Strengthening effect of different iron minerals for perfluorooctanoic acid and perfluorooctane sulphonic acid removal in constructed wetlands: Mechanisms of electron transfer and microbial effect

Perfluorooctanoic acid (PFOA) and perfluorooctane sulphonic acid (PFOS) pollution existed widely in constructed wetlands (CWs). Iron minerals have been proved to enhance PFOS and PFOA removal in CWs. However, the microbial extracellular electron transfer mechanisms mediated by iron minerals remain unclear. To strengthen the removal of PFOA and PFOS, and explore the mechanisms, in this study, four kinds of iron-based CWs were constructed with magnetite, siderite, sponge iron, and hematite. Results reflected that the addition of iron minerals increased the PFOS and PFOA removal by 7.27 %-16.42 % than that in control group. XRD and XPS results show that iron minerals can promote the formation of Fe(Ⅲ) (hydrogen)oxides that conducive to electron transfer. The electrochemical characterization of extracellular polymer (EPS) and microbial electron transport activity showed that iron minerals improve the capacitance and electron transfer properties of EPS. Microbial results showed that magnetite and siderite improved the diversity of microorganisms that associated with PFOA and PFOS degradation (e.g. Geobacter, Shewanella, and Pseudomonas). Furthermore, both magnetite and siderite can promote extracellular protein secretion (p < 0.05), and increase the abundance of genes that related to extracellular-related cytochrome synthesis pathway (K02275, K01784), EPS synthesis (pgi, pgm, ugp), and defluorine (K01560, K01561, K01563). This study provides insight into the mechanisms of the PFOA and PFOS removal process and contributes a new perspective to further enhance the treatment of new pollutants in CWs.