Innovative process using Fe⁰/CO₂ for the removal of nitrate from groundwater

In this study, the Fe⁰/CO₂ process was investigated for removing nitrate from aqueous solution, in terms of process efficiency, process operation mode, and post-treatment of the end product ammonium. The results show that nitrate at 30 mg/L could be removed from solution within 30 min under the conditions of 2 g/L Fe⁰ and 200 mL/min CO₂ flow rate. Additionally, nitrite was not detected in treated solution, whereas ammonium is the predominant nitrogen-containing species. The normalized residual nitrate concentration decreased with increasing nitrate concentration (2.18–24.19 mg N/L). Nitrate removal was inhibited significantly in the presence of humic acid. Comparing operation modes, NO₃⁻ reduction efficiency with increasing number of batch operations in Mode 2 (treated solution was emptied and refilled with freshly prepared solution for the next batch treatment, containing the same level of nitrate as the previous batch) is better than that with Mode 1 (treated solution was retained in the reactor and spiked with concentrated nitrate solution to raise nitrate concentration to a level close to the one in the previous batch). However, to guarantee satisfactory nitrate removal in batch operation mode, zero-valent iron supplementation needs to be taken into consideration. For example, the nitrate removal efficiency without Fe⁰ supplementation is decreasing in the third batch, compared with those with supplements of 0.25 and 1 g/L. According to a preliminary study, the undesired end-product ammonium can be removed from solution by about 95% within 22.5 h with an air flow rate of 500 mL/min and a solution pH around 12; the ammonium concentration decreased from 6.4 to 0.3 mg 12 N/L. Stripping time can be further shortened by increasing air flow rate and using an efficient air diffuser.