Pathways regulating the enhanced nitrogen removal in a pyrite based vertical-flow constructed wetland.

In this study, two vertical constructed wetland using natural pyrite (P-VFCW) and quartz sand (C-VFCW) as substrate were constructed, and the enhanced nitrate removal mechanism by pyrite was further investigated. Results showed that the nitrate removal efficiency (NRE) of P-VFCW was 4% higher than that of C-VFCW with a C/N of 0. Interestingly, the difference on NRE between CWs markedly enlarged with C/N and hydraulic retention time (HRT) simultaneously increasing. At a COD/N of 6 and HRT of 24 h, the effluent average NO-N and NO-N concentrations in P-VFCW and C-VFCW were 2.36 ± 2.64 mg/L/1.34 ± 1.28 mg/L, 9.20 ± 6.91 mg/L/5.57 ± 3.68 mg/L, respectively, revealing pyrite could promote heterotrophic denitrification and avoid nitrite accumulation. After the whole operation, a better growth of Canna indica occurred in P-VFCW. High-throughput sequencing implied that denitrifying bacteria (Comamonas), iron oxidation and reduction microorganism (Thiobacillus) and the rhizosphere microorganism differed in CWs.