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Adsorption coefficient


Penguin623
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Hi,

 

You can certainly implement adsorption in your reactive transport models using X1t/X2t. Currently available sorption models include the distribution coefficient approach (Kd), Freundlich and Langmuir isotherms, ion-exchange, and the Double Layer surface complexation model (with constant capacitance and non-electrostatic options). See chapters 9, 10, and 32 in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text for descriptions of the various sorption models and their implementation in speciation, reaction, and reactive transport models. The reference manuals which come with the GWB software will also come in handy. http://www.rockware.com/product/documentation.php?id=132

 

For future reference, this is the archive of old posts which have been moved from the main page. When starting a new thread, you should post on the main page to ensure a prompt response.

 

Hope to help,

 

Brian Farrell

Aqueous Solutions LLC

Makers of The Geochemist's Workbench

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Hi Brian,

 

Thank you very much for your reply. I really appreciate it. I can include Kd value I need in .dat file and add that file through File>>sorbing surface. But I did not find the Inert varialbe from Config>>Variables menu to set the sorbing mineral mass. If I do not include any minerals in the Initial, do I have to set the mass when using X1t/X2t?

 

Thank you very much for your great help!

 

 

Hi,

 

You can certainly implement adsorption in your reactive transport models using X1t/X2t. Currently available sorption models include the distribution coefficient approach (Kd), Freundlich and Langmuir isotherms, ion-exchange, and the Double Layer surface complexation model (with constant capacitance and non-electrostatic options). See chapters 9, 10, and 32 in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text for descriptions of the various sorption models and their implementation in speciation, reaction, and reactive transport models. The reference manuals which come with the GWB software will also come in handy. http://www.rockware.com/product/documentation.php?id=132

 

For future reference, this is the archive of old posts which have been moved from the main page. When starting a new thread, you should post on the main page to ensure a prompt response.

 

Hope to help,

 

Brian Farrell

Aqueous Solutions LLC

Makers of The Geochemist's Workbench

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The "inert" command is used in SpecE8 and React to specify unreactive mineral volume. Using the standard rock density of 2.65 g/cm3, SpecE8 and React will calculate the sorbed concentration using the total mass of equilibrium, kinetic, and inert minerals. Thus it is important to specify carefully the mass of minerals in the system. In X1t and X2t, the inert command is not used. Instead, the volume of rock is calculated in one of two ways:

 

1) If you do not set the porosity explicitly the program figures the porosity at each node from the volumes of fluid and “rock”, the latter being the sum of the volumes of the minerals in the nodal block. The minerals include both the equilibrium minerals swapped into the basis and the reactant minerals – those reacting according to kinetic rate laws and any set as simple

reactants.

 

2) For simulations in which you set an explicit value for porosity, the program when setting the initial conditions figures the difference at each node between the rock volume corresponding to the porosity value and the volume of minerals in the block. This volume is then carried through the simulation as “inert” or non-reactive volume.

 

So X1t and X2t will calculate sorbed concentrations like in SpecE8 and React (sum of equilibrium, kinetic, and inert mineral volume), but the inert volume is calculated rather than being specified. If you set porosity to 30%, for example, and do not add any minerals, then 70% of the volume is rock, and using density = 2.65 g/cm3 the sorbed concentration can be determined. If porosity is set to 30% and Quartz is set to 5%, then the inert volume is 65% and Quartz volume is 5%, leading to a total volume of 70% which goes into the calculation.

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Hi Brian,

 

Thank you very much for your reply. I really appreciate it. I have run one simulation on X2t for 5 days now and it is still calculating at the present. I am wondering usually how long is one simulation of X2t. And what affect the number of steps? It shows Step "9633000" now. Is that determined by Domain? I set the Domain as: X (3km), Y (3km), Z (1km) and both nx, ny (50). What unit is nx and ny? I put in the similar data and domain (but no Z) for X1t, it ran for only 5 min. How do I shorten the calculation time?

 

Thank you very much! Looking forward to your reply.

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Hi,

 

Simulation times in X2t can vary widely, ranging form seconds or minutes to hours or days, depending on the complexity. The spatial discretization, degree of heterogeneity of the domain (physical or chemical), the amount of species loaded and reactions to consider, surface reactions with electrostatic corrections, kinetic reactions, etc. will all have a big impact on your run times.

 

Nx and Ny are the number of nodal blocks in the x and y directions, used in the finite difference approximation to the transport equation. You'll either set Nx and overall domain length, or Nx and deltax, the size of an individual block, to define your domain.

 

When starting a new model, begin with the simplest possible setup. Only load what you think are the most important species, use a coarse grid, etc. When a run is over, you can look at the Run pane to see how much time was spent solving the transport vs. reaction equations.

 

Could you let me know which version of GWB you are running, and attach scripts for the two runs you were talking about? I'll try to take a look and see if I can figure anything out.

 

Hope to help,

Brian

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