Channel calcite dissolution

Model the carbonate precipitation in this carbonate channel by means of a Id transport with 40 cells of 10 m length each. Dispersivity is assumed with lm. Use the key words KINETICS and RATES and the BASIC program for calcite from the data set PHREEQC.dat describing the kinetics for both the calcite dissolution and the calcite precipitation. How much calcite precipitates each year within the channel s first 400 meter after the discharge How much C02 degasses at the same time ... 

Channel electrode methodology has been used to investigate calcite dissolution in aqueous solution at around pH 3 where the process is thought to be limited by the rate of transport of H+ to the crystal surface [197-199, 209, 220,223, 225,226, 230, 254-257]. Amperometric detection was used in the form of a platinum electrode held at a potential corresponding to the transport-limited reduction of H+ [10 3 M], The detector electrode was positioned downstream of a dissolving calcite crystal and, with the notation of the previous section, the cell geometry was defined by the parameters xu = 0.469 cm, xA = 0.512 cm. Figure 65 shows a plot of [H+1°°)... 

The mass transfer by convection and diffusion within the channel is precisely calculable. Numerical methods are used to fit measured concentrations at the detector electrode to the fluid flow in the cell, and to the reaction kinetics at the surface and in solution [24]. The technique was extensively developed during a study of the dissolution of calcite in dilute aqueous acid [25], and has latterly been applied to a number of organic reactions. 

Figure 2. Unstable growth of a dissolution channel in the transport-controlled reaction regime. Flow is from bottom to top, with the cement-bearing portion of the aquifer shown as shaded. Note the convergence of flow into the dissolution channel. Times are based on the dissolution of calcite using an undersaturation of 1 x 10 M calcite. The Darcian velocity upstream of the front is 1 x 10 ms ( O.Smi/r" ), the dispersion coefficient is 1 x 10 m s U...

The dissolution kinetics of ionic solids is currently an intense area of research activity [186-212], This arises from the obvious environmental and technological significance, as well as the intrinsic interest in the underlying fundamental principles. In this section, we establish the merits of employing channel electrode methodology in this general area. The experiments described have general applicability, but will be illustrated by reference to our work on calcite. The extension to other systems is self-evident and involves no new principles. 

An additional merit of the channel electrode set-up is that the exposed calcite surface can be viewed in situ, by light microscopy, allowing the crystallographic nature of the dissolution process to be linked directly to kinetic measurements. Furthermore, it should be noted that ex-situ studies on the morphology of crystal surfaces subjected to dissolution in the channel set-up are likely to provide greater information, by nature of the non-uniform accessibility of the crystal surface to protons, than might be anticipated from the same measurements on a crystal surface exposed to a uniform flux of electrolyte (vide infra). 

Plate 1. Differential interference contrast micrograph (DIC) of the upstream zone of a cleaved calcite crystal subjected to dissolution by lO-3M HC1 in the channel electrode system. The bar represents 67 pm. 

In this section, we report results on the dissolution of calcite in PMA using the channel electrode technique. Dissolution experiments were carried out with unbuffered polymaleic acid solutions using the channel elec-... 

In this section, we briefly provide preliminary results on the morphology of etch pits obtained on cleaved calcite crystals by dissolution in unbuffered aqueous solutions of maleic acid (3 mM) under both stationary conditions and through channel electrode experiments. Kinetic and morphological studies on the effect of maleic acid on the dissolution of calcium carbonate... 

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