[Question about Eh-pH diagram]

Brian:

 

Thanks a lot!

 

Yiran

[Question about Eh-pH diagram]

Brian:

 

I just found H+ can also pair with e- as the pair for Eh or pe. However, when choosing it, it was swapped out of the pH basis and pH cannot be set. A little confused.

 

Thanks!

 

Yiran

[Question about Eh-pH diagram]

Hi, Brian:

 

Thanks a lot. For the last message, what I meat was the state and the forms of iron in my reaction system. For example, lepidocrocite (a kind of FeOOH) was used as the electron acceptor, hydrogen was used as the electron donor. When the reaction occurs, I don't feel the Fe(III) and Fe(II) ratio can be simply expressed as Fe+++/Fe++ because the majority of Fe(III) stay in the solid phase. However, when using diagram species for lepidocrocite <--> Fe++, I could only define activity as 1 but could not input Fe(II) concentrations (e.g., Fe++=10 mM). Which forms for these iron species I should set to reflect the conditions in my reaction system?

 

Thanks!

 

Yiran

[Question about Eh-pH diagram]

Brian:

 

Thank you so much. Your suggestion is very helpful. Just to follow up, you mentioned some alternative species "such as O2(g), the e-, H2(aq), H2(g), or even activity ratios, such as Fe+++/Fe++". So far, I only found choice of Eh or pe under O2 pair (maybe I miss something else. I am wondering if there is another redox pair better fitting anaerobic condition. Is e- available for pairing with H2? Also, for Fe+++/Fe++, can we do this for pairing with ferric solid mineral with Fe++ as mineral is way less soluble than Fe+++ itself?

 

Thanks a lot!

 

Yiran

[Question about Eh-pH diagram]

Dear GWB：

 

I have a question about making Eh-pH diagram to predict iron secondary minerals formed during iron reduction under anaerobic condition. My questions include：

 

1） When we choose the pair for Eh, is oxygen-e^- the only choice? I could find other selections for Eh, but oxygen does not seem correct for anaerobic reaction, which typically exhibit negative Eh values.

 

2) For some of the solid phases, the ones showed up in the Eh-pH diagram were not observed in our study, while some of our observed ones did not appear. Is there any way I can check, modify or add some mineral information? Is solubility the only information needed for such analyses?

 

Any suggestion is highly appreciated!

 

Yiran

[Add new mineral for calculating saturation index of secondary mineral]

Hi Yiran,

 

Can you clarify what program you're using and what type of errors you encountered?

 

If you set your temperature equal to one of the principal temperature for which you have data, 25 C for example, then the programs will use those log Ks directly. If you set temperature to some other value, though, the program will interpolate (but not extrapolate) between principal temperatures. If the program can't interpolate to find a value for a particular reaction, then it won't load that reaction. So if you have data at 25 C and 60 C and set your system to 37 C, the program will interpolate between the two known values. Because you don't have a value at 60 C, though, the program has to ignore your mineral. One solution is to use the "extrapolate" option (from the Config - Alter log K dialog) to consider all reactions at all temperatures. This can be a little dangerous, though. A potentially better strategy would be to assume that the log K for your green rust doesn't change much over a small temperature range and simply set the log K at 60 C equal to the value you set for 25 C. This way, the program will load that mineral at any temperature between 25 C and 60 C, using the same value for the log K, but will not consider it outside that range. For more information, please see 6.33, extrapolate, in the GWB Reference Manual.

 

Hope this helps,

Brian

Thanks a lot, Brian. I will check it. The second solution sounds reasonable to me. I used the 25 degree temperature logK for green rust, but it did not show on the secondary minerals after I calculate the saturation index. I think what you mentioned may explain it.

 

Will try again and thanks a lot!

 

Yiran

[Add new mineral for calculating saturation index of secondary mineral]

Hi Yiran,

 

Unfortunately I don't know which value is closer to reality. It's common for some variability to exist in thermodynamic data like this. When you get around to the free energy calculations, try to compare a few different sources to see how much variability there is.

 

FYI, if you're interested in the effect of a particular mineral's stability on your calculation results, you can use the "alter log K" function to temporarily adjust the value that the GWB programs use. Just go to Config -> Alter log K to access the dialog.

 

Best,

Brian

Brian:

 

Thanks a lot for your suggestion. I will search for more thermodynamic information. For the logK you suggested, I have another question for it. I was trying to set the logK for one of the new minerals (carbonate green rust), since I could not find H and S for it, I was not able to use Van't Hoff equation to calculate logK at other temperatures. So, I input the value at 25 degree and left others 500. When I was calculating for the reactions at 37 degrees, the system reported errors and did not give any result. Is that mean the setting temperature have to be the same as the reaction temperature?

 

Thank you again!

 

Yiran

[Add new mineral for calculating saturation index of secondary mineral]

Hi Yiran, glad that helped.

 

Best,

Brian

Hi, Brian:

 

I have a follow-up question about calculating the equilibrium constant for the custom minerals. I did some calculations of lepidocrocite based on the Van't Hoff equation, the thermodynamic values I found for this mineral and other components for the dissolution reaction FeOOH+3H+--->Fe+++ +2H2O. The calculated Keq seems very small (logKeq=-0.00046 at 25 degree C), much lower than the value reported for dissolution constant (logKsp=log([Fe+++]/[H+]^3)) reported in literature (logKsp=1.39 reported in Schwertmann, Solubility and dissolution of iron oxides, Plant and Soil, 130, 1-25). For this reaction, my understanding about the Keq and Ksp is that they are the same. I am curious which values I should use. Because for another mineral I was planning to add, there is no solubility information but free energy, so I have to use the energy method to calculate equilibrium constant and am concerned about the accuracy.

 

Please advise and thanks a lot!

 

Yiran

[Add new mineral for calculating saturation index of secondary mineral]

Thank you, Brian. It is very helpful and your answer is applicable to my another question.

 

Yiran

[Add new mineral for calculating saturation index]

Dear all:

 

I am trying to calculate secondary mineral formation in the microbial iron reducing systems. One of the minerals we are interested is green rust with formula [Fe²⁺₄Fe³⁺₂(OH)₁₂][CO₃]·3H₂O, which is not in the mineral list for GWB. The GWB has Fe(OH)2 in the system, which is also another kind of green rust, but is different from what we observed. Does anyone has suggestion how to create it ([Fe²⁺₄Fe³⁺₂(OH)₁₂][CO₃]·3H₂O) in the system so that it can be calculated?

 

Millions of thanks!

 

Yiran

[Add new mineral for calculating saturation index of secondary mineral]

Hi, Brian:

 

I want to calculate saturation index for the secondary minerals formed from reduction of lepidocrocite (gamma-FeOOH). I found the logK information for different minerals in the thermo file, but lepidocrocite is not there and the logK values were given to different temperatures (e.g., 0-300 degree C) in the system. Now, I found solubility for lepidocrocite at room temperature in another paper, is there any way I can integrate such information into the system and fill the logK values for other temperatures?

 

Please advice. Thanks a lot!

 

Yiran