Chemical leaching, modeling

Two different approaches have been taken by researchers to determine the secondary mineralogy of CCBs (1) direct observation, which is accomplished via analysis of weathered ash materials, and (2) prediction, based on chemical equilibrium solubility calculations for ash pore-waters and/or experimental ash leachate or extractant solutions. Because the secondary phases are typically present in very low abundance, their characterization by direct analysis is difficult. On the other hand, predictions based on chemical equilibrium modelling or laboratory leaching experiments may not be reliable indicators of element leachability or accurately indicate the secondary phases that will form under field conditions (Eighmy et al. 1994 Janssen-Jurkovicova et al. 1994). 

Sorption is most commonly quantified using distribution coefficients (Kd), which simplistically model the sorption process as a partitioning of the chemical between homogeneous solid and solution phases. Sorption is also commonly quantified using sorption isotherms, which allow variation in sorption intensity with triazine concentration in solution. Sorption isotherms are generally modeled using the empirical Freundlich equation, S = K CUn, in which S is the sorbed concentration after equilibration, C is the solution concentration after equilibration, and Kt and 1 In are empirical constants. Kd and K are used to compare sorption of different chemicals on one soil or sorbent, or of one chemical on several sorbents. Kd and K are also commonly used in solute leaching models to predict triazine interactions with soils under various environmental conditions. 

A branched pore leaching model as applied to release of water-soluble carbon from soil incorporates reaction to soluble compounds coupled with pore diffusion within the solids and leaching into the bulk solution. Application of such a model appears to describe hemicellulose hydrolysis reasonably well but not significantly better than chemical reaction only or simple leaching models. 

These results could suggest that what has been traditionally been described as "biphasic" behavior may reflect a combination of chemical reaction and mass transfer effects, with each limiting xylan reaction and removal at different stages or modes of operation. This effect might be better described by a model that incorporates reaction of solids to form soluble species as a function of temperature and acid concentration coupled with a second mass transfer step that is affected by flow. On this basis, we plan to investigate whether the pore leaching model could be simplified and adapted in this way to better describe hemicellulose hydrolysis. 

Simple models are used to Identify the dominant fate or transport path of a material near the terrestrial-atmospheric Interface. The models are based on partitioning and fugacity concepts as well as first-order transformation kinetics and second-order transport kinetics. Along with a consideration of the chemical and biological transformations, this approach determines if the material is likely to volatilize rapidly, leach downward, or move up and down in the soil profile in response to precipitation and evapotranspiration. This determination can be useful for preliminary risk assessments or for choosing the appropriate more complete terrestrial and atmospheric models for a study of environmental fate. The models are illustrated using a set of pesticides with widely different behavior patterns. 

Flutson, J.L. and Wagner, R.J. (1992) Leaching Estimation and Chemistry Model. A Process Based Model of Water and Solute Movement, transformation, Plant Uptake and Chemical Reactions in the Unsaturated Zone. Version 3. Dept, of Soil, Crop and Atmospheric Sciences, Series No. 92-3, Cornell University, Ithica, New York. 

Simulation and predictive modeling of contaminant transport in the environment are only as good as the data input used in these models. Field methods differ from laboratory methods in that an increase in the scale of measurement relative to most laboratory methods is involved. Determination of transport parameters (i. e., transmission coefficients) must also use actual contaminant chemical species and field solid phase samples if realistic values are to be specified for the transport models. The choice of type of test, e.g., leaching cells and diffusion tests, depends on personal preference and availability of material. No test is significantly better than another. Most of the tests for diffusion evaluation are flawed to a certain extent. 

Apatite is being considered as a barrier that will prevent the leakage of radioactive nuclei from the radioactive waste storage. Because of the similarity in the chemical and spectral features REE have been chosen as a model of the fission products of the actinides. For this reason it is of importance to recognize whether the elements are incorporated in the bulk of the barrier, or adsorbed on the surface where they can be subjected to leaching out (Martin et al. 1996 Martin et al. 1999a Martin et al. 1999b). 

The use of chemical modelling to predict the formation of secondary phases and the mobility of trace elements in the CCB disposal environment requires detailed knowledge of the primary and secondary phases present in CCBs, thermodynamic and kinetic data for these phases, and the incorporation of possible adsorp-tion/desorption reactions into the model. As noted above, secondary minerals are typically difficult to identify due to their low abundance in weathered CCB materials. In many cases, appropriate thermochemical, adsorption/desorp-tion and kinetic data are lacking to quantitatively describe the processes that potentially affect the leaching behaviour of CCBs. This is particularly tme for the trace elements. Laboratory leaching studies vary in the experimental conditions used (e.g., the type and concentration of the extractant solution, the L/S ratio, and other parameters such as temperature and duration/ intensity of agitation), and therefore may not adequately simulate the weathering environment (Rai et al. 1988 Eary et al. 1990 Spears Lee, 2004). 

Wagenet, R.J. and J.L. Hutson (1989). LEACHM Leaching estimation model - a process based model for water and soute movement, transformation, plant uptake and chemical reactions in the unsaturated zone. Continuum Vol. 2. Water Resources Institute, Cornell University Ithaca, NY. 

Possible major sources of BFRs are wastewater effluent and flue gases from BFR factories and other facilities processing BFRs (Watanabe and Sakai, 2003). Additives such as PBDEs may leach from the surface of products treated with these compounds, in contrast to TBBPA that are reactive and chemically bound within the material matrices (de Wit, 2002). Based on a model estimate, Danish EPA concluded that PBDE source is evaporation from products in use (DEPA, 1999). Thus, indirect releases from products should also be considered in emission inventories. 

---