Particle sedimentation

The thickness of dry developer s layer is substantially smaller in a zone imbibed by a penetrant due to the process of particles sedimentation. Reduced thickness h of imbibed zone can be 80% smaller than the thickness h of dry one. It must be taken into account in the calculations of PT characteristics in the frame of the theory [1-3]. 

Particle Shape. Whereas the Stokes particle is assumed to be a sphere, very few real soHds are actually spherical. Flat and elongated particles sediment slower than spheres. For maximum sedimentation rate, the particle should be as spherical as possible. 

Pollutant Distribution. Of particular importance for the aquatic ecosystem is the distribution of volatile substances, eg, gases and volatile organic compounds, between the atmosphere and water, and the sorption of compounds at soHd surfaces, eg, settling suspended matter, biological particles, sediments, and soils (41,42). 

Changing kinematic viscosity, v, to dynamic viscosity, the velocity of particle sedimentation in the laminar regime is ... 

Figure 2.7 Particle sedimentation rates in slurry settling (schematic)...

Sedimentation coefficients are a measure of the velocity witli which a particle sediments in a centrifugal force field. Sedimentation coefficients are typically expressed in Svedbergs (.symbolized S), named to honor The Svedberg, developer of the ultracentrifuge. One S equals 10 . sec. 

Stokes law is rigorously applicable only for the ideal situation in which uniform and perfectly spherical particles in a very dilute suspension settle without turbulence, interparticle collisions, and without che-mical/physical attraction or affinity for the dispersion medium [79]. Obviously, the equation does not apply precisely to common pharmaceutical suspensions in which the above-mentioned assumptions are most often not completely fulfilled. However, the basic concept of the equation does provide a valid indication of the many important factors controlling the rate of particle sedimentation and, therefore, a guideline for possible adjustments that can be made to a suspension formulation. 

Solution of equation (10) which involves sedimentation in the presence of mixing and that of equation (11) which contains the sedimentation term only, are exponential in nature. The major conclusion which arises from this is that the logarithmic nature of the activity-depth profiles by itself is not a guarantee for undisturbed particle by particle sediment accumulation, as has often been assumed. The effects of mixing and sedimentation on the radionuclide distribution in the sediment column have to be resolved to obtain pertinent information on the sediment accumulation rates. (It is pertinent to mention here that recently Guinasso and Schink [65] have developed a detailed mathematical model to calculate the depth profiles of a non-radioactive transient tracer pulse deposited on the sediment surface. Their model is yet to be applied in detail for radionuclides. )... 

The introduction of estrogens and progestogens into the environment is a function of the way several factors are combined. The manufactured quantity and the dosage applied (amount, frequency, and duration) combined with the excretion efficiency of the compound and its metabolites, the capability of adsorption and desorption on soil, and the metabolic decomposition in sewage treatment are examples of necessary factors to assess environmental exposure. In general the fate and effect of a substance in the environment is dependent on the distribution into the different natural systems, such as air, water, and solids (soil, particles, sediment, and biota). Information on the physical and chemical properties (Ku, Kd, and Kim vapor pressure) of a compound may help determine whether it is likely to concentrate in the aquatic, terrestrial, or atmospheric... 

Regardless of the rotor speed and maximum velocity, sedimentation (or flotation) will not occur in a solution of equal density to the sample. Iso-density methods use this lack of movement in a manner comparable to a pH gradient in iso-electric focusing techniques. The methods are a combination of sedimentation and flotation, achieved by using a density gradient that straddles the density of the particles concerned. On centrifugation, the particles sediment until they reach a solvent zone with the same density. This results in the development of a zone for each type of particle present in the sample. 

There are two important effects due to particle sedimentation which may affect the rate of filtration. First, if the sediment particles are all settling at approximately the same rate, as is frequently the case in a concentrated suspension in which the particle size distribution is not very wide, a more rapid build-up of particles will occur on an... 

