Enhancing effects and mechanisms by reductive iron on constructed wetland with perfluorooctanoic acid exposure: Comparison between zero-valent and ferrous iron.

Reductive iron caused efficient enhancement of constructed wetlands (CWs) under toxic stress like perfluorooctanoic acid (PFOA), while effects and mechanisms brought by different iron have not been illustrated. In this study, impacts of zero-valent (Fe(0)) and ferrous iron (Fe(II)) on CW systems with PFOA exposure were compared. Higher ammonium removal above 98 % was discovered with Fe(0) addition than Fe(II), according with more expression of hydroxylamine dehydrogenase [EC:1.7.2.6]. On the contrary, Fe(II) was more prone to stimulate activities of ammonia monooxygenase and nitrite oxidoreductase, consisting with higher abundance of Nitrosomonas and Nitrospira than Fe(0). Fe(II) had more potential to improve denitrification, with total nitrogen removal 2.64–6.83 % higher compared to Fe(0) group. It was aligned with boost of dominant denitrifying bacteria like Zoogloea (53.48 %), which probably resulted from promotion of metabolism pathways related to nitrite reduction. Phosphorus removal with Fe(II) addition was 4.59–19.04 % higher than Fe(0). Higher TP removal in Fe(II) group could result from more enhancement of physicochemical processes, compared with Fe(0). Moreover, decrease of reactive oxygen species production by citrate and activation of mitochondrial metabolism was explanation for alleviation of PFOA stress with reductive iron introduction, particularly Fe(II). As for plants, Fe(0) led to higher chlorophyll contents and lower activity of antioxidant enzymes like catalase and malondialdehyde. Current work offered new information about comparative effects of reductive iron on CWs under PFOA exposure. • Fe(0) caused higher NH 4 ⁺-N removal due to up regulation of [EC:1.7.2.6]. • Denitrification could be promoted by Fe(II) with higher abundance of Zoogloea. • Physicochemical enhancement by Fe(II) caused superior TP removal than Fe(0). • Increase of chlorophyll and drop of CAT and MDA were sharper with Fe(0) addition. [ABSTRACT FROM AUTHOR]