Nitrate removal to its fate in wetland mesocosm filled with sponge iron: Impact of influent COD/N ratio

Sponge iron (s-Fe 0 ) is a porous metal with the potential to be an electron donor for denitrification. This study aims to evaluate the feasibility of using s-Fe 0 as the substrate of wetland mesocosms. Here, wetland mesocosms with the addition of s-Fe 0 particles (CW-Fe) and a blank control group (CW-CK) were established. The NO 3 − -N reduction property and water quality parameters (pH, DO, and ORP) were examined at three COD/N ratios (0, 5, and 10). Results showed that the NO 3 − -N removal efficiencies were significantly increased by 6.6 to 58.9% in the presence of s-Fe 0 . NH 4 + -N was mainly produced by chemical denitrification, and approximately 50% of the NO 3 − -N was reduced to NH 4 + -N, at the COD/ratio of 0. An increase of the influent COD/N ratio resulted in lower ehermical denitrification and higher bio-denitrification. Although chemical denitrification mediated by s-Fe 0 led to an accumulation of NH 4 + -N at COD/N ratios of 0 and 5, the TIN removal efficiencies increased by 4.5%–12.4%. Moreover, the effluent pH, DO, and ORP values showed a significant negative correlation with total Fe and Fe (II) ( P < 0.01). High-throughput sequencing analysis indicated that Trichococcus (77.2%) was the most abundant microorganism in the CW-Fe mesocosm, while Thauera , Zoogloea , and Herbaspirillum were the primary denitrifying bacteria. The denitrifiers, Simplicispira , Dechloromonas , and Denitratisoma , were the dominant bacteria for CW-CK. This study provides a valuable method and an improved understanding of NO 3 − -N reduction characteristics of s-Fe 0 in a wetland mesocosm.