Effects of organic degradation products on the migration behaviour of radionuclides in cementitious materials

The impact of phthalates, which might be present in a cementitious repository as degradation products of superplasticisers or PVC additives, on the migration behaviour of An(III)/Ln(III) was investigated in batch sorption experiments on hardened cement paste. In the absence of organics, An(III)/Ln(III) sorb strongly on HCP with distribution ratios, R_d, of 10⁵ to 10⁶ L kg⁻¹. Above a no-effect level of ~ 10^–3 mol L⁻¹ phthalate, a distinct decrease of the An(III)/Ln(III) sorption was observed with sorption reduction factors (i.e., the ratio between the R_d values without and with organic ligand) of 100 to 1000 at phthalate concentrations of 10^–1 mol L⁻¹. This sorption reduction is attributed to the effect of Ca complexation with phthalate on the stability of calcium silicate hydrate (C–S–H) phases as main sorbing phase. These results and ongoing diffusion experiments indicate an increase in the mobility of An(III)/Ln(III) in cementitious barriers with increasing phthalate concentrations.