Solution mixture speciation

[srikaush]

Hello,

 

I am trying to calculate the supersaturation with respect to magnetite of two solutions after they are mixed. The first solution contains only ferrous chloride while the second solution consists of sodium nitrite (that helps with oxidation of Fe(II) to Fe(III)), acetic acid and ammonium hydroxide. Now, when I feed my experimental conditions into the GSS worksheet (with measured pH of each solution) and mix them and then speciate the mixture, I do not get supersaturation values with respect to magnetite and the reason is attributed to the absence of Fe(III) in the initial solutions. I have also tried decoupling Fe(II) and Fe(III) but to no avail. Is this because the nitrite to nitric oxide reduction reaction in some way disregarded in this set up of conditions?

 

Another related issue- When I ignore the sodium nitrite in the second solution and instead assume all of it is used up to oxidize corresponding concentration of Fe(II) and I define the first solution as a mixture of Fe(II) and Fe(III), I get a supersaturation with respect to magnetite albeit with a charge imbalance. I would like to know if the charge imbalance makes the speciation and supersaturation numbers invalid.

 

Any help is appreciated! Thanks in advance.

 

Kaushik

[katezat]

Hello Kaushik,

 

Can you please attach your GSS file (.gss) so I can take a better look at your issue.

 

Thank you,

 

Katelyn

Aqueous Solutions LLC

[srikaush]

Hello Katelyn,

 

I have attached the gss files of the two cases I mentioned in my previous post. The first is the case I am essentially interested in. The second case is something that I tried that yielded supersaturation w.r.t. magnetite. Howevemr I don't feel the second case is accurate for definition of my system.

 

Thanks in advance for the help!

 

Kaushik

SolMix_Fe(II).gss

SolMix_Fe(III).gss

[srikaush]

Hello,

 

I am just reposting as I still need help on this. Any help is appreciated.

 

Thank you.

 

Kaushik

[katezat]

Hello Kaushik,

 

Thank you for sharing your GSS files and your patience. In general, in order for Magnetite to be considered in a calculation, a model must be set up so that Fe++ and Fe+++ are both allowed to form from your set of basis species. You could accomplish this with either an equilibrium model or a disequilibrium model. In the equilibrium model, you just add the basis species and a measure of oxidation state (such as DO, EH, pe, etc.) which would distribute mass among any coupled redox states. In the disequilibrium approach, you disable redox coupling reactions and thus constrain basis and redox species separately.

 

In SolMix_Fe(II) you’re only considering a single oxidation state of iron, hence, no Magnetite considered. In the other spreadsheet, Fe++ is decoupled from Fe+++ and both are included in your basis set, so Magnetite is a possibility. But with decoupled reactions, the only way to transfer mass from one oxidation state to another is with kinetics. I don’t think GSS is designed for what you’re trying to do. It seems like you’re trying to model a reaction process, so what you really need is a reaction path simulation like you could create with program React. I think you could set up an equilibrium or kinetic model that would work well for your purposes.

 

Another problem I noticed is that you’re limited somewhat by the thermo dataset that you’ve chosen. The Minteq dataset has very few redox coupling reactions. Fe++ is coupled to the basis species Fe+++, but there is no coupling reaction between different oxidation states of nitrogen. NO3-, NO2-, N2, NH4+, etc. all exist as separate Basis species. As a result, there’s no way that you can tie Fe++ oxidation to NO2- reduction. Even if you could, the dataset doesn’t have your desired reduced product nitric oxide. You can, of course, use a different dataset or modify the one you’re using.

 

It sounds like you could really benefit from using our reaction path modeling program React, included in our Standard package. Please visit http://gwb.com/requests.php to request a free 30 day demo or quote.

 

Kind regards,

 

Katelyn

Aqueous Solutions LLC

[srikaush]

Hello,

 

I have downloaded the specified demo of GWB standard and I have been trying to use the React module for simulating my experimental set up. However, I have not had luck with it. I am still confused as to how I can specify two separate fluids (with different aqueous species and pH @ room temperature) that come together at a particular temperature (different from room temperature) to react. Please let me know how I can do this. I would like to know the saturation index with respect to magnetite.

 

Thanks in advance!

 

Kaushik

[katezat]

Hello Kaushik,

 

In general, when you have a fluid analysis at room temperature but you’re interested in its chemistry at a different temperature, you set up a simple polythermal reaction path. To do so, open React and specify the composition of your sample. You’ll set the initial temperature to the value at which the fluid was analyzed, and the final temperature to your temperature of interest. Select Run > Go to trace the path. If you’d like, you can save the transformed fluid (the endpoint of your polythermal heating path) as a new fluid by going to Run > Pickup > System > Fluid, then saving your script.

 

You can use the pickup feature to simulate the mixing of two fluids. Define any fluid (we’ll call it fluid 1) on the Basis pane and run your model. Once complete, select Run > Pickup > Reactants > Fluid so that fluid 1 is now a reactant. When you do this, the Basis pane is vacated and you're free to specify the composition of your second fluid. Define your second fluid (you can read in a saved script if you’d like) and run your model to mix fluid 1 (the reactant) into fluid 2.

 

For more information on polythermal reaction paths, see section 3.4 of the GWB Reaction Modeling Guide. For the pickup feature, see section 3.9. Finally, for an interesting fluid mixing example (a “Flash diagram”), see section 3.7 or visit our fluid mixing webpage.

 

Kind regards,

 

Katelyn

Aqueous Solutions LLC