Zone sedimentation

The rate at which cells sediment depends on their density and size. It is therefore relatively easy to separate cells of different types which differ in density but more difficult to separate cells of the same type (but at different stages of the cell cycle), as in this case the rate of sedimentation (mm/h) is roughly equal to r2/4 where r is the cell radius in /im. A 2-fold increase in volume (the maximum to be expected) results in only a 1.6-fold increase in sedimentation rate. 

In order to stabilise the sedimenting cells, it is necessary to include a gradient and those most commonly used are serum or Ficoll (Pharmacia Ltd.) (Boone et al., 1968 Miller and Phillips, 1969 Macdonald and Miller, 1970 Warmsley and Pasternak, 1970). Sucrose has also been used (Sinclair and Bishop, 1965 Morris et al., 1967 Shall and McClelland, 1971 Shall, 1973b) but tends to lower cell viability, unless great care is taken to maintain isotonicity. 

Boone et al. (1968) centrifuge cells through a discontinuous 10-20% Ficoll gradient made up in Eagle s minimum essential medium modified for suspension (i.e. lacking calcium and bicarbonate and containing 10 times the normal phosphate concentration). They use an A-1X zonal centrifuge rotor and spin for 1 h at 1000 r.p.m. at 20°C, and obtained clear separation of different cell types (HeLa and rabbit thymocytes). 

Pretlow et al. (1978) showed that a shallow gradient of Ficoll (2.7-5.5%) centrifuged at low speed (about lOOOg) for about 100 min, allowed clear, isokinetic separation of spheres differing 2-fold in volume. The density change in the gradient was small (1.017-1.027 g/ml) and viscosity effects negligible. 

The gradient is formed from two solutions. Al contains either (a) 10% w/v sucrose in complete medium which has the NaCl concentration reduced by 146 mM to maintain constant osmotic pressure, or (b) 30% foetal calf serum in PBS A2 contains either (a) 2.72% w/v sucrose in compelte medium which has the NaCl concentration reduced by 40 mM or (b) 15% foetal calf serum in PBS. The volumes, and hence heights, of the two solutions must be the same (about 250 ml). 

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Laane, R.W.P.M., Sonneveldt, H.L.A., Van der Weijden, A.J. and Groenveld, G. (1999). Trends in the spatial and temporal distribution of metals (Cd, Cu, Zn an Pb) and organic compounds (PCBs and PAHs) in Dutch coastal zone sediments from 1981-1996 a model case study for Cd and PCBs. Journal of Sea Research, 41, 1-17. 

The tube method involved insertion of precleaned acrylic tubes (5-cm diameter) into the littoral zone sediments at a lake-water depth of about 1 m (about 15 cm into sediments). Lake water within the tube was removed by using a peristaltic pump and Teflon line. Once the tube was purged of lake water, groundwater was allowed to fill it to a depth of about 25 cm. The tube was then purged three times with groundwater before obtaining the sample. 

Juracek, K.E. and E.M. Thurman (1997). Assessing aquifer contamination risk using immunoassay Trace analysis of atrazine in unsaturated zone sediments. J. Environ. Qual., 26 1080-1089. 

Gaseous H2S can remediate certain redox-sensitive pollutants by reduction, often with concomitant immobilization resulting from the production of insoluble materials. For example, H2S is pumped in vadose zone sediments/soils to reduce and immobilize toxic chromates ... 

Once in the weathered zone (sediments, soils), the Fe of the Fe oxides may be remobilized under anaerobic conditions by microbial reduction ... 

Figure 7.28 Zone sedimentation in a suspension, (a) The various zones are delineated, showing the clear layer at the top of the suspension and the sedimented layer at the bottom immediately above this layer is a region in which the particles are crowded and begin to be compressed to form sediment (b) The height of the interface between the clarified zone and the suspension as a function of time.

