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The obtained results are inconsistent for absorption and ion exchanges


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I was trying reconstruct the examples in Pages 150-154 of the book Geochemical and Biogeochemical Reaction Modelling (Second Edition).

These examples are about absorption and ion exchanges. The results I got are different from those in the book.

I have attached the SpecE and database files.

 

Could you please check and give me some help?

 

CaMgNa_Ix.sdat fresh water.sp8 Freundlich-SeO4--.sdat Kd—SeO4-.sdat Langmuir-SeO4--.sdat sea water.sp8 SeO4--Freundlich.sp8 SeO4--Kd.sp8 SeO4--Langmuir.sp8

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Hello Teng,

I noticed a few problems with your scripts.

I think you missed the pH entry for seawater speciation calculation. Please check your scripts to make sure that they match the examples. You can find the input files for the examples from the textbook in the GWB subfolder Script --> GBRM scripts.

With regards to Fig. 9.1, SpecE8 can calculate the sorbed concentration of selenate to the solid at one selenate fluid concentration at a time. You can enter in a different selenate concentration into the basis pane of SpecE8 and get the sorbed concentration after speciation. You can also calculate the sorbed fraction at various selenate concentration in the fluid by setting up a titration reaction path in React. You can set up the basis composition as you had done in SpecE8 and in the “Reactants” pane, add in a Simple aqueous species and react in 25 umol of selenate. You can then use Gtplot to plot selenate in concentration (umol/kg) vs. selenate sorbed (umol). Note that to get the same units on the y-axis as Fig. 9.1, you would have to divide the sorbed concentration by the mass of the sediment. In these examples, an inert volume used of 189 cm^3 and the density is 2.65 g/cm^3 so you can calculate the total mass of the sediment. I recommend copying and pasting the numerical results from the React simulation and perform the conversion. To do so, go to Edit -> Copy as -> Spreadsheet in Gtplot and then paste the data into Excel.

Also note that the examples of interest in the Geochemical and Biogeochemical Reaction Modeling text use thermo.tdat as the thermodynamic database. You might need to change your thermo dataset so that you are using the same one for your calculations.

Hope this helps,

Jia Wang

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Thanks very much for telling me that there are input files in the GBRM scripts. It is really helpful. 

However,  even I use the Ch09-SeO4 in the example file. The obtained >L: SeO4-- are 2.465e-7 molality, different from the book (0.49 * 10-3 umol/g, i.e., 4.9 * 10-7 molality)

Is it because of my understanding incorrect?

Best wishes

Teng Deng


Geimage.png.bbf9cc891fe90dba065b0627427fb050.png

 

 

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Hello Teng,

The value reported in chapter 9 is in units of umol of SeO4-- sorbed per gram of solids. You would need to take the moles of sorbed species and divide by the mass of solids in the system.

That would be: 2.465e-7 moles SeO4-- sorbed / 500 grams solid = 4.93e-10 moles/gram =  .493e-3 umoles/gram

The reported values in molality value of 2.465e-07 is moles of SeO4-- sorbed per kg of solvent.

Hope this helps,
Jia Wang

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