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Cyanide Decomposition


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CN_Average_Barren.reaHi Tom or Brian,

I am trying to model cyanide decomposition using equilibrium approach with adding simple reactants. The process is described here


Minteq database seems have species I need including metal-CN ones (I cannot attach it due to your uplaoda limits). However, species like CN-, OCN-, and HN4+ are more like components in the database or using “redox” terminology of GWB decupled. Therefore reaction 6.32 and 6.38 does not work . Please, could you propose easy way to achieve the objective without getting into kinetics and catalysis?

May be you advise how fix database to make it work.

Thanks a lot!



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Hi Nikolay,


The Minteq dataset has a number of cyanide and cyanate species, but the problem is that CN- and OCN- are both listed as basis species, as are CO3-- and NH4+. As a result, there is no way for the species to achieve equilibrium with each other. What you want is for these species to be coupled in the thermo dataset. If you look at thermo.com.V8.R6+.dat, for example, you'll see that CN- is listed as a redox couple (formed from HCO3- and NH4+). I think you'll want to modify the Minteq dataset to include a coupling reaction like that, plus a second coupling reaction for OCN-.


If you were to modify the thermo dataset in this way, you'd see that decoupling the relevant redox pairs in the GWB programs would be just like using the Minteq dataset as it is now. If you wanted, you could use kinetic rate laws to describe the rate of redox transformations. But since you just want an equilibrium model for now, you would leave them coupled and use the simple reactant approach.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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You're welcome. Just to be clear, you'd need to modify the database the same way to use kinetics.


It actually wouldn't be too difficult to modify the thermo dataset. You shouldn't need to touch any of the aqueous species' reactions. All you'd need to do is replace two basis species (CN-, OCN-) with two identically named redox species. For each redox species, you need a reaction and a log K. You could use Rxn with thermo.com.V8.R6+.tdat loaded to balance a reaction between CN-, O2(aq), CO3--, and NH4+, and to find the log K at your temperatures of interest. You may need to search the literature for a reaction and log K between OCN- and CN-, or perhaps between OCN-, NH4+, and CO3--.


It's always a good idea to rename a dataset when you modify it in some way. If you need the original, though, you can always download it from our website. Once you do modify it to include the coupling reactions, you can just decouple the reactions in SpecE8 (so that you can add CN-, OCN-, CO3--, and NH4+ to the basis pane) and compare your results with the same model using the Minteq dataset. Since CN- and OCN- are basically "permanently decoupled" in thermo_minteq.tdat, you should get the same results.




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We have a new thermo data editor called TEdit in GWB10. It makes adding new species much easier than by editing a text file, and you can easily transfer entries from one thermo dataset to another. There's a new section in the GWB Essentials Guide describing its use, and we also have a section of tutorials on our website for editing thermo data. If you're using a text editor with an older version of GWB, it's a good idea to copy existing entries to use as templates and to make sure you're following the conventions described in the Thermo Datasets Appendix to the GWB Reference Manual.

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