The impact of nanoplastics (NPs) on the performance of membrane capacitive deionization (MCDI) systems

This study investigates the impact of nanoplastics (NPs) on fouling behavior and desalination performance in membrane capacitive deionization (MCDI), where their effects remain poorly understood. Under zero-voltage conditions, NPs preferentially accumulated on the anion-exchange membrane (AEM) due to electrostatic attraction, while deposition on the cation-exchange membrane (CEM) was limited by repulsion. In contrast, under applied voltage conditions, enhanced NP accumulation was observed on the CEM. This behavior is consistent with electric-field-driven modification of interfacial interactions, including compression of the electrical double layer (EDL), which reduces repulsive forces between NPs and the membrane surface. This fouling increased ion-transport resistance and reduced salt adsorption capacity (SAC) by approximately 51.5% after 120 cycles. Charge-based SAC, salt desorption capacity (SDC), and coulombic efficiency (CE) remained nearly ideal, indicating reversible capacitive charge storage. However, conductivity-based analysis revealed pronounced discrepancies between SAC and SDC that intensified over cycling. This mismatch arises from residual salt storage and interfacial trapping, which hinder ion release during desorption. Furthermore, increasing NP concentration and applied voltage accelerated fouling and advanced the onset of SAC decline by up to 80% (from 160 to 32 cycles). Fluorescence imaging showed that, while the AEM showed consistently high fouling under all conditions, CEM fouling depended strongly on NP concentration and applied voltage. These findings demonstrate distinct fouling mechanisms on AEM and CEM surfaces and provide insight into how NP accumulation influences long-term MCDI performance.