Dissolution-precipitation model

In the synthesis of single-waUed nanotubes, on the other hand, it is not the vaporization of starting material, but the diffusion of carbon units through the catalyst that is found to be the rate-determining step. The formation of SWNT advances according to the so-called dissolution-precipitation model (DP-model) that postulates a sequence of three steps as discussed below ... 

Dissolution-precipitation models. Dubinina and Lakshtanov (1997) developed a kinetic model that describes isotopic fractionation between a mineral and fluid involved in one of three types of dissolution-precipitation processes (Fig. 11). Type I (mineral synthesis) considers successive dissolution of an unstable phase, A, of uniform isotopic composition and precipitation (crystallization) of phase B. Type II (Ostwald ripening) involves the partial dissolution of phase B which has a non-uniform isotopic composition... 

The kinetic mass transfer model developed to take into consideration the geochemical evolution of the Cigar Lake ore deposit was mainly done by simulating the evolution of the Al-Si system in the Cigar Lake ore deposit system. To this aim the system formed by kaoli-nite, gibbsite and illite as main aluminosilicate solid phases was considered and kinetics for the dissolution-precipitation processes were taken from the open scientific literature (Nagy et al. 

The mass balance implied by hypothesis 3 is shown in Table 10. The net result of the assumptions built into this model is to decrease the amount of organic matter oxidized to carbon dioxide and to increase the amount of DIC from shell material dissolution. This, in turn, decreases the amount of shell material dissolution/calcite cement precipitation needed to achieve isotope balance. Between Olanta and MRN-77, the amount of dissolution/precipitation needed for isotope balance is 2.0 mmol CaC03 kg of H2O, and 25 mmol CaC03 from MRN-77 to HO-338. This, in turn, implies that l-13vol.% of the aquifer would be cemented by calcite, which is roughly in line with observed calcite cementation. 

There is reason to believe that such rate information for simple homogeneous reactions and for the least complex heterogeneous reactions, including mineral dissolution/precipitation, can be more usefully applied to studies of natural systems than has so far been the case. The laboratory rate data and theoretical models often do not apply directly to complex natural systems. However, they at least provide guidelines and direct our study toward a better understanding of natural rates. 

Purposes These models can speciate an aqueous solution, just as the speciation models do. They can also simulate changes in solution chemistry caused by mass transfer processes, such as dissolution/precipitation, ingassing/outgassing, ion exchange/adsorption, evaporation, boiling temperature and pressure changes, and mixing of two waters. 

Thus, we have completed all the steps to set up a UTCFIEM model. This example is a typical alkaline flood case in terms of compositions in the system. If magnesium is included, we simply add all of the lines related to calcium and modify those lines for magnesium. A case with clay and silica dissolution/ precipitation is briefly discussed in the next section. 

Once a solid mineral phase is introduced into this simple model and cation exchange or dissolution/precipitation reactions are allowed for, the relationships among pH, and added acidity or basicity are fundamentally changed. For example, the H" ions can react with the acid mineral soil to produce soluble AF. Figure 5.6 showed that the solubility of AF in the B and C horizons of some acid mineral soils is described fairly well by the solubility product of crystalline gibbsite ( so = [AF ][0H ] = 10 ), so that the approximate relationship between [AF ] and [H ] is... 

The instantaneous nucleation-growth-precipitation model [39] assumes that the film is formed directly on the substrate, without previous dissolution however, it was observed that active dissolution of the metal occurs. Therefore, Equations 8.11 through 8.13 were examined and rewritten considering metal dissolution, that is, terms corresponding to dissolution were added to the mathematical expressions ... 

The fact that the experimental data fit both models, the dissolution-precipitation and the instantaneous nucleation-growth-overlap models, confirms the assertion made at the beginning of this chapter, that is, some models are complementary to each other. 

Hypothetical reaction pathways chosen to model the L2 leachate-Uinta Sandstone system are illustrated in Figure 5. As a first approximation, dissolution/precipitation reactions affecting the mass balance of Na, K, Mo, SO4, and Cl were not considered. Instead, based upon the solubility controls discussed in the previous sections of this paper, the working hypothesis for the simulations is that the recarbonation of L2 leachate drives the reactions toward equilibrium. Along the path toward equilibrium, recarbonation is accompanied by the precipitation and dissolution of sepiolite, calcite, and an inferred hydrated magnesium carbonate mineral such as hydromagnesite. 

Figure 11. Schematic representation of the processes Types I-III described by Dnbininia and Lakshtanov (1997) in their models of isotopic exchange promoted by dissolution-precipitation reactions.

In Dubinina and Lakshtanov s (1997) Type II model, dissolution-precipitation proceeds by the Ostwald ripening after mineral synthesis. Upon completion of mineral synthesis (Type I), the phase B consists of an aggregate of crystals of different grain sizes. The recrystallization of these mineral grains will take place by Ostwald ripening,... 

Figure2 Ohnishi et al. (1985) and Chijimatsu et al. (2000)) and reactive-mass transport model (inside the box named Chemical in Figure 2). This is a system of governing equations composed of Equations (l)-(9), which couple heat flow, fluid flow, deformation, mass transport and geochemical reaction in terms of following primary variables temperature T, pressure head y/, displacement u total dissolved concentration of the n master species C< > and total dissolved and precipitated concentration of the n" master species T,. Here we set master species as the linear independent basis for geochemical reactions, and speciation in solution and dissolution/precipitation of minerals are calculated by a series of governing equations for geochemical reaction. Now we adopt equilibrium model for geochemical reaction (Parkhurst et al. (1980)), mainly because of reliability and abundance of thermodynamic data for geochemical reaction.

On mass transport and geochemical reaction, we will advance our model to take into account degassing, reaction of ionic exchange, surface complexation and kinetic reaction of dissolution/precipitation. 

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