Sedimentation colloids

Fig. 11. Complexation of metal ions by organic matter in suspended sediments, bottom sediments, colloidal and dissolved phases...

Marchesi J.R., W.A. House, G.F. White, N.J. Russell, and I.S. Earr (1991). A comparative study of the adsorption of linear alkyl sulphates and alkylbenzene sulphonates on river sediments. Colloids and Surfaces 53 63-78. 

Now, because the water-borne radioactive element is predominantly associated with the colloids, we no longer have a need for the distribution coefficient. There will still be a partitioning because the major portion of the radioactive elements will still be adsorbed to the sediment. This is a separate equilibrium partitioning coefficient, requiring a new experiment on the clay sediments and the colloids present. The partitioning colloid-clay ratio would most likely be dependent on the surface areas of each present in the sediments. A separate size distribution analysis has resulted in a sediment-colloid surface area ratio of 99 1 for the sediment. This results in a colloid retardation coefficient oiRc = 100 rather than Ri = 4.2 x 10 or i 2 = 6 x 10. ... 

Like sediments, colloids are often important in sorbing and transporting arsenic in soils (Sadiq, 1997 Waychunas, Kim and Banheld, 2005). Colloids may consist of clay minerals, organic matter, calcium carbonate, and various aluminum, manganese, and iron (oxy)(hydr)oxides (Sadiq, 1997). Important iron (oxy)(hydr)oxides include goethite, akaganeite (/J-FeO(OH)), hematite, ferrihydrites, and schwertman-... 

The treatment of sedimentation and diffusion is a little more complicated when the particles under consideration are charged. The smaller counter-ions (see Chapter 7) tend to sediment at a slower rate and lag behind the sedimenting colloidal particles. A potential is thus set up which tends to restore the original condition of overall electrical neutrality by accelerating the motion of the counter-ions and retarding the motion of the colloidal particles. 

Burgess, R. M., R. A. McKinney, W. A. Brown, J. G. Quinn. 1996. Isolation of marine sediment colloids and associated polychlorinated biphenyls A comparison of ultrafiltration and reverse phase chromatography. Environmental Science and Technology 30 1923. 

Kathiresan, K., Manivannan, S., Nabeel, M.A., and Dhivya, B. 2009. Studies on silver nanoparticles synthesized by a marine fungus, Penicillium fellutanum isolated from coastal mangrove sediment. Colloids and Surfaces B Biointerfaces, 71 133-7. 

Burgess RM, McKinney RA, Brown WA. 1996. Enrichment of marine sediment colloids with polychlorinated biphenyls Trends resulting from PCB solubility and chlorination. Environ Sci Technol 30 2556-2566. 

The various solution species are in "equilibrium" with the solid phase soil or sediment colloids. Undefined species are sorbed to the solid phase. While "equilibrium" is implied by the arrows, the net reaction represents very slow dissolution of Pu02. 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Anotlier standard metliod is to use a (high-speed) centrifuge to sediment tire colloids, replace tire supernatant and redisperse tire particles. Provided tire particles are well stabilized in tire solvent, tliis allows for a rigorous purification. Larger objects, such as particle aggregates, can be fractionated off because tliey settle first. A tliird metliod is (ultra)filtration, whereby larger impurities can be retained, particularly using membrane filters witli accurately defined pore sizes. 

In most colloidal suspensions tire particles have a tendency to sediment. At infinite dilution, spherical particles with a density difference Ap with tire solvent will move at tire Stokes velocity... 

In practice, sedimentation is an important property of colloidal suspensions. In fonnulated products, sedimentation tends to be a problem and some products are shipped in the fonn of weak gels, to prevent settling. On the other hand, in applications such as water clarification, a rapid sedimentation of impurities is desirable. 

Piazza R, Bellini T and Degiorgio V 1993 Equilibrium sedimentation profiles of screened charged colloids a test of the hard-sphere equation of state Rhys. Rev. Lett. 71 4267-70... 

Suspended matter in raw water suppHes is removed by various methods to provide a water suitable for domestic purposes and most industrial requirements. The suspended matter can consist of large soflds, settleable by gravity alone without any external aids, and nonsettleable material, often colloidal in nature. Removal is generally accompHshed by coagulation, flocculation, and sedimentation. The combination of these three processes is referred to as conventional clarification. 

Tyj)e of dryer Applicable with dry-product recirculation True and colloidal solutions emulsions. Examples inorganic salt solutions, extracts, milk, blood, waste liquors, rubber latex, etc. Pumpable suspensions. Examples pigment slurries, soap and detergents, calcium carbonate, bentonite, clay sbp, lead concentrates, etc. does not dust. Recirculation of product may prevent sticking Examples filter-press cakes, sedimentation sludges, centrifuged sobds, starch, etc. 

Figure 18-82 illustrates the relationship between solids concentration, iuterparticle cohesiveuess, and the type of sedimentation that may exist. Totally discrete particles include many mineral particles (usually greater in diameter than 20 Im), salt crystals, and similar substances that have httle tendency to cohere. Floccnleut particles generally will include those smaller than 20 [Lm (unless present in a dispersed state owing to surface charges), metal hydroxides, many chemical precipitates, and most organic substances other than true colloids. 

Modes of Operation There is a close analogy between sedimentation of particles or macromolecules in a gravitational field and their elec trophoretic movement in an electric field. Both types of separation have proved valuable not only for analysis of colloids but also for preparative work, at least in the laboratoiy. Electrophoresis is applicable also for separating mixtures of simple cations or anions in certain cases in which other separating methods are ineffectual. 

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

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