Sorbing Tracers Research Articles

Process controlling the slow sorption of quinoline onto clay during transport in columns

The sorption of quinoline onto clay-modified alumina was studied in order to determine 1) the cause of the observed slow approach to equilibrium, and 2) the validity of describing retarded transport with an effective K d approach. Possible rate-limiting mechanisms include aqueous diffusion into the porous particles, Knudsen diffusion, and chemical kinetics. Column experiments with nonsorbing ( 3H) and sorbing ( 45Ca, CaCl 2) tracers all exhibited fast sorption, indicating that diffusion processes are fast and ion exchange can be fast. In contrast, a slow approach to equilibrium is observed with quinoline. Quinoline sorption kinetics are therefore controlled by a specific quinoline/montmorillonite interaction (slow ion exchange or near-surface diffusion) because ordinary diffusion would have affected all of the solutes. The slow sorption may be caused by steric hindrance because the quinoline molecule is bulky and has only a single exchange site. Quinoline sorption to the surface was found to be generally nonlinear; but linearity below 10 mg/L enabled the exchange reaction to be represented as a first-order reversible reaction. The average desorption rate (k b) for quinoline was 7.0 × 10 −4/sec. It increased with increasing pH, indicating chemical control. Although most larger-scale field systems are likely to be essentially at equilibrium, the importance of determining the sorption rate-limiting mechanism will aid in designing remediation strategies, which can involve short-time manipulations of conditions over smaller scales.

Observation of Time Dependent Dispersion in Laboratory Scale Experiments with Intact Tuff

The migration of radionuclides through intact tuff was studied using tuff from Yucca Mountain, Nevada. The tuff samples were both highly zeolitized ash-fall tuff from the Calico Hills and densely welded devitrified tuff from the Topopah Springs member of the Paintbrush tuff. Tritiated water and pertechnetate were used as conservative tracers. The sorbing tracers {sup 85}Sr, {sup 137}Cs, and {sup 133}Ba were used with the devitrified tuff only. Greater tailing in the elution curves of the densely welded tuff samples was observed that could be fit by adjusting the dispersion coefficient in the conventional Advection Dispersion Equation, ADE. The curves could be fit using time dependent dispersion as was previously observed for sediments and alluvium by Dieulin, Matheron, and de Marsily. The peak of strontium concentration was expected to arrive after 1.5 years based on the conventional ADE and assuming a linear K{sub d} of 26 ml/g. The observed elution had significant strontium in the first sample taken at 2 weeks after injection. The peak in the strontium elution occurred at 5 weeks. The correct arrival time for the strontium peak was achieved using a one dimensional analytic solution with time dependent dispersion. The dispersion coefficient as a function ofmore » time used to fit the conservative tracers was found to predict the peak arrival of the sorbing tracers. The K{sub d} used was the K{sub d} determined by the batch method on crushed tuff. 23 refs., 9 figs., 2 tabs.« less

Analysis of Tracer Movement in Single Fracture in Granite Core (INTRAVAL Case 2).

This paper presents an analysis of INTRAVAL (An International Project to Study Validation of Geosphere Transport Models) test case 2, which is based on laboratory experiments on the tracer movement in a single fracture in granite cores. We have applied two kinds of mathematical models for simulations of tracer experiments; a constant aperture model that aperture of fracture is constant in the fracture plane and a variable aperture channel model that apertures of fracture are varied with a given probability distribution. Calculated breakthrough curves of non-sorbing and sorbing tracers were compared with experimental ones. Analysis of non-sorbing tracer shows that fracture flow velocities do not have linear relationship to injection flow rates, it may be explained by the variable aperture model taking into account of the stagnant water in the fracture. Those of sorbing tracers show that calibrated matrix retardations of the constant aperture model are approximately two times larger than the variable aperture model. This feature indicates that the variable aperture model gives longer travel time of nuclide than the constant aperture model if the retardations are same, and channeling is important phenomenon in safety evaluations of waste disposal.

Laboratory Radionuclide Migration Experiments at a Scale of 1 m

AbstractResults are presented from a migration experiment carried out in a natural fracture in a 1 m × 1 m × 0.6 m quarried granite block. Near-uniform flow in the fracture was achieved by controlling the groundwater flow from the fracture laterally across the exit face of the fracture. The longitudinal dispersion was determined from the elution profile of uranine, a non-sorbing tracer. The effect of matrix diffusion was determined by reducing the linear velocity of the transport solution from approximately 3 to 0.75 cm/h. The velocity of a sorbing tracer, Cs-137, was compared to that of non-sorbing I-131 and with that calculated from data obtained for Cs-137 in static sorption experiments on similar fracture surfaces. Information on channeling in the fracture was obtained by autoradiographing the fracture surface of the separated block after the termination of the experiment. Quantitative information on the sorbed Cs-137 inventory by two-dimensional gamma scanning of the fracture surfaces is underway.

Analysis of Some Laboratory Tracer Runs in Natural Fissures

Tracer tests in natural fissures performed in the laboratory are analyzed by means of fitting two different models. In the experiments, sorbing and nonsorbing tracers were injected into a natural fissure running parallel to the axis of a drill core. The models take into account advection, dispersion, diffusion into the rock matrix, and sorption onto the surface of the fissure and on the microfissures inside the matrix. For the second mechanism, one of the models considers hydrodynamic dispersion, while the other model assumes channeling dispersion. The models take into account time delays in the inlet and outlet channels. The dispersion characteristics and water residence time were determined from the experiments with nonsorbing tracers. Surface and volume sorption coefficients and data on diffusion into the rock matrix were determined for the sorbing tracers. The results are compared with values independently determined in the laboratory. Good agreement was obtained using either model. When these models are used for prediction of tracer transport over larger distances, the results will depend on the model. The model with channeling dispersion will show a greater dispersion than the model with hydrodynamic dispersion, assuming constant dispersivity.

Migration in a Single Fracture in Granitic Rock

ABSTRACTAn in situ experiment to study migration in a single fracture in granitic rock has been performed. Both sorbing and nonsorbing tracers have been used. Results from a tracer run with a nonsorbing tracer (Uranine) are presented. The breakthrough curves have been explored with a model which takes into account hydrodynamic dispersion as well as diffusion into the rock matrix. Diffusivities and porosities of the rock matrix measured in the laboratory are used to interpret the field data. A short description of the experimental design is given. This investigation is a part of the STRIPA PROJECT phase I.

Tracer movement in a single fissure in granitic rock: Some experimental results and their interpretation

Radionuclide migration was studied in a natural fissure in a granite core. The fissure was oriented parallel to the axis in a cylindrical core 30 cm long and 20 cm in diameter. The traced solution was injected at one end of the core and collected at the other. Breakthrough curves were obtained for the nonsorbing tracers, tritiated water, and a large‐molecular‐weight lignosulphonate molecule and for the sorbing tracers, cesium and strontium. From the breakthrough curves for the nonsorbing tracers it could be concluded that channeling occurs in the single fissure. A ‘dispersion’ model based on channeling is presented. The results from the sorbing tracers indicate that there is substantial diffusion into and sorption in the rock matrix. Sorption on the surface of the fissure also accounts for a part of the retardation effect of the sorbing species. A model which includes the mechanisms of channeling, surface sorption, matrix diffusion, and matrix sorption is presented. The experimental breakthrough curves can be fitted fairly well by this model by use of independently obtained data on diffusivities and matrix sorption.