Calculate amount of dissolved Pb in equilibrium with Hydrocerussite

[singh118]

How to specify in PHREEQC that an element concentration is controlled by a mineral E.g. - Instead of specifying Pb concentration, specify that Pb is controlled by Hydrocerussite).

Please see the attachment of my input in PHREEQC and see if it makes sense.

[Charlie]

Hi Vijay,
To answer your question directly, you have several ways to do it.
1/ You know that your solution is at equilibrium with Hydrocerussite (provided that it is in the database you use) you will specify
SOLUTION 1
Pb 1 Hydrocerussite

1 is a dummy number in the sense that it is not used, PHREEQC will calculate the value of Pb at equilibrium with Hydrocerussite.

2/ use equilibrium phase
EQUILIBRIUM_PHASES 1
Hydrocerussite 0 10
END

In this case you will dissolve Hydrocerussite up to equilibrium or up to the 10 mol that you just wrote.
The main difference as you see is that in this system you will add all the element contained within Hydrocerussite.

Cheers!
Julien

[Charlie]

Hi Vijay,
These are good suggestions from Julien.

The first suggestion can be used to assess the solubility of hydrocerussite and Pb (for different solutions and conditions i.e. pH)

Using the second suggestion, include:

SELECTED_OUTPUT
-file example.xls
-equilibrium_phases Hydrocerussite
-totals Pb

The selected output file will now contain total moles hydrocerussite remaining, and moles of hydrocerussite dissolved, so you can see how much hydrocerussite was used in the reaction. It will also report the total molality of aqueous Pb species.

[Charlie]

THREAD ORIGINALLY FROM LINKEDIN GROUP DISCUSSION..

Hello Julien and Charles,

Thanks for your comments.

I was already using suggestion 2 mentioned by Julien:
2/ use equilibrium phase
EQUILIBRIUM_PHASES 1
Hydrocerussite 0 10
END

But someone at our lab has mentioned that along with this there is also a need to do the following: (This looks very similar to Julien's #1 suggestion)
Instead of indicating a Pb concentration, specify (in the same table where you would input concentration) that the Pb is controlled by hydrocerrusite.

Do I really need to enter both suggestions in PHREEQC??


Hi Vijay,
I think you don't need lines other than the equilibrium phases block as you have written above - unless there are other sources of Pb besides the hydrocerussite..in which case specify these other phases in the equilibrium phases list. Cheers, Charlie



If I am looking for dissolved Pb concentrations in equilibrium with Hydrocerrusite, in PHREEQC - Is it a good assumption to only consider Pb(2+) molality or Total Pb should be taken into consideration?? I am thinking only Pb(2+) as per the reaction equation of hydrocerrusite shown below.

(Pb3(OH)2(CO3)2) + 2H+ = 3Pb+2 + 2CO3-2 + 2H2O


It depends on the objective really - in general if you are assessing the predicted concentrations against a regulatory water quality standard then you would use total rather than a specific species. If you are making a calculation regarding toxicity risks then you might need to consider speciation. See page 126, Zhu and Anderson (2002) if you have a copy, for an introduction about bioavailability and toxicity (Pb and As).
Delete 4 hours ago
 Julien Declercq


Hi Vijay,
As mentioned by Charles above, both approaches will give you a concentration of Pb in equilibrium with Hydrocerrusite.
The thing is PHREEQC is just calculator, you do need to have a good conceptualisation of what you want to do before writting your code.

If you mention why you need the Pb contration we might be able to help you better.
If for instance you are trying to calculate a solution to precipitate Hydrocerussite in the lab, simply dissolve it to equilibrium (Solution 1, temp and composition you want, Equilibrium 1 cerussite, equilibrium with atmospheric CO2, hydrocerussite 0 10)
It will also tell you if other Pb phases will be at equilibrium and might precipitate before the phases you want.

If you are looking at Pb contamination from Pb bearing minerals, then it might be worth allow other phases to dissolve/precipitate etc...

And remember that PHREEQC values are only indicative, the degree of validity of the results depend on the database you use and the associated errors.

Cheers!