Selective removal of nitrate ions by controlling the applied current in membrane capacitive deionization (MCDI)

The use of membrane capacitive deionization (MCDI) to selectively remove nitrate ions from a mixed solution of chloride and nitrate ions was investigated in this study. Adsorption equilibrium experiments were conducted to determine the selectivity of the AMX membrane toward the nitrate ion. Desalination experiments were also performed on a mixed solution (5 mM NaCl+2 mM NaNO3) using a MCDI cell consisting of carbon electrodes, a Neosepta AMX membrane, and a CMX membrane. The AMX membrane had a selectivity coefficient (image) of 4.37, indicating high selectivity of the AMX membrane toward the nitrate ion. The desalination experiments were performed by applying a constant current to the MCDI cell until the cell potential reached 1.0 V. The total amount of adsorbed ions was not affected by changes in the applied current. However, the mole fractions of the nitrate ions relative to all the adsorbed ions were inversely proportional to the applied current: 31.7, 33.7, and 41.0% at 30, 20, and 10 mA applied current, respectively. This result is attributed to the increase in the mole fraction of nitrate ions adsorbed by the AMX membrane at lower applied currents.