Nanoparticles and their influence on radionuclide mobility in deep geological formations

This article gives an overview of the current status of knowledge concerning the role of nanoparticles (inorganic and organic) in deep geological host rocks and the potential influence of these nanoparticles on radionuclide migration in far-field systems. The manuscript is not intended to be a full review paper or overview paper concerning nanoparticles, here the intention is to refer to recent publications but to highlight the progress made in the 6th framework project IP FUNMIG (Fundamental processes of radionuclide migration) and the open literature over the past 5 a concerning the process understanding of nanoparticle related issues in the three host rock formations investigated, namely: claystones, crystalline rocks and salt rock overburden. The results show inter alia that the inorganic nanoparticle concentration in deep groundwaters of advection dominated systems rarely exceeds 1 mg L−1 and is expected to be in the ng L−1 range in diffusion controlled systems. For organic nanoparticles DOC concentrations up to tens of milligrams in diffusion-controlled indurated clays with molecular sizes mostly <500 Da have been found. Fulvic acid type organics have been identified in crystalline environments and plastic Clay formations (Boom Clay) with molecular sizes ⩽300 kDa. Additional sources of inorganic nanoparticles from the repository near-field (compacted bentonite) were identified and the initial erosion rates were determined. The results indicate under stagnant conditions ∼38 mg cm−2 a−1 for bi-distilled water, ∼20 mg cm−2 a−1 for glacial melt water (Grimsel groundwater) and very low rates ∼0.02 mg cm−2 a−1 for 5 mM CaCl2 contact water. The low critical coagulation concentration (CCC) indicative for purely diffusion controlled coagulation of 1 mM L−1 Ca2+ found in bentonite nanoparticle stability analysis matches the low nanoparticle mobilization from compacted bentonite found in these systems.