M. Ji, X.G. Meng, C. Christodoulatos
Stevens Institute of Technology,
United States
Keywords: PFOS, adsorption, hydrophobic effect, in-situ ATR-FTIR, magnetic, non-magnetic
Summary:
Zero valent iron (ZVI) is a cost-effective reductant that has been widely used to treat heavy metals and organic pollutants, but limited research has investigated the contribution of different parts of ZVI to the removal of per/polyfluoroalkyl substances (PFAS) from water. This study investigated the contribution of the magnetic and non-magnetic parts of micron-scale ZVI (mZVI), as well as the surface oxide layer of mZVI to the removal of perfluorooctane sulfonic acid (PFOS, C8F17SO3H. The results showed that PFOS was effectively removed from 50 mg/L to 6 mg/L by 10 g/L of mZVI within 8 hours at neutral pH in the closed system. Low F- concentration (⁓ 1 mg/L) was detected in the suspension solution after treatment, suggesting that defluorination was not the main removal mechanism. The in situ ATR-FTIR spectra results suggested that PFOS was removed by mZVI mainly through the formation of outer-sphere surface complexation. The magnetic portion of mZVI, especially the Fe0 without the oxides layer on the surface, should be the main adsorbent. Adsorption isotherm study showed that the areal adsorption capacity of mZVI (21 m2/g) was 26 times higher than that of activated carbon (AC) (0.813 m2/g). The study of pH effect and zata potential showed that the high removed PFOS (35 mg/L) at pH>7 was obtained, indicating that the adsorption was not due to the electrostatic interaction but hydrophobic effect, it can overcome the electrostatic repulsion between PFOS anion and negatively charged mZVI surface. The presence of high NaCl concentration (0.5 M) could almost completely remove 50 mg/L PFOS using mZVI, which demonstrated that the hydrophobic effect played a key role in the adsorption process. The results of this study will benefit the development of mZVI based high specific surface area material for PFOS treatment.