Sedimenting particles, dynamic

Cochran JK, Krishnaswami S (1980) Radium, thorium, uranium and °Pb in deep-sea sediments and sediment pore waters from the north equatorial Pacific. Am J Sci 280 849-889 Cochran JK, Masque P (2003) Short-lived U/Th-series radionuchdes in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Colley S, Thomson J, Newton PP (1995) Detailed °Th, Th and °Pb fluxes recorded by the 1989/90 BQFS sediment trap time-series at 48°N, 20°W. Deep-Sea Res 42(6) 833-848... 

There are many equations which allow calculation of sediment transport rate within a water body, or sediment flux (see for example Task Committee of Computational Modeling of Sediment Transport Processes, 2004 for a review). However, these equations tend to be for a uniform sediment distribution, which is far from the variable source supply of material seen in events when the majority of sediment is moving. It is also generally considered that a particular flow has a maximum capacity to transport sediment, although the concentration this relates to depends again on sediment characteristics. Hence tliere are examples in China where sediment concentrations can reach several tens of thousands of parts per million for very fine particles, whereas a flow may become saturated with sand-sized particles at far lower concentrations. Rivers are often considered to be either capacity- or supply-limited in terms of their sediment transporting dynamics. However, in practice for most rivers, most of the time, sediment transport is limited by a complex and dynamic pattern of sediment supply. 

Both image analysis and electrical zone sensing observe and respond to each particle. Laser diffraction, sedimentation, and dynamic light scattering data are obtained from ensembles of dispersed particles in suspension. 

The sedimentation rate (a) is given by the following relations u=V.glf(p-p ) = m.glf (l-p lp)=M.glNf. f (l-p lp) =2r. g 9r) (p-Po). where V = volume of sedimentation particles, p = their density, p = density of dispersion medium, /= friction coefficient, M=molar weigh of particles, = Avogadro s number, rj = dynamic viscosity of dispersion medium. 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

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

The dynamic methods involve the placement of particles to be measured in an environment which is subsequently disturbed. The members belonging to the particle set react differently to these imposed environmental impulses. These different reactions are observed and therefrom deductions are made as regards the size characteristics. As examples of dynamic methods mention may be made of sieving, streaming, elutriation, and sedimentation. 

Particle Transport. Because many organic chemicals bind with aquatic particulate matter, particle transport can determine the fate of compounds. Sediment transport has been of interest to the engineering profession for many years. Many discussions of the dynamics of fluvial sediment transport have appeared in the literature (11, 12). As with hydrodynamic transport, one strategy for environmental modeling is to "piggy-back the transport of sorbed chemicals on a model of transport of the sediment phase. 

Particle size distributions of natural sediments and soils are undoubtedly continuous and do not drop to zero abundance in the region of typical centrifugation or filtration capabilities. Additionally, there is some evidence to indicate that dissolved and particulate organic carbon in natural waters are in dynamic equilibrium, causing new particles or newly dissolved molecules to be formed when others are removed. Experiments with soil columns have shown that natural soils can release large quantities of DOC into percolating fluids [109]. 

A different approach which also starts from the characteristics of the emissions is able to deal with some of these difficulties. Aerosol properties can be described by means of distribution functions with respect to particle size and chemical composition. The distribution functions change with time and space as a result of various atmospheric processes, and the dynamics of the aerosol can be described mathematically by certain equations which take into account particle growth, coagulation and sedimentation (1, Chap. 10). These equations can be solved if the wind field, particle deposition velocity and rates of gas-to-particle conversion are known, to predict the properties of the aerosol downwind from emission sources. This approach is known as dispersion modeling. 

It is our objective in this chapter to outline the basic concepts that are behind sedimentation and diffusion. As we see in this chapter, gravitational and centrifugal sedimentation are frequently used for particle-size analysis as well as for obtaining measures of solvation and shapes of particles. Diffusion plays a much more prevalent role in numerous aspects of colloid science and is also used in particle-size analysis, as we see in Chapter 5 when we discuss dynamic light scattering. The equilibrium between centrifugation and diffusion is particularly important in analytical and preparative ultracentrifuges. 

In Section 23.1, this procedure will be applied to just one completely mixed water body. This control volume may represent the lake as a whole or some part of it (e.g., the mixed surface layer). Section 23.2 deals with the dynamics of particles in lakes and their influence on the behavior of organic chemicals. Particles to which chemicals are sorbed may be suspended in the water column and eventually settle to the lake bottom. In addition, particles already lying at the sediment-water interface may act as source or sink for the dissolved chemical. In Section 23.3, two-box models of lakes are discussed, particularly a model consisting of the water body as one box and the sediment bed as the other. Finally, in Section 23.4, one-dimensional vertical models of lakes and oceans are discussed. 

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