Calcination of hydrotalcite to enhance the removal of perfluorooctane sulfonate from water

Abstract

Perfluorinated compounds (PFCs), due to their thermal and chemical stability and broad usage in industry, have been detected in surface and ground water. As emerging compounds, their fate in the environment and removal from water attracted great attention in the last 10 years. In this study, we evaluated the performance of calcinated hydrotalcite (CHT) for the removal of PFCs using perfluorooctane sulfonate (PFOS) as an example. Our results showed that CHT can sorb up to 1.4 times of its mass for PFOS and the removal was fast, with equilibrium being achieved in 30 min. The PFOS removal was less sensitive to pH in pH 4–11 range. At low loading levels, only small fractions of CHT were intercalated by PFOS and the 3R polytype of hydrotalcite (HT) was largely maintained. As the PFOS uptake increased beyond 300 mg/g, intercalation of PFOS took place. The structure of HT changed to 1 layer stacking beyond 600 mg/g. Electrostatic interaction was responsible for the PFOS uptake at low loading levels and hydrophobic interaction started to play a role among PFOS molecules as its loading level increased. Crystalline PFOS decomposed at 513 °C, but its decomposition temperature decreased to 443 °C once sorbed on CHT, due to the breakdown of HT at about 390 °C. The CHT was regeneratable after calcination of PFOS-saturated HT at 450 °C for 3 h with the full PFOS removal capacity restored. The finding from this study showed the superior feature of using CHT for the removal of PFCs over other methods.