REMOVAL OF PERFLUOROSURFACTANTS FROM AQUEOUS SOLUTIONS BY COPRECIPITATION WITH Mg–Al LAYERED DOUBLE HYDROXIDES AND THERMAL DECOMPOSITION OF RESULTING PRECIPITATES

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In recent years, perfluorosurfactants (PFS) such as perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) have drawn considerable attention as a persistent environmental substance. Therefore, the development of the effective removal and decomposition techniques of PFS anions is urgently required. Layered double hydroxides (LDHs) are the layer compound containing anion in their interlayer spaces. Further, there is considerable interest in designing LDH host layer as adsorbent for anionic pollutants and a catalyst for thermal decomposition of intercalated pollutants. In this study, the characteristics of Mg–Al LDH with respect to the removal of PFS anions from aqueous solutions and the thermal decomposition of PFS anions in the interlayer space were quantitatively investigated. The experiments of the removal of PFS anions were performed with the coprecipitation method using Mg(NO₃)₂ and Al(NO₃)₃ mixed solution. At lower concentrations (1–5 mmol·dm⁻³), both PFOS and PFOA ions were perfectly removed as mainly in the form of LDH, whereas at higher concentrations (10–20 mmol·dm⁻³), PFOS ion was removed as the mixture of LDH and its slightly soluble sodium salt, and PFOA ion were removed simply as LDH. The optimum condition of the PFS removal was as follows: Mg/Al molar ratio of 2, PFS/Al molar ratio of 0.10–1.0, pH of 9–10, and temperature of 25–40°C. Another interesting feature was that the high PFS removal was hardly influenced by the coexisted inorganic anion such as Cl⁻ and SO₄²⁻. The thermal decomposition of the resulting PFS/Mg–Al LDH was examined in the temperature range of 25–800°C. The intercalated PFOS ion decomposed at approximately 40°C lower than that of its naked sodium salt, whereas PFOA ion decomposed at approximately 240°C via the catalytic action of metal ions in the LDH host layer. Furthermore, the activation energies of the thermal decomposition of PFS anions were calculated using TG and DTA methods. Thus, a new method using Mg–Al LDH is proposed for the removal and thermal decomposition of PFS anions.