The Influences Of Fe²⁺ On Denitrification In Subsurface Flow Constructed Wetlands And Its Mechanism

The use of fertilizers and pesticides caused the situation of nitrogen pollution became increasingly serious. The eutrophication is aggravated. Nitrate pollution is closely related to human health and ecosystem, thus nitrate removal in water is a hot spot in water pollution. Constructed wetlands(CWs) as a kind of ecological and economical treatment technology has been widely used in denitrification process of wastewater at home and abroad. Fe2+ can participate in denitrification as electron donor. Some microorganisms can use the electron which is given by oxidizing Fe2+ to Fe3+ to deoxidize NO3- or NO2-. In this study, Fe2+ was added into the in?uent water of constructed wetlands and feasibility that Fe2+ participated in denitrification of CWs was investigated. The effect of the four kinds of factors including initial concentration of Fe2+, carbon nitrogen ratio(COD/N), the initial nitrogen level and hydraulic residence time(HRT) on the denitrification that Fe2+ participated in also were investigated. According to the difference of denitrification effect, the denitrification potentials, the number of microbial and microbial community function diversity in CWs in different denitrification ways, the biological and chemical impacts of external Fe2+ on denitrification were presented. The main conclusions are as follows:(1) Fe2+ as electron donor of denitrification in CWs was considered to be feasible. The external Fe2+ remarkably improved denitrification in CWs, with the increasing of initial concentration of Fe2+, the removal capability of NO3--N presented a rising trend under the condition of the COD/N being 2. However without carbon source, Fe2+ had no significant impact on denitrification in CWs, showing the positive impact on denitrification occurred in the presence of organic carbon. Under the different initial level of nitrate, both COD/N and the initial concentration of Fe2+ had significant effects on denitrification, and the influence of the COD/N on nitrate removal rate was greater th an the effect of initial concentration of Fe2+. With the increase of HRT, nitrate removal rate increased gradually. Under the different initial levels of nitrate, both the initial concentration of Fe2+ and HRT had significant influence on denitrification. The influence of the HRT on nitrate removal rate was greater than the effect of initial concentration of Fe2+ at low nitrate level, while the result was opposite at high nitrate level.(2) The nitrate removal efficiency and the denitrification potential(DPN) were different in different denitrification ways and the nitrate removal rate and the DPN had a good positive correlation. Nitrogen removal efficiency was poor without carbon source while Fe2+ promoted the denitrification with the COD/N of 2. CWs with COD/N ratios of 6 and 4 coupled with external 30 mg/L Fe2+ showed the similar nitrate removal rate and denitrification potentials, showing there was synergy between Fe2+ and carbon source in denitrification. Denitrification potential was affected by the number of denitrifying bacteria and dissolved oxygen. Denitrification potential positively correlated with the number of denitrifying bacteria, and negatively correlated with dissolved oxygen.(3) The number of bacteria was different in different CWs that in different denitrification ways. The number of bacteria in CW without carbon source was low and that in CW without carbon source but Fe2+ was no obvious increase. The number of denitrifying bacteria and iron bacteria in CWs added Fe2+ were increased significantly at the COD/N ratio of 2. The number of denitrifying bacteria in CW with COD/N ratio of 6 was higher than that in CW with COD/N ratio of 4 coupled with Fe2+ while the iron bacteria was lower greatly.(4) The difference of functional diversity of microbial community between different CWs was assessed with substrate utilization patterns gathered with Biolog Eco PlatesTM. The results showed that the total microbial activity were lower in CWs without carbon source while the total microbial activity were higher in CWs with carbon source. In the presence of carbon source, the extra Fe2+ promoted the total microbial activity. Carbon source and Fe2+ also affected the microbial community diversity indices as the microbial community richness, evenness and dominance index increased significantly in CWs added carbon source. Carbon sources be utilized by microbe in CWs under different in?uent wastewater were different. The utilizations of carbohydrates by microbe in all CWs were high while the utilizations of amino acid by microbe in CWs with Fe2+ and carbon source were higher than in CWs only with carbon source. Principal component analysis results showed that the carbon source and Fe2+ were the main factors causing the difference of the microbial community. Carbon source was thought of as the first main factor, Fe2+ as the second main factor.