Site-Specific Retention of Colloids at Rough Rock Surfaces

The spatial deposition of polystyrene latex colloids (d = 1 mu m) at rough mineral and rock surfaces was investigated quantitatively as a function of Eu(III) concentration. Granodiorite samples from Grimsel test site (GTS), Switzerland, were used as collector surfaces for sorption experiments. At a scan area of 300 X 300 mu m(2), the surface roughness (rms roughness, Rq) range was 100-2000 nm, including roughness contribution from asperities of several tens of nanometers in height to the sample topography. Although, an increase in both roughness and [Eu(III)] resulted in enhanced colloid deposition on granodiwite surfaces, surface roughness governs colloid deposition mainly at low Eu(III) concentrations (<= 5 x 10(-7) M). Highest deposition efficiency on granodiorite has been found at walls of intergranular pores at surface sections with roughness Rq = 500-2000 nm. An about 2 orders of magnitude lower colloid deposition has been observed at granodiorite sections with low surface roughness (Rq < 500 nm), such as large and smooth feldspar or quartz crystal surface sections as well as intragranular pores. The site-specific deposition of colloids at intergranular pores is induced by small scale protrusions (mean height = 0.5 +/- 0.3 mu m). These protrusions diminish locally the overall DLVO interaction energy at the interface. The protrusions prevent further rolling over the surface by increasing the hydrodynamic drag required for detachment. Moreover, colloid sorption is favored at surface sections with high density of small protrusions (density (D) = 2.6 +/- 0.55 mu m(-1), asperity diameter (phi) = 0.6 +/- 0.2 mu m, height (h) = 0.4 +/- 0.1 mu m) in contrast to surface sections with larger asperities and lower asperity density (D = 1.2 +/- 0.6 mu m(-1), phi = 1.4 +/- 0.4 mu m, h = 0.6 +/- 0.2 mu m). The study elucidates the importance to include surface roughness parameters into predictive colloid-borne contaminant migration calculations.