PhreeqcUsers Discussion Forum

Conceptual Models => Equilibrium assumptions => Topic started by: lourdsa on September 02, 2019, 03:36:00 PM

Title: Precipitation at Equilibrium
Post by: lourdsa on September 02, 2019, 03:36:00 PM
Hi,

I was hoping this can be clarified for me.

When I run the EQUILIBRIUM PHASES data block with 0 SI and 0 initial moles with respect to the following minerals:

USE SOLUTION 1
EQUILIBRIUM_PHASES 1
    Fe(OH)3(a) 0 0
    Goethite 0 0
    Hematite 0 0
    Hausmannite 0 0
    K-mica    0 0
    Manganite 0 0
    Pyrolusite 0 0
    Talc      0 0
    Chlorite(14A) 0 0
    Quartz 0 0
END

Only Hematite, K-Mica, Pyrolusite, Quartz and Talc are indicated as precipitates. However, for these minerals to precipitate, the model adjusts the redox from the initial value of -3.8 pe to 12.4 pe.

My questions are:
1)   Why does Hematite precipitate and not Fe(OH)3(a) or Goethite. I understood that Hematite precipitation is more favourable at high temperatures ?

2)   Since PHREEQc adjusts the redox to oxidising conditions, does this indicate precipitation of these minerals are unlikely to occur at existing conditions (i.e. at the initial conditions of -3.8 pe, 11 atm, 286 K)?

3)   Iron oxides (hematite and goethite) and iron hydroxides (Fe(OH)3(a)) are indicated to precipitate relatively instantaneously, but only after Fe (2) is oxidized to Fe (3). In a previous post you had mentioned that there is sufficient dissolved oxygen (0.03 mg/l) to drive the reaction and oxidise all the iron, however give the slightly reducing conditions measured (-3.8 pe) are there other conditions which would reduce the precipitate rate or stop the reaction?

4)   As you mentioned, an extreme case is that all of the iron would precipitate quickly, so the rate of Fe accumulation is equal to the rate of Fe inflow (flow rate times concentration). Should I assume that this would be total Fe concentration, not just Fe (3+) ?

5)   Do equilibrium phases take kinetics into consideration? I understand that equilibrium phases indicate quartz precipitation is 1.3E-04 moles at a redox of 12.4 pe, 286 K, 1 atm. This is equivalent to around 9,000 kg/year assuming a flow rate of 40 l/s. However, this seems unlikely, given the apparent slow precipitation rate and requirement for high temperatures. Would you agree?

Any help/advice on the script and how to progress this further will be greatly appreciated.

Thank you,
Kind regards,
LA

Title: Re: Precipitation at Equilibrium
Post by: John Mahoney on September 02, 2019, 06:35:08 PM
1)   Why does Hematite precipitate and not Fe(OH)3(a) or Goethite.  Hematite is the most stable Iron oxide phase in the database so in an Equilibrium_phases block it will select the most stable

2)   Since PHREEQc adjusts the redox to oxidising conditions, does this indicate precipitation of these minerals are unlikely to occur at existing conditions (i.e. at the initial conditions of -3.8 pe, 11 atm, 286 K)?  Probably but I would need to check it it using a Eh-pH diagram The small amount of oxygen is enough to drive the redox conditions to near the top of an Eh-pH diagram.   You do not appear to have any other reductants to halt this process but I would consider is the small amount of dissolve oxygen really there or could it be contamination when the DO was measured.  I never trust Very low DO readings.

3)   Iron oxides (hematite and goethite) and iron hydroxides (Fe(OH)3(a)) are indicated to precipitate relatively instantaneously, but only after Fe (2) is oxidized to Fe (3). In a previous post you had mentioned that there is sufficient dissolved oxygen (0.03 mg/l) to drive the reaction and oxidise all the iron, however give the slightly reducing conditions measured (-3.8 pe) are there other conditions which would reduce the precipitate rate or stop the reaction?  (SEE ABOVE DISCUSSION)

4)   As you mentioned, an extreme case is that all of the iron would precipitate quickly, so the rate of Fe accumulation is equal to the rate of Fe inflow (flow rate times concentration). Should I assume that this would be total Fe concentration, not just Fe (3+) ?  Difficult to say This gets into a Dahmkohler number issue.  where reaction rates and flow rates (and residence times) need to be considered altogether).Try running the flow model at different flow rates.

5)   Do equilibrium phases take kinetics into consideration? I understand that equilibrium phases indicate quartz precipitation is 1.3E-04 moles at a redox of 12.4 pe, 286 K, 1 atm. This is equivalent to around 9,000 kg/year assuming a flow rate of 40 l/s. However, this seems unlikely, given the apparent slow precipitation rate and requirement for high temperatures. Would you agree? EQP block are instantaneous so they do not consider kinetics.  you would need to use just the Kinetics/rates approach.