Toward an atomic-scale understanding of competitive adsorption between ions and molecules near aqueous interfaces

Capacitive deionization as a promising electrochemical technology has been tried to recover valuable nitrogen resource from urine solution. Such urine solution is usually complex in component, leading to competitive adsorption between target ions and organic molecules. To elucidate this competitive behavior at the atomic scale, we used the molecular dynamics simulation method under constant electrode potential to simulate NaCl solutions under different concentrations of urea. The presence of urea enhances the adsorption of anions at the anode but weakens the adsorption of cations at the cathode. In addition, urea molecules tend to replace water molecules near the two poles, and its electrostatic distribution characteristics cause urea molecules to adsorb parallel to the anode and perpendicular to the cathode, thus affecting the ion adsorption behavior. Furthermore, the urea molecules at the anode as H-bond donors favor the stability of the network, hence accommodating more anions, but the urea molecules with the hydrogen segment orienting to the cathode impede their H-bond donating and further push the cation adsorption layer away from the cathode. This work reveals atomistic details of the competitive adsorption between target ions and organic molecules, thereby supporting the improvement of CDI in treating complex wastewater.