Affected interactions and co-transport of cadmium sulfide quantum dots with Pb²⁺ by surface functionalization

Quantum dots (QDs), emerging semiconductor nanomaterials, have been detected in various environmental media and can adsorb co-existing contaminants (e.g., Pb²⁺). Surface modifications aimed at enhancing the performance of QDs can significantly affect their physicochemical properties, but their effects on QDs environmental behavior remain unclear. Herein, we investigated the adsorption and co-transport behaviors of aminated (NQD), hydroxylated (OQD), and carboxylated cadmium sulfide QDs (CQD) with Pb²⁺ via batch adsorption and quartz sand column experiments. The influence of ionic strength (IS) and cation valence on the co-transport of QDs and Pb²⁺ was examined. Our experimental findings revealed that Pb²⁺ inhibited the mobility of OQD and CQD but enhanced the transport of NQD due to the surface complexation and cation bridging effects. This promoting effect was weakened with increasing IS and cation valence, indicating the involvement of non-Derjaguin-Landau-Verwey-Overbeek forces. Furthermore, OQD and CQD with high mobility and strong affinity to Pb²⁺ effectively promoted the transport of Pb²⁺, with CQD exhibiting a more pronounced effect than OQD. Conversely, NQD reduced Pb²⁺ efflux due to their lower mobility and stronger adsorption to Pb²⁺. These results provide valuable insights into the role of surface modifications on QDs and their interactions with co-existing contaminants in subsurface environments.