Pharmaceutical removal at low energy consumption using membrane capacitive deionization

The performance of the membrane capacitive deionization (MCDI) system was evaluated during the removal of three selected pharmaceuticals, neutral acetaminophen (APAP), cationic atenolol (ATN), and anionic sulfamethoxazole (SMX), in batch experiments (feed solution: 2 mM NaCl and 0.01 mM of each pharmaceutical). Upon charging, the cationic ATN showed the highest removal rate of 97.65 ± 1.71%, followed by anionic SMX (93.22 ± 1.66%) and neutral APAP (68.08 ± 5.24%) due to the difference in electrostatic charge and hydrophobicity. The performance parameters (salt adsorption capacity, specific capacity, and cycling efficiency) and energy factors (specific energy consumption and recoverable energy) were further evaluated over ten consecutive cycles depending on the pharmaceutical addition. A significant decrease in the specific adsorption capacity (from 24.6 to ∼3 mg-NaCl g^-1) and specific capacity (from 17.6 to ∼2.5 mAh g^-1) were observed mainly due to the shortened charging and discharging time by pharmaceutical adsorption onto the electrode. This shortened charging time also led to an immediate drop in specific energy consumption from 0.41 to 0.04 Wh L^-1. Collectively, these findings suggest that MCDI can efficiently remove pharmaceuticals at a low energy demand; however, its performance changes dramatically as the pharmaceuticals are present in the target water.

A new standard metric describing the adsorption capacity of carbon electrode used in membrane capacitive deionization

A new standard metric has been developed to express the actual adsorption capacity of the carbon electrode in consideration of the electrode reactions. The adsorption experiments were carried out by changing the cell potentials (0.6-1.6 V) in the MCDI unit cell. The point at which the electrode reactions occur was determined from the change in instantaneous charge efficiency during the adsorption process. Then, the total charge supplied to the carbon electrode at this point was defined as the maximum allowable charge (MAC). The MAC values were constant at 59 C/g irrespective of cell potentials. In addition, the salt adsorption capacity (SAC_MAC) and the charge efficiency at MAC were approximately 16 mg/g and 91%, respectively, regardless of the cell potentials. Furthermore, the equivalent circuit analysis for the MCDI cell revealed that Faradaic reactions rarely occur at the MAC. The MAC is the maximum charge that can be supplied to the carbon electrode without electrode reactions. It is also unaffected by the cell components and operating conditions of the MCDI cell. Therefore, the MAC is expected to be a useful metric to objectively express the actual adsorption capacity of the carbon electrode.