Diffusive flux controlling factors

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Xu and Ruppel (1999) solved the coupled mass, heat, and momentum equations of change, for methane and methane-saturated fluxes from below into the hydrate stability region. They show that frequently methane is the critical, limiting factor for hydrate formation in the ocean. That is, the pressure-temperature envelope of the Section 7.4.1 only represents an outer bound of where hydrates might occur, and the hydrate occurrence is usually less, controlled by methane availability as shown in Section 7.4.3. Further their model indicates the fluid flow (called advection or convection) in the amount of approximately 1.5 mm/yr (rather than diffusion alone) is necessary to produce significant amount of oceanic hydrates. 

Michael Wasielewski of Northwestern University asked Thomas Moore about the type of light fluxes being used to investigate the solar flux. He also asked, Since we all know that photosynthesis has control mechanisms that actually modify electron flow, based on light flux, what kind of prospectus or perspective do we have for control mechanisms in such systems Moore explained that one of the factors that seems to limit natural photosynthesis is the diffusion of carbon dioxide into the system for fixing, so it is important in photosynthesis to throttle back the powerful oxidant when carbon dioxide is limiting. There is a control mechanism called nonphotochemical quenching that is related to the... 

The interpretation of measured flame profiles by means of the continuity equations may be approached in one of two ways. The direct experimental approach involves the use of the measured profiles to calculate overall fluxes, reaction rates, and hence rate coefficients. Its successful application depends on the ability to measure the relevant profiles, including concentrations of intermediate products. This is not always possible. In addition, the overall fluxes in the early part of the reaction zone may involve large diffusion contributions, and these depend in turn on the slopes of the measured profiles. Thus accuracy may suffer. The lining up on the distance axis of profiles measured by different methods is also a problem, and, in quantitative terms, factor-of-two accuracy is probably about the best that may normally be expected from this approach at the position of maximum rate. Nevertheless, examination of the concentration dependence of reaction rates in flames may still provide useful preliminary information about the nature of the controlling elementary processes [119—121]. Some problems associated with flame profile measurements and their interpretation have been discussed by Dixon-Lewis and Isles [124]. Radical recombination rates in the immediate post-combustion zones of flames are capable of measurement with somewhat h her precision than above. 

The first term in Eq. (42) describes the diffusion-limited flux, while the second term l/ l + a) is a correction factor for slow reaction kinetics. Because a is defined as the ratio D /k h, the value of is known and of Djn can be determined independently, the value of k can be determined using Eq. (42). Direct determination offe and from flux measurements as a function of the membrane thickness may be obtained varying the membrane thickness. As a result a is obtained also. If a < 1, the transport is mainly limited by diffusion of the complex while the transport is primarily controlled by the reaction rate in the case that a > 1. 

The effective diffusion coefficient depends on the particle porosity, the pore diameter, the tortuosity, and the nature of the diffusing species. For gas-filled pores, the above factors can be allowed for to make a reasonable estimate of the effective diffusivity in the gas phase. However, diffusion of adsorbed molecules along the pore walls, called surface diffusion, often contributes much more to the total flux than diffusion in the gas phase. This is particularly evident in the adsorption of water vapor on silica gel and the adsorption of hydrocarbon vapors on carbon, where the measured values of correspond to internal and external coefficients of comparable magnitude or even to external film control, For adsorption of solutes from aqueous solutions, surface migration is much less important, and the internal diffusion resistance generally dominates the transfer process. 

Pick s Laws and Membrane Flux Since compounds of environmental significance cross biomembranes only by passive diffusion, it is of interest to define those factors that control the flux through the membrane in order to assess which properties of the chemical may be significant. Pick s first law states that the flux, J, in mass per unit area per unit time is proportional to the concentration differential. 

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