Highly enhanced removal of nutrients and benzo[a]pyrene in a siphon constructed wetland with magnetite: Performance and mechanisms

Abstract

Due to its limited dissolved oxygen concentration, constructed wetland (CW) could not remove nutrients and benzo[a]pyrene (B[a]P) effectively. Therefore, in this study, a siphon was applied in CW (Si-CW) to create tidal flow to achieve efficient removal of nutrients and B[a]P, and the removal mechanism was explored. The results showed that the NH4+ and TP removal values for the Si-CW were 55.95% ± 7.75% and 20.79% ± 8.39%, which exceeded those for the conventional CW (CCW). Alternative anaerobic and aerobic environments formed by siphon also enabled magnetite in Si-CW to form the iron cycle (Fe2+/Fe3+ cycle), which promoted the occurrence of simultaneous nitrification and denitrification and hydrogen-utilizing autotrophic denitrification and improved nitrogen removal significantly. The mass balance showed that microbial degradation was the main mechanism of B[a]P removal (39.0% – 54.0%). Si-CW possessed better oxygen conditions and iron cycles, favoring the propagation and growth of B[a]P-degrading bacteria (i.e., Pseudoxanthomonas) while enhancing the proportion of microbial degradation of B[a]P. Metagenomic analysis indicated that Si-CW had a higher TC.FEV.OM gene expression of Pseudoxanthomonas, which could regulate the siderophores produced by Pseudoxanthomonas through the metabolic pathway (K02014) of the iron complex outer-membrane receptor protein to achieve efficient B[a]P removal.