Elemental Uptake Processes

General mechanisms of uptake, assimilation, distribution and transport have been suggested by a number of authors (Bowling, 1976 Baker, 1983 Marschner, 1983 Az-piazu et al., 1986 Davies and Jeffcoat, 1990 Fergusson, 1990 Streit and Stumm, 1993). 

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Soil solution is the aqueous phase of soil. It is in the pore space of soils and includes soil water and soluble constituents, such as dissolved inorganic ions and dissolved organic solutes. Soil solution accommodates and nourishes many surface and solution reactions and soil processes, such as soil formation and decomposition of organic matter. Soil solution provides the source and a channel for movement and transport of nutrients and trace elements and regulates their bioavailability in soils to plants. Trace element uptake by organisms and transport in natural systems typically occurs through the solution phase (Traina and Laperche, 1999). 

Bioaccumulation is a complicated process that couples numerous complex and interacting factors. In order to directly relate the chemical speciation of an element to its bioavailability in natural waters, it will be necessary to first improve our mechanistic understanding of the uptake process from mass transport reactions in solution to element transfer across the biological membrane. In addition, the role(s) of complex lability and mobility, the presence of competing metal concentrations and the role(s) of natural organic ligands will need to be examined quantitatively and mechanistically. The preceding chapter... 

Figure 9.24 b compares the isotope ion distribution of natural 40Ca+ and doped 44Ca+ in a cross section of a root of a Norway spruce. These measurements were carried out by SIMS using 69Ga+ primary ions (lOkeV, 0.1 nA, Ionoptika) with a lateral resolution < 0.1 pm.80 The investigations can be used to study the mechanism of mineral element uptake and kinetics of transport processes in plants as a function of time. About 20 min after the start of the tracer experiment a barrier was observed for the transport of enriched isotope 44Ca+ in the middle of the root.80... 

Caution must be used in applying the findings of this report to the actual uptake processes occurring in the nuclear fireball. There the situation is much more complicated. Not only are there rapid changes of temperature and pressure and violent turbulence, but the radioactive elements themselves constitute a small fraction of the total vaporized material. It is difficult to estimate how these factors would affect the uptake rates. 

Type of study A = analytical study, M = monitoring study, P = study on the effect of food processing/preparation, O = other studies, including investigations on element uptake/distribution/biotransformation and biological activity. 

Commonly, restrictions are placed on the transport fluxes (corresponding to the various uptake processes). For example, when specific information regarding an uptake rate is not known, the maximal uptake rate is set by using the constraints defined by Eq. (10) (fi is defined as the maximal uptake rate or b. element). However, often in experimental systems, the uptake rates have been measured, and the flux value can be fixed by using the formalism described by Eq. (10). Transport reactions for metabolites that are not present in a simulated culture condition are constrained to zero (0 < vj < 0). 

I.I. Effect of Residence Time on Desorption Some researchers found that trace elements [Ni, Pb, As(V)] reacted with metal oxides and pyrophyllite over longer times resulted in either irreversible or reversible sorption mechanisms. Violante et al. (2003) studied the effect of residence time on the sorption of Zn onto ferrihydrite in the presence of Cu. As Cu has a greater affinity than Zn for tire surfaces of ferrihydrite, Cu was added from 1 to 336 hours after Zn at a Zn/Cu molar ratio of 2. Zinc sorption increased, particularly when Cu was added 6 to 336 hours after Zn. A possible explanation of these findings is that trace elements initially sorbed on the surfaces of variable-charge minerals slowly form precipitates with time. As discussed before, sorption is considered to be the predominant sorption mechanism responsible for trace element uptake on mineral surfaces within the first few hours, while surface precipitation is considered to be a much slower process, occurring on a time scale of hours to days (McBride, 1994 Scheidegger et al., 1997 Sparks, 1999 Borda and Sparks, Chapter 3, this volume). Clearly, Cu added many hours or days after Zn addition cannot replace Zn ions that have formed precipitates on the surfaces of the ferrihydrite. 

The rate of transfer of solutes between soil and overlying water column and from one physical or chemical state to another is defined as flux. The dimensions of flux are M T where M is the mass of material transferred by flux, L is the distance or length, and T is the time. The processes associated with flux are advection, diffusion, and dispersion. Diffusive and advective flux between soil and overlying water and elemental uptake by rooted wetland vegetation are the major transport... 

The biogeochemical processes that generally describe the interaction of elements with particles are quite well known dissolution, flocculation, ion exchange, sorption, (co)precipitation, electron transfer, and biological uptake. In aquatic environments these reactions often occur simultaneously and competitively. In order to utilize marine tracers effectively, we must understand how elements are associated with particles and sediments. 

DeMaster DJ, Kuehl SA, Nittrouer CA (1986) Effects of suspended sediments on geochemical processes near the mouth of the Amazon river - examination of biological silica uptake and the fate of particle-reactive elements. Cont Shelf Res 6 107-125... 

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