Transport and retention mechanism of CdS nanoparticles in layered heterogeneous porous media: Coupled regulation of interfacial heterogeneity and ionic strength.

Engineered nanoparticles are increasingly used in industry and consumer products, raising concerns about their release and mobility in subsurface environments such as stratified aquifers. Understanding how physicochemical conditions and pore structure heterogeneity jointly govern nanoparticle transport is essential for predicting environmental fate and supporting risk assessment. This study aims to elucidate the transport and retention mechanisms of cadmium sulfide nanoparticles (CdS NPs) in layered porous media, with emphasis on the roles of structural heterogeneity and hydrochemical conditions. Saturated column experiments combined with high-resolution X-ray micro-computed tomography (μCT) were used to quantify nanoparticle spatial distributions, and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was employed to interpret interfacial interactions. Results indicate that layered heterogeneity significantly governs nanoparticle transport pathways by modifying local flow velocities and shear forces. Higher ionic strength and the presence of divalent calcium ions (Ca2+) further enhanced nanoparticle retention through electrical double layer compression and cation bridging. Together, these findings demonstrate a coupled mechanism whereby porous media architecture and aqueous chemistry jointly control nanoparticle deposition by regulating energy barriers and redistributing local flow. This study provides mechanistic insights and a basis for predicting nanoparticle transport in subsurface environments.