Annika S. Fjordbøge

The stability of gold nanoparticles (AuNPs) stabilized electrostatically with citrate or (electro)sterically by commercially available amphiphilic block copolymers (PVP-VA or PVA-COOH) was studied under various physicochemical conditions. Subsequently, the mobility of the AuNPs in porous media (sand) was investigated in column studies under environmental relevant physicochemical conditions. Electrostatically stabilized AuNPs were unstable under most physicochemical conditions due to the compression of the electrical double layer. Consequently, aggregation and deposition rapidly immobilized the AuNPs. Sterically stabilized AuNPs showed significantly less sensitivity towards changes in the physicochemical conditions with high stability, high mobility with negligible retardation, and particle deposition rate coefficients ranging an order of magnitude (1.5 × 10⁻³ to 1.5 × 10⁻² min⁻¹) depending on the type and amount of stabilizer, and thereby the surface coverage and attachment affinity. The transport of sterically stabilized AuNPs is facilitated by reversible deposition in shallow energy minima with continuous reentrainment and blocking of available attachment sites by deposited AuNPs. The stability and mobility of NPs in the environment will thereby be highly dependent on the specific stabilizing agent and variations in the coverage on the NP. Under the given experimental conditions, transport distances of the most mobile AuNPs of up to 20 m is expected. Due to their size-specific plasmonic properties, the easily detectable AuNPs are proposed as potential model or tracer particles for studying transport of various stabilized NPs under environmental conditions.

Field investigations were carried out to determine the occurrence of tetrachloroethene ( PCE ) dense nonaqueous phase liquid ( DNAPL ), the source zone architecture and the aquitard integrity at a 30‐ to 50‐year old DNAPL release site. The DNAPL source zone is located in the clay till unit overlying a limestone aquifer. The DNAPL source zone architecture was investigated through a multiple‐lines‐of‐evidence approach using various characterization tools; the most favorable combination of tools for the DNAPL characterization was geophysical investigations, membrane interface probe, core subsampling with quantification of chlorinated solvents, hydrophobic dye test with Sudan IV , and Flexible Liner Underground Technologies ( FLUTe ) NAPL liners with activated carbon felt ( FACT ). While the occurrence of DNAPL was best determined by quantification of chlorinated solvents in soil samples supported by the hydrophobic dye tests (Sudan IV and NAPL FLUTe ), the conceptual understanding of source zone architecture was greatly assisted by the indirect continuous characterization tools. Although mobile or high residual DNAPL ( S _t > 1%) only occurred in 11% of the source zone samples (intact cores), they comprised 86% of the total PCE mass. The dataset, and associated data analysis, supported vertical migration of DNAPL through fractures in the upper part of the clay till, horizontal migration along high permeability features around the redox boundary in the clay till, and to some extent vertical migration through the fractures in the reduced part of the clay till aquitard to the underlying limestone aquifer. The aquitard integrity to DNAPL migration was found to be compromised at a thickness of reduced clay till of less than 2 m.