for all adsorbents, with added ligand-exchange mechanisms involving the associated Fe- and Al-minerals in FeG and RemB, respectively. Desorption of adsorbed PFOA is greatest in methanol, compared to water, toluene, or hexane, which provides estimates of the binding strength and reversibility from an environmental-partitioning perspective; i.e. risk of remobilisation of bound PFOA owing to rainfall events is low, but the presence of polar organic solvents may increase leaching risk. Iron-mineral-functionalisation of GO enhances the amount of PFOA adsorbed (by 30%) as well as the binding strength, which highlights the advantage of combining mineral and C-phases. Successful sorption of a range of PFASs from a contaminated-site water sample highlights the potential of using ‘mixed’ adsorbents like FeG and RemB in situ for PFAS-remediation, as they provide avenues for enhanced sorption through multiple mechanisms.