Characteristics of nitrate reduction by zero-valent iron powder in the recirculated and CO2-bubbled system

In this study, the Fe0/CO2 process was investigated for removing nitrate from aqueous solution under different operating conditions such as CO2 bubbling rate (0-400 mL/min), Fe0 dosage (1-6 g/L), initial nitrate concentration (6-23 mg N/L), batch mode, and fresh Fe0 supplementing (0-1 g/L). Results show that the bubbling of CO2 flow rate at 200 mL/min was sufficient for supplying H+ into solution to create an acidic environment favorable to nitrate reduction reaction. It was found that sigmoidal model equation describes the S-curve behaviors of nitrate reduction, ferrous accumulation and ammonium formation satisfactorily, and the parameter t1/2 of the proposed model equation serves as a powerful tool for the comparison of nitrate reduction rate. Sustainability test demonstrates that Fe0 powder began to deteriorate after three batches operation. Concerning the operating modes, the batch mode with the treated solution emptied and freshly refilled outperforms the one, which was operated by retaining the treated solution and spiking concentrated nitrate into it for the next batch treatment. To guarantee satisfactory nitrate removal using the former mode, supplement of appropriate amount of Fe0 needs to be optimized