Nitrate reduction pathway of iron sulphides based MFC-CWs purifying low C/N wastewater: Competitive mechanism to inorganic and organic electrons

The hybrid system of microbial fuel cell and constructed wetlands (MFC-CWs) based on iron sulphides substrates in treating low ratio of carbon to nitrogen (C/N) wastewater has been paid more attention because the inorganic electrons supplied were involved in nitrate reduction. However, the relationships in the distribution of inorganic and organic electrons between nitrate reduction and electrogenesis remain unclear. Thus, three MFC-CWs with activated carbon (GAA), pyrite (GHH) and pyrrhotite (GPP) respectively were constructed to compare the nitrate reduction and electrogenesis in polishing the low C/N wastewater. Results showed that the amount of electrons involving nitrate reduction in GPP and GHH increased by 69.97% and 22.61% compared with GAA. And coulombic yields of GPP and GHH were 28 and 19 times higher than that of GAA, respectively. Thauera was regarded as one of the most important functional genera for autotrophic denitrification and electrogenesis. Sulfate and C/N were identified as key environmental factors affecting functional micro-flora. Furthermore, the structural equation model analysis showed that more electrons were allocated to reduce nitrate rather than electrogenesis in GPP and GHH systems. Both organic and inorganic electrons contributed significantly to the complete denitrification in GPP. However, a large part of inorganic electrons also participated in the dissimilatory nitrate reduction to ammonium process within GHH. The significant differences attribute to the competitive balance between inorganic electrons represented by sulfate and organic electrons represented by C/ N. These differences indicated the clues and revelation for practical applications of pyrite and pyrrhotite based CWs or MFC-CWs.