Experimental and Theoretical Insight of Perfluorooctanoic Acid Destruction by Alkaline Hydrothermal Treatment Enhanced with Zero-Valent Iron in Biochar
Min Yang, Zhongcheng Du, Hongjia Bao, Xiaolei Zhang, Qiang Liu, Wenshan Guo, Huu Hao Ngo, Long D. Nghiem
Abstract
This study demonstrates the complementarity between alkaline-thermal treatment and zero-valent iron immobilized in biochar (BC-ZVI) for enhanced degradation of perfluorooctanoic acid (PFOA). New and complementary research techniques, including 19 F-NMR and density functional theory (DFT) analysis were used to evaluate defluorination efficiency and elucidate possible degradation mechanisms. The defluorination of PFOA was governed primarily by alkaline strength while BC-ZVI could enhance reaction kinetics by 2.4 times. Adsorption of the intermediates to BC-ZVI resulted in near-complete removal of all fluorinated compounds (with the exception of perfluoracetic acid) in the aqueous phase within 15 min of reaction. After 4 h of alkaline hydrothermal treatment (240 °C and 250 mM NaOH) with BC-ZVI, defluorination efficiency of PFOA of 99.7% was observed. Perfluoracetic acid was the only persistent byproduct but accounted only for less than 1% of the initial PFOA. DFT analysis also showed that strong alkaline conditions could decrease the reaction energy barrier by 73.9% due to the decarboxylation of PFOA via nucleophilic substitution. In addition to Kolbe decarboxylation, the profile of intermediate products and DFT analysis results suggest that PFOA was degraded by other mechanisms leading to near-complete and effective defluorination.