Mechanical column experiments modeling

Breakthrough curves from column experiments have been used to provide evidence for diffusion of As to adsorption sites as a rate-controlling mechanism. Darland and Inskeep (1997b) found that adsorption rate constants for As(V) determined under batch conditions were smaller than those necessary to model breakthrough curves for As(V) from columns packed with iron oxide coated sand the rate constants needed to model the breakthrough curves increased with pore water velocity. For example, at the slowest velocity of 1 cm/h, the batch condition rate constant was 4 times smaller than the rate constant needed to model As adsorption in the column experiment. For a velocity of 90 cm/h, the batch rate constant was 35 times smaller. These results are consistent with adsorption limited by diffusion of As(V) from the flowing phase to sites within mineral aggregates. Puls and Powell (1992) also measured more retardation and smaller rate constants for As(V) at slower flow velocities where there was sufficient time for diffusion to adsorption sites. 

Laboratory column experiments were used to identify potential rate-controlling mechanisms that could affect transport of molybdate in a natural-gradient tracer test conducted at Cape Cod, Mass. Column-breakthrough curves for molybdate were simulated by using a one-dimensional solute-transport model modified to include four different rate mechanisms equilibrium sorption, rate-controlled sorption, and two side-pore diffusion models. The equilibrium sorption model failed to simulate the experimental data, which indicated the presence of a ratecontrolling mechanism. The rate-controlled sorption model simulated results from one column reasonably well, but could not be applied to five other columns that had different input concentrations of molybdate without changing the reaction-rate constant. One side-pore diffusion model was based on an average side-pore concentration of molybdate (mixed side-pore diffusion) the other on a concentration profile for the overall side-pore depth (profile side-pore diffusion). 

The content of iron was used in place of an iron surface in the simplified model and linear relationships were presumed between mineral precipitation and passivation of iron. Only a small iron content was used in each cell to achieve results in acceptable simulation times. The model results in PCE concentration profiles which are typically observed in column experiments (Fig. 13.6), even if the assumptions cannot be verified as the real mechanisms of passivation. Nevertheless, the simulation shows a migration... 

Although evidence from natural systems is useful to constrain reaction mechanisms and minerals to be incorporated into such models, time-dependent information is generally lacking. A series of laboratory column experiments have been conducted as blind test cases in order to test the capabilities of two of the currently available, coupled models to predict product solids and output fluid compositions with time. The experiments reacted single minerals of importance to the radioactive waste disposal programmes in the UK, Sweden and Switzerland, with simplified young (Na-K-Ca-OH) and evolved (Ca(OHE) synthetic cement porewater leachates. 

Fig. 3 Mechanism of pH-zone-refining CCC. (a) Preparation for the model experiment (b) chemohydrodynamic equilibrium in the separation column (c) elution profile of three major analytes.

In order to form a bridge between the laboratory (chemical) experiments and the theoretical (mathematical) models we refer to Table I. In a traditional approach, experimental chemists are concerned with Column I of Table I. As this table implies there are various types of research areas thus research interests. Chemists interested in the characteristics of reactants and products resemble mathematicians who are interested in characteristics of variables, e.g. number theorists, real and complex variables theorists, etc. Chemists who. are interested in reaction mechanism thus in chemical kinetics may be compared to mathematicians interested in dynamics. Finally, chemists interested in findings resulting from the study of reactions are like mathematicians interested in critical solutions and their classifications. In chemical reactions, the equilibrium state which corresponds to the stable steady states is the expected result. However, it is recently that all interesting solutions both stationary and oscillatory, have been recognized as worthwhile to consider. 

We see from Fig. lib that the Raman spectrum changes rather weakly in the employed temperature range (see, e.g., the experimental RS in Fig. 4 and the calculated RS in the right column of Fig. 1 lb). In view of Table VII, such behavior of the RS is obtained if the fitted model parameters g ,p , and y , pertaining to the transverse-vibration mechanism (d), exhibit a substantial change in the temperature interval of our interest. Due to the demonstrated steepness of the RS with respect to temperature, the agreement with experiment of the employed molecular model is attained for rather definite values of these parameters. We conclude that simultaneous application of the dielectric and Raman spectroscopy allows us to increase reliability of the employed molecular model. 

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