Figure 2 Is a plot of 6 D-H2O versus 6 D-CH for samples obtained from littoral zone sediments of several freshwater lakes and from several shallow (1 m or less water depth) areas of the Tampa Bay estuary. Also shown on Figure 2 are lines that describe predicted 6D-H2O/6D-CH Isotopic pairs resulting from varying the relative contributions to methane production of the acetate dissimilation and CO2 reduction pathways. This model was originally proposed by Woltemate et al. (22) and used In that study to estimate that methyl group transfer (from acetate or other methyl group donors such as methanol) was responsible for about 76% of total methane production In the sediments of Wurmsee, a shallow lake near Hannover, FRG.

Volkman J.K., Johns R.B. (1977) The geochemical significance of positional isomers of unsaturated acids from an intertidal zone sediment. Nature 261, 693-4. 

Schmaljohann, R., Drews, M., Walter, S., Linke, P., von Rad, U. and Imhoff, J.F. (2001) Oxygen minimum zone sediments in the northeastern Arabian Sea off Pakistan a habitat for the bacterium Thioploca. Marine Ecology Progress Series, 211, 27—42. 

It can be added to Table 8.6 that the average corrosion rate over the whole surface of steel was about 0.1 mm/year in the worst case (tidal zone sediment), and about 0.02 mm/year in average for all steel samples. Corresponding data for copper, zinc and lead were <0.02 mm/year. One can draw various conclusions, e.g. that steel corrodes relatively uniformly in tidal zone sediment, and that copper in general corrodes much more uniformly than the other materials. 

Fig. 1. Profiles of pigment distribution after rate-zone sedimentation. Cell-free extracts were layered onto 5-35% (wl/wt) sucrose gradients prepared over a 4 ml cushion of 60% sucrose and centrifuged at 4 C for 240 min in a Beckman SW27 rotor at 27,000 rpm. The density and antenna BChl concentrations were determined from the refractive index and deconvolution of the absorption spectrum (16) respectively.

However, if the interparticle distances are reduced in such a way that coarse particles cannot pass finer ones due to steric effects, then all particles are forced to settle with the same velocity and the sedimentation becomes equivalent to the flow through a fixed bed (zone sedimentation). As a result, the dispersed phase subsides with a sharp interface between the suspension phase and the supernatant. Zone sedimentation occurs for high particle concentrations or is a result of particle aggregation because aggregates immobilise liquid and, thus, increase the apparent volume of the dispersed phase (Fitch 1979). 

Zone sedimentation of aggregating suspensions coincides with the formation of a particle network (gel) in the lower part of the sedimentation vessel. This gel is gradually consolidated under its own weight, which reduces its porosity and the sedimentation velocity. The gel is, therefore, called compression or compaction zone, it eventually comprises aU particles of the suspension. The compression zone has a much higher porosity than the sediment of a weU-stabrhsed suspension (Sobisch et al. 2006). For fast aggregation and very fine particles, the gelation can finish even before a visible sedimentation of the dispersed phase occurs. 

Waterfront structures are exposed to a variety of marine environments. The resistance of materials to each of these environments may vary considerably, as weU as appHcabil-ity of various forms of corrosion control in mitigating the anticipated corrosion. The waterfront environment can be divided into five exposure zones sediment, immersion, intertidal, splash/spray, and atmospheric. In most cases, a single type of material will be used for the sediment, immersion, and intertidal zones. In some cases another material may be used for the splash and spray and atmospheric zones of the structure. An example of this would be the use of a reinforced concrete deck over steel pilings. Due to differences in corrosion activity between these zones, the corrosion performance of many materials is substantially different when exposed to two or more of these zones. Figure 1, taken from Ref 4, shows the result of a classical experiment where the corrosion of a continuous strip of... 

Zonierung, Stufung zone electrophoresis Zonenelektrophorese zone melting/ zone refining Zonenschmelze(n) zone sedimentation/ zonal sedimentation/ band sedimentation Zonensedimentation zwitterion Zwitterion... 

Warner, J, R., Soeiho, R., Birnboim, H. C., Girard, M., and Darnell, J. E. (1966). Rapidly labeled HeLa cell nuclear RNA. I. Identification by zone sedimentation of a heterogeneous fraction separate from ribosomal precursor RNA. J. Mol. Biol. 19, 349-361. 

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