Transport of Radionuclides and Colloid Through Quartz Sand Columns

Doktorsavhandling, 1995

Studies of the transport of radionuclides in the geosphere are one of the main tasks for performance assessment of a nuclear waste repository. A possible enhanced transport path for the radionuclides is the formation and transport of non-depositing colloids with a high affinity for radionuclides. The stability of colloids (silicate, aluminium oxide, bentonite and granite) have been investigated with respect to pH and ionic strength. None of the investigated colloids were stable in an aqueous solution with a ionic strength above 0.05, and at pH close to their point of zero charge. The values for the surface area-related sorption coefficient for the sorption of radionuclides on colloidal fractions were in the same range as on crushed samples. Goethite (.alpha.-FeOOH) has been used as a model colloid to study the transport through a quartz sand packed column over an extended period of time. The transport has been studied as a function of pH, goethite colloid concentration and flow rate. The transport of goethite has been modelled using a multilayer model, where convective, diffusional and force induced fluxes were taken into consideration. The transport of goethite colloid was studied at pH 4 and pH 10. The transport of goethite colloid follows the theoretical predictions based on electrostatic attraction/repulsion forces between the surfaces of goethite and quartz. At pH 4 the transport was delayed because of goethite deposition on quartz. No breakthrough of goethite colloid was observed until most of the available sites on quartz were occupied. When breakthrough occurred only a small fraction of the quartz surface was covered. Furthermore, the degree of the coverage decreased with increasing flow rate, indicating a kinetic effect. No dependence on goethite colloid concentration in the terms of the amount of deposition could be observed at the low goethite concentrations used (6 to 30 g/m3). No detachment of deposited goethite colloid could be detected, except when the physicochemical properties of the eluent were significantly changed that a net repulsion between goethite colloid and quartz developed. At pH 10, when both the surface of the goethite colloid and the quartz were negatively charged, no significant deposition was observed. The alteration of the transport of Na, Cs, Pm, Th, I (as IO3-) and Tc (as TcO4-) in the presence of goethite colloid was investigated at pH 4 and pH 10. The transport of the elements examined was altered if the interaction of the element and the goethite colloid was strong. The co-transport of the elements with goethite colloid could be predicted by using data from sorption experiments, if the available surface area was used in the calculations of the distribution coefficient. However, when no goethite colloid was present, and even if sorption experiments indicated a strong sorption of the element onto quartz, a significant fraction of the element was transported through the column with no retardation.