Colloid diffusion in crystalline rock: An experimental methodology to measure diffusion coefficients and evaluate colloid size dependence

Colloids present high sorption for many solutes and are considered potential contaminant carriers in geological environments. Experimental quantitative data are required for an adequate description of colloid-mediated contaminant transport within natural media. In this study, a methodology applying the nuclear ion beam technique Rutherford Backscattering Spectrometry (RBS) was used to measure colloid diffusion in crystalline rock and to analyze the effects of colloid size. It was found that colloids diffused within granite, colloid diffusion being both size-dependent and size-limited. Smaller colloids showed faster diffusion while diffusion was hindered for larger colloids of 250 nm. The measured apparent diffusion coefficients (Da) ranged from 7E-18 m2/s for 2 nm to 1.5E-18 m2/s for 100 nm colloids. These diffusion coefficients measured for gold colloids in granite are about five orders of magnitude lower than values found for weak or non-sorbing solutes, under the same experimental conditions. The experimental evidence of colloid diffusion in granite is described here for the first time.