Precipitation, kinetic simulation

The concentrations in pore water at a depth of only one centimeter below the sediment surface are considered as stationary. What release rates applying to phosphate must we postulate for the bacteria if it is known from experiments that the precipitation kinetics of phosphate minerals are in a certain range, and if the measured concentration profile in pore water is to be simulated correctly ... 

Figure 6, Results of a one-year kinetic simulation of urea hydrolysis for the Snake River Plain Aquifer, (a) pH. (b) Dissolved urea, dissolved inorganic carbon (as HC0 ), calcium, and ammonium. (c) Amount of calcium and ammonium on aquifer matrix exchange sites, (d) Amount of calcite precipitated.

Precipitation kinetics have been obtained by Monte Carlo simulations for four different supersaturations of the solid solution == 5, 6, 7, and 8 at.%) at T = 1000 K. At this temperature, the equilibrium concentration is Cl = 2.1 at.%. The simulation box contains 125000 lattice sites and its starting configuration is a completely disordered (random) solid solution. 

Little is known about the kinetics of dissolution, precipitation, and oxidation-reduction reactions in the natural environment. Consequently, simulating the kinetics of even more complicated injection- zone chemistry is very difficult. 

Comparing the development here to the accounting for the kinetics of mineral precipitation and dissolution presented in the previous chapter (Chapter 16), we see the mass transfer coefficients v and so on serve a function parallel to the coefficients v , etc., in Reaction 16.1. The rates of change in the mole number of each basis entry, accounting for the effect of each kinetic redox reaction carried in the simulation, for example,... 

In the previous two chapters (Chapters 26 and 27), we showed how kinetic laws describing the rates at which minerals dissolve and precipitate can be integrated into reaction path and reactive transport simulations. The purpose of this chapter is to consider how we can trace the reaction paths that arise when redox reactions proceed according to kinetic rate laws. 

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. 

Simulations of THC processes were performed using the TOUGHREACT code (Xu and Pruess, 2001 Xu et al 2003). TOUGHREACT is based on TOUGH2 (Pruess, 1991) and in addition to the coupling between heat, water, and vapor flow for multiphase systems, it solves aqueous and gaseous species transport, kinetic and equilibrium mineral-water reactions, and feedback of mineral precipitation/dissolution on porosity, permeability, and capillary pressure. 

Reactive transport simulation is more realistic because of the consideration of SrSOj and BaS04 precipitation in reservoir (Bertero et al., 1988), predicting a lower scaling in the production well than the conservative transport simulation (Figure 6). This can be justified by the fact that in a reactive transport simulation part of the mineral that could cause scale in the production well has already precipitated in the reservoir. For kinetic reasons, they are called potentially precipitated minerals because the local equilibrium assumption at the production well is not necessarily valid. Produced water can be SrSO and BaS04... 

The kinetic parameters for DOC biodegradation and solid carbonate precipita-tion/dissolution are given in Table 11.5. For calcite precipitation and dissolution rate constant were assigned for both solid carbonates. The simulation time was 100 years with a time step length of 10 d. 

Figure 11.5 compares simulated and sampled Fe and Mn distributions when the model is only slightly extended from the state, presented by Holzbecher et al. (2001), i.e. in which Fe is included as additional redox process of minor priority. Both measured Fe and Mn concentrations are well reproduced by the model. Thus observed Fe and Mn concentrations in the Oderbruch can be simulated when the redox model is supplemented only by a kinetic precipitation/dissolution approach. 

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