Diffusion pore water

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

Global uranium flux calculations have typically been based on the following two assumptions (a) riverine-end member concentrations of dissolved uranium are relatively constant, and (b) no significant input or removal of uranium occurs in coastal environments. Other sources of uranium to the ocean may include mantle emanations, diffusion through pore waters of deep-sea sediments, leaching of river-borne sediments by seawater," and remobilization through reduction of a Fe-Mn carrier phase. However, there is still considerable debate... 

Figure 7. Sediment eontains derived both seavenged from the water eolumn during particle settling and contained in solid material. Ra produced in the sediments is highly soluble in pore waters and diffuses into the overlying water or is advected across the sediment-water interface by discharging groundwater. Rn is produced within the water column from dissolved Ra and within the underlying sediments.

There are two main sources of Rn to the ocean, (1) the decay of sediment-bound Ra and (2) decay of dissolved Ra in the water column. Radon can enter the sediment pore water through alpha recoil during decay events (see discussion in Porcelli and Swarzenski 2003). Since radon is chemically inert, it readily diffuses from bottom sediments into overlying waters. The diffusion of radon from sediments to the water column gives rise to disequilibrium between Rn and Ra, with ratios of ( Rn/ Ra) >1 due to the addition of excess Rn. This excess Rn is unsupported and so is rapidly diminished by decay. Therefore, the excess Rn signal is only resolvable where significant transport of Rn away from the sediment can occur over timescales that do not significantly exceed the half-life of Rn. 

I consider a system in which organic matter is oxidized at a steady rate that is a specified function of depth in uniform calcium carbonate sediments. The oxidation of organic matter increases the total dissolved carbon in the pore water of the sediment. The resultant increase in acidity causes the dissolution of calcium carbonate and a consequent increase in alkalinity as well as another increase in total dissolved carbon. The total dissolved carbon and alkalinity are transported by diffusion between different depths in the sediment. 

Post depositional mobility by diffusion or by microorganism transport in pore water. 

The elements deposited within the sediment matrix show that mobilization processes may be occurring in the upper layers. At Station SIN 3, figure 4d for example, the element deposited (pg-cm-2) in the topmost layers decreases, often much more than in the concentration (Mg g 1). This may be due to organic matter decomposition and/or to environmental chemical reactions of solubility and precipitation of the given element. The metal must have been removed rapidly from the water column since the sediment concentration is shown to decrease rapidly with distance from the shipyard (Stations SIN 3 and SIN 2). Lead may not be mobilized significantly after deposition since any diffusion in the pore water would tend to "smooth" the concentration profile with time. 

The maximum deposition of Zn, Cu, Ni, and Cr appear to be at about the mid 1960 s, whereas the maximum for Pb appears to be just following World War II. The shape of the deposition profile curve for Zn, Ni, Cr, and possibly Cu indicates that these elements may be migrating downward in the sediments, possibly in the pore water. The decrease in deposition at the sediment-water interface indicates diffusion of the metals out of the sediment... 

Typically, functional porins are homotrimers, which assemble from monomers and then integrate into the outer membrane. The general porins, water-filled diffusion pores, allow the passage of hydrophilic molecules up to a size of approximately 600 Daltons. They do not show particular substrate specificity, but display some selectivity for either anions or cations, and some discrimination with respect to the size of the solutes. The first published crystal structure of a bacterial porin was that of R. capsulatus [48]. Together with the atomic structures of two proteins from E. coli, the phosphate limitation-induced anion-selective PhoE porin and the osmotically regulated cation-selective OmpF porin, a common scheme was found [49]. Each monomer consists of 16 (3-strands spanning the outer membrane and forming a barrel-like structure. 

Fig. 3 a - c. Schematic diagram illustrating the decreasing source method for diffusion transport determination of any organic pollutant in solution or leached from complex mixtures, as follows a column setup b pollutant concentration vs time in source and collection reservoirs during the test c pollutant concentration in solid-pore water with depth from source after the test... 

Because of the similarity of transport in biotilms and in stagnant sediments, information on the parameters that control the conductivity of the biofilm can be obtained from diagenetic models for contaminant diffusion in pore waters. Assuming that molecular diffusion is the dominant transport mechanism, and that instantaneous sorption equilibrium exists between dissolved and particle-bound solutes, the vertical flux ( ) through a stagnant sediment is given by (Berner, 1980)... 

Fuel cell operation entails (1) coupled proton migration and water fluxes in the PEM, (2) circulation and electrochemical conversion of electrons, protons, reactant gases, and water in CLs, and (3) gaseous diffusion and water exchange via vaporization/condensation in pores and channels of CLs, GDLs, and EEs. All components of an operating cell have to cooperate well in order to optimize the highly nonlinear interplay of these processes. It can be estimated that this optimization involves several 10s of parameters. 

In seawater, physical processes that transport water can also cause mass fluxes and, hence, are another means by which the salinity of seawater can be conservatively altered. The physical processes responsible for water movement within the ocean are turbulent mixing and water-mass advection. Turbulent mixing has been observed to follow Pick s first law and, hence, is also known as eddy diffusion. The rate at which solutes are transported by turbulent mixing and advection is usually much faster than that of molecular diffusion. Exceptions to this occur in locations where water motion is relatively slow, such as the pore waters of marine sediments. The effects of advection and turbulent mixing on the transport of chemicals are discussed further in Chapter 4. 

Some gases have subsurfece sources that are related to physical phenomena, such as inputs from the introduction of hydrothermal fluids in bottom waters or release from warming sediments. The latter is a source of methane, which can occur in sediments in a solid phase called a clathrate hydrate. Biogeochemical reactions in sediments can also produce gases that diffuse from the pore waters into the deep sea. 

Redox processes also tend to be separated in time and space due to the relative sluggishness of solute transport. For example, molecular diffusion is the major mechanism by which solutes can be transported through the pore waters of sediments. In many cases this process is slower than the chemical reaction rates and, thus, prevents... 

Diagrams of benthic sampiing equipment, (a) Pore water peepers used for collecting pore water in situ via moiecuiar diffusion across a 0.4- j,m polycarbonate membrane. These probes were 36 inches iong. Each of the six ports on the peeper held 6mL of equilibration fluid. The probes were deployed for 1 to 2.5 y to ensure equilibrium had been reached with the pore waters, (b) Benthic flux chamber. This chamber covered a bottom area of SSScm. Source From Aller, R. C., et al. (1998). Deep-Sea Research, 45, 133-165. 

Reaction rates of nonconservative chemicals in marine sediments can be estimated from porewater concentration profiles using a mathematical model similar to the onedimensional advection-diffusion model for the water column presented in Section 4.3.4. As with the water column, horizontal concentration gradients are assumed to be negligible as compared to the vertical gradients. In contrast to the water column, solute transport in the pore waters is controlled by molecular diffusion and advection, with the effects of turbulent mixing being negligible. 

Sedimentation rates range from centimeters per year in coastal sediments down to millimeters per thousand years on the deep-sea floor. Thus, the effects of molecular diffusion are generally greater than that of advection in shaping pore-water concentration profiles. 

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