Enhanced sorption of perfluorooctane sulfonate and perfluorooctanoate by hydrotalcites

Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) are recalcitrant compounds that are toxic to humans and ecosystems. Hydrotalcite-like compounds have emerged as promising PFOS and PFOA sorbents due to their high anion exchange capacity and high specific surface area. In this study, hydrotalcite (HT) compounds were intercalated with nitrate and carbonate ions (HT-CO3 and HT-NO3) and their specific surface area and particle aggregate size modified by treatment with acetone (AHT-CO3 and AHT-NO3). Sorption experiments with AHT-NO3 indicated that sorption equilibrium was reached faster with PFOS (< 5 min) compared to PFOA (< 20 min), and that sorption capacity for PFOS (1,610.0 mg/g) was substantially higher compared to PFOA (909.0 mg/g). The sorption yields are explained by anion exchange occurring at higher PFOS and PFOA concentrations, along with surface adsorption, and by PFOS having a higher affinity for the HT interlayer. In comparison to AHT-NO3, PFOS and PFOA sorption by AHT-CO3 was slower (equilibration times ≥ 20 min) and the maximum capacities were generally lower, because anion exchange is hindered by the affinity of the carbonate ion for the HT interlayer. As such, surface adsorption dominated across a large PFOS and PFOA concentration range (up to 2,000 mg/L). The presence of non-ionic species (trichloroethylene) did not affect the sorption capacity, while alkaline pH conditions and the presence of other anionic species (dodecyl sulfate) reduced the sorption capacity of AHT compounds towards PFOS and PFOA. Compared to untreated HT compounds, acetone treated HT exhibited improved sorption properties towards PFOA and PFOS removal from groundwater, particularly AHT-CO3. Overall, AHT compounds outperform activated carbon sorbents in terms of PFOA and PFOS sorption kinetics and capacities, thus could be promising new sorbents for PFAS removal from contaminated waters.