ARS | Publication request: Coupled effects of solution chemistry and hydrodynamics on the mobility and transport of quantum dot nanomaterials in the Vadose Zone

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2010
Publication Date: N/A

Interpretive Summary: Laboratory experiments were performed to characterize the mobility and transport of quantum dot (QD) nanomaterials in the vadose zone. Coupled effects of solution ionic strength, QD aggregate size, surface tension, contact angle, infiltration rate and initial water content of the porous media on QD mobility and transport were investigated. As solution ionic strength increased, QD retention increased; however, retention was significantly suppressed as non-ionic surfactant infiltration solution was used. Non-ionic surfactant limited QD aggregation and enhanced QD mobility and transport. The dominant phenomena for mobility and transport of QD in unsaturated porous media shifted from meso-scale processes, where infiltration by preferential flow allowed rapid transport, to pore scale processes governed by gas-water interfaces when chemical transport conditions shifted, as impacted by electrostatic forces (affecting surface potential and QD aggregate size) ad capillary forces. Organic chemicals, such as surfactants, present in the subsurface environment impact not only the surfaces of the QDs but also QD aggregation leading to enhanced mobility and transport. Therefore, under changing and heterogeneous physiochemical subsurface environments, QDs may be extremely mobile and transported by preferential flow in the vadose zone, potentially reaching groundwater.

Technical Abstract: To investigate the coupled effects of solution chemistry and vadose zone processes on the mobility of quantum dot (QD) nanoparticles, laboratory scale transport experiments were performed. The complex coupled effects of ionic strength, size of QD aggregates, surface tension, contact angle, infiltration and water content on the mobility and transport of QDs were demonstrated. As ionic strength increased, the QD retention increased; however this retention was significantly suppressed in the presence of non-ionic surfactant in the infiltration solution, regardless of the chemical transport conditions. Non-ionic surfactant limited the formation of QD aggregates by impacting steric forces and enhanced the solubility and transport of QDs in porous media. When changing from favorable to unfavorable chemical transport conditions, the dominating phenomena for mobility and transport of QDs in the vadose zone shifted from the meso scale process, where infiltration by preferential flow allowed rapid transport of QDs, to the pore scale process governed by gas-water interfaces (GWI) that impacts the mobility of QDs. Retention of QDs was controlled by electrostatic and capillary forces, with the latter being the most influential. GWI were found to be the dominant mechanism and site for QD removal compared with solid-water interfaces (SWI) and pore straining.