Removal of perfluorooctanoic acid from water using calcined hydrotalcite – A mechanistic study

Calcined hydrotalcite (CHT) was evaluated for its potential removal of perfluorooctanoic acid (PFOA) from water in this study. The uptake of PFOA by CHT could be as high as 1587 mg/g (ca. 3.8 mmol/g), slightly larger than the anion exchange capacity (AEC) of the hydrotalcite (HT). Such a high removal was fast and pH independent, suggesting the versatile use of CHT. Due to the structural memory effect of HT, the removal involved adsorption of PFOA during HT recovery and intercalation of PFOA into the interlayer of restructured HT at low and high initial concentrations, respectively. Limited by the specific surface area and AEC, the intercalated PFOA would form a vertical bilayer or admicelle conformation. As such, the HT intercalated with PFOA became one-layer stacking with a basal spacing of 2.04 nm in contrast to the 3R polytype of recovered HT having a layer thickness of 0.78 nm, as confirmed by X-ray diffraction, thermogravimetric, and infra-red analyses. Due to its high PFOA removal capacity and large partitioning coefficient, the amount of CHT used, thus, the disposal of PFOA-laden solid could be minimized.