Effect of activated carbon-supported nano-iron material on nitrogen removal and N₂O emissions from constructed wetlands

Constructed wetlands are an effective technology for the advanced treatment of tailwater in sewage treatment plants. However, the carbon-to-nitrogen ratio (C/N) in the tailwater is relatively low, and chemical oxygen demand (COD) is difficult to utilize, leading to incomplete nitrification and denitrification reactions when traditional constructed wetlands are used to treat this type of tailwater. Moreover, a large amount of N₂O gas (a greenhouse gas) is produced and escapes into the atmosphere, resulting in poor ecological benefits. In response to this situation, activated carbon-supported nano-iron material Fe-AC was prepared, and three groups of constructed wetlands (CW1, CW2, CW3) were designed and constructed. Pure gravel (blank control), gravel +3% Fe-AC, and gravel +8% Fe-AC were added, respectively, to simulate the low C/N water quality of the tail water of the sewage treatment plant. The principle of micro-electrolysis formed by the potential difference between the iron and carbon was utilized to enhance the denitrification in constructed wetlands. The influence of different dosages of Fe-AC on the denitrification of constructed wetlands and the reduction of N₂O emissions was explored, and the influence mechanism was analyzed. The results showed that Fe-AC could spontaneously form primary batteries in constructed wetlands to release electrons and H, serving as electron donors to promote nitrogen conversion and enhance the activity of microorganisms. Under the conditions of simulated influent COD of 28 mg/L and TN of 18 mg/L in the constructed wetland, i.e., C/N of 1.5, the effluent COD of CW1, CW2 and CW3 was stable at 6.0, 4.5 and 4.0 mg/L, and the removal rates reached 72%, 83% and 85%, respectively. The total nitrogen concentrations were stabilized at 14.0, 0.5 and 0.4 mg/L, and the removal rates reached 22%, 97% and 97%, respectively. The constructed wetland with Fe-AC added had a significant removal effect on COD and TN. The N₂O emissions of CW1, CW2 and CW3 were 1 330.87, 629.86 and 576.49 μg/(m²·h), respectively. Compared with the blank control group (CW1), the N₂O reduction rates of CW2 and CW3 reached 52.72% and 56.52%, respectively. The addition of Fe-AC had a significant effect on reducing N₂O emissions in constructed wetlands. The relative abundance of Bacteroidetes and methylotrophic genera within CW2 and CW3 supplemented with Fe-AC was significantly increased, and other functional bacteria related to denitrification and organic matter degradation were also screened and optimized. Based on the operation effects of the three groups of constructed wetlands, adding 3% Fe-AC to CW2 can meet the requirements of improving the denitrification effect of the constructed wetlands and reducing the emission of N₂O gas, which is more economical and reasonable than 8% Fe-AC in CW3.