Precipitation of Calcite using REACT

[thais98]

Hello! My knowledge of geochemistry is kinda poor and I've recently started using the GWB software with the Professional Demo. I need to run some reactions to see the amount of calcite that precipitates when reacting the water sample rich in CO2 with basalt minerals, such as anorthite, albite and diopside. So, what i'm doing is: adding those minerals as kinetics in the reactants and also, the kinetics of calcite (the one that I want to precipitate). I'm setting the concentration of calcite to 0 and rate constant of 5.01e-5 (i'm not sure if that's correct) . However, when I run the reaction, and go to the graphic, I don't see the mineral calcite precipitating, only the ones that I added a concentration. I was using PHREEQC before and regarding this step (not the kinetics) was much easier to understand.

I'm attaching the file, so if someone could take a look at it , i'd be happy  

 

amostra 8- reagindo com rocha.rea

[Jia Wang]

Hello,

I took a quick look at your file and I didn't find any issues that stood out. When I ran your input file, I observed Calcite precipitation. Perhaps you attached the wrong file? Or perhaps the mineral is not selected in Gtplot to display on your plot? To check, double-click on the interior of your plot to bring up the configuration dialog, make sure that the desired minerals are selected and click Apply. For more information on configuring Gtplot, please refer to section 6.2 XY Plot Configuration of the Reaction Modeling User Guide.

I see that you have supplied a pre-exp factor and activation energy for the Arrhenius equation instead of a rate constant for kinetic calcite. You can view the calculated rate constant, based on the values provided under Reactant properties in XY plot configuration. 

If this doesn't help, can you provide screenshots of where you encountered the issue?

Hope this helps,
Jia Wang

[thais98]

Hello, thank you for the reply. Oh I see, so the file that I sent had a nucleus density of 1000 cm2/cm3, and this one precipitated calcite. One of the things that I'm not understanting is what exactly is the nucleus density. Because when I ran the file with nucleus density of 0 cm2/cm3 there was no calcite precipitated, but instead aragonite. I'm not sure what would be the correct value to the nucleus density, whether or not I can input any value > 0 to precipitate calcite.

Regarding the pre-exp factor and Activation Energy of calcite, I selected those values from another article, since I'm not sure how I can calculate it. I'm not sure if I need to do calculations to get those values or are they already established? You can see that I have many doubts regarding this software xD

Thank you 

[Jia Wang]

Hello,

If you are designating a kinetic mineral without any initial mass in the system, you would need to specify a nucleus density value to provide an initial surface area for mineral growth. The built-in rate law for a kinetic mineral is rate = Surface area * rate constant * (1-Q/K). Since there is not any mass for the mineral to begin with, the nucleus density is used to prescribe a minimum value for a supersaturated mineral’s surface area over the course of the calculation. Once the surface area of the minerals exceeds that of the nucleus density, then the program will use the surface area of the actual mineral precipitated. In general, having the nucleus density will allow your kinetic mineral to precipitate but the quantity you set will affect your overall rate of reaction. If you have some idea of the proceeding dissolution or precipitation rate of your mineral, then you can experiment with this parameter. Also note that the GWB allows you to add a critical saturation index to be reached before the nucleus density becomes available for growth. For more information on kinetics of mineral dissolution and precipitation, please see section 4.2 in the GWB Reaction Modeling User Guide. You might be particularly interested in section 4.2.4 Nucleation. 

With regards to the rate constant, you can use the values selected from literature that are suitable for your system. The rate can vary over orders of magnitude depending on the conditions of your system and the reaction of interest. These values are typically derived from experiments. The software does not make any assumption regarding the rate constant and simply use the information the user provided to calculate the rate. By providing the pre-exp factor and activation energy like you did, the program solves the rate using the Arrhenius equation, allowing the rate constant to vary if you are conducting a polythermal reaction. Alternatively, you can provide a rate constant for the kinetic reaction from literature. This is up to the user and the process the model is trying to simulate. 

Hope this helps,
Jia Wang

[thais98]

Hello, thank you for the reply!

I was able to understand a bit more after your explanation , thank you very much !

[thais98]

Hello, sorry for sending more messages here 😅

I was trying to increased the moles of rock reacting to the fluid to see if the amount of calcite precipitanting would be higher, but apparently no. The maximum of SI that it reaches is 0. I'd like to know if there's something that is limiting the precipitation of calcite or if the software limits SI to 0 and how I can solve this problem. 

Thank you