The whole column, apart from a small portion of the sediment, is sam pled in this way. The chemical checks were carried out every 15 days, determining pH, Conductivity, C.O.D., Ammoniacal Nitrogen, Orthophosphate Phosphorus and, furthermore, each month Total phosphorus, Total Kjeldahl Nitrogen, Total Solids and Volatile Solids. The methods utilized are those indicated in the Standard Methods (A.P.H.A., 1980). The amount of matter particles sedimented in the tanks was estimated with the use ol appropriate sampling devices located at the bottom of the tanks and withdrawn after a variable permanence of 40 to 60 days. 

Theory. The velocity (v) of particle sedimentation during centrifugation depends on the angular velocity co of the rotor, its effective radius (teff, the distance from the axis of rotation), and the particle s sedimentation properties. These properties are expressed as the sedimentation coef cient S (1 Svedberg, = 10 s). The sedimentation coef cient depends on the mass M of the particle, its shape (expressed as the coef cient of friction, f), and its density (expressed as the reciprocal density v, partial specific volume ). 

When dispersions are analyzed where nonsolvated, sphere-like particles sediment, the sedimentation coefficients s Me independent of concentration at low solid contents and, therefore, it is possible to determine the particle size distribution in dispersion from the distribution of the sedimentation coefficients. 

Because chlorite is an anion, sorption of chlorite ions onto suspend particles, sediment, or clay surfaces is expected to be limited under enviromnental conditions. Thus, chlorite ions may be mobile in soils and leach into groundwater. However, chlorite (ions or salts) will undergo oxidation-reduction reactions with components in soils, suspend particles, and sediments (e.g., Fe, Mn ions see Section 6.3.2.2). Thus, oxidation-reduction reactions may reduce the concentration of chlorite ions capable of leaching into groundwater. 

HSCT emissions may also interact with polar stratospheric clouds, PSCs, in much the same way as with particles (Pitari et al., 1993). That is, reaction of a number of nitrogenous species on PSCs leads to the formation of HN03, which can remain adsorbed on or in the PSC. The larger cloud particles sediment to lower altitudes in the stratosphere, redistributing NO, or into the troposphere, permanently removing NOr... 

All ribosomes have two subunits, and each subunit contains several protein chains and one or more chains of RNA (ribosomal RNA, or rRNA). In the ribosome from E. coli, the smaller of the two subunits is known as the 30S subunit and the larger is referred to as the 50S subunit. (The unit S stands for Svedberg, a measure of how rapidly a particle sediments in a centrifuge.) The two subunits combine to form the active 70S ribosomal assembly. The special RNA molecules that are a part of the ribosome are quite distinct from messenger or transfer RNA molecules, and they play important roles in forming the overall ribosomal quaternary structure and in aligning mRNA and tRNA molecules during protein biosynthesis. 

Sedimentation potential This is due to the electric field created by charged particles sedimenting in a liquid. This situation is the opposite of electrophoresis. 

In Chapter 21 on box models no distinction was made between a compound being present as a dissolved species or sorbed to solid surfaces (e.g., suspended particles, sediment-water interface). In Boxes 18.5 and 19.1, and also in Illustrative Example 19.6, we learned that several of the transport and transformation processes may selectively act on either the dissolved or the sorbed form of a constituent. For instance, a molecule sitting on the surface of a sedimentaiy particle at the lake bottom does not feel the effect of turbulent flow in the lake water, while the dissolved chemical species is passively moved around by the currents. In contrast, a molecule sorbed to a suspended particle (e.g., an algal cell) can sink through the water column because of gravity, unlike its dissolved counterpart. 

Relative organic carbon content of suspended particles Sedimentation rate (estimated)... 

Particle sedimentation. The removal rate of particles, ls = vs//i = 2md l/5m = 0.4 d-1, is fairly large, but only a minor fraction of disulfoton is sorbed to the particles ... 

Finally, some distortion of the particle size distribution is caused by deviations from Stokes law during particle sedimentation. These deviations are a function of the particle size such that the retardation of Stokesian settling becomes larger with increasing particle size. This effect can be evaluated theoretically, however, and corrections can be made to determine size distributions experimentally. 

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