Contaminants mass transport

Contaminant mass transport in an air stripper is schematically shown in Figure 18.12. The removal process can be described mathematically by a mass balance for the contaminant assuming that there is no change in the accumulated contaminant in the stripper under steady-state conditions ... 

The governing equations for mass flow, energy flow, and contaminant flow in a room will be the continuity equation, Navier-Stokes equations (one in each coordinate direction), the energy equation, and the mass transport equation, respectively. 

Requirements regarding laboratory liquid-liquid reactors are very similar to those for gas-liquid reactors. To interpret laboratory data properly, knowledge of the interfacial area, mass-transfer coefficients, effect of contaminants on mass-transport processes, ionic characteristics of the system, etc. is needed. Commonly used liquid-liquid reactors have been discussed by Doraiswamy and Sharma (1984). 

The bio availability of organic compounds in soils/sediments to microbes, plants, and animals is important from the perspective of remediation and risk assessment. Cleanup technology (ex situ or in situ) of contaminated soils and bottom sediments requires mass transport of contaminants through the solid materials, which in turn depends on sorption/desorption kinetics. 

Before moving on to true mass transport issues it is worthwhile to point out that the quantity vep(Y - Yp) is in fact the water load per unit of time (whichever units of time v is expressed in.) Further that with an a-priori design process we may not know the required cross sectional area for flow and hence it may be more convenient to multiply the above relationship by and thus obtain the adsorbable contaminant input rate ... 

Vinten et al. (1983) demonstrated that the vertical retention of contaminated suspended particles in soils is controlled by the soil porosity and the pore size distribution. Figure 5.8 illustrates the fate of a colloidal suspension in contaminated water during transport through soil. Three distinct steps in which contaminant mass transfer may occur can be defined (1) contaminant adsorption on the porous matrix as the contaminant suspension passes through subsurface zones, (2) contaminant desorption from suspended solid phases, and (3) deposition of contaminated particles as the suspension passes through the soil. 

The reactions in zero-valent iron are heterogeneous due to the strong dependence of the reaction rate on the surface area of the iron (Burris et al., 1995). The surface reaction proceeds in four steps. First, the reactant undergoes mass transport from the groundwater to the iron surface (Matheson and Tratnyek, 1994). Second, the contaminant is absorbed onto the surface of the iron, where the chemical reaction occurs. Third, the reaction products desorb from the surface, which allows the site to become available for another reaction (Burris et al., 1996a). Finally, the products of the reaction return to the groundwater. Rate limitation could occur at any step. Where it may not be the sole limitation, mass transport plays an essential role in the kinetics of dechlorination (Matheson and Tratnyek, 1994). 

Nitroaromatic compounds (NACs) are one of the widespread contaminants in the environments. Sources of NACs are numerous they originate from insecticides, herbicides, explosives, pharmaceuticals, feedstock, and chemicals for dyes (Agrawal and Tratnyek, 1996). Under anaerobic conditions, the dominant action is nitro reduction by zero-valent iron to the amine. Other pathways do exist, such as the formation of azo and azoxy compounds, which is followed by the reduction of azo compounds to form amines. Also, in addition to the possibility of azo and azoxy compounds, phenylhydrox-ylamine may be an additional intermediate (Agrawal and Tratnyek, 1996). Nitrobenzene reduction forms the amine aniline. Known for its corrosion inhibition properties, aniline cannot be further reduced by iron. Additionally, it interferes with the mass transport of the contaminant to the surface of the iron. The overall reaction is as follows ... 

The overall reaction occurring at an Fe° surface involves a series of steps including (1) mass transport to the reactive site, (2) chemical reaction at the surface (e.g., sorption, electron transfer, etc.), (3) desorption, and (4) mass transport to the bulk solution (recall Fig. 7). Any one of these steps can limit the rate of contaminant removal by Fe°, so the observed... 

Because rates of reduction by Fe° vary considerably over the range of treatable contaminants, it is possible that there is a continuum of kinetic regimes from purely reaction controlled, to intermediate, to purely mass transport controlled. Fig. 9 illustrates the overlap of estimated mass transport coefficients (kmt) and measured rate coefficients (kSA). The values of kSA are, in most cases, similar to or slower than the kmi values estimated for batch and column reactors. The slower kSA values suggest that krxu < kml, and therefore removal of most contaminants by Fe° should be reaction limited or only slightly influenced by mass transport effects (i.e., an intermediate kinetic regime). 

Mass transport is understood to mean the molecular diffusion in, out and through plastic materials like that shown schematically in Fig. 1-3. This figure represents most applications where there is a layer of plastic material separating an external environmental media from an inner product media. The product can be a sensitive medium with a complex chemical composition, e.g. food, that must be protected from external influences such as oxygen and contaminants. It can also be an aggressive chemical that must not escape into the surrounding environment. Because this plastic material barrier layer usually includes low molecular weight substances incorporated into the polymer matrix, there are many applications in which the transport of these substances into the product and environment must be minimized. 

Advection is the transport of dissolved contaminant mass due to the bulk flow of groundwater, and is by far the most dominant mass transport process [2]. Thus, if one understands the groundwater flow system, one can predict how advection will transport dissolved contaminant mass. The speed and direction of groundwater flow may be characterized by the average linear velocity vector (v). The average linear velocity of a fluid flowing in a porous medium is determined using Darcy s Law [2] ... 

So from a simple mass transport point of view, hair is less susceptible to contamination by exogenous mechanisms than urine by endogenous mechanisms. But then hair, but not urine, is cleansed by normal hygienic practices and by special laboratory wash procedures. The latter are further strengthened by special kinetic analysis of the wash data. And, finally, the deposition of exogenous drugs onto hair, unlike their accumulation in urine, is not associated with the formation of metabolites. 

Chapter 8 addresses the treatment of contaminated air streams using photocatalysis. Special attention is given to the distinction between reaction kinetics and mass transport processes. The reviewed studies show the evolution from the early days of Ti02 photocatalysis, where the aim was to understand the basic process parameters, to today s development of phenomenological models assisting in the scaling-up of units. 

FRANZ, R., in Plastic Packaging Materials for Food Barrier Function, Mass Transport, Quality Assurance, and Legislation, ed. Piringer, O.-G., and Bauer, A. L., 2000. CASTLE, L., MAYO, A. and GILBERT, J., 1989. Migration of plasticizers from printing inks into foods. Food Additives and Contaminants, 6 (4), pp 437 443. 

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