Enhanced removal of organic, nutrients, and PFCs in the iron-carbon micro-electrolysis constructed wetlands: Mechanism and iron cycle

The poor removal capacity of the wastewater treatment plants (WWTPs) on perfluorinated compounds (PFCs) results in the desirability of advanced treatment. Therefore, this study mainly discusses the enhanced removal mechanism of iron-carbon constructed wetlands on chemical oxygen demand (COD), phosphorus, nitrogen, perfluorooctanoic acid (PFOA), and perfluorobutanesulfonic acid (PFBS). The results showed that the removal efficiencies of most pollutants increased with the extension of hydraulic retention time (HRT) (1d, 2d, 3d). During the 3-day HRT, the average removal efficiencies in F1 (iron-carbon wetland, 100 μg/L PFCs) for COD, total phosphorus (TP), total nitrogen (TN), PFOA, and PFBS were 61.88 %, 67.92 %, 58.26 %, 78.12 %, and 80.60 %, respectively. Compared with C1 (common gravel wetland, 100 μg/L PFCs), the corresponding removal efficiencies increased by 4.68 %, 24.53 %, 10.07 %, 16.52 %, and 13.16 %, respectively. However, the removal effect of F2 (iron-carbon wetland, 200 μg/L PFCs) became worse due to the high concentration of PFCs. Further study found that the efficient removal of phosphorus in F1 was the precipitation and coagulation of Fe(III) and Fe(II). Meanwhile, the removal of nitrogen mainly relied on autotrophic and heterotrophic denitrification. Simultaneously, the combination with Feammox (the process of anaerobic ammonium oxidation coupled to iron reduction) and other pathways promoted nitrogen removal and iron cycling in F1. Further mass balance calculation of PFOA and PFBS in iron-carbon wetland found that among all matrixes (coarse sand, gravel, and iron-carbon), the contribution rate of iron-carbon to the removal PFCs was the highest. In addition, the enrichment of microorganisms related to Fe(II)-dependent autotrophic denitrification and the cultivation of iron-reducing bacteria reconfirmed the iron cycle.