A multi-constituent site blocking model for nanoparticle and stabilizing agent transport in porous media

A quantitative understanding of the potential influence of engineered and natural stabilizing agents on the transport behavior of engineered nanomaterials will be crucial to assessing their environmental fate. Column transport experiments conducted with CdSe/ZnS quantum dot nanocrystals (QD) stabilized with poly-(acrylic-acid)-octylamine (PAA-OA) yielded retention profiles that exhibited increased retention with distance from the inlet, a trend which could not be reproduced by existing nanoparticle transport models. To address this shortcoming, a new Multi-Constituent Site-Blocking (MCB) model was developed and implemented to simulate coupled transport and retention of nanoparticles and stabilizing agents. Mass balance equations for solution constituents are linked through a Langmuir-type blocking term that accounts for the surface area occupied by each constituent. The model successfully reproduced experimental observations of delayed QD breakthrough and retention profiles. These results support the hypothesis that stabilizing agents present in the nanoparticle suspension inhibit deposition, facilitating enhanced nanoparticle mobility in the columns. Inter-model comparisons and model sensitivity analyses examine the dependence of nanoparticle mobility on the relative concentration and adsorption properties of the stabilizing agents. Modeling results indicate that both synergistic and competitive interactions between nanomaterials and stabilizing agents should be accounted for in order to accurately predict nanoparticle transport behavior in subsurface environments.