Unacclimated activated sludge improved nitrate reduction and N2 selectivity in iron filling/biochar systems

Iron (Fe)-based denitrification is a proven technology for removing nitrate from water, yet challenges such as limited pH preference range and low N2 selectivity (reduction of nitrate to N2) persist. Adding biochar (BC) can improve the pH preference range but not N2 selectivity. This study aimed to improve nitrate reduction and N2 selectivity in iron filling/biochar (Fe/BC) systems with a simplified approach by coupling unacclimated microbes (M) in the system. Factors such as initial pH, Fe/BC ratio, and Fe/BC dosage on nitrate removal efficiency and N2 selectivity were evaluated. Results show that the introduction of microbes significantly enhanced nitrate removal and N2 selectivity, achieving 100 % nitrate removal and 79 % N2 selectivity. The Fe/BC/M system exhibited efficient nitrate reduction at pH of 2–10. Moreover, the Fe/BC/M system demonstrated an improved electrochemical active surface area (ECSA), lower electron transfer resistance and lower corrosion potential, leading to enhanced nitrate reduction. The high i0 value in Fe/BC/M system means more Hads could be generated, thus improving the N2 selectivity. This study provides valuable insights into a novel approach for effective nitrate removal, offering a potential solution to the environmental challenges posed by excessive nitrate in wastewater, surface water and ground water. [Display omitted] •Developed an innovative Fe/BC/M system for effective nitrate removal.•Unacclimated microbes enhanced nitrate reduction and selectivity significantly.•Achieved 100 % nitrate removal efficiency and over 79 % N2 selectivity.•Assessed key factors for nitrate reduction and selectivity.•Mechanism of enhanced nitrate removal and N2 selectivity was discussed.