Interfacial interaction mechanisms between sediment-microplastics heterogeneous particulate matter (HPM) and cadmium (Cd) in saline environment

As microplastics enter aquatic environments, it remains unclear whether interfacial interactions between heterogeneous particulate matter (HPM, co-existing sediment and microplastics) and heavy metals in saline waters follow the same mechanisms as single particles, and how these interactions affect metal mobility and ecological risk. This study elucidates the interfacial mechanisms between HPM and cadmium (Cd) under various salinity conditions, highlighting the critical role of microplastics within HPM. Results show that: (1) 5 PSU and 15 PSU serve as critical salinity thresholds influencing interfacial interactions–low salinity (<5 PSU) enhances surface properties, promoting rapid initial adsorption (0–30 min) but limited retention, whereas high salinity (>15 PSU) significantly inhibits Cd adsorption; (2) Microplastics increase the salinity sensitivity of aquatic systems by modifying interfacial interaction to enhance the inhibitory effect, with maximum inhibition rate (IR = 41.29 %) observed at 7.5 PSU; (3) Salinity influence function (F_s), according to the salinity divergence coefficient (SDC), accurately characterizes the salinity effect and the microplastics contribution to Cd adsorption on HPM (R² = 0.92), supporting robust predictors of heavy metal fate. These findings offer theoretical insight into heavy metal transport in salt-affected aquatic environments.