Particle, chemical transport

Ionization, sorption, volatilization, and entrainment with fluid and particle motions are important to the fate of synthetic chemicals. Transport and transfer processes encompass a wide variety of time scales. Ionizations are rapid and, thus, usually are treated as equilibria in fate models. In many cases, sorption also can be treated as an equilibrium, although somtimes a kinetic approach is warranted (.2). Transport processes must be treated as time-dependent phenomena, except in simple screening models (.3..4) ... 

This technique has been used in the preparation of metal alloys ceramics and composite materials. To this end a chemical precursor converted to the gas phase is decomposed at either low or atmospheric pressure to produce the nano-structured particles which, transported in a carrier gas, are collected on a cold substrate. 

PCBs are relatively insoluble, viscous, and display a strong tendency toward sorption on solid particles. Their transport in the surface and movement through the subsurface is limited by their chemical and physical characteristics. Manufacturers normally marketed PCBs as mixtures of biphenyls. The combination of the various biphenyls in the mixture controlled the properties of the mixture. 

Note that when we consider the situation at equilibrium, the exact pathways involved in PCB uptake and depuration are not important to the end result (e.g., whether the chemical transport into the organism occurred via the dissolved phase or by direct ingestion of sediment particles and/or diet organisms). 

The rate of deposition and clearance of uranium-containing particles from the lung depends upon its chemical form and particle size. As previously discussed in the adsorption section, most of the larger uranium particles are transported out of the respiratory system by mucocilhary action, or swallowed and eliminated in the feces. Smaller particles with higher solubilities are more rapidly absorbed into the systemic circulation but can then be excreted in the urine. 

A membrane can be defined as a barrier between two phases. A molecule or particle is transported from one phase to another through the membrane because a driving force acts on that molecule or particle. This driving force can be chemical potential gradient, e.g., concentration gradient, pressure difference, or electrical potential, or combination of these (10). There is a proportional relationship between the flux and the driving force ... 

Turbulent diffusion is an important mode of chemical transport in both surface water and air. In the subsurface environment, groundwater flow normally lacks the eddy effects that characterize surface water and air movements because typical groundwater velocities are so much lower. Nevertheless, groundwater must take myriad detours as it moves from one point to another, traveling over, under, and around soil particles, as shown in Fig. 1-7. These random detours cause mixing, thus the net transport of a chemical from... 

Mud, silt and sandy sediments form mainly by weathering—the breakdown and alteration of solid rock. Usually, these sedimentary particles are transported by rivers to the oceans, where they sink onto the seabed. Here, physical and biological processes and chemical reactions (collectively known as diagenesis) convert sediment into sedimentary rock. Eventually these rocks become land again, usually during mountain building (orogenesis). 

Having a model that has a good theoretical basis, that has been validated in laboratory experiments, and that is consistent with field observations, it is advisable to make some predictions about particle deposition in systems of interest. An example is presented in Figure 3, adapted from the work of Tobiason (1987). The travel distance in an aquifer required to deposit 99% of the particles from a suspension is termed Lgg and is plotted as a function of the diameter of the suspended particles for two different values of a(p, c), specifically 1.0 (favorable deposition) and 0.001 (deposition with significant chemical retardation of the particle-collector interaction, termed unfavorable deposition ). Assumptions include U = 0.1 m day"1, T= 10°C, dc = 0.05cm, e = 0.40, pp= 1.05 gem"3, and H=10 2OJ. These results indicate the dependence of the kinetics of deposition on the size of the particles in suspension that has been predicted and observed in many systems. Small particles are transported primarily by convective Brownian diffusion, and large particles in this system are transported primarily by gravity forces. A suspended particle with a diameter of about 3 /im is most difficult to transport. Nevertheless, in the absence of chemical retardation, a travel distance of only about 5 cm is all that is needed to deposit 99% of such particles in a clean aquifer, that is, an aquifer that has not received and retained previous particles. 

The partial deep leaching of both prometryn and napropamide on our field site may indicate that an overlooked mechanism of adsorbed chemical transport is operating, which could involve either a mobile organic complex or conceiveably attachment to fine colloidal suspended particles. These hypotheses should he investigated not only on different field sites hut also under controlled laboratory conditions. Our research group at Riverside is planning such experiments in the future. 

In multimedia box models, the environmental fate of a chemical is described by a set of coupled mass-balance equations for all boxes of the model. These equations include terms for degradation, inter-media exchange such as settling and resuspension of particles, and transport with air and water flows [19,20]. Equations for different boxes are coupled by inter-media exchange terms (linking different environmental media) and terms for trans-... 

Because of the larger specific surface area of finer particles, chemicals with strong surface affinity tend to be enriched in fine fractions of particulate samples if surface adsorption is the controlling mechanism (Wei et al. 2009). In addition, particle size is the major factor determining the atmospheric behavior of particle-bound SVOCs, including the removal from the air, residence time, transport potential, and photolytic transformation. Fine-particle-bound SVOCs, like those in the gas phase, are susceptible to long-range transport via the air flow thus are able to disperse to remote areas such as the north and south poles. So far, studies have... 

Winterberg, M. and Tsotsas, E., 2000a. Correlations for Effective Heat Transport Coefficients in Beds Packed with Cylindrical Particles. Chemical Engineering Science, 55(23) 5937-5943. 

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