Transport von Kolloiden durch wassergesättigte, geklüftet poröse Medien am Beispiel von synthetischem nanopartikulärem Silber

Colloids are ubiquitous in the surface as well as in the subsurface aquatic environment. Colloids are known from porous, fissured and karst aquifers. Not only shallow but also deep saline groundwater contains colloids. Due to the increased application of engineered nanoparticles in consumer products and their release into the environment, a new potential class of colloids is generated, which effectiveness, stability and mobility within the environment, and in groundwater in particular, is hardly investigated yet. The research concerning the environmental impact of engineered nano-materials is only weakly developed if compared to the amount of new developed nano-sized materials and products. In this study laboratory experiments regarding the mobility and transformation of engineered nano-sized silver (AgNP) in three different water saturated sandstones with variations in structure and texture are discussed. In addition to undisturbed matrix samples, partially fractured rock samples and samples with distinct single fissure are considered. The applicability of established colloid transport models to the experimental AgNP transport results at variable solute strength and fluid velocity is investigated. Eluted fluid samples are mainly analysed using asymmetric flow field-floss fractionation with coupled UV/Vis and static laser light scattering concerning concentration and particle size. Particle attachment on rock surfaces is analysed using scanning electron microscopy with integrated energy dispersive X-ray detector at selected samples. The experimental and modelling results yields dependencies of AgNP transport on: Rock texture, Porosity and pore size, Hydraulic conductivity, Mineralogical composition and Solute chemistry. The partially fractured sandstones have increased AgNP mobility although the rock matrix has high impact on transport behaviour. There is evidence that AgNP is mobilized via colloid-facilitated transport on rock colloids. AgNP mobility in single fissured rock is comparably high. AgNP recoveries of up to 98% are measured. In almost all rock samples dependencies between recovery and applied AgNP mass was observed, which is also dependent on solute chemistry. This indicates limited deposition sites for specific hydrochemical conditions. Although AgNP deposition in the filter material of the experimental setup varies and the investigation of the associated processes need further research. AgNP specific deposition sites in the rock samples are heterogeneous distributed. At the matrix samples, DLVO theory is critical and the applicability of the filtration theory is limited. For the sandstone with small pore sizes and porosities, filtration theory is not applicable. For the single-fissured samples hydrodynamic forces are obviously dominating and the particle-rock interaction depends strongly on the porosity and permeability of the matrix. The macroscopic visible heterogeneous distribution of AgNP on the fissure surfaces indicate the inadequate numerical modeling approaches of AgNP deposition in natural rock samples. The DLVO theory as global estimator for AgNP deposition has limited applicability due to the unknown surface charge distribution. The experimental results show that engineered nano-scaled silver is highly mobile in fissured-porous sandstones with respect to the considered boundary conditions and an associated distribution within sandstones aquifers may occur. The findings indicate further experiments at field scale and under realistic boundary conditions considering also organic compounds in the future