Transport of citrate and polymer coated gold nanoparticles (AuNPs) in porous media: Effect of surface property and Darcy velocity

With the release of nanoparticles (NPs) into the subsurface, it is imperative to better understand the fate and transport of NPs in porous media. Three types of stable AuNPs were used as model NPs to investigate the impact of surface coatings (type and coverage) and water velocity on the NP transport in a porous media (column studies). The NPs were electrostatic stabilized citrate AuNPs and sterically stabilized AuNPs with amphiphilic block co-polymer (PVA-COOH) in two particle/polymer ratios (weak vs. strong stabilization). The citrate AuNPs transport was sensitive to ionic changes in the mixing front of the plume, where destabilization occurred, and will therefore depend on the size/type of release. Blocking of deposition sites by aggregates was seen to facilitate transport, whereby a higher flow velocity (larger shadow zone) also resulted in better transport. The polymeric surface coating had great impact with steric repulsion as a main force contributing to the transport of NPs in the porous media. Sufficient polymer coating was crucial to obtain highly unfavorable attachment conditions (very low α) where the enhanced NP mobility was independent of the water velocity (comparable to solute tracer). Without sufficient steric stabilization, the transport and recovery was significantly reduced compared to the solute tracer, but increased with increasing water velocity. This highlights the importance of sufficient surface coating to achieve enhanced mobility, but also the increased risk of spreading to down-gradient receptors. For the (weakly) sterically stabilized NPs, the loss of polymer through ligand exchange with the porous media negates transport.