Selective ion adsorption with pilot-scale membrane capacitive deionization (MCDI): arsenic, ammonium, and manganese removal

A pilot-scale membrane capacitive deionization (MCDI) unit was used for the selective removal of arsenic (up to 300 μg L–1 As(V)), ammonium (25 mg L–1 NH4+), and manganese (6 mg L–1 Mn2+) in experiments with brackish water. Tests were carried out using a commercially available MDCI module and different initial salt concentrations (total dissolved solids, TDS = 0–2 g L–1 NaCl) to investigate the removal capacity and behavior of the MCDI unit with different operational parameters such as applied current, voltage, flow rate, and experimental settings such as pH. Selectivity and adsorption behavior is described and a comparison with lab-models is presented to validate the results obtained in real-life scale and application. While the adsorption capacity of the module decreased with higher TDS, specific ion adsorption improved with ionic mobility and greater ion charge. Removal of NH4+ and Mn2+ at given concentrations was significantly higher than for As(V) but rejection of As(V) could be significantly increased (30%–89.5%) by raising the initial pH value above its pKa = 6.94. Depending on selected operational settings and feed water characteristics, the total energy consumption of the MCDI unit (only electrodes) ranged between 0.89 and 2.74 kWh m–3 with an overall optimum at