Weathering transport control

Groundwater environments can be represented as a simple flow-through system. For the situation where chemical weathering of mineral grains is transport controlled, the weathering rate of a mineral should be directly dependent on the rate of throughput of water. For the situation where rates are controlled by surface... 

Among the theories proposed, essentially two main mechanisms can be distinguished these are that the rate-determining step is a transport step (e.g., a transport of a reactant or a weathering product through a layer of the surface of the mineral) or that the dissolution reaction is controlled by a surface reaction. The rate equation corresponding to a transport-controlled reaction is known as the parabolic rate law when... 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

In this chapter, we have tried to review the recent literature on trace elements in rivers, in particular by incorporating the results derived from recent ICP-MS measurements. We have favored a field approach by focusing on studies of natural hydrosystems. The basic questions which we want to address are the following What are the trace element levels in river waters What controls their abundance in rivers and fractionation in the weathering - - transport system Are trace elements, like major elements in rivers, essentially controlled by source-rock abundances What do we know about the chemical speciation of trace elements in water To what extent do colloids and interaction with solids regulate processes of trace elements in river waters Can we relate the geochemistry of trace elements in aquatic systems to the periodic table And finally, are we able to satisfactorily model and predict the behavior of most of the trace elements in hydrosystems ... 

Equation (2.101) corresponds to transport-controlled kinetics (cf. Stumm 1990). White and Claassen conclude that after long times in natural water/rock systems parabolic rates tend to become linear. Helgeson et al. (1984) show that feldspar dissolution rates are linear if the feldspar is pretreated to remove ultrafine reactive particles. In other words initial parabolic rates are probably an artifact of sample preparation. It seems likely that, in general, the dissolution or weathering of most silicates in natural water/rock systems obeys zero-order kinetics. 

Under experimental conditions, most rock-forming silicate minerals are sufficiently nonreactive that transport of solutes toward, or away from, the dissolving surface does not control the rate. The rate of the overall reaction is controlled by reactions at the interfaces, that is, the second sequential step. In soil, solute transport is sufficiently rapid that silicate weathering is controlled by the reaction kinetics at the mineral-solution interface (Berner 1978). 

Overall, weathering controls the chemistry of material that is transported into the sediment and that which stays behind in the soil. As an example, consider a general weathering reaction for an aluminosilicate (Stumm and Morgan, 1995) ... 

Figure 9-3 portrays a hypothetical model of how chemical weathering and transport processes interact to control soil thicknesses. The relationship between soil thickness and rate at which chemical weathering can generate loose solid material is indicated by the solid curve. The rate at which transport processes can potentially remove loose solid weathering products is indicated by horizontal dotted lines. The rate of generation by chemical weathering initially increases as more water has the opporhmity to interact with bedrock in the soil. As soil thick-... 

When soil thickness is at the stable value (F), erosion is transport limited. Chemical weathering is also transport limited. This is, however, not because of reaction kinetics instead this limitation is primarily controlled by physical factors, most probably, restricted access of water to the primary minerals. 

In regions where erosion is transport limited, weathering rates are controlled by the supply of reactive fluids to unstable minerals. This is controlled by soil properties, regional base level, and ultimately, sea level. 

Modeling of the transport of the long-lived nuclides, especially U, require knowledge of the input at the water table as a boundary condition for aquifer profiles. There are few studies of the characteristics of radionuclides in vadose zone waters or at the water table. Significant inputs are likely to occur to the aquifer due to elevated rates of weathering in soils, and this is likely to be dependent upon climatic parameters and has varied with time. Soils may also be a source of colloids and so provide an important control on colloidal transport near recharge regions. 

In the following pages, we will summarise the main processes controlling the fractionation of radionuclides during weathering and transfers into surface waters. Subsequently, we will present the main results obtained on surface weathering and transport in the river waters. Throughout this chapter, we will use parentheses to denote activity ratios. 

The text deals with the microbial and chemical process engineering of sewer networks. It emphasizes dry-weather processes and not the wet-weather impacts that are primarily controlled by physical processes. Under such conditions, the physical in-sewer processes in terms of, for example, hydraulics, sediment and biofilm erosion and solids transport are important. A quite different approach must be applied when wet-weather conditions in sewers dominate. However, wet-weather performance of sewers also requires that sediment deposition be dealt with during dry-weather periods. 

As was mentioned in the introduction to this chapter "diffusion-controlled dissolution" may occur because a thin layer either in the liquid film surrounding the mineral or on the surface of the solid phase (that is depleted in certain cations) limits transport as a consequence of this, the dissolution reaction becomes incongruent (i.e., the constituents released are characterized by stoichiometric relations different from those of the mineral. The objective of this section is to illustrate briefly, that even if the dissolution reaction of a mineral is initially incongruent, it is often a surface reaction which will eventually control the overall dissolution rate of this mineral. This has been shown by Chou and Wollast (1984). On the basis of these arguments we may conclude that in natural environments, the steady-state surface-controlled dissolution step is the main process controlling the weathering of most oxides and silicates. 

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