Particle retention on granite as a function of residence time and particle size using a synthetic fracture flow cell
P 4.8 in Final repositories and underground disposal sites
The interaction of monodisperse fluorescent carboxylated polystyrene particles (25 nm and 1000 nm in diameter) with a cut granite surface (Grimsel granodiorite; Switzerland, ) and with acrylic glass is investigated both experimentally and numerically focusing on the effect of residence time, colloid size, collector material and fracture orientation on particle retention. Long particle residence time between 1 h and 24 h are accomplished by stop-flow experiments. Additionally to the colloid experiments, conservative solute tracer (Amino-G) experiments are performed to characterize the flow and transport conditions. A cylindrical parallel plate type fracture flow cell (diameter 38 mm; aperture 0.75 mm) has been designed for the transport experiments. The artificial fracture of the flow cell is sandwiched between the acrylic glass and/or the granite. At the flow cell outlet the breakthrough curves are obtained continuously by means of fluorescence spectroscopy. All experiments are conducted at pH 5 under low ionic strength (1 mM NaCl). Using colloid probe technique and atomic force microscopy (AFM) special surface information of the granite and acrylic glass discs are obtained in dependence of the particle size attached to the cantilever. Results show earlier first arrivals and more pronounced tailings in the measured breakthrough curves for both colloid types compared to the conservative tracer. A positive correlation between residence time and particle retention is observed in all experiments. Using 1000 nm particles, the results show that the fracture material has no significant effect on particle retention. For the same fracture orientation the increase in particle retention is similar for both acrylic glass and granite disc. However, in experiments with a horizontal fracture orientation the particle retention is higher than in experiments with vertical orientation. In contrast to that, results of experiments using 25 nm particles show an effect of the collector surface material on particle retention and no effect of fracture orientation, respectively. The increase of particle retention with time was higher in experiments with inserted granite disc than with inserted acrylic glass disc. These findings lead to the assumption that the 1000 nm particles undergo sedimentation and are not affected by the existent surface roughness due to the bigger particle size. On the contrary, 25 nm particles will not undergo sedimentation within 24 h but they are affected by surface inhomogeneity, such as surface roughness. For example, an increased roughness will direct enhance diffusivity of the smaller particles (25 nm) into scratches or cracks on granite/acrylic glass surface compared to bigger particle sizes (1000 nm). These experimental findings are corroborated by the results of AFM measurements and by 2-D numerical simulations using the software COMSOL Multiphysics®.
 Schäfer, Huber, Seher, Missana, Alonso, Kumke, Eidner, Claret, Enzmann (2012) Appl. Geochem. 27(2), 390.