Untangling the nitrate removal pathways for a constructed wetland- sponge iron coupled system and the impacts of sponge iron on a wetland ecosystem.

Sponge iron (s-Fe) is a potential alternative electron donor for nitrate reduction. To gain insight into the mechanism of denitrification in a constructed wetland- sponge iron coupled system (CW-Fe system), the removal performance and reduction characteristics of nitrate in constructed wetlands (CWs) with and without s-Fe application were compared. Results indicated that s-Fe intensified the removal of nitrate with a 6h-HRT. The nitrate removal efficiency was improved by 16-76 % with various influent NO-N concentrations (10-30 mg L) and at a chemical oxygen demand(COD)/N ratio of 5. The rates of chemical denitrification were positively correlated with the dosage of s-Fe and negatively correlated with the influent COD concentration. 16S rDNA sequencing revealed that hydrogen-utilizing autotrophic denitrifier of Hydrogenophaga was highly enriched (accounting for 10 % of the total OTUs) only in CW-Fe system. The micro-environment created by s-Fe was suitable for heterotrophic denitrifiers of Thauera, Tessaracoccus and Simplicispira. The determination of physiological indicators for plants showed that the application of s-Fe causes abiotic stress to wetland plants (Canna indica L.). Nevertheless, s-Fe can be used as a substrate for CWs, since it allows a high-efficiency removal of nitrate by mediating chemical denitrification and hydrogen-driven autotrophic denitrification.