Colloid diffusion coefficients in compacted and consolidated clay barriers: Compaction density and colloid size effects

Abstract

An experimental methodology applying the nuclear ion beam technique Rutherford Backscattering Spectrometry (RBS) is used to measure colloid diffusion profiles within three different types of clay: consolidated Opalinus clay (Switzerland), Callovo-Oxfordian clay (France) and FEBEX bentonite (Spain) compacted at different densities. The RBS technique is widely applied in materials science and it was selected because it allows the measurement of concentration profiles at short range distances (μm). The effects of colloid size, clay type and clay density were analyzed with negatively charged Au colloids of 2, 20 and 40 nm. Apparent diffusion coefficients ( D a) for gold colloids could be measured and D a values ranged from (10 −18 to 10 −19 m 2/s). The larger diffusion coefficient was measured for 2 nm colloids in the Opalinus clay with D a(Au 2 nm) = (2.1 ± 0.5) × 10 −18 m 2/s. The accessible porosity for colloids is even lower than that measured for anions, since not only anion exclusion but also size exclusion hinders diffusion. For example, 40 nm colloids did not accede at all to bentonite compacted at higher densities.