Association colloids naturally occurring

The lack of a method to determine the spatial distributions of permeability has severely limited our ability to understand and mathematically describe complex processes within permeable media. Even the degree of variation of intrinsic permeability that might be encountered in naturally occurring permeable media is unknown. Samples with permeability variations will exhibit spatial variations in fluid velocity. Such variations may significantly affect associated physical phenomena, such as biological activity, dispersion and colloidal transport. Spatial variations in the porosity and permeability, if not taken into account, can adversely affect the determination of any associated properties, including multiphase flow functions [16]. 

Celis, R. Cornejo, J. Hermosin, M.C. (1998) Textural properties of synthetic clay-ferrihy-drite associations. Clay Miner. 33 395-407 Celis, R. Hermosin, M.C. Cox, L. Cornejo, J. (1999) Sorption of 2,4-Dichlorophenoxyacetic acid by model particles simulating naturally occurring soil colloids. Environ. Sci. Techn. 31 1200-1206... 

Experimental studies. Sorption of radionuclides by colloids is affected by the same solution composition parameters discussed in the previous section on sorption processes. The important parameters include pH, redox conditions, the concentrations of competing cations such as Mg " " and K, and the concentrations of organic ligands and carbonate. The high surface area of colloids leads to relatively high uptake of radionuclides compared to the rock matrix. This means that a substantial fraction of mobile radionuclides could be associated with carrier colloids in some systems. The association of radionuclides with naturally occurring colloids and studies of radionuclide uptake by colloids in laboratory systems give some indication of the potential importance of colloid-facilitated radionuclide transport in the environment as discussed below. 

The most commonly used colloids are 5% albumin, naturally occurring plasma protein, and 6% hetastarch, a synthetic colloid formulation. These solutions offer more rapid restoration of intravascular volume because they produce greater intravascular volume expansion per quantity of volume infused. Colloids produce less peripheral edema than crystalloid, but there is no significant clinical impact. The use of colloid solutions and blood products may be particularly important if there is significant blood loss associated with sepsis or if the patient had severe preexisting anemia. 

About a century ago, when the unique properties of natural polymers were recognized, the term colloid was proposed to distinguish them from materials that could be obtained in crystalline form. It was soon recognized that certain crystalline substances could be transformed into colloids and the concept of a colloidal state of matter was developed. Collodial particles were considered to be built up of a large number of small molecules, by physical association. This concept, which was extended to cover the naturally occurring polymers, was to a very great extent responsible for the delay in the development of a polymer science. 

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