Observed equilibrium partition and second-order kinetic interaction of quantum dot nanoparticles in saturated porous media

This study integrated batch experiments and theoretical calculations to understand the equilibrium adsorption and kinetic interaction of CdSeS/ZnS alloyed quantum dots nanoparticles (QDNPs) in sand porous media under different ionic strengths (ISs; 0.001–0.2 M NaCl). Our experimental results showed that equilibrium was reached for QDNP concentration between solid phase and bulk solution due to reversible adsorption of the QDNPs on sand surfaces. Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy calculations showed that the repulsive energy barriers were low and primary energy wells were shallow (i.e., comparable to the average kinetic energy of a colloid) at all tested solution ISs. Hence, the QDNPs could mobilize into and simultaneously escape from the primary wells by Brownian diffusion, resulting in the reversible adsorption. Additional batch experiments confirmed that a fraction of adsorbed QDNPs was released even without any perturbation of system conditions. The release was more evident at a lower IS because the primary energy wells spanned more narrowly at low ISs and thus the nanoparticles have a higher possibility to escape out. The batch kinetic experiments showed that the adsorption of QDNPs followed first- and second-order kinetic interactions at low and high ISs, respectively. These results indicate that the well-known colloid filtration theory that assumes irreversible first-order kinetics for colloid deposition is not suitable for describing the QDNP adsorption. The findings in our work can aid better description and prediction of fate and transport of QDNPs in subsurface environments.