Geology & GeoMetallurgy

Geology & GeoMetallurgy 2017-04-04T06:58:01+00:00
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Hydrocerussite Element Concentration (9 replies)

Gruppen
2 years ago
Gruppen 2 years ago

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)

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

May I suggest that you re-post your question on phreeqcusers.org? That way I might be able to offer you a suggestion for your question, and others that use phreeqc might also see your question and contribute.

Helena Russell
2 years ago
Helena Russell 2 years ago

To answer your question directly, you have several ways to do it.

•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

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

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 the entire element contained within Hydrocerussite.

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

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

Also, if you 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.

Gruppen
2 years ago
Gruppen 2 years ago

Thanks for your comments.

I was already using suggestion 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 to1 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?

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

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 hydrocerussitein which case specify these other phases in the equilibrium phase’s list.

Gruppen
2 years ago
Gruppen 2 years ago

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

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

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).

Helena Russell
2 years ago
Helena Russell 2 years ago

As mentioned 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 conceptualization of what you want to do before writing your code.

If you mention why you need the Pb contraction 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 allowed other phases to dissolve/precipitate etc.

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

Victor Bergman
2 years ago
Victor Bergman 2 years ago

If you use the EQUILIBRIUM_PHASES block, and you equilibrate your solution with a solid, in this case hydrocerussite, without specifying what you want to modify to reach saturation with this mineral, then you will fix the total Pb concentration, but you will also modify the alkalinity of your water, given that the equilibrium reaction involves carbonate and protons.

If what you want to do is to see what would be the concentration of Pb if controlled by hydrocerussite solubility, without modifying the concentration of other groundwater components, then you can use the EQUILIBRIUM_PHASES block, indicating as equilibrium phase Hydrocerussite, indicating the target saturation index for your solid (0.0 in this case) and indicating also what reactant you want to modify to reach the saturation index (Pb in this case). The line will look as follows:

EQUILIBRIUM_PHASES 1

Hydrocerussite 0.0 Pb 10.0

And completely in agreement with previous comments, pay attention to the TDB you use and conceptualize which is the problem you want to assess.

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