Macropore flow

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Downward movement of triazines may occur from percolating water carrying them to lower soil depths. Within well-structured soils with abundant macropores, triazines have been reported to move to deeper depths than in nonstructured soils with fewer pores. Increased permeability, percolation, and solute movement can result from increased porosity -especially in no-tillage systems where there is pore connectivity at the soil surface. Triazines can move to shallow ground-water by macropore flow in sandy soil if sufficient rainfall occurs shortly after they are applied (Ritter et al, 1994a, b). 

We calculated the wetting (WC) and drainage front characteristics (DC) from the four sets of numerical data in /-mode. The points of the WC correspond to the path of the applied input flux signal in the (z, t) plane. The DC are calculated as the paths of the onset of the decay input signal in the same plane. The calculated curves are shown in Fig. 4-9 for the four numerical LG experiments (dots in the figure). No drainage characteristics exists for simulation 1, where no macropore flow occurred. 

Di Pietro, L., and F. Lafolie. 1991. Water flow characterization and test of a kinematic wave model for macropore flow in a highly contrasted and irregular double porosity medium. J. Soil Sci. 42 551-563. 

Kinetics may play an important role in pesticide adsorption (28, 39). Rapid transport of pesticide in large soil pores (rocks, void root channels, worm holes, etc.) could take place, thus exposing the pesticide to only a fraction of the adsorption sites. Since the organic matter content decreases with depth, the occurrence of macropore flow could result in pesticide movement beyond the surface layers where most of the CM resides. 

Below 30 an, the calibrated and calculated for aldicarb are similar. At that depth, phenomena such as undissolved granules, upwardly moving evaporation water, and macropore flow are much less likely to occur. Therefore, the calibration of aldicarb is more straightforward and a function of soil organic matter. 

Instantaneous equillibrium between sorbed and soluble phases is Inaccurate desorption is slower than adsorption. "Macropore" flow has been defined as rapid and deep Infiltration of rainfall at the onset of a storm as the rain follows preferential flowpaths In the soil. Hence, not all the aldicarb In the surface zones will "see" the water as It flows by, and desorb according to the equilibrium assumption. It Is difficult to determine which, If any, of these possibilities explain the malntainance of high aldicarb concentrations near the soil surface. Nonetheless, the high assigned partition coefficient Is the way In which the model artificially duplicates this behavior. 

Quisenberry, V.L., Phillips, R.E., and Zeleznik, J.M. Spatial distribution of water and chloride macropore flow in a well-structured soil. Soil-Science-Society-of-America-Joumal 58,1294-1300. 1994. 

Silica monoliths are prepared by sol-gel technology starting from alkoxide precursors, such as tetraethoxy-silane or tetramethoxy-silane [23]. The structure of a silica monolith is characterized by an interconnected silica skeleton and bimodal distribution of 1-3-pm macropores (flow through pores) and 10-20 nm mesopores. 

The influence of the physical heterogeneity on the flow regime non-imiform flow field, channel/macropore flow... 

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