Enhancement of immobilized biochar/FeS on nitrogen removal in constructed wetland at low temperature

Constructed wetland (CW) is an eco-friendly and economically treatment for the deep nitrogen removal. Microbial denitrification, a key process in CW, is temperature-sensitive, with decreased nitrogen removal efficiency at low temperature. To enhance the nitrogen removal efficiency at low temperature, this study explored the use of functional substrates, focusing on iron sulfide (FeS) nanoparticles and biochar (BC). The results showed that immobilized BC/FeS significantly improved nitrate (NO3-N) and total nitrogen (TN) removal in CW at low temperature (8–13 °C), which reached 93.59 ± 2.07 and 80.06 ± 3.00 %, respectively, with 30 % immobilized BC/FeS. Increasing the dosage of immobilized BC/FeS accelerated denitrification rate and reduced nitrite (NO2-N) accumulation. Autotrophic denitrification was the primary pathway, with the contribution of autotrophic denitrification increasing from 64.06 % (10 % immobilized BC/FeS) to 83.25 % (30 % immobilized BC/FeS). Sulfurimonas was the dominant denitrifying bacteria identified as a cold-tolerant denitrifying bacteria. The study underlines the potential of composite electron donor substrates to enhance nitrogen removal in CW, providing a sustainable solution to nitrogen pollution in cold climates.