Diffusion in leaching

Schwartzberg, H.G. and Chao, R.Y. 1982. Solute Diffusivities in Leaching Processes, Food Technol., February 1982, pp. 73-86. 

Effective Diffusivity in Leaching Particles. In Example 12.8-1 a time ofleaching of the solid particle of 3.11 h is needed to remove 80% of the solute. Do the following calculations. 

Diffusion in porous solids is usually the most important factor con-troUing mass transfer in adsorption, ion exchange, drying, heterogeneous catalysis, leaching, and many other applications. Some of the... 

High-speed agitators These are suitable for leaching reactions that are limited by diffusion in the leach liquor. An agitator of this type requires a propeller or a turbine to provide high-speed agitation... 

The film diffusion stages (i) and (v) are similar to the external mass transfer stages in leaching and may be dealt similarly (see Section 2.1.3). Solution flow conditions for resin touching the fixed beds are likely to be in the laminar flow range. 

The good correlation of the results of vapor diffusion and leaching experiments for butylate, alachlor, and metolachlor with their physical properties has given support to the value of physical property measurements to predict pesticide movement in the soil. 

Summarising, the paint-seawater mechanism includes the following rate-influencing steps hydrolysis and erosion of the active TBT-polymer binder, effective diffusion in the leached layer of dissolved pigment species and TBTC1,... 

In leaching field-aged residues of Atrazine and Metolachlor from a soil column, a model with a single diffusion parameter underestimated desorption at early times and overestimated desorption at late times. 

Enzymes can also be entrapped within the pores of a matrix or network. Small molecules can diffuse in and out of the matrix, whereas the macromolecular enzyme is maintained within the network. Precise control of the pore size is not possible, and therefore mass transfer limitations and enzyme leaching always cause problems [59]. 

The leachate sampling frequency had little effect on leach Vate until the semi-annual frequency was reached. The change in leach rate, as reflected in Figures 3a, 3b, 4, 5, and 6, was dramatic in changing from monthly to semi-annual sampling. The apparent mechanism shifted from lattice alteration to diffusion release this shift illustrates the important role of dissolved species in lowering the leach i Ate. These results are consistent with the work of El-Shamy — and Paul. —... 

In leaching, there usually is a very rapid leach rate that occurs when the mineral is on the surface of the particle, but many times the internal diffusion of the solid through the solid particle becomes controlling, and mixer variables do not affect the leaching rate beyond that point. In studying the effect of mixing on leaching processes, it is normally desirable to run separate experiments with... 

Optical microscope observations have shown that zircons often exhibit extremely complex microstructures (on the scale of 1-100 /tm), in which changes of birefringence correlate with the distribution of U and Th (Chakoumakos et al. 1987). However, no attempts appear to have been made to relate these microstructures to other impurities (such as water-related species) and crystal defects, both of which may significantly influence the processes of metamictization and recrystallization. Such a study involving TEM might also provide important information about the diffusion and leaching of radioactive impurities (and the products of their decay), processes that have important implications for ceramic nuclear-waste disposal and for techniques of age determination based on measurements of Pb/U isotopic ratios. 

Whatever method is adopted, leaching consists of a number of consecutive diffusional or mass transfer processes. The solvent first penetrates the raw material and dissolves the soluble elements. These diffuse in the opposite direction to the surface of the solid matrix and through the liquid layers at its surface to reach the bulk solution. These processes are under the influence of an overall concentration gradient, the concentration being lowest in the bulk solution. Any of these processes may be responsible for limiting the rate... 

Precipitation of insulin in pumps due to the formation of amorphous particles, crystals or fibrils of insulin can lead to changes in release pattern or to blockage which prevents insulin release. Amorphous or crystalline precipitates can be caused by the leaching of divalent metal contaminants or lowering of pH (due to CO2 diffusion or leaching of acidic substances), but can be prevented. More difficult to solve is the tendency of insulin to form fibrils as illustrated in Fig. f f. T4. 

The Rooting-Zone Soil Root-zone soil includes the A horizon below the surface layer. The roots of most plants are confined within the first meter of soil depth. In agricultural lands, the depth of plowing is 15-25 cm. In addition, the diffusion depth, which is the depth below which a contaminant is unlikely to escape by diffusion, is on the order of a meter or less for all but the most volatile contaminants. Soil-water content in the root zone is somewhat higher than that in surface soils. The presence of clay in this layer serves to retain water. Contaminants in root-zone soil are transported upward by diffusion, volatilization, root uptake, and capillary motion of water transported downward by diffusion and leaching and transformed chemically primarily by biodegradation or hydrolysis. 

Cross-linking of protein in situ avoids leaching out of proteins that may diffuse in water or alcohol. Many low-molecular-weight antigens (peptides) are extracted by non-cross-linking fixatives such as alcohol, or methanol-based solutions, but they are well preserved in tissue by formalin. 

---